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Surface figuring of glass substrates by local deposition of silicon oxide with atmospheric pressure plasma jet
Surface & Coatings Technology 205 (2011) S351–S354
Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Surface figuring of glass substrates by local deposition of silicon oxide with atmospheric pressure plasma jet Manuela Janietz ⁎, Thomas Arnold Leibniz-Insitut für Oberflächenmodifizierung, Permoserstraße 15, D-04318 Leipzig, Germany
a r t i c l e
i n f o
Available online 4 April 2011 Keywords: Atmospheric pressure plasma Plasma jet machining HMDSO Silicon oxide deposition Shape forming
a b s t r a c t We investigated the local deposition of silicon oxide on silicon wafers and glass substrates utilizing a 2.45 GHz microwave-driven plasma jet, which was operated in ambient air with helium and small admixtures of hexamethyldisiloxane (HMDSO). The deposited material was characterized by FTIR, ellipsometry, wear measurements and ERDA. It has been shown that silicon oxide layers are homogeneous, stoichiometric and free of carbon. An established dwell time method allowed precise local deposition of films and analogous structures which were transferred into the substrate bulk material using ion beam techniques or mechanical polishing. Shapepreserving transfers have been successfully conducted. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Silicon dioxide (SiO2) is widely used as interlayer dielectric in microelectronics devices and finds application as barrier film against permeation of chemically reactive substances or scratch-protective coating [1,2]. These films are often prepared in low pressure plasma systems, which is costly and time-consuming. Plasma enhanced CVD processes at atmospheric pressure using DC arc discharges [3] or glow discharges [4] are developed for large area industrial use. Additionally, various atmospheric pressure plasma jets allow local treatment [5]. At our institute different plasma jet sources have been developed to cover a wide range of surface modification functions and complete the variety of ultra-precision surface finishing techniques [6]. These plasma jets were used to etch materials such as quartz or silicon carbide using fluorinecontaining gases. In this process fluorine radicals react with the substrates to form volatile compounds [7]. But not all optical glasses can be processed in this way, as e.g. Zerodur® and BK7 contain components that do not form volatile fluorides and instead are left behind and roughen the surface. However, plasma jet deposition is applicable for deterministic surface machining of such materials. In our studies silicon oxide has been deposited on different substrates by a microwave (2.45 GHz) excited plasma jet fed with He/ O2/HMDSO gas mixture. The properties of these films have been investigated depending on plasma process parameters. Furthermore, we produced films with sinusoidal thickness variation which were used as a mask during the subsequent transfer
⁎ Corresponding author. Tel.: + 49 341 2352173; fax: + 49 341 2353309. E-mail address: [email protected] (M. Janietz). 0257-8972/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2011.03.127
into the substrate within a second step. For this purpose we tested two techniques. On the one hand, a broad argon ion beam was used to etch the sample with the deposited structure, on the other hand, the sample was processed with a mechanical polishing robot. Afterwards it was checked whether the structure could be found in the substrate.
2. Experimental The schematic configuration of the plasma jet source is shown in Fig. 1. It consists of two coaxial tubes connected with a coaxial cable supplying microwave power. Through the inner capillary plasma process gas helium has been passed with a flow of 600 sccm for a stable discharge. Also, oxygen (10 sccm) was passed through the capillary and dissociated in the active plasma zone. Nitrogen (800 sccm) has been injected in the cavity between the inner conductor and the outer tube to shield the plasma jet from the surrounding atmosphere. To deposit films the vapour of the siliconcontaining precursor monomer HMDSO (C6H18Si2O) has been mixed with a carrier gas and added to the peripheral nitrogen flow in order to avoid dust formation or coating inside the capillary. The precursors’ quantity has been controlled by the flow of the carrier gas oxygen (5 sccm). The plasma has been excited by a pulsed microwave (repetition rate 7.1 kHz) with an average power of 5.5 W at 2.45 GHz and operated steadily in ambient air. Mounting the plasma jet source on a three-axis-motion system allowed us to move the plasma jet over the substrate, which could additionally be heated by a commercially available heating plate. Film thickness has been controlled by the velocity of the jet while it was moved along a meandering course over the sample. All experiments have been carried out on one-side polished Si(100) wafers or quartz plates.
