Abstract:
The present invention provides a mega-boule for use in fabricating microchannel plates (MCPs). The mega-boule includes a cross-sectional surface having at least first, second and third areas, each area occupying a distinct portion of the cross-sectional surface. The first and second areas include a plurality of optical fibers, transversely oriented to the cross-sectional surface, each optical fiber having a cladding formed of non-etchable material and a core formed of etchable material. The third area is disposed interstitially between and surrounding the first and second areas, and includes non-etchable material.

Description:
TECHNICAL FIELD  
       [0001]     The present invention relates to microchannel plates (MCPs) for use with image intensifiers, and more specifically, to a device and method for fabrication of multiple MCPs using a mega-boule wafer.  
       BACKGROUND OF THE INVENTION  
       [0002]     Microchannel plates are used as electron multipliers in image intensifiers. They are thin glass plates having an array of channels extending there through and are located between a photocathode and a phosphor screen. An incoming electron from the photocathode enters the input side of the microchannel plate and strikes a channel wall. When voltage is applied across the microchannel plate, these incoming or primary electrons are amplified, generating secondary electrons. The secondary electrons then exit the channel at the back end of the micrcochannel plate and are used to generate an image on the phosphor screen.  
         [0003]     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.  
         [0004]     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.  
         [0005]     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.  
         [0006]     Several thousands of the cut lengths of single fiber  10  are then stacked into a graphite mold and heated at a softening temperature of the glass to form hexagonal array  16 , as shown in  FIG. 2 . As shown, each of the cut lengths of fiber  10  has a hexagonal configuration. The hexagonal configuration provides a better stacking arrangement.  
         [0007]     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.  
         [0008]     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.  
         [0009]     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 .  
         [0010]     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 .  
         [0011]     Each bundle  16  that forms the outermost layer of fibers in tube  22  is replaced by a support rod  24 . This is preferably done by positioning one end of a support rod  24  against one end of a bundle  16  and then pushing support rod  24  against bundle  16 , until bundle  16  is out of tube  22 . The assembly formed when all of the outer bundles  16  have been replaced by support rods  24  is called a boule, and is generally designated as  30  in  FIG. 3 .  
         [0012]     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.  
         [0013]     In order to form the microchannels, cores  12  of optical fibers  10  are removed, by etching with dilute hydrochloric acid. After etching the boule, 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 .  
         [0014]     Additional process steps include beveling and polishing of the glass boule. After the plates are etched to remove the core rods, the channels in the boule are metallized and activated.  
         [0015]     As described, the current method of manufacturing an MCP includes stacking multiple bundles, and then placing the stacked bundles within a sheath of rim glass. The supporting rods of non-etchable fibers are then used to fill the interstitial space between the bundles of etchable fibers and the rim glass (tube  22 ) to form a boule. The boule is then sliced at an angle into thin wafers to produce a bias angle. The wafers are then etched, hydrogen fired to form a conduction layer, and metallized to provide electrical contact.  
         [0016]     After the boule is sliced into wafers, each wafer is handled individually. A typical size of the wafer is approximately 1 inch diameter. This is much smaller than the wafer size of current semiconductor processing tools and necessitates use of custom fabrication processing tools. Handling each boule wafer individually leads to large amounts of touch labor for a part very sensitive to particle contamination. The yield of these wafers are, therefore, reduced.  
         [0017]     The present invention addresses the need for fabricating MCPs using more efficient fabrication methods and for methods that are less subject to contamination and reduced yield.  
       SUMMARY OF THE INVENTION  
       [0018]     To meet this and other needs, and in view of its purposes, the present invention provides a mega-boule for use in fabricating microchannel plates (MCPs). The mega-boule includes a cross-sectional surface having at least first, second and third areas, each area occupying a distinct portion of the cross-sectional surface. The first and second areas include a plurality of optical fibers, transversely oriented to the cross-sectional surface, each optical fiber having a cladding formed of non-etchable material and a core formed of etchable material. The third area is disposed interstitially between and surrounding the first and second areas, and includes non-etchable material.  
