Abstract:
An infrared sensor includes: a package; an infrared detecting element formed on the package, the infrared detecting element including a thermal detector and an absorber formed on the thermal detector which is configured to absorb infrared light rays of a specific wavelength that are detected by conversion of the infrared light rays into heat; and a cap formed on the package to cover the infrared detecting element, the cap including: a body having front and rear surfaces, through which the infrared light rays transmit; and a shielding film, with a window formed therein, provided on at least one of the front and rear surfaces of the body, the infrared light rays being reflected by the shielding film other than a portion of the shielding film having the window, and every one of the infrared light rays passing through the window of the cap impinging on the absorber.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present disclosure is related to an infrared sensor and an array thereof, and more particularly, to a wavelength selective thermal type infrared sensor and an array thereof. 
     2. Description of the Related Art 
     A conventional wavelength selective thermal type infrared sensor has an optical filter, provided on an infrared detector, to detect infrared light rays of a specific wavelength. An optical filter which transmits infrared light rays of a specific wavelength by the use of plasmon resonance is used as an optical filter, for instance (See especially JP 2007-248382 A). 
     SUMMARY OF THE INVENTION 
     A conventional wavelength selective thermal type infrared sensor using an optical filter has the following problems. Firstly, the structure of the infrared sensor becomes complex, since the optical filter is provided in addition to an infrared detector. Secondly, the detection efficiency decreases, since a part of the infrared light rays is absorbed during the rays passing through the optical filter. Thirdly, the detection efficiency depends on the incident angle of the infrared light ray, since the amount of the infrared light rays passing through the optical filter depends on the incident angle of the infrared light rays. And fourthly, a plurality of optical sensors having transparency of different wavelength are provided to detect a plurality of infrared light rays in different wavelength regions. 
     With regard to these problems, instead of an infrared sensor with an optical filter, a thermal type infrared sensor has been studies. The thermal type infrared sensor includes a periodically repeating structure on an infrared detector for generating plasmon resonance of infrared light rays of a specific wavelength to increase the absorption of the infrared light rays of the specific wavelength. 
     The thermal type infrared sensor, however, has a problem of decreasing the sensitivity (S/N ratio) of the infrared sensor, because the infrared light lays impinge on the side surface of the infrared detector having no periodically repeating structure and also on the support legs, resulting in the absorption of the infrared light lays having a wavelength other than the specific wavelength. 
     The object of the present disclosure is to provide a high-sensitivity thermal type infrared sensor in which an absorber for infrared light rays having a specific wavelength is formed on the infrared sensor instead of an optical filter. Accordingly, the absorption of infrared light rays having a wavelength other than the specific wavelength can be prevented. 
     The present disclosure an infrared sensor for detecting infrared light rays of a specific wavelength, the infrared sensor including: 
     a) a package; 
     b) an infrared detecting element formed on the package, the infrared detecting element including:
         a thermal detector; and   an absorber formed on the thermal detector which is configured to absorb infrared light rays of a specific wavelength, wherein the infrared light rays of the specific wavelength are detected by the conversion of the infrared light rays into heat; and       

     c) a cap formed on the package to cover the infrared detecting element, the cap including:
         a body having a front and rear surfaces, through which the infrared light rays transmit; and   a shielding film, with a window formed therein, provided on at least one of the front and rear surfaces of the body, wherein the infrared light rays are reflected on the shielding film other than the portion of the window, wherein       

     every one of the infrared light rays passing through the window of the cap impinge on the absorber. 
     The present disclosure is also an infrared sensor for detecting infrared light rays of a specific wavelength, the infrared sensor including: 
     a) a package; 
     b) an infrared detecting element formed on the package, the infrared detecting element including:
         a thermal detector; and   an absorber formed on the thermal detector which is configured to absorb infrared light rays of a specific wavelength, wherein the infrared light rays of the specific wavelength are detected by the conversion of the infrared light rays into heat;       

     c) a shielding structure, with a window formed therein, covering the infrared detecting element, wherein the infrared light rays pass through the window; and 
     d) a cap formed on the package to cover the shielding structure, wherein 
     every one of the infrared light rays passing through the window of the shielding structure impinge on the absorber. 
