Abstract:
An intensified hybrid solid-state sensor includes an imaging device comprising a solid-state sensor assembled with an image intensifier cathode, microchannel plate (MCP), and body envelope. This device combines the best functions of the image intensifier, good signal-to-noise ratio and high logarithmic gain, with the electronic read-out functions either of a Complementary Metal Oxide Semiconductor (CMOS) or charged coupled device (CCD). Applications for this invention are primarily night vision systems where good low light sensitivity and high gain are required.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention is directed to an intensified hybrid solid-state sensor. More particularly, the present invention relates to an image intensifier using a CMOS or CCD sensing device connected in close physical proximity to a microchannel plate (MCP) and photo cathode.  
         BACKGROUND OF THE INVENTION  
         [0002]    The present invention relates to the field of image intensifying devices using solid-state sensors, such as a CMOS or CCD device. Image intensifier devices are used to amplify low intensity light or convert non-visible light into readily viewable images. Image intensifier devices are particularly useful for providing images from infrared light and have many industrial and military applications. For example, image intensifier tubes are used for enhancing the night vision of aviators, for photographing astronomical bodies and for providing night vision to sufferers of retinitis pigmentosa (night blindness).  
           [0003]    There are three types of known image intensifying devices in prior art; image intensifier tubes for cameras, all solid-state CMOS and CCD sensors, and hybrid EBCCD/CMOS (Electronic Bombarded CCD or CMOS sensor).  
           [0004]    Image intensifier tubes are well known and used throughout many industries. Referring to FIG. 1, a current state of the prior 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 the intensifier tube  10  shown in FIG. 1, infrared energy impinges upon a photo cathode  12 . The photo cathode  12  is comprised of a glass faceplate  14  coated on one side with an 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 the vacuum housing  22  through a negative electron affinity (NEA) coating  20  present on the GaAs active layer  18 .  
           [0005]    A microchannel plate (MCP)  24  is positioned within the vacuum housing  22 , adjacent the NEA coating  20  of the photo cathode  12 . Conventionally, the MCP  24  is made of glass having a conductive input surface  26  and a conductive output surface  28 . Once electrons exit the photo cathode  12 , the electrons are accelerated toward the input surface  26  of the MCP  24  by a difference in potential between the input surface  26  and the photo cathode  12  of approximately 300 to 900 volts. As the electrons bombard the input surface  26  of the MCP  24 , secondary electrons are generated within the MCP  24 . The MCP  24  may generate several hundred electrons for each electron entering the input surface  26 . The MCP  24  is subjected to a difference in potential between the input surface  26  and the output surface  28 , which is typically about 1100 volts, whereby the potential difference enables electron multiplication.  
           [0006]    As the multiplied electrons exit the MCP  24 , the electrons are accelerated through the vacuum housing  22  toward the phosphor screen  30  by the difference in potential between the phosphor screen  30  and the output surface  28  of approximately 4200 volts. As the electrons impinge upon the phosphor screen  30 , many photons are produced per electron. The photons create the output image for the image intensifier tube  10  on the output surface  28  of the optical inverter element  31 .  
           [0007]    Image intensifiers such as those illustrated in FIG. 1 have advantages over other forms of image intensifiers. First, intensifiers have a logarithmic gain curve. That is, the gain decreases as the input light level is increased. This matches the human eye response particularly when bright lights are in the same scene as low lights. Most solid-state devices have a linear response; i.e., the brighter the light the brighter the output signal. The result is that bright lights appear much brighter to a viewer of a solid-state system and tend to wash out the scene. Solid-state sensors can be modified to produce a gain decrease as input light is increased, however, this requires changing the amplifier gain, using shuttering, or using anti-blooming control.  
           [0008]    Another advantage of image intensifiers is the ability to function over a large range of input light levels. The power supply can control the cathode voltage and thereby change the tube gain to fit the scene. Thus tubes can function from overcast starlight to daytime conditions.  
