Patent Publication Number: US-2022221635-A1

Title: Infrared filter and thermal infrared sensing device including the same

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority of Taiwanese Invention Patent Application No. 110100929, filed on Jan. 11, 2021. 
     FIELD 
     The disclosure relates to a light filter, and more particularly to an infrared filter and a thermal infrared sensing device including the same. 
    
    
     
       BACKGROUND 
         FIG. 1  illustrates a conventional thermal infrared sensing device  9 , such as a thermographic camera. The conventional thermal infrared sensing device  9 , which is sensitive to an infrared light (L) having a wavelength ranging from 8 μm to 14 μm, includes a housing  91  defining a vacuum chamber  911 , a thermal infrared image detector  92  disposed within the vacuum chamber  911 , and a filter  93  that is mounted to the housing  91  and that is arranged in front of the thermal infrared image detector  92  in an incident direction of the infrared light (L). The vacuum chamber  911  is used for preventing heat loss from the thermal infrared image detector  92  when an atmospheric pressure is higher than a pressure in the vacuum chamber  911 . The filter  93  might be a germanium (Ge) substrate or a silicon (Si) substrate. 
       Referring to  FIG. 2 , the Ge substrate has a relatively high transmittance to the infrared light (L) that has a wavelength ranging from 3 μm to 14 μm. Hence, the Ge substrate is suitable to serve as the filter  93 . Nevertheless, the Ge substrate is too expensive to be commercialized, and is brittle, hence, is unsuitable for mass production. Therefore, the Si substrate is more often used to serve as the filter  93  due to cost consideration. However, the Si substrate has a relatively low transmittance to the infrared light (L) having a wavelength ranging from 8 μm to 14 μm. 
       In order to improve the low transmittance of the Si substrate to the infrared light (L) having a wavelength ranging from 8 μm to 14 μm, reduction of a thickness of the Si substrate is proposed.  FIG. 3  illustrates the spectral transmittance of Si substrates that have different thickness and that are formed by different methods. The curves A and C represent the spectral transmittance of the Si substrates formed by a floating zone method and having a thickness of 5 mm and 20 mm, respectively. Curves B and D represent the spectral transmittance of the Si substrates formed by a Czochralski method and having a thickness of 5 mm and 20 mm, respectively. It can be seen that the thinner the Si substrate is, the higher the transmittance to the infrared light (L) having the wavelength of 8 μm to 14 μm is. Nevertheless, the thinner Si substrate tends to be deformed or bended, resulting in breaking of the Si substrate due to the atmospheric pressure being higher than the pressure in the vacuum chamber  911 . 
     
    
    
