KR19990079366A - Tifty liquid crystal display element - Google Patents

Abstract

In the TFT liquid crystal display device using the common electrode 10 to be floated, the capacitance of the floating common electrode with respect to the adjacent signal line 20, the scanning line 21, and other floating common electrodes is minimized, To improve image quality. In the present invention, the capacitance generated between the common electrode and the black matrix floating by using a black matrix made of a resin instead of the metal black matrix is eliminated, and the area of the floating common electrode is made smaller than the pixel electrode, The coupling capacitance between the electrode and the signal line and the coupling capacitance between the floating common electrode and the scanning line are minimized. In addition, the time constant which is the product of the resistance of the floating common electrode and the capacitance of the floating common electrode and the peripheral wiring is made larger than the time constant of the scanning line, so that the falling voltage is reduced in all the pixels.

Description

Tifty liquid crystal display element

In the TFT liquid crystal display device having the common electrode 10 to be floated, the capacitance of the common electrode to be floated with the adjacent signal line 20, the scanning line 21, and other floating common electrodes is minimized, -through voltage and leakage shift voltage) is small, and as a result, crosstalk and flicker are eliminated to improve the image quality.

1 is a cross-sectional view of a TFT. The gate electrode 2 of the TFT is connected to the scanning line 21, the source electrode 8 is connected to the signal line 20, and the drain electrode 9 is connected to the pixel electrode 5. The capacitance between the pixel electrode 5 and the common electrode 5 of the storage capacitance maintains the voltage applied to the liquid crystal layer in the selection period due to the storage capacitance and the falling of the liquid crystal layer when the scanning line is turned from on to off Reduce the voltage.

A color filter glass substrate 110 opposed to the TFT substrate is provided with a common electrode to which a specific voltage is applied so that information is displayed on the pixel with a voltage applied between the pixel electrode and the common electrode.

FIG. 2 is an equivalent circuit for explaining the operation of the conventional TFT liquid crystal display device. The TFT is usually n-channel. During the selection period, a voltage higher than 10 V is applied to the gate electrode connected to the scanning line, so that the resistance of the channel between the drain electrode and the source electrode is reduced, so that the voltage across the signal line is caught in the liquid crystal layer through the pixel electrode. During the non-selection period, a voltage 5 to 10 V lower than the signal line is applied to the gate electrode connected to the scanning line, so that the drain electrode and the source electrode are electrically disconnected to maintain the charge accumulated in the liquid crystal layer C _LC during the selection period. Voltage is applied to the liquid crystal layer of each pixel through the signal line while sequentially applying the on voltage to the scanning lines. When the rms voltage applied to the liquid crystal layer between the pixel electrode and the common electrode is adjusted, the linearly polarized light passing through the polarizing plate passes through the liquid crystal layer to change the polarization state. The light is selectively transmitted through the plate to display information as brightness of the pixel. The polarity of the voltage applied to the liquid crystal layer is changed every frame to prevent the electrochemical reaction of the liquid crystal molecules by adjusting the waveform of the voltage applied to the signal line and the common electrode.

The biggest problem in the TFT liquid crystal display device as shown in FIG. 2 is a feedthrough voltage generated when the voltage of the scanning line is turned off and the charge of the channel enters the pixel. Residual DC is caught in the liquid crystal layer due to this voltage, The picture quality has dropped.

Fig. 3 shows the scanning line drive waveform, the signal voltage, and the voltage waveform across the liquid crystal layer. If the filling rate is 100%, the voltage V _LC (T) of the liquid crystal layer immediately before the TFT is turned off is the voltage V _D across the signal line. The voltage V _LC (t) applied to the liquid crystal layer due to the parasitic capacitance Cgs between the gate electrode 2 and the drain electrode 9 of the TFT changes to the off state as the voltage of the gate electrode changes from Vgh to Vgl, It changes like a formula.

The polarity of the voltage is alternately applied to the liquid crystal layer every frame, ΔV _P The voltage applied to the liquid crystal layer is lowered in the + frame, and the voltage applied to the liquid crystal layer in the - frame is lowered ΔV _P . The voltage of the signal line is V _D = 5 V, the voltage drop ΔV _P The voltage applied to the liquid crystal layer during the (+) frame is V _D (+) = 4V, and during the (-) frame, V _D (-) = 6V. The difference in absolute voltage across the liquid crystal layer during the (+) and (-) frames is 2 · ΔV _P to be. This difference in voltage causes a difference in brightness, which causes a flicker on the screen. Since the falling voltage differs depending on the line delay of the scanning line or the temperature or the capacitance of the pixel, the falling voltage can not be completely compensated.