           [0009]    However, image intensifier/I 2  cameras suffer from numerous disadvantages. The electron optics of the phosphor screen produces a low contrast image. This results in the object looking fuzzier to the human observer, or solid-state sensor, when viewed through an image intensifier. Although this deficiency has been somewhat reduced with further image intensifier development, solid-state imagers generally have better performance.  
           [0010]    Another disadvantage with image intensifier/I 2  cameras is “halo.” Halo results from electrons being reflected off either the MCP or the screen. The reflected electrons are then amplified and converted into light in the form of a ring around the original image. In image tubes, the halo from electrons reflected from the MCP has been reduced to a negligible effect for the most recent production tubes. However, the halo from the screen section still exists, although not to the degree of the cathode halo. Nevertheless, the screen halo is still a significant defect in imaging systems when a CCD or CMOS array is coupled to the image intensifier. This is because these arrays are more sensitive than the eye to the low light levels in the screen halo.  
           [0011]    Another disadvantage is that image intensifiers do not have a method of providing electronic read-out. Electronic read-out is desired so that imagery from thermal sensors may be combined with intensified imagery with the result that the information from both spectra will be viewed at the same time. One solution has been to create an I 2  camera by coupling a CCD or CMOS array to an image intensifier tube. When a solid-state device is coupled to an image tube the resultant camera has all performance defects of the image tube that is low contrast, often poor limiting resolution due to coupling inefficiencies and the added cost of the image tube to the camera.  
           [0012]    Solid-state devices typically include CCD or CMOS sensors. They function by directly detecting the light, electronically transferring the signal to solid-state amplifiers, then displaying the image on either a television type tube or display such as a liquid crystal display. FIGS. 2 a  and  2   b  illustrate a flow chart and schematic diagram for a typical CCD sensor.  
           [0013]    CCD and CMOS sensors are solid-state devices; that is, there is no vacuum envelope and the output is an electronic signal that must be displayed elsewhere and not within the sensor. The solid-state devices operate with power of 5-15 volts. The light is detected in individual pixels as labeled “s” and translated into electrons that are stored in the pixel until the pixel is read out to the storage register. From the storage register the electronic information contained in multiple pixels is then transferred to a read out register and then to output amplifiers and then to a video display device such as a cathode ray tube.  
           [0014]    The disadvantages of an all solid-state device are poor low light level performance, potential blooming from bright light sources, poor limiting resolution, and high power consumption. The poor low light performance is due to dark current and readout noise resulting in low signal-noise ratios. If a signal gain mechanism were provided prior to read-out this issue would be negated, as sufficient signal would exist to overcome the noise sources. Solid-state device architectures usually do not permit an amplification section prior to read-out. The poor limiting resolution is due to large pixel sizes usually chosen in an attempt to collect a large signal and thereby increase the signal to noise ration. These disadvantages have effectively prevented the use of solid-state sensors in night vision applications. The advantages of solid-state devices are better image contrast as compared to the image intensifier/I 2  camera, the availability of electronic read-out, and lower cost, particularly when the solid-state sensor is a CMOS array.  
           [0015]    As can be seen, the strengths and weaknesses of image intensifiers and solid-state sensors compliment each other and theoretically a combination of both devices would give better performance. One such combination proposed as an alternative to image intensifiers/I 2  cameras and solid-state sensors, is the electron bombarded CCD/CMOS sensor (EBCCD/CMOS). This device consists of the photo-cathode and body envelope of the image tube, and either a CCD or CMOS sensor integrated into this envelope. An illustrative example of an EBCCD/CMOS sensor is shown in FIG. 3. A high voltage is applied between the cathode and solid-state sensor so that the resulting electrons are amplified within the silicon in the solid-state sensor by electron bombardment.  
