Modi cation of the refractive index and the dielectric constant of silicon dioxide by means of ion implantation

Transcription

1 Nuclear Instruments and Methods in Physics Research B 166±167 (2000) 171±176 Modi cation of the refractive index and the dielectric constant of silicon dioxide by means of ion implantation J.W. Swart *, J.A. Diniz, I. Doi, M.A.B. de Moraes CCS, FEEC and IFGW ± UNICAMP, State University of Campinas, R. Pandia Calogeras 90, Campinas, SP, Brazil Abstract The modi cation of silicon dioxide lms by means of ion implantation of uorine and carbon was studied. 19 F and 12 C ions were separately and sequentially implanted in 250 nm thick thermal SiO 2 lms with energies ranging from 10 to 50 kev and uences in the interval to cm 2. Metal/oxide/semiconductor (MOS) capacitors were fabricated on half side of the wafers. The implanted SiO 2 /Si samples were characterized by means of ellipsometry and Fourier transform infrared (FTIR) spectroscopy. The MOS capacitors were used to determine the relative dielectric constant. Our results indicate a considerable reduction of the dielectric constant and refractive index. The refractive index was reduced from 1.46 to 1.29 when only uorine was implanted or when uorine with a higher dose was implanted in combination with carbon. For the same conditions, a relative dielectric constant of 3.4 was obtained and a shift in the Si±O bond stretching mode from 1085 to 1075 cm 1 was observed by FTIR spectroscopy. Ó 2000 Elsevier Science B.V. All rights reserved. PACS: 61.72Ww; 68.55Jk; f Keywords: Dielectric constant; Ion implantation; Silicon oxide 1. Introduction * Corresponding author. Tel.: ; fax: address: jacobus@led.unicamp.br (J.W. Swart). Insulating lms of low dielectric constant are needed for multilevel interconnection systems for ULSI IC fabrication. For this purpose, SiO 2 lms doped with uorine have been proposed [1±7]. However, thermodynamic constraints impose limitations to the introduction of uorine during the deposition process and the absorption of H 2 O by uorine species in SiOF lms [1±7]. This absorption may cause hydrolysis reaction with formation of OH and HF bonds, leading to the corrosion of the oxide layer [1]. An interesting alternative has been proposed which uses CF 4 plasma ion implantation following the silicon dioxide deposition [1]. This CF 4 plasma ion implantation may form C±F and C±O bonds, which retard the hydrolysis reaction, improving lm stability [1]. This, however, presents two drawbacks: (a) as low ion implantation energy is used (2 kev), a twolayer lm is formed where only the top one presents a low dielectric constant; (b) the absence of independent control of the ion implantation of the di erent chemical species within the plasma. These X/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S X(99)

2 172 J.W. Swart et al. / Nucl. Instr. and Meth. in Phys. Res. B 166±167 (2000) 171±176 drawbacks can be overcome by the use of standard ion implantation employing higher energies. Although plasma ion implantation is preferred based on economical considerations, a study by ion implantation of di erent chemical species and energies will be very valuable for the determination of the best process parameters such as ion species and their uence and energy. In this work, the modi cation of silicon dioxide lms by means of 19 F and 12 C ion implantation was studied. The ions were separately and sequentially implanted in 250 nm thick thermal SiO 2 lms with variation of ion energy and uence. The lm characterization by means of capacitance± voltage (C±V) measurements, ellipsometry and Fourier transform infrared (FTIR) spectroscopy was obtained. 2. Experimental The silicon oxide layers were formed on p-type single-crystal Si(1 0 0) wafers with resistivities ranging from 11 to 22 X cm. The substrates were cleaned by the RCA method and thermally oxidized at 1000 C in wet O 2 and annealed at 1000 C for 30 min in nitrogen. The modi cation of silicon dioxide lms was performed by means of ion implantation of uorine and/or carbon. The 19 F and 12 C ions were separately or sequentially implanted in 250 nm thick thermal SiO 2 lms with energies ranging from 10 to 50 kev and uences in the interval of to cm 2. For control, some samples were not implanted. Table 1 shows details of the F and C implantation performed on SiO 2 /Si. Metal/oxide/silicon capacitors were formed on half side of the wafers by e-beam deposition of 0.15 lm thick aluminum lm, followed by sintering in forming gas at 430 C for 20 min. The Al electrodes were patterned with 150 lm diameter dots. The wafer backside was etched in bu ered HF to remove the oxide and a 0.15 lm thick Al lm was subsequentily deposited. C±V measurements at 1 MHz were performed to determine the Table 1 The F and C implantation performed on SiO 2 /Si and ellipsometry, FTIR spectroscopy and C±V results Ion implantation Ellipsometry FTIR C±V Samples # Ions Fluence (cm 2 ) Energy (kev) T ox a (nm) n ox b Si±O c (cm 1 ) e ox d 4 F Control oxide ± ± F F F F C F C F C F C F C C C Control oxide ± ± a The oxide thickness. b The oxide refractive index. c The peak position of Si±O bond stretching vibration mode. d The oxide dielectric constant.

