Abstract:
A photocathode for an image intensifier tube includes a faceplate, a glass plate disposed opposite the faceplate, and a span having one end attached to the glass plate and the other end attached to the faceplate for forming a sealed chamber between the faceplate and the glass plate. A semiconductor layer is bonded to a surface of the glass plate, where the surface is disposed outside of the sealed chamber. The semiconductor layer forms a photocathode. A thermal electric cooler (TEC) is disposed inside the sealed chamber for cooling the photocathode. The faceplate is formed from sapphire material. The glass plate is formed from high conductivity glass. The span is formed from either high conductivity glass or low conductivity glass. The faceplate and the glass plate form a path for light to impinge upon the semiconductor layer, and the photocathode of the semiconductor layer is configured to convert the light into electrons for emission toward an electron gain device.

Description:
FIELD OF THE INVENTION 
     The present invention relates, in general, to image intensifier tubes and, more specifically, to a photocathode structure subjected to cooling. 
     BACKGROUND OF THE INVENTION 
     Night vision systems are used in a wide variety of military, industrial and residential applications to enable sight in a dark environment. For example, night vision systems are utilized by military aviators during nighttime flights. Security cameras use night vision systems to monitor dark areas and medical instruments use night vision systems to alleviate conditions such as retinitis pigmentosis (night blindness). 
     Image intensifier devices are employed in night vision systems to convert a dark environment to an environment perceivable by a viewer. More specifically, the image intensifier device within the night vision system collects tiny amounts of light in a dark environment, including the lower portion of the infrared light spectrum present in the environment, which may be imperceptible to the human eye. The device amplifies the light so that the human eye can perceive the image. The light output from the image intensifier device can either be supplied to a camera, external monitor or directly to the eyes of a viewer. The image intensifier device is commonly employed in vision goggles that are worn on a user&#39;s head for transmission of the light output directly to the viewer. Accordingly, since the goggles are worn on the head, they are desirably compact and light weight for purposes of comfort and usability. 
     Image intensifier devices include three basic components mounted within a housing, i.e. a photocathode (commonly called a cathode), a microchannel plate (MCP), and a phosphor screen (commonly called a screen, fiber-optic or anode). The photocathode detects a light image and converts the light image into a corresponding electron pattern. The MCP amplifies the electron pattern and the phosphor screen transforms the amplified electron pattern back to an enhanced light image. 
     Referring to  FIG. 1 , a current state of the art Generation III (GEN III) image intensifier tube  10  is shown. Examples of the use of such a GEN III image intensifier tube in the prior art are exemplified in U.S. Pat. No. 5,029,963 to Naselli, et al., entitled REPLACEMENT DEVICE FOR A DRIVER&#39;S VIEWER and U.S. Pat. No. 5,084,780 to Phillips, entitled TELESCOPIC SIGHT FOR DAYLIGHT VIEWING. The GEN III image intensifier tube  10  shown, and in both cited references, is of the type currently manufactured by ITT Corporation, the assignee herein. In intensifier tube  10  shown in  FIG. 1 , infrared energy impinges upon photocathode  12 . The photocathode  12  is comprised of glass faceplate  14  coated on one side with antireflection layer  16 , a gallium aluminum arsenide (GaAlAs) window layer  17  and gallium arsenide (GaAs) active layer  18 . Infrared energy is absorbed in GaAs active layer  18 , thereby resulting in the generation of electron/hole pairs. The produced electrons are then emitted into vacuum housing  22  through a negative electron affinity (NEA) coating  20  present on GaAs active layer  18 . 
     A microchannel plate (MCP)  24  is positioned within vacuum housing  22 , adjacent NEA coating  20  of photocathode  12 . Conventionally, MCP  24  is made of glass having a conductive input surface  26  and a conductive output surface  28 . Once electrons exit photocathode  12 , the electrons are accelerated toward input surface  26  of MCP  24  by a difference in potential between input surface  26  and photocathode  12  of approximately 300 to 900 volts. As the electrons bombard input surface  26  of MCP  24 , secondary electrons are generated within MCP  24 . The MCP  24  may generate several hundred electrons for each electron entering input surface  26 . The MCP  24  is subjected to a difference in potential between input surface  26  and output surface  28 , which is typically about 1100 volts, whereby the potential difference enables electron multiplication. 
