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Influence of barrier and cap layer deposition on the properties of capped and non-capped porous silicon oxide
Microelectronic Engineering 55 (2001) 45–52 www.elsevier.nl / locate / mee
Influence of barrier and cap layer deposition on the properties of capped and non-capped porous silicon oxide a, b a a a S.E. Schulz *, H. Koerner , C. Murray , I. Streiter , T. Gessner a
Chemnitz University of Technology, Centre of Microtechnologies, D-09107 Chemnitz, Germany b Infineon Technologies AG, High Frequency Products, D-81730 Munich, Germany
Abstract Novel highly porous SiO 2 xerogels are being developed as low dielectric constant materials. For successful integration into DAMASCENE structures, the attractive electrical properties of these materials must not degrade as further cap and barrier layers are deposited and patterned. The influence of the deposition of PECVD SiO 2 cap and sputtered and MOCVD TiN barrier layers on the electrical properties of low k xerogel films was examined. FTIR was used to show that the pore surface methyl groups formed during HMDS treatment survive cap deposition. Electrical results indicate only small changes to the dielectric constant, leakage current density and field breakdown voltage after the cap was deposited. The deposition of the barrier layer was found to increase the dielectric constant of the xerogel by about 10–15% but not when the xerogel was capped first. 2001 Elsevier Science B.V. All rights reserved. Keywords: Xerogel; Dielectric; Integration
1. Introduction The trend towards feature size reduction which has been the cornerstone of IC development for decades is not sustainable using currently employed materials due to the problem of interconnect RC delay [1]. This is governed by the conductivity of the metalisation and the dielectric constant of the insulator and becomes dominant as feature size decreases to the submicron area. Highly porous SiO 2 xerogel material is a promising candidate to replace its currently used dense counterpart due to its lower dielectric constant and thermal stability [2]. These materials are especially suitable when the pore size is in the nanometer range and therefore well below the geometrical dimensions applied in metalisation. Numerous challenges remain to be overcome before the integration of xerogel materials in DAMASCENE architecture can be achieved. These include, among others, ensuring the stability of the advantageous electrical characteristics of the xerogel films during the deposition of other layers. *Corresponding author. Tel.: 149-371-531-3651; fax: 149-371-531-3131. E-mail address: [email protected] (S.E. Schulz). 0167-9317 / 01 / $ – see front matter PII: S0167-9317( 00 )00427-5
2001 Elsevier Science B.V. All rights reserved.
46
S.E. Schulz et al. / Microelectronic Engineering 55 (2001) 45 – 52
Fig. 1. Integration of xerogel in DAMASCENE process.
The DAMASCENE scheme requires the deposition of isolating SiO 2 cap layers to protect the xerogel during CMP and conducting barrier layers such as TiN to prevent Cu migration (Fig. 1). The influence of the deposition of these materials on the electrical and chemical properties of low k xerogel dielectric films is the subject of this work.
2. Experimental The xerogel deposition process consists of several distinct steps and has been described in detail elsewhere [3,4]. The TEOS:water ratio used in the precursor for this work was R 5 7. Spin on was performed when the precursor viscosity was in the region of 10–15 mPa s, which has been found to produce highly homogeneous films. The precursor was then spun on to a number of 4-inch diameter silicon wafers at various spin speeds from 1000 to 1500 rpm for 10 s. After a gelation period the films were dried by exposure to air and were then annealed to 4508C for 1 h. The films were finally placed in hexamethyl disilazane (HMDS) vapour for 24 h. This has the effect of replacing highly polar hydroxyl groups on the surface of the pores with less polar methyl groups. This treatment has been found to improve the electrical characteristics of the resulting films [2]. Whenever possible, the xerogel-coated wafers were broken into two halves — one which was used as a control and the other which underwent further processing. This consisted of 10 nm and 50 nm PECVD SiO 2 cap layer deposition from SiH 4 and N 2 O gas using low and high power plasma. TiN barrier layers (100 nm) were also deposited on xerogel and SiO 2 capped xerogel using two methods: (a) DC magnetron sputtering and (b) MOCVD pyrolysis of TDMAT employing an H 2 / N 2 densifying plasma for the initial and final 20 nm. The temperatures experienced by the films during PECVD, sputtering and MOCVD were 300, ,100 and 4508C, respectively. A shadow mask was used during some sputter depositions to produce dot structures while lithography and etching of blanket films was used for other sputtered and MOCVD TiN samples. After etching the TiN, the photoresist was not removed from the dots to prevent any influence from the resist stripper. Film thicknesses were measured using white light interferometry. Each xerogel film was measured
