Abstract:
Hybrid dual layer filter can be employed can be employed as filters. A multispectral imager comprises a two layer filter array monolithically integrated onto detector array, a top layer of pigment based filter and a lower layer of plasmonic nano-optic filter to make a low cost and narrow bandwidth filter without side leaking or side peaks. Multispectral imager comprises a microlens array, a mosaic patterned optical filter array underlying the microlens array and including a two-dimensional repetition of a unit mosaic pattern, and a pixelated detector array underlying the mosaic patterned optical filter array. The unit mosaic pattern comprises an array of composite filter elements having different peaks in a respective transmittance spectrum. Each composite filter element comprises a pigment based filter portion and a plasmonic nano-optic filter portion.

Description:
BACKGROUND 
     Demand for a low-cost snapshot multi spectral imager has been increasing for various applications, which include accurate color reproduction, machine/robot vision, plant and vegetation research, food processing, counterfeit detection, early stage diagnosis of cancer, medical in-vivo imaging, and defense applications (point/stand-off optical spectral detection systems for remote sensing). Especially, accurate color reproduction is highly desired for a growing number of smart displays equipped with color camera modules and color displays. 
     A typical multispectral imager essentially consists of either rotating filter wheels, mechanically diced thin-film dichroic filters mounted in front of an image sensor, or multiple cameras with bulk dichroic filters. Even for those touted as commercial systems, there is no real volume production pathway with significant price or reduced complexity enhancements for as few as tens or hundreds of units. 
     There is a low cost color filter array used for typical CCD or CMOS image sensor which is a negative type photosensitive material that can be patterned with UV light. It consists of pigments to define the spectrum of the color filter, a dispersant polymer for pigment dispersion, an initiator to generate the radical for the polymerization reaction, a monomer to be polymerized and an alkaline soluble polymer to control the development property. The photo-polymerization starts with the radicals generated when the initiator is exposed to UV light. When the radical gets in contact with the monomers, the polymerization starts and forms the high molecular weight polymer insoluble for the developer. The un-exposure area is not polymerized and is removed during the development process. As a result, the pattern profile is formed. This type of color filter array requires coating, pre-bake, exposure, development, rinse and post-bake multiple times to make a mosaic pattern. Although this type of filter can be made at low cost for one filter, the cost increases as number of filter type of an array increases. Also the bandwidth is broad and the band selectivity is limited to a visual range. 
     Recently, it has been found that certain nanostructures work as an optical filter, and has a strong advantage compared to prior technologies. Multiple, or almost infinite number of, optical filters can be made on a single layer, at no additional cost. The spectral shapes of these plasmonic nano-optical filters can be controlled. It can be narrowed but usually at an expense of the transmission power. Also usually unwanted second peaks or third peaks are generated at relatively high transmission power. For the above reasons, low cost and clean spectral filters are not readily available for multi-spectral imager application so far. 
     SUMMARY OF THE INVENTION 
     Hybrid dual layer filter can be employed can be employed as filters. A multispectral imager comprises a two layer filter array monolithically integrated onto detector array, a top layer of pigment based filter and a lower layer of plasmonic nano-optic filter to make a low cost and narrow bandwidth filter without side leaking or side peaks. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic representation in a vertical cross-sectional view of a FSI (front side illumination) multispectral imager with dual layer filter arrays according to an embodiment of the present disclosure. 
         FIG. 2  is a schematic representation in a vertical cross-sectional view of a BSI (back side illumination) multispectral imager with dual layer filter arrays according to an embodiment of the present disclosure. 
         FIG. 3A  is a schematic representation in a perspective view of a multispectral imager with monolithically integrated multispectral filter array with a mosaic pattern. 
         FIG. 3B  is a schematic representation of a spectral response of a multispectral filter array. 
         FIGS. 4A, 4B and 4C  show spectral responses of different types of multispectral band pass filters for multispectral image. 
         FIGS. 5A, 5B, and 5C  show examples of spectral responses of plasmonic nanofilters. 
         FIGS. 6A and 6B  show examples of spectral responses of pigment based RGB CMY filters. 
         FIG. 7A  shows a schematic representation of an overlay of spectral responses of a plasmonic nanofilter with pigment based RED color filter. 
