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Effects of plasma on excimer lamp based selective activation processes for electroless plating
Applied Surface Science 138–139 Ž1999. 62–67
Effects of plasma on excimer lamp based selective activation processes for electroless plating D.J. Macauley, P.V. Kelly ) , K.F. Mongey, G.M. Crean National Microelectronics Research Centre, Lee Maltings, Prospect Row, Cork, Ireland
Abstract Several photoselective activation processes for electroless plating have been reported in recent years using excimer lamps to selectively photodecompose chemical coatings. In this work, optical emission spectroscopy ŽOES. of a plasma existing in a process chamber during operation of a 222 nm KrCl) excimer lamp is presented. This plasma is shown to be generated by the excimer lamp power supply rather than by the UV excimer lamp radiation and is pressure dependent in the range from 4.0 = 10y4 mbar to atmospheric pressure. The effects of this plasma on previously reported pressure dependence of these photo-selective activation processes for electroless plating are discussed. The plasma assisted nature of the selective decomposition process for electroless plating is demonstrated. The conditions required for a true photolytic process under excimer lamp radiation are reported. q 1999 Elsevier Science B.V. All rights reserved. PACS: 81.15.y z; 82.40.Ra; 82.50.y m; 82.50.Fv Keywords: Methods of deposition of films and coatings; film growth and epitaxy; Plasma reactions Žincluding flowing afterglow and electric discharges.; Photochemistry and radiation chemistry; Photolysis, photodissociation, and photoionisation by infrared, visible, ultraviolet radiation
1. Introduction Selective activation processes for electroless plating using excimer lamps to selectively decompose activation precursors w1–7x generally involve the deposition of a metal Žusually palladium. salt w1–5x or organometallic coating w6,7x on a substrate and its decomposition to the metal under exposure to an excimer lamp. The mechanism of decomposition under an excimer lamp has been determined w1,3x to be photolytic. In this paper, evidence is presented that a plasma generated in the excimer lamp reaction )
Corresponding author. Tel.: q353-21-904183; Fax: q35321-270271; E-mail: [email protected]
chamber by the r.f. power supply of these lamps decomposes the precursor, even in the absence of the excimer lamp radiation. Such pressure dependent plasmas have been previously reported w6,8x to be generated by excimer lamp power supplies. The pressure dependence of the optical emission intensity of the principal emission peak of this plasma is found to correlate with the observed pressure dependence of the decomposition reaction.
2. Experimental The excimer lamp reactor used in this work consisted of a stainless steel low vacuum chamber con-
0169-4332r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 9 8 . 0 0 4 9 6 - 6
D.J. Macauley et al.r Applied Surface Science 138–139 (1999) 62–67
Fig. 1. Schematic of excimer lamp experimental set-up Žshowing the irradiation of the set of four control experiment substrates detailed in Table 1..
taining a single cylindrical KrCl) excimer lamp mounted horizontally over a sample holder Žschematic shown in Fig. 1.. The lamp ground electrode was a metal gauze sheath covering approximately 90% of the lamp length. The lamp was powered by an r.f. power supply at a frequency of 175 kHz, 3.5 kV r.m.s. at forward powers in the range 0–100 W on an internal coil electrode within the excimer lamp inner deionised cooling water tube. The power absorbed by the lamp is over 90% at the optimum supply frequency, when the lamp is struck by the application of a higher than normal voltage spike, and is 20–30% of the forward power prior to striking the lamp. Coatings of each of two proprietary photoactivators in tetrahydrofuran solutions, and palladium acetate in acetone solution, were made by spin-coating the solutions of concentration 0.01 M onto 96% alumina ceramic substrates for process testing and onto Suprasil II synthetic silica substrates for UV absorption spectroscopy experiments. The coatings were exposed to between 10 and 25 mW cmy2 of excimer lamp radiation, with a Foturan glass mask Žopaque at wavelengths shorter than 300 nm. having etched apertures in a test pattern, or with no mask, or, in the case of a series of control experiments, with the masking arrangement as described in Fig. 1. The substrate temperature did not exceed 608C after 30 min irradiation at 25 mW cmy2 at a chamber pressure of 0.1 mbar. Optical emission spectroscopy of the plasma glow observed in the lamp chamber was performed by collecting the plasma optical emission from the re-
63
gion directly above the sample holder into an optical fibre bundle coupled to the glass viewport of the chamber Žopaque at 222 nm. and transmitting it through a calibrated single-grating monochromator having a spectral resolution of 4 nm, into a photomultiplier tube read in photovoltaic mode by termination in a 50 V resistor. The chamber base pressure using the rotary pump was 0.1 mbar and using the turbomolecular pump was 5 = 10y4 mbar. The pressure was controllable by backfilling with air using a mass flow controller. Ultraviolet absorption spectroscopy was performed using a Uvikon spectrophotometer in the wavelength range 190–580 nm.
