| Summary: | 碩士 === 大葉大學 === 電機工程學系碩士班 === 93 === The grain size in polycrystalline diamond films is determined by many factors including growth period, ratio of reacting gases, and nucleation mechanism.
In this study, using CH4 and H2 as working gas, microwave assisted CVD was utilized to deposit polycrystalline diamond films on p-type (111) Si substrates. In the deposition of polycrystalline diamond films, conventional process can be divided into 4 stages: etching, carburization, biased enhanced nucleation (BEN), and growth. In the BEN stage, Si substrate was negatively biased to attract positively charged hydrogen and carbon ions towards the substrate and subsequently bonded with each other to form silicon carbide, diamond nuclei, and carbonated layer. And the CH4/H2 ratio was intentionally raised to 1:40 so that the carbon concentration in the deposition chamber was accordingly increased. In this study, it was found that carburization step can be erased. The SiC interface layer required for nucleation of diamond can also be achieved in the subsequent BEN step with larger CH4 concentration instead. Our experimental results also showed that better diamond films can be achieved if the growth phase was divided into two steps: one step with lower pressure, followed by another step with higher pressure. One possible explanation is that mean free path at low pressure is large, electrons can thus obtain enough energy from electric field to ionize working gases so that there was plenty ionized species and radicals to accelerate the growth process.
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