sputter depth profiling of nanoscale interfaces by optimizing ... - NIO

Proceedings of 12th ISMAS Symposium cum Workshop on Mass Spectrometry

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Sputter Depth Profiling of Nanoscale Interfaces by Optimizing Depth Resolution in Secondary Ion Mass Spectrometry B. R. Chakraborty National Physical laboratory, New Delhi-110012, India Email: [email protected] Abstract In SIMS depth profiling, the aim is to determine the local composition of the material as a function of its depth beneath the original surface. To characterize interfaces ion sputtering and identification of depth distribution of different species are carried out. This technique is frequently used because they are applicable to almost any kind of optimum depth resolution in the atomic mono-layer region over a wide range up to several micrometers. In this paper, optimization of depth resolution by changing different parameters used during sputtering will be discussed. This will be further applied to show the depth profile of some typical nanoscale interfaces of semiconductor device materials. Introduction For any semiconductor device the product reliability or device failure depends highly on impurity or contamination in surfaces and interfaces, which can be controlled through surface characterization. A detailed elemental identification, geometrical position and the electronic and vibronic structure of atoms at surfaces and interfaces are what is meant by surface characterization. Significant developments have taken place in last three decades for the characterization of solids, largely using new experimental techniques. New methods for the control and preparation of interfaces have emerged from the vast amount of engineering and development carried out in recent years considering the never ending demands of miniaturization to produce advanced integrated circuitry and other devices (VLSI, ULSI) with astonishing increase in device density and performance. Although Silicon coupled with the unique properties and compatibility of its oxide, SiO2, has been used as the main electronic material, other semi-conducting compounds like GaAs, GaN, SiGe are also emerging as the basic electronic materials for device fabrication in different technological applications. In the present paper attempts have been made to characterize the interfaces of different semiconductor device structures based on Si, GaAs, GaN and SiGe using sputter depth profiling by secondary ion mass spectrometry (SIMS). Since the intermediate layers are of nanoscale, it is necessary to optimize the depth resolution of the equipment to get the best results. Certified reference material like Ta2O5/Ta has been used to optimize the instrumental parameters for depth profiling before investigating the device structures. Ion sputtering and depth distribution of different species To characterize a surface adequately it is often necessary to measure the following quantities: (1) the type of atomic species present on the surface; (2) the arrangement of structure of the surface atoms; (3) the electronic structure (Mainly valence electrons) of the surface atoms;(4) the motion of the atoms like diffusion or erosion; (5) the nature and distribution of defects; (6) the surface topography; (7) spatial distribution of impurity atoms

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Proceedings of 12th ISMAS Symposium cum Workshop on Mass Spectrometry

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on the surface and (8) the depth distribution of different atomic species in the vicinity of interfaces. Among the various techniques developed for characterizing interfaces, surface and interface analysis methods in combination with ion sputtering are frequently used because they are applicable to almost any kind of optimum depth resolution in the atomic mono-layer region over a wide depth range unto several micrometers. This depth resolution by sputtering critically depends on an appropriate choice of experimental conditions and through understanding of the large number of parameters, which generate characteristic deviations between the measured profile and the original shape of the in-depth distribution of composition. Various phenomena, the most important of which are due to ion beam induced changes of surface roughness and composition, limit the experimentally achievable depth resolution. The influence of sample characteristics and experimental parameters like ion beam energy and incident angle as well as sample rotation on depth resolution and its dependence on the sputtered depth serves as a guide line for optimized profiling condition. 1.1

