KR20020058390A - LCD with micro-lens - Google Patents

Abstract

PURPOSE: A liquid crystal display to which a micro-lens is attached is provided to minimize a variation in transmissivity caused by brightness and alignment error for all gray levels. CONSTITUTION: An active element(20) is formed at each pixel, and a slit pattern(21) or a floating electrode is placed, corresponding to a pixel electrode(5) or a common electrode(6). A micro-lens(3) for focusing lights on a transmission area of the pixel is attached to a liquid crystal cell. A nematic liquid crystal(4) having positive dielectric anisotropy is inserted into the liquid crystal cell. The lights that passed through the micro-lens pass the common electrode or the central lines of the slit pattern or floating electrode.

Description

LCD with microlenses {LCD with micro-lens}

In the present invention, the process tolerance of the liquid crystal cell is increased in the liquid crystal display device with the microlens 3 attached thereto. In particular, the change in transmittance due to brightness and alignment error is minimized with respect to the gray level. It was set as.

1 is a schematic diagram of a liquid crystal projector. The liquid crystal projector separates the white light emitted from the light source 42 into red, blue and green, passes through the liquid crystal panels 10R, 10G, and 10B, which modulate the respective colors, merges again, and then enlarges the field lens 44. On the screen 41. The white light from the light source is divided into red, blue, and green in a dichroic mirror (43), passes through a liquid crystal panel that controls light transmittance, and reassembles in a color mirror, and passes through the projection lens (44) to the screen (41). Is projected on. In FIG. 1, the lowercase alphabetic characters of the color mirror 43 represent the reflecting color. Red is r, green is g, and blue is b. In FIG. 1, the capital letters of the liquid crystal panel 10 indicate modulation of light of each color. The liquid crystal display device that modulates red is R, the liquid crystal display device that modulates green is G, and the liquid crystal display device that modulates blue is B. Liquid crystal projectors are small in size, can easily adjust the size of the screen, and have excellent color reproduction, and are widely used as large display devices such as HDTV or seminar presentation screens.

In the liquid crystal display device used in the liquid crystal projector, an active element that applies and cuts a voltage to the liquid crystal layer is attached to each pixel. Due to the ease of gray scale implementation and fast response characteristics, among the active matrix LCDs, TFT liquid crystal display elements are most often used as display elements of liquid crystal projectors.

FIG. 2 is a cross-sectional view of a conventional TFT liquid crystal display element for a liquid crystal projector with a microlens 3 attached thereto, and FIG. 3 is a plan view of various electrodes of the TFT liquid crystal cell pixel of FIG. The operation principle of the TFT liquid crystal panel is as follows. The gate electrode of the TFT 20 in each pixel is connected to the scan line 8, the source electrode to the signal line 7, and the drain electrode to the pixel electrode 5, respectively. There is a liquid crystal layer 4 between the common electrode 6 and the pixel electrode 5. In the selection period, a voltage higher than that of the signal line is applied to the gate electrode connected to the scan line, and the connection resistance of the channel between the drain electrode and the source electrode is reduced, so that the voltage applied to the signal line is applied to the liquid crystal layer through the pixel electrode. In the non-selection period, a voltage lower than that of the signal line is applied to the gate electrode connected to the scan line, and the drain electrode and the source electrode are electrically disconnected to maintain the charge accumulated in the liquid crystal layer during the selection period. While scanning the scan lines sequentially, each pixel electrode is charged through the signal lines to apply voltage to the liquid crystal layer. When the rms (root means square) voltage applied to the liquid crystal layer between the pixel electrode and the common electrode is adjusted, the polarization state changes as the linearly polarized light passes through the liquid crystal layer and the light passes through the liquid crystal layer. Display information as. The liquid crystal mode of the liquid crystal projector mainly uses a 90 ^o TN mode in the case of a transmissive type, an electrically controlled birefringence (ECB) mode in which the liquid crystal is aligned, or a TN mode having a twist angle of less than 90 ^o .

