正バイアス印加マイクロ波プラズマCVDによる高配向ダイヤモンド膜の形成に関する研究

摘要

Diamond films of good crystal quality have been expected in various phases ofapplications, especially for electron devices at high temperatures and at high electricfields. Single crystalline diamond films have been realized by homoepitaxial growth ondiamond substrates. Heteroepitaxial growth of diamond films have also been attemptedon various substrates, but the films are composed in general of diamond grains. SiC isone of the substrates appropriate for this purpose because of its similar chemicalaffinity with diamond. However, the growth of diamond films on Si substrates is highlydesired for the application to Si LSI, not only because of cost but also a possibility ofintegration into Si LSI. There is however a large lattice mismatch between diamond andsilicon and a large difference of the surface energy. So far, various methods have beenreported on the synthesis of diamond films on Si substrates, e.g., filament method, RFand microwave plasma CVD (MPCVD) methods. In general, heteroepitaxial growth ofdiamond films on Si forms polycrystalline diamond films composed of diamond grainswith different crystal orientations. B-doped diamond films have also been grown andthe reported Hall mobilities were by one order of magnitude lower than those obtainedfor homoepitaxial diamond films grown on natural diamond substrates. The lowmobility of heteroepitaxial diamond films may be ascribed to bad crystallinity of grains,small grain size and diverse grain orientation. Therefore, formation of highly orienteddiamond films of large grain sizes on Si substrates has been of great interests forelectron device applications.The BEN (Bias Enhanced Nucleation) method has been widely used to increase thenucleation sites (~1010 /cm2) on Si and other substrates in heteroepitaxial growth ofdiamond films by MPCVD. It is a pre-treatment of the surface of substrates under anegative bias at relatively high CH4 concentrations prior to diamond growth. After theBEN treatment, the growth of diamond films is in general carried out under no bias.In the process of MPCVD of diamond films, hydrogen atoms generated in largequantity in the plasma play an important role. They accelerate the gas phase reactionand promote the formation of radicals, and help stabilize the growing sp3-bondeddiamond surface. They also etch the non-diamond components such as graphite andamorphous carbon. Radicals such as methyl radicals (CH3*) also enhance the growth ofdiamond films. On the other hand, ions impinging on the substrates would causephysical etching of growing diamond grains and also form new nucleation sites ongrains, resulting in diversely oriented growth of diamond grains. Consequently, it isexpected that a reduction of ions impinging into the substrate as well as an increase ofradicals on the substrate surface would improve the grain size, the growth rate, thecrystal quality and the degree of the grain orientation. In the MPCVD diamond, it wasreported that the surface applied a positive bias was smooth, whereas the surfaceapplied a negative bias was rough probably due to ion damage.This study deals with the effects of a positive bias to the Si (100) substrates duringthe MPCVD diamond growth process on the growth and quality of diamond films. It isshown that the positive bias enhances the grain size, the growth rate and the (100)grain orientation. It is also shown that the positive bias increases the hole Hall mobilityof B-doped diamond films grown on Si substrates.