Using Infrared Spectroscopy Structures Energy ...

Structural and Compositional Characterization of High Energy Separation by Implantation of Oxygen Structures Using Infrared Spectroscopy D. I. Siapkas, N. Hatzopoulos, C. C. Katsidis, T. Zorba, C. L. Mitsas and P. L. F. Hemment J. Electrochem. Soc. 1996, Volume 143, Issue 9, Pages 3019-3032. doi: 10.1149/1.1837142 Email alerting service

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J. Electrochem. Soc., Vol. 143, No.9, September 1996 The Electrochemical Society, Inc.

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Structural and Compositional Characterization of High Energy Separation by Implantation of Oxygen Structures Using Infrared Spectroscopy D. I. Siapkas, N. HatzopouIos, C. C. Katsidis, 1. Zorba, and C. 1. Mitsos

Solid State Section, Department of Physics, Aristotle University of Thessaloniki, 54006 Thessaloniki, Greece P. i. F. Hemment

Department of Electronic and Electrical Engineering, University of Surrey, Guildford, Surrey G U2 5XH, England ABSTRACT

Silicon was implanted with 2 MeV O ions with doses covering the range from 1 X 1017 to 2 X 1018 O cm2, at an implantation temperature of 700°C. Subsequently, samples were capped and annealed at 1300°C. Infrared reflectance spectroscopy has been used in order to characterize the as-implanted and annealed samples. The optical modeling of the multilayer structures and the data reduction procedure are given in detail. The thickness, chemical composition, crystallinity, interface macroscopic roughness, and refractive index profiles are quantified. It is shown that infrared reflectance spectroscopy is a quick, nondestructive, analytical, and precise method for characterizing highenergy separation by implantation of oxygen (SIMOX) structures. Cross correlation with H beam Rutherford backscattering/channeling, secondary ion mass spectroscopy, and cross-sectional transmission electron microscopy results, gives good agreement. The formation of oxide in the high energy region follows the same basic rules as in the standard SIMOXcase. No anomalous oxygen diffusion was observed during annealing and a buried layer formed during annealing even for the lowest dose. It is found that the microstructure of the annealed samples is strongly dependent on the implantation conditions such as beam current density and that even for the highest dose of 2 X 101 0 cm', a continuous stoichiometric silicon dioxide layer has not formed after annealing. Infroduction Separation by implantation of oxygen (SIMOX) technol-

ogy in the medium energy range (150 to 400 keV) is well studied and understood.' Lately, the availability of a new generation of megaelectron voltage (MeV) implanters has sparked a new interest in high energy implantation and its potential applications.2 High energy (MeV) SIMOX technology could find applications in the bipolar metal oxide On leave from Research Centre Rossendorf, Institute for Ion Beam Physics and Materials Research, 01314 Dresden, Germany.

semiconductor (MOS)-silicon on insulator (SOl) technology,3 could be used as an alternative to standard SIMOX technology in order to reduce both the implantation temperature and the induced damage during implantation,4'5

and could be potentially used for the fabrication of Si-

based optical waveguides. Since the late 1970s high energy SIMOX structures have been mainly analyzed by cross-sectional transmission electron microscopy (XTEM), Rutherford backscattering spectroscopy (RBS), and nuclear reaction analysis (NRA).4-15 Besides all this activity, up to now no nondestructive tech-

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niques have been reported for the characterization of high energy SIMOX, although the infrared dielectric response of semiconductive epitaxial,'6 ion beam doped,18 and ion beam synthesized19'2° thin films, has often been used in

order to obtain information concerning their structural and electrical properties. Since MeV implantation of oxygen in silicon creates structures with thickness in the order of micrometers, the most suitable nondestructive analytical technique is infrared (IR) reflectance spectroscopy. The large penetration depth of the JR radiation enables even the thickest of the samples produced by ion implantation to be evaluated with no need for beveling. Moreover, the strong interaction of infrared radiation with atomic vibra-

tions and free carriers enables the investigation of the nature of the formed silicon oxide (stoichiometry and stress) and, especially in the case of intentional doping, of

the Si layer electrical transport properties (free carrier density and mobility), respectively.

Figure la demonstrates the modification of the JR

reflectance spectrum of a single-crystal Si wafer (bulk Si),

after implantation of 1.2 MeV O ions to a dose of 2 >< 1018 cm2 and subsequent annealing in two steps at 1300°C for 6 h21 (SJMOX, in Fig. la). In the SIMOX spectrum in

Fig. la, the peaks seen centered at 455 and 1075 cm' are attributed to vibronic resonances in Si dioxide, and so it is concluded that buried silicon oxide has been formed. Between the higher longitudinal optical (LO) frequency of silicon dioxide and the fundamental bandgap of Si, large amplitude oscillations are present in the reflectance spectrum which are the result of interference of multiplied light reflected between the front surface and the buried dioxide layer. The presence and shape of these large ampli-

tude fringes indicates that the refractive index of the

buried layer is substantially different from that of silicon as well as that of a well-defined interface between the Si overlayer and the buried layer which has been created

after high temperature annealing. It is only at wave numbers less than 1500 cm' that the dispersion, caused by sil-

icon dioxide atomic vibrations, distorts the periodicity

and amplitude of the interference fringes. An even larger distortion to the SIMOX spectrum interference fringes can be seen when free carrier excitations

are present in the Si overlayer. Figure lb shows the

reflectance spectrum of the SIMOX sample whose spec-

trum can be seen in Fig. la, after implantation with 1.5 MeV AC ions to a dose of 6 )< 1015 cm2 and subsequent

annealing at 1100°C for 0.5 h (in order to activate the As atoms21). As a consequence of the arsenic activation, excitations attributed to the generated free carriers, appear in the low frequency part of the spectrum in Fig. ib, thus distorting the former periodicity of the SIMOX spectrum in Fig la. In Fig. lb, the resonances due to atomic vibrations of the dioxide appear completely screened by the free carrier excitations and a reflectance plasma edge appears in the spectrum. From the far-JR reflectance measurements, where free carrier excitations are dominant, it is possible

to determine electrical transport parameters with the

same, or even better, accuracy as with electrical measurements but without electrodes and contacts.2'21 In order to analyze JR spectra taken from SIMOX samples such as those seen in Fig- la, the following procedure

is followed: the reflectivity of the sample is calculated (based on a generalized matrix method24), as a result of a detailed multilayer modeling of the sample structure. A regression analysis program then fits the calculated spectrum to the experimental data. This yields the best fit parameters (such as thickness, composition, crystallinity, interface roughness, and refractive index depth profiles) for the model under trial. Different structural models are tried, and the one that gives the lowest unbiased estimator value (see later, the section on Regression analysis) is finally selected as the best fit model. The examination of the JR spectra is limited to wave numbers greater than 1500 cm where it can be assumed that dispersion caused

by the silicon dioxide absorption band is negligible. 0.9

Bulk Si SIMOX

Moreover, since the Si wafers examined are only lightly doped, the resonances attributed to free carrier excitations

are confined to the spectral region around 50 cm and

below. This means that free carrier effects on the spectra above 1500 cm will also be negligible. As a consequence the interference fringe pattern is the main spectral feature above 1500 cm' and is studied in detail. This paper reports detailed results for annealed SIMOX

0.6

samples, coming out of a general project studying the growth, formation, and characterization of high energy

SIMOX structures. Preliminary results of this work have already been reported.25 Proton beam RBS, secondary ion mass spectroscopy (SIMS), and XTEM, have been used to compare with, and to provide complementary information to JR data. In the as-implanted samples the oxygen distribution, as well as the refractive index depth profile, are seen to follow a skewed Gaussian shape up to the dose just enough for stoichiometry.21 In the annealed samples, oxy-

0.3

0

0.9

gen redistribution and segregation during annealing

change the shape of the oxygen distribution to that of a 11)

distinguishable layer with a roughly constant oxygen concentration and fairly abrupt interfaces (SIMS results, later on). The refractive index profile again follows closely the shape of the oxygen distribution. Furthermore, roughness of the layer interfaces is detected due to the scattering of the JR radiation at these interfaces.

