Brian Douglas Jackson. A thesis submitted in conforrnity with ... Knsten Coupland, Jianhao Yang, David Moore, and Sola Ness. They helped in a variety of ways.
Pulsed-Laser Deposition of Silicon Dioxide Thin-Films Using the Molecular Flourine Laser
Brian Douglas Jackson
A thesis submitted in conforrnity with the requirements for the degree of Master of Applied Science Graduate Department of Electrical and Computer Engineering University of Toronto
O Copyright by Brian Douglas Jackson 1997
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Thin-Films Using ;he Molecular Fluorine Laser Brian Douglas Jackson Master of Applied Science Degree, 1997 The Department of Electrical and Computer Engineering The University of Toronto
Abstract The short-wavelength extension of pulsed-laser deposition (PLD)to the 157-nm
F2laser may enable low-temperature growth of silica films for electronics and photonics applications. This thesis examines the effects of laser fluence, background gas, and substrate temperature on the properties of SiOz films grown for the first time using the F2-laser. The deposited films were characterized by atomic force microscopy, x-ray photoelectron spectroscopy and Fourier-transform infrared spectroscopy. The strong absorption of 157-nm radiation in fused silica enabled the growth of virtually particulate-free Si02 films by F2-PLD, in contrast to results with longer wavelength lasers. Stoichiometric films were produced in ambient oxygen (4x10'' Ton), which compensated for oxygen loss due to Siondissociation by the 7.9-eV laser photons. Akhough the process parameters were not fully optimized, F2-PLD produced films with comparable properties to the best-previous PLD-grown SiO2 films, but inferior to films deposited by VUV-assisted CVD.
1 would first like to thank my thesis supervisor, Professor Peter Herman, for his
ongoing advice during the coarse of this project. Additionally, 1 would like to recognize the financial contributions of NSERC and the Ontario Laser and Lightwave Research Centre. Several individuals provided assistance through the use of diagnostics instruments: Professor Stefan Zukotynski, of the Department of Electrical and Cornputer Engineering, allowed me the use of his FTIR spectrometer and mechanical profilorneter; Dr. Rana Sohdi, of the Surface Science Lab in the Centre for Biornaterials, provided ongoing advice regarding the use of the x-ray photoelectron spectrometer in his lab; and finally, Prof. Cynthia Goh, of the Department of Chemistry, provided the use of her atomic force microscope. 1 would also like to thank my fellow graduate students in the lab: Keith Beckley, Knsten Coupland, Jianhao Yang, David Moore, and Sola Ness. They helped in a variety of ways. Additionally, a high school CO-opstudent under my supervision, Tony Yoo, was a great help in designing and building Our deposition system. Finally, 1 would like to thank my fiancée, Trish, for her ongoing patience and encouragement in the face of adversity.
Abstract ...........................................................................................................................
**
ii
... .. List of TabIes ................................................................................................................... vil ... List of Figures .................................................................................................................. viii Acknowledgements ............................................................................................................
iii
1. Introduction ............................................................................................................. 1
1.1
Project Motivation ............................................................................................. 1
1.2 Thesis Outline ..................................................................................................2
.
2
Background Information .............................................................................................. 4 2.1
2.2
Pulsed-Laser Deposition ....................................................................................4 2.1.1
Basic Mechanisms .................................................................................. 4
2.1.2
Advantages and Disadvantages of PLD .................................................6
2.1.3
Applications of PLD.............................................................................. 8
Laser Ablation .................................................................................................. 10 2.2.1
Overview ..............................................................................................10
2.2.2 Photothermal Ablation ......................................................................... 10
2.3
2.2.3
Photochernical Ablation .......................................................................13
2.2.4
Particul@e Generation During Laser Ablation ..................................... 15
2.2.5
Minimization of Particulate Generation ............................................... 17
Silicon Dioxide Thin-Films..............................................................................18 2.3.1
Applications .........................................................................................18
2.3.2 Growth Techniques .............................................................................. 20
2.4
Pulsed-Laser Deposition of Silicon Dioxide ....................................................21 2.4.1 Motivation for PLD of Silicon Dioxide ...............................................21 2.4.2 Past Results of SiOz Growth by PLD ................................................... 22 2.4.3 Vacuum-Ultraviolet F2-Laser Ablation of Silicon Dioxide .................25
3. Experimental Procedures ........................................................................................... 30
3.1
Overview .......................................................................................................... 30
3.2
Film Deposition ................................................................................................ 31 3.2.1 The Molecular Fluorine Laser ..............................................................31 3.2.1.1 Basic Principles of Operation ................................................ 31 3.2.1.2 Ruorine Operation ................................................................. 33 3.2.1.3 Argon Fluoride Operation...................................................... 34
3.2.2 Initial Experimental Configuration ......................................................34 3.2.3 Follow-up Experimental Configuration ...............................................37 3.2.3.1 F2-PLDDeposition Chamber Design ..................................... 37 3.2.3.2 Experimental Procedure......................................................... 39 3.3
Film Characterization ....................................................................................... 41 3.3.1 Surface Roughness .............................................................................4 1 3.3.1 .1 Atornic Force Microscopy ................................................... 4 2 3.3.2 Chernical Composition and Structure ................................................ 4 2 3.3.3.1 X-ray Photoelectron Spectroscopy...................................... 4 2 3.3.3.2 .Fourier-Transfomi Infrared Spectroscopy .............................47 3.3.3 Film Thickness ..................................................................................... 5 0 3.3.3.1 Mechanical Profilometry ....................................................... 5 0 3.3.3.2 Fourier-Transform Infrared Spectroscopy ............................. 5 0 3.3.3.3 X-ray Photoelectron Spectroscopy......................................51 3.3.4 Film Density ......................................................................................... 51
4. ExperimentalResults ................................................................................................
52
4.1
Overview .......................................................................................................... 52
4.2
Surface Roughness ...........................................................................................
53
4.4
4.5
Chernical Structure .......................................................................................... 62 4.4.1
Wide-Range FïIR Transmission Spectra (450-5200 cm-') ................. 62
4.4.2
Si-O-Si Asymmetric Stretching Mode Peak Spectra ........................... 65
Film Thickness, Deposition Rate. and Density ................................................ 69
5 . Discussion .................................................................................................................. 72 5.1
Overview .......................................................................................................... 72
5.2
Deposition Rate ................................................................................................
5.3
Particulate Reduction in F2-PLD versus ArF-PLD ..........................................75
5.4
Chernical Composition and Structure .............................................................. 78
72
5.4.1
Correlation of XPS and FTIR Data ......................................................78
5.4.2
5.4.3
.................................. 80 Oxygen-Deficiency of SiO, Films Deposited in Vacuum or Argon .... 82
5.4.4
Surface Oxygen-Deficiency of Films Deposited in Dry Air or O2.......84
5.4.5
Effect of Deposition Parameters on IR ASM Peak Parameters ...........86
Carbon Contamination of the Deposited Films
89 6 . Scientific and Commercial Significance ....................................................................
6.1
Overview .......................................................................................................... 89
6.2
Cornparison With Previous Results ................................................................. 89
6.3
Scientific Importance .......................................................................................
6.4
Cormnercialization of F2-PLD of SiOz...........................................................94
7 . Conclusions............................................................................................................
92
96
8. References................................................................................................................100
Appendix A - F2-PLD Charnber Optical System ......................................................108
2- 1 Controllable Parameters in PLD ................................................................................. 6 2-2 Previous Examples of SiO2Thin-Film Growth by PLD .......................................... 23 3- 1 Fluorine Laser Operating Characteristics ................................................................. 33
3-2 Argon Fluoride Laser Operating Characteristics ...................................................... 34 3-3 Initial Experimental Parameters ............................................................................... 37
3-4 Follow-up Experimental Parameters ........................................................................ 41 3-5 XPS Peak Energies and Sensitivity Factors.............................................................. 43 3-6 Significant Infrared Features in Si02Films .............................................................. 48
4-1 Deposition Conditions for Analyzed Samples.......................................................... 52 4-2 EIemental Ratios From 0"and 60"XPS Scans ......................................................... 57
4-3 XPS O 1s and Si2p Peak Widths and Separations..................................................... 61 4-4 Relative % Absorption of FTIR Spectral Features in PLD Si02 Films ....................64
4-5 Si-O-Si ASM Peak Parameters of PLD Sioz Films ................................................. 68 4-6 PLD SiOs Film Thickness and Density ....................................................................70 5- 1 157-nm and 193-nm Radiation Interaction with Fused Silica ..................................76
5-2 Summary of XPS and FTIR Data for F2-and ArF-PLD of Si02..............................79
5-3 Range of Possible Oxygen-Deficient Surface Layer Parameters ..............................85 6-1 Cornparison of Si-O-Si ASM Peak Parameters With Previous Results ................... 89 A-1 Effect of Lens Birefringence on the Optical Imaging System ................................ 109
vii
Pulsed-Laser Deposition Geometry ..........................................................................4 Ablation-Rate Dependence on Laser Fluence for a Typical Insulator ..................... 12 F2- and ArF-Laser-Ablated UV-Grade Fused Silica ............................................... 26 Optical Absorption Spectmm of UV-Grade Fused Silica .......................................26
F2-and ArF-Laser Ablation Rates in UV-Grade Fused Silica ................................ 27 Schematic of the Initial Deposition Chamber ......................................................... 35 Schematic of the Very-High Vacuum F2-PLDDeposition Chamber ...................... 38 Typical XPS Survey Spectrum of a Carbon-Contaminated SiO. Film ...................44 Angle-Resolved XPS of a Carbon-Contarninated Silica Film ................................45 Sample Si-O-Si ASM Peak Spectrum in an SiOz Thin-Film ..................................49
AFM Image of an ArF-PLD Sioz Film ................................................................... 54 AFM Image of an F2-PLD SiOa Film ..................................................................... 54 0" and 60" XPS Survey Scans of Si02 Film Sample A............................................55 O" XPS Survey Scans of Si02 Film Samples F. G. and H .......................................56 0 1 s Photoelectron Peaks for F2-PLD SiOî Film Sarnples K and L ........................ 60 Si2p Photoelectron Peaks for F2-PLD SiOz Film Samples K and L .......................60 450-5200 cm-' IR Transmission Spectra of PLD Si02 Samples F and N ...............63 700- 1300 cm-' Nomalized Transmission Spectra for Sample L ............................66 900-1300 cm4 Transmission Spectra for Sarnples H.K. L. and M .......................67 Deposition Rates for F2-PLD of SiOz...................................................................... 73
viii
1.1 Project Motivation Pulsed-laser deposition (PLD)is a highly flexible thin-film growth technique which has been successfully applied to a wide range of materials [l]. The energetic nature of the depositing species [2] enhances the growth process [3], potentially enabling the deposition of high quality films on low-temperature substrates.
Additionally,
sequential ablation of multiple target materials allows accurate control of the film stoichiometry, enabling the growth of heterostructures and the deposition of films with well-defined doping profiles. These two characteristics of PLD motivate the application of this technique to the growth of SiO2 thin-films.
In particular, low-temperature
(c450°C) growth of high quality SiOÎ is required for applications in the semiconductor electronics industry [4]. Likewise, the fabrication of planar optical waveguides and active gain media for future optical integrated circuits will be enabled by the growth of high quality silica films with independently controlled concentration profiles of two, or more, dopant ions. Past research efforts [5,6,7,8] have exarnined the application of a variety of conventional lasers to the deposition of silica films, with only iimited success. In particular, the best previous SiOl films grown by PLD [5], obtained by 193-nm ArF-laser ablation of silicon or silicon monoxide targets in an
0 2
background, were of significantly
lower quality than films produced by VUV-lamp-assisted chernical vapour deposition [9].
produced films contaminated by 50-100 nm particulates [ 5 ] . The observed generation of particulates dunng ArF-laser ablation of fused silica onginates with the weak optical absorption of the 6.3-eV photons by silicon dioxide, which has a bandgap of
- 9-eV.
In contrast, the 7.9-eV photons of the F2-laser are
strongly absorbed by permanent defects within the bandgap of fused silica and transient defects generated during laser-irradiation of the silica surface. Previous work in this lab
has shown [IO] that this strong radiation-matenal interaction causes F2-laser ablation of fused silica to produce smooth etch patterns, with significantly less cracking and debris generation than is observed for ablation at 193-nm.
The significant reduction in debris-generation observed in F2-laser ablation of fused silica, relative to ArF-laser ablation, motivates the extension of pulsed-laser deposition to the 157-nm wavelength to enable the deposition of particulate-free silica films from a bulk silica target. Furtherrnore, the 7.9-eV photons of the F2-laser are expected to interact strongly with the laser-generated vapour plume, creating the highly excited species needed for low-temperature growth of high quaIity silica films.
1.2 Thesis Outline This thesis describes the application of the F2-laser to PLD of Si02 films: Chapter 2 presents a review of pulsed-laser deposition, laser ablation, the applications of SiOz thin-films, existing Si02 thin-film growth techniques, and past results of PLD
a 1 1 u MSC;I
~ I U I ~ L I W IUI I ~ U ~ C S IU ~ L C ; ~ . 1111s11iait;n5~ G A ~ I I U SU ~ U I ILILE;~ L U J C ; L ; LI w m v a i i u I l
presented above, and provides a background to later discussions of the observed results. The experimental setup and procedures for F2-PLD of Si02 films are described in Chapter 3, as are the diagnostic techniques used to characterize the deposited films. Chapter 4 presents the measured properties of F2-PLD Si02 films for a range of deposition pararneters. The particular film properties which have been measured are surface roughness, chernical composition and structure, thickness, and density. The observed correlation between the deposition pararneters and the resulting film properties are discussed in Chapter 5.
In particular, the observed reduction in
particulate contamination in F2-PLD is explained, as is the observed improvement in film quality resulting from increases in the oxygen background pressure, reductions in the laser fluence, and mcreases in the substrate temperature. Chapter 6 compares the F2-PLD results with previously published silica film properties, demonstrating that the results presented here are comparable to the best previous results of SiOa film-growth by PLD. Additionally, the scientific and commercial significance of this work is discussed in light of the observed results. Finally, Chapter 7 summarizes the key points of this thesis and discusses the wide
range of potential future work leading from this project.
2.1 Pulsed-Laser Deposition 2.1.1 Basic Mechanisms In its simplest f o m , pulsed-laser deposition (PLD) c m be described in terms of three stages: Iaser ablation, vapour-plume transport, and film growth. Figure 2-1, below, presents the basic geometry of the PLD process.
In the laser ablation process, high-power laser pulses are focused ont0 the surface of a bulk target, vaporizing a thin (- 10-1000 nm) layer of material in 10-50 ns. As the
1-
Substrate
/"
Film
-
Figure 2-1 Pulsed-Laser Deposition Geometry A focused laser pulse vaporizes the surface of a bulk target, creating a plume which explosively expands towards the deposition substrate. The rapid plume expansion compresses the background gas, creating a shock-wave which slows the plume expansion, reducing the energy of the depositing species.
highly energetic atoms, ions, electrons, and molecules. The ablation process, and its dependence upon material and laser properties, are described in more detail in Section 2.2. Once formed, the initially-dense vapour plume expands into the surrounding background gas according to the laws of hydrodynamics. This expansion produces a highly forward-peaked velocity distribution, with typical on-axis kinetic energies of 10-400 eV per atom or ion [Il]. As the plume expands, it compresses the background gas, creating a shock-wave which slows the plume expansion [12,13,14].
Depending
upon the target-to-substrate distance and the background gas pressure, the expanding vapour reaches the substrate with typical energies of 0.1-100 eV per atom [2], depositing sub-monolayer thicknesses of material per pulse. The kinetic energy of the condensing vapour is transferred to the target surface, supplying thermal and kinetic energy to the deposition process. This energy encourages the formation of smooth, high-quality films by enhancing the mobility of atoms on the surface of the film and discouraging the growth of islands on the film surface [3]. Each stage of the pulsed-Iaser deposition process is a highly complex process which is critically dependent upon a number of controllable parameters, the rnost significant of which are outlined in Table 2-1, on the following page. As a whole, the pulsed-laser deposition process is highly tunable, allowing high quality deposition of a wide range of materials.
