Abstract:
An improved liquid crystal filling device is shown. Prior to joining a substrate with another substrate between which the liquid crystal is to be charged, the liquid crystal is dropped on the substrate and then the other substrate is superimposed on the substrate under pressure. Sandwiched between the substrates, the liquid crystal spreads at high temperature.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to an improved manufacturing process for liquid crystal device, more paticular to liquid crystal filling device. 
     2. Prior Art of the Invention 
     There are well known many kinds of compact display in which liquid crystals are made of use. In such a device, liquid crystal is charged in the very thin cavity formed between opposed substrates. One of the substrates is made light transmissive and the other is made reflective so that opitical characteristics as seen from the transmissive side can be changed by controlling the electric field applied on the liquid crystal between the substrates. Contiguous to the liquid crystal and on each substrate, a pair of electrodes are formed with the liquid crystal in between. At least one of the electrode is separated into a large number of parts with which the liquid crystal constitutes A4 size display having 400×200 picture elements in cooperation with polarizing plate. Liquid crystal displays of this kind have been used broadly in microcomputors, ward processers, televisions and the like in order to cope with demands to make the device thin. 
     Out of processing steps to manufacture the display, charging step for filling the thin cavity without involving bubbles requires dexterity. Prior art charging steps are carried out as follow. Namely, the pair of substrates is laminated with spacers in between and the periphery of the pair is sealed off by adhesive except for an access to the interior of the thin cavity thus formed between the substrates. Then the access of the substrates are dipped in liquid crystal in a vacuum chamber having been evacuated sufficiently. Thereafter, the pressure in the vacuum chamber is increased to the atomospheric pressure whereupon the liquid crystal is injected into the cavity of about 5 to 10 thick in virtue of capillary effect. There are shortcomings, however, in the above process in respect of the following items. 
     (i) Workers must have skill and dexterity to carry out the charging process because of high viscosity of liquid crystal, especially of (chiral) smectic which is broadly used in this field. 
     (ii) It takes substantial time to fill the cavity with liquid crystal, paticularlly when one desires filling of a very thin cavity less than 4 micron thick such as ferroelectric liquid crystal (FLC) forming memory media of 0.5 to 3 micron. 
     (iii) Because of high temperature during the filling process (for instance, it takes 8 to 10 hours at 120° C. to charge FLC to the cavity of A4 size), the sealing structure is likely to be degraded and enter into liquid crystal as an impuruty. 
     (iv) The spacers tend to move during injection of the liquid crystal and flock apart from the access. 
     (v) It is wastful that a large part (approx. 90%) of the expensive liquid remains uncharged out of the cavity in vain. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide an improved liquid crystal filling device having high reliability. 
     It is another object of the invention to provide an improved liquid crystal filling device capable of forming a liquid crystal layer of uniform thickness. 
     It is further object of the invention to provide an improved filling device capable of the fast charging of liquid crystal. 
     It is further object of the invention is to provide an economic filling device in which an amount of liquid crystal is efficiently utilized. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1 (A) through 1(E) explanatory views showing manufacturing process according to the invention. 
    
    
     BRIEF DESCRIPTION OF THE INVENTION 
     In view of the conventional shortcomings, the inventors employ the filling method of liquid crystal in which an amount of the liquid crystal is disposed on one of substrates prior to joining the substrates to constitute a cavity in between and thereafter the liquid crystal is let spread by being sandwiched between the substrates under pressure. According to this filling process, all the parts of the liquid crystal are disposed within the cavity between the substrates. For instance, 0.2 cc of liquid crystal is sufficient to prepare a A4 size display (20 cm×30 cm). This is very economic. Further such a process requires processing time only about one hour to complete the filling. 
     So far, it has been believed as an important matter to carry out a filling process without applying any stress to the liquid crystal layer. Because of this, peripheral sealing means is functioning also as a strut protecting the liquid crystal from being subjected to stress. 
     The inventors have confirmed that a liquid crystal layer such as of smectic phase is less influenced by stress applied thereon because of high viscosity of the liquid crystal and not degraded even after undergoing rough treatment such as of the invention. 
     According to the invention, since filling of liquid crystal is made before completion of joining of substrates and sealing, orientation of a liquid crystal layer can be accomplished by slightly offsetting one of the substrates in regard to the other by 1 to 10 4  micron meters at somewhat high temperature. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIGS. 1(A) through 1(E), a liquid crystal filling device and process according to the invention are shown. 
