Abstract:
A liquid crystal panel development method for a panel including first and second substrates and first and second electrodes with a display voltage applied to liquid crystal interposed therebetween. The method involves applying a voltage signal between the electrodes with an AC voltage component of amplitude Vac and a DC voltage component Vdc, changing Vac and Vdc to measure the range of optimal DC component variation ΔVdc, and determining a structure or material of the liquid crystal panel such that the range of optimal DC component variation ΔVdc becomes less than a given value. ΔVdc=|Vdcb —Vdcw|, where Vdcb is the Vdc value at the minimum range of transmittance variation when Vdc is changed with Vac being fixed for displaying black, and Vdcw is the Vdc value at the minimum range of transmittance variation when Vdc is changed with Vac being fixed for displaying white.

Description:
This is a divisional of application Ser. No. 11/312,912, filed Dec. 20, 2005, now U.S. Pat. No. 7,136,131, which is a divisional of application Ser. No. 10/747,517, filed Dec. 29, 2003, now U.S. Pat. No. 7,095,473, which prior application was a divisional of application Ser. No. 09/927,005, filed Aug. 9, 2001, now U.S. Pat. No. 6,819,384. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a liquid crystal display (LCD) panel capable of reducing a persistence degree and a development method thereof. 
   2. Description of the Related Art 
   Each of  FIGS. 31 and 32  is a schematic sectional view showing a structure of one pixel of an LC panel.  FIG. 31  shows a state where no voltage is applied, and  FIG. 32  shows a state where a voltage is applied. 
   The LCD panel includes substrates  10  and  20  opposing to each other, and a sealed-in nematic liquid crystal  30  having an anisotropic dielectric positive constant. In the substrate  10 , a flat electrode  12 , a dielectric layer  13  and a vertically oriented layer  14  are formed on a face of a transparent insulating substrate  11 , for example, a glass substrate, and on the other face thereof, a polarizer  15  is formed. In the substrate  20 , a common electrode  23  is formed on one face of a transparent substrate  21 , for example, a glass substrate, an insulating layer  24  is formed thereon, a pixel electrode  25  is formed on the insulating layer  24 , and further an insulating layer  26  and a vertically orientated later  27  are formed thereon. On the other face of the substrate  21 , a polarizer  28  is formed. Transmission axes of the polarizers  15  and  28  perpendicularly cross over each other. 
   When backlight in the direction shown by arrows in  FIG. 31  enters into the LCD panel, the light is transformed into linearly polarized light by the polarizer  28 . When the flat electrode  12 , the common electrodes  23  and the pixel electrode have the same potential, the liquid crystal  30  effects no change in the plane of polarization of the linearly polarized light, and therefore the linearly polarized light cannot be transmitted through the polarizer  15 , resulting in a dark state. 
   When, as shown in  FIG. 32 , the flat electrode  12  and the common electrode  23  has the same potential but the pixel electrode  25  is applied with a potential different from the both former electrodes, an electric field arises. Dotted lines of  FIG. 32  show the lines of electric force. Liquid crystal molecules are inclined relative to an incident light direction under influence of the electric field to cause birefringence, and part of the light can transmit through the polarizer  15 , resulting in a bright state. 
   Since the common electrode  23  and the pixel electrode  25  are made of an opaque metal, behaviors of liquid crystal molecules over the electrodes are not problematic in terms of display. 
   If the flat electrode  12  does not exist, liquid crystal molecules between the pixel electrode and the common electrode  23  tend to reduce inclination thereof, which will produces the drop region of transmittance. The flat electrode  12  makes the electric field between the common electrode  23  and the pixel electrode  25  asymmetric so as to contributes to prevent the transmittance from locally dropping. The dielectric layer  13  reinforces the lateral component of the electric field in the liquid crystal  30  to make it possible for the liquid crystal  30  to be driven with lower applied voltage. The common electrode  23  and the pixel electrode  25  each are stripe electrodes extending in the direction perpendicular to the sheet of  FIG. 32 , and alternately formed on the top and bottom surfaces of the insulting layer  24 . The insulating layer  24  is for preventing common electrodes and pixel electrodes from short-circuiting at positions where the both overlap as will be described later. The insulating layer  26  is for reducing the persistence degree. 