0.6 C C) C)

0.3

Experimental 2 MeV oxygen ions ('O) were implanted into , p-

0

1000

Wavenumber (cm') Fig. 1. IR reflectance spectra of single-crystal Si implanted with 0 ions at 1.2 MeV energy to a dose of 2 x 1018 crn2: (a} annealed in two steps at 1300°C for 6 h, (b) implanted with AC ions at 1.5 MeV to a dose of 6 x 10, cm2 and annealed at 1100°C for 0.5 h.

type, device grade Si wafers, using the UVEE 2 MV Van de Graaff heavy ion accelerator at the University of Surrey.21

After electrostatically scanning the beam on the defining aperture, the current on the Faraday cups was 1 to 20 iA, giving a current density in the range 0.25 to 5 p.A cm2. The oxygen dose ranged from 1 x loll to 2 x 1018 0 cm2. To carry out high temperature implants, an HVEE heated sample holder was used which accepted only one wafer

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.1 Electrochem. Soc., Vol. 143, No.9, September 1996 The Electrochemical Society, Inc.

at a time. Wafers were clamped onto a thin mica sheet on a metal block which was resistively heated and heat was transferred to the wafer by conduction. Prior to implantation the wafers were preheated and then maintained at a constant temperature throughout the implant. The temperature could be controlled over the range 300 to 700°C, where the temperature was monitored by a thermocouple

the layer, and (ii) that the interfaces are parallel, optically

water cooled in order to dissipate excess heat. To establish the relationship between the indicated temperature from a

refractive index of the buried layer and by using a gener-

inserted in the heating block. The sample holder was

control thermocouple and the actual wafer temperature, another thermocouple was glued (using liquid silica) onto the surface of a wafer. Temperature values from the wafer thermocouple were on average 100°C less than those indicated by the control thermocouple. The value of current density routinely used was 2.5 1sA cm2. Power loadings for the combination of current and voltage used (P = I x V) were between 2 and 10 W cm2, resulting to a maximum

temperature between 250 and 520°C, respectively, for beam heating only. For routine power loading implants (up

to 5 W cm2) and for wafer temperatures greater than

500°C, the wafer temperature remained the same either with or without beam heating. During the high power loading (10 W cm2) implants, the wafer temperature was about 30% higher28 with than without beam heating. So, by specifically choosing the heater temperatures according to the power loadings, it was possible to have a wafer temperature of 700°C for all samples. Samples were capped with Si02 and annealed at 1300°C for 6 or 12 h. Infrared reflection measurements were made using a

Fourier spectrophotometer Model Bruker IFS 113v (extended), with a working frequency range of 10 to 20,000

cm', at the University of Thessaloniki. The angle of incidence was near normal, and the accuracy was better than 0.3%. A specially designed sample holder was used to take reflection measurements. A fresh front aluminized mirror was used as a reference. The samples were heavily lapped from the back side to avoid any reflection from the back side of the substrate. The H beam RBS/Channeling was performed using 1 MeV energy protons with a scattering angle of 150°, at the University of Surrey. The XTEM and SIMS analyses were performed at Imperial College, using a JEOL 200 CX operating at 200 key and the ATOMIKA Model 6500 ion micro-

brobe, respectively. For SIMS, the primary beam was 10 key Cs ions, the currents were between 0.5 and 2 p.A, while the beam was raster scanned to produce a 600 x 600 p.m crater in the sample. Underlying Theory and Optical Data Analysis Optical modeling of multilayer structures.—The analysis of the multilayer thin film optical spectra was accomplished by using a multiple wavelength method29'3° by which curve fitting procedures with the use of specific dispersion relations of the optical constants were employed. Other methods often used in the analysis of optical spec-

flat with maximum layer thickness on the order of the wavelength of the incident radiation thus excluding the treatment of incoherently reflected and transmitted light.

The extraction of meaningful information from the lB spectra of SIMOX structures requires the lifting of these constraints in the model. This has been done by using a two-half joined Gaussian-shaped distributions for the

alized matrix method which allows the treatment of coherent, partially coherent, and incoherent multiply

reflected beams inside the multilayer media in the same way.24 This incoherence results from the incorporation of macroscopic surface and interface roughness effects, as well as the effect of a thick but finite substrate. Consider the multilayer structure shown in Fig. 2(a) composed of n layers with complex refractive indexes = n1 — ik1 and n + 1 interfaces. The Fresnel reflection and transmission coefficients of the ith interface are r and t1, respectively, defined in terms of the electric field amplitudes EIR, EIL, ER, and EL, where 1? and L signify right and

left going waves and the prime distinguishes between

waves on the right and on the left of the interface. The optical admittance of the ith interface is defined as the ratio of the tangential components of the E and H field amplitudes as —N. cosO. for s-waves or N1 /cosO1 for p - waves

E1L

iii

[1]

01 being the angle of incidence of radiation and the Fresnel

reflection and transmission coefficients are written in

-'

terms of the optical admittance (F) as

r=

— 2Y1_1

1Ry+

y: -y. ______

tiL =

[2a]

2Y.1

[2b]

The application of the boundary conditions at the ith

interface yields the relation between the field amplitudes to the right and to the left of the interface which cast in matrix form is

i( \ E1 IL) t (EIR') =

1

IR

cL

ttiR IL— llRliL) IL

[3]

The 2 )< 2 matrix relating the separated field amplitudes is called the refraction matrix W1111 and is given above in its

most general form.24 The usual form of the refraction

tra include single wavelength methods31'32 by which a system of nonlinear equations is solved with no assumptions

on the relationship between the index of refraction n and the extinction coefficient k, and direct methods33'34 by which the optical constants are determined by the position and the height of the interference fringes resulting from multiple reflections within the structure. The calculation of the reflectance and transmittance of a multilayer system such as a high energy SIMOX structure is readily han-

dled by matrix methods35 whereby a system transfer

matrix is defined, transforming the electromagnetic field amplitudes on one side of the multilayer system to those on the other. In such a matrix method, or any other method for that matter, in order to deconvolute the interference fringes resulting from multiple reflections within the structure, two simplifying assumptions are usually made, namely, (i) that the layers composing the structure are homogeneous and isotropic, imposing a constant refractive index within

i-i

I

n

,t÷1

(a)

E substrate

I

1.1

n

n+1

(b)

Fig. 2. Optical multilayers composed of n + 1 interfaces and n layers used in calculating the reflectivity of: (a) a system composed of n coherent layers and (b) a system composed of n — 1 colrnrent layers and an additional thick (incoherent) layer.

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J. Electrochem. Soc., Vol. 143, No. 9, September 1996 The Electrochemical Society, Inc.

matrix cited does not take into account that, in general, the term tiRtiL — rIRrIL is a complex quantity of unit magnitude which has a phase associated with it. Thus the refraction matrix that most often appears has the form264° 1 'iR

=

tr1

[4]

The fact that the field amplitudes inside the ith layer are not independent but are related by a phase difference ip = 2irwN1d1 (where w is the wave number and d1 is the layer thickness) due to an optical path difference is represented by the matrix relation 0

= e"'

E:_1R

EIR

0 e'' E1L

Ef_l,L

[5]

where the 2 X 2 diagonal matrix is referred to as the phase

matrix V. The repeated application of the above transformations

in which the superscript (o) quantities refer to Fresnel coefficients of smooth surfaces and s = 2rZ.

