Surface Chemistry of Fluorine-Containing Molecules Related to CVD Process on Silicon Nitride: SiF 4 , XeF 2 , and HF
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SURFACE CHEMISTRY OF FLUORINE-CONTAINING MOLECULES RELATED TO CVD PROCESS ON SILICON NITRIDE: SiF4, XeF2, AND HF* DUANE A. OUTKA Sandia National Laboratories, Livermore, CA 94551
ABSTRACT The reactivity of several fluorine-containing molecules on a polycrystalline silicon nitride (Si 3 N 4 ) surface is studied under ultrahigh vacuum (UHV) conditions using temperature programmed desorption (TPD) and Auger electron spectroscopy (AES). The chemistry of fluorine on Si 3 N4 is of interest in understanding the high temperature chemical vapor deposition (CVD) of Si 3 N4 , which uses SiF 4 as a starting material. XeF 2 is reacted with a Si 3N4 surface to prepare and characterize various surface SiFx (1 < x < 3) species. These are identified by the chemical shift induced by the fluorine atoms in the Si (LMM) Auger peak and by changes in the TPD. Of these species, SiF 2 is stable to the highest temperature. SiF 2 is also formed by the reaction of SiF 4 with a Si 3 N4 . Because SiF 2 is so stable, its decomposition is proposed as a rate-determining step in the CVD deposition of Si 3N4 from SiF 4 . Gaseous HF, which is a product of the CVD process, does not dissociate on Si 3 N4 and is therefore unlikely to cause the etch-like marks on the Si 3N4 coating that are observed under certain conditions. INTRODUCTION Silicon nitride (Si 3 N4 ) is being developed as a high-temperature oxidation barrier for lightweight, carbon-composite parts in turbine engines [1]. Currently, such parts can be coated in a small-scale CVD reactor from starting materials of silicon tetrafluoride (SiF 4 ) and ammonia (NH 3). The CVD reaction is conducted at 1700 K and the overall reaction is: 3SiF4 (g) + 4NH3 (g) ---> Si 3 N4 (s) + 12HF(g).
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The silicon-fluorine chemistry is of particular interest in this CVD process because SiF4 is a starting material and the Si-F bond is the strongest bond that must be broken, up to 167 kcal mol-1 [2]. This study examines the reactivity of a variety of fluorine-containing molecules on the Si 3N4 surface in order to gain insight into this CVD process. No previous UHV surface studies involving bulk Si 3 N4 have been reported. EXPERIMENTAL The sample was a polycrystalline P3-Si 3N 4 obtained from Allied Signal Aerospace company. It was 1.3 cm in diameter and 1.5 mm thick. Groves (approx. 0.38 mm wide) were cut into the edge of the sample with a diamond saw in order to hold the sample and thermocouple. For heating, a 0.3-mm tungsten wire was formed into a flat zig-zag filament and placed against the back of the sample. This was stiff enough to hold the sample when the ends of the filament were bent to lie in the groves cut into the sample. For temperature measurement, a chromel-alumel thermocouple (0.125 mm diameter wire) was spot welded to a 0.125 mm thick Ta foil that was folded and pressed into the grove cut into the sample. The Si 3 N4 sample was cleaned by bombardment with 5 keV argon ions. Then the sample was heated to 1300 K or higher. Si 3 N4 begins to thermally decompose at these temperatures, and heating pr
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