Abstract:
An improved packaged liquid crystal display (LCD) assembly is described. A recess is used to house a support material while the LCD cell  609  is positioned at least partially within the containment structure. A plurality of spaced apart stabilizers are attached from the sides of the LCD cell  609  to the substrate without transmitting residual stresses induced during fabrication and operation. A support material is dispensed in the recess such that it provides support for the LCD cell  609  without transmitting residual stresses from the substrate. The described arrangements permit an LCD assembly which minimizes the amount of forces and stresses that lead to optical defects. The stabilizers, in addition to supporting the cell, also act to contain the encapsulating material used to protect the bonding wires. The support material, in addition to minimizing transmission of stresses, also provides improved heat dissipation from the LCD cell  609 . In another embodiment, a method for constructing the LCD assembly is described.

Description:
RELATED APPLICATIONS 
   This application is related to application Ser. No. 09/130,631 filed Aug. 8, 1998, which is incorporated herein by reference. 
   FIELD OF THE INVENTION 
   The present invention relates, generally to liquid crystal display assemblies and, more particularly, relates to miniature liquid crystal display assemblies constructed to reduce optical defects. 
   BACKGROUND OF THE INVENTION 
   In the recent past, substantial research and development resources have been directed toward small scale Liquid Crystal Display (LCD) and light valve technologies. These miniature LCD assemblies are typically employed in high resolution projection displays, such as a reflective LCD projectors, SXGA formats (1,280×1,024 pixel resolution) and even HDTV formats (above 1,000 line resolution), or the like. 
   Briefly, as shown in  FIGS. 1 and 2 , a conventional small scale LCD assembly  20  is illustrated including a die  21  having a pixel array  22 . This pixel array  22  is typically composed of rows and columns of electrically conductive pathways each forming an individual pixel (not shown). Each pixel can be individually changed to an “on” condition by selecting the appropriate row and column of pixel array  22 . Positioned around or concentrated on one end of the pixel array are a plurality of die bond pads  23  which are internally connected to the pixel array  22  to enable operational control thereof. Selection of the appropriate pixel is controlled by control circuitry, either included within the die  21  or external to the die  21 . In either configuration, external control signals may be used to control the functions of the die  21 . 
   As best viewed in  FIGS. 2 and 3 , a transparent glass plate  24  is typically placed over the die  21  and the pixel array  22 , such that a portion of the glass plate  24  overhangs the die  21 . The glass plate  24  is usually affixed to die  21  through an adhesive seal  25  which together cooperate to define a sealed volume encompassing the pixel array  22 . This sealed volume is then commonly filled with a solution  26  of liquid crystal material such as Twisted Nematic Liquid Crystals (TNLC). To facilitate grounding of the glass plate  24 , a conductive coating (not shown) may be deposited over the undersurface  28  thereof. 
   The die  21  is typically rigidly or semi-rigidly mounted to a substrate  27  for mounting support and to facilitate heat conductive dissipation for the die. A conductive adhesive  29  ( FIG. 3 ), such as a conductive epoxy, is generally applied to the undersurface  28  of the die  21  to adhere the die directly to the top surface of the substrate  27 . In this manner, a heat conductive pathway is created directly between the die and the substrate to dissipate heat generated by the die. 
   The substrate  27  generally includes a plurality of substrate bond pads  30  which are typically wire bonded to the die bond pads  23  through bonding wires  31 . Finally, an encapsulating material  32  is applied to seal die  21  to substrate  27 . The encapsulating material  32  ( FIG. 3 ) normally encapsulates the bonding wires  31  and the internal elements of die  21  without obscuring a view of the pixel array  22  through the glass plate  24 . 
   By activating the appropriate pixels, the corresponding liquid crystals in the TNLC, deposited in sealed volume, are caused to either align or rotate through an appropriate polarizer. Upon alignment, light is permitted to pass through the aligned crystals and the adjacent glass plate, thus appearing light in color. In contrast, when the liquid crystals are rotated, light is prevented from passing therethrough and, hence the glass plate  24 , so that the corresponding pixel appears dark in color. 
   One important aspect in the proper operation of these small scale LCD or light valve assemblies is the maintenance of proper distance uniformity (typically about 2–4 μm) between the pixel array and the undersurface  33  of the glass plate. Variances in these distances may often times cause the pixel array to function improperly or cause operational failure. 
