Radical Detection in Deposition Plasmas by Threshold Ionization Mass Spectroscopy

Neutral radicals produced in SiH4 and H2-SiH4 RF discharges are detected by threshold ionization mass spectrometry. Relative radical densities are found similar in all discharges, showing a minor amount (5–9 %) of Si2H2 besides SiH3 and neglectable amounts of other radicals. Absolute radical densities are also determined by calibration of the mass spectrometer with the background gas signal. Introduction Hydrogenated amorphous (a-Si:H) and microcrystalline (μc-Si:H) silicon, and their carbon and germanium alloys, are used for most large-area semiconductor devices, such as liquid-crystal displays and photovoltaics [Solar Energy Mat., 2003]. These thin films are deposited from the hydrogenated molecular gases of Si, C and Ge and dopants P and B (e.g. SiH4). The chemical vapour deposition process uses DC, RF, VHF or microwave discharge, or a high-temperature “hot-wire” surface to dissociate the feed gas. In the deposition process, neutral radicals yield most film growth, but cations [Perrin et al., 1989] and very small Si particles [Childs et al., 2000] also contribute to the growth of the semiconductor film. Better understanding of the plasma enhanced chemical vapour deposition (PECVD) process has both theoretical and practical importance and motivates further studies of deposition plasmas. Here we report studies of neutral radical species at the substrate during RF-discharge deposition of a-Si:H from SiH4 and H2–SiH4. We use threshold ionization mass spectroscopy (TIMS) where radicals are selectively ionized by an electron beam with well-defined energy above the radical ionization threshold, but below the dissociative ionization threshold of the parent molecule. This method can identify minor amounts of all the SixHy radicals in the presence of their stable parent gases (SixH2x+2). It also measures radical densities at the substrate surface, where they contribute to film growth. In contrast, optical detection methods yield radical densities in the bulk plasma [Kae-Nune et al., 1995]. Previous TIMS radical measurements have been done in Ar-SiH4 and low pressure SiH4 DC discharges [Robertson and Gallagher, 1986], in an Ar-H2-SiH4 cascaded arc plasma source [Kessels et al., 2000 and 2001] and in low pressure SiH4 RF discharges [Kae-Nune et al., 1995], [Perrin et al., 1998]. In contrast, the present measurements are carried out in RF SiH4 and H2 diluted SiH4 mixtures at much higher pressures as they are used for producing many amorphous silicon devices today. Experimental setup The experimental apparatus is shown in figure 1. The flowing-gas RF plasma reactor is similar to industrial reactors used for production of high quality amorphous and microcrystalline silicon solar cells. The cylindrical 80 mm (dia.) x 20 mm (gap) discharge chamber is driven at 13.56 MHz. Neutral species and positive ions are sampled through an orifice in the grounded electrode (1 mm dia. with SiH4 and 0.4 mm with H2-SiH4). In the first differential chamber a pair of grids deflects positive ions escaping from the discharge, while allowing the neutral species to reach the ionizer. In the following differential pumping stage, the extracted species are ionized by a magnetically confined electron beam, and in a third differentially pumped chamber the ions are analyzed by a quadrupole mass spectrometer. The mass spectrometer can be operated at 2 MHz, to detect silane and disilane species, and 5 MHz to detect H and H2. WDS'06 Proceedings of Contributed Papers, Part II, 91–95, 2006. ISBN 80-86732-85-1 © MATFYZPRESS