Patent Application
20080013017
Prior to description of the embodiments, common structure, method, etc., that is, structure and fabrication method of a liquid crystal panel, and movement of liquid crystal molecules of a liquid crystal panel are described. Here, according to the present invention, a transmissive region involves a phase difference of about λ/2 if a liquid crystal layer is not applied with a voltage, and a reflective region involves a phase difference of about λ/4 if a liquid crystal layer is not applied with a voltage. Further, the present invention is applicable to all liquid crystal modes that allow application of horizontal electric field to liquid crystal (for example, an IPS mode or an FFS mode). The following description about the structure and fabrication method of a liquid crystal panel and movement of liquid crystal molecules is given based on an array structure of the FFS mode.
A region including the transmissive electrode 22 (transmissive region) transmits light from a backlight source and a region including the reflective electrode 23 (reflective region) reflects external light. Then, the transmissive electrode 22 and the reflective electrode 23 constitute a common electrode 24. Further, a protective film 25 is formed to cover the common electrode 24. Further, a comb-like pixel electrode 26 is formed on the common electrode 24 through a protective film 25 as an insulating film. A driving voltage for driving a liquid crystal is applied to the pixel electrode 26 through a TFT (not shown).
Further, an oriented film 50a is formed between the TFT substrate 20 and the liquid crystal layer 10. Further, an oriented film 50b is formed between the CF substrate 30 and the liquid crystal layer 10. The oriented film 50 is a film for aligning liquid crystal molecules in the liquid crystal layer 10. This film is subjected to rubbing to thereby align liquid crystal molecules in a predetermined direction.
In the transflective liquid crystal display of the present invention, a cell gap should be set in each of the transmissive region and the reflective region. To define a cell gap in each region, a gap control layer 11 is formed on the CF substrate 30 side of the liquid crystal layer 10. Further, the gap control layer 11 is formed opposite to the reflective electrode 23. The gap control layer 11 controls a phase difference in the reflective region and the transmissive region.
According to the present invention, a phase difference of the liquid crystal layer 10 of the reflective region is about λ/4 if the liquid crystal layer 10 is not applied with a voltage, and a phase difference of the liquid crystal layer 10 of the transmissive region is about λ/2 if the liquid crystal layer 10 is not applied with a voltage. Incidentally, the gap control layer 11 may be formed on the TFT substrate 20 side as well as the CF substrate 30 side of the liquid crystal layer 10 as shown in
Further, the TFT 40 is provided in each pixel. The TFT 40 is electrically connected with the pixel electrode 26. A gate electrode (not shown) of the TFT 40 is formed on a gate line (scanning line) 42. The ON/OFF control over the TFT is executed in accordance with a signal input from a gate terminal. A source electrode 43 of the TFT 40 is connected with a source line (signal line) 44. Further, a drain electrode 45 of the TFT 40 is connected with the pixel electrode 26.
If a voltage is applied to the gate electrode of the TFT 40, a current flows through the source line 44. Further, a level of voltage applied to the source electrode 43 is controlled as desired to thereby change an actual voltage applied to the liquid crystal (driving voltage). A voltage applied to the liquid crystal can be controlled with the source electrode 43. Thus, as for driving conditions of the liquid crystal, intermediate transmittance of the liquid crystal can be freely set.
The above pixels are arranged in matrix in a display region on the TFT substrate 20. Therefore, plural gate lines 42 extend in parallel. Further, plural source lines 44 extend in parallel. A region surrounded by two adjacent gate lines 42 and two adjacent source lines 44 corresponds to a pixel. Further, in the TFT 40, a semiconductor thin film is formed on the gate insulating film. The semiconductor thin film includes a source region connected with the source electrode 43, a drain region connected with the drain electrode 45, and a channel region formed between the source region and the drain region.
