An effective approach for reducing surface roughness of PMMA by thermal radiation induced local reflow

Roughness including surface roughness and line edge roughness (LER) of lithographically generated patterns in resists has been one of the most important issues in semiconductor industry because it limits the further reduction of the minimum feature size in lithography. Despite enormous amount of attempts have been made to reduce the roughness by changing developing condition and developers, ultrasonic agitation in the development, heat reflow, etc., the progress is still far away from satisfaction. In this paper, we report, for the first time, a novel and effective approach to reduce the surface roughness on PMMA surface by a process termed as thermal radiation induced local reflow (TRILR). The basic working principle of the process is first comprehensively described. PMMA surfaces with various scales of roughness are deliberately created by a greyscale electron beam lithography (EBL). Reductions of the roughness by the TRILR process are quantitatively characterized by an atomic force microscope (AFM). Optical spectra taken on the PMMA surface with different processing conditions also demonstrate the significant improvement of treated surfaces. It is believed that this novel method should be applicable to all the positive tone resists which can undergo a glass transition process at high temperatures.