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Substrate pretreatment for improvements in structural and mechanical properties of zinc oxide coatings on glass
Thin Solid Films 469–470 (2004) 70 – 74 www.elsevier.com/locate/tsf
Substrate pretreatment for improvements in structural and mechanical properties of zinc oxide coatings on glass G.T. West*, P.J. Kelly Institute of Materials Research, University of Salford, Salford, Greater Manchester M5 4WT, UK Available online 27 September 2004
Abstract Reactively sputtered zinc oxide coatings were deposited onto float glass substrates following exposure of the substrates to an oxygen ion beam. A Hall-effect linear ion source was used to produce ion beams at different nominal beam voltages in the range 0–1500 V, within an otherwise standard unbalanced magnetron sputtering system. Coatings were deposited onto the pretreated glass under identical operating conditions and were then characterised in terms of their structural and mechanical properties. A series of consistent trends were found revealing improvements in these properties with increasing ion beam voltage. For example, a reduction in the surface roughness of the deposited film, as measured via atomic force microscopy (AFM), was observed. Critical load scratch testing and mechanical wear testing demonstrated significant improvements in mechanical durability, with peak values occurring for glass that had been pretreated with a beam voltage of 1000 V. These improvements in structural and mechanical properties were obtained without any effect on the optical properties of these coatings, which were measured via spectrophotometry. X-ray diffraction analysis of these coatings indicated that they all had a strong (002) texture, but that the position and width of this peak varied with substrate pretreatment. This work demonstrates the ability to improve film properties through the pretreatment of substrates with the linear ion source. D 2004 Elsevier B.V. All rights reserved. Keywords: Ion bombardment; Zinc oxide; Magnetron sputtering
1. Introduction Zinc oxide films are commonly used in the large-area glass coating industry for their optical properties—refractive index, n 550i2.0, and for their effect on the growth of subsequent films within a multilayer coating [1–3]. These films are commonly deposited onto large-area glass panes via dc magnetron sputtering. One application is for the lowemissivity glazing market. However, this type of coating is susceptible to damage in storage and transit, and also to environmental degradation [4,5]. Previous work by the authors has shown that the use of an ion beam to pretreat the glass surface prior to the deposition of titania films led to significant improvements in the mechanical properties of the
subsequent coatings [6]. This study sets out to establish whether zinc oxide coatings on glass also show these improvements, and aims to further investigate their causes. A standard magnetron sputtering system was enhanced via the inclusion of a Hall-type ion source. Float glass substrates were exposed to oxygen ion beams of different voltages for equal time periods, and subsequently coated with 100 nm zinc oxide films under identical conditions using pulsed-dc reactive magnetron sputtering. The structural, optical and mechanical properties of the coatings were then analysed by a variety of techniques and compared to those of coatings grown on untreated glass.
2. Experimental * Corresponding author. Tel.: +44 1612954784; fax: +44 1612955108. E-mail addresses: [email protected] (G.T. West)8 [email protected] (P.J. Kelly). 0040-6090/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2004.06.190
A stainless steel chamber, evacuated by means of a rotary pump and oil diffusion pump, was fitted with an Advanced
G.T. West, P.J. Kelly / Thin Solid Films 469–470 (2004) 70–74
