METHOD FOR FABRICATING A PIXELIZED PHOSPHOR

Abstract: A process for making a pixelized phosphor structure involves the steps of forming an integral, non-layered mold by creating a plurality of openings in a substrate, and depositing a phosphor into the openings of the mold, thereby filling each of the openings of the mold with phosphor, and thereby forming a pixelized phosphor structure comprising an array of discrete phosphor elements. A second process for making a pixelized phosphor structure involves the steps of forming a first integral, non-layered mold by creating a plurality of first openings in a substrate, forming a series of one or more generations of replicas of the master mold, each of the one or more generations of replicas having a plurality of second openings, depositing a phosphor into the second openings of one of the one or more generations of replicas, thereby filling each of the second openings of the one of the one or more generations of replicas with phosphor, and thereby forming a pixelized phosphor structure comprising an array of discrete phosphor elements. A third process for making a pixelized phosphor structure involves the steps of providing a sheet of a first phosphor, creating a plurality of discrete holes in the first phosphor, each of the discrete holes extending through the first phosphor and having two openings, and depositing a second phosphor onto the first phosphor to form a pixelized phosphor structure, at least a portion of the second phosphor being deposited into the plurality of discrete holes in the first phosphor.

Description: METHOD FOR FABRICATING A PIXELIZED PHOSPHOR Field of the Invention This invention relates to a process for the fabrication of a phosphor and more particularly, it relates to a process for the fabrication of pixelized phosphors. Discussion of Related Art In the field of X-ray detection it is well-known to employ so-called intensifying screens to increase the radiation available for detection purposes. Such screens contain an X-ray luminescent material which is selected to emit a relatively large number of light photons for each X-ray photon striking the material. This effectively amplifies the X-rays to be detected since both the X-rays themselves and light emitted by X-ray-induced emission from the luminescent material are available for detection on film or other detection mediums or devices, such as arrays of light-sensitive electronic sensors. The primary incentive to use such intensifying screens in medical applications is to reduce the amount of X-ray radiation which is required to produce a given exposure, thereby reducing the radiation risk to which a patient or operator is exposed. It is known that such intensifying screens, while increasing the amount of radiation available for detection, also have the effect of reducing the sharpness of the resultant image. In general, image distortion in luminescent screens or structures is caused by the diffusion of light within the luminescent material which causes a blurring of the image with consequent loss of definition and contrast. This diffusion of light is brought about by two fundamental physical processes. First, as the ionizing (e.g., X- ray) radiation is converted into light, the direction of emission of light is random so that it is emitted approximately equally in all directions. The second effect is that the high energy radiation is penetrating, the degree of penetration being dependent upon the energy of the impinging radiation and the nature of the material being penetrated. The higher the energy, the deeper the penetration. A lower density material will also lead to a deeper penetration. Thus, it is seen that as visible light is generated along a path through the screen and normal to its surface, light will also be radiating in all other directions. Some of the light radiated at an angle off the normal to the surface of the screen will reach the film or other detecting means and result in a diffuse image. As a result, the design of such intensifying screens has involved a trade-off between screens of large thickness, which result in increased luminescent radiation for a given X-ray level, but which also produce decreased image sharpness, and screens of less thickness, which result in improved image sharpness relative to the thicker screens, but which also require more X-ray radiation to produce acceptable film images, thereby increasing the X-ray dosage to which the patient must be exposed. In practice, the thicker or high speed screens are utilized in those applications which do not require maximum image sharpness, thereby reducing the patient exposure to X-rays, while medium speed and slow speed screens are utilized when increased image resolution is required. These latter screens employ thinner phosphor layers and may incorporate dyes to minimize transverse propagation of light by attenuating such rays more than a normal ray which travels a shorter path. In general, detail or slow speed screens require approximately 8 times the X- ray dosage of high speed screens. Several patents have proposed solutions to the problem of reducing the amount of scattered luminescent radiation which reaches the film or other detector from such screens. These patents have suggested a cellularized or pixelized approach to the construction of such screens, the structure generally consisting of volumes of luminescent material separated by wall members. The wall members are disposed generally parallel to the direction of X-ray travel and their purpose is to reflect light emitted by the luminescent material and thereby prevent scattered light from reaching the detection means. One such approach is taught in U.S. Pat. No. 3,041,456, in which a rectangular body of plastic having a luminescent phosphor dispersed therein is sliced into thin slices which are then coated on one or both sides with a reflective material. These coated slices are then bonded back together and sliced again in a direction transverse to that of the first slicing. These coating and bonding operations are repeated to produce a double laminated body from which screens of the desired thickness may be obtained. The approach of this patent, while being theoretically attractive, presents significant problems in manufacturing because of the requirement to repeatedly handle and align extremely small pieces of the phosphor without damage or contamination. An alternative approach is suggested in U.S. Pat. No. 3,643,092. The structures proposed there employ adjacent walls having a corrugated member disposed therebetween so as to form a plurality of chambers extending in the direction of X-ray travel. At least a portion of each of these chambers is filled with a luminescent phosphor which reacts to X-ray radiation in the manner described above to produce light. The chamber structure is such that the walls thereof, formed by the planar wall members and the corrugated member, confine and/or reflect emitted light so as to limit the amount of scattered radiation reaching the detection means. The structures proposed in this patent, like that of U.S. Pat. No. 3,041,456, are attractive in theory, but present problems in fabrication because of the requirement to handle the small and fragile components. Other literature has suggested that chemical etching or milling be employed to produce grooves in a phosphor material, the grooves then being filled or plated with a highly reflective material to form light reflecting walls. However, this type of etching or milling produces surfaces which are relatively rough, so that even though subsequently plated or coated, they do not provide a good reflective surface. Such relatively rough surfaces have the effect of producing multiple reflections so that much of the light is lost through severe scattering. An additional disadvantage of such chemical milling or etching is that the walls produced must be at least 0.003-0.010 inches thick in order to provide sufficient strength for handling of the structure. Walls of this thickness are discernible and result in corresponding lines appearing in the image on the film, thereby reducing the resolution. Additionally, walls of this thickness reduce the amount of available phosphor by a corresponding amount, thus reducing the light output from the structure. Further, these structures have the disadvantage that the circumference of walls are continuous and rigid so that when the phosphor cures after being poured or impregnated into the cells, shrinkage or expansion may occur. This often results in fracturing of the phosphor with a resultant poor light transmission due to the separated interface at the fracture. U.S. Pat. No. 3,936,645 discloses a cellularized luminescent structure which is fabricated by utilizing a laser to cut narrow slots in the luminescent material in both the X and Y directions. The slots are then filled with material which is opaque to either light or radiation or both. U.S. Pat. No. 5,153,438 discloses a structured scintillator material wherein the gaps between the individual scintillator elements are preferably filled in with a reflective material such as titanium dioxide, magnesium oxide, etc., in order to maximize the portion of light within each element that is collected by its associated photosensitive cell. In this patent, the individual elements are formed by preferential deposition of the phosphor over structures existing on the surface of the substrate. U.S. Patent No. 5,302,423 discloses a process for fabricating a pixelized phosphor involving the steps of depositing a phosphor on a support; exposing the deposited phosphor to a source of electromagnetic radiation through a mask, thereby ablating the phosphor segmentally, resulting in a series of structures in both the X and Y directions to produce an array of pixelized phosphors separated by slots; and filling the resulting slots between the pixelized phosphors with phosphor material of the same or different composition as used in the first step such that each of