Abstract: A photolithographic method for manufacturing a CMOS semiconductor device having retrograde twin well with high aspect ratio (of thickness of the photoresist to width of the mask line or thickness of the resulting structure to its own width) is disclosed. The method is applicable for high energy implantation or other processes requiring thick photoresists with high aspect ratios, with one embodiment comprising the following: (a) forming a thick film of positive photoresist on a silicon substrate (b) prebaking (softbaking) the thick film using non-typical conditions (c) exposing the thick film using UV light (d) post exposure baking (PEB) the thick film using non-typical conditions (e) developing the exposed, thick film (f) stabilizing the thick film using non-typical conditions According to this method, a thick film of photoresist with high aspect ratios can be accurately and advantageously formed.