JP2809701B2 - Active matrix type liquid crystal display device - Google Patents

Description

Description of the Invention [Object of the Invention] (Industrial application field) The present invention relates to an active matrix type liquid crystal device, and more particularly to an active matrix type liquid crystal display device which performs gradation display.

(Prior Art) In recent years, active matrix liquid crystal display elements have been receiving attention as display elements capable of high-resolution display. In particular, an active matrix type liquid crystal display device including a field effect transistor (FET) as a switching element is suitable for gradation display, and is being applied to televisions and the like.

The operation mode of the liquid crystal display element is TN type, DS type, GH type, DA
There are a P type, a thermal writing type and the like, and a TN type is generally used for an active matrix type liquid crystal display element.

FIG. 10 shows an equivalent circuit of one pixel of the active matrix type liquid crystal display element. A field effect type thin film transistor 6 is disposed at the intersection of the address line 2 and the signal line 4, and the gate 6G of the thin film transistor 6 is connected to the address line 2 by the drain.
6D is connected to the signal line 4, and the source 6S is connected to the pixel electrode 8. A liquid crystal layer 12 is sandwiched between the pixel electrode 8 and the counter electrode 10. On the surfaces of the pixel electrode 8 and the counter electrode 10 which are in contact with the liquid crystal layer 12, an alignment layer or the like for aligning liquid crystal molecules is formed. These alignment layers and the like often function as the capacitors 14 and 16 shown in FIG. The voltage applied to the liquid crystal layer 12 is controlled by the voltage applied to the address line 2 and the signal line 4, but the voltage actually applied to the liquid crystal layer 12 is applied between the pixel electrode 8 and the counter electrode 10. The obtained voltage is divided by a series connection of the capacitor 14, the liquid crystal layer 12, and the capacitor 16.

The relationship between the liquid crystal applied voltage and the transmittance (VT characteristic) in a normally white (black display on a white background) mode of a TN type liquid crystal display element is shown in FIG. 11 as an example.
A voltage at which the transmittance starts to change (referred to as a threshold voltage Vth) and a voltage at which the change in the transmittance is almost finished (saturation voltage Vs
for example, there is a difference of about 1 to 2V. FET
In the active matrix type liquid crystal display element using the switching element as a switching element, gradation display is performed by providing several voltage levels between Vth and Vsat.

(Problems to be Solved by the Invention) As described above, since the difference between Vth and Vsat is not so large as about 1 to 2 V, the difference between the applied voltage levels decreases as the number of gradations increases, and the control of the applied voltage There is a problem that becomes difficult. That is, when, for example, 64 gradations are displayed by the active matrix type liquid crystal display element, a band-like unevenness in density may occur on the entire screen due to a slight variation in the output of the driving IC. This is because the variation in the output voltage of the driving IC is much larger than the voltage difference between the gradations, and this variation is visually recognized as a difference in light and shade, which significantly impairs the display quality. Further, when the gradation display is performed by such a method, there is a problem that the transmittance changes when the viewing angle is changed is large and the viewing angle range is narrow due to the operation principle of the TN type liquid crystal display element.

Also, in a liquid crystal display element based on an operation mode other than the TN mode, such as the GH mode or the DAP mode, the control of the applied voltage becomes more difficult as the number of gradations increases.

SUMMARY OF THE INVENTION An object of the present invention is to provide an active matrix type liquid crystal display element which solves the above problems and has good display quality and capable of multi-gradation display.

[Means for Solving the Problems] The present invention relates to a matrix wiring composed of a plurality of address lines and a plurality of signal lines, a field effect transistor disposed at an intersection of the matrix wiring, A pixel electrode connected to the signal line via a field-effect transistor;
In an active matrix liquid crystal display device having a counter electrode facing the pixel electrode and a liquid crystal layer sandwiched between the pixel electrode and the counter electrode, between each pixel electrode and the pixel electrode and the liquid crystal layer, or An active matrix type liquid crystal display device characterized in that a dielectric layer whose surface is flattened is provided between a counter electrode and a liquid crystal layer, and the dielectric layer partially has a region having a different dielectric constant. .