S352
M. Janietz, T. Arnold / Surface & Coatings Technology 205 (2011) S351–S354
gas inlet cavity
gas inlet capillary
deposited layers by light microscope showed that the material is particle-free.
pulsed microwave
3.1. Characterization
grounded shield pipe electrode
plasma jet liquid precursor substrate heater Fig. 1. Schematic experimental setup. Excitation of plasma jet by microwave energy (2.45 GHz).
Film properties have been investigated by Fourier Transform Infrared Spectroscopy (FTIR) using a Bruker IFS 55 spectrometer. Transmission spectra have been accumulated 32 times with 4 cm− 1 resolution in the range from 7500 to 560 cm− 1. A silicon wafer similar to those used as substrate was the reference for the spectra. Film thickness and refractive index were obtained by an ex-situ ellipsometer Woollam M-2000VI, which provides a spectral range from λ = 370 nm to λ = 1700 nm with up to 588 wavelength channels. Spectra were recorded at angles of incidence of 70° and 75°. Elemental depth profiles were measured using elastic recoil detection analysis (ERDA) using 200 MeV 197Au15+ ions. The density of the films has been estimated by X-ray reflectivity (3003 PTS by Seifert). Wear tests were performed with the Universal Nanomechanical Tester (UNAT) from ASMEC using a diamond sphere applying a load of 5 to 100 mN. Interferometric measurements have been carried out with an Aspheric Stitching Interferometer from QED Technologies.
3. Results and discussion In preliminary tests measurements of static deposition profiles that have been produced at various distances from the sample, information on the tool profile have been obtained. As the precursor is mixed into the plasma peripherally, the deposition profile shows a local minimum in the center at distances up to 4 mm between nozzle and substrate. At larger distances the profile becomes bell-shaped as shown in Fig. 2. Moreover, the deposition rates decrease with increasing working distance, so we have chosen 5 mm as a standard parameter. The full width at half maximum (FWHM) is 1.4 mm. The assessment of
a)
b)
300
height in nm
2.5 mm 200
100
0 -3
-2
-1
0
1
2
3
position in mm Fig. 2. Profile (a) and microscope image (b) of plasma jet footprint deposited at 5 mm working distance.
Two normalized infrared absorption spectra over the range of 4000– 700 cm− 1 for films deposited at substrate temperatures of 50 and 550 °C, respectively, are shown in Fig. 3. Both films have approximately the same thickness of 400 nm to be comparable to each other. All IR spectra show a slope base line, which is caused by optical interference at the thin oxide layer [8], and was subtracted for better clearness. Characteristic bands at 800 and 1075 cm− 1[9] corresponding to bending or stretching siloxane bridges, respectively, have been observed. The intense Si\O\Si main peak shows a broad shoulder at higher wave numbers, which is attributed to larger angle Si\O\Si bonds in a cage structure [10]. This nanoporosity leads to reduction of density. No bands of carbon bonds like Si\CH3, that would appear at 1273 and 2840–2980 cm− 1[11], were visible in the spectra, although HMDSO has six methyl groups. Vibrations of hydroxyl groups appear at about 925 cm− 1 and 3200–3650 cm− 1. The latter band is very broad and can be assigned to H bonded stretching OH at 3350 cm− 1 and free stretching OH at 3650 cm− 1 that is found on the film surface. The main differences between these two spectra lay in the appearance of such hydroxyl group related peaks at 950 cm− 1 and 3350 cm− 1 and the half width of the Si\O\Si vibration at 1075 cm− 1. OH groups are incorporated into the layers by absorption of water: Si\O\Si + H2O → Si\OH+ Si\OH. Atmospheric water preferentially reacts with siloxanes with smaller bond angles, as they are expected to be more reactive than relaxed groups [12]. The FWHM of the stretching vibration represents the angle distribution of the siloxane bonds and therefore narrows with OH incorporation. ERDA depth profiles prove that the samples are free of carbon, except for a 10–20 nm thick surface contamination film. Nitrogen could not be found. The ratio of silicon and oxygen is constant within a small range (2%) and corresponds to stoichiometric silicon dioxide. Beside silicon and oxygen the films contain hydrogen, which can be quantified by ERDA. The film deposited at room temperature has a hydrogen content of 7%, while it is only 3%, when the substrate is heated during deposition. In XRR measurements the density of the material could be estimated from the critical angle of total reflection. It ranged between 1.8 g/cm³ (unheated