Patent Publication Number: US-2022228920-A1

Title: Microbolometer with filtering function

Description:
The present patent application claims priority from the French patent application filed on 5 Jun. 2019 and assigned application no. FR1905965, the contents of which is hereby incorporated by reference. 
     TECHNICAL FIELD 
     The present disclosure relates generally to the field of infrared imaging, and in particular to a microbolometer and to a method of manufacturing a microbolometer. 
     BACKGROUND ART 
     Microbolometers are a type of uncooled infrared (IR) camera used to capture thermal images of an image scene. Such IR cameras generally comprise an arrangement of IR-sensitive detectors forming a pixel array. Each pixel of the pixel array converts a measured temperature at the pixel into a corresponding electrical signal, generally a voltage, which is in turn converted by an ADC (analog to digital converter) into a digital output signal. 
     Each pixel of a microbolometer comprises a membrane suspended over a substrate. The membrane comprises an absorption layer that absorbs energy from the IR light hitting the pixel, causing its temperature to rise as a function of the intensity of the IR light. The membrane for example also comprises a thermal layer which has the property that its resistance is modified by this temperature rise, and the pixel can thus be read by detecting the change in resistance of this thermal layer, which is thermally linked to the absorption layer. 
     It is generally desirable that a microbolometer has a relatively high sensitivity, generally implying a high absorption rate, for the targeted range of wavelengths. It is also desirable that the device is relatively compact and the cost relatively low. 
     However, there is a technical problem in reducing the dimensions and/or cost of a microbolometer without decreasing its sensitivity. Indeed, the smaller the pixel pitch of a microbolometer, the lower the amount of absorbed power, and the more difficult it is to engineer arms for supporting the absorption layer having relatively high thermal resistance. 
     SUMMARY OF INVENTION 
     It is an aim of embodiments of the present disclosure to at least partially address one or more problems in the prior art. 
     According to one embodiment, there is provided a microbolometer comprising an array of pixels, each pixel comprising one or more detection cells, each detection cell comprising an absorption layer, wherein: the pitch of the detection cells in at least one direction in the plane of pixel array is between 5 and 11 μm; a pixel fill factor FF of the absorption layer of the one or more detection cells in each pixel is in a range 0.10 to 0.50; and a sheet resistance Rs of the absorption layer of each detection cell is between 16 and 189 ohm/sq. 
     According to one embodiment, a ratio Rs/FF of each pixel of the array is between 200 and 600 ohm/sq. 
     According to one embodiment, each pixel has a pixel fill factor in the range 0.10 to 0.40. 
     According to one embodiment, each pixel has a pixel fill factor in the range 0.20 to 0.40. 
     According to one embodiment, the pitch of the detection cells in at least one direction in the plane of pixel array is between 8 and 9 μm. 
     According to one embodiment, each pixel of the array has:
         a pixel fill factor FF equal to or greater than 0.40 and less than 0.50 and a sheet resistance Rs of the absorption layer of at least 75 ohm/sq; or   a pixel fill factor FF equal to or greater than 0.30 and less than 0.40 and a sheet resistance Rs of the absorption layer of at least 50 ohm/sq; or   a pixel fill factor FF equal to or greater than 0.20 and less than 0.30 and a sheet resistance Rs of the absorption layer of at least 25 ohm/sq; or   a pixel fill factor FF equal to or greater than 0.10 and less than 0.20 and a sheet resistance Rs of the absorption layer of at least 16 ohm/sq.       

     According to one embodiment, a ratio Rs/FF is in a range 377 ohm/sq plus or minus 20%. 
     According to one embodiment, the absorption layer of each detection cell is a metal layer. 
     According to one embodiment, the absorption layer is formed of TiN and has a thickness of between 10 and 115 nm. 
     According to one embodiment, the pixel array comprises a substrate, and each pixel of the pixel array comprises a reflective layer formed on the substrate and a membrane suspended over the reflective layer, a quarter-wave cavity being formed between the membrane and the reflective layer in each pixel, and the membrane comprising the absorption layer and a thermal layer. 
     According to one embodiment, the absorption layer has a surface area of less than 75 percent of the surface area of the membrane. 
     According to one embodiment, the quarter-wave cavity has a height in the range 1.5 to 3.5 μm. 
     According to one embodiment, the pitch of the detection cells in at least one direction in the plane of pixel array is less than four times the height of the quarter-wave cavity. 
     According to a further aspect, there is provided a method of manufacturing a microbolometer array comprising: forming an array of pixels, each pixel having one or more detection cells, wherein forming said array comprising: forming the detection cells to have a pitch of between 5 and 11 μm in at least one axis in the plane of the pixel array; and forming each detection cell to comprise an absorption layer having a pixel fill factor FF in a range 0.10 to 0.50 and a sheet resistance Rs of between 16 and 189 ohm/sq. 
     According to yet a further aspect, there is provided a microbolometer comprising an array of pixels, each pixel comprising one or more detection cells, each detection cell comprising an absorption layer forming a quarter-wave cavity having a height h of between 1.5 and 5 μm, wherein the pitch of the detection cells in at least one axis in a plane of the pixel array is in the range 2.4h to 3.6h. 
     According to one embodiment, a pixel fill factor FF of the absorption layer of the one or more detection cells in each pixel is in a range 0.20 to 0.70. 
     According to one embodiment, a pixel fill factor FF of the absorption layer of the one or more detection cells in each pixel is in a range 0.10 to 0.50. 
     According to one embodiment, the pitch of the detection cells is in the range 4 to 15 μm. 
     According to one embodiment, the pitch of the detection cells is in the range 5 to 11 μm. 
     According to one embodiment, the absorption layer is a metal layer having a sheet resistance of 189 ohm/sq or less. 
     According to one embodiment, the absorption layer is a metal layer having a sheet resistance of 126 ohm/sq or less. 
     According to one embodiment, the absorption layer is formed of TiN. 
     According to one embodiment, the cavity height is of between 1.5 and 3.5 μm. 
     According to one embodiment, each pixel of the array has:
         a pixel fill factor FF equal to or greater than 0.40 and less than 0.50 and a sheet resistance Rs of the absorption layer of at least 75 ohm/sq; or   a pixel fill factor FF equal to or greater than 0.30 and less than 0.40 and a sheet resistance Rs of the absorption layer of at least 50 ohm/sq; or   a pixel fill factor FF equal to or greater than 0.20 and less than 0.30 and a sheet resistance Rs of the absorption layer of at least 25 ohm/sq; or   a pixel fill factor FF equal to or greater than 0.10 and less than 0.20 and a sheet resistance Rs of the absorption layer of at least 16 ohm/sq.       