Energy (Hall-type) linear ion source [7] and a 250150 mm2 rectangular planar magnetron which were used for pretreatment of the substrate and deposition of zinc oxide, respectively. A detailed description of the coating rig is given elsewhere [6]. The ion source (Advanced Energy 38 cm Linear Ion Source) was powered by a modified Advanced Energy Pinnacle supply, and the magnetron by an Advanced Energy MDX supply and SPARC-LE 20 pulsing unit. Substrates were mounted on a centrally located rotating holder. Oxygen ion beams were employed at varying voltage, such that each float glass substrate was exposed for 10 min to an ion beam of 500, 1000, or 1500 V nominal discharge voltage (equating to average ion energies of 250, 500 and 750 eV) prior to the deposition (under identical conditions) of a ZnO film. The zinc oxide coatings were grown via reactive magnetron sputtering to approximately 100-nm thickness. The deposition conditions were a single magnetron with a 99.5% pure zinc target at 1 A (290 V, 0.3 kW), pulsed at 20 kHz in an argon/oxygen atmosphere at 0.167 Pa (1.25 mTorr). Optical Emissions Monitoring was used for feedback control of the oxygen flow and regulated at 35% of the full metal zinc signal. Unmodified glass substrates were also used for ZnO growth to provide baseline data for comparison. Following pretreatment and coating of the glass substrates, the resulting zinc oxide films were characterised with respect to their structural, optical and mechanical properties. A Siemens D5000 diffractometer was used for X-ray diffraction analysis of the zinc oxide films using CuKa1 radiation at k=1.5405 from 108 to 908 incidence. Critical load scratch testing was carried out using five single pass scratches at progressively increasing loads from 1 to 40 N with a 200 Am Rockwell diamond stylus for each sample (Teer Coatings ST3000). Critical loads to failure were recorded. The same stylus was used to generate bidirectional reciprocating wear tracks at a constant 5 N load for 30 cycles, and the cross-sectional
71
area of the wear tracks measured via a Dektak profilometer. The surface morphology of the zinc oxide coatings was examined via atomic force microscopy (AFM), using a Digital Instruments nanoscope. An Aquilla Instruments nkd8000 spectrophotometer was employed to derive data for transmission and reflection across a spectrum from 400 to 1000 nm wavelengths. Optical modelling software was used to derive values of refractive index, n and absorption coefficient, k from the measured data.
3. Results The XRD data shows a clear peak in all samples at a d-spacing in the region of 0.26 nm, corresponding to the 002 orientation of crystalline zinc oxide (0.2602 nm). An example spectrum is shown in Fig. 1. The exact position of the peak varies between coatings grown on differently modified substrates (different ion beam voltages), as do the peak widths at full-width half maximum (FWHM), from which the crystallite sizes have been estimated using the Scherrer equation [8–10]. These data are given in Figs. 2 and 3. Fig. 4 displays the mechanical data derived via critical load scratch testing and reciprocating wear testing. It is clear from this graph that the critical load required for coating failure varies with the level of ion bombardment experienced by the substrate during pretreatment. All substrates that received ion source pretreatment prior to deposition showed improvement in coating durability, with increasing loads required for failure, up to a maximum value resulting from pretreatment at 1000 V. However, this effect was lessened when pretreatment voltage was increased to 1500 V. The level of wear exhibited by the coatings reduced with increasing ion beam voltage during pretreatment, with a minimum wear cross section at 1000 V pretreatment, and decreasing wear resistance at higher voltages.
Fig. 1. XRD data for ZnO grown on unmodified glass — all samples grown on modified glass also exhibited the same (002) orientation.
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Fig. 4. Mechanical properties of ZnO films grown on ion beam modified glass.
Fig. 2. (002) Diffraction peak shape for ZnO coatings grown following varying substrate modification.
Values of Ra roughness are given in Fig. 5, and sample AFM plots of coating surfaces are given in Fig. 6. The roughness of the zinc oxide coatings was higher following 500 V ion beam pretreatment of the substrate than for those grown on untreated substrates. The roughness of the coatings deposited following treatment at higher voltages reduced to a level below that of zinc oxide grown on untreated glass. No significant variation was exhibited in optical properties by the zinc oxide coatings, whether grown on unmodified glass or that modified at any beam voltage. Transmission values for all coatings were 90% at 550 nm, and consistent values of 2.0 were derived for refractive index, n at 550 nm, via computer modelling. Fig. 7 gives a
Fig. 3. Effect of substrate surface modification on subsequent film crystalline structure.
typical transmission/reflection spectrum, and a typical n and k spectral variation is shown in Fig. 8.
