1. Field of the Invention
The present invention relates to a semiconductor device having circuits structured by electric field effect transistors (FETs), for example, thin film transistors (TFTs), and to a method of manufacturing the semiconductor device. The present invention relates, for example, to a semiconductor device, typically a liquid crystal display panel, and to an electronic device in which such a semiconductor device is mounted as its component.
Note that, throughout this specification, the term electro-optical device indicates general devices for performing shading display by changing an electrical signal, and that liquid crystal display devices and display devices using electroluminescence (EL) are included in the category of electro-optical device.
Note also that, throughout this specification, the term element substrate indicates general substrates on which active elements such as TFTs and MIMs are formed.
2. Description of the Related Art
Techniques of structuring thin film transistors using semiconductor thin films (having thicknesses on the order of several nm to several hundred nm) formed on a substrate having an insulating surface have been focused upon in recent years. The thin film transistors are being widely applied to electronic devices like ICs and semiconductor devices, and in particular, their development has accelerated rapidly as switching elements of liquid crystal display devices.
Liquid crystal display devices are known to be roughly divided into active matrix types and passive matrix types.
A high-grade image can be obtained with active matrix liquid crystal display devices using TFTs as switching elements. Active matrix applications are generally to notebook type personal computers, but they are also expected to be used in televisions for a home and in portable information terminals.
The active matrix liquid crystal display devices are generally driven by line inversion drive. With the line inversion drive, for example source line inversion drive, the polarity of voltages applied to adjacent source lines differs, as shown in FIGS. 37A and 37B, and the polarity of the voltage applied to each source line changes each frame. FIGS. 37A and 37B show the polarity of voltages applied to pixels during the source line inversion drive. Drive in which the polarity of the voltage differs for each adjacent source line is referred to as the source line inversion drive. Drive in which the polarity of the voltage differs for each adjacent gate line is referred to as gate line inversion drive.
FIG. 10 shows schematically a cross section of a pixel portion of a liquid crystal display device. An electric field formed between pixel electrodes 102a and 102b formed on a substrate 101, and an opposing electrode 103 formed on an opposing substrate 104, as shown in FIG. 10, is referred to as a vertical direction electric field 105 in this specification. Further, an electric field formed between the adjacent pixel electrodes 102a and 102b is referred to as a horizontal direction electric field 106 in this specification.
Liquid crystals in the vicinity of the pixel electrodes orient themselves along the horizontal direction electric field if the line inversion drive is performed, the liquid crystal orientation in edge portions of the pixel electrodes becomes nonuniform, and disclinations develop. In order to obtain a good quality black level, light shielding films for covering the disclinations are necessary. However, the aperture ratio drops if the disclinations are covered by the light shielding films. It is necessary to come up with a scheme in which a good quality black level can be obtained, and as little disclination as possible develops when displaying a high aperture ratio, bright image. Note that, in this specification, liquid crystal orientation irregularities developing due to differences in the direction of the pre-tilt angle, and differences in the twist direction, at liquid crystal orientation film interfaces are referred to as xe2x80x9cdisclinationsxe2x80x9d. Further, regions having different brightnesses produced due to an irregular orientation state of the liquid crystals when a polarization plate is formed is referred to as xe2x80x9clight leakagexe2x80x9d.
In particular, the occupied ratio of pixels in which disclinations and light leakage developing due to horizontal direction electric fields is large enough that it cannot be ignored in liquid crystal display devices in which the pixels are formed at a very fine pitch, such as that of a projecting liquid crystal display device. Further, these disclinations and light leakage are expanded when projected onto a screen with the projecting liquid crystal display device, and therefore whether or not the light leakage and disclinations can be suppressed is vital in maintaining contrast.
An object of the present invention is to provide an element structure such that liquid crystal disclination and light leakage can be stopped in an active matrix liquid crystal display device.
The following measures are taken in order to solve the above stated problems with the conventional technique.
Overlapping edge portions of pixel electrodes with predetermined height convex portions.
FIG. 2 is a cross sectional diagram of a simulation model. The present invention utilizes moving disclinations and light leakage, which are caused when a voltage is applied to liquid crystal 202, to edge portions of pixel electrodes by forming edge portions of pixel electrodes 203a and 203b on a first substrate (not shown) so as to overlap with convex portions 204 formed on a level surface as shown in FIG. 2. An opposing electrode 201 is formed in an opposing substrate 207.