     According to one embodiment, the ratio Rs/FF of each pixel of the array is between 200 and 600 ohm/sq. 
     According to one embodiment, the ratio Rs/FF of each pixel of the array is within 20 percent of 377 ohm/sq. 
     According to one embodiment, each detection cell comprises a membrane comprising the absorption layer, a thermal layer and a dielectric layer. 
     According to yet a further aspect, there is provided a method of fabricating a microbolometer, the method comprising forming an array of pixels, each pixel comprising one or more detection cells, wherein forming the array comprises: forming each detection cell to comprise an absorption layer forming a quarter-wave cavity having a height h of between 1.5 and 5 μm; and forming the detection cells to have a pitch, in at least one axis in the plane of the pixel array, in the range 2.4h to 3.6h. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The foregoing features and advantages, as well as others, will be described in detail in the following description of specific embodiments given by way of illustration and not limitation with reference to the accompanying drawings, in which: 
         FIG. 1  schematically illustrates image capture circuits of an IR camera according to an example embodiment; 
         FIG. 2  is a cross-section view of a pixel of a microbolometer according to an example embodiment; 
         FIG. 3  is a plan view of part of a microbolometer array according to an example embodiment; 
         FIG. 4  is a graph showing an absorption rate of an absorption layer of a microbolometer pixel as a function of the pixel fill factor for eight different sheet resistances of the absorption layer in the case of a pixel pitch of 17 μm, a Fabry-Perot cavity height of 2.5 μm and for a light wavelength of 10 μm; 
         FIG. 5  is a perspective view of a pixel of a microbolometer according to an example embodiment of the present disclosure; 
         FIG. 6  is a cross-section view of part of the pixel of  FIG. 5  according to an example embodiment of the present disclosure; 
         FIG. 7  is a graph showing an absorption rate of an absorption layer of a microbolometer pixel as a function of the pixel fill factor for eight different sheet resistances of the absorption layer in the case of a pixel pitch of 8.5 μm, a Fabry-Perot cavity height of 2.5 μm and for a light wavelength of 10 μm; 
         FIG. 8  is a plan view of part of a microbolometer array according to an example embodiment of the present disclosure; 
         FIG. 9  is a plan view of part of a microbolometer array according to a further example embodiment of the present disclosure; 
         FIG. 10  is a graph showing an absorption rate of an absorption layer of a microbolometer pixel as a function of the pixel fill factor for many different sheet resistances of the absorption layer in the case of a pixel pitch of 8.5 μm, a Fabry-Perot cavity height of 2.5 μm and for a light wavelength of 10 μm; 
         FIG. 11  is a graph showing an absorption rate gain of an absorption layer of a microbolometer pixel as a function of the pixel fill factor for many different sheet resistances of the absorption layer in the case of a pixel pitch of 8.5 μm, a Fabry-Perot cavity height of 2.5 μm and for a light wavelength of 10 μm; 
         FIG. 12  is a graph showing in more detail a region of the graph of  FIG. 11  corresponding to a gain of 20% or more; 
         FIG. 13  is a graph representing in more detail a region of the graph of  FIG. 11  corresponding to a gain of 50% or more; 
         FIG. 14  is a graph representing an absorption rate of an absorption layer of a microbolometer pixel as a function of a ratio of sheet resistance/fill factor for different fill factors; 
         FIG. 15  is a graph showing in more detail a region of the graph of  FIG. 14  corresponding to an absorption rate of over 0.9; 
         FIG. 16  is a graph showing an absorption rate of an absorption layer of a microbolometer pixel as a function of the pixel fill factor for eight different sheet resistances of the absorption layer in the case of a pixel pitch of 11 μm, a Fabry-Perot cavity height of 3.5 μm and for a light wavelength of 13 μm; 
         FIG. 17  is a graph showing an absorption rate of an absorption layer of a microbolometer pixel as a function of the pixel fill factor for eight different sheet resistances of the absorption layer in the case of a pixel pitch of 5 μm, a Fabry-Perot cavity height of 1.5 μm and for a light wavelength of 6 μm; 
         FIG. 18  is a graph showing an absorption rate of an absorption layer of a microbolometer pixel as a function of the pixel fill factor for eight different sheet resistances of the absorption layer in the case of a pixel pitch of 5 μm, a Fabry-Perot cavity height of 2.5 μm and a light wavelength of 10 μm; 
         FIG. 19  is a graph showing an absorption rate of an absorption layer of a microbolometer pixel as a function of the pixel fill factor for eight different sheet resistances of the absorption layer in the case of a pixel pitch of 6 μm, a Fabry-Perot cavity height of 2.5 μm and a light wavelength of 10 μm; 
         FIG. 20  is a graph showing an absorption rate of an absorption layer of a microbolometer pixel as a function of the pixel fill factor for eight different sheet resistances of the absorption layer in the case of a pixel pitch of 7 μm, a Fabry-Perot cavity height of 2.5 μm and a light wavelength of 10 μm; 
         FIG. 21  is a graph showing an absorption rate of an absorption layer of a microbolometer pixel as a function of the pixel fill factor for eight different sheet resistances of the absorption layer in the case of a pixel pitch of 8 μm, a Fabry-Perot cavity height of 2.5 μm and a light wavelength of 10 μm; 
         FIG. 22  is a graph showing an absorption rate of an absorption layer of a microbolometer pixel as a function of the pixel fill factor for eight different thicknesses of the absorption layer in the case of a pixel pitch of 9 μm, a Fabry-Perot cavity height of 2.5 μm and a light wavelength of 10 μm; 
         FIG. 23  is a graph showing an absorption rate of an absorption layer of a microbolometer pixel as a function of the pixel fill factor for eight different sheet resistances of the absorption layer in the case of a pixel pitch of 10 μm, a Fabry-Perot cavity height of 2.5 μm and a light wavelength of 10 μm; 
         FIG. 24  is a graph showing an absorption rate of an absorption layer of a microbolometer pixel as a function of the pixel fill factor for eight different sheet resistances of the absorption layer in the case of a pixel pitch of 11 μm, a Fabry-Perot cavity height of 2.5 μm and a light wavelength of 10 μm; 
         FIG. 25  is a graph showing an absorption rate of an absorption layer of a microbolometer pixel as a function of the pixel fill factor for eight different sheet resistances of the absorption layer in the case of a pixel pitch of 12 μm, a Fabry-Perot cavity height of 2.5 μm and a light wavelength of 10 μm; 
         FIG. 26A  is a graph showing an absorption rate of an absorption layer of a microbolometer pixel as a function of the light wavelength, for seven different pixel fill factors, for a pixel pitch of 8.5 μm and for a Fabry-Perot cavity height of 2.5 μm; 
         FIGS. 26B to 26G  are graphs showing an absorption rate of an absorption layer of a microbolometer pixel as a function of the light wavelength, for seven different pixel fill factors,  FIGS. 26B and 26C  corresponding to a Fabry-Perot cavity height of 2.5 μm and pixel pitches of 6 μm and 9 μm respectively;  FIGS. 26D and 26E  corresponding to a Fabry-Perot cavity height of 1.5 μm and pixel pitches of 3.6 μm and 5.4 μm respectively;  FIGS. 26F and 26G  corresponding to a Fabry-Perot cavity height of 5 μm and pixel pitches of 12 μm and 18 μm respectively; 
         FIGS. 26H to 26R  are graphs showing an absorption rate, of an absorption layer of a microbolometer pixel having a Fabry-Perot cavity height of 2.5 μm, as a function of the light wavelength, for seven different pixel fill factors, the figures respectively showing examples with pixel pitches of 5 μm, 5.5 μm, 6 μm, 6.5 μm, 7 μm, 7.5 μm, 8 μm, 8.5 μm, 9 μm, 9.5 μm and 10 μm; 
         FIG. 27  is a graph showing a rate of non-absorbed diffracted power as a function of the light wavelength and for seven different pixel fill factors and for a pixel pitch of 8.5 μm and for a Fabry-Perot cavity height of 2.5 μm; 
         FIGS. 28 and 29  are graphs showing a spatial distribution of the electric field and magnetic field respectively in a pixel of a microbolometer having an absorption layer of 6 nm in thickness, a pixel fill factor of 0.3, and for a light wavelength of 6 μm; 
         FIGS. 30 and 31  are graphs showing a spatial distribution of the electric field and magnetic field respectively in a pixel of a microbolometer having an absorption layer of 6 nm in thickness, a pixel fill factor of 0.3, and for a light wavelength of 8 μm; 
         FIGS. 32 and 33  are graphs showing a spatial distribution of the electric field and magnetic field respectively in a pixel of a microbolometer having an absorption layer of 6 nm in thickness, a pixel fill factor of 0.3, and for a light wavelength of 10 μm; 
         FIGS. 34 and 35  are graphs showing a spatial distribution of the electric field and magnetic field respectively in a pixel of a microbolometer having an absorption layer of 6 nm in thickness, a pixel fill factor of 0.3, and for a light wavelength of 12 μm; 
