Abstract:
An apparatus includes a resonator portion responsive to electromagnetic radiation within a frequency range, a converter portion responsive to radiation received by the resonator portion for emitting electromagnetic radiation within a different frequency range, and a detector portion which detects the radiation emitted by the converter portion. According to one feature, the resonator portion, the converter portion and the detector portion are respective parts of an integrated circuit. According to a different feature, the radiation emitted by the converter portion is infrared energy.

Description:
TECHNICAL FIELD OF THE INVENTION 
   This invention relates in general to techniques for detecting radiation and, more particularly, to techniques for detecting radiation in one frequency range using a detector which operates in a different frequency range. 
   BACKGROUND OF THE INVENTION 
   There are existing devices that can accurately and efficiently detect and/or image infrared (IR) radiation. These devices are utilized in a variety of applications, including military and commercial applications. However, it is possible for environmental and other factors to interfere with efficient and accurate detection of a target or scene using IR energy. For example, the presence of clouds, smoke, rain, or even camouflage netting can make it difficult for an IR detector to accurately and reliably detect the IR radiation emitted from or scattered by a target or scene which is of interest. On the hand, a target or scene of interest will usually emit or scatter not only IR radiation, but also radiation at significantly lower frequencies. Radiation at these lower frequencies can readily penetrate clouds, smoke, rain, camouflage netting and other comparable conditions or structures, and is thus less susceptible to adverse influence from various environmental conditions. However, detecting and/or imaging this lower frequency radiation can present some problems. 
   In this regard, existing IR detectors (which are often called bolometers) are highly sensitive devices that are capable of measuring a temperature change caused by the absorption by the detector of received radiation. Although these devices are optimized for IR radiation, radiant energy in other frequency ranges can theoretically be absorbed and produce a measurable temperature change within an IR detector. This is advantageous from the perspective that, as noted above, radiation with a frequency well below the frequency of IR radiation is less susceptible to certain environmental influences. 
   Consideration has therefore been given to the idea of using an IR detector to detect such radiation. However, in the case of radiation with a frequency well below that of IR radiation, the wavelength of the radiation is much longer than the size of a typical bolometer. Consequently, in order to efficiently capture the energy of this radiation, it is necessary to provide a resonant antenna-like structure for the low frequency radiation. In this regard, in order to efficiently detect lower frequency radiation through use of an antenna, the electromagnetic energy received by the antenna must be efficiently coupled into the IR detector, so that the IR detector undergoes a measurable temperature change. 
   But optimized bolometers usually have a thermally sensitive material (such as amorphous silicon) with an electrical resistance which is several thousand ohms per square. This resistance would also correspond to the electromagnetic load resistance presented by the bolometer to the antenna. In contrast, resonant antenna configurations suitable for the low frequency radiation of interest will typically have a relatively low impedance, on the order of a few ohms to a few hundred ohms. Therefore, the concept of using a resonant antenna-like structure with a bolometer has been hampered by the fact that the need to maximize the sensitivity of the high-resistance bolometer tends to conflict with the need to match the bolometer impedance to the low-impedance antenna resonator. 
   SUMMARY OF THE INVENTION 
   From the foregoing, it may be appreciated that a need has arisen for a method and apparatus for capturing radiation in one frequency range and then detecting this energy with a detector that operates in a different frequency range. According to one form of the invention, a method and apparatus are provided to address this need in the context of an arrangement which includes an integrated circuit having a section with a resonator portion, a converter portion coupled to the resonator portion, and a detector portion disposed in the region of the converter portion. In this context, the method and apparatus involve: causing the converter portion to respond to radiation received by the resonator portion within a first frequency range by emitting electromagnetic radiation within a second frequency range substantially different from the first frequency range; and detecting with the detector portion the radiation emitted by the converter portion within the second frequency range. 