Complete incoherence of reflected and transmitted waves in multilayer thin film structures usually results from reflection of the beams from the back side of a thick substrate when no special precautions are taken to eliminate this effect, such as artificially roughening the back

face of the substrate by lapping or by covering it with black velvet cloth which, of course, excludes any transmittance data from being acquired. The calculation of the system Fresnel coefficients in this case is carried out in two steps. First the transfer matrix of the n — 1 coherent layers is calculated (Fig. 2b) reducing them to a single effective interface. Following, the multiplication of

sin_i by the phase matrix U,, of the substrate and the refraction matrix W,,1,, of the n + lth interface is performed yielding the system transfer matrix as 5 = sin—i U,, W,,101

[10]

for the n + 1 interfaces and n layers of a multilayer system transforms the separated field amplitudes to the right of the system to those on the left. The product matrix result-

the elements of which produce the required Fresnel coefficients corrected for any substrate back reflectance. Such an algorithm has been developed by us in FORTRAN 77 code running on a 486 PC compatible machine.

referred to as the system transfer matrix S so that

Dispersion of optical constants.—In the previously described multilayer analysis, an unlimited number of uniform layers can be used to simulate the examined

ing from the above procedure is again a 2 X 2 matrix

1EQL)

= (s si2')1E+iz s2, s22fiE+iL

[6]

Then the Fresnel coefficients of the multilayer are given in terms of the transfer matrix elements as

r ——

— S21

[7a]

Si!

1 tR

11 —

—— l2

[7b]

[7c]

SI'

tL

DetS

[7d]

11

from which the reflectances R, B' and transmittances T, T' are obtained as the magnitudes of the complex vectors rE, rL and tR, tL, respectively.

So far no reference has been made concerning the polarization of the incident light (except in the definition of Y3 or the degree of coherency of the multiply reflected beams inside the multilayer system so that the above formalism for calculating the multilayer Fresnel coefficients works equally well for all kinds of incident radiation and multilayer systems. Partial incoherence produced by macroscopic surface or interface roughness can be introduced by modifying the Fresnel coefficients of the respective interfaces by terms which represent the phase differences in the reflected and transmitted beams due to a Gaussian distribution of irregularities of height 5k and of rms height Z. These modified coefficients for a rough surface are41'42

r0R = re2"°' = ar = br r0 = rOLe toR = te_V2

o)2rij—n,)2 = ct

toL = te_h/2 2,n)l =

[8a]

l$b] [8c]

[3d]

and for a rough ith interface

r1 = rfe2"'"°' = ar

[9a]

rIL = rje_2(lim)O = t3r

[9b]

= te_hh2 2(n,_n_i)1 =

[9c]

tiL = tje_1/2 )1(n,_1_n,)2 = 'ytjj

19d]

tIR

implanted region. Each layer is defined by its thickness d,

and by its complex refractive index N, or its complex dielectric constant E, which is the square of the complex refractive index.43 In the general case

= —

t— 1'

A€(w.ro) 2

2

UI —17U)

____________ __________ 2

(U +ry.,U)

+€_ [11]

In the study of the spectra of annealed doped SIMOX structures (Fig. 1) all three terms should be considered and their parameters should be given a depth distribution.21'22 The first term is a sum of the contributions to the disper-

sion due to the interaction of ions or atoms of the solid with the electromagnetic field, referred to as lattice dis-

persion. The atomic vibrational parameters wTO, -y, and & have their usual spectroscopic meaning.22 The second term is due to intraband carrier transitions w: plasma frequen-

cy, -yr: free carrier damping) within the conduction or valence band while the third term, €,,, is due to bound elec-

trons. Far below the fundamental bandgap of Si the last term, €, is usually assumed to be a constant real number,

independent of energy. This assumption will be satisfied if bound-electron absorption occurs at wave numbers considerably greater than the spectral region involved in the reflectivity measurements. In our analysis, as a more accu-

rate representation of on the low-frequency side of the fundamental absorption edge, the bound electron contribution is expressed via a Sellmeier equation. This contribution, taking into account that the level of absorption is very low, can be considered to be real with a slight dependence upon energy as being expressed by the Sellmeier

equation. The above are justified since the analysis was limited to the spectral region 1500 to 7000 cm', that is,

below the fundamental energy gap, Eg = 1.12 eV

(9000 cm') of Si. Also in this spectral region, which is above the highest LO frequency (1240 cmj of the buried oxide layer atomic vibrations, the lattice contribution becomes of little importance. In the case of undoped SIMOX structures studied here, only unintentional doping of the Si wafers is present, confining the free carrier plas-

ma resonances to the spectral region around 50 cm' and below. So, the free carrier contribution to the dielectric function (w) of the Si overlayer and Si substrate is negli-

gible (ton = 0), and only the bound electron contribution is considered in the analysis. For the real part of the refractive index, n0, of the recrystallized Si overlayer as well as for the Si substrate, n3, a Sellmeier equation of the form n0 = [Asr + Bsr/(flsr —wi]

[12]

3023


was used. The above equation also expresses the real part of the dielectric function, E1 n2 — k2, since it is assumed

that k 0.

At this point it is worth noticing that as w approaches zero the refractive index and the dielectric function are both reduced in value. At w = 0 the dielectric function takes the constant value n2 = (A + B/a2) = €. which is the constant third term in Eq. 11. The values of the constants A, B, and il for the substrate

Si, found to give the best fits were: A5 = 4.1485, B = 5.8951 cm2, fl5 27974 cm'. In the case of recrystallized Si, since r is slightly higher than n5, it was convenient to introduce a fitting parameter fr to express r as the product of fr and n5,4425 that is nr = frfls = fr [A5 + BJ(fl2 —

This leads to a significant reduction of the number of parameters using onlyf and the substrate constants A5, B5, and a, defined above. The dispersion of the refractive index for Si02 (n502) was

taken from the three-term Sellmeier equation given by Malitson45 —

St02

1

= A2

m

+

m3X2 A2 —

m

+

m5X2 A2 —

m

[13]

where A is the wavelength expressed in microns, and m1 =

0.6961663, m2 = 0.0684043, m3 = 0.4079426, m4 = 0.1162414, m5 = 0.8974794, m6 = 9.896161 are given con-

stants. Equation 13 generates data which are equal to

those reported by Philip46 to the third decimal point for the wavenumber region of interest here.

Models for the refractive index depth profile—As-

implanted state.—The refractive index (n) of the Si overlayer was assumed to be recrystallized (nr) after implanta-

tion at high temperatures, while the substrate is

considered as bulk Si (n,). By making the assumption that all of the implanted oxygen is bonded in an Si02 configuration with neighboring Si atoms, one can correspond the oxygen depth profile to the (n) depth profile for the region of the buried layer, as the (n) depth profile is influenced by the amount of Si02 in

the buried layer. The oxygen depth profile in the asimplanted samples is known from the literature to be

skewed Gaussian in form, and so an assumption that the (n) profile in this region is of the same shape, appears to be reasonable. For doses (1) lower than the saturation dose l55t (the dose for which the peak oxygen concentration exceeds the level, 4.67 X 1022 cm3, for stoichiometric Si02), which is 2.0 x 1018 O cm2 for 2 MeV oxygen in Si, see Ref. 25, a skewed Gaussian profile is chosen as the fitting function for the refractive index.

(rrR/10), having different indexes of refraction, n, as determined by the equation = n8— [n —

exp [(—1/2){[--Ki.R + ôx(j

—

2)]/.RJ2]

[14]

where n is the refractive index at the tail of the halfGaussian (which can be either r or n, depending on whether it is the surface or substrate side half-Gaussian), can be either R1 or R2 and j is a counter for the layers comprising the multilayer structure. The Gaussian distribution is approximated by the layers from j 2 to j = m — 1. The adjustable parameters defining the distribution are R,,,,,, and n(R,,,,). Figure 3 shows a multilayer structure of m layers which approximates an implanted sample having a Gaussianshaped refractive index profile in the implanted region.

