JP4651530B2 - O-plate compensator, its manufacturing method and nematic liquid crystal display - Google Patents

Description

  The present invention relates to liquid crystal display (LCD) designs and, more particularly, maximizes the field of view of such displays by maintaining minimal changes in relative gray levels and high contrast ratios over a wide range of viewing angles. It is related to technology. These goals are achieved by making and manufacturing LCDs using O-plate compensator technology.

  In liquid crystals, the liquid crystals are useful for electronic displays because the polarization passing through the liquid crystal layer is affected by the birefringence of the liquid crystal layer, which can be changed by applying a voltage to the liquid crystal layer. By using this effect, the transmission or reflection of light from external sources, including ambient light, can be controlled with much less power than that required for luminescent materials used in other types of displays. As a result, liquid crystal displays are now commonly used in various applications such as digital watches, calculators, portable computers, and many other types of electronic devices. These applications highlight some of the advantages of LCD technology, including very light weight, low power consumption and a very long operating life.

  The information content of many liquid crystal displays is represented in the form of numerous numbers or strings generated by segment electrodes arranged in a pattern on the display. These electrode segments are connected to the electronic drive circuit by individual leads. By applying a voltage to the appropriate combination of segments, the electronic drive circuit controls the light transmitted through the segments.

  Graphics and television displays can be achieved by using a matrix of pixels in the display connected between two sets of vertical conductors by an XY sequential addressing mechanism. A more sophisticated addressing mechanism applied primarily to twisted nematic liquid crystal displays uses an array of thin film transistors to control the drive voltage at individual pixels.

  Contrast and relative grayscale intensity stability are important attributes in determining the quality of a liquid crystal display. The main factor limiting the contrast that can be achieved with a liquid crystal display is the amount of light that leaks into the dark display. Furthermore, the contrast ratio of the liquid crystal device also depends on the viewing angle. The contrast ratio of a typical liquid crystal device is maximal only within a narrow viewing angle range centered around normal incidence and decreases as the viewing angle increases. This loss of contrast ratio is caused by light leaking into the black pixel element at a large viewing angle. Even in color liquid crystal displays, such leakage causes severe color changes for both saturated and grayscale colors.

  In a typical prior art twisted nematic liquid crystal display, in addition to the color change caused by dark state leakage, the optical anisotropy of the liquid crystal molecules is a large variation in gray level transmission as a function of viewing angle, i.e. brightness- The field of view with acceptable gray scale stability is very limited because it causes changes in the voltage curve. This variation is often so severe that extreme gray angles cause some of the gray levels to reverse their transmission levels. These limitations are particularly important for applications that require very high quality displays such as evionics where it is important to view the cockpit display from both pilot and co-pilot seat positions. Such high information content displays require that relative gray level transmission be as small as possible with respect to viewing angle. If a liquid crystal display capable of providing a high-quality and high-contrast image for a wide field of view can be provided, it would be a significant advance in the art.

  FIGS. 1A and 1B show a conventional normally white twisted nematic liquid crystal display 100 that includes a polarizer 105, an analyzer 110 whose polarization axis is perpendicular to the polarization axis of the polarizer 105, a light source 130 and an observer 135. ing.

  In the normally white configuration of FIGS. 1A and 1B, the “non-selected” region 115 (no applied voltage) is in the bright state and the “selected” region 120 (activated by the applied voltage) is in the dark state. In the selection region 120, the liquid crystal molecules are tilted and rotated to align with the applied electric field. Once this orientation is complete, all liquid crystal molecules in the cell will be oriented so that their long axis is perpendicular to the major surface of the cell. This configuration is known as homeotropic orientation.

  Since the liquid crystals used in twisted nematic displays exhibit positive birefringence, the arrangement known as this homeotropic configuration will exhibit the optical symmetry of a positive birefringent C-plate. As is well known in the art, the C-plate is a uniaxial birefringent plate whose anomalous axis (ie, its optical axis or c-axis) is perpendicular to the plate surface (parallel to the direction of normal incident light). is there. Therefore, the liquid crystal of a normally white display will be isotropic to normal incident light that would be blocked by crossed polarizers in the selected state.

  One reason for the loss of contrast when the viewing angle is increased in a normally white display is that the homeotropic liquid crystal layer is not isotropic to light that is not perpendicular. Light propagating through the layer at a non-perpendicular angle appears in two modes due to the birefringence of the layer, and phase delay is introduced between these modes and increases with the incident angle of light. This incident angle dependence of the phase causes ellipticity in the polarization state that is incompletely lost by the second polarizer, thereby causing light leakage. In order to correct this effect, the optical compensation element must also have a C-plate symmetry, although it is negatively birefringent (ne <n0). Such a compensator provides a phase delay with the opposite sign to that caused by the liquid crystal layer, thereby restoring the original polarization state, so that the light passing through the activated region of the layer is more complete by the output polarizer. To be able to be blocked. However, C-plate compensation does not affect gray scale variation with respect to viewing angle, and the present invention addresses this.

In describing the orientation of the various compensation elements of the display, it is convenient to refer to the normal axis perpendicular to the display, indicated by dotted line 370. In the case of a normally white display, the polarizer 300 (its polarization direction is the plane of 315 in the drawing) and the analyzer 305 (its polarization direction is toward the plane of 320 in the drawing) are 90 ° apart from each other. Oriented to be (Polarizer 300 and analyzer 305 both polarize the electromagnetic field. However, typically, the term “polarizer” refers to the polarizer element closest to the light source, and the term “analyzer” refers to the LCD. The first transparent electrode 325 and the second transparent electrode 330 are respectively opposite to the liquid crystal layer 310 so that a voltage can be applied to the liquid crystal layer by the voltage source 335. Placed on the glass plates 340 and 345 adjacent to the surface. As described below, the inner surfaces of the glass plates 340 and 345 proximate to the liquid crystal layer 310 can be physically or chemically treated to affect the desired liquid crystal alignment, such as by buffing.

