JP3447730B2 - Display device - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an active type display device, and more particularly to an active type liquid crystal display device,
This is so that a clear gradation level can be set.

[0002]

2. Description of the Related Art Liquid crystal compositions have different dielectric constants in the horizontal and vertical directions with respect to the molecular axis because of their material properties. Therefore, liquid crystal compositions may be aligned horizontally or vertically with respect to an external electric field. It can be done easily. The liquid crystal electro-optical device uses the anisotropy of the dielectric constant to control the amount of transmitted light or the amount of dispersion of light to perform ON / OFF display.

FIG. 2 shows the electro-optical characteristics of nematic liquid crystal. When the applied voltage is Va (point A 201),
2 when the amount of transmitted light is almost 0% and Vb (point B 202)
About 0%, about 70% in the case of Vc (C point 203),
In the case of Vd (D point 204), it becomes about 100%. That is, if only points A and D are used, black and white two-gradation display can be performed, and if points at which the electro-optical characteristics rise such as points B and C are used, halftone display can be performed.

Conventionally, in the case of gradation display of a liquid crystal electro-optical device using a TFT, gradation display is performed by changing the voltage applied to the gate of the TFT or the voltage applied between the source and the drain to adjust the voltage in an analog manner. Was there.

[0005]

However, it is difficult to manufacture a large number of matrix-configured TFT elements so that all the TFT elements have uniform characteristics. The fine adjustment was very difficult.

[0006]

Therefore, the present invention proposes means for supplying a clear gradation display level to liquid crystal by performing digital gradation display instead of conventional analog gradation display. Is.

Therefore, in an active matrix type liquid crystal display device, gradation display of a display device using a display drive system having a display timing related by a unit time t for writing in an arbitrary pixel and a time F for writing one screen is The signal during the writing time of the time t is time-divided without changing the time F, whereby the average value of the electric field applied to the liquid crystal of the pixel at the time t is changed according to the division ratio, and the gradation is changed. It is characterized by making it possible to display.

For the purpose of explanation, a 4 × 4 matrix as shown in FIG. 3 is used. In the case of the conventional display device, as shown in FIG. 4, the strength of the electric field of the signal lines 210 to 213 in the data direction determines the electric field applied to the pixel electrodes as shown at 220 to 223, and the transmittance of the liquid crystal is thereby determined. Is determined.
Needless to say, the reference numerals in FIGS. 3 and 4 correspond to each other.

In the present invention, such analog gradation control is not performed, but as shown in FIG. 1, the signal during the writing time of the unit time t225 for writing to an arbitrary pixel is set as time division, and the number of divisions is set. The gradation of minutes can be displayed.

At this time, the electric field change 2 during the writing time
When 27, 229, and 231 change as shown in FIG. 1, the average values 228, 230, and 232 are obtained in the non-writing time, and clear gradation display is possible. A more detailed description will be given below with reference to examples.

[0011]

[Embodiment 1] In this embodiment, a liquid crystal display device will be described using a liquid crystal display device having a circuit configuration as shown in FIG. FIG. 5 shows an actual arrangement configuration of electrodes and the like corresponding to this circuit configuration. For simplicity of description, only the portion corresponding to 2 × 2 is shown. The actual drive signal waveform is shown in FIG. In order to simplify the explanation, the explanation will also be made on the signal waveform in the case of the 4 × 4 matrix configuration.

First, a method of manufacturing the liquid crystal display device used in this embodiment will be described with reference to FIG. In FIG. 6A, a silicon oxide film as a blocking layer 51 is formed on a glass 50, such as quartz glass, which can withstand a heat treatment at 700 ° C. or less, for example, about 600 ° C. Fabricate to a thickness of ~ 300 nm. The process conditions were an atmosphere of 100% oxygen, a film forming temperature of 15 ° C., an output of 400 to 800 W, and a pressure of 0.5 Pa. The film formation rate using quartz or single crystal silicon for the target was 3 to 10 nm / min.

A silicon film was formed thereon by LPCVD (Low Pressure Vapor Phase) method, sputtering method or plasma CVD method. When forming by the reduced pressure vapor phase method, it is 1 more than the crystallization temperature.
450-550 ° C, which is low by 00-200 ° C, for example, 530 ° C
CVD of disilane (Si ₂ H ₆ ) or trisilane (Si ₃ H ₈ )
The film was supplied to the apparatus to form a film. The reactor pressure is 30-300
It was Pa. The film formation rate was 5 to 25 nm / min. N
To control the threshold voltage (Vth) of the TFT, boron is used in an amount of 1 × 10 ¹⁵ to 1 × 10 ¹⁸ cm ^-3 using diborane.
It may be added during the film formation as a concentration of.

