Abstract:
An improved liquid crystal device is described. The distance between a pair of substrates is kept constant by means of spacers interposed therebetween. The spacers consist of two kinds of spacers. One kind of spacers is fusable and functions to prevent the distance from expanding. The other kind of spacers is non-fusable and functions to prevent the distance from contracting.

Description:
This is a divisional application of Ser. No. 260,605, filed Oct. 21, 1988, now U.S. Pat. No. 4,973,138. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a liquid crystal device and manufacturing method for the same. 
     In recent years, ferroelectric liquid crystal materials have attracted the interest of researchers due to their quasi-hysteresis characteristics. The performance of this type liquid crystal device depends largely on the distance between a pair of substrates in which liquid crystal material is disposed. If there is substantial disparity in distance between the substrates, the response times of respective constituent cell elements are dispersed, and degradation of display performance and irregular color result. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a liquid crystal device in which a liquid crystal layer is formed with a constant thickness. 
     It is another object of the present invention to provide a method of manufacturing liquid crystal devices without causing coming-off of spacers which function to maintain the distant between a pair of substrates constant, and without causing the occurance of void spaces in a liquid crystal layer formed between the pair of substrates. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross sectional view showing a liquid crystal device in accordance with the present invention. 
     FIG. 2 is a diagram showing the points of a liquid crystal device at which distances were measured in order to evaluate the disparity of the substrate gap of the liquid crystal device in accordance with the present invention. 
     FIG. 3 is a schematic diagram showing an apparatus for disposing a liquid crystal material between a pair of substrates of a liquid crystal device. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to FIG. 1, a liquid crystal display is illustrated in accordance with the present invention. In the figure, the display comprises a pair of 310 mm×220 mm glass substrates 1, transparent electrode arrangements 2 formed on the opposed insides of the substrates 1 by a known sputtering, orientation control films made of polyimide covering the inside surfaces of the substrates over the electrode arrangement, fusible spacers 5 and non-fusible spacers 4 both interposed between the substrates 1, a sealing member 7 for preventing the loss of liquid crystal material and a liquid crytal layer 9 made of a ferroelectric liquid crystal material disposed between the substrates 1. For operation, crossed Nicols 8 are placed on the both external sides of the displays. The electrode arrangements are provided with two sets of parallel electrode strips in an orthgonal relationship by a known photolithography. Adjacent strips are spaced by distances of 20 microns. Generally speaking, the average diameter of the fusible spacers is 3-5 times larger than that of the non-fusible spacers. 
     The non-fusible spacers 4 are made of a plurality of segments of a glass fiber. The fusible spacers 5 are made of plurality of epoxy particles of 5.5 micron diameter (Toraypearl, a trademark of spacers manufactured by Toray Co.). The spacers 4 and 5 are distributed between the substrates 1 by the following procedure. Glass fiber segments of 2.5 microns average diameter and 10 microns average length are mixed with a freon (CFCl 2 ) at 2 gram/liter and sufficiently dispersed by applying ultrasonic vibration. The dispersed fiber freon mixture is sprayed on the surface of a glass substrate. In the same way, the epoxy particles are mixed with and dispersed in freon at a proportion of 1.0 gram/liter, and distributed on the surface of the substrate. On the other hand, an opposed glass substrate is provided at the periphery thereof with a sealing member of a thermosetting resin such as an epoxy adhesive by screen press printing. The two substrates are joined and heated at 180° C. for two hours under a pressure of 2 Kg/cm 2  in order to fix the fusible spacers 5 and the sealing member 7. Then, an amount of ferroelectric liquid crystal material (CS-1011 manufactured by Tisso Co.) is interposed between the substrates by vacuum charging method. 
     By virtue of the two kinds of spacers, the distance between the substrates is maintained constant. The fusible spacers prohibit the expansion of the distance while the non-fusible spacers prohibit the contraction of the distance. In this description, &#34;non-fusible&#34; is also meant &#34;non-contracting.&#34; 
     The distributed densities of these spacers are controlled by adjusting the spraying time. Average distance (gap) in microns and standard deviation (S.D.) were measured in cases with varied densities of spacers 4 and 5. Following are experimental results. 
     