Note that, in this specification, the convex portions are formed selectively below the pixel electrodes. Regions in which the pixel electrodes overlap with upper edge portions of the convex portions are referred to as first regions (a) of the pixel electrodes. Regions in which the pixel electrodes are formed in side portions of the convex portions are referred to as second regions (b) of the pixel electrodes. Regions in which the pixel electrodes are formed on a level surface, and which contact the second regions of the pixel electrodes, are referred to as third regions (c) of the pixel electrodes.
Further, the height (h) of the convex portions is the maximum value of the length of a vertical line formed from the upper edge portions of the convex portions to the level surface on which the convex portions are formed.
In addition, a cell gap (d) is the distance from the opposing electrode formed on the opposing substrate (second substrate) to the third region of the pixel electrode.
An inter-pixel electrode distance (s) is the distance between the first regions of mutually adjacent pixel electrodes.
Conventionally, if there are convex portions in the orientation surface of liquid crystals, then the orientation of the liquid crystal is disordered and light leakage develop at the convex portions, and therefore it is thought that a liquid crystal orientation surface should be as level as possible. However, the applicants of the present invention found that when the first regions of the pixel electrodes formed on the convex portions having a predetermined height, and the second regions of the pixel electrodes formed on the side portions of the convex portions having a predetermined height, are present, simulation results on the liquid crystal orientation show that liquid crystal orientation irregularities caused by the horizontal direction electric field in driving a liquid crystal display device can be reduced. Specifically, locations at which disclinations and light-leakage appear are in the edges of the pixel electrodes during black display.
This phenomenon is explained by the schematic diagrams of FIGS. 1A to 1C which show the principles of the present invention. The liquid crystal orientation method is taken as a TN method. FIGS. 1A to 1C show liquid crystal orientations when driving a liquid crystal display device by a 5 V or xe2x88x925 V video voltage with line inversion drive. An orientation film is not shown in the figures.
First, as shown in FIG. 1A, if edge portions of the pixel electrodes 203a and 203b formed on the first substrate (not shown in the figures) are formed on the convex portion 204, then it becomes more difficult for disclinations to develop, compared to the case where no convex portion presents. Nevertheless, if the height of the convex portion is low, then the influence of a horizontal direction electric field formed between the pixel electrode 203a and the pixel electrode 203b is strong with respect to a vertical direction electric field formed between the pixel electrodes 203a and 203b and the opposing electrode 201 formed on the second substrate (not shown in the figures). Liquid crystal molecules 208 in the vicinity of the convex portion orient with an inclination in a diagonal direction with respect to the substrate surface. Thus, light leakage can be seen below a cross Nicol polarization plate. Further, a pre-tilt angle of liquid crystal molecules 209 in the vicinity of the interface of the orientation film is determined by rubbing directions 205 and 206, and therefore a disclination 210 having high light strength is formed at the point where the direction of the horizontal direction electric field in the vicinity of the interface differs from the direction of the liquid crystal molecules in the vicinity of the interface due to rubbing.
However, the higher the convex portion becomes, the more the position at which the disclination 210 seen in FIG. 1A appears changes to the edge of the pixel electrode. In addition, if the convex portion 204 overlapping with the edge portions of the pixel electrodes 203a and 203b is made higher, then a vertical direction electric field formed between a first region 215 of the pixel electrode and the opposing electrode 201 becomes stronger with increasing the height of the convex portion, as shown in FIG. 1B, and the influence of the horizontal direction electric field becomes weaker. Several vertical direction electric fields are formed almost in the substrate surface by an electric field formed between a second region 212 of the pixel electrode, among the pixel electrodes 203a and 203b, formed in the side portion of the convex portion and the opposing electrode 201. TN method liquid crystals are positive type liquid crystals, and therefore the longitudinal axes of liquid crystal molecules 211 orient parallel to the electric fields. Disclination and light leakage in the vicinity of the convex portion is thus reduced.
