Precise control of critical dimension(CD) loss (defined as the length of the top of contact hole minus the bottom of resist in this paper) and etched profile of contact holes is a key technology in the fabrication of Ultra Large Scaled Integrated Circuit(ULSI). In case of fine contact hole etching, small CD loss and vertical profile is essential. We have found out that N2 addition to Ar/CHF3/CF4 sharpens etched profile with CD loss kept small. And N2 addition also increases etch rate without a heavy deterioration of selectivity of SiO2 versus heavily doped n-type poly cry stall ine Si(n+ poly Si). Mechanisms of changes in etching characteristics have been investigated and discussed with the emphasis on adlayer formed on etched surface.