the pixelized phosphors on the support are separated by a width of from about 0.5-25 microns. Although the foregoing disclosed processes are satisfactory for their intended use, continued improvements in processes for making pixelized phosphor structures are constantly sought after and are in demand within the field of direct digital radiography as well as areas of the imaging arts. It was against this background that such improved processes were sought. Summary of the Invention In accordance with the present invention, there are provided improved processes for the fabrication of pixelized phosphors. In a first embodiment, the present invention provides a process for making a pixelized phosphor structure, comprising the steps of (a) forming an integral, non-layered mold by contacting a single-layer substrate with radiation to create a plurality of openings in the substrate, and (b) depositing a first phosphor into the openings of the mold, thereby filling each of the openings of the mold with the first phosphor, and thereby forming a pixelized phosphor structure comprising an array of discrete phosphor elements. In a second embodiment, the present invention provides a process for making a pixelized phosphor structure, comprising the steps of (a) forming an integral, non-layered mold by applying a mechanical tool to a substrate to create a plurality of openings in the substrate, (b) depositing a first phosphor into the openings of the mold, thereby filling each of the openings of the mold with the first phosphor, (c) forming a continuous phosphor layer on top of the mold, thereby forming a pixelized phosphor structure comprising an array of discrete phosphor elements projecting from the continuous phosphor layer, and (d) disengaging the pixelized phosphor structure from the mold. The foregoing first and second embodiments of the present invention are quite efficient and advantageous as they involve no scribing or drilling of holes in the phosphor and are adaptable to an on-line continuous procedure, both of which contribute to improving the overall manufacturing speed of the process. Additionally, the inventive processes do not entail the difficulties encountered with ablation of phosphors with lasers. In a third embodiment, the present invention provides a process for making a pixelized phosphor structure, comprising the steps of (a) forming an integral, non-layered master mold by contacting a single-layer substrate with radiation to create a plurality of first openings in the substrate, (b) forming a series of one or more generations of replicas of the master mold, each of the one or more generations of replicas having a plurality of second openings, (c) depositing a first phosphor into the second openings of one of the one or more generations of replicas, thereby filling each of the second openings of the one of the one or more generations of replicas with the first phosphor, and thereby forming a pixelized phosphor structure comprising an array of discrete phosphor elements. In a fourth embodiment, the present invention provides a process for making a pixelized phosphor structure, comprising the steps of (a) forming an integral, non-layered mold by applying a mechanical tool to a substrate to create a plurality of first openings in the substrate, (b) forming a series of one or more generations of replicas of the master mold, each of the one or more generations of replicas having a plurality of second openings, (c) depositing a first phosphor into the second openings of one of the one or more generations of replicas, thereby filling each of the second openings of the one of the one or more generations of replicas with the first phosphor, and thereby forming a pixelized phosphor structure comprising an array of discrete phosphor elements. The processes of the third and fourth embodiments of the present invention facilitate a multi-step mold- forming operation. That is, the substrate worked by either radiation or a mechanical tool can be used to directly produce the pixelized phosphor or, alternatively, to form one or more generations of replicas which are then used to produce pixelized phosphors. While the one-step operation offers the advantages of speed and simplicity, the multi-step operation allows materials which are not amenable to deformation by electromagnetic or ionizing radiation (e.g., metal) to be used as the second or third mold from which the pixelized phosphor is produced. In addition, the use of replica generation allows the first, "master" mold, which requires exacting initial fabrication, to be copied with much less chance of damage. In either case, the present mold-forming methods are fast and efficient, and allow discrete phosphor structures having high aspect ratios (ranging from about 1:1 to 20:1) to be produced. High aspect ratios are desirable as they allow for increased resolution at a given dosage of radiation. In this manner, a high quality image can be produced while minimizing the patient's exposure to X-ray radiation. In a fifth embodiment, the present invention provides a process for making a pixelized phosphor structure, comprising the steps of (a) providing a sheet of a first phosphor, (b) creating a plurality of discrete holes in the first phosphor, each of the discrete holes extending through the first phosphor and having two openings, and (c) depositing a second phosphor onto the first phosphor to form a pixelized phosphor structure, at least a portion of the second phosphor being deposited into the plurality of discrete holes in the first phosphor. The foregoing fifth embodiment of the present invention is advantageous due to the ease with which the deposition of phosphor to fill the holes can be accomplished. The holes, which extend throughout the phosphor sheet, permit phosphor to be deposited by forcing a phosphor, or a phosphor mixture, through the holes without having any dead-ended cavities with air gaps which may not otherwise be completely filled by such a technique. Optionally, a layer of a light- reflecting or light-absorbing material may be deposited on the walls of the holes of the first phosphor. In this application: "aspect ratio," means a ratio of the height or depth of a feature relative to the width of that feature; "generations of replicas" means a series of replica molds wherein each successive replica mold forms a negative replica, or "mirror image," of the immediately preceding replica mold in the series; "pixelized phosphor" means a sheet of adjoining phosphor elements which are optically isolated from one another; "slot" means an empty space or gap, such as a groove, formed in a substrate, that separates one phosphor element from another; "cavity" means a hole formed in a substrate; "opening" means a slot or cavity formed in a substrate; "array" means a collection of elements arranged in a predetermined order; "sensor" means a electronic device for converting electromagnetic radiation into a corresponding electrical signal (e.g., a photodiode or photo- conductor) ; "continuous phosphor layer" means phosphor deposited onto a mold structure after filling the cavities of the mold, to provide a layer which bridges the structured regions; "discrete phosphor structures" means polygonal projections of phosphor extending from a substantially flat surface; and "planarizing" means removing excess phosphor or light-reflecting or light-absorbing material from one or both major surfaces of the pixelized phosphor. Other aspects, advantages, and benefits of the present invention are apparent from the detailed description, the examples, and the claims. Detailed Description of the Preferred Embodiments In accordance with the first and second embodiments, the present invention provides a process for making a pixelized phosphor structure, comprising the steps of (a) forming an integral, non-layered mold by forming a plurality of openings in a single-layer substrate, and (b) depositing a phosphor into the openings of the mold, thereby filling each of the openings of the mold with phosphor, and thereby forming a pixelized phosphor structure comprising an array of discrete phosphor elements. The resulting pixelized phosphor structure can be disengaged from the mold, or left in the mold. The inventive method may optionally include the following additional steps: (c) forming a continuous phosphor layer on top of the mold such that the array of discrete phosphor elements projects from the continuous phosphor layer; (d) depositing a layer of a light-reflecting or light-absorbing material on walls of the discrete phosphor structures; (e) depositing a phosphor of the same or different composition as the phosphor used in step (b) in spaces between the phosphor structures; and/or (f) planarizing at least one major surface of the phosphor sheet. The openings formed in the mold may comprise a plurality of cavities, the cross-section of which can correspond to a square, a circle, a rectangle, a hexagon or any other discrete polygonal structure. Alternatively, the openings may comprise a plurality of slots extending longitudinally along the substrate. One set of slots may be formed to intersect substantially perpendicularly with another set of slots to define discrete phosphor structures therebetween. Preferably, the aspect ratio of the discrete phosphor structures defined by the slots or cavities will be in the range of about 1:1 to 20:1 and, more preferably, in the range of about 2:1 to 8:1. If the discrete phosphor structure has a nonuniform width, the width can be taken to mean the average width. Preferably, the slots or cavities in the mold are separated by a width of from about 5 to 200 microns and, more preferably, from about 15 to 100 microns. One method for forming the mold, in accordance with the first embodiment, is a one-step process in which the mold is formed directly by contacting a substrate with electromagnetic or ionizing radiation of sufficient energy to create a plurality of slots or cavities in the substrate. The substrate may be