Image sensor and method for fabricating the image sensor

An image sensor, comprising:

• a substrate having a first surface and a second surface opposite to the first surface;
• a transfer gate structure disposed over the first surface of the substrate;
• a floating region in the substrate and adjacent to a first side of the transfer gate structure;
• a light sensing region in the substrate and adjacent to a second side of the transfer gate structure opposite to the first side of the transfer gate structure;
• an interlayer dielectric (ILD) layer disposed over the first surface of the substrate and covering the transfer gate structure and the light sensing region;
• a metallization stack disposed over the ILD layer;
• and
• a light-reflective layer disposed over the metallization stack, the metallization stack being vertically between the ILD layer and the light-reflective layer, wherein the light-reflective layer has a plurality of protrusions extending toward the substrate.

An image sensor, comprising:

• a semiconductor substrate having a first surface and a second surface opposite to the first surface, wherein the first surface of the semiconductor substrate has a plurality of recessed structures;
• a transfer gate structure disposed on the first surface of the semiconductor substrate;
• a light sensing region in the semiconductor substrate and adjacent to the transfer gate structure;
• a light-reflective metal layer over the first surface of the semiconductor substrate and covering the light sensing region, the light-reflective metal layer having a plurality of first protrusions matching shapes of the recessed structures;
• and
• a first dielectric layer over the second surface of the semiconductor substrate, the first dielectric layer having a plurality of second protrusions, wherein a width of the first protrusions of the light-reflective metal layer decreases toward a first direction, and a width of the second protrusions of the first dielectric layer decreases in a first direction toward a second direction opposite to the first direction.