News Column

Researchers Submit Patent Application, "Liquid Crystal Display", for Approval

July 30, 2014



By a News Reporter-Staff News Editor at Electronics Newsweekly -- From Washington, D.C., VerticalNews journalists report that a patent application by the inventors Furukawa, Tomoo (Osaka-shi, JP); Hisada, Yuhko (Osaka-shi, JP); Asada, Katsushige (Osaka-shi, JP), filed on August 3, 2012, was made available online on July 17, 2014.

The patent's assignee is Sharp Kabushiki Kaisha.

News editors obtained the following quote from the background information supplied by the inventors: "Active matrix liquid crystal displays utilizing active elements such as thin film transistors (TFTs) are widely spread as display devices because they are thin and light and provide high-definition images that compare with images provided by CRT displays. Roughly two display formats mentioned below are known as display formats of such liquid crystal displays.

"One is a vertical electric field mode. In this mode, an electric field in a direction substantially perpendicular to the substrate surface drives a liquid crystal layer, and modulates light incident on the liquid crystal layer, thereby performing display. Exemplary vertical electric field liquid crystal modes include TN (Twisted Nematic) mode and MVA (Multi-domain Vertical Alignment) mode.

"The other is a transverse electric field mode. In this mode, an electric field in a direction substantially parallel to the substrate surface drives a liquid crystal layer. Exemplary transverse electric field liquid crystal modes include IPS (In-plane Switching) mode and FFS (Fringe Field Switching) mode.

"Specific examples of the IPS mode include a liquid crystal display including first and second substrates, plural gate wirings and plural data wirings, a thin film transistor, a common wiring, a common electrode, a pixel electrode, and a liquid crystal layer (see Patent Literature 1). In the liquid crystal display, the plural gate wirings and the plural data wirings on the first substrate are orthogonal to each other and define a unit pixel that is divided into first, second, and third regions. The thin film transistors are formed at the intersections of the gate wirings and the data wirings. The common wiring is parallel with the gate wirings and branched into the common electrode that is bent in the first, second, and third regions respectively at first, second, and third angles. The pixel electrode is connected to a drain electrode of the thin film transistor and formed parallel with the common electrode. The liquid crystal layer is arranged between the first substrate and the second substrate that faces the first substrate.

"Specific examples of the FFS mode include a transverse electric field-type liquid crystal display panel that has slit openings having a doglegged shape formed by connecting slit openings extending in different directions (see Patent Literature 2). Another example is a liquid crystal display including an electrode that has a straight portion, a first slit, and a second slit. From one side of the straight portion extending in a first direction, the first slit extends in a second direction that is different from the first direction. From the other side of the straight portion, the second slit extends in a third direction that is different from the first and second directions (see Patent Literature 3)."

As a supplement to the background information on this patent application, VerticalNews correspondents also obtained the inventors' summary information for this patent application: "Technical Problem

"A transverse electric field-type liquid crystal display includes an electrode (e.g., pixel electrode) in which plural slits are formed in parallel with each other, and a transverse electric field is generated using the electrode. The panel transmittance thereof varies in accordance with the width (S) of the slit, the width (L) of (a linear portion of) the electrode between the slits, and the sum of them (S+L). Generally, a smaller sum of L and S indicates a higher panel transmittance. Due to limitations in the production process, L and S each cannot be smaller than a predetermined value, so that the actual panel transmittance is limited. The reason for this is that there is a limitation to the minimum width of a pattern formed on a photomask that is used in the photolithography step for forming slits and to the minimum width of a pattern that can be actually formed by a device used in the photolithography step.

"The following will describe problems related to a conventional FFS mode with reference to a FFS-mode liquid crystal display according to Comparative Embodiment 1 studied by the present inventors. FIG. 10 is a schematic plan view of an active matrix substrate in a FFS-mode liquid crystal display according to Comparative Embodiment 1. Embodiments similar to Comparative Embodiments are shown in FIG. 9 of Patent Literature 2 and FIG. 6 of Patent Literature 3.

