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CATALYTIC OXIDATION OF SILICON NITRIDE THIN FILMS USING POTASSIUM* J. W. ROGERS, JR., D. S. BLAIR, and C. H. F. PEDEN Sandia National Laboratories, Albuquerque, NM 87185-5800

ABSTRACT Thin silicon nitride films on a Si(100) substrate have been oxidized using potassium in a low thermal budget process. The presence of potassium on the SisN4 surface greatly lowers the temperature-time requirements for oxidation as compared with direct thermal oxidation. Introduction Silicon nitride (SiaN4) thin films are used extensively in the semiconductor industry for diffusion masks, surface passivation layers, and masking layers for selective oxidation. Si3N4 is used in this broad range of applications because of its chemical stability and desirable electrical properties. However, SisN4 thin films are known to oxidize at elevated temperatures and surface oxidation has been exploited to produce silicon oxynitride whose electronic properties can be tailored by judicious control of the oxygen content of the film. These films show potential for fabrication of radiation hardened microelectronics. Unfortunately the high temperatures necessary to thermally oxidize the SisN4 will produce dopant diffusion and resulting degradation of device performance. One goal of an ongoing program in our laboratory is to find alternate low temperature synthetic routes to the formation of SiaN4 and silicon oxynitride thin films. Several recent studies suggest [1-3] that certain alkali metals (Cs, Na, and K) act as "catalysts" and enhance the oxygen uptake during direct oxidation of Si to Si02 . We have investigated the use of an alkali catalyst to oxidize thin films of SiSN4 on Si at low temperature. A comparison has been made between the low temperature catalytic oxidation of SisN4 and direct thermal oxidation at elevated temperatures (1125 to 1275 K). Experiment SiaN4 thin films were prepared by low-pressure, direct thermal nitridation of a Si(100) crystal at 1025 K using hydrazine as the nitridant gas [4]. This procedure yielded 5-25 A thick films of pure, stoichiometric SiaN4. Unlike silicon nitride prepared by chemical vapor deposition methods, these films were free from oxygen contaminants. Potassium (K), from a heated alkali metal source, was then deposited at 300 K in submonolayer to multilayer quantities onto the S 3N4 film for a predetermined period of time in an ambient of 1 x 10 Torr of 02. The substrate was then heated to 900 K for less than two minutes to facilitate removal of the potassium and oxidation of the SisN4. X-ray Photoelectron Spectroscopy (XPS) was used with standard operating conditions [4] to follow the extent of oxidation and the chemical state of surface intermediates after various treatments.

*This work, performed at Sandia National Laboratories, was supported U.S. Department of Energy under contract number DE-AC04-76DP00789.

Mat. Res. Soc. Symp. Proc. Vol. 131. '1989 Materials Research Society

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Results and Discussion Following deposition of K onto the Si1N4 surface, examination of the surface by XPS sugg