JP2004012795A - Reflection type liquid crystal display element and liquid crystal display device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To eliminate a battery effect to be a cause of asymmetry of a liquid crystal response, to dispense with offset voltage to be applied to driving voltage and to secure high reliability even when long term driving is performed, in a reflection type liquid crystal display element. <P>SOLUTION: A coating layer 43 consisting of a conductive thin film of the same material as a transparent electrode is formed on the surface side opposed to the transparent electrode of a pixel electrode 42A. Thereby, the battery effect between electrodes opposed to each other is extinguished, asymmetry of the liquid crystal response is canceled and offset voltage conventionally needed is not needed. As a result, long term reliability when the reflection type liquid crystal display element is driven can be enhanced. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a reflective liquid crystal display device having a reflective pixel electrode and a liquid crystal display device such as a reflective liquid crystal projector using the same.
[0002]
[Prior art]
2. Description of the Related Art Conventionally, liquid crystal display elements have been used in various display devices such as projection displays (projectors). The liquid crystal display element is also called a liquid crystal panel or a liquid crystal cell. Liquid crystal display elements are roughly classified into a transmissive type and a reflective type. Both types have a configuration in which liquid crystal is sealed between a pixel electrode substrate and an opposing substrate facing the pixel electrode substrate. In the transmission type liquid crystal display device, a transparent electrode made of, for example, ITO (Indium Tin Oxide) is provided on both the pixel electrode substrate and the counter substrate.
[0003]
On the other hand, in recent years, as liquid crystal projectors have been developed with higher definition, smaller size and higher brightness, reflection type liquid crystal display devices can be reduced in size and definition and can be expected to have high light use efficiency. It has been attracting attention and has actually been put to practical use. In this reflective liquid crystal display element, a transparent electrode made of, for example, ITO is provided on the counter substrate side, and a reflective pixel electrode (hereinafter, also simply referred to as “reflective electrode”) is provided on the pixel electrode substrate side. A reflection type liquid crystal display element used for a liquid crystal projector is generally of an active type, and for example, a pixel electrode substrate formed by forming a semiconductor switching circuit of a C-MOS (Complementary-Metal Oxide Semiconductor) type on a silicon substrate. Used. The reflection electrode is arranged on the silicon driving element substrate. The reflective electrode has a function of reflecting light incident from the counter substrate side and a function of applying a voltage to the liquid crystal. As a material of the reflective electrode, a metal material mainly containing aluminum (Al), which is generally used in an LSI (Large Scale Integrated) process, is used.
[0004]
In this reflective liquid crystal display device, a voltage is applied to the liquid crystal by the transparent electrode and the pixel electrode provided on each substrate. At this time, the liquid crystal changes its optical characteristics according to the potential difference between the opposing electrodes, and modulates the incident light. This light modulation enables gradation expression, and the modulated light is used for image display.
[0005]
[Problems to be solved by the invention]
By the way, in the liquid crystal display element, in order to prevent ions existing in the liquid crystal from burning on one of the substrates during driving, the polarity of the driving voltage is inverted ± every predetermined period, and a voltage is applied between the electrodes. A general driving method is generally used. FIG. 7 shows a conceptual diagram of a driving voltage according to this driving method. As shown by the thick solid line in the figure, if the absolute values of the voltages of the respective polarities applied between the counter electrodes are the same at V1, there is no difference in the effective voltage applied to the liquid crystal. No phenomenon such as burn-in occurs. However, in practice, especially in the case of a reflective liquid crystal display element, there is a difference in the effective voltage applied to the liquid crystal when the applied voltage is plus and minus. This is due to the fact that the electrode material used for each substrate is different in the reflective liquid crystal display element.
[0006]
That is, in the reflection type liquid crystal display element, as described above, ITO is generally used as the transparent electrode, and an aluminum metal film slightly mixed with copper or the like is used for the opposing pixel electrode. In this case, since the standard electrode potentials of the ITO and aluminum electrodes themselves are different, a battery effect occurs in an element using these dissimilar metal electrodes. The standard electrode potential of aluminum is -1.66 V. When this aluminum electrode and the ITO electrode are combined, a considerably large battery effect occurs between the electrodes. In the simple electrode system, the standard electrode potential refers to the internal potential of the electrode phase with respect to the solution phase when no current flows at the interface between the electrode and the solution and the electrode reaction is in an equilibrium state.