Here, a normal Gaussian profile is shown but usually joined half-Gaussians are used. The extinction coefficient, that is, the imaginary part of the refractive index can also be given a Gaussian depth

dependence but as discussed in the introduction it is

assumed for k] to be approximately zero. Annealed state.—In the models for the annealed SIMOX samples the Si overlayer can be recrystallized Si, while the substrate is assumed to be c-Si. When the parameter f, is close to 1, then the Si in the overlayer is considered to be of bulk quality. When fr is greater than 1, the Si in the overlayer is considered to contain extended defects and finally, if f was to be lower than 1, the overlayer material would be considered as a mixture of c-Si and Si02 precipitates. The buried layer was approximated by considering lay-

ers of constant or graded composition, made up from a mixture of c-Si and Si02. For layers with an anticipated high content of Si (and low proportion of Si02) the (n)

wave number dispersion was given by multiplying n, with an adjustable coefficient, s. For layers with an anticipated low proportion of Si (and high content of Si02) the value of was multiplied by s to give the dispersed (n) for the layer. In this way, the number of fit parameters required for each layer was reduced. For a layer of constant refractive index the adjustable parameters were the thickness, d and s, which is expected to take values < 1 for Si-rich layers and values> 1 for Si02-rich layers. The model includes as adjustable parameters the rms heights Z, of surface or/and interface irregularities, which can be viewed as macroscopic roughness. Z is initially set

to zero and only when the fit quality does not improve

The skewness of the (n) profile is simulated by using two

joined half-Gaussians. The depth at which the two half-

Gaussians join, which occurs at the minimum of the

refractive index, is called R,,,, while the standard deviations for the surface and substrate side haif-Gaussians are called zR1 and R2, respectively. From these two standard deviations and Rm,,, the first two moments of the range distribution, R and are calculated according to well-

known equations.47

The tail of a Gaussian is, by definition, approached

asymptotically, but in our case it had to be reached within

a certain finite thickness. By truncating each half-

Gaussian at K standard deviations, it was found after con-

vergence tests that the resulting step in the refractive

index profile had a negligible influence on the calulated spectrum (less than 0.003 for the difference in absolute reflectance values, which is the experimental accuracy), when K was set equal to four or in some cases even three. The whole profile is divided into k(R1 + LR2)/&x m — 2 uniform layers (usually 60 to 80) of equal thickness, 6x

depth

Rp-ARp Rp Rp±ARp

Fig. 3. A muhilayer structure of m layers used to model an asimplanted sample having a Gaussian shaped refractive index depth

profile in the implanted region. The Gaussian peak at R, has a standard deviation AR,, and is approximated by m - 2 layers whose refractive index variation is given by Eq. 14 in the text.

3024


(after several models have been tried), it is allowed to be

fitted. The buried layer was approximated by considering an increased number of layers from one to the necessary number for an acceptable fit. We have found that in most cases a five-layer model with double interfaces or, at the most, a seven-layer model for the refractive index profile is suffi-

vs

€+2€

€2+2€ [16] Whenthis is solved for the volume fraction of t (Si03) it gives

=

cient (Fig. 4)48-50

Effective medium models for the calculation of the Si02

volume fraction.—In order to be able to compare the resifits of the optical analysis with those obtained by other

independent techniques it is necessary to convert the

depth profiles of the refractive index into depth profiles of

the 5i03 fraction or to the oxygen atomic concentration (cm3). This was achieved by using the effective medium (EM) models44'5153 to derive a value for the fraction of Si03

in the mixture of Si and Si03 in the sample under investigation. According to these models, the complex dielectric constant of a host medium, (€), and those of several spherical inclusions, of types 1, 2, ..., (ci, €2, ...,) are connected to the effective complex dielectric function of the mixture, €, with an equation of the generic form

_____ = 1 €+2€h

+v2 €2€h

are the volume fractions of material of in total volume. Several approximations types 1, 2, exist to describe specific materials configurations. In the program developed for the analysis of experimental data

where v1, v2,

generated in this project it was possible to employ one of the following: the Lorentz-Lorenz approximation (LL)

where h is set equal to 1; the Maxwell Garnett direct the approximation [MG(1)] where is set equal to

Maxwell Garnett inverse approximation [MG(2)] where €h is set equal to e; the Bruggeman effective medium approximation (EMA) where €h is set equal to €. As the samples

under investigation consisted of only two materials (Si and Si03) it was found to be adequate to use the EMA model to obtain values for the volume fraction of 5i03. In this case Eq. 15 becomes

(€—€2)(2€+€5) —

[17]

= 1. v2 Thus it was possible to calculate the depth profile of the Si05 fraction (and consequently Si) by relating the value for ( at a certain wavelength, found by fitting the spectra, to the known values for the dielectric functions of the two constituent materials, €2 for Si and €5 for 5i03. The value for (€) used is not necessarily the same as that of fused silica (Eq. 13), since small deviations from stoichiometry, excess strain, or densification can change (increase) it.04'44 Therefore, (€) was given as a product of n010, with 1.006, which was found to be the best fit for the (n) of the buried where v5 +

5i02 layer in a standard, annealed SIMOX sample (200 keV, 1.8 >< 10u + cm2, Ta = 1300°C, ta = 6h 56) The oxygen concentration (N0) depth profile can be calculated from the Si03 volume fraction, v, as follows

[15]

€2+2€h

€1+2€h

=0

+8)2

N0 =

vdrN

[18]

where d is the density of 5i02 (taken as 2.33) in g cm3, r (= 2) is the ratio of oxygen atoms to silicon atoms in the Si02 molecule, M (= 60) is the molecular weight of Si03 in g/mol and Na,, is the Avogadro number (6.022 X 1023) in atom/mole. Regression anal ysis.—The analysis is based on a nonlin-

ear least square regression routine which minimizes the function =

, (fmode1()..fexP)2

[19]

where f5 is the set of measured values, ff°°46' (u) is the calculated (using a specific model) set of values correspond-

Model 3

Model 2 Model 1

5-layer model

3-layer model 81

vi Vi

c-Si

--

LI

VI

- - 'a -SI

..'or 5102

V2

Sb2

7 c-SI

82

V2

La

t3

V2

82

V5

Vs

ts

V4

t4

V5

V4

85

V5

6

t4

bulk sIlIcon

rtcw\ Pig. 4. Abrupt interface models used in cakulaling the reflectivity of the annealed high energy SIMOX structures. Model was used for low dose (1 x 1017 cm2) samples and Model 3 for high dose (2 x io' cmi samples.

3025


ing to fr", & (a1, a2, ..., a) are the model fit parameters. The accuracy of the final fit is expressed by the unbiased estimator Q2 of the fit quality Q2

—-— n—p

[20]

where n is the number of experimental points and p is the number of parameters used in the fitting procedure. The specific minimization method used in this work was a grid search, using a modified version of the GRIDLS algorithm

described in Ref. 57, yielding the highest stability by imposing physically meaningful constraints to all the model parameters. It was shown by Kim and Vedam58 that

the method of "grid search," as described by Ref. 57 is a very efficient way of fitting optical data. The basic drawback of the method is its slow convergence in the case of strongly dependent parameters which is partially compensated for by the rather simple calculations involved as

spectrum. For the annealed sample, the scattering centers almost disappear in the same region, giving a spectrum

comparable to that of bulk Si. So, an estimation of the

buried layer thickness is not possible for both samples. The questions remaining, concerning the redistribution of oxygen and any anomalous diffusion during annealing, as well as the thickness and oxygen concentration depth profiles are answered by IR spectroscopy. For the samples

prior to, as well as after annealing, interference fringes were observed (Fig. 6) in both cases, indicating the presence of a buried layer, with a refractive index different from that of crystal Si. Figure 6a shows the experimental JR reflectance spectrum (filled circles) and the corresponding best fit, for the

as-implanted sample. The fit was obtained assuming a two-half-joined Gaussian-shaped distribution for the

for the values of the rms height of surface/interface

refractive index of the buried layer.25 The fitting results can be seen in Table I. Figure 6b shows the experimental IR reflectance spectrum (filled circles) for the annealed sample together with various fits. By using a three-layer model (long dashed line), consisting of a Si overlayer, a buried mixed (Si-rich) layer, and a Si substrate, a best fit with a Q2 value equal to 3.1 was obtained. Comparing the fit of the three-layer model and the experimental spectrum it is clear that at high wave numbers the fit is not as good as at lower wave numbers, since the decrease of the fringe amplitude is faster in the experimental than the calculated spectrum. Since absorption is negligible at these wave numbers, we are led to conclude that some light scattering mechanism is present in the sample. The most probable explanation is the presence of macroscopic roughness at the buried layer interfaces with the Si overlayer and the substrate, which would be a prominent cause of light scattering inside the material. This macroscopic interface roughness is a consequence of a discontinuous combination of regions of different refractive index arranged in a geometrical pattern which has comparable dimensions to the wavelength of the IR radiation (larger than 1.4 p.m). When the model was modified to allow macroscopic roughness at the interfaces of the buried layer (see the section on Optical modeling of multilayer structures) the

roughness the error was about 50% of the best fit parameter values.

