Abstract:
A micromechanical device including an improved sensing element and improved bending elements is described. Sensing elements include multi-layered structures which are thinner, lighter, and flatter than structures presently known within the related arts. Bending elements include structures which separately respond to illumination by an infrared source so as to twist a sensing element. Micromechanical pixels may be arranged to form two-dimensional arrays of infrared sensitive pixels. Arrays of micromechanical pixels are applicable to imaging devices for use within the fields of security and surveillance, firefighting, automotive safety, and industrial monitoring.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is based upon and claims priority under 35 U.S.C. § 119(e) from U.S. Provisional Application No. 60/929,518 filed Jun. 29, 2007, entitled High Sensitivity Radiation Detector and Radiation Imaging Device, the contents of which are hereby incorporated in their entirety by reference thereto. 
     
    
     FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT 
       [0002]    None. 
       BACKGROUND OF THE INVENTION 
       [0003]    1. Field of the Invention 
         [0004]    The present invention generally relates to a micromechanical device for use within infrared imaging devices. Specifically, the invention is a micromechanical pixel including improved sensing and bending elements which separately and in combination increase the sensitivity and decrease the response time of the pixel. 
         [0005]    2. Description of the Related Art 
         [0006]    Infrared imaging devices enable a user to view an object via the infrared band of the spectrum, which is otherwise invisible to the human eye. Infrared imaging devices are applicable to security and surveillance, firefighting, automotive safety, and industrial monitoring because the peak thermal emission of objects in such applications is centered within the infrared region. However, the high cost of infrared imaging devices remains a challenge, thereby limiting their use. 
         [0007]    The radiation detectors employed within imaging devices are either photon detectors or thermal detectors. 
         [0008]    Photon detectors produce an image when incident radiation is absorbed within a sensing material via interactions with electrons bound to lattice or impurity atoms or with free electrons. An output signal, in the form of a voltage or current change, is produced by changes in the electronic energy distribution. The materials used in photon detectors, typically HgCdTe and InSb, exhibit very high quantum efficiency in the infrared band. However, photon detectors must be cryogenically cooled, thus increasing the weight, volume, and power consumption of presently known devices. Furthermore, materials which are highly quantum efficiency are notoriously difficult to process and costly. As such, imaging devices based on photon detector technologies are limited to specialized applications within the fields of national defense and astronomy. 
         [0009]    Thermal detectors produce an image when incident radiation is absorbed by a thermally-sensitive material that alters some physical property of the material, examples including resistance or capacitance. The alteration of the physical property is typically detected by a readout integrated circuit (ROIC), which generates an output signal. Thermal detectors operate at room temperature, thus avoiding the cooling required by and complexity of photon detector devices; however, the performance of thermal detectors, as measured by their noise equivalent temperature difference (NETD), is approximately ten times less sensitive than photon detectors. The thermal sensitivity of detector materials, examples including vanadium oxide or amorphous silicon, is characteristically in the range of 2%/K to 3%/K. The bias of an interrogation pulse from a ROIC, which controls detector responsivity, is restricted to prevent self-heating of a pixel. While less costly than photon detector devices, thermal detector devices are affordable within the fields of industrial monitoring and firefighting, yet too costly for most consumer and many industrial applications. 
         [0010]    Thermal imaging devices employing passive thermal bending, composed of bi-layer micro-cantilevers for temperature and radiation sensing and electrical, capacitive, or optical readout, are described within the related arts. For example,  FIG. 1   a  shows an exemplary bi-layer cantilever  50  including a first layer  1  contacting and attached to a second layer  2  which are thereafter attached to a stationary support  51 . The first layer  1  has a coefficient of thermal expansion different from that of the second layer  2 .  FIG. 1   b  shows the same bi-layer cantilever  50  after the first layer  1  and second layer  2  are heated by infrared radiation  52  causing the bi-layer cantilever  50  to bend. This approach to thermal imaging eliminates the monolithic integration of a pixel-level ROIC, further eliminating electronic noise and simplifying device fabrication. However, the sensitivity of presently known cantilevers is poor because of their low absorption efficiency and large mass. 
         [0011]    An improved micromechanical thermal imaging device is described by Ishizuya et al. in U.S. Pat. Nos. 6,080,988, 6,339,219, 6,469,301, and 6,835,932. Referring now to  FIGS. 2-4 , a micromechanical pixel  3  is shown including a sensing element  4  disposed between and separated from a pair of bending elements  5   a ,  5   b  adjacent to a substrate  9 . The sensing element  4  includes an optical absorption cavity  7  bounded by an absorber layer  8  and a reflector  6  which are spaced apart by and attached to a support post  29 , as shown in  FIG. 3 . Each u-shaped bending element  5   a ,  5   b  is composed of a pair of bi-layer cantilevers  10   a ,  11   a  and  10   b ,  11   b . Each paired arrangement of bi-layer cantilevers  10   a ,  11   a  and  10   b ,  11   b  is separated by a thermal isolation region  12   a ,  12   b  having a low thermal conductance. Each bi-layer cantilever  10   a ,  10   b ,  11   a ,  11   b  is composed of a high expansion layer  33  which contacts and is attached to a portion of a low expansion layer  34 , as shown in  FIG. 4 , opposite of the substrate  9 . The low expansion layer  34  of the innermost bi-layer cantilevers  11   a ,  11   b  is attached to the sides of the absorber layer  8 , as represented in  FIG. 2 . Bending elements  5   a ,  5   b  are attached to the substrate  9  via a pair of anchor posts  13   a ,  13   b  so that a gap  49  is provided between the sensing element  4  and substrate  9  and between the bending elements  5   a ,  5   b  and substrate  9 . The height of the gap  49  may be adjusted by making the length of the innermost bi-layer cantilevers  11   a ,  11   b  shorter than the outermost bi-layer cantilevers  10   a ,  10   b.    
         [0012]    In the absence of infrared illumination, the outermost bi-layer cantilevers  10   a ,  10   b  negate the deflection of the innermost bi-layer cantilevers  11   a ,  11   b , thus producing a net bending of zero so as to maintain zero tilt along the sensing element  4 , regardless of the change in ambient temperature. When illuminated via an infrared source, the optical absorption cavity  7  receives and converts infrared radiation into heat which is conducted into the innermost bi-layer cantilevers  11   a ,  11   b , resulting in additional bending with respect to the outermost bi-layer cantilevers  10   a ,  10   b  and causing the sensing element  4  to tilt with respect to the plane of the substrate  9 . Proper function of the device in  FIG. 2  requires the micromechanical pixel  3  to be backside illuminated  32 , whereby infrared radiation is transmitted through the substrate  9 . High sensitivity is achieved via an efficient, yet lightweight, sensing element  4  and thin bi-layer cantilevers  10   a ,  10   b ,  11   a ,  11   b . However, the micromechanical pixel  3  in  FIG. 2  suffers from several deficiencies, which limit sensitivity and contribute to sensor noise, including a low fill factor, poor reflector flatness, and mechanical complexity. 
         [0013]    The micromechanical pixel  3  described in  FIGS. 2-4  is applicable to a variety of detectors. For example,  FIG. 5  shows an exemplary optical readout device  28  described by Ishizuya et al. in U.S. Pat. No. 6,339,219 which includes an infrared lens system  15 , an infrared detection array  16 , a first lens system  19 , an aperture plate  22 , a second lens system  24 , and an imager  25  arranged in the order described. Within the front end of the apparatus, rays from a source  14  pass through the infrared lens system  15  and are thereafter directed onto the infrared detection array  16 . The infrared detection array  16  includes a focal plane array  17  composed of micromechanical pixels  18  which are mechanically responsive to the thermal loading induced by the infrared rays. Within the back end of the apparatus, micromechanical pixels  18  reflect the incident light  20  from a visible light source  23 , one example being a light emitting diode (LED), so that the reflected light  21  passes through the first lens system  19  which compresses the reflected light  21  allowing it to pass through the pinhole  53  along the aperture plate  22 . The reflected light  21  then passes through the second lens system  24  which expands the reflected light  21  so as to impinge a focal plane array  27  composed of receptor pixels  26  within the imager  25 , examples being a complementary metal oxide semiconductor (CMOS) device or charged-coupled device (CCD). Thereafter, the resultant image is communicated to a video display device. 
         [0014]    The detector in  FIG. 5  employs an optical system to simultaneously measure the deflections of all micromechanical pixels  18  so as to project a visible image of spatially-varying infrared radiation directly onto a commercial-off-the-shelf visible CMOS or CCD imager. The number of receptor pixels  26  within the CMOS or CCD array is generally chosen to be more than the number of micromechanical pixels  18 . In operation, an image produced by the detector in  FIG. 5  is of uniform intensity over the entire array of receptor pixels  26  when no illumination is present because of the canceling effect of the paired arrangement of bi-layer cantilevers  10   a ,  11   a  and  10   b ,  11   b , as described above for  FIGS. 2-4 . When illuminated by an infrared source, a sensing element  4  tilts within each micromechanical pixel  18  and deflects light away from the pinhole  53 , thus projecting darker receptor pixels  26  with intensities which are proportional to the radiation level. The detector effectively converts infrared radiation into intensity change at a visible or near-infrared readout wavelength. 
         [0015]    The micromechanical pixel  3  in  FIG. 2  produces design related noise including: (1) noise caused by the radiative heat exchange between each pixel and its environment, referred to as background fluctuations; (2) noise caused by the dynamic heat exchange between each pixel and the substrate, referred to as thermal fluctuations; (3) noise from mechanical energy stored in the cantilever continuously exchanged with thermal energy, referred to as thermomechanical noise; and (4) noise caused by the random arrival rate of photons at the CMOS/CCD imager, referred to as shot noise. Since all noise sources are probabilistic, the total NETD for a micromechanical IR imager is equal to the square root of the sum of the squares of the contributing noise sources and is given by 
         [0000]        NETD   TOT   =√ {square root over ( NETD   BF   2   +NETD   TF   2   +NETD   TM   2   +NETD   SN   2 )},  (1) 
         [0000]    where the subscripts BF, TF, TM, and SN refer to the NETD due to background fluctuations, thermal fluctuations, thermomechanical noise, and shot noise, respectively. The background fluctuation NETD is given by 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       NETD 
                       BF 
                     
