Ion implantation causes damage to photoresist and brings about the formation of a carbonized layer on the surface of the resist. The relationship between the extent of damage and the etch resistance of the resist was investigated. Four ion species, namely, Si, P, As, and BF2 were used. Heavier ions and higher implant energies generally increase the extent of damage and the density of the carbonized layer. Compared to a fresh resist, the formation of a dense carbonized layer increases the etch resistance whereas the formation of a porous carbonized layer decreases the etch resistance.