A PROCESS FOR THE FABRICATION OF A LIQUID CRYSTAL DISPLAY DEVICE

Abstract: A process is described for the fabrication of a liquid crystal display device (10) comprising a cell formed from a pair of plates (12, 14) facing one another and between which the liquid crystal is disposed, a plurality of electrodes formed on said plates, and a plurality of control conductive paths (16) for controlling the electrodes, formed substantially outside the cell on at least one plate (12) and connectable to the control circuit (20) for the display device (10). The process according to the invention comprises the operation of applying by printing a protective anti-corrosion coating layer (40) on the plate (12) carrying the control conductive paths (16) in a predetermined configuration such as to cover at least these conductive paths (16). This operation is performed after formation of the electrodes and the control conductive paths (16) on the associated plates (12, 14) and before formation of the cell by coupling the plates (12, 14) and introduction of the liquid crystal.

Description: A process for the fabrication of a liquid crystal display device The present invention relates to a process for the fabrication of a liquid crystal display device according to the preamble of Claim 1. A liquid crystal display device (more commonly known by the acronym LCD) is formed from two glass plates facing one another and separated by a gap within which a liquid crystal substance is contained. On the inner surface of the glass plates a transparent conductive layer is deposited, typically ln 2 0 3 and/or Sn0 2# constituting the electrodes and configured in such a way as to forπr the symbols which it is intended to display (alphanumeric characters, graphic points, icons) . The resultant heterogeneous structure in which portions of glass substrate alternate with electrode portions made of conductive material is covered by a dielectric coating layer (Si0 2 ) and by a further alignment layer. The alignment layer, generally of polyimide resin, is in contact with the liquid crystal substance and contributes to promote an univocal orientation of the molecules over the entire contact surface . On the outer surface of the glass plates is disposed a respective polarizing layer orientated in such way that the directions of polarization of the two layers are orthogonal to one another (in the case of liquid crystals of the twisted nematic type) , or at other angle (for applications of the super twisted nematic type) . As is known in the art, reflective or backlighted display devices can be fabricated and, depending on the arrangement adopted, it is envisaged that in correspondence with the rear plate of the device, there will be a further reflecting layer or a convenient illumination device respectively. A display device thus formed is coupled to a control system via a connection portion formed on one of the glass plates in an area projecting with respect to the facing plate, and comprising a plurality of control conductive paths which can be connected to the. electrode portions . In a first embodiment an integrated microcircuit is mounted directly on the plate carrying the connection portions and acts as a processor for the control instructions coming from the external systems such as an electrode control circuit for the device (display device of the COG, i.e. chip-on-glass type) . To the microcircuit are connected the control conductive paths and a series of connection terminals intended for connection to an external control system. In an alternative embodiment the location of a control circuit directly on the plate is not envisaged, and therefore the connection portion comprises exclusively the electrode control conductive paths, directly coupled to the connection terminals for connection to the external control system. The fabrication process of a liquid crystal display device comprises a first series of overall operations performed on a pair of glass sheets on which a plurality of devices is defined (of the order of a hundred for each pair of sheets with current technology) , and a second series of operations performed after coupling of the original sheets in facing position and cutting or separation of the cells constituting the individual devices . The first series of operation comprise, in succession, the phases of washing and sterilization of the glass sheets, the formation of the electrode portions and connection portions using conventional techniques of deposition, photolithography and etching, the deposition of the dielectric coating layer covering the electrode portions, the printing of the alignment layer and its subsequent treatment (abrasion) in correspondence with the surface in contact with the liquid crystal . To allow coupling of the glass sheets in such a way as to form a gap in which to introduce the liquid crystal, while nevertheless guaranteeing the electrical connection between the plates which combine to form each display device, spacer elements formed by an adhesive substance in which are embedded microspheres of conductive material are conveniently deposited by silk screen printing on one of the sheets . Then the glass sheets are connected together, followed by the cutting operations for separation of the cells constituting the individual devices . When separation of the cells has taken place, each of these is filled with the liquid crystal substance and subsequently sealed, then the polarizing layers are applied to the outer faces of the device in contact with the glass plates and, if it is intended that the final product should be of the COG type, the integrated microcircuit is assembled in correspondence with the connection portion of the associated plate. At the end of this series of operations, and before final packaging, there is again performed an operation of covering and sealing of the control conductive paths exposed outside the cell to ensure a protective anti-corrosion coating. The function of the protective coating is to avoid any corrosion process on the electrode control conductive paths, the effects of which are often the cause of irreversible damage on the final . product . Contaminating agents can in fact reach the conductive paths during the product packaging phases or in everyday use since the device, depending on the specific applications, can be exposed to temperature