Influence of the Electrode Spacing on the Plasma Characteristics and Hydrogenated Amorphous Silicon Film Properties Grow

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Influence of the Electrode Spacing on the Plasma Characteristics and Hydrogenated Amorphous Silicon Film Properties Grown in the DC Saddle Field PECVD System Keith R. Leong, Nazir P. Kherani, and Stefan Zukotynski Department of Electrical and Computer Engineering University of Toronto, Toronto, Ontario, M5S 3G4, Canada ABSTRACT A new plasma deposition system was built with the capability of varying the electrode spacing in the DC Saddle Field plasma enhanced chemical vapor deposition system. An ion mass spectrometer was installed just below the substrate holder to sample the ion species travelling towards the substrate. Silane plasma and amorphous silicon film studies were conducted to shed light on the impinging ion species, ion energy distributions, and film properties with varying electrode spacing. The results indicate that decreasing the distance between the substrate and cathode leads to a reduction in the high energy ion bombardment. INTRODUCTION Hydrogenated amorphous silicon (a-Si:H) is a versatile optoelectronic material that is used in devices such as thin film transistors and solar cells. Hydrogenated amorphous silicon has been deposited by a wide range of methods, some of which include radio frequency (rf) Plasma Enhanced Chemical Vapor Deposition (PECVD) [1], Hot Wire CVD [2], microwave plasma [3], DC PECVD [1], and DC Saddle Field PECVD [4]. However, the majority of systems used to deposit low defect density, device quality, a-Si:H are capacitively coupled rf PECVD systems. Further, the optimization of the properties of a-Si:H films has largely proceeded by experimental trial and error. The inter electrode spacing has been found to be a critical parameter. Weakliem et al. [5] reported that increasing the electrode spacing increases the hydrogen content of the resulting film and the number density of ions consisting of four or five silicon atoms within the plasma. Ross and Jaklik [6] found that plasma polymerization increases with increasing inter electrode spacing. This was thought to be due to higher order radicals (SinHm) [7]. With decreasing electrode spacing the silane dissociation rate increases because of a higher power per silane molecule [8, 9]. This increased dissociation rate was also observed at different pressures [7]. The DC Saddle Field (DCSF) PECVD system utilizes a central semitransparent grid anode held at a positive potential. The anode is sandwiched between two semitransparent grid cathodes. The cathodes can be electrically biased, floating, or grounded. Beyond each cathode lies a grounded substrate holder. Once the plasma is ignited, electrons are forced by the electric field towards the anode. Due to the transparency of the anode, a fraction of the electrons will miss the anode and pass through to the other side. By symmetry this process continues and the electrons tend to oscillate about the anode. Thus, the electron path length is increased and its interaction with the precursor gas leads to effective excitation, ionization and dissociation. It has been shown that the plasma ign