Further, the CF substrate 30 is divided into two regions: a reflective region 31 and a transmissive region 32. That is, transmissive region 32 of the CF substrate 30 is formed on the transmissive region of the TFT substrate 20 as a region including the transmissive electrode 22. The reflective region 31 of the CF substrate is formed on the reflective region of the TFT substrate 20 as a region including the reflective electrode 23 and the gap control layer 11. Incidentally, 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 as shown in
Referring next to
Subsequently, the gate line 21, the gate electrode of the TFT 40, the gate terminal, the common line, and the reflective electrode 23 as a reflector are formed. First, a metal film is formed on the substrate through sputtering, followed by a photoengraving process for applying a resist as a photosensitive resin through spin coast, and performing exposure and development thereof. After that, patterning is executed through etching to thereby form the gate line 42, the gate electrode of the TFT 40, the gate terminal, the common line, and the reflective electrode 23. The reflective electrode 23 is formed only in the reflective region.
Here, the transmissive electrode 22 and the reflective electrode 23 partially overlap with each other. As a result, the transmissive electrode 22 and the reflective electrode 23 are brought into contact with each other and electrically connected together. Further, the common line is integrally formed with the reflective electrode 23. Further, the common line is formed in parallel to the gate line 42 between adjacent gate lines 42 to connect common electrodes of adjacent pixels. Therefore, a common potential is applied to the transmissive electrode 22 and the reflective electrode 23 that constitute the common electrode 24 through the common line.
Next, an amorphous silicon film as a semiconductor thin film and a gate insulating film are formed by various CVD methods such as a plasma CVD method, and a pattern of the semiconductor thin film is formed through a photoengraving process and an etching process. In this case, a contact hole for short-circuiting the common line to the source line 44 outside a display region is formed in the gate insulating film on the common line. Further, the gate insulating film may be formed to cover the common electrode 24 or not to cover the common electrode 24. After that, a conductive film for forming a source line is formed through sputtering. Then, the source line 44, the source electrode 43, the drain electrode 45, and the source terminal are formed through photoengraving process and etching process. Further, a conductive pattern for short-circuiting plural common lines is formed on the contact hole.
It is desirable that the patterns of the source line 44, the source electrode 43, the drain electrode 45, and the source terminal be used as a mask to remove the underlying semiconductor thin film through etching to insulate the adjacent source lines 44 from each other. After that, an insulating film is formed of Si3N4, SiO2, or a mixture or laminate thereof through various CVD methods such as plasma CVD to thereby form the protective film 25.
To electrically connect between the gate terminal and the source terminal, a contact hole is formed in the gate insulating film and the protective film 25. At this time, to establish continuity with the drain electrode 45 of the TFT 40, a contact hole is also formed in the protective film 25 on the drain electrode 45. After that, a transparent conductive film of ITO, SnO2, InZnO, or the like, a laminate thereof, or a transparent conductive layer of mixture thereof is formed through sputtering, evaporation, coating, CVD, printing, and sol-gel method. The comb-like pixel electrode 26 is formed through photoengraving process and etching process. The pixel electrode 26 is electrically connected with the drain electrode 45 of the TFT 40 through the contact hole. Therefore, a driving voltage for driving a liquid crystal is applied to the pixel electrode 26 through the source line 44. Incidentally, a potential may be applied in a reverse direction. That is, a common potential may be applied to the comb-like electrode, and a pixel potential may be applied to a lower layer. In this case, the comb-like electrode is connected with the common line.
Next, an assembly process of the liquid crystal panel 1 composed of the thus-manufactured TFT substrate 20 and opposite CF substrate 30 is described. The two substrates are coated with a polyimide resin, for example, JALS-3003 available from JSR as an oriented film 50 for aligning liquid crystal molecules, followed by rubbing with cloth. The liquid crystal is parallel-aligned. As the rubbing direction of the TFT substrate 20 and the CF substrate 30, there are a parallel direction and a non-parallel direction. In this example, the rubbing direction is a non-parallel direction.