         FIGS. 36 and 37  are graphs showing a spatial distribution of the electric field and magnetic field respectively in a pixel of a microbolometer having an absorption layer of 6 nm in thickness, a pixel fill factor of 0.3, and for a light wavelength of 14 μm; 
         FIGS. 38 and 39  are graphs showing a spatial distribution of the electric field and magnetic field respectively in a pixel of a microbolometer having an absorption layer of 22 nm in thickness, a pixel fill factor of 0.3, and for a light wavelength of 6 μm; 
         FIGS. 40 and 41  are graphs showing a spatial distribution of the electric field and magnetic field respectively in a pixel of a microbolometer having an absorption layer of 22 nm in thickness, a pixel fill factor of 0.3, and for a light wavelength of 8 μm; 
         FIGS. 42 and 43  are graphs showing a spatial distribution of the electric field and magnetic field respectively in a pixel of a microbolometer having an absorption layer of 22 nm in thickness, a pixel fill factor of 0.3, and for a light wavelength of 10 μm; 
         FIGS. 44 and 45  are graphs showing a spatial distribution of the electric field and magnetic field respectively in a pixel of a microbolometer having an absorption layer of 22 nm in thickness, a pixel fill factor of 0.3, and for a light wavelength of 12 μm; and 
         FIGS. 46 and 47  are graphs showing a spatial distribution of the electric field and magnetic field respectively in a pixel of a microbolometer having an absorption layer of 22 nm in thickness, a pixel fill factor of 0.3, and for a light wavelength of 14 μm. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Like features have been designated by like references in the various figures. In particular, the structural and/or functional features that are common among the various embodiments may have the same references and may dispose identical structural, dimensional and material properties. 
     For the sake of clarity, only the operations and elements that are useful for an understanding of the embodiments described herein have been illustrated and described in detail. In particular, the circuits used for measuring pixel resistances in a microbolometer array have not been described in detail, and nor have the methods for processing captured pixel data in order to generate thermal images. 
     In the following disclosure, unless indicated otherwise, when reference is made to absolute positional qualifiers, such as the terms “front”, “back”, “top”, “bottom”, “left”, “right”, etc., or to relative positional qualifiers, such as the terms “above”, “below”, “higher”, “lower”, etc., or to qualifiers of orientation, such as “horizontal”, “vertical”, etc., reference is made to the orientation shown in the figures, or to a microbolometer as orientated during normal use. 
     Unless specified otherwise, the expressions “around”, “approximately”, “substantially” and “in the order of” signify within 10%, and preferably within 5%. 
     In the following description, the following terms will be considered to have the following definitions. 
     “absorption layer”: a layer that absorbs energy from IR light in a pixel of a microbolometer. In the embodiments of the present disclosure, this layer is formed of a metal such as TiN, Ti or Pt. 
     “pixel fill factor”: the ratio between the surface area of the absorption layer and the pixel surface area. In the present disclosure, the values of the pixel fill factor are provided with an accuracy of two decimal places. 
     “pixel pitch” or “detection cell pitch”: the interval at which the pixels/detection cells are formed in a microbolometer array. The pixel pitch may correspond to the width of each pixel or detection cell in the x or y direction, or the distance from the edge of one pixel or detection cell to the corresponding edge of an adjacent pixel or detection cell. In the present disclosure, the pitches of pixels and of detection cells are provided in micrometers with an accuracy of one decimal place, and in some cases of two decimal places. 
     Throughout the present disclosure, values of sheet resistance are assumed to be accurate to within 5 percent, and thicknesses expressed in nanometers are assumed to be accurate to the closest nanometer. 
       FIG. 1  schematically illustrates an image capture device  100  of an IR camera according to an example embodiment. The device  100  comprises an array  102  of pixels  104  forming a microbolometer that is capable of operating at ambient temperature. The array for example comprises N columns and M rows, where N and M are for example each equal to 2 or more, and can equal over a thousand. An output of the array  102  is coupled to a read circuit  106 , which is for example a read out integrated circuit (ROIC). The circuit  106  for example includes one or more analogue to digital converters for converting signals captured in the microbolometer array  102  into digital signals S. The digital signals S are for example provided to a processing device (P)  108  that performs processing of the raw image data in order to generate thermal images I. For example, the device  100  is capable of capturing still thermal images or thermal images forming a video stream. 
       FIG. 2  is a cross-section view of one of the pixels  104  of the microbolometer array  102  of  FIG. 1  in more detail. The pixel  104  comprises a membrane  202  suspended over a reflective surface  204  formed on a substrate  206 . For example, the membrane  202  is supported by arms  208  that also provide thermal insulation between the substrate  206  and the membrane  202 . 
     The membrane  202  for example comprises, across at least part of its surface area, an absorption layer  210 , a thermal layer  212 , and dielectric layers  214 , one of which isolates the absorption and thermal layers  210 ,  212  from each other, and a further two of which sandwich the layers  210 ,  212 . 
     The absorption layer  210  is for example formed of metal, such as TiN, Ti, Pt or another metal. The absorption layer  210  absorbs the energy from infrared light hitting the pixel and is thus heated. This heat is transferred to the thermal layer  212 , which is formed of a material having an electrical resistance that varies as a function of temperature. Contacts C 1  and C 2  close to edges of the thermal layer  212  permit the resistance of the thermal layer of each pixel to be measured by read out circuitry (not illustrated in  FIG. 2 ), via the arms  208 . 
     The thickness Em of the membrane  202  is typically in the range 100 nm to 1 μm. 
     The space between the membrane  202  and the surface of the reflective layer  204  defines a quarter-wave cavity  216 , also known as a Fabry-Perot cavity. Generally, the cavity  216  is filled with air or under a partial vacuum. The height h of this cavity is chosen to achieve a relatively high absorption rate of infrared light at a desired wavelength range by the absorption layer  210  of the membrane  202 . In particular, the height h of the cavity is chosen to equal λtg/4, where λtg is a target wavelength. 
     The height h is for example in the range 0.50 to 5.0 μm, and in some embodiments in the range 1.5 to 3.5 μm. In the following, unless stated otherwise, the examples and simulations are based on cavity heights of 2.5 μm, which provides relatively high absorption of light wavelengths centered around 10 μm. Indeed, the spectrum around this wavelength is generally of the most interest for thermal IR applications. However, in alternative embodiments different wavelengths of light could be targeted. 
       FIG. 3  illustrates part of the microbolometer array  102  of  FIG. 1  in more detail according to an example embodiment. In  FIG. 3 , a dashed grid represents the limits of the pixels of the array, and solid rectangles represent the surface area of the absorption layers  210  of each pixel. 
     An x axis in  FIG. 3  is defined as corresponding to the direction, in the plane of the microbolometer array, of the rows of pixels, and a y axis in  FIG. 3  is defined as corresponding to the direction, in the plane of the microbolometer array, of the columns of pixels. 
     The pixels in the microbolometer array for example have a pixel pitch Px in the x direction and pixel pitch Py in the y direction, the pixel pitches Px and Py for example being substantially identical. In the following, when reference is made to a particular value of the pixel pitch, unless stated otherwise, it is assumed that this is the pitch in both the x and y directions. An area Apix of the pixel is equal to Px.Py. The absorption layers  210  for example have widths Wx in the x direction that are shorter than the pitch Px, and widths Wy in the y direction that are shorter than the pitch Py. The surface area Aabs of the absorption layer is thus equal to Wx.Wy. A pixel fill factor FF of the absorption layer within each pixel can be defined as Aabs/Apix. 
     First Aspect 
     In order to provide a relatively high absorption rate of the microbolometer, and thus a high sensitivity, the fill factor FF is generally chosen to be as close to 1 as possible, as will now be described with reference to  FIG. 4 . 
       FIG. 4  is a graph representing the absorption rate (ABSORPTION RATE) of a microbolometer pixel, such as the pixel  104  of  FIGS. 1 to 3 , having a pixel pitch of 17.0 μm, as a function of the fill factor (FF), for eight different sheet resistances of an absorption layer formed of TiN and for a light wavelength of 10 μm. The sheet resistances and thicknesses of the TiN absorption layers corresponding to the curves  401  to  408  of  FIG. 4  are indicated in the following table: 
     