   A different form of the invention relates to the context of an arrangement which includes a resonator portion, a converter portion coupled to the resonator portion, and a detector portion disposed in the region of the converter portion. In this context, the method and apparatus involve: causing the converter portion to respond to radiation received by the resonator portion within a selected frequency range by emitting infrared radiation, the selected frequency range being substantially different from a frequency range of infrared radiation; and detecting with the detector portion the infrared radiation emitted by the converter portion. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A better understanding of the present invention will be realized from the detailed description which follows, taken in conjunction with the accompanying drawings, in which: 
       FIG. 1  is a diagrammatic fragmentary top view of part of an integrated circuit which includes an array of detector sections; 
       FIG. 2  is a block diagram of one of the detector sections of the apparatus of  FIG. 1 ; 
       FIG. 3  is a diagrammatic fragmentary perspective view of one of the detector sections of  FIG. 1 , in an enlarged scale; 
       FIG. 4  is a diagrammatic sectional view taken along the section line  4 — 4  in  FIG. 3 ; 
       FIG. 5  is a diagrammatic fragmentary top view of part of an alternative embodiment of the detector section of  FIG. 3 ; 
       FIGS. 6–9  are each a diagrammatic fragmentary sectional view similar to  FIG. 4 , but showing a respective different alternative embodiment; 
       FIG. 10  is a block diagram showing a detector section which is an alternative embodiment of the detector section of  FIG. 2 . 
   

   DETAILED DESCRIPTION 
     FIG. 1  is a diagrammatic fragmentary top view of an apparatus which is part of an integrated circuit  10 . The integrated circuit  10  includes a two-dimensional array of detector sections, nine of which are visible at  11 – 19  in  FIG. 1 . The nine detector sections  11 – 19  are provided on a substrate  21 , and represent a portion of a larger array of detector sections. Each of the detector sections  11 – 19  generates a respective pixel in images produced by the array. 
   In the embodiment of  FIG. 1 , the detector sections  11 – 19  are each configured to detect electromagnetic radiation which impinges on the integrated circuit  10  from externally thereof, and which is within a frequency range that is well below the frequency range of infrared (IR) radiation. Stated differently, each of the detector sections  11 – 19  is configured to detect radiation having wavelengths which are substantially longer than the wavelengths of infrared radiation. For example, infrared radiation typically has wavelengths within a range of approximately 2 to 15 microns, whereas the detector sections  11 – 19  are each configured to detect radiation with significantly longer wavelengths, such as radiation with wavelengths ranging from roughly 100 microns to roughly 2,000 microns. The detector sections  11 – 19  could, for example, each be configured to be optimally responsive to radiation having a wavelength of about 1,000 microns. However, although some specific wavelength values are given here by way of example, the present invention can be used with incident radiation having other frequencies and wavelengths. 
   In the embodiment of  FIG. 1 , the detector sections  11 – 19  are all effectively identical. Therefore, only one of these detector sections will be described here in greater detail, which is the detector section  11 . More specifically,  FIG. 2  is a block diagram of the detector section  11 . The detector section  11  includes two spaced and electrically conductive antenna elements  31  and  32 , which are each approximately triangular, and which are arranged to form a dipole antenna configuration of a type commonly known as a bow-tie antenna. Although the antenna elements  31 – 32  of  FIG. 2  define a resonator which is a bow-tie antenna, it would alternatively be possible to utilize any other suitable resonator configuration. 
   The distance  34  between the outer edges of the antenna elements  31  and  32  is selected to be approximately one-half of the wavelength λ for the center frequency of the frequency range that the detector section  11  is intended to detect. This permits the bow-tie antenna  31 – 32  to serve as a resonator with respect to frequencies near that center frequency, in order to optimize reception of those frequencies. With reference to  FIG. 1 , it will be noted that the center-to-center spacing of the detector sections  11  in both horizontal and vertical directions is a dimension which is slightly greater than the dimension  34  in  FIG. 2 . 