-
'l'able 2-1 Controllable Yarameters in YLU Stage
Controllable Parameter
Laser Ablation
Laser (wavelength, pulse-length, pulseenergy, fluence (energyhea), and repetition rate) Target (composition, density, and rotation) Dual-Bearn Ablation
Plume Expansion
Ambient Gas (species and pressure) Target-to-Substrate Distance Particulate Filtering Plasma Excitation
Film Growth
Substrate (material, temperature, and translationlrotation) Energetic-Bearn Assist
2.1.2 Advantages and Disadvantages of PLD Pulsed-laser deposition has a number of attractive features:
Tunability - The quality of P m - g r o n films is critically dependent upon a number of controllable parameters, permitting the process to be tuned to suit a wide range of applications.
Low Substrate-Temperature Growth - A significant portion of the energy required for smooth, crystalline film growth may be supplied by the kinetic energy of the condensing vapour, reducing the need for elevated substrate temperatures.
O
- -
-
-
-
-
- - 1--------
- --
-
L
Y
grow films of multi-component alloys from elemental targets, and to grow complex heterostructures.
Reactive Deposition - Ablation in the presence of a reactive background gas or ion-beam permits the deposition of complex oxides and nitrides which are difficult to grow using conventional ultra-high v&uum techniques (i.e. SiO,N,, and Carbon Nitride).
Non-equilibrium Process - The energetic nature of the condensing vapour and the pulsed growth process enables the growth of material phases which are not therrnodynamically allowed [15,16], as is evidenced by recent examples of carbon nitride growth by ion-beam-assisted PLD [17]. Although these advantages are undisputed, three distinct disadvantages of PLD remain:
Deposition Area
- The highly forward-directed plume expansion causes the deposited
film area to be significantly smaller than in conventional film-growth techniques such as molecular beam epitaxy and chemical vapour deposition. However, this drawback c m be overcome by scanning the laser andor substrate relative to the target. For exarnple, recent work [18j has demonstrated the deposition of yttria films with thickness uniforrnities of +/- 3.4% over 8" substrates,.
Particulate Generation - As described in more detail in Section 2.2.4, the laser ablation process often produces particulates, ranging in size from 10-nm to several prn in
diameter, which can contaminate the growing film. Although interesting applications exist for the controlled deposition of particulates, their presence represents a major contamination problern for most applications. Ongoing research shows that the amount of debris deposited on the film surface can be significantly reduced by a combination of mechanical filtering and process parameter tuning [l9,20,2 1,221.
Defect Generation
- Highly-energetic condensing atoms and ions may penetrate the
surface of the deposited film creating defects in the substrate and film 131. In electronics applications, in particular, these may limit the attainable film quality [23]. Adjusting the pressure of the background gas controls the average velocity of the incident particles, thus controlling defect generation.
2.1.3 Applications of PLD Pulsed-laser deposition has been used to deposit an extraordinarily wide range of materials, as evidenced b; a recent bibliography of PLD references [24]. Historically, the most significant application of PLD has been in the area of high temperature superconducting thin films. The demonstration that PLD could be used to deposit YBCO films with zero resistivity at
- 85 K [25] sparked a significant amount of
high temperature superconductivity research over the past decade, and has stirnulated research in PLD in general.
h addition to superconducting films, PLD of cornplex ceramic oxides has also been shown to produce high quality ferroelectric 126,271, magnetoresistive [28,29], and
-
-
-
recent demonstration of colossal magnetoresistivity in films of Lao.67C~.33Mn0, [28]. The most interesting aspect of these developments is that it is now possible to use PLD to combine two or more types of ceramic oxide in heterostructures, enabIing novel device applications [3 1,32,33].
In the area of semieonducting films, PLD has been used to deposit a wide range of materials, including SiGe- [34] and GaAs-based [35] alloys, II-VI compounds [36], and Group IiI-Nitrides [37,38,39]. Again, the ability to tailor device properties by the growth
of heterostmctures is interesting, as is the possibility of introducing dopants during growth through the use of controlled pressures of reactive background gas [23].
PLD has also been used to deposit a number of optical materials, including: Zn0 as a piezo-electric, piezo-optical, transparent conductor [40]; TiOl for use as antireflection coatings on Silicon 1411; rare-earth-doped phosphate glasses for optical waveguide applications [42]; and hafnia, yttria, and zirconia for optical multilayer structures [43]. The final area of significant PLD research is in the field of hard coatings, such as boron nitride [38], carbon nitride [17,44], and diamond-like carbon [45]. In these cases, the energetic nature of the depositing vapour appears to be responsible for enabling the growth of the desired hard phases, which are not thermodynamically stable under the growth conditions of conventional techniques [16,17].
2.2.1 Overview In the ablation process, a high-power laser is directed ont0 the surface of a target, depositing energy by a combination of photothermal and photochernical processes. Given
a sufficient intensity of incident laser light, a combination of thermal vaponzation and electronic bond-breaking &es place, leading to the removal of nm- to pm-thick layers of material per laser pulse.
In the sections that follow, the basic mechanisms of photothermal ablation (thermal vaporization) and photochernical ablation (electronic bond-breaking) are explained in more detail. Additionally, the generation of particulates during the laser ablation process is discussed, as are techniques for rninimizing the particulate contamination of pulsed-laser-deposited films.
2.2.2 Photothermal Ablation When high-power laser pulses are focused ont0 the surface of a material, optical absorption leads to localized heating of the surface layer of the target. In a thermal mode1 of ablation, this absorption of energy leads to the melting and vaporization of the target surface.
Two parameters control the depth of energy-deposition in the target and, thus, the thickness of the ablated layer
- the
optical absorption coefficient and the thermal
.
diffusivity of the target material. Assuming linear absorption, the optical pulse energy
to the inverse of the opticaI absorption coefficient:
Additionally, as the surface layer heats up, energy is carried into the bulk of the target by
thermal diffusion, with a characteristic depîh of:
where D is the thermal diffusivity,
is the laser pulse length, and z, is the carrier lifetime
in the target material [46]. For transparent materials, such as fused silica, thermal diffusion is negligible relative to the optical absorption depth, and the absorbed energy per unit volume can be approximated by:
where Uabs(z)is the density of absorbed energy, Fo is the incident laser fluence (energy per pulse per unit area), and R is the reflectivity of the target material. Assuming that the abIated depth is defined by the depth at which the absorbed energy is equal to the energy required to vaporize the target material, UVp,Equation 2-3 can be inverted to give the ablated depth per laser pulse, dvap.This equation then takes the form of the Beer-Lambert Law shown in Figure 2-2 and Equation 2-4:
Assumed Piameters: ' . ,
1
.
10
Fluence (~/crn')
-
Figure 2-2 Ablation-Rate Dependence on Laser Fhence for a Typical Insulator The ablation rate for insulating materials is logarithrnically dependent upon the laser fluence. Thus, there is a fluence threshold, below which ablation does not occur. Above threshold, the ablation rate is inversely proportional to the absorption coefficient of the target material.
where, Fth,the threshold fluence for ablation, is defined as:
Thus, the ablation depth per pulse varies as the natural logarithm of the laser fluence, with a slope q u a 1 to the inverse of the optical absorption coefficient.
mode1 for the laser ablation process:
@
Thermal diffusion increases both the threshold fluence and the etch-rate.
*
The optical and thermal properties of the target material change as it is heated to the melting and boiling temperatures. At high pulse-rates, the surface of the target does not cool to room-temperature between pulses, reducing the threshold fluence and increasing the etch-rate.
*
Photochernical effects can significantly alter the laser-material interaction through defect formation and photo-decomposition in the bulk target and the vapour plume (see section 2.2.3).
2.2.3 PhotochemicaI Ablation In a wide range of laser ablation processes, the thermal mode1 presented in the previous section does not fully explain the observed effects. In particular, when laser photon energies are comparable to the energies of specific bonds andlor defects in the target material, direct photo-chernical process may take place. Examples of this type of process include: laser-induced desorption, the formation of defects on the surface or in the bulk, and photo-dissociation andor photo-ionization of the gas phase.
Laser-induced desorption describes the process by which individual atoms, ions, or molecules are rernoved from a surface by laser fluences significantly below the ablation threshold.
Examples of laser-induced desorption include desorption from
surface defects and photo-dissociation at the surface of an oxide or nitride. For example, the work of Dickinson et al. [47,48] has shown that photons with energies below the bandgap of the bulk target may induce ion andior neutral atom or molecule emission frorn pre-existing surface defects. In the case of ArF-laser irradiation of Mg0 [48], it is believed that the incident radiation photo-ionizes low-energy defects on the target surface, creating positive ions which are ejected from the surface by Coulomb forces. Alternatively, the work of Kurosawa et al., [49] has shown that above bandgap irradiation of Si3N4 by 193-nm radiation creates silicon-rich surfaces in which silicon crystallites form. In these cases, the high energy photons directly dissociate the Si-N bonds, leading to the observed desorption of nitrogen.
Defect and Electron-Hole Pair Formation When "high" energy photons irradiate the surface of a bulk target, coupling of radiation into the material may cause bonds to be modified or broken, forrning defects or electron-hole pairs. These defects may be semi-permanent or permanent, as in the case of
the duration of the laser pulse, as discussed below. An example of the effect that defect and electron-hole pair formation can have on the laser ablation process is discussed in recent work by Sugioka et al. [50]. In this experiment, Raman scattering of a 266-nm laser bearn was used to create a bearn consisting of
- 45% 266-nm radiation plus a coincident spectrum of Stokes and anti-
Stokes shifted beams with wavelengths ranging from 594-nm to 133-nm. When this multi-wavelength bearn was focused ont0 a fused silica target, the VUV components of the beam were found to create both metastable oxygen-deficient defect sites and shortlived electron-hole pairs in the silica target.
The electron-hole pairs, in particular,
strongly absorbed the 266-nm fundamental bearn, ailowing high quaIity ablation to occur at fluences significantly below the threshold for ablation with 266-nm radiation alone. 2.2.4 Particdate Generation During Laser Ablation As mentioned in Section 2.1.2, film contamination by particulates generated during the laser ablation process is one of the prirnary disadvantages of pulsed-laser deposition.
A number of possible mechanisms have been proposed for particulate
generation, including:
explosive-boiling, subsurface super-heating, shock-induced
droplet emission, target erosion, and clustering.
Explosive Boiling - The phenomena of explosive boiling describes a rapid liquid-to-gas phase transition during the absorption of the laser pulse. The sudden rise in pressure
carry liquid droplets with it [5 1,521. Subsurface Super-Heating
-
Subsurface absorption by defects in the bulk ancilor
evaporative cooling of the surface can lead to a situation in which a buried layer vaporizes before the surface. The explosive expansion of this trapped vapour could eject liquid droplets a d o r solid particles from the target surface 1531. Shock-Induced Droplet Emission - The explosive force with which the vapour plume expands from the target surface generates shock-waves within the target. These shockwaves may eject droplets from the liquid surface of the target 1211. Target Erosion - It is well documented that repeated ablation at the sarne target site can lead to the formation of surface ripples and cones [54]. Although the mechanism for cone formation is not definitively known, one possibility is that inhomogeneous etching results from the interference of the incident bearn with waves scattered from defects. Once cones and ripples are formed, thermal andfor mechanical shock rnay eject droplets and/or solid particles from the sharp edges of the structure. Clustering - If the vapour plume expands into a background gas of sufficiently high pressure, collisions may cause atoms and ions in the plume to coalesce into nm-scale clusters, as is observed in the formation of C60,or buckeyballs, when graphite is ablated in a high-pressure helium jet [55].
Particulate generation, and the resulting contamination of films by the particulates, can be minirnized by a number of techniques which can be grouped into 4 main categories: target optimization, laser optimization, velocity filtering, and diffusive deposition. Target Optimization
- Particulate generation is minirnized by using ablation targets of
high density and opticaf quality [19]. Alternatively, recent experiments have shown that ablation from a Iiquid target produces significantly fewer particulates than ablation from a solid target [56].
Laser Optirnization
- Near-threshold ablation fluences have been shown to produce
significantly fewer particulates than high fluences [20].
AdditionalIy, the laser
wavelength can be tuned to reduce particulate contamination of the film - strong optical absorption in the bulk minimizes the generation of particulates [16,19], while strong plume absorption may result in vaporization of particulates within the plume [57,58]. VeIocity Filtering - The generated particulates travel with characteristic velocities which are orders-of-magnitude lower than the plasma expansion velocity [59]. Thus, properlytimed mechanical shutters may be used to intercept the particulates without greatly affecting the plasma expansion [2 11. Diffusive Deposition - Due to collisions with the background gas, the vapour plume can diffuse around objects, while the heavier particulates tend to travel in straight lines.
absorbs the partides, while a significant portion of the vapour diffuses around the mask to reach the substrate 1221. Alternatively, orienting the substrate plane perpendicular to the target plane causes the particulates to drift past the substrate and deposit on the chamber walls. The disadvantages of these techniques are reduced deposition rates and a significant reduction in the kinetic energy of the depositing vapour [60].
Silicon Dioxide Thin-Films 2.3.1 Applications Silicon dioxide films are used in a variety of applications in the electronics and photonics industries, with a corresponding range of desired properties. For electronics applications, the siIicon dioxide insulating Iayer is only 7-nm thick in 0.25-pm silicon-based MOS structures [4]. These very thin layers require tight control of the film thickness (I0.7-nm) and the absence of mounds, particdates, or pinholes on the film surface (0.5 defects/cm2). Additionally, low deposition temperatures (< 450°C) are required to minimize dopant migration between already-deposited layers
[61] and to permit deposition on temperature-sensitive aluminum interconnects [4]. Other requirements placed upon these films include ultra-high punty, low oxide-charge densities and uniforrn deposition over large areas. A second application of silica films which is rapidly growing in importance, is as the basis for optical waveguides in optical integrated circuits. In this case, a planar
index-of-refraction-modifying ions.
Co-doping of this structure with optically-active
ions, such as erbium oxide, allows the formation of an optically-active waveguide - the basis for optical devices such as amplifiers and lasers for telecommunications applications. A third application of silicon dioxide films is in the formation of rnultilayer
dielectric stacks for use as wavelength-selective optical filters and mirrors. Because silica has a relatively Iow index-of-refraction (n-1.46) compared with other transparent oxides, such as titanium oxide (nL2.4) and yttria (Y203, n-1.8), it is useful as the low index-ofrefraction material in these structures. Silicon dioxide films for optics and photonics applications must be of high purity and have low surface roughness. In particular, although pure fused silica is transparent over a wide spectral range (0.17-8.0 pm),impurities produce absorption bands which may interfere with desired applications. For example, OH' impurities in otherwise-pure fused silica produce an absorption band at
-
1.4-prn which interferes with the 1.55-pm
telecommunications wavelength band [62].
Additionally, particulates, pinholes, and
surface roughness scatter light. For example, according to (631,the surface scattering coefficient (s) scales with the ratio of surface roughness (O) to optical wavelength (h):
Silicon dioxide layers are grown by a wide variety of techniques:
Oxidation - Oxide layers in silicon electronics have historically been grown by the hightemperature oxidation of the surface of a silicon wafer, or a previously-grown silicon film. Due to the maturity of silicon processing technology, high purity and precise thicknesses are routinely obtained by this technique. Unfortunately, the production of high quality films at reasonable rates requires temperatures of 800-1200°C, leading to significant problems with dopant diffusion [61]. Rapid thermal oxidation (RTO) is used to minimize the thermal load on the silicon wafer, however, temperatures of
- 950°C are
needed for up to 2 minutes, which remains too long for some applications [4]. UV larnps have been used to assist RTO through the creation of highly reactive atomic oxygen. However, the deposition temperatures needed (450-550°C) for this process remain higher than would be desired, especially for deposition on aluminum interconnect layers [4J.
