JPH0895062A - Liquid crystal display element - Google Patents

Abstract

PURPOSE: To provide a liquid crystal display element of which liquid crystal can be oriented in a favorable state with little turbulence and driven by low voltage. CONSTITUTION: A recessed part 31 is formed on a substrate 11 for forming TFT of a liquid crystal display element, and the TFT 14 is formed in the recessed part 31. A picture element electrode 13 connected to the active element 14 is formed on the main face of the base plate 11. A flattening film 17 is formed on the active element 14 and the picture element electrode 13 while filling the recessed part 31, and an orientation film 18 is formed thereon. The maximum inclination (ϕ) due to the recessed and projecting part of the surface of the orientation film 18 is made smaller than the cone angle (θ) of ferroelectric liquid crystal 23, and the turbulence of orientation due to the recessed and projecting part is restrained at minimum.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a ferroelectric liquid crystal display device (including an antiferroelectric liquid crystal display device) having ferroelectricity, and more particularly, to an active matrix type ferroelectric device with little disorder in orientation and easy to manufacture. The present invention relates to a dielectric liquid crystal display device.

[0002]

2. Description of the Related Art FIG. 8 shows a part of a conventional active matrix type ferroelectric liquid crystal display device. This liquid crystal display device includes a TFT substrate 1, a counter substrate 2 arranged to face the TFT substrate 1, a ferroelectric liquid crystal 3 sealed between the TFT substrate 1 and the counter substrate 2, and It is composed of polarizing plates 4 and 5 which are sandwiched.

The TFT substrate 1 has a pixel electrode 6 and a TFT.
(Thin film transistor) 7 are arranged and formed in a matrix. A counter electrode 8 that faces the pixel electrode 6 is formed on the counter substrate 2.

Alignment films 9 and 10 are formed on the TFT substrate 1 and the counter substrate 2, respectively, and the facing surfaces of the alignment films 9 and 10 are subjected to alignment treatment such as rubbing. The ferroelectric liquid crystal display element having such a structure has characteristics that it has a high-speed response and a wide viewing angle as compared with a normally used TN liquid crystal display element and the like.

[0005]

In the structure of FIG. 8, since the TFT 7 is thick, a convex portion corresponding to the TFT 7 is formed on the surface of the alignment film 9 in contact with the ferroelectric liquid crystal 3. The liquid crystal molecules of the ferroelectric liquid crystal in the vicinity of the convex portion have the convex alignment film 9
Orientation is based on the surface of. Therefore, the alignment state of the ferroelectric liquid crystal 3 is disturbed near the convex portion. The disorder of the alignment in the vicinity of the convex portion affects the alignment of the liquid crystal around it. Therefore, a defect occurs in the alignment structure of the ferroelectric liquid crystal 3, which causes display unevenness and contrast reduction.
In particular, in the case of a ferroelectric liquid crystal, the liquid crystal has a layer structure of a smectic phase, and the disorder of the alignment may cause a defect in the layer structure itself, which may cause display unevenness and a decrease in contrast.

In order to solve such a problem, the TFT 7
Also, a film (planarization film) for flattening the interface between the alignment film 9 and the ferroelectric liquid crystal 3 is formed between the pixel electrode 6 and the alignment film 9. However, the T
Since unevenness occurs depending on the shape of the FT 7, an extremely thick flattening film is required to make the surface of the alignment film 9 substantially flat. Further, since the thick flattening film is formed on the pixel electrode 6, the gap between the pixel electrode 6 and the ferroelectric liquid crystal 3 becomes wide, and the voltage drop in the flattening film causes the voltage to be applied to the ferroelectric liquid crystal 3. The applied voltage becomes relatively small and the drive voltage becomes high.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a liquid crystal display device capable of orienting a liquid crystal in a good state with little disorder. Another object of the present invention is to provide a liquid crystal display device capable of driving liquid crystal with a low voltage.

[0008]

In order to achieve the above object, a liquid crystal display element of the present invention comprises: a first substrate having a depression formed on one principal surface; and a depression in the first substrate. At least a part of the active elements arranged in a matrix, the pixel electrodes arranged in a matrix and connected to the corresponding active elements, and the pixel elements formed on the active elements and the pixel electrodes, A first in which a depression and an unevenness corresponding to the active element are formed
Alignment film and a second alignment film that faces the first substrate.
Substrate, a counter electrode formed on a surface of the second substrate facing the first substrate and facing the pixel electrode, a second alignment film formed on the counter electrode, A chiral smectic phase liquid crystal having a spiral structure and sealed between the first and second alignment films, the first alignment film having a spiral axis of the spiral structure on its surface. It is characterized in that the maximum tilt angle is smaller than the angle formed by the axis of the spiral and the director of the liquid crystal molecules.