                     = 
                     
                       
                         
                           2 
                            
                           
                             ( 
                             
                               
                                 4 
                                  
                                 
                                   f 
                                   2 
                                 
                               
                               + 
                               1 
                             
                             ) 
                           
                         
                         
                           
                             ɛτ 
                             0 
                           
                            
                           η 
                            
                           
                             
                                
                               P 
                             
                             / 
                             
                                
                               T 
                             
                           
                         
                       
                        
                       
                         
                           
                             2 
                              
                             
                               k 
                               B 
                             
                              
                             σ 
                              
                             
                                 
                             
                              
                             
                               B 
                                
                               
                                 ( 
                                 
                                   
                                     T 
                                     D 
                                     5 
                                   
                                   + 
                                   
                                     T 
                                     B 
                                     5 
                                   
                                 
                                 ) 
                               
                             
                           
                           A 
                         
                       
                     
                   
                   , 
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
         [0000]    where f is the f-number of the lens, ε is the pixel emissivity, τ 0  is the transmission of the optics, η is the pixel absorption efficiency, dP/dT is the differential irradiance, k B  is Boltzmann&#39;s constant, σ is the Stefan-Boltzmann constant, B is the thermal bandwidth, T D  is the detector temperature, T B  is the background temperature, and A is the active pixel area. 
         [0016]    The NETD due to thermal fluctuations is given by 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       NETD 
                       TF 
                     
                     = 
                     
                       
                         2 
                          
                         
                           ( 
                           
                             
                               4 
                                
                               
                                 f 
                                 2 
                               
                             
                             + 
                             1 
                           
                           ) 
                         
                          
                         
                           T 
                           D 
                         
                          
                         