A new structure is proposed to reduce the falling voltage. The common electrode 10 floated by the number of scanning lines is placed on the color filter substrate 110 on the sixth scanning line without changing the TFT substrate. A conventional common electrode is divided in parallel to the pixels of the scanning line by the number of scanning lines to form a divided common electrode and is placed in a floating (high) impedance state. That is, if left unconnected to the external driving circuit, it becomes floating.

4 is an equivalent circuit diagram of a TFT liquid crystal display element having a floating common electrode. The polarity of the voltage applied to the signal line is adjusted to induce the voltage to the floating common electrode. FIG. 5 is a view showing that OV is induced in the floating common electrode. FIG. When the scanning line is turned on, the same gradation of the same scanning line is divided into two grades, and the odd and even voltages are applied with + voltage and - voltage, respectively. Considering only two pixels of the same gray level, OV is induced in the floating common electrode as shown in the right figure of FIG. The voltage induced in the floating common electrode can be obtained by Kichhoff's law. If the voltage applied to the n-th signal line is V _s (n), the capacitance of the liquid crystal layer is C _LC (n), and the voltage induced in the common electrode divided during the selection period is V _M , V _s (n) - V _M. V _M can be found from Kichhoff's law. The sum of the incoming and outgoing currents to the divided common electrode is O, so we can write as follows.

(2)

Since the reaction time of a TN type liquid crystal display device is usually 20 to 40 ms dC _LC (n) / dt≈0 (2), V _M can be written in the following approximate expression.

, The odd-th gray scale by dividing the same into two categories at the time when the scanning line on the even-numbered voltage is the + - give a voltage is applied, (3) where _M is a Ⅴ 0Ⅴ is derived. If the voltage of the waveform added in common to all the signal lines in the equation (3) is V ', then the voltage applied to the n-th signal line is V _s (n)'.

V _S (n) '= V _S (n) + V' (4)

(4) is substituted into equation (3), the voltage induced on the common electrode is given by V _M '.

Therefore, the voltage applied to the liquid crystal layer is the same as the conventional one. That is, the liquid crystal does not react to a voltage that is common to all the pixels. Thus, the pixel having the structure of the floating common electrode when the scan line changes from the on to off in this logic is not affected by the voltage change of the scan line _gh Ⅴ -Ⅴ _gl. Therefore, the falling voltage is almost negligible.

6 is a cross-sectional view of a conventional color filter which removes a falling voltage by using a floating common electrode. The color filter glass substrate 110 is mainly made of chromium (Cr) or a metal black matrix 15 , And red (11), green (12), and blue (13) color filters. A planarizing film 14 is formed by spin coating an organic material usually composed of polyimide or acrylic. The common electrode 10 is placed on the planarizing film. The metal black matrix can not control the alignment of the liquid crystal molecules between the pixels and the pixels due to the lateral electric field, thereby blocking the light leakage or the light entering the a-Si layer.

FIG. 16 is a plan view of the metal black matrix. The metal black matrix is also floated, because the capacitance between the adjacent signal lines and the scan line and the metal black matrix results in a specific voltage being not an ideal equivalent circuit of the fourth aspect. 7 is an equivalent circuit when the scanning line of the TFT liquid crystal display element using the conventional floating common electrode is off. The signal line and the floating common electrode are connected by the off resistance of the TFT, the capacitance between the floating common electrode and the BM is C _FLOAT-BM , the capacitance of the scanning line and the signal line of the metal black matrix is C _BM-SCAN And C _BM-DATA . As a result of the capacitance coupled with the various types of metal black matrixes, the voltage induced in the floating common electrode can not be well controlled. If the voltage that is constantly added to all the pixels is V 'as in Equation (4), then the voltage induced in the floating common electrode is a very complicated equation, not a formula of V _M '. Therefore, a voltage change occurs in the liquid crystal layer in the voltage change V _gh- V _gl of the scanning line when the scanning line changes from on to off.

The present invention eliminates the capacitance between the common electrode and the black matrix by using a black matrix 16 made of a resin instead of the metal black matrix 15, . By adjusting the pattern of the floating common electrode 10, the coupling capacitance between the floating common electrode and the signal line and the coupling capacitance between the floating common electrode and the scanning line are minimized, and the aperture ratio is increased. In addition, the optimum condition was determined by computer simulation of the voltage induced in the liquid crystal layer according to the time constant which is the product of the resistance of the floating common electrode and the capacitance of the floating common electrode and the peripheral wiring.

