Silicon-on-insulator (SOI) structures implanted with 200 or 400 keV N+ ions at a dose of 7.5 × 1017cm−2 were studied by spectroscopic ellipsometry (SE). The SE measurements were carried out in the 300–700 nm wavelength (4.13-1.78 eV photon energy) range. The SE data were analysed by the conventional method of using appropriate optical models and linear regression analysis. We applied a seven-layer model (a surface oxide layer, a thick silicon layer, upper two interface layers, a thick nitride layer and lower two interface layers) with good results. The fitted parameters were the layer thickness and compositions. The results were compared with data obtained from Rutherford backscattering spectroscopy (RBS) and transmission electron microscopy. The sensitivity of our optical model and fitting technique was good enough to distinguish between the silicon-rich transition layers near the upper and lower interfaces of the nitride layer, which are unresolvable in RBS measurements

Fried, M.

Gyulai, J.

Hanekamp, L.J.

Khanh, N.Q.

Laczik, Z.

Lohner, T.

Nijs, J.M.M. de

Silfhout, A. van

English

de Nijs, J.M.M.

van Silfhout, A.

NARCIS 

Non-destructive characterization of nitrogen-implanted silicon-on-insulator structures by spectroscopic ellipsometry

University of Twente Research Information

Materials Science and Engineering, B2 (1989) 131 - 137 131 Non-destructive Characterization of Nitrogen-implanted Silicon-on- insulator Structures by Spectroscopic Ellipsometry* M. FRIED and T. LOHNER Central Research Institute for Ph3,sies, t7.0. Box 49, Budapest H-1525 (Hungary) J. M. M. DE NIJS, A. VAN SILFHOUT and L. J. HANEKAMP Department of Applied Physics, Twente Technical University, P.O. Box 217, 7500 A E Enschede (The Netherlands) N. Q. KHANH Central Research lnstitute jor Physics, P.O. Box 49, Budapest H-1525 (Hungary) Z. LACZIK Central Research lnstitute jor Physics, 17. O. Box 49, Budapest tt-1525 (Hungary), and Department of Metallurgy and Science of Materials, University of OxJbrd, Parks Road, Oxford, OX1 3PH (U.K.) J. GYULAI Central Research Institute./or Physies, P.O. Box 49, Budapest tt-1525 (Hungary) (Received June 2.1988) Abstract 1. Introduction Silicon-on-insulator (SO1) structures implanted with 200 or 400 keV N ÷ ions at a dose of 7.5×1017cm 2 were studied by spectroscopic ellipsometry (SE). The SE measurements were carried out in the 300- 700 nm wavelength (4.13-1.78 eV photon energy) range. The SE data were analysed by the conventional method of using appropriate optical models" and linear regression analysis. We applied a seven-layer model (a surface oxide layer, a thick silicon layer, upper two inter- ]'ace layers, a thick nitride layer and lower two interface layers) with good results. The fitted para- meters were the layer thicknesses and composi- tions. The results' were compared with data obtained from Rutherford backscattering spectros- copy (RBS) and transmission electron microscopy. The sensitivity of our optical model and fitting technique was good enough to distinguish between the silicon-rich transition layers near the upper and lower interfaces" of the nitride layer, which are unresolvable in RBS measurements. *Paper presented at the Symposium on Deep Implants: Fundamentals and Applications at the E-MRS Spring Meeting, Strasbourg, May 31 -June 2, 1988. High dose implantation of reactive ions, such as oxygen, nitrogen or both, into silicon wafers has been successfully used to synthesize silicon-on- insulator (SOI) structures suitable for very-large- scale integration and radiation hard applications [1, 2]. The advantages of SOI devices have been noted by many investigators [3-5]. Parameters affecting the SOI microstructures have been extensively studied by Rutherford backscattering spectroscopy (RBS), transmission electron microscopy (TEM) imaging, secondary ion mass spectrometry and Auger electron spectroscopy [6-10]. Quite