Trench isolation structure for image sensors

Abstract

Various embodiments of the present disclosure are directed towards an image sensor, and a method for forming the image sensor, in which an inter-pixel trench isolation structure is defined by a low-transmission layer. In some embodiments, the image sensor comprises an array of pixels and the inter-pixel trench isolation structure. The array of pixels is on a substrate, and the pixels of the array comprise individual photodetectors in the substrate. The inter-pixel trench isolation structure is in the substrate. Further, the inter-pixel trench isolation structure extends along boundaries of the pixels, and individually surrounds the photodetectors, to separate the photodetectors from each other. The inter-pixel trench isolation structure is defined by a low-transmission layer with low transmission for incident radiation, such that the inter-pixel trench isolation structure has low transmission for incident radiation. The low-transmission layer may, for example, be or comprise metal and/or some other suitable material(s).

1 - 20 . (canceled) 
     
     
         21 . An image sensor, comprising:
 a pixel comprising a photodetector in a substrate;   an outer trench isolation structure extending into the substrate, wherein the outer trench isolation structure has a pair of outer isolation segments respectively on opposite sides of the photodetector at a boundary of the pixel; and   an inner trench isolation structure comprising a pair of inner isolation segments respectively on the opposite sides of the photodetector, wherein the inner trench isolation structure is between the outer isolation segments;   wherein the outer trench isolation structure comprises a low-transmission layer configured to block incident radiation regardless of incident angle, wherein the inner trench isolation structure is dielectric, and wherein a top surface of the low-transmission layer is elevated relative to a top surface of the substrate.   
     
     
         22 . The image sensor according to  claim 21 , further comprising:
 a dielectric layer covering the photodetector and fully defining the inner trench isolation structure, wherein the dielectric layer has a top surface level with the top surface of the low-transmission layer.   
     
     
         23 . The image sensor according to  claim 22 , wherein the top surface of the substrate has a saw-toothed profile overlying the photodetector, and wherein the dielectric layer conforms to the saw-toothed profile. 
     
     
         24 . The image sensor according to  claim 22 , wherein the top surface of the dielectric layer is continuous from a first one of the outer isolation segments to a second one of the outer isolation segments. 
     
     
         25 . The image sensor according to  claim 21 , wherein the outer trench isolation structure comprises a dielectric liner layer separating the low-transmission layer from the substrate, and wherein the dielectric liner layer has a bottom surface underlying the low-transmission layer and level with a bottom surface of the substrate. 
     
     
         26 . The image sensor according to  claim 21 , wherein a height of the outer trench isolation structure is greater than a height of the substrate. 
     
     
         27 . The image sensor according to  claim 21 , wherein the substrate comprises a region between and directly contacting the outer trench isolation structure and the inner trench isolation structure, and wherein the region has a symmetrical profile and decreases in width continuously from a bottom surface of the substrate to the top surface of the substrate. 
     
     
         28 . An image sensor, comprising:
 a pixel comprising a photodetector in a substrate;   an outer trench isolation structure extending into the substrate, wherein the outer trench isolation structure has a pair of outer isolation segments respectively on opposite sides of the photodetector at a boundary of the pixel; and   an inner trench isolation structure comprising a pair of inner isolation segments respectively on the opposite sides of the photodetector, wherein the inner trench isolation structure is between the outer isolation segments;   wherein the outer trench isolation structure comprises metal, wherein the inner trench isolation structure is dielectric and extends completely through the substrate, and wherein the outer and inner trench isolation structures have different heights.   
     
     
         29 . The image sensor according to  claim 28 , wherein the outer trench isolation structure has a greater height than the inner trench isolation structure. 
     
     
         30 . The image sensor according to  claim 28 , wherein a width of the inner trench isolation structure decreases continuously from a top surface of the inner trench isolation structure to a bottom surface of the inner trench isolation structure. 
     
     
         31 . The image sensor according to  claim 28 , further comprising:
 an interconnect structure underlying the substrate and comprising a plurality of wires and a plurality of vias that are alternatingly stacked, wherein the plurality of vias comprises a first via extending to the pixel, and wherein an interface between the first via and the pixel is level with a bottom surface of the inner trench isolation structure.   
     
     
         32 . The image sensor according to  claim 28 , wherein the photodetector comprises a first contact region, a pair of second contact regions, and a guard ring in the substrate, wherein the first contact region has a first doping type, wherein the pair of second contact regions have a second doping type opposite the first doping type and are respectively on opposite sides of the first contact region, and wherein the guard ring separates the first contact region from the second contact regions. 
     
     
         33 . The image sensor according to  claim 32 , further comprising:
 a periodic structure overlying the photodetector and extending into a top surface of the substrate, wherein the periodic structure has a width beginning at a first location and ending at a second location, and wherein the first and second locations respectively and directly overlie the second contact regions.   
     
     
         34 . The image sensor according to  claim 28 , wherein the outer trench isolation structure comprises a metal core and a dielectric liner layer separating the metal core from the substrate, and wherein a thickness of the dielectric liner layer is 10-100 nanometers. 
     
     
         35 . An image sensor, comprising:
 a pixel comprising a photodetector in a substrate; and   a trench isolation structure extending into the substrate, wherein the trench isolation structure has a pair of isolation segments respectively on opposite sides of the photodetector at a boundary of the pixel;   wherein the trench isolation structure comprises a metal layer with a reflectance greater than about 80%.   
     
     
         36 . The image sensor according to  claim 35 , wherein the metal layer comprises silver. 
     
     
         37 . The image sensor according to  claim 35 , wherein the metal layer comprises copper, and wherein the trench isolation structure comprises an aluminum oxide layer extending along a sidewall of the metal layer and a bottom surface of the metal layer. 
     
     
         38 . The image sensor according to  claim 35 , wherein the photodetector is a single-photon avalanche diode. 
     
     
         39 . The image sensor according to  claim 35 , further comprising:
 an additional trench isolation structure extending into the substrate and separating the trench isolation structure from the photodetector, wherein the metal layer and the additional trench isolation structure respectively have a metal sidewall and a dielectric sidewall facing each other, and wherein a bottom edge of the metal sidewall is elevated relative to a bottom edge of the dielectric sidewall, and wherein a top edge of the metal sidewall is elevated relative to a top edge of the dielectric sidewall.   
     
     
         40 . The image sensor according to  claim 35 , further comprising:
 a dielectric layer spaced from the trench isolation structure and comprising a pair of dielectric segments, wherein the pair of dielectric segments are respectively on the opposite sides of the photodetector and are between the isolation segments, wherein the pair of dielectric segments have individual bottoms that are closer to a bottom surface of the substrate than to a top surface of the substrate, and wherein the dielectric layer extends continuously along the top surface of the substrate from one of the dielectric segments to another one of the dielectric segments.