Abstract:
A microchannel plate (MCP) is formed from a boule. The MCP includes a plate having opposing end surfaces formed of acid resistant glass and acid etchable glass, and multiple channels extending longitudinally between the opposing end surfaces. The multiple channels are formed by circumferential walls of the acid resistant glass that surround the acid etchable glass. A respective circumferential wall forms a curved surface extending longitudinally between the opposing end surfaces. The curved surface is configured to reduce light from passing from one end surface to the other end surface. The acid resistant glass has a lower softening temperature than the acid etchable glass. As a result, the acid etchable glass may be subjected to a bending process, without reducing the diameter size of the microchannels that are formed after the bending process.

Description:
FIELD OF THE INVENTION 
     This invention relates, in general, to microchannel plates (MCPs) for use in image intensifier tubes, and in particular, to a microchannel plate having curved channels. 
     BACKGROUND OF THE INVENTION 
     Image intensifier tubes are used in night/low light vision applications to amplify ambient light into a useful image. A typical image intensifier tube is a vacuum device, roughly cylindrical in shape, and generally includes a body, photocathode and faceplate, microchannel plate (MCP), and output optic and phosphor screen. Incoming photons are focused on the glass faceplate by external optics, and strike the photocathode that is bonded to the inside surface of the faceplate. The photocathode converts the photons to electrons, which are accelerated toward the MCP by an electric field. The MCP has many microchannels, each of which functions as an independent electron amplifier, and roughly corresponds to a pixel of a CRT. The amplified electron stream, emanating from the MCP, excites the phosphor screen and a resulting visible image is passed through output optics to any additional external optics. The body holds these components in precise alignment, provides electrical connections, and also forms a vacuum envelope. 
     In general, fabrication of a microchannel plate starts with a fiber drawing process, as disclosed in U.S. Pat. No. 4,912,314, issued Mar. 27, 1990 to Ronald Sink, which is incorporated herein by reference in its entirety. For convenience, FIGS. 1-4, disclosed in U.S. Pat. No. 4,912,314 are included herein and discussed below. 
     In  FIG. 1 , there is shown a starting fiber  10  for the microchannel plate. Fiber  10  includes glass core  12  and glass cladding  14  surrounding the core. Core  12  is made of glass material that is etchable in an appropriate etching solution. Glass cladding  14  is made from glass material which has a softening temperature substantially the same as the glass core. The glass material of cladding  14  is different from that of core  12 , however, in that it has a higher lead content, which renders the cladding non-etchable under the same conditions used for etching the core material. Thus, cladding  14  remains after the etching of the glass core. A suitable cladding glass is a lead-type glass, such as Corning Glass 8161. 
     The optical fibers are formed in the following manner: An etchable glass rod and a cladding tube coaxially surrounding the rod are suspended vertically in a draw machine which incorporates a zone furnace. The temperature of the furnace is elevated to the softening temperature of the glass. The rod and tube fuse together and are drawn into a single fiber  10 . Fiber  10  is fed into a traction mechanism in which the speed is adjusted until the desired fiber diameter is achieved. Fiber  10  is then cut into shorter lengths of approximately 18 inches. 
     Several thousands of the cut lengths of single fiber  10  are then stacked into a mold and heated at a softening temperature of the glass to form hexagonal array  16 , as shown in  FIG. 2 . The cut lengths of fiber  10  together form a hexagonal configuration. The hexagonal configuration provides a better stacking arrangement. 
     The hexagonal array, which is also known as a multi assembly or a bundle, includes several thousand single fibers  10 , each having core  12  and cladding  14 . Bundle  16  is suspended vertically in a draw machine and drawn to again decrease the fiber diameter, while still maintaining the hexagonal configuration of the individual fibers. Bundle  16  is then cut into shorter lengths of approximately 6 inches. 
     Several hundred of the cut bundles  16  are packed into a precision inner diameter bore glass tube  22 , as shown in  FIG. 3 . The glass tube has a high lead content and is made of a glass material similar to glass cladding  14  and is, thus, non-etchable by the etching process used to etch glass core  12 . The lead glass tube  22  eventually becomes a solid rim border of the microchannel plate. 