FIG. 8 is a structure of a color filter of a TFT liquid crystal display element using a common electrode for light of the present invention. The underlying structure is the same as in Fig. 6. A black matrix 16 made of resin is coated on the color filter glass substrate 110 and there are red 11, green 12, and blue 13 color filters. Spin coating of organic materials usually made of polyimide or acrylic makes a planarizing film. The common electrode 10 is placed on the flattening film. The resin black matrix 16 can not adjust the alignment of the liquid crystal molecules between the pixel and the pixel due to the lateral electric field, thereby blocking the light leakage or the light entering the a-Si layer. Resin Black Matrix is a polyimide series containing black pigment or dye. Resin Black Matrix absorbs and blocks light, and metal black matrix reflects and blocks light. Therefore, the resin black matrix has a low reflectance, but it usually has to be thicker by 2 to 3 탆 in order to absorb light completely.

In the case of using a black matrix made of a resin, since the C _FLOAT-BM is 0 in FIG. 7, the TFT liquid crystal display element using the floating common electrode is equivalent to the fourth circuit. (pattern). The floating common electrode can be distinguished by the display portion 101 overlapping the pixel electrode and the joint portion 12 overlapping the signal line. FIG. 9 is a pixel plan view of a TFT liquid crystal display element having a floating common electrode, and shows relative positions of a signal line, a pixel electrode, and a floating common electrode. The TFT substrate has a scanning line 21, a drain electrode 9, an a-Si filler 7, and a signal line 20, and a common electrode floated on the common electrode substrate (color filter substrate). The display region 101 of the floating common electrode 10 is located inside the pixel electrode 5. [ Figure 10 is a detailed view of the scheme. A length d1 in which the pixel electrode overlaps with the floating common electrode in the vertical directions A and D is denoted by d1 and a length of overlapping of the pixel electrode and the floating common electrode in the horizontal direction B and C is d3 between the signal line and the pixel electrode of the TFT substrate And the distance between the scanning line and the pixel electrode is d4.

If d2 and d4 are small, the aperture ratio becomes high, but the probability of short connection between the pixel electrode, the signal line, and the scanning line increases, and the yield decreases. d2 and d3 vary from manufacturer to manufacturer, but usually from 3 to 6 microns.

The tolerances of d2 and d3 are usually within 1 to 2 μm due to the misalignment of the photomask. d1 and d3 depend on the adhesion tolerance between the TFT substrate and the common electrode substrate. If the line width (W) of the joint portion of the floating common electrode is small, the joint between the floating common electrode and the signal line can be suppressed. The electric capacity is reduced and the image quality is improved, but the yield is reduced due to short circuit.

If d1 and d3 are larger than the cementation tolerance, the light does not leak. Even if d1 and d3 are smaller than the cementation confinement, the resin black matrix should be covered by the cementation tolerance. Therefore, d1 and d3 should be designed to be 2 to 6 μm more than the cementation tolerance. If the joint tolerance is 2 탆, d 1 and d 3 are set to about 4 to 8 탆. In this case, the line width of the floating common electrode is as narrow as 8 to 16 탆 compared with the line width of the pixel electrode. The line width of the floating common electrode varies greatly depending on the cementing tolerance.

The coupling capacitance between the signal line and the floating common electrode is a function of the capacitance connected to the side electric field between the display portion of the floating common electrode and the signal line and the capacitance due to the vertical electric field between the signal line and the junction of the floating common electrode Assuming that the coupling capacitance between the signal line and the floating common electrode is C _sc and the storage capacitance of the pixel is C _ST , the equivalent circuit of the pixel when the scanning line is off corresponds to the seventeenth same. The falling voltage is given by the following equation.

Conservation capacitance (C _ST) and the parasitic capacitance (C _GD) to create larger than about 10 times, polo putting the common electrode and the capacitance signal line is forming (C _DC) to the capacitance of the liquid crystal layer (C _LC) of less than about 1/30 The voltage drop is about 1/300 of the variation of the voltage of the scanning line. In the conventional case, the storage capacitance C _ST is made about 20 to 30 times the parasitic capacitance C _GD , and the aperture ratio is lowered. Since the variation range of the voltage of the scanning line is about 30 V, the lowering voltage calculated by equation (6) is about 0.1 V.

If the ratio of C _DC to C _LC is about 1:30, the capacitance between the display portion of the floating common electrode and the signal line is 60: 1, and the capacitance between the joint portion of the floating common electrode and the signal line is about 60: 1 . 12 shows the ratio of C _DC and C _LC computed by a finite element method according to the distance between the pixel electrode and the display portion of the floating common electrode.