As the resolution of LCD projectors of LCD projectors is getting higher and higher, 0.7-inch XGA class is commercialized, and 0.5-inch XGA liquid crystal display devices will be developed in the future. The higher the resolution, the lower the aperture ratio and the lower the luminance (brightness). In order to increase the luminance, the microlens 3 is attached to the liquid crystal cell as shown in FIG. 2 is a cross-sectional view of a liquid crystal display element for a liquid crystal projector with a microlens 3 attached thereto. The microlens increases the effective aperture ratio by sending light coming into a region that cannot modulate light such as BM (Black Matrix), signal line, or scanning line to the pixel electrode 5. 3 is a plan view of various electrodes of the TFT liquid crystal cell pixel of FIG. 2, in which a scanning line 8, a signal line 7, and a pixel electrode 5 are arranged.

Since it is difficult to make the thickness of the liquid crystal cell constant in the process of making the liquid crystal cell, the thickness of the liquid crystal cell varies to some extent depending on the position of the same liquid crystal panel. In addition, since a large amount of infrared light is emitted from the light source, the thickness of the liquid crystal cell varies due to the difference in thermal expansion according to the position of the same liquid crystal panel. When the temperature difference between the upper and lower glass substrates 1 and 2 of the liquid crystal panel is 1 ° C., the change in the thickness of the liquid crystal cell is about 0.1 μm. Although the difference in transmittance is small depending on the thickness of the liquid crystal cell when the screen is bright, the lower the gray scale, the larger the change in transmittance depending on the thickness of the liquid crystal cell. FIG. 7 is a graph showing transmittance for each gray level for a change in thickness of a liquid crystal cell. FIG. The thickness of the liquid crystal cell was 4.0 µm, (a) was 4.4 µm, and (d) was transmittance for each gray level when weaved at 3.6 µm. It can be seen that the difference is more than 40% depending on the thickness. In order to prevent the deterioration of the image quality caused by the thickness unevenness of the liquid crystal cell, a slit pattern 21 or floating in which each electrode is etched to the pixel electrode 5 or the common electrode 6 which applies a voltage to the liquid crystal layer Attempts have been made to place the electrodes 22. In the case of attaching microlenses, the overall design should be made in consideration of the alignment error of the bonding process. According to the present invention, the focus of the microlens is placed on the slit pattern or the center symmetry line of the floating electrode to minimize the change in transmittance due to the alignment error of the bonding process.

1 is a schematic diagram of a liquid crystal projector.

2 is a conventional liquid crystal display device having a microlens attached thereto.

3 is a plan view of various electrodes of a conventional TFT liquid crystal cell pixel.

4 is a liquid crystal display device of the present invention with a microlens attached thereto.

5 is a plan view of various electrodes of a pixel having a slit pattern.

6 is a plan view of various electrodes of a pixel with a floating electrode.

7 is a graph showing transmittance of each gray scale with respect to a thickness change of the liquid crystal cell of the present invention.

8 is a graph showing the relative transmittance change with respect to the alignment error of the microlens.

9 is an explanatory diagram for obtaining a voltage distribution of a slit pattern.

※ Explanation of codes for main parts of drawing

1,2 Glass substrate 3 Microlens 4 Liquid crystal layer 5 Pixel electrode

6 Common electrode 7 Signal line 8 Scan line 10 Liquid crystal display

20 TFT 21 Slit Pattern 22 Floating Electrode

40 Cross sections of transmitted light 41, 42 Symmetry lines

The present invention relates to a liquid crystal display device with a microlens with a large process tolerance of the liquid crystal cell. In particular, the change in transmittance due to brightness and alignment error is minimized with respect to the gray level. As a result, the image quality and yield are improved. According to the present invention, a slit pattern 20 or a floating electrode 21 in which each electrode is etched is disposed on a pixel electrode 5 or a common electrode 6 that applies a voltage to the liquid crystal layer, thereby changing the thickness of the liquid crystal cell. By using the lateral electric field properties that follow, the degradation of the image quality due to the change in transmittance and the alignment error of the bonding process is minimized.

Liquid crystal alignment when the slit pattern is placed on the pixel electrode has been studied since Lien of IBM published in SID in 1992 {SID 92 DIGEST p33}. Until now, the liquid crystal was made into a multi domain mainly using a slit pattern, and thus, the liquid crystal was mainly limited to a portion having a wider viewing angle. The inventors of the present patent filed a patent in 1999 to increase the process tolerance of the thickness of the TN liquid crystal cell using a slit pattern (Patent Application No. 10-1999-0016499).