     In order to protect fibers  10  of each bundle  16 , during processing to form the microchannel plate, a plurality of support structures are positioned in glass tube  22  to replace those bundles  16  which form the outer layer of the assembly. The support structures may take the form of hexagonal rods of any material having the necessary strength and the capability to fuse with the glass fibers. Each support structure may be a single optical glass fiber  24  having a hexagonal shape and a cross-sectional area approximately as large as that of one of the bundles  16 . The single optical glass fiber, however, has a core and a cladding which are both non-etchable. The optical fibers  24 , or support rods  24 , are illustrated in  FIG. 3 , as being disposed at the periphery of assembly  30  and surrounding the plurality of bundles  16 . The support rods are also known as filler fibers. 
     The support rods may be formed from one optical fiber or any number of fibers up to several hundred. The final geometric configuration and outside diameter of one support rod  24  is substantially the same as one bundle  16 . The multiple fiber support rods may be formed in a manner similar to that of forming bundle  16 . 
     The assembly formed when all support rods  24  have been placed around the ends of bundles  16  is called a boule, and is generally designated as  30  in  FIGS. 3 and 5 . 
     Boule  30  is fused together in a heating process to produce a solid boule of rim glass and fiber optics. The fused boule is then sliced, or diced, into thin cross-sectional plates. The planar end surfaces of the sliced fused boule are ground and polished. 
     In order to form the microchannels, cores  12  of optical fibers  10  are removed, by etching with dilute hydrochloric acid. After etching the thin plates, the high lead content glass claddings  14  remains to form microchannels  32 , as illustrated in  FIG. 4 . Also, support rods  24  remain solid and provide a good transition from the solid rim of tube  22  to microchannels  32 . After the plates are etched to remove the core rods, the channels in the plate are metalized and activated. 
     The current method of manufacturing an MCP also includes dicing the boule at an angle into thin wafers to produce a bias angle. The wafers are then etched, hydrogen fired to form a conduction layer, and metalized to provide electrical contact. After the boule is sliced into wafers, each wafer is handled individually. A typical size of the wafer is approximately 1 inch diameter. 
     The microchannels of an MCP each form a generally straight bore extending from input to output surfaces of the MCP. As shown schematically in  FIG. 11 , MCP  110  includes input surface  111  and output surface  112 . The microchannels, designated as  113 , are inclined at a bias angle with respect to the opposing input output surfaces. However, each microchannel forms a bore that is substantially centered about a straight axial line extending between the input and output surfaces. 
     Curved microchannels have been considered as a way of increasing gain of an MCP. Such curved channels have been very tricky and expensive to produce. No known MCP is produced with curved channels, although curved channel electron multipliers have been produced for testing purposes. Two methods are known for making a curved channel MCP. Both methods are described below with respect to  FIGS. 6 and 7 . 
     The first method for making a curved channel MCP is shown in  FIG. 6 . As shown, MCP  63  is heated and placed between two horizontally sliding plates, top plate  61  and bottom plate  62 . Each plate is notched to receive approximately one-half of the height of MCP  63 . The top and bottom plates are brought together to completely nestle the MCP. Next, the top plate is slid horizontally with respect to the lower plate. This causes shearing of one end surface of the MCP with respect to the other end surface of the MCP, thereby providing curves to the microchannels. This method requires exceptional temperature control, very accurate movement of the shearing plates, and probably does not produce adequate uniformity for an imaging application. 
     The second method of making a curved MCP is shown in  FIG. 7 . As shown, MCP  73  is sandwiched between two heated plates  71  and  72 . The two closed plates are spun in a counter-clockwise direction (for example). The spinning of the plates produces a centripetal force which pushes the center of the MCP outward. With the exterior surfaces of the MCP fixed by the notches in plates  71  and  72 , it is believed that the result is curved channels in the MCP. Like the first method, this method requires accurate temperature control. This method also substitutes the difficulty of high-speed rotary motion for the problem of high accuracy linear motion. It will be understood, however, that the goal of each of these methods is higher gain, and not reduced light transmission. 