           [0016]    The advantages of the EBCCD/CMOS device are that it provides electronic readout. But the disadvantages are numerous. First, the intra-scene dynamic range is compressed. This means that overall contrast within the scene, when bright objects are next to dark objects, is reduced compared to an image intensifier/I 2  camera and all solid-state device. Secondly, the sensor suffers “halo” degradation of the image around bright lights due to electrons reflected off of the solid-state sensor. This halo exists in regular image tubes; however, technological improvements have reduced the halo to the point of non-existence. Thirdly, the very high voltage required to operate the device (2-10 kV) damages the silicon surface causing decay in performance over time.  
           [0017]    Therefore, it is an object of the present invention to provide an intensified hybrid solid-state sensor that combines the functions of the image intensifier, good signal-to-noise ratio and high logarithmic gain, with the electronic read-out functions either of a complementary Metal Oxide Semiconductor (CMOS) or charged coupled device (CCD).  
         SUMMARY OF THE INVENTION  
         [0018]    The present invention provides an intensified hybrid solid-state sensor. The solid-state sensor, according to the present invention, includes an imaging device comprising a solid-state sensor assembled with an image intensifier cathode, microchannel plate (MCP), and body envelope. This device combines the best functions of the image intensifier, good signal-to-noise ratio and high logarithmic gain, with the electronic read-out functions either of a complementary Metal Oxide Semiconductor (CMOS) or charged coupled device (CCD). Applications for this invention are primarily night vision systems where good low light sensitivity and high gain are required. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]    In order that the invention will become more clearly understood it will be disclosed in greater detail with reference to the accompanying drawings, in which:  
         [0020]    [0020]FIG. 1 is a schematic illustration of a typical image intensifying tube;  
         [0021]    [0021]FIG. 2A is flow chart for a typical CCD sensor;  
         [0022]    [0022]FIG. 2B is a schematic diagram of a typical CCD imaging surface;  
         [0023]    [0023]FIG. 3 is a cross-sectional view of a typical Electron Bombarded CCD device;  
         [0024]    [0024]FIG. 4A is a cross-sectional view of an intensified hybrid solid-state sensor according to the present invention;  
         [0025]    [0025]FIG. 4B is a schematic representation of an intensified hybrid solid-state sensor according to the present invention;  
         [0026]    [0026]FIG. 5A is a schematic illustration of a microchannel plate (MCP) and a back thinned CCD for use in the present invention;  
         [0027]    [0027]FIG. 5B is a schematic illustration of a microchannel plate (MCP) and a standard CCD for use in the present invention;  
         [0028]    [0028]FIG. 5C is a perspective view of a CMOS-type image sensor for use with the present invention;  
         [0029]    [0029]FIG. 6A is a perspective view of MCP channels having round profiles and a CMOS well;  
         [0030]    [0030]FIG. 6B is a perspective view of MCP channels having square profiles and a CMOS well;  
         [0031]    [0031]FIG. 7A is a schematic top view of a large pixel/small MCP channel pitch per unit area of the sensor surface according to the present invention;  
         [0032]    [0032]FIG. 7B is a schematic top view of a one-to-one pixel to MCP channel per unit area of the sensor surface according to the present invention  
         [0033]    [0033]FIG. 7C is a schematic top view of a small CMOS pixel pitch/large MCP channel per unit area of the sensor surface according to the invention; 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0034]    [0034]FIG. 4B is a schematic representation of an intensified hybrid solid-state sensor device  41  according to the present invention. Sensor  41  comprises a standard image tube photo cathode  54 , a microchannel plate (MCP)  53  and a solid-state imaging sensor  56 . Solid-state imaging sensor  56  can be any type of solid-state imaging sensor. Preferably, solid-state imaging sensor  56  is a CCD device. More preferably, solid-state imaging sensor  56  is a CMOS imaging sensor. FIG. 5A illustrates a back-thinned CCD imaging device as imaging sensor  56 ′. In this embodiment, MCP  53  is connected with a back thinned CCD sensor  56 ′. Back-thinned CCD  56 ′ includes an electron receiving surface, such as diffusion collection area  56   a ′ and a readout area  62 . FIG. 5B illustrates an alternative standard CCD imaging device including MCP  53  connected to a standard CCD  56 ″. CCD  56 ″ includes an oxide cover  63  and plurality of collection wells  64 . FIG. 5C illustrates sensor  56  as a CMOS sensor, including a CMOS substrate  56 ′″ and a plurality of collection wells  65 .  