3. Results and discussion 3.1. ObserÕation of pressure dependence of decomposition reaction Fig. 2 shows the UV absorption spectra of a proprietary photoactivator coating on synthetic silica before and after exposure to the KrCl) excimer lamp at 10 mW cmy2 , without any mask, for 5 min as a function of chamber pressure. The unexposed photochemical is observed to have a ligand-to-metal charge transfer ŽLMCT. absorption peak near 222 nm. The absorption spectrum was not significantly changed when the excimer lamp exposure was carried out at 10 mbar pressure, indicating that decomposition had not occurred. Some evidence of decomposition was observed when the exposure was carried out at 1 mbar, as seen in the reduction of the
Fig. 2. UV spectra in transmission of photoactivator coating on a Suprasile II fused silica wafer before and after 5 min exposure to a 222 nm KrCl) excimer lamp fluence of 10 mW cmy2 at pressures of 0.1, 1.0 and 10 mbar.
64
D.J. Macauley et al.r Applied Surface Science 138–139 (1999) 62–67
Fig. 3. UV spectra in transmission of palladium acetate coating on a Suprasile II fused silica wafer before Žas deposited. and after 5 min exposure to a 222 nm KrCl) excimer lamp fluence of 25 mW cmy2 at pressures of 0.15 and 0.01 mbar.
LMCT peak, but it is evident that decomposition is far from complete. By contrast, when the same exposure is carried out at 0.1 mbar, the decomposition was almost complete, as evidenced by the flatter, metal-like absorption spectrum and the loss of the
LMCT peak. Similar pressure dependent results were observed for the decomposition of palladium acetate coatings, under these conditions. Esrom et al. w1x previously reported that the decomposition of a thin film of palladium acetate
Fig. 4. Photograph of the plasma optical emission glow at the ungauzed end of the excimer lamp.
D.J. Macauley et al.r Applied Surface Science 138–139 (1999) 62–67
65
deposited on 96% alumina ceramic under exposure to a Xe) excimer lamp at a centre wavelength of 172 nm was dependent on the pressure in the chamber and was enhanced by almost an order of magnitude on reducing the chamber pressure to 1 mbar. A metal mask used to pattern the UV exposure in that work had physical apertures at the sites which were to be exposed to the excimer lamp. Esrom et al. attributed this effect to the more efficient removal of the volatile reaction products from the coating after decomposition. In this work, we observed that the decomposition of palladium acetate under a 222 nm excimer lamp was complete ŽFig. 3. after 5 min exposure at a pressure of 0.15 mbar, However, when a chamber pressure of 0.01 mbar was used for the same exposure, the decomposition reaction failed to occur at the lower pressure, as evidenced by a similar UV absorption spectrum to that obtained from the palladium acetate coating as deposited ŽFig. 3.. This result indicates that the removal of the volatile products is not the reason for the enhanced reaction rate observed at 0.1 mbar. 3.2. EÕidence of pressure dependent plasma During operation of the excimer lamp, residual visible Žblue. radiation from the lamp can be observed. When the reactor chamber was in the pressure regime at which the decomposition reaction is observed, a second Žviolet. glow is observed, initially close to the excimer lamp, but gradually extending into the chamber as the pressure is reduced. It proved possible to generate this glow ŽFig. 4. by powering the excimer lamp without striking the lamp, so that it could be studied in isolation. Optical emission spectroscopy ŽOES. of this glow existing in the process chamber, when a 222 nm KrCl) excimer lamp is supplied with 100 W r.f. power without the excimer discharge being struck, is presented in Fig. 5a. Spectral lines characteristic of a plasma discharge in the chamber are evident. Fig. 5b shows the intensity of the most intense plasma optical emission peak at 354 nm as a function of chamber pressure. Its intensity clearly peaks at approximately 0.1 mbar, with a similar pressure dependence to that observed for the decomposition reaction.