"A liquid crystal display according to Comparative Embodiment 1 includes an active matrix substrate (array substrate) 510, a counter substrate (not illustrated) facing the array substrate, and a horizontal alignment-type liquid crystal layer (not illustrated) provided between the substrates. The array substrate 510 includes, as illustrated in FIG. 10, a data bus line 513, a gate bus line 551, a TFT 553, a common electrode 515, an insulating film (not illustrated) formed on the common electrode 515, and a pixel electrode 517 formed on the insulating film. The initial alignment direction, namely, the alignment direction under application of no voltage, of liquid crystal molecules is set to the vertical direction of FIG. 10.

"Each pixel electrode 517 has plural slits 530 formed in parallel with each other. The common electrode 515 faces the slits 530. Control of the voltage to be applied between the pixel electrode 517 and the common electrode 515 allows control of alignment of liquid crystal molecules, more specifically, rotation of liquid crystal molecules. The pixel electrode 517 includes plural linear portions 517 formed in parallel with each other and connecting portions 519 and 520 connecting the linear portions to each other.

"Each slit 530 has a symmetrical shape to the top and bottom, and includes the straight portions (main portions) 531 and 532, a portion (V portion) 533 that connects the main portions 531 and 532 to each other and is formed of two straight portions combined in a V shape, and linear portions (auxiliary portions) 534 and 535 respectively provided between the main portion 531 and the connecting portion 519 and between the main portion 532 and the connecting portion 520. As mentioned above, each slit 530 bends between the auxiliary portion 534 and the main portion 531, between the main portion 531 and the V portion 533, between the main portion 532 and the V portion 533, and between the main portion 532 and the auxiliary portion 535. In addition, the V portion 533 has one bent portion. Accordingly, each slit 530 has five bent portions. The V portion 533 and the auxiliary portions 534 and 535 each have a larger angle of tilt relative to the vertical direction than the main portions 531 and 532.

"The V portion 533 and the auxiliary portions 534 and 555 are subsidiary portions. The alignment of most liquid crystal molecules in the liquid crystal layer is controlled in regions including the main portions 531 and 532. The rotation direction of liquid crystal molecules in the region including the main portion 531 is opposite from that in the region including the main portion 532, and therefore, the alignments of liquid crystal molecules collide with each other between the regions. For facilitating the alignment of liquid crystal molecules between the regions, the V portion 533 which has a comparatively large angle of tilt relative to the vertical direction is provided between the regions. In the vicinity of the connecting portion 519, the alignment of liquid crystal molecules may be disturbed by the electric field generated from the connecting portion 519. For suppressing alignment disorder of liquid crystal molecules in the vicinity of the connecting portion 519, the auxiliary portion 534 which has a comparatively large angle of tilt relative to the vertical direction is provided between the connecting portion 519 and the main portion 531. The auxiliary portion 535 is provided for the similar reason. As a result, even when a local pressure is applied to the screen of the liquid crystal display from outside (e.g., when the screen is pressed by fingers) to cause locally disordered alignment of liquid crystal molecules, namely, locally disturbed display, such a defect is quickly recovered.

"Comparative Embodiment 1, however, has problems mentioned below.

"The slits 530 are provided in parallel with each other and have the same planar shape. In this case, the V portion 533 and the auxiliary portions 534 and 535 each have a width S narrower than the width S of the main portions 531 and 532. In addition, the width L of the linear portion 518 of the pixel electrode 517 is narrower in portions forming the V portion 533 and the auxiliary portions 534 and 535 than in portions forming the main portions 531 and 532. If the widths L and S in regions including the main portions 531 and 532 are minimized to the allowable limits in terms of the process for the purpose of increasing the panel transmittance, the widths L and S in the region including the V portion and in the regions including the auxiliary portion 534 and 535 may be below the allowable limits. In such a case, various defects caused by variation in process may occur. Specifically, luminance may vary from one panel to another, or the display may be non-uniform even within one panel. Such defects are presumably caused by disordered alignment of the mask, variation in width of the pattern, and the like.

