Ion implantation-caused damage depth profiles in single-crystalline silicon studied by Spectroscopic Ellipsometry and Rutherford Backscattering Spectrometry

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Pergamon

Vacuum/volume 50/number 3–4/pages 293 to 297/1998 © 1998 Elsevier Science Ltd All rights reserved. Printed in Great Britain 0042–207X/98 $19.00+.00

PII : S0042–207X(98)00056–6

Ion implantation-caused damage depth profiles in single-crystalline silicon studied by Spectroscopic Ellipsometry and Rutherford Backscattering Spectrometry P Petrik,a* O Polga´r,b T Lohner,b M Fried,b N Q Kha´nhb and J Gyulai,a a Joint Chair for Experimental Physics of the Technical University Budapest and of the KFKI Research Institute for Materials Science, H-1525 Budapest, P.O.B. 49, Hungary ; b Joint Chair for Experimental Physics of the Technical University Budapest and of the KFKI Research Institute for Materials Science, H-1525 Budapest, P.O.B. 49, Hungary received for publication 16 January 1998

Damage created by ion implantation of Ar+ ions into single crystalline silicon is characterized using Spectroscopic Ellipsometry (SE) and Rutherford Backscattering Spectrometry (RBS). To create buried disorder, Ar+ ions with an energy of 100 keV were implanted into the samples. Ion doses were varied from 5×1013 atom/cm2 to 6.75×1014 atom/cm2. Damage depth profiles have been investigated using RBS combined with channeling, and SE. For the analysis of the SE data optical models were used, which consist of a stack of layers. The result proves the applicability of spectroscopic ellipsometry for the characterization of ionimplantation-caused damage. As an independent cross-checking method, Rutherford Backscattering Spectrometry was used. The RBS results basically supported the optical model of SE. © 1998 Elsevier Science Ltd. All rights reserved

Introduction Ion implantation caused damage formation has been intensively studied\ motivated by both fundamental and technological reasons[ Widely used measurement techniques are the Trans! mission Electron Microscopy "X!TEM#\ Rutherford Back! scattering Spectrometry "RBS# and optical re~ection and transmission spectroscopy[ Ellipsometry has been proved to be a very e}ective tool for the characterization of ion!implanted semiconductors\ because it is a non!destructive\ non!contact\ rapid and sensitive measurement technique[ The depth dis! tribution of the disorder obtained by RBS combined with the channeling e}ect can be used to construct realistic optical models and to cross!check the ellipsometry results[0\1 The applicability of SE for the non!destructive determination of damage depth pro_les in ion!implanted semiconductors has been demonstrated by several research groups[2Ð7 In the present paper we report on an SE and RBS study of medium mass "Ar¦# ion!implantation induced damage pro_les[ Experimental 099 keV Ar¦ ions were implanted at room temperature into 4[0Ð 5[8 Vcm\ p!type\ ð099Ł single!crystalline silicon using current  Corresponding author[

densities of 269Ð399 nA:cm1[ The implanted dose was varied from 4×0902 ion:cm1 to 5[64×0903 ion:cm1[ The implantation was performed at the Central Research Institute for Physics\ Budapest[ The ellipsometry spectra were obtained at the Fraunhofer Institute fur Integrierte Schaltungen in Erlangen\ Germany\ using a SOPRA ES3G rotating polarizer spectroscopic ellipsometer in the spectral range of 149Ð649 nm at an angle of incidence of 64[05>[ Rutherford backscattering and channeling techniques with 0[4 MeV He¦ ions were used to determine the buried disorder[ The detector was placed to detect ions scattered through 86>\ i[e[ with a glancing exit angle of 6> to the surface[ In this geometry\ the depth resolution is better than 4 nm[8 To evaluate the spectra we used the RBX program written by Kotai\09 which can handle channeled spectra too[ Spectroscopic ellipsometry provides tan C and cos D spectra\ where C and D are the ellipsometric angles that describe the re~ection of the polarized light[ The evaluation of the SE measurement data was carried out using the method of assuming appropriate optical models and _tting the calculated spectra on the measured one varying the model parameters with linear regression[ The best _t model parameters are obtained in terms of their 84) con_dence limits and the unbiased estimator "s# of the mean square deviation\ 293