     SUMMARY 
     Therefore, an object of the disclosure is to provide an infrared filter that can alleviate or eliminate at least one of the drawbacks of the prior art. 
     According to an aspect of the disclosure, an infrared filter includes a silicon substrate and a metallic reticulated structure that is disposed on the silicon substrate and that is formed with a plurality of holes for transmitting an infrared light. 
     According to another aspect of the disclosure, a thermal infrared sensing device includes a housing defining a vacuum chamber, a thermal infrared image detector disposed within the vacuum chamber, and the abovementioned infrared filter coupled to the housing in position away from the thermal infrared image detector with the metallic reticulated structure facing the thermal infrared image detector. The infrared filter allows an incident infrared light to transmit therethrough so as to be detected by the thermal infrared image detector. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other features and advantages of the disclosure will become apparent in the following detailed description of the embodiments with reference to the accompanying drawings, of which: 
       FIG. 1  is a cross-sectional view of a conventional thermal infrared sensing device; 
       FIG. 2  is a graph illustrating transmittance curves of a germanium substrate and a silicon substrate at various wavelengths, respectively; 
       FIG. 3  is a graph respectively illustrating transmittance curves of silicon substrates each having different thickness and being formed by different methods at various wavelengths; 
       FIG. 4  is a perspective view illustrating an embodiment of an infrared filter of the disclosure; 
       FIG. 5  is a cross-sectional view of an embodiment of a thermal infrared sensing device including the infrared filter of the disclosure; and 
       FIG. 6  is a perspective cutaway view of the thermal infrared sensing device of  FIG. 5 . 
     DETAILED DESCRIPTION 
     Before the disclosure is described in greater detail, it should be noted that where considered appropriate, reference numerals or terminal portions of reference numerals have been repeated among the figures to indicate corresponding or analogous elements, which may optionally have similar characteristics. 
     Referring to  FIG. 4 , an embodiment of an infrared filter  100  of the disclosure includes a silicon substrate  1 , and a metallic reticulated structure  2  that is disposed on the silicon substrate  1 . The metallic reticulated structure  2  includes a metallic frame  21 , and a plurality of metallic wires  22  that are connected to the metallic frame  21  and that are interlaced with each other to form a plurality of holes  23  for transmitting an infrared light (L). 
     The silicon substrate  1  has a thickness of less than 0.5 mm, and may be fabricated by a floating zone method or a Czochralski method. In this embodiment, the silicon substrate  1  is fabricated by the Czochralski method for reducing cost. 
     The metallic reticulated structure  2  is made of a metal or a metallic alloy. In this embodiment, the metallic reticulated structure  2  is made of titanium. 
     The metallic frame  21  is disposed on a boundary of a surface of the silicon substrate  1 . The metallic frame  21  has a shape conforming with that of the silicon substrate  1 . 
     Each of the holes  23  has a dimension selected from a square, a rectangle, a rhombus, a circle, and a hexagon, but is not limited thereto, as long as the holes  23  allow the infrared light (L) to pass therethrough. Hence, an arrangement of the metallic wires  22  is varied to comply with the dimension of the holes  23 . In some embodiments, the metallic wires  22  are connected to and criss-cross between any two opposite sides of the metallic frame  21 , such that the holes  23  have a dimension of a square or a rectangular. In this embodiment, a dimension of each of the holes  23  is square, and the holes has a length of 50 μm and a width of 50 μm, but is not limited thereto. 
     In this embodiment, a mechanical strength of the infrared filter  100  is enhanced by disposing the metallic reticulated structure  2  on the surface of the silicon substrate  1 . Thereby, a deformation or bending of the silicon substrate  1  is reduced when an external force is applied to another surface of the silicon substrate  1  opposite to the metallic reticulated structure  2 . In order to verify the improvement in the mechanical strength of the infrared filter  100  of the disclosure, a deformation and a corresponding stress of each of a comparative example of a conventional infrared filter and Examples 1 to 5 of the infrared filter  100  of the disclosure are listed in Table 1. The values of deformation and the corresponding stress are obtained using an ANSYS simulation software. 
     The infrared filter  100  of Example 1 includes the silicon substrate  1  and the metallic reticulated structure  2  disposed thereon. The metallic reticulated structure  2  is made of titanium, has a thickness of 1 μm, and is formed with the holes  23 . Each of the holes  23  has a square dimension with a length of 50 μm and a width of 50 μm. Examples 2 to 5 of the infrared filter  100  are similar to that of Example 1 except that the metallic reticulated structures  2  of Examples 2 to 5 of the infrared filter  100  have thickness of 2 μm, 4 μm, 8 μm, and 16 μm, respectively. The conventional infrared filter of the comparative example is structurally similar to the infrared filter of Example 1 except that the conventional infrared filter does not include the metallic reticulated structure  2 . 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
             
            
               
                   
                   
               
               
                   
                   
                 Comparative 
               
               
                   
                   
                 example 
               
               
                   
                 Examples 
                 1 
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                 1 
                 2 
                 3 
                 4 
                 5 
                 Without 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Metallic 
                 Material 
                 Ti 
                 Ti 
                 Ti 
                 Ti 
                 Ti 
                 metallic 
               
               
                 reticulated 
                 Thickness (μm) 
                 1 
                 2 
                 4 
                 8 
                 16 
                 reticulated 
               
