Ferroelectric liquid crystal device and method of manufacturing the same

Abstract: A ferroelectric liquid crystal device (81) has transparent plates (83a, 83b), on which transparent electrodes (85a, 85b) are formed, arranged opposed to each other to form a space therebetween, with ferroelectric liquid crystals (95) sealed in the space. An inversion preventing member (89b) is provided on the main surface of the electrodes (85b) of one plate (83b) at an outside edge of an area where the opposing electrodes overlap with each other, for preventing undesired inversion of spontaneous polarization of the liquid crystal molecules in the pixel portions. The state of display of the pixel portions is thereby made stable.

Description: The present invention relates to a ferroelectric liquid crystal device and, more specifically, to the structures of its pixel portions and non-pixel portions. The present invention further relates to a method of manufacturing such a ferroelectric liquid crystal device. Fig. 23 is a perspective view showing schematically a conventional ferroelectric liquid crystal device. Transparent electrodes 3a are arranged spaced apart from each other on a main surface of a transparent plate 2a. Transparent electrodes 3b are arranged spaced apart from each other on a main surface of a transparent plate 2b. The transparent plates 2a and 2b are positioned opposing to each other to form a space. The transparent electrodes 3a and the transparent electrodes 3b intersect with each other in three dimensions. Areas at which the transparent electrodes 3a and 3b intersect with each other constitute pixels portions. The remaining portions are the non-pixel portions. Display patterns are formed by combining the pixel portions. Ferroelectric liquid crystal is sealed in the space formed by the transparent plates 2a and 2b. The ferroelectric liquid crystal is not shown in Fig. 23. The conventional ferroelectric liquid crystal device 1 will be further described with reference to Figs. 24 and 25. Fig. 24 is a cross sectional view taken in a direction of an arrow XXIV of Fig. 23. Fig. 25 is a cross sectional view taken in a direction of an arrow XXV of Fig. 24. As shown in Fig. 24, the ferroelectric liquid crystal device 1 has a structure that liquid crystal molecules 6 are sandwiched by the transparent plate 2a on which transparent electrodes 3a are formed and a transparent plate 2b on which transparent electrodes 3b are formed. The portion represented by 7a shows that the left end portion of the liquid crystal molecule 6 is projected upward from the surface of the sheet. The portion represented by 7b shows that the right end portion of the liquid crystal molecule 6 is projected upward from the surface of the sheet. The reference character 8 indicates spontaneous polarization of the liquid crystal molecule 6. The head of the arrow corresponds to the positive polarity, while the tail of the arrow corresponds to the negative polarity. The reference character 9 shows a line indicating a boundary between a layer of the liquid crystal molecule 6 and a layer of the liquid crystal molecule 6. The line represented by 9 will be hereinafter called as a boundary. An insulating film 4a is formed on the transparent electrode 3a. An alignment film 5a is formed on the insulating film 4a. An insulating film 4b is formed on the transparent electrode 3b. An alignment film 5b is formed on the insulating film 4b. The insulating films 4a, 4b and alignment films 5a and 5b are not shown in Fig. 23. The alignment films 5a and 5b serve to make the orientation of the liquid crystal molecules 6 parallel to the main surface of the transparent plates 2a and 2b. However, the boundaries 9 are not always made parallel to each other dependent on the alignment films 5a and 5b. Rubbing is done on the transparent plates 2a and 2b, so that the boundaries 9 are made parallel to each other. Fig. 26 shows rubbing on the transparent plate 2a on which the alignment film is formed. A rubbing cloth 11 is attached around a rubbing roller 12. The rubbing roller 12 is rotated in the direction of the arrow, and the transparent plate 2a is fed in contact with the rubbing cloth 11. Consequently, a plurality of linear grooves are formed on the transparent plate 2a. These grooves are called rubbings. The boundaries 9 are aligned with these grooves. The intensity of polarity of the alignment film is considered to be changed by the rubbings. Rubbing is also done on the transparent plate 2b on which the alignment film is formed. The ferroelectric liquid crystal device has the following characteristics. Namely, bistability, memory effect and high responsiveness. Bistability will be described at first. As shown in Fig. 25, the liquid crystal molecule 6 is stable when the longitudinal axis thereof is inclined by  from the normal 10 of the boundary 9 and when it is inclined by - . Namely, the liquid crystal molecule 6 is in a bistable state. How the liquid crystal molecule 6 is set in a bistable state will be described with reference to Figs. 24, 25, 27 and 28. In Fig. 27, only the orientation of the liquid crystal molecules 6 is different from that in the ferroelectric liquid crystal device 1 shown in Fig. 24. Fig. 28 is a cross sectional view taken along the direction of an arrow XXVIII of Fig. 27. As shown in Figs. 27 and 28, if the polarity of the alignment film 5a is different from that of the alignment film 5b, for example if the polarity of the alignment film 5a is negative and that of the alignment film 5b is positive, the liquid crystal molecules 6 will be as shown in Figs. 27 and 28, because the liquid crystal molecules 6 have spontaneous polarization 8. As shown in Fig. 28, the liquid crystal molecules 6 are stable with the longitudinal axes being inclined by  from the normal 10. When a positive voltage is applied to the transparent electrode 3a and a negative