A photolithographic process for patterning a photoresist layer over a substrate. A positive photoresist layer is formed over the substrate. The positive photoresist layer contains a photoacid generator and a photobase generator. The positive photoresist layer is exposed to light through a photomask so that the photoacid generator in the photoresist layer is changed into photoacid and the photobase generator is changed to photobase. The photomask has a first pattern region and a second pattern region that correspond with a first region and a second region of the photoresist layer. The first pattern region has a duty ratio greater than the second pattern region so that the first region is exposed to a higher light intensity than the second region. Finally, the positive photoresist layer is developed. The reaction threshold for turning the photobase generator into photobase is adjusted according to the exposure strength between the first region and the second region. Therefore, the quantity of photobase generated in the first region is much greater than in the second region. Ultimately, the diffusion of photoacid within the first region is blocked without affecting the development of the positive photoresist layer in the second region.