         [0019]     In another aspect, the invention includes a method of forming a plurality of microchannel plates (MCPs). The method includes the steps of: (a) providing a bundle of optical fibers, wherein each optical fiber includes a cladding formed of non-etchable material and a core formed of etchable material; (b) stacking a plurality of the bundles to form at least first and second cross-sectional areas, defining first and second mini-boules, respectively; (c) stacking non-etchable material interstitially between and surrounding the at least first and second mini-boules; and (d) fusing the plurality of bundles and the stacked non-etchable material for forming the plurality of MCPs in the at least first and second cross-sectional areas.  
         [0020]     The method may also include the steps of: (e) dicing the fused bundles and non-etchable material to form multiple mega-boule wafers, each mega-boule wafer defining a batch die; (f) activating, and metallizing each mega-boule wafer for forming the plurality of MCPS; and (g) extracting from each mega-boule wafer the plurality of MCPs.  
         [0021]     In yet another aspect, the invention includes a method of forming a batch die for forming multiple microchannel plates (MCPs). The method includes the steps of: (a) providing etchable and non-etchable optical materials; and (b) stacking the etchable and non-etchable optical materials to form a stack having a cross-sectional surface including at least first, second and third areas. The first and second areas are stacked with the etchable optical material and the third area is stacked with the non-etchable optical material, and the third area is disposed interstitially between and surrounding the first and second areas. The method may also include forming the first, second and third areas distinctly and separately from each other.  
         [0022]     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 DRAWING  
       [0023]     The invention is best understood from the following detailed description when read in connection with the accompanying drawing. Included in the drawing are the following figures:  
         [0024]      FIG. 1  is a partial view of a fiber used in fabricating microchannel plates in accordance with the present invention;  
         [0025]      FIG. 2  is a partial view of a bundle of fibers shown in  FIG. 1  for use in fabricating microchannel plates in accordance with the present invention;  
         [0026]      FIG. 3  is a cross-sectional view of a packed boule in accordance with the prior art;  
         [0027]      FIG. 4  is a partial cut-away view of a microchannel plate;  
         [0028]      FIG. 5  is a flow diagram illustrating a method for fabricating microchannel plates using a mega-boule wafer, in accordance with the present invention;  
         [0029]      FIG. 6  is a cross-sectional view of a monolithic stack, including a cross-sectional view of a mega-boule cut from the monolithic stack, in accordance with the present invention;  
         [0030]      FIG. 7  is a cross-sectional view of a 4-inch semiconductor mega-boule wafer, illustrating that ten standard 18 mm MCPs may be extracted from the batch die, in accordance with the present invention;  
         [0031]      FIG. 8  is a cross-sectional view of a 4-inch semiconductor mega-boule wafer, illustrating that 14 standard 16 mm MCPs may be extracted from the batch die, in accordance with the present invention;  
         [0032]      FIG. 9  is a cross-sectional view of a 4-inch semiconductor mega-boule wafer, illustrating that 28 rectangular MCPs may be extracted from the batch die, in accordance with the present invention;  
         [0033]      FIG. 10A  is a schematic cross-sectional view of opposing arched-presses configured to press the monolithic stack of  FIG. 6  into a circular geometry, in accordance with the present invention;  
         [0034]      FIG. 10B  is a schematic cross-sectional view of opposing linear presses configured to press the monolithic stack of  FIG. 6  into a rectangular geometry, in accordance with the present invention; and  
         [0035]      FIG. 11  is a side view of the monolithic stack of  FIG. 6  being diced into multiple mega-boule wafers, in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0036]     The present invention relates to forming a plurality of MCPs by using a method amenable to conventional wafer fabrication tools. More specifically, an embodiment of a method of the present invention is shown in  FIG. 5 , and is generally designated by reference numeral  50 . As will be explained, the method forms a batch die for making multiple MCPs from a single large wafer. The single large wafer, referred to as a mega-boule wafer, is sized to be accommodated by conventional wafer fabrication tools.  
         [0037]     Referring now to  FIG. 5  and beginning with step  51 , fibers of glass core and glass cladding are formed by method  50 . Starting fiber  10  is shown in  FIG. 1  and includes glass core  12  and glass cladding  14 . Core  12  is made of material that is etchable, so that the core may be subsequently removed by etching a mega-boule wafer, in accordance with the present invention. Glass cladding  14  is made of glass that is non-etchable under the same conditions that allow etching of core  12 . Thus, each cladding remains after the etching process, and becomes a boundary for a microchannel that forms upon removal of a corresponding core.  