   One problem with conventional rigid display device constructions where the substrate  27 , the glass plate  24  and the silicon die  21  are all attached are optical defects due to warping. Since the structures are composed of different materials or composites that have different coefficients of expansion, they expand at different rates and cause each other to warp. As a result of this deformation, depending in part upon the construction processes, significant residual stresses may be induced upon the cell. At a minimum, these internal stresses cause optical defects such as variations in color uniformity and fringes, and variations in the cell gap thickness which may cause optical shadows. As these optical defects may be produced by deformations as small as 0.25 microns, minor stresses may substantially reduce optical quality. 
   This is especially true since the undersurface  28  of the die  21  is typically rigidly affixed or attached directly to the substrate  27 . For example, when the substrate  27  and the die  21  are both composed of a silicon material, upon heating, the glass plate  24  expansion tends to negatively bow or warp ( FIG. 4 ) at a rate greater than that of the substrate  27 . Upon more extensive high temperature thermal cycling during operation, additional occurrence of optical fringes and optical non-uniformity may even further compromise the performance of the LCD assembly. 
   In contrast, when the die  21  is composed of a silicon material and the substrate  27  is composed of a more conductive material, such as aluminum, upon heating, the substrate expansion tends to positively bow or warp ( FIG. 5 ) the substrate at a rate greater than that of the die  21  and glass plate  24 . As viewed in the cross-sectional view of  FIG. 5 , central thinning of the cell is caused which results in defects such as discoloration and the appearance of optical shadows. 
   Another cause of optical defects due to stress occurs during construction of the small scale LCD assembly  20 . Commonly, the encapsulating material  32  used to protect the bonding wires  31  may surround the glass plate  24  and the silicon die  21 . As the encapsulating material  32  is cured, differences in thermal expansion between the encapsulating material  32  and the glass plate  24  or the silicon die  21  may lead to peripheral deformation of the glass plate  24  or the silicon die  21 , leading to further stressing and optical defects. 
   In view of the foregoing, it should be apparent that improved LCD assembly and construction techniques would be desirable. 
   SUMMARY OF THE INVENTION 
   An improved packaged liquid crystal display (LCD) assembly is described in which the cell liquid crystal cell is suspended. More specifically, a plurality of spaced apart stabilizers are attached from the sides of the LCD cell to the substrate without transmitting residual stresses induced during fabrication and operation. The stabilizers additionally do not adhere the bottom surface of the LCD cell to the containment structure. 
   In one preferred embodiment, a support material is dispensed in the containment chamber such that it provides support for the LCD cell without transmitting residual stresses from the substrate. In some embodiments, the LCD cell is dispensed on top or partially within the support material. 
   The LCD assembly generally includes a liquid crystal cell including a die having a pixel array, a transparent plate attached to the die, and a liquid crystal material disposed in a gap region between the die and the transparent plate. A containment chamber is used to house a support material while the LCD cell is positioned at least partially within the containment structure. 
   The described arrangements have numerous advantages and permit an LCD assembly which minimizes the amount of forces and stresses that lead to optical defects. The stabilizers, in addition to supporting the cell, also act to contain an encapsulating material used to protect the bonding wires. The support material, in addition to minimizing transmission of stresses, also allows improved heat dissipation from the LCD cell. 
   In another embodiment, a method for constructing the LCD assembly is described. As a result of the reduced temperature sensitive curing involved in the present invention, cycle time, or the time required to construct the LCD assembly, is reduced to less than five hours. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings and in which like reference numerals refer to similar elements and in which: 
       FIG. 1  is a top perspective view of a prior art Liquid Crystal Display (LCD) assembly illustrating a die rigidly mounted to a substrate. 
       FIG. 2  is a top plan view of the prior art LCD assembly of  FIG. 1 . 
       FIG. 3  is an enlarged, fragmentary, side elevation view, in cross-section, of the prior art LCD assembly taken substantially along the plane of the line  3 — 3  in  FIG. 2 . 
       FIG. 4  is a fragmentary, side elevation view, in cross-section, of the prior art LCD assembly of  FIG. 3 , and illustrating delamination of the transparent plate from the die resulting from a negative bow configuration. 