         [0035]    For various reasons, CCD based image sensors are limited or impractical for use in many applications. First, CCDs require at least two polysilicon layers with a buried-channel implant to-achieve their high performance, meaning that they cannot be fabricated using standard CMOS fabrication processes. Second, the level of integration that can be achieved with CCD based imagers is low since they cannot include the devices necessary to integrate them with other devices in an application. Finally, the circuits used to transfer data out of the image array to other devices on the system board, such as Digital Signal Processors (DSPs) and other image processing circuits, have a large capacitance and require voltages higher than the other circuits. Since the currents associated with charging and discharging these capacitors are usually significant, a CCD imager is not particularly well suited for portable or battery operated applications.  
         [0036]    As such, less expensive image sensors fabricated out of integrated circuits using standard CMOS processes are desirable. Essentially, with a CMOS type imager sensor, a photo diode, phototransistor or other similar device is employed as a light-detecting element. The output of the light-detecting element is an analog signal whose magnitude is approximately proportional to the amount of light received by the element. CMOS imagers are preferred in some applications since they use less power, have lower fabrication costs and offer higher system integration compared to imagers made with CCD processes. Moreover, CMOS imagers have the added advantages that they can be manufactured using processes similar to those commonly used to manufacture logic transistors. While the preferred embodiment of the invention incorporates a CMOS sensor as the imaging sensor  56 , any solid-state imaging sensor would work and is within the scope of this patent.  
         [0037]    Referring again to FIG. 4B, photo cathode  54  can be a standard photo cathode as used in any known type of image intensifying device. Photo cathode  54  can be, but is not limited to, a material such a GaAs, Bialkali, InGaAs, and the like. Photo cathode  54  includes an input side  54   a  and an output side  54   b . MCP  53  can be, but is not limited to a silicon or glass material, and is preferably about 10 to 25 mm thick. MCP  53  has a plurality of channels  52  formed between an input surface  49  and output surface  50 . Channels  52  can have any type of profile, for example a round profile  52 ′ (FIG. 6A) or a square profile  52 ″ (FIG. 6A.) MCP  53  is connected to electron receiving surface  56   a  of imaging sensor  56 .  
         [0038]    Preferably, output surface  50  of MCP  53  is physically in contact with electron receiving surface  56   a  of imaging sensor  56 . However, insulation may be necessary between MCP  53  and imaging sensor  56 . Accordingly, a thin insulating spacer  55  may be inserted between output surface  50  of MCP  53  and electron receiving surface  56   a  of imaging sensor  56 . Insulating spacer  55  can be made of any electrical insulating material and is preferably formed as a thin layer, no more than several microns thick, deposited over electron receiving surface  56   a  of imaging sensor  56 . For example, insulating spacer may be, but is not limited to, an approximately 10 μm thick film Alternatively, insulating spacer  55  could be a film formed on the output surface  50  of MCP  53  (not shown).  
         [0039]    CMOS imaging sensor  56  includes electron receiving surface  56   a  and output  56   b . The increased number of electrons  48  emitted from MCP  53  strike electron receiving surface  56   a . Electron receiving surface  56   a  comprises a CMOS substrate  56 ′″ and a plurality of collection wells  65  (FIG. 5C). Electrons  48  (See FIG. 4B) collected in collection wells  65  are processed using standard signal processing equipment for CMOS sensors to produce an intensified image signal that is sent through output  56   b  to an image display device  46 .  