     FIG. 33  shows an electrode pattern of one pixel, formed in the substrate  20  of  FIG. 31 .  FIGS. 34 and 35  are patterns of the pixel electrode  25  and the common electrode  23 , respectively, of  FIG. 33 . 
   In  FIG. 33 , a data line DL 1  and a scan line SL 1  cross over each other with an insulating layer interposing therebetween. Each of the pixel electrode  25  and the common electrode  23  has a stripe section and a peripheral section connecting ends of the stripe section. The lines of the stripe section are inclined 45 degrees to each of the scan line SL 1  and the data line DL 1 . 
   When the potential of the scan line SL 1  goes high, a TFT  29  is turned on to apply the potential of the data line DL 1  onto the pixel electrode  25  and generate an electric field between the stripe electrodes of the pixel electrode  25  and the common electrode  23 . The longitudinal direction of the upper half of the stripe electrodes is different from that of the lower half of the stripe electrodes by 90 degrees as shown in  FIG. 33 , whereby the LCD panel has wider range of viewing angles than in a case where the both halves of the stripe electrodes are parallel to each other. 
   The common electrode  23 A has peripheral protrusions which are connected to the common electrodes of adjacent pixels not shown. 
     FIG. 36(A)  is an enlarged partial view near a crossover of a stripe electrode and the peripheral section of  FIG. 33 .  FIG. 36(B)  shows the lines of electric force with dotted lines near the crossover when a voltage is applied between the pixel electrode  25  and the common electrode  23 . 
   A peripheral section of the pixel or common electrode has crossover portions to stripe electrodes of the common or pixel electrodes with the insulating layer interposing therebetween since a pixel has a rectangular shape, and each of the pixel electrode  25  and the common electrode  23  has stripe electrodes in parallel to each other and has a continuous shape. For example, a side  251  of the pixel electrode  25  is connected to a side  252  of the peripheral section, and a side  231  of the common electrode  23  is parallel to the side  251 , while the side  231  crosses over the side  252  at an acute angle. 
     FIG. 37  is a schematic sectional view showing inclination of liquid crystal molecules between the pixel electrode  25  and the common electrode  23  of one pixel of the LCD panel when a voltage is applied therebetween. 
   In  FIG. 32 , a structure between the pixel electrode  25  and the liquid crystal  30  is different from that between the common electrode  23  and the liquid crystal  30 , which causes persistence. 
   In  FIG. 36(B) , since the side  252  crosses over the side  231  at an acute angle, an electric field therebetween near the crossover is stronger than that between the parallel sides. Further, a direction of electric field strength near the crossover is different from that between the parallel sides. Due to such conditions, a transmittance-voltage characteristic near the crossover is different from that between the parallel portion, causing not only degradation of an image quality but also persistence. 
   In  FIG. 37 , since the insulating layer  26  exists above the pixel electrode  25 , application of an electric field in this portion is useless and effective application of the electric field to the liquid crystal  30  is prevented. If the insulating layer  26  is omitted in order to solve this problem, it causes more persistence since the insulating resistance of the vertically oriented layer  27  is low. If the pixel electrode  25  is exposed to the liquid crystal  30 , not only is the degree of persistence enhanced, but liquid crystal molecules also decompose. Further, since the top surface of a pixel electrode  25  is flat, it is not possible to effectively apply an electric field to the liquid crystal  30  in relation to transmittance, which prevents achieving higher contrast display. 
   In development of an LCD panel, measurement of a persistence degree is performed at each trial when a structure or material of the LCD panel is changed in order to reduce the persistence degree to a value lower than a given value, and it takes, for example, 48 hours to measure the persistence degree in each trial, which makes a development term thereof longer. 