[0007]
For this reason, even when voltages having the same absolute value in each polarity are externally applied as shown by a thick solid line in FIG. 7, an electromotive force is generated by a battery effect, and an asymmetric voltage is applied to the liquid crystal. As a result, the reflectivity of the device differs depending on the polarity of the applied voltage, causing flicker and accumulating an internal voltage in the device, causing problems such as image sticking. If an aluminum electrode is used in place of the ITO transparent electrode and the opposite electrode is the same aluminum electrode, the battery effect is canceled and the above-described asymmetry does not occur. However, this is not practical because light does not pass through the element. Naturally, in a normal transmissive liquid crystal device in which opposing electrodes are formed of ITOs, such an asymmetry problem does not occur because the electrodes are of the same kind. Therefore, this asymmetry is an essential problem of the reflection type liquid crystal element.
[0008]
In order to eliminate the asymmetry of the reflectivity, in the reflection type liquid crystal display element, a drive voltage is multiplied by a DC offset voltage ΔV, and as shown by a thick broken line in FIG. Must be applied. For example, when aluminum is used as the material of the reflective electrode and ITO is used as the transparent electrode, the effective voltage difference between the polarities of the liquid crystal becomes 1 V or more, and this is applied as the offset voltage ΔV. However, if the value of the offset voltage ΔV is too large, not only the asymmetry cannot be completely removed, but also the offset voltage ΔV gradually changes from the initial set value during long-term driving, and as a result, the The internal voltage is accumulated in the device and burn-in occurs. For this reason, the reliability during long-term driving decreases. Further, in order to apply the offset voltage ΔV, it is necessary to provide a circuit for the offset voltage ΔV, which complicates the electric circuit. Therefore, it is not preferable that a reflective liquid crystal display element has a battery effect.
[0009]
On the other hand, for example, in JP-A-9-244068 and JP-A-10-54995, as a reflective electrode material, a metal having a standard electrode potential sufficiently lower than that of aluminum, for example, tungsten (W), titanium (Ti), or titanium nitride ( It has been shown that the use of (TiN) can alleviate the above-mentioned problem of the voltage difference, avoid the battery effect, and reduce the offset voltage.
[0010]
However, when tungsten, titanium or titanium nitride is used as the reflective electrode material, a sufficient reflectivity cannot be obtained as compared with commonly used aluminum, and thus there is no suitable electrode material in this regard. Therefore, it is desired to develop a technique capable of reducing the offset voltage without impairing the light reflection function as a reflection electrode.
[0011]
The present invention has been made in view of such a problem, and an object of the present invention is to eliminate a battery effect which causes asymmetry of liquid crystal response, to eliminate an offset voltage applied to a driving voltage, and to perform long-term driving. It is an object of the present invention to provide a reflective liquid crystal display element and a liquid crystal display device that can ensure high reliability even in such cases.
[0012]
[Means for Solving the Problems]
A reflective liquid crystal display element according to the present invention includes a pixel electrode substrate having a reflective pixel electrode made of a metal material, a counter substrate having a transparent electrode provided to face the pixel electrode, and a pixel electrode substrate. And a liquid crystal injected between the substrate and the opposing substrate, and the surface of the pixel electrode facing the transparent electrode is covered with a conductive thin film of the same material as the transparent electrode.
[0013]
The liquid crystal display device according to the present invention performs an image display using light modulated by the above-mentioned reflective liquid crystal display device according to the present invention.
[0014]
In the reflective liquid crystal display element and the liquid crystal display device according to the present invention, the surface of the pixel electrode facing the transparent electrode is covered with a conductive thin film of the same material as the transparent electrode, so that the battery effect between the facing electrodes is obtained. Disappears. This eliminates the asymmetry of the liquid crystal response, and eliminates the need for the offset voltage conventionally required for the drive voltage.