substrate) and 2.1 g/cm3 (substrate heated to 550 °C during deposition). Ellipsometric parameters Ψ and Δ have been measured and modeled with a Cauchy film to obtain refraction index, film thickness and inhomogeneity. Deposition rates have been estimated by means of film thickness, speed and tool profile of the jet. The maximum deposition rate of 0.4 mm3/h has been achieved at room temperature. When the substrate is heated during the process, deposition rates decrease as already observed by Babayan [13]. Furthermore, the expected linear dependencies have been observed: deposition rate increases with increasing average microwave power (3–9 W) and precursor flow (0.5–10 sccm). The determined refractive indices lie between 1.42 and 1.45 (at λ = 550 nm) and are therefore slightly lower than the tabulated value of silicon oxide. This can be explained by the incorporation of hydroxyl groups, as water has a lower refractive index than silicon oxide. Wear measurements have shown that the mechanical stability of the films significantly increases with increasing substrate temperature during deposition, although the films are softer than thermal silicon oxide even at a deposition temperature of 500 °C (Fig. 4). 3.2. Application: transfer by ion beam etching or polishing Deposition by plasma jet is the first step of a two-step procedure for structuring different materials like e.g. quartz lenses. For this purpose
M. Janietz, T. Arnold / Surface & Coatings Technology 205 (2011) S351–S354
S353
50 °C 550 °C
absorbance in a.u.
-OH Si-O-Si
Si-CH3
Si-O-Si
H bonded OH free stretching OH -CH3
4000
3000
2000
1000
wavenumber in cm-1 Fig. 3. FTIR spectra of films deposited at different substrate temperatures: 50 °C (solid line) and 550 °C (dashed line).
test structures have been deposited using the dwell time method [14]. Subsequently the structures have been proportionally transferred into the substrate material by a homogeneous removal using an ion beam. We investigated, whether the produced material is suitable for such an application. Interferometric measurements prove, that a test structure of 230 nm height, which has been obtained under the standard conditions described in Section 2, could be completely transferred into a quartz disc at a size ratio of 1:0.8 by means of a broad argon ion beam (FWHM=18 mm) that was scanned over the sample with a line feed of 1 mm. The ion beam energy was 800 eV at a total current of 50 mA. The quartz substrate was etched at a rate of 10 nm/s, while the etching rate of the deposited silicon oxide was about 25% higher, due to its lower density. Also the mechanical polishing transfer has been realized using a 7-axes polishing robot from Satisloh/Zeeko. Material was removed by an inflatable, rotating polishing head (1000 rpm) that was scanned over the sample with a mixture of rare earth oxides slurry at 1 bar back pressure. This requires a higher mechanical stability of the silicon layers than necessary for ion beam transfer. Material deposited at standard parameters was nearly completely removed during the polishing process. Only material deposited at elevated temperatures (~400 °C) could achieve a sufficiently high stability. The size ratio between deposited structure and structured substrate after polishing was 1:0.7, the shape has been preserved as shown in Fig. 5.
a)
b)
4. Conclusion With the presented plasma jet silicon oxide could be deposited at a maximum deposition rate of 0.4 mm3/h using HMDSO as a precursor. Analyses showed that the layers are stoichiometric and free of carbon residues. However, hydroxyl groups are incorporated and lead to a reduction of density and mechanical stability. A hybrid technique using ion beam or even polishing tools for transferring a deposited structure is very promising in terms of precise surface modification of glass materials, as the spatial resolution of the plasma jet is combined with the high homogenous removal rate of such tools. Shape-preserving transfers at size ratios of 1:0.8 and 1:0.7, respectively, have been demonstrated.
Acknowledgements The author would like to thank her colleagues from IOM, especially Dr. J. W. Gerlach, who carried out the ERDA measurements at the LMU München in Garching and H. Paetzelt for polishing. This work has been supported by the German Ministry of Education and Science within the framework of the InnoProfile program ‘Ultra precision machining using atomic particle beams’.
c)
20 µm Fig. 4. Wear measurements (75 mN load) showing the mechanical stability of films deposited on unheated (a) and heated substrates (500 °C, (b)) compared to thermal silicon oxide (c).
