Preparation of Thin Composite Coatings by Sol-Gel Techniques
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PREPARATION OF THIN COMPOSITE COATINGS BY SOL-GEL TECHNIQUES, J. Martinsen, R. A. Figat and M. W. Shafer, IBM T. J. Watson Research Center, New York, 10598
INTRODUCTION The formation and characterization of thin ceramic coatings is currently an active area in materials science. Specifically, there has been increased interest in these materials as dielectric and optical layers for advanced microelectronic circuitry [1].This is in addition to their continued use and evaluation as protective coatings in a variety of applications [2]. One method of forming these coatings is via the sol-gel technique. However, only a few papers have been published in this area [3-7]. Dense silicon dioxide, aluminium oxide and various glass compositions have been deposited in thin film form on various substrates. Further, silicon dioxide films, formed by the sol-gel method, have been nitrided in ammonia at high temperatures and partially converted to an oxynitride [8-11]. These films were shown to be highly non-homogeneous, with a predominance of nitrogen at the surface. Despite the desirability of dense uniform well bonded coatings of the nitrided ceramics, i.e. Si3N4 and the oxynitrides, no method yet exists for the formation of nitrides by the sol-gel method. This paper discusses some results of a completely different strategy for formation of silicon nitride films and the incorporation of nitrogen into ceramic coatings. The approach involves dispersing a solid particulate phase, i.e. Si3N41 in a sol and using a spinning technique to deposit the coatings. We report the preparation of a number of multi-component films on fused quartz and sapphire substrates, as well as a microstructural investigation of these films as a function of firing temperature. Experimental Methods The silica sols were prepared by combining (by volume) 39% tetraethoxysilane (TEOS), 55% ethanol, and 6% water, which corresponds to a 2:1 molar ratio of water to silicon. A small amount of nitric acid was added to catalyze the hydrolysis, with a resulting final pH of 2.1-2.8. The mixture was heated to reflux for 5 hours, cooled, and then allowed to age for one week. The alumina sols were prepared by mixing (by weight) 20% aluminum isopropoxide and 800/. water, which corresponds to a 45:1 molar ratio of water to aluminum. The reaction was catalyzed with a small amount of nitric acid (pH - 2.4), and then heated to 95°C for 4 hours. No aging was necessary for this material. The silica borate sol is prepared by first reacting a mixture of 40% of TEOS, 57% ethanol, and 3% water (catalyzed with nitric acid) at 80'C for 16 hours. To this is added enough boron isopropoxide so that the molar ratio of silicon to boron is 9:1. Enough water is also added to attain a water to Si + B molar ratio of 2:1. The mixture is then again heated at 900 for 2 hours, cooled, and is then ready for use.
Mat.
Res.
Soc.
symp.
Proc. Vol.
32 (1984)QElsevier Science Publishing Co.,
Inc.
146
Silicon nitride slurries are prepared by combining the appropriate amounts of a-silicon nitride (UB
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