Next, as shown in FIG. 1C, if the height of the convex portion 204 is made higher, then the liquid crystals on the pixel electrode 203b form an electric field having a direction diagonal with respect to the substrate surface, the same direction as the pretilt angle, between the opposing electrode and the second region of the pixels. Thus, liquid crystal molecules 213 orient diagonally with respect to the substrate surface, along the electric field, and a light leakage having a width which cannot be ignored is formed in the vicinity of the convex portion. On the pixel electrode 203a, an electric field having a direction diagonal with respect to the substrate surface and formed between a second region 216 of the pixel electrode and the opposing electrode is opposite to the direction of the pre-tilt angle of the liquid crystals, and therefore it is difficult for liquid crystals 217 to follow the electric field, and formation of light leakage becomes relatively difficult. However, the width of the light leakage formed due to the electric field formed between the opposing electrode and the second region 212 of the pixel electrode 203b becomes wider, and overall, the aperture ratio drops.
It can be understood from the above that the optimal value of the height of the convex portion exists in order to reduce both the disclination of black display and the width of the light leakage when the edge portions of the pixel electrodes are formed on the convex portion. If the convex portion is too tall, then overall, the width of the light leakage will become larger (see FIG. 1C.) The structures of FIG. 1A and FIG. 1B can increase the aperture ration. The subsequent simulation results substantiate these principles.
The optimal value of the height of the convex portion can be considered to be determined with a cell gap (namely, an element for determining the strength of the vertical direction electric field) as a parameter.
The applicants of the present invention performed simulations, and confirmed the optimal value of the convex portion height.
Disclination and light leakage due to horizontal direction electric fields become problems in particular when the pixel area is small, and the proportion of the pixel occupied by the disclination and the light leakage is large enough that it cannot be ignored. In other words, mainly for cases where the device is used as a projection liquid crystal display device. Projection liquid crystal display devices have a small pixel pitch, and inevitably the distance between the pixel electrodes is small at 4.0 xcexcm or less. The applicants of the present invention performed simulations focusing on inter-pixel electrode distances of equal to or less than 4.0 xcexcm in order to reduce the disclination and the light leakage in the projection liquid crystal display device.
A simulation model is shown in FIG. 2. The opposing electrode 201, the liquid crystals 202, the convex portion 204, and the pixel electrodes 203a and 203b become the structural elements of the simulation model in FIG. 2. The simulation model of FIG. 2 was taken as one unit, and periodically repeated.
The simulation parameters are as follows:
cell gap, d: 4.5 xcexcm, 3.0 xcexcm;
distance between pixel electrodes, s: 2 xcexcm, 4 xcexcm;
convex portion height, h: 0 xcexcm, 0.2 xcexcm, 0.3 xcexcm, 0.4 xcexcm, 0.5 xcexcm, 0.7 xcexcm, 1.0 xcexcm, and 1.5 xcexcm; and
pixel pitch, p: 18 xcexcm, 43 xcexcm.
Fixed conditions in the simulations are as follows:
width of first region of pixel electrode, o: 1.0 xcexcm;
electric potential of the pixel electrode 203a: +5 V;
electric potential of the pixel electrode 203b: xe2x88x925 V; and
electric potential of the opposing electrode 201: 0 V.
In order to generalize the relationship between the distance s between pixel electrodes, the cell gap d, and the height h of the convex portion on which the edge portion of the pixel electrode is formed, the simulation was performed with cell gaps of 4.5 xcexcm and 3.0 xcexcm. ZLI4792 was used for the liquid crystals for both cell gaps, 4.5 xcexcm and 3.0 xcexcm, and the orientation was found by computation.
The pre-tilt angle of the liquid crystals was set to 6.0xc2x0, and the chiral pitch was set to left handed at 70 xcexcm. The rubbing direction 205 and 206 are shown in FIG. 2. The twist angle is 90xc2x0. The orientation of the liquid crystal is TN method.
Further, in order to increase the number of evaluations and grasp the tendencies, the simulations were performed as stated above with two pixel pitches.
Liquid crystal orientation simulation software from Syntech Corp. entitled LCD Master 2SBENCH was used, and simulations of the liquid crystal orientation ware run. 2SBENCH shows the liquid crystal orientation by a two-dimensional planer surface composed of the cell cap direction and the substrate surface direction.