       
         
           
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Curve 
                 TiN Thickness 
                 Sheet Resistance 
               
               
                 Reference 
                 (nm) 
                 (ohm/sq) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 401 
                 5 
                 377 
               
               
                 402 
                 10 
                 189 
               
               
                 403 
                 15 
                 126 
               
               
                 404 
                 20 
                 94 
               
               
                 405 
                 25 
                 75 
               
               
                 406 
                 30 
                 63 
               
               
                 407 
                 35 
                 54 
               
               
                 408 
                 40 
                 47 
               
               
                   
               
            
           
         
       
     
     It can be seen from  FIG. 4  that an absorption rate of over 0.9 can be achieved using relatively thin absorption layers of 5 or 10 nm in thickness, and using a fill factor of over 0.65. The best absorption rate is obtained using an absorption layer of 5 nm in thickness and with a fill factor approaching 0.90. However, the achievable fill factor for pixels having a pitch of 17.0 μm is generally around 0.60. 
     In order to reduce the dimensions of the microbolometer array while maintaining the number of pixels, it would be desirable to reduce the pixel pitch. However, reducing the pixel pitch is difficult to achieve without significantly reducing the fill factor. Furthermore, the curves of  FIG. 4  indicate that reducing the fill factor would lead to a significant reduction in the absorption rate, which is undesirable. 
     The present inventor has found that, surprisingly, when the pixel pitch in at least one axis, and preferably both axes, is reduced to a value in the range 5 to 11 μm, and preferably to a value in the range 8 to 9 μm, a high absorption rate can still be achieved by using a relatively low fill factor of between 0.10 and 0.50, and selecting an absorption layer  210  having a sheet resistance of between 16 and 189 ohm/sq, and in some embodiments of between 16 and 130 ohm/sq. 
     Reducing the sheet resistance of the absorption layer implies increasing its thickness. For example, for an absorption layer of TiN, the thickness is of at least 10 nm in order to achieve a sheet resistance of less than 200 ohm/sq, and of at least 15 nm in order to achieve a sheet resistance of less than 130 ohm/sq. 
     Such an increase in the thickness of the absorption layer can increase the thermal conduction between the absorption layer and the substrate, which could lead to poor performance. In particular, with reference to the pixel of  FIG. 2 , increasing the thickness of the layer  210  will increase the thermal conduction to the substrate  206  via the arms  208 , leading to reduced sensitivity. A modified pixel having a relatively low fill factor and relatively low thermal conduction with the substrate will now be described in more detail with reference to  FIG. 5 . 
       FIG. 5  is a perspective view of a pixel  500  of a microbolometer according to an example embodiment of the present disclosure. 
     The pixel  500  for example comprises a membrane  502  suspended over a reflective layer  504  formed on a substrate  506 . 
     The membrane  502  is for example supported by a pair of arms  518 ,  520 , which are respectively anchored to two pillars  508  formed at opposite corners of the pixel. Each pillar  508  comprises a base portion  510 , from which extends a column  512 . The columns  512  of the two pillars pass through respective end portions  514 ,  516  of the arms  518 ,  520  and into caps  522 . The arms  518 ,  520  link the pillars to the membrane  502 , and provide, in particular, the functions of mechanically supporting the membrane  502 , of providing an electrical connection between the membrane  502  and the ROIC (not illustrated in  FIG. 5 ) formed for example in the substrate  506 , and of providing thermal insulation between the membrane  502  and the substrate  506 . In order to provide good thermal insulation, the arms  518 ,  520  are for example relatively long, providing relatively high thermal resistance. Indeed, in the example of  FIG. 5 , the arms  518 ,  520  extend parallel to, and separate from, opposite edges of the membrane  502 , and are attached to opposite corners of the membrane  502  via linking portions  524 . Thus the arms  518 ,  520  for example extend substantially the length of one edge of the membrane  502 . In some embodiments, each of the arms  518 ,  520  has a length of at least 50 percent of the pixel pitch. 
     The membrane  502  for example comprises, in a portion of its surface area, a stack  526  comprising a thermal layer  528  and an absorption layer  530 , these layers for example being insulated from each other by dielectric layers (not illustrated in  FIG. 5 ). For example, the stack  526  occupies less than 75% of the surface area of the membrane  502 , including the surface of the arms  518 ,  520 . The region of the membrane  502  surrounding the stack  526  forms a support layer for the stack  526 , and also provides an electrical connection between points close to the edges of the thermal layer  528  and the arms  518 ,  520  such that the resistance of the thermal layer  528  can be measured by the read out circuitry. For example, dashed lines in  FIG. 5  represent a footprint of electrical connections  531 ,  532  respectively extending from the arms  518 ,  520  to opposite edges of the thermal layer  528 . These electrical connections  531 ,  532 , and the arms  518 ,  520  are for example formed by a metallic layer of TiN, Ti, Pt, or another metal, sandwiched between dielectric layers, formed for example of silicon nitride, silicon dioxide, silicon oxynitride, or other electrically insulating material. 
     While in the embodiment of  FIG. 5  the absorption layer  530  is formed over the thermal layer  528 , in alternative embodiments the order could be inversed, the absorption layer  530  for example being formed on an underside of the membrane  502 . 
     A cavity  533  between the reflective layer  504  and the membrane  502  forms a quarter-wave cavity of height h. In one example, this height h is of 2.5 μm in order to target light wavelengths of around 10 μm, although in alternative embodiments different heights, for example in the range 1.5 to 3.5 μm could be used to target different wavelengths of light. 