any material which is capable of deformation (e.g., by ablation or photolithography) by electromagnetic or ionizing radiation and is suitable for receiving deposited phosphor. Examples of suitable materials include plastics (e.g., polycarbonate or acrylic), silicone, photoresist, thermoplastic elastomer, rubber, or polyimide. The substrate may be structured in accordance with the techniques for structuring a phosphor sheet as disclosed in U.S. Patent No. 5,302,423, the disclosure of which is hereby incorporated by reference herein. In applying those techniques to the formation of a mold, openings can be formed in the substrate by ablation with electromagnetic high energy radiation such as a laser (e.g., an excimer or C0 2 laser), or by high energy ionizing radiation such as x-rays, gamma rays, or electron beams. The electromagnetic or ionizing radiation can be pulsed and/or a mask can be used to achieve the desired pattern of openings in the substrate, thereby forming the mold. The use of electromagnetic or ionizing radiation to form the mold may, in some cases, result in an unacceptable degree of surface roughness on the walls of each of the slots or cavities. To reduce or eliminate surface roughness, a unique projection imaging laser ablation technique can be employed, in accordance with the present invention. A description of the projection laser ablation technique of the present invention also can be found in commonly assigned, copending United States patent application, entitled "METHOD AND APPARATUS FOR STEP AND REPEAT EXPOSURE," to Patrick Fleming, Andrew Ouderkirk, and Eric Borchers, bearing serial no. , and Attorney Docket No. 51613USA4A, filed on the same day as the present application. The entire content of the above-identified patent application is incorporated herein by reference. According to this technique, a mask defining a pattern that corresponds to the pattern of openings to be formed in the substrate is provided. A laser beam is passed through the mask to selectively ablate areas of the substrate, thereby removing substrate material to form the slots or cavities. A very high fluence generally is required to remove the substrate material. Therefore, the area of the pattern defined by the mask preferably is made quite small, on the order of one square centimeter or less. The substrate and small mask are sequentially moved relative to one another such that the mask is placed over different areas of the substrate. The different areas of the substrate are exposed via the mask with the laser beam to form openings, for example, in the form of slots or cavities. A larger mask, sufficient to cover the entire area to be imaged, alternatively can be provided and scanned with the laser beam. Because the manufacture of a large mask is very expensive, the use of a smaller mask is preferred. The projection imaging laser ablation technique of the present invention provides two options. According to the first option, the small mask is illuminated at each position on the substrate with only a partial laser beam exposure. Thus, the substrate receives only a partial laser beam exposure via the mask. The partial laser beam exposure is insufficient to completely ablate the substrate to a depth necessary to form the desired slots or cavities. Rather, the mask must be repeatedly subjected to the partial laser beam exposure to transmit a cumulative exposure sufficient to achieve the desired depth. The repeated laser beam exposures preferably are not applied successively at the same mask position. Instead, the substrate and mask are moved relative to one another such that the mask is placed over a plurality of mask positions at which the substrate is subjected to a partial laser beam exposure via the mask. Ordinarily, the substrate will be moved by a translation stage relative to the mask, which is fixed. The substrate and mask then are moved relative to one another to return the mask to each position a number of times for repeated partial laser beam exposures until the desired slot or cavity depth has been achieved. Thus, the plurality of exposures of the substrate to the laser beam radiation has a cumulative effect of ablating each opening to the desired depth. The first option reduces surface roughness caused by debris generated by the ablation of the substrate material. In particular, a soot-like debris tends to accumulate around the slots or cavities due to laser ablation. If the slots or cavities are closely spaced, some of the debris generated by ablation of one position of the substrate inevitably falls into areas of a subsequently ablated portion of the substrate. The debris alters the ablation characteristics of the substrate, relative to a clean substrate. The altered ablation characteristics of the substrate cause roughened surfaces on the walls of the slots or cavities. By applying only partial laser beam exposures in repeated steps according to this first option, however, the debris accumulation in each step will be reduced. The reduced accumulation of debris reduces or eliminates the alteration of the ablation characteristics of the substrate. As a result, repeated partial laser beam exposures can be used to achieve the desired depth without an undue degree of roughness. According to the second option, the substrate is given a partial laser beam exposure insufficient to completely ablate the substrate to a desired depth for formation of slots or cavities. Thus, the second option substantially corresponds to the first option. To expose the next position on the substrate, however, the substrate and mask are moved relative to one another by only a partial mask dimension from the last position exposed. Consequently, the next position subjected to laser beam exposure partially overlaps with the previous position. As in the first option, the repeated partial laser beam exposures applied according to the second option serve to reduce surface roughness caused by ablation-generated debris. In addition, the second option reduces surface roughness caused by laser beam nonuniformity. Specifically, no laser beam can be made to perfectly illuminate the mask. Moreover, as the laser beam is made more uniform, the cost of the laser optics increases and the efficiency of the laser decreases. The nonuniformity of the laser beam causes the illumination pattern transmitted to the substrate via the mask to be similarly nonuniform. The relative movement of the mask and substrate by only a partial mask dimension for successive partial laser beam exposures causes slots or cavities formed in the substrate to be illuminated via different areas of the mask pattern. As a result, the nonuniformity produced across the mask tends to "average out," such that the overall effect of repeated partial laser beam exposures and partial mask dimension movements is relatively uniform. The slots or cavities may also be formed in the substrate by using conventional photolithographic techniques. In this case, the substrate would be a photoresist material, preferably a negative dry photoresist material, and would be imaged with ultraviolet light through an appropriate mask (e.g., a chromium mask) and then rinsed with a developer. Sheets of dry photoresist are commercially available in thickness of 50-150 microns from, e.g., Hercules, Inc. and Dyna Chem, Inc. Another method for formation of the mold, in accordance with the second embodiment, involves formation of the openings in the substrate by the use of mechanical tools, such as drilling tools or diamond- turning tools. A diamond-turning tool will form slots in the substrate. Therefore, if cavities are desired in the pixelized phosphor structure, a mirror-image replica of the original diamond-turned substrate ordinarily must be formed. Examples of suitable materials for manufacture of the mold when a mechanical tool is used include metals, such as copper, electroless nickel, brass, or aluminum, plastic (e.g., polycarbonate or acrylic) , silicone, thermoplastic elastomer, rubber, or polyimide. One preferred pattern formed on the substrate by either radiation or mechanical means is a "checkerboard" pattern having alternating, substantially square cavities extending in two perpendicular and intersecting directions. Again, the formation of square cavities using diamond-turning will require the formation of slots in a first substrate, followed by replication of the resulting master mold to form a mold having the square cavities. In this manner, approximately half of the total volume of the mold will be comprised of cavities while the remaining half will be comprised of the non-ablated portions of the mold substrate material. Another preferred pattern formed on the substrate by either radiation or mechanical means is a "staggered checkerboard" pattern having alternating, substantially square cavities extending in two perpendicular and intersecting directions, wherein the cavities are separated from one another by a portion of the substrate. In this manner, less than half of the total volume of the mold will comprise cavities. In the event that the walls of the slots or cavities exhibit an undesirable surface roughness, the smoothness of the walls of the slots or cavities can be improved by coating the walls with a layer of lacquer, followed by a thin layer of silver using electroless deposition. In the case where slots are formed in the sheet, the slots are generally formed in two perpendicular directions. The lacquer can be applied to the walls of the structure by, for example, dip coating in a solution of lacquer and lacquer thinner. The lacquer-coated structure then can be dried to remove the solvent. The thickness of the lacquer coating can be readily controlled by varying the ratio of lacquer to lacquer thinner. The lacquer-coated walls then can be coated with a thin layer of silver using, for example, electroless plating. The resulting lacquer/silver coated walls are smoother than uncoated walls. Another method of forming the mold involves a multi-step process, in