Mechanical packaging and thermal management of flat mirror arrays

Abstract: A liquid crystal element, a packaging structure providing thermal and alignment control, a display device including the same, and methods of fabrication and assembly are provided. The liquid crystal element includes: a semiconductor wafer, having microcircuitry and an array of reflective pixels; a layer of electro-optical responsive liquid crystal medium, of uniform thickness, disposed on the reflective pixels; a transparent conductive layer positioned on the liquid crystal, being substantially parallel to the reflective layers, to ensure a uniform thickness of the liquid crystal; and an insulative transparent layer provided on the conductive layer. The liquid crystal element is laminated to an optically flat substrate to limit the out-of-plane distortions thereof. The structure formed by element and substrate are disposed in a substrate holder which is mounted to a wiring board, and coupled to voltage sources for actuating the liquid crystal. During mounting, an aligning fixture is used to ensure proper orientation of the element relative to the related optical elements. Once the element is positioned, a heat sink is coupled to the rear surface of the substrate holder to dissipate heat.

Description: FIELD OF THE INVENTION The present invention relates to the construction, mechanical packaging, and thermal management of flat mirror arrays for reflection projection displays, and more specifically to the assembly of substantially flat liquid crystal semiconductor spatial light modulator on a surface, in alignment with the optical and electrical components and such thermal sinking devices as are necessary for use with liquid crystal display elements. BACKGROUND OF THE INVENTION Projection apparatus which utilize the optical characteristics of liquid crystals are well known in the art. A generalized liquid crystal display consists of a source of light, having frequency components of the full visible spectrum, which is directed at a series of filters which separate the light into red, blue, and green component beams. The component beams are then directed to interact with an optically reactive liquid crystal medium, which alters the component beams in accordance with specific image information. The altered beams are reconstituted and either transmitted directly onto a screen, or may be projected through a lens. The interaction of the component beams with the optically reactive liquid crystal elements, and the elements themselves, may be categorized as either transmissive or reflective. In a transmissive liquid crystal element, a quantity of an electrically active liquid crystal medium is positioned between a pair of spaced optically transparent surfaces. Each of the transparent surfaces further includes an inner surface electrode which permits the application of a voltage across the liquid crystal, therein altering the optical characteristics of the medium, for example, from transparent to absorptive (or scattering) in the frequency of the specific beam. It is understood that the image information necessary to provide a meaningful picture is provided by the appropriate variation of the voltage across the element. Reflective liquid crystal elements are similar to transmissive ones, but for the fact that only one of the spaced surfaces is optically transmissive. The other surface is reflective. In such an element the component beam of light is directed through the transmissive surface and the liquid crystal, reflects off the far reflective surface, passes back again through the liquid crystal medium and the transmissive surface, and thereby may be properly encoded with image information in accordance by variance of the transmissive state of the liquid crystal. There are many critical design requirements of reflective liquid crystal elements, which must be met in order to construct a specific device which is functional for the variety of applications for which reflective liquid crystal displays are intended. Three of these requirements are, thermal control, reflective surface flatness, and consistent alignment with other optical elements. The first of these, which is thermal control, is a requirement common to both transmissive and reflective liquid crystal elements. The transmission, scattering, and absorption of incident light by the liquid crystal medium, as well as the resistive heating associated with the voltage drop thereacross, entails a buildup of heat. Inasmuch as the performance of liquid crystals are substantially sensitive to temperature, and may be disrupted by such temperature fluctuations as associated with the above mentioned heating effects, cooling means must be provided. A number of methods and structures have been taught in the art which are capable of providing sufficient cooling for liquid crystals such that the use of colour liquid crystal displays has been widespread. For example, U.S. Patent No. 5,170,195 to Akiyama et al. teaches a cooling device comprising a container filled with a liquid coolant fluid and outwardly extending heat fins. The container, which is constructed of transparent materials, is disposed in contact with the face of a liquid crystal display element, therein receiving the heat from the element and providing a