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Microwave PECVD for large area coating
Surface & Coatings Technology 200 (2005) 21 – 25 www.elsevier.com/locate/surfcoat
Microwave PECVD for large area coating M. LiehrT, M. Dieguez-Campo Applied Films GmbH & Co KG, Siemensstrasse 100, D-63755 Alzenau, Germany Available online 11 March 2005
Abstract Microwave Plasma Enhanced Chemical Vapour Deposition (PECVD) of thin films is the method of choice when highest deposition rates and/or high fragmentation of precursor material is desirable. However, large area applications have always suffered from poor film thickness uniformity and unacceptable variations of thin film properties. Coaxial plasma line sources in various arrangements have proven their ability to generally overcome all former technological limitations inherent in many other types of microwave plasma sources, which prevented microwave PECVD from becoming a mainstream technology in the field of large area coatings. In this article, the advantages and the potential of the coaxial plasma line sources and suitable vacuum processes are described. Improvements of the coaxial plasma lines are introduced which are indispensable for large area plasma applications and which have been investigated in several laboratory and production prototype vacuum process systems. A variety of thin films have been deposited, ranging from organic and ceramic silicon dioxide to nanocrystalline diamond and microcrystalline silicon for photovoltaic applications. D 2005 Elsevier B.V. All rights reserved. Keywords: Organometal plasma; Microwave plasma; Silicon dioxide; Silicon thin film; Solar cell
1. Introduction PECVD processes can be driven by a wide variety of power sources which range from pure DC to AC at frequencies from acoustic (kHz), radio and vhf frequencies to the radar range (GHz) and combinations thereof. The choice of power source is commonly subject to simple rules: plasma density and process rates increase with higher frequencies, plasma ion energies drop with increasing frequency. However, when it comes to process uniformity over large areas (N ~1 m2) the situation becomes more complicated. As long as the wavelength of the applied power is larger or comparable to linear dimensions of the plasma processing reactor free space wave propagation is not possible and all wave modes will have to rely on conductive components (waveguide, electrodes) inside the reactor. The wavelengths of industrial microwave frequencies are usually much smaller than the dimensions of large area plasma processing vessels. Since metallic surfaces are not as effective to distribute microwave power in a plasma T Corresponding author. Tel.: +49 6023 926530; fax: +49 6023 926070. E-mail address: [email protected] (M. Liehr). 0257-8972/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2005.02.061
discharge, other and generally more complicated methods of injecting microwave power into plasma discharges have been developed. Amongst them, slotted line antennas [1,8,9] with discrete radiating elements, commonly used in radar applications have been tried but have not proven to be industrially viable. Fluctuating power reflections may occur from the atmosphere-to-vacuum interfaces if they are small in size resulting in high-density plasma discharges close to their vacuum side surfaces.
2. The coaxial line plasma source The breakthrough on the way to a spatially sustained microwave discharge over areas much larger than the wavelength of the applied microwave power was certainly the both brilliant and simple idea to replace slotted waveguides and their troublesome discrete radiating elements by making the electrically conductive plasma part of a coaxial transmission line [2,3,10]. Microwave power is supplied to a vacuum vessel by a coaxial transmission line in the transversal electromagnetic (TEM) wave mode. Inside the vacuum vessel a tube manufactured from a dielectric
22
M. Liehr, M. Dieguez-Campo / Surface & Coatings Technology 200 (2005) 21–25
Coaxial line plasma source with inner metallic pipe and ceramic vacuum interface tube Carrier gas supply line Coaxial microwave power input
Plasma
Feedstock gas supply line
Substrate
Fig. 1. The principal layout of the coaxial line plasma source in semi-remote plasma arrangement.