In the transmissive region of the liquid crystal panel 1 of the present invention, if no voltage is applied between the pixel electrode 26 and the common electrode 24, liquid crystal molecules are uniaxial-oriented in the direction substantially vertical to the figure plane. In this case, however, if a voltage is applied between the pixel electrode 26 and the common electrode 24, an electric field is generated in the liquid crystal layer 10, torsional deformation of the liquid crystal molecules occurs. To control the direction of torsional deformation accompanying voltage application, the rubbing direction is set at the angle of about 10 to 20 degrees to the teeth arrangement direction of the comb-like pixel electrode 26.
A seal member is applied around a display region on the TFT substrate 20 with a dispenser, and the substrates are bonded such that the oriented films 50 face each other. The sealing member is cured while heated under an appropriate pressure. In this way, the cell gap of the transmissive region is adjusted to 3.2 μm, and the cell gap of the reflective region is adjusted to 1.6 μm. Then, a liquid crystal material having a double refractive index of 0.088 (wavelength: 589.3 nm, 20° C.), for example, MLC6418 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 1.
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 1 on both of the TFT side and CF side. Further, a back light unit as an illuminating device is provided outside the TFT substrate to complete the liquid crystal display.
Next, movement of the liquid crystal molecules 60 of the liquid crystal panel 1 is described.
The pixel electrode 26 and the common electrode 24 are formed on the TFT substrate 20 including the TFT 40. The liquid crystal layer 10 is formed between the CF substrate 30 including a color filter and the TFT substrate 20. Here, description is made of a P (positive) type liquid crystal where the liquid crystal molecules 60 are aligned toward the direction parallel to the electric field application direction.
The oriented film 50 is formed between the TFT substrate 20 and the liquid crystal layer 10 and between the CF substrate 30 and the liquid crystal layer 10. An oriented film 50b of the CF substrate 30 is given orientation property in the direction D1 through rubbing. The direction D1 extends from the front side to the rear side of the figure plane. Further, the oriented film 50a of the TFT substrate 20 is given orientation property in the direction D2 through rubbing. The direction D2 extends from the rear side to the front side of the figure plane.
Thus, a liquid crystal used for the liquid crystal layer 10 is uniaxially oriented substantially in parallel to the TFT substrate 20 and the CF substrate 30 if no voltage is applied as shown in
In contrast, if a voltage is applied between the comb-like pixel electrode 26 and the common electrode 24, as shown in
A liquid crystal display 100 according to a first embodiment of the present invention includes the TFT substrate 20 and the CF substrate 30 as two substrates sandwiching the liquid crystal layer 10 in the above liquid crystal panel 1.
Further, in the liquid crystal display 100 of this embodiment, a biaxial retarder 201 (first about-λ/2-wave plate) having an in-plane phase difference of about λ/2, a biaxial retarder 202 (first about-λ/4-wave plate) having an in-plane phase difference of about λ/4, and a polarizing plate 203 (first polarizing plate) are arranged on the outer surface of the TFT substrate 20 in the stated order from the TFT substrate 20 side. Further, a biaxial retarder 301 (second about-λ/4-wave plate) having an in-plane phase difference of about λ/4 and a polarizing plate 302 (second polarizing plate) are arranged on the outer surface of the CF substrate 30 in the stated order from the CF substrate 30 side. The polarizing plate is intended to absorb light oscillating in one direction and allow transmission of light oscillating in the other direction to thereby generate linearly-polarized light. As a result, it is possible to provide a liquid crystal display of wide view angle, which is free of tone reversal, in a liquid crystal mode for applying a horizontal electric field to a liquid crystal at low cost.
A circularly polarizing plate used for a general transflective liquid crystal display is a so-called wide-band circularly polarizing plate, in which a λ/4-wave plate, a λ/2-wave plate, and a polarizer are arranged in this order from a panel side opposite to the liquid crystal layer of the substrate. The present invention provides a transflective liquid crystal display, but its structure is absolutely different from general circularly polarizing plates. That is, the biaxial retarder 201 having an in-plane phase difference of about λ/2 or the biaxial retarder 301 having an in-plane phase difference of about λ/4 is directly bonded to the glass substrate. Further, the λ/4-wave plate and the λ/2-wave plate are interchanged in position on the TFT substrate 20 side. The λ/2-wave plate is not provided between λ/4-wave plate and the polarizing plate on the CF substrate 30 side.