voltage is applied to the transparent electrode 3b, the direction of the spontaneous polarization 8 of those liquid crystal molecules 6 positioned between the transparent electrodes 3a and 3b is inverted as shown in Fig. 24. Referring to Fig. 25, the liquid crystal molecules 6 on the transparent electrode 3b have their longitudinal axes inclined by -  from the normal 10. However, when the application of the voltages is stopped, the state shown in Figs. 27 and 28 are regained. The reason for this is that the alignment film 5a has negative polarity and the alignment film 5b has positive polarity. Therefore, the liquid crystal molecules 6 are not stable when the longitudinal axes are inclined by -  from the normal 10. In that case, the bistable state is not realized. If the polarity of the alignment film 5a and the polarity of the alignment film 5b are made the same and the intensities of the polarities are made approximately the same, then stable state as shown in Fig. 24 and 25 is realized even when a positive voltage is applied to the transparent electrode 3a and a negative voltage is applied to the transparent electrode 3b and thereafter the application of voltages is stopped, as shown in Figs. 24 and 25. In that case, even if the polarities of the alignment film 5a and of the alignment film 5b are the same, the liquid crystal molecules 6 are influenced by the stronger polarity after the application of voltages is stopped, when the intensity of polarity is different. Then, a negative voltage is applied to the transparent electrode 3a shown in Fig. 24 and a positive voltage is applied to the transparent electrode 3b. Then the liquid crystal molecules 6 will be at the state shown in Figs. 27 and 28. Even when the application of voltages is stopped, the state of the liquid crystal molecules 6 shown in Figs. 27 and 28 is maintained. Namely, the bistable state of the liquid crystal molecules 6 can be realized by making the polarity of the alignment film 5a the same as that of the alignment film 5b and by making the intensities of polarities approximately equal to each other. The ferroelectric liquid crystal device 1 has memory effect, since the orientation of the liquid crystal molecules 6 is maintained even after the application of voltages is stopped. In addition, since the liquid crystal molecule 6 has both positive and negative polarities called spontaneous polarization, the orientation of the liquid crystal molecule 6 is immediately changed when the voltage is applied thereto. Namely, the ferroelectric liquid crystal device 1 has high responsiveness. The operation of the ferroelectric liquid crystal device 1 will be described with reference to Figs. 24, 25, 27 and 28. Referring to Fig. 24, when a positive voltage is applied to the transparent electrode 3a and a negative voltage is applied to the transparent electrode 3b, the liquid crystal molecules 6 are arranged as shown in Figs. 24 and 25. Thereafter, at the state of Figs. 24 and 25, a negative voltage is applied to the transparent electrode 3a and a positive voltage is applied to the transparent electrode 3b. Then, orientation of those liquid crystal molecules 6 which are positioned between the transparent electrode 3a and the transparent electrode 3b is changed as shown in Figs. 27 and 28. The liquid crystal molecules 6 positioned on the non-electrode portion 18 are not influenced by the voltages, so that the orientation thereof is not changed. A deflecter (polarizer) transmitting light only in the direction shown by the arrow D of Fig. 25 is attached to the transparent plate 2a shown in Fig. 24, and a deflecter (polarizer) transmitting light only in the direction shown by the arrow E of Fig. 25 is attached below the transparent plate 2b. When light is projected from above, an area where the transparent electrode 3a intersects with the transparent electrode 3b in three dimensions is set to a dark state, while an area where the non-electrode portion 18 is positioned is set to a bright state. The reason for this will be described with reference to Fig. 25. Since the angle of light deflected in the direction D is the same as the angle of the liquid crystal molecules 6 between the transparent electrodes 3a and 3b, the light directly passes through the liquid crystal molecules 6. The light deflected in the direction D can not pass through the deflecter transmitting light only in the E direction, so that the area where the transparent electrodes 3a and 3b intersect three dimensionally is set to a dark state. Meanwhile, the angle of light deflected in the direction D is different from the angle of the liquid crystal molecules 6 positioned on the non-electrode portion 18. Therefore, the light is elliptically deflected when it passes through the liquid crystal molecules 6. Part of the elliptically deflected light passes through the deflecter transmitting light only in the direction E, so that the non-electrode portion 18 is set to the bright state. A method of driving the ferroelectric liquid crystal device 1 will be described in the following. Fig. 29 is a partial plan view showing pixel portions and non-pixel portions of the ferroelectric liquid crystal device. Electrodes 19a and 19b correspond to the transparent electrodes 3a shown in Fig. 23, while electrodes 21a and 21b correspond to transparent electrodes 3b shown in Fig. 23. Areas where the electrodes 19a, 19b and 21a and 21b intersect with each other are the pixel portions 23a, 23b, 23c and 23d, respectively. The remaining areas are the non-pixel portions 25. A pulse such as shown by 27a of Fig. 30 is applied to the electrode 19a. A pulse such as shown by 27b of Fig. 30 is applied to the electrode 19b. A pulse such as shown by 27c of Fig. 30 is applied to the electrode 21a. Then, a voltage having the pulse of 27d, that is, the voltage of the pulse 27a - the voltage of the pulse 27c, is applied to