Depletion Fraction of Silane and Dominant Neutral Radical in RF Glow Discharge in Silane
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DEPLETION FRACTION OF SILANE AND DOMINANT NEUTRAL RADICAL IN RF GLOW DISCHARGE IN SILANE Quixun Lin, Xuanying Lin, Zeng Xu, Yunpen Yu Department of Physics, Shantou University, Guangdong, China 515063 E-mail:xylin@stumis%hkucnt.hku.hk Shaoqi Peng Department of Physics, Zhongshan University, Guangdong, China 510275 ABSTRACT A quadripole mass spectrometer was installed on the line of a PCVD system to measure the depletion fraction of silane and the monosilicon radicals in the discharges. The paper presents a method of measuring the depletion fraction of silane when setting the mass spectrometer at the high ionization voltage. The experimental results were compared and accordant with those reported before [1,2,3] I. INTRODUCTION Materials of hydrogenated amorphous silicon (a-Si:H) have been widely used in photovoltaic cells, thin film transistors, displays, televisions, electrophotography and various kinds of image sensors [4]. A high quality a-Si:H film has been prepared at high deposition rate up to 2.Onm/s by means of radio frequency (rf) glow discharge in pure silane [5], this gives a wide capability for reducing the production price of the a-Si:H devices. The film quality is dependent on the discharge technology conditions, which control the space procedures in the discharge chamber and the surface procedures on the substrate. Diagnosing the discharge parameters for understanding the deposition mechanism and searching for the optimum deposition conditions for improving the quality of a-Si:H film have become a concerned problem. Langmuir probe and floating double probes device have been used for measuring the electron density, electron energy distribution function and electron temperature, but the ionization degree of silane gas in the discharge at low pressure and low temperature is only in the range of 10-5 - 10-4 . A large part of silane gas is depleted on forming the monosilicon SiHn (n=0-3). It has become widely accepted that neutral radical species are responsible for most of the a-Si:H deposition from discharges. Therefore diagnosing monosilicon radicals is very important for understanding the deposition mechanism. Several groups have been studying the neutral radicals [6,7,8], most of them used a beam ionizer and mass spectrometer to diagnose the radicals in DC glow discharge to simulate the situations in rf glow discharge system. The paper reports the depletion fraction of silane gas and the species of neutral radicals in pure silane rf glow discharges at different technology conditions measured by a quadripole mass spectrometer, which was set up on the line of a plasma chemical vapor deposition (PCVD) system. II. EXPERIMENTAL APPARATUS
AND METHODS
The experimental apparatus is shown in Fig.1 schematically. The model of quadripole mass spectrometer is LZL-203. The mass spectrometer chamber is composed of three parts: ionization chamber, focussing and accelerating electron lenses and a quadripole mass analyzer. The operation pressure in mass spectrometer chamber is less than lxl0-2 Pa. The analyzed gas w
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