     Furthermore, the present disclosure is an infrared sensor array in which the infrared sensors are arranged in a matrix format. 
     According to the infrared sensors of the present disclosure, the infrared light rays other than those wanted to be detected will not be absorbed by the infrared detecting element. Therefore, the thermal detector of the infrared detecting element with high sensitivity can be realized. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a wavelength selective infrared sensor according to embodiment 1 of the present disclosure; 
         FIG. 2  is a cross sectional view of the wavelength selective infrared sensor taken along line II-II in  FIG. 1 ; 
         FIG. 3  is a plane view of the wavelength selective infrared detecting element of  FIG. 1 ; 
         FIG. 4  is a cross sectional view of the wavelength selective infrared detecting element taken along line IV-IV in  FIG. 3 ; 
         FIG. 5  is a cross sectional view of another wavelength selective infrared sensor according to embodiment 1 of the present disclosure; 
         FIG. 6  is a cross sectional view of another wavelength selective infrared sensor according to embodiment 1 of the present disclosure; 
         FIG. 7  is a cross sectional view of a wavelength selective infrared sensor according to embodiment 2 of the present disclosure; 
         FIG. 8  is a cross sectional view of a wavelength selective infrared sensor according to embodiment 3 of the present disclosure; 
         FIG. 9  is a cross sectional view of another wavelength selective infrared sensor according to embodiment 3 of the present disclosure; 
         FIG. 10  is a cross sectional view of another wavelength selective infrared sensor according to embodiment 3 of the present disclosure; and 
         FIG. 11  is a cross sectional view of another wavelength selective infrared sensor according to embodiment 3 of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiment 1 
       FIG. 1  is a perspective view of a wavelength selective infrared sensor according to embodiment 1 of the present disclosure, generally denoted at  100 .  FIG. 2  is a cross sectional view of the wavelength selective infrared sensor  100  taken along line II-II shown in  FIG. 1 . 
     As illustrated in  FIGS. 1 and 2 , the wavelength selective infrared sensor  100  includes a package  10  having a bottom portion  11  and a side wall portion  12  and formed of a ceramic material such as aluminum oxide. A cap  30  is fixed on the package  10  by an adhesive material such as a solder, a resin or a silver paste. The cap  30  includes a body  31  which transmits infrared light rays and a shielding film  32  which is formed on the front and rear surfaces of the body and transmits no infrared light rays. The body  31  is formed of silicon or germanium, for instance. The shielding film  32  is formed of a metal such as aluminum or gold. A portion of the body  31  which is not covered by the shielding film  32  serves as a transmission window  33  for an infrared light ray. 
     The transmission window  33  is formed by selective etching of the shielding film  32  using photolithographic technique after forming the shielding film  32  on the front and rear surfaces of the body  31  by evaporation or sputter. Instead, the shielding films  32  may be formed selectively on the body  31  by evaporation or sputter using a metal mask. 
     As illustrated in  FIG. 2 , a wavelength selective infrared detecting element  50  is formed on the bottom portion  11  of the package  10 . Although  FIG. 2  shows two infrared detecting elements  50  formed side by side on the bottom portion  11 , the number of the infrared detecting elements  50  is not limited to two. It is noted that  FIG. 2 , as well as  FIGS. 5-11 , shows a schematic view of the infrared detecting elements  50 , and the detail of the infrared detecting elements  50  is illustrated in  FIGS. 3 and 4 . The wavelength selective infrared detecting element  50  is filled with an inert gas or the like, and then sealed. 
       FIG. 3  is a plane view of the wavelength selective infrared detecting element generally denoted at  50 , and  FIG. 4  is a cross sectional view thereof taken along line IV-IV shown in  FIG. 3 . The wavelength selective infrared detecting element  50  is a thermal type infrared sensor such as a resistance type bolometer sensor of vanadium oxide (VO x ) or a SOI diode type bolometer sensor using thermal behavior of a PN diode. 
