Abstract:
The present invention provides an improved seal for sealing a liquid crystal display (LCD) device. An improved seal is formed between a transparent plate and a die having a pixel array. The improved seal is configured to encircle the pixel array of the die when the die and the transparent plate are joined. The die and the transparent plate are joined together such that the improved seal is disposed between the transparent plate and the die. In one embodiment, the improved seal is a hybrid seal. The hybrid seal includes a first seal encircling the pixel array of the die and adhesively coupling the transparent plate and the die. The hybrid seal further includes a second seal encircling the second seal. In another embodiment, the first seal lacks a characteristic necessary for an effective seal. The second seal possesses the characteristic, such that the hybrid seal possesses the necessary characteristic.

Description:
BACKGROUND OF THE INVENTION 
     The present inventions relate generally to liquid crystal displays (LCDs). More specifically to methods and apparatuses for sealing LCD devices. 
     Today, small scale LCD devices, such as light valves are becoming more prominent. Small scale LCD devices, and light valves are typically manufactured according to certain specifications to ensure that they function properly. One criteria is that the pixel array is appropriately sealed to prevent contamination and ensure proper operation. 
     Referring to FIG. 1, a typical small scale LCD device  10  includes a die  20  containing a pixel array  22 . Pixel array  22  is typically composed of rows and columns of electrically conductive pathways. At the intersection of a row and a column of the electrically conductive pathways is a pixel. Each pixel can be turned on individually by selecting the appropriate row and column of pixel array  22 . Selection of a pixel is controlled by control circuitry, either included within the die  20  or external to the die  20 . In both cases, external control signals may be used to control the functions of the die  20 . Bond pads  25 , are usually placed around pixel array  22 , and typically connected to the pixel array  22  to allow control of the operation of the pixel array  22 . 
     Bond pads  25  are electrically coupled to pixel array  22  by circuitry that is normally internal to the die  20 . A glass plate  30  is typically placed over die  20  and pixel array  22 , such that the glass plate  30  overhangs the die  20 , and may cover an area of die  20  that could have contained bond pads  25 . The placement of bond pads  25  are often concentrated to one or two sides of die  20 , such that the glass plate  30  does not cover the bond pads  25 . 
     Die  20  is typically mounted to a substrate  80 . Substrate  80  includes a plurality of substrate pads  85 . Bond pads  25  typically are wire bonded to the substrate pads  85  by bonding wires  90 . 
     Typically, an adhesive seal  50  is formed between glass plate  30  and die  20  and surrounding pixel array  22 . The sealed area between glass plate  30  and pixel array  22  is commonly filled with a solution of liquid crystal materials  60 . After die  20  is properly affixed with glass plate  30 , die  20  is attached and coupled to substrate  80 , and sealed. Seal  50  contains the liquid crystal material  60  from leaking. 
     In typical LCD devices seal  50  often times does not maintain its integrity, which causes deformation and irregularities in the LCD devices. The failure of seal  50  to adequately adhere glass plate  30  to die  20  may cause artifacts  70 . The artifacts may extend into the active area of the pixel array, causing visual aberrations in the final LCD device. Such artifacts render the LCD device defective. 
     FIGS. 2,  3  and  4  illustrate cross-sectional views of an LCD device  12 . The cross-sections depict a glass plate  30  bonded to a die  20  by a seal  50 . The interior is filled with liquid crystal material  60 . 
     FIG. 2 illustrates the initial shape of seal  50  immediately after glass plate  30  is attached to die  20 . Seal  50  sufficiently contains liquid crystal material  60 . During the process of handling LCD device  12  many factors contribute to the deformation of seal  50 , and LCD device  12  itself. 
     After glass plate  30  is attached to die  20  and LCD device  12  is filled with liquid crystal material  60  further processing steps require handling the LCD device  12 . For example, the LCD device  12  is placed on a substrate (such as substrate  80  of FIG.  1 ). Also, LCD device  12  is subjected to functional testing. During these processing steps, LCD device  12  may undergo temperatures and stresses that may cause seal  50  to deform. 
     Referring to FIG. 3, many seals do not maintain their integrity during handling and/or testing of the LCD device. In typical LCD devices seal  50  is tensile. That is, seal  50  has a tendency to expand. The tensile nature of  50  becomes more troublesome during the heating and handling of the LCD device. 
