Patent Application
20070263145
Prior to description of the embodiments, common structure and method, that is, structure and manufacturing method of a liquid crystal panel is described.
In the TFT substrate, each pixel includes a transmissive electrode (transparent electrode) 1 corresponding to a transmissive region and a reflective electrode 2 corresponding to a reflective region. A region including the transmissive electrode 1 (transmissive region) transmits light from a backlight unit and a region including the reflective electrode 2 (reflective region) reflects external light. Then, to drive a pixel electrode composed of the transmissive electrode 1 and the reflective electrode 2, each pixel includes a TFT (thin film transistor) 3. Then, a drain electrode of the TFT 3 is electrically connected with the transmissive electrode 1 and the reflective electrode 2. The gate electrode of the TFT 3 is connected with a gate line (scanning line) 4 to execute on/off control over the TFT 3 based on a signal input from the gate terminal. A source electrode of the TFT 3 is connected with a source line (signal line) 5. If a voltage is applied to the gate electrode, a current flows thorough the source line 5. A voltage applied to the source electrode is appropriately controlled to thereby change actual voltage applied to liquid crystal (driving voltage). The voltage applied to liquid crystal can be adjusted with the source electrode. Thus, as for driving conditions of liquid crystal, an intermediate transmittance of liquid crystal can be freely determined.
The present invention is not only applied to such simple structure that the transmissive region and the reflective region are upper and lower regions of a pixel like the TFT substrate as shown in
Next, a manufacturing process of the liquid crystal panel is described. Referring first to
Subsequently, an insulating film 8, a semiconductor thin film 9, and an ohmic contact film 10 are successively formed by various CVD methods such as a plasma CVD method, and patterns of the semiconductor thin film 9, and the ohmic contact film 10 are formed through a photoengraving process and an etching process. The insulating film 8 is made of SiNx or SiOy and used as a gate insulating film. As the semiconductor thin film 9, for example, amorphous silicon (a-Si) or polysilicon (p-Si) is used. The ohmic contact film 10 is an n type semiconductor, and an n-a-Si film or n-p-Si film obtained by doping a small amount of phosphorus (P) into a-Si or p-Si is used as the ohmic contact film 10. As a result, the structure of
After that, a conductive film 11 is formed through sputtering, which is used for forming a source line, and then a photoengraving process and an etching process are performed. As a result, a source line, a source electrode, a drain electrode, a reflective electrode, and a source terminal are formed. The patterns of the source line, the source electrode, the drain electrode, the reflective electrode, and the source terminal are used as a mask, and the underlying ohmic contact film 10 is removed through etching. It is desirable to insulate adjacent source lines from each other. A central portion of the ohmic contact film 10 in the TFT portion is removed by this process to expose the semiconductor thin film 9. Through the above processes, the structure of
After that, a protective film 12 is formed with an insulating film made of Si3N4, SiO2, etc., or mixture or laminate thereof, by various CVD methods such as plasma CVD. To ensure continuity with the gate terminal and the source terminal, a contact hole is formed in the insulating film 8 and the protective film 12. At this time, a contact hole is formed also in the protective film 12 to obtain continuity with the drain electrode of the TFT. As a result, the structure of
After that, a transparent conductive film 13 is formed of ITO (Indium Tin Oxide), SnO2, InZnO, etc. through sputtering, evaporation, coating, CVD, printing, and a sol-gel method. The transparent conductive film 13 may be a transparent conductive layer made of a mixture or laminate of these materials. Then, a transmissive electrode is formed through a photoengraving process and an etching process. As a result, the structure of
Next, an assembly step of the liquid crystal panel composed of the thus-manufactured TFT substrate and CF substrate opposite thereto is described. The two substrates are coated with a polyimide resin, for example, AL-22501 available from JSR as an oriented film for aligning liquid crystal molecules, followed by rubbing with cloth. The rubbing direction of the TFT substrate is opposite and parallel to the rubbing direction of the CF substrate, and liquid crystal is parallel-aligned to the TFT substrate and CF substrate. A sealing material is applied around a display region on the TFT substrate with a dispenser, and the substrates are bonded such that the oriented films face each other. At this time, a spacer may be sprayed between the TFT substrate and the CF substrate. The sealing member is cured while heated under an appropriate pressure. A cell gap of the transmissive region is adjusted to 3.8 μm, and a cell gap of the reflective region is adjusted to 2.0 μm. Then, a liquid crystal material having a double refractive index of 0.065 to 0.070, for example, MJ042545 available from melc is filled in between the substrates with vacuum infusion or the like. After the injection of the liquid crystal, an injection port is sealed to complete the liquid crystal panel.