  As is well known in the LCD art (eg, “The Molecular Physics of Liquid-Crystal Devices” by Kahn, Physics Today, pp. 66-74, 1982). The inner surface of the plates 340 and 345 (the surface adjacent to the layer 310) is coated with a surface treatment agent for aligning liquid crystals such as polyimide, buffed, and the buffed directions are perpendicular to each other. , The orientation vector of the liquid crystal material is the buffed direction (known as the “rubbing direction”) in the region of the layer 310 adjacent to each of the plates 340 and 345 in the absence of voltage. Tend to be oriented. Further, the orientation of the liquid crystal axis (ie, the orientation vector) is from the first major surface adjacent to the plate 340 (ie, at the 310/340 interface) to the plate 345 (ie, at the 310/345 interface). Twist smoothly with respect to the normal axis at a 90 ° angle along the path of layer 310 to the second major surface.

  In the absence of an applied electric field, the polarization direction of the incoming polarization will be rotated 90 ° as it passes through the liquid crystal layer. Thus, when a glass plate and a liquid crystal layer are placed between crossed polarizers such as polarizer 300 and analyzer 305, the light polarized by the polarizer and traversing the display, as illustrated by ray 350, is It will be oriented in the polarization direction of the analyzer 320 and will therefore pass through the analyzer.

However, when a sufficient voltage is applied to the electrodes 325 and 330, the applied electric field tends to align the alignment vector of the liquid crystal material parallel to this electric field. In this state of the liquid crystal material, the light passed by the polarizer 300 as indicated by the light beam 355 will be cut off by the analyzer 305. Thus, the activated pair of electrodes creates a dark area in the display, and light that passes through the area of the display without being exposed to a given field creates a bright area. As is well known in the LCD display art, an appropriate pattern of electrodes activated in selected combinations can be used in this manner to display alphanumeric or graphical information. As will be described further below, one or more compensator layers, such as layers 360 and 365, may be included in the display to improve display quality.
<Characteristics of normally white twisted nematic LCD>
FIG. 4 shows a calculated plot of the tilt of the liquid crystal alignment vector as a function of position in a 90 ° twisted nematic cell liquid crystal layer (cell gap normalized to 1). When no voltage is applied (curve 400), when a typical selected state voltage is applied (curve 405), and some intermediate voltages selected to produce linearly spaced gray levels 2 shows a typical distribution of molecular tilt angles in the case where is applied (curves 410, 415, 420, 425, 430, 435).

  FIG. 5 shows a related plot showing the calculated twist angle of liquid crystal molecules as a function of position in the cell for the same cell. In the absence of applied voltage, the twist is evenly distributed throughout the cell (straight curve 500). Under fully selected voltage, the twist is distributed as shown by the outer S-shaped curve 505. The twist distribution with respect to the gray level is indicated by an intermediate curve between the two curves 500 and 505.

As shown in FIGS. 4 and 5, when a fully selected voltage is applied, almost all of the twist that occurs in the liquid crystal molecules and a significant portion of the tilt occurs in the central region of the cell. Because of this phenomenon, the continuous variation of molecular orientation in the cell can be divided into three regions, each of which is characterized by its own optical symmetry. Thus, the central regions 440 (FIG. 4) and 510 (FIG. 5) are nominally homeotropic in a fully selected state and can be considered close to the characteristics of the C-plate. Regions 445, 450 (FIG. 4) and 515, 520 (FIG. 5) near each surface of the cell each act as an A plate whose anomalous axis is oriented in the rubbing direction of the adjacent substrate. Since the molecules in regions 445, 450, 515 and 520 are essentially untwisted, the molecules are essentially oriented in their respective rubbing directions on each side of the liquid crystal layer. In addition, because the molecular twist angles in regions 445 and 515 tend to be perpendicular to the twist angles of the molecules in regions 450 and 520, the effect of these two regions on the light passing through the cell is counteracted and the central The C plate area will have a dominant influence.
<C-plate compensation>
As is well known in the art, a negative C-plate compensator is designed to correct for the angle-dependent phase shift caused by propagation through the approximate C-plate region in the center of the LCD cell. Such a compensator is effective to the extent that the optical symmetry of the central region dominates the selected state of the liquid crystal cell, i.e. the molecules are oriented in a given field. This means that negative C-plate compensation works best at this time, as a strong field is used for the activation state, which approximates the homeotropic approximation more accurately. Using a C-plate has been shown to significantly reduce dark state leakage over a wider field of view, thereby improving contrast and reducing color desaturation.
<Stability of gray scale>
A C-plate compensator can be used to improve contrast, but this does not improve grayscale stability. The problem of maintaining the contrast grayscale brightness difference with respect to the field of view is essentially assigned between the selected (black for normally white display) and unselected (white for normally white displays) states. It relates to the change of the brightness level with respect to the level. This phenomenon is generally illustrated using a transmission or brightness-voltage (BV) electro-optic response curve for a display assigned 8 gray levels from level 0 (selected black state) to level 7 (unselected white state). . Gray levels from 0 to 7 are selected by assigning these levels a set of voltages linearly spaced in brightness along the BV curve between selected and non-selected voltages. .

