Evaluation of resist-film property by scan and spin coating

It is vital to control Critical Dimension (CD) within a wafer and pattern profile in photolithography process. We have previously reported our evaluation results with chemically amplified resists that one of the causes of pattern profile fluctuation is a change in resist film composition before exposure such as non-uniform distributions of additives (photo-acid generator (PAG), quencher) concentration and casting solvent etc.; and thus resist film property control is essential to suppress these factors [1]. This is also true for thin resist film in finer line process and with top surface imaging. In the same paper [1], we have reported that a straightforward method to understand a change in film property is to check the amount of thickness loss by developing unexposed film after post apply bake (PAB). This method can be applied to thin film as well. We created unexposed films with the thickness range of 50-900nm by changing the total solid content (TSC) in resist and spin speed at resist coating. The amount of thickness loss significantly increased with sub-200nm thickness; the film property of which was quite different from that of 200nm-or-over thickness. Moreover, pattern line edge roughness (LER) as well as pattern surface roughness was prominent with 100nm-thick film even when we used a resist which has an ability to create good patterns on film with a thickness of 400nm. This is because the film quality diminishes in bulk below a certain thickness, while the property on the surface or interface layer predominates. Then we studied Scan coating to control thin film property. In Spin coating, chemical liquid dispensed on static wafer is spread by spinning the wafer and solvent is evaporated to form a film. On the other hand, in Scan coating, wafer remains static even after chemical liquid is dispensed and the wafer is dried under reduced pressure [2]-[7]; which means the thinner evaporation rate is slower than that in Spin coating and film property control may be easier. We, therefore, expect that Scan coating is a possible method to control CD and pattern profile. In this study, we compared the process performance and film property of KrF resist films by Scan and Spin coatings and examined the film composition control.