   SUMMARY OF THE INVENTION 
   Accordingly, it is an object of the present invention to provide a liquid crystal display panel capable of reducing a persistence degree. 
   In one aspect of the present invention, there is provided a liquid crystal panel comprising: first and second substrate; and liquid crystal interposed between the first and second substrates; the first substrate comprising: an insulating substrate; first and second electrodes, formed over the insulating substrate, for a display voltage to be applied therebetween; and a first insulating layer covering the first and second electrodes; wherein the first electrode is disposed higher than the second electrode in relation to a direction from the insulating substrate toward the second substrate, and the first and second electrodes overlap each other with a second insulating layer being interposed therebetween at an overlapping portion, wherein a thickness of the first insulating layer on the first electrode is substantially equal to the insulating layer on the second electrode. 
   With this configuration, when the voltage signal is applied between the first and second electrodes, electric states over the first and second electrodes are almost the same, whereby persistence is reduced in comparison with a case where the thicknesses are different from each other as shown in  FIG. 31   
   Other aspects, objects, and the advantages of the present invention will become apparent from the following detailed description taken in connection with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic circuit diagram of a liquid crystal display device for use in a method of the present invention. 
       FIG. 2  is an illustration of a persistence degree. 
       FIG. 3  is a graph showing a voltage waveform, having an AC amplitude Vac and a DC component Vdc, applied on a liquid crystal pixel. 
       FIG. 4  are graphs showing a measured transmittance waveform of the liquid crystal pixel in a case where the AC amplitude Vac is 2 V and the DC component Vdc is −3 V. 
       FIG. 5  is a graph showing a measured transmittance waveform of the liquid crystal pixel in a case where the AC amplitude Vac is 2 V and the DC component Vdc is −2V. 
       FIG. 6  is a graph showing a measured transmittance waveform of the liquid crystal pixel in a case where the AC amplitude Vac is 2 V and the DC component Vdc is −1V. 
       FIG. 7  is a graph showing a measured transmittance waveform of the liquid crystal pixel in a case where the AC amplitude Vac is 2 V and the DC component Vdc is −0.5 V. 
       FIG. 8  is a graph showing a measured transmittance waveform of the liquid crystal pixel in a case where the AC amplitude Vac is 2 V and the DC component Vdc is 0 V. 
       FIG. 9  is a graph showing a measured transmittance waveform of the liquid crystal pixel in a case where the AC amplitude Vac is 2 V and the DC component Vdc is 0.5 V. 
       FIG. 10  is a graph showing a measured transmittance waveform of the liquid crystal pixel in a case where the AC amplitude Vac is 2 V and the DC component Vdc is 1 V. 
       FIG. 11  is a graph showing a measured transmittance waveform of the liquid crystal pixel in a case where the AC amplitude Vac is 2 V and the DC component Vdc is 2 V. 
       FIG. 12  is a graph showing a measured transmittance waveform of the liquid crystal pixel in a case where the AC amplitude Vac is 2 V and the DC component Vdc is 3 V. 
       FIG. 13  is a graph showing a measured relationship between the DC component Vdc and the variation width ΔT of liquid crystal pixel transmittance in a case where the AC amplitude Vac is 2 V. 
       FIG. 14  is a graph showing a measured relationship between the AC amplitude Vac, and the value of DC component Vdc at which the variation width ΔT of liquid crystal pixel transmittance is the minimum. 
       FIG. 15  is a graph showing a measured relationship between a persistence degree and the range of optimal DC component variation ΔVdc. 
       FIG. 16  is a schematic sectional view showing a structure of a liquid crystal pixel capable of reducing a persistence degree in a state where no voltage is applied, of a second embodiment according to the present invention. 
       FIG. 17  is a schematic sectional view showing the liquid crystal pixel of  FIG. 16  in a state where a voltage is applied. 
       FIGS. 18(A)-18(F)  are schematic sectional views showing a fabrication process of the substrate  20 A of  FIG. 16 . 
       FIG. 19  is a plane view showing an electrode pattern of a liquid crystal pixel capable of reducing a persistence degree, of a third embodiment according to the present invention. 
