Funneled light pipe for pixel sensors

1. A semiconductor structure fabrication method, comprising:

providing a structure that includes (i) a semiconductor substrate, (ii) a photo collection region on the semiconductor substrate, and (iii) a BEOL (Back End Of Line) layer on the photo collection region and the semiconductor substrate;

etching the BEOL layer so as to form a funneled cavity in the BEOL layer, wherein a cross-section of the funneled cavity has a tapered shape;

after said etching the BEOL layer so as to form the funneled cavity in the BEOL layer is performed, further etching the BEOL layer through the funneled cavity so as to form a cylindrical cavity in the BEOL layer, wherein the cylindrical cavity are directly above the photo collection region and directly beneath the funneled cavity;

forming a funneled light pipe in the cylindrical cavity and the funneled cavity;

after said forming the funneled light pipe is performed, removing a top portion of the funneled light pipe from the funneled cavity;

after said removing the top portion is performed, forming a color filter region in the funneled cavity; and

forming a micro-lens on top of the color filter region,

wherein the color filter region comprises a transparent material whose refractive index is higher than a refractive index of a material of the BEOL layer,

wherein the BEOL layer comprises M interconnect layers, M being a positive integer, and

wherein the entire color filter region resides in a top interconnect layer of the M interconnect layers.

2. The method of claim 1 , further comprising, after said etching the BEOL layer through the funneled cavity is performed and before said forming the funneled light pipe in the cylindrical cavity and the funneled cavity is performed, forming a light reflective layer on side walls of the cylindrical cavity and the funneled cavity.

3. The method of claim 1 , wherein the light reflective layer comprises aluminum.