     As the multiplied electrons exit MCP  24 , the electrons are accelerated through vacuum housing  22  toward phosphor screen  30  by a difference in potential between phosphor screen  30  and output surface  28  of approximately 4200 volts. As is the electrons impinge upon phosphor screen  30 , many photons are produced per electron. The photons create the output image for image intensifier tube  10  on the output surface of optical inverter element  31 . 
       FIG. 2  is a schematic representation of image intensifier tube  41 . The tube includes photocathode  54 , microchannel plate (MCP)  53  and imaging sensor  56 . Imaging sensor  56  can be any type of solid-state imaging sensor, such as a CCD device, or a CMOS imaging sensor. 
     Photocathode  54  can be, but is not limited to, a material such as GaAs, Bialkali, InGaAs, and the like. Photocathode  54  includes input side  54   a  and output side  54   b . MCP  53  has a plurality of channels  52  formed between an input surface and an output surface. 
     An electric biasing circuit  44  provides a biasing current to image intensifier tube  41 . Electric biasing circuit  44  includes a first electrical connection  42  and a second electrical connection  43 . First electrical connection  42  provides a biasing voltage between photocathode  54  and MCP  53 . Second electrical connection  43  applies a biasing voltage between MCP  53  and imaging sensor  56 . In this configuration, photocathode  54 , MCP  53 , and imaging sensor  56  are maintained in a vacuum body or envelope  61  as a single unit, in close physical proximity to each other. 
     Still referring to  FIG. 2 , in operation, light  58 ,  59  from an image  57  enters image intensifier tube  41  through input side  54   a  of photocathode  54 . Photocathode  54  changes the entering light into electrons  48 , which are output from output side  54   b  of photocathode  54 . Electrons  48  exiting photocathode  54  enter channels  52  of MCP  53 . Secondary electrons are generated within the plurality of channels  52  of MCP  53 . The MCP  53  may generate several hundred electrons in each of channels  52  for each electron entering through the input surface. Thus, the number of electrons  47  exiting channels  52  is significantly greater than the number of electrons  48  that entered channels  52 . The intensified number of electrons  47  exit channels  52  and strike the electron receiving surface of imaging device  56 . The imaging device transforms the electrons into a light image which may be stored in memory or viewed on display  46 . 
     SUMMARY OF THE INVENTION 
     To meet this and other needs, and in view of its purposes, the present invention provides a photocathode for an image intensifier tube including a faceplate, a glass plate disposed opposite the faceplate, and a span having one end attached to the glass plate and the other end attached to the faceplate, for forming a sealed chamber between the faceplate and the glass plate. A semiconductor layer is bonded to a surface of the glass plate, where the surface is disposed outside of the sealed chamber. The semiconductor layer forms a photocathode. A thermal electric cooler (TEC) is disposed inside the sealed chamber for cooling the photocathode. The faceplate is an annular structure; the glass plate is an annular structure, and the span is an annular bracket extending between the glass plate and the faceplate for providing a separation distance between the faceplate and the glass plate. The faceplate is formed from a sapphire material, or other optically transparent material of high thermal conductivity. The glass plate is formed from high conductivity glass. The span is formed from either high conductivity glass or low conductivity glass. Preferably, the span is formed from low conductivity glass or other low conductivity material. 
     The faceplate and the glass plate form a path for light to impinge upon the semiconductor layer, and the photocathode of the semiconductor layer is configured to convert the light into electrons for emission toward an electron gain device. The electron gain device is a microchannel plate (MCP). 