     In the example of  FIG. 5 , the membrane  502  is substantially square, but includes cutouts  534 ,  536  at opposite corners permitting space for the pillars  508 . The stack  526  is for example formed in a portion of the membrane  502  between these cutouts, and thereby extends nearly the full length of the membrane  502 . 
     Of course, while  FIG. 5  provides one example of implementation of a microbolometer having an absorption layer of relatively low sheet resistance, it will be apparent to those skilled in the art that many different implementations would be possible, for example omitting the base portions  510  and/or the caps  522 , and/or using different forms for the arms  518 ,  520  and electrical connections  531 ,  532 . 
       FIG. 6  is a cross-section view taken along a line A-A′ of  FIG. 5  passing through the membrane  502  and through the arms  518 ,  520 . 
     As illustrated in  FIG. 6 , the membrane  502  is for example formed of a dielectric layer  602 , in which are formed the electrical connections  531 ,  532 , for example by metal deposition. The thermal layer  528  and absorption layer  530  are for example electrically insulated from each other by a dielectric layer  604 , and regions close to the edges of the underside of the thermal layer  528  for example contact top surfaces of the electrical connections  531 ,  532 , which are otherwise covered by dielectric material. In one embodiment, the absorption layer  530  is a layer of TiN having a thickness of at least 10 nm, and in some cases of at least 15 nm. The thermal layer  528  for example has a thickness of around 100 nm. 
     It should be noted that while the electrical connections  531 ,  532  add to the absorbent surface area of the pixel, given the relatively low thickness and high sheet resistance of these metal layers, the present inventor has found that they have a relatively small impact on the effective optical fill factor of the pixel, and can thus be ignored. 
       FIG. 7  is a graph representing the absorption rate (ABSORPTION RATE) of an absorption layer, as a function of the fill factor (FF) of a microbolometer pixel, such as the pixel  500  of  FIG. 5 , and having a pitch of 8.5 μm, for eight different sheet resistances of the absorption layer  530 . As with the example of  FIG. 4 ,  FIG. 7  is based on a quarter-wave cavity having a height of 2.5 μm and a light wavelength of 10 μm. The absorption layer  530  is assumed to be formed of TiN, and the sheet resistances of the absorption layer  530  corresponding to the curves  701  to  708  of  FIG. 7  are the same as those of the curves  401  to  408  of Table 1 respectively. 
     It can be seen in  FIG. 7  that, in the case of the curves  702  to  708 , the best absorption rates can be achieved when the fill factor is equal to 0.50 or lower, and 0.40 or lower in the case of the curves  703  to  708 . 
     It can be seen from  FIG. 7  that, below a fill factor of 0.20, the absorption rate falls rapidly, even for the thickest absorption layers. Therefore, in some embodiments, the fill factor could be chosen to be equal to or greater than 0.20. 
     Thus it can be seen from  FIG. 7  that a microbolometer having a relatively low pixel pitch and a high absorption rate can be achieved by reducing the fill factor of the absorption layers of each pixel to the range 0.1 to 0.5, and also reducing the sheet resistance of the absorption layers to less than 189 ohm/sq, and in some embodiments to less than 130 ohm/sq. 
     Examples of microbolometer arrays having fill factors of 0.5 or less will now be described in more detail with reference to  FIGS. 8 and 9 . 
       FIG. 8  illustrates part of a microbolometer array  800  according to one example embodiment. Like in  FIG. 3 , a dashed grid represents the limits of the pixels  804  of the array and solid rectangles represent the surface area of each absorption layer  530 . However, with respect to the example of  FIG. 3 , the pixel fill factor FF has been reduced in the array of  FIG. 8 , for example to a value in the range 0.10 to 0.50. Furthermore, the absorption layers  530  are not square, but rectangular in  FIG. 8 , the widths Wx in the x axis being lower than the widths Wy in the y axis. The pixels are for example square, the pixel pitch Px in the x direction being equal to the pixel pitch Py in the y direction, although in some embodiments these pitches could be different. The pitches Px and Py are each for example in the range 5.0 to 11.0 μm. 
     In the example of  FIG. 8 , each pixel  804  of the array corresponds to a detection cell comprising a single absorption layer  530 . In alternative embodiments, each pixel may comprise more than one detection cell, as will now be described in relation with  FIG. 9 . 
       FIG. 9  illustrates part of a microbolometer array  900  of a microbolometer according to a further example embodiment. In the example of  FIG. 9 , a dashed grid represents the limits of the pixels  904  of the array and solid rectangles represent the surface area of the absorption layers  530 . Each pixel  904  comprises more than one detection cell, each detection cell having a corresponding absorption layer  530 . In the example of  FIG. 9 , each pixel  904  comprises a two-by-two arrangement of four detection cells. The detection cells of each pixel  904  are for example coupled together such that they generate a single pixel value, and thus each group of detection cells can be considered to form a single pixel of the array. 
     The pixel fill factor FF in the case of  FIG. 9  becomes equal to the ratio Aabs/Apix, where Apix is the surface area of each pixel, and Aabs is the combined surface area of the absorption layers  530  in each pixel. 
     The relevant pitch in the case of  FIG. 9  is no longer the pixel pitch, but the pitch of the detection cells, in other words of the absorption layers  530 . These pitches are labelled Px and Py in the x and y directions respectively in  FIG. 9 . For example, these pitches Px, Py are each measured from an edge of one absorption layer  530  to the corresponding edge of the adjacent absorption layer  530 . 
       FIG. 10  is a graph similar to that of  FIG. 7 , for the same types of microbolometer pixels, but showing further curves for sheet resistances down to 16 ohm/sq. Indeed, the curves  1001  to  1008  of  FIG. 10  represent absorption layers having the same sheet resistances as those of the curves  401  to  408  of Table 1. The curves  1009  to  1023  correspond to the sheet resistances and TiN thicknesses indicated in the following table. 
     