         ______________________________________  Density   Density   AverageSample (spacer 5)            (spacer 4)                      Gap (u)                             S. D. Defects______________________________________A       50-100   10-15     2.45   0.50  fewB      100-150   10-15     2.49   0.01  fewC      150-200   10-15     2.53   0.09  muchD      100-150   1-5       2.45   0.20  fewE      100-150   20-25     2.55   0.05  much______________________________________ 
    
     As shown in the above results, samples B and D were good devices. Particularly, sample B was confirmed to have a uniform substrate gap, few defects and little irregular color. Although a glass fiber is used to make the non-fusible spacers in this embodiment, alumina or suitable organic material can be used instead. 
     Next, a second embodiment will be described. This embodiment is suitable for the case that the main problem is void spaces which occur in the liquid crystal layer because of the contraction of the liquid crystal material ensuing from cooling completion of liquid crystal disposing. In this case, it is undesired to perfectly prohibit the change of the distance between the pair of substrates. The liquid crystal layer contracts due to the differential temperature during manufacture, and if the distance change is impossible by virtue of the spacers, void spaces have to occur in the layer to compensate the contraction of the liquid crystal material. In turn, the liquid crystal layer urges the distance between the pair of substrates to increase during disposing process thereof. 
     The spacers used for the second embodiment are made of a resilient fusible material which allows contraction and expansion to some extent. The construction and the manufacturing process are substantially same as those of the first embodiment. Uniformity of the distance between the substrates and the orientation of liquid crystal material were evaluated with varied densities (/mm 2 ) of spacers. Followings are the experimental results. 
     
         ______________________________________   Density of    Uniformity                           Orientation ofSample  Spacers       of Distance                           liquid crystal______________________________________1       1.2           bad       good2       2.8           not so good                           good3       3.1           good      good4       4.5           good      good5       6.9           good      good6       9.1           good      good7       12.1          good      good8       14.8          good      good9       15.5          not so good                           good10      17.3          not so good                           not so good11      19.5          bad       not so good______________________________________ 
    
     In case of sample 1, spacers came off from the surface of the substrate during disposing of liquid crystal into the space between the pair of substrates and eventually the distance became unequal. Sample 2 followed suit but the number of coming-off spacers was smaller than that of the sample 1. In samples 9, 10 and 11, the distances were increased. In samples 10 and 11, the orientation of liquid crystal was disturbed. Followings are data of the distances measured in microns at the points as shown in FIG. 2 for samples 1, 5 and 10. Accordingly, the desirable density of spacers is 3 to 15 mm -2 . 
     
         ______________________________________Point   Sample 1     Sample 5 Sample 10______________________________________A       2.4          2.1      2.3B       2.6          2.2      2.4C       2.5          2.2      2.4D       2.4          2.1      2.6E       3.2          2.2      2.7F       3.0          2.3      2.9G       2.9          2.2      2.9H       3.1          2.3      2.8I       3.0          2.2      2.5J       2.7          2.2      2.3K       2.5          2.2      2.4L       2.3          2.1      2.3______________________________________ 
    
     The conspicuous difference between the first and the second embodiments is the number of the spacers. The spacer 5 of the first embodiment is such that no expansion is allowed, and therefore coming-off of the spacers is very likely and eventually the number of the spacers must be increased. In case of the second embodiment, the spacers can absorb the expansion of the distance between the substrates and therefore the neccessary number of the spacers is small. 
     Uniformity and void spaces were evaluated in cases with varied degrees of elasticity of the spacers. The results are as below. 
     
         ______________________________________     Degree ofSample    Elasticity  Uniformity                           Void Space______________________________________1          5.0 (%)    good      exist2          8.3        good      exist3         11.4        good      none4         18.4        good      none5         22.5        good      none6         30.2        good      none7         33.7        good      none8         36.4        not so good                           none9         40.5        bad       none10        48.3        bad       none______________________________________ 
    
     From the above results, the preferred degree of elasticity ranges from 10% to 35%. Samples 8, 9 and 10 were not acceptable because of expansion of the distance due to the high degrees of elasticity. Followings are data of the distances measured in microns at the points as shown in FIG. 2 for samples 1, 5 and 10. 
     