       FIG. 5  is a fragmentary, side elevation view, in cross-section, of the prior art LCD assembly of  FIG. 3 , and illustrating a positive bow configuration. 
       FIG. 6  is a top perspective view of an exemplary Liquid Crystal Display (LCD) assembly in accordance with one embodiment of the present invention in which a plurality of stabilizing members are used to support the LCD cell. 
       FIG. 7  is a top plan view of a substrate assembly that is used to package the LCD assembly of  FIG. 6 . 
       FIG. 8  is an enlarged, side elevation view of a substrate assembly  700  that is used to package the LCD assembly of  FIG. 6  taken substantially along the plane of the line  8 — 8  in  FIG. 7 . 
       FIG. 9  is the substrate assembly of  FIG. 8  further including a thermal grease. 
       FIG. 10  is the substrate assembly of  FIG. 9  further including an LCD cell disposed partially in the thermal grease. 
       FIG. 11  is the substrate assembly of  FIG. 7  further including an LCD cell disposed partially in the thermal grease. 
       FIG. 12  is the substrate assembly of  FIG. 11  further including a plurality of stabilizers attached to the transparent plate of the LCD cell. 
       FIG. 13  is the LCD assembly of  FIG. 6  including a plurality of wire bonds between the die and the substrate. 
       FIG. 14  is the LCD assembly of  FIG. 13  further including the encapsulating material. 
       FIG. 15  is a top view of the LCD assembly of  FIG. 14  including the encapsulating material. 
       FIG. 16  is a flow chart, in accordance with one embodiment of the present invention, which illustrates the construction of an LCD assembly. 
   

   DETAILED DESCRIPTION 
   In the following detailed description of the present invention, numerous specific embodiments are set forth in order to provide a thorough understanding of the invention. However, as will be apparent to those skilled in the art, the present invention may be practiced without these specific details or by using alternate elements or processes. In other instances well known processes, procedures, components, and circuits have not been described in detail so as not to unnecessarily obscure aspects of the present invention. 
   Referring initially to  FIGS. 6–9 , a small scale liquid crystal display (LCD) assembly  600  according to one embodiment of the present invention will be described. The liquid crystal display  600  includes a substrate  602  having a recess  604  that acts as a containment chamber for receiving a die  606  attached to a transparent plate  608 . A liquid crystal material is disposed between the die  606  and the transparent plate  608 . Generally, the die  606 , the transparent plate  608  and the liquid crystal material and other components between the die and transparent plate are collectively referred to as an LCD cell  609 . The die  606  includes a pixel array  610  and a plurality of die bond pads  612 . The pixel array  610  is responsible for producing the images that are shown on the liquid crystal display. The die bond pads  612  allow electrical communication with external devices. A cantilevered ground trace  622  passively contacts the transparent plate  608  to further ground the LCD cell. 
   In an embodiment of the present invention, a support material or thermal grease  620  is disposed in the recess  604 . The support material  620  is arranged to support the LCD cell  609  in a floating relationship. A plurality of spaced apart stabilizers  624  are used to support the LCD cell  609 . In one embodiment, the stabilizers  624  connect the substrate  602  to the sides of the LCD cell  609 , as for example to the sides of transparent plate  608 . The stabilizers  624  are formed from an elastic material and provide the principle means of holding the LCD cell  609  in place. With the described arrangement, the LCD cell  609  is substantially insulated from stresses and/or warpage induced by the substrate  602 . 
   In the embodiment shown, a pair of the stabilizers  624  also act, along with a barrier  626 , to retain an elastomeric encapsulating material (not shown in  FIG. 6 ) used to protect bonding wires  616  that electrically couple the die to external elements. As described in more detail below, these barriers effectively limit the encapsulating material to one end of the LCD cell  609 . Thus, the primary elements that mechanically couple the LCD cell to substrate are the stabilizers  624 . The only other elements that provide any additional mechanical coupling are the cantilevered ground trace  622  and the encapsulating material. The cumulative effect of the described structure minimizes the residual stress on the LCD cell. The reduced stresses reduce the possibility of externally induced warpage occurring within the LCD assembly  600 , during both construction and operation of the device, which in turn reduces the probability of internal stress induced optical defects, including variations in color uniformity and fringes, and optical shadows. 
