Optical and mass spectrometric measurements of dissociation in low frequency, high density, remote source O2/Ar and NF3/Ar plasmas

Abstract

Remote plasma sources are widely used in applications such as chamber cleaning and flowable chemical vapor deposition. In these processes, it is desirable that the dissociation rate of feed gases be as high as possible and stable. Here, the authors present results on radical densities and gas dissociation fractions for a 400 kHz toroidal transformer-coupled plasma source (MKS Instruments), operating at a power density of 5–50 W/cm3 with feed gas mixtures of O2 or NF3 in Ar and pressures of 0.4 or 2.0 Torr. Radical densities and feed gas dissociation percentages in the plasma were measured by optical emission spectroscopy combined with Ar actinometry. In the plasma, O2 was about 60% dissociated in dilute O2 mixtures (10%–20%). Dissociation decreased with the increasing addition of O2, dropping to 10% dissociation for 90% O2 in the feed gas. NF3 was >95% dissociated for all NF3/Ar mixtures. Little or no dependence on the flow rate was found. Plasma products flow into an anodized Al downstream chamber that is probed by vacuum ultraviolet (VUV) absorption spectroscopy and line-of-sight molecular beam mass spectrometry. In the downstream chamber, O recombined on the walls to form O2 (detected by VUV O2 absorption). The measured downstream O/O2 ratio was a strongly increasing function of an increasing flow rate reproduced by a downstream global model with O wall recombination probability of γO between 0.001 and 0.002. NF3 does not reform in the downstream chamber, as verified by VUV absorption and mass spectrometry. No NF or NF2 was detected, and F mostly recombined to form F2 at the back of the downstream chamber, along with N2. The F2, F, and N2 product absolute number densities were consistent with the 3:1 F:N mass balance of the NF3 feed gas.