Further, in the transflective liquid crystal device of this embodiment, biaxial retarders 202 and 301 having an in-plane phase difference of about λ/4 are provided adjacent to the inner side of the polarizers 203 and 302 (liquid crystal layer side). The biaxial retarders 202 and 301 having an in-plane phase difference of about λ/4 are arranged such that polarizing axes extend in the direction substantially parallel or substantially vertical to polarizing axes of the polarizing plates 203 and 302. This is to realize crossed nicols (orthogonality) of the polarizing plates 203 and 302 by use of the biaxial retarders 202 and 301 having an in-plane phase difference of about λ/4.
If two orthogonal polarizing plates are used, crossed nicols cannot be realized, so a black area is isolated at the view angle other than the front view angle. Further, if there is a large angular difference between the polarizing axes of the polarizing plates 203 and 302 and the polarizing axes of the biaxial retarders 202 and 301 having an in-plane phase difference of about λ/4, a phase difference occurs in the biaxial retarders 202 and 301 having an in-plane phase difference of about λ/4, which influences a design for attaining a black-and-white display in both of the transmissive mode and the reflective mode. Therefore, in the transflective liquid crystal device of this embodiment, the biaxial retarders 202 and 301 having an in-plane phase difference of about λ/4 are provided adjacent to the inner side of the polarizers 203 and 302 (liquid crystal layer side) such that polarizing axes extend in the direction substantially parallel or substantially vertical to the polarizing axes of the polarizers 203 and 302.
Further, it is preferred to use a biaxial retarder as the retarder. This is to compensate for change in light phase difference and keep a predetermined difference because an optical path length in the liquid crystal layer varies depending on view angle (light output angle).
Further, in the transflective liquid crystal device of this embodiment, the biaxial retarder 201 having an in-plane phase difference of about λ/2 is provided on the inner side of the biaxial retarder 202 having an in-plane phase difference of about λ/4 on the TFT substrate 20 side (liquid crystal layer side). The biaxial retarder 201 having an in-plane phase difference of about λ/2 is provided to attain a black-and-white display in both of the transmissive mode and the reflective mode in the horizontal electric field type device.
At this time, the odd number of biaxial retarders having an in-plane phase difference of about λ/2 should be provided. Hence, in the transflective liquid crystal device of this embodiment, a single biaxial retarder having an in-plane phase difference of about λ/2 is provided on the inner side of biaxial retarder 202 having an in-plane phase difference of about λ/4 on the TFT substrate 20 side (liquid crystal layer side). Further, to realize an in-plane phase difference of about λ/2 in a wide band, it is necessary to set the optimum axial angle of the retarder. Further, the reason the wave plate having an in-plane phase difference of about λ/2 is a biaxial retarder is to keep a phase difference of λ/2 at a given view angle.
Display characteristics of the liquid crystal display panel are determined based on phase differences in various retarders (biaxial retarders), Nz coefficients, a slow axis angle, an absorbing axis angle of the polarizing plate, cell gaps in the reflective region and the transmissive region, an axial angle of the liquid crystal layer (an angular difference between the rubbing direction of the substrate 1 and the rubbing direction of the substrate 2), and physical properties of a liquid crystal material (refractive index). Desired electro optical characteristics can be obtained by appropriately setting these parameters. The Nz coefficient is a value defined by Nz=(nx−nz)/(nx−ny). The refractive index in the slow axis direction in the retarder plane is represented by nx, the refractive index in the direction orthogonal to nx in the retarder plane is represented by ny, and the refractive index in the vertical direction of the retarder is represented by nz.
Values of the above parameters that contribute to optical design of this embodiment are summarized in Table 1 below. A retardation of the retarder is calculated based on the wavelength of 550 nm, and the retardation of a liquid crystal is calculated based on the wavelength of 589.3 nm.