     As illustrated, a common problem of the tensile nature of seal  50  is the delamination of liquid crystal material  60  from glass plate  30 . Heating and/or handling of the LCD device may cause thermal excursion of the liquid crystal material. The liquid crystal material will typically tend to expand. Due to the tensile nature of seal  50 , seal  50  may not contain the expanding liquid crystal material and expand outward, as illustrated. The liquid crystal material delaminates from the glass plate because the seal does not prevent the thermal excursion of the liquid crystal material. 
     A similar problem that causes artifacts in the LCD device is the deformation of the glass plate and/or the die. FIG. 4 illustrates a LCD device  12  with a deformed glass plate  30 . Glass plate  30  (or die  20 ) may deform or bow during heating and/or handling of the LCD device. The deformation often times causes the glass plate to separate from the liquid crystal material  60 . Again, seal  50  may not prevent the problem due to its tensile nature. The delamination of the glass plate from the liquid crystal material similarly causes artifacts in the LCD device that may render the device defective. 
     Therefore, current methods of sealing a LCD device may not sufficiently prevent delamination of the glass plate from the liquid crystal material. A seal that is capable of preventing the delamination of liquid crystal material from the glass plate is desired. 
     SUMMARY OF THE INVENTION 
     The present invention provides an improved seal for sealing a liquid crystal display (LCD) device. An improved seal is formed between a transparent plate and a die having a pixel array. The improved seal is configured to encircle the pixel array of the die when the die and the transparent plate are joined. The die and the transparent plate are joined together such that the improved seal is disposed between the transparent plate and the die. 
     In one embodiment, the improved seal is a hybrid seal. The hybrid seal includes a first seal encircling the pixel array of the die and adhesively coupling the transparent plate and the die. The hybrid seal further includes a second seal encircling the second seal. 
     In another embodiment, the first seal lacks a characteristic necessary for an effective seal. The second seal possesses the characteristic, such that the hybrid seal possesses the necessary characteristic. 
     These and other advantages of the present inventions will become apparent to those skilled in the art upon a reading of the following descriptions of the invention and a study of the several figures of the drawing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a typical small scale LCD device. 
     FIG. 2 illustrates a LCD device with a seal immediately after a glass plate is attached to a die. 
     FIG. 3 illustrates the LCD device of FIG. 2 with the liquid crystal material delaminating from the glass plate. 
     FIG. 4 illustrates the LCD device of FIG. 2 with a deformed glass plate. 
     FIG. 5 illustrates a LCD device with an improved seal in accordance with an embodiment of the present invention. 
     FIG. 6 a  illustrates a LCD device with a hybrid seal in accordance with another embodiment of the present invention. 
     FIG. 6 b  illustrates the placement of a hybrid seal in accordance with an embodiment of the present invention. 
     FIG. 7 illustrates a hybrid seal in accordance with an embodiment of the present invention. 
     FIG. 8 illustrates a mounted LCD device in accordance with an embodiment of the present invention. 
     FIG. 9 illustrates a LCD device in accordance with an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention provides an improved seal for use in LCD devices. The seal more effectively resists the effects of thermal excursion of liquid crystal material within an LCD device, thereby preventing artifacts. In one embodiment, a hybrid seal is utilized to provide a compressive seal that is better able to contain the liquid crystal material. In alternate embodiments, hybrid seals are utilized to meet all the necessary requirements of an effective seal. 
     Conventional seals may exhibit tensile behavior because a single sealant material may not meet all the criteria of an adequate seal. Among the criteria, a seal should (1) not react with the liquid crystal material; (2) have a high glass transition temperature (the temperature at which the sealant becomes fluid); (3) provide an effective barrier against humidity; (4) curable at a low temperature such that heating does not cause the liquid crystal material to deteriorate; and (5) prevent the delamination of the liquid crystal material. 
     Often times a single material may not be able to meet even these limited number of criteria. A single sealant may meet one or more of the criteria, but not all the criteria. Thus, conventional sealants have typically lacked the ability to prevent the delamination of the liquid crystal material in order to meet some of the other criteria. 
     However, the present invention provides an effective seal without compromising on any of the criteria. In one embodiment of the present invention, a hybrid seal is utilized to provide an effective seal. A hybrid seal is comprised of two sealants. A first seal provides one or more of the features of an effective seal and a second seal provides any other criteria not met by the first seal. 