     SUMMARY OF THE INVENTION 
     To meet this and other needs, and in view of its purposes, the present invention provides a microchannel plate (MCP) formed from a boule. The MCP includes a plate having opposing end surfaces formed of acid resistant glass and acid etchable glass, and multiple channels extending longitudinally between the opposing end surfaces. The multiple channels are formed by circumferential walls of the acid resistant glass that surround the acid etchable glass. A respective circumferential wall forms a curved surface extending longitudinally between the opposing end surfaces. The curved surface is configured to reduce light from passing from one end surface to the other end surface. The acid resistant glass has a lower softening temperature than the acid etchable glass. 
     Another embodiment of the present invention includes a boule for forming multiple MCPs. The boule includes core rods formed of acid etchable glass, and cladding glass, surrounding the core rods, formed of acid resistant glass. The core rods and the cladding glass extend longitudinally between ends of the boule, and the core rods are smoothly curved between the ends of the boule. The core rods have a lower softening temperature than the cladding glass. The softening temperature of the core rods is at least 25 degree Centigrade lower than the softening temperature of the cladding glass. As an example, the softening temperature of the core rods is approximately 550 degrees Centigrade and the softening temperature of the cladding glass is approximately 580 degrees Centigrade. The core rods are substantially parallel to each other between the ends of the boule. A core rod forms a portion of a circle intersecting a chord, and the chord is approximately 8 inches in length and the furthest distance from the chord to the circle is approximately 0.4 inches. 
     Yet another embodiment of the present invention is a mold for bending a boule for making multiple MCPs. The mode includes a structure having a longitudinal direction and a transverse direction, and a notch formed in the structure, extending in the longitudinal direction between ends of the structure. The notch forms a U-shape, oriented in the transverse direction. The U-shape includes a portion of a first circle configured to receive and cradle a boule. The notch forms a portion of a second circle, oriented in the longitudinal direction, configured to impart a bend in the boule having a curved surface similar to the second circle. The structure is configured to receive the boule in a heated state having a first temperature effective in softening cladding glass in the boule, and having a temperature lower than a second temperature effective in softening core rods in the boule. 
     Still another embodiment of the present invention is a method for curving a boule having core rods and cladding glass surrounding the core rods. The method includes the steps of: heating the boule to a first temperature, wherein the first temperature is effective in softening the cladding glass; and bending the boule and, in turn, bending the core rods. The method also includes the steps of: placing the boule on a mold having a curved surface; and bending the boule after heating to the first temperature, so that the boule conforms to the curved surface. Another step includes dicing the boule to obtain multiple MCPs. 
     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 partial view of a fiber used in fabricating microchannel plates. 
         FIG. 2  is a partial view of a bundle of fibers shown in  FIG. 1  for use in fabricating microchannel plates. 
         FIG. 3  is a cross-sectional view of a packed boule. 
         FIG. 4  is a partial cut-away view of a microchannel plate. 
         FIG. 5  is a perspective view of a boule. 
         FIG. 6  is a cross-sectional view of an MCP sandwiched between two plates, used for forming a shearing force to bend the channels of the MCP. 
         FIG. 7  is another cross-sectional view of an MCP sandwiched between two plates, used for forming a centripetal force to bend the channels of the MCP. 
         FIG. 8  is a functional block diagram of an image intensifier system, in accordance with an embodiment of the present invention. 
         FIGS. 9A ,  9 B and  9 C are different views of a mold used for providing a curvature to the boule shown in  FIG. 5 , in accordance with an embodiment of the present invention. 
         FIG. 10A  is a partial cross-sectional view of a boule, before the microchannel etchable rods are subjected to being curved. 
         FIG. 10B  is a partial cross-sectional view of the boule of  FIG. 10A , after the microchannel etchable rods are subjected to being curved, in accordance with an embodiment of the present invention. 
