JPH0618849A - Active matrix type liquid crystal display device and quantitative evaluation method of its internal residual voltage - Google Patents

Abstract

(57) [Summary] [Object] To quantitatively evaluate an internal residual voltage generated by a DC voltage and obtain a quantitative evaluation method of an internal residual voltage of a matrix type liquid crystal display device which suppresses a burn-in phenomenon. [Configuration] Two levels of an optimum common voltage and a DC voltage that minimize the frequency component of the luminance of 1/2 of the frame frequency are taken, and the frequency analysis of the optical characteristics is performed in synchronization with the change of the common voltage to obtain the DC voltage. The change in the luminance frequency component induced by the voltage was evaluated, and the relationship between the luminance frequency component immediately after the common voltage change and the common voltage after the change was obtained. Perform quantitative evaluation. [Effect] The liquid crystal display device can be quantitatively evaluated in a short time.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention quantitatively evaluates an internal residual voltage generated by an external DC voltage in an active matrix type liquid crystal display device, and selects and optimizes a constituent material of the liquid crystal display device obtained according to the evaluation method. The present invention relates to an active matrix type liquid crystal display device designed for structure and a quantitative evaluation method of its internal residual voltage.

[0002]

2. Description of the Related Art Constituent materials and structural design of conventional active matrix liquid crystal display devices are focused on improving display characteristics such as contrast ratio, response characteristics, viewing angle characteristics and reliability characteristics at high temperature operation. The optimization has been done. For example,
An example in which the viewing angle dependence of luminance can be improved by optimizing the refractive index and cell gap value of a liquid crystal material is shown in Spring National Convention 2-81 of the Institute of Electronics, Information and Communication Engineers 1988.

Further, it has been found that by providing a high resistance insulating film (for example, a silicon nitride film) on the electrode for applying a signal voltage to the liquid crystal, the DC voltage component applied to the liquid crystal is reduced and the high temperature reliability is improved. ing. This film will be called a passivation film.

Further, it has been found that display characteristics and yield improvement can be achieved by providing a storage (storage) capacitor connected in parallel with the liquid crystal capacitor. As a result of these achievements, the product performance of the active matrix type liquid crystal display device has been significantly improved and has come to the market mainly for pocket TVs.

[0005]

However, recently, as the field of application of the active matrix type liquid crystal display device has expanded, more severe display performance than ever has been required. For example, in a display device (for example, for measuring instruments and industrial equipment) that displays the same pattern for a long time, a phenomenon in which the pattern remains after being displayed for a long time becomes a problem.

It has been known that the cause of this image sticking phenomenon occurring in the active matrix type liquid crystal display device is caused by the DC voltage component applied to the liquid crystal and the internal residual voltage generated by the DC voltage component. In a thin film transistor (hereinafter referred to as TFT) type liquid crystal display device, a DC voltage V _dc generated as the brightness of liquid crystal changes is given by the following equation 1.

[0007]

[Equation 1]

In this _equation 1, C _gd is the coupling capacitance between the gate and source of the TFT, C _st is the above-mentioned storage capacitance, and C 1
_c1 and C1 _C2 are liquid crystal capacitors in two states having different brightness. V _g is the amplitude of the gate voltage. Therefore, the DC voltage can be reduced by increasing the holding capacitance C _st value and decreasing the coupling capacitance C _gd value.

However, it is difficult to make this value completely zero. For example, C _gd = 0.08 PF, C _st = which are typical values for a TFT type liquid crystal display device.
1PF, C1 _c1 = 0.1PF, C1 _C2 = 0.2PF
Substituting into, V _dc = 0.13V. Even when such a slight DC voltage is applied, it may cause a burn-in phenomenon after the pattern is displayed for several hours.
Therefore, in order to suppress this image sticking phenomenon, D
Even if the C voltage is applied to the liquid crystal, it is necessary to adopt a structure and a constituent material that are unlikely to generate an internal residual voltage.

However, since the internal residual voltage characteristics are determined by the combined effect of the liquid crystal, the alignment film material and the passivation film structure, it is difficult to improve the characteristics only by evaluating the individual material characteristics. Therefore, evaluation using an actual liquid crystal display device is required, but if pattern display is simply performed for a long time, the experiment takes a very long time. Further, it was difficult to quantitatively evaluate the internal residual voltage that causes the phenomenon by directly observing the image sticking phenomenon itself.

The present invention has been made to solve the above problems, and an active matrix type liquid crystal display capable of quantitatively evaluating a residual voltage which causes a burn-in phenomenon in a short time using an actual liquid crystal display device. The purpose is to obtain a quantitative evaluation method of the internal residual voltage of the device, and to obtain a matrix type liquid crystal display device with an optimum constituent material and an optimum structure that can reduce the burn-in phenomenon that has become clear according to this quantitative evaluation method. Further, it is an object of the present invention to provide an active matrix type liquid crystal display device which has a low internal residual voltage, is not affected by a DC voltage component in a signal, and is excellent in high temperature reliability.