the pixel portion 23a, as shown in Fig. 30. The pulse 27d exceeds the threshold value 29. Therefore, the orientation of the liquid crystal molecules arranged in the pixel portion 23a is changed. A voltage of the pulse 27e, that is, the voltage of the pulse 27b - the voltage of the pulse 27c, is applied to the pixel portion 23b. The pulse 27e does not exceed the threshold value 29. Therefore, the liquid crystal molecules in the pixel portion 23b are kept as they are. As shown in Fig. 30, the pulse 27d applied to the pixel portion 23a does not always exceed the threshold value. Such a driving method is called multiplexing driving. The conventional ferroelectric liquid crystal device has the following two drawbacks. The first of these will be described with reference to Figs. 31, 32 and 33. Fig. 31 is a vertical sectional view of a conventional ferroelectric liquid crystal device. Fig. 32 is a cross section of the ferroelectric liquid crystal device shown in Fig. 31 taken from the direction of the arrow XXXII. Fig. 33 is a partial plan view of the display surface of the ferroelectric liquid crystal device shown in Fig. 31. The reference characters in Figs. 31 and 32 denote the same portions denoted by the same reference characters of Figs. 24 and 25. As shown in Fig. 31, the distance between the alignment films 5a and 5b positioned on the non-electrode portion 18 (portion where there is no electrode) is greater than that between the alignment films 5a and 5b on the transparent electrodes 3a and 3b, since the transparent electrodes 3a and 3b are not formed on the non-electrode portion 18. Therefore, the difference of intensity of the polarities between the alignment films 5a and 5b positioned on the non-electrode portion 18 is further reduced than the difference of intensity of polarities between the alignment films 5a and 5b on the transparent electrodes 3a and 3b. Consequently, as shown in Figs. 31 and 32, when liquid crystal is sealed, liquid crystal molecules 6 inclined by  from the normal 10 and the molecules inclined by -  from the normal 10 may possibly be mixed on the non-electrode portion 18. When such a ferroelectric liquid crystal device is used, there are portions 35a transmitting light and portions 35b not transmitting light are mixed on the non-pixel portion 31 which is the non-electrode portion, as shown in Figs. 33. Since there are portions 35a transmitting light and portions 35b not transmitting light mixed, the state of display on the non-pixel portion 31 will be uneven. The reference character 33 represents pixel portions. The second drawback of the conventional ferroelectric liquid crystal device will be described with reference to Figs. 34, 35 and 36. The same reference characters in Figs. 34 and 35 represent the same portions as represented in Fig. 29. Referring to Fig. 34, the liquid crystal molecules in the pixel portions 23a and 23c are of the same orientation as the liquid crystal molecules 6 on the non-electrode portion 18 shown in Fig. 25. The liquid crystal molecules in the pixel portions 23b and 23d are of the same orientation as the liquid crystal molecules 6 on the electrode portion 3b shown in Fig. 25. Even if the voltage applied to the pixel portions 23a, 23b, 23c and 23d does not exceed the threshold value, regions 37 in which spontaneous polarization of the liquid crystal molecules is inverted are generated after a time lapse of a prescribed period, as shown in Fig. 35. The reason for this may be the fact that the pulse 27d or the pulse 27e shown in Fig. 30 is always applied to the pixel portions 23a, 23b, 23c and 23d. Inversion of the spontaneous polarization of the liquid crystal molecules spreads in the direction shown by the arrow A in Fig. 35. The reason will be described. Fig. 36 is a perspective view showing arrangement of liquid crystal layers in the pixel portion 23a. As shown in Fig. 36, a large number of liquid crystal layers 39 having lateral "V" shape are arranged in the pixel portion 23a. The liquid crystal layers 39 are arranged in the same direction. Since inversion of the spontaneous polarization starts from the liquid crystal molecule in the liquid crystal layer 39 near the side shown by the letter B, the inversion of the spontaneous polarization proceeds from B to C. The direction from B to C is the same as the direction shown by the arrow A in Fig. 35. A ferroelectric liquid crystal device in which the state of display of the pixel portions can be made stable is disclosed in EP-A-0 324 433, published on July 19, 1989. The ferroelectric liquid crystal device shown in EP-A-0 324 433 will be described with reference to Figs. 1, 2, 3 and 4. As shown in Fig. 1, a plurality of electrodes 43 are arranged spaced apart from each other on a main surface of a plate 41. A molybdenum film 45 is formed on one side edge of each of the electrodes 43. Two of such plates 41 are prepared for manufacturing the ferroelectric liquid crystal device. Figs. 3 and 4 are plan views of a plate used for manufacturing the ferroelectric liquid crystal device. As shown in Fig. 3, electrodes 43a are arranged spaced apart from each other on the main surface of the plate 41a. Molybdenum films 45a are formed on one side edge of each of the electrodes 43a. As shown in Fig. 4, electrodes 43b are arranged spaced apart from each other on the plate 41b. The direction of extension of the electrodes 43b is orthogonal to the direction of extension of the electrodes 43a shown in Fig. 3. Molybdenum films 45b are formed on one side edge of the electrodes 43b. The plate 41b shown in Fig. 4 is arranged to be opposed to the plate 41a shown in Fig. 3. The edge portion represented by G of the plate 41b faces the edge portion represented by D of the plate 41a, while the edge portion represented by F of the plate 41b faces the edge portion represented by E of the plate 41a. Fig. 2 is a plan view of a pixel