Role of Low-Energy Ion Impact in Plasma Deposition of Cubic Boron Nitride Films

Abstract

Cubic boron nitride (cBN) is potentially expected as the semiconductor for high temperature condition due to the large band gap, high breakdown field, and high resistance to oxidation. cBN films have been synthesized from vapor phase using a variety of physical vapor deposition such as sputtering and ion beam deposition rather than plasma-enhanced chemical vapor deposition. However, high-energy (>50 eV) ion bombardment for cBN formation results in low-quality cBN films and thus obstructs their electrical applications. In our previous studies, high-quality cBN films were deposited under low-energy ion bombardment (<40 eV) by plasma-enhanced chemical vapor deposition using the chemistry of fluorine. The mean ion energy for cBN formation was reduced greatly, then the resulting films consisted of micron-sized grains with crystallographic morphology. In this study, the deposition mechanism of our cBN films is examined in terms of ion energy, ion flux, and ion to depositing boron flux ratio onto the substrate. The ion energy and the ion to boron flux ratio are determined from the sheath potential and the ratio of incident ion flux to net deposited boron flux, respectively. For negative substrate biases where sp2-bonded BN phase only or no deposit is formed, both the ion energy and the ion to boron flux ratio are high. For positive substrate biases where cBN phase is formed, the ion energy and the ion to boron flux ratio are estimated in the range of a few eV to 35 eV and 100 to 130, respectively. The impact of negative ions is presumed to be negligible due to their low kinetic energy. The impact of positive ions with high ion to boron flux ratios is primarily responsible for reduction of the ion energy for cBN film deposition. [1] K. Teii and S. Matsumoto, Thin Solid Films 576, 50-54 (2015).