"From the standpoint of suppressing such defects, if the widths L and S in the region including the V portion 533 and in the regions including the auxiliary portions 534 and 535 are set within the allowable limits, the widths L and S in the regions including the main portions 531 and 532 are large, causing deterioration in display performance, for example, reduction in the panel transmittance.

"As described above, suppression of defects caused by variation in process and improvement in display performance are hardly achieved at the same time in a conventional FFS mode.

"The present invention has been devised in consideration of the state of the art, and aims to provide a liquid crystal display that enables suppression of defects caused by variation in process and improvement in display performance.

"Solution to Problem

"The present inventors have intensively studied about a liquid crystal display that enables suppression of defects caused by variation in process and improvement in display performance to find out the following fact. In a liquid crystal display, a plurality of slits are provided in parallel with each other, the plurality of slits each having a first straight portion that has a first end and a second end and extends in a first direction, a second straight portion that is connected to the first end of the first straight portion and extends in a second direction, and a bent portion bent in a connecting region of the first straight portion and the second straight portion. The plurality of the first straight portions having the second ends aligned along the same straight line. On an assumption of a first slit being an endmost slit among the plurality of slits in the pixel (here, a slit next to the first slit bends in a manner that the first straight portion and the second straight portion of the slit come closer to the first slit), a slit more distant from the first slit has a shorter first straight portion. This configuration reduces the difference between the widths L and S in the region including the first straight portion and the widths L and S in the region including the second straight portion. In this case, even when the width L and S in the region including the second straight portion are close to the allowable limits, the widths L and S in the region including the first straight portion are prevented from falling below the allowable limits. As a result, variation in the slit pattern caused by variation in process is suppressed, and the transmittance can be increased. In this manner, the present inventors succeeded in solving the above problems to complete the present invention.

"One aspect of the present invention is a liquid crystal display (hereinafter, also referred to as a liquid crystal display according to the present invention) including: a first substrate; a second substrate facing the first substrate; and a liquid crystal layer that is positioned between the first substrate and the second substrate and contains liquid crystal molecules, the first substrate including a first electrode, an insulating film provided on the first electrode, and a second electrode provided on the insulating film, the second electrode having a plurality of slits formed within a pixel, the first electrode facing the plurality of slits, the plurality of slits being parallel with each other, the plurality of slits each having a first straight portion that has a first end and a second end and extends in a first direction, a second straight portion that is connected to the first end of the first straight portion and extends in a second direction, and a bent portion bent in a connecting region of the first straight portion and the second straight portion, a plurality of the first straight portions having the second ends aligned along the same straight line, on an assumption of a first slit being an endmost slit among the plurality of slits in the pixel, a slit next to the first slit bending in a manner that the first straight portion and the second straight portion of the slit come closer to the first slit, a slit more distant from the first slit having a shorter first straight portion.

"The configuration of the liquid crystal display of the present invention is not especially limited by other components as long as it essentially includes such components. The straight line is not a physical part but is a virtual line.

"The following will discuss preferable embodiments of the liquid crystal display according to the present invention. The following embodiments may be employed in combination.

"When an alignment direction of the liquid crystal molecules under application of no voltage is set as an initial alignment direction, the first direction and the second direction are preferably each different from the initial alignment direction. This configuration allows more effective control of the alignment of liquid crystal molecules.

"Preferably, the first direction and the initial alignment direction form an angle larger than an angle formed by the second direction and the initial alignment direction, and the first straight portion is shorter than the second straight portion. This configuration enables the first straight portion and the second straight portion to function more effectively as a supplemenraty portion (auxiliary portion) and a main portion, respectively.