The distance between the pixel electrode and the signal line is 2, 4, and 6 μm. As shown in the figure, the distance between the pixel electrode and the signal line does not greatly affect the ratio of C _DC and C _LC . The width of the signal line was 6 mu m, the width and length of the pixel electrode were 70 mu m and 220 mu m, respectively, and the liquid crystal was ZLI-4792 manufactured by Merck, Germany. In order to set the ratio of the capacitance C _DC and C _LC of the display portion of the floating common electrode to the capacitance C _DC of the signal line to 1:60, the pixel electrode and the common electrode must be separated by about 4 μm or more as shown in FIG. The condition for setting the ratio of the capacitance C _DC and C _LC of the signal line to the joint of the floating common electrode to 1:60 can be easily obtained from the formula of the parallel plate capacitance described below.

The area of the parallel plate electric capacity is A, the electrode distance is d, and the electric capacity C when the dielectric constant of the dielectric is ε is as follows.

(7)

The area of the portion overlapping the signal line in the joint portion and the overlapping portion of the pixel electrode and the display portion may be 1:60 or more. The ratio of the capacitances C _DC and C _LC of the pixel electrode to the joint of the floating electrode can be made larger than 1:60 if the width of the joint portion of the floating electrode is about 40 μm based on the TFT liquid crystal display element. The width of the signal line is 7 占 퐉, and the width of the joint portion of the floating common electrode is about 40 占 퐉, so that the area is about 280 占 퐉 ² . Since the width of the pixel electrode is about 70 mu m and the length is about 240 mu m, the area is 16,800 mu m ² . In this case, the area ratio is about 1: 70.

Since the scanning line and the floating common electrode do not overlap each other and the line width of the pixel electrode in the scanning line direction is small, the capacitance between the scanning line and the floating common electrode is about 1/4 or less of the capacitance between the signal line and the floating common electrode Thus, the shape of the floating common electrode can be applied to a signal line.

The voltage induced in the liquid crystal layer varies depending on the resistance of the floating common electrode and the electric capacitance between the floating common electrode and the peripheral wiring. In consideration of this, the resistance of the floating electrode must be determined. Figure 13 is a modem shot in a simulation. The signal delay of the scanning line occurs due to the resistance R and the capacitance C of the scanning line. This signal delay greatly affects the falling voltage of the TFT. The falling voltage depends on the signal delay of the gate line. FIG. 17 shows a gate pulse waveform having a time constant of 0.05 μs and 3.0 μs and an amplitude of 30 V. FIG.

In FIG. 17, the waveform of the gate electrode having a time constant (RC) of the scanning line (RC) of 0.05 s is very short because the conversion time from the gate voltage to the on-time is very short, so that the charge of the channel when the scanning line is on is proportional to the parasitic capacitance ratio . However, in the scanning line signal parasitic with a time constant of 3.0 μs, the conversion time from 0n to off is very gentle, and when the voltage of the scanning line is changed from on to off, the charge of the channel escapes to the signal line to some extent from the equilibrium state, To the pixel. The time constant of the floating common electrode is larger than the time constant of the scanning line, so that a stable voltage is applied to the liquid crystal layer without a falling voltage.

If the time constant of the floating common electrode is faster than the time constant of the scanning line, the off waveform of the scanning line in the floating common electrode reacts differently depending on the position of the floating common electrode and feed back each other. FIG. 14 shows the voltage of the liquid crystal layer after the scanning line is turned off by varying the time constant RC of the floating common electrode of FIG. 13.

At this time, the time constant of the scanning line was simulated by giving a value of O μs in the pad portion of the scanning line and 2 μs in the portion apart from the pad.

It can be seen that the voltage of the liquid crystal layer is stabilized when the time constant of the floating common electrode is 2 mu sec or more. 15 shows the residual DC voltage which is a difference between the voltages of the liquid crystal layer generated when the positive voltage and the negative voltage are applied. When the time constant of the floating common electrode is 1.0 mu sec or more, it is stable to some extent. 0V. ≪ / RTI >

Since the floating common electrode is ITO, the surface resistance of ITO is thinned to 10000 Ω / □. Assuming that the width and length of the floating common electrode are about 0.3 mm and 20 cm, the resistance in the cross-sectional direction of the floating electrode is about 7 k ?. If the minimum capacitance of a pixel is 0.1 pF, there are 3000 pixels, so the total capacitance is 0.3 nF. In this case, the time constant of the floating common electrode is about 2 mu s. The time constant of the floating common electrode is controlled mainly by the sheet resistance of the ITO which is the material of the pixel electrode. To increase the time constant, the thickness of the pixel electrode is reduced to increase the resistance.