variation, humidity, organic contaminating impurities (fine dust, sweat, fingerprints, etc) . In the prior art the protective anti-corrosion coating is constituted by a thick layer of silicone substance applied by means of an automatic delivery system which required manual labour for the preliminary masking of the areas which must remain accessible once the sealing is completed (essentially the connection terminals to the external control system) and in the application of the substance itself. In particular the repetition of the said operations for each individual device derived from the pair of original sheets is onerous. A further disadvantage is the prolonged waiting times for polarization of this substance, which increase the time taken for the fabrication process of the final product. A further disadvantage of the known process is that the particular coating technique used involves the application of the substance in quantities which it is difficult to regulate and this creates mechanical interference with the packaging wrapper of the final product or requires large tolerances for coupling between the parts, which counteracts any advantageous reduction in the dimensions of the final product intended to receive such a display device . The object of the present invention is to provide a process for the fabrication of liquid crystal display devices such as to obviate the previously explained problems, avoiding the disadvantages of the prior art . In particular, the object of the invention is to provide a fabrication process able to allow the application of a protective coating of reduced dimensions such as not to affect the dimensions of the products intended to receive the display device thus fabricated. A further object of the invention is to provide a fabrication process in which the application of a protective coating to the control conductive paths can be performed automatically in parallel on a plurality of devices obtainable from a single pair of original glass sheets, and with a reduced use of manual labour. According to the present invention these objects are achieved by a process for the fabrication of liquid crystal display devices having the characteristics set out in Claim 1. The process according to the invention is characterized by the fact that the application of the protective anti- corrosion coating takes place by transfer of a layer of resin, from a dedicated printing die (mask) on which is set out the specific configuration of the areas to be covered, to the original glass sheet carrying the control conductive paths of a plurality of devices. More specifically, this operation consists in a printing process in which the transfer of the protective resin takes place by means of a print drum, which has an associated mask of photopolymeric resin which defines the areas to be covered. The application of the protective coating by printing is performed in parallel on a plurality of devices formed on the original sheet of glass, before separation into individual cells. The process according to the invention has the advantage of a greater precision in the covering and protection of the areas of the plate on which the control conductive paths are located, and in particular makes it possible to define a priori the configuration of this covering in such a way as to leave free areas carrying alignment marks or symbols essential for assembly of the integrated controlling microcircuit according to COG techniques, such as the connection terminals for connection to the external control system. Replacing the operation of application of the protective substance by the delivery system according to the current art - and the necessary laborious operation for masking the connection terminals - with the application procedure by printing according to the invention, performed in parallel on a plurality of devices, advantageously obtains a significant reduction in the cost of labour and process time. Moreover, the display devices thus fabricated advantageously have reduced mechanical dimensions. In fact, the printing process of the invention allows the application of a protective coating the thickness of which is about three orders of magnitude less than the thickness of the layer deposited according to the prior art, and therefore this layer no longer constitutes a factor influencing the dimensions of the devices thus fabricated, and consequently, the consumer products intended to include them. Further characteristics and advantages of the invention will be explained in more detail in the following detailed description of an embodiment given by way of non-limitative example with reference to the attached drawings, in which: Figure 1 is a perspective view of a liquid crystal display device without the protective anti-corrosion layer; Figure 2 is a perspective view of a liquid crystal display device fabricated with the process of the prior art; Figure 3 is a sectional view taken on the line III-III of Figure 2; Figure 4 is a perspective view of a liquid crystal display device fabricated using the process according to the invention; Figure 5 is a sectional view taken on the line V-V of Figure 4; Figure 6 is a schematic representation of the print system used in the process according to the invention; and Figure 7 is a schematic representation of a print die (mask) of the system of Figure 6. With reference to Figure 1 there is schematically shown a liquid crystal display device 10 in which the main components are shown. Such a device (hereinafter more briefly indicated with the acronym LCD) is formed from two plates of glass 12, 14 facing one another and separated by a gap in which a liquid crystal substance (not shown) is disposed. One of the two plates (in the drawing the rear plate 12) has an area projecting with respect to the opposite plate and includes a plurality of control conductive paths 16 connected to the electrode portions present on the inner surfaces of the plates 12, 14, directly or via spacer elements disposed between the plates (indicated with the reference 18 and visible in Figures 3 and 5) in which are embedded microspheres of conductive material, respectively. An integrated microcircuit 20 is mounted directly on the plate 12 and is connected to the control conductive paths 16. Alignment marks 22 are provided for guiding the assembly of the microcircuit 20 on the