accordance with the third and fourth embodiments, wherein a first, "master" mold is made by either contacting a subsrate of a first material with electromagnetic or ionizing radiation of sufficient energy to create a plurality of first openings in the sheet, or applying a mechanical tool to the substrate to form the openings. Thereafter, a series of one or more generations of replicas of the master mold are made. Generations of replicas means that each replica is formed as a mirror image of the preceding replica. For example, a first generation, replica mold can be formed by depositing a second material onto the master mold. The first generation replica mold forms a negative replica, or "mirror image" of the first, master mold, having a plurality of second openings. The first generation replica mold can be disengaged from the master mold. The second openings of the first generation, negative replica mold, which may comprise slots or cavities depending on the openings formed in the master mold, can be used to form the pixelized phosphor (i.e., phosphor material can be deposited onto the second openings of the first generation replica mold to form the pixelized phosphor) . A second generation, positive replica mold of the master mold alternatively can be formed from the first generation replica mold, if desired, by depositing a third material into the negative replica mold. The second generation replica mold is a mirror image of the first generation replica mold, and therefore a positive replica of the master mold. The resulting second generation positive replica mold will have another plurality of second openings corresponding to the first openings in the master mold. After disengaging the third material from the first generation negative replica mold, phosphor can be deposited in the second openings in the resulting second generation positive replica mold to form the pixelized phosphor structure. The second and third materials used to form the first generation negative replica mold and the second generation replica mold, respectively, can be the same. Formation of the master mold for the multi-step process can be performed as described above, i.e., the materials from which the mold is made and the techniques used to structure it are identical to those described in conjunction with the above one-step mold- making method. To form the first generation negative replica mold, the second material is deposited onto the master mold by any suitable method such as electroplating or knife coating, or by any of the methods described below for depositing phosphor onto a mold. The third material similarly can be deposited in the first generation negative replica mold by the same methods to form the second generation positive replica mold. The second and third materials from which the first generation, negative replica mold and the second generation, positive replica mold, respectively are made can include such materials as metal (e.g., plated nickel), plastic (e.g., polycarbonate or acrylic), silicone, a photoresist, thermoplastic elastomer, rubber, or polyimide. The first generation replica mold can be left in the master mold or disengaged, i.e., separated, from the master mold. Separation of the first generation replica mold from the master mold can sometimes be difficult. In this case, the master mold can be dissolved away from the first generation replica mold by using an appropriate solvent. For example, when the second material is nickel, methylene chloride can be used to dissolve away an acrylic master mold, or a 50% KOH solution can be used to dissolve away a polyimide master mold. Alternatively, a release coating, e.g., petroleum jelly, wax, or silicone, can be applied to the master mold prior to formation of the first generation replica mold to facilitate separation of the master mold from the first generation replica mold. Similarly, a release coating can be applied to the first generation, negative replica mold prior to formation of the second generation, positive replica mold to facilitate separation of the second generation replica mold from the first generation replica mold. If successive generations are both formed from a metal, a passivation layer should be applied to the preceding mold to facilitate release. During deposition of the phosphor material onto the mold, the openings of the mold should be filled with the phosphor material. Additionally, if the phosphor structure is to be removed from the mold, a continuous phosphor layer should be deposited on top of the mold which interconnects the phosphor in the openings. The resultant structure is an array of discrete phosphor structures projecting from the continuous phosphor layer. Any conventional phosphor may be utilized in the present invention. Non-limiting examples of such phosphors include: phosphors represented by BaS0 4 :A x (where A is at least one element selected from Dy, Tb, and Tm, and x satisfies 0.001 < x < 1 mol %) as disclosed in Japanese Patent Publication No. 80487/1973; phosphors represented by MgS0 4 :A x (where A is either Ho or Dy, and x satisfies 0.001 < x < 1 mol %) as disclosed in Japanese Patent Publication No. 80488/1973; phosphors represented by SrS0 4 :A x (where A is at least one element selected from Dy, Tb and Tm, and x satisfies 0.001 < x < 1 mol %) ; as disclosed in Japanese Patent Publication No. 80489/1973; phosphors composed of Na 2 S0 4 , CaS0 4 or BaS0 4 containing at least one element selected from Mn, Dy and Tb as disclosed in Japanese Patent Publication No. 29889/1976; phosphors composed of BeO, LiF, MgS0 4 or CaF 2 as disclosed in Japanese Patent Publication No. 30487/1977; phosphors composed of Li 2 B 4 0 :Cu or Ag as disclosed in Japanese Patent Application No. 39277/1978; phosphors represented by either Li 2 0* (B 2 0 2 ) x :Cu (where x satisfies 2 < x < 3), or Li 2 0* (B 2 0 2 ) x :Cu, Ag (where x satisfies 2 < x < 3) , disclosed in Japanese Patent Publication No. 47883/1979; phosphors represented by SrS:Ce, Sm; SrS:Eu, Sm; La 2 0 2 S:Eu, Sm; and (Zn,Cd)S:Mn, X (where X is halogen) as disclosed in U.S. Pat. No. 3,859,527; phosphors represented by ZnS:Cu or Pb; barium aluminate phosphors represented by BaO- (Al 2 0 3 ) x :Eu (where x satisfies 0.8 < x < 10) and alkali earth metallosilicate phosphors represented by M II O x Si0 2 :A (where M n is Mg, Ca, Sr, Zn, Cd or Ba; A is at least one element selected from Ce, Tb, Eu, Tm, Pb, Tl, Bi and Mn; and x satisfies 0.5 < x < 2.5) as disclosed in Japanese Patent Publication No.12142/1980; alkali earth fluorohalide phosphors represented by (Baι- x - y Mg x Ca y ) FX:eEu 2+ (where X is at least one of Br and Cl; and x, y and e satisfy 0 < x + y < 0.6, xy ≠ 0, and 10 "6 < e < 5 x 10 "2 , respectively) ; phosphors represented by LnOX:xA (where Ln is at least one element selected from La, Y, Gd, and Lu; X is Cl and/or Br; A is Ce and/or Tb; and x satisfies 0 < x < 0.1) as disclosed in Japanese Patent Publication No. 12144/1980; phosphors represented by (Baι- x M n x ) FX:yA (where M is at least one element selected from Mg, Ca, Sr, Zn, and Cd; X is at least one element selected from Cl, Br and I; A is at least one element from Eu, Tb, Ce, Tm, Dy, Pr, Ho, Nd, Yb, and Er; x and y satisfy O < x < 0.6 and 0 < y < 0.2, respectively) as disclosed in Japanese Patent Publication No. 12145/1980; phosphors represented by BFX:xCe, yA (where X is at least one element selected from Cl, Br, and I; A is at least one element selected from In, Tl, Gd, Sm, and Zr; and x and y satisfy 0 < x < 2 x 10 "1 and 0 < y < 5 x 10 "2 , respectively) as disclosed in Japanese Patent Publication No. 84389/1980; rare-earth element- activated divalent metal fluorohalide phosphors represented by M n FX-xA:yLn (where M 11 is at least one element selected from Mg, Ca, Ba, Sr, Zn, and Cd; A is at least one oxide selected from BeO, MgO, CaO, SrO, BaO, Zno, Al 2 0 3 , Y 2 0 3 , La 2 0 3 , ln 2 0 3 , Si0 2 , Ti0 2 , Zr0 2 , Ge0 2 , Sn0 2 , Nb 2 0 5 , Ta 2 0 5 , and Th0 2 ; Ln is at least one element selected from Eu, Tb, Ce, Tm, Dy, Pr, Ho, Nd, Yb, Ev, Sm, and Gd; X is at least one element selected from Cl, Br and I; and x and y satisfy 5 x 10 "5 < x < 0.5 and 0 < y < 0.2, respectively) as disclosed in Japanese Patent Publication No. 160078/1980; phosphors represented by either xM 3 (P0 4 ) 2 *NX 2 :yA or M 3 (P0 4 ) 2 :yA. (where each of M and N is at least one element selected from Mg, Ca, Sr, Ba, Zn, and Cd; X is at least one element selected from F, Cl, Br, and I; A is at least one element selected from Eu, Tb, Ce, Tm, Dy, Pr, Ho, Nd, Yb, Er, Sb, Tl, Mn, and Sn; and x and y satisfy 0 < x < 6 and 0 < y < 1, respectively) ; phosphors represented by either nRX 3 • mAX' 2 :xEu or nReX 3 , mAX , 2 :xEu, ySm (where R is at least one element selected from La, Gd, Y, and Lu; A is at least one element selected from Ba, Sr, and Ca; each of X and X 1 is at least one element selected from F, Cl, and Br; x and y satisfy 1 x 10 " " < x < 3 x 10 "1 and 1 x 10 "4 < y < 1 x 10 "1 , respectively; and n/m satisfies 1 x 10 "3 < n/m < 7 x 10 "1 ) ; alkaline halide phosphors represented by M I X-aM II X-bM III X:cA (where M 1 is at least one alkali metal selected from Li, Na, K, Rb, and Cs; M 11 is at least one divalent metal selected from Be, Mg, Ca, Sr, Ba, Zn, Cd, Cu, and Ni; M 111 is at least one trivalent metal selected from Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Al, Ga, and In; each of X, X' and X" is at least one halogen selected from F, Cl, Br, and I; A is at least one element selected from Eu, Tb, Ce, Tm, Dy, Pr, Ho, Nd, Yb, Er, Gd, Lu, Sm, Y, Tl, Na, Ag, Cu, and Mg; and the values a, b and c satisfy 0 < a < 0.5, 0 < b < 0.5 and 0 < c < 0.2 respectively) as disclosed in Japanese Patent Publication No. 148285/1982; phosphors represented by cerium-doped Lutetium Oxyorthosilicate Lu 2 (i- X )Ce 2 χ (Si0 4 ) 0 as mentioned in the IEEE Transactions of Nuclear Science, vol. 34, no. 4, 1992, pp. 502-505; phosphors represented by neodymium doped in Yttrium Orthosilicate (Nd 3+ :Y 2 Si0 