                           
                             
                               k 
                               B 
                             
                              
                             BG 
                           
                         
                       
                       
                         
                           τ 
                           0 
                         
                          
                         η 
                          
                         
                             
                         
                          
                         A 
                          
                         
                           
                              
                             P 
                           
                           / 
                           
                              
                             T 
                           
                         
                       
                     
                   
                   , 
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
         [0000]    where G is the thermal conductivity. 
         [0017]    The NETD due to thermomechanical noise is equal to 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       NETD 
                       TM 
                     
                     = 
                     
                       
                         
                           2 
                            
                           
                             ( 
                             
                               
                                 4 
                                  
                                 
                                   f 
                                   2 
                                 
                               
                               + 
                               1 
                             
                             ) 
                           
                            
                           G 
                         
                         
                           
                             ητ 
                             0 
                           
                            
                           A 
                            
                           
                               
                           
                            
                            
                            
                           
                             
                                
                               P 
                             
                             / 
                             
                                
                               T 
                             
                           
                            
                           
                               
                           
                            
                           ℜ 
                         
                       
                        
                       
                         
                           
                             
                               k 
                               B 
                             
                              
                             
                               T 
                               D 
                             
                              
                             B 
                           
                           
                             kQ 
                              
                             
                                 
                             
                              
                             
                               ω 
                               0 
                             
                           
                         
                       
                     
                   
                   , 
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
         [0000]    where l is the length of the bimaterial cantilever,           is the pixel responsivity (defined as the change in pixel deflection angle per degree Kelvin), k is the stiffness of the cantilever, Q is the cantilever Q-factor, and ω 0  is the cantilever resonant frequency. 
         [0018]    The NETD due to shot noise is given by the expression 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       NETD 
                       SN 
                     
                     = 
                     
                       
                         
                           
                             ( 
                             
                               
                                 4 
                                  
                                 
                                   f 
                                   2 
                                 
                               
                               + 
                               1 
                             
                             ) 
                           
                            
                           G 
                         
                         
                           
                             ητ 
                             0 
                           
                            
                           A 
                            
                           
                             
                                
                               P 
                             
                             / 
                             
                                
                               T 
                             
                           
                            
                           
                               
                           
                            
                           Δ 
                            
                           
                               
                           
                            
                           P 
                         
                       
                        
                       
                         
                           
                             2 
                              
                             qPB 
                           
                           
                             ℜ 
                             c 
                           
                         
                       
                     
                   
                   , 
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
         [0000]    where P is the visible light power received by a CMOS/CCD pixel, ΔP is the change in light power per degree Kelvin (where ΔP∝         ), q is the elementary charge, and            c  is the responsivity of the CMOS/CCD imager. 
         [0019]    The dominant source contributing to the NETD in a micromechanical pixel is typically the shot noise NETD. The shot noise NETD may be lowered by increasing the responsivity           or lowering the absolute shot noise. It may be appreciated, therefore, that there remains a need for further advancements and improvements, thus facilitating a micromechanical pixel with improved thermal sensitivity and response time. 
         [0020]    Accordingly, what is required is a micromechanical pixel with enhanced responsivity without adversely affecting thermal properties of the pixel. 
         [0021]    What is also required is a micromechanical pixel with enhanced thermal response time without adversely affecting the responsivity of the pixel. 
       SUMMARY OF THE INVENTION 
       [0022]    An object of the present invention is to provide a micromechanical pixel with enhanced responsivity without adversely affecting thermal properties of the pixel. 
         [0023]    Another object of the present invention is to provide a micromechanical pixel with enhanced thermal response time without adversely affecting responsivity of the pixel. 
         [0024]    The present invention is a micromechanical device for infrared sensing with improved thermal sensitivity and thermal response time. Performance of sensing element within the micromechanical device is improved by increasing its absorption efficiency and greater control of light reflected therefrom. Performance of the bending elements within the micromechanical device is improved by increasing their sensitivity to thermal loading and isolating the sensing element and innermost bi-layer cantilever from the outermost bi-layer cantilever and substrate. The design features described below may be implemented alone or combined within a pixel, as described in the Detailed Description of the Invention. 
         [0025]    The thermal sensitivity of a micromechanical pixel is directly related to the absorption efficiency of infrared radiation within the sensing element. The distance between the reflector and absorber layer within a sensing element is typically designed to serve as a quarter-wavelength resonant cavity to enhance absorption in a specific infrared band; however, absorption peaks in both the long-wavelength infrared band (λ=8-14 μm) and mid-wavelength infrared band (λ=3-5 μm) are possible due to harmonic effects. Long and mid wavelength bands are preferred for infrared imaging because atmospheric transmission is very high within these wavelengths. Broadband infrared anti-reflection coatings, which minimize reflections in the infrared band of interest, may be applied to both the top and bottom surfaces of the substrate to maximize absorption by the sensing element. 
         [0026]    The responsivity of a micromechanical pixel is related to the contrast in intensity of light that passes through a pinhole aperture, or the intensity changes from bright to dark during pixel deflection, after the light is reflected by a reflector within a sensing element. As such, the reflector, typically a highly reflective metal including, but not limited to, aluminum and gold, must tightly focus the reflected light, thus requiring strict tolerance on the flatness of the reflector to prevent stray reflections or scattering. To achieve acceptable contrast, the radius of curvature of the reflector should be larger than approximately 2 cm. When the optical absorption cavity is disposed between the absorber layer and reflector, the reflector is freely suspended via an attachment post at the center of the reflector, as described above. To satisfy the flatness criterion, the reflector thickness must be greater than ˜0.5 μm, thus creating a large thermal mass within the pixel which slows the thermal response time. Accordingly, the reflector thickness must be decreased to minimize the thermal response time of the pixel. 
         [0027]    The sensitivity of a micromechanical pixel is related to the bending of bi-layer cantilevers in response to heat transferred from the absorber layer. As such, any increase in the bending sensitivity of the bi-layer cantilevers causes a corresponding increase in the reflected intensity contrast over same temperature change. In general terms, the bending sensitivity of a bi-layer cantilever is proportional to the difference in thermal expansion coefficients of the two constituent materials and inversely proportional to the cantilever thickness. The constituent materials composing a bi-layer cantilever include a metal having a high thermal expansion coefficient, typically aluminum or gold, and a dielectric having a low thermal expansion coefficient, typically Si 3 N 4  or SiO 2 , although other materials are possible. Accordingly, a material having a higher thermal expansion coefficient and a smaller thickness enhances the thermal sensitivity of the bending elements. 
         [0028]    The sensitivity and response time of a micromechanical pixel is related to degree of isolation offered by the thermal isolation region between the innermost bi-layer cantilevers and the outermost bi-layer cantilevers. The ideal thermal resistance within the isolation region represents a trade-off between thermal sensitivity and thermal response time. A large thermal resistance allows heat to accumulate within the pixel, which enhances thermal sensitivity, but a small thermal resistance allows heat to be more quickly removed from the pixel so as to reduce the thermal response time. Accordingly, thermal resistance within the thermal isolation region must be tailored to optimize both sensitivity and response time. 
         [0029]    The micromechanical pixel from  FIG. 2  facilitates designs variations within the pixel structure so as to enhance the responsivity of the pixel without affecting the thermal properties thereof. Likewise, the thermal response time of the micromechanical pixel from  FIG. 2  is characterized by the thermal time constant τ, defined by 
         [0000]    
       
