INSTITUTE OF PHYSICS PUBLISHING PLASMA SOURCES SCIENCE AND TECHNOLOGYPlasma Sources Sci. Technol. 13 (2004) 8–14 PII: S0963-0252(04)67711-5Plasma enhanced chemical vapourdeposition of hydrogenated amorphoussilicon at atmospheric pressureM Moravej1, S E Babayan2,GRNowling1,XYang1andR F Hicks11Chemical Engineering Department, University of California, Los Angeles, CA 90095, USA2Surfx Technologies LLC, 10624 Rochester Ave., Los Angeles, CA 90024, USAE-mail: [email protected] 10 January 2003, in final form 13 August 2003Published 11 November 2003Online atstacks.iop.org/PSST/13/8 (DOI: 10.1088/0963-0252/13/1/002)AbstractAmorphous hydrogenated silicon films were grown using an atmosphericpressure helium and hydrogen plasma with silane added downstream of thesource. A maximum deposition rate of 120 ± 12 Å min−1was recorded at asubstrate 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. Thedeposition rate increased rapidly with the silane and hydrogen partialpressures, up to 0.1 and 7.0 Torr, respectively, then remained constantthereafter. By contrast, the deposition rate decreased exponentially as theelectrode-to-substrate distance was increased from 5.0 to 10.5 mm. The totalhydrogen content of the films ranged from 2.5 to 8.0 ± 1.0 at%. Theseresults together with a model of the plasma chemistry indicate that H atomsand SiH3radicals play an important role in the deposition process.1. IntroductionAmorphous hydrogenated silicon is widely used in solarcells and thin film transistors (TFT) for flat panel displays[1–10]. This material is normally grown on glass substratesat temperatures below 500˚C by plasma enhanced chemicalvapour deposition (PECVD). Radio-frequency capacitivedischarges are often used for this process, although inductivelycoupled plasmas (ICP), electron cyclotron resonance (ECR)sources, and helicon waves have been explored for thispurpose as well [1–4, 11]. Capacitive discharges exhibitelectron densities in the range 109–1011cm−3with averageelectron temperatures near 3.0 eV. By contrast, ICP, ECR,and helicon wave sources generate higher electron densities,between 1010and 1013cm−3, but are more difficult to designand operate for large-area PECVD applications [11].We have developed an atmospheric pressure plasmathat exhibits physical and chemical characteristics similar tolow-pressure discharges [12–18]. The plasma is generatedby flowing helium and a reactive gas between two closelyspaced metal electrodes, in which one of the electrodes isconnected to a radio frequency power source. The dischargeis capacitive and is sustained by bulk ionization of the gassuspended between thesheaths. For a pure helium atmosphericpressure plasma, we have measured an electron density of3.0 × 1011cm−3, an average electron temperature of 2.0 eV,and a neutral temperature of 120˚C [12, 13]. Recently, ithas been shown that this plasma source may be scaled up touniformly treat large substrate areas [19]. The high-pressureoperation combined with the reasonably high electron densitysuggests that this gas discharge may have certain advantagesfor materials processing.In this paper, we examine the deposition of amorphoushydrogenated silicon using an atmospheric pressure heliumand hydrogen plasma with downstream addition of silane. Theeffectoftheprocessvariablesonthea-Si : H deposition rate andhydrogen content in the films has been determined. Inaddition,a numerical model of the plasma chemistry has been developedto identify which reactive species are likely to be involved inthe deposition process. A comparison of the model results withthe experimental data indicates that ground-state hydrogenatoms and SiH3radicals are the most abundant reactiveintermediates.0963-0252/04/010008+07$30.00 © 2004 IOP Publishing Ltd Printed in the UK 8PECVD of hydrogenated amorphous silicon2. Experimental methodsA schematic of the plasma source used in the PECVDexperiments is shown in figure 1. The source consisted of twoaluminium electrodes, 33 mm in diameter, separated by a gap1.6 mm wide. Both electrodes were perforated to allow heliumand hydrogen to flow through them. The upper electrode wasconnected to a radio frequency power supply (13.56 MHz),while the lower electrode was grounded. A third aluminiumplate was installed beneath the lower electrode. It containedan internal network of channels and holes that mixed silanewith the plasma afterglow. Located 5.0–10.5 mm furtherdownstream was a rotating sample stage and heater, both withadjustable heights.The films were deposited by the following procedure: aCorning 1737 glass substrate was rinsed with acetone andmethanol, and placed on the sample holder. Then, heliumand hydrogen were fed to the plasma source at flow ratesof 40.0 l min−1and 0.0–920 cm3min−1, respectively. Aftera 10 min purge, the sample was heated to a temperaturebetween 100˚C and 450˚C, and the discharge ignited with45 W of RF power. Then, after one additional minute, thePECVD reaction was started by feeding 5.0% SiH4in He at0.31–48.0 cm3min−1. Growthwas carried out for 10 min, afterwhich the silane flow was stopped, and 1 min later the plasmawas 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, thefilms were deposited at 300˚C, 6.3 Torr H2, 0.3 Torr SiH4, andan electrode-to-substrate distance of 6.0 mm. At these standardconditions, the growth rate was 62.0 ± 5.0 Å min−1.The thickness of the films was measured with a TencorAlpha-Step 200. A step was created by first protecting halfof the film with a silicone sealant (GE Translucent RTV 108).Then, the unmasked region was etched in heated potassiumhydroxide solution. After etching, the silicon sealant wasremoved by rinsing it with acetone and methylethylketone.The uniformity across the deposited region in several filmswas found to be 3.2% of 1σ , and was determined by takingthe standard deviation of 30 points across the substrate. Foreach sample, the thickness reported is an average of five pointstaken along the substrate. To calculate the deposition rate,the measured thickness was divided by the 10 min reactionCeramicSpacerRFPowerWaferStageWaferHe/H2SiH4SiH4HeaterExhaustPlasmaZoneFigure 1. Schematic of the atmospheric pressure PECVD chamber.time. In separate experiments, it was confirmed that the rateremained