4. Discussion The shift towards larger interplane lattice spacing (d) for films grown on the modified substrates is an indication of more compressive residual stresses, with a maximum spacing, and hence stress, observed for coatings grown on glass modified by a 1000 V oxygen ion beam. The reduction in the lattice spacing for samples modified at 1500 V suggests a reduction in compressive stress for these films. Grain sizes estimated from X-ray diffraction also show a trend with pretreatment beam voltage. Approximate grain sizes reduced significantly with substrate pretreatment. A minimum grain size of 16.3 nm occurred at 1000 V beam voltage, compared with a maximum grain size of 46.2 nm for untreated glass. AFM measurements of coating surface roughness show a minimum roughness for coatings grown on pretreated samples at 1000 V beam voltage, although the roughness of coatings grown on untreated substrates was lower. This may be a result of differences in substrate surface roughness, and not just crystallite size.
Fig. 5. Ra roughness values for zinc oxide grown on glass modified at varying voltage.
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Fig. 6. Surface morphology of zinc oxide grown on unmodified, and 1500 V modified glass from 1Am2 AFM plots (with maximum feature height of 20 nm).
Mechanical properties, measured by independent techniques, exhibited similar trends with pretreatment voltage to that of the structural data. Pretreatment significantly improves both wear resistance, and the required load for failure during scratch testing. Both mechanical properties show a maximum benefit at 1000 V beam voltage, reducing with increases in voltage beyond this. Although it is reasonable to assume that the modifications to coating structures are linked to the improvements in mechanical durability, defining a precise causality is not attempted here. Observed reductions in surface roughness of the coatings may also be attributable to differences in crystalline structure, i.e. smaller grain size, although the trend is not as clear. These changes in structural and mechanical properties of the coatings resulting from pretreatment of the substrate were not accompanied by any detrimental effect on their optical performance. At present, insufficient data are available to provide an explanation as to why pretreatment of substrate surfaces leads to such significant improvements in durability. It is
speculated that greater nucleation density for films grown on modified substrates may be the cause of differences in their crystalline structure, and hence their mechanical properties. Changes in nucleation and growth of the coatings may be a result of a change in surface chemistry or surface morphology of the glass, or a result of removing contaminants/corrosion products from the surface as a consequence of ion beam pretreatment. However, a more comprehensive understanding of the properties of the ion beam and of the effects of bombardment on glass substrates is necessary to adequately explain the processes involved in changing the coating growth, structure and durability.
Fig. 7. Typical transmission and reflection spectra for ZnO coatings on glass.
Fig. 8. Typical variation in n and k with wavelength for ZnO coatings on glass.
5. Conclusions This study has shown that the structural and mechanical properties of zinc oxide coatings deposited on float glass can be modified via pretreatment of the substrate with an oxygen ion beam. The effect has been shown to vary with nominal beam voltage, with a maximum occurring at 1000 V. For example, the wear rate of coatings pretreated at this voltage was reduced by a factor of three in comparison with
74
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coatings deposited on untreated substrates, and the critical load to failure was increased by 50%. The mechanism for these changes is not fully understood; however, further work is in progress, including a study of the ion beam species incident at the substrate, and the changes in surface chemistry resulting from this bombardment.
Acknowledgements The authors acknowledge the support provided by Pilkington Technology Centre, UK, and EPSRC for funding this research, and Advanced Energy Industries for use of the Linear Ion Source.
References [1] C. May, J. Strumpfel, D. Schulzes, Proc. 43rd Tech. Conf. Soc. Vac. Coaters, Denver, 2000, pp. 137 – 142. [2] K. Suzuki, Thin Solid Films 351 (1999) 8. [3] C. May, F. Milde, G. Teschner, Proc. 45th Tech. Conf. Soc. Vac. Coaters, Lake, 2002, pp. 153 – 158. [4] E. Ando, M. Miyazaki, Thin Solid Films, 392 (2) 289. [5] M. Miyazaki, E. Ando, J. Non-Cryst. Solids, 178, 245. [6] G.T. West, P.J. Kelly, Thin Solid Films 447–448 (2004) 20. [7] N. Capps, D. Carter, G. Roche, Semicond. Int. 23 (8) (2000) 16. [8] Y. Qu, et al., J. Vac. Sci. Technol., A 12 (4) (1994) 1507. [9] B. Window, F. Sharples, N. Savvides, J. Vac. Sci. Technol., A 6 (4) (1988) 2333. [10] K.K. Shih, D.A. Smith, J.R. Crowe, J. Vac. Sci. Technol., A 6 (3) (1988) 1681.