  FIG. 6 shows a normally white 90 ° twisted nematic display when the horizontal viewing angle is incremented by 10 ° from 0 ° to 40 ° while the vertical viewing angle is fixed at 0 °. It is the plot which computed the BV (transmittance-driving voltage) curve. (The change in the BV curve with respect to the horizontal angle does not depend on whether the horizontal shift is to the left or to the right until the first order.) Note that the gray level will be selected. The areas of each curve almost overlap each other for various horizontal angles. This means that if gray levels are selected to be linearly spaced at 0 °, these gray levels will remain very linear even at larger horizontal viewing angles. means.

  The problem with grayscale stability arises mainly when the viewing angle in the vertical direction changes. This is a series of BV curves for a normally white 90 ° twisted nematic display when the horizontal viewing angle remains fixed at 0 ° and the vertical viewing angle varies from -10 ° to + 30 °. It is shown in FIG. It can be observed that at angles less than 0 ° (measured from the normal), the BV curve moves to the right (greater voltage) and falls monotonically from its maximum but does not reach zero.

At angles above the normal, the curve moves to the left and the value after the first minimum is restored (rebound). This effect can be explained by considering the perspective when the viewer looking at the display sees the display from above, at the normal, and from below the normal, as shown in FIG. An important feature to note is the relationship between the tilt of the average liquid crystal alignment vector at the center of the cell and the light traveling towards the viewer as the voltage across the cell is increased.

  For example, increasing the voltage across the cell causes the average liquid crystal orientation vector at the center of the cell to tilt from a parallel (with respect to polarizer 300 and analyzer 305) orientation 815 toward the homeotropic orientation 825. For those who view at normal incidence 800, the retardation is highest at the unselected state voltage and lowest at the selected state voltage. When the retardation is approximately 0, the polarization state of the light is not changed and the light is blocked by the analyzer. Therefore, the viewer can see that the brightness monotonously decreases to 0 as the voltage increases.

  Next, consider the case of the viewing direction in the positive vertical direction (in the case of a person looking above the normal incidence 805). At some intermediate voltage, the average orientation vector 820 faces the viewer and the retardation is minimal. Here, the viewer sees that brightness first decreases with voltage and reaches a minimum at the point where retardation is minimized, then increases.

  For a negative vertical viewing direction (for a viewer looking below normal incidence 810), the average orientation vector always shows a large anisotropy for the light beam—even at the highest voltage. Thus, the viewer can see a monotonous decrease in brightness. In addition, the average liquid crystal orientation vector is always oriented at a greater angle with respect to the light ray for the viewer 810 viewing from below the normal than for the viewer 800 viewing at normal incidence. Therefore, the anisotropy is greater and the brightness level is always higher in the negative vertical viewing direction than in normal incidence.

This dependence of LCD brightness on viewing angle has a significant impact on grayscale stability. In particular, the gray level luminance variation with respect to the viewing angle may be extreme. (Note that the voltage selected to produce 50% gray level in the 0 ° curve in FIG. 7 causes a dark state in the + 30 ° curve and approaches a completely white state at −10 °.)
<Grayscale compensation by O-plate>
A birefringent O-plate compensator can be used to eliminate gray level inversion and improve gray scale stability. The principle of an O-plate compensator as described in US Patent Application No. 223,251 is that its main optical axis is oriented at a substantially oblique angle with respect to the plane of the display (hence the name “O-plate”). A positive birefringent material is used. By “substantially oblique” is meant an angle that is significantly greater than 0 ° and significantly less than 90 °. O-plates are used, for example, at angles of 30 ° to 60 °, typically 45 ° with respect to the display surface. Furthermore, an O plate using a uniaxial or biaxial material can be used. The O-plate compensator can be placed at various locations between the polarizer and analyzer layers of the LCD.

  In general, the O plate compensator may include not only the O plate but also the A plate and / or the negative C plate. As is well known in the art, the A-plate is a birefringent layer and its extraordinary axis (ie its c-axis) is oriented parallel to the surface of the layer. Therefore, its a-axis is oriented perpendicular to the surface (parallel to the direction of normal incident light) and is therefore referred to as the A plate. A-plates can be manufactured by using uniaxially stretched polymer films such as polyvinyl alcohol or other appropriately oriented organic birefringent materials.

The C plate is a uniaxial birefringent layer whose anomalous axis is oriented perpendicular to the surface of the layer (parallel to the direction of normal incident light). A negative birefringent C-plate can be obtained, for example, by using a uniaxially compressed polymer (see, eg, US Pat. No. 4,701,028 to Clark et al.), A stretched polymer film. Alternatively, it can be produced by using a physical vapor grown inorganic thin film (see, eg, US Pat. No. 5,196,953 to Yeh et al.).

  Oblique deposition of thin films by physical vapor deposition is known to produce films with birefringent properties (eg, “Thin Retardation Plate by Oblique Deposition (Thin) by Motohiro and Taga” Film Retardation Plate by Oblique Deposition) "Applied Optics (Appl. Opt.), Vol. 28, No. 3, pp. 2466-2482, 1989). By further exploiting the tilt orientation of the symmetry axis, the Motohiro process can be improved or enhanced to produce O-plates. Such components are inherently biaxial. Due to the growth characteristics of these components, a fine columnar structure is produced. The column angle is tilted towards the direction of vapor flow. If the deposition angle (measured from the normal) is 76 °, for example, the column angle will be about 45 °. The column creates an oval cross section as a result of shadowing. This elliptical cross section results in a biaxial characteristic of the film. Birefringence is entirely due to the fine structure of the film with respect to size and symmetry and is called shape birefringence. These phenomena in thin films have been extensively studied and explained by Macleod (“Structure-Related Optical Properties of Thin Films”, J. Vac. Sci. Technol. A, Vol. .4, No. 3, pages 418-422, 1986).