(Operation) The present invention substantially sets a voltage difference between a voltage Vth at which the transmittance starts to change and a voltage Vsat at which the change of the transmittance almost ends in the relationship between the liquid crystal applied voltage and the transmittance (VT characteristic). In this case, the gradation can be easily displayed. In the present invention, basically, in one pixel, regions where electric field strengths applied to the liquid crystal layer are different from each other are provided while applying the same voltage between the pixel electrode and the counter electrode. That is, as shown in FIGS. 3A and 3B, regions where the transmittance characteristics (VT characteristics) with respect to the voltage applied between the electrodes are different from each other are provided in one pixel. If each pixel is sufficiently small compared to the entire liquid crystal display element,
The VT characteristic curve of the pixel appears to the observer as shown in FIG. 3c, which is the sum of the characteristics of FIGS. 3a and 3b. As a result, Vt
This means that the difference between h and Vsat is substantially enlarged, and multi-tone display becomes easier.

Such effects are not limited to TN type liquid crystal display elements,
, DAP type, ferroelectric liquid crystal device, and other liquid crystal display devices that respond optically to applied voltage.

Further, according to the present invention, the narrowness of the viewing angle range when performing gradation display is also reduced. This effect will be described in the case of a TN type liquid crystal display element. FIG. 4 shows a change in the VT characteristic depending on the viewing angle direction. FIG. 4a shows a conventional active matrix liquid crystal display device, and FIG. 4b shows an active matrix liquid crystal display device according to the present invention. Is shown. In a liquid crystal display device with a TN operation mode, the relationship between the applied voltage and the transmittance is changed by changing the viewing angle direction.
It changes from a solid line to a broken line. Therefore, in the conventional example, as shown in FIG. 4A, the transmittance changes greatly when the viewing angle direction is changed. On the other hand, as shown in FIG. 4b, in the active-trics type liquid crystal display device of the present invention, the voltage of the VT characteristic due to the change in the viewing angle direction when a plurality of regions having different characteristics are viewed as one pixel. Since the shift width is almost the same as that of the conventional example, the change in the transmittance in the viewing angle direction is small due to the gentle change in the transmittance due to the applied voltage, and the viewing angle range is wide.

Note that the number of regions where electric field strengths applied to the liquid crystal layer are different from each other is not limited to two, and more regions may be provided. Further, it is preferable that areas having different electric field intensities have substantially equal areas. Further, the electric field strength may be changed gradually within one pixel without clearly separating regions having different electric field strengths. When a plurality of regions are provided, or when the electric field intensity is gradually changed, the viewing angle range can be further widened.

In the present invention, in order to form a region in which the electric field intensity is partially different in one pixel, the surface between each pixel electrode and the liquid crystal layer or between the counter electrode and the liquid crystal layer is formed for each pixel electrode. Is provided with a planarized dielectric layer, and this dielectric layer has regions where the dielectric constants are partially different.

FIG. 5 shows an equivalent circuit of one pixel. As is clear from the comparison with FIG. 10, one pixel is divided into, for example, two regions A,
B. 8A is a pixel electrode corresponding to the area A, 8B is a pixel electrode corresponding to the area A, 10A is a counter electrode corresponding to the area A, and 10B is a counter electrode corresponding to the area A. 12A,
Reference numeral 12B denotes a liquid crystal layer corresponding to each of the regions A and B, and reference numerals 14A and 14B denote capacitors formed by the liquid crystal layers 12A and 12B and the pixel electrodes 8A and 8B via an alignment film or the like. 16A and 16B are capacitors formed by the liquid crystal layers 12A and 12B and the counter electrodes 10A and 10B via an alignment film or the like.