The present invention minimizes the capacitance generated by the floating common electrode with the adjacent signal line (20), the scanning line (21), and other floating common electrodes, thereby improving the image quality without crosstalk and flicker. In addition, the time constant of the floating common electrode is made larger than the time constant of the scanning line, thereby reducing the feedback effect caused by the difference in the falling voltage between the pixels when the scanning line is off. The TFT liquid crystal display element of the present invention can reduce the area of the storage capacitance, thereby realizing a bright screen because of high aperture ratio.

The above identifier is missing.

Sectional view of the first-degree TFT

2, the equivalent circuit of the conventional TFT liquid crystal display device

3 is a graph showing a relationship between a falling voltage

4 is an equivalent circuit of a TFT liquid crystal display element having a floating common electrode;

In the fifth method, a constant voltage is induced to the floating common electrode

6 is a cross-sectional view of a conventional color filter of a TFT liquid crystal display element having a floating common electrode;

Seventh Embodiment An equivalent circuit of a TFT liquid crystal display element having a floating common electrode using a metal black matrix

8 is a sectional view of a color filter of a TFT liquid crystal display element having a floating common electrode of the present invention

9 is a plan view of a pixel of a TFT liquid crystal display element having a common electrode floated

10 shows the details of FIG. 9

11 shows the shape of the floating common electrode

12 shows the relationship between the capacitance between the floating common electrode and the signal line and the capacitance of the pixel

13 shows the RC equivalent circuit of the floating common electrode

14 shows the voltage variation of the liquid crystal layer when the time constant of the common electrode is varied

15 shows the change in residual DC voltage of the liquid crystal layer when the time constant of the common electrode is varied

16 shows a top view of the black matrix of FIG. 6

17 shows an equivalent circuit of a unit pixel of a TFT liquid crystal display element having a floating common electrode

Signal delay of the 18th degree scanning line

[Description of Drawings]

1 TFT Glass substrate 2 Gate electrode 3 Gate insulating film

4 Capacitance common electrode 5 Pixel electrode 6 n +

7 a-Si 8 source electrode 9 drain electrode

10 Floating common electrode 11 Red color filter 12 Green color filter

13 blue color filter 14 planarization film 15 metal black matrix

16 Resin Black matrix 20 Signal line 21 Scanning line

101 Display of the floating common electrode

102 Joint of floating common electrode

110 Color filter glass substrate

The above identifier is missing.

The above identifier is missing.

Claims (8)

There is a resin bracket matrix 16 on a glass substrate opposed to the TFT glass substrate 1 and a common electrode 10 floated so as to oppose the pixel electrodes connected to the respective scanning lines, ) Is smaller than that of the pixel electrode 5 and the polarity of the signal line voltage is adjusted so that a specific voltage is induced to the floating common electrode, Display element. The TFT liquid crystal display device according to claim 1, wherein a joint portion (102) of the floating common electrode overlapping with the signal line when the signal line (20) is vertically projected on the floating common electrode (10) The TFT liquid crystal display device according to claim 1, wherein the upper, lower, left and right line widths of the display area of the common electrode to be floated are 8 to 16 占 퐉 narrower than the upper and lower, A common electrode 10 floated so as to oppose the pixel electrode 5 connected to each scanning line 21 is provided on the TFT glass substrate 1 and the area of the display region 101 of the common electrode to be floated is set to The TFT liquid crystal display device according to claim 1, wherein the polarity of the voltage of the signal line (20) is adjusted so as to induce a specific voltage to a common electrode that is smaller than the pixel electrode and displays screen information by a voltage difference between the common electrode and the pixel electrode. 5. The TFT liquid crystal display device according to claim 4, wherein a joint common line of the floating common electrode overlapping with the signal line when vertically projecting the signal line and the common electrode to be floated is 40 [ 5. The TFT liquid crystal display according to claim 4, wherein the line widths of the upper, lower, right, and left sides of the display region of the common electrode to be floated are narrower by 8 to 16 占 퐉 than upper and lower and left and right line widths of the pixel electrode. The common electrode 10 which is floated so as to face the pixel electrode 5 connected to the scanning line 21 is placed on the glass substrate opposed to the TFT glass substrate 1 and the time constant of the floating common electrode 10 is set to the scanning line The polarity of the signal line voltage is adjusted so as to induce a constant voltage to the common electrode which is larger than the time constant of the liquid crystal display element 21 and the screen information is displayed by a voltage difference between the common electrode and the pixel electrode to be floated. 8. The TFT liquid crystal display according to claim 7, wherein the time constant of the floating common electrode is longer than 2 mu s.