         ______________________________________Point   Sample 1     Sample 5 Sample 10______________________________________A       2.1          2.1      2.2B       2.9          2.2      2.5C       2.1          2.3      2.4D       2.2          2.3      2.6E       2.1          2.3      2.8F       2.1          2.2      3.0G       2.2          2.2      2.8H       2.1          2.1      2.7I       2.0          2.2      2.7J       2.1          2.3      3.0K       2.1          2.2      2.7L       2.2          2.2      2.4______________________________________ 
    
     In practice, spacers having varied degrees of elasticity can be prepared by making combination use of a mixture of plural kinds of spacers having different degrees of elasticity. For instance, an effective 27.5% degree is obtained by use of first spacers of 5% degree and second spacers of 50% degree at 1:1 proportion. Followings are data of the distances measured in microns at the points as shown in FIG. 2 in cases with the combination use, the use of the first spacers only and the use of second spacers only. 
     
         ______________________________________Point  Combination  First Spacer                          Second Spacer______________________________________A      2.1          2.0        2.2B      2.1          2.1        2.4C      2.2          2.0        2.5D      2.1          2.1        2.9E      2.2          2.1        2.8F      2.1          2.0        2.5G      2.2          2.0        2.7H      2.1          2.0        2.6I      2.2          2.1        2.4J      2.2          2.0        2.5K      2.1          2.1        2.3L      2.2          2.0        2.5______________________________________ 
    
     The expansion of the mated substrate and ensuing excess charge of liquid crystal material can be avoided by employing the following method for disposing liquid crystal material. 
     FIG. 3 is a schematic diagram showing an apparatus for disposing liquid crytal material between a pair of substrates of liquid crystal device. The apparatus comprises a vacuum chamber 10, a vacuum pump 19 for evacuating the vacuum chamber, a press 12 equipped with torque screws 15, a pair of plates 12 and 14 having opposed smooth inside surfaces which are finely oriented parallel with each other, an accumlator 7 containing liquid crystal material therein and adapted to pour the liquid crystal material into the space between the pair of substrates and a heater 16 located just below the plate 14. 
     A liquid crystal device in advance of disposing procedure is placed between the pair of plates 12 and 14 and pressed by rotating the torque screws 15 at 1  Kg/cm 2 . In this condition, the pressure in the chamber 10 is reduced by the vacuum pump 19 to 1×10 -4  Torr. The temperature of the device is elevated at the same time to 150° C. by means of the heater. Then, an amount of liquid crystal material such as a ferroelectric liquid crystal is poured into an inlet port provided at the periphery of the liquid crystal device, and the pressure in the chamber is returned to an atmospheric pressure by leaking N 2  gas thereinto at 20 cc/min. Followings are data of the distances measured in microns at the points as shown in FIG. 2 in cases that liquid crystal is disposed into the liquid crystal device with and without application of a pressure and without a pressure. 
     
         ______________________________________Point     Pressed Device                 Non-pressed Device______________________________________A         2.1         2.0B         2.4         2.0C         2.1         2.1D         2.2         2.1E         3.4         2.0F         2.2         2.0G         2.2         2.1H         3.5         2.1I         2.2         2.0J         2.1         2.0K         2.4         2.0L         2.1         2.1______________________________________ 
    
     The present invention is effective particularly when used in ferroelectric liquid crystal devices. In the liquid crystal devices of this type, substantial volume change takes place during operation and the operational life time is greatly dependent of the resistance to the volume change which may appear in the form of microvibrations. The use of spacers made of a resilient material can absorb such microvibrations. The resilient material may be organic resilient materials or suitable inorganic materials. 
     While several embodiments have been specifically described, it is to be appreciated that the present invention is not limited to the particular examples described and that modifications and variations can be made without departure from the scope of the invention as defined by the appended claims.