cooling effect therefor. Such a cooling system, however, is considerably more bulky than would be otherwise desired. In addition, the manufacturing of such a device necessitates substantially more complexity than is desirable for a competitively marketed display system. U.S. Patent No. 5,170,194, to Kurematsu et al. comprises a liquid crystal display element, which is a reflective type, consisting of a top optically transmissive surface, a quantity of voltage sensitive liquid crystal material, and a rear reflective surface which forms the front surface of a heat sink element. Stated alternatively, the reflective surface of the reflective liquid crystal element is the aluminum surface of a heat sink element. The cooling of the element is, therefore, provided directly to the liquid crystal by virtue of its contact with the heat sink. While the Kurematsu et al. device is a functional solution to the problem of overheating in reflective liquid crystal elements, the manufacturing of liquid crystal elements having a bulky heat sink attached directly to the liquid crystal element has considerable difficulties related to alignment, surface preparation, and in the ultimate assembly thereof. It is, therefore, an impractical solution. The second design requirement, which is the flatness of the reflective surface, is, by definition, a problem associated with reflective surfaces only. CMOS technology has been identified as an ideal fabrication method for constructing the high performance transistor arrays, which entails the building up of material on the surface of high quality single crystal silicon. Lattice mismatching, as well as thermal expansion characteristics can generate significant stresses in the underlying substrate material (the silicon), therein causing significant bowing of the mirrored surface. The bowing, which translates to a non-planarity of the surface, causes both (1) a non-uniform thickness of the liquid crystal layer between the bowed reflective surface and the planar transmissive top layer, and (2) variations in the path length of the reflected light from different parts of the element, and of the array. These effects compromise the electro-optic properties of the elements and/or array. Finding a solution to the above, and therein meeting the flatness requirement, contributes considerably to meeting the third requirement of the reflective liquid crystal elements which is providing for consistent alignment of the reflected component beam with other optical elements. In addition, the liquid crystal elements must be practically positionable in their array such that the reflective surfaces are properly oriented with respect to both their respective optics, and with respect to one another. The gross alignment requirements demonstrate a further reason why the structure taught by Kurematsu et al. in U.S. Patent No. 5,170,194 is unfit for practical use in a high volume fabrication scheme. The size, shape, and weight of the heat sink element, which is also the surface providing the reflectivity of the element, makes the proper alignment of each element difficult. The uniformity of tolerances necessary for a functional display are incompatible with such bulky elements. K. Kasahara et al., "A Liquid-crystal TV display panel using a MOS array with gate bus drivers", Biennial Display Research Conference, October 1980, 96-101 discloses a liquid crystal display structure according to the preamble of claim 1. It is, therefore, a principal object of the present invention to provide a reflective liquid crystal display element having sufficient cooling means to prevent disruption of the proper function of the liquid crystal material. It is further an object of the present invention to provide a mirror array, for use in a reflective liquid crystal display element, having a substantially flat surface, therein providing for proper reflective angulation of the light directed thereon. It is still further an object of the present invention to provide a liquid crystal display element having a substantially uniformly layered reflective surface, liquid crystal medium, and transmissive layer so that the electro-optic response of the element is not compromised. It is another object of the invention to provide a liquid crystal display element which is sufficiently positionable such that the manufacture of a plurality thereof, set forth in an array which is uniformly in proper alignment with their additional optical elements, is practical and feasible within a commercially viable fabrication scheme. SUMMARY OF THE INVENTION These and other objects of the present invention are achieved by the fabrication of a reflective liquid crystal display element as claimed in claim 1. Preferably, the display element has a substantially flat mirror surface, a corresponding uniform liquid crystal, a separately mounted thermal heat sink, and an enhanced packaging so that the elements may be set forth in an array which is properly aligned with optical elements associated therewith. Preferably, with respect to the fabrication of the substantially flat mirror surface, the structure has a semiconductor substrate, for example a silicon wafer, having circuitry disposed on the surface thereof, which has formed thereon an optically reflective surface layer. The semiconductor