Abstract:
This disclosure describes a temperature controlled photodetector. The disclosed detector can reach a temperature at which responsivity is maximized within a short time and with little wasted power. Furthermore, the photodetector prevents thermal gradients from developing across the detector so that the whole detector region has equivalent responsivity.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of priority to provisional application Ser. No. 60/890,798 filed on Feb. 20, 2007, hereby incorporated by reference in its entirety. 
     
    
     BACKGROUND OF THE DISCLOSURE 
       [0002]    Photodetector response is a function of temperature. Each photodetector has a temperature at which responsivity is maximized. However, this optimum temperature may not be equivalent to atmospheric temperature. Often, atmospheric temperature is lower than the optimal temperature, especially when a photodetector is used at high altitude or in severe weather conditions. By heating the photodetector, the detector&#39;s temperature can be thermally stabilized at or around the optimum temperature, thus maximizing photodetector responsivity under any atmospheric conditions. 
         [0003]    The semiconductor substrate in a photodetector is often mounted in a package. To heat the photodetector, heaters have been mounted outside of the enclosure. In order to raise the detector temperature the packaging must also be heated. Much of this heat does not reach the semiconductor substrate. This wastes power and means that significant time elapses while the semiconductor substrate temperature is raised to the optimal temperature. Furthermore, if there are other electronics along the thermally path between the heater and the semiconductor substrate, these electronics may be damaged during heating. 
         [0004]    Consumer electronics, such as digital cameras, cell phones, and laptop computers could benefit from photodetectors that consume little power and reach optimal detection efficiencies quickly. Various weapons systems would also benefit from low power consumption and the ability to quickly raise the photodetector temperature to its optimum. 
       SUMMARY 
       [0005]    Against this background, a temperature controlled photodetector with a heater in contact with a semiconductor substrate and being thermally isolated from the packaging is herein disclosed. The disclosed photodetector is capable of reaching optimal temperature within a matter of seconds even when atmospheric temperature is around fifty-five degrees Celsius. 
         [0006]    This disclosure describes a temperature controlled photodetector comprising a semiconductor substrate having a photosensitive region, a heating element in contact with the semiconductor substrate, the semiconductor substrate being thermally isolated from a substrate mount, and an enclosure comprising a window, attached to the substrate mount, and enclosing the semiconductor substrate and the heating element. 
         [0007]    This disclosure further describes a temperature controlled photodetector comprising a semiconductor substrate having a photosensitive region; a heating element in contact with the semiconductor substrate wherein the heating element comprises an electrically-resistive material generating heat when a current is passed through it; wherein the heating element has a heating element area larger than a photosensitive area of the photosensitive region; a thermally-isolating annular element in contact with the substrate mount and the semiconductor substrate; a temperature sensor in contact with the semiconductor substrate; a hermetically-sealed enclosure comprising a window transparent in the optical and near infrared, attached to the substrate mount, and enclosing the semiconductor substrate, the heating element, the thermally isolating element, and the temperature sensor; at least one electrically conductive lead extending from the heating element through the enclosure and the substrate mount; at least one electrically conductive lead extending from the photosensitive region through the enclosure and the substrate mount; and at least one electrically conductive lead extending from the semiconductor substrate through the enclosure and the substrate mount. 
         [0008]    These and various other features as well as advantages will be apparent from a reading of the following detailed description and a review of the associated drawings. Additional features are set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the described embodiments. The benefits and features will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    The following drawing figures, which form a part of this application, are illustrative of embodiments of the apparatus or device described below and are not meant to limit the scope of the disclosure in any manner, which scope shall be based on the claims appended hereto. 
           [0010]      FIG. 1  illustrates a cross-sectional view of the photodetector  100 . 
           [0011]      FIG. 2  illustrates a plan view from the top of the photodetector  100 . 
           [0012]      FIG. 3  illustrates an embodiment of the photodetector  100  wherein the heating element comprises a serpentine electrical lead, as seen from the bottom. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]      FIG. 1  illustrates a cross-sectional view of the photodetector  100 . The photodetector  100  comprises a semiconductor substrate  102 . In an embodiment, the semiconductor substrate  102  is P-type silicon. Other semiconductor substrates, such as N-type silicon, Gallium-Arsenide, and Indium-Phosphide may also be used. The semiconductor substrate  102  may be substantially thinner in the vertical dimension than in the lateral dimension as illustrated in  FIG. 1 . 