     In FIG. 1(A), a pair of substrates is with opposed insides thereof on which are provided with paterned electrode for defining picture elements. The substrates 1 are made of glass pane such as of coning 7059. If color display is desired, color filter may be disposed between the electrode and the substrate or between the electrode and the liquid crystal layer. A contiguous surfice of the electrode to liquid crystal is made an orientation treated surfice. Although, in the figures, only a pair of substrates is shown, descriptions are dispensed with for the electrodes, filters, oriented films, shadow masks, active elements or so on, fearing confusion between element in the view. 
     The electrode of the lower substrate 1 is oriented at its upper surfice. On the oriented surface is placed an amount of liquid crystal 2 such as S8 (octyl-oxy-benzylidene-amino-methylebuthyl-benzoate) at one or more spot. In addition, on a number of spots in diverse positions of the periphry of the upper substrate 1 are put amounts of thrmosetting resin, e.g., epoxy resin especially at the corner portion. 
     The substrates 1 are carried in a vacuum chamber 100 comprising an upper member 10&#39; and a lower member 10 and separated into first and second spaces 4 and 5 by a silicon rubber sheet 6. The two substrates are clamped togather in air tight manner. After removing the upper member 10&#39; from the lower member 10, the lower substrate 1 is placed on a heater provided in the first chamber 4. Then, the temperature of the substrate 1 is raised to a prescribed temperature (e.g., 120° C.) chosen between room temperature and 150° C. at which the liquid crystal has sufficiently low viscosity, normally higher than 70° C., so that the amount of the liquid crystal can be extended. A number of spacer may be placed apart from each other at an appropreate step of above. 
     With the upper substrate 1 held above the lower substrate 1 at a suitable distance, e.g., 1 mm to 10 mm, the first member 10 is fitted with the second member 10&#39;. The contact between the first and second member 10 and 10&#39; is sealed off by a O-ring. The first and second spaces 4 and 5 are entirely separated in air-tight manner by the flexible sheet 6. 
     Thereafter, both the first and second spaces 4 and 5 are evacuated simultaneously by a vacuum pump 14 with valves 12 and 12&#39; opened and valves 13 and 13&#39; closed. Upon superposing the upper substrate 1 on the second substrate 1 with confirming coincidence of the pair of substrates 1, the pressure of the second space 5 is let increase gradually to the atmospheric pressure by leak through the valve 13&#39;. By virtue of differential pressure between the first and second spaces, the silicon rubber 6 extends downward and suppresses the upper substrate 1 to the lower substrate 1 under the atomospheric pressure of 1 kg/cm 2 . If necessary, further pressure may be added by introducing pressurized nitrogen. 
     In this manner, the amount of liquid crystal on the substrate 1 is extended under the uniform pressure between the subatrates and constitutes a laminate structure with the pair of the substrates 1. In the same manner, the sealing resin is elongated at the corner region of the substrate 1 so that the substrates 1 are joined with an area of 1 to 15 mm 2  surrounded by the elongated resin. 
     To avoid overextention of the liquid crystal and contact of the liquid crystal with the sealing resin, a fiber barrier 18 of 1 to 3 micron may be placed around the liquid crystal, especially at the corner. 
     In case where the spacers are employed, the gap between the substrates can be precisely controlled by choosing the diameter of the spacers. The diameter of the spacer exactly corresponding to the gap between the substrates 1. 
     In case where the spacers are not used, the gap between the substrates 1 can be adjusted by precisely controlling the differential pressure between the first snd second spaces 4 and 5 and temperature of the substrate 1. 
     The amount of the liquid crystal has to be suitably prescribed to avoid falling short of prevailing throughout the desired area while overflow is avoided by virtue of the barrior and the sealing. 
     After completion of the filling process, the heater 3 is turned off and the pressure of the first space 4 is increased to the atomospheric pressure. Then, the second member 10&#39; is removed from the first member 10 and the laminated substrates 1 thus processed are taken out. For security, the periphery of the substrate may be covered with prastic sealing material 9 as shown FIG. 1(E). According to the invention, filling process can be completed within only about one hour far short compared to the prior art process which takes several hours to complete a filling. 
     It will be obvious that the device may be readly modified without departing from the spirit of the invention as described below. 
     In place of the hard glass substrate, a flexible substrate can be used, e.g., glass sheet of 0.3 to 0.6 mm thickness cured with chemical reinforcing treatment, a transmissive heat-proof organic resin sheet such as polyimide, PAN, PET, or the like. 
     The step of holding the upper member 10&#39; over and apart from the lower member 10 prior to applying pressure is not indispensible. In the step, the upper member 10&#39; can be placed on the lower member making partial contact in stead. 
     As a partition member between the first and second spaces, any flexible sheet can be employed instead of the silicon rubber sheet 8 as long as it has heat resistance at least against 500° C.