       FIG. 20  is a plane view showing the pixel electrode of  FIG. 19 . 
       FIG. 21  is a plane view showing the common electrode of  FIG. 19 . 
       FIG. 22(A)  is an enlarged partial view near a crossover of a stripe electrode and the peripheral section of  FIG. 19 . 
       FIG. 22(B)  is a diagram showing the lines of electric force with dotted lines near the crossover when a voltage is applied between the electrodes of  FIG. 22(A) . 
       FIG. 23  is a plane view showing an electrode pattern of a liquid crystal pixel capable of reducing a persistence degree, of a fourth embodiment according to the present invention. 
       FIG. 24  is a plane view showing the common electrode of  FIG. 23 . 
       FIG. 25  is a plane view showing an electrode pattern of a liquid crystal pixel capable of reducing a persistence degree, of a fifth embodiment according to the present invention. 
       FIG. 26  is a plane view showing the common electrode of  FIG. 25 . 
       FIG. 27  is a plane view showing an electrode pattern of two liquid crystal pixels adjacent to each other, of a sixth embodiment according to the present invention. 
       FIG. 28  is an enlarged sectional view taken along line A-A of  FIG. 27 . 
       FIGS. 29(A)-29(C)  are schematic sectional views showing a fabrication process of a substrate on the back light incident side. 
       FIGS. 30(A)-30(B)  are schematic sectional views showing the fabrication process following  FIG. 29 . 
       FIG. 31  is a schematic sectional view showing a structure of one pixel of an LCD panel compared to the present invention in a state where no voltage is applied. 
       FIG. 32  is a schematic sectional view showing the pixel of  FIG. 31  in a state where a voltage is applied. 
       FIG. 33  is a plane view showing an electrode pattern of one pixel formed in the substrate  20  of  FIG. 31 . 
       FIG. 34  is a plane view showing the pixel electrode of  FIG. 33 . 
       FIG. 35  is a plane view showing the common electrode of  FIG. 33 . 
       FIG. 36(A)  is an enlarged partial view of the pattern near a crossover between electrodes of  FIG. 33 . 
       FIG. 36(B)  is a diagram showing the lines of electric force with dotted lines when a voltage is applied between the electrodes of  FIG. 36(A) . 
       FIG. 37  is a schematic sectional view showing inclination of liquid crystal molecules between a pixel electrode and a common electrode of one pixel of a prior art LCD panel when a voltage is applied therebetween. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring now to the drawings, wherein like reference characters designate like or corresponding parts throughout several views, preferred embodiments of the present invention are described below. 
   First embodiment 
   First of all, there will be described a development method capable of decreasing the development term of the LCD panel which has a structure or employs material capable of reducing a persistence degree. 
     FIG. 1  is a schematic circuit diagram of a liquid crystal display device for use in a method according to the present invention.  FIG. 1  shows a case where a pixel array has a matrix with 3 rows and 6 columns for simplicity. 
   The circuit itself is the same as that of the prior art. A data line DL 1 , a scan line SL 1 , TFT  29 , a pixel electrode  25 , a common electrode  23 , and a flat electrode  12  in  FIG. 1  are formed as shown in  FIG. 31  for example. The flat electrode  12  is provided for all the pixel electrodes. The scan lines are connected to the output of a scan driver  31 , and the data lines are connected to the output of a data driver  32 . A control circuit  33  controls the data driver  32  on the basis of a pixel clock CLK and a horizontal sync signal HSYNC, and also provides a video signal VS to the data driver  32 , and further controls the scan driver  31  on the basis of the horizontal sync signal HSYNC and a vertical sync signal VSYNC. Rows of a pixel array are line-sequentially selected by the scan driver  31 , and display data (a gradation voltage set) is provided to the pixels of a selected row from the data driver  32 . 
     FIG. 2  is an illustration of a persistence degree. 