portion of the ferroelectric liquid crystal device. The region represented by B in Fig. 36 is on the right side of the pixel 47, and the region C of Fig. 36 is on the left side of the pixel portion 47. Therefore, recess of the lateral "V" shape of the liquid crystal layer 39 shown in Fig. 36 faces the side of the molybdenum film 45b. If the pixel portion 47 is structured in this manner, the liquid crystal molecules in the pixel portion 47 are not inverted unless the voltage applied to the pixel portion 47 exceeds the threshold value. The reason for this may be the fact that there is a molybdenum film 45b on the side B where the liquid crystal molecules tend to be inverted. However, the unevenness of display of the non-pixel portion, which was the first drawback, could not be eliminated by the ferroelectric liquid crystal device disclosed in EP-A-0 324 433. In addition, as shown in Fig. 1, the distance between the electrodes 43 is small, so that there is a possibility of short circuit between electrodes 43, as the molybdenum film 45 may possibly be in contact with the adjacent electrode 43. In one aspect, the present invention provides the ferroelectric liquid crystal device defined by claim 1. In another aspect, the present invention provides the method, defined by claim 6, for manufacturing a ferroelectric liquid crystal device. According to the present invention, the provision of an inversion preventing member which is electrically insulative ensures a stable display state for the pixel portions by preventing inversion of spontaneous polarization while overcoming the above-mentioned problem of short circuit between adjacent electrodes. An embodiment of the present invention will be described in detail, by way of example only, with reference to the accompanying drawings, in which: Fig. 1 is a perspective view of a plate included in an example of a conventional ferroelectric liquid crystal device; Fig. 2 is a plan view of a pixel portion of the example of the conventional ferroelectric liquid crystal device; Fig. 3 is a plan view of one plate included in the example of the conventional ferroelectric liquid crystal device; Fig 4. is a plan view of the other plate included in the example of the conventional ferroelectric liquid crystal device; Fig. 5 is a cross sectional view of an embodiment of the present invention; Fig. 6 is a plan view with one and the other plates included in the embodiment stacked; Fig. 7 is a plan view of one plate of the embodiment; Fig. 8 is a plan view of the other plate of the embodiment; Fig. 9 is a diagram of waveforms of pulses applied to the pixel portion; Figs. 10 to 13 are plan views of a pixel portion; Fig. 14 is a plan view of one plate included in a first modification of the embodiment; Fig. 15 is a plan view of the other plate included in the first modification of the embodiment; Fig. 16 is a plan view showing one and the other plates included in the first modification of the embodiment stacked; Fig. 17 is a plan view of the one and the other plate included in a second modification of the embodiment stacked; Fig. 18 is a plan view of one plate included in the second modification of the embodiment; Fig. 19 is a plan view of the other plate included in the second modification of the embodiment; Fig. 20 is a plan view of a pixel portion of the first modification of the embodiment; Fig. 21 is omitted in this specification; Fig. 22 is a partial plan view of one and the other plates included in a third modification of the embodiment stacked; Fig. 23 is a perspective view schematically showing a conventional ferroelectric liquid crystal device; Fig. 24 is a cross sectional view taken along the direction of the arrow XXIV of Fig. 23 in a first state; Fig. 25 is a cross sectional view taken along the direction of the arrow XXV of Fig. 24; Fig. 26 shows rubbing of a plate; Fig. 27 is a cross sectional view taken along the line XXIV of Fig. 23 in a second state; Fig. 28 is a cross sectional view taken along the direction of the arrow XXVIII of Fig. 27; Fig. 29 is a partial plan view showing pixel portions and non-pixel portions of a conventional ferroelectric liquid crystal device; Fig. 30 is a diagram of waveforms of pulses applied to the electrode portion; Fig. 31 is a cross sectional view of a conventional ferroelectric liquid crystal device in which the display state of non-pixel portions is uneven; Fig. 32 is a cross sectional view taken along the direction of the arrow XXXII of Fig. 31; Fig. 33 is a plan view of non-pixel portions in which the display state is uneven; Figs. 34 and 35 are partial plan views showing pixel portions and non-pixel portions of a conventional ferroelectric liquid crystal device; and Fig. 36 is a perspective view showing arrangement of liquid crystal layers in a pixel portion. An embodiment of the ferroelectric liquid crystal device in accordance with the present invention will be described in the following. Fig. 5 is a cross sectional view of the embodiment of the ferroelectric liquid crystal device in accordance with the present invention. As shown in Fig. 5, transparent plates 83a and 83b are arranged opposing to each other to form a space therebetween. A plurality of transparent electrodes 85b are arranged spaced apart from each other on the transparent plate 83b. The thickness of the transparent electrodes 85b is 300 to 5000Å. An insulating film 87b is formed to cover the transparent electrode 85b on the transparent plate 83b. The insulating film 87b is formed of SiO 2 . The insulating film 87b is formed by sputtering. The thickness of the insulating film 87b is 300 to 5000Å. A black mask 89b is formed on the insulating film 87b positioned between the transparent electrodes 85b. Portions of the black mask 89b extend to the outside edge of the transparent electrode 85b on the main surface of the transparent electrode 85b. Portions of the black