Role of N2 addition on CF4/O2 remote plasma chemical dry etching of polycrystalline silicon

Abstract

The remote plasma chemical dry etching of polycrystalline silicon was investigated using various CF4/O2/N2 gas compositions. The effects of O2 and N2 addition on the etch rate and surface chemistry were established. Admixing O2 to CF4 increases the gas phase fluorine density and increases the etch rate by roughly sevenfold to a maximum at an O2/CF4 ratio of 0.15. The addition of small amounts of N2 (N2/CF4=0.05) can again double this etch rate maximum. Strong changes in surface chemistry were also seen as a result of N2 addition to CF4/O2. Real-time ellipsometry and atomic force micro-roughness measurements reveal that nitrogen addition at low O2/CF4 ratios leads to the smoothing of surfaces, but to increased oxidation at high O2/CF4 ratios. Based on etch rate data and gas phase species analysis, we propose that NO plays an important role in the overall etching reaction. Variable tube lengths separated the reaction chamber from the discharge. These tubes were lined with either quartz or Teflon liners. In general, etch rates diminished with quartz tube length. At the longer transport tube lengths (e.g., 125 cm), using a Teflon lining material strongly increases the etch rate for pure CF4/O2 discharges as compared to the quartz. For discharges containing N2, the etch rate is more than doubled. This can be explained by the low recombination rate of atomic fluorine on Teflon and the subsequent high density of F atoms that reach the process chamber, even for long transport tube lengths. In situ ellipsometric measurements reveal postplasma surface modifications for certain etching chemistries. Comparisons of these results to x-ray photoemission measurements reveal a dependence of the stability of the postprocessing surface reaction layer on the etching conditions and hence the thickness and composition of the layer, i.e., whether the layer is comprised of volatile (SiFx-like) or involatile (SiOy-like) species. Thicker, more SiOy-like reaction layers create a barrier for the diffusion and subsequent desorption of the volatile products and a postplasma removal of a portion of the reaction layer is observed. Thinner, more SiFx-like layers leave a fluorine deficient surface in the postplasma stage which results in increased tendency to postplasma layer growth. The etching of silicon is not always limited by the arrival rate of atomic fluorine for our processing conditions.