material acting as an atmosphere-to-vacuum interface replaces the outer conductor of the coaxial line, resulting in a discharge of high radial symmetry circumjacent the tube surface as indicated in Fig. 1. The so created dplasma linesT show similar characteristics to coaxial lines. High wave damping by the plasma results in non-uniform plasma density distributions and related process rates at the substrate. Measures have to be introduced to control the attenuation of the propagating power, such as a partial outer shielding or the replacement of the straight inner conductor by a helical line. Fig. 2 shows a coaxial line plasma source with a tapered outer shielding made from metal resulting in a more even plasma density distribution. Although partial shielding is an easy way to control the plasma uniformity it also destroys the radial symmetry of the plasma line source. Keeping the radial symmetry would be essential if the plasma source were used for bi-directional coating of two substrates simultaneously, minimizing self coating of inner reactor walls and doubling the throughput, strongly resulting in reduced cost of ownership and system footprint. A helical transmission line can be designed so that the length of one turn equals one wavelength of the applied microwaves (operating in the bend fireQ mode, i.e. radiating primarily along the axis of symmetry). The electrical field of the microwaves across the diameter then rotates around the axis of symmetry. A substantial amount of the microwave power will experience circular polarization following the helicity of the helix. Initial test with
helical structures, however, revealed that some space is required between the outside of the helical turns and the dielectric interface tube or else the intended effect cannot be observed. Taking into account the helix diameter being determined by the wavelength of the microwaves this would have resulted in large dielectric interface tube diameters causing too much tensile force on the brittle tube walls. Eventually, the diameter of the helix could be reduced by filling the core of the helix with dielectric materials such as quartz or alumina. Also, the dielectric filling helps to concentrate more microwave power in the helical core and therefore strengthen the intended purpose of the helical line. Fig. 3 shows a helical line with an alumina rod in the core and a tapered coaxial-to-helical transition stage. By feeding microwave power to the coaxial line plasma source from both ends [4] devices of up to 3000 mm in length have been built [5]. Large area microwave plasma sources can be built by arranging arrays of parallel coaxial line plasma sources as shown in Fig. 4. T-junctions are ideal for microwave power splitting since they are quite easy to manufacture and are of compact built. But every T-junction reflects about 1/7 of the incident power. Unacceptable levels of reflected microwave power would have to be anticipated for cascaded Tjunctions in particular but the inherent reflection of a Tjunction can been overcome by adequate design changes employing finite element based computer modeling. Fig. 5 shows an 8-line plasma source array before and after a coating process and the EcoVeRaR coater. Here, the
Metal shields (tapered)
Uniform plasma Fig. 2. Coaxial line plasma source (side view) with tapered metal shields for improved plasma uniformity. The shield behaves like outer conductors of a coaxial transmission line.
M. Liehr, M. Dieguez-Campo / Surface & Coatings Technology 200 (2005) 21–25
23
Length: 400 mm (approx.), OD: 24 mm
Helical line (soft copper)
Dielectric core (alumina)
Dielectric tube (vacuum interface) Fig. 3. Helical line plasma source with alumina core designed for end fire mode. Propagating power shows circular polarization. Helical plasma source works best in wide dielectric interface tubes.
Cascaded coaxial power divider
Planar plasma source array
Rectangularto-coaxial transformer
Impedance matched rectangular waveguide
Substrate
Microwave power input Fig. 4. Planar plasma source array with four parallel coaxial line plasma sources operated with two power sources employing symmetrical, cascaded coaxial 1:4 power dividers.
Microwave generators and waveguide components Substrate in vertical carrier
Door with pumping port and substrate heating
Coaxial line plasma source array (16 lines for 8 microwave generators) Fig. 5. Eight coaxial line plasma sources with metal shields forming a planar array of 0.5 m2, mounted in a laboratory coater, seen from below (left) and above (middle). The new EcoVeRaR coating system uses 16 coaxial lines, 8 microwave generators and can coat two substrates (1 m2) simultaneously.
24
M. Liehr, M. Dieguez-Campo / Surface & Coatings Technology 200 (2005) 21–25
f = 100 Hz
f = 10 Hz
eventually leads to continuous plasma conversion rates with decreasing ripple as indicated in Fig. 6.
Plasma-light signal Microwave signal
f = 1.000 Hz
3. Results The coaxial line plasma source is generally used in a semi-remote process arrangement as shown in Fig. 2. The precursor gas is supplied to the vacuum process chamber somewhere in-between the coaxial line plasma source or plasma source array and the substrate and its flow is directed towards the substrate. Other gaseous components are supplied from beyond the coaxial line plasma source so that they have to pass through the plasma region where they experience the impact of energetic electron bombardment and UV radiation.
f = 10.000 Hz Plasma-light signal Microwave signal
Fig. 6. Plasma afterglow effect during pulsed power operation. Light emission from a SiH4/H2 plasma measured with a fast photodiode as a function of the pulse frequency f at 15% duty cycle.