[0009]

According to the above construction, the maximum tilt angle of the first alignment film having irregularities corresponding to the active element is the angle formed by the axis of the spiral and the director of the liquid crystal molecule (cone angle).
Formed smaller than. According to such a configuration, the first
The disturbance of the alignment due to the inclination of the surface of the alignment film can be absorbed by the distortion of the spiral structure for one layer of liquid crystal molecules in contact with the first alignment film. Therefore, the disorder of orientation does not spread over a wide range and is limited to a narrow range. Therefore, it is possible to provide a liquid crystal element with less disordered orientation.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A ferroelectric liquid crystal display device according to embodiments of the present invention will be described below with reference to the drawings. (First Embodiment) First, the structure of the entire ferroelectric liquid crystal display device according to the first embodiment of the present invention will be described with reference to FIGS. FIG. 1 is an overall sectional view of the liquid crystal display element of the first embodiment, and FIG. 2 is a plan view of a transparent substrate (TFT substrate) on which pixel electrodes and thin film transistors are formed.

This liquid crystal display element is of an active matrix type, and as shown in FIG. 1, a pair of insulating transparent substrates (for example, glass substrates) 11 and 12 facing each other and arranged in parallel, and transparent. It is composed of a liquid crystal cell 27 composed of a liquid crystal 23 arranged between the substrates 11 and 12, and polarizing plates 25 and 26 arranged so as to sandwich the liquid crystal cell 27.

As shown in FIGS. 1 and 2, the transparent substrate (TF
Pixel electrodes 13 made of a transparent conductive material such as ITO and TFTs (thin film transistors) 14 connected to the pixel electrodes 13 are arranged in a matrix on a T substrate 11. As shown in FIG. 3, the TFT substrate 11 has grooves 31 formed in a lattice shape, and the TFTs 14 are formed by being embedded in the grooves 31.

Gate lines 15 are arranged between the rows of the pixel electrodes 13 and data lines 16 are arranged between the columns of the pixel electrodes 13. The gate electrode of each TFT 14 is connected to the corresponding gate line 15, the drain electrode is connected to the corresponding data line 16, and the source electrode is connected to the corresponding pixel electrode 13. The gate line 15 and the data line 16 are also embedded in the groove 31 formed in the TFT substrate 11.

A flattening film 17 made of SiO ₂ or the like is formed on the pixel electrode 13, the TFT 14, the gate line 15, the data line 16 and the like. In addition, the flattening film 17
Are also filled in the groove 31 formed in the TFT substrate 11. A first alignment film 18 made of polyimide or the like is formed on the flattening film 17. The surface of the first alignment film 18 is subjected to an alignment treatment such as a rubbing treatment.

A transparent counter electrode 20 is formed on the inner surface of the transparent substrate (counter substrate) 12 so as to face each pixel electrode 13 and to which a constant reference voltage is applied. Counter electrode 20
A second alignment film 21 is formed on the above. The surface of the second alignment film 21 is also subjected to the alignment treatment.

The TFT substrate 11 and the counter substrate 12 are adhered to each other at the outer peripheral edge portion thereof via a frame-shaped sealing material 22. Liquid crystal 23 is enclosed in a region surrounded by the TFT substrate 11, the counter substrate 12, and the sealing material 22. Liquid crystal 23
For example, the chiral smectic C phase has a spiral pitch smaller than the cell gap and is sealed in a liquid crystal cell in a state of having a spiral structure, and the chiral smectic C phase spiral structure is distorted by an applied voltage.
rmed Helix Ferroelectric) Liquid crystals are preferred. Distance between the TFT substrate 11 and the counter substrate 12 (more accurately, distance between the first alignment film 18 and the second alignment film 21 = liquid crystal layer thickness d)
Is held at a constant value by the gap material 24.

According to the liquid crystal display device having the above structure, the TFT
14 is formed in the groove 31 of the TFT substrate 11, that is, embedded in the TFT substrate 11. Further, by filling the groove 31, the TFT 14 and the pixel electrode 13
A flattening film 17 is formed on the above, and a first alignment film 18 is further formed thereon. Therefore, as compared with the case where the groove 31 is not formed, the first alignment film 1 is more easily formed.
8 can be sufficiently flattened.

