Systems, methods, and apparatus for sensitive thermal imaging

Abstract

The high-pixel-count uncooled thermal imaging arrays disclosed herein have liquid crystal (LC) microcavity transducers separate from the read-out integrated circuit (ROIC). The transducer converts incident infrared (IR) radiation in birefringence changes that can be measured with visible light. In other words, the system uses the temperature sensitivity of the LC birefringence to convert the IR scene to a visible image. Measurements on sample arrays indicate that the LC material quality is similar to that of bulk samples and has good noise performance. Additionally, high-fill-factor arrays on fused-silica substrates may be processed to enable optimization of conditions for greatly improved temperature sensitivity. An additional IR absorber layer may be integrated into the process to tune the structure for the infrared.

1 . A method for fabricating a liquid crystal cell, the method comprising:
 defining a first inner surface of the liquid crystal cell with a first inorganic layer;   texturing the first inner surface to provide a first alignment surface for aligning liquid crystal material with respect to the first inorganic layer;   depositing a sacrificial layer on the first inner surface;   texturing a surface of the sacrificial layer to form a negative alignment surface;   depositing a second inorganic layer on the negative alignment surface to form a second alignment surface for aligning the liquid crystal material with respect to the second inner surface;   forming a hole in the second inorganic layer;   removing the sacrificial layer via the hole;   disposing the liquid crystal material in the liquid crystal cell via the hole; and   sealing the hole in the second inorganic layer to yield a liquid crystal cell containing the liquid crystal material aligned with respect to the first alignment surface and the second alignment surface.   
     
     
         2 . The method of  claim 1 , wherein texturing the first inner surface comprises diamond lapping the first inner surface in a first direction. 
     
     
         3 . The method of  claim 2 , wherein texturing the surface of the sacrificial layer comprises diamond lapping the surface in the first direction. 
     
     
         4 . The method of  claim 2 , wherein texturing the surface of the sacrificial layer comprises diamond lapping the surface in a second direction different than the first direction. 
     
     
         5 . The method of  claim 1 , wherein depositing the second inorganic layer comprises forming a cavity having a thickness of about 0.5 microns to about 10 microns. 
     
     
         6 . The method of  claim 1 , wherein depositing the second inorganic layer comprises forming a cavity having a width of about 2 microns to about 100 microns. 
     
     
         7 . The method of  claim 1 , wherein sealing the hole comprises filling the hole with a fluoropolymer. 
     
     
         8 . The method of  claim 1 , wherein the first inorganic layer is disposed on another sacrificial layer, the other sacrificial layer being disposed on a substrate, and further comprising:
 removing the other sacrificial layer to form at least one thermal leg supporting the liquid crystal cell above the substrate.   
     
     
         9 . A liquid crystal transducer comprising:
 a substrate;   an array of liquid crystal cells, each liquid crystal cell in the array of liquid crystal cells comprising:
 a first textured inner surface; 
 a second textured inner surface opposite the first textured inner surface; and 
 liquid crystal material disposed between and aligned with the first textured inner surface and the second textured inner surface; and 
   a plurality of thermal legs, each thermal leg in the plurality of thermal legs in physical contact with the substrate and with a respective liquid crystal cell in the array of liquid crystal cells to support the respective liquid crystal cell and to thermally isolate the respective liquid crystal cell from the substrate.   
     
     
         10 . The liquid crystal transducer of  claim 9 , wherein the liquid crystal material in at least one liquid crystal cell in the array of liquid crystal cells has a thickness of less than about  2  μm. 
     
     
         11 . A photolithographic method for fabricating a liquid crystal transducer, the method comprising:
 forming a first silicon dioxide layer on a substrate;   depositing a first sacrificial layer on top of the silicon dioxide layer;   depositing a first silicon nitride layer on top of the first sacrificial layer to form at least one thermal leg and a first surface of a liquid crystal cavity;   depositing a second silicon dioxide layer onto the first silicon nitride layer;   forming a first alignment surface on the first silicon dioxide layer;   depositing a second sacrificial layer on the first alignment surface;   forming a second alignment layer on the second sacrificial layer;   depositing a second silicon nitride layer on the second alignment layer to form a second surface and sides of the liquid crystal cavity;   forming a hole in the second silicon nitride layer;   removing the second sacrificial layer via the hole;   disposing liquid crystal material in the liquid crystal cavity via the hole; and   removing the first sacrificial layer.   
     
     
         12 . A liquid crystal transducer, comprising:
 a substrate;   a liquid crystal cell defining at least one sealed cavity;   a thermal leg, in physical contact with the substrate and the liquid crystal cell, to support the liquid crystal cell and to thermally isolate the liquid crystal cell with respect to the substrate;   liquid crystal material, disposed within the at least one sealed cavity, to vary a birefringence of the liquid crystal cell in response to a change in temperature; and   an alignment layer, disposed on at least one interior surface of the at least one sealed cavity, to align the liquid crystal material with respect to the interior surface of the at least one sealed cavity.   
     
     
         13 . The liquid crystal transducer of  claim 12 , wherein the liquid crystal cell has an optical thickness selected to resonate at a wavelength between about 2 μm and about 14 μm. 
     
     
         14 . The liquid crystal transducer of  claim 12 , wherein at least one of the thermal leg or the substrate comprises silicon nitride.