A semiconductor structure with a stop layer for planarization process therein and a method for forming the same is disclosed. The method includes the steps of: forming a trench in a substrate and between active areas; filling the trench with isolation layer; doping the isolation layer with an element to form a doped isolation region; annealing the doped isolation region; and planarizing the annealed and doped isolation region and measuring a planarization depth thereof. The coefficients of thermal expansion (CTEs) of the stop layer, the dielectric layer, and the active area are different.