mask 89b which are positioned between the transparent electrodes 85b serve as light intercepting members. Portions of the black mask 89b which are positioned on the transparent electrodes 85b serve as the inversion preventing member. Colour mosaics produced FUJI HANTO Electronics Technology Corp. were used as the black mask 89b. The formation of the black mask 89b was carried out in the following manner. First, colour mosaics were formed by using a spin coater on the entire surface of the insulating film 87b. The thickness of the colour mosaic was 8500Å. A photoresist was formed on the entire surface of the colour mosaic. The colour mosaic was selectively etched by using the photoresist as a mask. The photoresist left on the colour mosaic was removed. Thus the formation of the black mask 89b was completed. The alignment film 91b is formed to cover the black mask 89b on the insulating film 87b. The alignment film 91b is of polyimideamide. PSI-XS014 produced by CHISSO Corp. was used as the alignment film 91b. The formation of the alignment film 91b is done by using a spin coater. The thickness of the alignment film 91b is 500Å. Rubbing is done on the main surface of the alignment film 91b. Transparent electrodes 85a, an insulating film 87a, a black mask 89a and an alignment film 91a are formed in this order on the transparent plate 83a. The material and the thickness of each of the transparent plate 83a, the transparent electrodes 85a, the insulating film 87a, the black mask 89a and of the alignment film 91a are the same as those of the transparent plate 83b, the transparent electrodes 85b, the insulating film 87b, the black mask 89b and the alignment film 91b. Rubbing is also done on the main surface of the alignment film 91a. The direction of rubbing is the same as that on the main surface of the alignment film 91b. Ferroelectric liquid crystal 95 is sealed between the alignment films 91a and 91b. The type of the ferroelectric liquid crystal 95 is chiral smectic C. ZLI-4237-000 produced by E. Merck was used as the ferroelectric liquid crystal 95. A sealing member 97 is provided around the space formed by transparent plate 83a and 83b. A filling inlet is provided at a portion of the sealing member 97, which is not shown in Fig. 5. The ferroelectric liquid crystal 95 is introduced to the cell from the filling inlet. Filling is done in vacuum. The filling inlet is closed by using a resin. The resin was acrylic UV hardening type resin. The reference numeral 99 in Fig. 5 represents a portion of the resin. A deflecting plate 82a is provided on the transparent plate 83a. The deflecting plate 82a transmits light only in the direction of D in Fig. 25. A deflecting plate 82b is provided below the transparent plate 83b. The deflecting plate 82b transmits light only in the direction of E of Fig. 25. The thickness of the liquid crystal layer (represented by K) between the black mask 89b on the transparent electrode 85b and the black mask 89a on the transparent electrode 85a is 1.0µm. The thickness of the liquid crystal layer between the portion of the transparent electrode 85b on which the black mask 89b is not provided and a portion of the transparent electrode 85a on which the black mask 89a is not provided, is 1.8µm. The method of positioning black masks 89a and 89b will be described with reference to Figs. 6, 7 and 8. As shown in Fig. 7, a plurality of transparent electrodes 85b are arranged spaced apart from each other on the main surface of the transparent plate 83b. Black masks 89b are arranged between the transparent electrodes 85b. Portions of the black mask 89b extend to the left side edge of the transparent electrode 85b. As shown in Fig. 8, a plurality of transparent electrodes 85a are arranged spaced apart from each other on the main surface of the transparent plate 83a. The direction of extension of the transparent electrodes 85a is orthogonal to the direction of extension of the transparent electrodes 85b shown in Fig. 7. Black masks 89a are arranged between the transparent electrodes 85a. Portions of the black mask 89a extend to both side edges of the transparent electrode 85a. Fig. 6 shows a state in which the transparent plate 83a shown in Fig. 8 is positioned over the transparent plate 83b shown in Fig. 7. Pixels such as shown in Figs. 10, 11, 12 and 13 are formed to test the effects of the embodiment of the ferroelectric liquid crystal device in accordance with the present invention. Fig. 13 shows a pixel portion of the embodiment of the ferroelectric liquid crystal device in accordance with the present invention. Fig. 12 also shows a pixel portion of a ferroelectric liquid crystal device in accordance with the invention. The pixel portion 113 shown in Fig. 10 will be described. The pixel portion 113 is a region where an electrode 109a intersects an electrode 109b in three dimensions. Black masks 110a and 110b are provided on both sides of the electrode 109a. Black masks 111a and 111b are provided on both sides of the electrode 109b. The pixel portion 121 of Fig. 11 will be described. The pixel portion 121 is a region where an electrode 117a intersects with an electrode 117b in three dimensions. Black masks 119a and 119b are provided on both sides of the electrode 117b. A portion of the black mask 119b extends over the electrode 117b, which is represented by 122. Black masks 120a and 120b are provided on both sides of the electrode 117a. A portion of the black mask 120a extends over the electrode 117a, which portion is represented by 124a. A portion of the black mask 120b extends over the electrode 117a, which portion is represented by 124b. The pixel portion 129 shown in Fig. 12 will be described. The pixel portion 129 is a region where an electrode 125a intersects with an electrode 125b in three dimensions. Black masks 126a and 126b are provided on both sides of the electrode 125b. A portion