Substrate pretreatment for improvements in structural and mechanical properties of zinc oxide coatings on glass G.T. West*, P.J. Kelly Institute of Materials Research, University of Salford, Salford, Greater Manchester M5 4WT, UK Available online 27 September 2004
Abstract Reactively sputtered zinc oxide coatings were deposited onto float glass substrates following exposure of the substrates to an oxygen ion beam. A Hall-effect linear ion source was used to produce ion beams at different nominal beam voltages in the range 0–1500 V, within an otherwise standard unbalanced magnetron sputtering system. Coatings were deposited onto the pretreated glass under identical operating conditions and were then characterised in terms of their structural and mechanical properties. A series of consistent trends were found revealing improvements in these properties with increasing ion beam voltage. For example, a reduction in the surface roughness of the deposited film, as measured via atomic force microscopy (AFM), was observed. Critical load scratch testing and mechanical wear testing demonstrated significant improvements in mechanical durability, with peak values occurring for glass that had been pretreated with a beam voltage of 1000 V. These improvements in structural and mechanical properties were obtained without any effect on the optical properties of these coatings, which were measured via spectrophotometry. X-ray diffraction analysis of these coatings indicated that they all had a strong (002) texture, but that the position and width of this peak varied with substrate pretreatment. This work demonstrates the ability to improve film properties through the pretreatment of substrates with the linear ion source. D 2004 Elsevier B.V. All rights reserved. Keywords: Ion bombardment; Zinc oxide; Magnetron sputtering
1. Introduction Zinc oxide films are commonly used in the large-area glass coating industry for their optical properties—refractive index, n 550i2.0, and for their effect on the growth of subsequent films within a multilayer coating [1–3]. These films are commonly deposited onto large-area glass panes via dc magnetron sputtering. One application is for the lowemissivity glazing market. However, this type of coating is susceptible to damage in storage and transit, and also to environmental degradation [4,5]. Previous work by the authors has shown that the use of an ion beam to pretreat the glass surface prior to the deposition of titania films led to significant improvements in the mechanical properties of the
subsequent coatings [6]. This study sets out to establish whether zinc oxide coatings on glass also show these improvements, and aims to further investigate their causes. A standard magnetron sputtering system was enhanced via the inclusion of a Hall-type ion source. Float glass substrates were exposed to oxygen ion beams of different voltages for equal time periods, and subsequently coated with 100 nm zinc oxide films under identical conditions using pulsed-dc reactive magnetron sputtering. The structural, optical and mechanical properties of the coatings were then analysed by a variety of techniques and compared to those of coatings grown on untreated glass.
2. Experimental * Corresponding author. Tel.: +44 1612954784; fax: +44 1612955108. E-mail addresses: [email protected] (G.T. West)8 [email protected] (P.J. Kelly). 0040-6090/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2004.06.190
A stainless steel chamber, evacuated by means of a rotary pump and oil diffusion pump, was fitted with an Advanced
G.T. West, P.J. Kelly / Thin Solid Films 469–470 (2004) 70–74
Energy (Hall-type) linear ion source [7] and a 250150 mm2 rectangular planar magnetron which were used for pretreatment of the substrate and deposition of zinc oxide, respectively. A detailed description of the coating rig is given elsewhere [6]. The ion source (Advanced Energy 38 cm Linear Ion Source) was powered by a modified Advanced Energy Pinnacle supply, and the magnetron by an Advanced Energy MDX supply and SPARC-LE 20 pulsing unit. Substrates were mounted on a centrally located rotating holder. Oxygen ion beams were employed at varying voltage, such that each float glass substrate was exposed for 10 min to an ion beam of 500, 1000, or 1500 V nominal discharge voltage (equating to average ion energies of 250, 500 and 750 eV) prior to the deposition (under identical conditions) of a ZnO film. The zinc oxide coatings were grown via reactive magnetron sputtering to approximately 100-nm thickness. The deposition conditions were a single magnetron with a 99.5% pure zinc target at 1 A (290 V, 0.3 kW), pulsed at 20 kHz in an argon/oxygen atmosphere at 0.167 Pa (1.25 mTorr). Optical Emissions Monitoring was used for feedback control of the oxygen flow and regulated at 35% of the full metal zinc signal. Unmodified glass substrates were also used for ZnO