         
           
             
               
                 
                   τ 
                   = 
                   
                     C 
                     G 
                   
                 
               
               
                 
                   ( 
                   6 
                   ) 
                 
               
             
           
         
       
     
         [0000]    where C is the heat capacitance of the pixel. For a micromechanical pixel, the heat capacitance is the sum of all components, especially the absorber and reflector since these have by far the largest volumes. The thermal response time of a pixel is minimized by reducing its heat capacitance, either by shrinking the pixel dimensions or choosing materials with lower specific heat capacities. 
         [0030]    It will be appreciated by those skilled in the art that the description herein, including the disclosure provided by the illustrative claims section, is illustrative and explanatory of this invention, but is not intended to be restrictive thereof or limiting of the advantages, applications, and uses which can be achieved by this invention. 
         [0031]    Several exemplary advantages are noteworthy. For example, the present invention is simpler to fabricate, more robust thus enabling tighter manufacturing tolerances and higher uniformity, more responsive to thermal inputs, and more optically flat than the related arts. Furthermore, the present invention achieves a higher fill factor, higher absorption efficiency, greater bending, and greater tilt than the related arts. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0032]    The invention will now be described in more detail, by way of example only, with reference to the accompanying drawings, in which: 
           [0033]      FIG. 1   a  is perspective view of an exemplary bi-layer cantilever element composed of a layer having a high thermal expansion coefficient and a layer having a low thermal expansion coefficient. 
           [0034]      FIG. 1   b  is a perspective view of the bi-layer cantilever element from  FIG. 1   a  showing resultant bending after the cantilever is heated by infrared radiation. 
           [0035]      FIG. 2  is a perspective view of a micromechanical pixel including a sensing element, a pair of bi-layer micro-cantilevers, and a substrate. 
           [0036]      FIG. 3  is a front elevation view of the micromechanical pixel in  FIG. 2  showing arrangement of sensing element and substrate. 
           [0037]      FIG. 4  is a side elevation view of a bi-layer micro-cantilever for the micromechanical pixel in  FIG. 2  showing arrangement of layers with high and low thermal expansion coefficients. 
           [0038]      FIG. 5  is a schematic diagram showing an exemplary imaging device including an array of micro-cantilever pixels within an optical readout camera and an array of receptor pixels within an imager. 
           [0039]      FIG. 6   a  is a front elevation view of an embodiment of the present invention including a multi-layer reflector. 
           [0040]      FIG. 6   b  is a front elevation view of an alternate embodiment of the device in  FIG. 6   a  wherein the sensing element is front illuminated by inverting the order of the absorber and reflector and optical readout occurs through the substrate. 
           [0041]      FIG. 7  is a front elevation view of an embodiment of the present invention showing reflector, absorber, and partially absorbing layers contacting in a layered arrangement. 
           [0042]      FIG. 8  is a front elevation view of an alternate embodiment of the device in  FIG. 7  wherein the sensing element is front illuminated and having a reflective coating applied to the top surface of the substrate opposite of the partially absorbing layer so that optical readout occurs through the substrate. 
           [0043]      FIG. 9   a  is a front elevation view of an alternate embodiment of the present invention wherein a readout circuit with capacitive sensor contacts the substrate opposite of an absorbing layer so as to allow the capacitive sensor to measure the gap between substrate and sensing element. 
           [0044]      FIG. 9   b  is a side elevation view of a bending element showing an electrically conductive layer with low thermal conductance applied to the low expansion layer. 
           [0045]      FIG. 10  is an exemplary absorption spectrum plot for one exemplary implementation of the device in  FIG. 7 . 
           [0046]      FIG. 11  is a top elevation view of an embodiment of the present invention including bending elements arranged in an opposing fashion so that they deflect in opposed directions. 
           [0047]      FIG. 12  is a side elevation view of one bending element showing arrangement of the low and high expansion layers so that the high expansion layer is disposed opposite of the substrate. 
           [0048]      FIG. 13  is a top elevation view of an alternate embodiment of the device in  FIG. 11  wherein the bending elements are joined to the sensing element at its center. 
           [0049]      FIG. 14  is a top elevation view of an alternate embodiment of the device from  FIG. 11  wherein the bending elements are joined to the sensing element in an offset arrangement. 
           [0050]      FIG. 15  is a top elevation view of an alternate embodiment of the device in  FIG. 11  wherein one bending element is a bi-layered element and the other bending element is a single layer element. 
           [0051]      FIG. 16  is side elevation view of a bending element wherein an ultra-high expansion polymer is attached to a low-expansion dielectric. 
           [0052]      FIG. 17  is a side elevation view of a bending element wherein an ultra-high expansion polymer is attached to a high-expansion metal which is thereafter attached to a low expansion dielectric. 
           [0053]      FIG. 18  is an exemplary deflection-temperature plot for a bi-layered cantilever without and with a polymer coating. 
           [0054]      FIG. 19   a  is top elevation view of an alternate embodiment of the present invention including a pair of interlocking bending elements, wherein sensing element and substrate are not shown. 
           [0055]      FIG. 19   b  is a front elevation view of a micromechanical device showing the interlocking elements in  FIG. 19   a  disposed between a sensing element and substrate. 
           [0056]      FIGS. 20   a - 20   d  are sectional views from the device in  FIG. 19   a  showing arrangement of high and low thermal expansion layers. 