[0012]

According to a method of quantitatively evaluating an internal residual voltage of an active matrix type liquid crystal display device according to the present invention, a 1/2 fF component (fF) of luminance in a steady state of a common voltage of the active matrix type liquid crystal display device. Is the optimal common voltage V _como that minimizes the frame frequency and V _como + V _dc (V _dc is the DC applied voltage) to which the DC voltage is applied, and the frequency of the optical characteristics is synchronized with the change in the common voltage. The time change of the 1/2 fFHz component of the luminance induced by the DC voltage is evaluated by performing the analysis, and the common voltage is changed from V _como to V _com = V _como + V _dc.
½ fF component M (1/2 fF) of the brightness immediately after changing to
And the common voltage V _com are obtained, and the obtained result is compared with the evaluation result to quantitatively evaluate the internal residual voltage.

Further, by using the above quantitative evaluation method of internal residual voltage, selection of liquid crystal and alignment film material capable of suppressing generation of internal residual voltage and optimization of structural design are performed.

Further, the passivation film on the thin film transistor array is removed only on the pixel electrode, and is left on other parts such as the thin film transistor, gate and source wiring.

[0015]

In the quantitative evaluation method of the internal residual voltage as described above, the frequency component which is half the frame frequency of the luminance when the common voltage V _como is changed to V _com and the DC voltage is applied to the liquid crystal display device from the outside. Of the internal voltage is quantified by observing the change over time, then obtaining the relationship between the frequency component and the common voltage immediately after changing the common voltage, and comparing the characteristics with the results of the above time change observation. .

Further, by using such a quantitative evaluation method of the internal residual voltage, the constituent materials and the structural design in which the internal residual voltage is less likely to occur are optimized.

Further, by optimizing the film structure of the passivation film on the thin film transistor and removing the passivation film only on the pixel electrode, the internal residual voltage is reduced and, at the same time, the DC voltage of the gate and source lines is reduced. Suppresses the effect on the liquid crystal.

[0018]

【Example】

Example 1. FIG. 1 is a block diagram showing the configuration of an evaluation system applied to the method for quantitatively evaluating the internal residual voltage of this active matrix type liquid crystal display device. In FIG.
Reference numeral 11 denotes a liquid crystal display device. Among signals input to the liquid crystal display device 11, a gate and source control signal 21 is input from a normal control circuit 12 and is input to a common electrode (counter electrode) as usual. Only the common signal 22 is input from the newly provided common signal generation circuit 13.

The optical characteristics of the liquid crystal display device 11 are detected by a photomultiplier tube (photomultiplier) 14, and the frequency analysis is performed by FF.
A T (fast Fourier transform) analyzer 15 is used. Further, the signal generator 16 outputs the common control signal 23 that determines the timing at which the common signal changes to the common signal generation circuit 13, and
A trigger signal 24 that determines the start of observation of the FFT analyzer 15 is output to this FFT analyzer 15.

FIG. 2 is a time chart showing the timing relationship of the gate, source and common signals generated in the evaluation system of FIG. As shown in FIG. 2 (a),
The source signal V _s is inverted at the frame time tF period, and the AC amplitude voltage V _sa and the DC offset voltage V _so are halftone (50
% Brightness). The gate signal V _g is shown in FIG.
As shown in (b), it is always fixed to the on-state voltage V _g +. This is to eliminate the influence of variations in the characteristics of the TFT elements provided in each liquid crystal display device.

The common voltage V _com shown in FIG. 2C is V
Two levels of _com1 and _Vcom2 are taken, and the periods of time at the levels are t ₁ and t ₂ , respectively. The measurement trigger signal of the TFT analyzer 15 is generated in synchronization with the level change of the common signal.

Next, a method of measuring the internal residual voltage of the liquid crystal display device using the above evaluation system will be described. In the steady state, the optimum common voltage that minimizes flicker is V _como . Here, the intensity of flicker is represented by a 1/2 fF component obtained by frequency analysis of luminance. However, fF (= 1 / tF) is the frame frequency.

Since the case of fF = 60 Hz is considered here, the flicker is a 30 Hz component of luminance. The above common voltage V _com1 and V _com2 values are given by the following (1)
Equation, set as in equation (2). V _com1 = V _como + V _dc (1) V _com2 = V _como (2) Here, the DC voltage V _dc corresponds to the DC applied voltage.