growth to provide baseline data for comparison. Following pretreatment and coating of the glass substrates, the resulting zinc oxide films were characterised with respect to their structural, optical and mechanical properties. A Siemens D5000 diffractometer was used for X-ray diffraction analysis of the zinc oxide films using CuKa1 radiation at k=1.5405 from 108 to 908 incidence. Critical load scratch testing was carried out using five single pass scratches at progressively increasing loads from 1 to 40 N with a 200 Am Rockwell diamond stylus for each sample (Teer Coatings ST3000). Critical loads to failure were recorded. The same stylus was used to generate bidirectional reciprocating wear tracks at a constant 5 N load for 30 cycles, and the cross-sectional
71
area of the wear tracks measured via a Dektak profilometer. The surface morphology of the zinc oxide coatings was examined via atomic force microscopy (AFM), using a Digital Instruments nanoscope. An Aquilla Instruments nkd8000 spectrophotometer was employed to derive data for transmission and reflection across a spectrum from 400 to 1000 nm wavelengths. Optical modelling software was used to derive values of refractive index, n and absorption coefficient, k from the measured data.
3. Results The XRD data shows a clear peak in all samples at a d-spacing in the region of 0.26 nm, corresponding to the 002 orientation of crystalline zinc oxide (0.2602 nm). An example spectrum is shown in Fig. 1. The exact position of the peak varies between coatings grown on differently modified substrates (different ion beam voltages), as do the peak widths at full-width half maximum (FWHM), from which the crystallite sizes have been estimated using the Scherrer equation [8–10]. These data are given in Figs. 2 and 3. Fig. 4 displays the mechanical data derived via critical load scratch testing and reciprocating wear testing. It is clear from this graph that the critical load required for coating failure varies with the level of ion bombardment experienced by the substrate during pretreatment. All substrates that received ion source pretreatment prior to deposition showed improvement in coating durability, with increasing loads required for failure, up to a maximum value resulting from pretreatment at 1000 V. However, this effect was lessened when pretreatment voltage was increased to 1500 V. The level of wear exhibited by the coatings reduced with increasing ion beam voltage during pretreatment, with a minimum wear cross section at 1000 V pretreatment, and decreasing wear resistance at higher voltages.
Fig. 1. XRD data for ZnO grown on unmodified glass — all samples grown on modified glass also exhibited the same (002) orientation.
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Fig. 4. Mechanical properties of ZnO films grown on ion beam modified glass.
Fig. 2. (002) Diffraction peak shape for ZnO coatings grown following varying substrate modification.
Values of Ra roughness are given in Fig. 5, and sample AFM plots of coating surfaces are given in Fig. 6. The roughness of the zinc oxide coatings was higher following 500 V ion beam pretreatment of the substrate than for those grown on untreated substrates. The roughness of the coatings deposited following treatment at higher voltages reduced to a level below that of zinc oxide grown on untreated glass. No significant variation was exhibited in optical properties by the zinc oxide coatings, whether grown on unmodified glass or that modified at any beam voltage. Transmission values for all coatings were 90% at 550 nm, and consistent values of 2.0 were derived for refractive index, n at 550 nm, via computer modelling. Fig. 7 gives a
Fig. 3. Effect of substrate surface modification on subsequent film crystalline structure.
typical transmission/reflection spectrum, and a typical n and k spectral variation is shown in Fig. 8.
4. Discussion The shift towards larger interplane lattice spacing (d) for films grown on the modified substrates is an indication of more compressive residual stresses, with a maximum spacing, and hence stress, observed for coatings grown on glass modified by a 1000 V oxygen ion beam. The reduction in the lattice spacing for samples modified at 1500 V suggests a reduction in compressive stress for these films. Grain sizes estimated from X-ray diffraction also show a trend with pretreatment beam voltage. Approximate grain sizes reduced significantly with substrate pretreatment. A minimum grain size of 16.3 nm occurred at 1000 V beam voltage, compared with a maximum grain size of 46.2 nm for untreated glass. AFM measurements of coating surface roughness show a minimum roughness for coatings grown on pretreated samples at 1000 V beam voltage, although the roughness of coatings grown on untreated substrates was lower. This may be a result of differences in substrate surface roughness, and not just crystallite size.