3.1. Silicon thin films from silane-hydrogen plasmas double sided plasma source consists of an array of sixteen parallel coaxial plasma lines powered by 8 microwave generators each feeding into four coaxial lines by means of 1:4 coaxial power splitter in a double cascade arrangement. The microwave power from each generator is coupled into a standard rectangular R-26 waveguides transporting the microwaves through a circulator, a directional coupler and a tapered waveguide section to the waveguide-to-coaxial transformer. C/w power levels of up to 15 kW per generator have been successfully tested during the deposition of hard carbon coatings. Stub or EH tuners are not necessary as long as the waveguide transmission line impedances are matched. For the coating or treatment of temperature sensitive substrate materials like polymers low net power input is preferable. However, in order to sustain a flat but bclosedQ plasma cloud over the area of interest a certain minimum power is necessary or else the plasma becomes unstable and non-uniform. This low power threshold can be overcome by using pulsed power. By providing a surplus of power to the plasma source for a short time a stable and uniform discharge can be sustained. With increasing pulse frequency the plasma afterglow effect becomes more prominent and
Fig. 7 shows a glass substrate of roughly 1 0.5 m2, which has been uniformly coated by microcrystalline silicon for solar cell applications. The crystalline morphology can reliably be controlled by adjustment of the silane–hydrogen (SiH4/H2) ratio and the distance between precursor gas inlet and the substrate surface as could be verified by Raman spectroscopy, shown in Fig. 7. The precursor gas silane is supplied downstream and hydrogen above the plasma source. Microwave plasmas are of particular interest for the formation of microcrystalline silicon since the required silane dilution ratio has to be very high and substantial deposition rates require a high plasma density. 3.2. Silicon dioxide formation using organometal–oxygen plasmas
arbitrary units
Microwave PECVD can be used to its full potential when it comes to plasma polymerization processes. Pure ceramic and borganicQ silicon dioxide layers (several microns) for barrier or anti scratch applications by microwave PECVD from organic precursor material have been produced. The
300
400
500
600
700
raman shift (1/cm) Fig. 7. Uniform silicon film on float glass (left). Raman spectroscopy (right) shows different morphologies of the deposited silicon. Microcrystalline material shows a sharp peak at higher wave numbers.
M. Liehr, M. Dieguez-Campo / Surface & Coatings Technology 200 (2005) 21–25 Signal A = InLens
1µm
MAG = 12.00 K X EHT = 2.00 kV
25
100
Date :3 Mar 2004 Time :17:59
C 90
N O
Atomic concentration
80
oxygen
Si
70 60 50 40
silicon
30 20
carbon
10 0 0
1500
3000
4500
6000
7500
9000 10500 12000 13500 15000 16500 18000 19500 21000
Time of sputtering
Fig. 8. SEM micrograph of 2 Am thick, almost pure SiO2 film deposited by an HMDSO–oxygen plasma on float glass. XPS depth profile (right) of the same film.
precursor material hexamethyldisiloxane or HMDSO, with the chemical formula (CH3)3–Si–O–Si–(CH3)3, is supplied downstream and oxygen from the far side, very similar to the silane–hydrogen system. Fig. 8 shows a micrograph of a deposited 2 Am thick SiO2 film on glass and an XPS spectrum which has been taken from the same film. Surprisingly, the ratio of carbon in this film is extremely low. Of course, SiO2 films with a much higher organic content can be deposited by changing the HMDSO/oxygen ratio, gas flow rate, pressure and adjusting the microwave power level accordingly. Experimental results on the HMDSO/O2 system with a linear microwave plasma source of smaller size using Fourier transform infrared spectroscopy can be found under Ref. [6]. 3.3. Nanocrystalline diamond films from a methane–hydrogen plasma Transparent nanocrystalline diamond films of 1 Am thickness have been deposited at 450 8C on Pyrex and float glass 30 30 cm2 in size. XRD spectra have revealed grain sizes between 10 and 20 nm. A transmittance of 70% at 700 nm was observed [7].