As described above, while the common voltage V _com value input to the liquid crystal display device 11 is changed, the brightness 3
Observe the time change of 0 Hz component (frame frequency is 60 Hz). An example of this observation is shown in FIGS. Figure 3,
FIG. 4 shows a 30 Hz component M (30) of luminance in a liquid crystal display device using two different types of alignment films and liquid crystal material systems (unit:
9 shows a time change with a change in the common voltage V _com level of dBV).

Here, the DC voltage V _dc = 3V, t ₁ = t ₂
= 500 seconds. In both cases, a strong 30 Hz component is seen when V _com = V _com1 (DC voltage applied state). However, in V _com = V _com2 (DC voltage removed state), a remarkable 30 Hz component is continuously observed in LCD-1 shown in FIG. 3 and then _gradually decreases, whereas it is shown in FIG. In LCD-2, the value is reduced to a value lower than immediately after the DC voltage is removed.

The 30 Hz component of luminance observed in the LCD-1 (FIG. 3) in the state where no DC voltage is applied is induced by the internal residual voltage generated by the DC voltage application for a long period before that. all right. Therefore, the behavior of the internal residual voltage that causes the burn-in phenomenon can be examined by examining the time change of the 30 Hz component of the luminance.

Next, the common voltage is changed from V _como to V _com = V
_30Hz component of brightness and V immediately after changing to _como + V _dc
sought a _com relationship. The result is shown in FIG. Here, the 30 Hz component M (30) of brightness is the DC component M of brightness
(0) is standardized. By comparing the characteristics of FIG. 5 with the M (30) characteristics after removal of V _dc in FIGS. 3 and 4, it becomes possible to quantitatively evaluate the internal residual voltage V _int .

FIG. 6 shows changes in the internal residual voltage V _int of LCD-1 (FIG. 3) and LCD-2 (FIG. 4) obtained by this method with time t ₁ . In FIG. 6, white circles and black circles indicate the case of LCD-1, and white triangles and black triangles indicate the case of LCD-2. As apparent from FIG. 6, the LCD-1, with increasing time t _1, though while the internal residual voltage V _int is increased rapidly, the LCD-2, also an increase in time t ₁ No
It can be seen that there is only a slight increase.

Further, the comparison of internal residual voltage V _int value at time t ₁ = 2000 seconds internal residual voltage V _int is nearly saturated, 10 when the internal residual voltage V _int value in LCD-1 is the LCD-2 It's more than doubled. The inventors actually evaluated the burn-in characteristics of the two types of liquid crystal display devices after long-time pattern display.

As a result of this evaluation, it was confirmed that the image sticking phenomenon was significantly suppressed in LCD-2 and the pattern disappeared instantly even after the pattern was displayed for 8 hours. Therefore, by using this method, it is possible to quantitatively evaluate the internal residual voltage that causes the burn-in phenomenon, and it is possible to select the liquid crystal and the alignment film material in which the internal residual voltage is less likely to occur.

Example 2. FIG. 7 shows the results of examining the difference in the dependency of the saturation value V _sat of the residual voltage on the DC voltage V _dc due to the passivation film structure, using the above method. In FIG. 7, white circles indicate LCD-1 and white triangles indicate LCD.
-3 (position A), the black triangle mark indicates the case of LCD-3 (position B), LCD-3 is liquid crystal, and the alignment material is LCD.
It is the same as -1, and is a liquid crystal display device in which the passivation film is partially removed. The position A in FIG. 7 corresponds to the passivation film, and the position B corresponds to the portion without the passivation film.

As described above, the V _sat value in the portion without the passivation film is reduced to about 60% of that in the portion with the passivation film. Therefore, in order to reduce the internal residual voltage, it is better to remove the passivation film on the pixel electrode. However, under normal driving conditions, DC voltages of about 1 to 2 V and 10 to 20 V are applied between the source and gate signals and the common signal, respectively. Therefore, the passivation film cannot be removed over the entire surface of the TFT array.

Therefore, as shown in FIG. 8, the TF has a structure in which there is no passivation film on the pixel electrode and the passivation film covers the other portions.
T was manufactured. That is, FIG. 8 is a sectional view of the TFT for explaining the passivation film structure in the second embodiment. In FIG. 8, 101 is a pixel electrode formed on a transparent insulating substrate (not shown) via a dielectric film (not shown). Gate electrode 102, gate wiring 10
4, the source wiring 105 is also formed on this dielectric film.

A gate insulating film 111, an amorphous Si film 112, and an insulating film 113 are sequentially deposited on the upper surfaces of these, and the insulating film 113 is patterned to connect the source electrode 103 to the source wiring 105 and the drain electrode 10.
6 is connected to the pixel electrode 101, and finally a passivation film 114 is formed and patterned, and as described above,
There is no passivation film on the pixel electrode 101, and it is formed on other portions.