5 ) as mentioned in IEEE Journal of Quantum Electronics, vol. 26, no. 8, August 1990, pp.1405-1411 and in European Patent Application No. 0,253,589; phosphors represented by Gd 2 0 2 S:R where R is at least one element selected from Tb, Eu, Pr, and Tm; and phosphors represented by thermoluminescent materials such as CsI:Na, LiF, and the like. The presently preferred phosphors are ones composed of alkali metal halides or Gd 2 0 2 S:R. In some instances, the phosphor can contain a binder such as, for example, an epoxy, an acrylate, a phenolic, and the like. A preferred binder is ACMAS UV radiation curable binder, as described in United States Patent No. 5,411,806, the entire content of which is incorporated herein by reference. The phosphor can be deposited onto the mold using a number of different methods. A first method is vacuum evaporation. In this process, a vacuum evaporating apparatus into which a mold has been placed is evacuated to a level of 10 "6 Torr or so. Then, at least one aforementioned phosphor is vaporized by resistive heating, electron beam heating, or the like to produce a layer of the phosphor with a desired thickness formed on the surface of the mold. The layer containing a phosphor can also be formed by repeating the vaporizing procedures a number of times. In addition, a covacuum evaporation can be conducted using a plurality of resistive heaters or electron beams. It is also possible to heat or cool the deposited layer during vaporization, if necessary, or to heat-treat the deposited layer after vaporizing. After the vacuum evaporating operation, the phosphor-containing layer is optionally provided with a protective layer on its side opposite to the mold. A second method is a sputtering technique. In this process, a sputtering apparatus in which a mold has been placed is evacuated to about 10 "6 Torr. Then, an inert gas such as Ar or Ne is introduced into the apparatus to raise the inner pressure up to a level of about 10 ~3 Torr. Afterwards, at least one aforementioned phosphor is sputtered to produce a layer of the phosphor with a desired thickness deposited in and on the surface of the mold. The phosphor layer can also be formed by repeating a plurality of sputtering procedures. After the sputtering operation, the phosphor layer can be provided with a protective layer on its side opposite the mold if necessary. A third method is chemical vapor deposition (CVD) . In this method, the phosphor layer is obtained on a mold by decomposing the intended phosphor or organometallic compound containing the raw material of the phosphor using thermal energy, high-frequency power, and the like. A fourth method is a spraying technique. In this method, the phosphor layer is obtained by spraying phosphor powder onto a tacky layer of a mold. Alternatively, the phosphor powder can be co-deposited onto the mold along with a tacky material. A fifth method is a baking method. In this method, an organic binder-containing phosphor powder dispersed therein is coated on a mold which is then baked and thus, the organic binder is volatilized, and a phosphor layer without binder is obtained. A sixth method is a curing method. In this method, an organic polymerizable binder containing phosphor powder dispersed therein is coated on a mold which is then subjected to conditions which initiate and complete polymerization of the binder, thereby forming a solid composite mass of polymerized binder and phosphor. A seventh method is a spray pyrolysis method. In this method, the phosphor is formed by spraying a solution of base elements suspended in a suitable volatilizable carrier onto a heated mold which causes the vaporization of the carrier during deposition of the phosphor. An eight method is a calendering method. In this method, the phosphor is mixed with a binder and deposited into the openings of the substrate with a knife coater, for example, and then passed through a pair of calendering rolls defining a contact surface therebetween. Successive passes through the calendering rolls serve to compress the phosphor into the openings, substantially eliminating voids between the phosphor and the walls of the mold. After deposition of the phosphor, the phosphor can be left in the mold or the mold can be disengaged from the phosphor. To assist in disengagement, the top as well as openings of the mold will have preferably been coated with a release material, such as a silicone, petroleum jelly, or wax, prior to deposition of the phosphor onto the mold. Upon disengagement from the mold, the continuous phosphor layer on top of the mold becomes the base of the resultant pixelized phosphor with an array of discrete phosphor structures projecting from the continuous phosphor layer. The discrete phosphor structures have a height which may range from about 50- 500 microns, and a width which may range from about 25- 170 microns. More preferably, the height ranges from 50-200 microns and the width ranges from 50-85 microns. Optionally, a thin light reflecting layer, such as aluminum or silver, can then be formed on the walls of the discrete phosphor structures. A sputtering, evaporation, electroless plating, electroplating, or other thin film deposition technique can be utilized. Also, optionally, a light-absorbing material can be deposited on the walls of the discrete phosphor structures. Preferably, the layer of light-reflecting or light-absorbing material ranges in thickness from about 1000 to about 10,000 Angstroms. Alternatively, the phosphor can be left in the mold, provided the mold is made from a material that is substantially transparent to X-ray radiation. A phosphor material of the same or different composition as used to form the discrete phosphor structures can optionally be deposited into the slots or cavities between the projecting phosphor structures. Using a phosphor material of a different composition than that used previously may enhance the containment of light within a single pixel since the differences in the index of refraction will cause light traversing within a pixel to be reflected back into the pixel when the index of refraction within the pixel is greater than that exterior to the pixel. The resulting pixelized phosphor structure and thin metal film, if utilized, can then be planarized on one or both major surfaces by any suitable method such as mechanical abrasion, ion milling, chemical etching, plasma etching, and mechano-chemical lapping. If desired, the resulting planarized pixelized phosphor sheets can then be disposed on a support to provide structural strength and abrasion resistance. The support for the phosphor can be various polymeric materials such as polyimides and polyesters, glass, tempered glass, quartz, metals, and the like. Especially preferable materials are, for example, plastic film such as cellulose acetate, polyester, polyethylene terephthalate, polyamide, polyimide, cellulose triacetate or polycarbonate film, or metallic sheets such as aluminum, steel, or copper. The pixelized phosphor sheet can be disposed on a substrate consisting of a sensor array or on a multitude of sensor arrays which can be described as being a "sub-module". A collection of sub-modules can be assembled by butting them together in an "edge-to- edge" manner to form a complete, large-size radiographic imaging panel. The process of forming the phosphor can also be made on the large-size radiographic imaging panel. The sensor array can be made of amorphous silicon, single crystal silicon, cadmium telluride, copper indium diselenide, and other sensor materials known to one skilled in the art. In the case of single crystal silicon, the sensor array can be a conventional sensor array on a silicon wafer from about 300 to about 700 microns in thickness. Additionally, the sensor array can be on a thinned silicon wafer, preferably from about 10-50 microns in thickness and more preferably, from about 10-20 microns in thickness. A sensor array on a sufficiently thinned silicon wafer has the advantage of being transparent to light so that the phosphor can illuminate the sensor array through the silicon, from the side opposite to the light-detecting sensor. The use of thinned out wafers, butted edge-to- edge, accomplishes a high fill factor to effectively collect the light from the phosphor. This manner of illumination is termed "back-illumination". Alternatively, the pixelized phosphor sheet can also be disposed on a fiber optic element. The fiber optic element can be composed of a large bundle of individual optical fibers which are joined parallel to each other so that an image projected into one end of the bundle will be transmitted uniformly to the other end of the bundle maintaining a one-to-one correspondence of the relative positions of different portions of the image. The light transmitting surface of this bundle of fiber optics can be sufficiently smoothed by polishing so as to permit the formation of a patterned surface which can then be coated with uniform deposition of a phosphor. Also alternatively, the resulting phosphor sheet can be disposed on a conventional silver halide-based photographic film such as those used in conventional radiography. In a fifth embodiment, the present invention provides a process for making a pixelized phosphor structure, comprising the steps of (a) providing a sheet of a first phosphor, (b) creating a plurality of discrete holes in the first phosphor, each of the discrete holes extending through the first phosphor and having two openings, and (c) depositing a second phosphor onto the first phosphor to form a pixelized phosphor structure, at least a portion of the second phosphor being deposited into the plurality of discrete holes in the first phosphor. A layer of light- reflecting or light-absorbing material can be formed on the walls of the holes. The sheet of phosphor can be made by any of the techniques well known to those skilled in the art of making phosphor screens for use in traditional radiography. As is conventional, the phosphor is made