   For example, a case is considered where display data of each pixel has a 64-step gradation, white corresponds to the sixty-fourth gradation and black corresponds to the first gradation. The persistence degree is defined as follows: 
   (A) A fixed pattern including white and black is displayed, for example, for 48 hours. 
   (B) Immediately thereafter, the halftone of the thirty-second gradation is displayed, and the brightness Bmw and Bmb in respective regions in which the white and black was displayed in the step (A) are measured. The persistence degree PD is calculated with the following formula:
 
 PD= 100( Bmw−Bmb )/ Bm  %
 
   where Bm is a smaller one of the Bmw and Bmb. 
   In the step (B), in order that no persistence can be recognized by a human, a persistence degree has to be less than 6% under ordinary illumination in a room and less than 3% in dark room. 
   The persistence degree is different according to a structure or material of an LCD panel. In development of an LCD panel, if the persistence degree is measured at each trial when a structure or material of an LCD panel is changed in order to reduce the persistence degree to a value lower than a given value, and it takes, for example, 48 hours to measure the persistence degree in each trial, which makes a development term thereof longer. Therefore, it is effective to search a physical quantity having a high correlation with the persistence degree and which can be measured in a short time. 
   Liquid crystal pixel is applied with an AC voltage of rectangular wave in order to prevent its degradation.  FIG. 3  shows a voltage waveform applied between the pixel electrode  25  and the common electrode  23 , and between the pixel electrode  25  and the flat electrode  12  of  FIG. 32 , wherein a frequency is 30 Hz. 
   The voltage waveform has a DC component in order to prevent flickers from arising under the application of the only AC voltage, that is, in order to avoid a cyclical change in transmittance. The amplitude of the AC voltage of rectangular wave and the DC voltage component are indicated by Vac and Vdc, respectively. 
   The LCD panel transmittance was measured each time the DC component Vdc was altered stepwise with the AC amplitude being fixed.  FIGS. 4 to 12  show variations in transmittance in cases where the DC component Vdc was set at −3 V, −2 V, −1 V. −0.5 V, 0 V, 0.5 V, 1 V, 2 V and 3 V, respectively, with the AC amplitude Vac being fixed at a black display voltage 2V. As shown in  FIG. 4 , the range of transmittance variation is indicated by ΔT. 
     FIG. 13  is a graph showing a relationship between the DC component Vdc and the range of transmittance variation ΔT in a case where the AC amplitude Vac is 2 V. It is estimated from this graph that the value of the DC component Vdc at which the range of transmittance variation ΔT is the minimum is −0.38 V. 
   Likewise, measured are the value of the DC component Vdc at which the range of transmittance variation ΔT assumes the minimum ΔTmn in cases where the AC amplitude Vac is a white display voltage 7 V and a halftone display voltage (2+7)/2=4.5 V.  FIG. 14  shows the results obtained by the measurement, wherein ΔVdc denotes the range of optimal DC component variation. The DC component Vdc is fixed in an actual liquid crystal display device. Therefore, as the range of optimal DC component variation ΔVdc decreases, flickers become weaker. 
     FIG. 15  is a graph showing a relationship between the persistence degree and the range of optimal DC component variation ΔVdc, obtained by measuring the persistence degree and the range of optimal DC component variation ΔVdc each time the structure or material of an LCD panel is changed. It can be seen that there is a very high correlation between the persistence degree and the range of optimal DC component variation ΔVdc. Further, it is found that in order to lower the persistence degree less than the above described 6%, the range of optimal DC component variation ΔVdc has to be less than 0.5 V, and in order to lower the persistence degree less than the above described 3%, the range of optimal DC component variation ΔVdc has to be less than 0.2 V. 
   Since the range of optimal DC component variation ΔVdc can be measured in a short time with ease, by use of ΔVdc it is possible to reduce the development term of an LCD panel with the persistence degree being less than a given value. 
   Note that it was confirmed that there is a high correlation between the range of optimal DC component variation ΔVdc and the persistence degree even in LCD panels having structures where the pixel electrode  25  and the common electrodes  23  are employed without the flat electrode  12  and the pixel electrode  25  and the flat electrode  12  are employed without the common electrodes  23 , and therefore there will be a similar correlation therebetween in LCD panels having other structures. 