may be diced into a multiplicity of properly sized elements, each of which is laminated to an optically flat surface. The lamination of the mirrored semiconductor elements is preferably done with a spin-on or spray-on liquid adhesive which cures in place, or alternatively with an adhesive film. In either of these cases, or additional alternative means which may be found compatible, it is understood that the joint must be capable of both surviving all such thermal cycles and solvent treatments as are necessary to fabricate the liquid crystal element, and efficiently transferring heat such that the temperature gradient associated with the cooling means (such as are described hereinafter) is sufficiently small, therein protecting the functionality of the liquid crystal. In order that the thermal expansion characteristics of the semiconductor do not cause secondary warping of the mirrored surfaces, subsequent to the lamination thereof, the substrate is preferably chosen to have a similar coefficient of thermal expansion. An ideally suitable material for the optically flat surface, for use with a silicon semiconductor substrate, is polished ground glass. Ceramic, silicon, or metal substrate materials may also be used as the optically flat substrate. With respect to the fabrication of a uniformly thick layer of liquid crystal on the mirrored substrate, the preparation of the substrate as described above provides a flat surface onto which the liquid crystal is disposed. The spacing of a flat transmissive layer, such as glass, in parallel alignment with the substrate layer provides a uniformly thick region of liquid crystal. This uniform thickness is provided by an accurately positioned edge seal which secures the substrate and the transmissive top layer together in spaced relation to one another. With respect to the provision of a sufficient heat sink, which provides cooling to the liquid crystal elements, the present invention further comprises a substrate holder on which the liquid crystal element rests. Any gaps between the holder and the element are filled with a thermal paste. Once the assembly of element and holder are affixed to the printed wiring board to which it is mounted for display purposes (the mounting process and the corresponding superior alignment characteristics of this invention are set forth hereinbelow), a heat sink is attached to the rear surface of the substrate holder, whereby the heat buildup within the element can be efficiently removed. BRIEF DESCRIPTION OF DRAWINGS Figure 1 is a side cross-section conceptual view of a reflective liquid crystal display element; and Figure 2 is a side cross-sectional view of the reflective liquid crystal display structure of the present invention wherein the mirror array is joined to an optically flat substrate. Figure 3 is a perspective view of a substrate holder of the present invention. Figure 4 is a side cross-section view of the element, substrate, and substrate holder mounted to a wiring board, wherein a heat sink is coupled to the rear of the assembly. Figure 5 is a perspective view of the wiring board, having the substrate holder loosely mounted thereto, therein corresponding to one step of the assembly process. DETAILED DESCRIPTION OF PREFERRED EMBODIMENT Referring now to Figure 1, a side cross-section conceptual view of a reflective liquid crystal display element 12, is shown. A reflective layer 2 is deposited on a base material 4, which provides a backing for the reflective layer 2. In the present invention the backing layer 4 further comprises microcircuitry for providing a voltage, as will be more fully set forth below with respect to Figure 2. It is desirable that the reflective layer 2 comprise a conductive material. If the reflective layer 2 is not conductive, it is necessary to provide a transparent conductive layer (not shown) across the reflective surface. On top of the reflective layer is disposed a liquid crystal fluid medium 6 which is electro-optically active. In practice, one type of liquid crystal medium 6 permits the polarized light to be reflected without any rotation of the polarization orientation when there is an absence of voltage thereacross, and reflects the light and rotates the polarization when a given voltage is applied thereacross. By suitably using an analyzer in the reflected light path, the displayed image may selectively presented as bright or dark depending upon which polarization the analyzer allows to pass. In a different embodiment, which does not employ polarized light, the incident light may be diffusely scattered in correspondence with the application of a voltage while permitting the light to go through when no voltage is applied. The reverse situation of the liquid crystal medium causing scattering with no voltage and being transparent under applied voltage is also a variation. This liquid crystal layer 6 is generally maintained at a specific thickness, as described more fully hereinbelow, also with respect to Figure 2, so as to ensure constant optical properties and effects across the breadth of the element. Above the liquid crystal medium 6 are sequentially positioned a transparent conductive layer 8 and a transparent insulator layer 10, respectively. The transparent conductive layer 8 may be a separate and distinct