           [0057]      FIGS. 21   a - 21   d  are sectional views of alternate embodiments of the bi-layer cantilever showing various non-planar structures. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0058]    Embodiments of the present micromechanical device  65  are described with reference to the micromechanical pixel  3  shown in  FIG. 2 . However, the present invention is applicable within a wide variety of micromechanical pixels that rely on mechanical deflection in response to infrared absorption to generate an optically readable signal. 
         [0059]    Accordingly, the description of and drawing for embodiments for the present invention provided below describe and show design features of the new micromechanical device  65  without further reference to features the embodiment may have with other micromechanical devices. The embodiments described below may be fabricated via micromechanical methods and processes understood in the art. 
         [0060]    Referring now to  FIG. 6   a , the sensing element  4  of a micromechanical device  65  with backside illumination  32  includes a layered reflector  35 , an optical absorption cavity  7 , and an absorber layer  8 . The layered reflector  35  is constructed to have a multi-layered structure including a pair of outer layers  36 ,  38  disposed about and contacting an inner layer  37 . Outer layers  36 ,  38  are composed of a highly reflective metal, examples including, but not limited to, aluminum, gold, silver, copper, chromium, nickel, platinum, tantalum, titanium, and alloys thereof, to accommodate optical readout. The inner layer  37  is composed of a MEMS compatible dielectric, examples including, but not limited to, Al 2 O 3 , HfO 2 , MgO, SiC, Si 3 N 4 , SiO 2 , TiN, and ZrO 2 . In preferred embodiments, the layered reflector  35  should be planar in extent and have a radius of curvature greater than approximately 2 cm to ensure a flatness which minimizes stray reflections and scattering. In some embodiments, the layered reflector  35  is preferred to have a thickness not more than approximately 0.5 μm to ensure flatness and thermal response time required for most applications. 
         [0061]    Outer layers  36 ,  38  and inner layer  37  are typically thin films of uniform thickness. Outer layers  36 ,  38  may have the same thickness so as to form a layered reflector  35  of symmetric extent or different thicknesses so as to form a layered reflector  35  of asymmetric design. Both embodiments may offset the presence of stress gradients that develop within the layered reflector  35  during fabrication of the micromechanical device  65 . 
         [0062]    Outer layers  36 ,  38  and inner layer  37  are layered to form a single structure via thin-film deposition methods understood in the art. The layered reflector  35  distributes stresses more evenly throughout the thickness of the structure than a single layer element, so as to prevent warp along the layered reflector  35 . 
         [0063]    The thin film outer layers  36 ,  38  and inner layer  37  may develop an intrinsic stress on the order of −300 MPa to +300 MPa during fabrication of the layered reflector  35  and the micromechanical device  65 . This stress may be reduced via a low-temperature anneal cycle at a temperature less than 400° C. Thin films having a thickness of at least 0.5 μm were also found to mitigate stress related warp; however, it is desired to minimize film thickness to minimize the heat capacity of the micromechanical device  65 . 
         [0064]    In most embodiments, it is not possible to completely remove stress gradients within the layered reflector  35 . The layered reflector  35  may be used to balance and offset stresses across the median plane of the structure to avoid stress induced warp. A layered reflector  35  with balanced internal stresses further allows the thickness thereof to be less than that of a single layer structure composed of a metal, without compromising the flatness of the element. 
         [0065]    The layered reflector  35  shown in  FIG. 6   a  reduces the heat capacity of a micromechanical device  65  and the thermal response thereof because the layered reflector  35  is thinner than a single layer structure and the dielectric inner layer  37  has a lower specific heat capacity than metals which typically comprise a layered reflector  35 . 
         [0066]    A partially absorbing layer  41  may be applied to the absorber layer  8  so as to maintain a high absorption efficiency within the micromechanical device  65 . The partially absorbing layer  41  is a thin-film layer, which enhances absorption within the sensing element  4 . In preferred embodiments, the thickness of the partially absorbing layer  41  is approximately 10 nm. 
         [0067]    The absorption characteristics of the partially absorbing layer  41  are directly related to the sheet resistance of the composition comprising the layer. In preferred embodiments, the partially absorbing layer  41  should be an alloy with an adjustable sheet resistance, one example being NiCr, to facilitate the optimization of absorption within a specific infrared band or for a specific pixel design. 
         [0068]    Referring again to  FIG. 6   a , the substrate  9  may in some embodiments include an anti-reflection coatings  30 ,  31 , examples including, but not limited to, IRX/Ge/YF 3 , IRX/Ge/BaF 2 , and IRX/Ge/ZnS which are transmissive of the infrared regions of interest, applied to the surfaces  54   a ,  54   b  along the top and/or bottom of the substrate  9 . Anti-reflection coatings  30 ,  31  are applied via deposition methods understood within the art. The substrate  9  is likewise transmissive of infrared wavelengths of interest. 
         [0069]    Referring now to  FIG. 6   b , the micromechanical device  65  is shown so as to facilitate use with applications having frontside illumination  47 . In this embodiment, absorption and reflection losses caused by the transmission of infrared radiation through the substrate  9  are avoided, thus improving the absorption of infrared light by the micromechanical device  65  and facilitating a higher responsivity. Frontside illumination  47  heats the sensing element  4  via infrared radiation. Compared to the sensing element  4  in  FIG. 6   a , the sensing element  4  in  FIG. 6   b  is inverted so that the infrared radiation reaches the absorber layer  8  first. The optical absorption cavity  7  is situated between the partially absorbing layer  41  and reflector  35 , which may consist of only a single highly reflective metal layer or, as shown in  FIG. 6   b , a layered reflector  35 . 