         [0038]     As discussed before, a suitable cladding glass is a lead-type glass, such as Corning Glass 8161. In subsequent stages of the inventive process, using conventional fabrication tools on the mega-boule wafer, the lead oxide is reduced to activate the inner surfaces of each of the glass claddings, so that they are capable of emitting secondary electrons.  
         [0039]     As described in U.S. Pat. No. 4,912,314, which is incorporated herein by reference in its entirety, optical fibers  10  are formed in the following manner: An etchable glass rod and a cladding tube coaxially surrounding the glass 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 . The fiber is fed into a traction mechanism, where the speed is adjusted until the desired fiber diameter is achieved. Fiber  10  is then cut into shorter lengths of approximately 18 inches.  
         [0040]     The method next enters step  52  and forms multiple hexagonal arrays of fibers  10  to define multiple bundles  16 , as shown in  FIG. 2 . Several thousands of the cut lengths of a single fiber  10  are stacked into a graphite mold and heated at the softening temperature of the glass in order to form each hexagonal array, wherein each of the cut lengths of fiber  10  has a hexagonal configuration. It will be appreciated that the hexagonal configuration provides a better stacking arrangement. In addition to the hexagonal configuration, other configurations may also be used, such as a triangular configuration and a rhombohedral configuration.  
         [0041]     The hexagonal array  16 , which is also referred to as a multi assembly or as a bundle, includes several thousand single fibers  10 , each having core  12  and cladding  14 . This 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. The bundle  16  is then cut into shorter lengths of approximately 6 inches.  
         [0042]     Several hundred of the cut bundles  16  are then stacked by step  53  of the inventive method to form individual larger stacks, each having a predetermined cross-sectional area. Each larger stack of the predetermined cross-sectional area containing the bundles is referred to herein as a mini-boule. The stacking continues in steps  54  and  55  by also stacking non-etchable glass (also referred to herein as support rods) so that the non-etchable glass surrounds each mini-boule. Multiple mini-boules may be stacked together, and multiple support rods may be stacked between the mini-boules and stacked to surround the peripheries of each of the mini-boules. In this manner, each mini-boule is separated from each other mini-boule by the support rods. The stacking may continue in this manner, until a cross-sectional area of a predetermined size is reached. The predetermined cross-sectional size is a function of a size that may be accommodated by conventional wafer fabrication tools. The multiple mini-boules and the interstitially placed support rods are referred to herein as a mega-boule.  
         [0043]     As best shown in  FIG. 6 , mega-boule  62  includes multiple mini-boules  66  with interstitial area  64  comprised of multiple non-etchable support rods. The non-etchable support rods separate and surround each mini-boule  66 . The non-etchable support rod  24  has a high lead content and is made of a glass material which is similar to glass cladding  14  and is, thus, non-etchable by the process used to etch away glass core  12 . The non-etchable glass has a coefficient of expansion which is approximately the same as that of fibers  10 . The non-etchable glass of support rods  24 , after the method of the invention is completed, eventually becomes a solid rim border of each fabricated microchannel plate.  
         [0044]     It will be appreciated that the non-etchable support rods provide a support structure to protect each mini-boule  66 . Each support rod may take the form of a hexagonal rod (for example) of any material having the necessary strength and the capability to fuse with the etchable glass fibers. The material of the support rods have a temperature coefficient close enough to that of the etchable glass fibers to prevent distortion of the latter during temperature changes.  
         [0045]     In one embodiment, each support rod may be a single optical glass fiber  24  ( FIGS. 3 and 6 ) of hexagonal shape (for example) and of cross-sectional area approximately as large as that of one of the bundles  16 . Of course, the single optical fiber may have a core and a cladding which are both non-etchable under the aforementioned conditions. The optical support fibers  24  are schematically illustrated in  FIG. 6 . Both the core and the cladding of support rods  24  are made of the same high lead content glass material as the material of glass claddings  14  of fibers  10 . These support rods  24  form a cushioning layer and a separation space between each mini-boule  66  formed on mega-boule  62 .  
