Abstract:
An EL panel on a rigid substrate is thinned in selected areas, or overall, to provide a backlight for keypads and other applications that would otherwise require a more flexible panel or additional structure. Lamp materials are deposited on one side of a rigid substrate and then substrate material is ablated with a suitable tool, working from the opposite side of the substrate as the lamp materials. The depth of cut can be constant or variable, enabling one to tailor the flexibility of an area to the desired tactile response for a keypad or to provide clearance in close quarters. The invention is compatible with known process for making an EL panel.

Description:
BACKGROUND OF THE INVENTION 
   This invention relates to a thick film, inorganic, electroluminescent (EL) panel and, in particular, to an EL panel having a substrate that has more than one thickness in cross-section. 
   As used herein, and as understood by those of skill in the art, “thick film” refers to one type of EL lamp and “thin film” refers to another type of EL lamp. The terms only broadly relate to actual thickness and actually identify distinct disciplines. In general, thin film EL lamps are made by vacuum deposition of the various layers, usually on a glass substrate or on a preceding layer. Thick film EL lamps are generally made by depositing layers of inks on a substrate, e.g. by roll coating, spraying, or various printing techniques. The techniques for depositing ink are not exclusive, although the several lamp layers are typically deposited in the same manner, e.g. by screen printing. A thin, thick film EL lamp is not a contradiction in terms and such a lamp is considerably thicker than a thin film EL lamp. 
   As used herein, an EL “panel” is a single sheet including one or more luminous areas, wherein each luminous area is an EL “lamp.” An EL lamp is essentially a capacitor having a dielectric layer between two conductive electrodes, one of which is transparent. The dielectric layer can include phosphor particles or there can be a separate layer of phosphor particles adjacent the dielectric layer. The phosphor particles radiate light in the presence of a strong electric field, using relatively little current. 
   In the context of a thick film EL lamp, and as understood by those of skill in the art, “inorganic” refers to a crystalline, luminescent material that does not contain silicon or gallium as the host crystal. (A crystal may be doped accidentally, with impurities, or deliberately. “Host” refers to the crystal itself, not a dopant.) The term “inorganic” does not relate to the other materials from which an EL lamp is made. 
   EL phosphor particles are typically zinc sulfide-based materials, including one or more compounds such as copper sulfide (Cu 2 S), zinc selenide (ZnSe), and cadmium sulfide (CdS) in solid solution within the zinc sulfide crystal structure or as second phases or domains within the particle structure. EL phosphors typically contain moderate amounts of other materials such as dopants, e.g., bromine, chlorine, manganese, silver, etc., as color centers, as activators, or to modify defects in the particle lattice to modify properties of the phosphor as desired. The color of the emitted light is determined by the doping levels. Although understood in principle, the luminance of an EL phosphor particle is not understood in detail. The luminance of the phosphor degrades with time and usage, more so if the phosphor is exposed to moisture or high frequency (greater than 1,000 hertz) alternating current. 
   Various colors can be produced by mixing phosphors having different dopants or by “cascading” phosphors. A copper-activated zinc sulfide phosphor produces blue and green light under an applied electric field and a copper/manganese-activated zinc sulfide produces orange light under an applied electric field. Together, the phosphors produce what appears to be white light. It has long been known in the art to cascade phosphors, i.e. to use the light emitted by one phosphor to stimulate another phosphor or other material to emit light at a longer wavelength; e.g. see U.S. Pat. No. 3,052,810 (Mash). It is also known to doubly cascade light emitting materials. U.S. Pat. No. 6,023,371 (Onitsuka et al.) discloses an EL lamp that emits blue light coated with a layer containing fluorescent dye and fluorescent pigment. In one example, the pigment absorbs blue light and emits green light, while the dye absorbs green light and emits red light. 
   A modern (post-1985) EL lamp typically includes transparent substrate of polyester or polycarbonate material having a thickness of about seven mils (0.178 mm.). A transparent, front electrode of indium tin oxide or indium oxide is vacuum deposited onto the substrate to a thickness of 1000 Å or so. A phosphor layer is screen printed over the front electrode and a dielectric layer is screen printed over phosphor layer. A rear electrode is screen printed over the dielectric layer. It is also known in the art to deposit the layers by roll coating. 
