Optimization of substrate reflectivity, resist thickness, and resist absorption for CD control and resolution

Abstract

Improvement of CD control can be achieved by reduction of substrate reflectivity effects. On highly reflective substrates such as metals, dyed resists are used most of the time. Especially for poly gate level patterning, the use of Bottom Anti Reflective Coatings has become common practice. While originally organic BARCs dominated, interest is gradually shifting towards inorganic BARCs of the SiOxNy type. Their highly conformal deposition flOW really allows for tuning towards zero reflectivity, even on substrates with topography. Furthermore, the use of inorganic BARC as a hard mask for etching allows for a thinner resist layer. This reduction of the resist thickness is advantageous for obtaining high resolution.It should be realised, however, that while resist thickness reduction improves resolution, it increases CD swing effects. Also, increased resist absorption reduces CD swing, but negatively influences resolution on substrates with a low reflectivity. Thus, while resist absorption, resist thickness and substrate reflectivity can be used as parameters to optimise process performance, optimum conditions for CD control and resolution are generally different.The subject of this paper is how to determine optimum values for resist absorption, resist thickness and substrate reflection. We quantify the effect of these parameters on both CD control and resolution. Furthermore, requirements for BARC parameter variations are discussed. Finally, practical boundary conditions on increasing resist absorption a:nd thickness for better CD control are determined.