     FIG. 5 illustrates an LCD device  112  with an improved seal in accordance with an embodiment of the present invention. LCD device  112  includes a transparent plate  130  and a LCD die  120 . A hybrid seal  147  seals the perimeters of transparent plate  130  and die  120 , enclosing liquid crystal material  160 . 
     Hybrid seal  147  includes a first seal  150  and a second seal  155 . In one embodiment, the first seal  150  is a conventional type of seal that is not reactive with the liquid crystal material and provides an effective barrier against humidity. However, the first seal may also exhibit tensile behavior during heating and/or handling. Second seal  155  provides a compressive seal that has a tendency to compress the first seal  150  when heated, to prevent the first seal from expanding. 
     The first seal  150  is placed immediately surrounding the liquid crystal material. The second seal  155  provides an additional seal in order to remedy any deficiencies of the first seal. Since the second seal does not contact the liquid crystal material the second seal need not be unreactive with the liquid crystal material. The exclusion of that criteria creates more alternatives for the selection of the material of the second seal. 
     In the illustrated embodiment, the second seal provides a compressive seal that maintains the integrity of the hybrid seal. During heating and/or handling the second seal prevents the first seal from expanding. In turn, the liquid crystal material is prevented from delaminating from the transparent plate through thermal excursion. 
     Also, the second seal helps to prevent the transparent plate and/or the die from bowing or deforming. Since the second seal is compressive, the second seal exerts greater adhesive forces on the transparent plate and the die. The adhering force helps to prevent bowing and deformation. 
     Thus, a hybrid seal may be used to provide an effective seal that meets all the necessary criteria. The use of different sealants provides greater flexibility in the selection of the materials of the sealants. No one seal need meet all the necessary criteria since any deficiency may be remedied by the other seal. 
     While the illustrated embodiment focuses on the tensile/compressive aspect of a seal, the present invention applies to the other criteria of a seal. By way of example, one seal may react with the liquid crystal material, have a low glass transition temperature or ineffective against humidity. The other seal may adequately provide the needed attribute. Among some of the criteria, desireable aspects of the hybrid seal includes in one embodiment, a low curing temperature of about less than 70° C., and a glass transition temperature greater than about 70. 
     Of course, the placement of the seal also dictates which one of the criteria the seals may lack. For example, the first (inner) seal should always be non-reactive with the liquid crystal material since the first seal directly contacts the liquid crystal material. 
     Ideally both seals should have a low curing temperature. However, the second (outer) seal may have a higher curing temperature than the first seal since the second seal is not located directly adjacent to the liquid crystal material. Further, the present invention is not limited to just two seals. Any number of seals may be used to provide a hybrid or composite seal that meets all the necessary requirements of a seal. Thus, the outermost seals, furthest from the liquid material, may have significantly higher curing temperatures. 
     FIG. 5 illustrates an embodiment of the present invention with a hybrid seal placed at the very periphery of transparent plate  130 . However, the hybrid seal may be placed at any suitable location around the pixel array of LCD die  120 . 
     FIG. 6 a  illustrates a LCD device with a hybrid seal in accordance with another embodiment of the present invention. The LCD device includes a hybrid seal  147  that is placed within the circumference of transparent plate  130 . 
     FIG. 6 b  illustrates the placement of a hybrid seal in accordance with an embodiment of the present invention. In one embodiment, second seal  155  is placed directly surrounding first seal  150  within the perimeter of the transparent plate  130  or die  120  before the transparent plate and die are attached together. In alternate embodiments, the seals need not be physically in contact with each other upon application. The seals may be comprised of epoxy-like materials that expand when the transparent plate is attached to the die. 
     Seals  150  and  155  may be comprised of any suitable types of material so long as the combination of the seals provides an effective seal within an LCD device. By way of example, Three Bond 51, 96, 98 and 184 by the Three Bond Corporation; Norland 68 by the Norland Corporation; Silicone E 17303 by the General Electric Corporation; and Loctite 395 by Loctite, Inc., may be utilized in accordance with the present invention. 
     Combinations of the sealants may be used to provide a more effective seal than only using a single material. In one embodiment, a first seal comprised of Three Bond 51 and a second seal comprised of Three Bond 184 provides a superior seal. 