  Uniaxial O-plate components can also be used to improve the gray scale stability of twisted nematic liquid crystal displays. These components can be manufactured by using appropriately oriented organic birefringent materials. One skilled in the art will recognize other means of manufacturing uniaxial and biaxial O plates.

  FIGS. 9 and 10 illustrate one effect that the O-plate compensator layer can have on a normally white twisted nematic display. FIG. 9 shows the effect of the O-plate compensator layer on the electro-optic response of the display at various horizontal viewing angles, and FIG. 10 shows the effect of the O-plate compensator layer on the vertical viewing angle of the display. Is shown. In this particular embodiment, one A plate layer is placed on both sides of the liquid crystal cell adjacent to the liquid crystal layer, and an O plate layer is placed between the polarizer layer and the A plate / liquid crystal / A plate layer stack. Is done. The variation of the BV curve with respect to the horizontal and vertical viewing angles is greatly reduced compared to the uncompensated case shown in FIGS.

  The elimination of grayscale inversion by using an O-plate compensator layer occurs in the following manner. In the positive vertical viewing direction, the retardation of the O-plate increases with the viewing angle and tends to offset the decreasing retardation of the liquid crystal layer. When the observer is looking down on the axis of the average liquid crystal alignment vector, the presence of the O-plate prevents the layer between the two polarizers from becoming isotropic. Accordingly, the rebound of the BV curve shown in FIG. 7 is reduced and moves to a higher voltage outside the grayscale voltage range as shown in FIG.

  In the negative vertical viewing direction, an O-plate and A-plate combination whose optical axes are nominally 90 ° to each other is negative with the optical axis oriented perpendicular to the plane containing the O-plate and A-plate axes. It tends to exhibit birefringence characteristics similar to those of the birefringence retarder. The direction of the retarder axis is nominally parallel to the average liquid crystal orientation in the central region of the cell when driven with a voltage between the selected and unselected states. Thus, the presence of an O-plate oriented in this manner tends to cancel the birefringence of the liquid crystal layer, lowering the BV curve or equivalently moving the BV curve in the direction of a lower voltage (ie, left). Let Similar effects occur in the positive and negative horizontal viewing directions.

The overall effect of incorporating an O-plate compensator in this manner is to eliminate large rebounds in the grayscale voltage region and to shift the BV curve from left to right as the viewing angle changes from negative to positive. It is to reduce.

The orientation of the compensator optical axis can be carefully chosen so that the combined retardation effect cancels each other in the normal viewing direction and minimizes rebound in the horizontal viewing direction. More than one O-plate combination can be used as long as each orientation meets these requirements. Furthermore, the negative C-plate can increase the contrast ratio in a large field of view, although in some configurations may be accompanied by some degradation in grayscale stability.
<O-plate technology>
The liquid crystal layer, the compensator layer, the polarizer layer, and the analyzer layer can have various orientations with respect to each other in the liquid crystal display. Some of the possible configurations considered and described in US Patent Application No. 223,251 are repeated in Table 1 below, where “A” indicates the A plate and “C” indicates the C plate. “O” indicates an O plate, “LC” indicates a liquid crystal, and “OxO” indicates a cross O plate. Crossed O-plates are adjacent O-plates whose azimuth angles Φ235 nominally intersect, with a first azimuth angle oriented from 0 ° to 90 ° and a second azimuth angle from 90 ° to 180 °. Oriented.

  The projection of the principal axis onto the surface of the display with respect to the orientation of the liquid crystal orientation vector can vary depending on the embodiment. For example, when using two O plates, the projection of the O plate axis may be 45 ° with respect to the orientation of the average liquid crystal alignment vector near the center of the liquid crystal cell, and the projection of the O plate axis is relative to the alignment of the liquid crystal alignment vector It may be substantially parallel.

  Crossed O-plate (OxO) designs that are further compensated with A-plates provide additional design flexibility. The choice of A-plate value is not critical because such a design can be adjusted by changing the relative orientation of the A-plate. Therefore, it is possible to obtain a desired solution using the retardation value of a commercially available A plate.

  The flexibility that the O-plate compensation mechanism provides to the display designer allows performance to be tailored to specific display production requirements. For example, with simple configuration and parameter changes, isocontrast optimized for viewing from the left or right, isocontrast optimized for viewing from an extreme vertical angle, or from above normal It is possible to achieve isocontrast optimized for viewing at large angles on the left and right sides. In addition, configurations and parameters can be adjusted to improve both contrast and grayscale stability for a particular field of view, or to further optimize the other at the expense of one. Further, a negative birefringent A plate may be used instead of the positive birefringent A plate. In this case, the negative birefringent A plate will be oriented so that its extraordinary axis is perpendicular to the proper orientation for the positive birefringent A plate. As those skilled in the art of liquid crystal display design will understand, when using a negative A-plate, additional changes must be made to other components of the compensator to optimize performance. It will be.

  When viewed at an angle close to the normal of the twisted nematic liquid crystal display surface, this twisted nematic liquid crystal display gives a high quality output, but with a large viewing angle, the image quality decreases, the contrast is poor, and the gray scale is uniform. Not. This occurs because the phase retardation effect of the liquid crystal material on the light passing through it essentially changes with the tilt angle of the light, and the image quality deteriorates when the viewing angle is large. However, by introducing one or more optical compensation elements with the liquid crystal cell, this unwanted effect due to the angle is substantially corrected, thereby providing a higher contrast at a larger viewing angle than otherwise possible. And a stable relative gray level intensity can be maintained.

  The goal of the present invention is to disclose a positive birefringent O-plate compensator and its manufacturing method that makes it possible to significantly improve the grayscale characteristics and contrast ratio of a liquid crystal display over a wide range of viewing angles. It is.