Chernical Vapour Deposition
- CVD is a technique commonly used
in the growth of
silica-based layers for electronics and optical applications. Precursor gases (Le. SiC12H2 and N20for SiOz growth) are supplied to a heated surface, where they react to produce a film of the desired material. The temperatures required for thermally-activated CVD of high quality silica films reach 700-900 OC, depending upon the particular precursor gases used [4,6 1,641. Plasma-enhanced CVD allows the deposition temperature to be reduced to 250-400 OC, however, ion-bombardment has been found to produce defects in the deposited films which limit the film quality [64]. Photon-assisted CVD has been used to
are lower than with other CVD techniques [64,9].
Sputtering - Electric-arc-discharge-, ion-beam-, and plasma-discharge-sputtering are used to deposit many types of optical layers, including SiOz [65,66]. Typically, energetic electrons andor ions bombard the surface of a bulk target, leading to the ernission of atoms, ions, and electrons from the target surface. The sputtered species travel through the surrounding background and deposit on a nearby substrate. As in the case of pulsedlaser deposition, the sputtered atoms and ions are energetic, allowing low-temperature deposition and the use o f a reactive background gas. However, unlike in the case of the . photon beam in pulsed-laser deposition, the beams of sputtering electrons and ions originate frorn a source interna1 to the deposition chamber, potentially leading to contamination of the deposited film.
2.4 Pulsed-Laser Deposition of Silicon Dioxide 2.4.1 Motivation for Pulsed-Laser Deposition of Silicon Dioxide A need remains for a low-temperature growth technique which can produce Sion films of comparable quality to high-temperature-grown films, at commercially-viable growth-rates. The potential for pulsed-laser deposition to produce high quality films at low temperatures raises the prospect that an optimized PLD process may fil1 this void. The relative ease with which PLD may produce silica films with several
independently-controlled doping profiles raises the prospect for the growth of high quality
control of the index-of-refraction to define a planar optical waveguide, while additional erbium oxide doping will create a waveguide which is optically-active in the 1550-nm telecommunications wavelength band. The additional possibility for low-temperature growth may be of benefit for applications requiring growth on temperature-sensitive substrates such as previously-fabricated electronic or photonic circuits. The third motivation for studying PLD of Si02 films is that silicon dioxide, due to its wide bandgap, forms a test case for the deposition of other optical materials. As will be discussed in the following section, the high optical transparency of silicon dioxide at most conventional laser wavelengths makes pulsed-laser deposition from a bulk fused silica target difficult. Thus, information gained in studying the deposition of silica films should be useful in future work studying the deposition of other wide-bandgap materials.
2.4.2 Past Results of SiOz Growth by PLD Table 2-2 summarizes the published examples of silicon dioxide film growth by pulsed-laser deposition. Lasers ranging from the IO-pm CO2 laser to the 193-nm Argon Fluoride laser have been-used, producing Si02 films by both direct deposition from a silicon dioxide target and reactive deposition from a silicon or silicon monoxide target. For each study, Table 2-2 lists the laser used, the ablation target material, and the range of particulate sizes found on the surfaces of the deposited films, where this information is available.
-
Table 2-2 Previous Examples of SiOz Thin-Film Growth b y PLD Reference
'
Ablating Laser
Ban and Kramer, 1970 [67]
Ruby (480-nm)
SiOs
Bykovskii et al., 1978 [68]
TEA CO2(10-pm)
Si02
Wolf, 1992 [6]
Nd:YAG (1.O6-pm)
Slaoui et al., 1991 1691 Slaoui et al., 1992 [70] Fogarassy et al., 1992 [SI Chen et al., 1993 [8] Baeri et al., 1995 [7]
Argon Fluoride (193-nm)
2 0 Nd:YAG (532-nm) plus Oz-plasmadischarge
.
1 1
---
unknown unknown
Si02,
0.1 - 1.O pm
Si, S i 0
negligible
Si
1
negligible
Xenon Chloride (308-nm)
To date, the best SiOz thin-films grown by PLD were produced by Fogarassy et al. [5,69,70]. In this work, an Argon Fluoride laser was used to ablate bulk silicon and silicon monoxide targets in a background of O2 (- 100-mTorr), producing stoichiometric Si02 films 151.
Optimization of the deposition parameters produced films with
reasonably good electrical and optical properties, however, this required substrate temperatures of
- 450°C andor a post-deposition anneal at - 800°C [69,70]. These high
temperatures preclude thè application of this process to several potential electronics applications [4]. The quality of films deposited at low temperatures is expected to be improved by increasing the kinetic energy of the depositing species, thus reducing the need for additional thermal excitation in the growth process. This increase in kinetic energy may
between the plume species and the background gas. For the case of ablation of a silicon or silicon monoxide target in a background of 0 2
gas, reduced ambient pressures (below 100-mTorr) were found [SI to produce
substoichiometric SiO, films.
Reduction of the oxygen pressure below 100-mTorr
requires the presence of highly reactive oxygen species such as atomic oxygen. For exampIe, Chen et al. [8] used an RF-driven oxygen plasma to assist 532-nm PLD from a silicon target, reducing the pressure required for stoichiometric growth to 1-mTorr (a
lOOx reduction relative to growth in
0 2
gas). However, this plasma-assisted technique
increased the systern corn;>lexity,and the films produced were of lower quality than those produced by Fogarassy et al. using ArF-PLD without a plasma-assist [5,69,70]. The use of a fused silica target offers a significant potential advantage over the use of a Si or S i 0 target by enabling the growth of stoichiometric films in low background pressures. For exarnple, Fogarassy et al. obtained stoichiometric SiOa films by ArF-laser ablation of fused silica in a 10'~Torr vacuum. However, the films grown to-date by PLD
from a fused silica target have been contarninated by significant quantities of 0.1- 10 Fm particulates [5,6,7]. This -particulatecontamination is the result of the high transparency of bulk fused silica at the IR and UV wavelengths used - a situation which is known to lead to particulate generation during the laser ablation process 181. These previous studies also demonstrated a significant trend towards reduced particulate size and density as the ablating laser wavelength was reduced from 308-nm to 193-nm. A further short-
vacuum-ultraviolet absorption bands in bulk fused silica.
2.4.3 Vacuum-Ultraviolet F2-LaserAblation of Silicon Dioxide The general trend of previous results towards reduced particulate contamination at shorter wavelengths indicates that a vacuum-ultraviolet laser is an attractive candidate for
PLD of fused silica films from a bulk target. Of the lasers available which emit radiation in the vacuum-ultraviolet (VUV), only the 157-nm molecular fluorine laser produces sufficient pulse energies (10's of mJ) at sufficient repetition rates (1-100 Hz) for application to PLD. Past work by Herman et al. [10,71] has shown that F2-laser ablation produces high quality etch patterns in ultraviolet-grade fused silica, while ArF-laser ablation generates cracking and debris around the etch sites, as is demonstrated in Figure 2-3. This effect was attributed to an increase in the strength of the laser-material interaction in fused silica in moving from the 193-nrn wavelength of the ArF laser to the 157-nm wavelength of the F2 laser. Although the bandgap of SiOz is
- 9-eV, defects and blurring of the bandedge in
fused silica result in significant linear absorption of the 7.9-eV F2-laser photons, as is shown in Figure 2-4. In contrast, the 6.3-eV energy of the ArF-laser photons lies well within the bandgap of fused silica, and the 193-nm radiation-material interaction occurs through a weak two-photon process.
ArF-Laser Ablation
F2-Laser Ablation
-
Figure 2-3 ArF- and F2-Laser-Ablated UV-Grade Fused Silica Optical microscope images of ArF- and F2-laser ablated holes in fused silica. show a significant quantity of debris surrounding the ArF-laser-ablated hole 62 shots). This debris is not present around the F2-laser-ablated hole (3.5-~/cm~, ( 2 - ~ / c m50 ~ , shots). Both holes are -100x200-pm2. Adapted from [IO].
Absorption Coefficient
-
Abs. Cocf. (157-nrn) = 14 cmA-1 % Abs. of Irnm of UV-GradeSilica
8 Abs. (157-nrn) = 75%
0.1
0.11
0.12
0.13
0.14
0.15
0.16
0.17
Wavelengtli (pni)
-
Figure 2-4 Optical Absorption Spectrum of UV-Grade Fused Silica The optical absorption coefficient and the resulting optical absorption of 1-mmof UV-grade fused silica are shown. Due to defects and bandedge blurring, fused silica is absorbing at 157-nrn (ais7 - 14-cm-') [72]. In contrast, fused silica is highly transparent at the 193-nm (no absorption data availabie at 193-nm).
ine nign transparency or msea siiica ro ~YJ-nrnraaiarion
causes mr-iswt;~
abIation of Si02 to be difficult to control, especially near the fluence threshold, where incubation effects and a sudden onset of ablation (see Figure 2-5) have been found to make the process unpredictable [71].
These effects result in poor ablated-surface
morphologies and, in some cases, the generation of particulates [IO]. In contrast, the relatively strong Iinear absorption of 157-nm radiation in fused silica causes the F2-laser ablation process to be characterized by a srnooth onset of ablation, smoother ablated surfaces, and the absence of particulate debris [711.
-
Figure 2-5 F2- and ArF-Laser Ablation Rates in UV-Grade Fused Silica Ablation rate as a function of laser fluence for ArF- and IFz-laser ablation of UV-grade fused silica [71]. Note the sharp ablation onset at the 4-5 ~ / c m ~ threshold for ArF-laser ablation. In contrast, F2-laser ablation has a smooth onset at a threshold fluence of l.o-~/crn~.
PLD is expected to enable the low-temperature growth of high quality silicon dioxide films with precisely controlled dopant concentration profiles for applications in the electronics, optics, and photonics industries (see Section 2.4.1). However, despite a number of studies using a range of lasers [5,6,7,8],the low-temperature growth of highquality silica films by PLD has yet to be demonstrated (see Section 2.4.2). Deposition of silicon dioxide from a fused silica target is expected to produce stoichiometric films in relatively low ambient pressures, enhancing low-temperature growth, as compared with reactive deposition in high background pressures of oxygen. However, to-date, silica films grown by PLD from a fused silica target have been found to be contaminated by 0.1-10 pm particulates generated during the ablation process [5,6,7]. This particulate generation can be directly linked to the weak optical absorption of fused silica at the laser wavelengths used. In contrast, high-energy photons from the F2 Iaser have been shown to enable well-controlled laser ablation of fused silica, with minimal debris generation [10,71]. Thus, Fz-laser ablation is expected to enable the deposition of particulate-free silicon dioxide films from a fused silica target. Further, the high photon energy is also expected to result in a strong absorption of the F2-laser radiation by the laser-ablated products, producing a highly excited vapour plume. The combination of this strong plume-heating and growth in low ambient pressures will produce the energetic species needed for low-temperature growth of high-quality films.
& . S v
W.--=
vi
* L
.
YY
V-
".---Y.*
UIY..iUI
1"
Y . . .. .
Y 4
-Il..-----
--- -- ---- -------------
information which may be learned about the 157-nm radiation-material interaction by examining the effects of laser parameters on the properties of the deposited films. This basic knowledge may be applied in ongoing work studying F2-laser photosensitivity and ablative micromachining of silica-based materials [711. Further, this study is only the second published example of PLD using a vacuumultraviolet laser. Thus, this short-wavelength extension of the PLD process will lay the groundwork for future studies examining the deposition of other high bandgap materials from appropriate bulk targets. For example, the one previous F2-PLD study, by Fujii et al. (731, examined the growth of fluoropolyrner thin-films by F2-laser ablation of a TeflonTM target.
3.1 Overview The deposition experiments described Section 3.2 have proceeded in two stages:
In the initial experiments, films were deposited in a moderate-vacuum ablation chamber, with the goal of demonstrating the deposition of debris-free silica films using the fluorine laser. For this purpose, films were deposited using the fluorine and the argon fluoride lasers, allowing the 157-nm results to be contrasted with results obtained using the longer 193-nrn wavelength. As a follow-up to this experiment, a new, very-high-vacuum deposition chamber was built to allow the determination of the effects of a range of deposition parameters on the properties of the deposited films. Both stages of the experiment were performed using the lab-built molecular fluorine laser described in Section 3.2.1. The deposition charnbers and the pararneter ranges studied in the two stages of the experiments are described in Sections 3.2.2 and
3.2.3. The deposited films where characterized by several techniques: Atomic force microscopy (AFM), described in Section 3.3.1.1, provided twodimensional maps of the surface of the deposited films.
measure the chemical composition of the deposited films. Fourier-transform infrared (FTIR) spectroscopy was used to analyze the chernical structure of the films, as described in Section 3.3.2.2. Film thickness and density were determined by combinations of FïIR, ion-milling during XPS,and mechanical profilometry, as is outlined in Sections 3.3.3 and 3.3.4.
3.2 Film Deposition 3.2.1 The Molecular Fluorine Laser 3.2.1.1 Basic Principles of Operation The molecular fluorine laser produces pulsed vacuum-ultraviolet (VUV) radiation at 157-nm. This corresponds to photon energies of 7.9-eV, which is above the bandgap of most common materials, enabling novel material processing applications. The design of the laser used in these experiments [74] is based upon standard excimer laser technology, in which a high-voltage pulsed-power supply is used to drive a transverse electric discharge in the laser cavity. This discharge excites the laser gas mixture (-0.2% fluorine in 9.5-atm. of helium), forming an excited fluorine molecular state which radiatively de-excites to the lower laser state by emitting a 7.9-eV photon. Unlike standard excimer laser transitions, the lower laser state is weakly bound, and the Iaser is self-terminating, producing 15-ns pulses. To overcome the effects of lower-state
pressures (9.5-atm.) with a small electrode gap spacing to provide intense electron-impact pumping of the laser gas.
.
The resulting high laser gain permits the use of a single-pass optical resonator, in which the front laser-optic is a magnesium-fluoride window providing only 4% reflection.
The back laser optic is a concave, aluminum-coated (second surface),
magnesium-fluoride mirror with a radius of curvature of 10-rn to partially offset the divergence of the laser beam. Unfortunately, the interactions of fluorine gas and ultraviolet radiation with the intemal components of the laser vesse1 causes the gradual build-up of impurities in the gas mixture.
Due to the short wavelength of the F2-laser, even relatively srnaIl
concentrations of these impurities in the laser gas may strongly absorb the beam energy, leading to the gradual decay of the laser energy over time. This effect is counteracted by slowly cycling the gas mixture through a cryogenic gas purifier (API C-2000-HP) which removes the impurities, extending the gas fil1 lifetimes by a factor of 3-4.
The operational characteristics of the fluorine laser used in these experiments are summarized in Table 3-1, -below:
-
Table 3-1 Fluorine Laser Operating Characteristics [74]
1
Pulse Energy
1
40-50 ml
1
Wavelength
1
157-nm
1. 1
Pulse Length
15-11s
Spectral Width
0.002-nm
Pulse Rate BeamArea Gas-Fil1 Lifetirne
1
1
1-2 Hz -3x12mm2
- 10,000shots
The pulse rate of 1-2 Hz is limited by the lack of interna1 gas circulation in the laser vesse1 and the capacity of the high-voltage power supply system - in particular, the spark gap switch. Additionally, as discussed in the previous section, the gas-fil1 lifetime is limited by the sensitivity of the fluorine laser to contaminants which strongly absorb the 157-nm radiation. Without a gas purifier, fil1 Iifetimes of
- 3000 shots are typical.
The pulse energy, pulse rate, and gas-fil1 lifetimes of this laser are al1 significantly lower than for commercially-available UV-excimer lasers (- 500-mJ,200-Hz, 108shots), making the application of this F2laser to PLD particularly challenging.
By f i h g the laser vessel with different gas mixtures, different wavelength
radiation can be producedl In particular, a 6-atm. gas fil1 of 0.2% fluorine, 4% argon, and 96% helium was used for an argon fluoride laser, producing radiation at 193-nm. The characteristics of the ArF laser used in these experiments are outlined in Table 3-2.