Basic principle of depth profiling by sputtering

In depth profiling, the aim is to determine the local composition of the material as a function of its depth beneath the original surface, which may be expressed as Xi = f(z) where Xi represents the composition and z is the depth from the surface. But in actual experiment we measure the signal intensity Ii in counts/s as a function of time t, or Ii = f(t). This intensity-time relation has to be converted to composition-depth relation knowing the parameters of conversion for intensity to composition and time to depth. Even after such a calibration the profile obtained deviates from the original in-depth distribution due to ion beam induced alteration of surface composition and topography. This deviation is usually defined as the depth resolution Δz, which gives the idea of accuracy in determine the depth profile. It is therefore essential to optimize Δz for the achievement of a high resolution depth profiling. This optimization leads to minimizing all the effects, which tend to increase Δz. IUPAC and ASTM E-42 committee have adopted the definition of Δz by “the depth range over which a signal increases (or decreases) by a specific amount when profiling an ideally sharp interface between two media. By convention, the depth resolution corresponds to the distance over which a 16% to 84% (or 84% to 16%) change in signal is measured” [1]. The depth resolution Δz may be expressed in terms of the standard deviation б by the relation Δz = 2 [2]. Assuming depth resolution function of approximately Gaussian shape and mutual independence, contributions from different physical phenomena add up in quadrature to give

Where

Δz

=

(Δzo2+ Δzs2+ Δzm2+2+Δzr2+ Δzλ2+ΔzΙ2+ ...)1/2

Δzo

=

lateral inhomogeneity of depth distribution in the sample

Δzs

=

surface roughening by sputtering statistics

Δzm

=

atomic mixing

Δzr

=

ion beam induced roughening

Δzλ

=

information depth

12th ISMAS-WS-2007, March 25-30, 2007, Cidade de Goa, Dona Paula, Goa

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Proceedings of 12th ISMAS Symposium cum Workshop on Mass Spectrometry

ΔzI

=

inhomogenity of ion beam intensity

For optimizing instrumental parameters a depth profiling is performed with a certified reference material of Ta2O5/Ta, for which Δz ≤ 2nm should be attained when 1-3 keV Ar+ ions are used for sputtering. Besides instrumental factors, sputtering induced profile distortions like changes of the sample surface topography and surface composition contribute heavily on depth resolution Dz. Sample rotation introduced by Zalar in 1985 [3] removes to a great extent the ion beam induced roughness as can be seen from Fig. 1(a). [4]. The atomic mixing contribution Δzm is influenced by parameters like primary ion mass, beam energy and incident angle. This is demonstrated in Fig. 1(b)[4].

(a)

(b)

Fig.1.(a) Depth resolution Δz as a function of sputter depth of Ni/Cr multilayer using 3keV Ar+ with sample rotation, (b) Variation in decay length, as a function of ion mass, energy and incident angle, contributing to atomic mixing, Δzm. ( Results from Ref. 8).

2.

Experimental

The samples were characterized for their interfaces by a SIMS equipment which is of quadrupole type [Model MIQ 256, CAMECA-RIBER]. The base pressure was maintained at 10-10mbar during the experiment. The choice of primary ion (or mass) like Ga+, O2+ or Cs+, their energy, fluence (ions/cm2) and angle of incidence were varied from sample to sample for achieving the optimum depth resolution. An electronic window gating of 10% was used to minimize the crater edge effect. Further detail of the equipment is given elsewhere [5]. Different experimental parameters used during the depth profiles are given in Table-I and Table-II. The depth calibration was carried out using a Tencore Alpha-Step 500 surface profilometer at the crators created during depth profiling.

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Proceedings of 12th ISMAS Symposium cum Workshop on Mass Spectrometry

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Fig 2. SIMS depth profile of Ta2O5/Ta for TaO (197) signal using(a) O2+ ion, (b) Ga+ ion and (c) Cs+ ion as primary beam. Calculationof Δz for (a) and (c) are shown.