[0015]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
[0016]
[Description of reflective liquid crystal display device]
As shown in FIG. 1, the reflection type liquid crystal display element 21 according to the present embodiment is formed by injecting a liquid crystal material between the opposing substrate 30 and the pixel electrode substrate 40 which are arranged to face each other. Liquid crystal layer 35 provided.
[0017]
The liquid crystal material forming the liquid crystal layer 35 is, for example, a vertical alignment type nematic liquid crystal generally called a vertical alignment liquid crystal. Note that the vertical arrangement refers to a state in which the initial molecular orientation of the liquid crystal is arranged perpendicular to each substrate surface. Generally called vertical alignment liquid crystal.
[0018]
The counter substrate 30 includes a glass substrate 31, and at least a transparent electrode layer 32 and an alignment film 33 are laminated on a surface of the glass substrate 31 on the liquid crystal layer 35 side. As the alignment film 33, for example, a film obtained by rubbing (aligning) a polyimide-based organic compound or an obliquely deposited film of an inorganic material such as silicon dioxide (SiO ₂ ) is used. The surface of the alignment film 33 on the liquid crystal layer 35 side is subjected to a rubbing treatment in order to arrange the liquid crystal molecules in a predetermined arrangement state. The transparent electrode layer 32 is configured such that a transparent electrode having a light transmitting function is provided on the entire surface. As a material of the transparent electrode, generally, ITO which is a solid solution substance of tin oxide (SnO ₂ ) and indium oxide (In ₂ O ₃ ) is used. A common potential (for example, ground potential) is applied to the transparent electrode in all pixel regions.
[0019]
The pixel electrode substrate 40 includes a substrate 41 made of, for example, a silicon material. On the surface of the substrate 41 on the liquid crystal layer 35 side, at least a reflective electrode layer 42, a covering layer 43, and an alignment film 44 are laminated. It is configured. On the substrate 41, a switching element (not shown) for selectively applying a voltage to each pixel electrode 42A of the reflective electrode layer 42 is provided.
[0020]
Like the alignment film 33 of the counter substrate 30, the alignment film 44 is, for example, a film of a polyimide-based organic compound or an obliquely deposited film of an inorganic material such as silicon dioxide, and its surface is subjected to a rubbing treatment. Have been.
[0021]
The reflective electrode layer 42 is configured by arranging a plurality of reflective pixel electrodes 42A made of a metal material in a matrix. The thickness of the reflective electrode layer 42 is, for example, 50 nm to 500 nm. A drive voltage is applied to the pixel electrode 42A by a switching element (not shown) provided on the substrate 41. The switching element is provided corresponding to each pixel electrode 42A, and is configured by, for example, a C-MOS type or a MIS (Metal Insulator Semiconductor) type field effect transistor (FET (Field Effect Transistor)).
[0022]
As the metal material of the pixel electrode 42A, a material having a high reflectance in the visible region is preferable. Generally, aluminum, more specifically, an aluminum metal used for wiring in an LSI process to which copper or silicon is added at several wt% or less. A membrane is used. However, a material other than aluminum may be used as the metal material of the pixel electrode 42A as long as sufficient reflectance can be obtained.
[0023]
The covering layer 43 is formed by overcoating (covering) the entire surface of the reflective electrode layer 42 facing the transparent electrode of each pixel electrode 42. The thickness of the coating layer 43 is, for example, not less than 10 nm and less than 300 nm. The coating layer 43 is formed of the same conductive thin film as the opposing transparent electrode layer 32, for example, ITO. This coating layer 43 is provided to eliminate the asymmetry of the liquid crystal response. The most characteristic feature of the present reflection type liquid crystal display element 21 is that the conductive thin film of the same material as the transparent electrode facing the pixel electrode 42 is overcoated on the pixel electrode 42.
[0024]
Next, the operation and operation of the reflective liquid crystal display element 21 configured as described above will be described.
[0025]
In the reflection type liquid crystal display element 21, the incident light L <b> 1 incident from the counter substrate 30 side and passing through the liquid crystal layer 35 is reflected by the reflection function of the pixel electrode 42 </ b> A provided on the reflection electrode layer 42. The light L1 reflected by the reflective electrode layer 42 is emitted through the liquid crystal layer 35 and the counter substrate 30 in a direction opposite to that at the time of incidence. At this time, the liquid crystal layer 35 changes its optical characteristics according to the potential difference between the opposing electrodes, and modulates the passing light L1. This light modulation enables gradation expression, and the modulated light L2 is used for image display.