         [0070]    Substrate  9  materials include, but are not limited to, silicon and glass. Optical readout now occurs through the substrate  9 , necessitating the substrate  9  to be transmissive in the visible or near-infrared region. To maximize infrared absorption in the sensing element  4 , thin film coatings  30 ,  31  composed of materials that are transmissive in the visible or near infrared region and reflective in the MWIR and/or LWIR regions are applied to both surfaces  54   a ,  54   b  of the substrate  9 ; however, the thin film coatings  30 ,  31  should allow for optical readout. One exemplary thin film coating  30 ,  31  is indium tin oxide (ITO). Anti-reflection coatings designed for the visible or near-infrared regions may also be applied to both surfaces  54   a ,  54   b  to increase transmission of the readout light. 
         [0071]    Referring now to  FIG. 7 , the sensing element  4  of a micromechanical device  65  includes a reflector  6 , an absorber layer  8 , and a partially absorbing layer  41  arranged and contacting in the order described. In this embodiment, the thickness of the reflector  6  may be reduced to decrease the thermal response time of the sensing element  4  without compromising the flatness thereof. The optical absorption cavity  7  is now situated between the partially absorbing layer  41  and substrate  9 . 
         [0072]    The reflector  6  is desired to have good reflectivity and low absorptivity in the visible or near-infrared regions to facilitate optical readout. Reflector  6  materials may include, but are not limited to, aluminum, gold, silver, copper, chromium, nickel, platinum, tantalum, titanium, and alloys thereof. The thickness of the reflector  6  may be approximately 0.2 μm because it is now supported by the absorber layer  8 . In preferred embodiments, the thickness of the reflector  6  should be less than that of the absorber layer  8 , so as to minimize thermally-induced bending within the sensing element  4 . 
         [0073]    Referring again to  FIG. 7 , backside illumination  32  of the micromechanical device  65  may in some embodiments require an anti-reflection coating  31  along the surface  54   b  of the substrate  9  closest to the infrared source to maximize transmission. The anti-reflection coating  31  should be transmissive of the infrared regions of interest. It is not necessary to apply an anti-reflection coating to the other surface  54   a  along the substrate  9 , because higher reflectivity is desired within the optical absorption cavity  7 . 
         [0074]    Referring now to  FIG. 10 , the absorption spectrum is shown for an exemplary micromechanical device  65  including the structure in  FIG. 7 . The micromechanical device  65  is comprised of a sensing element  4  including a partially absorbing layer  41  composed of NiCr, an absorber layer  8  composed of SiN, a reflector  6  composed of NiCr/Au, a substrate  9  composed of silicon, and an anti-reflection coating  31  composed of IRX/Ge/ZnS. The optical absorption cavity  7  is a gap or space dimensioned to form a half-wavelength resonance cavity.  FIG. 10  indicates that absorption is achievable within both MWIR and LWIR bands via a single pixel design. 
         [0075]    Referring now to  FIG. 8 , the sensing element  4  of the micromechanical device  65  is shown where the absorber layer  8  is in contact with the reflector  6 . In this embodiment, the optical absorption cavity  7  is situated between the partially absorbing layer  41  and substrate  9 . Frontside illumination  47  heats the sensing element  4  via infrared radiation. The combination of absorber layer  8  and reflector  6  into a single stack facilitates the 2 cm flatness requirement via a lower total thickness than otherwise achievable when the layers are separated. The lower heat capacity resulting from the lower material volume within the combined stack facilitates a quicker response time. 
         [0076]    The reflector  6  must transmit medium-wavelength infrared (MWIR) and long-wavelength infrared (LWIR) and reflect in the visible or near-infrared regions. Exemplary materials comprising the reflector  6  include Yb 2 O 3 , Y 2 O 3 , Zr 2 O 3 , and Hf 2 O 3 . The absorber layer  8  may be composed of Al 2 O 3 , HfO 2 , MgO, SiC, Si 3 N 4 , SiO 2 , TiN, and ZrO 2 . The partially absorbing layer  41  may be composed of NiCr to enhance absorption. In some embodiments, the reflector  6  may be eliminated through careful design of the absorber layer  8  and partially absorbing layer  41 . 
         [0077]    Referring now to  FIG. 9   a , the micromechanical device  65  is shown so as to facilitate use with applications having capacitive readout to detect the tilt within a sensing element  4 , rather than the optical readout as described for embodiments in  FIGS. 6   a ,  6   b ,  7 , and  8 . The sensing element  4  contains a conductive layer  56  which contacts and is attached to an absorber layer  8 . The absorber layer  8  may be composed of materials including, but not limited to, Al 2 O 3 , HfO 2 , MgO, SiC, Si 3 N 4 , SiO 2 , TiN, and ZrO 2 . The micromechanical device  65  also contains a capacitive sensor with readout circuit  48  which resides on along one surface  54   a  of the substrate  9 . The conductive layer  56  and capacitive sensor with readout circuit  48  facilitate measurements of the gap  55  between the substrate  9  and sensing element  4 , which is dependent on the intensity of infrared radiation absorbed by the pixel. 
         [0078]    The conductive layer  56  serves as one plate of a parallel-plate capacitor. The conductive layer  56  may be composed of a metal or metal alloy which is electrically conductive or a partially-absorbing metal or alloy, examples including, but not limited to, NiCr, to enhance absorption. The second plate of the parallel-plate capacitor is located within the capacitive sensor with readout circuit  48  attached to the substrate  9  using thin-film deposition methods understood in the art. The second plate may be composed of a metal or metal alloy. In this embodiment, the bending elements  5   a ,  5   b  also contain an electrically conductive layer  56  along its length, as represented in  FIG. 9   b , to electrically connect the sensing element  4  to the readout circuit disposed along the substrate  9 . Since the thermal isolation region  12   a ,  12   b  along a bending element  5   a ,  5   b  must have low thermal conductance, the conductive layer  56  must also have low thermal conductance. Exemplary compositions for the conductive layer  56  include indium tin oxide and titanium nitride. 