The simulation results are as shown below. FIGS. 3 to 8 are partial blowup diagrams of the simulation results.
FIGS. 3 to 8 are simulations in which the height of the convex portion was changed at conditions of the gap d set to 4.5 xcexcm, the inter-pixel spacing distance s set to 2.0 xcexcm, and the pixel pitch p set to 18 xcexcm. The transmittivity in each coordinate, calculated from equipotential lines, liquid crystal directors, and index of refraction anisotropy, is shown. The pixel electrode 203a has coordinates from 1 to 17 xcexcm, and the pixel electrode 203b has coordinates from 19 to 35 xcexcm, and FIGS. 3 to 8 show blowups of the points at which there is light leakage and disclination in the vicinity of the convex portion. The liquid crystal is a positive type, and therefore lines of electric force can be considered to have nearly the same direction as the director of the liquid crystals.
The orientation of the liquid crystals on a pixel electrode having a xe2x88x925V electric potential is explained below.
A horizontal electric field is formed up through a region entering the inside of the pixel electrode when there is no convex portion, as shown in FIG. 3. Further, disclinations develop in regions in which the direction of the horizontal electric field and the direction of the liquid crystal pre-tile angle are opposite.
When the height of the convex portion is 0.3 xcexcm, the vertical direction electric field becomes stronger due to the first region of the pixel electrode formed in the upper edge portion of the convex portion, and therefore the position of the disclination moves toward the outside of the pixel electrode, as shown in FIG. 4, compared with the case of no convex portion in FIG. 3.
If the height of the convex portion becomes taller at 0.7 xcexcm, as shown in FIG. 5, then there are many liquid crystals which become oriented perpendicular to the substrate along the lines of electric force due to the effect of the horizontal direction electric field becoming stronger, and due to the effect of the electric force lines, due to the second region of the pixel electrode formed in the side portion of the convex portion and the opposing electrode, possessing components nearly perpendicular with respect to the substrate surface. The disclination in the vicinity of the convex portion becomes less.
When the height of the convex portion becomes higher at 1.0 xcexcm, the width of the disclination in the vicinity of the convex portion drops by just 0.2 xcexcm compared with the case in which the convex portion height is 0.7 xcexcm, as shown in FIG. 6.
As shown in FIG. 7, the lines of electric force formed by the second region of the pixel electrode formed in side portion of the convex portion, and by the opposing electrode, possess an angle order of 60xc2x0, with respect to the substrate surface, in the vicinity of the convex portion, which becomes at 1.5 xcexcm. The liquid crystals orient along the electric force lines, and light leakage is formed near the convex portion.
Note that the angle of the electric force lines with respect to the substrate surface is estimated from the distribution of equipotential lines.
In FIG. 8, the height of the convex portion increases to 3.0 xcexcm, and the vertical direction electric field becomes stronger. The liquid crystals on the third region of the pixel electrode of the edge portion of the convex portion therefore orient nearly perpendicular to the surface of the substrate. However, the electric field formed between the second region of the pixel electrode formed in the side portion of the convex portion, and the substrate surface, possesses an angle on the order of 30xc2x0, with respect to the substrate surface, and the liquid crystals orient along the lines of electric force. Broad light leakage is consequently formed in the vicinity of the convex portion.
The orientation of the liquid crystals of FIG. 4 corresponds to the schematic diagram of FIG. 1A. The orientation of the liquid crystals of FIG. 5 and FIG. 6 corresponds to the schematic diagram of FIG. 1B, and the orientation of the liquid crystals of FIG. 7 and FIG. 8 corresponds to the schematic diagram of FIG. 1C. Namely, it is confirmed that the light leakage of the liquid crystals becomes larger if the height of the convex portion exceeds an upper limit.
In order to improve the display quality, a systematic simulation was performed. The distance between one edge portion and another edge portion of the disclination and the light leakage influencing the aperture ratio was paid attention to.
Data was also taken regarding the light leakage and the disclination width of a pixel electrode having a xe2x88x925 V electric potential. This is because the light leakage and the disclination of a pixel electrode having a xe2x88x925 V electric potential greatly influences the display quality due to high light strength.