               
                 structure 
                 Width/length (μm) 
                 50/50 
                 50/50 
                 50/50 
                 50/50 
                 50/50 
                 structure 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Pressure 
                 0.5 
                 Deformation 
                 −0.83 
                 −0.79 
                 −0.71 
                 −0.58 
                 −0.41 
                 −0.85 
               
               
                 (atm) 
                   
                 (μm) 
               
               
                   
                   
                 Stress (MPa) 
                 26.6 
                 25.1 
                 23.5 
                 19.8 
                 14.2 
                 26.4 
               
               
                   
                 1 
                 Deformation 
                 −1.66 
                 −1.57 
                 −1.41 
                 −1.16 
                 −0.82 
                 −1.71 
               
               
                   
                   
                 (μm) 
               
               
                   
                   
                 Stress (MPa) 
                 53.2 
                 50.2 
                 47 
                 39.7 
                 28.4 
                 52.8 
               
               
                   
                 2 
                 Deformation 
                 −3.32 
                 −3.14 
                 −2.82 
                 −2.32 
                 −1.65 
                 −3.42 
               
               
                   
                   
                 (μm) 
               
               
                   
                   
                 Stress (MPa) 
                 106.3 
                 100.4 
                 94.1 
                 79.3 
                 56.9 
                 105.5 
               
               
                   
                 4 
                 Deformation 
                 −6.73 
                 −6.37 
                 −5.72 
                 −4.71 
                 −3.34 
                 −6.93 
               
               
                   
                   
                 (μm) 
               
               
                   
                   
                 Stress (MPa) 
                 215.5 
                 203.4 
                 190.6 
                 160.8 
                 115.2 
                 213.8 
               
               
                   
                 5 
                 Deformation 
                 −8.41 
                 −7.96 
                 −7.15 
                 −5.89 
                 −4.18 
                 −8.66 
               
               
                   
                   
                 (μm) 
               
               
                   
                   
                 Stress (MPa) 
                 269.4 
                 254.2 
                 238.3 
                 200.9 
                 144 
                 267.3 
               
               
                   
                 6 
                 Deformation 
                 −10.1 
                 −9.56 
                 −8.58 
                 −7.07 
                 −5.01 
                 −10.39 
               
               
                   
                   
                 (μm) 
               
               
                   
                   
                 Stress (MPa) 
                 323.2 
                 305.1 
                 286 
                 241.1 
                 172.8 
                 320.7 
               
               
                   
                 8 
                 Deformation 
                 −13.5 
                 −12.7 
                 −11.4 
                 −9.42 
                 −6.68 
                 −13.85 
               
               
                   
                   
                 (μm) 
               
               
                   
                   
                 Stress (MPa) 
                 423.1 
                 406.8 
                 381.3 
                 321.5 
                 230.4 
                 427.6 
               
               
                   
                 10 
                 Deformation 
                 −16.8 
                 −15.9 
                 −14.3 
                 −11.8 
                 −8.35 
                 −17.31 
               
               
                   
                   
                 (μm) 
               
               
                   
                   
                 Stress (MPa) 
                 538.7 
                 508.5 
                 476.6 
                 401.9 
                 288 
                 534.5 
               
               
                   
                 100 
                 Deformation 
                 −168 
                 −159 
                 −143 
                 −118 
                 −83.5 
                 −173.14 
               
               
                   
                   
                 (μm) 
               
               
                   
                   
                 Stress (MPa) 
                 5387 
                 5085 
                 4766 
                 4019 
                 2880 
                 5345.2 
               
               
                   
                 150 
                 Deformation 
                 −252 
                 −239 
                 −214 
                 −177 
                 −125 
                 −259.71 
               
               
                   
                   
                 (μm) 
               
               
                   
                   
                 Stress (MPa) 
                 8081 
                 7627 
                 7149 
                 6028 
                 4320 
                 8017.8 
               
               
                   
                 200 
                 Deformation 
                 −336 
                 −319 
                 −286 
                 −236 
                 −167 
                 −346.28 
               
               
                   
                   
                 (μm) 
               
               
                   