Fig. 6b is the best fit (Q = 7 X 10) obtained assuming one

compared to more sophisticated methods such as the

Levenberg-Marquardt algorithm.49 The processing time on a 486 DX 50 MHz personal computer to achieve a typical fit is below 5 mm for the most complex cases. The errors o

associated with the set of parameters, introduced by the least squares procedure were calculated during the fit from the last three points along the grid search. The experimental accuracy of the measurement was estimated to be —0.003 and so the smallest meaningful Q2

value for our analysis (Q) is that of 3 x iO- (which corresponds to .R= IRcaic —

Rm,asI

—0.003). Computer fits to the

data yielded typical Q2 values of —1O to iO, corre-

sponding to an average value of LR — 0.005 to 0.002. The experimental accuracy also imposes an uncertainty in the

values for the best fit parameters. In general the uncertainty for all thickness and refractive index values was nm and respectively. From the uncertainty in the (n) values, a corresponding uncertainty in the v1 (± 1%) and N0 (± 5 X 1020 cm5) values occur. The uncertainty on N0 due to an Si02 density variation of 0.1 is 2 X 1021 cm5.

The errors associated to the fit procedure were smaller (about 25 to 50%) than the uncertainty values. Especially

Results and Discussion

quality of the fit improved. The short dashed line in "rough" interface (the upper), while the solid line is the

In order to demonstrate the versatility of the IR technique the analysis of the samples implanted with the

extreme doses, i.e., the maximum and the minimum in the dose range, are presented in detail.

(x

In Fig. 5, channeled RBS spectra from the sample implanted with a dose of 1 X 10' 0 cm2 before and after annealing can be seen, together with the random

RBS spectrum from the annealed sample. The channeled RBS spectrum from bulk Si is also presented for direct comparison. Protons (1 MeV), rather than He, were used to analyze these thick structures because of their higher penetrating capacity and their ability to give clear light element peaks due to their smaller stopping power.59 On the other hand, a great deal of depth resolution is lost, which results in a rather large experimental error in thickness determination (100 nm). Only the random spectrum from the annealed sample is presented for reasons of clarity since the random spectra for both as-implanted and annealed samples were found to be almost identical. No resolvable Si dip or oxygen peak is visible and thus no definite values for the Si overlayer or the buried layer thickness could be extracted, let alone any information concerning the oxygen distribution. Nevertheless an estimation of the Si overlayer thickness can be obtained from the channeled spectra. For both samples a Si overlayer of bulk Si crystalline quality, about 1.9 im thick (ch. 287 to ch. 244), can be distinguished. For

the as-implanted sample the scattering yield increases rapidly in the region between ch. 243 and ch. 226 where it

reaches a value which is lower than that of the random

rJD

C

C)

150

200

250

Channel number

Fig. 5. 1 MeV H RBS channeled spectra from the sample implant-

ed with 2 MeV oxygen ions with a dose of 1 x 10' OF cm2 (long

dashed line) and the same sample annealed at 1300°C for 6 h (solid line), together with the random spectrum of the annealed

sample (short dashed line). A crystal Si channeled spectrum (squares) is included for comparison.

J. Electrochem.Soc., Vol. 143, No. 9, September 1996 O The Electrochemical Society, Inc. 2 MeV

tion noted just above). The best fit results for all three models can be seen in Table 11. In Fig. 7 the refractive index profiles used in the simulation of the IR spectra of Fig. 6, can be seen. The (n) of the Si overlayer of both as-implanted and annealed samples is 3.45 (at 3000 em-') which is higher but very close to the value for the Si substrate (3.43), in accordance to channeling results. These depth profiles can be turned into oxygen concentration profiles by using one of the many effective medium theories. Taking the Bruggeman's EMA model, much referenced in previous optical studies of SIMOX structure^,^^.^^^^^ which assumes the presence of two mixed phases one inside another (see the section on effective medium models for the calculation of the SiO, volume fraction), the SiO, component depth profiles of the mixture were found (Fig. 8). From that the oxygen concentration depth profiles seen in Fig. 9 were obtained (as discussed in the section noted immediately above) for both the as-implanted and annealed samples. As a direct comparison, the SIMS profiles from the same samples are included in Fig. 9. The overall agreement is quite good considering that SIMS is much more sensitive ~ oxygen ) con(about 1016cm-3 ") than IR (5 X loz0~ m - in centration but at the same time less accurate in depth determination. The SIMS profile for the as-implanted sample peaks at a depth of 2.163 km (R,,,), with an oxygen,concentration value similar to that obtained by IR. Certainly the assumption that the as-implanted oxygen distribution has a Gaussian shape is true, although the SIMS profile is more peaked (higher kurtosis) than that of IR, which incorporates the normal kurtosis value of a Gaussian (p = 3). Also, the negative skewness implied by the IR results is verified by SIMS. Looking at both as-implanted and annealed profiles the redistribution and segregation of the oxygen from the Si overlayer to the buried oxide layer is clearly evident. The abruptness of the annealed sample interfaces is something that again agrees very well with the IR results. The thickness of the Si overlayer, which was found by SIMS to be 12.0 pm is larger from the IR thickness (1.94 pm) by 0.06 pm, which is about the difference between the SIMS and IR obtained values of R,, (0.068 km). A systematic error in the sputter time to depth conversion (of the order of 3% in the Si thickness) could explain these discrepancies. A difference also exists between the SIMS and IR profiles for the thickness of the buried oxide layer. It can be explained by considering the use of roughness in the IR model. The rough areas (which in the IR model are not considered to contain any oxygen) above and below the buried layer,

0 ' in Si

1 x 10''at.cm-~ as-implanted

..... exptl. ----

..

calc.

exptl. z,=O.z~=o zl=50nm,zz=0 zl=50nm,zz=40nrn

.- - .

- -- - -----1 ,,

10

3500

5500

Wavenumber (ern-')

Fig. 6. Experimental IR reflectance spectm (filled circles) in he range 1500 ta 7000 cm-' from Si sam s implanted with 2 MeV oxygen ions ta a dose of 1 x 10" a) and annealed at 13W°C for 6 h (b), compared to theoretical spectra (lines) obtained after fitting the data. For the annealed sample the model includes either no rough interfaces to the buried layer (long dashed line), or only one rou h interface with an rms value of 50 nm (shortdashed line) and r interfaces are rough with finafly assunru that both the buried h e upper one having an rms value?, = 50 nm and the lower one ta having an nns value of z, = 40 nm ( d i d line).

?

best fit (Q = 1 x achieved assuming a model with both interfaces of the buried layer to be macroscopically "rough." The values of the macroscopic roughness obtained are expressed as the rms value for the thickness of the interface irregularities, z, and z, for the upper and lower interface, respectively (see as described in the sec-

Table I. Fitting results obtained for the Si samples implanted with 2 MeV oxygen ions with doses of 0.1 and 2.0 x 10" 0' cm-2 [ n ( U values are given at 3000 cm-'; refractive index of unimplanted Si at 3000 cm-' is 3.431. The parameter uncertainties rising from the experimental accuracy of IR are also indicated.