When the sputtering method is used, the back pressure before the sputtering is set to 1 × 10 ^-5 Pa or less, the single crystal silicon is used as the target, and the atmosphere is mixed with 20 to 80% of hydrogen in argon. For example, argon is 20% and hydrogen is 80%.
The film forming temperature was 150 ° C., the frequency was 13.56 MHz, the sputter output was 400 to 800 W, and the pressure was 0.5 Pa.

When a silicon film is formed by the plasma CVD method, the temperature is, for example, 300 ° C., and monosilane (SiH ₄ ) or disilane (Si ₂ H ₆ ) is used. These were introduced into a PCVD apparatus, and high-frequency power of 13.56 MHz was applied to form a film.

The films formed by these methods are
5 × 10 oxygen^{twenty one}cm^-3The following is preferable. This acid
When the elemental concentration is high, it is difficult to crystallize and the thermal annealing temperature is
Higher or longer thermal anneal times must be provided.
If it is too small, the backlight will turn off the light.
The current will increase. Therefore 4 × 10¹⁹~ 4 x 10 ^{twenty one}
cm^-3And the range. 4 × 10 for hydrogen²⁰cm^-3And silicon 4
× 10^{twenty two}cm^-3Was 1 atom%. Also,
To promote crystallization of the source and drain
Therefore, the oxygen concentration is 7 × 10¹⁹cm^-3Below, preferably 1 × 10¹⁹
cm^-3The following is to form the channel of the TFT that makes up the pixel
5 × 10 oxygen only in the region by ion implantation²⁰~ 5 x 10
^{twenty one}cm^-3You may add so that it may become. Then the peripheral circuit
This TFT does not emit light because it does not emit light.
With less carrier and higher carrier mobility
Squeezing is effective for high frequency operation.
It

An amorphous silicon film having a thickness of 50 to 500 nm, for example, 150 nm is formed by the method as described above, and then 12 to 70 at a temperature of 450 to 700 ° C.
Heat treatment was performed at a medium temperature in a non-oxide atmosphere for an hour, for example, the temperature was kept at 600 ° C. in a hydrogen atmosphere. Since the amorphous silicon oxide film is formed on the surface of the substrate below the silicon film, no specific nuclei are present in this heat treatment, and the whole is uniformly annealed. That is, it has an amorphous structure at the time of film formation, and hydrogen is simply mixed therein.

The annealing causes the silicon film to shift from an amorphous structure to a highly ordered state, and a part thereof assumes a crystalline state. In particular, a region having a relatively high degree of ordering after the film formation of silicon tends to be crystallized and become a crystalline state. However, since silicon existing between these regions is bonded to each other, the silicon members pull each other. Peak 522 of single crystal silicon as measured by laser Raman spectroscopy
Peaks shifted to the lower wave number side than cm ^-1 are observed. The apparent particle size is 5 to 50 when calculated from the full width at half maximum.
nm and microcrystals, but in reality, there are many highly crystalline regions with a cluster structure, and each cluster has a semi-amorphous structure in which silicon is bonded (anchoring) with each other. Could be formed.

As a result, the coating film is in a state in which it may be said that there is substantially no grain boundary (hereinafter referred to as GB). Since the carriers can easily move between the clusters through the anchored portions, the carrier mobility is higher than that of polycrystalline silicon in which so-called GB is clearly present. That is, hole mobility (μh) = 10 to ²⁰⁰ cm ² /
Vsec, electron mobility (μe) = 15 to 300 cm ² / V
sec is obtained.

On the other hand, when the film is polycrystallized by a high temperature anneal of 900 to 1200 ° C. instead of the anneal at a medium temperature as described above, segregation of impurities in the film occurs due to solid phase growth from the nucleus. , GB have a large amount of impurities such as oxygen, carbon, and nitrogen, and have a large mobility in the crystal, but they form a barrier in GB and hinder the movement of carriers there. As a result, it is difficult to obtain a mobility of 10 cm ² / Vsec or more. That is, in this embodiment, the silicon semiconductor having the semi-amorphous or semi-crystal structure is used for the reason as described above.

A silicon oxide film is formed thereon as a gate insulating film 54 with a thickness of 50 to 200 nm, for example 100 nm. This was performed under the same conditions as the production of the silicon oxide film as the blocking layer. During this film formation, a small amount of fluorine may be added to immobilize sodium ions.

After this, 1-5 × 10 ²¹ cm of phosphorus is placed on the upper side.
^-3 concentration silicon film or this silicon film with molybdenum (Mo), tungsten (W), MoSi ₂ or
A multilayer film with WSi ₂ was formed. This was patterned with a second photomask to obtain FIG. 6 (B). In FIG. 6 (C), the source for NTFT - scan 20, was added by ion implantation using the gate electrode 9 as a mask at a dose of 1~5 × 10 ¹⁵ cm ^-2 of phosphorus as a drain 18. Further, when aluminum (Al) is used as the gate electrode material, the self-alignment method can be applied by patterning this with a second photomask and then anodizing the surface. Since the contact hole can be formed at a position closer to the gate, the characteristics of the TFT can be further improved by reducing the mobility and the threshold voltage.