4. Conclusion The coaxial line plasma source now appears to be the only industrially viable way of microwave PECVD for large area coatings in the range of a square meter and beyond after
having been constantly improved and matured over the recent years and with suitable microwave power generators commercially available now. It is the combination of such a high-energy plasma source and the flexibility of plasma chemistry involving organic materials that allows the production of materials so diverse by their nature like pure ceramic and organic (polymeric) thin films—by mainly changing gas flow ratios.
References [1] A.W. Rudge, K. Milne, A.D. Lover, P. Knight (Eds.), The Handbook of Antenna Design, vol. 2, Peter Peregrinus Ltd., 1983, p. 1, Chap. 9. [2] E. R7uchle, Lecture at Third International Workshop on Microwave Discharges: Fundamentals and Applications. Abbaye de Fontevraud, France, 20–25 April 1997. [3] H. Ranke, H. Bickmann, W. Kemmner, E. R7uchle, T. Magg, German patent publication No. DE 4136297 A1, 6 May 1993. [4] E. R7uchle, W. Petasch, F.K. R7uchle, H. Mqgge, German Patent DE 19503205 C1, 2 May 1995. [5] H. Muegge, Muegge Electronic GmbH, private communication. [6] M. Walker, K.-M. Baumg7rtner, J. Feichtinger, M. Kaiser, A. Schulz, E. R7uchle, Vacuum 57 (2000) 387. [7] Y. Koga, K. Tsugawa, M. Shelby, M. Liehr, Proceedings of the 14th European Conference on Diamond, Diamond-like Materials, Carbon Nanotubes, Nitrides and Silicon Carbides, Salzburg, Austria, 2003 (September 7–12). [8] Z. Zakrzewski, M. Moisan, Plasma Sources Sci. Technol. 4 (1995) 379. [9] A.J. Sangster, A.H.I. McCormick, IEE Proceedings, vol. 136, Pt. H, No. 1, 1989 (Feb). [10] W. Petasch, E. R7uchle, J. Weichart, H. Bickmann, Surf. Coat. Technol. 74–75 (1995) 200.
Microwave PECVD for large area coating M. LiehrT, M. Dieguez-Campo Applied Films GmbH & Co KG, Siemensstrasse 100, D-63755 Alzenau, Germany Available online 11 March 2005
Abstract Microwave Plasma Enhanced Chemical Vapour Deposition (PECVD) of thin films is the method of choice when highest deposition rates and/or high fragmentation of precursor material is desirable. However, large area applications have always suffered from poor film thickness uniformity and unacceptable variations of thin film properties. Coaxial plasma line sources in various arrangements have proven their ability to generally overcome all former technological limitations inherent in many other types of microwave plasma sources, which prevented microwave PECVD from becoming a mainstream technology in the field of large area coatings. In this article, the advantages and the potential of the coaxial plasma line sources and suitable vacuum processes are described. Improvements of the coaxial plasma lines are introduced which are indispensable for large area plasma applications and which have been investigated in several laboratory and production prototype vacuum process systems. A variety of thin films have been deposited, ranging from organic and ceramic silicon dioxide to nanocrystalline diamond and microcrystalline silicon for photovoltaic applications. D 2005 Elsevier B.V. All rights reserved. Keywords: Organometal plasma; Microwave plasma; Silicon dioxide; Silicon thin film; Solar cell
1. Introduction PECVD processes can be driven by a wide variety of power sources which range from pure DC to AC at frequencies from acoustic (kHz), radio and vhf frequencies to the radar range (GHz) and combinations thereof. The choice of power source is commonly subject to simple rules: plasma density and process rates increase with higher frequencies, plasma ion energies drop with increasing frequency. However, when it comes to process uniformity over large areas (N ~1 m2) the situation becomes more complicated. As long as the wavelength of the applied power is larger or comparable to linear dimensions of the plasma processing reactor free space wave propagation is not possible and all wave modes will have to rely on conductive components (waveguide, electrodes) inside the reactor. The wavelengths of industrial microwave frequencies are usually much smaller than the dimensions of large area plasma processing vessels. Since metallic surfaces are not as effective to distribute microwave power in a plasma T Corresponding author. Tel.: +49 6023 926530; fax: +49 6023 926070. E-mail address: [email protected] (M. Liehr). 0257-8972/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2005.02.061
discharge, other and generally more complicated methods of injecting microwave power into plasma discharges have been developed. Amongst them, slotted line antennas [1,8,9] with discrete radiating elements, commonly used in radar applications have been tried but have not proven to be industrially viable. Fluctuating power reflections may occur from the atmosphere-to-vacuum interfaces if they are small in size resulting in high-density plasma discharges close to their vacuum side surfaces.