     The infrared detecting element  50  includes a substrate  51  of silicon, for instance. The substrate  51  has a concavity  60 . A thermal detector  54  is supported over the concavity  60  by two supporting legs  53 . 
     The thermal detector  54  is formed of a dielectric material such as silicon oxide, and includes a detecting film  56  and a wire layer  57  connected to the detecting film which are embedded in the dielectric material. The detecting film  56  may be a vanadium oxide (VO x ) film forming a bolometer or a crystalline silicon layer forming a pn junction, for instance. The wire layer  57  is formed of a titanium alloy, for instance. 
     The supporting leg  53  is formed of silicon oxide, for instance, and includes a wire layer  57  embedded in the silicon oxide supporting leg. On the substrate  51  surrounding the concavity  60 , a dielectric layer  52  of silicon oxide for instance is formed. A wire layer  58  of aluminum for instance is formed in the dielectric layer  52 . A detecting film  56  is connected to a wire layer  58  through the wire layer  57 . The electric signal detected by the detecting film  56  is transmitted to an external element through the wire layers  57  and  58 . 
     On the thermal detector  54 , an absorber  70  is formed. The absorber  70  is formed of a metal film having periodically aligned concave portions  71 , where an infrared light ray having a particular resonance wavelength is selectively absorbed based on surface plasmon resonance. The concave portions  71  do not extend to the other side of the metal film, and are arranged in one direction or in orthogonal directions. 
     The absorber is formed of a metal film such as gold, silver or aluminum, where surface prasmon resonance is easily generated. When an infrared light ray having a wavelength of 10 μm is absorbed, the concave portions  71 , each having a side length of 6 μm and a depth of 1.5 μm, are arranged in a matrix format with a repeating period length of 10 μm, for instance. The horizontal sectional view of the concave portion  71 , which is shown as a square, may be a circle, a rectangle, an ellipse or the like. 
     As illustrated in  FIG. 4 , the transmission window  33  is formed in the cap  30  so that every one of the infrared light rays passing through the transmission window  33  impinge on the absorber  70 , when the package  10  with the infrared detecting element  50  thereon is covered by the cap  30 . That is the transmission window  33  is so designed that incident infrared light rays with a critical angle θ (an incident light rays with an incident angle more than angle θ is totally reflected) passing through the edge of the transmission window  33  impinge on the absorber  70 . 
     In the wavelength selective infrared sensor  100  according to embodiment 1 of the present disclosure, the cap  30  with the transmission window  33  is formed on the package  10 , so that every one of the infrared light rays passing through the transmission window  33  impinge on the absorber  70  formed on the thermal detector  54 , and do not impinge on the side wall of the thermal detector  54  nor on the supporting leg  53 . Thus no infrared light rays will be absorbed on the side wall of the thermal detector  54  nor on the supporting leg  53 , only the infrared light ray of the specific wavelength is absorbed by the absorber  70 . 
     Consequently a high-sensitivity wavelength selective infrared sensor  100  with improved S/N ratio can be provided which eventually reduces noise signal components caused by the absorption of the unwanted infrared light rays having wavelength other than the specific wavelength. 
     Furthermore, a wavelength selectivity infrared sensor of low cost and simple structure can be provided, since complex MEMS technology is not used in the manufacturing process, and no optical filter is used. The detection efficiency does not depend on the incident angle, since no optical filter is used. 
       FIGS. 5 and 6  are cross sectional views of variations of wavelength selective infrared sensors according to embodiment 1 of the present disclosure, generally denoted by  110  and  120  respectively. In  FIGS. 5 and 6 , the same numerals as those used in  FIG. 2  denote the same or corresponding elements. 
     In the wavelength selective infrared sensor  110 , only the rear surface of the body  31  of the cap  30  is partially covered by the shielding film  32 , and the transmission window  33  for infrared light rays is defined by a portion where no shielding film  32  is formed. In the wavelength selective infrared sensor  120 , only the front surface of the body  31  of the cap  30  is partially covered by the shielding film  32 , and the transmission window  33  for infrared light rays is defined by a portion where no shielding film  32  is formed. 