by coating a support with a composition comprising phosphor particles, a polymeric binder and a solvent. The coating is dried to remove the solvent, leaving an adherent layer of phosphor particles dispersed in the binder. The phosphor sheet can also be made by coating a composition comprising particles of a phosphor dispersed in a cross-linked polymeric matrix formed by heat-curing or radiation-curing. The coating may further include an unsaturated cross-linkable polymer, a polymerizable acrylic monomer, a thermoplastic polyurethane elastomer, and an initiator. The coating techniques used include knife coating, roll coating, gravure coating, extrusion coating, curtain coating and the like. A plurality of holes with two openings can be created in the phosphor sheet by any suitable method such as by ablating through the phosphor layer with a laser (e.g., an excimer laser or C0 2 laser), or by applying a mechanical tools, such as a drill, to the phosphor sheet. The other steps of the inventive process can be practiced as disclosed earlier herein. In addition, at least one major surface of the resultant pixelized phosphor can be planarized, also as disclosed earlier herein. The following non-limiting examples further illustrate the present invention. EXAMPLE 1 An acrylic sheet was opened with 100 micron diameter holes to a depth of 200 microns using a C0 2 laser. The operating conditions of the Coherent model 42 C0 2 laser were: 46 W power (CW) , pulse length of 0.1 ms, pulse spacing of 0.003 inches, feed rate of 100 inches/minute, cavity pressure of 23 mbar using a gas mixture of 14% C0 2 with the balance N 2 . The holes were then coated with 1000 angstroms of Ag using electroless plating, followed by Ni electroplating with an upper continuous layer as a mechanical support. The acrylic was dissolved away using methylene chloride to give a nickel mold having surface features (posts) of a height of 200 microns, with a base diameter of 100 microns and a top diameter of 30 microns. The areas between the posts of the resulting nickel mold were filled with a phosphor to form a structured phosphor sheet. EXAMPLE 2 A 125 mm thick sheet of phosphor (Gd 2 0 2 S:Tb) was coated on a sheet of mylar. A C0 2 laser was used to open 100 micron diameter holes which extended throughout the entire sheet of phosphor. The conditions used for the laser were the same as in Example 1, except that the pulse spacing was changed to 0.006 inches. EXAMPLE 3 Holes were opened on a silicone sheet, using the same laser and operating conditions as in Example 1. The holes had a diameter of 100 microns with a 250 micron center-to-center distance, and a depth of 150 microns. A solution of 1.5% petroleum jelly in dichloromethane was mixed in a bath of hot water and coated onto the surface of the holes in the silicone sheet as a release layer. Thereafter, a silicone rubber (Silastic™ J from Dow Corning) was poured into the holes. The sheet was placed in a vacuum chamber and a vacuum was applied for 3-4 minutes to ensure that no air pockets remained trapped in the holes. The silicone rubber was then cured for 24 hours at room temperature and for 1 hour at 65°C. After curing, the silicone rubber was disengaged from the silicone sheet and used as a mold for the formation of a pixelized phosphor structure. EXAMPLE 4 Phosphor comprising Gd 2 0 2 S:Tb of a thickness of 75 microns was coated on a polyester film which was coated with 2000 angstroms of aluminum. Holes 100 microns in diameter with a center-to-center spacing of 250 microns were opened in the phosphor with a C0 2 laser. The holes extended all the way down to the aluminum coating. The C0 2 laser operating conditions were: 48 W power (CW) , feed rate of 100 inches/minute, pulse spacing of 0.006 inches, pulse length 0.1 ms, cavity pressure 16 mbar, 14% C0 2 gas. The holes were then coated with 3000 angstroms of silver using a thermal evaporation technique. The upper layer of silver on the phosphor was then removed by lapping. A second layer of Gd 2 0 2 S:Tb phosphor was then knife coated, in vacuum, onto the structured phosphor sheet. EXAMPLE 5 An acrylic sheet was opened with 100 micron diameter holes to a depth of 200 microns using the conditions in EXAMPLE 1. The holes were filled with Gd 2 0 2 S:Tb phosphor by knife coating in a vacuum environment, leaving a continuous layer of phosphor on the upper surface. The acrylic was then etched away, using methylene chloride, and a structured phosphor consisting of phosphor posts was obtained. EXAMPLE 6 A mold for producing pixelized phosphor was created using a projection laser ablation technique with partial mask dimension movement, creating a mold having consistent sizes and shapes of features. An imaging mask with 2116 openings was obtained from Microphase Labs of Colorado Springs, Colorado. The openings were squares measuring 100 microns on an edge, with a 130 micron center-to-center spacing, arranged in a square matrix, 46 openings on an edge. The mask was mounted at the object plane of a laser ablation imaging system with 2x reduction optics, resulting in a projected image of 50 micron square openings on 65 micron centers at the image plane. The ablation system was powered by an excimer laser operating at a wavelength of 248 microns. Kapton™ E polyimide film, commercially available from DuPont, 75 microns thick and laminated to a 25 micron thick copper foil to form a substrate, was mounted on an x-y translation stage located at the image plane of the laser system. The laser power settings resulted in a power output of 800 mJ/cm 2 at the substrate, and it was determined that approximately 352 pulses were required to ablate the polyimide film down to the adhered copper layer. A translation of two center-to-center distances per pulse in a first direction traversing the substrate was chosen, followed by a change of three center-to- center distances in another direction, perpendicular to the first direction, before repeating the substrate traversal. The imaging mask was positioned so that the rows and columns of features were coincident with the x and y axes of the translation stage, and the substrate was positioned such that the projected image of the mask was in the upper left corner of the substrate. After each pulse of the laser, the translation stage was shifted to the left a distance of two features (130 microns), and the process was continued until the far edge of the substrate was imaged, completing the first pass of the ablation. The stage was then translated back to the original position, and translated up a The mold was then coated with 1000 Angstroms of silver, using electroless deposition. A coating comprising a mixture of T16 phosphor (Gd 2 0 2 S:Tb), was mixed with ACMAS UV radiation curable binder in the ratio of 11:1, as described in United States Patent No. 5,411,806. This mixture was then applied to the mold and cured using the technique described in United States Patent No. 5,411,806. EXAMPLE 8 The nickel replica of EXAMPLE 6 was replicated in polycarbonate as described in EXAMPLE 7, resulting in a rectangular array of square holes, 50 microns on an edge with a center-to-center spacing of 65 microns and a depth of 75 microns. EXAMPLE 9 An acrylic film was embossed using a nickel mold having a series of adjacent parallel 90 degree vee- grooves, measuring 300 microns in width and 175 microns in depth. The acrylic film was coated with a solution consisting of 0.6 grams of petroleum jelly in 56 grams of 1, 2-dichloroethane, and baked at 100°C for twenty minutes to remove the solvent. A mixture of T6 phosphor (Gd 2 0 2 S:Tb) was mixed with ACMAS binder, coated into the mold, and cured as described in EXAMPLE 7. The thickness of the phosphor layer excluding the vee- groove pattern was approximately 175 microns. The cured phosphor replica was removed from the mold surface. -37- EXAMPLE 10 A substrate having a thickness of approximately 500 microns was provided. The substrate was made of Fotoform™ material, commercially available from Corning Glass Corporation, of Corning, New York. As obtained from Corning, the substrate had 85 micron by 85 micron square cavities with center-to-center spacing of 115 microns. The substrate was dip coated in a solution of 10% acrylic lacquer and 90% lacquer thinner and allowed to dry overnight. The coating had a thickness of 2 microns. The lacquer-coated walls then were coated with a 1,000 Angstrom silver layer, using electroless plating. The resulting substrate was filled with a mixture of phosphor as described in EXAMPLE 9 . The phosphor-filled substrate was exposed to X-ray radiation and used to image a sheet of film. The film imaged with the lacquer/silver coated phosphor was observed to produce improvements in light output and image sharpness, relative to film imaged with a phosphor without the lacquer/silver coating. Having described the exemplary embodiments of the invention, additional advantages and modifications will readily occur to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. Therefore, the specification and examples should be considered exemplary only, with the true scope and spirit of the invention being indicated by the following claims. -38- distance of three features (195 microns) . The process of pulsing and translating the stage two features to the left was then repeated across the substrate, to complete the second pass of the ablation. The process of translating up three features and then pulsing and translating across the substrate was continued until the entire surface of the substrate was traversed. In this manner, a central rectangular region of the substrate was imaged with square holes, 75 microns in depth. This central region was bounded on all edges by a region 46 holes wide, in which the depth varied from 75 microns near the central region, down to approximately zero at the edge of the imaged region. The holes in the central region were characterized, however, by having consistent sizes, depths, and shapes. A nickel replica of this structure