2. The coaxial line plasma source The breakthrough on the way to a spatially sustained microwave discharge over areas much larger than the wavelength of the applied microwave power was certainly the both brilliant and simple idea to replace slotted waveguides and their troublesome discrete radiating elements by making the electrically conductive plasma part of a coaxial transmission line [2,3,10]. Microwave power is supplied to a vacuum vessel by a coaxial transmission line in the transversal electromagnetic (TEM) wave mode. Inside the vacuum vessel a tube manufactured from a dielectric
22
M. Liehr, M. Dieguez-Campo / Surface & Coatings Technology 200 (2005) 21–25
Coaxial line plasma source with inner metallic pipe and ceramic vacuum interface tube Carrier gas supply line Coaxial microwave power input
Plasma
Feedstock gas supply line
Substrate
Fig. 1. The principal layout of the coaxial line plasma source in semi-remote plasma arrangement.
material acting as an atmosphere-to-vacuum interface replaces the outer conductor of the coaxial line, resulting in a discharge of high radial symmetry circumjacent the tube surface as indicated in Fig. 1. The so created dplasma linesT show similar characteristics to coaxial lines. High wave damping by the plasma results in non-uniform plasma density distributions and related process rates at the substrate. Measures have to be introduced to control the attenuation of the propagating power, such as a partial outer shielding or the replacement of the straight inner conductor by a helical line. Fig. 2 shows a coaxial line plasma source with a tapered outer shielding made from metal resulting in a more even plasma density distribution. Although partial shielding is an easy way to control the plasma uniformity it also destroys the radial symmetry of the plasma line source. Keeping the radial symmetry would be essential if the plasma source were used for bi-directional coating of two substrates simultaneously, minimizing self coating of inner reactor walls and doubling the throughput, strongly resulting in reduced cost of ownership and system footprint. A helical transmission line can be designed so that the length of one turn equals one wavelength of the applied microwaves (operating in the bend fireQ mode, i.e. radiating primarily along the axis of symmetry). The electrical field of the microwaves across the diameter then rotates around the axis of symmetry. A substantial amount of the microwave power will experience circular polarization following the helicity of the helix. Initial test with
helical structures, however, revealed that some space is required between the outside of the helical turns and the dielectric interface tube or else the intended effect cannot be observed. Taking into account the helix diameter being determined by the wavelength of the microwaves this would have resulted in large dielectric interface tube diameters causing too much tensile force on the brittle tube walls. Eventually, the diameter of the helix could be reduced by filling the core of the helix with dielectric materials such as quartz or alumina. Also, the dielectric filling helps to concentrate more microwave power in the helical core and therefore strengthen the intended purpose of the helical line. Fig. 3 shows a helical line with an alumina rod in the core and a tapered coaxial-to-helical transition stage. By feeding microwave power to the coaxial line plasma source from both ends [4] devices of up to 3000 mm in length have been built [5]. Large area microwave plasma sources can be built by arranging arrays of parallel coaxial line plasma sources as shown in Fig. 4. T-junctions are ideal for microwave power splitting since they are quite easy to manufacture and are of compact built. But every T-junction reflects about 1/7 of the incident power. Unacceptable levels of reflected microwave power would have to be anticipated for cascaded Tjunctions in particular but the inherent reflection of a Tjunction can been overcome by adequate design changes employing finite element based computer modeling. Fig. 5 shows an 8-line plasma source array before and after a coating process and the EcoVeRaR coater. Here, the
Metal shields (tapered)
Uniform plasma Fig. 2. Coaxial line plasma source (side view) with tapered metal shields for improved plasma uniformity. The shield behaves like outer conductors of a coaxial transmission line.