However, in order to completely flatten the surface of the first alignment film 18, a very thick flattening film 17 is required, which is practically difficult to manufacture. On the other hand, DHF (De
The molecules of the liquid crystal 23 are formed as shown in FIG. 5, and the long axis direction (director) of the liquid crystal molecule is rotated by a certain angle for each layer of the layer structure having a smectic phase, and as a whole, Form a spiral structure. This spiral structure is not fixed, and the orientation can be maintained in an appropriately distorted state according to an externally applied electric field, magnetic field, contact surface inclination, or the like.

Therefore, as shown in FIG. 4, the maximum tilt angle φ of the surface of the first alignment film 18 with respect to the spiral axis of liquid crystal molecules (or the inner surfaces of the transparent substrates 11 and 12 arranged in parallel).
Is less than or equal to the angle (cone angle) θ with respect to the helical axis of the liquid crystal molecules, as shown in FIG.
Distortion of the orientation due to the inclination of the surface can be suppressed by the distortion of the spiral structure of one layer of liquid crystal molecules, and it is possible to prevent the disturbance of the orientation from spreading to a wide region.

In the case of the structure shown in FIG.
The first alignment film 1 formed by the TFT 14
The unevenness of 8 can be suppressed to be relatively small. further,
Since it is not necessary to completely flatten the surface of the first alignment film 18, it is possible to reduce the thickness of the flattening film 17 and improve the characteristics of the liquid crystal display element.

Next, a method of manufacturing the liquid crystal display device having the above structure will be described with reference to FIGS. First, the manufacturing process on the TFT substrate 11 side will be described. First, a groove 31 is formed on the TFT substrate 11 made of glass or the like by using an excimer laser or a hydrofluoric acid type etching solution in a plane in FIG. 3 and in a cross section in FIG. TFT substrate 11 in which groove 31 is formed
A metal film of chromium, aluminum or the like having a thickness of about 50 nm to 200 nm is formed on the entire surface of
This metal film is patterned using a photolithography method or the like to form the gate electrode 33 and the gate line 15 in the groove 31.
Form within.

The entire thickness of the TFT substrate 11 is 200 nm to 4 nm.
A gate insulating film 34 made of a 00 nm silicon nitride film (SiN) or the like is formed by CVD or the like. The gate insulating film 34 is also deposited on the main surface 11A of the TFT substrate 11,
It is formed commonly to all the TFTs 14. Gate insulating film 3
An i-type (intrinsic) semiconductor layer (for example, an intrinsic amorphous silicon layer or an intrinsic polysilicon layer) 35 is deposited on the surface 4 and patterned into the element shape of each TFT 14.

A thickness of 100 nm to 2 is formed on the entire surface of the TFT substrate 11.
A 00 nm silicon nitride film is formed and patterned to form a blocking layer 36 for protecting the channel region of the i-type semiconductor layer 35 from etching. Then, an n-type high-concentration silicon layer is formed and patterned to form an ohmic contact layer 37.

A transparent conductive film such as ITO is formed by sputtering or the like, and is patterned to form the pixel electrode 13 connected to the ohmic contact layer 37. Then, a chromium layer 38 having a thickness of 30 nm to 70 nm and an aluminum layer 39 having a thickness of 150 nm to 250 nm are sequentially deposited on the entire surface of the TFT substrate 11. Next, the chrome layer 38 and the aluminum layer 39 are patterned to form the data line 16 integrally formed with the source electrode SE, the drain electrode DE, and the drain electrode DE connected to the pixel electrode 13 and the ohmic contact layer 37. .

Then, using a spin coater or the like, Si-
(OH) ₄ or the like is applied to the entire surface of the TFT substrate 11 and heated to form a flattening film 17 made of SiO ₂ . CV
Unlike D, sputtering, etc., by using the spin coating method, the inside of the groove 31 can be filled and the planarizing film 17 having a relatively flat surface can be formed. The flattening film 17 is
It may be formed by spin-coating polyimide or the like. It may also be formed by printing or the like. The surface of the flattening film 17 does not need to be perfectly flat, and the maximum tilt angle φ of the surface of the first alignment film 18 formed later is an angle with respect to the helical axis of liquid crystal molecules (or the surface of the TFT substrate 11). (Cone angle) Select a value that is less than θ.

