Atmospheric Pressure PECVD of Nitride for photovoltaic applications

B.Dresler1, J.Roch1, G. Mäder1, I. Dani, S. Kaskel1

1 Fraunhofer Institute for Material and Beam Technology, IWS, Dresden, Germany

1. Introduction 3. Results Amorphous hydrogenated silicon nitride (a-SiNx:H) With both silicon precursors typical deposition rates of thin films have been studied extensively because of their 0.08 - 0.22 nm*m/s (dynamic) were achieved. The refrac- electrical and optical properties that make them very tive index of the films can be adjusted between 1.9 and suitable as antireflection and passivation layer on c-Si 2.4 at a wavelength of 550 nm. solar cells. Usually, the silicon nitride layers are deposited The influence of different types of ammonia injection by plasma enhanced chemical vapor deposition (PECVD) into the plasma reactor was studied. A direct injection of [1, 2]. Also hot wire CVD [3] is a promising method for NH3 into the plasma source results in case of in a SiN deposition. However, these processes run at low low deposition rate and in films containing a low amount pressure, making it difficult to establish an in-line tech- of nitrogen. In case of TMS the direct ammonia injection nology [4]. With a deposition at atmospheric pressure the leads to the formation of layers. Injection establishment of a real in-line production technique would of ammonia into the downstream plasma zone (remote) be possible, especially when combined with similar proc- results in nitrogen-rich silicon nitride layers for both pre- ess technologies, for example plasma-chemical etching. cursors. Main parameter for increasing the quality of the films, 2. Experimental indicated by the index of refraction, is the ratio of the The deposition of silicon nitride using silane and precursor flow to the flow of ammonia downstream the tetramethylsilane (Si(CH3)4, TMS) as alternative silicon plasma source, because the remotely injected ammonia is precursor was studied. An atmospheric pressure micro- the main nitrogen supplier of the film. wave plasma source based on the concept of a cylindrical Elastic recoil detection (ERD) analysis shows, that the resonator with annular slots (CYRANNUS®) operating at films have an oxygen content less than 3 %. The content a frequency of 2.45 GHz was evaluated for the continuous of of the silane based films is with approx. 16 % deposition of silicon nitride on 6” silicon solar wafers. much higher than that of the reference. The con- Experiments for silicon nitride deposition were carried tent is insignificant (Fehler! Verweisquelle konnte nicht out varying the process parameters in a wide range (pre- gefunden werden.). cursor, composition and total flow of the plasma gas, sub- The results of the FTIR in-situ gas phase investigations strate temperature and velocity). Typical experimental will be presented at the conference in July 2009. parameters are summarized in Table 1. Fourier Trans- formed Infrared (FTIR) in-situ gas phase investigations Table 2 Composition of films deposited with AP- PECVD have been provided to detect reaction products of the compared to a conventionally deposited reference deposition process, to achieve a high precursor conversion Precursor Si O N C H rate and to get information on gas phase chemistry in- volved in the deposition process. [at %] [at %] [at %] [at %] [at %] SiH 38.5 2.28 42.6 0.12 16.5 Table 1 Typical parameters for silicon nitride deposition 4 with atmospheric pressure microwave PECVD TMS 35.9 2.77 48.1 0.54 12.7

Parameter Parameter range SiH4 (ref- 40.9 1.00 43.9 0.30 13.9 Total plasma gas flow 80 … 145 erence)

(N2 + Ar (+ NH3 / H2)) [slm] References Power input [kW] 6 … 10 [1] B. Swatowska, T. Stapinski, Vacuum 82 (2008) Substrate temperature [°C] 430 942–946 [2] A. El amrani, I. Menous, L. Mahiou, R. Tadjine, A. SiH4 flow [sccm] 6 … 20 Touati, A. Lefgoum, Renewable Energy 33 (2008) TMS flow [g/h] 1 … 10 2289–2293 [3] R.E.I. Schropp , R.H. Franken, H.D. Goldbach, Z.S. NH3 flow [sccm] 1 … 100 Houweling, H. Li, J.K. Rath, J.W.A. Schüttauf, R.L. Stolk, V. Verlaan, C.H.M. van der Werf, Thin Solid Films 516 (2008) 496–499