Table II. Structural data obtained for the Si sample implanted with 2 MeV oxygen ions with a dose of 1 x 10" 0+cm-2 and subsequently annealed at 13W°C for 6 h, by various analytical techni ues. (t: thickness in pm; n: refractive index at 3000 cm-'; L: mtio of channeled ta random yield from the near surface; &: bockgroun%oxygen concentration; N-: mean value of o gen concentration in the buried layer; I,:rms value of the upper intehce roughness; z,: nns d u e of the lower interface roughness). pammeter uncertain*. rising from the experimental accuracy of the various techniques are also indicated.

X

IR S i overlayer Buried layer

S i substrate

t = 1.94 t 0.002 n = 3.45 t 0.014 t = 0.22 + 0.005

RBS t t

=

xm =

1.9 + 0.1 = 0.04 0.3 + 0.1

SIMS t

=

XTEM t

2.0 Ir 0.02

N, = 1 x 10" cm-3 t

=

0.35

+ 0.02

t

=

=

1.98 + 0.1

0.27

+ 0.014

3027


1 022

2 MeV O in Si I x 1017 cm2 as-implanted A

SIMS IR

0 0 0 ()

annealed • SIMS IR

1020 >

A4A

o

A

• •

. • • .'•

1019

2.0

0

Depth (in) Fig. 7. Refractive index depth profiles obtained after the analysis of infrared reflectance spectra of Si samples implanted with 2 MeV

oxygen ions to a dose of 1 x 10 cm2, (solid line) and after annealing at 1300°C for 6 h (dashed line).

have a total rms height (Z) of 0.09 p.m, which corresponds

to a total interfacial region thickness (height of irregularities zh) of 0.12 p.m. Adding the 0.22 p.m which has a constant oxygen concentration, with the 0.12 p.m of the interfacial regions makes a total of 0.34 p.m for the buried layer

0.5

1.0

I 1.5

.

I

2.0

.

I

2.5

Depth (l.tm) Fig. 9. Comparison of the oxygen concentration depth profiles obtained by JR (lines) and SIMS (symbols) analysis, for Si samples implanted with 2 MeV oxygen ions to a dose of 1 x 1 0' cm2 (solid line: 1k; triangles: SIMS) and annealed at 1300°C for 6 h (dashed line: IR; squares: SIMS).

thickness,

which now agrees well with the thickness

(0.35 p.m) obtained by SIMS.

The buried layer being noncontinuous could account for the apparently false indication from the SIMS profile that

the oxygen concentration has a maximum at the lower interface of the buried layer. An average value for the oxy-

gen concentration in the buried layer is 4 X 1021 cm3.

2 MeV 0 in Si

SIMS gives a retained dose (after annealing) of 1.14 )< 1017

1 x 10'7at.cm2

12

as—implanted

annealed

I)

0

cm2, which is about the same with the JR retained dose (1.1 x iO cm2), while there is no significant retained dose difference before and after annealing. So, both techniques show that no anomalous diffusion is occurring, away from any possible surface influence, and that in high energy SIMOX structures, mechanisms such as oxygen segregation and coalescence of Si02 precipitates operate, exactly as it has been seen for standard SIMOX structures.62

In order to check the validity of the fitting result, that rough interfaces are present in the buried oxide layer, an annealed sample was analyzed by XTEM. The micrograph seen in Fig. 10 verifies the JR results concerning the Si overlayer's good crystal quality and its thickness value (1.98 p.m thick) as well as the presence of a 0.27 p.m thick buried layer consisting of amorphous Si02 precipitates, mixed with crystal Si. These precipitates which are octahedral in shape are linked by a network of dislocations between them. No resolvable interfaces exist but rather an intermixing of the two materials (Si and Si02) is evident.

8

0 Q

w

There is no doubt that this situation can be very well 2.0 2.5 Depth (i.tm) Fig. 8. Depth profiles of the Si02 component obtained after the

use of an EMA model on the data presented in Fig. 7, for the Si sam-

ples implanted with 2 MeV oxygen ions to a dose of 1 x 10' cm2, (solid line) and after annealing at 1300°C for 6 h (dashed line).

described by the macrosopic roughness models used in the JR model equations for the reflectivity calculations (Eq. 9a-9d). Table II presents the structural characteristics of the annealed sample as found by IR, RES, SIMS, and XTEM techniques. In Fig. 11, the channeled RBS spectra for the high dose

sample (2 X 1018 0 cm2) for both as-implanted and annealed samples, as well as for bulk Si, can be seen. Only

the random RBS spectrum from the annealed sample is shown (short dashed line) for reasons of clarity. It was

3028


Comparing the channeled spectra from the as-implanted (long dashed line) and annealed (solid line) samples, the crystal recovery (up to bulk Si levels) for almost half of the

Si overlayer can be clearly seen after annealing. For the as-implanted sample, the region between ch. 250 and ch. 232 (just above the buried layer) displays a high scattering yield which does not reach that of the random spectrum. This means that this region cannot be amorphous, polycrystalline, or stoichiometric Si02. It is very probable that it consists of a mixture of damaged Si and Si02 precipitates. In Fig. 12, the IR experimental (filled circles) and calculated (lines) spectra for the as-implanted (Fig. 12a) and annealed (Fig. 12b) high dose (2 X 1018 0 cm2), samples,

can be seen. For the as-implanted sample the model

described in the low dose sample case was used. The best

fit values for the adjustable parameters can be seen in

Fig. 10. XTEM micrograph from the sample implanted with 2 MeV

oxygen ions to a dose of 1 x 1017 cm2, annealed at 1300°C for

oh.

found after simulation that the yield of the oxygen peak is the result of non-Rutherford scattering of the 1 MeV W beam from the buried oxygen atoms (high scattering cross section of protons by oxygen atoms affected by nuclear resonance scattering). So, information about the oxygen distribution profile had to be extracted63 from the Si dip region, where the yield comes from silicon atoms in the buried layer. It is apparent that the Si signal in the region of the Si dip has an thverted Gaussian shape. This is mainly due to the limited depth resolution of the 1 MeV H beam with which the RBS analysis was performed. It would take a layer of constant composition, thicker than 0.5 p.m, in order to see a flat area where it is now a Gaussian.

Table I. The value of n(Rmax) (at 3000 cm') was 1.44. The value for stoichiometric fused silica, which has been found among all known types of 5i02, to achieve the best fitting, is at the same wave number equal to a = 1.41 (from Ref. 44). The fit quality is again very good indicating that the only assumption of the model, the refractive index profile being of a two-half-joined Gaussian shape is true.

For the annealed sample, a three-layer model (like the one used in the low dose sample case) was tried for the fitting of the experimental spectrum (Fig. 12b, long dashed line, Q2 = 1.27). The fit is good for wave numbers only up to 4000 cm1. An attempt to use two more layers (five layer

model), assumed to consist of a mixture of Si and Si02, yielded a better fit (Fig. 12b, short dashed line, Q2 = 5 X 10-). Still, an even better fit was achieved when two more mixed layers were added in the configuration (Fig. 12b, solid line, Q2 = 6 >< 10). The addition of more mixed layers or surface/interface roughness did not result in any observable improvement of the fit and so this seven-layer

0.9

2 MeV, 2.13 x 10

as-implanted (x 10) 16

.2 MeVOinSi 2 x 1018cm2

annealed random channel. as-i m planted

a)

0.6 0 C

(U 4-. 0 a)

9- 0.3 a)

channel.

12

0

Cl)

C

0 .9-8 V

a)

f1)

t

0.6 0 C

>-

(U a.)