-
Table 3-2 Argon Fluoride Laser Operating Characteristics
1
Pulse Energy
1
90-100 ml
1
1
Pulse Length
1
15-ns
1
I
Pulse Rate
I
1-2 Hz
Gas-Fil1 Lifetime
I
3.2.2 Initial Experimental Configuration Because 157-nm radiation is absorbed in air, al1 experiments were done with evacuated beam-tubes connecting the laser vessel and the deposition charnber. Additionally, in order to maximize the energy transmitted to the target, a single magnesium-fluoride focusing lens was used for al1 experiments.
157-nm or 193-nm Laser Beam
f---
I 4-----
Star Tech Energy Detector
w
Rotating Beamsplitter
. Turbopump
Argon or Dry Air Supply Line Evacuated Beamline
MgF2 Lens on 2-Stage
-
*.
8
Silicon Substrate Ablation Plume
UV-Grade Fused Silica Target
Exhaust to Rotary-Vane Pump
id
X-Y Stage
-
Figure 3-1 Schematie of the Initial Deposition Chamber The original deposition chamber, depicted in Figure 3-1, is a general-purpose vacuum chamber evacuated by a 50-Ysturbomolecular pump (Turbovac 50) backed by a 3-CFM dual-stage rotary-vane mechanical pump (Trivac D4A). With this combination, the base pressure obtained during these experiments was
-
6x10-~ Torr.
During
deposition, a micrometering valve was used to control the ambient pressure in the range of 16'- 1o - Torr ~ of argon or dry air. An aperture (- 3.0x7.5-mm2) was placed in the bearn tube to select a unifonn portion of the beam. This aperture was imaged by an 8.6-cm magnesium-fluoride focusing lens, producing typical image sizes of -220~480-~m~ on the fused silica target.
to be controlled with 10-20% accuracy. The target was slowly scanned relative to the laser beam (- 10-pdpulse) to allow fresh material to be exposed during ablation, a technique which is generally known to reduce particulate generation resulting from target texturing. The substrates used for îhese experiments were
- 1x1-cmZ pieces of a sjlicon
(100) wafer which was polished on both sides to make infrared transmission measurements possible.
The substrates were positioned 1.5-2.5 cm from the target,
along the line of the target normal, with four substrates mounted symmetrically on the holder for each experiment. In some cases, a narrow (- 1-mm) strip of aluminum was stretched over one of the substrates to create a trough in the deposited film which could be profiled to determine the film thickness (see sub-section 3.3.3.1). Early results showed hydrocarbon contamination to be a significant problem, and the chamber was therefore baked before some deposition trials, producing base pressures
as low as 2.5x10-~Torr. Unfortunately, bake-out was complicated by the long g l a s beam tubes (- 1.5-m), large O-ring surfaces, and external welds in the vacuum chamber, and contamination remained a problem.
The deposition conditions studied in these initial experiments are summarized in Table 3-3, on the following page:
20-mJ
Energy on Target
5-10 mT
Fluence on ~ a r g e t
3-5 ~ / c r n ~
1-1.5 Hz
Laser Repetition-Rate
1 Base Pressure
1
2 . 5 ~ 1 0 -' ~8x10-~Torr
1
1oe5- 4x 10' Torr argon or dry air
Ambient Pressure
1.5-2.5 cm
Target-to-SubstrateDistance
1 Substrate Temperature
8- 10 ~ / c r n ~
1
25°C
1
3.2.3 Follow-up Experimental Configuration Based upon the results of the initial experiments, a new very-high-vacuum deposition chamber was built to eliminate film contamination and to allow a wider range of deposition parameters to be controlled. Using this new deposition charnber, a set of follow-up experirnents were perforrned to study the effects of laser fluence, ambient gas pressure and species, and substrate temperature on the quality of F2-PLD silica films.
3.2.3.1 F2-PLDDeposition Chamber Design
A new deposition chamber was designed and built for this project, as shown in Figure 3-2, on the following page. In general, the chamber was designed to produce as low a vacuum as possible, within the time and price constraints of this project. Al1 large flanges were sealed with copper gaskets, with a small number of VitonTM O-ring and TeflonTM-taped pipe-thread seals confined to feedthroughs (dl-rnetal-sealed parts can be
smcon suDsrrare on Heated Holder
-
M .
-"...-.--
Beamtube
Welded Bellows Vacuum-Sealed
C Turbopump
Substrate Shield
-
Figure 3-2 Schematic of the Very-High Vacuum F2-PLDDeposition Chamber extremely expensive). Al1 interna1 metal parts were stainless steel, with TeflonTM sleeves used to prevent the binding of several metal-on-metal rotating contacts. The magnesium-fluoride focusing lens aiso served as the entrance window to the chamber, separating the deposition-chamber vacuum from the lower-grade beamtube
vacuum, while also minimizing the number of optical elements to maximize energy throughput.
This lens was mounted on the end of a welded bellows, allowing a
feedthroughs allowed the user to:
shield the substrate during pre-deposition laser-
cleaning of the target, position one of two targets in the path of the laser, and rotate the targets dunng ablation. The substrate was heated to a maximum tested temperature of 450°C by a tungsten-wire resistive heater, and the temperature was monitored by a type-K thennocouple mechanically attached to the substrate holder. Viewports were included to allow visual observation of the ablation plume during deposition. 3.2.3.2 Experimental Procedure The new deposition charnber was connected by a sealed beamtube to the existing ablation chamber, which was, in turn, connected to the laser. Thus, the entire bearnpath
was a sealed system which was evacuated during deposition by a dual-stage rotary-vane pump (Trivac D433,attached to the ablation chamber) to maintain optical transparency. After initial evacuation, argon flow was used to maintain a pressure of
- 1-Torr in the
beamtubes to minimize oil backstreaming from the mechanical pump. The deposition charnber was evacuated with a turbomolecular pump (Turbovac 50
- 50-Vs or Turbo-V250 - 250-Vs) backed by a liquid-nitrogen-trapped, 3-CFM dual-stage rotary-vane pump (Trivac D4A), producing base pressures of
- 2 . 5 ~ or1 1~ .~4 ~ Torr. 1 ~ ~
Prior to al1 but one deposition experiment, the charnber was baked to
- 80-90°C for
16-24 hrs to accelerate the outgassing process and reduce pump-down tirnes, allowing samples to be produced every other day. A micrometering valve was used to allow argon, dry air, or oxygen to flow through the chamber, with the turbopump running, producing
after initial assembly, and then re-evacuated pior to most deposition experiments to minirnize potential contamination from oil or water vapour in the supply lines.. A 5.0-cm focal length magnesium-fluoride lens focused the bearn ont0 the silicon
dioxide target. Unfortunately, the lens used for this experiment was birefringent, causing the horizontal and vertical polarizations to have focal lengths differing by
- 0.15-cm.
This birefringence made it difficult to accurately define the laser fluence, as is described in more detail in Appendix A. However, two image positions were defined and used in this experiment
- one
corresponding to a highly peaked fluence profile, with a
maximum fiuence s I O - J / C ~and ~ , the second corresponding to a flatter profile, with a fluence of
- 3-4 ~/cm*.
.
The target was mounted on a stainless-steel holder which waç slowly rotated (-
0.2-RPM)during the experiment to reduce target texturing resulting from repeated
ablation at the same site. Immediately prior to deposition, an annular region of the target
was laser-cleaned using a defocused laser fluence of
- 1-1.5 Ucm2 (appropriate lens
position defined by visual observation of the reduction of the ablation plume size near threshold). The distance from the target to the substrate was set at 2.5-cm for al1 experiments. One 1x 1-cm2 substrate plus one 2x 1-cm2 substrate were used for each trial. Pnor to being loaded into the deposition chamber, the substrates were ultrasonically cleaned with acetone, followed by methanol. Additionally, in most cases, after chamber bakeout,
----
----------
-.---
-----
--
---
-
.-
- - -- - - - - - -
-
- -- -
-
- -
contaminant~from the substrate surface. During deposition, the temperature of the silicon substrates was allowed to cool to 25°C or maintained at 400°C. The range of pamheters studied in this stage of the experiment are summarized in Table 3-4, below:
-
Table 3-4 Follow-up Experimental Parameters Energy on Target Peak FIuence on Target Laser Repetition-Rate 2.5~ 1O-' or 1 . 4 1~o ' ~ Torr
Base Pressure
vacuum to 1x 1v3Ton argon, air, O2
Ambient Pressure Target-to-SubstrateDistance Substrate Temperature
3.3 Film Characterization 3.3.1 Surface Roughness The surface roughness of the deposited films was studied using atomic force microscopy .
Atomic Force Microscopy (AF'M)was used to measure the RMS surface roughness of the deposited films, and to measure and count the particulates on the surface of the deposited films. AFM is a high-resolution scanning-probe rnicroscopy technique which produces a two-dimensional map of surface height down to the atomic-scale. The scanning-probe is mounted on a cantilever which flexes due to the inter-atomic forces between the tip of the probe and the sarnple surface. Monitoring the deflection of the cantilever with a laser beam permits the height of the surface to be mapped as the probetip is scanned over a 1-100 pm range in the x- and y-directions. The vertical heightresolution of this technique ranges from several microns down to a fraction of a nanometer. The primary advantages of AFM over scanning electron microscopy are: the ability to study insulating surfaces and the production of quantitative height data.
3.3.2 Chernical Composition The chemical composition of the deposited films has been analyzed using x-ray photoelectron spectroscopy (XPS)and Fourier-transform infrared spectroscopy (FTIR): 3.3.2.1 X-ray Photoelectron Spectroscopy
X-ray photoelectron spectroscopy (XPS)is a common chemical analysis technique which provides quantitati~emeasurements of the elemental ratios in the surface-region (to a 5-10 nm depth) of a deposited film, plus information about the local chemical state. X-rays directed ont0 the sample cause core electrons to be ejected from atoms near the film surface. As shown in Table 3-5, the ejected electrons have energies which are
energy analyzer to record a spectrum with peaks corresponding to the each element in the
film, as shown in Figure 3-3 for a carbon-contaminated silica film.
-
Table 3-5 XPS Peak Energies and Sensitivity Factors [75] Peak
Stmcture
Energy (eV)
Sensitivity Factor
Silicon 2p
si (si4
99.3
0.40
carbon 1s
(
si'+
100.3 [76]
siZ+
101.1 [76]
--
C (graphite)
1
284.5
1
0.34
The sensitivity factors given in Table 3-5, together with the areas under the spectral peaks of the analyzed elements, are used to calculate the relative elemental concentrations within
- 5% accuracy.
For example, the ratio of oxygen to silicon in a
sarnple can be determined using the equation:
where 1, is the measured peak area, Fx is the sensitivity factor, and C , is the atomic concentration of element x.
-1000
-800
-600
-400
-200
O
Binding Energy (eV)
-
Figure 3-3 Typical XPS Survey Spectrum of a Carbon-Contarninated SiO, Film The identification of the peak-energies in this survey spectmrn allows the elements present in the sample to be identified as O, C, and Si. Additional higher-resolution scans of the Ols, Cl s and Si2p peaks are used to determine the chemical composition of the film from the relative peak areas.
Also, as shown in Table 3-5, the energy levels of the core electrons shift by up to
5%, depending upon the binding-state of the atom.
Therefore, the energy of a
photoelectron emitted from a particular atom is dependent upon the local chemical state. For example, the Si2p peak-energy shifts from 99.3-eV to 103.5-eV in moving from bulk silicon (sio) to SiOz (si4*), with three intermediate States having intermediate energies. Thus, a substoichiometric oxide will likely have a mixture of oxidation States, allowing the position and width of the Si2p peak to be used as measures of the stoichiometry of the deposited films.
Unfortunately, sample-charging during x-ray irradiation makes
measurement of the absolute Si2p peak-energy difficult. However, past work by Tao et al. [77] has shown that the energy separation between the 0 1 s and Si2p peaks is also a
sample charging, and thus, is easily measured. The depth of sensitivity of XPS is ejected photoelectrons with a range of h
- 10-nm,due to an exponential decay of the
- 2.7-nm [78]. Thus. tilting the sample relative
to the energy analyzer adjusts the depth-of-sensitivity, permitting the measured elemental concentrations to be corrected for thin layers of surface contamination which can accumulate during sarnple storage and transport to the analysis chamber. For example, in the film structure shown in Figure 34, a carbon-contaminated film of SiO, is covered by a carbon overlayer of thickness dc. In this case, the predicted intensities of the oxygen, carbon, and silicon peaks as a function of the angle to the
Surface Normal
Detected Electrons
Incident
/
l
Film: (1-(2"3;x)'z~:C + z SiO, -
-
._._____.._-_._-____.
1
1
._-,
Figure 3-4 Angle-Resolved XPS of a Carbon-Contaminated Silica Film Tilting the sample changes the angle, 8, between the detected photoelectron trajectories and the sample normal. Combining measurements at two, or more, angles, allows the effect of a thin overlayer of contamination to be accounted for in determining the elemental concentrations in the bulk of the film.
where K is a geometrical factor. Thus, measurements of the oxygen, carbon and silicon peak intensities at 0' and 60' to the surface normal may be used to calculate the thickness of the carbon overlayer and the concentration of carbon in the underlying film:
And, knowing z, the corrected concentrations of O, C, and Si in the film are:
{co= XZ, cc = l - ( l + x ) z , Cs =
)corn.tc,,
(3-5)
Ion-milling of the sample surface enables depth-profiling of the elernental ratios in the sample film. This also allows surface contamination to be removed prior to analysis. However, accurate analysis of a sputtered depth-profile requires that the measured
- - -
-
- - - - - -- - - -
-
A
Y
another. In particular, the sputtering of oxides, such as SiO2, often produces an oxygendeficient surface [79].Because ion milling is a destructive technique, it was only used on a limited number of sarnples. Additionally, due to the difficulty in correcting for the effect of preferential sputtering, the measured depth-profiles were only used for the detennination of film thickness, as will be discussed in sub-section 3.3.3.3.
3.3.2.2 Fourier-Transform Infrared Spectroscopy Fourier-transforrn infrared spectroscopy (FTIR) probes the vibrational structure of a sarnple material, perrnitting the analysis of the chemical structure of a film which has been deposited on an infrared-transparent substrate, such as polished silicon.
The
transmission of 2-20 pm radiation through the sample was referenced to the transmission of a blank silicon substrate, providing the transmission spectrum of the deposited film. Absorption peaks in the analyzed film can be assigned according to published peak positions [6,9,70,80,8 1,821, allowing the chemical composition of the deposited films to be determined. Table 3-6 summarizes the primary spectral features of relevance to the transmission of silica films on silicon substrates. Of the features listed, only the 1080-cm-', 800-cm-', and 450-cm-' peaks appear in high quality fused silica. The SiOH,
H20,and OH features are indicators of contamination during deposition and/or film porosity, which can trap airborne water vapour. Additionally, the 880-cm-' Si203feature is an indicator of oxygen-deficiency in the film.
Ref.
Wavenumber (cm-')
Wavelength (ilml
Structure
3600
2.8
SiOH
porosity
1701
3350
3.0
SiOH
porosity
[~OI
3000
3.3
OH
porosity
[O]
1630
6.1
H20
porosity
~701
1150-1200
8.3-8.7
Si02
disorder
WI
1050-1090
9.3
Si02
Si-O-Si asymmetric stretching mode (ASM)
940
10.6
SiOH
porosity
880
11.4
Si20î
substoichiometry or SiH
800
12.5
Si02
Si-O bending mode
vol
450
22.2
Si02
Si-O rocking mode
~701
Notes
[6,70,80, 8 1,821
vol [9,8 11
The strongest silicon dioxide peak, at 1050-1090 cm-', shown in Figure 3-5, corresponds to the asymmetric stretching mode (ASM)of the Si-O-Si structure. Shifts in the peak-wavenumber and width of this peak are indicators of film quality. In particular,
shifts in the peak to lower wavenurnber have been attributed to combinations o c substoichiometry, contamination, porosity, and stress [6,70,80,8 11. effects also cause the peak to broaden from the
Likewise, these
- 70-75 cm-' FWHM for a high-quality
oxide [9,83]. As an extreme example, Fogarassy et al. have shown that in thermallyevaporated SiO, the main peak is centred at 980cm", with a width of
- 175-cm-'[82].