   Second Embodiment 
     FIGS. 16 and 17  are schematic sectional views showing a structure of a liquid crystal pixel capable of reducing the persistence degree, of a second embodiment according to the present invention, wherein  FIG. 16  shows a state where no voltage is applied and  FIG. 17  shows a state where a voltage is applied. 
   The structure of a substrate  20 A is different from that of the substrate  20  of  FIG. 31 . The other structure is the same as that of  FIG. 31 . 
     FIG. 18  are schematic sectional views showing a fabrication process of the substrate  20 A. In  FIG. 18 , the right end portions of (A) to (F) indicate a place where a common electrode  23 A and a pixel electrode  25 A are stacked with an insulating layer  24 A interposing therebetween. 
   (A) A common electrode  23 A made of metal is formed on a transparent insulating substrate  21  by photolithography. 
   (B) an insulating layer  24  is coated on the substrate  21 . 
   (C) A pixel electrode  25 A is formed on the insulating layer  24  by photolithography. 
   (D) The insulating layer  24  is etched with the pixel electrode  25 A as a mask and the only portion thereof under the pixel electrode  25 A is left. 
   (E) An insulating layer  26 A is coated on the substrate  21 . 
   (F) A vertically oriented layer  27  is coated on the insulating layer  26 A. 
   By fabricating the substrate  20 A in such a way, as shown in  FIG. 16 , the thicknesses of the insulating layer  26 A over the pixel electrode  25 A is substantially equal to that over the common electrode  23 A. Therefore, electric states over and near the common electrode  23 A and over and near the pixel electrode  25 A are almost the same as shown in  FIG. 17  in a case where an AC voltage of rectangular wave is applied between the pixel electrode  25 A and the common electrode  23 A, and the persistence is reduced in comparison with an LCD panel having the structure of  FIG. 31 . In other words, the range of optimal DC component variation ΔVdc of  FIG. 15  decreases, and thereby the persistence degree becomes lower. 
   The insulating layers  24 A and  26 A are made of, for example, SiNx,SiO2, resist or acrylic resin. In a trial, SiNx was used as the insulating layers  24 A and  26 A, JALS 204 made by JSR Co. as the vertically oriented layer  27 , and ZLI4535 made by Merck Japan Co. as the liquid crystal  30 , and the persistence degree reducing effect of the trial article was confirmed. 
   Third Embodiment 
     FIG. 19  is a plane view showing an electrode pattern of a liquid crystal pixel capable of reducing the persistence degree, of a third embodiment according to the present invention, which is analogous to  FIG. 33 . 
   The electrode pattern is formed, for example, in the substrate  20 A of  FIG. 16  or the substrate  20  of  FIG. 31 . 
     FIGS. 20 and 21  are plane views showing the pixel electrode  25 A and the common electrode  23 A of  FIG. 19 , which are analogous to  FIGS. 34 and 35 , respectively. 
   A peripheral section of the pixel or common electrode has crossover portions to stripe electrodes of the common or pixel electrodes with the insulating layer interposing therebetween since a pixel has a rectangular shape, and each of the pixel electrode  25 A and the common electrode  23 A has stripe electrodes in parallel to each other and has a continuous shape. For example, a side  251  of the pixel electrode  25 A is connected to a side  252  of the peripheral section, and a side  231  of the common electrode  23 A is parallel to the side  251 , while the sides  252  and  232  are connected to the side  251  and  231 , respectively, crosses over each other. 
     FIG. 22(A)  is a partial enlarged view near a crossover of electrodes.  FIG. 22(B)  shows the lines of electric force with dotted lines when a voltage is applied between the pixel electrode  25 A and the common electrode  23 A. 
   Since the sides  252  and  232  cross over each other at an obtuse angle, concentration of the lines of electric force decreases, and thereby it is suppressed for an electric field strength to become larger in comparison with a case where the sides  252  and  232  cross over each other at an acute angle as shown in  FIG. 36(A) . 