  The compensator design of the present invention that includes a positive birefringent O-plate layer allows the gray scale characteristics and contrast ratio of a liquid crystal display (LCD) to be significantly improved over a wide range of viewing angles.

  An O-plate compensator comprising an organic liquid crystal polymer and a method for manufacturing the same are disclosed herein. The compensator is a uniaxial birefringent thin film whose anomalous axis is oriented obliquely with respect to the film surface. (Note that this birefringent thin film may be weakly biaxial.) The oblique orientation of the liquid crystal orientation vector parallel to the anomalous axis of the film is obliquely deposited SiO, which is a mechanically rubbed orientation material. This is accomplished by placing an organic thin film on a specially prepared surface for aligning liquid crystals such as This film can be placed from a solution of a liquid crystal polymer or from a reactive liquid crystal monomer having a nematic phase. Any solvent that can be used during the manufacturing process is evaporated and the organic film is kept at the temperature of its nematic phase. If reactive monomers are used, the film is then photopolymerized. Alternative examples of organic O-plates include the use of smectic A and smectic C materials. A manufacturing technique using these materials will be described.

Several exemplary embodiments of the invention are described below. For the sake of clarity, the features of the actual implementation are not described in full here. Of course, the development of any such real implementation (as in any development project) will have specific goals for the developer that will vary from implementation to implementation, such as following system-related constraints and business-related constraints. It will be appreciated that many implementation-specific decisions must be made to achieve the secondary goals. Further, it will be appreciated that such development work would be complex and time consuming, but would be a normal business of device engineering for those skilled in the art having the benefit of this disclosure.
<Introduction>
FIG. 11 shows an exemplary embodiment of a liquid crystal display (LCD) system according to the present invention using one O-plate compensator 1100 disposed between the polarizer 300 and the liquid crystal layer 310. The O plate layer 1100 includes a positive birefringent liquid crystal polymer layer whose optical symmetry axis is oriented at an angle of about 20 ° to about 70 ° with respect to the surface of the liquid crystal polymer layer 310. Alternatively, the O-plate compensator layer may be disposed between the liquid crystal layer 310 and the analyzer 305, or may be disposed at both locations.

  The decision on where to place the O-plate compensator is a pure design choice and is generally based on the optical requirements of the display to be compensated and the manufacturing and cost constraints of the display system. The

In general, the compensator may include not only the O plate but also the A plate and / or the negative C plate. As is well known in the art, the A-plate is a birefringent layer and its extraordinary axis (ie its c-axis) is oriented parallel to the surface of the layer. Therefore, its a-axis is oriented perpendicular to the surface (parallel to the direction of normal incident light) and is therefore referred to as the A plate. A-plates can be manufactured by using uniaxially stretched polymer films such as polyvinyl alcohol or other appropriately oriented organic birefringent materials.
<Nematic example>
Another exemplary embodiment shown in FIG. 12A includes a hard glass substrate 1200, an alignment layer 1205, a polymerizable high pretilt liquid crystal monomer layer 1207 having a nematic phase, a high pretilt liquid crystal / alignment layer interface 1207/1205, and a nematic phase. A polymerizable O-plate liquid crystal monomer layer 1210, a liquid crystal O-plate / high pretilt liquid crystal layer interface 1210/1207, an air layer 1215, and a liquid crystal / air interface 1210/1215 are shown. The liquid crystal is composed of a polymerizable monomer compound and contains about 0.5% of lrgacure-651, which is a photoinitiator. Further, the liquid crystal has a temperature dependent tilt angle of about 40 ° at the liquid crystal / air interface 1210/1215.

  The alignment layer 1205 is generated by coating the surface of the substrate 1200 with a polyimide material that creates a low liquid crystal tilt angle. The alignment surface is then rubbed to create a temperature dependent polar tilt angle 1217 of about 8 ° at the high pretilt liquid crystal / alignment layer interface 1207/1205, and a uniform specific orientation tilt direction. The liquid crystal tilt angle 1217 may be in the range of 5 ° to 20 °.

A thin film of the liquid crystal monomer 1207 is formed on the alignment layer 1205 by using spin coating from a solution in which liquid crystal is dissolved in a solvent. Other methods of producing organic liquid films include, for example, dip, meniscus, and slot-die coating. The chemical structure of a liquid crystal monomer conventionally known as C6M is shown in FIG. 12B. The concentration of C6M is about 8.5%. The solvent is monochlorobenzene. Finally, the coating solution contains about 0.05% of the photoinitiator lrgacure-651. As known to those skilled in the art, C6M
Other materials may be used in place of lrgacure-651 and monochlorobenzene. It is believed that the above concentrations can vary within the solubility range of the solute.

After forming the film, the solvent is evaporated and the temperature of the liquid crystal layer 1207 is adjusted to produce a thin film of liquid crystal material in the nematic phase. Next, about 360 nanometers (nm) to achieve a total exposure, typically 4-10 J / cm ₂ , sufficient to polymerize the monomer into a polymer film in which the order of the liquid crystal phase is maintained. ) Is irradiated to the liquid crystal film.

  Other possible alignment layer materials may be used in place of layers 1205 and 1207 to provide a pretilt angle of about 40 °. Such materials may include, for example, a mixture of homogeneous and homeotropic alignment materials that are subsequently rubbed.

  A thin film of liquid crystal monomer 1210 is produced on the polymerized high pretilt liquid crystal layer 1207 by using a spin coating.

