Dielectric structure overlying image sensor element to increase quantum efficiency

Abstract

Various embodiments of the present disclosure are directed towards a pixel sensor. The pixel sensor includes a substrate having a front-side opposite a back-side. An image sensor element comprises an active layer disposed within the substrate, where the active layer comprises germanium. An anti-reflective coating (ARC) structure overlies the back-side of the substrate. The ARC structure includes a first dielectric layer overlying the back-side of the substrate, a second dielectric layer overlying the first dielectric layer, and a third dielectric layer overlying the second dielectric layer. A first index of refraction of the first dielectric layer is less than a second index of refraction of the second dielectric layer, and a third index of refraction of the third dielectric layer is less than the first index of refraction.

What is claimed is: 
     
         1 . A pixel sensor, comprising:
 a substrate having a front-side opposite a back-side;   an image sensor element comprising an active layer disposed within the substrate, wherein the active layer comprises germanium; and   an anti-reflective coating (ARC) structure overlying the back-side of the substrate, wherein the ARC structure comprises a first dielectric layer overlying the back-side of the substrate, a second dielectric layer overlying the first dielectric layer, and a third dielectric layer overlying the second dielectric layer, wherein a first index of refraction of the first dielectric layer is less than a second index of refraction of the second dielectric layer, and wherein a third index of refraction of the third dielectric layer is less than the first index of refraction.   
     
     
         2 . The pixel sensor of  claim 1 , wherein the first dielectric layer, the second dielectric layer, and the third dielectric layer each comprise a dielectric material different from one another. 
     
     
         3 . The pixel sensor of  claim 1 , wherein a thickness of the first dielectric layer is greater than a thickness of the second dielectric layer, and wherein a thickness of the third dielectric layer is greater than the thickness of the first dielectric layer. 
     
     
         4 . The pixel sensor of  claim 1 , wherein the image sensor element is configured to generate electrical signals from near infrared (NIR) radiation. 
     
     
         5 . The pixel sensor of  claim 1 , wherein the first dielectric layer comprises tantalum oxide, the second dielectric layer comprises titanium oxide or silicon carbide, and the third dielectric layer comprises silicon dioxide. 
     
     
         6 . The pixel sensor of  claim 1 , further comprising:
 an isolation structure disposed within the substrate, wherein the isolation structure extends from the back-side of the substrate to the front-side of the substrate, and wherein the image sensor element is spaced laterally between sidewalls of the isolation structure.   
     
     
         7 . The pixel sensor of  claim 1 , further comprising:
 an interconnect structure disposed along the front-side of the substrate, wherein the interconnect structure includes an interconnect dielectric structure, a plurality of conductive vias, and a plurality of conductive wires; and   a reflector disposed within the interconnect dielectric structure and directly underlying the image sensor element.   
     
     
         8 . The pixel sensor of  claim 1 , further comprising:
 a grid structure overlying the ARC structure;   a light filter overlying the grid structure; and   a micro-lens overlying the light filter.   
     
     
         9 . The pixel sensor of  claim 1 , wherein the substrate comprises a first material different from germanium. 
     
     
         10 . An integrated circuit (IC), comprising:
 a first IC die comprising a first substrate and a first interconnect structure overlying the first substrate;   a second IC die overlying the first IC die, wherein the second IC die comprises a second substrate and a second interconnect structure underlying the second substrate, wherein the first and second IC dies contact at a bond interface between the first and second interconnect structures;   a plurality of image sensor elements disposed within the second substrate;   a grid structure overlying the plurality of image sensor elements, wherein each image sensor element is spaced laterally between sidewalls of the grid structure; and   an anti-reflective coating (ARC) structure disposed between the second substrate and the grid structure, wherein the ARC structure comprises a first dielectric layer, a second dielectric layer, and a third dielectric layer each with an index of refraction that is different from one another, wherein the second dielectric layer overlies the first dielectric layer and the third dielectric layer overlies the second dielectric layer, and wherein the first dielectric layer comprises a first metal oxide and the second dielectric layer comprises a second metal oxide different from the first metal oxide.   
     
     
         11 . The IC of  claim 10 , wherein the first metal oxide is tantalum oxide and the second metal oxide is titanium oxide. 
     
     
         12 . The IC of  claim 10 , wherein a first index of refraction of the first dielectric layer is less than a second index of refraction of the second dielectric layer, wherein a third index of refraction of the third dielectric layer is less than the second index of refraction. 
     
     
         13 . The IC of  claim 12 , wherein the ARC structure further comprises a passivation layer disposed between the first dielectric layer and the second substrate, wherein an index of refraction of the passivation layer is equal to the third index of refraction. 
     
     
         14 . The IC of  claim 13 , wherein a thickness of the third dielectric layer is greater than a thickness of second dielectric layer, wherein the thickness of the second dielectric layer is greater than a thickness of the passivation layer. 
     
     
         15 . The IC of  claim 12 , wherein the second index of refraction is within a range of about 2.4 to 2.6 and the third index of refraction is less than about 1.55. 
     
     
         16 . The IC of  claim 10 , further comprising:
 a first isolation structure extending from a front-side surface of the second substrate to a first point above the front-side surface, wherein each image sensor element is spaced laterally between sidewalls of the first isolation structure; and   a second isolation structure extending from a back-side surface of the second substrate to a second point below the back-side surface, wherein the second point is below the first point such that the second isolation structure contacts the first isolation structure.   
     
     
         17 . An integrated circuit (IC), comprising:
 a first IC die comprising a first substrate and a first interconnect structure overlying the first substrate;   a second IC die overlying the first IC die, wherein the second IC die comprises a second substrate, a second interconnect structure underlying the second substrate, and a plurality of image sensor elements disposed in the second substrate, wherein the first and second interconnect structures are disposed between the first and second substrates, wherein the image sensor elements respectively comprise a germanium layer disposed in the second substrate; and   an anti-reflective coating (ARC) structure disposed on a back-side surface of the second substrate, wherein the ARC structure comprises a first dielectric layer, a second dielectric layer, and a third dielectric layer, wherein the second dielectric layer is disposed between the first and third dielectric layers, wherein a thickness of the first dielectric layer is greater than a thickness of the second dielectric layer and is less than a thickness of the third dielectric layer.   
     
     
         18 . The IC of  claim 17 , wherein materials of the first, second, and third dielectric layers are different from one another. 
     
     
         19 . The IC of  claim 17 , wherein the ARC structure further comprises a passivation layer disposed between the first dielectric layer and the second substrate, wherein a thickness of the passivation layer is less than the thickness of the second dielectric layer. 
     
     
         20 . The IC of  claim 17 , further comprising:
 an upper bond pad structure disposed on the second interconnect structure and laterally offset from the plurality of image sensor elements; and   a liner layer extending along opposing sidewalls of the ARC structure to the upper bond pad structure.