JP3469148B2 - Reflection type liquid crystal electro-optical device and method of manufacturing the same - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electro-optical device using a nematic liquid crystal having high-speed response and high contrast. Conventionally, a TN (Twisted Nematic) type liquid crystal electro-optical device has been used as a display element of a timepiece, a calculator or the like. FIG. 5 shows the configuration of the TN liquid crystal electro-optical device.
This will be briefly described with reference to FIG. By injecting a nematic liquid crystal having a positive dielectric anisotropy between substrates 51 and 52 which are aligned at an angle of 90 ° to each other, twist alignment of liquid crystal molecules 53 occurs. When an electric field is applied to the liquid crystal, the major axis of the liquid crystal molecules is oriented perpendicular to the substrate due to the interaction between the electric field and the dielectric anisotropy. The state (twist) of the liquid crystal molecules when no voltage is applied to the liquid crystal and the state when the voltage is applied are identified using the polarizing plate 54. Alternatively, conversely, there is a method in which a nematic liquid crystal having a negative dielectric anisotropy is interposed between one of the substrates subjected to the vertical alignment processing. In recent years, research on ferroelectric liquid crystals has been very advanced. The structure of an optical device using ferroelectric liquid crystal is about 2 μm, which is much smaller than that of a TN type liquid crystal device. I do. The ferroelectric liquid crystal molecules have two stable states without applying an electric field, and the molecules are oriented to one stable state by applying an electric field. By applying an electric field in the opposite direction, the molecules are oriented to another stable state. By distinguishing the state of the two liquid crystals using a polarizing plate, light and dark were displayed. [0005] In the case of an optical device using this ferroelectric liquid crystal, the response time is very fast, approximately several tens of microseconds, and therefore, applications to various fields have been expected. [0006] Alternatively, there is an active type in which switching elements such as TFTs and MIMs are arranged in each pixel. Further, there is an STN type liquid crystal in which a twist angle of a nematic liquid crystal is set to 180 ° to 270 °. [0008] However, the above T
Since the N-type liquid crystal electro-optical device has a very slow response time of several tens of milliseconds and a poor response to an applied voltage, its application range is limited to applications other than small-area display such as watches and calculators. . In order to further increase the response speed, a method of shortening the substrate interval is considered. When the substrate interval is shortened, the time from the ON state to the OFF state of the liquid crystal (referred to as rise time) is increased. The time from the OFF state to the ON state (referred to as a fall time) does not become short. In addition, it is difficult to make the liquid crystal be twisted at 90 ° between one substrate and the other. Further, it is possible to increase the response speed by increasing the voltage at the time of driving. However, since the range of the appropriate drivable voltage is determined by the liquid crystal, it is easy to increase the voltage. Can not. Although the response time of an electro-optical device using a ferroelectric liquid crystal is certainly fast, there are many problems. The first problem is that it is very difficult to control the alignment of the liquid crystal. Conventionally, in addition to rubbing, oblique evaporation of silicon oxide, a method of applying a magnetic field, and a temperature gradient method have been performed, but no uniform alignment can be obtained at present under any of the methods. Therefore, high contrast cannot be obtained. Second, a liquid crystal exhibiting a smectic phase can be used as a ferroelectric liquid crystal. Therefore, the ferroelectric liquid crystal has a layer structure peculiar to the smectic liquid crystal. Once this layer structure is broken by an external force, it does not return even if the external force is removed. In order to restore this, it is necessary to heat and once make a phase transition to an isotropic phase, so that a ferroelectric liquid crystal having a layered structure that collapses due to a small external impact is not practical. Third, the ferroelectric liquid crystal accumulates electric charges at the interface with the alignment film due to the spontaneous polarization of the liquid crystal itself, and forms an electric field in a direction opposite to the polarization of the liquid crystal. If you display it, the next time you try to display a different screen,