9- 0.3 a)

4

0

I MeVH, lO1sC, 170°

0

100

150 200 250 Channel number

300

2000

4000 Wavenumber

6000

(cm)

Fig. 12. Experimental infrared reflectance spectra (filled circles) in

Fig. 11. 1 MeV H RBS channeled spectra from the sample implanted with 2 MeV oxygen ions to a dose of 2 x 1018 O cm2 (long dashed line) and annealed at 1300°C for 6 h (solid line) together with the random spectrum of the annealed sample (short dashed line). A crystal Si channeled spectrum (squares) is included for comparison.

the range from 1500 to 7000 cm from Si samples implanted with 2 MeY oxygen ions to a dose of 2 x 1018 cm2 (a) and annealed at 1300°C for 6 h (b), compared to theoretical spectra (lines) obtained after fitting the data to a Gaussian distribution model (a) and ta a layer model (b) assuming three layers (long dashed line), five layers (small dashed line), and seven layers (solid line).

3029


0

Table Ill. Layer thickness (jm) and composition percentage x (%) of Si02 component for the three models used in the IR spectra simulation of the 2 MeV, 2 x 1018 cm2, annealed sample. The parameter uncertainties rising from the experimental accuracy of IR are also indicated.

Three layers (Q2 = 1.27)

Sample

1.841

CSi/fr

c-Si + xSiO2 c-Si + xSiO2 c-Si + xSiO2 c-Si + xSiO2 c-Si + xSiO2

0.002/1.010

0.627

0.005

0.005 + 79 1

Five layers (Q° = 5 )< 10) 1.828

0.002/1.015

0.051 0.193 0.314

0.005 + 88 0.005 + 70

0.005 1 1

0.005 + 80 1

c-Si substrate

Seven layers (Q2 = 6 X 10°) 1.855

0.002/1.005

0.129 0.005 + 76

0.005

0.083 + 0.005 + 56 0.111 0.005 + 80

1 1 1

0.136 0.143

1

0.005 + 55 1 0.005 + 82

model is taken as the best fit for the annealed sample. The fitting results for the three models can be seen in Table III. In Fig. 13, the refractive index depth profiles used for

In order to establish the best implantation and annealing experimental conditions for the fabrication of high energy SIMOX structures of high structural quality, so that they would be suitable for use as Si-based optical

annealed samples seen in Fig. 12, are presented. The same procedure (EMA model, as described above) as for the low dose samples, was followed in order to turn the refractive index depth profiles of Fig. 13, into Si02 component depth profiles. The results can be seen in Fig. 14. It can immedi-

waveguiding structures, a set of samples was prepared and analyzed by JR reflectance with the same methodology as described above. In Fig. 16, the JR experimental (filled circles) and best fit

ately be seen that during annealing the oxygen redistributes at regions above and below the profile peak. A distinct layer has formed which contains two regions of high Si proportion and three regions of high Si02 proportion.

implanted with 2 MeV oxygen ions, with a dose of 1 >< 1018

with a beam current density of 5 A cm2, in order to

Here, the Si rich regions can be considered to be Si islands,

reduce the implantation time, while sample B (Fig. 16b) was implanted with a beam current density of 1 p.A cm2.

common for standard SIMOX structures.47'64-66

Subsequently, both were annealed at 1300°C for 6 h.

the fitting of the IR spectra from the as-implanted and

In Fig. 15, a comparison of oxygen concentration depth

profiles for the as-implanted and annealed samples obtained by JR and RBS is presented. Very good agreement exists for the as-implanted sample, while for the annealed

sample the low depth resolution of RBS does not allow sharp changes in oxygen concentration levels to be detected. The thickness values for the buried layer are almost the

same, and the values for the Si overlayer thickness are within the RBS accuracy. After 6 h annealing at 1300°C, a

nonstoichiometric, noncontinuous buried layer was formed, which contains regions of high Si content.

calculated (solid lines) spectra for three Si samples O cm, can be seen. Sample A (Fig. 16a) was implanted

Sample C (Fig. 16c), was implanted with the same current density as sample B but annealed twice, at 1300°C for 6 h, making a total annealing time of 12 h.

The differences seen between the three JR spectra in Fig. 16 denote possible structural differences between the samples. The small differences in the maxima and minima wave number positions indicate a difference in Si overlayer thickness. The difference in the fringe amplitude indicates that the buried layers are not of the same thickness or refractive index. The fitting of the experimental spectra yields the oxygen concentration depth profiles for all three samples seen in Fig. 17. Higher beam current density (sample A, Fig. 17a),

results in the presence of a two-band structure in the

buried layer. A small band of low oxygen concentration can be seen at a mean depth of 1.65 p.m, above the main

2 MeV, 2.0 x 1018cm2 as-implanted annealed

>< II)

80

> -I-, 0 Co 9-

C

a) C

00. E 0C.)

0

40

U)

1.0

1.5

2.0

2.5

Depth ().tm) Fig. 13. Refractive index depth profiles obtained after the analysis of infrared reflectance spectra of Si samples implanted with 2 MeV oxygen ions to a dose of 2 x 1018 cm2 (solid line) and after annealing at 1300°C for 6 h (dashed line).

0— 1.0

1.5

2.0

2.5

Depth (rim) Fig. 4. Depth profiles of the S102 component obtained after the use of an [MA model on the data presented in Fig. 13, for the Si samj)les implanted with 2 MeV oxygen ions to a dose of 2 X 1018 cm (solid line) and after annealing at 1300°C for 6 h (dashed line).

3030

J. Elect rochem. Soc., Vol. 143, No.9, September 1996 The Electrochemical Society, Inc.

cn

E C.)

1)

Ci

C

0C)

13

0

C

Co

a)

C U)

U)

0)

0

1.0

1.5

2.0

2.5

Depth (jim) Fig. 15. Comparison of the oxygen concentration depth profiles obtained by JR and RBS analysis for Si samples implanted with 2 MeV oxygen ions to a dose of 2 X 1018 cnf2 (circles: IR, triangles: RBS) and annealed at 1300°C for 6 h (dashed line: JR, solid line:

2000

4000

6000

Wavenumber (cm1) Fig. 16. Experimental infrared reflectance spectra (filled circles) in

the range from 1500 to 7000 cm from Si samples implanted with

2MeVoxygenionstoadoseofl x 10'8cm2andwith(a)abeam

RBS).

current density of 5 pA cm2 and annealed at '1300°C for 6 h, (b) a beam current density of'1 pA cm2 and annealed at 1300°C for 6

buried layer. It could be assumed to be a band of Si02 pre-

cipitates, which nucleated and grew heterogeneously at a

h, and (c) a beam current density of 1 pA cm2 and annealed at 1300°C for 12 h, compared to theoretical spectra (solid linesl obtained after fitting the data to theoretical models described in the text.

vacancy-rich region. The main buried layer consists of five regions, two of which have a higher oxygen concentration.

ing. We suggest that an increase of dose and annealing for

preferential nucleation site such as a high strain or a

The other three can be considered as Si-rich regions, the well-known Si islands. So, a nonstoichiometric, noncontinuous buried layer is the outcome of the high current density implantation. For lower beam current density, (sample B, Fig. 17b) the

Si02 precipitate band above the buried layer does not appear. Possibly the lower current did not facilitate the

creation of the nucleation sites which attracted the oxygen in the previous case. The back interface Si island region has now disappeared but two Si-rich regions still remain. Finally, a further annealing for 6 h at 1300°C (sample C, Fig. l7c), is responsible for the disappearance of the Si islands region from the front interface of the buned layer, meaning that the 5i02 precipitates have sufficiently coa-

lesced in to the buried layer. Now, three regions can

describe the buried layer. None of them is stoichiometric although two are of high oxygen concentration. There still is a Si-rich region in the middle of the buried layer. It is apparent that the high beam current density results in the presence of Si02 precipitates both at the front and at the back interface of the buried layer. These precipitates have nucleated at the numerous nucleation sites created by the radiation damage induced by the high beam current. When the current was lowered these areas became smaller (front interface) or disappeared (back interface). The high beam current also resulted in the appearance of a separate band of Si02 precipitates. The increase of the annealing time to 12 h resulted in the complete dissolution of the Si02 precipitates at the interfacial regions. Still the buried layer contains Si islands, and it is doubtful if an increase of annealing time or temperature would be beneficial for this dose. It is evident then that the dose of 1 X 1018 0 cm2, is not high enough for a buried continuous and stoichiometric layer to form, even after 12 h anneal-

