Method for fabricating image sensor with uniform photosensitizing sensitivity

A method for fabricating an image sensor is provided. A substrate is provided, and then a plurality of photoresist patterns is formed on the substrate. The photoresist patterns are arranged in a first array and defined by a plurality of photomask patterns arranged as a photomask pattern array, wherein a top view of each photoresist pattern has a substantially square shape and a distance between two neighboring photoresist patterns decreases from a center of the first array toward an edge of the first array. Besides, each photomask pattern includes a transparent portion and an opaque portion, wherein an area proportion of the transparent portion included in a photomask pattern increases from the center toward the edge of the photomask pattern array. Then, a thermal reflow step is performed to convert the photoresist patterns into a plurality of microlenses arranged in a second array.