These are performed through the gate insulating film 54. However, in FIG. 6B, the silicon oxide on the silicon film may be removed using the gate electrode 9 as a mask, and then phosphorus may be directly ion-implanted into the silicon film.

Next, heating anneal was performed again at 600 ° C. for 10 to 50 hours. The source 20 and the drain 18 of the NTFT were manufactured as N ⁺ by activating impurities. A channel forming region 21 is formed under the gate electrode 9 as a semi-amorphous semiconductor.

In this way, it is possible to fabricate an NTFT without applying a temperature above 700 ° C. in all steps, even though it is a self-aligned method. Therefore, it is not necessary to use an expensive substrate such as quartz as the substrate material,
This process is extremely suitable for the large-pixel liquid crystal display device of the present invention.

In this embodiment, the thermal anneal is shown in FIG.
It was performed twice in (C). However, the anneal of FIG. 6A may be omitted depending on the desired characteristics, and both may be combined with the anneal of FIG. 6C to reduce the manufacturing time. In FIG. 6D, the interlayer insulator 65 was formed as a silicon oxide film by the above-described sputtering method. The silicon oxide film may be formed by using the LPCVD method, the photo CVD method, or the atmospheric pressure CVD method. For example, it is formed to have a thickness of 0.2 to 0.6 μm, and then a window 66 for an electrode is formed using a photomask. Further, aluminum is formed on all of them by a sputtering method to form leads 71, 72 and contacts 67,
After forming 68 using a photomask, an organic resin 69 for flattening the surface, for example, a translucent polyimide resin was applied and formed, and electrode holes were formed again using the photomask.

To form a TFT as shown in FIG. 6 (G), and to connect the output end thereof to the electrode of one pixel of the liquid crystal device as a transparent electrode, ITO is formed by the sputtering method.
(Indium tin oxide film) was formed. It was etched with a photomask to form the pixel electrode 17 and the contact 73 between the pixel electrode and the drain electrode. This ITO film is formed at room temperature to 150 ° C., and 200 to 400 ° C.
Of oxygen or air in the atmosphere.

In this way, the NTFT 13 and the electrode 17 of the transparent conductive film which is the pixel electrode are formed on the same glass substrate 50. The resulting TFT has a mobility of 40 (cm
² / Vs) and Vth were 5.0 (V). In this way, the first substrate was obtained.

Next, as a second substrate, ITO (indium tin oxide film) was also formed by sputtering on a substrate in which a silicon oxide film was laminated to a thickness of 200 nm on soda lime glass by sputtering. This ITO is room temperature to 150
The film was formed at a temperature of 200 ° C., and was achieved by oxygen at 200 to 400 ° C. or an anneal in the atmosphere.

A polyimide precursor was printed on the substrate by the offset method, and baked at 350 ° C. for 1 hour in a non-oxidizing atmosphere such as nitrogen. After that, a known rubbing method was used to modify the surface of the polyimide, and at least in the initial stage, means for aligning liquid crystal molecules in a certain direction was provided to obtain a second substrate.

Then, the liquid crystal composition exhibiting ferroelectricity was sandwiched between the first substrate and the second substrate, and the periphery was fixed with an epoxy adhesive. A TAB-shaped drive IC was connected to the leads on the substrate, and a polarizing plate was attached to the outside to obtain a transmissive liquid crystal display device.

Pixel A when the drive waveform shown in FIG. 1 is added,
The display state of F and I is shown in FIG. It was found that a clear gradation display can be obtained.

[Example 2] In this example, a first substrate and a second substrate were obtained by the same steps as in the previous example 1.
However, no polyimide alignment film was formed on the second substrate. In addition, since it is used as a viewfinder for a video camera, the pixel pitch is 60 μm and the matrix configuration is 200 × 300 dots.

In this example, a dispersion type liquid crystal display device was prepared in which a nematic liquid crystal composition was dispersed in an acrylic organic resin as the liquid crystal composition. 62% by weight of nematic liquid crystal was dispersed in an epoxy-modified acrylic resin having ultraviolet curability, sandwiched between a first substrate and a second substrate, and then 10
A UV light source with an output of 00 mW was cured by irradiation for 20 seconds.

When the number of time divisions for gradation display is 16,
The liquid crystal display device is capable of displaying 4096 colors in total with 16 gradations for each color. The drive waveform at that time is shown in FIG.