3 J.W. Swart et al. / Nucl. Instr. and Meth. in Phys. Res. B 166±167 (2000) 171± relative dielectric constant. A BOONTON 72-B capacimeter was used. This equipment was calibrated to obtain the capacitance measurements with an error lower than 1%. The thickness (T ox ) of the oxide lms was measured by ellipsometry. A Rudolph Research Auto EL NIR-3 ellipsometer was employed. A xed wavelength of nm and an incidence angle of 70 were used. Each determined value of the refractive index and thickness corresponds to average of ve measures in di erent positions on the surface samples with experimental error lower than 1%. A complex refractive index N s of ) i was used for the Si substrate in the calculations. The chemical bonds in unimplanted and implanted samples were identi ed by FTIR spectrometry using a FTS-40 BIO-RAD system. 3. Results and discussions Table 1 shows the values of oxide thickness and refractive index obtained by ellipsometry. The average values of the thickness of the oxides implanted with C and/or F ions were found to be 240 nm. The refractive index varied from 1.46 to The values of Table 1 were used to plot the curves of Figs. 1 and 2. These curves are based on small amount of data points. In this way, the interpolating curves can only be considered as general trends and the interpolated values cannot be used as absolute values. Fig. 1 shows the oxide refractive index as a function of (a) F ion energies, (b) F or C ion uences, (c) F and C ion uence ratios ([F]/[C]) for C ion uence of cm 2 and (d) F and C ion uence ratios ([F]/[C]) for C ion uence of cm 2. For samples implanted with a F ion energy of 10 kev and uences of cm 2, the refractive index was 1.39, while with an energy of 40 kev and for the control oxide, the refractive indexes were 1.46 (Fig. 1(a)). This indicates that low F ion energies and uences can produce lms with a lower refractive index due to implantation damages near the oxide surface. The refractive index was reduced from 1.46 to 1.29 or from 1.46 to 1.41 when only uorine or carbon were implanted (Fig. 1(b)), respectively, or from 1.46 to 1.29 when uorine with higher dose was implanted in combination with carbon with ion uence of cm 2 (Fig. 1(c)). The refractive index behavior presented in Fig. 1(b) for samples implanted with F was similar to that shown in Fig. 1(c), indicating the reduced in uence of C ion implantation for this uence ( cm 2 ). For a xed carbon uence of cm 2 (Fig. 1(d)), the refractive index as a function of F and C ion uence ratios ([F]/[C]) was increased from 1.41 to 1.45 for ratios between 0 and 0.1, and was decreased from 1.45 to 1.39 for ratios between 0.1 and 0.2. So, a higher in uence of C ion implantation for this uence ( cm 2 ) was observed. This, increasing of the refractive index, could be due to the formation of the C±O and C±F bonds. This formation can eliminate unstable Si±F or O±F bonds and retard the hydrolysis reaction [1]. FTIR spectroscopy was performed on the oxides (Table 1 and Fig. 1) to evaluate whether the Si±O absorption main peak position is shifted in relation to that of the control oxide. Table 1 presents the values of the Si±O stretching mode peak position, while Fig. 1 shows the Si±O bond absorption main peak position as a function of (a) F ion energies, (b) F or C ion uences, (c) F and C ion uence ratios ([F]/[C]) for C ion uences of cm 2 and (d) F and C ion uence ratios ([F]/[C]) for C ion uences of cm 2. Main absorption peaks due to Si±O stretching vibrations at 1085 cm 1 for control oxides and between 1083 and 1076 cm 1 for oxides implanted with uorine and/or carbon were observed, indicating the modi cation in the oxide stoichiometry, as observed previously with the oxide refractive index. C±V characteristics of Al/SiO 2 /Si specimens were performed to evaluate the lm dielectric constants under strong accumulation conditions at 1 MHz. Fig. 2 presents the oxide dielectric constants as a function of (a) F ion energies, (b) F or C ion uences, (c) F and C ion uence ratios ([F]/ [C]) for C ion uences of cm 2 and (d) F and C ion uence ratios ([F]/[C]) for C ion uences of cm 2. In general, the dielectric constant behavior (Fig. 2) is similar to that presented by the refractive index (Fig. 1). For F ion energy of 10 kev and uences of cm 2, the dielectric constant was 3.5, while for an energy of 40 kev