   Further, with respect to a line SA passing through between the sides  251  and  231 , the sides  251  and  252  are symmetrical to the sides  231  and  232 , respectively, resulting in that the direction of electric field vector between the sides  252  and  232  is parallel to that between the sides  251  and  231 . 
   Accordingly, rapidly changing distribution of the transmittance near electrode crossover is alleviated, with the result that display image quality is improved and persistence degree is reduced. This holds at other electrode crossovers in a similar way. 
   Trial liquid crystal panels were fabricated in which the electrode patterns of  FIG. 19  and  FIG. 33  were employed both with the other conditions being the same as those of the above described trial example, and it was confirmed that the liquid crystal panel employing the electrode pattern of FIG.  19  has a lower persistence degree than that employing the electrode pattern of  FIG. 33 . 
   Fourth Embodiment 
     FIG. 23  is a plane view showing an electrode pattern of a liquid crystal pixel capable of reducing the persistence degree, of a fourth embodiment according to the present invention, which is analogous to  FIG. 19 .  FIG. 24  is a plane view showing the common electrode  23 B of  FIG. 23 , while the pixel electrode  25 A is the same as that of  FIG. 23 . 
   In the peripheral section of the common electrode  23 B, cutoff portions  23 B 1  to  23 B 8  are formed with ensuring one body of the common electrodes  23 B. The positions of the cutoff portions  23 B 1  to  23 B 8  are each near crossovers between the common electrode  23 B and the pixel electrode  25 A. 
   In a case where non of these cutoff portions exist, an electric field arises in a non-display region between these portion and corresponding portions of the pixel electrode  25 A when a voltage is applied, which affects orientation of liquid crystal molecules in a display region near the non-display region. This adverse influence is removed by the cutoff portions, resulting in improving a display image quality and reducing the persistence degree in comparison with that of the third embodiment. 
   Fifth Embodiment 
     FIG. 25  is a plane view showing an electrode pattern of a liquid crystal pixel capable of reducing the persistence degree, of a fifth embodiment according to the present invention, which is analogous to  FIG. 33 .  FIG. 26  is a plane view showing the common electrode  23 C of  FIG. 25 , while the pixel electrode  25  is the same as that of  FIG. 34 . 
   In the common electrode  23 C, cutoff portions  23 B 1  to  23 B 8  are formed with ensuring one body of the common electrodes  23 C, resulting in improving a display image quality and reducing the persistence degree in comparison with the structure of  FIG. 23  for the same reason as that of the above described fourth embodiment. 
   Sixth Embodiment 
     FIG. 27  is a plane view showing an electrode pattern of two liquid crystal pixels adjacent to each other, of a sixth embodiment according to the present invention, wherein the both pixels have the same pattern. 
   The frame sections of a common electrode  23 D and a pixel electrode  25 D overlap each other with an insulating layer interposing therebetween. The stripe electrode section of the common electrode  23 D are formed under and between stripe electrodes of the pixel electrode  25 D, and therefore the line density of the stripe electrode sections of the common electrode  23 D is two times greater than that of the pixel electrode  25 D. 
     FIG. 28  is an enlarged sectional view taken along line A-A of  FIG. 27 . 
   Different points from the liquid crystal pixel of  FIG. 32  are that the stripe electrodes of the pixel electrode  25 D are convex in cross section, and an insulating layer  26 D is formed only on the stripe electrodes of the pixel electrode  25 D and no insulating layer is formed on display areas between stripe electrodes of the common electrode  23 D and the pixel electrode  25 D. A vertically oriented layer  27  is thinner than the insulating layer  26 D, therefore it is depicted with a thick line in  FIG. 28 . 