         [0040]    An electric biasing circuit  44  provides a biasing current to sensor  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 photo cathode  54  and MCP  53 . The biasing voltage from first electrical connection  42  is preferably set so as to be less than the biasing voltage than the EBCCD/CMOS sensor cathode to CCD voltage, i.e., 2-10 kV. For example, one preferred biasing voltage could be similar to that of image tubes, such as ˜1400V. Second electrical connection  43  applies a biasing voltage of between MCP  53  and CMOS sensor  56 . Preferably, the biasing voltage applied through second electrical connection  43  is significantly less than the image tube—screen voltage of about 4200V of the prior art devices (FIG. 1). For example, the biasing voltage applied through second electrical connection  43  could be, but is not limited to ˜100V. FIG. 4A illustrates one potential configuration of the sensor  41 . In this configuration, photo cathode  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.  
         [0041]    Referring to FIG. 4B, in operation, light  58 ,  59  from an image  57  enters intensified hybrid solid-state sensor  41  through input side  54   a  of photo cathode  54 . Photo cathode  54  changes the entering light into electrons  48 , which are output from output side  54   b  of photo cathode  54 . Electrons  48  exiting photo cathode  54  enter channels  52  through input surface  49  of MCP  53 . After electrons  48  bombard input surface  49  of MCP  53 , secondary electrons are generated within the plurality of channels  52  of MCP  53 . MCP  53  may generate several hundred electrons in each of channels  52  for each electron entering through input surface  49 . 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  through output side  50  of MCP  53 , and strike electron receiving surface  56   a  of CMOS imaging device  56 .  
         [0042]    [0042]FIG. 6 illustrates how the increased number of electrons  47  exit channels  52  and strike a particular collection well  65 ′ of CMOS imaging sensor  56 . As can be seen from this illustration, a relationship exists between the collection wells  65 ′ and the number of channels  52  which emit electrons  47 . In general, adjacent channels  52  of MCP  53  are separated by a predetermined channel pitch  52   a . FIG. 6 illustrates a channel pitch  52   a  that results in more than one channel  52  per collection well  65 ′.  
         [0043]    FIGS.  7 A- 7 C illustrate three different alternatives of CMOS well/channel pitch relationships according to the invention. FIG. 7A illustrates one relationship between channel pitch  52   a  and a CMOS collection well  65 ′. In this case, channel pitch  52   a  is relatively small, while the size of CMOS well  65 ′ is relatively large. This permits several electrons  47  from two or more channels  52  to strike CMOS collection well  65 ′. FIG. 7B illustrates another CMOS well/channel pitch relationship. In this embodiment, channel pitch  52   a  and the size of CMOS collection well  65 ′ are approximately in a one-to-one relationship. As such electrons  47 ′ from a single channel  52  strike a single collection well  65 ′. FIG. 7 c  illustrates another CMOS well/channel pitch relationship where channel pitch  52   a  is relatively large and the size of CMOS collection well  66  is relatively small. In this case electrons  47 ″ from a single channel  52  strike multiple collection wells  66 . While each of these structures provide various advantages, the relationship illustrated in FIG. 7A is preferred for the present invention.  
         [0044]    As a result, the intensified hybrid solid-state sensor operates in different conditions than any of the other prior art concepts. The result is that the MCP  53  can be mounted directly on the CMOS sensor  56  giving the hybrid device similar contrast to the all solid-state device but with low halo, good signal-to-noise ratio, and logarithmic gain of the image tube. Since operating voltages are lower, the hybrid device can be gated like an image tube allowing operation from overcast starlight condition to daytime operation. The hybrid sensor has better halo from the lack of physical gap between MCP  53  and CMOS sensor  56 . This lack of physical separation in the two components is also why contrast is improved when compared to the EBCCD/CMOS or image intensified camera. The hybrid device also has the logarithmic gain curve of the image tube. Unlike the EBCCD/CMOS sensor, the hybrid sensor can be gated due to the low cathode voltages.  
         [0045]    The above detailed description of a preferred embodiment of the invention sets forth the best mode contemplated by the inventor for carrying out the invention at the time of filing this application and is provided by way of example and not as a limitation. Accordingly, various modifications and variations obvious to a person of ordinary skill in the art to which it pertains are deemed to lie within the scope and spirit of the invention as set forth in the following claims.