         [0079]    The sensing element  4  absorbs infrared radiation deflecting the sensing element  4  and decreasing the distance between the two parallel plates, thereby altering the capacitance in the parallel-plate capacitor. The capacitive sensor with readout circuit  48  measures the resultant gap  55 . In this embodiment, the bi-layer cantilevers  10   a ,  10   b ,  11   a ,  11   b  provide automatic compensation to changes in ambient temperature. The present embodiment eliminates shot noise which is common to optical readout devices. 
         [0080]    Referring now to  FIG. 11 , a micromechanical device  65  is shown whereby the bending elements  5   a ,  5   b  are disposed so as to be separately responsive such that the left bending element  5   b  deflects towards the substrate  9  and the right bending element  5   a  deflects away from the substrate  9 . It is likewise possible for the order of deflection noted above to be reversed. Unlike the micromechanical pixel  3  shown in  FIG. 2 , which has the sensing element  4  tilt in the direction parallel to the bi-layer cantilevers  10   a ,  10   b ,  11   a ,  11   b , the present embodiment enables the sensing element  4  to twist in the direction perpendicular to the bi-layer cantilevers  10   a ,  10   b ,  11   a ,  11   b . The resultant device enhances the overall tilt angle of the sensing element  4 , thus improving thermal sensitivity. The sensing elements  4  and substrate  9  described herein are applicable to this embodiment. 
         [0081]    Opposed functionality of the bending elements  5   a ,  5   b  is achieved by placing the high expansion layers  33  along the top surface  57  of the low expansion layers  34  along the left bending element  5   b , as represented in  FIG. 4 , and the high expansion layers  33  along the bottom surface  58  of the low expansion layers  34  along the right bending element  5   a , as represented in  FIG. 12 . The bending elements  5   a ,  5   b  contact and are attached to the sensing element  4  at one end thereof in a symmetric fashion. 
         [0082]    Opposed functionality of the bending elements  5   a ,  5   b  may also be achieved by placing the high expansion layer  33  along the top surface  57  of the low expansion layer  34  for half of its length and along the bottom surface  58  of the low expansion layer  34  for the other half of its length. The bending elements  5   a ,  5   b  deflect in an s-shape in this configuration. This configuration prevents the bending elements  5   a ,  5   b  from contacting the substrate that may occur if they were to deflect downward. 
         [0083]    The responsivity of a micromechanical device  65  is proportional to the tilt angle of the sensing element  4 , which is equal to the difference between the angles of tilt along the outer bi-layer cantilevers  10   a ,  10   b  at ambient temperature and the inner bi-layer cantilevers  11   a ,  11   b  heated by incident infrared radiation. The sensing element  4  tilts at the same angle as the inner bi-layer cantilevers  11   a ,  11   b , or Δθ, when the bending elements  5   a ,  5   b  are mutually responsive so as to deflect in the same direction, as provided by the device in  FIG. 2 . Assuming a small deflection angle, the change in angle, Δθ, of a bi-layer cantilever  10   a ,  10   b ,  11   a ,  11   b  in response to a change above ambient temperature ΔT is given by the expression 
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         [0000]    where l b  is the length of the bi-layer cantilevers  10   a ,  10   b ,  11   a ,  11   b  and t 1  and t 2  are the thicknesses, E 1  and E 2  are the Young&#39;s modulus, and α 1  and α 2  are the thermal expansion coefficients of the two materials comprising the cantilevers. 
         [0084]    Referring again to  FIG. 11 , the sensing element  4  twists at an angle determined by the out-of-plane deflection of the opposed bending elements  5   a ,  5   b . Assuming a small angle, the out-of-plane deflection of the bending elements  5   a ,  5   b  is l b Δθ and the angle of twist is equal to 2l b Δθ/w, where w is the width between the bending elements  5   a ,  5   b  or the width of the sensing element  4 , and the factor of 2 results from the opposed deflections of the bending elements  5   a ,  5   b . The degree of enhancement achievable by bending elements  5   a ,  5   b  with opposed functionality is approximately 2l b /w, corresponding to a 2 to 4 times increase in the responsivity of a typical micromechanical pixel  3 . 
         [0085]    Referring now to  FIG. 13 , a micromechanical device  65  is shown wherein the bending elements  5   a ,  5   b  from  FIG. 12  are arranged to contact and attach to the sensing element  4  at an offset  60  from an edge  59   a  along the sensing element  4 . In preferred embodiments, the offset  60  should be approximately one-half of the total height (h) of the sensing element  4 , although other arrangements are possible. This embodiment increases the rigidity of the micromechanical device  65 , however, the bending moment between bending elements  5   a ,  5   b  and sensing element  4  is reduced. The sensing elements  4  and substrates  9  described herein are applicable to this embodiment. 
         [0086]    The twisting of the sensing element  4 , as described above, produces torsion within the bending elements  5   a ,  5   b . The degree of responsivity enhancement may be limited by the torsional rigidity of the bending elements  5   a ,  5   b . In some embodiments, it might be desirous to reduce the thickness of the contact structure between each bending element  5   a ,  5   b  and the sensing element  4  so as to further reduce the torsional rigidity. In other embodiments, it might be advantageous to include a spring-like or hinge-like connection between each bending element  5   a ,  5   b  and sensing element  4 . In yet other embodiments, it could be advantageous to have the contact be composed of a material having a low Young&#39;s modulus. 
         [0087]    Referring now to  FIG. 14 , a micromechanical device  65  is shown wherein the bending elements  5   a ,  5   b  from  FIG. 12  are arranged to contact and attach to the sensing element  4  in an asymmetric arrangement at an offset  42  along the sensing element  4 . While  FIG. 14  shows the left bending element  5   b  aligned with the top edge  59   a  and the right bending element  5   a  aligned with the bottom edge  59   b , other arrangements are possible whereby one or both bending elements  5   a ,  5   b  are located along the sensing element  4  at a distance from the respective edge  59   a ,  59   b . The sensing elements  4  and substrates  9  described herein are applicable to this embodiment. 