     At least one cantilever bracket is attached to the glass plate at one end, and forms a seat for the annular TEC at another end. The at least one cantilever bracket is formed of copper material to provide thermal conductivity between the TEC and the glass plate. The seat includes an indentation formed in the at least one cantilever bracket for receiving the annular TEC. The at least one cantilever bracket is bonded at an end to the glass plate. Standoffs are formed on top of the glass plate for providing a separation distance between the glass plate and the opposing faceplate. 
     Another embodiment of the present invention is a photocathode structure having a sealed chamber formed by walls, a bottom wall providing an exterior surface to the sealed chamber, a photocathode layer disposed on the exterior surface, and a TEC disposed within the sealed chamber for cooling the photocathode layer. The TEC is in thermal contact with the photocathode layer by way of high conductivity material. The high conductivity material includes glass and at least one copper bracket attached to the glass. 
     Yet another embodiment of the present invention is an image intensifier tube including a photocathode structure, an electron sensing device, and an electron gain device disposed between the electron sensing device and the photocathode structure. The photocathode structure includes: a sealed chamber formed by walls, a bottom wall providing an exterior surface to the sealed chamber, a photocathode layer disposed on the exterior surface, and a TEC disposed within the sealed chamber for cooling the photocathode layer. The TEC is in thermal contact with the photocathode layer by way of a high conductivity material, which includes glass and at least one copper bracket attached to the glass. 
     It is understood that the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       The invention may be understood from the following detailed description when read in connection with the following figures: 
         FIG. 1  is a cross-sectional diagram of a conventional image intensifier tube. 
         FIG. 2  is a functional block diagram of a conventional image intensifier system. 
         FIG. 3  is a cross-sectional diagram of a first set of components used in assembling a photocathode structure, in accordance with an embodiment of the present invention. 
         FIG. 4  is a cross-sectional diagram of a second set of components used for assembling a photocathode structure, in accordance with an embodiment of the present invention. 
         FIG. 5  is a cross-sectional diagram of an assembled photocathode structure, using the sets of components shown in  FIGS. 3 and 4 , in accordance with an embodiment of the present invention. 
         FIGS. 6A ,  6 B and  6 C are cross-sectional diagrams and perspective diagrams, respectively, showing portions of an assembled photocathode structure, in accordance with an embodiment of the present invention. 
         FIG. 7  is a plot of wafer temperature versus TEC power, showing performance results of using an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention provides a photocathode structure that is cooled in temperature to reduce generation of dark currents. It is known that a photocathode generates dark currents, when its temperature increases during operation in an image intensifier tube or in a solid state image intensifier. The dark currents of the photocathode is temperature dependent. Lowering the temperature is one method of reducing dark currents. 
     Lowering the temperature, however, requires electrical power, whose usage is preferably minimized, especially during operation of a night vision goggle device. In conventional photocathodes (such as shown in  FIG. 1 ), the entire image intensifier system is cooled, by immersing the device in an exterior tube. The exterior tube results in an inefficient usage of electrical power, because a large mass is required to be temperature cooled. For example, the tube body, the MCP and the photo-anode structure are unnecessarily cooled. 
     As will be explained, the present invention advantageously concentrates on cooling primarily only the photocathode structure. The present invention advantageously uses a vacuum formed between the photocathode structure and the MCP to obtain a high thermal resistance, so that the amount of heat re-entering the photocathode structure is reduced. The present invention also reduces the amount of material comprising the photocathode structure, in order to reduce the number of paths for re-entrant heat flowing into the photocathode structure. Furthermore, the present invention replaces the reduced amount of material comprising the photocathode with a vacuum, which forms a high thermal resistance. 
     Referring now to  FIGS. 3 ,  4  and  5 , there is shown a cooled photocathode structure, in accordance with an embodiment of the present invention.  FIGS. 3 and 4  show two separate sets of components of the photocathode structure and  FIG. 5  shows an integrated and assembled photocathode structure. 