The voltage applied to the liquid crystal layer is controlled by the voltage applied to the address lines and the signal lines, similarly to the conventional active matrix type liquid crystal display device. The voltage actually applied to the liquid crystal layer is obtained by dividing the voltage applied between the pixel electrode and the counter electrode by the series connection of the capacitors 14A and 14B, the capacitance of the liquid crystal layers 12A and 12B, and the capacitors 16A and 16B. It will be pressed. Therefore, the capacitors 14A and 14A
By making B or the capacitance of the capacitors 16A and 16B different, the electric field strength applied to the liquid crystal layers 12A and 12B can be made different between the region A and the region B.

Here, in order to make the capacitances of the capacitors 14A, 14B, 16A, 16B different, the dielectric constant or the thickness of the dielectric layer between the liquid crystal layers 12A, 12B and the electrodes 8A, 8B, 10A, 10B is changed. It can be realized by making them different. As the dielectric layer, an alignment film itself for aligning liquid crystal is used, or a separately provided dielectric layer is used.

Note that the magnitude of the capacitance of the capacitor needs to be designed in consideration of the physical properties of the liquid crystal material, particularly the dielectric constant and the thickness of the liquid crystal layer. That is, since the capacitor and the liquid crystal layer form a series circuit, the voltage applied between the pixel electrode and the counter electrode is distributed to the capacitor and the liquid crystal layer. Further, since the distribution ratio is determined by the respective impedances, it is necessary to calculate the impedance of the liquid crystal layer in advance and then determine the capacitance of the capacitor. It should also be noted that the impedance of the liquid crystal layer changes depending on the applied voltage because the liquid crystal itself has dielectric anisotropy.

(Example) Hereinafter, an example of the active matrix type liquid crystal display device of the present invention will be described with reference to FIG. 1 and FIG.

Address lines 20 made of metal are formed at predetermined intervals on the glass substrate 18, and the surface thereof is covered with an insulating gap 22. Further, signal lines 24 made of metal are formed at predetermined intervals so as to intersect with the address lines 20, and a thin film transistor 26 and a pixel electrode 28 made of ITO are formed at each intersection. The thin film transistor 26 includes a gate electrode 26G, a drain electrode 26D, and a source electrode 26S made of metal, a semiconductor film 28 made of, for example, amorphous silicon, and low-resistance semiconductor films 30a and 30b. The gate electrode 26G is connected to the address line 20, and the drain electrode 26D is connected to the signal line 24.
The source electrode 26S is connected to the pixel electrode 28. Further, an insulating film 32 made of, for example, SiNx is formed so as to cover a part of the thin film transistor 26, the signal line 24, and the pixel electrode 28. For example, the insulating film 32 is formed so as to cover substantially half of the pixel electrode 28, and the inside of the square indicated by the reference numeral 33 in the pixel electrode 28 in FIG.

This insulating film 32 is formed as follows. First, the entire surface of the array substrate on which the matrix wiring, the thin film transistor 26, the pixel electrode 28, etc. are formed is about 1 μm
To form a SiN _x film with a thickness of Next, a resist film is formed on the SiN _x film to be left by the optical engraving method, and when dry etching is performed, only the SiN _x film covered by the resist film remains. Thereafter, if the resist film is peeled off, a predetermined portion of the SiN _x film remains. An orientation film 34a made of, for example, polyimide is formed to a thickness of 1000 A so as to further cover the insulating film 32 thus formed.

On the other hand, the glass substrate 36 has, for example, a lattice-shaped light-shielding film
38 is formed on it, for example, IT
An opposing electrode 40 made of O is formed, and an alignment film 34b made of, for example, polyimide is further formed thereon.

Further, a liquid crystal layer 42 made of, for example, a nematic liquid crystal is interposed between the glass substrate 18 and the glass substrate 36, and has a thickness of about 5 μm. The liquid crystal molecules are aligned with the alignment films 34a and 34b.
By the action of, the liquid crystal molecules are oriented substantially parallel to the substrate, and the direction of the liquid crystal molecules is twisted by 90 degrees between the alignment films 34a and 34b.
It is a so-called TN type liquid crystal display element.