The voltage distribution in the slit pattern can be obtained using a simple equation. 9 is an explanatory diagram for obtaining an electric field and a voltage distribution induced in a slit pattern. The voltage distribution in the slit pattern A assumes that a very small fine electrode in the A position is circular as shown in FIG. Electrodes with various voltage distributions (V1, V2, V3 ....) are placed around the microelectrode, and knowing the capacitances (C1, C2, C3 ...) formed by each electrode and the microelectrode , The induced voltage V (A) of the A portion can be obtained by the following equation.

In FIG. 9, it is assumed that V1 and V3 are voltages of the pixel electrode, and V2 is the voltage of the common electrode. If the voltage distribution of the microelectrode portion of A is different from the voltage distributions (V1, V3; V1 = V3) of the pixel electrode, a horizontal electric field of a voltage corresponding to the difference is applied between the A region and the pixel electrode. As the thickness of the liquid crystal cell decreases, the capacitance C2 between the microelectrode and the common electrode increases, so that V (A) moves toward the voltage of the common electrode. On the contrary, when the thickness of the liquid crystal cell increases, the storage capacitance C2 decreases, resulting in a decrease in the V (A). Moves toward the voltage distribution of the pixel electrode. If the dielectric anisotropy of the liquid crystal is positive and the lateral and vertical electric fields are hung simultaneously, the force of the liquid crystal molecules to align horizontally becomes stronger when the lateral electric field is counted. Get stronger. If the liquid crystal cell's thickness d is reduced, the horizontal electric field becomes larger, so that the refractive index anisotropy of the liquid crystal becomes larger. Therefore, refractive index anisotropy becomes small. Therefore, the transmittance of the liquid crystal cell is a function of Δnd. Since Δn and d move in opposite directions, the change in Δnd is reduced when there is a slit pattern, so that the change in transmittance of the liquid crystal panel with respect to the thickness change of the liquid crystal cell is reduced.

Even when the floating electrode 21 is inside the pixel electrode instead of the slit pattern, the voltage induced to the floating electrode is expressed by Equation (1). The voltage of the floating electrode can be known by calculating the storage capacitance between the floating electrode and the peripheral electrode rather than the minute region. Similarly, in the case of the floating electrode, when the thickness d of the liquid crystal cell decreases, the voltage induced in the floating electrode moves toward the voltage of the common electrode, thereby increasing the horizontal electric field, thereby increasing the refractive index anisotropy of the liquid crystal, and conversely, the thickness d of the liquid crystal cell. When is increased, the refractive index anisotropy is reduced because the voltage induced by the floating electrode moves toward the voltage of the pixel electrode. Since the brightness of the liquid crystal cell is a function of Δnd, and Δn and d move in opposite directions, the transmittance of the liquid crystal panel is insensitive to the change in the thickness of the liquid crystal cell, similarly to the case of the slit pattern, even with the floating electrode.

4 is a cross-sectional view of a TFT liquid crystal display element used in the liquid crystal projector of the present invention. FIG. 4 differs only in the shape of the pixel electrode from FIG. 2, and the other parts are the same. A cross-sectional view of the pixel electrode of FIG. 4 is shown in FIG. The slit pattern is designed such that the center of the cross section 40 of the light passing through the microlens is at a symmetry point where the X-axis symmetry line 41 and the Y-axis symmetry line 42 of the slit pattern meet. The symmetry line bisected in the major axis direction of the slit pattern is the center symmetry line. In Fig. 5, the X-axis symmetry line 41 is the central symmetry line.

6 is a cross-sectional view of a pixel electrode containing a floating electrode. The slit pattern is designed so that the center of the cross section 40 of the light passing through the microlens is at the symmetry point where the X-axis symmetry line 41 and the Y-axis symmetry line 42 of the floating electrode meet. Such conditions can minimize the change in transmittance with respect to alignment error. If the microlens cannot focus on the point where the symmetry lines of the X and Y axes meet due to the structure of the pixel or the microlens, one point is selected from the center symmetry line.

