A method for forming photoresist patterns includes providing a substrate, forming a bi-layered photoresist on the substrate, and performing a photolithography process to pattern the bi-layered photoresist. The bi-layered photoresist includes a first photoresist layer and a second photoresist layer positioned between the first photoresist layer and the substrate. The first photoresist layer has a first refraction index and the second photoresist layer has a second refraction index, and the second refraction index is larger than the first refraction index.