   Referring next to  FIGS. 7–15 , the packaging of a small scale liquid crystal display (LCD) assembly  600  according to one embodiment of the present invention will be described in more detail.  FIGS. 7 and 8  illustrate a substrate assembly  700  that is used to package the LCD cell. The substrate assembly  700  includes a substrate  602  which is used as the base of the LCD assembly  600 . Any suitable type of substrate may be utilized in accordance with the present invention. The substrate  602 , may take any suitable form including simple substrate blocks, chip carriers, leaded chip carriers, as well as other types of substrates which can be used for packaging the LCD cell  609 . In the described embodiment, the substrate  602  is simply a machined aluminum block. Alternately, the substrate  602  may include Alloy Ash 42, a ceramic alloy, a combination of materials (ie. plastic and metal), or any material with a coefficient of thermal expansion which substantially facilitates less induced stress in the die  606 . The substrate  602  includes a recess  604  sized to accommodate the LCD cell  609 . The recess  604  is peripherally larger than the periphery of the die  606  so that the walls of the recess do not rigidly contact the die  606 . 
   A thin printed circuit flex tape  618  is attached to the top surface  802  of the substrate  602  to serve as a connection to external circuitry. The printed circuit flex tape  618  may be formed from any suitable material such as polyimide tape and includes a plurality of tape bond pads  614  positioned to permit wire bonding to the die bond pads  612  of the LCD cell  609 . The flex tape  618  further includes a conductive ground trace  704  which is electrically connected to the cantilevered ground trace  622  to facilitate grounding the transparent plate  608 . 
   The cantilevered ground trace  622  is a pre-shaped thin metal strip which permits passive contact between the ground trace  704  and the transparent plate  608 . The spring tension of the thin metal strip is such that it does not induce substantial stresses in the transparent plate  608  and thus does not cause optical defects in the LCD assembly  600 . In operation, the cantilevered ground trace  622  serves to ground the transparent plate  608 . 
   The substrate assembly  700  also includes a barrier  626  which is used in subsequent containment of the encapsulating material. In the embodiment shown, the barrier  626  is attached to the top surface of the substrate  602  and surrounds the external bond pads  614  and a portion of the recess  604 . Barrier  626  may be formed from a variety of materials such as a molded plastic or other material capable of acting as a dam for the encapsulating material. In this embodiment, the barrier  626  is attached to the substrate  602  prior to placement of the LCD cell  609 . Alternately, the barrier  626  may be attached to the substrate subsequent to the placement of the LCD cell  609 . 
   A thermal grease is placed in the substrate recess  604  to support the liquid crystal cell in a “floating” relationship relative to the substrate  602 . That is, the liquid crystal cell is not adhered or otherwise attached to the bottom surface of the recess. In this manner, forces and residual stresses induced by the substrate  602  will not be transmitted directly to the die  606 , and thus stress induced optical distortions can be significantly reduced. As best illustrated in  FIG. 9 , the thermal grease  620  is placed in the recess  604  to a depth  902 . The depth of the thermal grease may be widely varied, however, by way of example, gap distances between the undersurface of the die  606  and the recess bottom  904  in the range of approximately 0.3 mm to about 0.8 mm work well. 
   Since the thermal grease  620  contacts both the die  606  and the substrate  602 , it forms a thermally conductive pathway to facilitate heat dissipation from the LCD cell  609  (and particularly the die) to the substrate  602 . Preferably, the thermal grease  620  has a high thermal conductivity. A wide variety of materials may be used as the thermal grease  620 . By way of example, Dow Corning 340 silicone heat sink compound works well. Additional properties which may be advantageous for the thermal grease  620  include constant viscosity, thermal stability, low thermal expansion and insensitivity to curing practices. 
   After the grease  620  has been placed in the recess  604 , the LCD cell  609  is then placed in the recess  604  as shown in  FIGS. 10 and 11 . The LCD cell  609  typically includes the die  606 , the transparent plate  608 , and a liquid crystal material disposed therebetween. The die  606  includes a pixel array  610  and plurality of die bond pads  612 . The composition of transparent plate  608  may be of any suitable material such as glass and plastic, or the like, which provides substantial rigidity and a suitable adhesive surface for the stabilizers  624 . While the transparent plate  608  is rectangular in this embodiment, it will be understood that the transparent plate  608  may be any geometric shape sufficient to cover the pixel array  610  of the die, while further sufficiently mounting to the LCD cell  609  via the stabilizers  624 . 
