Use of dark mirror coating to suppress stray light in an optical sensor assembly

Abstract

An optical sensor assembly is provided in which a dark mirror coating is used to suppress stray light in the form of both unwanted reflections from non-optically active regions of the sensor assembly surface and unwanted transmission of light into the surface region of the sensor assembly. The sensor assembly includes an image sensor positioned in a substrate adjacent to substrate surface areas that are not optically active. A dark mirror coating covering those surface areas significantly reduces reflections from non-optically active surface regions and improves image sensor performance in terms of signal-to-noise ratio and reduction in the appearance of “ghost” images, in turn enhancing the accuracy and precision of the sensor. The dark mirror coating may in the alternative, or in addition, be positioned underneath an optical filter, depending on the structure, material, and requirements of a particular sensor assembly.

We claim: 
     
         1 . An optical sensor assembly comprising:
 a substrate having a surface;   an optical image sensor at the substrate surface having an aperture for detecting and receiving light and providing a signal in response thereto, wherein the aperture defines an optically active surface region, with the remainder of the substrate surface defining a non-optically active surface region;   a dark mirror coating disposed over at least a portion of the substrate surface but not substantially in the region of the aperture, wherein the dark mirror coating is configured as an absorptive anti-reflective coating to reduce reflections from the non-optically active surface region and to reduce transmission through the non-optically active surface region and into the substrate, and comprises an initial absorbing layer provided on the substrate surface and an outwardly facing dielectric layer on the initial absorbing layer.   
     
     
         2 . The OSA of  claim 1 , wherein the initial absorbing layer comprises a metallic material selected from aluminum, gray metals, and alloys thereof. 
     
     
         3 . The OSA of  claim 2 , wherein the initial absorbing layer comprises two or more absorbing sublayers. 
     
     
         4 . The OSA of  claim 1 , wherein the outwardly facing dielectric layer comprises two or more dielectric sublayers. 
     
     
         5 . The OSA of  claim 1 , further including an optical thin film filter disposed over the optical image sensor and configured to filter light so that only light in a selected wavelength range passes through the filter and reaches the sensor. 
     
     
         6 . The OSA of  claim 5 , wherein the dark mirror coating is disposed over at least a portion of the optical thin film filter. 
     
     
         7 . The OSA of  claim 5 , wherein the dark mirror coating is disposed under at least a portion of the optical thin film filter. 
     
     
         8 . The OSA of  claim 5 , wherein a first dark mirror coating is disposed over at least a portion of the optical thin film filter and a second dark mirror coating is disposed under at least a portion of the optical thin film filter. 
     
     
         9 . The OSA of  claim 1 , wherein the optical sensor assembly comprises two or more optical sensors at the substrate surface each having an aperture for detecting and receiving light and providing a signal in response thereto. 
     
     
         10 . The OSA of  claim 9 , wherein an optical thin film filter is disposed over each optical sensor. 
     
     
         11 . The optical sensor assembly of  claim 1 , wherein the optical thin film filter includes terminal tapering regions and the dark mirror coating is formed over the tapering regions so as to cover both a portion of the optical thin film filter and a portion of uncoated substrate surface. 
     
     
         12 . An optical sensor assembly comprising:
 a substrate having a surface;   an optical image sensor at the substrate surface having an aperture for detecting and receiving light and providing a signal in response thereto, wherein the aperture defines an optically active surface region, with the remainder of the substrate surface defining a non-optically active surface region;   a dark mirror coating disposed over at least a portion of the substrate surface but not substantially in the region of the aperture, wherein the dark mirror coating is configured as an absorptive anti-reflective coating to reduce reflections from the non-optically active surface region and to reduce transmission through the non-optically active surface region and into the substrate, and further wherein the dark mirror coating comprises   a first pair of layers provided on the substrate surface, the first pair of layers including a first substantially non-absorbing layer disposed directly on the substrate surface and a first absorbing layer adjacent to and overlying the first substantially non-absorbing layer, optionally at least one additional pair of layers including a substantially non-absorbing layer and an absorbing layer configured such that the absorbing layers and the substantially non-absorbing layers alternate, and an outwardly facing dielectric layer serving as the surface of the dark mirror coating.   
     
     
         13 . The optical sensor assembly of  claim 12 , wherein at least one absorbing layer is comprised of two or more absorbing sublayers. 
     
     
         14 . The optical sensor assembly of  claim 12 , wherein at least one substantially non-absorbing layer is comprised of two or more substantially non-absorbing sublayers. 
     
     
         15 . The optical sensor assembly of  claim 12 , wherein each absorbing layer is comprised of a metallic absorbing material selected from aluminum, gray metals, and alloys thereof. 
     
     
         16 . The optical sensor assembly of  claim 12 , wherein the absorbing material is selected from tantalum, niobium, titanium, nickel, chromium, silicon, and alloys thereof. 
     
     
         17 . The optical sensor assembly of  claim 12 , wherein the absorbing material is selected from tantalum, niobium, and alloys thereof. 
     
     
         18 . The optical sensor assembly of  claim 12 , wherein each substantially non-absorbing layer is comprised of a dielectric material. 
     
     
         19 . The optical sensor assembly of  claim 18 , wherein the dielectric material is selected from SiO 2 , Ta 2 O 5 , NbTaO 5 , Nb 2 O 5 , TiO 2 , NbTiO x , Al 2 O 3 , Si 3 N 4 , Cr 2 O 3 , MoO 3 , and combinations thereof. 
     
     
         20 . The optical sensor assembly of  claim 12 , wherein when the dark mirror coating comprises at least one additional pair of layers, the absorbing layers may comprise the same material or different materials and the substantially non-absorbing layers may comprise the same material or different materials.