S.E. Schulz et al. / Microelectronic Engineering 55 (2001) 45 – 52
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at ten different points on the wafer to produce a map of thickness homogeneity for each wafer and batch. FTIR was employed to monitor changes in xerogel chemical features. Transmission and reflection FTIR was used to examine the effects of HMDS post-treatment on the xerogel films, and to look at the effects of cap layer deposition on the chemical composition of the film. The wafer substrates user for FTIR were 100 mm diameter3600 mm thick, 10–20 V cm, two-sided polished Si. Double side polishing is required for accurate transmission and better reflectance measurements. Electrical characteristics were measured using both Hg probe and needle probe CV stations capable of measuring the capacitance of the films Cox , the leakage current density Jleak . The field breakdown voltage Vb could be measured using Hg probe only. 3. Results
3.1. FTIR investigations 3.1.1. Effects of HMDS treatment on xerogel Fig. 2 shows the reflectance spectrum of a xerogel sample before and after exposure to HMDS. The most distinctive effects are observed at about 3750 cm 21 where the isolated (–OH) absorption peak is seen to nearly disappear after HMDS treatment. Also, the strong antisymmetric CH stretching vibration in Si–CH 3 is observed after treatment at 2960 cm 21 . Both these results are consistent with
Fig. 2. FTIR spectra of xerogel before and after HMDS treatment.
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the expected reaction of HMDS with highly porous xerogels: i.e. the replacement of hydrogen in surface hydroxyl groups by non-polar –Si(CH 3 ) 3 groups.
3.1.2. Effects of PECVD SiO2 cap layer deposition on HMDS treated xerogel SiO 2 cap layers of 10 and 50 nm were deposited by means of PECVD onto bare wafers and xerogel films. The PECVD process used to deposit the cap layers produces temperatures in the films of about 3008C. The onset of –CH 3 thermal oxidation was found by Yang et al. [5] to be in the region of 3108C, with the –CH 3 peak diminishing to near zero at 5008C. The transmission spectra in Fig. 3 of the cap layers on xerogel shows no significant variation from the non-capped xerogel layer. The antisymmetrical and symmetrical stretching vibrations of –CH 3 at about 2960 and 2900 cm 21 , respectively, shown in the Fig. 4, reveal that there is no major reduction of the peak intensity after cap layer deposition. This indicates that the deposition process has not removed many of the methyl groups introduced during HMDS treatment and so the hydrophobic nature of the porous xerogels can survive cap layer deposition at these temperatures. 3.2. Electrical results The homogeneity of the film thickness was to ,5%. The total error in the calculation of dielectric constant values due to errors in film capacitance, spot area and film thickness measurement was calculated to be 10%, although experience suggests repeatability to within about 5%.
Fig. 3. Transmission spectra of xerogel films with various SiO 2 cap layers.
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Fig. 4. –CH 3 absorption peaks for capped and non-capped xerogel films.
3.2.1. Cap layer deposition The variation of dielectric constant k for various treatments is shown in Fig. 5. The treatments are shown in Table 1; it can be seen that the only significant impact on the k value of the films comes with 50 nm SiO 2 deposition and possibly with N 2 O exposure. The influence of these treatments on Jleak and Vb are shown in Figs. 6 and 7. The leakage currents for the samples are generally lower than 1?10 211 A cm 22 before treatment. Leakage current densities are observed to increase with cap deposition by about an order of magnitude for the 50 nm high power PECVD SiO 2 film. Field breakdown voltages before treatment of 3.1–3.8 MV cm 21 are slightly reduced after treatment. In general the cap deposition process does not seriously degrade the electrical qualities of the xerogel films.
Fig. 5. Influence of cap treatment on xerogel k value.
