Atom- and Radical-Surface Sticking Coefficients Measured Using Resonance Enhanced Multiphoton Ionization (REMPI)
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ATOM- AND RADICAL-SURFACE STICKING COEFFICIENTS MEASURED USING RESONANCE ENHANCED MULTIPHOTON IONIZATION (REMPI) ROBERT M. ROBERTSON* AND MICHEL J. ROSSI** *Present Address: Applied Materials Corporation, Santa Clara CA 95054 **Department of Chemical Kinetics, SRI International, Menlo Park CA 94025
ABSTRACT Sticking coefficients - of neutral transient species at ambient temperature were measured using in situ Resonance Enhanced Multiphoton I for I and CF3 It Ionization (REMPI) of the transients in a Knudsen cell. on a stainless steel surface were 0.16 and >0.5, respectively, whereas for CF 3 on the same surface was measured to 0.5. INTRODUCTION Due to the ever shrinking dimensions of thin film devices and VLSI chips, it is essential to control the different processes involved in VLSI chip production. In order to achieve control over these processes, fundamental understanding in terms of elementary reactions occurring in the gas phase and on the solid substrate is required. Our approach involves selective generation of one and only one atomic or molecular species using photolytic methods and to investigate its interaction in situ with a given surface under experimental conditions that come as close as possible to process conditions. In this paper we present results on the sticking coefficients of several atomic and molecular species on surfaces of practical interest. The use of the term "sticking" is ambiguous in the case of wall deactivation of vibrationally excited species, because the process may not correspond to mass accomodation but rather to energy transfer or energy accomodation. We use this term only to describe the disappearance of the detected species due to heterogeneous interaction with the surface of interest.
EXPERIMENTAL The experimental apparatus has been described in detail elsewhere [1], so that only a brief description will be given here. The Very Low Pressure Photolysis (VLP$) reactor is a Knudsen cell, which is part of a flowing gas experiment. The pressure inside the VLP4Z reactor is in the mTorr range (give or take a decade), so that the molecules undergo predominantly gaswall collisions. The molecules of interest are generated from appropriate precursors either via IR-MPD using a pulsed CO2 TEA laser or via single photon UV photolysis using a pulsed excimer laser. The measurement principle rests on the fact that one compares the rate of the process of interest with the rate of a "reference" process. By calibrating this "reference" process for the atom or free radical of interest, one can then put this relative comparison of rates on an absolute basis. The Knudsen cell used in our investigations uses the gas-wall collision rate as a reference process for decay processes on the tens of ps time scale, whereas slow decay processes are measured against the effusion of the species out of the Knudsen cell. Consequently, fast transient decay
Mat. Res. Soc. Symp. Proc. Vol. 131. C1989 Materials Research Society
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processes are monitored in situ using time dependent resonance enhanced multipho
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