Next, a solution of aromatic polyamic acid or the like is applied onto the flattening film 17 by using a spin coater or the like, and 1
It heats at 00-350 degreeC and forms a polyimide type polymer film. The formed polyimide-based polymer film is subjected to alignment treatment such as rubbing to complete the first alignment film 18 having irregularities and having the maximum inclination angle φ of the surface equal to or less than the angle θ with respect to the helical axis of the liquid crystal molecules.

On the other hand, for the counter substrate 12, a transparent conductive layer such as ITO is deposited and patterned to form the counter electrode 20.
Then, a second alignment film 21 is formed on the counter electrode 20, and an alignment treatment such as rubbing is performed on the second alignment film 21.

After that, the two transparent substrates 11 and 12 are bonded to each other through the sealing material 22 and the spacer 24, and the transparent substrates 11 and 1 are joined together.
A liquid crystal cell 27 is formed by injecting the liquid crystal 23 between the two using a vacuum injection method or the like.

In the configuration shown in FIG. 4, the groove 31 has a T
It is desirable that the apex of the FT 14 and the height of the gate insulating film 34 be substantially equal to each other, that is, the depth of the TFT 14 minus the thickness of the gate insulating film 34. Further, although the TFT 14, the gate line 15 and the data line 16 are formed in the groove 34, holes for forming the TFT 14 are formed in a matrix, the TFT 14 is formed in the hole, and the gate line 15 and the data line 16 are formed. May be arranged on the surface of the TFT substrate 11. Also,
The shape of the depressions such as grooves and holes, the method of forming the depressions, etc. are arbitrary.

(Second Embodiment) In the first embodiment, the flattening film 17 is formed on the pixel electrode 13 and the distance between the pixel electrode 13 and the liquid crystal 23 is large.
The voltage drop at 7 becomes large and the drive voltage becomes high.

Therefore, for example, as shown in FIG. 7, by forming the pixel electrode 13 on the flattening film 17, the voltage drop in the flattening film 17 can be removed and the drive voltage can be lowered. . The pixel electrode 13 is connected to the source electrode SE of the TFT 14 through the contact hole 41 formed in the flattening film 17, and the first alignment film 18 is formed on the flattening film 17 and the pixel electrode 13. In this case, the pixel electrode 1
Since the first alignment film 18 is directly formed on the
The alignment film 18 is likely to have irregularities corresponding to the ends of the pixel electrodes 13. Therefore, for example, the end portion of the pixel electrode 13 is formed in a tapered shape so that the maximum inclination angle φ of the surface of the first alignment film 18 is equal to or less than the cone angle θ of the liquid crystal 23.

As described above, in the above embodiment, T
The TFT 1 is formed in the groove (concave portion) 31 formed on the FT substrate 11.
4 is formed, and the flattening film 1 is formed on the TFT 14 and the pixel electrode 13.
7 is formed, the surface of the first alignment film 18 formed on the flattening film 17 is almost flat even if the flattening film 17 is relatively thin. Therefore, the alignment disorder of the liquid crystal 23 can be prevented. In addition, the pixel electrode 13 is the TFT substrate 1.
Since it is formed on the first flat portion (main surface) 11A, the distance between the pixel electrode 13 and the liquid crystal 23 is small, and the drive voltage can be lowered. The flattening film 17 may be arranged as needed. For example, if the polyimide forming the first alignment film 18 fills the groove 31, and the maximum inclination angle φ of the surface is equal to or less than the cone angle θ, it is not necessary to dispose the flattening film 17.

The structure and shape of the TFT 14 are not limited to those shown in the first and second embodiments. For example, a channel etch type TFT that does not use a channel blocking film may be used. Further, an active element such as MIM may be formed in the recess instead of the TFT. Further, in the first and second embodiments, the DHF liquid crystal is used as the liquid crystal 23, but the SBF (Short pitch Bi) having no memory property is used.
A ferroelectric liquid crystal such as a stable ferroelectric liquid crystal, an antiferroelectric liquid crystal having a ferroelectric phase and an antiferroelectric phase, or the like may be used. Besides, the present invention is not limited to the embodiments, and various modifications and applications are possible.

[0034]

As described above, according to the present invention, the surface of the alignment film on the side of the substrate on which the active element is formed can be easily flattened. Furthermore, since it is not necessary to completely flatten the surface of the alignment film, it is easy to manufacture the alignment film, and it is possible to suppress disorder of alignment and display a high-quality image.