         FIG. 11  is a pictorial of an MCP having straight bores. 
         FIG. 12  is a pictorial of an MCP having curved bores, in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     An image intensifier includes an MCP disposed between a photocathode and an image sensing device. For example, as schematically shown in  FIG. 8 , image intensifier tube  80  includes MCP  91  disposed in vacuum housing  83  between photocathode  90  and image sensing device  92 . 
     As shown, light energy  82  reflected from object  81  impinges upon photocathode  90 . Photocathode  90  receives the incident energy on input surface  94  and outputs the energy, as emitted electrons, on output surface  95 . The output electrons, designated as  85 , from photocathode  90 , are provided as an input to an electron gain device, such as MCP  91 . The MCP includes input surface  86  and output surface  87 . As electrons bombard input surface  86 , secondary electrons are generated within microchannels  88  of MCP  91 . The MCP generates several hundred electrons for each electron entering input surface  86 . 
     Although not shown, it will be understood that MCP  91  is subjected to a difference in voltage potential between input surface  86  and output surface  87 , typically over a thousand volts. This potential difference enables electron multiplication. Electrons  89 , outputted from MCP  91 , impinge upon solid state electron sensing device  92 . Electron sensing device  92  may be a CMOS imager, for example, and includes input surface  93  and output surface  96 , as shown in  FIG. 8 . 
     In general, electron sensing device  92  includes a phosphor screen on input surface  93 . The output signals from electron sensing device  92  may be provided to image display  84  by way of a bus, or may be stored in a memory (not shown). 
     For reasons explained below, in an embodiment of the invention, MCP  91  includes curved microchannels  88 . 
     Conventional microchannels of an MCP each form a generally straight bore extending from its input surface to its output surface. As shown schematically in  FIG. 11 , MCP  110  includes input surface  111  and output surface  112 . The microchannels, designated as  113 , are inclined at a bias angle with respect to the opposing input and output surfaces. Furthermore, each microchannel forms a bore that is substantially centered about a straight axial line extending between input and output surfaces  111  and  112 . 
     The inventor has discovered that as a result of the straight microchannels, light  114  shown in  FIG. 11  is reflected from or generated by a phosphor screen (not shown), re-enters microchannels  113 , and exits the microchannels. Because light  114  propagates as photons from surface  112  to the other surface  111  without reflecting off the channel walls, light  114  is substantially unattenuated at the output surface of microchannels  112 . 
     The photons, after exiting surface  111 , impinge upon a photocathode (not shown) and are converted into electrons that emanate from the photocathode surface. These electrons are again amplified by the MCP. The phosphor screen converts the amplified electrons from the MCP into light. The phosphor screen is covered with an aluminum reflector layer, but this tends to have a multitude of small holes, and bleeds a small amount of light back towards the MCP. The MCP permits a small amount of light to pass through, and thus some screen light is able to re-activate the photocathode. This represents spatially-disconnected noise, and degrades the tube image. 
     Due to the intricacies of the screen process, the aluminum reflector layer is difficult to produce without holes. Additionally, there are known tradeoffs to the aluminum reflector thickness and its method of deposition, so reducing light leakage through changes in the screen process is likely to degrade phosphor efficiency, MTF and/or SNR. 
     In order to reduce light transmission through MCP  91 , the inventor has discovered that curved microchannels, as shown in  FIGS. 8 and 12 , reduce the light transmission. Because light  124  ( FIG. 12 ) propagates from surface  87  to surface  86  by reflecting off the walls of microchannels  88 , light  124  is attenuated at surface  86 . The light must make multiple reflections off the channel walls, thereby losing intensity after each reflection. Although light  124  may be re-activated by photocathode  90  into electrons  85  ( FIG. 8 ) and may again be amplified by MCP  91 , the resulting re-activated electrons are substantially reduced. Thus, curved microchannels  88  are effective in reducing re-activated electrons and in reducing spatially-disconnected noise. 