The axial angle is increased (+) in the counterclockwise direction with the right direction (3 o'clock direction) set to reference angle (0 degrees). That is, 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°. The biaxial retarder is available from Nitto Denko Corporation, and the Nz coefficient is not limited to 0.4 or 0.5 as shown in Table 1 but may be selected from a range of 0 to 0.8.
Further, an adhesive for bonding the circularly polarizing plate (laminate of the polarizer 302 and the λ/4-wave plate 301) on the CF substrate 30 side and the glass substrate or an adhesive for bonding the polarizer 302 in the circularly polarizing plate on the CF substrate 30 side and the λ/4-wave plate 301 may be a disp. adhesive. In particular, the disp. adhesive is preferably used for bonding the glass substrate and the circularly polarizing plate. This is because, at the time of reflecting light incident on the liquid crystal panel, light reflected in one direction can diffuse in all directions due to the disp. adhesive.
As a result, the visibility in the reflective mode can be improved. To reflect light incident on the liquid crystal panel 1, the reflective electrode 23 is formed on the TFT substrate 20 in the liquid crystal panel 1. Further, a mirror reflector may be provided on the rear side of the liquid crystal panel 1 in place of the reflective electrode 23. That is, light reflected by the reflective electrode 23 or mirror reflector can diffuse with the disp. adhesive. The disp. adhesive is obtained by randomly mixing beads having a refractive index different from that of an adhesive into the adhesive, and thus has a function of diffusing transmitted light. As the disp. adhesive, for example, haze 60 is used.
Further, an anti-reflection film may be formed through evaporation or continuous sputtering on the circularly polarizing plate on the CF substrate 30 side. Hence, display quality of the liquid crystal display in the reflective mode can be further increased.
As described above, in the liquid crystal display of this embodiment, the above-described phase difference film and polarizer are provided on the outer side of the liquid crystal panel. Thus, it is possible to provide a liquid crystal display capable of transmissive display at wide view angle and black-and-white display in both of the transmissive mode and the reflective mode without involving tone reversal at low cost. Hence, display quality can be improved.
Further, in the liquid crystal display 200 of this embodiment, a biaxial retarder 211 having an in-plane phase difference of about λ/2 (first about-λ/2-wave plate), an biaxial retarder 212 having an in-plane phase difference of about λ/2 (second about-λ/2-wave plate), a biaxial retarder 213 having an in-plane phase difference of about λ/4 (first about-λ/4-wave plate), and a polarizing plate 214 (first polarizing plate) are provided on the outer surface of the TFT substrate 20 in this order from the TFT substrate 20 side. Further, a biaxial retarder 311 having an in-plane phase difference of about λ/2 (third about-λ/2-wave plate), a biaxial retarder 312 having an in-plane phase difference of about λ/4 (second about-λ/4-wave plate) and a polarizing plate 313 (second polarizing plate) are provided on the outer surface of the CF substrate 30 in this order from the CF substrate 30 side. In this way, it is possible to provide a liquid crystal display of wide view angle at low costs in the liquid crystal mode for applying a horizontal electric field to the liquid crystal without involving tone reversal. This is because a single biaxial retarder having an in-plane phase difference of about λ/2 is provided on the TFT substrate 20 side and the CF substrate 30 side to keep a phase difference of λ/2 at wider view angle.
Values of the above parameters that contribute to optical design of the second embodiment are summarized in Table 2. The retardation of the retarder is calculated based on the wavelength of 550 nm, and the retardation of the liquid crystal is calculated based on the wavelength of 589.3 nm.
The axial angle is increased (+) in the counterclockwise direction with the right direction (3 o'clock direction) set to reference angle (0 degrees). That is, 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°. The biaxial retarder is available from Nitto Denko Corporation, and the Nz coefficient is not limited to 0.4 or 0.5 as shown in Table 1 but may be selected from a range of 0 to 0.8.
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 |
|---|---|---|---|
| 2006-182689 | Jun 2006 | JP | national |