A circularly polarizing plate with a retarder as described in detail in the following embodiments is bonded to the outer surface of the thus-manufactured liquid crystal panel on both of the TFT substrate side and CF substrate side. Further, a back light unit as an illuminating device is provided outside the TFT substrate to complete the liquid crystal display device. The transmissive region transmits light from a back light unit. In the reflective region, external light incident on the liquid crystal panel is reflected by reflective electrode. As a result, the incident external light can be reflected to the display surface side.
The first polarization control element 27 and the second polarization control element 28 each include at least a polarizing plate and a retarder. The polarizing plate absorbs light oscillating in one direction and allows transmission of light oscillating in the other direction to thereby generate linearly-polarized light. In this embodiment, a first polarizer 20 and a second polarizer 25 constitute the polarizing plate. The retarder involves a special phase difference of λ/2 or λ/4. The retarder of the phase difference of λ/2 or λ/4 is referred to as λ/2-wave plate or λ/4-wave plate. The retarder is used for optical compensation and used for widening a viewing angle.
The second polarization control element 28 is provided on the display surface side of the CF substrate 15. The second polarization control element 28 includes a second polarizer 25, a second λ/2-plate 24, a biaxial retarder 23, and a liquid crystal film 22 in this order from the display surface side to constitute the circularly polarizing plate. Accordingly, incident external light is linearly polarized by the second polarizer 25. Further, light is circularly polarized by the second λ/2-plate 24, the biaxial retarder 23, and the liquid crystal film 22 and then incident on the CF substrate 15. Further, the second polarization control element 28 and the CF substrate 15 are bonded with a light diffusion layer 21.
On the other hand, a first polarization control element 27 is provided on the rear side of the TFT substrate 14. The first polarization control element 27 includes a first polarizer 20, a first λ/2-plate 19, and a first λ/4-wave plate 18 in this order from the rear surface side to constitute the circularly polarizing plate. Accordingly, light from a back light unit is linearly polarized by the first polarizer 20. Further, the light is circularly polarized by a first λ/2-plate 19 and a first λ/4-wave plate 18 and incident on the TFT substrate 14.
The circularly polarizing plate is a so-called wide-band circularly polarizing plate composed of a laminate of a λ/4-wave plate, a λ/2-plate, and a polarizing plate. The wide-band circularly polarizing plate is intended to convert incident light into circularly polarized light to widen a selective wavelength band. The first λ/4-wave plate 18 may be a biaxial retarder and designed to further wide viewing angle. At this time, Nz coefficient of the biaxial retarder is desirably 0 to 0.8. Nz coefficient is defined as follows: Nz=(nx−nz)/(nx−ny). In this case, a refractive index in a slow axis direction in the retarder plane is represented by nx, a refractive index in a direction vertical to nx in the retarder plane is represented by ny, and a refractive index in a vertical direction of the retarder is represented by nz. A parameter for controlling polarized conditions of light emitted in the forward direction of the retarder is a phase difference and slow axis angle in the retarder plane, and absorbing axis angle of the polarizing plate.
In this embodiment, the reflective region includes a liquid crystal layer 16, a reflective electrode 2, and a gap control layer 17. Incidentally, the other components such as an oriented film and a color filter are not shown. In this embodiment, the reflective electrode 2 is partially formed on the TFT substrate 14, and a gap control layer 17 is partially formed below the CF substrate 15. Further, the reflective electrode 2 and the gap control layer 17 face each other. Light incident from the CF substrate 15 side is reflected by the reflective electrode 2.
In a transflective type liquid crystal display device using the circularly polarizing plate, a cell gap should be set for each of the transmissive region and the reflective region. That is, the gap control layer 17 is formed to adjust a cell gap D2 in the transmissive region and a cell gap D1 in the reflective region. The gap control layer 17 for defining a cell gap may be formed on the TFT substrate 14 side or the CF substrate 15 side. Needless to say, the gap control layer 17 may be formed on both of the TFT substrate 14 side and the CF substrate 15 side. In this embodiment, a description is made of the structure that the gap control layer 17 is formed on the CF substrate 15 side. Further, a liquid crystal material for the liquid crystal layer 16 is uniaxially aligned substantially in parallel to the TFT substrate 14 and the CF substrate 15 if not applied with a voltage, or moved to rise if applied with a voltage.
A light diffusion layer 21 is formed between the CF substrate 15 and the second polarization control element 28. In this embodiment, for example, a dispersion adhesive is used for the light diffusion layer 21. This is because beads of different refractive indexes are randomly mixed into an adhesive material to impart a diffusion function. If the haze value of the dispersion adhesive is 60 or more, reflected light can be well diffused.