   The inks used include a binder, a solvent, and a filler, wherein the filler determines the nature of the ink. A typical solvent is dimethylacetamide (DMAC). The binder is typically a fluoropolymer such as polyvinylidene fluoride/hexafluoropropylene (PVDF/HFP), polyester, vinyl, epoxy, or Kynar 9301, a proprietary terpolymer sold by Atofina. A phosphor layer is typically screen printed from a slurry containing a solvent, a binder, and zinc sulphide particles. A dielectric layer is typically screen printed from a slurry containing a solvent, a binder, and particles of titania (TiO 2 ) or barium titanate (BaTiO 3 ). A rear (opaque) electrode is typically screen printed from a slurry containing a solvent, a binder, and conductive particles such as silver or carbon. 
   As long known in the art, having the solvent and binder for each layer be chemically the same or chemically similar provides chemical compatibility and good adhesion between adjacent layers; e.g., see U.S. Pat. No. 4,816,717 (Harper et al.). It is not easy to find chemically compatible phosphors, dyes, binders, fillers, solvents or carriers and to produce, after curing, the desired physical properties, such as flexibility, and the desired optical properties, such as color and brightness. 
   An EL lamp constructed in accordance with the prior art is relatively stiff, even those only three mils (0.076 mm.) thick, making the lamp unsuited to some applications requiring greater flexibility, such as keypads. Layer thickness and stiffness are not directly related. The material from which the layer is made affects stiffness. Typically, EL lamps are made from the materials listed above. An EL lamp backlighting a keypad, for example, typically has holes under the keys to avoid affecting the actuation of a key. 
   Relatively flexible EL lamps are known in the art. U.S. Pat. No. 5,856,030 (Burrows) discloses an EL lamp made on a UV cured urethane layer on a release paper. The release paper provides substantial structural support while the lamp layers are applied from an ink containing a vinyl gel. There are several difficulties with this approach. Unlike panels made on substrates that are seven mils thick, or so, EL panels made on thin sheets from flexible materials, e.g. urethane one to five mils thick, do not keep their shape but bend or curl. This makes it extremely difficult to automate the assembly of panels into end products, e.g. a keypad for a cellular telephone or as the luminous structure in a three dimensional molded object. Another problem is the number of extra layers that must be deposited compared to an EL lamp made on a polyethylene or polycarbonate substrate. The extra layers increase processing time, increase the chance for error, and often require additional equipment, which is expensive. Yet another problem is the fact that the thin urethane layers may not provide the proper resiliency for keypads. In other words, an additional structure must be provided for tactile feedback, which further increases cost and the chance for defects. 
   U.S. Pat. No. 6,280,599 (Terada et al.), and the corresponding divisional U.S. Pat. No. 6,551,440, disclose a thin film EL lamp on a glass substrate having a portion of the substrate etched by hydrofluoric acid to reduce the separation of a light emitting layer from a filter layer. Glass is, obviously, a rigid substrate, more rigid than polyester or polycarbonate, that breaks rather than deforms. One could define rigidity in numerical terms but those of skill in the art do not usually operate on that basis. As used herein, a rigid material has approximately the same bending characteristics as a polyester sheet having a thickness of seven mils (0.178 mm.). As used herein, a flexible material has approximately the same bending characteristics as a sheet of polyurethane having a thickness of three mils (0.076 mm). The invention relates to relatively rigid substrates. 
   In view of the foregoing, it is therefore an object of the invention to provide a thick film, inorganic, EL panel that is made using conventional materials and processes and that is thinner in some areas than in other areas. 
   Another object of the invention is to provide a thick film, inorganic, EL panel that is made using conventional materials and processes on a substrate that is later reduced in thickness. 
   A further object of the invention is to provide a process for thinning all or part of the substrate of a substantially completed EL lamp. 
   Another object of the invention is to provide a thick film, inorganic, EL panel that is made using conventional materials and processes and having reduced thicknesses in preselected areas, wherein the reduced thicknesses optimize the lamp for providing tactile feedback when backlighting a keypad. 