M. Liehr, M. Dieguez-Campo / Surface & Coatings Technology 200 (2005) 21–25
23
Length: 400 mm (approx.), OD: 24 mm
Helical line (soft copper)
Dielectric core (alumina)
Dielectric tube (vacuum interface) Fig. 3. Helical line plasma source with alumina core designed for end fire mode. Propagating power shows circular polarization. Helical plasma source works best in wide dielectric interface tubes.
Cascaded coaxial power divider
Planar plasma source array
Rectangularto-coaxial transformer
Impedance matched rectangular waveguide
Substrate
Microwave power input Fig. 4. Planar plasma source array with four parallel coaxial line plasma sources operated with two power sources employing symmetrical, cascaded coaxial 1:4 power dividers.
Microwave generators and waveguide components Substrate in vertical carrier
Door with pumping port and substrate heating
Coaxial line plasma source array (16 lines for 8 microwave generators) Fig. 5. Eight coaxial line plasma sources with metal shields forming a planar array of 0.5 m2, mounted in a laboratory coater, seen from below (left) and above (middle). The new EcoVeRaR coating system uses 16 coaxial lines, 8 microwave generators and can coat two substrates (1 m2) simultaneously.
24
M. Liehr, M. Dieguez-Campo / Surface & Coatings Technology 200 (2005) 21–25
f = 100 Hz
f = 10 Hz
eventually leads to continuous plasma conversion rates with decreasing ripple as indicated in Fig. 6.
Plasma-light signal Microwave signal
f = 1.000 Hz
3. Results The coaxial line plasma source is generally used in a semi-remote process arrangement as shown in Fig. 2. The precursor gas is supplied to the vacuum process chamber somewhere in-between the coaxial line plasma source or plasma source array and the substrate and its flow is directed towards the substrate. Other gaseous components are supplied from beyond the coaxial line plasma source so that they have to pass through the plasma region where they experience the impact of energetic electron bombardment and UV radiation.
f = 10.000 Hz Plasma-light signal Microwave signal
Fig. 6. Plasma afterglow effect during pulsed power operation. Light emission from a SiH4/H2 plasma measured with a fast photodiode as a function of the pulse frequency f at 15% duty cycle.
3.1. Silicon thin films from silane-hydrogen plasmas double sided plasma source consists of an array of sixteen parallel coaxial plasma lines powered by 8 microwave generators each feeding into four coaxial lines by means of 1:4 coaxial power splitter in a double cascade arrangement. The microwave power from each generator is coupled into a standard rectangular R-26 waveguides transporting the microwaves through a circulator, a directional coupler and a tapered waveguide section to the waveguide-to-coaxial transformer. C/w power levels of up to 15 kW per generator have been successfully tested during the deposition of hard carbon coatings. Stub or EH tuners are not necessary as long as the waveguide transmission line impedances are matched. For the coating or treatment of temperature sensitive substrate materials like polymers low net power input is preferable. However, in order to sustain a flat but bclosedQ plasma cloud over the area of interest a certain minimum power is necessary or else the plasma becomes unstable and non-uniform. This low power threshold can be overcome by using pulsed power. By providing a surplus of power to the plasma source for a short time a stable and uniform discharge can be sustained. With increasing pulse frequency the plasma afterglow effect becomes more prominent and
Fig. 7 shows a glass substrate of roughly 1 0.5 m2, which has been uniformly coated by microcrystalline silicon for solar cell applications. The crystalline morphology can reliably be controlled by adjustment of the silane–hydrogen (SiH4/H2) ratio and the distance between precursor gas inlet and the substrate surface as could be verified by Raman spectroscopy, shown in Fig. 7. The precursor gas silane is supplied downstream and hydrogen above the plasma source. Microwave plasmas are of particular interest for the formation of microcrystalline silicon since the required silane dilution ratio has to be very high and substantial deposition rates require a high plasma density. 3.2. Silicon dioxide formation using organometal–oxygen plasmas
arbitrary units
Microwave PECVD can be used to its full potential when it comes to plasma polymerization processes. Pure ceramic and borganicQ silicon dioxide layers (several microns) for barrier or anti scratch applications by microwave PECVD from organic precursor material have been produced. The
300
400
500
600
700
raman shift (1/cm) Fig. 7. Uniform silicon film on float glass (left). Raman spectroscopy (right) shows different morphologies of the deposited silicon. Microcrystalline material shows a sharp peak at higher wave numbers.