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Table 1 Various film treatments Sample
Treatment
Temp. (8C)
1,2 3,4 5,6 7,8 9,10
10 nm standard PECVD SiO 2 50 nm standard PECVD SiO 2 50 nm high power PECVD SiO 2 SiH 4 (no plasma) N 2 O (no plasma)
3001 3001 3001 300 300
Fig. 6. Influence of cap layer treatment on leakage current density.
3.2.2. Barrier layer deposition As mentioned above, the TiN barrier layers were deposited either by sputtering or by MOCVD. The films were deposited onto either xerogel directly or onto a 50-nm PECVD cap layer on xerogel. The dots required for characterisation were formed either by sputtering through a shadow mask or by lithography of blanket sputtered or MOCVD layers. Again the effects on the xerogel of each variant described in Table 2 were examined. The variation of k for each variant is shown in Fig. 8. In general it can be seen that the greatest change in k occurred for lithographically processed TiN depositions on bare xerogel (samples 5, 6 and 9). It was thought that TiN etching during the lithographic process might be responsible for the apparent increase in k. This was tested by exposing an evaporated aluminium dot on xerogel to the TiN etch. This did not produce an appreciable increase in k. Therefore these effects must be related to
Fig. 7. Influence of cap layer treatment on field breakdown voltage.
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Table 2 Barrier layer deposition parameters Sample
Deposition
Substrate
Patterning
Temp. (8C)
1,2 3,4 5,6 7,8 9
Sputtering Sputtering Sputtering MOCVD MOCVD
50-nm cap on xerogel Xerogel Xerogel 50 nm cap on xerogel Xerogel
Shadow Shadow Litho / etch Litho / etch Litho / etch
,100 ,100 ,100 450 450
the deposition itself. In comparison, deposition onto capped xerogel did not greatly increase the k value of the xerogel. Leakage current densities for shadow mask samples 1–4 were unchanged with respect to their untreated xerogel control samples and were in the 1?10 211 A cm 22 range. The presence of the photoresist on the TiN dots for the etched samples 5–9 produced wide variations in measured Jleak values. Resist removal using EKC 505 wet stripper, which was found to be benign for blanket xerogel films, produced unusually high k and Jleak results for etched dot samples. Annealing these samples under vacuum seemed to return the samples to their original values. This may be due to wet PR remover trapped under the dots themselves, although there is no reliable data to confirm this. O 2 plasma resist removal was most detrimental to the xerogel, increasing the Jleak by four orders of magnitude. There remains therefore a need for a reliable resist remover which does not harm the xerogel properties.
4. Conclusions FTIR investigations confirm that the deposition of SiO 2 cap layers on xerogel films does not remove the methyl groups produced after treating the xerogel in HMDS vapour. The deposition of SiO 2 cap layers on xerogel were found to have negligible effects on the dielectric constant, leakage current density and field breakdown voltage of the xerogel films. While TiN barrier
Fig. 8. Influence of barrier treatment on xerogel k value.
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deposition on SiO 2 / xerogel did not affect the dielectric constant of the xerogel, an increase in k was observed when TiN was deposited directly onto xerogel. This could lead to an increases in line-to-line capacitance in an integrated structure. More work is required to confirm this result. Acknowledgements The contributions of M. Rennau and M. Henker of the Centre for Microtechnology, T.U. Chemnitz in the electrical characterisation of the samples is gratefully acknowledged. The FTIR work investigations carried out by Dr. Friedrich of the Physics Department, T.U.Chemnitz, is also appreciated. References [1] [2] [3] [4] [5]
M. Bohr, in: Tech. Digest IEEE Int. Electron. Devices Meeting, 1995, p. 241. C. Jin, J.D. Luttmer, D.M. Smith, T.A. Ramos, MRS Bull. 22 (10) (1997) 39. C.J. Brinker, G.W. Scherrer, Sol Gel Science, Academic Press, 1990. T. Winkler, S.E. Schulz, T. Gessner, in: MRS Symp. Proc. ULSI XII, MRS Warrendale PA, 1998, pp. 347–351. H.-S. Yang, S.-Y. Choi, S.-H. Hyun, H.-H. Park, J.-K. Hong, Non Crystalline Solids 221 (1997) 151–156.