In this embodiment, the liquid crystal film 22 is a WV film available from FUJIFILM Corporation. The liquid crystal film 22 is used as a part of the λ/4-wave plate or λ/4-wave plate and as a viewing angle compensation film. The WV film is a liquid crystal film obtained by hybrid-aligning discotic liquid crystal compounds. Further, the biaxial retarder 23 is POLYCA available from Nitto Denko Corporation (in-plane phase difference=140 nm, Nz coefficient=0.1), and a first λ/4-wave plate 18, a first λ/2-plate 19, and a second λ/2-plate 24 are each ZEONOR available from Nitto Denko Corporation. These are bonded each other with an adhesive. The antireflection film 26 is, for example, AR3 available from Sony Chemical&Information Device Corporation. Further, the antireflection film 26 of a laminate film structure formed through continuous sputtering (a film of large refractive index and a film of small refractive index are alternately layered) produces a greater effect than the film of a single-layer.
A liquid crystal display device that uses the WV film as a retarder changes a phase difference under high-humidity or -temperature conditions, and the optimum black display voltage is changed. As a result, there arises a problem that a contrast ratio is lowered. Here, in this embodiment, the second polarization control element 28 subjected to antireflection (AR processing) through continuous sputtering is provided on the display surface side of the transflective type liquid crystal display device. As a result, change in optimum black display voltage is suppressed under high-humidity or -temperature conditions. Thus, reduction in contrast ratio can be suppressed down to 1/10 or less of that of the polarization control element not subjected to antireflection (AR processing).
As for the antireflection (AR processing), multilayered thin films of different refractive indexes are formed, and light reflected by the surface is canceled out by interference. Antireflection is processed by providing the antireflection film 26. The antireflection (AR processing) is applied only to the second polarization control element 28 on the CF substrate 15 side. That is, the antireflection film 26 is formed on the display surface side of the liquid crystal panel. In the related art of Japanese Unexamined Patent Application Publication No. 2005-107501, a WV film is formed on the TFT substrate side. In such structure, the TFT substrate side surface should be subjected to antireflection (AR processing) to suppress a phase difference change under high-humidity or -temperature conditions. This leads to a remarkable increase in cost. Thus, in this embodiment, the second polarization control element 28 including the liquid crystal film 22 is formed on the CF substrate 15 side. Then, the antireflection film 26 is formed through continuous sputtering on the second polarization control element 28 on the CF substrate 15 side. Hence, it is possible to prevent a contrast ratio from lowering even under high-humidity or -temperature conditions without increasing costs. That is, according to this embodiment, deterioration in display characteristics can be prevented.
Further, in this embodiment, the light diffusion layer 21 is used. It is best to form the light diffusion layer 21 between the second polarization control element 28 and the CF substrate 15, that is, between the liquid crystal film 22 and the CF substrate 15. This is because, if the light diffusion layer 21 is formed in the other portion, for example, between the liquid crystal film 22 and the biaxial retarder 23, between the biaxial retarder 23 and the second λ/2-plate 24, and between the second λ/2-plate 24 and the second polarizer 25, light reflected by the light diffusion layer 21 increases reflective luminance in black display.
The rate that the reflective luminance is increased in black display becomes large as the distance of the light diffusion layer 21 from the CF substrate 15 increases. This is because design values of the retarder (liquid crystal film 22, biaxial retarder 23, and second λ/2-plate 24) provided on the CF substrate 15 side are determined such that blackest black is obtained with respect to light reflected by the reflective electrode 2 in the liquid crystal cell. Light reflected by a portion other than the reflective electrode 2 is not considered. Hence, light reflected by a portion other than the reflective electrode 2 is transmitted through the second polarizer 25.
For example, if the light diffusion layer 21 is formed between the liquid crystal film 22 and the biaxial retarder 23, there is reflected light for which a residual phase difference in the liquid crystal layer 16 and the phase difference in the liquid crystal film 22 are not considered. Accordingly, the reflected light is partially transmitted through the second polarizer 25. As a result, a contrast ratio is lowered in a reflective mode. Further, if the light diffusion layer 21 is formed between the biaxial retarder 23 and the second λ/2-plate 24, there is a reflected light for which a residual phase difference in the liquid crystal layer 16, a phase difference in liquid crystal film 22, and a phase difference in biaxial retarder 23 are not considered. Thus, a contrast ratio is lower in the reflective mode. Here, if the light diffusion layer 21 is formed between the CF substrate 15 and the liquid crystal film 22, only a residual phase difference in the liquid crystal layer 16 is not considered. Therefore, it is possible to prevent a contrast ratio from lowering. Hence, it is best to form the light diffusion layer 21 between the CF substrate 15 and the second polarization control element 28, that is, between the CF substrate 15 and the liquid crystal film 22.