After coating, the liquid crystal monomer layer 1210 provides a uniform polar tilt angle 1220 of 40 ° and a uniform azimuth tilt direction at the liquid crystal O-plate / high pretilt liquid crystal layer interface 1210/1207 across the surface of the alignment layer 1205. Heated to a temperature within its nematic range. The goal is to achieve nominally the same tilt angle (1220) at the substrate / liquid crystal interface and the liquid crystal / air interface (1225). Since there is no preferred orientation orientation of the liquid crystal at the liquid crystal / air interface, the orientation at this interface is determined by the orientation orientation of the liquid crystal O-plate / high pretilt liquid crystal layer interface 1210/1207. Next, a liquid crystal monomer film at a wavelength of about 360 nm, which is sufficient to polymerize the monomer into a polymer film in which the order of the liquid crystal phase is maintained, is typically 4 to 10 J / cm _2. Irradiate. The nematic order present in the liquid crystal phase of the monomer is maintained by the polymer film produced by photopolymerization.

  The purpose of the high pretilt liquid crystal layer 1207 is to increase its pretilt angle to about 40 ° without changing the orientation of the liquid crystal O-plate layer 1210. This occurs because the intrinsic nematic / air tilt angles 1219 and 1225 of the C6M liquid crystal are about 40 °. As long as the thickness of the high pretilt liquid crystal layer 1207 is greater than about 100 nm, the liquid crystal molecules will move from a pretilt angle of about 8 ° at the high pretilt liquid crystal / alignment layer interface 1207/1205 to a tilt angle 1219 of 40 ° nematic / air. Will continue to spread / bend deformation. The nematic order present in the liquid crystal phase of the monomer is maintained by the polymer film generated by photopolymerization.

  In view of these procedures, the liquid crystal molecules of the liquid crystal O-plate layer 1210 are aligned at a liquid crystal O-plate / polymerized high pretilt liquid crystal layer interface 1210/1207 at a pretilt angle 1220 of 40 ° (adjacent nematic / air tilt angle 1219). Orientation). Since the same material is used for both the liquid crystal O-plate layer 1210 and the high pre-tilt liquid crystal layer 1207, the nematic / air tilt angle 1225 has almost the same value as the liquid crystal pre-tilt angle 1220, and the tilt angle of the liquid crystal O-plate layer 1210 is Uniform across the thickness of the layer. Furthermore, the surface of the polymerized high pretilt liquid crystal layer 1207 exhibits excellent wetting characteristics due to the liquid crystal molecules of the liquid crystal O-plate layer 1210.

  For most applications, the liquid crystal O-plate layer 1210 is much thicker (typically> 1.0 microns) than the high pretilt liquid crystal layer 1207. Therefore, the contribution of the high pretilt liquid crystal layer 1207 to the overall phase retardation characteristics of the O-plate layer 1210 is negligible. This process results in a thin film of liquid crystal polymer 1210 that is positively birefringent and has an axis of symmetry that is oriented with a polar tilt angle of about 40 °.

  The liquid crystal O-plate layer / air tilt angle 1225 and the liquid crystal pretilt angle 1220 may differ by about 60 °. Thereafter, the liquid crystal O plate layer 1210 may exhibit a spreading / bending deformation between the liquid crystal O plate / high pretilt liquid crystal layer interface 1210/1207 and the liquid crystal O plate / air interface 1210/1215. The liquid crystal O-plate layer 1210 can also exhibit azimuthal twist by incorporating chiral additives or by using chiral polymerizable liquid crystal monomers or chiral liquid crystal polymers.

  Alternatively, if the liquid crystal material is a fluid at room temperature, a thin film layer of this material can be deposited directly on the high pretilt liquid crystal layer 1207.

In another alternative embodiment, nematic liquid crystal material 1210 may be replaced with a polymerizable smectic A liquid crystal monomer material. The anchoring energy of the smectic A liquid crystal material at the liquid crystal O-plate / high pretilt liquid crystal layer interface 1210/1207 is significantly greater than the anchoring energy at the liquid crystal / air interface 1210/1215. Further, the bending deformation elastic constant of each smectic A phase material is very large. This means that the alignment of the liquid crystal alignment vector of the bulk film is uniform and is substantially determined by the tilt angle 1220 at the liquid crystal O plate / high pretilt liquid crystal layer interface 1210/1207. The advantage of using a polymerizable smectic A material in this process is that the resulting liquid crystal polymer film's polar tilt angle is not sensitive to the liquid crystal monomer tilt angle 1225 at the liquid crystal / air interface 1210/1215. .
<Examples Oriented on Smectic C Substrate>
An alternative embodiment is shown in FIG. As before, the compensator system comprises a hard glass substrate 1300, an alignment layer 1305, a polymerizable liquid crystal O plate monomer layer 1310, a liquid crystal O plate / alignment layer interface 1310/1305, an air layer 1315, and a liquid crystal / air interface 1310. / 1315 is included. However, in this example, the polymerizable liquid crystal layer 1310 has a temperature range of smectic C phase and a maximum smectic tilt angle 1320 of about 45 °.

  Such materials are preferred because liquid crystal materials having a smectic C-phase to nematic phase transition tend to have a large smectic tilt angle 1320 in the range of 10 ° to 45 °. This liquid crystal material consists of a polymerizable monomer compound and contains about 0.5% lrgacure-651.

  Similar to the nematic embodiment, an alignment layer 1305 is produced on the surface of the substrate. In a preferred embodiment, the alignment layer material is a thin film of SiO, disposed obliquely at a polar angle of about 60 °, and coated with egg lecithin, a thin film of homeotropic alignment material. This alignment surface provides a uniform specific orientation tilt direction as determined by the orientation SiO deposition angle and a liquid crystal pretilt angle 1325 of about 85 °.