"Preferably, the first direction and the initial alignment direction form an angle of 20.degree. to 40.degree., the second direction and the initial alignment direction form an angle of 3.degree. to 10.degree., the straight line is a first straight line, the bent portions of the plurality of slits are aligned along a second straight line, and the first straight line and the second straight line form an angle of 5.degree. to 15.degree.. This configuration effectively enables recovery from disordered alignment caused by pressing and increase in the transmittance at the same time. The second straight line is not a physical part but is a virtual line.

"The plurality of slits each may have a third straight portion being connected to the second end of the first straight portion and extending in a third direction, and the first straight portion and the third straight portion may form a V shape. This embodiment is suitable for a case where a V portion is provided at the center of a pixel.

"Preferably, when the alignment direction of the liquid crystal molecules under application of no voltage is set as an initial alignment direction, the third direction and the initial alignment direction form an angle of 20.degree. to 40.degree., the plurality of slits each further have a fourth straight portion and a second bent portion, the fourth straight portion being connected to an end of the third straight portion on the side not connected to the first straight portion and extending in a fourth direction, the second bent portion bent in a connecting region of the third straight portion and the fourth straight portion, the straight line is a first straight line, the second bent portions of the plurality of slits are aligned along a third straight line, the first straight line and the third straight line form an angle of 5.degree. to 15.degree., and the fourth direction and the initial alignment direction form an angle of 3.degree. to 10.degree.. This configuration effectively enables recovery from disordered alignment caused by pressing and increase in the transmittance at the same time. The third straight line is not a physical part but is a virtual line.

"The second electrode may include at least three linear portions adjacent to the plurality of slits and a connecting portion connecting the at least three linear portions to each other, and the first straight portion may be adjacent to the connecting portion. This embodiment is suitable for a case where an auxiliary portion is provided next to a connecting portion of an electrode.

"Advantageous Effects of Invention

"The present invention realizes a liquid crystal display that can achieve suppression of defects caused by variation in process and improvement in display performance at the same time.

BRIEF DESCRIPTION OF DRAWINGS

"FIG. 1 is a schematic plan view of an active matrix substrate of a FFS-mode liquid crystal display according to Embodiment 1.

"FIG. 2 is a schematic plan view of a common electrode provided on an active matrix substrate according to Embodiment 1.

"FIG. 3 is a schematic cross-sectional view of an active matrix substrate taken along an A-A' line in FIG. 1.

"FIG. 4 is an enlarged schematic plan view of a vicinity of a V portion of a pixel electrode according to Embodiment 1.

"FIG. 5 is a schematic plan view of an active matrix substrate of a FFS-mode liquid crystal display according to Embodiment 2.

"FIG. 6 is an enlarged schematic plan view of a vicinity of an auxiliary portion of a pixel electrode according to Embodiment 2.

"FIG. 7 is a schematic plan view of an active matrix substrate of a FFS-mode liquid crystal display according to Embodiment 3.

"FIG. 8 is a schematic plan view of an active matrix substrate of a FFS-mode liquid crystal display according to a modified example of Embodiment 3.

"FIG. 9 is a schematic plan view of a common electrode provided on an active matrix substrate of a modified example of Embodiment 3.

"FIG. 10 is a schematic plan view of an active matrix substrate of a FFS-mode liquid crystal display according to Comparative Embodiment 1."

For additional information on this patent application, see: Furukawa, Tomoo; Hisada, Yuhko; Asada, Katsushige. Liquid Crystal Display. Filed August 3, 2012 and posted July 17, 2014. Patent URL: http://appft.uspto.gov/netacgi/nph-Parser?Sect1=PTO2&Sect2=HITOFF&u=%2Fnetahtml%2FPTO%2Fsearch-adv.html&r=3875&p=78&f=G&l=50&d=PG01&S1=20140710.PD.&OS=PD/20140710&RS=PD/20140710

Keywords for this news article include: Electronics, Photolithography, Sharp Kabushiki Kaisha.

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Source: Electronics Newsweekly


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