was formed by plating up from the copper located at the bottom of the holes. EXAMPLE 7 A flexible sheet of acrylic (PMMA) , about 0.080 inches thick was patterned using a diamond-turning tool to form a master mold. The acrylic was wrapped securely around a drum, and a series of parallel grooves were cut into it. The sheet was then removed from the drum, rotated 90 degrees, and re-mounted on the drum. A second series of parallel grooves were then cut, resulting in an acrylic master mold having a series of posts protruding from the substrate. The acrylic master was then replicated in nickel, using the electroforming process described in United States Patent No. 5,317,805, the entire content of which is incorporated herein by reference, with an electroplating current of 20 A/ft 2 that resulted in a -35- deposition rate of 0.001 inch/hour. The resulting first generation replica sheet comprised a series of holes within the nickel surface, formed by the posts in the acrylic master mold. The replication process was again performed on the first generation replica sheet, again in nickel, to result in a second generation replica mold. Prior to plating the first generation mold with nickel to form the second generation mold, a passivation of two percent aqueous solution of potassium dichromate was applied to the first generation mold for thirty seconds at room temperature. The passivation was rinsed with distilled water, and replication was carried out. The resulting second generation replica mold was identical to the initial acrylic master mold, except that the material was nickel. A final replication of the second generation replica mold was accomplished by hot-pressing a sheet of polycarbonate with the second generation replica mold at a temperature of 170° C. The applied pressure was stepped up as follows: 7 tons for 1 minute, 10 tons for 1 minute, and 12.5 tons for 1 minute. The polycarbonate sheet was removed from the second generation nickel mold after cooling to room temperature. The resulting polycarbonate mold consisted of a square matrix of holes having a square cross-section which varied as a function of depth. The holes were 100 microns in depth, 50 microns on an edge at the surface of the polycarbonate and 33 microns on an edge at the bottom of the hole. The holes were separated by walls having a trapezoidal cross-section, and measuring 28 microns at the surface of the polycarbonate and 42 microns at the bottom of the holes. -36-

Claims: WHAT IS CLAIMED IS: 1. A process for making a pixelized phosphor structure, comprising the steps of: (a) forming an integral, non-layered mold by contacting a single-layer substrate with radiation to create a plurality of openings in said substrate; and (b) depositing a first phosphor into said openings of said mold, thereby filling each of said openings of said mold with said first phosphor, and thereby forming a pixelized phosphor structure comprising an array of discrete phosphor elements. 2. The process of claim 1, further comprising the step of forming a continuous phosphor layer on top of said mold such that said array of discrete phosphor elements projects from said continuous phosphor layer. 3. The process of claim 2, further comprising the step of disengaging said pixelized phosphor structure from said mold. 4. The process of claim 3, further comprising the step of filling areas between said discrete phosphor elements of the disengaged pixelized phosphor structure with a second phosphor. 5. The process of claim 4, wherein said second phosphor has a different composition than a composition of said first phosphor. 6. The process of claim 3, wherein said pixelized phosphor structure includes first and second major opposing surfaces, said process further comprising the step of planarizing at least one of the first and second major opposing surfaces of the phosphor structure, thereby forming at least one planarized surface on said pixelized phosphor structure. 7. The process of claim 6, further comprising the step of attaching said at least one planarized surface of said pixelized phosphor structure to a second substrate. 8. The process of claim 1, wherein an aspect ratio of each of said discrete phosphor elements is in a range of from approximately 1:1 to approximately 20:1, said aspect ratio representing a ratio of a height of each of said discrete phosphor elements to a width of each of said discrete phosphor elements. 9. The process of claim 8, wherein the aspect ratio of each of said discrete phosphor elements is in a range of 2:1 to 8:1. 10. The process of claim 1, wherein each of said discrete phosphor elements includes a wall, said process further comprising the steps of: forming a layer of lacquer on said wall of each of said discrete phosphor elements; and forming a layer of silver over said layer of lacquer. 11. The process of claim 1, wherein said step of forming said mold further includes the steps of: placing a mask over one of a plurality of portions of said substrate, wherein said mask defines a pattern of said openings; exposing said one of said portions of said substrate to said radiation via said mask, said radiation ablating areas of said one of said portions of said substrate to create said openings in said substrate, wherein each exposure of said one of said portions of said substrate to said radiation has an intensity insufficient to ablate each of said areas of said one of said portions of said substrate to a desired depth; and moving said substrate and said mask relative to one another such that said mask is placed over another of said portions of said substrate; exposing said another of said portions of said substrate to said radiation via said mask; and moving said substrate and said mask relative to one another such that said mask is placed over each of said portions of said substrate, and exposing each of said portions of said substrate to said radiation via said mask a plurality of times such that the plurality of exposures of said portions of said substrate to said radiation has a cumulative effect of ablating each of said areas to said desired depth. 12. The process of claim 11, wherein said step of moving said substrate and said mask relative to one another includes moving said substrate and said mask relative to one another by a distance less than a dimension of said mask. 13. The process of claim 1, wherein said step of depositing said first phosphor into said openings of said mold includes passing said mold between a pair of calendering rolls, said calendering rolls defining a contact area compressing said first phosphor into said openings of said mold, thereby reducing formation of voids within said openings. 14. The process according to claim 1, wherein a cross-section of each of said openings corresponds to one of a square, a circle, a rectangle, and a hexagon. 15. A process for making a pixelized phosphor structure, comprising the steps of: (a) forming an integral, non-layered master mold by contacting a single-layer substrate with radiation to create a plurality of first openings in said substrate; (b) forming a series of one or more generations of replicas of said master mold, each of said one or more generations of replicas having a plurality of second openings; (c) depositing a first phosphor into the second openings of one of said one or more generations of replicas, thereby filling each of said second openings of said one of said one or more generations of replicas with said first phosphor, and thereby forming a pixelized phosphor structure comprising an array of discrete phosphor elements. 16. The process of claim 15, further comprising the step of forming a continuous phosphor layer on top of said one of said one or more generations of replicas such that said discrete phosphor elements project from said continuous phosphor layer. 17. The process of claim 16, further comprising the step of disengaging said pixelized phosphor structure from said one of said one or more generations of replicas. 18. The process of claim 17, further comprising the step of filling areas between said discrete phosphor elements of the disengaged pixelized phosphor structure with a second phosphor. 19. The process of claim 18, wherein said second phosphor has a different composition than a composition of said first phosphor. 20. The process of claim 17, wherein said pixelized phosphor structure includes first and second major opposing surfaces, said process further comprising the step of planarizing at least one of the first and second major opposing surfaces of the phosphor structure, thereby forming at least one planarized surface on said pixelized phosphor structure. 21. The process of claim 20, further comprising the step of attaching said at least one planarized surface of said pixelized phosphor structure to a second substrate. 22. The process of claim 15, wherein said substrate comprises a first material, said step (b) of forming said series of one or more generations of replicas includes the steps of forming a negative replica of said master mold by depositing a second material into said master mold, and disengaging said second material from said master mold, and said step (c) of depositing said first phosphor includes depositing said first phosphor into said second openings of said negative replica. 23. The process of claim 15, wherein said substrate comprises a first material, said step (b) of forming said series of one or more generations of replicas includes the steps of forming a negative replica of said master mold by depositing a second material into said master mold, disengaging said second material from said master mold, forming a positive replica of said master mold by depositing a third material into said negative replica, and disengaging said third material from said negative replica, and said step (c) of depositing said first phosphor includes depositing said first phosphor into said second openings of said positive replica. 