Representative Results 2.1 Standard Ta2O5/Ta sample To optimize the depth resolution of the SIMS equipment, a certified reference material Ta2O5/Ta with thickness of the oxide film being 567Å was taken for depth profiling. Fig.2 shows the intensity vs depth in nm for three different primary sources viz. O2+, Ga+ and Cs+ at different primary energies but same incident angle of 45o to the surface normal. The

12th ISMAS-WS-2007, March 25-30, 2007, Cidade de Goa, Dona Paula, Goa

Proceedings of 12th ISMAS Symposium cum Workshop on Mass Spectrometry

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various experimental parameters for the depth profiling of Ta2O5/Ta using different ion sources are shown in Table-I. It is clear from the results obtained that the best depth resolution was obtained by Cs+ (Δz = 4.2 nm) compared to that obtained by using O2+ (Δz =18.8nm) as shown in Fig. 2., although the energies of both Cs+ and O2+ were the same i.e. 10 keV and the sputter rates were comparable. It was further observed that by reducing the energy of the O2+ ion beam, the Δz improves (not shown in figure) but was not better than the other two ion sources. The sputtering parameters thus found were used for the optimization of the other samples whose results will be discussed below. 2.2 Measurement of well widths of MOVPE grown InGaAs/InP quantum wells. InGaAs/InP quantum wells (4 wells with increasing thickness) were grown by MPVPE technique at IIT, Kharagpur, material Science Department and the well widths were measured by NPL SIMS using depth profiling [6]. The depth profiling for the QW structures was carried out with an O2+ primary beam. The beam energy and current was optimized at 7 keV and 50 nA at an incidence angle of 60o to the surface normal and the beam was rastered over an area of 150ux150u along with an electronic window electronic gating no0f 10% to avoid crater edge effect. With these parameters the Δz was calculated to be between 2-3 nm [Fig.3] in the first QW while it deteriorated with depth, as expected. The preferential sputtering rates of InP and InGaAs were also later normalized for the quantum will depth calculations using standard samples of InP and InGaAsP.

Fig.3. SIMS depth profile of InGaAs/InP QW showing only the variation of phosphorous (P) signal. The determination of Δz and well widths are indicated.

2.3 Detection of arsenic oxide at the interface of Gd2Ga3O15/GaAs Fabrication of GaAs MOS devices faces the trouble of surface oxidation of As which contribute to increase in interface state density within the band gap. Surface passivation using gadolinium gallium garnet (Gd2Ga3O15) has been tried to get only Ga-O on the top surface instead of the unwanted As-O species. The passivant Gd2Ga3O15 was deposited as a dielectric by e-beam evaporation on Si implanted n+GaAs. To detect the interface oxide layer of Ga & As, Cs+ beam was used at an energy of 11 keV with beam current of 60 nm at an angle of 45o

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Proceedings of 12th ISMAS Symposium cum Workshop on Mass Spectrometry

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to the surface normal. The beam was rastered on an area of 500µ x 500µ u with 10% window gating. Negative ions of As-O were monitored which showed a thin layer of 4 nm only at

Fig. 4. Interface oxide layer of As–O before and after annealing the Ga2O3(Gd2O3)/GaAs sample. The widths and the Δz value calculated for the oxide layers are indicated in nanometer.

the interface before annealing the sample and thin As-O layer broadens to 6 nm due to annealing. The Δz calculated in thin case was ~2.0 nm which is quite satisfactory with the above primary ion mass and energy. 2.4 Characterization of GaAs MIS devices at the interface The improvements in the electrical characteristics of Au/SixNy/n-GaAs MIS strucuters with NH3 plasma pre-treatment of GaAs prior to PECVD of a SixNy dielectric film followed by annealing, was characterized by SIMS depth profiling. It was revealed that the unpassivated sample showed a broad interface consisting of Ga-O and As-O species which narrow down considerably due to NH3 passivation. This thinning of oxide layer reduces the interface state

Fig. 5. Interface oxide layers of As–O and Ga–O before and after annealing the SixNy/n-GaAs structures. Determination of Dz is shown.