     Referring first to  FIG. 3 , there is shown a first set of components of a photocathode structure, generally designated as  62 . The first set of components is comprised of faceplate  63  and a thermal electric cooler (TEC)  64 . The faceplate  63  may be formed from sapphire material, for example, and may have an annular cross-section. The top annular surface of faceplate  63  is designated as  63 A and the bottom annular surface is designated as  63 B. It will be appreciated that faceplate  63  may be formed of any material having a high thermal conductivity (which, for example, may be greater than or equal to 33 W/m/k) and of any material providing a transparent window for light passing from top surface  63 A to bottom surface  63 B. 
     As shown in cross-section in  FIG. 3 , TEC  64  forms an annular ring. It will be appreciated, however, that TEC  64  may be one or more thermal coolers soldered or fastened to bottom surface  63 B of faceplate  63 , and does not need to be annular in shape. The one or more TECs  64  may be attached directly to the bottom surface of faceplate  63  using only one electrically insulating annular ceramic ring (not shown). 
     The faceplate  63  may include two contact ports for TEC power (not shown) and two contact ports for a thermistor (not shown). The thermistor may be used to control the on/off operation of the one or more TECs. The contact ports may be formed by drilling into faceplate  63 . The contact ports may be formed by a recess in the bottom surface of faceplate  63 , as shown by recess  65  in the faceplate. Of course, for an annular TEC, recess  65  may also be annular to completely receive the TEC. An indium sealant may be used for sealing any openings in recessed section  65  between the TEC and the faceplate. A high temperature solder material may also be used for assembling the TEC (one or more) with the faceplate. 
     It will be appreciated that a non-evaporable getter may be placed on the bottom surface of faceplate  63 . 
     Referring next to  FIG. 4 , there is shown a second set of components of a photocathode structure, generally designated as  66 . The second set of components is comprised of glass plate  67 , span  71 , one or more cantilevered brackets  69 ,  70 , and semiconductor layer  72 . 
     The span  71  and glass plate  67  may be formed from one type of glass or from two types of glass. As shown in  FIG. 4 , glass plate  67  is formed as a glass disk using high conductivity glass and span  71  is formed as an “L” shape using low conductivity glass. The glass plate  67  is bonded to span  71  forming a single “U” shape, when viewed in cross-section. As another embodiment, glass plate  67  and span  71  may be formed from one type of glass having high or low thermal conductivity. 
     As an example, the high conductivity glass may be BK7 having a thermal conductivity of 1.3 W/m/k. The low conductivity glass may have a thermal conductivity of 0.3 W/m/k. It is important, of course, that glass plate  67  be made from glass or other material that provides a transparent window for light to pass through the glass and impinge on semiconductor layer  72 , the latter converting the light into electrons. 
     The semiconductor layer  72  is bonded to glass plate  67  for providing the photocathode transformation of light (photons) into electrons. The electrons, of course, are then provided as an input to an MCP (such as MCP  53  shown in  FIG. 2 ). The semiconductor layer may include an active layer such as gallium arsenide (GaAs) and additional layers, such as an antireflection layer, a window layer of gallium aluminum arsenide (GaAlAs) and a negative electron affinity (NEA) coating disposed on the GaAs active layer (as described with respect to  FIG. 1 ). 
     It will be appreciated that after forming glass plate  67  and span  71 , the formed glass may be ground and polished. The semiconductor layer  72  is then bonded to glass plate  67 . Next, in a possible fabrication sequence, the surface of glass plate  67 , which is opposite to semiconductor layer  72  may be further ground and polished. The cantilevered brackets (one or more) may be finally attached to glass plate  67 . 
     As shown in  FIG. 4 , cantilevered brackets  69 ,  70  are bonded to the end disk surface of glass plate  67 . The bonding may be performed using frit or solder, for example. The cantilevered brackets may be formed of any conductive material having high thermal conductivity, such as copper. The cantilevered brackets may be formed as separate sections, as best shown in  FIG. 6B , and attached to the disk surface of glass plate  67  by way of a ring, as shown in  FIG. 4  designated as  75 . The ring  75  may be formed of materials identical to cantilevered brackets  69 ,  70 . It will be understood that ring  75  and cantilevered brackets  69 ,  70  may be a single piece of copper, for example. 