There is a problem that the previous display remains (referred to as "burn"). Fourth, the contrast ratio of an electro-optical device using a ferroelectric liquid crystal greatly depends on the tilt angle (or cone angle) of the liquid crystal. The value is 22.5 ° (45 °)
It is known that However, liquid crystals satisfying only the condition that the tilt angle (cone angle) is 22.5 ° (45 °) have already been synthesized, but other important conditions, such as the problem of the temperature range in which the liquid crystal exhibits ferroelectricity, Ferroelectric liquid crystals that can sufficiently satisfy the problem of responsiveness to an AC pulse and the like have not yet been developed. Therefore, at present, the problem of the temperature range is more important than the tilt angle. Therefore, the contrast ratio of an electro-optical device using a ferroelectric liquid crystal which is currently under study is not very large. At present, it is very difficult to apply a ferroelectric liquid crystal as a display device due to the problems described above. According to the present invention, there is provided a liquid crystal electro-optical device in which a nematic liquid crystal having a positive dielectric anisotropy is interposed between a pair of substrates. A liquid crystal alignment layer rubbed in an antiparallel direction is formed on the substrate, and a pretilt angle of molecules of the liquid crystal is 4 ° or less. Alternatively, a liquid crystal electro-optical device in which a nematic liquid crystal having a positive dielectric anisotropy is interposed between a pair of substrates, wherein a liquid crystal alignment layer rubbed in an antiparallel direction is formed on the substrates. And the magnitude of the polarity term of the surface tension of the liquid crystal alignment layer is not less than 10 dyne / cm, and further, a nematic liquid crystal having a positive dielectric anisotropy is interposed between the pair of substrates. Liquid crystal electro-optical device, wherein a liquid crystal alignment layer rubbed in an antiparallel direction is formed on the substrate, and the surface tension of the liquid crystal alignment layer is 4
0 dyne / cm or more. The present inventor has found that when the surface tension of the liquid crystal alignment layer increases, the pretilt of the liquid crystal decreases. The surface tension described in the present invention can be expressed by the following equation. (Equation 1) The method for determining the surface tension in the present invention is as follows. (1) First, liquids whose surface tensions are known are determined (two types), and (r _L ), (r _L ) ^d , and (r _L ) ^p for these two types of liquids are determined. Here, r _L is the surface tension, (r _L ) ^d is the dispersion term of the surface tension, and (r _L ) ^p is the polarity term of the surface tension. Table 1 is given as an example of the liquid having a known surface tension. [Table 1] (2) The contact angle is measured on the sample substrate. The liquid used at this time is the liquid i, j determined in (1), and the contact angle is X
_i and X _j . (3) Calculate the adhesion work (W _SL ) _i and (W _SL ) _j using Equation 2. (Equation 2) (4) Hereinafter, the polarity term (r _s ) ^p and the dispersion term (r _s ) ^d of the sample substrate are obtained by Expressions 3 and 4. (Equation 3) (Equation 4) [0021] than (5) or more, because (r _s) ^d, is (r _s) ^p determined, r _s = from ^{_{(r s) d + (r}} s) p, r s can also be determined. The liquid crystal used in the present invention may be a cholesteric (chiral nematic) liquid crystal, but a nematic liquid crystal is more preferable. In the present invention, while the distance between the substrates of the conventional TN type liquid crystal electro-optical device is approximately 8 μm,
The present invention uses a thin substrate spacing of approximately 5 μm or less, preferably 3.5 μm or less. In the present invention, the "anti-parallel direction" means that the rubbing direction of the substrate is substantially equal to 18 as shown in FIG.