Fig. 5. Ra roughness values for zinc oxide grown on glass modified at varying voltage.
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Fig. 6. Surface morphology of zinc oxide grown on unmodified, and 1500 V modified glass from 1Am2 AFM plots (with maximum feature height of 20 nm).
Mechanical properties, measured by independent techniques, exhibited similar trends with pretreatment voltage to that of the structural data. Pretreatment significantly improves both wear resistance, and the required load for failure during scratch testing. Both mechanical properties show a maximum benefit at 1000 V beam voltage, reducing with increases in voltage beyond this. Although it is reasonable to assume that the modifications to coating structures are linked to the improvements in mechanical durability, defining a precise causality is not attempted here. Observed reductions in surface roughness of the coatings may also be attributable to differences in crystalline structure, i.e. smaller grain size, although the trend is not as clear. These changes in structural and mechanical properties of the coatings resulting from pretreatment of the substrate were not accompanied by any detrimental effect on their optical performance. At present, insufficient data are available to provide an explanation as to why pretreatment of substrate surfaces leads to such significant improvements in durability. It is
speculated that greater nucleation density for films grown on modified substrates may be the cause of differences in their crystalline structure, and hence their mechanical properties. Changes in nucleation and growth of the coatings may be a result of a change in surface chemistry or surface morphology of the glass, or a result of removing contaminants/corrosion products from the surface as a consequence of ion beam pretreatment. However, a more comprehensive understanding of the properties of the ion beam and of the effects of bombardment on glass substrates is necessary to adequately explain the processes involved in changing the coating growth, structure and durability.
Fig. 7. Typical transmission and reflection spectra for ZnO coatings on glass.
Fig. 8. Typical variation in n and k with wavelength for ZnO coatings on glass.
5. Conclusions This study has shown that the structural and mechanical properties of zinc oxide coatings deposited on float glass can be modified via pretreatment of the substrate with an oxygen ion beam. The effect has been shown to vary with nominal beam voltage, with a maximum occurring at 1000 V. For example, the wear rate of coatings pretreated at this voltage was reduced by a factor of three in comparison with
74
G.T. West, P.J. Kelly / Thin Solid Films 469–470 (2004) 70–74
coatings deposited on untreated substrates, and the critical load to failure was increased by 50%. The mechanism for these changes is not fully understood; however, further work is in progress, including a study of the ion beam species incident at the substrate, and the changes in surface chemistry resulting from this bombardment.
Acknowledgements The authors acknowledge the support provided by Pilkington Technology Centre, UK, and EPSRC for funding this research, and Advanced Energy Industries for use of the Linear Ion Source.
References [1] C. May, J. Strumpfel, D. Schulzes, Proc. 43rd Tech. Conf. Soc. Vac. Coaters, Denver, 2000, pp. 137 – 142. [2] K. Suzuki, Thin Solid Films 351 (1999) 8. [3] C. May, F. Milde, G. Teschner, Proc. 45th Tech. Conf. Soc. Vac. Coaters, Lake, 2002, pp. 153 – 158. [4] E. Ando, M. Miyazaki, Thin Solid Films, 392 (2) 289. [5] M. Miyazaki, E. Ando, J. Non-Cryst. Solids, 178, 245. [6] G.T. West, P.J. Kelly, Thin Solid Films 447–448 (2004) 20. [7] N. Capps, D. Carter, G. Roche, Semicond. Int. 23 (8) (2000) 16. [8] Y. Qu, et al., J. Vac. Sci. Technol., A 12 (4) (1994) 1507. [9] B. Window, F. Sharples, N. Savvides, J. Vac. Sci. Technol., A 6 (4) (1988) 2333. [10] K.K. Shih, D.A. Smith, J.R. Crowe, J. Vac. Sci. Technol., A 6 (3) (1988) 1681.