     If made from a deformable material, such as copper, cantilevered brackets  69 ,  70  may be notched or recessed at their end portions to receive, hold or lock TEC  64 , as shown in  FIG. 5 . 
     The final assembly of the first and second sets of components  62  and  66  into an integrated photocathode structure is shown in  FIG. 5 , where the integrated photocathode structure is designated as  80 . In preparation for assembly, first set of components  62  ( FIG. 3 ) and second set of components  66  ( FIG. 4 ) may be subjected separately to a UHV (ultra-high vacuum) process. The first set of components  62  may undergo reduced temperature processing, whereas the second set of components  66  may be subjected to processing in a full temperature range. The reverse, however, may also be true. 
     The first and second sets of components may be press fitted during the UHV process using an indium bond to form a sealed evacuated chamber. The indium bond is designated as  81  and the sealed chamber is designated as  76 , as shown in  FIG. 5 . Two or more standoffs  68 A,  68 B may be provided on top of the disk end of glass plate  67  for supporting faceplate  63 . 
     The cantilevered brackets  69 ,  70  provide support for TEC  64 , as shown in  FIG. 5 . Although not shown, it will be appreciated that the cantilevered brackets may be notched or recessed to receive and hold TEC  64  in position. A bond may not be necessary to lock TEC  64  to cantilevered brackets  69 ,  70 . During the sealing process of first and second sets of components  62  and  66 , the cantilevered brackets may flex and take pressure away from TEC  64 . The flexing is very noticeable, when the cantilevered brackets and ring  75  are formed from a single piece of copper. 
     Referring next to  FIGS. 6A ,  6 B and  6 C, there is shown an assembled photocathode structure  80 .  FIG. 6A  is similar to  FIG. 5 , except that the photocathode structure is shown up-side down.  FIG. 6B  is a perspective view of photocathode structure  80 , with TEC  64  and span  71  ( FIG. 6A ) not shown.  FIG. 6C  is a cut-away view of photocathode structure  80 , with TEC  64  not shown. 
       FIG. 7  is a plot of wafer temperature)(C.°) versus TEC power (W). The two solid curves having the legend of “BK-7 spacer” implies that glass plate  67  and span  71  are formed from a single high thermally conductive material, such as BK-7. The two dashed curves having the legend of “low K spacer” implies that glass plate  67  is formed from a high thermally conductive material and span  71  is formed from a low thermally conductive material. The curves shown in  FIG. 7  are results of simulation taken at two different ambient temperatures (23° C. and 50° C.). It will be appreciated that the “low K spacer” (2 materials) provides a lower temperature than the “BK-7 spacer” for a fixed TEC power usage. 
     Accordingly, the present invention provides a low power method of cooling the photocathode by incorporating the TECs into a vacuum environment, such as chamber  76 . The vacuum chamber  76  is separate from photocathode layer  72 , in order to prevent poisoning of the photocathode surface, because the TEC cannot be processed at a high temperature. 
     Some penalty is paid by the present invention, due to an increased diameter of the cathode, which may be traded off between power usage versus size. In general terms, photocathode structure  80  may be sized for insertion into housing  22  of image intensifier tube  10  shown in  FIG. 1 . Of course, photocathode structure  12  is replaced by photocathode structure  80  of the present invention. 
     It will be observed that the vacuum chamber of photocathode structure  80  is separate from the vacuum chamber of housing  22 , in which the photocathode layer, MCP  24  and the input surface of anode  31  reside. 
     Although the invention is illustrated and described herein with reference to specific embodiments, the invention is not intended to be limited to the details shown. Rather, various modifications may be made in the details within the scope and range of equivalents of the claims and without departing from the invention.