Fig. 5. Ža. Optical emission spectrum of the plasma recorded in the excimer lamp processing chamber at a pressure of 0.1 mbar, in an air ambient. Žb. Variation of the optical emission intensity from the plasma at 354 nm with chamber pressure.
3.3. EÕidence that the decomposition process is plasma assisted Fig. 6 shows the results of a control experiment in which four coated samples were exposed at 0.1 mbar pressure as shown in Fig. 1, to the combinations of plasma and excimer lamp radiation described in Table 1. The results of the experiment are evident from Fig. 6 and are described in Table 1. Exposure of the coated sample to the plasma is found to be essential for decomposition, and exposure to the deep ultraviolet radiation through a transmitting synthetic silica plate in contact with the coating is found to inhibit the reaction under these conditions. The decomposition process observed is therefore concluded to be plasma-assisted and not photo-assisted. This plasma
D.J. Macauley et al.r Applied Surface Science 138–139 (1999) 62–67
66
Fig. 6. Photographs showing the four substrates after UV irradiation with the 222 nm KrCl) excimer lamp. The coating on substrate 3 has selectively decomposed without being exposed to the 222 nm excimer lamp UV irradiation Žsample numbers correspond to the exposure and masking arrangement shown in Fig. 1..
has been shown to be generated by the r.f. power supply rather than the 222 nm excimer lamp irradiation and to have a pressure dependence in the range from 5.0 = 10y4 mbar to atmospheric pressure which correlates well with the observed occurrence of decomposition of the photochemical coating.
Photoselective activation for electroless metallisation in the absence of the plasma can be performed using a reactor in which the excimer lamps are physically separated from the substrate by means of an ultraviolet transparent window provided that sufficient ultraviolet intensity and heat is supplied w9x.
Table 1 Exposure conditions and result of control experiment
4. Conclusions
Sample number
Exposure to: 222 nm UV Plasma UV
Plasma
Result: decomposition
1 2 3 4
Exposed Exposed Shielded Shielded
Shielded Exposed Exposed Shielded
None Complete Complete None
Exposed Exposed Exposed Exposed
Evidence is presented here that a pressure dependent plasma can be generated in the excimer lamp reaction chamber by the r.f. power supply used to produce the intense incoherent deep ultraviolet radiation from these lamps, and can drive chemical decomposition processes independently of the lamp radiation. The pressure dependence of the optical
D.J. Macauley et al.r Applied Surface Science 138–139 (1999) 62–67
emission intensity of the principal emission peak of this plasma is found to correlate with the observed pressure dependence of the organometallic decomposition reaction observed. A control experiment has demonstrated that the decomposition is not photolytic, but is in fact exclusively due to exposure to the chamber atmosphere with the plasma present. The phenomenon of plasma generation in reactor chambers equipped with excimer lamp sources means that good reactor design should physically separate the region of such sources from the workpiece or substrate by means of a UV transmitting window, such that the outer electrode of the lamp is not present in the same chamber compartment as the workpiece, in order to exclude plasma from the reactor proper.
Acknowledgements The authors would like to acknowledge Prof. Ian Boyd and Dr. Jun-Ying Zhang, University College
67
London, who supplied the lamp elements and lamp emission intensity calibrations. Part of this work Žto June 30 1997. was partly funded by the European Commission under Brite-Euram project 8073 PACE ‘Photo-Assisted Catalysis of Electroless Deposition’.
References w1x H. Esrom, J. Demny, U. Kogelschatz, Chemtronics 4 Ž1989. 202–208. w2x H. Esrom, Thin Solid Films 218 Ž1992. 231–246. w3x D.J. Macauley, P.V. Kelly, G.M. Crean, Mater. Res. Soc. Symp. Proc. 445 Ž1997. 45–50. w4x J.-Y. Zhang, H. Esrom, I.W. Boyd, Appl. Surf. Sci. 96–98 Ž1996. 399. w5x J.-Y. Zhang, I.W. Boyd, J. Mater. Sci. Lett. 16 Ž1997. 996. w6x J.-Y. Zhang, I.W. Boyd, Appl. Phys. A 65 Ž1997. 379. w7x J.-Y. Zhang, S.L. King, I.W. Boyd, Q. Fang, Appl. Surf. Sci. 96–98 Ž1996. 399. w8x H. Esrom, J.-Y. Zhang, U. Kogelschatz, Mater. Res. Symp. Proc. 236 Ž1992. 39. w9x D.J. Macauley. P.V. Kelly, K.F. Mongey, G.M. Crean, Paper G49, this symposium.