   The detector section  11  includes a resistor  36 , which has each end coupled to an apex of a respective one of the antenna elements  31  and  32 . The resistance of the resistor  36  is selected to be a very close match to the low impedance of the resonator defined by the antenna elements  31 – 32 . The electrical connection between each antenna element  31 – 32  and the associated end of the resistor  36  is configured to be relatively small, in order to provide a degree of thermal isolation between the resistor  36  and the antenna elements  31 – 32 , so as to minimize the extent to which heat generated within the resistor can flow into either antenna element. 
   The resistor  36  will inherently emit some level of infrared radiation, even when it is not receiving energy from the antenna elements  31 – 32 . But as the antenna elements  31 – 32  absorb radiation which is within the frequency range of interest, the energy from this received radiation will be absorbed by the resistor  36 , and the resulting electromagnetic current in the resistor will cause the resistor  36  to undergo Joule heating and to emit additional infrared radiation, as shown diagrammatically by several arrows  38 . It will be noted that, in effect, the resistor  36  takes received radiation which is within a first frequency range, and converts it into radiation within a second and significantly different frequency range, which in this embodiment is infrared radiation. 
   In  FIG. 2 , the infrared radiation emitted at  38  by the resistor  36  is detected by an infrared detector  41  of a known type, which is commonly called a bolometer. The bolometer  41  is coupled to a circuit  42  of a known type, which is commonly referred to as a readout circuit. In the integrated circuit  10  of  FIG. 1 , a respective separate readout circuit  42  is disposed within the substrate  21  below each of the detector sections  11 – 19 , but for clarity these readout circuits are not visible in  FIG. 1 . 
   Although as discussed above the detector sections  11 – 19  are all identical in the embodiment of  FIGS. 1–2 , the invention is not limited to this configuration. For example, the detector sections  11 – 19  in  FIG. 1  all have the same orientation, but it would alternatively be possible for their orientations to be different. For example, all of the detector sections in every other column could be rotated 90° clockwise from their illustrated positions. As another example, the orientations of the various detector sections could be random. It would also be possible for the detector sections to differ in structure. As one example, different detector sections could have different lengths λ/2 ( FIG. 2 ), so that different detector sections in a given array are responsive to radiation in respective different frequency ranges. This would permit the array to span a larger range of frequencies than would be practical for a single detector section. 
     FIG. 3  is a diagrammatic fragmentary perspective view which shows the detector section  11  of  FIG. 1  in an enlarged scale.  FIG. 4  is a diagrammatic fragmentary sectional view taken along the section line  4 — 4  in  FIG. 3 . It will be noted from  FIGS. 3–4  that the bolometer  41  is suspended at a location spaced vertically above the substrate  21 . In this regard,  FIG. 3  shows two electrically conductive posts  51  and  52 , which each extend vertically upwardly from the substrate  21 . As mentioned earlier, a readout circuit  42  is implemented within the substrate  21 , adjacent the top surface. For clarity, the readout circuit  42  is indicated diagrammatically in  FIG. 3  by a broken line. Each post  51  and  52  has its lower end coupled electrically to the readout circuit  42 . The upper ends of the posts  51  and  52  support a membrane  53  at a location spaced vertically above the substrate  21 . The space between the membrane  53  and the substrate  21  is approximately one-quarter of the wavelength of infrared radiation, for a reason which is explained later. 
   The membrane  53  includes a center portion  56 , as well as two L-shaped legs  57  and  58  that extend from diagonally opposite corners of the center portion  56  to the tops of the respective posts  51  and  52 . The membrane  53  is a multi-layer component. It has a bottom layer  61  which is made of a material such as silicon nitride, a bolometer layer  41  which is disposed on the bottom layer  61  and made of a thermally sensitive material such as amorphous silicon, two spaced electrodes  62  and  63  which are disposed on the bolometer layer  41 , and a top layer  66  which is made of a material such as silicon nitride. 