1300
1200
1100 Io00 900 Wavenumber (cm-')
800
700
-
Figure 3-5 Sample Si-O-Si ASM Peak Spectrum in an SiOzThin-Film The position and width of this peak are indicators of film quality, as is the strength of the high-wavenumber shoulder on the peak, defined here as the ratio of the absorption at the knee in the spectrum (B) to the peak absorption (A).
The high-wavenumber shoulder on the SiOz ASM peak (see Figure 3-5) has been related to disorder in the Si-O-Si lattice structure [70,80], and is weak relative to the ASM peak for a high-quality oxide layer. For example, VUV photochernical deposition has been shown [9,83] to produce films in which the absorption in the shoulder region is less than 20% of the main peak absorption. For the purpose of this thesis, the strength of this feature has been defined as the absorption at the knee in the peak spectrum (B), referenced to the ASM peak absorption (A), as seen in Figure 3-5. This definition is reasonable, provided that the ASM peak absorption is not too great (< 30%). Othenvise, deviations from the linear representation of the exponential absorption cause this definition to overestimate the relative strength of this feature.
Mechanical profilometry, FTIR, and XPS have al1 been used to estimate the thickness of the deposited films. 3.3.3.1 Mechanical Profilometry Profilometry was used to measure the thickness of some films deposited in the initial stage of the experiment. Masking a l-Zmm-wide strip of the substrate before deposition produced a trough which was scanned with a profilometer, providing thickness measurements down to
- 10-nm. The disadvantage of this method was that the mask may
have introduced changes to the local environment during deposition, potentially altering the results. For this reason, this technique was not employed in the case of the samples discussed in this thesis. As one alternative, it was found that in some cases, carefully scratching the film
surface produced a sharp trough which could be scanned with the profilometer. Unfortunately, this technique was only effective for films which were highly contaminated with carbon, and thus, of low quaiity.
3.3.3.2 Fourier-Transform Infrared Spectroscopy FTIR was used to estimate film thickness by comparing the integrated area of the
Si-O-Si ASM peak in the deposited films with areas calculated from tabulated infrared absorption data [72].
However, this technique measured the mass-density of the
deposited SiOî, requiring assumptions of the film density to infer the film thickness.
effective thickness of the Si02 in the film, assurning that the contamination did not introduce strong absorption features in the region of the SiO2ASM peak. Thus, porosity andor contamination may have caused this technique to underestimate the film thickness. 3.3.3.3 X-ray Photoelectron Spectroscopy Ion-mil1 depth-profiling during XPS was used to measure the thickness of the deposited films by measuring the time required to mil1 through the film, to the underlying substrate. The measured ion-milling time was proportional to the film thickness. After ion-milling, a mechanical profiler was used to measure the depth of the ion-milled holes, producing another estimate of film thickness.
3.3.4 Film Density
.
Film density was calculated by comparing film thickness measurement techniques. In particular, the thickness measured by profiling the XPS ion-milled holes was compared to the thickness calculated from FTIR. Because the latter calculation was
linearly dependent upon the film density, the ratio of the rneasured and calculated thicknesses was used to calcufate the film density, p ~ hfrom , the bulk density, pbulk:
PROFILER
where ~ F T I Rwas the thickness deterrnined from the Si02 ASM peak area, and ~ P R O F ~ L Ewas R the measured depth of the XPS ion-milled holes.
4.1 Overview The following sections presen t the combined experimental results from both deposition chambers. Table 4- 1 summarizes the deposition conditions for each analyzed sample. Samples A-E were deposited in the low-grade vacuum chamber, whiIe sarnples
F-N were deposited in the very-high vacuum chamber. Unless otherwise specified, the sarnples were deposited with the F2-laser, at a 2.5-cmdistance, and at room temperature.
-
Table 4-1 Deposition Conditions for Analyzed Samples Energy (mJ) (&O%)
Fiuence #of (~/cm*) Shots
Rate
(Hz)
Vacuum (Torr)
Tar.
Subst.
CIean
Bake
lx10"air
Y
-325°C
vacuum
Y
- 320°C
Ambient (Torr)
(&O%)
vacuum
air 2x10~ 4x10'~ o2
1 1 :ILC 1
400"
energy during deposition. The "FIuence" is the estimated on-target fluence based on the measured beam-energy, while the "Rate" is the total number of laser shots divided by the deposition time. The "Vacuum" column lists the pre-deposition base pressure, while the "Ambient" column lists the background gas pressure and species during deposition. The
"Tar. Clean" clean column identifies whether a pre-deposition laser-cleaning of the target was employed (Yes or No).
Finally, the "Subst. Bake" column identifies whether the
substrates were baked at high temperature before deposition, with the temperature listed corresponding to the temperature measured during baking.
4.2 Surface Roughness Figures 4-1 and 4-2 show atornic force microscope (AFM) images of ArF- and F2-PLD Si02 films, respectively.
The area of each image is 2.23~2.23-~m~. The
ArF-deposited film (sample F) is seen to be contarninated by particulates covering
- 1% of the deposited film.
- 50-100 nm diameter
In contrast, the F2-deposited film
(sample D) is virtually particulate-free. Discounting particulate contamination, the R M S surface roughness of these two films are similar -0.23-nm for the ArF- and 0.30-nm for the F2-deposited films, as compared to 0.16-nm for an uncoated substrate (not shown). Thus, both film samples are extremely smooth (- monolayer surface roughness). Both films were deposited in 4 x 1 0 ~Torr of argon, at laser fluences of
- 5 - ~ / c mfor~ the ArF- and F2-PLD films, respectively.
- 1 0 - ~ / c mand ~
The thickness of each film is
- 5 n m , as determined frbm R I R absorption spectra (see section 4.4.3 and 4.4.4).
-
Figure 4-1 AFM Image of an Ad?-PLDSi02Film ArF-PLD film grown by 1 0 - ~ / c mablation ~ in 4 x 1 0 ~Torr of argon, at 25°C. Note the 1% surface-coverage by 50-100 nm particulates. The RMS roughness in the boxed area is 0.23-nm. The image area is 2.23~2.23-pm2.
-
-
Figure 4-2 - AFM Image of an F2-PLD Sioz Film F2-PLD film grown by 5-Ucm2 ablation in 4x10-~Torr of argon, at 25°C. Note the absence of particulate contamination. The R M S roughness in the boxed area is 0.30-nm.The image area is 2.23x2.23-pm2.
The chernical composition of the deposited films was analyzed by x-ray photoelectron spectroscopy. For example, Figure 4-3, below, shows survey scans taken at
0"and 60" to the sample normal, for sample A, an F2-PLD film deposited in an ambient of 4x lo4 Torr of argon, with a background pressure of 7x 10-~ Torr (unbaked). As indicated,
the rneasured peaks correspond to oxygen, carbon and silicon photoelectrons, with the
relative intensity of the carbon peak increasing in the 60" scan, indicating the presence of
a carbon overlayer on the silica film.
- 1000.00
-800.00
-
-600.00 -400.00 Binding Energy (eV)
-200.00
0.00
Figure 4-3 0"and 60" XPS Survey Scans of SiOa Film Sample A XPS survey scans of F2-PLD film grown in 4 x 1 0 ~Torr of argon, with a base pressure of 7 x 1 To ~rr~(no chamber bakeout). Note the increase in the intensity of the carbon peak relative to the oxygen and silicon peaks for the 60" scan, indicating that there is-an overlayer of carbon on the film surface.
very-high vacuum chamber. These scans illustrate the effects of the base vacuum, and pre-deposition substrate baking and target cleaning on the measured carbon photoelectron signal. In particular, sample G, deposited after a 80-90°Cchamber bakeout, was found to have a significantly lower carbon peak intensity than sample F. Additionally, sample H was produced after baking the substrate and laser-cleaning the target, resulting in a further reduction of the carbon peak intensity relative to sample 1".
- 1000.00
-800.00
-600.00
-400.00
-200.00
0.00
Binding Energy (eV)
-
Figure 4-4 O" XPS Survey Scans of SiOz Film Sarnples F, G, and H XPS survey scans of three F2-PLD film samples grown by high fluence peak fluence) in 2x10~Torr of argon (F,G) and vacuum ablation (> 10-~/crn~ (H). Note the significant reduction in the carbon peak intensity observed as a result of chamber bakeout (G and H grown after 80-90°C chamber bakeouts) and target cleaning and substrate baking (H grown after laser cleaning the target and baking the substrates at - 320°C).
and Si2p peaks (after subtracting a local background) for 0" and 60"scans of each film.
These resuIts are summarized in Table 4-2. For each sample, the peak-area ratios were used to calculate the carbon content of the film, using Equations 3-4 and 3-5 in Section 3.3.2.1.
-
Table 4-2 Elemental Ratios From 0" and 60" XPS Scans
NIA.
- 50%
3.0
25
H (high F. vacuum)
2.4
3
1(high F, 2x 1o4 air)
1.1
1.5
J (high F, 1 x 1 air) ~ ~**
0.7
O
NIA.
l
K (IowF, vacuum) ** 1
1 M (L.F.,
air, 400°C)
** - note that samples J, K, and L were analyzed 1.5-2.5 weeks after growth, while al1 other samples were analyzed within 1 week of growth
Several ooservanons can De maae regardmg the data in .l'able 4-2. without bakeout, the films were highly contaminated with carbon (40-56 at. %) (samples A, B, and C, grown in the original charnber, and sample F, grown in the new chamber) in the new deposition-chamber, combining a predeposition bakeout, target clean and substrate bake reduced carbon contamination to
- 1%,or less (samples 1 to N) - target
cleaning and substrate bakeout were not possible in the original ablation chamber films deposited in vacuum (sarnples H and K) or argon (sarnples D and G) arnbients were significantly oxygen-deficient (Si01.55to Si01.7S) in vacuum, high fluence ablation (sample H) appeared to produce a more oxygendeficient film than low fluence ablation (sample K) (SiOi.65 versus Si01.75). However, this observation is inconclusive due to the delayed analysis of sarnple K deposition in dry air (samples 1, J, L, and M) or O2 (sample N) produced films with
- 10-20 % higher oxygencontent than deposition in vacuum or argon (sarnples D, G, and H) (SiO1.9-SiOi,95in air or O2versus Si01.55-Si01 in vacuum or argon) no significant difference was observed between films deposited in 2 x 1 0 ~Tom or 1x IO-^ Torr of dry air, or 4x l û 5Torr of O2(sarnples 1 versus J and L versus N) no statistically-significant difference was observed between films deposited using high or low fluence ablation in a dry air background (sample 1versus L)
intensity ratios in O" scans than in 60" scans
In addition to the relative peak intensities, the higher resolution peak spectra were used to compare the peak widths and the energy separation of the 01s and Si2p peaks for the PLD SiOz samples. For exarnple, Figures 4-5 and 4-6, on the following page, compare the Ols and Si2p photoelectron peak spectra for film samples K and L. The Si2p peak for the film deposited in a dry air arnbient (K) was observed to be 0.3-eV narrower than for the film deposited in vacuum (L). In addition, the separation of the Si2p and 0 1 s peaks was
- 0.1-eV srnaller for the sample deposited in dry air.
To account for sample
charging, the energy scale for each sample was calibrated by setting the energy of the C l s peak (not shown) to 285.0-eV. Table 4-3 summarizes the 0 1 s and Si2p peaks widths and the 01s-Si2p peak separations for the 14 PLD-grown Si02 samples and a bulk fused silica reference. From this data, several observations can be made: the Si2p peak width and the 01s-Si2p peak separations for carbon-contaminated films deposited in argon (films A, B, C, F, and G) were significantly larger than in bulk silica (0.4-0.6 eV wider Si2p peak, 0.4-0.9 eV larger separation) reducing carbon contamination (films D, E, and 1to N) reduced both the 01s and Si2p peak widths and the peak energy-separation (- 0.1-eV narrower 0 1 s peak,
- 0.1-0.2 e V narrower Si2p peak, 0.4-0.8 eV reduced separation)
-538.00
-536.00
-534.00 -532.00 Binding Energy (eV)
-530.00
-528.00
-
Figure 4-5 0 1 s PhotoeEectron Peaks for F2-PLD Si02 Film Samples K and L Ols peak spectra of F2-PLD Sioz films grown by 3-4 ~ / c r nablation ~ in vacuum (K) and 2 x 1 0 ~Torr of air (L). Note that the peak width was equal for deposition in air or vacuum (- 2.1-2.2eV)
-
-108.0
- 106.0
- 104.0
- 102.0
-100.0
-98.0
Binding Energy (eV)
-
Figure 4-6 Si2p Photoelectron Peaks for F2-PLDSiOz Film Samples K and L Si2p peak spectra of F2-PLD Si02 films grown by 3-4 ~ l c r nablation ~ in vacuum (K) and 2x10~Torr of air (L). Note the reduced peak width for deposition in air versus deposition in vacuum (2.1-eVversus 2.4-eV).
0 1 s Peak
F W H M (eV) (4.1-eV)
Si2p Peak FWHM (eV) (&O. &eV)
01s-Si2p Separation (eV) OI(. 1-eV)
1 B (1.5-cm distance) C (dry air ambient)
D (50°C bakeout)
F (no chamber bake)
1 G (no pre dep. clean)
1 air) 1 air) ** 1
H (high F, vacuum)
1 1(high 1 J (high F,
F, 2x 1 0 ~
K (low F, vacuum) **
1 L (low F, 2 x 1 0 ~air) ** 1 1 M (L.F., air, 1 400°C)
N (low F,4x10-'
02)
1 Bulk Fused Silica
(
** - note that sarnples J, K, and L were andyzed 1.5-2.5 weeks after growth, as compared with < 1 week for al1 other sarnples.
-
-
.
.
0 1 s peak-widths were generally comparable to that for the fused silica reference
-
(2.2-2.3 eV for A, B, C, and F; 2.0-2.1 eV for al1 others) films deposited in vacuum (samples H and K) had
- 0.2-0.4 eV broader Si2p peaks
than films deposited in air (samples 1, J, L, and M) or O2(sample N) the Si2p peak widths and 01s-Si2p peak separations for films deposited with low fluences in dry air (sample L and M) and 0 bulk silica reference (peak-width
2
(sample N) were similar to those for the
- 2.0-2.1 eV, peak separation - 429.3-429.4 eV)
4.4 Chemical Structure 4.4.1 Wide-Range F'TIR Transmission Spectra (450-5200 cm-') The chemical structure of the deposited films was analyzed using FTR transmission spectroscopy. Figure 4-7, on the following page, shows the transmission spectra for two deposited samples (F and N). In addition to the characteristic Si-O-Si features at
-
1050-cm" and
- 800-cm",
several other features are observed. The
oscillatory structures in the 1400-1900 cm-' and 3400-4000 cm-' ranges, and peaks at
- 2400-cm-' and 4700-cm"
decrease in magnitude as the spectrometer is allowed to
purge, and have been excluded frorn this analysis as an artifact of air. underlying film features are seen at
However,
- 3000-3600 cm-', 2900-cm-', and 1400-2200 cm".
Additionally, it is seen that the background is neither flat, nor constant, from scan to scan. This is believed to be due to absorption by contarninants on the backside of the sample, or
Sample
6000
5000
,
4000
3000
2000
O
1000
Wavenumber (cm")
-
Figure 4-7 450-5200cm" IR Transmission Spectra of PLD S i 9 Samples F and N IR transmission spectra of a contarninated F2-PLD film deposited in argon (F) and a noncontarninated film deposited in 4 x 1 0 ~Torr of Ot (N). Note the strong 3400-cm' absorption in the contaminated film. The strong, sharp absorption peak at 1050-cm" in both samples in the Si-O-Si ASM peak.