     In the wavelength selective infrared sensors  110  and  120 , every one of the infrared light rays passing through the transmission window  33  impinges on the absorber, and does not impinge on the side wall of the thermal detector nor on the supporting leg. Consequently high-sensitivity wavelength selective infrared sensors  110  and  120  with improved S/N ratio can be provided which eventually reduces noise signal components caused by the absorption of the unwanted infrared light rays having wavelength other than the specific wavelength. 
     In  FIG. 1 , although the infrared sensor array is formed by the infrared detecting elements  50  of  FIG. 3  aligned in a 2×3 matrix format, the infrared sensor array may be formed by the infrared detecting elements of other structure of the present disclosure. A plurality of infrared light rays having different wavelength can be simultaneously detected by the infrared detecting elements having different ranges of wavelength sensitivity arranged in a matrix format. 
     Embodiment 2 
       FIG. 7  is a cross sectional view of a wavelength selective infrared sensor according to embodiment 2 of the present disclosure, generally denoted by  200 . In  FIG. 7 , the same numerals as those used in  FIG. 2  denote the same or corresponding elements. 
     The wavelength selective infrared sensor  200 , in comparison with the wavelength selective infrared sensor  110  of embodiment 1, has a recess  34  formed in the transmission window  33  of the body  31  so as to decrease the thickness thereof. 
     The recess  34  is formed through the step of etching the body  31  with the shielding film  32  used as an etching mask. When the body  31  is made of silicon, the recess  34  can be formed by the wet etching using TMAH (Tetra Methyl Ammonium Hydroxide) solution as an etching solution. 
     The wavelength selective infrared sensor  200  according to embodiment 2 of the present disclosure has the body  31  whose thickness is decreased in the transmission window  33 . Thereby, the amount of infrared absorption during the infrared light rays passing through the body  31  is decreased, so that the infrared transmittance can be increased. Consequently the high-sensitivity infrared sensor  200  can be obtained. 
     It is noted that, in the optical sensor  200 , every one of the infrared light rays passing through the transmission window  33  impinges on the absorber formed on the thermal detector of the infrared detecting elements  50 , and does not impinge on the side wall of the thermal detector nor on the supporting leg. The recess  34  may be formed in the body of other infrared sensors  100  and  120 . 
     Embodiment 3 
       FIG. 8  is a cross sectional view of a wavelength selective infrared sensor according to embodiment 3 of the present disclosure, generally denoted by  300 . In  FIG. 8 , the same numerals as those used in  FIG. 2  denote the same or corresponding elements. 
     The wavelength selective infrared sensor  300 , in comparison with the wavelength selective infrared sensor  100  of embodiment 1, has a shielding structure  80  formed on the bottom portion  11  of the package  10  to cover the infrared detecting elements  50 , instead of the shielding film  32  of the cap  30 . The shielding structure  80  has a box shape configuration with four side faces and a top face in which a window  81  is formed. The shielding structure  80  is formed of metal such as aluminum. 
     The shielding structure  80  is formed by machine processing for instance. The window  81  is formed in such a manner that the infrared light rays that are not reflected on the cap  30  but pass through the cap  30  within the region of the windows  81  will impinge on thermal detector of the infrared detecting element  50 . 
     Thus no infrared light rays are absorbed on the side wall of the thermal detector nor on the supporting leg, only the infrared light ray of a specific wavelength is absorbed by the absorber. Consequently a high-sensitivity wavelength selective infrared sensor with improved S/N ratio can be provided which eventually reduces noise signal components caused by the absorption of the unwanted infrared light rays having wavelength other than the specific wavelength. 
       FIG. 9  is a cross sectional view of a variation of the wavelength selective infrared sensor according to embodiment 3 of the present disclosure, generally denoted by  310 . In  FIG. 9 , the same numerals as those used in  FIG. 2  denote the same or corresponding elements. 