The display characteristics of the liquid crystal panel are determined by a phase difference and slow axis angle of various retarders of the first polarization control element 27 and the second polarization control element 28 (first λ/4-wave plate 18, first λ/2-plate 19, liquid crystal film 22, biaxial retarder 23, and second λ/2-plate 24), absorbing axis angle of the polarizing plate, a phase difference and slow axis angle of the liquid crystal film 22, a cell gap of each of the reflective region and the transmissive region, twist angle (angular difference in rubbing direction between the TFT substrate 14 and the CF substrate 15) of the liquid crystal layer 16, and physical properties of a liquid crystal material (refractive index). Desired electrooptical characteristics can be obtained with these parameters. Values of the above parameters that contribute to optical design of this embodiment are summarized in Table 1. The retardation of the retarder or a refractive index of a liquid crystal material is calculated based on the wavelength of 550 nm.
As for the axial angle, the 3 o'clock direction is set to 0°, the 12 o'clock direction is set to 90°, the 9 o'clock direction is set to 180°, and the 6 o'clock direction is set to 270°.
In the case where the structures of the first polarization control element 27 and the second polarization control element 28, and design values of Table 1 are used, under high-humidity or -temperature conditions, the optimum black display voltage is not changed, and a contrast ratio is not lowered.
As described above, high display quality is obtained in this embodiment as compared with general transflective liquid crystal panels as shown in
In this embodiment, the light diffusion layer 21 is inserted between the second polarization control element 28 (a laminate of the second polarizer 25, the second λ/2-plate 24, the biaxial retarder 23, and the liquid crystal film 22) and the liquid crystal panel. That is, the light diffusion layer 21 is used as an adhesive for bonding the second polarization control element 28 to the glass substrate of the CF substrate 15. As a result, light reflected by the reflective electrode 2 in the liquid crystal panel can be diffused with the light diffusion layer 21. Further, as described above, the antireflection film 26 is formed on the second polarizer 25 to prevent a phase difference of the liquid crystal film 22 from changing under high-humidity or -temperature conditions. A reflective liquid crystal display device or transflective liquid crystal display device of this type enables high display quality in a reflective mode without decreasing a contrast ratio in a transmissive mode.
As for the axial angle, the 3 o'clock direction is set to 0°, the 12 o'clock direction is set to 90°, the 9 o'clock direction is set to 180°, and the 6 o'clock direction is set to 270°.
In this embodiment as well, it is best to form the light diffusion layer 21 between the liquid crystal film 22 and the CF substrate 15 for the same reason as the first embodiment.
In this embodiment, a viewing angle is not wider than that of the first embodiment but a cost is saved since the uniaxial retarder is used in place of the biaxial retarder. Further, the transflective type liquid crystal display device of this embodiment enables high display quality in a reflective mode without changing the optimum black display voltage and reducing a contrast ratio under high-humidity or -temperature conditions.
As described in this embodiment, the biaxial retarder 23 of the second polarization control element 28 may be the second λ/4-wave plate 29. Further, the biaxial retarder 23 may be used for one or both of the first polarization control element 27 and the second polarization control element 28.
To make the reflective electrode 2 uneven in section, an organic film such as an acrylic resin film is formed below the reflective electrode 2. To be specific, an organic film is formed below the transparent conductive film 13 of
In this embodiment, the structure of the first polarization control element 27 and the second polarization control element 28 bonded to the transflective type liquid crystal panel may be the same as the first embodiment. The transflective type liquid crystal display device of this embodiment enables a higher display quality in a reflective mode than that of the first and second embodiments without changing the optimum black display voltage under high-humidity or -temperature conditions and lowering a contrast ratio. Further, as described in the second embodiment, the second λ/4-wave plate 29 may be used in place of the biaxial retarder 23.
In the case of using an NH film for any existing transflective liquid crystal display device, the optimum black display voltage is changed and a contrast ratio is reduced under high-humidity or -temperature conditions due to a change in phase difference of the NH film. The NH film includes liquid crystal like the WV film. Thus, the antireflection film 26 is formed on the surface of the liquid crystal panel with the NH film to thereby prevent a contrast ratio from reducing under high-humidity or -temperature condition. That is, beneficial effects similar to those of the first embodiment are attained in this embodiment as well.
According to the above structure, the optimum black display voltage under high-humidity or -temperature conditions is not changed, and a contrast ratio is not lowered. Further, the light diffusion layer 21 is formed between the liquid crystal film 22 and the CF substrate 15 for the same reason as that of the first embodiment to thereby realize a transflective type liquid crystal display device having high display quality in a reflective mode.
From the invention thus described, it will be obvious that the embodiments of the invention may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended for inclusion within the scope of the following claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2006-135276 | May 2006 | JP | national |