Effects of plasma on excimer lamp based selective activation processes for electroless plating D.J. Macauley, P.V. Kelly ) , K.F. Mongey, G.M. Crean National Microelectronics Research Centre, Lee Maltings, Prospect Row, Cork, Ireland
Abstract Several photoselective activation processes for electroless plating have been reported in recent years using excimer lamps to selectively photodecompose chemical coatings. In this work, optical emission spectroscopy ŽOES. of a plasma existing in a process chamber during operation of a 222 nm KrCl) excimer lamp is presented. This plasma is shown to be generated by the excimer lamp power supply rather than by the UV excimer lamp radiation and is pressure dependent in the range from 4.0 = 10y4 mbar to atmospheric pressure. The effects of this plasma on previously reported pressure dependence of these photo-selective activation processes for electroless plating are discussed. The plasma assisted nature of the selective decomposition process for electroless plating is demonstrated. The conditions required for a true photolytic process under excimer lamp radiation are reported. q 1999 Elsevier Science B.V. All rights reserved. PACS: 81.15.y z; 82.40.Ra; 82.50.y m; 82.50.Fv Keywords: Methods of deposition of films and coatings; film growth and epitaxy; Plasma reactions Žincluding flowing afterglow and electric discharges.; Photochemistry and radiation chemistry; Photolysis, photodissociation, and photoionisation by infrared, visible, ultraviolet radiation
1. Introduction Selective activation processes for electroless plating using excimer lamps to selectively decompose activation precursors w1–7x generally involve the deposition of a metal Žusually palladium. salt w1–5x or organometallic coating w6,7x on a substrate and its decomposition to the metal under exposure to an excimer lamp. The mechanism of decomposition under an excimer lamp has been determined w1,3x to be photolytic. In this paper, evidence is presented that a plasma generated in the excimer lamp reaction )
Corresponding author. Tel.: q353-21-904183; Fax: q35321-270271; E-mail: [email protected]
chamber by the r.f. power supply of these lamps decomposes the precursor, even in the absence of the excimer lamp radiation. Such pressure dependent plasmas have been previously reported w6,8x to be generated by excimer lamp power supplies. The pressure dependence of the optical emission intensity of the principal emission peak of this plasma is found to correlate with the observed pressure dependence of the decomposition reaction.
2. Experimental The excimer lamp reactor used in this work consisted of a stainless steel low vacuum chamber con-
0169-4332r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 9 8 . 0 0 4 9 6 - 6
D.J. Macauley et al.r Applied Surface Science 138–139 (1999) 62–67
Fig. 1. Schematic of excimer lamp experimental set-up Žshowing the irradiation of the set of four control experiment substrates detailed in Table 1..
taining a single cylindrical KrCl) excimer lamp mounted horizontally over a sample holder Žschematic shown in Fig. 1.. The lamp ground electrode was a metal gauze sheath covering approximately 90% of the lamp length. The lamp was powered by an r.f. power supply at a frequency of 175 kHz, 3.5 kV r.m.s. at forward powers in the range 0–100 W on an internal coil electrode within the excimer lamp inner deionised cooling water tube. The power absorbed by the lamp is over 90% at the optimum supply frequency, when the lamp is struck by the application of a higher than normal voltage spike, and is 20–30% of the forward power prior to striking the lamp. Coatings of each of two proprietary photoactivators in tetrahydrofuran solutions, and palladium acetate in acetone solution, were made by spin-coating the solutions of concentration 0.01 M onto 96% alumina ceramic substrates for process testing and onto Suprasil II synthetic silica substrates for UV absorption spectroscopy experiments. The coatings were exposed to between 10 and 25 mW cmy2 of excimer lamp radiation, with a Foturan glass mask Žopaque at wavelengths shorter than 300 nm. having etched apertures in a test pattern, or with no mask, or, in the case of a series of control experiments, with the masking arrangement as described in Fig. 1. The substrate temperature did not exceed 608C after 30 min irradiation at 25 mW cmy2 at a chamber pressure of 0.1 mbar. Optical emission spectroscopy of the plasma glow observed in the lamp chamber was performed by collecting the plasma optical emission from the re-
63
gion directly above the sample holder into an optical fibre bundle coupled to the glass viewport of the chamber Žopaque at 222 nm. and transmitting it through a calibrated single-grating monochromator having a spectral resolution of 4 nm, into a photomultiplier tube read in photovoltaic mode by termination in a 50 V resistor. The chamber base pressure using the rotary pump was 0.1 mbar and using the turbomolecular pump was 5 = 10y4 mbar. The pressure was controllable by backfilling with air using a mass flow controller. Ultraviolet absorption spectroscopy was performed using a Uvikon spectrophotometer in the wavelength range 190–580 nm.