of the black mask 126a extends over the electrode 125b, which portion is represented by 128. Black mask 127a and 127b are provided on both sides of the electrode 125a. The pixel portion 137 shown in Fig. 13 will be described. The pixel portion 137 is a region where an electrode 133a intersects with an electrode 133b in three dimensions. Black masks 134a and 134b are provided on both sides of the electrode 133a. A portion of the black mask 134a extends over the electrode 133a, which portion is represented by 138a. A portion of the black mask 134b extends over the electrode 133a, which portion is represented by 138b. Black masks 135a and 135b are provided on both sides of the electrode 133b. A portion of the black mask 135a extends over the electrode 133b, which portion is represented by 136. Rubbing was done in the direction shown by L on all of the pixel portions shown in Figs. 10 to 13. Therefore, the area represented by B in Fig. 36 corresponds to the left side of the pixel portions. The area C of Fig. 36 corresponds to the right side of the pixel portion. A voltage large enough to invert the spontaneous polarization of the liquid crystal molecule was supplied to the pixel portions shown in Figs. 10 to 13, and the spontaneous polarization of the liquid crystal molecules was inverted. A voltage not large enough to invert the spontaneous polarization of the liquid crystal molecules was applied for a prescribed time period to see whether the spontaneous polarization of the liquid crystal molecules was inverted again or not. The voltage applied to the pixel portion is as shown in Fig. 9. The voltage is the voltage applied to the pixel portion during multiplex driving. The reference numeral 101 represents a switching pulse, which exceeds the threshold value 104. The spontaneous polarization of the liquid crystal molecules is inverted by the switching pulse 101. V PP of the switching pulse 101 is 20V, and the pulse width 105 of the switching pulse 101 is 200µsec. The reference numeral 103 represent a biasing pulse. V PP of the biasing pulse is 5V, and the pulse width of the biasing pulse 103 is 200µsec. After the application of the biasing pulse 103 for 15sec, photographs of the pixel portions were taken. The photographs were image processed by a computer and the pixel portions are divided into areas where the spontaneous polarization was inverted and areas where the spontaneous polarization was not inverted. The results are shown in Figs. 10 to 13. In the pixel portion 113 shown in Fig. 10, there is an area 115a in which the spontaneous polarization is inverted, and an area 115b in which the spontaneous polarization is not inverted. The area 115a in which the spontaneous polarization is inverted spreads from B to C. On the contrary, in the pixel portion 137 of the embodiment of the ferroelectric liquid crystal device in accordance with the present invention shown in Fig. 13, there is only an area 139b in which the spontaneous polarization is not inverted. The reason for this may be the fact that the black mask represented by 136 exists in the direction B, where the spontaneous polarization tends to be inverted. In the pixel portion 121 shown in Fig. 11, there is an area 123a in which the spontaneous polarization is inverted, and an area 123b in which the spontaneous polarization is not inverted. The reason for this may be the fact that there is no black mask extending over the electrode 117b in the B side where the spontaneous polarization tends to be inverted. In the pixel portion 129 shown in Fig. 12, there is an area 131a in which the spontaneous polarization is inverted and an area 131b in which the spontaneous polarization is not inverted. The black mask 127a does not extend over the electrode 125a. The black mask 127b does not extend over the electrode 125a. Therefore, inversions of the spontaneous polarization spread from the direction represented by M and from the direction represented by N. Therefore, it is understood that the black masks 138a and 138b shown in Fig. 13 contribute to prevent the inversion of the spontaneous polarization. The same experiment was carried out with the ratio of the biasing pulse and the switching pulse changed. The percentage of the area in which the spontaneous polarization was not inverted to the area of the pixel portion was calculated. The results are shown in Table 1. V b /V s Pixel Portion 113 Pixel Portion 121 Pixel Portion 129 Pixel Portion 137 1/4 56% 58% 65% 100% 1/3 32% 32% 35% 100% 1/2 5% 5% 7% 95% The reference character V b represents the voltage of the biasing pulse. The reference character V S represents the voltage of the switching pulse, which is always constant. As is apparent from Table 1, in the pixel portion 137 of the embodiment of the ferroelectric liquid crystal device in accordance with the present invention, the area of the region in which the spontaneous polarization of the liquid crystal molecules is not inverted is large, even when the values V s and V b are close to each other. The values V b and V s become close to each other as the number of electrodes is increased. Therefore, it is understood that the embodiment of the ferroelectric liquid crystal device in accordance with the present invention is effective even when the number of electrodes is increased. It is confirmed that the effect of preventing inversion of the spontaneous polarization of the liquid crystal molecules is especially notable when the thickness represented by P of the black mask 89b positioned on the transparent electrode 85b shown in Fig. 5 is not less than 0.2µm. The length of the black mask 89b shown by Q in Fig. 5 on the electrode portion 85b has no influence on the effect of preventing inversion of the spontaneous polarization of the liquid crystal molecules. In the second embodiment, the value Q is 2µm. Ferroelectric liquid crystal devices in which the thickness