  Next, a thin film of polymerizable liquid crystal monomer 1310 is placed on alignment layer 1305 by using a spin coating. Thereafter, the temperature of the liquid crystal film is raised to the nematic phase, and a uniform pretilt angle 1325 of about 85 ° is formed at the liquid crystal O plate / alignment layer interface 1310/1305. Thereafter, the temperature is slowly lowered, for example, at a rate of about 0.1 ° C./minute to obtain the smectic C phase of the liquid crystal material.

  This process results in the formation of a smectic layer that is parallel to the surface of the alignment layer with molecules initially tilted at a 0 ° smectic tilt angle 1320. When the temperature of the liquid crystal layer is lowered to the smectic C phase, the smectic tilt angle 1320 of the liquid crystal molecules increases. (The molecular azimuth tilt direction is determined by the azimuthal SiO deposition angle.) At temperatures just above the melting point of the material, the smectic tilt angle 1320 reaches a maximum of about 45 °. Those skilled in the art will recognize various other methods of forming a smectic layer parallel to the alignment layer.

  At the liquid crystal / air interface 1310/1315, there is no preferred orientation of the liquid crystal, so the orientation at this interface is determined by the orientation of the liquid crystal at the liquid crystal O plate / alignment layer interface 1310/1305. Further, in the smectic C material, the extreme tilt angle 1330 at the liquid crystal / air interface 1310/1315 does not affect the tilt angle of the bulk liquid crystal material.

Next, an ultraviolet light (chemical radiation) with a wavelength of about 360 nm sufficient to polymerize the monomer into a polymer film in which the order of the liquid crystal phase is maintained is typically about 4 to 10 J / cm ₂ for the liquid crystal monomer. The film 1310 is irradiated. In the nematic example, the liquid crystal ordering present in the monomeric smectic C phase is preserved in a polymer film produced by photopolymerization. This process results in a thin film of liquid crystal polymer that is positively birefringent and has an axis of symmetry that is oriented with a polar tilt angle 1320 of about 45 °.
<Example of smectic C electric field alignment>
Referring to FIG. 14, another exemplary liquid crystal display system has a hard glass substrate 1400, an alignment layer 1405, a high spontaneous polarization of ± 2-20 nC / cm ₂ and a maximum smectic tilt angle 1420 of about 45 °. Polymerizable liquid crystal monomer layer 1410 having a room temperature chiral smectic C phase, liquid crystal O plate / alignment layer interface 1410/1405, air layer 1415, liquid crystal O plate / air interface 1410/1415, and applied electric field (paper surface of FIG. 14 Direction 1440).

  Such materials are preferred because liquid crystal materials having a smectic C-nematic transition tend to have a large smectic tilt angle 1420 in the range of 10 ° to 45 °. The pitch of the liquid crystal (for example, the distance perpendicular to the smectic layer whose azimuthal orientation rotates 360 °) is about 100 micrometers (μm) or more. The liquid crystal also consists of a polymerizable monomer compound and contains 0.5% lrgacure-651.

  One skilled in the art will recognize that liquid crystals having different values of spontaneous polarization can be used. Furthermore, the liquid crystal material can be pitch compensated. That is, the liquid crystal material may consist of both left-handed and right-handed chiral smectic C molecules. The sign and magnitude of the spontaneous polarization of the left-handed molecule may be different from that of the right-handed molecule. The relative amounts of left-handed and right-handed molecules can be adjusted to give a relatively long pitch (eg, much larger than the thickness of the liquid crystal film) and still give a high value of spontaneous polarization of the liquid crystal mixture. .

  An alignment layer 1405 is placed on the surface of the substrate 1400 by using a spin coating. In this example, the alignment layer material is a thin film of egg lecithin, a homeotropic alignment material (eg, producing a liquid crystal tilt angle 1425 of about 90 °).

  Next, a thin film of polymerizable liquid crystal monomer 1410 is placed on alignment layer 1405, as discussed in the above example. Thereafter, an electric field is applied to the liquid crystal film 1410 having a 0 ° polar tilt angle 1425 and a specific azimuth angle. In FIG. 14, the electric field direction 1440 is perpendicular to the page.

  After the formation of the liquid crystal layer 1410, the temperature of the liquid crystal film is raised to the nematic phase, and uniform homeotropic alignment is formed at the liquid crystal O plate / alignment layer interface 1410/1405. Thereafter, the temperature is slowly lowered, for example, at a rate of about 0.1 ° C./minute to obtain the smectic C phase of the liquid crystal material. This process results in the formation of a smectic layer substantially parallel to the surface of the alignment layer where the molecules are initially tilted at a 0 ° smectic tilt angle 1420. Those skilled in the art will recognize various other methods of forming a smectic layer parallel to the alignment layer.

  When the temperature of the liquid crystal layer is lowered to the smectic C phase, the smectic tilt angle 1420 of the liquid crystal molecules increases. The orientation tilt direction of the molecule is generally perpendicular to the direction of the electric field and depends on the polarity of the electric field and the sign of spontaneous polarization. At temperatures just above the melting point of the liquid crystal material, the smectic tilt angle 1420 reaches a maximum of about 45 °.

  Since there is no preferred orientation of the liquid crystal at the liquid crystal / air interface 1410/1415, the orientation at this interface is determined by the orientation of the liquid crystal at the liquid crystal O plate / alignment layer interface 1410/1405. Furthermore, for smectic C materials, the extreme tilt angle 1430 at the liquid crystal / air interface 1410/1415 does not affect the tilt angle of the bulk liquid crystal material.