       
         
           
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                 Curve 
                 TiN Thickness 
                 Sheet Resistance 
               
               
                 Reference 
                 (nm) 
                 (ohm/sq) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 1009 
                 45 
                 42 
               
               
                 1010 
                 50 
                 38 
               
               
                 1011 
                 55 
                 34 
               
               
                 1012 
                 60 
                 31 
               
               
                 1013 
                 65 
                 29 
               
               
                 1014 
                 70 
                 27 
               
               
                 1015 
                 75 
                 25 
               
               
                 1016 
                 80 
                 24 
               
               
                 1017 
                 85 
                 22 
               
               
                 1018 
                 90 
                 21 
               
               
                 1019 
                 95 
                 20 
               
               
                 1020 
                 100 
                 19 
               
               
                 1021 
                 105 
                 18 
               
               
                 1022 
                 110 
                 17 
               
               
                 1023 
                 115 
                 16 
               
               
                   
               
            
           
         
       
     
     It can be seen from  FIG. 10  that providing microbolometer pixels having fill factors of between 0.10 and 0.50 and sheet resistances of between 16 and 189 ohm/sq can allow the pixel pitch to be significantly reduced (down to 8.5 μm in the example of  FIG. 10 ), while obtaining absorption rates of at least 0.3, or at least 0.45 in the case the sheet resistance is between 42 and 126 ohm/sq. 
     Furthermore, while lower sheet resistance levels of less than 50 ohm/sq may not reach absorption rates close to 1.0, for fill factors of between 0.10 and 0.30 they can provide significant gains with respect to the use of an absorption layer having a sheet resistance close to 377 ohm/sq, corresponding to the curve  1001 , as will now be explained in more detail with reference to  FIGS. 11 to 13 . 
       FIG. 11  is a graph showing curves  1102  to  1135  representing a gain in absorption with respect to the absorption layer represented by curve  1001  in  FIG. 10 . In particular, the curves  1102  to  1123  of  FIG. 11  represent the gain of the absorption layers corresponding to curves  1002  to  1023  of  FIG. 10 . Curves  1124  to  1135  in  FIG. 11  represent absorption layers having thicknesses in 5 nm increments from 120 to 190 nm. The curves  1126  to  1134  are not labelled in  FIG. 11 , but can be easily identified by their order between the curves  1125  and  1135 . 
     It can be seen from  FIG. 11  that detectable gains can be achieved when either:
         the pixel fill factor is equal to or greater than 0.40 and less than 0.50 and the sheet resistance is between 75 and 189 ohm/sq; or   the pixel fill factor is equal to or greater than 0.30 and less than 0.40 and the sheet resistance is between 47 and 189 ohm/sq; or   the pixel fill factor is equal to or greater than 0.20 and less than 0.30 and the sheet resistance is between 25 and 189 ohm/sq; or   the pixel fill factor is equal to or greater than 0.10 and less than 0.20 and the sheet resistance is between 16 and 189 ohm/sq.       

       FIG. 12  is a graph illustrating the curves  1102  to  1125  of  FIG. 11  in more detail, and in particular the gains of 20% or more. It can be seen from  FIG. 12  that gains of over 20% can be achieved when either:
         the pixel fill factor is equal to or greater than 0.30 and less than 0.40 and the sheet resistance is between 94 and 189 ohm/sq; or   the pixel fill factor is equal to or greater than 0.20 and less than 0.30 and the sheet resistance is between 38 and 189 ohm/sq; or   the pixel fill factor is equal to or greater than 0.10 and less than 0.25 and the sheet resistance is between 22 and 189 ohm/sq.       

       FIG. 13  is a graph illustrating the curves  1102  to  1115  of  FIG. 11  in more detail, and in particular the gains of 50% or more. It can be seen from  FIG. 13  that gains of over 50% can be achieved when the pixel fill factor is equal to or greater than 0.10 and less than 0.24 and the sheet resistance is between 34 and 189 ohm/sq. 
       FIG. 14  is a graph representing the absorption rate as a function of a ratio of the sheet resistance (Rs) over fill factor (FF) for fill factors of 0.10 (curve  1401 ) and 0.20 to 0.90 (curves  1402  to  1409 , of which only the curves  1402  and  1409  are labelled in  FIG. 14 ). As with previous examples, the curves of  FIG. 14  correspond to absorption layers formed of TiN with a pixel pitch of 8.5 μm, a quarter-wave cavity height of 2.5 μm and a light wavelength of 10 μm. 
       FIG. 15  illustrates the curves  1402  to  1409  of  FIG. 14  in more detail, for absorption rates of 0.90 and over. It can be seen that absorption rates of over 0.90 can be achieved when the fill factor is between 0.10 and 0.50 and the ratio Rs/FF is in the range 200 to 600 ohm/sq. It can also be noted from  FIG. 15  that the curves are substantially centered on the ratio of 377 ohm/sq, 377 ohms corresponding to the impedance of free space Z. Indeed, high absorption rates of over 0.93 can for example be observed when the ratio Rs/FF is equal to 377 plus or minus 40%, and even higher rates of over 0.95 can be observed when the ratio Rs/FF is equal to 377 plus or minus 20%. 
     The above examples are based on quarter-wave cavity heights of 2.5 μm. The principles described in relation with these examples could equally be applied to different quarter-wave cavity heights, for example for quart-wave cavity heights in the range 1.5 to 3.5 μm, as will now be explained with reference to  FIGS. 16 and 17 . 
       FIG. 16  is a graph representing the absorption rate (ABSORPTION RATE) as a function of the fill factor (FF) of a microbolometer pixel, such as the pixel  500  of  FIG. 5 , and having a pitch of 11 μm and a quarter-wave cavity height of 1.5 μm, for eight different sheet resistances of the absorption layer  530  and for a light wavelength of 13 μm. The absorption layer  530  is assumed to be formed of TiN, and the sheet resistances of the absorption layer  530 , corresponding to the curves  1601  to  1608  of  FIG. 16 , are the same as those of the curves  401  to  408  of Table 1 respectively. 
     It can be seen from  FIG. 16  that the results are similar to those of  FIG. 7 . 
       FIG. 17  is a graph representing the absorption rate (ABSORPTION RATE) as a function of the fill factor (FF) of a microbolometer pixel, such as the pixel  500  of  FIG. 5 , and having a pitch of 5 μm and a quarter-wave cavity height of 1.5 μm, for eight different sheet resistances of the absorption layer  530  and for a light wavelength of 6 μm. The absorption layer  530  is assumed to be formed of TiN, and the sheet resistances of the absorption layer  530 , corresponding to the curves  1701  to  1708  of  FIG. 17 , are the same as those of the curves  401  to  408  of Table 1 respectively. 
     It can be seen from  FIG. 17  that the gain demonstrated by the curves  1702  to  1708  with respect to the curve  1701  is more pronounced for fill factors in the range 0.10 to 0.50, but the best results are obtained when the sheet resistance is of less than 75 ohm/sq. 
     More generally, the principles described herein are for example applicable for any pixel pitch in the range 5 to 11 μm, where the quarter-wave cavity height is for example equal to λtg/4, where λtg is the light wavelength of interest (target wavelength), and the pitch is less than λtg. Thus, in the case of a quarter-wave cavity height of 1.5 μm, the pixel pitch is for example less than 6 μm, and in the case of a quarter-wave cavity height of 3.5 μm, the pixel pitch is for example anywhere in the range 5 to 11 μm. 
     The results of  FIGS. 7 and 10 to 13  are based on pixels having a pitch of 8.5 μm. The principles described in relation with these examples could be applied to pixels having a pitch anywhere in the range 5 to 11 μm, as will now be described in more detail with reference to  FIGS. 18 to 25 . 
       FIGS. 18 to 25  are graphs representing the absorption rate (ABSORPTION RATE) as a function of the fill factor (FF) of a microbolometer pixel, such as the pixel  500  of  FIG. 5 , having a quarter-wave cavity height of 2.5 μm and for a light wavelength of 10 μm, for eight different sheet resistances of the absorption layer  530 . The absorption layer  530  is assumed to be formed of TiN, and the sheet resistances of the absorption layer  530 , corresponding to the curves i 01  to i 08  of  FIGS. 18 to 25  (for i from 18 to 25), are the same as those of the curves  401  to  408  of Table 1 respectively. 
       FIG. 18  illustrates an example of a pixel pitch of 5 μm,  FIG. 19  an example of a pixel pitch of 6 μm,  FIG. 20  an example of a pixel pitch of 7 μm,  FIG. 21  an example of a pixel pitch of 8 μm,  FIG. 22  an example of a pixel pitch of 9 μm,  FIG. 23  an example of a pixel pitch of 10 μm,  FIG. 24  an example of a pixel pitch of 11 μm and  FIG. 25  an example of a pixel pitch of 12 μm. 
     As shown by  FIGS. 18 to 22 , for pixel pitches from 5 to 6 μm and fill factors from 0.10 to 0.40, the best results are obtained when the sheet resistance is between 94 and 189 ohm/sq, for pixel pitches from 6 to 7 μm and fill factors from 0.10 to 0.50, the best results are obtained when the sheet resistance is between 94 and 189 ohm/sq, and for pixel pitches from 7 to 9 μm and fill factors from 0.10 to 0.50, the best results are obtained when the sheet resistance is between 75 and 189 ohm/sq. 
     As shown by  FIGS. 22 to 25 , for pixel pitches from 9 to 10 μm and fill factors from 0.10 to 0.50, the best results are obtained when the sheet resistance is between 75 and 189 ohm/sq, and for pixel pitches from 10 to less than 12 μm and fill factors from 0.10 to 0.50, the best results are obtained when the sheet resistance is between 94 and 189 ohm/sq. 
     Second Aspect 
     A further difficulty in microbolometers is to filter out wavelengths of light outside the target range. In some embodiments, surface processing and/or coatings can be applied to one or more optical elements between the microbolometer array and the image scene in order to filter the received light. However, such techniques add to the cost. 
     The present inventor has found that a certain choice of pixel pitch, or of detection cell pitch, can lead to a filtering function, as will now be described with reference to  FIGS. 26A and 27 . 
       FIG. 26A  is a graph showing an absorption rate as a function of the light wavelength, for a pixel pitch/detection cell pitch of 8.5 μm, and for seven different pixel fill factors of a microbolometer pixel such as the pixel  500  of  FIG. 5 . For each fill factor, a thickness of the absorption layer is chosen that renders the ratio Rs/FF substantially equal to 377 ohm/square. In particular, the curves  2601  to  2607  in  FIG. 26A  are based on absorption layers formed of TiN having fill factors and sheet resistances as defined in the following table. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                 Curve 
                   
