Adhesive bonding and miscellaneous chemical manufacture – Methods – Surface bonding and/or assembly therefor
Reexamination Certificate
1999-04-16
2001-11-06
Crispino, Richard (Department: 1734)
Adhesive bonding and miscellaneous chemical manufacture
Methods
Surface bonding and/or assembly therefor
C156S252000, C156S240000, C156S241000, C156S247000, C156S289000, C349S001000, C349S092000, C349S155000, C349S187000, C428S195100, C427S163100, C427S163300, C427S058000, C427S108000
Reexamination Certificate
active
06312546
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
This invention relates to methods of manufacturing liquid crystal devices, and is concerned more particularly with the spacing apart of substrates in such a device. Although not limited to such an application, the invention is particularly applicable to the manufacture of ferroelectric liquid crystal devices.
DISCUSSION OF THE RELATED ART
The surface stabilised ferroelectric liquid crystal device (SSFLCD) possesses the advantage over other liquid crystal devices, such as the twisted nematic liquid crystal device, that it is a bistable device which can be switched between two states by switching pulses of alternate polarity and which will remain in one state in the absence of a switching pulse until a switching pulse of opposite polarity is applied to switch it to the opposite state. By contrast, in operation of a twisted nematic liquid crystal device, a drive signal must be applied continuously to maintain the device in one of its states. It is also a particular advantage of SSFLCD's that the individual switching elements can be passively addressed without requiring an active matrix in which a respective switching transistor is associated with each switching element. By contrast the addressing of conventional twisted nematic devices requires an active matrix in which a respective switching transistor, generally a thin film transistor (TTL), is associated with each switching element, and the fabrication of such an active matrix utilising thin film transistors requires a large number of fabrication steps which increase manufacturing complexity and cost.
As is well known, a conventional SSFLCD typically includes a cell in which a layer of chiral smectic ferroelectric liquid crystal material is contained between two parallel transparent substrates provided on their inside surfaces with electrode structures in the form of row and column electrode tracks which cross one another to form an addressable matrix of switching elements. Such switching elements are typically addressed on a line-by-line basis by applying data pulses in parallel to the column electrode tracks, each data pulse being either a switching pulse or a non-switching pulse, and by applying strobe pulses to the row electrode tracks so as to switch selected switching elements along each row from one state to the other under the effect of the electric field produced by the voltage difference between the data pulse and the strobe pulse applied to the relevant electrode tracks.
Furthermore rubbed alignment layers are provided on the inside faces of the substrates so as to overlie the electrode structures in order to impart a preferred alignment to the molecules of the liquid crystal material in the vicinity of the alignment layers. This ensures that the liquid crystal molecules, when in the chiral tilted smectic phase, are uniformly aligned in microlayers extending perpendicularly to the substrates with the molecules in each microlayer adopting a chevron geometry due to the alignment of the molecules with the alignment layers on both sides of the liquid crystal layer.
In order to ensure that the substrates are accurately aligned in parallel and with the required gap therebetween for containing the liquid crystal material, spacers are provided between the substrates so as to space the substrates apart, and in addition sealing strips are provided at the edges of the substrates in order to connect the substrates together and contain the liquid crystal material in the space between the substrates. The spacers may be in the form of glass or plastic beads which are distributed over the surface of one of the substrates so as to be held between the two substrates when the substrates are pressed together and sealed at their edges. However, since the spacer beads merely physically separate the two substrates rather than being connected to the substrates, they do not prevent some movement apart of the substrates in localised areas as a result of applied mechanical stress, and this can result in degradation of the performance of the device. A typical production process is described in J. Varney, “Automated Assembly of LCD's using UV Adhesives”, Japan Display '89, Oct. 16-18, 1989, Kyoto, Japan.
U.S. Patent Publication No. 5,710,097 discloses a method of applying spacers to a substrate in order to improve adhesion to the substrate surface. This method involves bringing a plastic laminate incorporating a spacer film into contact with the substrate surface, and then defining a pattern using a laser to cause local heating to transfer a spacer layer to the substrate only in those areas in which it is required. U.S. Patent Publication No. 5,593,802 discloses a method of forming spacers on a substrate surface by selectively exposing a laminate applied to the surface to ultraviolet light through a mask, the resulting exposed pattern being developed to leave spacers only where required. German Patent Publication No. 19649411A1 discloses a method of applying spacers to a substrate surface using gravity or static electricity to position the spacers in indentations on a master plate prior to depositing the spacers on the substrate surface by polarity reversal. However there are production difficulties in use of all these methods.
It is also known to use so-called active spacers which are in the form of glass or plastic beads coated with a material, or themselves formed of a material, which is intended to melt in response to applied heat. Such active spacers are introduced between the substrates in the manner already described, but an additional heating step is performed after the substrates have been assembled together with the spacers therebetween so as to cause the outer coatings of the spacers to melt and to subsequently set in such a manner as to physically bond the spacers to the two substrates. S. Kasahara et al., Fujitsu Sci. Tech. J., 30,2, pp.148-153 (December 1994) describes the use of active spacers in the form of low melting temperature lead-tin-fluorophosophate glass beads in association with conventional hard spacers, such as silicon dioxide beads. The soft beads are reduced to the size of the largest hard beads when sandwiched between the substrates and heated to an elevated temperature, and, on subsequent hardening when the temperature is reduced, serve to considerably strengthen the cell.
It is also known to provide spacer walls between the substrates at a suitable pitch and running substantially the whole length of the cell. Such walls not only space the substrates apart but also control the flow of liquid crystal material within the cell. These spacer walls are conventionally produced by depositing a layer of photosensitive polyimide on one of the substrates and masking and etching selective parts of the layer so as to leave material only where required to form the spacer walls, prior to assembly together of the two substrates (possibly after deposition of a suitable adhesive layer therebetween). However, since such formation of the spacer walls would affect the corresponding alignment layer, it is necessary either to apply the alignment layer after the spacer walls have been formed or alternatively to effect rubbing of the alignment layer after the spacers have been formed on top of the alignment layer. In either case, however, the formation of such walls involves additional fabrication steps, and in addition the walls may adversely affect the switching characteristics of the device.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a liquid crystal device manufacturing method which ensures that an assembly of two substrates spaced apart by spacers is produced in a straightforward manner.
According to the present invention there is provided a method of manufacturing a liquid crystal device having two substrates spaced apart by spacers, the method including the steps of applying the spacers to a transfer member such that the spacers are received on raised portions of the transfer member whilst not being received on intermediate recessed portions of the transfer
Bannister Robert William
Heath Ryan Michael
Crispino Richard
Lorengo J. A.
Renner , Otto, Boisselle & Sklar, LLP
Sharp Kabushiki Kaisha
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