a photo mask for use in forming a pattern , such as a photoresist pattern , is corrected to compensate for discrepancies in the transmissivity of the photo mask which results in the pattern having a distribution of critical dimensions that is too great or which deviates too much from the target critical dimension of the pattern . the photo mask includes a transparent substrate , a light - shielding layer pattern defining transmission sites on the transparent substrate , and at least some of which sites have a relatively low transmissivity . the method of correcting the photo mask includes doping a front surface of the transparent substrate of the photo mask with ions . a predetermined number of the sites can be doped to narrow the distribution of the critical dimensions of the pattern formed using the photo mask , or all of the transmission sites of the photo mask can be doped to make the average of the critical dimensions of the pattern closer to the target critical dimension of the pattern .