         [0014]      FIG. 2  illustrates a plan view from the top of the photodetector  100 . From this view a first surface of the semiconductor substrate  102  is seen. In  FIG. 3  the second surface of the semiconductor substrate  102  can be seen. In an embodiment, the second surface of the semiconductor substrate  102  may comprise a layer of silicon-dioxide as grown via thermal oxidation for instance. 
         [0015]    The semiconductor substrate  102  also comprises a photosensitive region  104 . The photosensitive region  104  can comprise the entire volume of the semiconductor substrate  102  or alternatively, only a portion of that volume. For instance, if the photosensitive region  104  is created via doping, then the photosensitive region  104  will comprise the volume of the semiconductor substrate  102  in which dopant particles have diffused into the silicon substrate  102 . Alternatively, the photosensitive region  104  may be formed via thin film deposition on the semiconductor substrate  102 . In another embodiment, the photosensitive region  104  may comprise a combination of diffused dopants and deposited thin films. 
         [0016]    The photo-sensitive region  104  comprises any photosensitive device, including, but not limited to, a photo-resistor, photo-diode, photo-transistor, avalanche photo-diode, charge-coupled device, or photo-conductor. The photo-sensitive region  104  can comprise a single region, or it may comprise multiple sub regions, such as a quadrant of four symmetric regions, for example. Although the photo-sensitive region may comprise multiple sub-regions, when an imaginary line is circumscribed around the photo-sensitive region, the two-dimensional area enclosed by this imaginary line will be referred to as a photo-sensitive area. 
         [0017]    The photodetector  100  also comprises a heating element  106 . The heating element  106  may be fixed to the second surface of the semiconductor substrate  102 . The heating element  106  may be fabricated separately from the semiconductor substrate  102 , and then affixed using epoxy or other means, or the heating element  106  may be fabricated directly onto the second surface of the semiconductor substrate  102  via metallic vapor deposition or other means. Alternatively, wafer bonding may be used to attach the heating element  106 . The heating element  106  may comprise any resistive material, including, but not limited to, gold, aluminum, copper, or polysilicon. In an embodiment, the heating element  106  has a serpentine shape, as illustrated in  FIG. 3 . The serpentine-shaped heating element  106  covers a large surface area while comprising a small cross section. As such, the heating element is highly resistive and more efficiently converts electric current to heat. The amount of current passed through the heating element  106  controls the heat dissipated and thus the speed with which the semiconductor substrate  102  changes temperature and the equilibrium temperature that the semiconductor substrate  102  reaches. 
         [0018]    An imaginary line circumscribing the heating element  106  encloses a surface area referred to as a heating element area. In an embodiment, the heating element area is larger than the photosensitive area. The larger the heating element area is compared to the photosensitive area the more negligible will be any thermal gradient created across the lateral dimension of the photo-sensitive region  104 . 
         [0019]    The photodetector  100  also may comprise a substrate mount  108 . Although the substrate mount  108  is not necessary to this disclosure, the substrate mount  108  may improve manufacturability. In an embodiment, the substrate mount  108  comprises an insulating material, such as PCB, FR-2, FR-3, FR-4, FR-5, FR-6, G-10, CEM-1, CEM-2, CEM-3, CEM-4, CEM-5, polyamide, Teflon, or other ceramics. 
         [0020]    The photodetector  100  also includes an enclosure  110 . The enclosure  110  may enclose any one or more of the following components: the semiconductor substrate  102 , the heating element  106 , a temperature sensor  118 , a thermally-isolating element  112 , and the substrate mount  108 . The enclosure  110  may be hermetically-sealed either creating a vacuum within the enclosure  110 , or the enclosure  110  may be filled with an inert gas, dried gas, or any other low-moisture gas. The enclosure  110  may be made of any material. In an embodiment, the enclosure  110  is made of metal. 
         [0021]    The enclosure  110  may also comprise a window  116 , transparent to the electromagnetic wavelengths for which the photodetector  100  is designed to detect. For instance, for an optical photodetector, the window may be made of glass, polymers, or other optically-transparent materials. Alternatively, for a near-infrared photodetector  100  the window  116  could be made of silicon or some other material transparent in the near-infrared. In an embodiment, the enclosure  110  comprises a material that is transparent to the wavelengths that are desired to be detected. In this embodiment a discrete window  116  may not be needed. 
         [0022]    In an embodiment, the optical window  116  may be shaped like a lens to focus photons onto the photo-sensitive region  104 , instead of allowing some photons to impinge on the non-detecting regions of the photodetector  100 . 