recently, spectroscopic ellipso- metry (SE) has been used with success in the investigation of oxygen-implanted SOI [11, 12]. However, nitrogen-implanted SOI is more diffi- cult, because of the stronger dependence of the microstructure on implantation dose. In this paper, we report the results of the characterization of SOI structures formed by a buried Si3N 4 layer. It is worth noting that the investigated samples are probably not the best for technological use. The emphasis in this paper is placed on the characterization method which may be the tool for future optimization experiments. In a previous paper [13], we applied multiple- angle-of-incidence (single-wavelength) ellipso- 0921-5107/89/$3.50 © Elsevier Sequoia/Printed in The Netherlands 132 metry (MALE) to determine the refractive indices and thicknesses of the same SOl layers using a relatively simple three-layer optical model. The results reported in that paper are shown together with the present results in Tables 3 and 4. 2. Experimental details Nitrogen ions were implanted at 200 keV (sample 1) and 400 keV (sample 2) into (100) p-type (12 f~ cm) single-crystal silicon using the 500 keV ion implanter at the Central Research Institute for Physics, Budapest. The samples were given a dose of 7 .5x1017N + cm -2 with a current density of 2 # A  cm "- on a 2 cm x 2 cm surface. External target heating during implanta- tion was applied at 600 °C. Post-implantation annealing was performed at t 300 °C for 2 h in argon for sample 1 and at 1200 °C for 2 h in N~ for sample 2. The silicon surface was covered with another piece of silicon in order to protect the sample surface from oxidation during high temperature annealing (face-to-face arrange- ment). RBS combined with channelling was performed with 2 MeV He + ions at a scattering angle of 165 ° . The SE measurements were performed on a rotating analyser ellipsometer in the 300-700 nm wavelength (4.13-1.78 eV photon energy) region at the Twente Technical University [14]. The cross-sectional TEM investigation was made at the University of Oxford. 3. Model variables Data were analysed using a FORTRAN pro- gram which calculates tan ~, and cos A values for an assumed input optical model of the sample and then minimizes the mean-square difference between measured and calculated tan V~ and cos A values by varying the designated model parameters according to the Fletcher-Powell [ 15] algorithm. With reference to the work of Aspnes et  al. [ 16], the quantity used to describe the agree- ment between the experimental data and the modelling process is defined as the unbiased esti- mator a. For the analysis of the samples the fol- lowing expression was used: u X ~ (COS A ~xp A t:al~'~z I -- COS i ' /:-1 +( tan  ~uj - t a n  Ni , [ J where n is the number of experimental measure- ments and p is the number o f  unknown mOdel parameters. The superscripts exp and catc refer to the experimental and calculated values respectively. We used three basic models (Fig. 1 ). The first (Fig. l(a)) consists of a silicon substrate, a nitride (mixed with silicon) layer, a silicon (mixed with a )  1. m o d e l  b )  2 . m o d e [  c)  3 . m o d e l  V 1 V2 V 3 oxide , - ' - s i l i c o n  si l icon • " "  ~ i t r ide n i t r ide  . "  / - . - "s i l i con bulk s i l icon v1 t l  v2 t2 v3 t 3 v4 v5 oxide I -  s i l icon s i l icon i / / / / "r~itr ide ni t r ide , - " s i l i c O n  / • . I / n l tnde  .. / ~ "s i l icon n i t r ide  , - " s i L i c o n  bulk s i l icon Vl V2 t~ t3 V4 14 v5 t t5 v6 v7 si l icon ~ . ~ "  -- h~'t r ide n i t r ide  , . '  si l icon n i t r ide  , - ' "  s i l icon n i t r ide  ~ ~ . "  ~silicon n i t r ide ,~ ' -  sihcon ni t r ide , " -s i l i con bulk s i l icon tz ~3 t4 t5 t6 17 Fig. 1. Optical  mode l s  used  for SE data  evaluation. T h e  t, values are the layer thicknesses,  and the z,, values are the vo lume frac- t ions of  the first componen t s  in each