3. Results and discussion 3.1. ObserÕation of pressure dependence of decomposition reaction Fig. 2 shows the UV absorption spectra of a proprietary photoactivator coating on synthetic silica before and after exposure to the KrCl) excimer lamp at 10 mW cmy2 , without any mask, for 5 min as a function of chamber pressure. The unexposed photochemical is observed to have a ligand-to-metal charge transfer ŽLMCT. absorption peak near 222 nm. The absorption spectrum was not significantly changed when the excimer lamp exposure was carried out at 10 mbar pressure, indicating that decomposition had not occurred. Some evidence of decomposition was observed when the exposure was carried out at 1 mbar, as seen in the reduction of the
Fig. 2. UV spectra in transmission of photoactivator coating on a Suprasile II fused silica wafer before and after 5 min exposure to a 222 nm KrCl) excimer lamp fluence of 10 mW cmy2 at pressures of 0.1, 1.0 and 10 mbar.
64
D.J. Macauley et al.r Applied Surface Science 138–139 (1999) 62–67
Fig. 3. UV spectra in transmission of palladium acetate coating on a Suprasile II fused silica wafer before Žas deposited. and after 5 min exposure to a 222 nm KrCl) excimer lamp fluence of 25 mW cmy2 at pressures of 0.15 and 0.01 mbar.
LMCT peak, but it is evident that decomposition is far from complete. By contrast, when the same exposure is carried out at 0.1 mbar, the decomposition was almost complete, as evidenced by the flatter, metal-like absorption spectrum and the loss of the
LMCT peak. Similar pressure dependent results were observed for the decomposition of palladium acetate coatings, under these conditions. Esrom et al. w1x previously reported that the decomposition of a thin film of palladium acetate
Fig. 4. Photograph of the plasma optical emission glow at the ungauzed end of the excimer lamp.
D.J. Macauley et al.r Applied Surface Science 138–139 (1999) 62–67
65
deposited on 96% alumina ceramic under exposure to a Xe) excimer lamp at a centre wavelength of 172 nm was dependent on the pressure in the chamber and was enhanced by almost an order of magnitude on reducing the chamber pressure to 1 mbar. A metal mask used to pattern the UV exposure in that work had physical apertures at the sites which were to be exposed to the excimer lamp. Esrom et al. attributed this effect to the more efficient removal of the volatile reaction products from the coating after decomposition. In this work, we observed that the decomposition of palladium acetate under a 222 nm excimer lamp was complete ŽFig. 3. after 5 min exposure at a pressure of 0.15 mbar, However, when a chamber pressure of 0.01 mbar was used for the same exposure, the decomposition reaction failed to occur at the lower pressure, as evidenced by a similar UV absorption spectrum to that obtained from the palladium acetate coating as deposited ŽFig. 3.. This result indicates that the removal of the volatile products is not the reason for the enhanced reaction rate observed at 0.1 mbar. 3.2. EÕidence of pressure dependent plasma During operation of the excimer lamp, residual visible Žblue. radiation from the lamp can be observed. When the reactor chamber was in the pressure regime at which the decomposition reaction is observed, a second Žviolet. glow is observed, initially close to the excimer lamp, but gradually extending into the chamber as the pressure is reduced. It proved possible to generate this glow ŽFig. 4. by powering the excimer lamp without striking the lamp, so that it could be studied in isolation. Optical emission spectroscopy ŽOES. of this glow existing in the process chamber, when a 222 nm KrCl) excimer lamp is supplied with 100 W r.f. power without the excimer discharge being struck, is presented in Fig. 5a. Spectral lines characteristic of a plasma discharge in the chamber are evident. Fig. 5b shows the intensity of the most intense plasma optical emission peak at 354 nm as a function of chamber pressure. Its intensity clearly peaks at approximately 0.1 mbar, with a similar pressure dependence to that observed for the decomposition reaction.
Fig. 5. Ža. Optical emission spectrum of the plasma recorded in the excimer lamp processing chamber at a pressure of 0.1 mbar, in an air ambient. Žb. Variation of the optical emission intensity from the plasma at 354 nm with chamber pressure.
