performing a first lithography process using a first pattern of a first photomask to form a first photoresist pattern on a front side of a device substrate, wherein the first pattern comprises substantially parallel lines with a first pitch and arranged along a first direction;

performing a first implantation process using the first pattern as a mask to form first isolation regions in the device substrate;

after performing the first implantation process, performing a second lithography process using a second pattern of a second photomask to form a second photoresist pattern on the front side of the device substrate, the second pattern being shifted from the first pattern by a distance less than the first pitch and in the first direction;

performing a second implantation process using the second photoresist pattern as a mask to form second isolation regions in the device substrate and spaced apart from the first isolation regions;

performing a third lithography process using a third pattern of a third photomask to form a third photoresist pattern on the front side of the device substrate, the third pattern being rotated from the first pattern by about 90 degrees;

performing a third implantation process using the third photoresist pattern as a mask to form third isolation regions in the device substrate and crossing over the first and second isolation regions; and

forming pixels between the first and second isolation regions.