The slit pattern and the floating electrode may be placed on the common electrode. When the slit pattern 20 and the floating electrode 21 are placed on the pixel electrode 5, the slit pattern 20 and the floating electrode 21 can be made simultaneously in the process of making the pixel electrode, and thus there is no additional process. However, when the slit pattern 20 and the floating electrode 21 are provided on the common electrode 6, the common electrode is exposed, so that an additional step of exposure may be performed in some cases.

Fig. 7 is a graph showing the transmittance of each gray scale with respect to the thickness change of the liquid crystal cell. The thickness of the liquid crystal cell was 4.0 µm, and (a) is 4.4 µm in thickness, and (d) is a transmission curve for each gray level in the conventional structure having a thickness of 3.6 µm. (B) and (c) are 4.4 [mu] m and 3.6 [mu] m thick, respectively, and the focal width of the microlenses was 4 [mu] m equal to the width of the slit pattern, and the width of the pixel electrode is 4 [mu] m. This is a change in transmittance for each gray level of a liquid crystal projector using a liquid crystal display element having the same structure. The standard is that the thickness of the liquid crystal cell is 4.0 μm. It can be seen that the degree of change in transmittance with respect to the thickness variation of the liquid crystal cell is reduced for all the gradation steps.

In case of attaching microlens, overall design should be made in consideration of alignment error. According to the present invention, the focus of the microlens is placed at the slit pattern or the symmetry point of the floating electrode, so that the change in transmittance due to the alignment error of the bonding process can be minimized. 8 is a rate of change of relative luminance with respect to the alignment error of the microlens in gradation step 96. The relative change in transmittance is shown by integrating the transmittance within a radius of 2 μm for every 0.25 μm around the central symmetry line. The integral value of the transmittance with a radius of 2 μm around X is 1 (x), and the integral value of transmittance with a radius of 2 μm around x is I (x). In the case of +1), the relative transmittance change rate R (x) in Fig. 8 is expressed by the following equation.

It can be seen that the relative transmittance change due to the alignment error is the smallest when the light passing through the microlens is based on the slit pattern or the center symmetry line of the floating electrode.

If the light passing through the microlens is out of the slit pattern, the effect of the slit pattern does not appear. Therefore, the design and process are adjusted so that the light passing through the microlens passes through the slit pattern or the floating electrode. When the width of the slit pattern or the floating electrode is large, the driving voltage increases and the contrast ratio decreases, so that the thickness of the slit pattern or the floating electrode is reduced. Since the thickness of the liquid crystal cell is about 4 μm, the width of the slit pattern is 4 μm most suitable in terms of driving voltage and contrast ratio. Under these conditions, in order for the light passing through the microlens to pass through the slit pattern, it must be placed within 2 m from the central symmetry lines 41 and 42.

The liquid crystal display device of the present invention can be applied as a display device of a liquid crystal projector that requires high brightness and high image quality, such as conference materials or HD TV.

Claims (4)

An active element 20 is attached to each pixel, and a slit pattern 21 or a floating electrode 22 is provided on the pixel electrode 5 or the common electrode 6, and the nimatic liquid crystal 4 having positive dielectric anisotropy is provided. Is injected, and a microlens 3 which collects light into the transmission region of the pixel is attached to the liquid crystal cell, and the light passing through the microlens is a slit pattern (or floating) of the common electrode 6 or the pixel electrode 5. And a central symmetry line (41; 42) of the electrode). The liquid crystal display device according to claim 1, wherein a distance between the center of the light passing through the microlens 3 and the slit pattern 21 or the central symmetry lines 41 and 42 of the floating electrode 22 is 2 m or less. . Light emitted from the light source 42 passes through the liquid crystal panel 10 to form an enlarged image on the screen 41, active elements are attached to each pixel of the liquid crystal panel, and liquid crystals having positive dielectric anisotropy are injected into the liquid crystal panel. And the light passing through the microlenses 3 of the pixels passes through the central symmetry lines 41 and 42 of the common electrode 6 or the slit pattern (or floating electrode) of the pixel electrode 5, respectively. Projector. The liquid crystal projector according to claim 3, wherein a distance between the center of the light passing through the microlens (3) and the slit pattern (21) or the central symmetry line (41; 42) of the floating electrode (22) is 2 m or less.