Method for making multi-step photodiode junction structure for backside illuminated sensor

A backside illuminated image sensor comprising:

• a substrate having a front surface and back surface, wherein the back surface is a light receiving surface;
• an inter-level dielectric layer formed over the substrate;
• at least one metal layer formed in the inter-level dielectric layer;
• a polysilicon layer formed on the substrate;
• a first doped region formed within the substrate;
• and
• a second doped region formed within the substrate over the first doped region such that the first doped region is nearer to the light receiving surface of the substrate than the second doped region, wherein the second doped region is formed to have an overlap of only a portion of the polysilicon layer;
• wherein the second doped region extends from a top surface of the substrate into at least a portion of the first doped region such that a distance between the light receiving surface of the substrate and the first doped region is less than 50 percent of the thickness of the substrate;
• and
• wherein the first and second doped regions have a first type conductivity.

A backside illuminated image sensor comprising:

• a substrate having a front surface and a back surface, wherein the back surface is light receiving surface;
• an inter-layer dielectric layer formed over the substrate;
• at least one metal layer formed in the inter-level dielectric layer;
• a polysilicon layer formed on the substrate;
• a first doped region formed within the substrate and extending to a first depth such that a distance between the light receiving surface of the substrate and the first doped region is less than 50 percent of the thickness of the substrate;
• a second doped region formed within the substrate over and contiguous to the first doped region such that the first doped region is nearer to the light receiving surface of the substrate than the second doped region, wherein the second doped region is formed to have an overlap of only a portion of the polysilicon layer;
• a third doped region formed within the substrate and extending to a second depth;
• and
• a fourth doped region formed within the substrate and extending to a third depth;
• wherein the second, third, and fourth doped regions extend from a top surface of the substrate;
• and
• wherein the first depth is greater than the second depth and the second depth is greater than the third depth.