Dual microwave-r.f. plasma deposition of functional coatings

Abstract

Plasma silicon nitride (P-SiN) and amorphous hydrogenated silicon (a-Si:H) films were deposited in a dual-frequency plasma, consisting of a microwave discharge with r.f. power simultaneously superimposed on the substrate holder. It is shown that the r.f.-induced negative substrate bias voltage V b substantially affects the deposition rate (between 15 and 30 Å s −1), the film composition and electrical properties. Ion fluxes bombarding the growing layers have been measured and found to be several times greater in the dual-frequency mode than in the “pure” r.f. mode. It is estimated that ionic species contribute about 30%–40% to the film growth rate. The increasing ion flux and energy with increasing V b enhance the formation of densely packed coatings which, in turn, reduces the dielectric loss tan δ of P-SiN, and the resistivity of a-Si:H by several orders of magnitude, when | V b| is raised from 0 to −800 V.