Back-side Illumination CMOS Image Sensor and Forming Method Thereof

A back-side illumination CMOS image sensor, comprising:

• a plurality of pixel units each comprising:
• a front-end structure, wherein the front-end structure comprises:
• a dielectric layer with a first and a second surfaces opposing to each other:
• a photodiode disposed on the first surface of the dielectric layer;
• a circuit layer bonded to the first surface of the dielectric layer;
• a deep trench isolation structure is patterned on the second surface of the dielectric layer defined by an opening of a mask layer;
• a first refraction layer disposed on the second surface of the dielectric layer including a bottom and side walls of the deep trench isolation structure;
• a reflection layer disposed directly on the first refraction layer at the bottom and side walls of the deep trench isolation structure;
• and
• a second refraction layer disposed on the second surface of the dielectric layer and filling the deep trench isolation structure;
• wherein a refractive index of the first refraction layer is smaller than a refractive index of the dielectric layer;
• and
• a pixel element bonded to the second surface of the dielectric layer.