Effects of processing conditions on the growth of nanocrystalline diamond thin films: Real time spectroscopic ellipsometry studies

Real time spectroscopic ellipsometry has been applied to characterize the preparation of similar to 2000-Angstrom thick nanocrystalline diamond films on Si substrates by microwave plasma-enhanced chemical vapor deposition. Diamond films prepared under different conditions, including variations in the substrate temperature and CH4 + H-2 + O-2 gas mixture, have been studied. In addition to providing accurate Si substrate surface temperatures for diamond film growth via an ellipsometry-based calibration, the real time approach yields information on the following processes: (i) generation of initial substrate damage by the seeding process used to enhance diamond nucleation, (ii) annealing of the seeding damage that occurs upon heating the substrate to the deposition temperature, and (iii) the structural evolution of the diamond him throughout the nucleation and bulk film growth regimes. In the nucleation regime, the following information on the diamond fim can be extracted: (i) the diamond mass thickness evolution with time, (ii) the sp(2) C and void volume fractions in the nucleating layer, and (iii) the thickness at which nuclei make contact, the latter providing an estimate of the nucleation density. In the bulk layer growth regime, the time evolution of the following information can be extracted: (i) the bulk layer thickness, (ii) the surface roughness layer thickness, (iii) the mass thickness of sp(2) C in the bulk layer, and (iv) the void volume fraction in the bulk layer. In the nucleation regime, we find that high quality diamond nanocrystals form under a wide range of gas compositions for substrate temperatures above 700 degrees C. It is proposed that nucleation is controlled by a disordered carbon phase embedded in the substrate surface during the seeding process, in which the Si wafer is abraded with diamond powder. Above 700 degrees C, the disordered C at the substrate surface is believed to recrystallize in the diamond phase upon exposure to an initial H-2 plasma prior to diamond film growth. A common feature of diamond growth under all deposition conditions is the incorporation of a large concentration of sp(2) C when diamond nuclei coalesce (typically after a thickness of 200-400 Angstrom). The maximum concentration occurs under conditions for which poorer quality bulk films are obtained, including low substrate temperatures (< 700 degrees C) and high CH4 flows ([CH4]/{[CH4] + [H-2]} > 0.02). The sp(2) C is trapped within similar to 300 Angstrom of the substrate interface, but under optimum conditions of growth (i.e., the diamond growth region of the C-H-O gas phase diagram), little additional sp(2) C forms with continued growth after the first 300 Angstrom. The formation of sp(2) C in the coalescence stage has been attributed to shadowing effects that lead to a reduction of atomic H relative to C-containing precursors arriving at shadowed surfaces. For the nanocrystalline diamond films studied here, it is found that films having the lowest volume fraction of sp(2) C at the end of the 2000 Angstrom deposition also exhibit the lowest void volume fraction and the smoothest surfaces.