   The transparent plate  608  passively contacts the cantilever ground trace  622 . As mentioned previously, the spring tension of the thin metal strip is preferably such that it does not induce substantial stress in the transparent plate  608 . A ledge portion  1102  of the transparent plate  608  is planarized to allow flexible contact with the cantilever ground trace  622 . In this manner, contact between the two members may be flexibly located along the planarized ledge. This arrangement is beneficial since the passive contact substantially reduces and minimizes any expansive effects along the axis extending generally perpendicular to the top surfaces of the transparent plate  608 . 
   The construction of the LCD cell  609  may be varied as will be appreciated by those skilled in the art. By way of example, one suitable LCD cell  609  construction is described in application Ser. No. 09/130,631 filed Aug. 8, 1998. Briefly, the small scale LCD assembly  600  includes a pixel array  610  formed on the die  606 . The die bond pads  612  are electrically coupled to the pixel array  610  through internal circuitry (not shown) to facilitate control of the pixel array  610 . An adhesive seal  1002  is formed around the pixel array  610  on the top surface of the die  606  and acts to adhere the transparent plate  608  to the die  606 . This adhesive seal  1002  seals a volume between the transparent plate  608 , die  606 , and within the perimeter of the adhesive seal  1002 . Pixel array  610  is enclosed within this sealed volume. In addition, precision conductive spacers  1004  may also be employed to facilitate a uniform spacing between the pixel array  610  and the transparent plate  608 . 
   To continue with construction of LCD assembly  600 , stabilizers  1202 ,  1204 ,  1206  and  1208  are attached to the LCD cell  609  as shown in  FIG. 12 . Collectively, the stabilizers  624  provide non-rigid support for the LCD cell  609  from the substrate  602 . In this embodiment, the stabilizers  624  are connected between the substrate and the sides of the transparent plate  608 . Preferably, the stabilizers connect to the side of the LCD cell  609  and not the undersurface or upper surface of the die  606  or the transparent plate  608 . The stabilizers are preferably sufficiently compliant such that they do not induce detrimental stresses in the LCD assembly during subsequent construction or operation. However, they must be sufficiently stiff to prevent movement of the LCD cell  609  within the recess. 
   The stabilizers  624  may be of any material that adheres to the substrate  602  and corresponding member of the LCD cell  609  and provides sufficient support. As an example, an epoxy or UV acrylate may be used. The stabilizers  624  may be of any shape and size to provide sufficient support and contain the encapsulating material. In one embodiment of the present invention, the stabilizers  624  take on a globular form in which the thickness of a stabilizer is less than the thickness of the transparent plate  608 . In another embodiment, the stabilizer material is such that it changes properties after a curing operation. For example, stabilizer  624  may change from a liquid material before curing to an elastomeric or rubbery material after curing. 
   It is noted that the thermal grease helps control the thickness of the stabilizers  624  since it prevents any stabilizer materials from reaching the underside of the die  606 . This protection is advantageous since any stabilizer material which adheres to the bottom surface of the die  606  may potentially induce stresses upon curing. Similarly, the thermal grease  620  prevents the encapsulating material used to protect the bonding wires  616  from attaching to the undersurface of the die as well. 
   In the embodiment shown in  FIG. 12 , a pair of stabilizers  624  are provided on each of two opposing sides of the LCD cell  609 . It should be apparent that the number, size and position of the stabilizers may be varied as long as they provide a substantially mechanically stable system. By way of example, six or eight stabilizers may be used in a variety of positions around the LCD cell  609 . Alternately, three stabilizers arranged in a balanced triangular arrangement may work in some situations as well (e.g., wherein two stabilizers are on one side of the recess and the third is situated on the opposing side of the recess  604 . It is noted that the stabilizers  624  are only placed on two sides of the LCD cell. As explained below, the encapsulating material secures a third side of the LCD cell. By not attaching the fourth side, induced stresses within the LCD cell may be avoided. 