Influence of barrier and cap layer deposition on the properties of capped and non-capped porous silicon oxide a, b a a a S.E. Schulz *, H. Koerner , C. Murray , I. Streiter , T. Gessner a
Chemnitz University of Technology, Centre of Microtechnologies, D-09107 Chemnitz, Germany b Infineon Technologies AG, High Frequency Products, D-81730 Munich, Germany
Abstract Novel highly porous SiO 2 xerogels are being developed as low dielectric constant materials. For successful integration into DAMASCENE structures, the attractive electrical properties of these materials must not degrade as further cap and barrier layers are deposited and patterned. The influence of the deposition of PECVD SiO 2 cap and sputtered and MOCVD TiN barrier layers on the electrical properties of low k xerogel films was examined. FTIR was used to show that the pore surface methyl groups formed during HMDS treatment survive cap deposition. Electrical results indicate only small changes to the dielectric constant, leakage current density and field breakdown voltage after the cap was deposited. The deposition of the barrier layer was found to increase the dielectric constant of the xerogel by about 10–15% but not when the xerogel was capped first. 2001 Elsevier Science B.V. All rights reserved. Keywords: Xerogel; Dielectric; Integration
1. Introduction The trend towards feature size reduction which has been the cornerstone of IC development for decades is not sustainable using currently employed materials due to the problem of interconnect RC delay [1]. This is governed by the conductivity of the metalisation and the dielectric constant of the insulator and becomes dominant as feature size decreases to the submicron area. Highly porous SiO 2 xerogel material is a promising candidate to replace its currently used dense counterpart due to its lower dielectric constant and thermal stability [2]. These materials are especially suitable when the pore size is in the nanometer range and therefore well below the geometrical dimensions applied in metalisation. Numerous challenges remain to be overcome before the integration of xerogel materials in DAMASCENE architecture can be achieved. These include, among others, ensuring the stability of the advantageous electrical characteristics of the xerogel films during the deposition of other layers. *Corresponding author. Tel.: 149-371-531-3651; fax: 149-371-531-3131. E-mail address: [email protected] (S.E. Schulz). 0167-9317 / 01 / $ – see front matter PII: S0167-9317( 00 )00427-5
2001 Elsevier Science B.V. All rights reserved.
46
S.E. Schulz et al. / Microelectronic Engineering 55 (2001) 45 – 52
Fig. 1. Integration of xerogel in DAMASCENE process.
The DAMASCENE scheme requires the deposition of isolating SiO 2 cap layers to protect the xerogel during CMP and conducting barrier layers such as TiN to prevent Cu migration (Fig. 1). The influence of the deposition of these materials on the electrical and chemical properties of low k xerogel dielectric films is the subject of this work.
2. Experimental The xerogel deposition process consists of several distinct steps and has been described in detail elsewhere [3,4]. The TEOS:water ratio used in the precursor for this work was R 5 7. Spin on was performed when the precursor viscosity was in the region of 10–15 mPa s, which has been found to produce highly homogeneous films. The precursor was then spun on to a number of 4-inch diameter silicon wafers at various spin speeds from 1000 to 1500 rpm for 10 s. After a gelation period the films were dried by exposure to air and were then annealed to 4508C for 1 h. The films were finally placed in hexamethyl disilazane (HMDS) vapour for 24 h. This has the effect of replacing highly polar hydroxyl groups on the surface of the pores with less polar methyl groups. This treatment has been found to improve the electrical characteristics of the resulting films [2]. Whenever possible, the xerogel-coated wafers were broken into two halves — one which was used as a control and the other which underwent further processing. This consisted of 10 nm and 50 nm PECVD SiO 2 cap layer deposition from SiH 4 and N 2 O gas using low and high power plasma. TiN barrier layers (100 nm) were also deposited on xerogel and SiO 2 capped xerogel using two methods: (a) DC magnetron sputtering and (b) MOCVD pyrolysis of TDMAT employing an H 2 / N 2 densifying plasma for the initial and final 20 nm. The temperatures experienced by the films during PECVD, sputtering and MOCVD were 300, ,100 and 4508C, respectively. A shadow mask was used during some sputter depositions to produce dot structures while lithography and etching of blanket films was used for other sputtered and MOCVD TiN samples. After etching the TiN, the photoresist was not removed from the dots to prevent any influence from the resist stripper. Film thicknesses were measured using white light interferometry. Each xerogel film was measured
