ROLE OF N-2 ADDITION ON CF4 O-2 REMOTE PLASMA CHEMICAL DRY-ETCHING OFPOLYCRYSTALLINE SILICON/

The remote plasma chemical dry etching of polycrystalline silicon was investigated using various CF4/O-2/N-2 gas compositions. The effects o f O-2 and N-2 addition on the etch rate and surface chemistry were est ablished. Admiring O-2 to CF4 increases the gas phase fluorine density and increases the etch rate by roughly sevenfold to a maximum at an O -2/CF4 ratio of 0.15. The addition of small amounts of N-2 (N-2/CF4=0. 05) can again double this etch rate maximum. Strong changes in surface chemistry were also seen as a result of N-2 addition to CF4/O-2. Real -time ellipsometry and atomic force micro-roughness measurements revea l that nitrogen addition at low O-2/CF4 ratios leads to the smoothing of surfaces, but to increased oxidation at high O-2/CF4 ratios. Based on etch rate data and gas phase species analysis, we propose that NO p lays 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 rate s diminished with quartz tube length. At the longer transport tube len gths (e.g., 125 cm), using a Teflon lining material strongly increases the etch rate for pure CF4/O-2 discharges as compared to the quartz. For discharges containing N-2, the etch rate is more than doubled. Thi s can be explained by the low recombination rate of atomic fluorine on Teflon and the subsequent high density of F atoms that reach the proc ess chamber, even for long transport tube lengths. In situ ellipsometr ic measurements reveal postplasma surface modifications for certain et ching chemistries. Comparisons of these results to x-ray photoemission measurements reveal a dependence of the stability of the postprocessi ng surface reaction layer on the etching conditions and hence the thic kness and composition of the layer, i.e., whether the layer is compris ed 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 remov al of a portion of the reaction layer is observed. Thinner, more Si-x- like layers leave a fluorine deficient surface in the postplasma stage which results in increased tendency to postplasma layer growth. The e tching of silicon is not always limited by the arrival rate of atomic fluorine for our processing conditions. (C) 1997 American Vacuum Socie ty.