S354
M. Janietz, T. Arnold / Surface & Coatings Technology 205 (2011) S351–S354 [3] I. Dani, V. Hopfe, D. Rogler, E. Lopez, G. Mäder, Vakuum in Forschung und Praxis 18 (2006) 30. [4] G.R. Nowling, M. Yajima, S.E. Babayan, M. Moravej, X. Yang, W. Hoffman, R.F. Hicks, Plasma Sources Sci. Technol. 14 (2005) 477. [5] M. Laroussi, T. Akan, Plasma Process. Polym. 4 (2007) 777. [6] T. Arnold, G. Böhm, R. Fechner, J. Meister, A. Nickel, F. Frost, T. Hänsel, A. Schindler, Nucl. Instrum. Meth. A 616 (2010) 147. [7] T. Arnold, G. Boehm, I.-M. Eichentopf, M. Janietz, J. Meister, A. Schindler, Vakuum in Forschung und Praxis 22 (2010) 10. [8] A. Goullet Goullet, C. Vallée, A. Granier, A.G. Turban, J. Vac. Sci. Technol. A 18 (2000) 2452. [9] V. Raballand, J. Benedikt, A. von Keudel, Appl. Phys. Lett. 92 (2008) 091502-1. [10] A. Grill, D.A. Neumayer, J. Appl. Phys. 94 (2003) 6697. [11] J. Schäfer, R. Foest, A. Quade, A. Ohl, K.-D. Weltmann, J. Phys. D: Appl. Phys. 41 (2008) 194010(1). [12] J.A. Theil, D.V. Tsu, M.W. Watkins, S.S. Kim, G. Lucovsky, J. Vac. Sci. Technol. A 8 (1990) 1374. [13] S.E. Babayan, J.Y. Jeong, V.J. Tu, J. Park, G.S. Sewyn, R.F. Hicks, Plasma Sources Sci. Technol. 7 (1998) 286. [14] T. Hänsel, F. Frost, A. Nickel, A. Schindler, Vakuum in Forschung und Praxis 19 (2007) 24.
Fig. 5. a) Profiles of test structure as deposited (solid line) and after transfer by polishing robot into Si(100) wafer (dashed line). b) Interferometric measurement before (above) and after transfer (below).
References [1] K. Teshima, Y. Inoue, H. Sugimura, O. Takai, Surf. Coat. Technol. 146–147 (2001) 451. [2] J. Schäfer, R. Foest, A. Quade, A. Ohl, K.-D. Weltmann, Plasma Process. Polym. 6 (2009) S519.
Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Surface figuring of glass substrates by local deposition of silicon oxide with atmospheric pressure plasma jet Manuela Janietz ⁎, Thomas Arnold Leibniz-Insitut für Oberflächenmodifizierung, Permoserstraße 15, D-04318 Leipzig, Germany
a r t i c l e
i n f o
Available online 4 April 2011 Keywords: Atmospheric pressure plasma Plasma jet machining HMDSO Silicon oxide deposition Shape forming
a b s t r a c t We investigated the local deposition of silicon oxide on silicon wafers and glass substrates utilizing a 2.45 GHz microwave-driven plasma jet, which was operated in ambient air with helium and small admixtures of hexamethyldisiloxane (HMDSO). The deposited material was characterized by FTIR, ellipsometry, wear measurements and ERDA. It has been shown that silicon oxide layers are homogeneous, stoichiometric and free of carbon. An established dwell time method allowed precise local deposition of films and analogous structures which were transferred into the substrate bulk material using ion beam techniques or mechanical polishing. Shapepreserving transfers have been successfully conducted. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Silicon dioxide (SiO2) is widely used as interlayer dielectric in microelectronics devices and finds application as barrier film against permeation of chemically reactive substances or scratch-protective coating [1,2]. These films are often prepared in low pressure plasma systems, which is costly and time-consuming. Plasma enhanced CVD processes at atmospheric pressure using DC arc discharges [3] or glow discharges [4] are developed for large area industrial use. Additionally, various atmospheric pressure plasma jets allow local treatment [5]. At our institute different plasma jet sources have been developed to cover a wide range of surface modification functions and complete the variety of ultra-precision surface finishing techniques [6]. These plasma jets were used to etch materials such as quartz or silicon carbide using fluorinecontaining gases. In this process fluorine radicals react with the substrates to form volatile compounds [7]. But not all optical glasses can be processed in this way, as e.g. Zerodur® and BK7 contain components that do not form volatile fluorides and instead are left behind and roughen the surface. However, plasma jet deposition is applicable for deterministic surface machining of such materials. In our studies silicon oxide has been deposited on different substrates by a microwave (2.45 GHz) excited plasma jet fed with He/ O2/HMDSO gas mixture. The properties of these films have been investigated depending on plasma process parameters. Furthermore, we produced films with sinusoidal thickness variation which were used as a mask during the subsequent transfer
⁎ Corresponding author. Tel.: + 49 341 2352173; fax: + 49 341 2353309. E-mail address: [email protected] (M. Janietz). 0257-8972/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2011.03.127
into the substrate within a second step. For this purpose we tested two techniques. On the one hand, a broad argon ion beam was used to etch the sample with the deposited structure, on the other hand, the sample was processed with a mechanical polishing robot. Afterwards it was checked whether the structure could be found in the substrate.