         [0088]    In this embodiment, the degree of tilt is increased over the device shown in  FIG. 2  by approximately 2l b /√{square root over (w 2 +l 2   p )}, where l p  is the length of the sensing element  4 . Accordingly, the responsivity of the present embodiment is 1.5 to 3 times greater than that of a micromechanical pixel  3  shown in  FIG. 2 . 
         [0089]    Referring now to  FIG. 15 , a micromechanical device  65  is shown wherein the right bending element  5   b  includes a pair of bi-layer cantilevers  10   b ,  11   b  and thermal isolation region  12   b , as described above, and the left bending element  5   a  is comprised of a low expansion layer  43  composed of a single material or composition. In this embodiment, the bending element  5   b  deflects towards or away from the substrate  9  enabling the sensing element  4  to tilt in the direction parallel to the bending elements  5   a ,  5   b  and to twist in the direction perpendicular to the bending elements  5   a ,  5   b . It is likewise possible for the arrangement of deflecting and non-deflecting members to be reversed. The sensing elements  4  and substrate  9  described herein are applicable to this embodiment. 
         [0090]    In this embodiment, the degree of tilt is increased over the device shown in  FIG. 2  by approximately l b /w, and approximately one-half that for the device in  FIG. 11 . 
         [0091]    Referring now to  FIG. 16 , the high expansion layer  33  within a bi-layer cantilever  45  may be replaced by an ultra-high expansion layer  44  comprised of a polymer, having a coefficient of thermal expansion greater than approximately 5×10 −5  K −1 . Exemplary ultra-high expansion polymers include, but are not limited to, styrene, acrylonitrile, pentafluorostyrene, methylmethacrylate, methacrylonitrile, benzonitrile, trimethylsilylacetylene, and trimethylsilylacetonitrile. The ultra-high expansion layer  44  contacts and is joined to a low expansion layer  34  composed of materials known within the art. 
         [0092]    Referring now to  FIG. 17 , the ultra-high expansion layer  44  may be directly joined to the high expansion layer  33  opposite of the low expansion layer  34  to form a tri-layer cantilever  46 .  FIG. 18  compares the deflection achieved by an exemplary bi-layer cantilever  10   a ,  10   b ,  11   a ,  11   b  and a tri-layer cantilever  46 . 
         [0093]    Referring now to  FIGS. 19   a ,  19   b , and  20   a - 20   d , a micromechanical device  65  is shown including a pair of u-shaped bending elements  5   a ,  5   b  which are oppositely disposed and interlocking. The first bending element  5   a  includes a pair of bi-layer cantilevers  10   a ,  11   a  separated by a thermal isolation region  12   a . The right bi-layer cantilever  10   a  includes a high expansion layer  33  disposed along the upper surface of a low expansion layer  34  and the left bi-layer cantilever  11   a  includes a high expansion layer  33  disposed along the lower or opposing surface of the low expansion layer  34 . The second bending element  5   b  includes a pair of bi-layer cantilevers  10   b ,  11   b  separated by a thermal isolation region  12   b . The left bi-layer cantilever  10   b  includes a high expansion layer  33  disposed along the lower surface of a low expansion layer  34  and the left bi-layer cantilever  11   b  includes a high expansion layer  33  disposed along the upper surface of the low expansion layer  34 . When interlocked in an opposing fashion, the high expansion layers  33  are disposed along the same surface of the bi-layer cantilevers  10   b  and  11   a  and bi-layer cantilevers  10   a  and  11   b . In some embodiments, the paired arrangement of high expansion layers  33  may be oppositely disposed, as shown in  FIG. 19   a , or on the same side as in  FIG. 2 . 
         [0094]    The outermost bi-layer cantilevers  10   a ,  10   b  are attached at their outermost end  63  to the substrate  9  via anchor posts  13   d ,  13   a , respectively. The inner most bi-layer cantilevers  11   a ,  11   b  are attached at their innermost end  62  to the planar surface  61  of the sensing element  4  via anchor posts  13   b ,  13   c , respectively. While a variety of arrangements are possible for the sensing element  4 , bi-layer cantilevers  10   a ,  10   b ,  11   a ,  11   b , and substrate  9 , it is preferred for the bi-layer cantilevers  10   a ,  10   b ,  11   a ,  11   b  to be disposed between the sensing element  4  and substrate  9 , as shown in  FIG. 19   b . The responsivity of the present embodiment is 3 to 6 times greater than that of a typical micromechanical pixel  3  shown in  FIG. 2 . The sensing elements  4  and substrates  9  described herein are applicable to this embodiment. 
         [0095]    Referring now to  FIGS. 21   a - 21   d , a variety of non-planar cantilevers, referred to as folded bi-layer cantilevers  64 , are described. Profiles may include, but are not limited to, triangular, square, trapezoidal, and curved, as shown in  FIGS. 21   a - 21   d , respectively. Folding is provided along the length (L) of the otherwise planar high expansion layer  33  and low expansion layer  34  comprising the bi-layer cantilevers  10   a ,  10   b ,  11   a ,  11   b ,  45 ,  46  described in  FIGS. 4 ,  12 , and  16 . Folding increases the effective length of the bi-layer cantilever  10   a ,  10   b ,  11   a ,  11   b ,  45 ,  46  without increasing the lateral length and pitch of a micromechanical device  65 . Folded bi-layer cantilevers  64  may be formed by three-dimensional patterning via standard micromechanical fabrication techniques. 
         [0096]    From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this invention, and without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions. 
         [0097]    The description above indicates that a great degree of flexibility is offered in terms of the invention. Although the present invention has been described in considerable detail with reference to certain preferred versions thereof, other versions are possible. Therefore, the spirit and scope of the appended claims should not be limited to the description of the preferred versions contained herein.