[0026]
By the way, to the pixel electrode 42A of the reflective electrode layer 42, for example, a drive voltage whose polarity is inverted ± is applied every predetermined period. At this time, in the conventional reflection type liquid crystal display device, the electrode material whose standard electrode potential is greatly different between the opposing electrodes is used. A voltage is generated, causing asymmetry in the response of the liquid crystal. For this reason, a DC voltage for correcting this is separately applied as an offset voltage and driven. If the magnitude of the offset voltage is large, not only is it difficult to completely correct the value, but the value also fluctuates or changes during long-term driving, which causes problems such as burn-in.
[0027]
On the other hand, in the present reflection type liquid crystal display element 21, the covering layer 43 is provided, and the pixel electrode 42 is overcoated with a conductive thin film of the same material as the opposing transparent electrode. The battery effect that occurs between them basically disappears. Since the conductive thin film is made of, for example, ITO, the incident light passes through the coating layer 43 as it is and does not affect the reflection function itself of the element. Therefore, in the present reflection type liquid crystal display element 21, the asymmetry of the liquid crystal response is eliminated while the reflection function is maintained, and the offset voltage conventionally required for the driving voltage is not required. As a result, high reliability can be obtained during long-term operation.
[0028]
Here, the reason why the battery effect is suppressed by overcoating the pixel electrode 42 with a conductive thin film of the same material as that of the opposing transparent electrode will be described below. The battery effect appears when there is a potential difference between opposing electrodes. Generally, the standard electrode potential of aluminum is -1.66 V, the sign is minus and the value is very large. For this reason, a large potential difference is generated between the transparent electrode such as ITO and the like, and as a result, the liquid crystal driving becomes asymmetric. If the opposing electrodes are made of the same material, the battery effect does not occur in principle, but if a liquid crystal cell is made up of ITO transparent electrodes, it becomes a transmissive panel and does not become a reflective device.
[0029]
In the present embodiment, while the pixel electrode 42A using an aluminum electrode or the like has the light reflection function required for the reflection type device as in the related art, the voltage application to the liquid crystal uses the same material as that of the opposing electrode. Through a transparent conductive thin film. Although the pixel electrode 42A exists in the liquid crystal cell, conductive films of the same material are provided at both ends of the liquid crystal layer 35 that substantially forms the cell. As a result, no potential difference occurs between both ends of the liquid crystal layer 35, and the battery effect is suppressed.
[0030]
Actually, a liquid crystal cell in which the coating layer 43 was provided on the pixel electrode 42 was manufactured, and the electromotive force was measured. As a result, it was observed that no electromotive force was generated in the reflective liquid crystal display element 21. It goes without saying that in a conventional liquid crystal cell without the covering layer 43, the electromotive force is measured by the battery effect.
[0031]
As described above, according to the reflective liquid crystal display element 21 of the present embodiment, the coating layer 43 made of the conductive thin film of the same material as the transparent electrode is provided on the surface of the pixel electrode 42A facing the transparent electrode. Since it is provided, the battery effect between the opposing electrodes can be eliminated. Therefore, the asymmetry of the liquid crystal response is eliminated, and the offset voltage conventionally required for the driving voltage becomes unnecessary. As a result, high reliability can be ensured even when long-term driving is performed. Further, a circuit for applying the offset voltage is not required.
[0032]
[Description of liquid crystal display device]
Next, an example of a liquid crystal display device using the reflection type liquid crystal display element 21 will be described. Here, an example of a reflection type liquid crystal projector using the reflection type liquid crystal display element 21 as a light valve as shown in FIG. 2 will be described.