         [0046]     In other embodiments of the invention, the support rods may have a cross sectional shape other than an hexagonal shape, so long as the resulting shape of the support rods does not produce interstitial voids. For example, support rods having a triangular shape or a rhombohedral shape are likely not to result in interstitial voids. Accordingly, these shapes may also be used.  
         [0047]     The glass rod and tube which forms the core and the cladding of support rod  24  are suspended in a draw furnace and heated to fuse the rod and tube together, and to soften the fused rod and tube sufficiently to form each support rod  24 . The so formed support rod  24  is then cut into lengths of approximately 18 inches and subjected to a second draw to achieve the desired geometric configuration and smaller outside cross-sectional diameter that is substantially the same as the outside cross-sectional diameter of bundle  16 . The support rods may also be formed from one optical fiber or any number of optical fibers up to several thousand fibers. The final geometric configuration and outside diameter of one support rod being substantially the same as one bundle  16 . It will be appreciated that the support rods may be replaced by any other glass rods of any size and shape, so long as the support rods are of material that is non-etchable and able to fuse upon heating with the etchable bundles.  
         [0048]     It will be appreciated that the cross-sectional area of mini-boule  66  may be stacked, as large as desired by a user, for providing a corresponding individual MCP of a predetermined active cross-sectional area. It will also be appreciated that the cross-sectional area of mini-boule  66  may define a circular surface, as shown in  FIG. 6 , or a cross-sectional area defining a different geometry, such as a rectangular surface, as shown in  FIG. 9 .  
         [0049]     After stacking the mega-boule to have a cross-sectional area of a predetermined size, the mega-boule is pressed into a monolithic stack in step  56 . The pressing step may be performed, while mega-boule  62  is suspended in a furnace. The furnace may be heated at an elevated temperature, so that bundles  16  of mini-boules  66  and support rods  24  of interstitial area  64  are softened. While mega-boule  62  is at its softening temperature point, the pressing step is effective in causing bundles  16  and non-etchable rods  24  (support fibers  24 ) to fuse together and form a monolithic stack.  
         [0050]     It will also be appreciated that the cross-sectional area of the monolithic stack may be circular, rectangular, or of any other geometry compatible with semiconductor wafer fabrication tools. For example, mega-boule  62  may be stacked to form a substantially circular cross-sectional geometry and, subsequently, pressed into a circular monolithic stack  100  by opposing arched-presses  101   a - 101   d , as exemplified in  FIG. 10A . As another example, mega-boule  62  may be stacked to form a substantially rectangular cross-sectional geometry and, subsequently, pressed into a rectangular monolithic stack  105  by opposing linear-presses  106   a - 106   d , as exemplified in  FIG. 10B .  
         [0051]     After the mega-boule is pressed into a monolithic stack, the pressed monolithic stack ( 100  or  105 ) is cut, in step  57 , to form a cross-sectional size compatible with semiconductor wafer fabrication tools. For example, the monolithic stack may be turned on a lathe, or some other machine, to produce a circular mega-boule of circumference  68 , as shown in  FIG. 6 .  
         [0052]     The cut monolithic stack is then sliced or diced, in step  58 , into multiple mega-boule wafers, as schematically depicted in  FIG. 11 . As shown, monolithic stack  110  is diced cross-sectionally to produce a plurality of mega-boule wafers  112 . Each mega-boule wafer  112  is now ready to be processed as a large batch die containing multiple MCPs. It will be appreciated that the large batch die (mega-boule wafer  112 ) is processed in the same manner as an individual MCP wafer is processed. Advantageously, however, the large batch die allows multiple MCPs to be concurrently produced with minimal human handling and contamination.  
         [0053]     The method of the invention then takes each mega-boule wafer, formed by dicing in step  58 , for further processing during step  59 . The mega-boule wafer is heated and etched to remove the glass cores (cores  12  in  FIG. 1 ). Since the glass claddings (claddings  14  in  FIG. 1 ) and the support glass fibers, or the support rods (rods  24  in  FIG. 6 ) have a higher lead content then the glass cores, they are non-etchable, under the same conditions used to etch the glass cores. Thus, the glass claddings and the support rods remain and become boundaries for the microchannels (microchannels  32  in  FIG. 4 ) formed in the mega-boule wafer. The etching process may be performed by using diluted hydrochloric acid.  