According to the above configuration, the insulating film is provided on the pixel electrode 28 side.
3. An active matrix type liquid crystal display device according to claim 2, wherein a region where the thickness of the dielectric is different from that of the liquid crystal layer, that is, a region where the capacitance of the capacitor is different from each other, is formed between the liquid crystal layer and the liquid crystal layer. Can be

When the active matrix type liquid crystal display element manufactured in this manner was displayed in 64 gradations, the transmittance was almost as designed at each gradation and the display quality was high. The change in transmittance when the viewing angle direction was changed was small, and the viewing angle range was wide.

In the above embodiment, capacitors having different capacitances are formed in each pixel on the pixel electrode side. However, as shown in FIG. 6, capacitors having different capacitances can be formed on the counter electrode side.

That is, a resin layer 44 of a predetermined shape made of a photosensitive resin may be formed on the opposing electrode 40 facing the pixel electrode 6 by the optical engraving method, and the alignment film 34b may be formed on the entire surface.

Further, as shown in FIG. 7, a plurality of times by several hundred A alignment film 34a made of polyimide in the configuration of FIG. 1, by forming a thickness of about 2 to 3 [mu] m, made of SiN _x insulating After flattening the step between the portion covered with the film 32 and the portion not covered with the film 32, etching is performed by O ₃ / UV so that the thickness of the alignment film 34a on the insulating film 32 becomes about 1000 A. It may be controlled. In this case, the relative permittivity of the used polyimide is about 3, while the relative permittivity of SiN _x is about 7, which is different from that of the polyimide. This means that capacitors having greatly different capacitances have been inserted.

In each of the above embodiments, the capacitance of a capacitor inserted in series in the liquid crystal layer is partially different in each pixel. However, as shown in FIG.
An insulating layer 46 is provided partially below the pixel electrode 28 so that the pixel electrode 28 partially projects, or as shown in FIG. Partially provided with an insulating layer 48, partially protruding the counter electrode 40,
A region in which the distance between the pixel electrode 28 and the counter electrode 40 is partially different in each pixel is formed, and even in an active matrix type liquid crystal display element, a region in which electric field strengths applied to the liquid crystal layer are different from each other in one pixel. Thus, the same effects as those of the above embodiments can be obtained.

Note that the present invention is not limited to the above-described embodiment, but can have various configurations within the spirit of the present invention.

[Effects of the Invention] According to the present invention, an active matrix liquid crystal display element having excellent controllability of gradation display, a wide viewing angle, and high display quality can be obtained.

[Brief description of the drawings]

FIG. 1 is a sectional view of an active matrix type liquid crystal display device of the present invention, FIG. 2 is a plan view showing one substrate of the active matrix type liquid crystal display device of FIG. 1, and FIG. 3 is a VT according to the present invention. FIG. 4 is a diagram illustrating characteristics, FIG. 4 is a diagram illustrating viewing angle dependency according to the present invention, FIG. 5 is an equivalent circuit diagram of one pixel of the active matrix type liquid crystal display device of the present invention, and FIGS. Sectional view showing another embodiment of the present invention.
FIG. 10 is an equivalent circuit diagram of one pixel of a conventional active matrix type liquid crystal display element, and FIG. 11 is a V-type of a TN type liquid crystal display element.
FIG. 4 is a diagram illustrating T characteristics.

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int.Cl. ⁶ , DB name) G02F 1/136 500

Claims (1)

(57) [Claims] A matrix wiring comprising a plurality of address lines and a plurality of signal lines; a field effect transistor disposed at an intersection of the matrix wiring; and a signal line connected to the signal line via the field effect transistor. Pixel electrode,
In an active matrix liquid crystal display element having a counter electrode facing the pixel electrode and a liquid crystal layer sandwiched between the pixel electrode and the counter electrode, for each of the pixel electrodes, the pixel electrode and the liquid crystal layer Active matrix, wherein a dielectric layer whose surface is flattened is provided between the counter electrodes and between the counter electrode and the liquid crystal layer, and the dielectric layer partially has a region having a different dielectric constant. Liquid crystal display device.