A polymer for filling gaps in a semiconductor substrate and a composition using the polymer are provided. According to the composition, holes having a diameter of 100 nm or less and an aspect ratio (i.e. a ratio between the diameter and height of the holes) of 1 or higher in semiconductor substrates can be substantially completely filled by common spin coating without formation of defects, e.g., air voids, the film can be dissolved by an aqueous alkaline solution (i.e. a developing solution) until a desired thickness is reached, the film is highly resistant to isopropyl alcohol (IPA) and plasma etching after curing by baking, and residue can be rapidly removed from the inside of the holes by ashing.