It means that it forms an angle of 0 °. Accordingly, since the liquid crystal is not caused to have a twist orientation of 90 ° unlike the conventional case, it is impossible to perform the display utilizing the light-emitting property as in the related art. Therefore, in the present invention, display utilizing the refractive index anisotropy of the liquid crystal is performed. In the present invention, nematic liquid crystal having a positive dielectric anisotropy is used, so that it is very easy to control the alignment of the liquid crystal. After the external force is removed even if the orientation is disturbed, the orientation is quickly returned to its original state, so that it is not necessary to heat to the isotropic phase or the nematic phase. In the present invention, the response time of the liquid crystal is much faster than that of the conventional TN type liquid crystal, and the rise time when an electric field is applied is approximately several tens of microseconds. It corresponds to the response time of the liquid crystal. Further, the fall time is also about 3 ms or less, and a liquid crystal display which has not been obtained conventionally can be obtained. FIG. 2 shows the relationship between the pretilt angle of the liquid crystal molecules and the fall time of the liquid crystal. As can be seen from FIG. 2, when the pretilt angle of the liquid crystal molecules is 4 ° or less, the fall time is 3 ms or less, and a very fast response can be obtained. FIG. 3 is a graph showing the polarity term of the surface tension of the alignment layer and the fall time of the liquid crystal. FIG.
As can be seen, when the magnitude of the polarity term of the surface tension of the liquid crystal alignment layer is 10 dyne / cm or more, the fall time is 3 msec, and a very fast response is obtained. FIG. 4 shows the relationship between the magnitude of the surface tension of the alignment layer and the fall time of the liquid crystal. As can be seen from FIG. 4, when the surface tension is 40 dyn / cm or more, the fall time is 3 ms or less, and a very fast response is obtained.
The liquid crystal used in the graphs of FIGS. 2 to 4 is ZLI-
4792 (Merck). Hereinafter, the present invention will be described with reference to examples. EXAMPLE 1 Example 1 A pair of soda glass (polished product)
Then, an ITO thin film was formed by a sputtering method and patterned by photolithography. Then, a polyimide having a large surface tension (trade name: LP, manufactured by Toray) is put on the electrode preparation surface in 100 parts.
After coating to a thickness of 0 ° and rubbing, one side was subjected to seal printing, and the other side was sprayed with a 2-micron spacer. Then, lamination was performed. At this time, the rubbing direction of the substrate was set to be antiparallel (180 °). Then, liquid crystal was injected by a vacuum injection method. After the injection of the liquid crystal, observation using a polarizing microscope revealed that the liquid crystal molecules were almost aligned in the rubbing direction over the entire liquid crystal layer. Then, a polarizing plate was attached to the liquid crystal cell so as to form crossed Nicols. At this time, the polarization axis of the polarizing plate on the light incident side was set at 45 ° with respect to the rubbing direction. Thus, the response speed was measured. The liquid crystal used was ZLI-4792 (manufactured by Merck). As a result, the rise time was 65 microseconds and the fall time was 2 ms. This is about 10 times to about 1000 times faster than the conventional TN type liquid crystal electro-optical device, and almost corresponds to the response time of the ferroelectric liquid crystal. The liquid crystal of the present liquid crystal panel had a pretilt angle of 2 °, a surface tension of 60 dyne / cm 2, and a polar term of 15 dyne / cm 2. Thereafter, a circuit for driving the liquid crystal was connected, and a backlight module was assembled to complete a transmissive liquid crystal panel. [Embodiment 2] In Embodiment 1, the case where a transmission type panel is manufactured has been described. In this embodiment, a reflection type panel will be described. In this case, two polarizing plates may be used in the same manner as in the transmission type, but display can be performed with only one polarizing plate, and a bright screen can be obtained as compared with a normal reflection type display. After forming ITO electrodes on a pair of substrates in the same manner as in Example 1, a polyimide thin film was obtained on the electrode forming surfaces of the two substrates in the same manner as in Example 1. Then, a rubbing treatment was performed on the polyimide production surface of one of the substrates using a cotton cloth. 1.4 μm SiO ₂ particles were sprayed as spacers. After measuring the distance between the sealed substrate and the cell bonded to the counter substrate by a known interference method, a nematic liquid crystal was injected by a vacuum injection method. The distance between the substrates was measured at five places and found to be 1.3 to 1.4 μm. After sealing the liquid crystal injection port, a polarizing plate was attached to the front surface of the panel, and a reflector was attached to the back surface. At this time, the angle of the polarizing axis of the polarizing plate was 45 ° with respect to the rubbing axis.
