Provided is a method for fabricating an image sensor device that includes providing a substrate having a front side and a back side; patterning a photoresist on the front side of the substrate to define an opening having a first width, the photoresist having a first thickness correlated to the first width; performing an implantation process through the opening using an implantation energy correlated to the first thickness thereby forming a first doped isolation feature; forming a light sensing feature adjacent to the first doped isolation feature, the light sensing feature having a second width; and thinning the substrate from the back side so that the substrate has a second thickness that does not exceed twice a depth of the first doped isolation feature. A pixel size is substantially equal to the first and second widths.