   The dimensions of the stabilizer  1202  may be widely varied in accordance with the needs of a particular system. By way of example widths on the order of 100 to 1800 mils, as for example 300 to 600 mils work well. By way of example, four stabilizers, each with a width  1212  of approximately 400 mils have been tested with success. It is understood that a single stabilizer may be wide enough such that it acts to provide the support of multiple stabilizers, such as stabilizers  1202  and  1204 . In this manner, even two opposing stabilizers may be used to provide sufficient rigidity for the LCD cell  609 . Preferably, the width  1212  is not so large as to induce stress of the LCD cell  609  within the width of the stabilizer as the stabilizer material cures. Thus, the width of stabilizers  624  may be varied so as to provide sufficient support for the LCD cell  609  but so large as to induce stress during curing of the stabilizer material. 
   After the LCD cell  609  has been attached to the substrate by the stabilizers, wire bonding is performed to electrically couple die bond pads  612  to the bond pads on flex tape  618  as best shown in  FIG. 13 . In the embodiment shown, the flex tape bond pads  614  are positioned adjacent just one end of the recess  602 , which corresponds to where the die bond pads  612  are situated when the die  606  is seated within the recess  604 . The bonding wires are then encapsulated using an appropriate encapsulating material  1400  ( FIGS. 14 and 15 ) to protect the bonding wires  616  and bond pads. A wide variety of encapsulating materials may be used, however it is desirable that the encapsulating material have good elasticity. 
   In the embodiment shown, the encapsulating material  1400  is poured into the LCD assembly  600  using a glob topping type approach as opposed to injection molding. The barrier  626  is used in conjunction with the stabilizers  624  ( FIGS. 14 and 15 ) to contain the encapsulating material  1400  at one end of the LCD cell  609 . The encapsulating material is preferably chosen such that its thermal expansion properties do not imposed significant stresses on the LCD cell  609  during subsequent curing. Examples of materials which may be used for encapsulating material  1400  include OE107 Epoxy (Epoxy Technology Corp.) and LCM 35 UV curable acrylate (Ablestik Corp. of Rancho Dominquez, Calif.). 
   The construction of LCD assembly  600  according to a specific embodiment of the present invention will now be described with reference to flowchart  1600  of  FIG. 16 . The substrate assembly that includes a recess suitable for holding a LCD cell  609  is formed (step  1602 ). A thermal grease  620  is placed within the recess  604  (step  1604 ). The LCD cell  609  is then set within the recess  604  such that the die  606  rests upon or is partially submerged in the thermal grease  620  (step  1606 ). After the LCD cell  609  has been placed within the recess, the stabilizers  624  are formed between peripheral edges of the LCD cell  609  and the substrate  602  to secure the LCD cell  609  in place (step  1608 ). 
   The stabilizer material is then at least partially cured in an oven using an ultraviolet belt or other suitable means (step  1610 ). In a specific embodiment of the present invention, curing is performed at 70 degrees Celsius for approximately two hours although any suitable curing approach may be used. The die bond pads are then wire bonded to the external bond pads (step  1612 ). After wire bonding is completed, the encapsulating material  1400  is then dispensed to protect the bonding wires (step  1614 ). The encapsulating material is then cured in an oven as required. A brief cold cure may also be performed at this time at a relatively low temperature, such as a fifteen minute cure at minus forty degrees Celsius. 
   Since there is considerably less curing of temperature sensitive materials then in conventional LCD assemblies, both the threat of warpage during curing and the cycling time (i.e. the time required to package a device) are significantly reduced. Although only a few embodiments of the present invention have been described in detail, it should be understood that the present invention may be embodied in many other specific forms without departing from the spirit or scope of the invention. Particularly, although the invention has been described primarily in the context of a recess in the substrate  602 , in alternative embodiments, the LCD containment structure may include walls on top of a substrate  602  or a wide variety of other structures. Additionally, barrier  626  used may be widely varied in accordance with the needs of a particular system. Barrier  626  may be further extended along the side of substrate to allow additional anchoring for the stabilizers. The barriers can be extended to form the walls of the containment chamer as well. Therefore, the present examples are to be considered as illustrative and not restrictive, and the invention is not to be limited to the details given herein, but may be modified within the scope of the appended claims.