2. Experimental The schematic configuration of the plasma jet source is shown in Fig. 1. It consists of two coaxial tubes connected with a coaxial cable supplying microwave power. Through the inner capillary plasma process gas helium has been passed with a flow of 600 sccm for a stable discharge. Also, oxygen (10 sccm) was passed through the capillary and dissociated in the active plasma zone. Nitrogen (800 sccm) has been injected in the cavity between the inner conductor and the outer tube to shield the plasma jet from the surrounding atmosphere. To deposit films the vapour of the siliconcontaining precursor monomer HMDSO (C6H18Si2O) has been mixed with a carrier gas and added to the peripheral nitrogen flow in order to avoid dust formation or coating inside the capillary. The precursors’ quantity has been controlled by the flow of the carrier gas oxygen (5 sccm). The plasma has been excited by a pulsed microwave (repetition rate 7.1 kHz) with an average power of 5.5 W at 2.45 GHz and operated steadily in ambient air. Mounting the plasma jet source on a three-axis-motion system allowed us to move the plasma jet over the substrate, which could additionally be heated by a commercially available heating plate. Film thickness has been controlled by the velocity of the jet while it was moved along a meandering course over the sample. All experiments have been carried out on one-side polished Si(100) wafers or quartz plates.
S352
M. Janietz, T. Arnold / Surface & Coatings Technology 205 (2011) S351–S354
gas inlet cavity
gas inlet capillary
deposited layers by light microscope showed that the material is particle-free.
pulsed microwave
3.1. Characterization
grounded shield pipe electrode
plasma jet liquid precursor substrate heater Fig. 1. Schematic experimental setup. Excitation of plasma jet by microwave energy (2.45 GHz).
Film properties have been investigated by Fourier Transform Infrared Spectroscopy (FTIR) using a Bruker IFS 55 spectrometer. Transmission spectra have been accumulated 32 times with 4 cm− 1 resolution in the range from 7500 to 560 cm− 1. A silicon wafer similar to those used as substrate was the reference for the spectra. Film thickness and refractive index were obtained by an ex-situ ellipsometer Woollam M-2000VI, which provides a spectral range from λ = 370 nm to λ = 1700 nm with up to 588 wavelength channels. Spectra were recorded at angles of incidence of 70° and 75°. Elemental depth profiles were measured using elastic recoil detection analysis (ERDA) using 200 MeV 197Au15+ ions. The density of the films has been estimated by X-ray reflectivity (3003 PTS by Seifert). Wear tests were performed with the Universal Nanomechanical Tester (UNAT) from ASMEC using a diamond sphere applying a load of 5 to 100 mN. Interferometric measurements have been carried out with an Aspheric Stitching Interferometer from QED Technologies.
3. Results and discussion In preliminary tests measurements of static deposition profiles that have been produced at various distances from the sample, information on the tool profile have been obtained. As the precursor is mixed into the plasma peripherally, the deposition profile shows a local minimum in the center at distances up to 4 mm between nozzle and substrate. At larger distances the profile becomes bell-shaped as shown in Fig. 2. Moreover, the deposition rates decrease with increasing working distance, so we have chosen 5 mm as a standard parameter. The full width at half maximum (FWHM) is 1.4 mm. The assessment of
a)
b)
300
height in nm
2.5 mm 200
100
0 -3
-2
-1
0
1
2
3
position in mm Fig. 2. Profile (a) and microscope image (b) of plasma jet footprint deposited at 5 mm working distance.