of the liquid crystal layer between the electrode portions is 1.8µm and the thickness of the liquid crystal layer between the non-electrode portions is 0.3µm, 0.5µm, 0.8µm and 1.6µm, respectively are formed. In the ferroelectric liquid crystal device in which the thickness of the liquid crystal layer between the non-electrode portions is not more than 0.5µm, a large number of disclinations are generated. Since liquid crystal molecules do not exist in the portions of disclinations, control of light can not be done at these portions. Therefore, the quality of display of the ferroelectric liquid crystal device in which a large number of disclinations are generated is significantly reduced. Black masks may be formed on all of 4 sides defining the shape of the pixel portion. The reason for this will be described with reference to Figs. 14, 15 and 16. As shown in Fig. 14, a plurality of transparent electrodes 149a are arranged spaced apart from each other on the main surface of the transparent plate 147a. No black mask is formed on the main surface of the transparent plate 147a. As shown in Fig. 15, a plurality of transparent electrodes 149b are arranged spaced apart from each other on the main surface of the transparent plate 147b. A black mask 151 is formed covering the transparent electrodes 149b on the main surface of the transparent plate 147b. Areas represented by 153 on the transparent electrodes 149b are not covered with the black mask 151. Fig. 16 shows a structure provided by stacking the transparent plate 147a shown in Fig. 14 and the transparent plate 147b shown in Fig. 15. The areas at which the transparent electrodes 149a intersect with the transparent electrodes 149b constitute pixel portions 150. The same experiment as carried out on the pixel portions shown in Figs. 10 to 13 was carried out on the pixel portion 150. The results are shown in Table 2 and Fig. 20. As shown in Fig. 20, there is only an area 152 in which the spontaneous polarization is not changed in the pixel portion 150. V b /V s Pixel Portion 150 1/4 100% 1/3 100% 1/2 95% As is apparent from Table 2, even when the values V b and V s become close to each other, the area in which the spontaneous polarization of the liquid crystal molecules is not inverted is large. Now, there are black masks 151 on all of the four sides defining the shape of the pixel portion 150 shown in Fig. 16. Therefore, the inversion of the spontaneous polarization of the liquid crystal molecule can be prevented no matter where the area represented by B in Fig. 36 is positioned on the pixel portion 150. The following is a method of forming black mask on four sides defining the shape of the pixel portion. The method will be described with reference to Figs. 17 and 19. As shown in Fig. 18, a plurality of transparent electrodes 143a are arranged spaced apart from each other on the main surface of a transparent plate 141a. Black masks 145a are formed between the transparent electrodes 143a. The black mask 145a extends to both side edges of the transparent electrode 143a. As shown in Fig. 19, a plurality of transparent electrodes 143b are arranged spaced from each other on the main surface of a transparent plate 141b. Black masks 145b are formed between the transparent electrodes 143b. The black mask 145b extends over both side edges of the transparent electrode 143b. Fig. 17 shows a structure provided by stacking the transparent plate 141a shown in Fig. 18 and the transparent plate 141b shown in Fig. 19. By forming black masks as shown in Figs. 18 and 19, pixel portions 142 can be surely provided when the transparent plate 141a and the transparent plate 141b are overlapped, even if the alignment therebetween is a little deviated. On the contrary, if the black masks are formed as shown in Figs. 14 and 15, the pixel portions 150 can not be formed unless the transparent electrodes 149a are positioned on the electrode portions 153 which are not covered with the black mask. Since the area of the portion of the electrode portion 153 not covered with the black mask is small, it is difficult to manufacture the ferroelectric liquid crystal device, when the black masks are formed as shown in Figs. 14 and 15. Any light intercepting material may be used for the black mask. Any of gelatin, polyimide, acryl polymer, methacrylate polymer and so on including any of dye, pigment, carbon powder and the like may be used as the black mask employed in the present invention, for instance. Inorganic oxide may also be used. Lift-off method or a method of selectively depositing black mask on the desired portion by using mask may be used as the method of forming the black mask. Rubbing of an organic alignment film such a polyimides, polyamides or polyvinyl alcohol, oblique incident evaporation of SiO 2 , method using a LB (Langmuir Blodgett) film may be used as a method of aligning the liquid crystal molecules. Another example of the ferroelectric liquid crystal device in accordance with the present invention will be described in the following. Fig. 22 shows the device. As shown in Fig. 22, transparent electrodes 155a are arranged spaced apart from each other. Black masks 159a are formed on side edges of the transparent electrodes 155a. Electrodes 155b are formed spaced apart from each other. Black masks 159b are formed on the side edges of the electrodes 155b. Areas where the electrodes 155a and 155b intersect with each other in three dimensions constitute pixel portions 160. In this example, although the state of display of the non-electrode portion can not be made uniform, the state of display of the pixel portions 160 can be made uniform. Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the scope of the present invention being limited only by the terms of the appended claims. There are described above novel features which the skilled man will appreciate give rise to advantages.