4 174 J.W. Swart et al. / Nucl. Instr. and Meth. in Phys. Res. B 166±167 (2000) 171±176 Fig. 1. The oxide refractive index and Si±O bond absorption main peak as a function of (a) F ion energies, (b) F ion uences, (c) C ion uences, (d) F and C ion uence ratios ([F]/[C]) for C ion uences of cm 2 and (e) F and C ion uence ratios ([F]/[C]) for C ion uences of cm 2. and for control oxide, the refractive indexes were 3.9 (Fig. 2(a)). The dielectric constant was reduced from 3.9 to 3.4 when only uorine or carbon (Fig. 2(b)) were implanted. The dielectric constant also was reduced from 3.8 to 3.6 when uorine with a higher dose was implanted in combination with carbon for a C ion uence of cm 2 (Fig. 2(c)), indicating the modi cation in the oxide stoichiometry. When uorine with a higher dose was implanted in combination with carbon for C ion uence of cm 2 (Fig. 2(d)), the dielectric constant presents a similar behavior as observed for the refractive index (Fig. 1(d)), with a maximum value for [F]/[C] ratio of 0.1. Our results, similar to values of oxides formed by introduction of uorine and/or carbon during the deposition process [2±7], indicate a considerable reduction of the dielectric constant and refractive index (Table 1, Figs. 1 and 2) for the following implantation conditions: F ion energy of 10 kev and uence of cm 2 (sample #4), F ion energy of 50 kev and uence of cm 2 (sample #11), F and C ion uence ratio ([F]/[C]) of 0.2 for C ion uences of cm 2 and F and C energies of 50 kev (sample #15).

5 J.W. Swart et al. / Nucl. Instr. and Meth. in Phys. Res. B 166±167 (2000) 171± Fig. 2. The oxide dielectric constant as a function of (a) F ion energies, (b) F ion uences, (c) C ion uences, (d) F and C ion uence ratios ([F]/[C]) for C ion uences of cm 2 and (e) F and C ion uence ratios ([F]/[C]) for C ion uences of cm 2. The above results are in accordance with previous publications [2±7], where uorine and carbon were introduced during the silicon oxide deposition process. In addition, our results show that: for low F ion implantation uences ( cm 2 ), the desired e ects are much more e ective if low ion energies are used. This indicates that a largely spread and diluted F concentration presents no e ect, while keeping some F concentrated near the surface, with concentration over a critical value, a bi-layer dielectric lm is obtained with the top one with low dielectric constant and refractive index; by using high energy ion implantation of F or C separately, to spread the ions into the whole silicon oxide layer, the dielectric constant and refractive index decrease monotonically with the ion uence; when co-implanting C and F at the same ion energy (50 kev), a cancellation e ect is observed leading to higher dielectric constant, with a maximum at a certain [F]/[C] ratio depending on the C ion uence. This co-implantation of C and F may form C±O and C±F bonds, eliminate unstable Si±F or O±F bonds, retard hydrolysis reaction and improve lm stability [1]; best results are obtained for high ion uences of F and/or C of the order of cm 2. When co-implanting F and C the best uence ratio [F]/ [C] corresponds to 0.2, with a C uence of cm 2.

6 176 J.W. Swart et al. / Nucl. Instr. and Meth. in Phys. Res. B 166±167 (2000) 171± Conclusions The modi cation of silicon dioxide lms by means of ion implantation of uorine and carbon was studied. 19 F and 12 C ions were separately or sequentially implanted in 250 nm thick thermal SiO 2 lms with energies ranging from 10 to 50 kev and uences in the interval to cm 2. Our results indicate a considerable reduction of the dielectric constant and the refractive index. The refractive index was reduced from 1.46 to 1.29 when only uorine was implanted or when uorine with higher dose was implanted in combination with carbon. For the same conditions, a relative dielectric constant of 3.4 was obtained and a shift in the Si±O bond stretching mode from 1085 to 1075 cm 1 was observed. Our results are similar to those obtained for the oxides formed by introduction of uorine during the deposition process [2±7]. The best condition to obtain the reduction of the dielectric constant is for oxides implanted with F and C, with a ratio of the ion uence ([F]/[C]) of 0.2, for C ion uences of cm 2 and F and C energies of 50 kev. Under this condition lms were obtained with low refractive index and dielectric constant. Acknowledgements The authors would like to thank Prof. Dr. N. I. Morimoto (LSI/EPUSP) for ellipsometric measurements and Prof. Dr. Ines Pereyra (LME/ EPUSP) for FTIR analysis. The work is supported by CAPES, FAPESP and FINEP of Brazil. References [1] S. Qin, Y.Z. Zhou, C. Chan, P.K. Chu, IEEE Electrons Dev. Lett. 19 (11) (1998) 420. [2] K. Kim, D. Kwon, G.S. Lee, Thin Solid Films 332 (1±2) (1998) 369. [3] S.M. Yun, H.Y. Chang, M.S. Kang, C.K. Choi, Thin Solid Films 341 (1±2) (1999) 109. [4] V. Pankov, J.C. Alonso, A. Ortiz, Japanese J. Appl. Phys. 37 (11) (1998) [5] Y.H. Kim, H.H. Kim, S.K. Lee, H.J. Kim, S.O. Kim, Y.S. Sohn, J. Korean Phys. Soc. 33 (1998) S179, Suppl. [6] J.H. Kim, S.H. Seo, S.M. Yun, H.Y. Chang, K.M. Lee, C.K. Choi, J. Electrochem. Soc. 143 (9) (1996) [7] A Grill, V. Patel, J. Appl. Phys. 85 (6) (1999) 3314.