A pixel of a complementary metal-oxide-semiconductor (CMOS) image sensor includes a semiconductor substrate having a first surface and a third surface formed by removing part of the semiconductor substrate from a second surface, an active region which is formed between the first surface and the third surface and which contains a photoelectric conversion element generating charges in response to light incident on the substrate at the third surface, and a trench-type isolation region formed from either of the first and third surfaces to isolate the active region from an adjacent active region. The trench-type isolation region is filled with first material in a process that leaves a void in the material, the void is filled or partially filled with second material, and then a layer of third material is formed over the resulting structure composed of the first and second materials.