12th ISMAS-WS-2007, March 25-30, 2007, Cidade de Goa, Dona Paula, Goa

Proceedings of 12th ISMAS Symposium cum Workshop on Mass Spectrometry

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density and the process of Fermi level pinning reverses showing better device characteristics [7]. The SixNy/GaAs structures were characterized by SIMS for the top layer composition and presence of surface impurities as well as the interface for identifying the oxide layers. In this case also Cs+ primary beam of 11 keV energy with 60 nA current was rastered over an area of 300µ x 300µ with 10% window gating in negative SIMS mode to get the best depth resolution. The resolution thus obtained was Δz ≈ 2.2 nm which could easily detect the thin layers of As-O & Ga-O based species at the interface. 2.4

Ga2O3(Gd2O3)/Si0.74Ge0.26/Si structures

High K-dielectrics on SiGe are used for fabrication of MIS structures for diminished device

Fig.6.. Interface oxide of SiO layer along with sharp substrate (SiGe) face showing the calculation of Δz.

size. GGG has been used to form the proper dielectric on strained SiGe substrate to prepare Ga2O3(Gd2O3)/Si0.74Ge0.26 on Si. The GGG/SiGe interface was characterized for its sharpness [8]. Fig.6.shows the overall SIMS depth profile showing the sharp fall in the intensity of Ga,GdO,Gd and O while one can see the sudden rise in the intensity of Si & Ge . The trace of SiO species suggests the presence of oxide layer at the interface over the SiGe layer. Fig.6 shows this layer in an enhanced form along with the sharp rise in Ge signal. The depth resolution in this system [see Table-II] was calculated to be Δz =1.91 nm [Fig. 6] which shows a very high depth resolution considering the high primary energy (10keV) used. 2.5

GaN/Si

During the synthesis of Wurtize GaN films by reactive hot wall vapour deposition technique for the fabrication of Au/GaN Schottky devices [9], the interface of GaN/Si were characterized by SIMS depth profiling. This could provide information about the sharpness of the interface besides the traces of impurities in the GaN film and also composional variation throughout the thickness of the film. Fig. 9 shows the depth profile of Ga,N and Si showing a sharp interface at around 80nm. Ga has been seen to diffuse deeper into the Si substrate, which may be due to recoil mixing while N was found negligible beyond the interface. The

12th ISMAS-WS-2007, March 25-30, 2007, Cidade de Goa, Dona Paula, Goa

Proceedings of 12th ISMAS Symposium cum Workshop on Mass Spectrometry

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Δz calculated [Fig. 7] in this case was a little high (Δz =6.04nm) compared to the previous data although similar experimental parameters for sputtering were used as shown in Table-II.

Fig. 7. SIMS depth profile of GaN/Si showing recoil mixing of Ga into Si substrate. The Dz calculation has been shown.

2.6

Ge/Si

Ion beam mixing (IBM), a phenomenon of atomic migration at the interface under the influence of energetic ions, has been of interest for the synthesis of non-conventional phases of silicides and other various compounds. Ge film (~125nm) was deposited on Si substrate to study

Fig.8. SIMS depth profile of Ge/Si system showing interface oxygen trace (shown by the hump) besides the sharpness of the interface and its Δz value.

IBM at the interface due to high energy (~MeV) ion beam interaction. To study the interface, SIMS depth profiling was used to identify impurities as well as oxide layers at the interface. Fig.8 shows the depth profile of Ge/Si where a small oxide layer is seen at the interface

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Proceedings of 12th ISMAS Symposium cum Workshop on Mass Spectrometry

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giving a hump to both Ge & Si signals. But the sharp interface can be seen from the very fast rise of Si signal. The recoil mixing of Ge inside the Si substrate can also be seen for another ~30nm. The depth resolution Δz obtained in this sputter process using O2+ ion beam with an energy of 7keV and sputter rate 4.67Å/sec. [see Table-II] was 2.28nm [Fig. 8] which is comparable to the lowest Δz values obtained using Cs+ ion with similar energy. 3