plate 12 to obtain a correct location thereof with respect to the paths 16. A series of connection terminals 24 is coupled to the microcircuit 20 and face outwardly of the device 10 for connection with external control systems (not shown) . In Figures 2 and 3 is shown, for comparison, an LCD 10 of the above-described type fabricated using the prior art technique set out in the introductory part of this specification. In both Figures can be seen the layer 30 of silicone substance constituting the protective anti-corrosion coating applied coarsely via an automatic delivery system. The deposition of the layer with the most precise technique available does not, however, enable to obtain a thickness less than 0.5-0.6 mm. The fabrication process according to the invention is described hereinafter and the device thus obtained is illustrated in Figures 4 and 5. A pair of glass sheets arranged for the production of a plurality of devices is preliminarily washed and sterilized i a clean room, and on one surface of each of these are defined corresponding electrode portions, the control conductive paths 16 and the connection terminals 24 through conventional techniques of deposition, photolithography and etching. The electrodes are configured in such a way as to form the symbols which it is intended to display (alphanumeric characters, graphic points, icons) as matrices of rows and columns or in the form of the desired characters on a first sheet and respective ground planes on the second sheet . Then the dielectric coating layer for separation from the liquid crystal is applied onto the glass substrate portions and onto the electrode portions . All this is performed according to criteria widely known in the art, which does not require to be described in this document, also because it is, in itself, not relevant for the purposes of understanding the invention. Thereafter, a layer of cross-linked resin as an anti- corrosion protective coating is applied by printing on predetermined areas of the original glass sheet (indicated 50 in Figure 6) carrying the control conductive paths 16 of the plurality of devices under formation. This occurs in a completely automatic manner on the production line in a clean room after a preliminary accurate washing and sterilization phase of the glass sheets with a solution of NaOH, and exposure to ultrasound, infrared rays and ultraviolet rays . In this way the protective layer is applied to a completely aseptic surface and without the intervention of external operators, avoiding any contamination of the electrode portions which are covered. With reference to Figures 6 and 7, the transfer of the protective coating substance from a delivery device 52 to the glass sheet 50 is effected by means of an intermediate drum member 54 with which is associated a dedicated mask 56 made of photopolymeric resin on which is formed the specific configuration of the areas to be covered (illustrated only partially in Figure 7) . The protective resin is supplied from the delivery device 52 in a rough (irregular) form to a series of distribution members, for example a pair of rollers 60, provided to form by spreading a layer of uniform thickness suitable to be transferred to a print die. The print die comprises the print drum 54 with which is associated the photopolymeric resin mask 56 made in the form of a flexible sheet wound at least partially around the drum, on which are formed (in the example described, in relief of about 0.8 mm) the print shapes 62 corresponding (in positive) to the predetermined configuration of the areas to be covered. The drum 54 is driven to rotate and itself drives the pair of counter-rotating rollers 60 in such a way as to obtain a uniform distribution of the resin and its transfer onto the mask 56. Printing onto the sheet 50 is obtained by making this slide tangentially with respect to the drum 54, for example on a motorized table which is caused to slide in the indicated direction. Thanks to the mask the areas of the sheet on which the control paths 16 lie are covered with extreme precision, leaving exposed the terminals of these paths intended for connection with the microcircuit 20, the alignment marks 22 for mounting of the microcircuit 20, and the portion where the external connection terminals 24 are located. The protective layer thus printed is indicated in Figures 4 and 5 with the reference 40 and has a thickness of about 0.1- 0.2 micron. After the application of the protective coating has taken place, printing of the alignment layer and its consequent treatment by abrasion are performed and, subsequently, coupling of the glass sheets conveniently spaced and the separation of the cells constituting the individual devices are effected. Then the operations of introduction of the liquid crystal, application of the polarizing layers, assembly of the integrated microcircuit and final packaging of the product are repeated on each cell according to known criteria already explained in the introductory part of this specification. In a preferred arrangement the technique advantageously allows the protective coating to extend partially towards the interior of the cell, in correspondence with the linking region of the plates, by choosing the most convenient configuration of the areas upon definition of the mask (the extension of the layer 40 beyond the spacer element 18 is shown in Figure 5) . Therefore, moisture, washing residues and other contaminating materials which tend by capillary action to become trapped in this area during subsequent manipulations in fabrication do not come into contact with the electrodes of the LCD. Display devices fabricated according to the invention lend themselves advantageously to be used in a wide range of consumer products (see, for example, the mobile telephone field) of ever smaller dimensions and intended for applications subject to attack by impurities, without being subject to irreversible damage by the corrosion of electrodes which can occur in particular conditions of use. Naturally, the principle of the invention remaining the same, the embodiments and details of construction can be widely varied with respect to what has been described and illustrated purely by way on non-limitative example, without by this departing from the scope of protection of the present invention as defined by the attached Claims .