24. The process of claim 15, wherein an aspect ratio of each of said discrete phosphor elements is in a range of from approximately 1:1 to approximately 20:1, said aspect ratio representing a ratio of a height of each of said discrete phosphor elements to a width of each of said discrete phosphor elements. 25. The process of claim 24, wherein the aspect ratio of each of said discrete phosphor elements is in a range of 2:1 to 8:1. 26. The process of claim 15, wherein each of said discrete phosphor elements includes a wall, said process further comprising the steps of: forming a layer of lacquer on said wall of each of said discrete phosphor elements; and forming a layer of silver over said layer of lacquer. 27. The process of claim 15, wherein said step of forming said master mold further includes the steps of: placing a mask over one of a plurality of portions of said substrate, wherein said mask defines a pattern of said openings; exposing said one of said portions of said substrate to said radiation via said mask, said radiation ablating areas of said one of said portions of said substrate to create said openings in said substrate, wherein each exposure of said one of said portions of said substrate to said radiation has an intensity insufficient to ablate each of said areas of said one of said portions of said substrate to a desired depth; and moving said substrate and said relative to one another such that said mask is placed over another of said portions of said substrate; exposing said another of said portions of said substrate to said radiation via said mask; and moving said substrate and said mask relative to one another such that said mask is place over each of said portions of said substrate, and exposing each of said portions of said substrate to said radiation via said mask a plurality of times such that the plurality of exposures of said portions of said substrate to said radiation has a cumulative effect of ablating each of said areas to said desired depth. 28. The process of claim 27, wherein said step of moving said substrate and said mask relative to one another includes moving said substrate and said mask relative to one another by a distance less than a dimension of said mask. 29. The process of claim 15, wherein said step of depositing said first phosphor into said second openings includes passing said second mold between a pair of calendering rolls, said calendering rolls defining a contact area compressing said first phosphor into said second openings, thereby reducing formation of voids within said second openings. 30. The process according to claim 15, wherein a cross-section of each of said openings corresponds to one of a square, a circle, a rectangle, and a hexagon. 31. A process for making a pixelized phosphor structure, comprising the steps of: (a) forming an integral, non-layered mold by applying a mechanical tool to a substrate to create a plurality of openings in said substrate; (b) depositing a first phosphor into said openings of said mold, thereby filling each of said openings of said mold with first phosphor; (c) forming a continuous phosphor layer on top of said mold, thereby forming a pixelized phosphor structure comprising an array of discrete phosphor elements projecting from said continuous phosphor layer; and (d) disengaging said pixelized phosphor structure from said mold. 32. The process of claim 31, wherein said pixelized phosphor structure includes first and second major opposing surfaces, said process further comprising the step of planarizing at least one of the first and second major opposing surfaces of the phosphor structure, thereby forming at least one planarized surface on said pixelized phosphor structure. 33. The process of claim 32, further comprising the step of attaching said at least one planarized surface of said pixelized phosphor structure to a second substrate. 34. The process of claim 33, further comprising the step of filling areas between said discrete phosphor elements of the disengaged pixelized phosphor structure with a second phosphor. 35. The process of claim 34, wherein said second phosphor has a different composition than a composition of said first phosphor. 36. The process of claim 31, wherein an aspect ratio of each of said discrete phosphor elements is in a range of from approximately 1:1 to approximately 20:1, said aspect ratio representing a ratio of a height of each of said discrete phosphor elements to a width of each of said discrete phosphor elements. 37. The process of claim 36, wherein the aspect ratio of each of said discrete phosphor elements is in a range of 2:1 to 8:1. 38. The process of claim 31, wherein said step of depositing said first phosphor into said openings of said mold includes passing said mold between a pair of calendering rolls, said calendering rolls defining a contact area compressing said first phosphor into said openings of said mold, thereby reducing formation of voids within said openings. 39. A process for making a pixelized phosphor structure, comprising the steps of: (a) forming an integral, non-layered master mold by applying a mechanical tool to a substrate to create a plurality of first openings in said substrate; (b) forming a series of one or more generations of replicas of said master mold, each of said one or more generations of replicas having a plurality of second openings; (c) depositing a first phosphor into the second openings of one of said one or more generations of replicas, thereby filling each of said second openings of said one of said one or more generations of replicas with first phosphor, and thereby forming a pixelized phosphor structure comprising an array of discrete phosphor elements. 40. The process of claim 39, further comprising the step of: (d) forming a continuous phosphor layer on top of said one of said one or more generations of replicas such that said array of discrete phosphor elements projects from said continuous phosphor layer. 41. The process of claim 40, further comprising the step of disengaging said pixelized phosphor structure from said one of said one or more generations of replicas. 42. The process of claim 41, further comprising the step of filling areas between said discrete phosphor elements of the disengaged pixelized phosphor structure with a second phosphor. 43. The process of claim 42, wherein said second phosphor has a different composition than a composition of said first phosphor. 44. The process of claim 41, wherein said pixelized phosphor structure includes first and second major opposing surfaces, said process further comprising the step of planarizing at least one of the first and second major opposing surfaces of the phosphor structure, thereby forming at least one planarized surface on said pixelized phosphor structure. 45. The process of claim 44, further comprising the step of attaching said at least one planarized surface of said pixelized phosphor structure to a second substrate. 46. The process of claim 39,. wherein said step (b) of forming said series of one or more generations of replicas includes the steps of forming a negative replica of said master mold by depositing a material into said master mold, said negative replica having said second openings, and disengaging said material from said master mold, and said step (c) of depositing said first phosphor includes depositing said first phosphor into said second openings of said negative replica. 47. The process of claim 39, wherein said step (b) of forming said series of one or more generations of replicas includes the steps of forming a negative replica of said master mold by depositing a first material into said master mold, said negative replica having a first plurality of said second openings, disengaging said first material from said master mold, forming a positive replica of said master mold by depositing a second material into said negative replica, said positive replica having a second plurality of said second openings, and disengaging said second material from said negative replica, and said step (c) of depositing said first phosphor includes depositing said first phosphor into said second plurality of said second openings of said positive replica. 48. The process of claim 39, wherein an aspect ratio of each of said discrete phosphor elements is in a range of from approximately 1:1 to approximately 20:1, said aspect ratio representing a ratio of a height of each of said discrete phosphor elements to a width of each of said discrete phosphor elements. 49. The process of claim 48, wherein the aspect ratio of each of said discrete phosphor elements is in a range of 2:1 to 8:1. 50. A process for making a pixelized phosphor structure, comprising the steps of: (a) providing a sheet of a first phosphor; (b) creating a plurality of discrete holes in said first phosphor, each of said discrete holes extending through said first phosphor and having two openings; and (c) depositing a second phosphor onto said first phosphor to form a pixelized phosphor structure, at least a portion of said second phosphor being deposited into said plurality of discrete holes in said first phosphor. 51. The process of claim 50, wherein said second phosphor is of a different composition than a composition of said first phosphor. 52. The process of claim 50, wherein said plurality of discrete holes in said first phosphor are formed by mechanically drilling said first phosphor. 53. The process of claim 50, wherein said pixelized phosphor structure includes first and second major parallel surfaces, said process further including the step of planarizing at least one of the first and second major surfaces of said pixelized phosphor structure, thereby forming at least one planarized surface.

Inventors: CALHOUN CLYDE D, DAHLQUIST JOHN C, FLEMING PATRICK R, KRINKE HARLAN L, TRAN NANG TRI, CALHOUN, CLYDE, D., DAHLQUIST, JOHN, C., FLEMING, PATRICK, R., KRINKE, HARLAN, L., TRAN, NANG, TRI
Applicants: MINNESOTA MINING & MFG, MINNESOTA MINING AND MANUFACTURING COMPANY
Application year: 1995
Main classification: H01J9
IPC codes: C09K11, G03F7, G21K4, H01J9