Hollow cathode sustained plasma microjets: Characterization and application to diamond deposition

Abstract

Extending the principle of operation of hollow cathode microdischarges to a tube geometry has allowed the formation of stable, high-pressure plasma microjets in a variety of gases including Ar, He, and ${\mathrm{H}}_2.$ Direct current discharges are ignited between stainless steel capillary tubes $\text{(d=178\hspace{0.17em}μ}\mathrm{m})$ which are operated as the cathode and a metal grid or plate that serves as the anode. Argon plasma microjets can be sustained in ambient air with plasma voltages as low as 260 V for cathode-anode gaps of 0.5 mm. At larger operating voltage, this gap can be extended up to several millimeters. Using a heated molybdenum substrate as the anode, plasma microjets in ${\mathrm{CH}}_4/{\mathrm{H}}_2$ mixtures have been used to deposit diamond crystals and polycrystalline films. Micro-Raman spectroscopy of these films shows mainly ${\mathrm{sp}}^3$ carbon content with slight shifting of the diamond peak due to internal stresses. Optical emission spectroscopy of the discharges used in the diamond growth experiments confirms the presence of atomic hydrogen and CH radicals.