Claims: CLAIMS 1. Process for the fabrication of a liquid crystal display device (10) comprising a pair of plates (12, 14) facing one another and coupled in such a way as to form a cell adapted to receive the liquid crystal; a plurality of electrodes formed on the said plates; and a plurality of control conductive paths (16) for controlling the electrodes, formed on at least one plate (12) and connectable to a control circuit (20) of the display device (10) , in which said plurality of conductive paths (16) is substantially external of the cell and provided with a protective coating (30; 40) , the process being characterized in that it includes the operations of: provision of the plates (12, 14); formation of the electrodes on each plate (12, 14) , and the control conductive paths (16) on at least one of the plates, in respective predetermined configurations; application by printing of a layer of protective coating (40) on the plate (12) carrying the control conductive paths (16) , in a predetermined configuration such as to cover at least said conductive paths (16) ; and subsequently coupling the plates (12, 14) in a facing position and introduction of the liquid crystal into the cell formed by the coupled plates . 2. A process according to Claim 1, characterized in that the operation of printing the protective coating layer (40)- ' is performed by transfer of a layer of protective substance from a print member (54) to the plate (12) carrying said conductive paths (16) by means of a mask (56) associated with said member (54) and defining the areas to be covered. 3. A process according to Claim 2 , characterized in that said layer of protective substance is obtained by spreading a quantity of substance preliminarily delivered in a coarse form, and is subsequently transferred to said print member (54) . 4. A process according to Claim 2, characterized in that said mask (56) is made of photopolymeric resin in the form of a flexible sheet carrying in relief a plurality of print shapes (62) corresponding to said areas to be covered. 5. A process according to any preceding claim, characterized in that the printing of the coating layer (40) is preceded by washing and sterilization of the plate (12) intended to receive said layer. 6. A process according to any preceding claim, characterized in that the protective substance constituting the coating layer (40) is a cross-linked resin. 7. A process according to any preceding claim, characterized in that the operation of printing the protective coating layer (40) is performed following deposition of a dielectric coating layer over the electrodes of said plate (12) . 8. A process according to any preceding claim, characterized in that the operation of printing the protective coating layer (40) precedes printing an alignment layer for promoting, in correspondence with the electrodes, a univocal orientation of the molecules of the liquid crystal . 9. A process according to Claim 2, characterized in that said mask is configured in such a way as to prevent printing on an area of the plate (12) in correspondence with a series of connection terminals (24) intended for connection to an external control circuit outside the display device (10) . 10. A process according to Claim 2 or Claim 9, characterized in that said mask is configured in such a way as to prevent printing on an area of the plate (12) in correspondence with a portion of said conductive paths (16) intended for connection to a control circuit (20) of the display device (10) capable of being mounted on said plate (12) . 11. A process according to any preceding claim, in which a plurality of plates (12) carrying said conductive paths is formed from a single original sheet (50) after a cutting operation, characterized in that the operation of printing the coating layer (40) is performed on said sheet (50) before the cutting operation.

Inventors: GERBAZ CORRADO, GERBAZ, CORRADO
Applicants: GERBAZ CORRADO, TECDIS S P A, GERBAZ, CORRADO, TECDIS S.P.A.
Application year: 2000
Main classification: G02F1
IPC codes: G02F1