FIG. 9 and FIG. 36 show the simulation results. FIG. 9 is a figure in which the height h of the convex portion, the light leakage and the disclination width x are graphed with respect to the cell gap d in the simulation model of FIG. 2. The term light leakage and disclination width x denotes the width of a region having high brightness due to disclination and light leakage formed on both sides of the convex portion.
FIG. 36 is a figure in which the height h of the convex portion, and the light leakage and the disclination width y are graphed with respect to the cell gap d in the simulation model of FIG. 2. The term light leakage and disclination width y denotes the width of a region having high brightness due to disclination and light leakage formed in one side of the convex portion, namely in an electrode side having a xe2x88x925 V electric potential.
Simulations were performed with the pixel pitch p set to 18 xcexcm and 43 xcexcm when the cell gap was 4.5 xcexcm. Further, the distance s between adjacent pixel electrodes was set to 2.0 xcexcm or to 4.0 xcexcm.
When the cell gap was 3.0 xcexcm, the distance s between adjacent pixel electrodes was set to 2.0 xcexcm or 4.0 xcexcm. The pixel pitch p was set to 18 xcexcm.
The relationship between the convex portion height, and the width of the light leakage and the disclination shows similar tendencies in both FIG. 9 and FIG. 36.
First, the relationship between the convex portion height and the light leakage and disclination width nearly did not change in accordance to pixel pitch. This is because formation of light leakage and disclination is a phenomenon caused by the horizontal direction electric field and the vertical direction electric field of the edge portions of the pixel electrodes.
Further, light leakage and disclination become relatively less with a smaller distance between adjacent pixel electrodes.
The regions of orientation irregularities of the liquid crystals, typically light leakage and disclination, are reduced along with increasing height of the pixel portion, and are not due to the pixel pitch p and the cell gap d, in both FIG. 9 and FIG. 36. If the height of the convex portion becomes too tall, there are conversely more regions of orientation irregularities. An optimal convex portion height is determined by the distance between the cell gap and the pixel electrode.
Considering the inflection point of the graph, it is preferable that the height of the convex portion in which the effect of a reduction in the liquid crystal regions having orientation irregularities appears significantly, be equal to or greater than 4.4% of the cell gap, and equal to or less than 22.5% of the cell gap when the call gap is 4.5 xcexcm.
When the cell gap is 3.0 xcexcm as well, and the distance s between the pixel electrodes is equal to or less than 2.0 xcexcm, a good effect of reducing, compared to a case in which there is no convex portion, the regions having orientation irregularities can be obtained by setting the height of the convex portion to be equal to or greater than 4.4% of the cell gap, and equal to or less than 22.5% of the cell gap.
If the height of the convex portion is less than 4.4% with respect to the cell gap, then the width of the light leakage and the disclination does not change much, even if the height of the convex portion is increased. If the height of the convex portion exceeds 22.5% to the cell gap, then the light leakage and the disclination width increases.
Further, orientation irregularities of the liquid crystals easily develop due to rubbing non-uniformity if the convex portion is high, and therefore reducing the height of the convex portion, thereby reducing the light leakage and the disclination width, is preferable to insure display quality. Consequently, for a case in which the cell gap is 4.5 xcexcm, the height of the convex portion may be suppressed to be greater than or equal to 4.4% of the cell gap, and less than or equal to 15.6% of the cell gap. Nearly equal light leakage and disclination reduction effects are obtained when the height of the convex portion is with the range of greater than or equal to 4.4%, and less than or equal to 22.5%, of the cell gap.
Further, when the cell cap is 3.0 xcexcm and the distance s between the pixel electrodes is equal to or less than 2.0 xcexcm, a good effect of reducing the light leakage and the disclination is obtained when the height of the convex portion is greater than or equal to 4.4% of the cell gap, and less than or equal to 15.6% of the cell gap, the same as when the height of the convex portion was set equal to or greater than 4.4%, and equal to or less than 22.5%, of the cell gap.
Conversely, there was more light leakage when the height of the convex portion was 22.5% when the distance s between the pixel electrodes was set to 4.0 xcexcm. Inclusive of when the distance between the pixel electrodes is 4.0 xcexcm, it is therefore preferable that the height of the convex portion be equal to or greater than 4.4%, and equal to or less than 15.6%, or the cell gap.