         FIG. 7B  shows a schematic representation of the resulting of spectral response of a dual layer filter made of a plasmonic nanofilter and pigment based RED color filter. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Unless otherwise specified, the words “a” or “an” as used herein mean “one or more”. The term “light” includes visible light as well as UV and IR radiation. The invention includes the following embodiments. 
     Referring to  FIG. 1 , a vertical cross-sectional view of a first exemplary multispectral imager  100  is shown, which can be employed to generate accurate-color, multispectral, and/or 3D images. The first exemplary multispectral imager  100  contains a microlens array  101 , a pigment based color filter array  102 , a plasmonic nanofilter array  103 , at least one metal interconnection layer  104 , and a pixelated photo detector array  105 . The number of different band pass filters for the mosaic pattern can be more than four. The vertical cross-sectional view of  FIG. 1  represents a front side illumination sensor structure. 
     As used herein, a plasmonic filter refers to a patterned metal film with subwavelength-size periodic hole arrays. A plasmonic filter acts as an optical filter due to the interference of surface plasmon polaritons (SPP) between adjacent holes. A plasmonic nanofilter refers to a plasmonic filter having patterned shapes of which at least one dimension is a nanoscale dimension (less than 1 micron). 
     Referring to  FIG. 2 , a vertical cross-sectional view of a second exemplary multispectral imager  200  is shown, which can be employed to generate accurate-color, multispectral, and/or 3D images. The second exemplary multispectral imager  200  contains a microlens array  201 , a pigment based color filter array  202 , and a plasmonic nanofilter array  203 , at least one metal interconnection layer  204 , and pixelated photo detector array  205 . The number of different band pass filters for the mosaic pattern can be more than four. The vertical cross-sectional view of  FIG. 1  represents a back side illumination sensor structure. 
     Referring to  FIG. 3A , an example of multispectral imager  300  is shown, including a filter array mosaic pattern  310  of multispectral imager, and a detector  320  with associated pixel array. The filters may be made of a layer or layers of highly conductive structured materials. The highly conductive structured material layer may include a periodic pattern or patterns of elements. The elements can have shapes and sizes configured such that a transmittance spectrum of the conductive layer has at least one pass band within the target wavelength range. 
       FIG. 3B  illustrates a schematic representation of a spectral response of an ideal multispectral filter array. 
     Referring to  FIGS. 4A, 4B and 4C , examples of spectral responses of different types of dichroic filters are shown. The respective pass wavelength ranges ( 410 ,  420 ,  431 ,  432 ,  433 ,  451 ,  453 ,  453 ,  454 ,  455 ,  456 ) are illustrated for each dichroic filters. Some of the dichroic filters show second peaks or second bands. A dichroic filter is an interference-based color filter that selectively passes light within a small wavelength range (a pass band) while reflecting light outside of the selective pass band. 
     Referring to  FIGS. 5A, 5B and 5C , examples of spectral responses of different wavelength plasmonic nano-optic filters in the visible and near infrared range  501 ,  502 ,  503 ,  504 ,  505 ,  506 ,  507 ,  508 , and  509  are shown. The filters may be made of a layer, or layers, of highly conductive structured materials. The highly conductive structured material layer(s) may include a periodic pattern, or patterns, of elements. The periodic pattern(s) of elements can have shapes and sizes that are configured such that a transmittance spectrum of the conductive layer has at least one pass band within the target wavelength range. The filters can show broad bandwidths and second and third peaks that are located outside the range of the first peak, i.e., outside the wavelength range within which transmission of light is desired for a given filter. 
     Referring to  FIGS. 6A and 6B , examples of different peak-wavelength pigment based filters in the visible range  601 ,  602 ,  603 ,  604 ,  605 , and  606  are shown. The filters show broad bandwidths and leakage in the longer wavelength ranges. 
     Referring to  FIG. 7A , a spectral response ( 701 ,  702 ) of a plasmonic nanofilter is overlaid with a spectral response  703  of a pigment based RED color filter. 