Conclusion

The optimization of SIMS depth profiling for identification of thin oxide layers was carried out by critical maneuvering of number of physical processes like recoil mixing, cascade mixing, penetration depth, etc. These processes were optimized by adjusting primary ion energy, ion mass, angle of incidence, and sputter rate to get the best depth resolution. Certified reference material like Ta2O5/Ta was used to optimize the experimental parameters for obtaining the best depth resolution. Except in the case of GaN/Si where Δz was higher (6 nm), in most of the other devices based on Si, GaAs or SiGe, the Dz was calculated to be 2–3 nm depth profiling and optimization of depth resolution has been tackled more as an art than science by many workers. While the best depth resolutionΔz ≈1nm [10] has been obtained by using inert gas ions in the energy range of 1-2 keV, it has been demonstrated by some experiments done at SIMS laboratory, NPL that Δz ≈2.0 nm could be achieved even using ions of higher energy and mass with suitable choice of parameters. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

ASTM E-42, Surface Analysis, E 673-91C, ASTM, Philadelphia (1992). D.Briggs and M.P.Seah, eds. Practical Surface Analysis, Vol.1 & 2, Wiley, Chichester(1990). A.Zalar, Thin Solid Films, 124(1985)223. S.Hoffmann, High Temp. Mat. Processes, 17 (1998)13. D.R.Sharma, B.R.Chakraborty, M.L.Das, Appl.Surf. Sci. 135(1997) 258. D.N.Bose, P.Bannerji, S.Bhunia, Y.Aparna, M.B.Chhetri and B.R.Chakraborty, Appl. Surf.Sc. 158(2000)16. J.J.Pancove, Air Force office of scientific Research, Technical Report no. AFOSR-TR, 80-1029, October 1980. S.Pal, S.K.Ray, B.R.Chakraborty, S.K.Lahiri, & D.N.Bose, J.Appl. Phys. 90(2001) 4103. B.Deb, A.Ganguly, S.Choudhuri, B.R.Chakraborty, A.K.Pal, Mat. Chem. Phys. 74(2002)282. M.P.Seah, S.J.Spencer, I.S.Gilmore, J.E.Johnstone, Surf.Interface Anal. 29 (2000) 73.

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Proceedings of 12th ISMAS Symposium cum Workshop on Mass Spectrometry

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BIO – DATA Name

:

Date of birth

Dr. Bibhash Ranjan CHAKRABORTY 1st August, 1950

:

Graduation

:

B.Sc. with Honours in Physics, from St. Xaviers College, Calcutta University, 1969.

Post-Graduation

:

M.Sc. Solid State Physics, I.I.T. Kharagpur, 1972.

Ph.D.

:

Indian Association for the Cultivation of Science,Kolkata 1972-1976.

Post Doc.

:

Technische Hochshule Darmstadt, (Germany) 1982-84.

Field of Specialization

:

Semiconductor materials, Vacuum science and Surface Physics including AES,XPS, ELS, LEED and SIMS.

International

:

Fellowships

DAAD (German) : 1982,1990,2003 UNDP : 1992, 1995

Visit abroad : 1)

Technische Hochschule, Darmstadt, Germany, 1982-84.

2)

Max-Planck Institute, Stuttgart, Germany, 1990.

3)

Pennsylvania State University, USA, 1992,U.N.D.P. Programme.

4)

University of Bourgogne, Dijon, 1993 and 1995, as visiting scientist.

5)

North Carolina State University, Raleigh, USA, 1995, U.N.D.P. Programme.

6)

Pennsylvania State University, State College, USA, 1997, Invited by the Surface Chemistry Dept. as a visiting scientist.

7)

University of Muenster, Germany, 2003.

Life Memberships

: 1.Indian Vacuum Society 2.Material Research Society of India 3.Indian Society for Mass Spectrometry

Special honour

:1. National Project Coordinator, UNDP Programme 1993-97 2. Visiting scientist at Pennsylvania State University USA,1997.

No. of publications in national & international refereed journals : 80 Present position

:

Dy. Head, Material Characterization division, and Scientist ‘F’ at the National Physical Laboratory, New Delhi.

12th ISMAS-WS-2007, March 25-30, 2007, Cidade de Goa, Dona Paula, Goa