In other words, under conditions of the distance between the pixel electrodes being equal to or less than 4.0 xcexcm, the height of the convex portion may be set equal to or greater than 4.4%, and equal to or less than 22.5%, of the cell gap when the cell gap is equal to or greater than 3.0 xcexcm, and equal to or less than 4.5 xcexcm. The height of the convex portion is preferably set greater than or equal to 4.4%, and less than or equal to 15.6%, of the cell gap.
The smaller the cell gap, the smaller the height of the convex portion needed for reducing the light leakage and the disclination width. For cases of the call gap being from 3.0 xcexcm to 4.5 xcexcm, good liquid crystal orientation can be obtained with a convex portion height of 15.6% or less. For cases of the cell gap being less than 3.0 xcexcm, it is therefore considered that the necessary convex portion height will be 15.6% or less to sufficiently reduce the light leakage and disclination width.
The height of the convex portion may be set equal to or less than 15.6% of the cell gap when the cell gap is less than or equal to 3.0 xcexcm. Of course, considering the inflection point of the graph, it can be expected that a good effect will also be obtained if the height of the convex portion is set equal to or less than 6.7% of the cell gap.
When the cell gap is 3.0 xcexcm, the light leakage and the disclination width are monotonically reduced as the convex portion becomes higher, provided that the height of the convex portion is equal to or less than 6.7% with respect to the cell gap. If the cell gap is made smaller at less than or equal to 3.0 xcexcm, it can be considered that the region in which the light leakage and the disclination width are monotonically reduced with increasing convex portion height is in a range in which the height of the convex portion does not exceed 6.7% with respect to the cell gap.
An upper limit to the height of the convex portion, or both upper and lower limits, can thus be determined. There is a concern that rubbing irregularities may occur if the fiber tips of a rubbing cloth are disordered, and therefore the determination of an upper limit for the height of the convex portion is necessary in order to manufacture a liquid crystal panel. Further, the optimal value of the height of the convex portion with respect to the cell gap tends to become smaller as the cell gap becomes smaller.
An optimal value for the height of the convex portion thus determined can be used not only for the TN method, but can also be widely used as means for hiding liquid crystal disclination in a normally white mode orientation method.
The optimal value of the convex portion height is one in which the lines of electric force formed by a horizontal direction electric field and a vertical direction electric field of an active matrix liquid crystal display device are suitably regulated, and as shown in FIG. 1B, one in which regions with generated electric force lines possessing components perpendicular to the surface of the substrate are increased in the edge portions of the pixel electrode.
Therefore, although the simulations were performed for a transmitting type liquid crystal display device, it can be considered possible to apply the present invention to a reflective type liquid crystal display device as well. This is because a voltage may also be applied to the pixel electrodes with a reflective type liquid crystal display device, and when orienting the liquid crystals by a vertical direction electric field, unnecessary electric fields directed diagonally with respect to the substrate surface are reduced, and light leakage and disclination of the edge portions of the pixel electrodes can be reduced.
Further, the simulation was made using the TN method, but the liquid crystal orientation method is not limited to the TN method. This is because, in an active matrix liquid crystal display device, unnecessary electric fields directed diagonally with respect to the substrate surface can be reduced by optimizing the convex portion height when orienting the liquid crystals by using a vertical direction electric field. For example, it is considered that it is possible to apply the present invention to methods such as an OCB (optically controlled birefringence) method, an STN method, and an EC method using homogeneously oriented cells.
Further, provided that orientation faults are not induced in the liquid crystals by the convex portion, it is thought that it is also possible to apply the present invention to an orientation method using sumectic liquid crystals. For example, it is possible to apply the present invention to a liquid crystal display device using ferroelectric liquid crystals or anti-ferroelectric liquid crystals. Further, by adding high molecular weight molecules with liquid crystal properties to these liquid crystals, it is thought that it is also possible to apply the present invention to liquid crystal display devices using materials hardened by irradiation of light (for example, ultraviolet light).