With such a structure, since the internal residual voltage is low, image sticking is unlikely to occur, and at the same time, it is not affected by the DC voltage component in the signal, so that a liquid crystal display device excellent in high temperature reliability is provided. realizable.

[0036]

Since the present invention has been made as described above, it has the following effects.

When the common voltage is changed and a DC voltage is externally applied to the liquid crystal display device, the time change of the frequency component of the luminance of half the frame frequency is observed, and the luminance frequency immediately after the common voltage is changed. By obtaining the relationship between the component and the common voltage and comparing the characteristics with the results of the above observations to quantify the internal residual voltage, the internal residual voltage that causes the burn-in phenomenon in a short time using an actual liquid crystal display device. It becomes possible to quantitatively evaluate the voltage, and it is possible to select the liquid crystal and the alignment film material in which the internal residual voltage is less likely to occur.

Further, by using the above evaluation method, the liquid crystal capable of suppressing the generation of the internal residual voltage, the selection of the alignment film material, and the optimization of the structural design are carried out, so that the active matrix type liquid crystal in which the internal residual voltage is hard to occur A display device can be obtained.

Furthermore, by optimizing the structure of the passivation film of the TFT by using the above evaluation method and adopting the TFT structure in which the passivation film is removed only on the pixel electrode, the internal residual voltage can be reduced and the gate can be reduced at the same time. The effect of the DC voltage of the source wiring on the liquid crystal can be suppressed, and an active matrix type liquid crystal display device excellent in high temperature reliability can be obtained.

[Brief description of drawings]

FIG. 1 is a block diagram showing a configuration of an evaluation system applied to a method for quantitatively evaluating an internal residual voltage of an active matrix type liquid crystal display device according to a first embodiment of the present invention.

FIG. 2 is a time chart showing a timing relationship between a gate signal, a source signal and a common signal generated in the evaluation system of FIG.

FIG. 3 is an explanatory diagram showing an observation example in which a change in the 30 Hz component of luminance is examined while changing the V _com value input to the liquid crystal display device for explaining the first embodiment.

FIG. 4 is an explanatory diagram showing an observation example in which a change in a 30 Hz component of luminance is examined while changing a V _com value input to a liquid crystal display device different from that in FIG. 3 for explaining the above-mentioned Example 1;

FIG. 5 shows a common voltage V for explaining the first embodiment.
It is explanatory drawing which shows the evaluation result example of the relationship between the 30 Hz component of brightness | luminance and _Vcom immediately after changing from _como to _Vcom = _Vcomo + _Vdc .

FIG. 6 is an internal residual voltage V for explaining Example 1 of the above.
is an explanatory diagram showing an evaluation example of a result of variation with time t ₁ of the _int.

FIG. 7 is an explanatory diagram showing an example of an evaluation result obtained by examining the difference in the DC voltage V _dc dependency of the saturation value V _sat of the internal residual voltage due to the passivation film structure in the active matrix type liquid crystal display device according to Example 2 of the present invention. is there.

FIG. 8 is a cross-sectional view of a TFT for explaining the passivation film structure in Example 2 of the above.

[Explanation of symbols]

 11 liquid crystal display device 12 control circuit 13 common signal generation circuit 14 photomultiplier tube 15 FFT analyzer 16 signal generator 101 pixel electrode 102 gate electrode 103 source electrode 104 gate wiring 105 source wiring 106 drain electrode 111 gate insulating film 112 amorphous Si film 113 Insulating film 114 Passivation film

Claims (3)

[Claims] 1. An optimum common voltage V at which a 1/2 fF component of luminance (fF is a frame frequency) is minimized when the common voltage of an active matrix type liquid crystal display device is in a steady state.
V _como + V _dc (V _{dc to como} and DC voltages are applied, D
C applied voltage), the frequency change of the optical characteristics is performed in synchronization with the change of the common voltage, and the time change of the 1/2 fFHz component of the luminance induced by the DC voltage is evaluated. The relationship between the 1/2 fF component M (1/2 fF) of the luminance immediately after the common voltage is changed from the optimum common voltage V _como to V _com = V _como + V _dd and the common voltage V _com , and the obtained result And a quantitative evaluation method of the internal residual voltage of the active matrix type liquid crystal display device, characterized by performing a quantitative evaluation of the internal residual voltage by comparing the above evaluation results. 2. An active method characterized by using the method for quantitatively evaluating an internal residual voltage according to claim 1, selecting a liquid crystal and an alignment film material capable of suppressing generation of an internal residual voltage, and optimizing a structural design. Matrix type liquid crystal display device. 3. In the above structural design, the passivation film on the thin film transistor array is removed only on the pixel electrode, and the passivation film is left on other parts such as the thin film transistor, gate and source wirings. The active matrix liquid crystal display device according to claim 2.