[Brief description of drawings]

FIG. 1 is a sectional view of a liquid crystal display element according to a first embodiment of the present invention.

FIG. 2 is a plan view of a transparent substrate on which a pixel electrode and a thin film transistor are formed.

FIG. 3 is a plan view for explaining a groove for forming a TFT.

FIG. 4 is an enlarged cross-sectional view of a TFT formation portion.

FIG. 5 is a schematic diagram for explaining a helical structure of liquid crystal molecules of DHF liquid crystal.

FIG. 6 is a diagram schematically showing an alignment state of DHF liquid crystal in the vicinity of the alignment film when the alignment film has an inclination.

FIG. 7 is an enlarged cross-sectional view of a TFT formation portion.

FIG. 8 is a cross-sectional view illustrating a configuration of a conventional active matrix type liquid crystal display element.

[Explanation of symbols]

11 ... TFT substrate, 12 ... counter substrate, 13 ... pixel electrode, 14 ... TFT (thin film transistor), 15 ... gate line, 16 ... data line, 17 ... planarization Membrane, 1
8 ... 1st alignment film, 20 ... Counter electrode, 21 ... 2nd alignment film, 22 ... Seal material, 23 ... Liquid crystal, 24 ... Gap material, 25 ... -Polarizing plate, 26 ... Polarizing plate, 27 ... Liquid crystal cell, 31 ... Groove, 33 ... Gate electrode, 34 ... Gate insulating film, 35 ... i-type semiconductor layer, 36 ... ..Blocking layer, 37 ... Ohmic contact layer, 38 ... Chrome layer, 39 ... Aluminum layer, 41 ... Contact hole, SE ... Source electrode, DE ... Drain electrode

Claims (6)

[Claims] 1. A first substrate having a depression formed on one main surface, active elements at least a part of which is formed in the depression of the first substrate and arranged in a matrix, and a matrix. And a pixel electrode connected to the corresponding active element, and formed on the active element and the pixel electrode.
A first alignment film on which the depressions and the irregularities corresponding to the active elements are formed; a second substrate arranged to face the first substrate; and a first substrate of the second substrate. And a second alignment film formed on the counter electrode, and a spiral structure is formed between the first and second alignment films. And a liquid crystal having a chiral smectic phase sealed therein, the first alignment film having a maximum tilt angle with respect to the axis of the spiral of the spiral structure on the surface of the first alignment film of the director of the spiral axis and the director of liquid crystal molecules. A liquid crystal display element, which is formed so as to be smaller than the angle formed. 2. The liquid crystal display element according to claim 1, wherein the recessed portion is formed of a groove, and a line connected to the plurality of active elements is formed in the groove. 3. A first substrate, an active element arranged in a matrix on the first substrate, at least a part of which is formed by embedding at least a part of the first substrate; A pixel electrode arranged in a matrix on the substrate and connected to the corresponding active element, and formed on the active element and the pixel electrode,
A first alignment film at least a part of which is formed to have an inclination; a second substrate arranged to face the first substrate; and a second substrate to face the first substrate of the second substrate. A counter electrode that is formed on a surface of the counter electrode and that faces the pixel electrode, a second alignment film formed on the counter electrode, and a spiral structure between the first and second alignment films. And a liquid crystal having a sealed chiral smectic phase, wherein the first alignment film is formed such that a maximum tilt angle with respect to the axis of the spiral is smaller than an angle formed by the axis of the spiral and a director of liquid crystal molecules. Liquid crystal display element characterized by the following. 4. The liquid crystal is composed of a liquid crystal material having ferroelectricity in which a helical structure of liquid crystal molecules in a chiral smectic phase is distorted in response to application of a voltage. The liquid crystal display device according to item 1. 5. A flattening film for flattening a contact surface between the liquid crystal and the first alignment film is formed on the active element and the pixel electrode, and the first alignment film is formed by: It is formed on the said planarization film, The liquid crystal display element of Claim 1, 2, 3 or 4 characterized by the above-mentioned. 6. A flattening film for flattening a contact surface between the liquid crystal and the first alignment film is formed on the active element and the pixel electrode, and the pixel electrode is flattened. The liquid crystal display element according to claim 1, wherein the liquid crystal display element is formed on a film and is connected to the active element through a contact hole formed in the flattening film.