[0033]
The reflection type liquid crystal projector shown in FIG. 2 is a so-called three-panel type which performs color image display using three liquid crystal light valves 21R, 21G, and 21B for each of red, blue, and green. This reflection type liquid crystal projector includes a light source 11, dichroic mirrors 12, 13 and a total reflection mirror 14 along an optical axis 10. The reflection type liquid crystal projector further includes polarization beam splitters 15, 16, 17, a combining prism 18, a projection lens 19, and a screen 20.
[0034]
The light source 11 emits white light including red light (R), blue light (G), and green light (B) required for displaying a color image, and is, for example, a halogen lamp, a metal halide lamp, or a xenon lamp. Etc.
[0035]
The dichroic mirror 12 has a function of separating light from the light source 11 into blue light and other color lights. The dichroic mirror 13 has a function of separating light that has passed through the dichroic mirror 12 into red light and green light. The total reflection mirror 14 reflects the blue light separated by the dichroic mirror 12 toward the polarization beam splitter 17.
[0036]
The polarization beam splitters 15, 16, and 17 are provided along the optical paths of red light, green light, and blue light, respectively. These polarization beam splitters 15, 16 and 17 have polarization separation surfaces 15A, 16A and 17A, respectively. The polarization separation surfaces 15A, 16A and 17A convert the incident color lights into two polarization components orthogonal to each other. It has the function of separating. The polarization separation surfaces 15A, 16A, and 17A reflect one polarization component (for example, an S-polarization component) and transmit the other polarization component (for example, a P-polarization component).
[0037]
The liquid crystal light valves 21R, 21G, 21B are constituted by a reflective liquid crystal display element 21. Color light of a predetermined polarization component (for example, S-polarization component) separated by the polarization splitting surfaces 15A, 16A, and 17A of the polarization beam splitters 15, 16, and 17 is incident on these liquid crystal light valves 21R, 21G, and 21B. It has become. The liquid crystal light valves 21R, 21G, and 21B are driven in accordance with a drive voltage given based on an image signal, modulate incident light, and direct the modulated light to the polarization beam splitters 15, 16, and 17. It has the function of reflecting.
[0038]
The synthesizing prism 18 has a function of synthesizing the color light of a predetermined polarization component (for example, a P-polarization component) emitted from the liquid crystal light valves 21R, 21G, and 21B and having passed through the polarization beam splitters 15, 16, and 17. . The projection lens 19 has a function of projecting the combined light emitted from the combining prism 18 toward the screen 20.
[0039]
In the reflection type liquid crystal projector configured as described above, the white light emitted from the light source 11 is first separated into blue light and other color lights (red light and green light) by the function of the dichroic mirror 12. Among them, the blue light is reflected toward the polarization beam splitter 17 by the function of the total reflection mirror 14. On the other hand, the red light and the green light are further separated into red light and green light by the function of the dichroic mirror 13. The separated red light and green light enter the polarization beam splitters 15 and 16, respectively.
[0040]
The polarization beam splitters 15, 16, and 17 separate the incident color lights into two polarization components orthogonal to each other on the polarization separation surfaces 15A, 16A, and 17A. At this time, the polarization separation surfaces 15A, 16A, and 17A reflect one of the polarization components (for example, the S-polarization component) toward the liquid crystal light valves 21R, 21G, and 21B.
[0041]
The liquid crystal light valves 21R, 21G, and 21B are driven in accordance with a drive voltage given based on an image signal, and modulate the incident color light of a predetermined polarization component on a pixel basis. At this time, since the liquid crystal light valves 21R, 21G, and 21B are constituted by the above-mentioned reflective liquid crystal display element 21, an offset voltage is not required, and the liquid crystal light valves 21R, 21G, and 21B are favorably driven by a drive voltage with good symmetry.
[0042]
The liquid crystal light valves 21R, 21G, and 21B reflect the modulated color lights toward the polarization beam splitters 15, 16, and 17, respectively. The polarization beam splitters 15, 16, 17 allow only a predetermined polarization component (for example, a P-polarization component) of the reflected light (modulated light) from the liquid crystal light valves 21R, 21G, 21B to pass therethrough, and travel toward the combining prism 18. Emit. The combining prism 18 combines the color lights of predetermined polarization components that have passed through the polarizing beam splitters 15, 16, and 17, and emits the light toward the projection lens 19. The projection lens 19 projects the combined light emitted from the combining prism 18 toward the screen 20. Thereby, an image corresponding to the light modulated by the liquid crystal light valves 21R, 21G, 21B is projected on the screen 20, and a desired image is displayed.