Claims: A ferroelectric liquid crystal device (81) having ferroelectric liquid crystal (95) sealed in a space formed between first and second plates (83a, 83b; 141a, 141b; 147a, 147b) arranged opposing to each other, comprising: a second electrode portion (85a; 143a; 149a) arranged on a surface of said second plate facing said first plate; a plurality of first electrode portions (85b; 143b; 149b) arranged spaced apart from each other on a surface of said first plate facing said second plate; and an inversion preventing member (89b; 145b; 151) provided on the main surface of said first electrode portion or said second electrode portion at at least an outside edge of an area where said first electrode portion and said second electrode portion overlap with each other, for preventing undesired inversion of spontaneous polarization of molecules of said ferroelectric liquid crystal, said outside edge lying at the side (B) of said area at which said inversion of spontaneous polarization tends to occur; characterised in that : said inversion preventing member is electrically insulative. A ferroelectric liquid crystal device according to claim 1, wherein: said second electrode portion comprises a plurality of second electrode portions arranged spaced apart from each other on said surface of the second plate and intersecting said first electrode portions; the intersection of said first and second electrode portions defines a said area of rectangular shape; and said inversion preventing member is provided at one side of the rectangle corresponding to said outside edge and at two sides thereof adjoining said one side. A ferroelectric liquid crystal device according to claim 2, wherein said inversion preventing member comprises: a first inversion preventing member provided at one side edge of said first electrode portions on the main surface of said first electrode portions; and a second inversion preventing member provided at the adjacent side edges of adjacent ones of said second electrode portions on the main surface of said second electrode portions. A ferroelectric liquid crystal device according to claim 1, wherein: said second electrode portion comprises a plurality of second electrode portions arranged spaced apart from each other on said surface of the second plate and intersecting said first electrode portions; the intersection of said first and second electrode portions defines a said area of rectangular shape; and said inversion preventing member is provided at all four sides of the rectangle, one side of the rectangle corresponding to said outside edge. A ferroelectric liquid crystal device according to claim 4, wherein said inversion preventing member comprises: a first inversion preventing member provided at the adjacent side edges of adjacent ones of said first electrode portions on the main surface of said first electrode portions; and a second inversion preventing member provided at the adjacent side edges of adjacent ones of said second electrode portions on the main surface of said second electrode portions. A method for manufacturing a ferroelectric liquid crystal device, comprising the steps of: forming a plurality of first electrode portions arranged spaced apart from each other on a surface of a first plate; forming a first electrically insulative inversion preventing member for preventing undesired inversion of spontaneous polarization of molecules of ferroelectric liquid crystal at one side edge of said first electrode portions on the main surface of said first electrode portions; forming a plurality of second electrode portions arranged spaced apart from each other on a surface of a second plate; forming a second electrically insulative inversion preventing member at the adjacent side edges of adjacent ones of said second electrode portions on a main surface of said second electrode portions; and arranging said first plate and said second plate so as to be opposed to each other such that a space is formed therebetween and said first electrode portions and said second electrode portions intersect each other; and sealing ferroelectric liquid crystal in said space; said first electrically insulative inversion preventing member being provided at a said side edge of the first electrode portions where said inversion of the spontaneous polarization tends to occur.

Inventors: ITOH NOBUYUKI, NARUTAKI YOZO, SHINOMIYA TOKIHIKO, ITOH, NOBUYUKI, NARUTAKI, YOZO, SHINOMIYA, TOKIHIKO
Applicants: SHARP KK, SHARP KABUSHIKI KAISHA
Application year: 1990
Main classification: G02F1
IPC codes: G02F1