Etching Properties of Hydrogenated Amorphous Silicon

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ETCHING PROPERTIES OF HYDROGENATED AMORPHOUS SILICON Y.S. Tsuo, Y. Xu, D.W. Baker, and S.K Deb Solar Energy Research Institute, Golden, Colorado 80401-3393 ABSTRACT We have studied wet-chemical and dry etching properties of doped and undoped hydrogenated amorphous silicon (a-Si:H) films with bonded hydrogen content varying from 0 to 20 at.%. Etching processes studied include (1) wet-chemical etching using solutions of KOH, isopropyl alcohol (IPA), and 1120, (2) hydrogen plasma etching, and (3) XeF 2 vapor etching. INTRODUCTION Etching is a material removal process that is the reverse of deposition. Etching is used extensively in semiconductor processing, especially in the fabrication of integrated circuits [1]. For thin-film hydrogenated amorphous silicon (a-Si:H) photovoltaic module fabrications, etching may be used in place of laser scribing. Etching is also used in fabricating a-Si:H thin-film transistor circuits. The ability to do selective etching is important in lithography operations to fabricate integrated circuits. In addition, because etching of weak bonds during chemical vapor deposition is important in achieving highquality a-Si:H, better understanding of the etching properties of a-Si:H may lead to better deposition techniques. Etching properties of a-Si-based alloys, such as a-Si:H, a-Sil xGex:H, and a-Sii.xCx:H, are more complicated than crystalline silicon. Factors that may affect the etching properties of a-Si alloys include alloy composition, hydrogen content and bonding configurations, optical band gap, and the Fermi level position, which depends on doping level and defect density. The microstructure of a-Si alloys, such as voids, columnar growth structure, microcrystallinity, etc., may also affect the etching property. Because a-Si alloys are usually deposited as thin films, we need also to consider the effects of etchants on the substrates. For our etching studies, a-Si:H films were typically 500-nm thick deposited on Coming code 7059 glass or n-type crystalline silicon substrates. The radio frequency (RF) diode glow discharge deposition conditions are 22-mW/cm 2 RF power density, 700-mtort Sill4 pressure, and 70-sccm SiH 4 flow rate. The substrate temperature is varied from room temperature to 500'C to obtain films of different hydrogen content. WET-CHEMICAL ETCHING For fast etch removal of a-Si alloy films, such as removing films deposited on the inside walls of a stainless steel deposition chamber, we use commercially available HNO 3/CH 3COOH/HF (3:1:2) etchants. For slow, more controlled etching of a-Si alloy films on glass substrates, we use a KOH-based solution. Haller et al. [2] studied the etch rates of undoped and phosphorous-doped a-Si:H deposited at 275 0 C and 125'C in KOHbased etchants. They found that the etch rate is faster for the 125°C films, which have higher hydrogen content than the 275 0 C films. The KOH-based etchants used by us are Mat. Res. Soc. Symp. Proc. Vol. 219. @1991 Materials Research Society

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mixtures of KOH, isopropyl alcohol (IPA), and H 2 0. An etchant w

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