[0043]
As described above, according to the reflective liquid crystal projector of the present embodiment, the covering layer 43 made of the conductive thin film of the same material as the transparent electrode facing the pixel electrode 42A is provided on the pixel electrode 42A. Since the reflective liquid crystal display element 21 is used as the liquid crystal light valves 21R, 21G, 21B, the offset voltage conventionally required for driving the liquid crystal light valves 21R, 21G, 21B is not required. This eliminates the need for a circuit for applying the offset voltage, and simplifies the drive circuit for the liquid crystal light valves 21R, 21G, 21B.
[0044]
【Example】
Next, specific characteristics of the reflection type liquid crystal display element 21 will be described as examples. Before describing the examples, the characteristics of a conventional reflective liquid crystal display element will be described as a comparative example.
[0045]
[Comparative example]
As a reflective liquid crystal display element (liquid crystal cell) for evaluation as a comparative example, a reflective liquid crystal display element (liquid crystal cell) using ITO for a transparent electrode material on an opposite substrate and aluminum for a pixel electrode on a pixel electrode substrate was prepared. The device for evaluation was manufactured as follows. First, a glass substrate on which an ITO transparent electrode is formed as a counter substrate, and a silicon substrate on which an aluminum electrode (aluminum film having a thickness of 150 nm) is formed as a pixel electrode substrate are washed, and then each is deposited on a vapor deposition apparatus. Then, a SiO ₂ film was formed as an alignment film by oblique deposition at a deposition angle of 45 ° to 55 °. The thickness of the alignment film was 50 nm. The pretilt angle of the liquid crystal was controlled to be about 3 °. After that, an appropriate number of glass beads having a diameter of 2 μm are scattered between the substrates on which the alignment films are formed, the two are adhered to each other, a vertical liquid crystal material having a negative dielectric anisotropy Δε is injected, and the reflection type liquid crystal is injected. A cell was prepared.
[0046]
In the device thus manufactured, the transmittance of the liquid crystal when a rectangular wave voltage (± V1) of 60 Hz as shown in FIG. 7 is applied as a driving voltage between the ITO transparent electrode and the pixel electrode. (Actually measuring the reflectance of the device because it is a reflection type, which is equivalent to measuring the transmittance of the liquid crystal) as the driving characteristics. The measurement was performed at a wavelength of 520 nm at room temperature.
[0047]
FIG. 3 shows the measurement results as the applied voltage (V) on the horizontal axis and the transmittance on the vertical axis (hereinafter, the change in the reflectance R with respect to the applied voltage V to the pixel electrode is referred to as VT characteristic). In FIG. 3, a black circle plot indicates a change in reflectance R (+) when a positive voltage is applied to the pixel electrode side, and a white triangle plot indicates a change when a negative voltage is applied. The change of the reflectance R (-) is shown. In FIG. 3, the figure is simplified by setting the applied voltage on the horizontal axis as an absolute value.
[0048]
As can be seen from the VT characteristic curve (VT curve), the VT curve is asymmetric (R (+), R (-) does not overlap with the polarity of the applied voltage. VT curve of the reflectance R (+) when the applied voltage is positive is lower than the VT curve of the reflectance R (-) when the applied voltage is negative. It is shifting. That is, when compared at the same applied voltage (absolute value), the characteristic that R (+)> R (-) was always obtained.
[0049]
Despite the same external voltage being applied to the liquid crystal cell in plus and minus, this asymmetric drive of the liquid crystal indicates that no symmetrical voltage is applied to the liquid crystal. However, this is due to a direct current battery effect generated between different electrodes of the ITO transparent electrode and the aluminum electrode. If the driving is continued in this state, the internal voltage is accumulated in the liquid crystal cell, thereby causing burn-in. Therefore, for practical use, it is necessary to apply the offset voltage ΔV by the voltage corresponding to the shift (for the effect of the battery) so that R (+) = R (−). In the case of this comparative example, ΔV = 0.6 V, and as shown in FIG. 7, driving is performed by applying a DC offset voltage to the signal voltage by ΔV. However, if the value of ΔV is not accurately set and applied, the above-described burn-in phenomenon is concerned in long-term driving, and the value of ΔV itself may change due to long driving, a change in environmental temperature, and the like. Therefore, it is essentially practically necessary to reduce or eliminate ΔV.
