Germanium-silicon light sensing apparatus II

A method for fabricating a photodetector, the method comprising:

• forming a recess in a first silicon substrate, wherein the recess comprises a plurality of silicon surfaces;
• forming a germanium absorption region in the recess, wherein one or more surfaces of the germanium absorption region are in direct contact with one or more of the plurality of silicon surfaces of the recess;
• forming a first interconnect layer over the first silicon substrate;
• forming circuitry on a second silicon substrate;
• forming a second interconnect layer over the second silicon substrate;
• bonding the first interconnect layer of the first silicon substrate with the second interconnect layer of the second silicon substrate, such that the circuitry is in between the germanium absorption region and the second silicon substrate;
• forming a filter layer and a lens layer;
• forming an antireflection coating layer between the lens layer and the first silicon substrate, wherein forming the antireflection coating layer comprises forming a silicon nitride layer or a silicon-high-k material layer between the lens layer and the first silicon substrate;
• and
• forming a spacer layer that separates the lens layer from the germanium absorption region by an effective focal length of the lens layer to allow focusing of an incident light to a diffraction-limited spot that is on an order of a square of an optical wavelength of the incident light.