M. Liehr, M. Dieguez-Campo / Surface & Coatings Technology 200 (2005) 21–25 Signal A = InLens
1µm
MAG = 12.00 K X EHT = 2.00 kV
25
100
Date :3 Mar 2004 Time :17:59
C 90
N O
Atomic concentration
80
oxygen
Si
70 60 50 40
silicon
30 20
carbon
10 0 0
1500
3000
4500
6000
7500
9000 10500 12000 13500 15000 16500 18000 19500 21000
Time of sputtering
Fig. 8. SEM micrograph of 2 Am thick, almost pure SiO2 film deposited by an HMDSO–oxygen plasma on float glass. XPS depth profile (right) of the same film.
precursor material hexamethyldisiloxane or HMDSO, with the chemical formula (CH3)3–Si–O–Si–(CH3)3, is supplied downstream and oxygen from the far side, very similar to the silane–hydrogen system. Fig. 8 shows a micrograph of a deposited 2 Am thick SiO2 film on glass and an XPS spectrum which has been taken from the same film. Surprisingly, the ratio of carbon in this film is extremely low. Of course, SiO2 films with a much higher organic content can be deposited by changing the HMDSO/oxygen ratio, gas flow rate, pressure and adjusting the microwave power level accordingly. Experimental results on the HMDSO/O2 system with a linear microwave plasma source of smaller size using Fourier transform infrared spectroscopy can be found under Ref. [6]. 3.3. Nanocrystalline diamond films from a methane–hydrogen plasma Transparent nanocrystalline diamond films of 1 Am thickness have been deposited at 450 8C on Pyrex and float glass 30 30 cm2 in size. XRD spectra have revealed grain sizes between 10 and 20 nm. A transmittance of 70% at 700 nm was observed [7].
4. Conclusion The coaxial line plasma source now appears to be the only industrially viable way of microwave PECVD for large area coatings in the range of a square meter and beyond after
having been constantly improved and matured over the recent years and with suitable microwave power generators commercially available now. It is the combination of such a high-energy plasma source and the flexibility of plasma chemistry involving organic materials that allows the production of materials so diverse by their nature like pure ceramic and organic (polymeric) thin films—by mainly changing gas flow ratios.
References [1] A.W. Rudge, K. Milne, A.D. Lover, P. Knight (Eds.), The Handbook of Antenna Design, vol. 2, Peter Peregrinus Ltd., 1983, p. 1, Chap. 9. [2] E. R7uchle, Lecture at Third International Workshop on Microwave Discharges: Fundamentals and Applications. Abbaye de Fontevraud, France, 20–25 April 1997. [3] H. Ranke, H. Bickmann, W. Kemmner, E. R7uchle, T. Magg, German patent publication No. DE 4136297 A1, 6 May 1993. [4] E. R7uchle, W. Petasch, F.K. R7uchle, H. Mqgge, German Patent DE 19503205 C1, 2 May 1995. [5] H. Muegge, Muegge Electronic GmbH, private communication. [6] M. Walker, K.-M. Baumg7rtner, J. Feichtinger, M. Kaiser, A. Schulz, E. R7uchle, Vacuum 57 (2000) 387. [7] Y. Koga, K. Tsugawa, M. Shelby, M. Liehr, Proceedings of the 14th European Conference on Diamond, Diamond-like Materials, Carbon Nanotubes, Nitrides and Silicon Carbides, Salzburg, Austria, 2003 (September 7–12). [8] Z. Zakrzewski, M. Moisan, Plasma Sources Sci. Technol. 4 (1995) 379. [9] A.J. Sangster, A.H.I. McCormick, IEE Proceedings, vol. 136, Pt. H, No. 1, 1989 (Feb). [10] W. Petasch, E. R7uchle, J. Weichart, H. Bickmann, Surf. Coat. Technol. 74–75 (1995) 200.