The simulations were performed with an angle 90xc2x0 (hereafter referred to as a convex portion taper angle) formed between the surface contacting the second region of the pixel electrode formed in the side surface of the convex portion, and the third region of the pixel electrode formed in the level surface. However, it is also possible to apply the present invention if the convex portion taper angle is less than 90xc2x0. As shown in the cross sectional diagram of FIG. 35 for the lines of electric force when the convex portion has a taper, the electric force lines are generated in a direction perpendicular to a conductor for cases in which the taper angle xcex8 of the convex portion 204 is less than 90xc2x0, and therefore the bend of electric force lines 218 formed between the opposing electrode 201 and second regions 219 of the pixel electrodes 203a and 203b becomes relaxed when the convex portion has a taper. Liquid crystals 220 orient very well perpendicular with respect to the substrate surface. A very large effect in reducing light leakage and disclination is therefore obtained, compared to when the taper angle is 90xc2x0, by using the relationship shown in the present invention between a convex portion height 221 and the cell gap when the convex portion has a taper.
Width of the first region of the pixel electrode on the upper portion of the convex portion
Next, changes in the orientation of liquid crystals when the width of the first region of the pixel electrode formed overlapping with the upper portion of the convex portion is investigated.
A simulation model is shown in FIG. 2. The opposing electrode 201, the liquid crystals 202, the convex portion 204, and the pixel electrodes 203a and 203b become the structural elements in FIG. 2.
The simulation parameters are as follows:
cell gap, d: 4.5 xcexcm;
distance between pixel electrodes, s: 2 xcexcm, 4 xcexcm;
convex portion height, h: 0 xcexcm, 0.5 xcexcm; and
width of first region of pixel electrode, o: xe2x88x921 xcexcm, xe2x88x920.5 xcexcm, 0 xcexcm, 0.5 xcexcm, 1.0 xcexcm.
The symbol for the width o of the first region of the pixel electrode, such as xe2x88x921.0 xcexcm, indicates that the pixel electrode is not formed on the convex portion, and that the edge portion of the pixel electrode is located at 1.0 xcexcm from the convex portion.
Fixed conditions in the simulations are as follows:
electric potential of the pixel electrode 203a: +5 V;
electric potential of the pixel electrode 203b: xe2x88x925 V;
electric potential of the opposing electrode 201: 0 V; and
pixel pitch, p: 18 xcexcm.
Simulation results are shown in the cross sectional diagrams of FIG. 11 to FIG. 15. The distance s between pixel electrodes is 2.0 xcexcm.
In FIG. 11, there is no convex portion. The convex portion and the pixel electrode do not mutually overlap in FIG. 12, and the edge of the pixel electrode is located 0.5 xcexcm from the edge of the convex portion. In other words, the second region of the pixel electrode and the first region of the pixel electrode do not exist in FIGS. 11 and 12. Light leakage from the edges of the pixel and the disclination width x shows no change at all at this point in FIG. 11 and FIG. 12.
The pixel electrode is formed in the side portion of the convex portion in FIG. 13. Namely, there is a second region of the pixel electrode. Compared to FIGS. 11 and 12, the position of the disclination on a pixel electrode having a xe2x88x925 V electric potential, moves 0.4 xcexcm to the pixel edge. The vertical direction electric field is made stronger, and the horizontal direction electric field becomes a little weaker due to the second region of the pixel electrode.
The first region of the pixel electrode formed in the edge portion on the convex portion, and the second region of the present invention formed in the side portion of the convex portion exist in FIG. 14. The width of the first region of the pixel electrode is 0.5 xcexcm. The vertical direction electric field becomes stronger, and the disclination on the pixel electrode having a xe2x88x925 V electric potential moves to the edge of the pixel electrode due to the first region of the pixel electrode.
The width of the first region of the pixel electrode is set to 1.0 xcexcm in FIG. 15, compared to 0.5 xcexcm in FIG. 14. The vertical direction electric field is made additionally stronger with respect to the horizontal direction electric field due to the 1.0 xcexcm width, and the disclination on the pixel electrode having a xe2x88x925 V electric potential moves to the edge of the pixel electrode.
It can thus be understood that there is a disclination reduction effect due to the existence of the first region of the pixel electrode and the second region of the pixel electrode.