     The wavelength selective infrared sensor  310 , in comparison with the wavelength selective infrared sensor  300 , has a recess formed in the body  31  of the cap  30 , so that the thickness of the body  31  is partially decreased. The recess  34  can be formed by wet etching of the body  31  using a photoresist as an etching mask, for example. The recess  34  is formed in such a manner that the infrared light rays that transmit through the recess  34  and pass through the window  81  of the shielding structure  80  will impinge on the thermal detector of the infrared detecting element  50 . 
     In the infrared sensor  310 , the amount of infrared absorption during the transmission of rays through the body  31  is decreased, so that the infrared transmittance can be increased. Consequently the high-sensitivity infrared sensor  310  can be obtained. 
     It is noted that the recess  34  may be formed on the front surface or both the front and rear surfaces, although, in the wavelength selective infrared sensor  310 , the recess  34  is formed on the rear surface of the body  31  of the cap  30 . 
       FIG. 10  is a cross sectional view of another variation of the wavelength selective infrared sensor according to embodiment 3 of the present disclosure, generally denoted by  320 . In  FIG. 10 , the same numerals as those used in  FIG. 2  denote the same or corresponding elements. 
     The wavelength selective infrared sensor  320 , in comparison with the wavelength selective infrared sensor  100  of embodiment 1, has a shielding structure  90  formed on the infrared detecting elements  50 , instead of the shielding film  32  of the cap  30 . The shielding structure  90  has a box shape with four side faces and a top face in which a window  91  is formed. The shielding structure  90  is formed of metal such as aluminum. 
     The shielding structure  90  is formed by the MEMS technique. According to MEMS technique, for example, a sacrificing layer is formed on the infrared detecting element  50  and then a metal layer is formed on top. Then, the sacrificing layer is removed to form the shielding structure  90 . The window  91  is formed in such a manner that the infrared light rays that are not reflected on the cap  30  but pass through the window  91  will impinge on the thermal detector of the infrared detecting element  50 . 
     Thus no infrared light rays are absorbed on the side wall of the thermal detector nor on the supporting leg, only the infrared light ray of a specific wavelength is absorbed by the absorber. Consequently a high-sensitivity wavelength selective infrared sensor with improved S/N ratio can be provided which eventually reduces noise signal components caused by the absorption of the unwanted infrared light rays having wavelength other than the specific wavelength. 
       FIG. 11  is a cross sectional view of a variation of yet another wavelength selective infrared sensor according to embodiment 3 of the present disclosure, generally denoted by  330 . In  FIG. 11 , the same numerals as those used in  FIG. 2  denote the same or corresponding elements. 
     The wavelength selective infrared sensor  330 , in comparison with the wavelength selective infrared sensor  320 , has the recess  34  additionally formed in the body  31  of the cap  30 , so that the thickness of the body  31  is partially decreased. The recess  34  can be formed by wet etching of the body  31  using a photoresist as an etching mask, for example. The recess  34  is formed in such a manner that the infrared light rays that transmit through the recess  34  and pass through the window  91  of the shielding structure  90  will impinge on the thermal detector of the infrared detecting element  50 . 
     In the infrared sensor  330 , as the thickness of the body  31  is decreased in the recess  34 , the amount of infrared absorption during the transmission of rays through the body  31  is decreased, so that the infrared transmittance can be increased. Consequently the high-sensitivity infrared sensor  330  can be obtained. 
     It is noted that the recess  34  may be formed on the front surface or both the front and rear surfaces, although, in the wavelength selective infrared sensor  330 , the recess  34  is formed on the rear surface of the body  31  of the cap  30 . 
     According to embodiments 1-3 of the present disclosure, a metal film having concave portions aligned periodically is used as an absorber  70 , however, other types of metal film may be used, such as a metal film having convex portions aligned periodically, or a metal film having a dielectric film formed thereon with metal patterns formed periodically over the dielectric film to establish a MIM structure, as long as infrared light rays having a specific wavelength are absorbed by the generation of prasmon resonance. Furthermore, the absorber  70  may take such a structure that silicon nitride or silicon oxide is used as a dielectric material, and the optical path length from the front surface of the absorber  70  to the detecting film  56  is made equal to one fourth of the specific wavelength.