material, such as a conductive polymer, however, it is preferable that the layer 8 comprise an electrically conductive oxide formed on the bottom side of the transparent insulator layer 10. The function of the reflective liquid crystal element 12 described hereinabove is best understood with respect to the additional features of such a display device. An external light source (not shown) is directed onto the element 12, through transparent layers 10 and 8 and through the liquid crystal medium 6. If a voltage is applied across the liquid crystal 6, therein causing the polarized light to be rotated, the reflected light passes through the analyzer, to the observer. If the voltage is not applied, the light is reflected without rotation and does not pass through the analyzer and therefore appears dark to the observer. For a colour display, each of the element corresponds to one of three colours: green, blue, or red. The composite of three images, each of a different colour are provided, therein forming a full colour image. Referring now to Figure 2, which shows a side cross-sectional view of the specific structure 7 of the present invention, which ensures that the underlying backing layer, which in this embodiment is a semiconductor wafer 4 having circuitry disposed thereon (not shown), remains uniformly flat is herein described. The wafer 4 has front and rear surfaces 3 and 5 respectively. After fabrication of a reflective layer 2, in the form of mirror pixels, on the front surface 3 of the semiconductor wafer 4, the wafer is diced to the required dimensions and laminated to a substrate 14 having an optically flat surface 15 to give the global flatness to the array. The substrate 14 must be sufficiently rigid to resist out-of-plane distortion of the wafer 4. The lamination of the wafer 4 to the optically flat substrate 14 may be achieved by a variety of means, however, in a preferred embodiment the lamination is provided by a spin-on or spray-on liquid adhesive layer that cures in place by light or heat such that additional fabrication steps are not required beyond the joining of the wafer 4 to the substrate 14. It is further understood that the joining means must be capable of surviving all of the thermal cycles and solvent treatments associated with the subsequent manufacture of a liquid crystal element. Preferred optically flat substrates 14 include, but are not limited to, ground polished glass, wherein the thermal expansion coefficient of the glass is equivalent to that of the wafer 4. In alternative designs, it may be preferable to use polished metal or ceramic materials as the substrate. It is further understood, that thick and rigid silicon may be utilized as a substrate material as well. A particularly suitable example of the proper fabrication of the structure described above comprises laminating a bowed silicon wafer 4 to a polished flat glass substrate with a thermosetting adhesive. In this example, the lamination is preferably carried out in a gas pressure cell where uniform gas pressure is applied over the whole surface at 200 degrees centigrade. In such an example, the degree of flatness of the reflective layer 2 on the surface of the wafer 4 may be maintained within the required tolerances for the intended liquid crystal application. Once the wafer 4, and the reflective layer 2 disposed thereon, have been joined properly, an edge sealing material 18 is disposed such that the liquid crystal 6 may be applied to the reflective surface in partitioned spacings. In addition, the sealing material 18 supports and secures to the conductive and insulative transparent layers 8,10, respectively, with the liquid crystal in between, therein holding the element together. The spacing of the transparent layers 8,10 from the reflective layer 2 must also be maintained at proper distance tolerances such that the liquid crystal layer has a uniform depth across the array. In other words, the reflective layer 2 and the transparent layers 8,10 must be substantially parallel to one another. The uniform thickness of the liquid crystal is critical to the proper performance of the display element because of two factors. The first is that the optical path of the incident light through, and back through, the liquid crystal layer must be the same distance at all points along the array in order to prevent non-symmetric angulation of the beams at the optical elements which merge the mono-chromatic beams together. The second factor compelling the maintenance of a constant thickness through the liquid crystal layer 6 is that the electro-optical characteristics of the liquid crystal are directly related to the voltage applied thereacross. Voltage is applied across the liquid crystal via terminals of an external voltage source 16, coupled to the conductive transparent layer 8 and the microcircuitry imprinted on the wafer 4. Equivalent voltage drops across two regions of liquid crystal, one of which is thicker than the other, cause different intensity of molecular alignment shifts. The voltage gradient across the thicker region is necessarily less than the gradient across the narrower region, therein causing a reduced optical effect in the liquid crystal. In the narrower region, a sharper voltage gradient causes a more intense orientation shift of the