layer. (Thus  the I - v i values are the vo lume fractions of the second components•  nitride) layer and a surface oxide (mixed with sili- con) layer. The second (Fig. l(b)) and the third (Fig. l(c)) models are enhanced with two and four additional interface layers respectively. Each layer is modelled using the Bruggeman effective- medium model with two end-member complex dielectric functions (or complex refractive index functions) of crystalline silicon (c-Si)[17] or SiO~ i18] or SigN 4 [19]. These functions are not neces- sarily the most appropriate for describing these layers but are the best obtainable from literature. However, they are good starting points. Each layer is described by the volume fractions of the constituents. If the volume fraction is less than zero or greater than unity, this means that the actual refractive index of the layer is less or greater than the basic value of unity. During the fitting process, not all the para- meters were varied. Firstly, we investigated only the surface layer considering only the 300-400 nm wavelength region. After having 133 fitted the parameters (the thickness and volume fraction) of the surface layer, we fitted the other parameters using the whole wavelength region, but now having fixed the parameters of the sur- face. The imaginary part of the complex refrac- tive index of single-crystal silicon (which is proportional to the light absorption) is greater than 0.4 in this (high photon energy) region, meaning that the optical penetration depth is less than 0.2 #m. This is supported by the presented simulations (Fig. 2). There is no difference in the curves up to 400 nm. The rapidly changing interference pattern is due to the silicon layer which is sandwiched between the surface film and the buried nitride layer. The structure of this pattern (the number and places of the oscillations, and the amplitudes) is very sensitive to the thicknesses. In c-Si the optical penetration depth of light depends on its wavelength. In general, the higher the wavelength, the deeper is the penetration. In t -  O a) I i [ b) I I -1 1 i 12 nm ox ide  350 nm Si - o  37Onto Si - • 180 nm n i t r i de  butk Si 300 400 500 600 700 WL[nm]  " ~ 0 "  0 u i 12nm ox ide  350 nm Si 180 nm n i t r i d e - o  200nm n i t r i d e -  * butk Si 301 400 500 WL[nm]  L 600 Fig. 2. Simulated SE spectra on the inserted optical models: (a) 20 nm variation in the silicon; (b) 20 nm variation in ~ 0 I -1 700 the nitride. 134 other words the measurements at longer wave- lengths contain more information in the buried layer. This fact is shown in Fig. 2(b). if the thick- ness of the buried nitride layer is changed by 20 nm, there is a great variation mainly above 500 nm, the difference increasing with increasing wavelength. The  main problem was the choice of the initial parameters. We must make a good guess for the thicknesses as otherwise the program can "miss" or, in other words, can find a false minimum. We used computer  simulations similar to that in Fig. 2. 4. Resu l t s  and d i s cus s ion  The  best fits are presented in Figs. 3 and 4. The  first model (Fig. l(a)) cannot satisfactorily describe the samples, although the tLficknesses can be determined with great precision. After having completed the models with interface layers (Figs. l(b) and lie)., the fittings became increasingly better. The  unbiased estimator o decreased by more than a magnitude. (We did no t  study the effect of more interface layers. More  layers mean more paramelers--each additional layer gives two more parameters and thus in k creases the computing time. The  most sophisti- cated model took more than 5 h on our IBM PC/AT computer  and the time increased as the square of the number  of parameters.) The  materials parameter~ which yielded the best-fit spectra to the experimental data are shown in Tables 1 and 2. The  most interesting results are the silicon-rich thin transition layers near the upper and lower interfaces of the nitride layer. This result is comparable with the Auger depth profile measuremem of Oosting e~ al .  [10] on a sample (160 keV. 7 x  I() [r N" cm ; 1200 °C for 1 h) similar to sample 1. I 'he  volume fraction values of these layers near to or less than zero can be interpreted in the fol- o 0.0 1.0 1 . m o d e t  l i i (i; = 0 . 0 4 8  4.13 , , , 1.78. 2. m o d e l  i i i ___ (3 = 0.008 x x x 3. m o d e l  G = 02001 i ] Epho ton  [ eV  ] 4.13 1.78 4.13 i I I i i , - -  C O [ C U [ O t ~  XXX exper imentoL - 1 £  . 300 400 5 0 0  6(~0 700 300 400  500 600 700 300 400  WL[nml  WL[nm] Fig. 3. Plot of cos A and tan 7; vs. wavelength of sample 1 fitted by the optical models in Fig. 1. 1.?8 1.0 I i F . . . .  , , , ~ - ,  ' 1 . 0  50O 600 700 WL lnm]  135 1.0 0.c 1 . m o d e l  i I 6 = 0 . 0 0 3 2  2.mode l  i i 6 = 0.0016 3.model  i _ _  i 6 = 0 . 0 0 0 2 5  4.13 1.78 4.13 1.0 ' ' ' - 1.0 Fig .  4. Ephoton [eV]  - -  caLcutctt ed xxx experimento[ 1.78 4,13 i 300 t, O0 500 600 700 300 400 500 600 700 300 400 500 600 WL[nm] WL[nm] WL[nm] P l o t  o f  co s  A a n d  t an  ~p ws. w a v e l e n g t h  o f  s a m p l e  2 f i t ( ed  by t he  o p t i c a l  m o d e l s  in  Fig .  1. 1.0 0.0 I. 7B - - ~ - - - -  d.O -1,0 700 T A B L E  1 B e s t  f i ts  o n  s a m p l e  1 M o d e l  1, o = 0 3 ) 4 8  M o d e l  2, o = O.(X)8 M o d e l  3, o = O.(X)I t, = 351 n m .  r :  = 1.02 t, = 3 5 2  n m ,  t,~ = 1 .00  t~ = 3 5 2  rim, t,: = 0 . 9 9  t 3 = 175 n m .  1' 3 = 0 . 9 6  t 3 = 3 5  rim, t, 3 =(1.41 t 4 = 130  n m ,  u 4 = 0 . 9 4  t 5 = 18 rim, 1, 5 = 0 . 0 5  t 3 = 2 7  n m ,  l , 3 - 0 . 6 6  l 4 = 6 n m ,  i, 4 = - (I.25 t5 = 130  n m ,  l,~ = 0 . 9 3  6, = 7  n m ,  l , ,  = - 0 . 1 8  t 7 = 12 n m ,  l, 7 = 0 . 6 7  T A B L E  2 B e s t  f i ts  o n  s a m p l e  2 M o d e l  1, o = 0 . 0 0 3 2  M o d e l 2 ,  o = O. (X) I6  M o d e l  3, o = 0 . ( X ) 0 2 5  t~ = 6 0 1  n m .  t, 2 = 0 . 9 9  t 2 = 6 0 1  n m ,  t,~ = 0 . 9 8  t2 = 6 0 1  n m ,  r :  = 1.00 t~ = 2 5 5  n m ,  t,~ = 0 . 7 5  t~ = 3 6  n m ,  t, 3 = 0 . 5 0  t 4 = 2 0 0  rim, t'4 = 0 . 7 3  t 5 = 2 4  r im,  t,, = 0 . 3 4  t 3 = 2 9  n m ,  l~  = 0 .71  t 4 = 10 n m ,  t~4 - 0.31 t~ = 2(J0 n m ,  l, 5 = [).73 t6 = 9 n m ,  t;6 = (J. 14 t 7 = 18 n m ,  t, 7 = 0 . 5 6  136 lowing manner. We use the dielectric function of single-crystal silicon. However, these layers prob- ably consist of small damaged silicon crystallites; so their dielectric functions tend towards that of amorphous silicon. Thus the use of the dielectric function of c-Si results in a volume fraction greater than the real value. The volume fraction of the thick nitride layer is a very useful parameter. It can show the quality of the buried insulator layer. For sample 2 the implanted dose is obviously too small. This is because the peak concentration of the as-implan- ted nitrogen profile is lower than that for sample 1 because of the higher range straggling (higher implantation energy). The results were compared with data from other independent measurements. In Figs. 5 and 6 the results from RBS and TEM are presented. We can conclude that the layer thickness data are in very good agreement (Tables 3 and 4). More- over, the TEM pictures show that the upper nitride-silicon interface is rougher and thicker than the lower interface. + 2000 keY He 8=165 ° t ;'k l 5Oh lOe a> e n e r g y  o £  s c a t t e r e d  i o n s [ k e Y /  + 2000 keV He 8=165 * 40N 2 C~ 2~- (b) Fig. 5. RBS sample 2. "["  -.