Abstract: Disclosed is a process for planarization of semiconductor structures having dielectric isolation regions. Specifically, the process is directed to planarization of an organic polyimide layer obtained following filling of deep trenches in a semiconductor substrate having high and low density trench regions with this material. After over-filling the trenches with the polyimide and obtaining a non-planar polyimide layer having a thickness much larger in the low trench density regions than that in the high density regions, a photoresist layer is applied thereover. The photoresist is then controllably exposed using a mask which is the complement or inverse of the mask used for imaging the trench patterns to obtain a thick blockout photoresist mask over the trenches and a thin wetting layer of photoresist over the remainder of the substrate.