Absolute intensities of the vacuum ultraviolet spectra in oxide etch plasma processing discharges

Abstract

In this article, we report the absolute intensities of ultraviolet light between 4.9 and 24 eV (250 to 50 nm) striking a silicon wafer in a number of oxide etch processing discharges. Our emphasis is on photons with energies greater than 8.8 eV, which have enough energy to damage SiO2. These discharges were in an inductively driven Gaseous Electronics Conference (GEC) reference cell which had been modified to more closely resemble commercial etching tools. Comparisons of measurements made through a side port in the cell and through a hole in the wafer indicate that the vacuum ultraviolet (VUV) light in these discharges is strongly trapped. For the pure halocarbon gases examined in these experiments (C2F6, CHF3, C4F8), the fluxes of VUV photons to the wafer varied from 1 to 3×1015 photons/cm2 s or equivalently from 1.5 to 5 mW/cm2. These measurements imply that 0.1%–0.3% of the rf source power to these discharges ends up hitting the wafer as VUV photons for our typical 20 mT, 200 W rf discharges. For typical “ashing” discharges containing pure oxygen, the VUV intensities are slightly higher—about 8 mW/cm2. As argon or hydrogen diluents are added to the fluorocarbon gases, the VUV intensities increase dramatically, with a 10/10/10 mixture of Ar/C2F6/H2 yielding VUV fluxes on the wafer 26 mW/cm2 and pure argon discharges yielding 52 mW/cm2. Adding a rf bias to the wafer had only a small effect on the VUV observed through a side port of the GEC cell.