   Since the stripe electrodes of the pixel electrode  25 D are convex in cross section, the top surface thereof is sloped toward both sides with the maximum height at the middle. In order to form such a convex shape, unlike  FIG. 32 , there is formed a stripe electrode of the common electrode  23 D under each stripe electrode of the pixel electrode  25 D, wherein this stripe electrode of the common electrode  23 D has a narrower width than that of the pixel electrode  25 D. In order to emphasize this convex shape, a channel protective layer  41  is partially removed with leaving portions over the stripe electrodes of the common electrode  23 D, wherein the channel protective layer  41  is formed when the TFT  29  of  FIG. 27  is formed, and has a width narrower than that of the underlying stripe electrode. 
   With such a structure having a convex shape in cross section, the lines of electric force becomes as shown with dotted lines in  FIG. 28  when a voltage is applied between the pixel electrode  25 D and the common electrode  23 D. That is, since the lines of electric force near a slope of the pixel electrode  25 D are normal to the slope, inclination of liquid crystal molecules relative to a normal to the surface of the substrate  21  becomes larger, which increases a transmittance in white display in comparison with the case of  FIG. 32 , thereby improving a display contrast. 
   Further, since the patterns of the pixel electrode  25 D and the insulating layer  26 D are the same as each other, and no insulating layer  26 D exists in display areas between the stripe electrodes of the pixel electrode  25 D and the common electrode  23 D, an electric field is more effectively used on liquid crystal molecules in comparison with the case of  FIG. 32 , resulting in improving a display contrast in comparison with the case of  FIG. 32  under the same applied voltage. 
   Furthermore, since liquid crystal molecules do not directly contact with the pixel electrode  25 D, decomposition of liquid crystal molecules are prevented, and the persistence degree is also reduced. 
     FIGS. 29 and 30  are schematic sectional views showing a fabrication process of a substrate  20 D on the back light incident side, and each section corresponds to one taken along line B-B of  FIG. 27 . There will be described below the fabrication process. 
   (A) A common electrode  23 D and a scan (gate) line SL 1  both made of metal are formed on a substrate  21  by photolithography. 
   (B) There are formed on the substrate  21  an insulating layer  24 , an intrinsic semiconductor layer  42 , and a channel protective layer  41 . (C) The channel protective layer  41  is partially removed with leaving portions only over the scan line SL 1  and the common electrode  23 D by photolithography. 
   (D) An n+ semiconductor layer  43 , a conductive layer  25 D and an insulating layer  26 D are formed on the semiconductor layer  42 , and these layers are etched into the same pattern not only to form the source S and drain D of TFT  29  over the scan line SL 1  but also to simultaneously form the stripe electrodes of the pixel electrode  25 D and the insulating layer  26 D over stripe electrodes of the common electrode  23 D. The conductive layer  25 D has conductive layers  25   a,    25   b  and  25   c,  for example, Ti/Al/Ti. If only the Al layer is used as the electrode  25 D, the Al diffuses into the n+ semiconductor layer  43 , therefore a Ti layer is used in order to avoid this diffusion, while if only the Ti layer is used, a resistance value becomes large, and therefore the Al layer is also used. The insulating layer  26 D is a silicon nitride layer or a silicon oxide layer formed by means of DVD. 
   Note that if the two layer structure of Ti/Al is used as the pixel electrode  25 D and aluminum nitride is used as the insulating layer  26 D, these layers can be continuously grown by a sputter device, resulting in reducing the number of steps of the fabrication process. Further, as the insulating layer  26 D, a photoresist used for patterning may be left over the stripe electrodes of the common electrode  23 D. 
   (E) A vertically oriented layer  27  drawn with a thick line in  FIG. 30  is coated on the insulating layers  24  and  26 . 
   According to the six embodiment, since by forming the TFT  29 , the stripe electrodes, each having a convex shape in cross section, of the pixel electrode  25 D and the insulating layer  26 D thereon are also formed simultaneously, there is no need to increase the steps of fabrication process in order to form the pixel electrode  25 D and the insulating layer  26 D. 
   Although preferred embodiments of the present invention has been described, it is to be understood that the invention is not limited thereto and that various changes and modifications may be made without departing from the spirit and scope of the invention.