     Referring to  FIG. 7B , the spectral response ( 704 ,  705 ) of a dual layer filter made of a plasmonic nanofilter and pigment based RED color filter is shown. The transmission spectra of the dual layer filter can be obtained by multiplying the transmittance spectra of the plasmonic nanofilter with the transmittance spectra of the respective pigment based filter within the same dual layer filter. 
     According to an aspect of the present disclosure, a multispectral imager is provided. The multispectral imager comprises a microlens array ( 101  or  201 ), a mosaic patterned optical filter array {( 102 ,  103 ) or ( 202 ,  203 )} underlying the microlens array and including a two-dimensional repetition of a unit mosaic pattern  310 , and a pixelated detector array ( 105  or  205 ) underlying the mosaic patterned optical filter array {( 102 ,  103 ) or ( 202 ,  203 )}. The unit mosaic pattern comprises an array of composite filter elements ( 150  or  250 ) having different peaks in a respective transmittance spectrum. Each composite filter element ( 150  or  250 ) comprises a pigment based filter portion ( 152  or  252 ) and a plasmonic nano-optic filter portion ( 153  or  253 ). 
     In one embodiment, the unit mosaic pattern  310  can be an m×n rectangular pattern, wherein m and n are independent integers greater than 1. In one embodiment, the unit mosaic pattern can comprise a combination of multiple hexagonal patterns that can be repeated in two directions. 
     In one embodiment, each plasmonic nano-optic filter portion ( 153  or  253 ) can comprise a conductive material layer including a periodic pattern of geometric shapes. In one embodiment, the plasmonic nano-optic filter portions ( 153 ,  253 ) within the unit mosaic pattern can comprise the same conductive material having different periodic patterns of geometrical shapes. In one embodiment, the conductive material can be an elemental metal or an intermetallic alloy of at least two elemental metals. In one embodiment, shapes and sizes of the geometrical shapes can be configured such that a transmittance spectrum of each second layer has at least one pass band within a respective pass band of the first layer within a same composite filter element. 
     In one embodiment, the multispectral imager can be configured to generate a multispectral image employing the mosaic patterned optical filter array. In one embodiment, each pigment based filter portion ( 152 ,  252 ) in the unit mosaic pattern can have a different composition from other pigment based filter portions ( 152 ,  252 ) in the unit mosaic pattern. 
     In one embodiment, the transmission spectra of each composite filter element ( 150  or  253 ) can be the same as the product of a respective pigment based filter portion ( 152  or  252 ) in the composite filter element ( 150  or  250 ) and a respective plasmonic nano-optic filters ( 153  or  253 ) in the composite filter element ( 150  or  250 ). 
     In one embodiment, at least one metal interconnect layer  104  can overlie the pixelated detector array  105 . In another embodiment, at least one metal interconnect layer  204  can underlie the pixelated detector array  204 . 
     In one embodiment, each pigment based filter portion ( 152  or  252 ) can overlie a respective plasmonic nano-optic filter portion ( 153 ,  163 ) within each composite filter element ( 150  or  250 ). In one embodiment, each composite filter element ( 150 ,  250 ) may comprise a portion of an optional upper transparent material layer overlying a respective plasmonic nano-optic filter portion ( 153  or  253 ), and a portion of an optional lower transparent material layer underlying the respective plasmonic nano-optic filter portion ( 153  or  253 ). In one embodiment, the pixelated detector array ( 104 ,  204 ) can comprise semiconductor photodetectors. 
     In one embodiment, a method of interpreting bio-chemical contents of an organism is provided. The multispectral imager of the present disclosure can be provided. A multispectral image of an organism can be taken. Health condition of the organism can be identified by correlating the multispectral image with spectral distribution data from organisms with previously characterized health conditions. In one embodiment, the organism can be a human, and the multispectral image can be taken from a part of a human body. 
     In one embodiment, a method of acquire a multispectral image is provided. The spectral imager of the present disclosure can be provided. A multispectral image can be taken employing the spectral imager. 
     Although the foregoing refers to particular preferred embodiments, it will be understood that the present invention is not so limited. It will occur to those of ordinary skill in the art that various modifications may be made to the disclosed embodiments and that such modifications are intended to be within the scope of the present invention. All of the publications, patent applications and patents cited in this specification are incorporated herein by reference in their entirety.