Angle. Thus, a circuit for driving the liquid crystal was connected, and a backlight module was assembled, whereby a reflective liquid crystal panel was completed. As in this embodiment, 1
Since display is performed by using two polarizing plates, a brighter image can be obtained as compared with a normal reflective liquid crystal panel. Embodiment 3 In this embodiment, a case where the present invention is applied to a color projector will be described with reference to FIG. Since the color filter is not used in the projector according to the present embodiment, a bright projector having higher transmittance than that of the conventional projector can be manufactured. In FIG. 6, a wavelength 633 is used as the red light source 61.
He-Ne laser with a peak near nm, green light source
An Ar laser having a peak around 515 nm as 62 and an Ar laser having a peak near 477 nm as blue light source 63 are used. And irradiate the liquid crystal panels 65, 66, and 67 produced in Example 1. The light that has passed through the liquid crystal panel is combined by an optical system 68 of which the amount of transmitted light is limited as ON, OFF or gray scale by the shutter function of the liquid crystal panel, and is further enlarged and displayed on a display screen 69. In this embodiment, since the light source has a single wavelength, the optical conditions when passing through the liquid crystal panel can be adjusted in each liquid crystal panel. Sharp display can be performed without blurring of the screen. The laser beam used in the present embodiment is, in addition to the above-mentioned gas laser, a laser beam using metal vapor of cadmium or zinc, a solid laser using ruby or the like, or another type of laser beam in the visible light range. Those having a peak wavelength can be used. In addition, it is also possible to use a device that can emit light in the visible light region by passing through a special optical system at a wavelength other than the normal emission wavelength, such as a second harmonic of a YAG laser. Normally, three laser beams having emission wavelengths close to the red, blue, and green wavelengths are used. However, colors are synthesized by using four or more laser beams having different wavelengths, and the colors are combined. Display can also be performed. As described above, the present invention is capable of displaying images in a new mode which has not been provided in the conventional liquid crystal electro-optical device. It is possible to obtain a liquid crystal electro-optical device in which the alignment control is very easy and the response speed, particularly the falling time, is very high. Further, according to the present invention, a large-screen display can be easily obtained.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows an example of the structure of a liquid crystal electro-optical device according to the present invention. FIG. 2 shows a graph of a pretilt angle and a response speed. FIG. 3 shows a graph of a surface tension polarity term and a response speed. FIG. 4 shows a graph of surface tension and response speed. FIG. 5 shows a structure of a conventional TN type liquid crystal panel. FIG. 6 shows a schematic diagram of a projection optical system according to the invention. [Description of Signs] 51, 52 Substrate 53 Liquid Crystal Molecules 54 Polarizer 61, 62, 63 Laser Light 64, 68 Optical System 65, 66, 67 Liquid Crystal Panel 69 Display Screen

Claims (1)

(57) anisotropy of the Claims 1 dielectric constant between a pair of substrates is a positive value
A reflection type liquid crystal electro-optical device was allowed intervening Matic crystal spacing of the pair of substrates is 5μm to 1 [mu] m, the pretilt angle of the liquid crystal is at 4 ° or less, the rise time of the liquid crystal is more 10μ seconds 100 μs not yet
A full, fall time of the liquid crystal is less than 3m seconds, the liquid crystal alignment layer formed on the substrate is rubbed in a direction antiparallel to a a surface tension 40 dyne / cm or more, The magnitude of the polarity term of the surface tension is 10d
A reflection-type liquid crystal electro-optical device, wherein the reflection-type liquid crystal electro-optical device holds yne / cm or more.