   SUMMARY OF THE INVENTION 
   The foregoing objects are achieved in this invention in which an EL panel on a rigid substrate is thinned in selected areas, or overall, to provide a backlight for keypads and other applications that would otherwise require a more flexible panel or additional structure. Lamp materials are deposited on one side of a rigid substrate and then substrate material is ablated with a suitable tool, working from the opposite side of the substrate as the lamp materials. The depth of cut can be constant or variable, enabling one to tailor the flexibility of an area to the desired tactile response for a keypad. The invention is compatible with known process for making an EL panel. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete understanding of the invention can be obtained by considering the following detailed description in conjunction with the accompanying drawings, in which: 
       FIG. 1  is a cross-section of an EL lamp constructed in accordance with the prior art; 
       FIG. 2  is a plan view of an EL panel constructed in accordance with the invention; 
       FIG. 3  is a three quarter view of a cellular telephone including an EL panel constructed in accordance with the invention; 
       FIG. 4  illustrates constructing an EL panel in accordance with a preferred embodiment of the invention; 
       FIG. 5  illustrates constructing an EL panel in accordance with the invention by grinding; 
       FIG. 6  illustrates constructing an EL panel in accordance with the invention by cutting; and 
       FIG. 7  illustrates constructing an EL panel in accordance with the invention by ablating. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  is a cross-section of an EL lamp constructed in accordance with the prior art. The various layers are not shown in proportion. In lamp  10 , substrate  11  supports transparent front electrode  12 , which is a thin layer of indium tin oxide or indium oxide. Phosphor layer  13  overlies the front electrode and dielectric layer  15  overlies the phosphor layer. Layers  13  and  15  are combined in some applications. Overlying dielectric layer  15  is opaque rear electrode  16 . Optional layer  18  may also be provided, e.g. for sealing or protecting lamp  10 . Typically, coated phosphor particles are used, eliminating the need for a sealing layer. None of the layers is drawn to scale. Optional layer  18 , for example, is 1 mil. (0.025 mm) thick, as are the phosphor layer and the dielectric layer. 
     FIG. 2  is a plan view of a panel having a plurality of areas of reduced thickness. Panel  20  includes a plurality of interstitial runs  21  between and around several areas, such as areas  22  and  23 . The areas have reduced thickness by reducing the thickness of the substrate from the major surface opposite the major surface the lamp materials. The interstitial runs are thicker than areas  22  or  23 . Region  24  is a strait of reduced thickness connecting two larger areas of reduced thickness. Panel  20  need not be reduced in thickness the same amount in each area. 
   The open areas may or may not be completely surrounded, depending upon the design of the panel. The open areas correspond to the lamps in a panel. The reduced thickness areas are more flexible than the remainder of panel  20  and do not interfere with the operation of an underlying membrane switch. Keys, which would be positioned above panel  20 , are completely backlit with no dark areas. 
     FIG. 3  is a perspective view of an electronic device, represented by cellular telephone  30 , which includes an EL panel constructed in accordance with the invention. Cellular telephone  30  has several backlit areas, such as keypad  31 , LCD (liquid crystal display)  32 , and function keys  33 ,  34 , and  35 . While all such areas could be backlit by a single EL panel, at least two panels are preferred, one for the LCD and one for the remaining areas. As a result, cellular telephone  30  is more easily constructed with fewer elements than in the prior art because a separate sheet for providing tactile feedback can be omitted. 
     FIG. 4  is a cross-section of an EL panel constructed in accordance with a preferred embodiment of the invention. In  FIG. 4 , EL panel  40  includes substrate  41  having lamp materials  42  on the lower surface thereof. Depending upon the sequence in which the lamps materials are deposited, panel  40  may emit light predominantly upwardly, if the layers are deposited as shown in  FIG. 1 , or may emit light downwardly. The direction is immaterial to the invention. For the structure illustrated in  FIG. 4 , panel  40  emits light predominantly upwardly. 
   Panel  40  overlies switch matrix  46  containing a plurality of membrane switches, such as switch  48 , in an array. The reduced thickness areas, such as area  43 , have substantially the same pattern as the membrane switches. Actuator  44  extends into reduced thickness area  43  to push a small portion of panel  40  downward to actuate switch  48 . The thickness, shape, and material of substrate  41  under and around actuator  44  determine the tactile feedback to the actuator. 
   The thickness of substrate  41  is designated as “t”. The removed thickness is designated “d”, for depth of cut, and the width of the cut is designated “w”. 
   Obviously, d is less than t. Preferably, (t−d) is equal to or greater than about one mil (0.025 mm). Depth d need not be constant from cut to cut or within a cut. Width w need not be constant from cut to cut or within a cut; i.e., a cut can have any desired shape. As further discussed below, the walls of the cut need not be perpendicular to the floor of the cut but can be tapered or otherwise shaped. 