S.E. Schulz et al. / Microelectronic Engineering 55 (2001) 45 – 52
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at ten different points on the wafer to produce a map of thickness homogeneity for each wafer and batch. FTIR was employed to monitor changes in xerogel chemical features. Transmission and reflection FTIR was used to examine the effects of HMDS post-treatment on the xerogel films, and to look at the effects of cap layer deposition on the chemical composition of the film. The wafer substrates user for FTIR were 100 mm diameter3600 mm thick, 10–20 V cm, two-sided polished Si. Double side polishing is required for accurate transmission and better reflectance measurements. Electrical characteristics were measured using both Hg probe and needle probe CV stations capable of measuring the capacitance of the films Cox , the leakage current density Jleak . The field breakdown voltage Vb could be measured using Hg probe only. 3. Results
3.1. FTIR investigations 3.1.1. Effects of HMDS treatment on xerogel Fig. 2 shows the reflectance spectrum of a xerogel sample before and after exposure to HMDS. The most distinctive effects are observed at about 3750 cm 21 where the isolated (–OH) absorption peak is seen to nearly disappear after HMDS treatment. Also, the strong antisymmetric CH stretching vibration in Si–CH 3 is observed after treatment at 2960 cm 21 . Both these results are consistent with
Fig. 2. FTIR spectra of xerogel before and after HMDS treatment.
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the expected reaction of HMDS with highly porous xerogels: i.e. the replacement of hydrogen in surface hydroxyl groups by non-polar –Si(CH 3 ) 3 groups.
3.1.2. Effects of PECVD SiO2 cap layer deposition on HMDS treated xerogel SiO 2 cap layers of 10 and 50 nm were deposited by means of PECVD onto bare wafers and xerogel films. The PECVD process used to deposit the cap layers produces temperatures in the films of about 3008C. The onset of –CH 3 thermal oxidation was found by Yang et al. [5] to be in the region of 3108C, with the –CH 3 peak diminishing to near zero at 5008C. The transmission spectra in Fig. 3 of the cap layers on xerogel shows no significant variation from the non-capped xerogel layer. The antisymmetrical and symmetrical stretching vibrations of –CH 3 at about 2960 and 2900 cm 21 , respectively, shown in the Fig. 4, reveal that there is no major reduction of the peak intensity after cap layer deposition. This indicates that the deposition process has not removed many of the methyl groups introduced during HMDS treatment and so the hydrophobic nature of the porous xerogels can survive cap layer deposition at these temperatures. 3.2. Electrical results The homogeneity of the film thickness was to ,5%. The total error in the calculation of dielectric constant values due to errors in film capacitance, spot area and film thickness measurement was calculated to be 10%, although experience suggests repeatability to within about 5%.
Fig. 3. Transmission spectra of xerogel films with various SiO 2 cap layers.
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Fig. 4. –CH 3 absorption peaks for capped and non-capped xerogel films.
3.2.1. Cap layer deposition The variation of dielectric constant k for various treatments is shown in Fig. 5. The treatments are shown in Table 1; it can be seen that the only significant impact on the k value of the films comes with 50 nm SiO 2 deposition and possibly with N 2 O exposure. The influence of these treatments on Jleak and Vb are shown in Figs. 6 and 7. The leakage currents for the samples are generally lower than 1?10 211 A cm 22 before treatment. Leakage current densities are observed to increase with cap deposition by about an order of magnitude for the 50 nm high power PECVD SiO 2 film. Field breakdown voltages before treatment of 3.1–3.8 MV cm 21 are slightly reduced after treatment. In general the cap deposition process does not seriously degrade the electrical qualities of the xerogel films.
Fig. 5. Influence of cap treatment on xerogel k value.