P Petrik et al : Silicon studied by SE and RBS

s

6

$

n 0 calc 1 s "cosDexp j −cosDj # "n−p−0# j0 0:1

calc 1 ¦"tanCexp j −tanCj #

%7

\

where n is the number of independent measurement values cor! responding to the di}erent wavelengths and p is the number of unknown model parameters "{{exp|| refers to measured data\ {{calc|| refers to calculated data#[ For the analysis of the SE data optical models were used\ which consist of a stack of homogeneous layers ] a native oxide layer at the surface\ a thin amorphous silicon layer under the native oxide layer modeling the anomalous surface amorphization\00 and 19 layers with _xed and equal thicknesses and damage levels described by a coupled half!Gaussian depth pro_le function[7 The calculated spectra were _tted to the measured ones varying the model parameters "thickness of the native oxide layer\ the thin amorphous silicon layer\ and the four parameters describing the Gauss!pro_le#[ The main problem of the _tting procedure is the choice of the initial values of the model parameters[ If these initial values are not chosen properly\ the program can _nd a false minimum[ The complex refractive index of each layer is calculated by the Bruggemann!E}ective Medium Approximation "B!EMA# using the complex dielectric function of the single crys! talline silicon and the implanted amorphous silicon01 as layer components[ The damage level is described by the amorphous silicon content of the layer[ Results and discussion Figure 0 shows the measured ellipsometry spectra of crystalline silicon implanted using 099 keV Ar¦ ions in the spectral range of 149Ð619 nm[ The implanted doses are varied between 0[94×0902 ions:cm1 to 5[64×0903 ions:cm1[ For ion doses below 2[1×0903 ions:cm1 tan C and cos D change systematically\ and the di}er! ences are small[ For ion doses above 1[10×0903 ions:cm1 the cos D and tan C curves change drastically with changing ion doses\ especially for cos D at wavelengths above 449 nm[ The insert shows the change of cos D in the spectral range of 179Ð249 nm[ The ellipsometry measurement data were _rst analyzed in the UV spectral region[ Here the penetration depth of the light is not more than 79 nm at the wavelength of 399 nm\ even for crystalline silicon[ Thus\ a simple optical model can be used taking into account the surface oxide\ the surface amorphous layer\00 and the bulk with the combination of crystalline silicon and _ne! grained polycrystalline silicon[02 Figure 1 shows this optical model together with the measured and simulated curves for implantation doses of 9[4\ 1[10\ and 5[64×0903 atom:cm1[ The calculated data _t well the measured ones[ Measured data of a virgin silicon sample is also plotted as a reference[ The model parameters as a function of implanted dose are shown in Table 0[ The thickness of the SiO1 layer slightly increases in the dose range of 9[4×0902 ions:cm1 to 3[54×0903 ions:cm1 with increasing dose "from 1[28 nm to 2[00 nm#[ The thickness of the a!Si layer also increases with increasing dose[00 The di}erence is highest above the dose of 1[10×0903 atom:cm1[ The p!Si content of the bulk material increases from 08[0) to 38[2)[ The standard deviation "s# and the 84) con_dence limits "the values behind the 2 sign# are acceptable for all doses[ 294

Figure 0[ Measured ellipsometry spectra of single crystalline silicon sam! ples implanted with 099 keV Ar¦ ions together with the measured spectra of a virgin sample[ The insert shows the cos D spectrum from 179 to 249 nm[

Figure 2 shows the measured and calculated spectra for four di}erent doses in the whole spectral range[ The optical model consists of a native oxide layer on the top of the sample\ a thin near!surface damaged layer below the top oxide layer\ a layer describing the damage in vicinity of the projected range\ and a single crystalline silicon bulk[ The layer describing the damage is divided into 19 sublayers with _xed and equal thicknesses\ each having the components of single crystalline silicon and implanted amorphous silicon[ The fraction of the amorphous silicon is described by a coupled half!Gaussian depth pro_le function[7 The curves in Fig[ 2 are shifted with 9[0 and 9[4 with increasing doses for tan C and cos D\ respectively[ The measured and calcu! lated spectra _t very well[ For cross!checking the SE results\ high depth resolution RBS measurements were made with the detector placed at 86> scat! tering angle for the series of the implanted samples and for a virgin sample used as a reference "Fig[ 3#[ The buried disorder in the depth of approximately 29Ð049 nm increases rapidly with increasing doses above 1[10×0903 atom:cm1[ The buried disorder layer is totally amorphous near 099 nm for doses above 2[1×0903 atom:cm1[ The surface amorphous layer is also clearly seen near the surface\ which proves the validity of the model used in the UV range investigation[ Figure 4 shows the buried depth pro_les calculated from SE