-
on the dicon reference. In several cases, scans have been repeated one or more tirnes after careful cleaning to obtain more accurate results. Table 4-4, on the following page, lists, for each sample, the peak absorption of each feature, relative to a local linear background.
-
Table 4-4 Relative % Absorption of FTIR Spectral Features in PLD SiOz Films
Sample
3400
2900
2000
1050
cm-'
cmd
cm-'
cm-'
880 cm-"
800 cm-'
(%)
(%) (120%)
(%) (*20%)
(W
(W
(%)
(*20%)
(~10%) ( ~ 2 0 % ) (&20%) 1
1.1
0.8
0.7
2.5
0.7
-
D (bakeout)
O.1
0.05
noise
4.4
0.2
0.7
E(Am
0.4
0.1
O.1
5.4
-
-
F (no bakeout)
0.4
0.2
noise
1.8
-0
O.3
G (no clean)
O.1
0.05
noise
7.2
-0
1
H (high F, vac.)
O. 1
0.1
0.2
5.6
-0
0.5
1(high F, 2x 104 air)
0.3
0.3
0.3
5.1
0.2
0.5
J (high F, 1 x 10-~ air)
0.3
0.2
0.2
13.4
0.6
1 .O
.
C (dry air)
K (low F, vacuum)
- < 0.1 0.2 0.4 8.4 -0 0.8 -
L (low F, 2x10~air)
< O.1
O.1
0.2
4.4
0.2
0.5
M (L.F., air, 400°C)
< 0.05
0.05
0.3
6.7
O.1
0.7
0.05
O.1
0.3
4.1
0.3
0.5
N (low F, 4x 10"
02)
From the data in Table 4-4,several observations can be made: the features at 1050-cm'' and 800-crn-~ correspond to the Si-O-Si asymmetric stretching mode [6,9,70,80,8 11 and Si-Obending mode [70,81], respectively
features previously observed at 3350-cm-' [70], 3000-cm" [6],and
- 1630-cm-' [70]
the peak at 880-cm" has been separately attrïbuted to Si203 [81] and SiH [9] the 3400-cm" feature was strongest in samples C and F (44% and 22% of the ASM peak strength, respectively), corresponding to films produced without a chamber bakeout. In contrat, the strength of this feature was
- 6-7% of the ASM peak height
in samples D and 1, and < 2% of the ASM peak height in al1 other samples. the absorption of the 800-cm" peak correlates strongly with the absorption of the l050-cm-' peak: A , , = ~ x A , ~ ~rn=O.llSf ~, 0.005
(4-1
no other sets of peaks are significantly correlated, and, other than the 800-cm-' and 1050-cm-' peaks, no sét of peaks correlates with the various deposition parameters.
4.4.2 Si-O-Si Asymmetric Stretching Mode Peak Spectra The measured IR transmission spectra have been analyzed to determine the peak absorption strength, wavenumber, width, and high-wavenumber shoulder height of the SiOt ASM absorption peak at 1027-1060 cm". This process has been complicated by the previously-mentioned variation in the background of the measured spectra. For exarnple, Figure 4-8, on the following page, shows three spectra collected for sample L, in which each spectmm has been normalized to the transmission at 1300-cm-'.
1300
1200
1100
1O00
900
800
700
Wavenumber (cm")
-
Figure 4-8 700-1300 cm-'Normalized Transmission Spectra for Sample L Cornparison of three ASM peak spectra for F2-PLD SiOa film sarnple L, showing the effect of contamination in altering the shape of the spectrum backgrounds, causing the measured peak heights to Vary by 8% (4.40-4.76%) These curves have been normalized to the transmission at 1300-cm"'.
-
Figure 4-9, on the following page, compares the 900-1300c r i ' transmission spectra for sarnples H, K,L, and M, demonstrating the effects of high versus low ablation fluences (H versus K), vacuum versus air ambient (K versus L), and 25°C versus 400°C deposition temperatures (L versus M). A linear background has been subtracted from each spectrum to simplify the determination of the ASM peak parameters.
1200
1100
1O00
900
Wavenumber (cm-')
-
Figure 4-9 900-1300cm1Transmission Spectra for Samples H, K, L, a n d M FIIR ASM peaks for F2-PLD Sioz samples grown by: > I O - J I C ablation ~~ in vacuum (H), 3-4 ~ / c m *ablation in vacuum (K), 3-4 ~/crn' ablation in 2 x 1 0 ~Torr of dry air at 25°C (L), and 2.5-4 Ucm2 ablation in 2 x 1 0 ~Torr of dry air at 400°C (M). Note the increase in the peak wavenumber, and the reduction in the peak width and the relative strength of the high-wavenumber shoulder for reduced fluence, deposition in air vs. vacuum, and increased substrate temperature.
Table 4-5, on the following page, surnmarizes the ASM peak features for each of the analyzed samples.
The shoulder strengths presented here are the ratio of the
absorption at the high wavenumber inflection point in the ASM peak spectrum to the
ASM peak area, as previously described in Figure 3-5 of Chapter 3.
-
-A ame 4-3 31-u-31ASM
(&-cm*')
(110 %)
Peak FWHM (cm") (&-cm-')
C (dry air)
1060
2.6
140
3.4
58
D (bakeout)
1030
4.4
112
6.1
32
E (ArF)
1027
5.4
131
8.1
46
3.0
61
Sample
Peak
reaK raramerers or rLu mu2 r( iims
Position (cm-')
Peak Strength
(W
Peak
Area (115 %)
Shoulder Strength (%) (&20%)
I
F (no bakeout)
1037
1.8
161
G (no clean)
1030
4.0
136
6.1
42
H (high F, vac.)
1030
7.2
122
10.5
38
1(high F,2x104 air)
1042
5.1
95
6.0
27
J ( h i g h F , l x l ~ * ~ a i r ) 1047
13.4
90
15.0
25
K (low F, vacuum)
1035
8.4
102
10.5
26
L (low F, 2 x 1 0 ~air)
1047
4.4
91
4.9
18
M (LF, air, 400°C)
1053
6.7
84
6.7
15
N (low F, 4x10-~02)
1046
4.1
88
4.3
20
Referring to Figure 4-9 and Table 4-5, several observations can be made: films deposited without a charnber bakeout (samples C, and F) had very broad peaks
(FWHM = 136-161 cm-') and very strong high-wavenumber shoulders (42-58 %)
84-122 cm', shoulder strength = 15-38 %) deposition in increasing pressures of dry air produced films with higher wavenumber peak positions, and reduced peak widths and high-wavenumber shoulder heights, relative to deposition in vacuum (high fluence: 1and J vs. H, low fluence: K vs. L) low fluence ablation produced films with 5-cm-' higher wavenumber peak positions, 11-27 cm-' narrower widths, and 9-12% lower shoulder heights, relative to high fhence ablation (samples K vs. H, and sarnples L vs. 1) deposition at 400°C (sarnple M) produced a 6-cm-' higher wavenumber peak position, 7-cm-' narrower width, and
- 3% lower shoulder height, relative to room-temperature
deposition (sample L) no significant difference was observed between the ASM peaks of films deposited in 2x 1o4 Torr of dry air (sample L) and 4x 10'' Torr of
a (samples N)
4.5 Film Thickness, Deposition Rate, and Density The thickness of several deposited films was measured by two techniques previously described in Section 3.3.4: calibration of the IR transmission peak-areas to tabulated absorption data [72], and profiling holes which were ion-milled through the
films during XPS.
Unfortunately, ion-milling was found to produce extremely
nonuniform etch patterns; making accurate depth determination difficult. The density
# of Laser Shots
IR Peak Area (*IO%)
IR Thicknes s(nm)
(*IO%)
Mill Time (4 (15s)
Mill Depth (nm) (~20%)
Density (% of
Bdk)
Dep. Rate (hhot)
(&O%)
(*IO%)
10,000
3.4
7
-
-
0.007
7300
6.1
13
-
-
0.018
5000
8.1
17
-
-
0.034
5000
3.O
6
-
-
0.012
G (nodean)
4300
6.1
13
-
-
0.030
H (H.F.,vac.)
7500
10.5
22
-
-
-
0.030
7800
6.0
12
-
-
0.0 17
8200
15.0
32
400
47
70
0.039
8300
10.5
22
360
20
110
0.027
L (L.F., 2 x 1 0 ~air)
8700
4.9
10
110
20
50
0.01 1
M (LF, air, 400°C)
7500
6.7
14
150
14
100
0.018
8000
4.3
9
120
11
80
0.01 1
1 F (no
balceout)
1 J (H.F.,1x 1o -air) ~
1 N (L.F.,
4x 10.'
09
was calculated by comparing the IR-calculated thickness and the profiled depth, while the
deposition rate was calculated using the IR data. Table 4-6 summarizes the IR-calculated thickness, ion-milling times, and ion-rnilled depths for the analyzed films. Although the large uncertainties in the data in Table 4-6 preclude any specific observations from being made about the effects of the deposition parameters on the deposition rate and film density, a few general observations can be made:
G-K and N (- 50% less), corresponding to a low laser energy (- 15-25% less) used for the deposition of samples F, L, and N the deposition rates for the ArF-PLD sample (E) and the F2-PLD sample deposited in 1 x 1 0 ~Torr ~ of dry air (J) were significantly higher than for al1 other samples
the average measured film density was
- 80% of the bulk density of fused silica.
However, no clear correlation can be made between the film density and the deposition parameters, with the available data set and the large errors the time required to ion mil1 through the film samples is linearly related to the IRmeasured film thickness: milling time {s} = 13.3 x IR thickness {nm}, R' = 0.91
(4-2)
5.1 Overview The following sections analyze the results presented in the Chapter 4, with the goal of explaining the major effects which have been observed. This discussion focuses on the film deposition rate, the elimination of particulates in F2-PLD of SiO2, and the dependence of the obsewed film properties on the deposition parameters.
5.2 Deposition Rate The measured deposition rates of 0.010-0.040
A per laser shot can be explained
by a brief examination of the known ablation rates of fused silica and typical film
thickness profiles for PLD. From previous work, the etch rate for F2-laser ablation of fused silica is known to be [71]:
where F is measured in Ucm2. Likewise, it is known that PLD film profiles are highly
forward-peaked, with typical distributions of the f o m [84]:
where, r is the radial position in the target plane and L is the target-to-substrate distance.
(Le. no redeposition occurs), and that al1 of the material which hits the substrate sticks (Le. the sticking coefficient = l), then equations 5-1 and 5-3 can be used to calculate the deposited film thickness per pulse, for a given set of deposition parameters, and an assumed power factor of n. For example, Figure 5-1, shows the predicted deposition rates for n = 8 and 14, for parameters corresponding to the high and low fluence conditions used in film deposition trials F to J and K to L, respectively. From this graph, it is seen that the measured deposition rate is less than the predicted rates.
However, the
discrepancy is less than a factor of two, which is reasonably small, given the simplicity of the model used.
O
5
10
15
20
On-Target Laser Energy (rd) Figure 5-1- Deposition Rates for F2-PLD of Si02 Predicted F2-PLD déposition rates versus on-target energy for a radial position of 5-mm on the target, a target-to-substrate distance of 2.5-cm, fluence distributions equivalent to the high and low fluence cases used in this work (F* > 1 0 - ~ / c mor~= 3-4 ~ / c r n ~and ) , power factors of n = 8 and 14. Note that the experimental data lies within a factor of 2 of this simple model.
here can be grouped into three primary categories: a reduced volume of ablated material, non-unitary transport efficiency of the ablated plume, and a non-unitary sticking coefficient.
A smaller-than-predicted ablation volume may be the result of non-
uniformities in the fluence distribution on the target. In particular, a significant portion of the on-target energy may lie in a low-fluence tail which contributes little towards ablation. Additionally, not al1 of the ablated material will reach the substrate -a portion will be reflected back ont0 the target surface due to collisions within the plume [85], and between the plume and the shock-wave generated in the background gas [12]. Finally, not al1 of the material that reaches the substrate will be incorporated into the growing film (Le. the sticking coefficient is less than 1). The non-uniformities in the bearn image may also account, at least in part, for the
- 40-50% drop in the deposition rate observed for three samples deposited with laser -
energies l5-25% below the average. If a significant percentage of the on-target energy lies in the low-fluence portion of the distribution, then the logarithrnic dependence of the etch-rate on laser fluence will cause relatively small changes in the laser energy to resuIt in large changes in the etch-rate. The sharply peaked film thickness distribution may also be partially responsible for the observed discrepancy between thickness measurement techniques, and between the predicted and measured thicknesses. Referring to Equation 5-2, if one assumes an intermediate value of n=l 1, it is found that the norrnalized film thickness drops from 1.O
positions may result in significantly different estimates of the film thickness. Finally, the sarne equations used to calculate the curves in Figure 5-1 c m also be used to estimate the deposition efficiency (i.e. the percentage of the ablated volume which is deposited on the substrate). For exarnple, for a 2.5-cm diameter substrate, the predicted eff~ciencyis
- 55 and 80% for n = 8 to 14.
hcreasing the substrate diameter tu 5.0-cm
increases the predicted efficiency to 90-99%. In practice, the previously discussed effects of non-unitary plume transport efficient and a non-unitary sticking coefficient will reduce the deposition efficiency.
Based upon the relationship between the observed and
predicted deposition rates, these effects rnight be expected to lead to efficiencies of
- 30-50 % for a 2.5-cm diameter substrate. 5.3 Particulate Reduction in F2-PLD versus ArF-PLD As was shown, in Section 4.2 and in previous work by Fogarassy et al. [SI, ArF-laser ablation of fused silica produces
-
1% surface-coverage of 50-100 nm
particulates which contaminate the surface of the deposited SiOz films. In contrast, F2-PLD has been shown (Section 4.2) to produce films which are virtually particulatefree. The fundamental reason for the observed reduction of particulate generation is the increased absorption of the 7.9-eV F2-laser photons relative to the 6.4-eV ArF-laser photons. Table 5-1, on the following page, presents the optical absorption coefficient for
1 " ' ,
-'"a
-a----
-- ".
a----
.a---
-1--
----
- 7
-
absorption coefficients and threshold fluences determined from ablation data [7 11.
-
Table 5-1 157-nm and 193-nm Radiation Interaction with Fused Silica
1721 (cd)
G~~ 1711
C711 (J/cm2)
Fth
ArF (193-nm, 6.4-eV)
transparent
I .OX los
> 5-J/cm2 **
F2(157-nm, 7.9-eV)
14
1 . 7 1~0'
1.O
**
sharp ablation onset observed at a threshold fluence 3-4x greater than the extrapolated threshold (- 1.6-J/cm2)
From Table 5-1, it is seen that the bulk optical absorption coefficient, which controls the initial absorption of energy in the target, and thus, the heat-affected depth, is orders-of-magnitude larger for 157-nm radiation than for 193-nm radiation. Additionally, the effective absorption coefficient, which controls the deposition of energy in the target during the latter stages of the laser pulse and the volume of ablated material, is 70% larger for 157-nm radiation than for 193-nm radiation. F2-laser ablation of fused silica is described by a logarithmic-dependence of the ~ .contrast, etch-rate on laser fluence, with a well-defined threshold fluence of 1. O - J / C ~In ArF-laser ablation has a very sharp ablation-onset at a threshold fluence which is 3-4 times larger than the extrapolated threshold fluence (> 5-J/cm2 versus was previously illustrated in Figure 2-5.