     The inventor considered different approaches to curving the channels of an MCP. One possible approach is heating and bending a boule, such as heating and bending boule  30  ( FIG. 5 ). Simply heating and bending a boule, however, may not be desirable. The fibers disposed adjacent to the outer circumferential edge of the boule may be more stretched than the fibers disposed adjacent to the inner portion of the boule. If the outer edge fibers stretch more than the inner portion of fibers, the outer edge channels would likely be reduced in diameter. Since channel gain of an MCP is a function of channel aspect ratio, for a fixed MCP thickness, the stretched channels would cause shading in an image tube. 
     The inventor discovered that a preferred approach to forming curved channels in an MCP is to bend a boule that is fabricated from two types of glass. In addition, one type of glass should have a higher forming temperature than the second type of glass. For example, the core rod (core  12  in  FIG. 1 ) should have a higher forming temperature than the clad glass (cladding  14  in  FIG. 1 ). For example, the softening temperature for the core rod may be approximately 580° C. and the softening temperature for the clad glass may be approximately 550° C. 
     The inventor discovered that the above 30° C. difference in the forming temperature is adequate to induce a curve in the boule and maintain the fibers in a rigid state without stretching the edge fibers. Thus, bending of boule  30  may be accomplished by heating the boule to the softening temperature of the clad glass and then bending the boule. Because the clad glass softens and shears, the boule is bent. The core rod, however, has a higher forming temperature and remains rigid at the lower softening temperature of the clad glass. As a result, the core rod resists stretching. 
     As shown in  FIGS. 10A and 10B , the core rods, designated as  100  (clad glass not shown), do not stretch after bending. The square ends  102   a  and  102   b  of boule  30  remain parallel after bending. Since the core rods do not stretch, the diameters of the resulting microchannels (after dicing and etching) are not reduced in diameter. The present invention thus reduces light transmission through the MCP without producing visible shading or FPN due to bending (or curving). 
     Fundamental to this process is the difference in softening temperature between the two types of glass used in fabricating the boule. The core rod must have a higher softening temperature so that it resists stretching while the clad shears. As an analogy, a bundle of uncooked spaghetti may be bent, even though the individual pieces cannot be stretched. The bending of the uncooked spaghetti occurs as the individual pieces slide relative to each other. 
     It will be appreciated that the present invention attempts to reduce light transmission through the microchannels of an MCP. This may be achieved by preventing light from passing through the MCP without also reflecting off the walls of the microchannels. Furthermore, the bending (or curving) of the microchannels may be slight. For example, simply offsetting the centers of the microchannels by one channel diameter results in at least two reflections of light off the channel walls. The at least two reflections produce light attenuation, which is a desired goal. Thus, the amount of curvature of the microchannels may be quite small. 
     Inherent in the present invention is a variation in sliced MCP bias angle, since the slicing angle is usually fixed with respect to the boule. This angular variation may be reduced by slicing the MCP at  900  to the bending axis, but this adds bias direction variation. 
     An exemplary structure for bending, or curving the boule is shown in  FIGS. 9A ,  9 B and  9 C. As shown mold  200  includes a structure having a longitudinal direction and a transverse direction. A notch is formed in the structure, extending in the longitudinal direction between ends of the structure. The notch forms a U-shape, oriented in the transverse direction. The U-shape has a portion of a first circle configured to receive and cradle a boule. The notch forms a portion of a second circle, oriented in the longitudinal direction and configured to impart a bend in the boule having a curved surface similar to the second circle. 
     The mold  200  is configured to receive the boule in a heated state having a first temperature effective in softening cladding glass in the boule, but having a temperature lower than a second temperature effective in softening core rods in the boule. 
     As an example of dimensions, mold  200  may have a length (L) of 8 inches, a height (H) of 1.25 inches, and a width (W) of 1.75 inches. The diameter of the notch (D) may be 1.125 inches and the curvature of the notch may form a minimum dimension C of 0.4 inches for a length (L) of 8 inches. 
     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.