   Cutting is based upon the inclined plane or wedge, in which the edge of a tool is forced between two portions of the material to be cut, forcing the portions apart. Some cutting tools produce a long strip of material, while others remove material in chips or chunks. Grinding tools remove chunks of material. A laser is somewhat different in that material is removed by changing the state of the material, from solid to liquid or from solid to gas. Somewhat similarly, material can also be removed by dissolving the material in a suitable solvent or etchant. As used herein, “ablate” is intended to mean removing material by cutting (including abrading) or by changing state (melting, dissolving, or evaporating). 
     FIG. 5  illustrates thinning a substrate by grinding. Preferably, panel  40  is held against platen  51  by a slight vacuum. Platen  51  includes upper plate  51  and lower plate  52  separated by a small amount to define plenum  54 , which is coupled to a source of vacuum (not shown). A plurality of holes, such as holes  55  and  56 , in upper plate  52  permit air to leak into plenum  54  when the holes are not covered. Also within plenum  54  are a plurality of tubes, such as tubes  57  and  58 , for conveying coolant to lower the temperature of at least upper plate  52  and anything laying on the upper plate. Tubes  57  and  58  are preferably part of a single, long, serpentine tube thermally coupled to upper plate  52 . Upper plate  52  is preferably made from metal, e.g. aluminum, but can be made from plastic because the amount of heat that must be conducted away from panel  40  is not great. Vacuum tables or platens are known per se in the art. 
   Grinding wheel  61  rotates in a substantially vertical plane about horizontal axis  61 . Grinding wheel  63  rotates in a substantially horizontal about vertical axis  64 . The axes can be rotated as desired. The wheels are made of alumina or other abrasive material and are preferably relatively fine grit (&gt;100). Suitable actuators for manipulating the wheels are not shown but are well known in the art. Any form of ablation produces heat. A suitable pressure is applied to remove material without excessive pulling or heating. Appropriate temperatures and pressures are readily determined empirically; i.e., using a test strip. 
   Grinding wheel  61  has a rectangular profile and produces a cut with substantially vertical wall and a flat floor. Grinding wheel  63  is tapered and produces tapered walls and a substantially flat floor. The wheels can be suitably shaped to produce any desired profile for the floor and walls of a cut. For example, a tapered wall can be merged with a floor that tapers downwardly toward the center to control the resiliency of the substrate for providing tactile feedback while actuating a switch. A tapered floor is obtained by changing the depth of cut as one scans across the area of reduced thickness. In addition or instead, tactile feedback can be controlled by narrow slits or cuts, such as slit  26  ( FIG. 2 ), made with a suitable tool. 
     FIG. 6  illustrates apparatus in which the cooling and hold-down functions are separated. Platen  71  provides a vacuum hold-down for panel  40 . Cooling is separately provided by tube  73 , which provides a suitable fluid, e.g. air, gas or gas mixture, or water. The cooling fluid also aids in removing cuttings. Cutting head  75  is a reamer or milling bit including a plurality of blades for removing material from substrate  41 . As with grinding wheels, cut  76  is shaped by shaping the cutting tool or by suitably manipulating the cutting tool. 
     FIG. 7  illustrates an alternative embodiment of the invention in which material is removed by changing state. Laser  81  locally heats substrate  41 , causing the surface material to vaporize in a small area. The process is repeated as the laser is scanned over the cut until the desired amount of material is removed. The shape of the cut is determined by the movement of the laser. A pulsed or continuous laser can be used. Cooling is provided by fluid from tube  82 , which also aids in removing cuttings (whether solid, liquid, or gaseous) from the cut. 
   The invention thus provides a thin, thick film, inorganic EL panel that is made using conventional materials and processes on a rigid substrate and is thinner in some areas than in other areas by removal of material from the substrate after the lamp layers are deposited. All or part of the substrate of a substantially completed EL lamp is thinned and the thinning can optimize tactile feedback when backlighting a keypad. 
   Having thus described the invention, it will be apparent to those of skill in the art that various modifications can be made within the scope of the invention. For example, a tacky sheet could be used instead of a vacuum table for supporting a panel during thinning. The motion of a tool over the surface of a panel is relative; i.e. a movable table is as effective as a robotic arm having a tool on the end thereof. A single tool or a plurality of tools can be used to pattern the substrate.