Excimer Laser Photodissociation Studies of Disilane AT 193 nm
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EXCIMER LASER PHOTODISSOCIATION STUDIES OF DISILANE AT 193 NM J. M. JASINSKI*, J. 0. CHU-, AND M. H. BEGEMANN** *IBM RESEARCH DIVISION, T. J. WATSON RESEARCH CENTER, YORKTOWN HEIGHTS, NY, 10598 **DEPARTMENT OF CHEMISTRY, VASSAR COLLEGE, POUGHKEEPSIE, NY, 12601
ABSTRACT
Optical emission, laser spectroscopic and mass spectrometric techniques have been used to study the ArF excimer laser induced photochemistry of disilane at 193 nm.
Evidence is found
for the formation of a number of photofragments from single and multiphoton dissociation. Effects due to secondary photolysis are observed at high excimer laser repetition rates. INTRODUCTION Disilane, Si 2 H6 , is a convenient silicon source for photchemical CVD of silicon containing thin films because it is the simplest silicon hydride which can readily be excited by standard photolysis sources such as 185 nm Hg lamps and 193 nm ArF excimer lasers [1]. In order to understand the mechanism of thin film growth in photo-CVD processes it is necessary to understand the gas phase photochemistry of disilane. In particular it is important to understand what the primary photoproducts are and whether thay are kinetically stable enough to diffuse to the film growth surface. The only previous study of disilane photochemistry in this spectral region was performed at 147 nm using light from a Xe resonance lamp [2]. The available information on the electronic spectroscopy and possible photodissociation pathways for disilane has recently been reviewed [3]. In this paper we summarize the results of optical emission, laser spectroscopic and mass spectrometric studies of the 193 nm photodissociation of disilane. We report absolute primary quantum yields for disilane loss and for the formation of one stable product, silane. We present direct spectroscopic evidence for the formation of silicon atoms, silylene and silylidyne from both single and multiphoton dissociation processes. We present indirect evidence that the single photon photochemistry is dominated by the formation of silicon monoradical and/or closed shell unsaturated silicon species and we provide evidence that as the time between laser pulses is decreased relative to the average residence time of the gas in our photolysis cell secondary photolysis processes dramatically change the photochemistry.
EXPERIMENTAL All experiments were conducted in a stainless steel flow cell under conditions of constant total pressure and constant total gas flow. In optical experiments, typically 40 - 200 mTorr of disilane diluted in 2 - 10 Torr of a buffer gas such as helium at total total flow rates of 50 300 sccm was photolyzed by the unfocused output of an ArF excimer laser at repetition rates of 2 - 40 Hz. A generalized schematic of the experimental arrangement is shown in Figure 1. Ultraviolet and visible emission was monitored at right angles to the excimer laser beam through a suprasil window with a monochrometer and photomultiplier. Infrared emission was Mat. Res. Soc. Symp. Proc. Vol. 131. t1989 Materials Research Society
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monit
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