                 TiN Thickness 
                 Sheet Resistance 
               
               
                   
                 Reference 
                 Fill Factor 
                 (nm) 
                 (ohm/sq) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 2601 
                 0.10 
                 50 
                 38 
               
               
                   
                 2602 
                 0.20 
                 25 
                 75 
               
               
                   
                 2603 
                 0.30 
                 17 
                 111 
               
               
                   
                 2604 
                 0.40 
                 13 
                 145 
               
               
                   
                 2605 
                 0.50 
                 10 
                 189 
               
               
                   
                 2606 
                 0.60 
                 8 
                 234 
               
               
                   
                 2607 
                 0.70 
                 7 
                 269 
               
               
                   
                   
               
            
           
         
       
     
     It can be seen from  FIG. 26A  that a filtering function is obtained having a cut-off frequency λr, defined as an absorption rate of less than 0.5, at a wavelength in a range (RANGE λr) of around 6 to 8 μm, depending on the fill factor. The present inventor has found that this filtering function is a result of diffraction that occurs when the pixel or detection cell pitch in either or both of the x and y directions approaches the lower cut-off wavelength defined by the quarter-wave cavity of each pixel. In the example of  FIG. 26A , the quarter-wave cavity has a height h of 2.5 μm, leading to a lower cut-off wavelength λc of around 6 μm. More generally, the lower cut-off wavelength λc defined by the quarter-wave cavity is equal to around 0.6λ0, where λ0 is equal to the targeted wavelength equal to four times the height h of the quarter-wave cavity. 
     It will be noted that, below the trough at around 6 μm, the absorption rate rises again to peak for a wavelength at around 4 μm. However, these lower wavelengths are for example removed by relatively inexpensive surface treatments or filtering layers applied to optical elements between the microbolometer array and the image scene. 
     More generally, the height h of the quarter-wave cavity of each pixel (see for example  FIGS. 2 and 5 ) is in the range 0.5 to 5 μm, leading to a targeted light wavelength of between 2 and 20 μm. However, the filtering performance is particularly apparent for quarter-wave cavities of at least 1.5 μm, corresponding to a targeted light wavelength of at least 6 μm. 
     A quarter-wave cavity of 1.5 μm in height leads to a lower cut-off wavelength λc of around 3.6 μm and a quarter-wave cavity of 5 μm in height leads to a lower cut-off wavelength λc of around 12 μm. In some embodiments, the height h of the quarter-wave cavity of each pixel is in the range 1.5 to 3.5 μm. 
     For example, in order to obtain a filtering function, the detection cell pitch is chosen to be in the range 0.9λc to 1.65λc, corresponding to the range [1.2λ0/2-1.8λ0/2] plus or minus 10% and preferably from λc to 1.5λc. Indeed, this corresponds to the range within which the quarter-wave cut-off frequency effect operates. The filtering performance is particularly apparent when the detection cell pitch is chosen to be in the range 2.4h to 3.6h, where h is the quarter-wave cavity height, and thus for a quarter-wave cavity height of 2.5 μm, the pixel pitch is in the range 6 to 9 μm. In some embodiments, according to the second aspect, the pitch of the detection cell is between 3.6 μm and 18 μm, and for example between 4 μm and 15 μm, and preferably between 5 and 11 μm. 
     It can be seen from  FIG. 26A  that the rejection, and thus the filtering, is enhanced as the fill factor decreases and the thickness of the absorption layer increases. In some embodiments, the absorption layer is formed of metal and has a sheet resistance Rs of less than 189 ohm/sq. 
     In some embodiments, the fill factor and sheet resistance of the absorption layer are chosen based on the same criteria as described above in relation with the first aspect. For example, the absorption layer has a pixel fill factor FF in a range 0.10 to 0.50 and a sheet resistance Rs in the range 16 to 189 ohm/sq. Furthermore, to obtain relatively high gains in absorption, the absorption layer is for example chosen such that:
         the pixel fill factor is equal to or greater than 0.40 and less than 0.50 and the sheet resistance is between 75 and 189 ohm/sq; or   the pixel fill factor is equal to or greater than 0.30 and less than 0.40 and the sheet resistance is between 47 and 189 ohm/sq; or   the pixel fill factor is equal to or greater than 0.20 and less than 0.30 and the sheet resistance is between 25 and 189 ohm/sq; or   the pixel fill factor is equal to or greater than 0.10 and less than 0.20 and the sheet resistance is between 16 and 189 ohm/sq.       