12 h at 1300°C (or even higher) is the best recipe for the fabrication of buried homogeneous and stoichiometric Si02 layers formed by high energy oxygen ion implantation. Conclusions In this paper the applicability of infrared reflection

spectroscopy to the analysis of samples produced by high energy (2 MeV) oxygen implantation into silicon is demonstrated. The thickness, chemical composition, crystallinity, and interface roughness together with the refractive index profiles, are quantified. It is clear that this quick, nondestructive, and inexpensive technique offers many advan-

tages, as it is analytical and precise, being particularly suitable for the analysis of thick samples such as those produced by high energy implantation. Correlation with

BBS/channeling results (as well as with SIMS and XTEM results) showed good agreement, concerning thickness as well as composition and crystallinity. Oxide formation follows the same basic mechanisms as in the standard SIMOX case. No anomalous diffusion was observed during annealing and the formation of a buried layer, even for the lowest dose, is demonstrated. It has also been shown that high beam current densities result after

annealing in noncontinuous, nonstoichiometric band

structures. The main difference between the standard and higher energy cases, is the wider standard deviation of the oxygen distribution, which produces a wider spread of the oxygen atoms and consequently of the oxide precipitate nuclei, in the as-implanted samples. During annealing the

relative remoteness of this growing nuclei hinders the processes of Ostwald ripening and coalescence and so

inhomogeneous layers form for the standard, for 200 keV SIMOX, annealing temperatures and times. A dose of 2 >
..in21 I'.,

16. C. L. Mitsas and D. I. Siapkas, Solid State Commun.,

0>(

83, 857 (1992).

17. C. L. Mitsas, E. K. Polychroniadis, and D. I. Siapkas, Semicond. Sci. Technol., 8, S356 (1993).

18. L. L. Lion, W. G. Spitzer, J. E. Fredrickson, and S-L. Kwun, J. App!. Phys., 59, 1927 (1986).

1022

19. G. K. Hubler, P. R. Malmberg, and T. P. Smith, ibid., 50, 7147 (1979). 1021

1.5

2.0

2.5

Depth (j.tm) Fig. 17. Oxygen concentration depth profiles obtained by IR analysis for Si samples implanled with 2 MeY oxygen ions with a dose of x 1 0' cm2 and with (a) a beam current density of 5 pA cm2 and annealed at 1300°C for 6 h, (b) a beam current density of 1 pA cm2 and annealed at 1300°C for 6 h, and (c) a beam current density of 1 pA cm2 and annealed at 1300°C for 12 h.

20. G. K. Hubler, P. H. Malmberg, C. A. Carosella, T. P. Smith, W. G. Spitzer, C. N. Waddell, and C. N. Phillippi, Radi. Effects, 48, 81 (1980).

21. C. C. Katsidis, D. I. Siapkas, W. Skorupa, N. Hatzopoulos, and D. Panknin, in Proceedings of the X International Conference on Ion Implantation Technology, Catania, Italy, 1994.

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23. C. C. Katsidis, D. I. Siapkas, D. Panknin, N. Hatzopoulos, and W. Skorupa, Microelectron. Eng., 28, 439 (1995).

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annealing at 1300°C for 6 h, but it is a inhomogeneous,

nonstoichiometric layer. For the samples implanted to a dose of 1 x 1018 O cm2 (Fig. 17), an improvement in the buried layer structure was seen when a higher annealing time was used. We then conclude that an annealing temperature even higher than 1300°C and annealing time longer than 6 h, are needed to obtain high quality high energy SIMOX structures. Acknowledgments The authors would like to thank Professor J. Stoemenos, Department of Physics, Aristotle University of Thessaloniki, for the very helpful discussions. We are grateful to Dr. R. J. Chater and Dr. Y. Li, Department of Materials, Im-

penal College, London, for providing us with the SIMS and XTEM data, respectively. Finally, we acknowledge the help of the staff of the D. R. Chick Laboratory, University of Surrey, M. Chapman, R. Watt, M. Browton, A. Cansell, and J. E. Mynard. This work was supported in part by the UK Science and Engineering Research Council.

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(1987).

Moisture Resistance of Annealed Borophosphosilucate

Glass Films for Very Large Scale Integrated

Circuit Applications Masaki Yoshimaru and Hideaki Matsuhashi Oki Electric Industry Company, Limited, VLSI Research and Development Center 550-1 Higashiasakawa, Hachioji, Tokyo 193, Japan ABSRACT

The moisture resistance of borophosphosilicate glass (BPSG) films annealed at 900°C was studied. The investigation focused on phosphorus leaching from, and water penetration into, films during exposure to a saturated water vapor at 120°C. Phosphorus leaching was measured using x-ray fluorescence spectroscopy and water penetration was measured using secondary ion mass spectroscopy. Water desorption from films after exposure to the water vapor was also measured using thermal desorption spectroscopy. These studies showed that phosphorus in films with a phosphorus concentration exceeding 9 weight percent (w/o) P leached from films during exposure to water vapor, but phosphorus in films with 9 w/o P or less showed scarcely any leaching, regardless of the boron concentration. In films with a phosphorus concentration of 9 w/o P or less, phosphorus suppressed, and boron enhanced, water penetration into the films, but boron had less of an effect with increased phosphorus concentration. A thermal desorption spectroscopy study showed that phosphorus decreases, and boron increases, the number of molecular water absorption sites. Apparently the change in the number of water absorption sites changes the degree of water penetration into films. Thus, BPSG films with a higher phosphorus concentration not exceeding 9 w/o P have higher moisture resistance, because films show lower water penetration and scarcely any phosphorus leaching.

Infroduction Borophosphosilicate glass (BPSG) film has been studied

widely as a glass-flow dielectric for planarization in the fabrication of very large scale integrated circuits (VLSIs). With increasing VLSI integrations, device surface topography must be made as smooth as possible because of the shallow depth of focus of subhalf micron lithography. This, in turn, requires enhanced BPSG fusibility. BPSG fusibility is enhanced by substantially increasing film boron and phosphorus content.18 Figure 1 shows the flowangle dependence of 800 nm thick phosphosilicate glass (PSG) and BPSG films on phosphorus [P205 mole percent (m/o)I and boron concentrations (B203 m/o) after 900°C annealing. Data points in Fig. 1 follow the same curved line indicating that phosphorus and boron enhance silicate glass fusibility to almost the same degree. It has been reported that PSG and borosilicate glass (BSG) with high phosphorus and boron content react easily with water,915 causing VLSI reliability problems. PSG with high phos-

phorus content causes aluminum corrosion in a moist ambient.'°1617 Water penetrating the PSG causes a thresh-

old voltage shift in a metal oxide semiconductor field effect transistors (MOSFET) during high-temperature stress tests.t3 The moisture resistance of BPSG films remains to be clarified, however.

The purpose of this study is to examine the effects of boron and phosphorus content on the moisture resistance in BPSG focusing on phosphorus leaching from films and water penetration into films. Deuterium oxide (D20) was used as a moisture source because water penetration of film is easily measured using secondary ion mass spectroscopy (SIMS).56192°

Experimental BPSG films were deposited on 6 in. silicon wafers at 400°C by atmospheric pressure chemical vapor deposition (CVD) using silane, phosphine, and diborane cooxidation. The oxygen-to-hydride ratio was maintained at a constant