         [0023]    The photodetector  100  may also comprise electrically-conductive leads  114  providing power for the heater, detector voltage, detector signal(s), and/or temperature sensor  118  signal(s). The electrically-conductive leads  114  may comprise electrical wire soldered at either end of each wire, wire bonds, connecting pins (as illustrated in  FIG. 1 ), bump bonds, or other means for electrical interconnect. The enclosure  110  and substrate mount  108  may either or both comprise means for the electrically-conductive leads  114  to extend through them. For instance,  FIG. 1  illustrates an embodiment wherein the electrically-conductive leads extend through holes fabricated in the enclosure  110  and the substrate mount  108 . These holes may be insulated, or the leads  114  may be insulated. In a non-illustrated embodiment, the leads  114  may extend through the sides, top, and/or bottom of the enclosure  110  and/or substrate mount  108 . 
         [0024]    The photodetector  100  also comprises a thermally-isolating element  112 . In an embodiment, the thermally-isolating element  112 , has an annular shape. The thermally-isolating element  112  supports the semiconductor substrate  102  on the substrate mount  108  while thermally isolating the substrate mount  108  from the semiconductor substrate  102 . The thermally-isolating element  112  may comprise any insulating material, such as PCB, glass, ceramic, rubber, or polymer. Preferably, the walls of the thermally-isolating element  112  will be thin, so as to minimize the cross-sectional area through which heat may conductively transfer between the substrate mount  108  and the semiconductor substrate  102 . The thermal path between the substrate mount  108  and the semiconductor substrate  102  is thus highly resistive to heat flow, and thus most of the heat generated by the heating element  106  will be transferred into heating the semiconductor substrate  102  rather than other components of the photodetector  100 . 
         [0025]    The photodetector  100  also may comprise a temperature sensor  118 . The temperature sensor  118  may comprise a Zener-diode, thermistor, micro-electro-mechanical systems device, resistance temperature detector, thermocouple, or any other low-thermal mass temperature monitoring device. Low-thermal mass prevents heat from being wasted trying to heat the temperature sensor  118  instead of the semiconductor substrate  102 . The temperature sensor  118  may be in contact with the semiconductor substrate  102 , and may be in contact with the photo-sensitive region  104 . The temperature sensor  118  can be used in a feedback loop in order to regulate the semiconductor substrate  102  temperature. In an alternative embodiment instead of a temperature sensor  118  the temperature can be regulated by utilizing a current-limiting or voltage-limiting circuit to control the heating element  106 . As such empirically-determined currents or voltages could be associated with given temperatures, and a desired temperature could be set by applying the associated current or voltage. 
         [0026]    Reference to the top, bottom, or sides of the detector are not limiting, but rather relative. For instance, the top of the detector could be the bottom in an application where the detector is installed with the window  116  facing downwards. Of course, in some applications the detector will be rotated and twisted such that the so-called top of the detector operates in a direction facing towards the earth one moment and towards the sky the next. 
         [0027]    Those skilled in the art will recognize that the apparatus of the present disclosure may be implemented in many manners and as such are not to be limited by the foregoing exemplary embodiments and examples. Any features of the disclosed embodiments may be combined into single or multiple embodiments, and alternate embodiments having fewer than, or more than, all of the features described herein are possible. The disclosed functionality may also be, in whole or in part, distributed among multiple components, in manners now known or to become known. Thus, myriad combinations are possible in achieving the functions, features, interfaces and preferences described herein. Moreover, the scope of the present disclosure covers conventionally known manners for carrying out the described features and functions, as well as those variations and modifications that may be made to the disclosed components as would be understood by those skilled in the art now and hereafter. 
         [0028]    While various embodiments have been described for purposes of this disclosure, such embodiments should not be deemed to limit the teaching of this disclosure to those embodiments. Various changes and modifications may be made to the elements and operations described above to obtain a result that remains within the scope of the apparatus described in this disclosure. For example, the temperature sensor  118  could be an optical temperature sensor  118  such that it need not be in contact with the semiconductor substrate  102 . In another example, power and electronics may all be contained within the enclosure  110  such that the electrically-conductive leads need not extend through the enclosure  110  and/or substrate mount  108 . 
         [0029]    Various adaptations, modifications, and extensions of the described device will be apparent to those skilled in the art and are intended to be within the scope of the invention as defined by the claims that follow.