P Petrik et al : Silicon studied by SE and RBS

Figure 1[ Measured and calculated ellipsometry spectra of single crys! talline silicon samples implanted with 099 keV Ar¦ ions for three di}erent doses using the spectral range below 259 nm "UV# together with the measured spectra of a virgin sample[ The optical model used is also shown in the _gure[

Figure 2[ Measured and calculated ellipsometry spectra of single crys! talline silicon samples implanted with 099 keV Ar¦ ions for four di}erent doses using the entire spectral range of the measurement[ The optical model used for the calculations is described in the text[ The vertical scales refer to the 0[94×0903 dose spectra ^ each of the other tan C and cos D spectra has been shifted by 9[0 and 9[4\ respectively\ with respect to the preceding one[

Table 0[ Parameters of the model used in the UV range as a function of the implanted dose[ The standard deviation "s# in the last column shows the quality of the _t Dose "×0903 at[:cm1#

DSiO1 "nm#

Da!Si "nm#

C p!Si ")#

s

9[49 9[64 0[94 0[41 1[10 2[19 3[54 5[64

1[2829[63 1[3229[64 1[3229[71 1[4829[73 1[6029[87 1[8429[00 2[0029[01 1[4629[18

9[1129[09 9[1429[00 9[2129[01 9[3929[01 9[3529[03 9[5529[06 9[8329[07 0[3629[37

08[022[8 14[322[8 15[323[2 21[423[1 39[924[9 34[024[7 38[225[1 43[4205[8

9[9949 9[9940 9[9945 9[9952 9[9955 9[9964 9[9978 9[9904

 Concentration of p!Si in the optical model[

and RBS measurements for six di}erent implanted doses[ The results of SE and RBS agree well[ The peak of the damage pro_le is at 099 nm\ which agree also with the X!TEM results "Fig[ 5#[ Above 2[1 atom:cm1 the damage layer becomes totally amorph! ous[

Conclusions Buried damage pro_les were characterized for 099 keV Ar¦ ions implanted into single crystalline silicon using doses of 9[4×0902 atom:cm1 to 5[64×0903 atom:cm1[ High depth resolution RBS technique was used as a cross!checking method[ The ellipsometry spectra were analyzed in the UV region "in order to obtain the thicknesses of the surface oxide and the surface amorphous lay! ers# and in the whole spectral range\ to determine the buried damage pro_les[ The results show that SE is a fast\ non!destruc! tive\ sensitive\ and precise method for measuring ion!implan! tation!caused disorder\ if a proper optical model is used[ There is a good agreement between the results of SE\ RBS\ and X! TEM[ Acknowledgements The authors would like to acknowledge the Analytical Depart! ment of the Fraunhofer Institut fur Integrierte Schaltungen and the Lehrstuhl fur Elektronische Bauelemente "Friedrich!Alex! ander Universitat Erlangen!Nurnberg\ Germany# for the possi! 295

P Petrik et al : Silicon studied by SE and RBS

Figure 3[ Random and aligned high depth resolution RBS spectra of single crystalline silicon samples implanted with 099 keV Ar¦ ions for four di}erent doses together with the measured spectra of a virgin sample[ The measurement was recorded with a detector placed at the scattering angle of 86>[

Figure 4[ Deduced damage depth pro_les for samples implanted with 099 keV Ar¦ into single crystalline silicon obtained using Spectroscopic Ellipsometry "−# and Rutherford Backscattering Spectrometry "#[ The implanted doses were varied from 6\4×0902 atom:cm1 to 5[64×0903 atom:cm1[ For the sample implanted with the dose of 3[54×0903 atom:cm1 X!TEM result is also shown[ For doses above 2[1×0903 atom:cm1 very good agreement between the SE and RBS results are obtained[

Figure 5[ X!TEM picture of a single crystalline silicon sample implanted with 099 keV Ar¦ ions using dose of 3[54×0903 atom:cm1[

bility of using the laboratories[ Support from OTKA Grant No[ T906233 and from Hungarian!German Intergovernmental Pro! ject "TeT\ No[ D03:85# are greatly appreciated[ P[ Petrik is indebted to the Foundation for Hungarian Science of the Hung! arian Credit Bank and to the Soros Foundation for _nancial support[ The authors thank the team operating the accelerator for their help in experimental procedures[ 296

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