-
1.6-~/cm~), as
Further, incubation, cracking and surface
swelling were found to degrade the quality of ArF-laser-ablated sites [7 11.
to particulate generation through several mechanisms which were previously described in Section 2.2.4:
explosive boiling, subsurface super-heating, shock-induced droplet
emission, and target texturing. Additionally, for near-threshold ArF-ablation of fused silica, the steep ablation onset will cause relatively small spatial nonuniforrnities in the laser beam to produce lzirge nonuniformities in the etch-rate and thermal expansions. These nonuniforrnities in etch-rate will generate large lateral stress gradients in the target which may directly eject droplets andor particles from the surface 1851. Additionally, the nonuniformities in etch-rate will accelerate the process of target texturing, leading to additional particulate formation.
In contrat with the 193-nm case, the stronger absorption of 157-nm radiation in the fused silica target will reduce the generation of particulates caused by explosive boiling, subsurface super-heating, shock-induced droplet emission, and target texturing. Additionally, the smooth onset of ablation at the threshold fluence will reduce particulate generation caused by nonuniformities in the spatiai distribution of the beam.
In addition to reducing the initial generation of particulates, it is possible that the high energy of the Fz-laser photons may lead to vaporization of particulates within the ablation plume. In particular, 157-nm radiation is known to be strongly absorbed by
-
molecular oxygen (a 0.2-cm-' at 1-Torr), which may be generated by the dissociation of SiOz during ablation. 193-nm radiation, on the other hand, is not strongly absorbed by O*. Absorption of the trailing edge of the laser pulse by molecular oxygen within the
than the 193-nm ablation plume. This heating could cause droplets and particulates which are generated during ablation to be vaporized within the plume. The elimination of particulates in F2-PLD SiOl films grown by ablation from a fused silica target is a major advantage of F2-PD, potentially enabling the growth of high-quality silica-based rnaterials.
5.4 Chernical Composition and Structure 5.4.1 Correlation of XPS and FTIR Data Referring to Tables 4-2, 4-3, and 4-5, the information obtained from XPS and
FTJR is summarized in Table 5-2, on the following page. From this data, it is seen that there is a strong correlation between the XPS and FTIR data. For example, comparing samples A-C and F with samples D, E, and H-N, it is seen that the oxygentontent, the 01s-Si2p peak separation, the ASM peak width, and the ASM shoulder height al1 increase with high levels of carbon contamination. Additionally, comparing sarnples 1, J,
L, M, and N with samples D, E, O, H, and F, it is seen that the Si2p peak width, 01s-Si2p peak separation, ASM peak wavenurnber, ASM peak width and ASM shoulder height al1 correlate with the XPS-measured oxygen-content of the deposited films.
Increased
oxygen-content reduces the Si2p peak width, the 01s-Si2p peak separation, the ASM peak-width, and the ASM shoulder height, and increases the ASM peak wavenumber.
Table 5-2 - Summary of XPS and FTIR Data for Fs-and ArF-PLD of Si02 FTIR ASM Peak Data
XPS Data
O ls-Si2p Separation (eV)
I
1
Position (cm")
I
ASM FWHM (cm-')
ASM Shoulder
ml
C ( 6 x 1 0 ~air)
D (bakeout, Ar)
F (no bakeout, Ar) G (no ctean, Ar)
H (H.F.,vacuum) 1(H.F., 2x10" air)
J (H.F.,1 x 1 0 ~air) ~
1L
(L.F., 2 x 1 0 ~air)
I I M (LF,air, 400°C)
N (L.F., 4x IO-' 02)
The strong correIation observed within the combined XPS and FîR data set increases the confidence in conclusions which are drawn with respect to the effect of
--=
-------=- -""""
-"
r"-r""--
"'-
-a
""
-- ----- - -------
--
- --
'
Y
delay between sarnple deposition and analysis varied from 2-15 days for XPS analysis and frorn 2 days to 2 months for FïR analysis, for each sarnple, at least one of the two anaIysis techniques was performed within 1 week of deposition. Thus, the observed correlation within the data indicates that the variation of the deposition parameters had a more significant impact on the rneasured film properties than the effect of sample ageing.
5.4.2 Carbon Contamination of the Deposited FiIms As was discussed in Section 4.3, films grown without a predeposition bakeout of the deposition chamber were found to be highly contaminated with carbon (- 50 at. %). It is believed that the source of this carbon was oil contamination of the vacuum system. The vacuum chamber used in the initial deposition experiments was constructed from flat stainless-steel plates joined by external welds. These extemal welds leave nmow cracks between the plates on the vacuum-side of the welds. These cracks, in turn, trapped oil which backstreamed from the mechanical pump during several years of medium-to-low vacuum operation without a liquid-nitrogen trap on the pump line.
The sensitivity of these experiments to contamination is explained by the low deposition rates measured in these experiments, and a brief examination of gas kinetics. It is known that the time required for a monolayer to form on an initially-bare is [86j: surface, in a background gas of pressure P and average molecular size 6,
where S is the sticking coefficient. Thus, assuming a sticking coefficient of 1, air
(d,= 3.72 A)forrns a monolayer in:
Additionally, because the mass of a molecule is approximately proportional to d3, the monolayer formation time can be expected to scale with d?
Thus, the contaminating oil
molecules, which are larger than air molecules, are expected to deposit at a faster rate. However, this is offset by the fact that, in practice, the sticking coefficient is c 1. As a general rule-of-thumb, it has been suggested [2] that to produce pure films, the following inequality should be maintained:
.
base pressure s < IO-' ~ o r-o r depostion rate A
For example, in the experiments performed in the original ablation chamber, the base pressure was 1.7x104
- 5x10.~ Torr, resulting in a base-pressure/deposition-rate quotient of
TOK-S/Afor the deposition rates observed (-
0.03-as). Experimentally, these
conditions were observed to result in significant levels of contamination in the deposited films, as would be expected. In cornparison, in the experiments performed in the veryhigh vacuum PLD chamber, the base pressure was
-
1 0 - ~ Torr, resulting in a base-
recommendation of Equation 5-5, explaining the observed purity of the deposited films.. 5.4.3 Oxygen-Deficiency of SiO, Films Deposited in Vacuum or Argon
Films grown by high fluence F2-laser ablation of fused silica in vacuum or argon were shown in Sections 4.3 and 4.4 to be significantly oxygen-deficient.
This
substoichiometry is likely the result of photo-dissociation of silicon dioxide by the high energy F2-photons. The atomic oxygen generated by this dissociation process would be much more volatile than silicon monoxide or atomic silicon. For this reason the oxygen rnay be consumed by interactions with, for example, background gas constituents or the walls of the deposition chamber. The bandgap of silicon dioxide is
- 9-eV, which is significantly larger than the
7.9-eV F2-photon energy. However, this laser photon energy is comparable to the energy levels of permanent defects which cause the band-edge of fused silica to blur to
- 8eV.
In addition to the defects present in the bulk phase, it is also known that transient defects are generated during the laser ablation process.
These defects cause the effective
absorption coefficient determined from ablation data to be a factor of -104 larger than the optical absorption coefficient at 157-nm [7 11. Thus, photo-dissociation in the bulk phase
may occur as the result of the high-energy F2-laser photons coupling into both the permanent and the transient defect sites.
In addition to the photo-dissociation of the bulk, it also possible that strong absorption of the incident laser energy by the vapour plume may cause further
reach 1000's of O C [59]. These high temperatures will result in the population of excited states from which 7.9-eV photons may photo-dissociate the vapour molecules.
For
example, if the excited states are uniformly distributed in energy, then at a temperature of
3000°C,a Boltzmann distribution will result in 1.4% of the electrons occupying states with excitation energies larger than 1.1-eV. Obviously this mechanism is dependent upon the existence of an appropriate range of excited states from which photon-excited transitions to the vacuum level are allowed. Section 4.3 showed that the oxygen-content of films deposited in vacuum is increased by reducing the ablation fluences (SiOi.6s for > 10-J/cm2 versus Si01.75for 3-4 J/cm2). This relative improvement in stoichiometry apparent in moving from high
fluence to low fluence can be explained by both bulk and plume photo-dissociation mechanisms.
In the case of bulk photo-dissociation, non-linearities in the ablation
process may increase the degree of dissociation with increases in the laser intensity. Likewise, higher fluence~will lead to longer laser-plume interaction times and higher plume temperatures which will enhance photo-dissociation within the plume. The observation that ArF-PLD in argon produced an oxygen-deficient film is contrary to the results presented by Fogarassy et al. [5]. In that work, it was shown that ArF-PLD in vacuum produced stoichiometric films. However, the base pressure in those experiments was relatively high (- IO-' Torr), and oxygen may have been incorporated into the film from the background gas.
for the deposition of superconducting YBCO films by KrF- and ArF-PLD [88]. In both cases, it was found that an O2 ambient was required to produce stoichiometric films, however, higher oxygen pressures were required in the case of ArF-PLD relative to
KrF-PLD. This result was attributed to increased photo-dissociation of C u 0 by the 6.4eV photons produced by the ArF-laser. Similariy, the work of Takigawa et al. [88] has shown that above-bandgap irradiation of Si02 by low fluence 126-nm radiation causes oxygen to desorb from the target surface, producing a silicon-rich layer on the target surface. This work also showed that low fluence irradiation by 146-nm radiation did not produce this effect. Thus, if photo-dissociation during 157-nm ablation does occur from the bulk phase, it may be the result of a non-linear effect due to the higher laser intensities used in the present experiments.
5Ae4 Surface Oxygen-Deficiency of Films Deposited in Dry Air or 4 As described briefly in Section 2.3, 0" XPS scans of films deposited in dry air or oxygen produced calculated stoichiometries of stoichiometry calculations of
- Si01.9, while 60" scans produced
- SiOi.s. This leads to the conclusion that the surfaces of
the deposited films must be oxygen-deficient, since the calibration of the energy analyzer is not dependent upon the measurement angle. Although the mechanism explaining this effect is not known, a quick calculation can be used to predict the range of possible film structures which could lead to this observation.
of thickness d and composition SiO,, then one predicts that the measured oxygen and silicon photoelectron intensities will be:
where h-2.7-nm [80], Fo=0.78, and Fsi=0.4, as discussed in Section 3.3.2.1. Thus, it is possible to calculate a range of points (d,x) for which the oxygen:silicon ratios calculated from the photoelectron intensity ratios are -1.9
and -1.8 for 0" and 60" scans,
respectively. Table 5-3, below, summarizes this range, complete with the calculated oxygen:silicon concentration ratios. As can be seen, based on the data available, is not possible to determine the exact structure of the oxygen-deficient layer, other than to Say that it is likely < 1.5-nm thick. A more definitive conclusion than this would require the
use of a more accurate analysis technique.
-
Table 5-3 Range of Possible Oxygen-Deficient Surface Layer Parameters
In Section 4.2.2, four deposition parameters were shown to strongly affect the quality of F2-PLD Si02 films, as evidenced by changes in the position and shape of the ASM peak in the IR transmission spectrum: vacuum quality, oxygen partial pressure
during deposition, laser fluence, and substrate temperature. The vacuum quality affects film properties through the effects of carbon contamination. As would be expected, reducing the contamination levels by improving the vacuum quality leads to improved film quality, as is demonstrated by reductions in the peak-width (161-cm-' to 122-cm-') and the high-wavenumber shoulder strength (61% to 38%). Laser fluence has also been shown to strongly affect the film quality. For the cases of both deposition in vacuum and deposition in 2 x 1 0 ~Torr of dry air, the low fluence case was found to produce significantly higher quality films than the low fluence case. In particular, for deposition in vacuum, the low fluence condition was found to produce a film with a higher peak-wavenumber (1035-cm1 versus 1030 cm-'), a smaller peak-width (102-cm-' versus 122-cm-'), and a reduced shoulder-height (26% versus 38%), relative to the high fluence condition. For deposition in vacuum, the improvement in film quality c m definitively be linked to the XPS-measured improvement in the oxygen-content of the film deposited with a reduced laser fluence. However, in the case of deposition in air, if there was an improvement in film stoichiometry, it was at a level below the sensitivity of the XPS analysis used. Another cause of the improvement in film
plume species. It is possible that high energy species in the plume, generated by an increase in laser-plume heating at high laser fluence, were responsible for creating defects in the film, reducing the film quality. Increasing the partial-pressure of O2 in the ambient gas from vacuum to 4x105 Torr has been sho& to improve the film quality, as indicated by increased ASM peak-wavenumbers, reduced peak-widths, and reduced shoulder-heights. However, it was found that deposition in 2 x 1 0 ~Torr of dry air and 4x10~' Torr of O2 produced statistically-equivalent results. This observation indicates that the primary effect of the ambient was to improve the film stoichiornetry by supplying oxygen to the depositing film. Although the ambient gas may, in some cases, affect the film quality by slowing the expansion of the ablated plume, this effect was not observed in this pressure range.
A further increase'in the ambient pressure to 1x10-~Torr of dry air was seen to further improve film quality. In this case, it is not possible to definitively determine which mechanism was responsible for the improvement. It is possible that the oxygen content of the film may have been improved by the increase in oxygen partial pressure. However, it is also possible that an increase in the number of collisions between the plume and the background acted to retard highly energetic plume species which would otherwise have generated defects in the deposited film. Finally, for deposition in 2 x 1 0 ~Torr of dry air, increasing the substrate temperature to 400°C was found to significantly improve the film quality. Possible
depositing film and a therrnally-driven increase in the reaction rate for a key step in the incorporation of oxygen from the background gas. For example, an increase in the surface mobility of oxygen adatoms could be responsible for an improvement in film stoichiometry by encouraging the diffusion of these atoms into oxygen vacancies in the lattice. Additionally, an .enhancement of the surface and volume mobility of the film atoms would help to heal defects created by the impact of energetic plume species with the film surface.
6.1 Overview This section discusses the significance of the work presented in this thesis, in the context of previous PLD studies and alternate approaches to Si02 film deposition. Additionally, the scientific importance and the potentid commercialization of F2-PLD of Si02are discussed.
6.2 Comparison With Previous ResnIts The width and .wavenumber position of the asyrnmetric stretching mode absorption peak at
- 1050-1090cm-' are strong indicators of the quality of a Si02 film
[9,69,80]. Table 6-1, below, compares the IR ASM peak parameters of the highest quality F2-PLD SiOz films grown in this work witb previous results obtained by ArF-PLD
-
Table 6-1 Comparison of Si-O-Si ASM Peak Parameters With Previous ResuIts Peak Position (cm-')
Peak FWHM (cm-')
Shoulder Height (cm-')
F2-PLD (25°C)
1047
91
18
F2-PLD (400")
1053
84
15
ArF-PLD [5,69,81 ]
1080
78-80
35
RF-assisted PLD (25°C)[8]
f 072
101
35
VUV-CVD (300°C)[9,83]
1065
70-75
20
Deposition Technique
are for low fluence ablation in 2 x 1 0 ~Torr of dry air, at temperatures of 25°C and 400°C. Likewise, the ArF-PLD result corresponds to
- 10-~/crn~ablation of silicon or silicon
monoxide in 100-mTorr of O2 at 450°C, and the RF-assisted PLD result corresponds to 0 . 4 - ~ / c m ~532-nm ablation of silicon in a 1-mTorr oxygen plasma at 40°C. Additionally, these PLD results are compared with the films produced by W V photoassisted CVD [9,83], a low-temperature (300°C) technique which has been shown to produce films comparable in quality to those grown by high-temperature oxidation. The study of F2-PLD of Si02 films was based on a coarse optirnization of the process parameters, leaving considerable room for future improvement in film quality. Nevertheless, Table 6-1 shows that the best F2-PLD SiO2 films are comparable with the best previous results for PLD of Si02 15,811. In particular, the ASM peak of the best F2-PLD film is marginally wider (- 5-cm-')than in the best ArF-PLD films. However, the high-wavenumber shoulder on the ASM peak of the F2-PLD film, which was linked to structural disorder in the films [69,80], is significantly weaker (- 50%) than in the ArF-PLD film, indicating a higher quality. Relative to previous work studying PLI3 of silica films from a bulk fused silica target, the F2-PLD process has been shown to produce particulate-free films, while results of ArF-PLD (this work and [5]), XeCl-PLD [7], and Nd:YAG PLD [ 6 ] ) were contaminated by 0.1 - 10 prn particulates. These particulates limited the quality which
-
-
-
- - ---- - -
-J
---- O - -
- *
--
a.--
-----
- - - - - - - P r
a
-
--O-----
improvement in moving to the Fzlaser. The need for further optimization of the F2-PLD process is a remaining issue, given that the F2-PLD films are of lower quality than those produced by VUV-assisted
CVD [9,83]. This evidence is provided by the larger (10-14 cm") ASM peak-widths in the F2-PLD films, relative to the WV-assisted CVD films. These CVD films were
shown to have peak-widths comparable to the generally accepted width of 70-cm-' for a high quality thermal oxide [9,83]. Significant room remains for optimization of the deposition parameters.