     The present invention may be used in combination with a variety of suitable liquid crystal materials. By way of example, polymer dispersed liquid crystals, twisted-newmatic liquid crystals or ferroelectric. 
     However, the present invention is not limited to amorphous materials for use as a sealant. FIG. 7 illustrates a hybrid seal  147 Ã in accordance with another embodiment of the present invention. Hybrid seal  147 Ã includes a first seal  150  that is comprised of conventional sealant materials, such as those listed above. Second seal  155 , on the other hand, formed from fusible material, such as a solder. Fusible seals are discussed in further detail in U.S. patent application Ser. No. 09/056,165, entitled “FUSIBLE SEAL FOR LCD DEVICES AND METHODS FOR MAKING SAME” by Ranjan J. Mathew and Hem P. Takiar, filed Apr. 6, 1998, which is incorporated herein by reference. 
     The fusible seal  155  provides even further structural stability to the LCD device. The structural characteristics of the fusible seal prevents any deformation of the first seal, thus preventing delamination of the liquid crystal material. Further, fusible seal  155  provides a good barrier against humidity and has a high glass transition temperature. While a fusible seal may require a higher curing temperature, localized fusing of the fusible seal, as discussed in the above-referenced application, may be utilized. Localized heating may also be used in conjunction with more conventional sealant materials. 
     After the transparent plate and the die are properly sealed with a hybrid seal, the LCD device may be mounted on a substrate. FIG. 8 illustrates a mounted LCD device  110  in accordance with an embodiment of the present invention. Mounted LCD device  110  includes the LCD device  112  of FIG. 5 attached to a substrate  180 . 
     The effects of heating and warping may be further alleviated by processing the LCD device after being mounted on a substrate. Any suitable substrate material may be utilized in accordance with the present invention. By way of example, aluminum, silicon, alumina (ceramic) and aluminum nitride may be used. In one embodiment an aluminum substrate adds further stability to hybrid seal  147  during heating and/or handling. 
     In another embodiment a cavity is formed within the substrate. The cavity helps to provide greater stability and prevents warping of the LCD device. The use of substrates and substrates with cavities are discussed in further detail in U.S. patent application Ser. No. 09/130,631, entitled “LIQUID CRYSTAL DISPLAY ASSEMBLY AND METHOD FOR REDUCING RESIDUAL STRESSES” by Ranjan J. Mathew and Seshadri Vikram, filed Aug. 6, 1998, which is incorporated herein by reference. 
     In addition to placing the LCD device on a substrate, another seal may be applied to the LCD device. In conventional LCD devices a glob coating is placed around the transparent plate and over the LCD die and substrate. The glob coating also covers the bond pads of the LCD die and the substrate pads of the substrate. The glob coating is typically applied after all functional testing and handling has been completed since the glob coating prevents such activities. 
     Rather than placing a glob coating over the entire LCD device and substrate without covering the transparent plate, another seal may be formed around the periphery of the transparent plate. The extra seal may be placed about the periphery of the transparent plate without interfering with any of the functional elements of the LCD device, such as the bond and substrate pads. 
     FIG. 9 illustrates a LCD device in accordance with an embodiment of the present invention. The LCD device includes a hybrid seal  147 ÃÃ, which has a first seal  150 , a second seal  155  and a third seal  165 . The third seal may be comprised of typical sealant materials, as discussed above, or materials typically used for glob coating. By way of example, third seal may be comprised of ultraviolet curable and thermally curable silicones or epoxies. The third seal adds further structural stability to the hybrid seal. The third seal also prevents significant warping of the transparent plate and the LCD die. 
     Thus, a hybrid seal may be utilized to remedy many deficiencies in conventional single material seals used in LCD devices. Hybrid seals provide added flexibility in the selection and use of existing materials rather than spending additional time and money to produce a single material sealant that meets all the requirements of an effective seal. Hybrid seals can meet all the requirements without extra cost in terms of development. 
     While this invention has been described in terms of several preferred embodiments, it is contemplated that alternatives, modifications, permutations and equivalents thereof will become apparent to those skilled in the art upon a reading of the specification and study of the drawings. It is therefore intended that the following appended claims include all such alternatives, modifications, permutations and equivalents as fall within the true spirit and scope of the present invention.