3.3. EÕidence that the decomposition process is plasma assisted Fig. 6 shows the results of a control experiment in which four coated samples were exposed at 0.1 mbar pressure as shown in Fig. 1, to the combinations of plasma and excimer lamp radiation described in Table 1. The results of the experiment are evident from Fig. 6 and are described in Table 1. Exposure of the coated sample to the plasma is found to be essential for decomposition, and exposure to the deep ultraviolet radiation through a transmitting synthetic silica plate in contact with the coating is found to inhibit the reaction under these conditions. The decomposition process observed is therefore concluded to be plasma-assisted and not photo-assisted. This plasma
D.J. Macauley et al.r Applied Surface Science 138–139 (1999) 62–67
66
Fig. 6. Photographs showing the four substrates after UV irradiation with the 222 nm KrCl) excimer lamp. The coating on substrate 3 has selectively decomposed without being exposed to the 222 nm excimer lamp UV irradiation Žsample numbers correspond to the exposure and masking arrangement shown in Fig. 1..
has been shown to be generated by the r.f. power supply rather than the 222 nm excimer lamp irradiation and to have a pressure dependence in the range from 5.0 = 10y4 mbar to atmospheric pressure which correlates well with the observed occurrence of decomposition of the photochemical coating.
Photoselective activation for electroless metallisation in the absence of the plasma can be performed using a reactor in which the excimer lamps are physically separated from the substrate by means of an ultraviolet transparent window provided that sufficient ultraviolet intensity and heat is supplied w9x.
Table 1 Exposure conditions and result of control experiment
4. Conclusions
Sample number
Exposure to: 222 nm UV Plasma UV
Plasma
Result: decomposition
1 2 3 4
Exposed Exposed Shielded Shielded
Shielded Exposed Exposed Shielded
None Complete Complete None
Exposed Exposed Exposed Exposed
Evidence is presented here that a pressure dependent plasma can be generated in the excimer lamp reaction chamber by the r.f. power supply used to produce the intense incoherent deep ultraviolet radiation from these lamps, and can drive chemical decomposition processes independently of the lamp radiation. The pressure dependence of the optical
D.J. Macauley et al.r Applied Surface Science 138–139 (1999) 62–67
emission intensity of the principal emission peak of this plasma is found to correlate with the observed pressure dependence of the organometallic decomposition reaction observed. A control experiment has demonstrated that the decomposition is not photolytic, but is in fact exclusively due to exposure to the chamber atmosphere with the plasma present. The phenomenon of plasma generation in reactor chambers equipped with excimer lamp sources means that good reactor design should physically separate the region of such sources from the workpiece or substrate by means of a UV transmitting window, such that the outer electrode of the lamp is not present in the same chamber compartment as the workpiece, in order to exclude plasma from the reactor proper.
Acknowledgements The authors would like to acknowledge Prof. Ian Boyd and Dr. Jun-Ying Zhang, University College
67
London, who supplied the lamp elements and lamp emission intensity calibrations. Part of this work Žto June 30 1997. was partly funded by the European Commission under Brite-Euram project 8073 PACE ‘Photo-Assisted Catalysis of Electroless Deposition’.
References w1x H. Esrom, J. Demny, U. Kogelschatz, Chemtronics 4 Ž1989. 202–208. w2x H. Esrom, Thin Solid Films 218 Ž1992. 231–246. w3x D.J. Macauley, P.V. Kelly, G.M. Crean, Mater. Res. Soc. Symp. Proc. 445 Ž1997. 45–50. w4x J.-Y. Zhang, H. Esrom, I.W. Boyd, Appl. Surf. Sci. 96–98 Ž1996. 399. w5x J.-Y. Zhang, I.W. Boyd, J. Mater. Sci. Lett. 16 Ž1997. 996. w6x J.-Y. Zhang, I.W. Boyd, Appl. Phys. A 65 Ž1997. 379. w7x J.-Y. Zhang, S.L. King, I.W. Boyd, Q. Fang, Appl. Surf. Sci. 96–98 Ž1996. 399. w8x H. Esrom, J.-Y. Zhang, U. Kogelschatz, Mater. Res. Symp. Proc. 236 Ž1992. 39. w9x D.J. Macauley. P.V. Kelly, K.F. Mongey, G.M. Crean, Paper G49, this symposium.