Next, data is added for when the distance s between the pixel electrodes is 4.0 xcexcm, and data is systematically taken. FIGS. 16A and 16B show the simulation results. FIG. 16A is a graph of the width o of the first region of the pixel electrode, and the light leakage and the disclination width x, versus the cell gap d, in the simulation model of FIG. 2. The term light leakage and disclination width x here indicates the width of a region having high brightness causes by disclination and light leakage formed in both sides of the convex portion.
FIG. 16B is a graph of the width o and the light leakage and the disclination width y, versus the cell gap d, in the simulation model of FIG. 2. The term light leakage and disclination width y here indicates the width of a region having high brightness causes by disclination and light leakage formed in one side of the convex portion, namely in the electrode side having an electric potential of xe2x88x925 V.
From FIGS. 16A and 16B, it can be understood that there is an effect in which disclination and light leakage are reduced if the width o of the first region of the pixel electrode is equal to or greater than 0.5 xcexcm, preferably equal to or greater than 1.0 xcexcm, without dependence on the distance s between the pixel electrodes.
The light leakage and the disclination width when the width of the first region of the pixel electrode is 0 xcexcm in FIGS. 16A and 16B shows a light leakage and disclination width of a state in which the pixel electrode is only formed in the side surface of the convex portion. Compared to cases in which the width of the first region of the pixel electrode is equal to or greater than 0.5 xcexcm, or equal to or greater than 1.0 xcexcm, the effect of reducing the light leakage and the disclination width is reduced. However, compared to the case in which the width of the first region of the pixel electrode is xe2x88x920.5 xcexcm, in which the pixel electrode does not contact the convex portion at all, the light leakage and the disclination width do drop.
An actual experiment in which the width over which the convex portion and the pixel electrode overlap was carried out. FIG. 33A is an upper surface diagram of a substrate having a convex portion, and FIGS. 33B and 33C are cross sectional diagrams of the substrate having the convex portion.
Pixel electrodes 301a denoted by slanted line portions in the upper surface diagram of FIG. 33A all have the same electric potential. Further, the pixel electrodes 301b denoted by vertical line portions all have the same electric potential. This is in order to connect adjacent pixel electrodes by a transparent conducting film 300 having a width of 3 xcexcm. Assuming line inversion driving, an electric potential of +5 V is imparted to the pixel electrodes 301a. In addition, an electric potential of xe2x88x925 V is imparted to the pixel electrodes 301b. A rubbing direction 302 for the substrate having a convex structure is shown in the figures. A rubbing direction for a substrate opposing the substrate having the convex structure is perpendicular to the rubbing direction 302.
A cross section of the upper surface diagram of FIG. 33A cut along the dashed line G-Gxe2x80x2 is shown in FIG. 33B. A cross section of the upper surface diagram of FIG. 33A cut along the dashed line H-Hxe2x80x2 is shown in FIG. 33C. Identical reference symbols are used for the same portions as in FIG. 33A. The edge portions of the pixel electrode 301a and 301b formed on the substrate 303 contact a convex portion 304. The distance between the adjacent pixel electrodes 301a and 301b was set constant at 2.0 xcexcm, and the liquid crystal orientation was confirmed by changing the width of the pixel electrode overlapping on the convex portion, namely by changing the width 305 of the first region of the pixel electrode. The width of the first region of the pixel electrode was set to xe2x88x921.0 xcexcm, 0 xcexcm, 0.5 xcexcm, and 1.0 xcexcm. The cell gap was 4.5 xcexcm, the height of the convex portion was 0.5 xcexcm, and the pixel pitch was 18 xcexcm.
Photographs of the liquid crystal orientation when the pixel electrode structure of FIGS. 33A to 33C was used are shown in FIGS. 34A to 34D. The adjacent pixel electrodes in the horizontal direction of the page have the same electric potential. The rubbing direction was perpendicular to the page. An effect of reducing the disclination width was found in the experiments as well if the width o of the first region of the pixel electrodes was equal to or greater than 0.5 xcexcm, preferably equal to or greater than 1.0 xcexcm. It was found that the disclination and the light leakage width extending in the horizontal direction of the page decreases along with increases in the width o of the first region or the pixel electrodes.