[0050]
Note that the above phenomenon similarly occurred when a polyimide film was used as the alignment film, and when a nematic liquid crystal material other than the vertically aligned liquid crystal was used.
[0051]
By the way, it is considered that the above asymmetry of the reflectances R (+) and R (−) is caused by the fact that the standard electrode potential of aluminum used as the pixel electrode is largely different from that of the ITO transparent electrode. Therefore, by covering the pixel electrode with a thin film of the same material as that of the ITO transparent electrode, asymmetry can be improved as in the following embodiments.
[0052]
[Example 1]
In this embodiment, aluminum (film thickness 150 nm) is used as the metal material of the pixel electrode 42A, and a conductive thin film (ITO thin film) of the same material as the opposite ITO transparent electrode is overcoated on the aluminum electrode by sputtering. The VT characteristic in the case of doing was examined. The thickness of the overcoated ITO thin film is 100 nm. The production of the element for evaluation is the same as that of the above-mentioned comparative example except that the covering layer 43 is provided on the pixel electrode 42A. The measurement conditions were the same as in the comparative example, and the change in the reflectance R of the liquid crystal when a rectangular wave voltage of 60 Hz was applied was measured. As in the comparative example, the VT curve is shown by simplifying the horizontal axis as the absolute value of the applied voltage.
[0053]
FIG. 4 shows the VT characteristics. The transmittance value indicates a value relative to the value of aluminum. FIG. 6 shows the measurement results together with the relationship between R (+) and R (−) (asymmetry) and the value of the offset voltage.
[0054]
As can be seen from this result, by covering the pixel electrode 42A with the ITO thin film, the state of the VT curve changes greatly as compared with the comparative example, and R (+) = R (-). No asymmetry of the reflectances R (+) and R (-) was observed, and no offset voltage was observed. That is, symmetrical driving was performed, and no problem such as image sticking was observed at all even after long-term driving.
[0055]
The effect of this example was similarly observed when a polyimide film was used as the alignment film, and when a nematic liquid crystal material other than the vertically aligned liquid crystal was used.
[0056]
[Example 2]
Next, the film thickness was changed to 10 nm, 30 nm, and 300 nm, and a liquid crystal cell overcoated with an ITO thin film was prepared and evaluated in the same manner as in Example 1. The production and measurement conditions of the element for evaluation are the same as those in Example 1.
[0057]
FIG. 6 shows the result. From these results, it was confirmed that the effect of the overcoating of the ITO thin film had a sufficient effect if it was 10 nm or more. Although less than 10 nm seems to have the effect of suppressing asymmetry, in this case, it is difficult to form a uniform thin film. On the other hand, when the film thickness exceeds 300 nm, crystal grains grow rapidly, so that the surface shape becomes rough and light is scattered. Therefore, for example, when applied to a liquid crystal projector, the light collection efficiency of the projection optical system is reduced, and as a result, the total brightness and light amount are reduced. Therefore, it can be said that the thickness of the ITO thin film to be overcoated is appropriately 10 nm or more and less than 300 nm.
[0058]
[Example 3]
Next, a liquid crystal cell overcoated with an ITO thin film was prepared in the same manner as in Example 1 while changing the film thickness in a range from 20 nm to 300 nm, and the reflectance was measured. The production and measurement conditions of the element for evaluation are the same as those in Example 1.
[0059]
FIG. 5 shows the measurement results with the horizontal axis representing the film thickness (nm) and the vertical axis representing the reflectance (%). The figure shows the relative reflectance assuming that the liquid crystal cell is composed of only the aluminum pixel electrode without overcoating the thin film, as 100%. From this measurement result, it was found that the overcoat of the ITO thin film hardly affected the reflectance of the liquid crystal cell.