Conventionally, as a method for performing gradation display, a method has been proposed in which a plurality of writings (display frames) are used, for example, 16 frames are used to perform gradation display according to the ON / OFF ratio. Was there. This means that if 8 out of 16 frames are turned on and the remaining 8 are turned off, the average transmittance is 50%,
Gradation display is performed. In addition, in the case where 4 frames of 16 frames are ON and the remaining 12 frames are OFF, gradation display is performed with an average transmittance of 25%. However, when this method is used, there is a possibility that the minimum number of confirmation frames, which is 30 frames that cannot be confirmed by human eyes, is interrupted, which is a factor that deteriorates the display quality.

However, in the present invention, the method of the present invention for increasing the driving frequency of the driver to perform gradation display enables gradation display without substantially lowering the frame frequency. It was possible to provide a high-quality screen that does not deteriorate the display quality without interrupting.

It is useful to increase the gradation display capability as compared with the conventional method by simultaneously performing the gradation display method of the present invention and the conventional gradation method. The present invention is a method of controlling an average voltage applied to a liquid crystal pixel when displaying one pixel, and has a feature that it is not necessary to make the liquid crystal respond completely at this time. Conventionally, it is difficult to directly add the voltage of V _b or V _C of FIG. 2 to the pixel, but by changing the average voltage applied to the pixel electrode, it is possible to directly change the voltage of V _b or V _C. This is because the same effect as that added can be obtained. In other words, the present invention is a method of controlling a liquid crystal that responds halfway.

Although the NTFT, that is, the N-channel type field effect transistor is used in the present specification, the PTF is used.
A T or P channel type field effect transistor may be used as a driving element.

[0040]

The gradation F of the display device using the display driving method having the display timing related to the unit time t for writing in any pixel and the time F for writing one screen does not need to change the time F. In addition, since the signal during the writing time of the time t is time-divided, it is possible to obtain clear gradation display which is digitally controlled, and it is possible to compensate the variation in the in-plane characteristics of the TFT. In addition, the display frequency does not decrease as compared with the gradation display method using a plurality of frames, and high-quality display is possible.

[Brief description of drawings]

FIG. 1 is a drive waveform according to an example of the present invention.

FIG. 2 shows electro-optical characteristics of nematic liquid crystal.

FIG. 3 is an N-channel TFT matrix circuit diagram.

FIG. 4 Drive waveform of a conventional example

FIG. 5 is a plan view of the device according to this embodiment.

FIG. 6 is a manufacturing process of this embodiment.

FIG. 7 is a display example according to the present embodiment.

FIG. 8 is a drive waveform according to the present embodiment.

[Explanation of symbols]

227, 229, 231 ... Change in electric field during writing time 228, 230, 232 ... Electric field applied to pixel electrode during non-writing time 225 ... Unit time for writing into pixel

─────────────────────────────────────────────────── --Continued from the front page (56) Reference JP-A-53-27483 (JP, A) JP-A-58-199564 (JP, A) JP-A-3-34434 (JP, A) JP-A-4-34 196171 (JP, A) (58) Fields surveyed (Int.Cl. ⁷ , DB name) G02F 1/1368 G02F 1/1345

Claims (4)

(57) [Claims] 1. A channel formation region, a source region and a drain region, a gate insulating film formed in contact with the channel formation region, formed on a first substrate, and formed in contact with the gate insulation film. First and second thin film transistors having a gate electrode, a planarizing organic resin film formed on the first and second thin film transistors, and formed on the planarizing organic resin film, and An electrode electrically connected to the first thin film transistor through a hole formed in an organic resin film for planarization, a second substrate facing the first substrate, the first substrate and the first substrate A liquid crystal sandwiched between a second substrate and an active matrix type display device for performing digital gradation display, wherein a pixel is formed by the first thin film transistor. The configured peripheral circuit by the second thin film transistor, the channel forming region is made in crystalline silicon showing the peak of the Raman spectrum from 522cm ^-1 to a lower wavenumber side, the channel formation region of the first thin film transistor The oxygen concentration is 5 × 10 ^{20 to} 5 × 10 ²¹ cm ⁻³ , and the oxygen concentration of the channel forming region of the second thin film transistor is lower than the oxygen concentration of the channel forming region of the first thin film transistor; By time-dividing the signal during the writing time of the unit time for writing in any pixel without changing the time for writing the screen, the digital value is calculated by the average value of the electric field applied to the liquid crystal in the unit time for writing in the arbitrary pixel. A display device characterized by performing gradation display. 2. The display device according to claim 1, wherein the organic resin film is a translucent polyimide resin film. 3. The display device according to claim 2, wherein a film is provided between the first substrate and the first and second thin film transistors. 4. The display device according to claim 3, wherein the film and the gate insulating film are made of the same material.