Silicon Nitride Coatings Formed using the Selective Area Laser Deposition (SALD) Technique
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ABSTRACT In this paper, the Selective Area Laser Deposition (SALD) technique was used to deposit silicon nitride material from the gas phase. Tetramethylsilane (TMS) and ammonia were chosen as precursors for silicon and nitrogen respectively. Effects of processing temperatures and gas ratios of TMS to total pressure (PTMS + PNH3) on the relative amounts of silicon nitride and the growth kinetics were studied. Further, surface morphology and electrical properties of the deposits were also examined. It is found that the as-deposited materials are mainly composed of amorphous phases and the heat-treated samples (at 1500'C for 8hrs) consist of c-Si 3N 4, c-SiC and n-SiC. The amount of cz-Si 3N 4 decreases with the increase of the TMS pressure. The volume growth rate of deposits also increases with the TMS pressure. The apparent activation energy for these processes is estimated as 1OOkJ/mol.
INTRODUCTION Si3N4 has been used in various structural fields due to its high mechanical strength, low thermal expansion coefficient, excellent thermal shock resistance, wear resistance and corrosion resistance [1]. The typical processing methods for silicon nitride structural components are powder metallurgy techniques such as hot pressing and sintering. Because silicon nitride is ceramic material, it is expensive and is also extremely difficult to form the complex shape either through sintering which depends on the die used, or hot-pressing and thereafter machining. Development of intelligent materials processing and manufacturing techniques, such as the
selective area laser deposition (SALD) [2,3], will make wider silicon nitride application possible. SALD is an approach to fabricate solid dense materials through gas phase deposition using a laser to locally heat the desired area. This makes possible to form three dimensional ceramic components with an arbitrary shape through a layer by layer sequence, to fabricate functionally graded materials which varies in chemical composition through changing precursors or adjusting pressure ratios of the precursors, and to directly embed in-situ sensors into the functional ceramic parts. Si3N,1 is also a critical film in semiconductor industry as an electrical insulation layer. Chemical vapor deposition (CVD) is the most popular approach to produce Si3N4 films or coatings. A wide variety of silicon containing precursors such as SiC14, SiH 2C12, Sil4, SiF 4 , etc., have been selected to deposit silicon nitride. Ammonia is almost the only choice for nitrogencontaining precursor. In this study, tetramethylsilane (TMS) is chosen as the silicon containing precursor. The reason is as follows. Firstly, TMS has been successfully used as a precursor for SiC vapor deposition and infiltration [4]. The SiC so-produced has been used as a macrocomponent to embed SiC-graphite thermal sensors with Si 3N 4 as an electrical insulator between the sensor and macro-components [6]. Thus, it would be desirable to deposit Si3N 4 also through the use of TMS so that contamination due to the change of gaseous prec
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