   The electrodes  62  and  63  are separated by a gap or slot  67  which extends across the middle of the center portion  56 . Each of the electrodes  62  and  63  has a portion which extends along a respective one of the legs  57  and  58 , and which is electrically coupled to the upper end of a respective one of the conductive posts  51  and  52 . The amorphous silicon in the bolometer layer  41  has a resistance which inherently varies in response to changes in temperature. The readout circuit  42  is coupled to the bolometer layer  41  through the conductive posts  51 – 52  and the electrodes  62 – 63 . The readout circuit  42  can electrically determine the resistance of the bolometer  41  between the electrodes  62 – 63  at any given point in time, which in turn allows the readout circuit  42  to determine the current temperature of the bolometer layer  41 . This in turn is a representation of the amount of heat that the bolometer layer  41  has absorbed, and the amount of absorbed heat corresonds to the amount of energy in the received radiation. 
   With reference to  FIG. 3 , the resistor  36  of  FIG. 2  is implemented in the form of a thin rectangular film which is disposed on the top surface of the substrate  21 . In the disclosed embodiment, the resistor  36  is made from titanium tungsten (TiW), but could alternatively be made from any other suitable material, including but not limited to nickel chromium (NiCr), tungsten, doped polysilicon, or various primary metals. As shown in  FIG. 3 , the antenna elements  31  and  32  are disposed on opposite sides of the resistor  36 , and are thin layers disposed on the top surface of the substrate  21 . The antenna element  31  has an apex which is electrically coupled to an edge on one side of the resistor  36 , and the antenna element  32  has an apex which is electrically coupled to an edge on the opposite side of the resistor  36 . In the disclosed embodiment, the antenna elements  31  and  32  are each made of a metal such as aluminum, gold, or copper, but could alternatively be made of any other suitable material. 
   With reference to  FIG. 4 , and as discussed earlier in association with  FIG. 2 , the antenna elements  31  and  32  collectively serve as a resonator which receives radiation within a selected frequency range that is substantially different from the frequency of infrared radiation. In particular, this received radiation has a frequency much lower than the frequency of infrared radiation. The energy from this received radiation is absorbed by the resistor  36 , and causes the resistor  36  to be heated and to emit infrared radiation at  38 . This infrared radiation travels upwardly and is absorbed within the membrane  53 , where it heats the bolometer layer  41  and thus changes its resistance. The readout circuit  42  can measure how this resistance changes over time, and can thus measure the amount of heat absorbed by the membrane  53 , which in turn represents the amount of the radiation of interest which has been received by the antenna elements  31  and  32 . 
   As mentioned above, the vertical spacing between the resistor  36  and the membrane  53  is approximately one-quarter of the wavelength of infrared radiation. This space effectively serves as a resonant cavity for infrared radiation, such that the thermal energy  38  produced by the resistor  36  is essentially trapped within this cavity until it can be absorbed by the bolometer  41 , thereby ensuring that the bolometer  41  has a high efficiency with respect to absorption of the infrared energy  38  produced by the resistor  36 . In the disclosed embodiment, this cavity is subject to a vacuum, but it would alternatively be possible to provide a gas within this cavity, such as nitrogen, freon, argon, helium, carbon dioxide, or any other suitable gas. As mentioned above, the resistor  36  in the depicted embodiment is made of titanium tungsten. This means that it has not only a suitable resistance characteristic, but that it also is reflective to infrared radiation. This reflectance helps to keep infrared radiation trapped within the resonant cavity until the radiation can be absorbed by the membrane  53 . 
   As mentioned above, the resistor  36  is configured to have a resistance which is matched to the impedance of the antenna elements  31  and  32 . Depending on the frequency of the incident radiation which is to be detected, the configuration and thus the impedance of the antenna elements  31  and  32  can be different. Consequently, although the rectangular resistor  36  of  FIG. 3  is suitable for some frequency ranges, and can be adjusted to some extent by varying its size, thickness and material, other approaches are also possible. 