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Table 1 Various film treatments Sample
Treatment
Temp. (8C)
1,2 3,4 5,6 7,8 9,10
10 nm standard PECVD SiO 2 50 nm standard PECVD SiO 2 50 nm high power PECVD SiO 2 SiH 4 (no plasma) N 2 O (no plasma)
3001 3001 3001 300 300
Fig. 6. Influence of cap layer treatment on leakage current density.
3.2.2. Barrier layer deposition As mentioned above, the TiN barrier layers were deposited either by sputtering or by MOCVD. The films were deposited onto either xerogel directly or onto a 50-nm PECVD cap layer on xerogel. The dots required for characterisation were formed either by sputtering through a shadow mask or by lithography of blanket sputtered or MOCVD layers. Again the effects on the xerogel of each variant described in Table 2 were examined. The variation of k for each variant is shown in Fig. 8. In general it can be seen that the greatest change in k occurred for lithographically processed TiN depositions on bare xerogel (samples 5, 6 and 9). It was thought that TiN etching during the lithographic process might be responsible for the apparent increase in k. This was tested by exposing an evaporated aluminium dot on xerogel to the TiN etch. This did not produce an appreciable increase in k. Therefore these effects must be related to
Fig. 7. Influence of cap layer treatment on field breakdown voltage.
S.E. Schulz et al. / Microelectronic Engineering 55 (2001) 45 – 52
51
Table 2 Barrier layer deposition parameters Sample
Deposition
Substrate
Patterning
Temp. (8C)
1,2 3,4 5,6 7,8 9
Sputtering Sputtering Sputtering MOCVD MOCVD
50-nm cap on xerogel Xerogel Xerogel 50 nm cap on xerogel Xerogel
Shadow Shadow Litho / etch Litho / etch Litho / etch
,100 ,100 ,100 450 450
the deposition itself. In comparison, deposition onto capped xerogel did not greatly increase the k value of the xerogel. Leakage current densities for shadow mask samples 1–4 were unchanged with respect to their untreated xerogel control samples and were in the 1?10 211 A cm 22 range. The presence of the photoresist on the TiN dots for the etched samples 5–9 produced wide variations in measured Jleak values. Resist removal using EKC 505 wet stripper, which was found to be benign for blanket xerogel films, produced unusually high k and Jleak results for etched dot samples. Annealing these samples under vacuum seemed to return the samples to their original values. This may be due to wet PR remover trapped under the dots themselves, although there is no reliable data to confirm this. O 2 plasma resist removal was most detrimental to the xerogel, increasing the Jleak by four orders of magnitude. There remains therefore a need for a reliable resist remover which does not harm the xerogel properties.
4. Conclusions FTIR investigations confirm that the deposition of SiO 2 cap layers on xerogel films does not remove the methyl groups produced after treating the xerogel in HMDS vapour. The deposition of SiO 2 cap layers on xerogel were found to have negligible effects on the dielectric constant, leakage current density and field breakdown voltage of the xerogel films. While TiN barrier
Fig. 8. Influence of barrier treatment on xerogel k value.
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deposition on SiO 2 / xerogel did not affect the dielectric constant of the xerogel, an increase in k was observed when TiN was deposited directly onto xerogel. This could lead to an increases in line-to-line capacitance in an integrated structure. More work is required to confirm this result. Acknowledgements The contributions of M. Rennau and M. Henker of the Centre for Microtechnology, T.U. Chemnitz in the electrical characterisation of the samples is gratefully acknowledged. The FTIR work investigations carried out by Dr. Friedrich of the Physics Department, T.U.Chemnitz, is also appreciated. References [1] [2] [3] [4] [5]
M. Bohr, in: Tech. Digest IEEE Int. Electron. Devices Meeting, 1995, p. 241. C. Jin, J.D. Luttmer, D.M. Smith, T.A. Ramos, MRS Bull. 22 (10) (1997) 39. C.J. Brinker, G.W. Scherrer, Sol Gel Science, Academic Press, 1990. T. Winkler, S.E. Schulz, T. Gessner, in: MRS Symp. Proc. ULSI XII, MRS Warrendale PA, 1998, pp. 347–351. H.-S. Yang, S.-Y. Choi, S.-H. Hyun, H.-H. Park, J.-K. Hong, Non Crystalline Solids 221 (1997) 151–156.