Properties of C4F8 inductively coupled plasmas. I. Studies of Ar/c-C4F8 magnetically confined plasmas for etching of SiO2

Abstract

Gas mixtures containing c-C4F8/Ar are commonly used for the plasma etching of dielectric materials such as SiO2. To quantify the dependence of fundamental plasma parameters of systems using these mixtures, inductively coupled plasmas in the pressure range of 6–20 mTorr, with and without magnetic confinement, were investigated. Measurements were also made in pure Ar and O2 to provide a comparison baseline. We found that use of magnetic confinement strongly influences the total ion flux, ion composition, and ion energy distributions in these discharges. Magnetic confinement increases the ion saturation current density, as measured with a Langmuir probe, most effectively at the lowest pressure examined here (6 mTorr). The increase in current density generally decreases as the pressure increases. Mass spectroscopic measurements of the ion flux showed that the CF+ ion flux dominates in C4F8 discharges, both with and without magnetic confinement. When Ar is added to C4F8 discharges with magnetic confinement, the CF2+ and CF3+ ion fluxes increase, and CF3+ becomes the dominant fluorocarbon ion. The ion energy distributions with and without magnetic field indicate that the plasma potential is generally lower for the magnetically confined discharges, although these trends are not monotonic when diluting C4F8 with Ar. Etching rates of SiO2, Si, and photoresist are higher with magnetic confinement, while etching yields are lower. The chemical compositions of passively deposited fluorocarbon films on Si, as measured by x-ray photoemission spectroscopy, differ little with and without magnetic confinement for varying C4F8/Ar gas compositions.