molecules. For these reasons, it is preferable that all of the layers be as substantially parallel as possible. As discussed above, assembly of the display requires that each element be accurately aligned to the colour combining optics, be properly coupled to its electrical voltage source, and have a structure which provides adequate temperature stabilization so that the performance of the element is not degraded by power dissipation within the element. Referring now to Figure 3, an alignment and support structure, which includes a substrate holder 20 is shown. The substrate holder 20 comprises a dish-like structure having a square perimeter conformation, wherein the interior 22 of the region is thinner than the perimeter. The four elongate sides 24 of the structure include regularly spaced screw holes 26 for mounting of the holder 20, and the structure 7 therein, to a wiring board (as described more fully with respect to Figure 5). Extending into the thinner region 22 of the substrate holder 20, are provided a plurality of support pads 28 on which the structure 7 is designed to rest. In the presently illustrated embodiment, three support pads 28 have been chosen, therein providing the ideal number of points on which to define planarity of the surface. The substrate holder 20 is ideally made of a good heat conducting material, such as a metal, so that the structure 7 will not be insulated as it builds up heat. In addition, the gap formed by the pads 28 between the substrate and the substrate holder is filled with a thermal paste which further increases the rate at which the dissipated power within the element can be removed by heat sinking means (as described more fully hereinbelow with respect to Figure 4). Referring now to Figure 4, a cross-sectional view of the liquid crystal display element 12 and the substrate holder 20 having a heat sink device 30 coupled to the rear side of the holder 20, and being mounted to a wiring board 40 is provided. As described above, the element 12 is disposed on the optically flat substrate 14, which is in turn coupled to the substrate holder 20 via an adhesive thermal paste 32. The substrate holder 20 is coupled to a wiring board 40 by a set of coupling screws 34 which are disposed through the wiring board into the screw holes 26 of the holder 20. The wiring board includes cut-out sections formed therein (see Figure 5), about which the substrate holders 20 are coupled, and in which cut-out sections the display elements 12 are positioned so that they are in light receiving position with the remainder of the optical elements in the display device. Referring now to Figure 5, the wiring board 40 is shown, having a liquid crystal element 12 mounted in a cut-out section 42 thereof, is shown in a perspective view. The region of the wiring board 40 which is adjacent to the cut-out section 42 includes a plurality of electrical connection pads to which the proper electrical connections (not shown) may be made to provide coupling to the driver chips 44 to drive the element 12. In the area surrounding the cut-out section 42, screw holes 46 are further provided which are mateable to the substrate holder 20 by screws 34, as described above. In order to make the electrical connection of the driver chips 44 to the wiring board 40, the driver chips 44 are first attached to the semiconductor wafer 4 which is known in the art. Then, the element 12, with driver chips 44 attached, must be precisely oriented on the wiring board 40. In this aligned position the electrical connection of the driver chips 44 to the wiring board 40 is made.

Claims: A liquid crystal display structure (12) comprising a semiconductor wafer having front and rear surfaces (3, 5), a reflective layer (2) on a portion of said front surface, liquid crystal medium (6) and transparent layers (8, 10) disposed on said reflective layer, and a flat and rigid substrate (14) laminated to said rear surface, characterized in that said flat and rigid substrate is optically flat and has a similar coefficient of thermal expansion to said wafer, said optically flat substrate being therefore sufficiently rigid to resist out-of-plane distortions of the wafer. The display structure as set forth in claim 1, wherein the reflective layer comprises an array of reflective pixels. The display structure as set forth in claim 1, wherein the optically flat substrate is ground and polished glass. The display structure of claim 1 further comprising: a substrate holder (20), into which the optically flat substrate is disposed; and a wiring board (40), on which the substrate holder is mounted. The liquid crystal display apparatus as set forth in claim 4, wherein said wiring board is coupled to a remote voltage source. The liquid crystal display apparatus as set forth in claim 4, further comprising a heat sink (30) coupled to said substrate holder whereby heat may be dissipated therefrom.

Inventors: CIPOLLA THOMAS M, COLGAN EVAN, MELCHER ROBERT L, MOK LAWRENCE S, NARAYAN CHANDRASEKHAR, SHI LEATHEN, YANG KEI-HSIUNG, CIPOLLA, THOMAS M., COLGAN, EVAN, MELCHER, ROBERT L., MOK, LAWRENCE S., NARAYAN, CHANDRASEKHAR, SHI, LEATHEN, YANG, KEI-HSIUNG
Applicants: IBM, INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1996
Main classification: G02F1
IPC codes: G02F1, H04N5, G09F9