In
particular, increased partial-pressures of O2may further improve the stoichiometry of the
F2-PLD films by the incorporation of background oxygen to fil1 oxygen vacancies in the
film structure. These higher arnbient pressures may also slow down energetic plume species which can create defects in the growing film. Beyond the effects of increased oxygen pressures, a wide range of growth temperatures and laser fluences remain to be tested.
Additionally, the uniformity of the on-target fluence distribution may be
improved, and the target-substrate geometry may be adjusted during the optimization process. Thus, it is expected that further work should result in the production of higher quality films.
.
The work presented here is of scientific interest for three general reasons: the potential for high quality Si02 film growth, the uniqueness of F2-laser PLD, and the fundamental information which may be learned about the interaction of 157-nm radiation with fused silica. Despite the wide range of materials which have been successfully grown by PLD, no group has successfully demonstrated PLD of extremely high quality silica films (see section 2.4.2). The work of Fogarassy et al. 15,811 produced films of reasonably good quality by ablation of a silicon or silicon monoxide target in 100-mTorr of O2 at 450°C. However, a 20-s 800°C post-deposition rapid thermal annealing step, was required to produce optimal-quality films 1811, limiting the potential applications of this technique to growth on temperature-insensitive substrates. This study has shown that that, after only a coarse optimization, F2-PLD produces films of similar quaiity to the best previous
ArF-PLD results [5,8 11, without a high-temperature anneding step. Due to the relatively small number of F2-lasers currently in use in labs worldwide, the work desci-ibed in this thesis is only the second-known demonstration of F2-PLD. However, F2-PLD is of general interest to the scientific community due to the novel applications that may be enabled by the strong interaction of the 7.9-eV F2-laser photons with most materials. This strong laser-material interaction will allow PLD to be extended to the ablation of targets which are transparent to radiation from conventional lasers. For example, in the work of Fujii et al. [73],the deposition of fluoropolymer thin-
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between the short wavelength radiation and the laser-ablated plume will allow the formation of highly excited plume species. An exarnple of one such potential application of F2-PLD which has not yet been explored is in the area of hard coatings, such as diarnond-Iike carbon. In that case, a strong interaction of the 157-nm radiation with the laser-ablated plume should produce the highly-excited plume species which are needed for the growth of the metastable phases in hard coatings 1173. F2-laser processing of fused silica and related wide-bandgap materials may lead to novel applications in the production of photonic and electronic devices. Current areas of interest include: fibre and planar-waveguide photosensitivity, laser rnicromachining of fused silica for the fabrication of optical waveguide devices, and laser micromachining of crystal quartz for applications in rnicrowave devices [7 11.
In order to optimize these processes, the interaction of the 157-nm radiation with fused silica needs to be thoroughly understood. Studying the fiIms deposited by F2-laser ablation of fused silica is one technique for studying the laser-material interaction. In particular, this work has shown that Silicon dioxide dissociates during F2-laser ablation, leading to the questions of where this dissociation occurs, and whether it is due to a purely photochemical process or a combination of thermal and photochernical effects.
Two major issues must be addressed before F2-PLD of Si02 may be used in a commercial product or process: deposition rate and process optimization. The deposition rates measured in these experiments (- 0.02-0.04 A/s over an area
- 2.5-cm in diameter) are much too low to be cornrnercially viable.
Commercially
available excimer lasers operate at up to 300 Hz, which represents a factor of 200-300 improvement over the pulse-rates used in these experiments. Additionally, optimization of the optical system and the laser design may increase the on-target-energy by a factor of
- 5, leading to a total deposition rate of - 150-200 n m h , averaged over a 20-cm diameter wafer. This rate remains relatively small, when compared with rapid thermal
CVD, for which deposition rates larger than 6.0 Clm/hr may be achieved [4]. Due to the low deposition rates predicted for F2-PLD of Sion, relative to more conventional techniques such as rapid thermal CVD,an optirnized PLD process must produce unique films not producible by the lower-cost conventional techniques. In particular, the areas in which F2-PLD of SiO2 may have an advantage over techniques such as CVD are: deposition at low temperatures (c 450°C), growth of films with complicated doping profiles, and the incorporation of SiO2 films into heterostnictures with other PLD-grown rnaterials. The required SiOl film thicknesses for electronics dielectnc layers are only 7-nm. Thus, the relatively modest deposition rates predicted for F2-PLD may be sufficient for
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very high. Although the quality of films presented in this thesis are comparable with the
best results produced by PLD [5,8 11, they are of lower structura1 quality than films produced by WV-lamp-assisted CVD [9,83].
Thus, further process optimization,
focussing upon the electronic properties of the deposited films, is needed before F2-PLD may be applied to the growth of Si02 for electronics applications. The work presented here provides direction for this effort by presenting an overview of the effects of the primary deposition parameters on the structural quality of the deposited films.
In the area of optical waveguide applications, the required films are much thicker (several pm) than those produced here (10-30 nm). However, the required structural quality of the deposited films is lower than that dernanded in electronics. Thus, it is possible that the present F2-PLD process rnay already produce films of sufficient quality for these applications, al'though fuaher analysis is needed to characterize the optical properties of the deposited films. Additionally, the slow deposition rates predicted for F2-PLD, relative to CVD techniques, may be offset by the relative simplicity of introducing controlled concentrations of dopants in the PLD-grown films.
Pulsed-laser deposition of Si02 films has been extended, for the first tirne, to the vacuum-ultraviolet 157-nm wavelength of the F2-laser. This short-wavelength extension is expected to enable the low-temperature growth of high quality silica films with controlled doping profiles-for applications in the electronics andior photonics industries. The work presented here has exarnined the effects of the laser fluence, background gas, and substrate temperature on the properties of the deposited films. A very-high-vacuum deposition chamber h a been built and implemented to meet the specific challenges of working with the F2-laser. AFM, XPS,and FTIR have been used to determine the surface roughness, chernical composition, and structural quality, respectively, of the deposited films.
Particular attention has been paid to the IR
absorption peak corresponding to the asyrnmetric stretching mode (ASM) of the Si-O-Si bond, which is known to be a strong indicator of the structural quality of the deposited films 19,801.
AFM has shown that the relatively strong absorption of 157-nm radiation by fused silica enables the growth of virtually particulate-free 15-nm-thick SiOz films from a bulk fused silica target, in sharp contrast to results obtained with conventional, longerwavelength lasers (this work and [5,6,7]). This particulate-free deposition from a fused silica target is expected to enable the growth of high quality silica films at low substratetemperatures.
silicon dioxide during ablation, causing films deposited in vacuum or argon to be significantly oxygen-deficient (Si01.55-1.75).This oxygen-deficiency has been observed to be reduced by the use of a low laser fluence (3-4 ~ / c r nversus ~ > 10-~/cm~).
The effect of dissociation during ablation was counteracted by deposition in a relatively low partial-pressure of oxygen (4x10" Torr), which significantly increased the oxygen-content of the deposited films (to SiOl.g.2.0). This resulted in a corresponding improvement in the film structure (- 20-25% reduction in the ASM peak width). Additionally, increasing the substrate temperature from 25°C to 400°C produced a further 8% reduction in the ASM peak width. The coarse optimization process described in this thesis has produced F2-PLD grown silica films (for ablation by 3-4 k m 2 in 2 x 1 0 ~Torr of dry air, at a,temperature of 400°C) which are comparable to the best previous Si02 films grown by PLD [5,81].
Although these best F2-PLD grown SiO2 films are structurally inferior to the highest quality oxides deposited by techniques such as VUV photo-assisted CVD [9,83], significant room remains for optimization of the F2-PLD process. Thus, it is expected that fine tuning of F2-PLD may yet produce very high quality, particulate-free silica films, at low substrate-temperatures (c450°C). Additionally, it is possible that films of lessthan-optimal structural quality will be acceptable for use in some applications, such as optical structures. ~ o w e v e rthese , applications may require optimization of other process characteristics, such as the deposition rate.
deposition parameters, depositing thicker films, examining additional film properties, growing doped-silica films, examining other materials systems, and studying the fundamental aspects of the interaction of 157-nm radiation with fused silica. Additional experiments are required to optimize the deposition process. The effects of higher partial-pressures of oxygen (> 2 x 1 0 ~TOITof 02)should be explored and a detailed examination of the effects of substrate temperature is needed. The on-target fluence should also be optimized, and made more uniforrn with a new lens that eliminates birefringence. Optimization of the F2-PLD process will also require the characterization of application-specific film properties, including:
electrical properties (i.e. dielectric
constant and breakdown voltage), optical properties (i.e. absorption and refractive index),
and mechanical properties (i.e. density, stress, chemical resistance, and adhesion to the substrate). Thick films (- 1-10 pm) are required for optical applications. The growth of these films should be facilitated by a new 20-50 Hz
F2
laser which is currently being
tested in the lab. The deposition of thicker films will also improve the accuracy of the
FTIR analysis of the deposited films by reducing the sensitivity of the technique to thin layers of surface contamination. With the application of a high repetition-rate laser to the deposition process, thick films for optical applications may be grown. At this stage, it will be necessary to study
oxide, in the deposited film. This will allow the formation of passive and active optical waveguiding regions which may be examined for their application to integrated optical circuits. As was mentioned in Section 6.3.2, there is interest in the application of F2-PLD to materials other than pure and doped silica. In particular in the growth of hard coatings such as diarnond-like carbon, a strong interaction between the high energy F2-laser photons and the ablated plume may create highly excited plume species, enabling the growth of high quality films of this important class of material. Additionally, F2-PLD may be applied to other wide bandgap materials, such as laser host crystals (YAG and
YLF) and fluorides (MgF2, CS2, and LiF) for potentially novel applications. Finally, the work presented here provides insight into the interaction of the high energy F2-photons with silica-based materials.
However, it also raises additional
questions. Of particular interest is the determination of whether the dissociation of silicon dioxide during ablation occurs in the bulk, the plume, or both. The answer to this question is of importance, not only in optimizing the F 2 - P D process, but also in the application of the F2-laser to photosensitivity and laser-ablative micromachining of silicabased materiah.
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~hotoele&-o~~pectroscop~, ~ e r k i n~lme'Corporation, Eden ~ i a r i (1 e 992).
"
76. Lu, Z.H., M.J. Graham, S.P. Tay, D.T. Jiang, and K.H.Tan, "Effects of growth temperature on the Si02/Si(100) interface structure", J. Vac. Sci. Technol. B 13 (4), 1626-1629 (1995). 77. Tao, Y., Z.H. Lu, M.J. Graham, and S.P. Tay, "X-ray photoelectron spectroscopy and x-ray absorption near-edge spectroscopy study of Si02/Si(100)", J. Vac. Soc. Technol. B 12 (4), 2500-2503(1994). 78. Yano, F., A. Hiraoka, T. Itoga, H. Kojima, K. Kanehori, and Y. Mitsui, "X-ray photoelectron spectroscopy study of submonolayer native oxides on HF-treated Si surfaces", J. Vac. Sci. Technol. A 13(6), 2671-2675 (1995). 79. Briggs, D., and M.P. Seah, Practical Surj6ace Analysis, John Wiley & Sons, Inc., Chichester, 169472 (1990). 80. Bensch, W., and W. Bergholz, "An Fï-IR study of silicon dioxides for VLSI microelectronics", Semicond. Sci. Technol. 5,421-428 (1990). 81. Slaoui, A., E. Fogarassy, C. Fuchs, and J.P. Stoquert, "Influence of arnbient gas and substrate temperature in preparation of silicon dioxide films by laser ablation", in Laser Ablation of Electronic Materials - Basic Mechanisms and Applications, E. Fogarassy and S. Lazare (Editors), Elsevier Science Publishers B .V., 35 1-361 (1992). 82. Fogarassy, E., C. Fuchs, A. Slaoui, S. de Unamuno, J.-P. Stoquert, W. Marine, and B. Lang, " h w temperature synthesis of silicon oxide and oxynitride films by reactive laser ablation", Mat. Res. Soc. Symp. Proc. Vol. 285,3 19-329 (1993). 83. Bergonzo, P., and I.W. Boyd, "Rapid photochernical deposition of silicon dioxide films using an excimer lamp", J. Appl. Phys. 76 (7),4372-4376 (1994). 84. Saenger, K.L., "AnguIar distribution of ablated material", in Pulsed Laser Deposition of Thin Films, D.B.Chrisey and G.K.Hubler, editors, John Wiley & Sons, Inc., New York, 199-227 (1994).
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J. Non-Crystal. Solids, 125, 107-1 16 (1990).
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In order to maxirnize the on-target laser energy, a single 5.0-cm focal length, 1.5"-O.D., magnesium-fluoride lens was used to focus the laser beam without the use of
an imaging aperture. Due to the relatively large size of the laser beam by the time it reaches the PLD charnber, approximately 50% of the beam energy is lost before the light reaches the lens. A coarse estimate for the lens transmission predicts that another 33%of the beam energy is lost in passing through the lens, producing an estimated on-target energy of 15-mJ for a typical 45-ml laser pulse. The 5.0-cm focal length of the lens allows large optical demagnifications to be obtained, yielding high laser fluences on target. However, the close proximity of the lens to the target causes it to be coated during regular P D operation, making periodic cleaning necessary (every 5-10 experiments). Unforhinately, this lens is birefiingent, with the optical axis in the plane of the lens, causing the horizontally and vertically polarized light to see different indices of refraction. In particular, the ordinary and extraordinary indices of refraction of MgF2 at 157-nm are 1.464 and 1.478, causing the lens to have focal lengths of 5.0-cm and 4.85cm for the two polarizations. This leads to a corresponding separation between the image planes, as outlined in Table 5-1 on the following page, making precise definition of laser fluence difficult.
The optical imaging problem is further complicated by the beam divergent, which causes the energy density at the object plane to be dependent upon the distance of the
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0t;am uivergence ais0 increases me slze
or m e rocus spot
relative to the diffraction Iimit for a collimated beam, and causes the focus plane to shift
- 5.05-cm for the ordinary polarization. The measured divergence of the beam is - 4-rnrad in the vertical plane and - Zmrad in the horizontal plane. away from the lens to
-
Table A-1 Effect of Lens Birefringence on the Optical Imaging System
I
Relative Lens Position 2.0-mm 3.5-mm
Ordinary Beam
F, - 15-3/cm2 F, 1-J/cm2
4.0-mm
. >25-~/cm~ focused, F
5.5-mm
imaged, F. = 2.8-J/cm2
Extraordinary Beam
I
focused, F. > 2 5 - ~ / c m ~ imaged, F, = 2.84/cm2
Fo- 1-J/cm2
-
F. 0.6-Ucm2
The result of diffraction and birefringence is that the image and focal planes of the two polarizations overlap. For example, Table 5-1, shows that in moving the lens doser to the lens, the focus plane for the ordinary polarization is reached before the image plane for the extraordinary polarization.
Thus, choosing to position the lens at 5.5-mm
produces a relatively well-defined fluence of
- 3.5-Jlcm2, however, the rnajority of the
energy in the extraordinary polarization is not used in ablation. Alternatively, positioning the lens at 3.5-mm ensures that the rnajority of the beam energy contributes to ablation, however, the fluence distribution on target is extrernely non-uniform.
A IMAGE,lnc ----
APPLIED
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