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INSTITUTE OF PHYSICS PUBLISHING PLASMA SOURCES SCIENCE AND TECHNOLOGY Plasma Sources Sci Technol 13 2004 8 14 PII S0963 0252 04 67711 5 Plasma enhanced chemical vapour deposition of hydrogenated amorphous silicon at atmospheric pressure M Moravej1 S E Babayan2 G R Nowling1 X Yang1 and R F Hicks1 1 2 Chemical Engineering Department University of California Los Angeles CA 90095 USA Surfx Technologies LLC 10624 Rochester Ave Los Angeles CA 90024 USA E mail rhicks ucla edu Received 10 January 2003 in final form 13 August 2003 Published 11 November 2003 Online at stacks iop org PSST 13 8 DOI 10 1088 0963 0252 13 1 002 Abstract Amorphous hydrogenated silicon films were grown using an atmospheric pressure helium and hydrogen plasma with silane added downstream of the source A maximum deposition rate of 120 12 min 1 was recorded at a substrate temperature of 450 C 6 3 Torr H2 0 3 Torr SiH4 778 Torr He 32 8 W cm 3 and an electrode to substrate spacing of 6 0 mm The deposition rate increased rapidly with the silane and hydrogen partial pressures up to 0 1 and 7 0 Torr respectively then remained constant thereafter By contrast the deposition rate decreased exponentially as the electrode to substrate distance was increased from 5 0 to 10 5 mm The total hydrogen content of the films ranged from 2 5 to 8 0 1 0 at These results together with a model of the plasma chemistry indicate that H atoms and SiH3 radicals play an important role in the deposition process 1 Introduction Amorphous hydrogenated silicon is widely used in solar cells and thin film transistors TFT for flat panel displays 1 10 This material is normally grown on glass substrates at temperatures below 500 C by plasma enhanced chemical vapour deposition PECVD Radio frequency capacitive discharges are often used for this process although inductively coupled plasmas ICP electron cyclotron resonance ECR sources and helicon waves have been explored for this purpose as well 1 4 11 Capacitive discharges exhibit electron densities in the range 109 1011 cm 3 with average electron temperatures near 3 0 eV By contrast ICP ECR and helicon wave sources generate higher electron densities between 1010 and 1013 cm 3 but are more difficult to design and operate for large area PECVD applications 11 We have developed an atmospheric pressure plasma that exhibits physical and chemical characteristics similar to low pressure discharges 12 18 The plasma is generated by flowing helium and a reactive gas between two closely spaced metal electrodes in which one of the electrodes is 0963 0252 04 010008 07 30 00 2004 IOP Publishing Ltd connected to a radio frequency power source The discharge is capacitive and is sustained by bulk ionization of the gas suspended between the sheaths For a pure helium atmospheric pressure plasma we have measured an electron density of 3 0 1011 cm 3 an average electron temperature of 2 0 eV and a neutral temperature of 120 C 12 13 Recently it has been shown that this plasma source may be scaled up to uniformly treat large substrate areas 19 The high pressure operation combined with the reasonably high electron density suggests that this gas discharge may have certain advantages for materials processing In this paper we examine the deposition of amorphous hydrogenated silicon using an atmospheric pressure helium and hydrogen plasma with downstream addition of silane The effect of the process variables on the a Si H deposition rate and hydrogen content in the films has been determined In addition a numerical model of the plasma chemistry has been developed to identify which reactive species are likely to be involved in the deposition process A comparison of the model results with the experimental data indicates that ground state hydrogen atoms and SiH3 radicals are the most abundant reactive intermediates Printed in the UK 8 PECVD of hydrogenated amorphous silicon A schematic of the plasma source used in the PECVD experiments is shown in figure 1 The source consisted of two aluminium electrodes 33 mm in diameter separated by a gap 1 6 mm wide Both electrodes were perforated to allow helium and hydrogen to flow through them The upper electrode was connected to a radio frequency power supply 13 56 MHz while the lower electrode was grounded A third aluminium plate was installed beneath the lower electrode It contained an internal network of channels and holes that mixed silane with the plasma afterglow Located 5 0 10 5 mm further downstream was a rotating sample stage and heater both with adjustable heights The films were deposited by the following procedure a Corning 1737 glass substrate was rinsed with acetone and methanol and placed on the sample holder Then helium and hydrogen were fed to the plasma source at flow rates of 40 0 l min 1 and 0 0 920 cm3 min 1 respectively After a 10 min purge the sample was heated to a temperature between 100 C and 450 C and the discharge ignited with 45 W of RF power Then after one additional minute the PECVD reaction was started by feeding 5 0 SiH4 in He at 0 31 48 0 cm3 min 1 Growth was carried out for 10 min after which the silane flow was stopped and 1 min later the plasma was extinguished All the runs were performed at 32 8 W cm 3 45 W 778 Torr helium and a rotation rate of 200 rpm Periodically to check the reproducibility of the process the films were deposited at 300 C 6 3 Torr H2 0 3 Torr SiH4 and an electrode to substrate distance of 6 0 mm At these standard conditions the growth rate was 62 0 5 0 min 1 The thickness of the films was measured with a Tencor Alpha Step 200 A step was created by first protecting half of the film with a silicone sealant GE Translucent RTV 108 Then the unmasked region was etched in heated potassium hydroxide solution After etching the silicon sealant was removed by rinsing it with acetone and methylethylketone The uniformity across the deposited region in several films was found to be 3 2 of 1 and was determined by taking the standard deviation of 30 points across the substrate For each sample the thickness reported is an average of five points taken along the substrate To calculate the deposition rate the measured thickness was divided by the 10 min reaction SiH4 He H2 SiH4 RF Power Plasma Zone Ceramic Spacer Wafer Wafer Stage Heater time In separate experiments it was confirmed that the rate remained constant over a 0 30 min growth period The hydrogen composition in the films was analysed by infrared spectroscopy using a Bio Rad FTS 40A with a DTGS


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