                   
                 Stress (MPa) 
                 10774 
                 10169 
                 9531 
                 8037 
                 5760 
                 10690.4 
               
               
                   
               
            
           
         
       
     
     As shown in Table 1, due to the metallic reticulated structure  2  being disposed on the silicon substrate  1 , each of the infrared filter  100  of Examples 1 to 5 has a lower deformation compared to that of the conventional infrared filter of the comparative example under different stress conditions. Furthermore, the thicker the metallic reticulated structure  2  is, the lower the deformation of the infrared filter  100  is. These results indicate that the mechanical strength of the infrared filter  100  can be enhanced by inclusion of the metallic reticulated structure  2  in a manner similar to reinforced concrete having concrete and reinforcing steel bars. 
     Referring to  FIGS. 5 and 6 , a thermal infrared sensing device  1000  of the disclosure includes a housing  200  defining a vacuum chamber  201 , a thermal infrared image detector  300  disposed within the vacuum chamber  201 , and the abovementioned infrared filter  100  coupled to the housing  200  and spaced apart from the thermal infrared image detector  300 . The metallic reticulated structure  2  of the infrared filter  100  faces the thermal infrared image detector  300 . The infrared filter  100  allows the incident infrared light (L) to transmit therethrough so as to be detected by the thermal infrared image detector  300 . 
     In this embodiment, the housing  200  has an aperture  202  that cooperates with the thermal infrared image detector  300  to define a field of view  400  with an angle (a), and the infrared filter  100  is coupled to the housing  200  in alignment with the aperture  202 , as shown in  FIG. 6 . The incident infrared light (L) within the field of view  400  is detectable by the thermal infrared image detector  300 . 
     In some embodiments, the thermal infrared image detector  300  may include a thermopile chip for receiving radiant energy of the incident infrared light (L) and converting the radiant energy to electrical energy. In some embodiments, the thermopile chip may include a semiconductor substrate (e.g., such as doped or undoped crystalline silicon, germanium, or the like), and a plurality of electronic devices formed thereon. 
     In some embodiments, the silicon substrate  1  of the infrared filter  100  serves as an optical filter selectively transmit the incident infrared light (L). 
     Due to the silicon substrate  1  having a thickness of less than 0.5 mm, the silicon substrate  1  has a higher transmittance to the infrared light (L) having a wavelength ranging from 8 μm to 14 μm, i.e., a wave band known as thermal infrared. The silicon substrate  1  having a higher transmittance to the infrared light (L) permits the thermal infrared image detector  300  to have a higher sensitivity. Furthermore, the metallic reticulated structure  2  disposed on the silicon substrate  1  enhances the mechanical strength of the infrared filter  100  and reduces the possibility of breakage or deformation of the silicon substrate  1  having a lower thickness, even if a higher external force is applied thereto. Additionally, compared with the germanium substrate, the silicon substrate  1  used in the infrared filter  100  greatly reduces the production cost of the thermal infrared sensing device  1000 . Besides, by using the Czochralski method to fabricate the silicon substrate  1 , the production cost of the same can be further reduced. 
     In the description above, for the purposes of explanation, numerous specific details have been set forth in order to provide a thorough understanding of the embodiments. It will be apparent, however, to one skilled in the art, that one or more other embodiments may be practiced without some of these specific details. It should also be appreciated that reference throughout this specification to “one embodiment,” “an embodiment,” an embodiment with an indication of an ordinal number and so forth means that a particular feature, structure, or characteristic may be included in the practice of the disclosure. It should be further appreciated that in the description, various features are sometimes grouped together in a single embodiment, figure, or description thereof for the purpose of streamlining the disclosure and aiding in the understanding of various inventive aspects, and that one or more features or specific details from one embodiment may be practiced together with one or more features or specific details from another embodiment, where appropriate, in the practice of the disclosure. 
     While the disclosure has been described in connection with what are considered the exemplary embodiments, it is understood that this disclosure is not limited to the disclosed embodiments but is intended to cover various arrangements included within the spirit and scope of the broadest interpretation so as to encompass all such modifications and equivalent arrangements.