         [0054]     The mega-boule wafer is then placed in an atmosphere of hydrogen gas, whereby the lead oxide of the non-etched lead glass is reduced to render claddings  14  as electron emissive. In this way, a semi-conducting layer is formed in each of the glass claddings and this layer extends inwardly from the surface that bounds each microchannel  32  ( FIG. 4 ).  
         [0055]     Because support rods  24  become boundaries for each mini-boule  66 , the active area of each microchannel plate is decreased. In this way, there are less channels to outgas. Additionally, since each MCP must be made to a predetermined outside diameter, so that it may be accommodated within an image intensifier tube, the area along the rim of each MCP is not used. The area along the rim is blocked by internal structures in the image intensifier tube. Therefore, support rods  24  may form a border of a predetermined area surrounding each mini-boule  66 . This border may be the area along the rim of each MCP which is blocked by the internal structures of the image intensifier tube. Thin metal layers are applied as electrical contacts to each of the planar end surfaces of the mega-boule wafer. This allows the establishment of an electric field across each MCP and provides entrance and exit paths for electrons excited by the electric field.  
         [0056]     After activation and metallization, each mega-boule wafer may be connected to a test fixture, whereby each MCP in the mega-boule wafer may be simultaneously tested for proper operation.  
         [0057]     If individual dies are required for producing each MCP, the mega-boule wafer may be processed, in step  60 , to extract individual MCPs from the mega-boule wafer. The extracting step may be performed by scribing using a laser. The scribing operation should preferably be free from particle generation, in order to minimize contamination of the multiple MCPs.  
         [0058]     Advantages of the present invention are many. The shape and size of the monolithic stack may depend on the type of semiconductor wafer fabrication tools available. The shape and size of the mega-boule wafer, which is diced from the monolithic stack, may also depend on the type of semiconductor wafer fabrication tools are available. Consequently, specialized tools may be avoided.  
         [0059]     Furthermore, handling and particle defects may be reduced, because the processing tools are automated and limit the amount of human interaction with the MCP dies. Throughput may be increased, because a higher packing density of MCP dies is possible on the mega-boule wafer. This increases the batch size.  
         [0060]     Moreover, tool fixture issues for different sizes of MCPs may be easily resolved, because the mega-boule wafer is the fixture that holds the individual MCP dies. Finally, different MCP formats may easily be incorporated into a production line, because the mega-boule wafer is the fixture, and different MCP sizes may be accommodated in a single mega-boule wafer. Peculiar tools for each MCP size may thus be avoided. Although the stacking steps and dicing step may be different for different size requirements of MCPs, the tooling is the same for processing a mega-boule wafer, as a batch die of a predetermined cross-sectional area. This reduces capital costs.  
         [0061]      FIGS. 7-9  show different batch sizes for a 4-inch semiconductor mega-boule wafer.  FIG. 7  illustrates that ten standard 18 mm MCPs, generally designated as  72 , may fit within mega-boule wafer  70 . The interstitial area, designated as  74 , is the non-etchable glass left after the desired ten MCPs are removed from the 4-inch mega-boule wafer  70 .  
         [0062]      FIG. 8  illustrates that 14 standard 16 mm MCPs, generally designated as  82 , may fit within 4-inch mega-boule wafer  80 . The interstitial area, designated as  84 , is the non-etchable glass left after the desired  14  MCPs are removed from the 4-inch mega-boule wafer  80 .  
         [0063]      FIG. 9  illustrates the flexibility of densely packing rectangular MCPs within 4-inch mega-boule wafer  90 . As shown, a batch size of 28 MCPs, generally designated as  92 , may fit within the 4-inch mega-boule wafer. The non-etchable glass left after the recantangular MCPs are removed is designated as  94 . It should be understood, however, that the present invention is not limited to 4-inch mega-boule wafers. Other sizes may be used consistent with semiconductor fabrication tools.  
         [0064]     Although illustrated and described herein with reference to certain specific embodiments, the present invention is nevertheless 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 spirit of the invention.