Two normalized infrared absorption spectra over the range of 4000– 700 cm− 1 for films deposited at substrate temperatures of 50 and 550 °C, respectively, are shown in Fig. 3. Both films have approximately the same thickness of 400 nm to be comparable to each other. All IR spectra show a slope base line, which is caused by optical interference at the thin oxide layer [8], and was subtracted for better clearness. Characteristic bands at 800 and 1075 cm− 1[9] corresponding to bending or stretching siloxane bridges, respectively, have been observed. The intense Si\O\Si main peak shows a broad shoulder at higher wave numbers, which is attributed to larger angle Si\O\Si bonds in a cage structure [10]. This nanoporosity leads to reduction of density. No bands of carbon bonds like Si\CH3, that would appear at 1273 and 2840–2980 cm− 1[11], were visible in the spectra, although HMDSO has six methyl groups. Vibrations of hydroxyl groups appear at about 925 cm− 1 and 3200–3650 cm− 1. The latter band is very broad and can be assigned to H bonded stretching OH at 3350 cm− 1 and free stretching OH at 3650 cm− 1 that is found on the film surface. The main differences between these two spectra lay in the appearance of such hydroxyl group related peaks at 950 cm− 1 and 3350 cm− 1 and the half width of the Si\O\Si vibration at 1075 cm− 1. OH groups are incorporated into the layers by absorption of water: Si\O\Si + H2O → Si\OH+ Si\OH. Atmospheric water preferentially reacts with siloxanes with smaller bond angles, as they are expected to be more reactive than relaxed groups [12]. The FWHM of the stretching vibration represents the angle distribution of the siloxane bonds and therefore narrows with OH incorporation. ERDA depth profiles prove that the samples are free of carbon, except for a 10–20 nm thick surface contamination film. Nitrogen could not be found. The ratio of silicon and oxygen is constant within a small range (2%) and corresponds to stoichiometric silicon dioxide. Beside silicon and oxygen the films contain hydrogen, which can be quantified by ERDA. The film deposited at room temperature has a hydrogen content of 7%, while it is only 3%, when the substrate is heated during deposition. In XRR measurements the density of the material could be estimated from the critical angle of total reflection. It ranged between 1.8 g/cm³ (unheated substrate) and 2.1 g/cm3 (substrate heated to 550 °C during deposition). Ellipsometric parameters Ψ and Δ have been measured and modeled with a Cauchy film to obtain refraction index, film thickness and inhomogeneity. Deposition rates have been estimated by means of film thickness, speed and tool profile of the jet. The maximum deposition rate of 0.4 mm3/h has been achieved at room temperature. When the substrate is heated during the process, deposition rates decrease as already observed by Babayan [13]. Furthermore, the expected linear dependencies have been observed: deposition rate increases with increasing average microwave power (3–9 W) and precursor flow (0.5–10 sccm). The determined refractive indices lie between 1.42 and 1.45 (at λ = 550 nm) and are therefore slightly lower than the tabulated value of silicon oxide. This can be explained by the incorporation of hydroxyl groups, as water has a lower refractive index than silicon oxide. Wear measurements have shown that the mechanical stability of the films significantly increases with increasing substrate temperature during deposition, although the films are softer than thermal silicon oxide even at a deposition temperature of 500 °C (Fig. 4). 3.2. Application: transfer by ion beam etching or polishing Deposition by plasma jet is the first step of a two-step procedure for structuring different materials like e.g. quartz lenses. For this purpose
M. Janietz, T. Arnold / Surface & Coatings Technology 205 (2011) S351–S354
S353
50 °C 550 °C
absorbance in a.u.
-OH Si-O-Si
Si-CH3
Si-O-Si
H bonded OH free stretching OH -CH3
4000
3000
2000
1000
wavenumber in cm-1 Fig. 3. FTIR spectra of films deposited at different substrate temperatures: 50 °C (solid line) and 550 °C (dashed line).