[0060]
Note that the present invention is not limited to the above embodiment, and various modifications can be made. For example, the reflection type liquid crystal display element of the present invention is not limited to a liquid crystal projector, and can be widely applied to other display devices and video display units in various portable electronic devices and various information processing terminals.
[0061]
【The invention's effect】
As described above, according to the reflective liquid crystal display element according to any one of claims 1 to 6, or the liquid crystal display device according to claim 7 or 8, the surface side of the pixel electrode facing the transparent electrode. Is covered with a conductive thin film of the same material as the transparent electrode, so that the battery effect between the opposing electrodes can be eliminated. As a result, the asymmetry of the liquid crystal response can be eliminated, so that the offset voltage applied to the driving voltage in the related art becomes unnecessary. Since the liquid crystal response can be made completely symmetric, high reliability can be ensured even when driving for a long time.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view illustrating a configuration of a reflective liquid crystal display device according to an embodiment of the present invention.
FIG. 2 is a configuration diagram illustrating an example of a liquid crystal display device configured using the reflective liquid crystal display device illustrated in FIG.
FIG. 3 is a characteristic diagram showing a relationship between a driving voltage and a transmittance when aluminum is used as a metal material of a pixel electrode (comparative example).
FIG. 4 is a characteristic diagram showing a relationship between a drive voltage and a transmittance when a pixel electrode is coated with ITO.
FIG. 5 is a characteristic diagram showing a relationship between a film thickness of a coating layer and reflectance.
FIG. 6 is an explanatory diagram collectively showing the asymmetry situation and the measurement results of the offset voltage in each example.
FIG. 7 is a waveform chart for explaining an example of a driving method in a liquid crystal display element.
[Explanation of symbols]
11 light source, 12, 13 dichroic mirror, 14 total reflection mirror, 15, 16, 17 polarizing beam splitter, 18 synthetic prism, 19 projection lens, 20 screen, 21 liquid crystal display element, 21R , 21G, 21B: liquid crystal light valve, 31: glass substrate, 32: transparent electrode layer, 33, 44: alignment film, 35: liquid crystal layer, 40: pixel electrode substrate, 41: substrate, 42: reflective electrode layer, 42A: Pixel electrode, 43 ... Coating layer.

Claims (8)

A pixel electrode substrate having a reflective pixel electrode made of a metal material,
A counter substrate having a transparent electrode provided to face the pixel electrode,
A reflective liquid crystal display device including a liquid crystal injected between the pixel electrode substrate and the counter substrate,
A reflective liquid crystal display device, wherein a surface of the pixel electrode facing the transparent electrode is covered with a conductive thin film of the same material as the transparent electrode. The transparent electrode is an indium-tin oxide film (ITO);
2. The reflective liquid crystal display device according to claim 1, wherein the pixel electrode is covered with a thin film of an indium tin oxide film. 2. The reflective liquid crystal display device according to claim 1, wherein a main component of the pixel electrode is aluminum. 2. The reflective liquid crystal display device according to claim 1, wherein a thickness of the conductive thin film coated on the pixel electrode is 10 nm or more and less than 300 nm. 2. The reflective liquid crystal display device according to claim 1, wherein the pixel electrode is driven by a switching device provided on a silicon substrate. 2. The reflective liquid crystal display device according to claim 1, wherein the liquid crystal is a vertically aligned liquid crystal. A liquid crystal display device comprising a reflective liquid crystal display element and displaying an image using light modulated by the reflective liquid crystal display element,
The reflective liquid crystal display element,
A pixel electrode substrate having a reflective pixel electrode made of a metal material,
A counter substrate having a transparent electrode provided to face the pixel electrode,
Comprising a liquid crystal injected between the pixel electrode substrate and the counter substrate,
A liquid crystal display device, wherein a surface of the pixel electrode facing the transparent electrode is covered with a conductive thin film of the same material as the transparent electrode. A light source,
Projection means for projecting light emitted from the light source and modulated by the reflective liquid crystal display element onto a screen,
The liquid crystal display device according to claim 7, wherein the liquid crystal display device is configured as a reflective liquid crystal projector.

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