~ ~ 100 ~ ; i " vwgir~ ~ 100 ~ ~ . • • " zh enercjy o£ scat tered ions[keY chanelling spectra of (a) sample l and (b) ~d IL:C'~ SUDSt rFLte ( ( Qrr el) Bright field mctg~, - -  Sur face  lop sit con ioyer ried nO.ride c',' DGrk f i e l d  !q 'a [ ;e  Fig. 6. Cross-sectional TEN piclurcs on smnple t. In the surface single-crystal silicon layel, three dislocations can be seen. TABLE 3 Collected data of sample 1. (multip!e,angle~of- incidence eilipsometry results are from ref. 13) f'arameter Value obtained by the foUowing tech- niques MAlE [13/ RBS (nm) TEM (nm) Thickness of top 355 silicon (nm) Thickness of buried 177 nitride (nm) Complex refractive 3.89-0.04i index of top silicon Complex refractive 2.03-0,00i index of nitride 35O 380 180 TABLE 4 Collected data of sample 2, (multiple,angle-of- incidence ellipsometry resultsare from ref. 13) Parameter Value Obtained by the.following techniqtws MAlE [13] RBS Thickness of top 603 silicon (nm) Thickness of buried 250 nitride (nm) Complex refractive 3.89-i).02i index Of top silicon Complex refractive 2.57-~}.00i index of nitride 600 5. Conclusion It was demonstrated that SE using linear regression analysis can give information not only about the thickness but also on the composition of each layer and interface of the SO1 structure formed by thermally activated redistribution of high dose ion-implanted nitrogen. The agreement between SE and other analytical techniques (RBS and cross-sectional TEM) is fairly good. The advantages of SE over conventional ellip- sometry originate from the fact that it utilizes the varying optical refraction and absorption with photon energy to obtain information about the multilayer system under investigation. it is worthwhile emphasizing that the consider- able sensitivity of SE to the interface properties, layer thicknesses and microstructure can give it an essential role in future optimization experi- ments. Acknowledgments This research was supported by Grant OTKA 1794 from the Hungarian Academy of Sciences and two scholarships (T.L. and Z.L.) from the Soros Foundation. We gratefully acknowledge the assistance of J. Waizinger, C. Seres, F. Gazdficska (all of the Cen- tral Research Institute of Physics) and J. Gyimesi (Microelectronics Company, Budapest). 137 References 1 H.W. Lam, R. F. Pinizzotto and A. F. Tasch, Jr., VLSI Handbook, Academic Press, New York, 1985, p. 503. 2 H. W. Lain, A. F. Tasch, Jr., and R. F. Pinizzotto, VLSI Electronics: Microstmcture Science, Vol. 4, Academic Press, New York 1982. 3 K. Izumi, M. Doken and H. Ariyoshi, Electron. Lett., 14 (1978) 593. 4 G. Zimmer and H. Vogt, IEEE 7)'ans. Electron. Devices, 30(1983111515. 5 S.L. Patridge, lEEk, Proc., 133 (1986)66. 6 R L. F. Hemment, R. 1L Peart, M. F. Yao, K. G. Stephens, R. J. Chafer, J. A. Kilner, D. Meekison, G. R. Booker and R. P. Arrowsmith, AppL Phys. Lett., 46 (1985) 952. 7 R-H. Chang, C. Slawinski, B.-Y. Mao and H. Lain, J. Appl. Phys., 61 (1987) 166. 8 W. Skorupa, K. Wollschlager, M. Woelskow, K.-H. Heinig and H. Bartsch, Proc. European Materials Research Socie O, Meet., Vol. XV, Les Editions de Physique, Paris, 1987, p. 629. 9 W. Skorupa, K. Wollschlager, U. Kreissig, R. Gr6tzschel and H. Bartsch, Nucl. lnstrurn. Methods B, 19 20 (1987) 285. 10 R H. Oosting, J. Petruzello and T. E McGee, J. Appl. Phys., 62 (1987) 4118. 11 J. Narayan, S. Y. Kim, K. Vedam and R. Manukonda, AppL Phys. Lett., 5l (1987) 343. 12 Z. Liang and D. Mo, Appl. Phys. Lett., 52 (1988) 105/). 13 N.Q. Khanh, M. Fried, G. Battistig, Z. Laczik. T. Lohner, E. J~iroli, V. Schiller and J. Gyulai, Physica Status Solidi A, 108 (1988) K35. 14 T. Holtslag, Ph.1). Thesis, Tweme Technical University, Enschede, 1986. 15 R. Fletcher and M. J. Powell, Cornput. J., 6 (1963) 163. 16 D. E. Aspnes, J. B. Theeten and F. Hottier, l~hys. Rev. B. 20 (1979 ) 3292. 17 D. E. Aspnes and A. A. Studna, Phys. Rev. B, 27(1983) 985. 18 H.R. Philipp, J. Phys. Chem. Solids. 32 ( 1971 ) 1935. 19 H.R. Philipp, J. Electrochern. Soc., 120 (1973) 295. 

https://research.utwente.nl/en/publications/06b7db3b-7692-4c77-93e9-d3e264169a0b

Non-destructive characterization of nitrogen-implanted silicon-on-insulator structures by spectroscopic ellipsometry

Abstract

Similar works

Full text

Available Versions

NARCIS

University of Twente Research Information