   In this regard,  FIG. 5  is a diagrammatic top view of an alternative embodiment with a resistor  81  that is an alternative embodiment of the resistor  36 . The resistor  81  is a thin layer of metal disposed on the top surface of the substrate  21 , and is configured as an elongate strip that extends from the apex of one antenna element  31  to the apex of the other antenna element  32 . The resistor  81  has a serpentine configuration with relatively small spaces between adjacent segments, so that a very high percentage of the space between the apexes of the antenna elements  31  and  32  is occupied by the resistor  81 . This permits the resistor  81  to provide a reflectance characteristic which is comparable to that of the resistor  36  of  FIG. 3 . The width, thickness and length of the resistor  81  can all be selectively varied in order to obtain a specific desired resistance characteristic, while still providing an appropriate degree of reflectivity. 
   As discussed above, the resistor  36  of  FIG. 3  and the resistor  81  of  FIG. 5  are each capable of serving as a reflector with respect to infrared radiation. As an alternative, it is possible to provide a reflector which is physically separate from the resistor. In this regard,  FIG. 6  is a diagrammatic fragmentary sectional view which is similar to  FIG. 4 , but which shows an alternative embodiment. The embodiment of  FIG. 6  is identical to the embodiment of  FIG. 4 , except for the provision of some additional structure. In particular, a layer  91  is provided on top of the resistor  36 , and is made of a material which is electrically insulating and thermally conductive. In  FIG. 6 , the layer  91  is made of silicon nitride, but it would alternatively be possible to use any other suitable material. A reflector layer  92  is provided on top of the layer  91 . In  FIG. 6 , the reflector layer  92  is made of aluminum, but it could alternatively be made of any other suitable material. 
   In  FIG. 6 , the resistor  36  generates and emits infrared energy in the same manner as in the embodiment of  FIG. 4 . This infrared energy passes through the thermally conductive layer  91  to the reflective layer  92 , and the reflective layer  92  emits the infrared energy upwardly through the resonant cavity, as indicated diagrammatically by the arrows  38 . The layer  92  also serves as a reflector for infrared energy which is within the cavity between the layer  92  and the membrane  53 , in order to optimize the absorption efficiency of the membrane  53 . It will be recognized that the layers  91  and  92  can also be used with a resistor configuration other than that of the resistor  36 , one example of which is the resistor  81  shown in  FIG. 5 . 
     FIG. 7  is a diagrammatic fragmentary sectional view which is similar to  FIGS. 4 and 6 , but which shows yet another alternative embodiment. In this regard,  FIG. 7  is similar to the embodiment of  FIG. 4 , except for the differences which are discussed below. In particular, the antenna elements  31  and  32  are still provided on the top surface of the substrate  21 , but the layer which serves as the resistor  36  is not on the top surface of the substrate  21 . Instead, the resistor  36  is disposed on the underside of the center portion  56  of the membrane  53 . Opposite side edges of the resistor  36  are each electrically coupled to the apex of a respective antenna element  31  or  32  by a respective vertical post or via  111 – 112 . In  FIG. 7 , the posts  111  and  112  are separate from and in addition to the posts shown at  51  and  52  in  FIG. 3 , and are spaced from each other and from the posts  51 – 52 . In  FIG. 7 , the resistor  36  produces infrared energy in the same manner as in the embodiment of  FIG. 4 , but this infrared energy is transmitted by direct conduction through the layer  61  to the bolometer layer  41 . Consequently, heat is transferred from the resistor  36  to the bolometer layer  41  with a very high degree of efficiency. 
     FIG. 8  is a diagrammatic fragmentary sectional view which is similar to  FIG. 4 , but which shows still another alternative embodiment. The embodiment of  FIG. 8  is similar to the embodiment of  FIG. 4 , except for differences which are discussed below. In  FIG. 8 , it will be noted that the layer serving as the resistor  36  is provided on top of the center portion  56  of the membrane  53 , and in particular is on top of the layer  66 . The antenna elements  31  and  32  are not provided on the top surface of the substrate  21 , but instead are supported at location which is vertically higher than the resistor  36 . 