test structures have been deposited using the dwell time method [14]. Subsequently the structures have been proportionally transferred into the substrate material by a homogeneous removal using an ion beam. We investigated, whether the produced material is suitable for such an application. Interferometric measurements prove, that a test structure of 230 nm height, which has been obtained under the standard conditions described in Section 2, could be completely transferred into a quartz disc at a size ratio of 1:0.8 by means of a broad argon ion beam (FWHM=18 mm) that was scanned over the sample with a line feed of 1 mm. The ion beam energy was 800 eV at a total current of 50 mA. The quartz substrate was etched at a rate of 10 nm/s, while the etching rate of the deposited silicon oxide was about 25% higher, due to its lower density. Also the mechanical polishing transfer has been realized using a 7-axes polishing robot from Satisloh/Zeeko. Material was removed by an inflatable, rotating polishing head (1000 rpm) that was scanned over the sample with a mixture of rare earth oxides slurry at 1 bar back pressure. This requires a higher mechanical stability of the silicon layers than necessary for ion beam transfer. Material deposited at standard parameters was nearly completely removed during the polishing process. Only material deposited at elevated temperatures (~400 °C) could achieve a sufficiently high stability. The size ratio between deposited structure and structured substrate after polishing was 1:0.7, the shape has been preserved as shown in Fig. 5.
a)
b)
4. Conclusion With the presented plasma jet silicon oxide could be deposited at a maximum deposition rate of 0.4 mm3/h using HMDSO as a precursor. Analyses showed that the layers are stoichiometric and free of carbon residues. However, hydroxyl groups are incorporated and lead to a reduction of density and mechanical stability. A hybrid technique using ion beam or even polishing tools for transferring a deposited structure is very promising in terms of precise surface modification of glass materials, as the spatial resolution of the plasma jet is combined with the high homogenous removal rate of such tools. Shape-preserving transfers at size ratios of 1:0.8 and 1:0.7, respectively, have been demonstrated.
Acknowledgements The author would like to thank her colleagues from IOM, especially Dr. J. W. Gerlach, who carried out the ERDA measurements at the LMU München in Garching and H. Paetzelt for polishing. This work has been supported by the German Ministry of Education and Science within the framework of the InnoProfile program ‘Ultra precision machining using atomic particle beams’.
c)
20 µm Fig. 4. Wear measurements (75 mN load) showing the mechanical stability of films deposited on unheated (a) and heated substrates (500 °C, (b)) compared to thermal silicon oxide (c).
S354
M. Janietz, T. Arnold / Surface & Coatings Technology 205 (2011) S351–S354 [3] I. Dani, V. Hopfe, D. Rogler, E. Lopez, G. Mäder, Vakuum in Forschung und Praxis 18 (2006) 30. [4] G.R. Nowling, M. Yajima, S.E. Babayan, M. Moravej, X. Yang, W. Hoffman, R.F. Hicks, Plasma Sources Sci. Technol. 14 (2005) 477. [5] M. Laroussi, T. Akan, Plasma Process. Polym. 4 (2007) 777. [6] T. Arnold, G. Böhm, R. Fechner, J. Meister, A. Nickel, F. Frost, T. Hänsel, A. Schindler, Nucl. Instrum. Meth. A 616 (2010) 147. [7] T. Arnold, G. Boehm, I.-M. Eichentopf, M. Janietz, J. Meister, A. Schindler, Vakuum in Forschung und Praxis 22 (2010) 10. [8] A. Goullet Goullet, C. Vallée, A. Granier, A.G. Turban, J. Vac. Sci. Technol. A 18 (2000) 2452. [9] V. Raballand, J. Benedikt, A. von Keudel, Appl. Phys. Lett. 92 (2008) 091502-1. [10] A. Grill, D.A. Neumayer, J. Appl. Phys. 94 (2003) 6697. [11] J. Schäfer, R. Foest, A. Quade, A. Ohl, K.-D. Weltmann, J. Phys. D: Appl. Phys. 41 (2008) 194010(1). [12] J.A. Theil, D.V. Tsu, M.W. Watkins, S.S. Kim, G. Lucovsky, J. Vac. Sci. Technol. A 8 (1990) 1374. [13] S.E. Babayan, J.Y. Jeong, V.J. Tu, J. Park, G.S. Sewyn, R.F. Hicks, Plasma Sources Sci. Technol. 7 (1998) 286. [14] T. Hänsel, F. Frost, A. Nickel, A. Schindler, Vakuum in Forschung und Praxis 19 (2007) 24.
Fig. 5. a) Profiles of test structure as deposited (solid line) and after transfer by polishing robot into Si(100) wafer (dashed line). b) Interferometric measurement before (above) and after transfer (below).
References [1] K. Teshima, Y. Inoue, H. Sugimura, O. Takai, Surf. Coat. Technol. 146–147 (2001) 451. [2] J. Schäfer, R. Foest, A. Quade, A. Ohl, K.-D. Weltmann, Plasma Process. Polym. 6 (2009) S519.