     Furthermore, in some embodiments, a ratio Rs/FF is in the range 200 to 600 ohm/sq, or to 377 plus or minus 40% or even to 377 plus or minus 20%. 
       FIG. 27  is a graph showing a rate of non-absorbed diffracted power as a function of the light wavelength. The seven curves  2701  to  2707  of  FIG. 27  correspond to the same absorption layers as the curves  2601  to  2607  respectively of  FIG. 26A . The non-absorbed diffracted power is calculated as the input power, minus the absorbed power, minus the reflected power. 
     While the graph of  FIG. 27  shows some artifacts, such as negative power at around the wavelength of 5 μm, the graph is still representative of the general behavior of the absorption layer. The graph of  FIG. 27  confirms in particular that power is filtered out in the wavelengths 6 to 8 μm, i.e. at around the lower cutoff wavelength λc, equal to around 7 μm, of the quarter-wave cavity. 
     Furthermore, the graph of  FIG. 27  confirms that, as the fill factor is reduced and the thickness of the absorption layer increased, the filtering effect is enhanced. 
       FIGS. 26B to 26G  are graphs showing an absorption rate of an absorption layer of a microbolometer pixel as a function of the light wavelength, for the same seven different pixel fill factors and absorption layer thicknesses as in  FIG. 26A , the corresponding curves being labelled with the same references as in  FIG. 26A . 
       FIGS. 26B and 26C  correspond to a Fabry-Perot cavity height of 2.5 μm and pixel pitches of 6 μm and 9 μm respectively. 
       FIGS. 26D and 26E  correspond to a Fabry-Perot cavity height of 1.5 μm and pixel pitches of 3.6 μm and 5.4 μm respectively. 
       FIGS. 26F and 26G  correspond to a Fabry-Perot cavity height of 5 μm and pixel pitches of 12 μm and 18 μm respectively. 
       FIGS. 26H to 26R  are graphs showing an absorption rate, of an absorption layer of a microbolometer pixel having a Fabry-Perot cavity height of 2.5 μm, as a function of the light wavelength, for the same seven different pixel fill factors and absorption layer thicknesses as in  FIG. 26A , the corresponding curves being labelled with the same references as in  FIG. 26A . The  FIGS. 26H to 26R  respectively show examples with pixel pitches of 5 μm, 5.5 μm, 6 μm, 6.5 μm, 7 μm, 7.5 μm, 8 μm, 8.5 μm, 9 μm, 9.5 μm and 10 μm, and demonstrate that the filtering function is particularly apparent when the pixel pitch is in the range 2.4h to 3.6h, corresponding to the range 6 μm to 9 μm, and particularly for fill factors of at least 0.2. 
     It should be noted that if plots were made similar to those of  FIGS. 26H to 26R , but with quarter-wave cavity heights of 1.5 μm, 5 μm, or anywhere in between, similar curves would be obtained, but with the x-axis scaled in view of the targeted wavelengths. 
     Common Aspects 
       FIGS. 28 to 47  are graphs showing the spatial distribution of electric and magnetic fields in a pixel, such as the pixel  104  or  500  described above, in the case of a pixel pitch of 8.5 μm, and a quarter-wave cavity height h of 2.5 μm. With reference also to  FIG. 2 , in the vertical scale shown in the graphs of  FIGS. 28 to 47 , the reflective layer  204  is positioned at approximately 0 μm, and the absorption layer  210  is positioned at approximately 2.5 μm. 
     The electric fields are for example represented by contours corresponding to electric field magnitudes expressed in V/m in the graphs. The magnetic fields are for example represented by contours associated with magnetic field magnitudes expressed in A/m in the graphs. 
     The graphs of  FIGS. 28 to 37  show the spatial distribution of the electric and magnetic fields in a pixel of a microbolometer having an absorption layer of 6 nm in thickness and a pixel fill factor of 0.30. Among these figures,  FIGS. 28 and 29  respectively represent the distribution of electric and magnetic fields in the case of light at a wavelength of 6 μm,  FIGS. 30 and 31  respectively represent the distribution of electric and magnetic fields in the case of light at a wavelength of 8 μm,  FIGS. 32 and 33  respectively represent the distribution of electric and magnetic fields in the case of light at a wavelength of 10 μm,  FIGS. 34 and 35  respectively represent the distribution of electric and magnetic fields in the case of light at a wavelength of 12 μm, and  FIGS. 36 and 37  respectively represent the distribution of electric and magnetic fields in the case of light at a wavelength of 14 μm. 
     The graphs of  FIGS. 38 to 47  show the spatial distribution of the electric and magnetic fields in a pixel of a microbolometer having an absorption layer of 22 nm in thickness and a pixel fill factor of 0.30. Among these figures,  FIGS. 38 and 39  respectively represent the distribution of electric and magnetic fields in the case of light at a wavelength of 6 μm,  FIGS. 40 and 41  respectively represent the distribution of electric and magnetic fields in the case of light at a wavelength of 8 μm,  FIGS. 42 and 43  respectively represent the distribution of electric and magnetic fields in the case of light at a wavelength of 10 μm,  FIGS. 44 and 45  respectively represent the distribution of electric and magnetic fields in the case of light at a wavelength of 12 μm, and  FIGS. 46 and 47  respectively represent the distribution of electric and magnetic fields in the case of light at a wavelength of 14 μm. 
     It can be seen that, at the light wavelength of 6 μm ( FIGS. 28, 29, 38 and 39 ), the effects of the Fabry-Perot cavity are dominant, while in the case of the relatively thick absorption layer ( FIGS. 38 and 39 ), there is a diffractive effect resulting in high decentralized field strengths. 
     For light at the wavelength of 8 μm ( FIGS. 30, 31, 40 and 41 ), the diffraction effect is dominant, particularly for the relatively thick absorption layer ( FIGS. 40 and 41 ). This phenomenon is also amplified as the fill factor falls. Furthermore, the field magnitude is strongly amplified for the relatively thick absorption layer. 
     For light at wavelengths of 10 μm, 12 μm and 14 μm, in the case of the relatively thin thickness of the absorption layer ( FIGS. 32 to 37 ), Fabry-Perot coupling dominates, whereas for the thicker absorption layer ( FIGS. 42 to 47 ), the Fabry-Perot coupling is replaced by a phenomenon characterized by a concentration of the electric field at the edges of the absorption layer, and a confinement of the magnetic field under the absorber. 
     An advantage of the embodiments described herein in relation with the first aspect is that a relatively high absorption rate can be achieved while using a relatively compact microbolometer array. 
     An advantage of the embodiments described herein in relation with the second aspect is that a filtering function can be obtained, thereby relaxing the constraints on optical filters of the microbolometer. 
     Furthermore, the present inventor has found that the reduction in the pixel fill factor to the range of 0.10 to 0.50 and the increase in the thickness of the absorption layer does not lead to an increase in the cross-talk between pixels, based on simulations carried out for absorption layers of 6 and 18 nm in thickness. 
     Various embodiments and variants have been described. Those skilled in the art will understand that certain features of these embodiments can be combined and other variants will readily occur to those skilled in the art. For example, it will be apparent to those skilled in the art that, while the example of absorption layers formed of TiN have been detailed, the principles described herein could be applied to other materials, including different metals.