   The structure which physically supports the antenna elements  31  and  32  is indicated diagrammatically in  FIG. 8  by broken lines  118  and  119 . Persons skilled in the art will readily understand the type of structure which would be needed to support the antenna elements  31  and  32 . Two conductive posts or vias  121  and  122  each extend vertically from a respective opposite side of the resistor  36  to an apex of a respective antenna element  31  or  32 . The vias  121  and  122  provide a path for electrical energy, but do not support the weight of the antenna elements  31 – 32 . Instead, the weight of the antenna elements  31 – 32  is supported by the support structure  118 – 119 . It should be noted that, in  FIG. 8 , the antenna elements  31  and  32  are spaced from the substrate and the readout circuitry in the substrate, thereby reducing the extent to which the antenna elements  31  and  32  may possibly be influenced by electromagnetic radiation generated by the operation of the readout circuit  42 . The vias  121 – 122  are each configured to be relatively small in cross section, in order to provide a degree of thermal isolation between the resistor  36  and the antenna elements  31 – 32 , so as to minimize the extent to which heat generated within the resistor can flow into either antenna element. 
     FIG. 9  is a diagrammatic fragmentary sectional view which is similar to  FIG. 4 , but which shows yet another alternative embodiment. The embodiment of  FIG. 9  is similar to the embodiment of  FIG. 4 , except for differences which are discussed below. In the embodiment of  FIG. 9 , the antenna elements  31 – 32  and the resistor  36  are all provided at a location spaced vertically above the membrane  53 , in a manner so that they are all on effectively the same vertical level. Structure supporting the antenna elements  31 – 32  and the resistor  36  at this level is indicated diagrammatically in  FIG. 9  by broken lines  118  and  119 . The infrared radiation  38  emitted by the resistor  36  travels downwardly to the membrane  53 . Since the resistor  36  has a degree of reflectivity, a resonant cavity is formed above the membrane  53 , between the membrane  53  and the resistor  36 . 
     FIG. 10  is a block diagram showing a detector section which is an alternative embodiment of the detector section of  FIG. 2 . The detector section of  FIG. 10  is identical to the detector section of  FIG. 2 , except that an amplifier  151  of a known type has been inserted between the antenna elements  31 – 32  and the resistor  36 . In particular, the amplifier  151  has two input terminals, each of which is electrically coupled to the inner apex of a respective one of the two antenna elements  31  and  32 . Further, the amplifier  151  has two output terminals, each of which is coupled to a respective end of the resistor  36 . The amplifier  151  can be selected so that the input impedance of the amplifier is matched to the impedance of the antenna, and so that the output impedance of the amplifier is matched to the resistance of the absorber. Where impedances are appropriately matched in this manner, the overall efficiency of the detector section is optimized. Further, use of the amplifier  151  avoids the need to try to directly match the impedance of the antenna elements as closely as possible to the impedance of the absorber. 
   The present invention provides a number of advantages. One such advantage results from the provision of a surrogate absorber in the form of a resistor having a resistance which is matched to the low-impedance antenna elements, so that the bolometer resistance can be optimized for best sensitivity without regard to the antenna elements. In effect, the invention permits separate control over the functions of maximizing heat generation based on detected energy, and optimizing sensitivity of the infrared detector. A further advantage is that an accurate and sensitive detector of a known type, which operates in one frequency range such as that of infrared energy, can be used to detect and/or image radiation from a significantly different frequency range. This is particularly advantageous in a context involving the detection or imaging of radiation which has frequencies well below the frequencies of infrared energy. 
   Although several selected embodiments have been illustrated and described in detail, it will be understood that various substitutions and alterations are possible without departing from the spirit and scope of the present invention, as defined by the following claims.