Next, the liquid crystal monomer film 1410 is typically irradiated with ultraviolet light (chemical radiation) having a wavelength of 360 nm sufficient to polymerize the monomer to form a polymer film in which the order of the liquid crystal phase is maintained, at 4 to 10 J / cm _2. Irradiate. As in the case of the above-described embodiment, the liquid crystal order present in the monomer is maintained by the polymer film generated by photopolymerization. This process results in a thin film of liquid crystal polymer that is positively birefringent and has an axis of symmetry that is oriented with a polar tilt angle 1420 of about 45 °.
<Possible variations>
For each of the exemplary embodiments described above, many variations are possible, and these variations will be apparent to those skilled in the art of liquid crystal display devices. For example, other possible substrate materials may include polymer films. The polymerizable liquid crystal monomer material may contain as a component a molecule that contains a large number of reactive functional groups and thus can act as a crosslinker. Other extreme tilt angles of the liquid crystal / alignment layer interface can be achieved by appropriate selection of reactive liquid crystals, alignment material changes, rubbing conditions, and the like. In addition, non-reactive liquid crystal materials can be combined with polymerizable liquid crystals. The resulting liquid crystal polymer film will have plastic or gel properties. The liquid crystal material may also contain polymers (eg, liquid crystal polymers) or oligomers that increase the viscosity of the liquid crystal mixture and improve its film-forming properties.

  In addition, the liquid crystal polymer is expected to have a birefringence of about 0.05 to about 0.25 depending on the specific chemical structure of the liquid crystal and the polymerization temperature. For values of birefringence in this range, useful O-plate compensators will have a thickness of 0.20-10 μm. Moreover, O-plate compensators made from polymerizable smectic C liquid crystal materials are expected to be slightly biaxial.

  It will be appreciated by those skilled in the art having the benefit of this disclosure that various modifications may be made to the embodiments shown above without departing from the inventive concepts described herein. Accordingly, it is not only the above-described exemplary embodiments, but also the claims, that define the rights claimed for this invention only.

It is a figure which shows the operation | movement of a normally white type 90 degree twisted nematic liquid crystal display as a whole. It is a figure which shows the coordinate system used in order to specify the orientation of a component in description of this invention. It is a schematic sectional drawing of a 90 degree twisted nematic transmission type normally white type liquid crystal display. FIG. 7 is a plot of the orientation vector tilt angle (expressed in degrees along the vertical axis) as a function of position (as a fraction of depth along the horizontal axis) in a 90 ° twisted nematic liquid crystal cell. . FIG. 5 is a diagram showing relevant plots for the cell shown in FIG. 4 and showing the twist angle of the liquid crystal molecules as a function of the position of the liquid crystal molecules in the cell. FIG. 6 shows a plot of calculated brightness versus voltage (BV) electro-optic curves in various horizontal viewing directions for a typical twisted nematic display without the benefit of grayscale compensation by an O-plate. FIG. 6 shows a plot of calculated brightness versus voltage (BV) electro-optic curves in various vertical viewing directions for a typical twisted nematic display without the benefit of O-scale grayscale compensation. It is a figure which shows the perspective of the viewer regarding the orientation of the average orientation vector of a liquid crystal. FIG. 6 shows calculated brightness vs. voltage electro-optic curves in various horizontal viewing directions for a normally white twisted nematic liquid crystal display with the benefit of O-plate compensation according to the present invention. FIG. 6 shows calculated brightness versus voltage electro-optic curves in various horizontal viewing directions for a normally white twisted nematic liquid crystal display with the benefit of O-plate compensation according to the present invention. FIG. 4 illustrates an exemplary liquid crystal display using an organic thin film O-plate compensator according to the present invention. FIG. 6 illustrates an exemplary embodiment of the present invention using a nematic liquid crystal material. It is a figure which shows the chemical structure of C6M which is the conventional liquid crystal monomer. FIG. 6 illustrates another exemplary embodiment of the present invention using a smectic C liquid crystal material in combination with a substrate alignment technique. FIG. 6 shows another exemplary embodiment of the present invention using a smectic C liquid crystal material in combination with a field alignment technique.

Claims (13)

Compensator for a liquid crystal display, wherein the compensator is a positively birefringent polymerized polymer formed from reactive liquid crystal monomers and whose mean principal optical axis is oriented at an oblique angle with respect to the normal of the display A liquid crystal material, and (a) first, second and third interfaces;
(B) a first liquid crystal layer comprising continuously deformed liquid crystal molecules having a first tilt angle at the first interface and a second tilt angle at the second interface;
(C) a second liquid crystal layer having a third tilt angle that is substantially uniform between the second interface and the third interface;
Including
The compensator, wherein the first liquid crystal layer and the second liquid crystal layer are coupled at the second interface, and the third interface is an air interface.   The compensator according to claim 1, wherein the thickness of the first liquid crystal layer exceeds 100 nm.   The compensator of claim 1, wherein the thickness of the second liquid crystal layer is greater than 1.0 microns and thicker than the first liquid crystal layer.   The compensator of claim 1, wherein the first tilt angle is in the range of 5 to 20 degrees.   The compensator of claim 1, wherein the third tilt angle is in the range of 20 to 70 degrees.   The compensator of claim 1, further comprising an alignment layer coupled to the first liquid crystal layer at the first interface.   The compensator of claim 6, further comprising a glass substrate coupled to the alignment layer on a side opposite to the first interface.   The compensator of claim 1, wherein the first liquid crystal layer is composed of a polymerized nematic liquid crystal material.   The compensator of claim 1, wherein the second liquid crystal layer is composed of a polymerized nematic liquid crystal material.   The compensator according to claim 1, wherein the second liquid crystal layer is made of a polymerized smectic liquid crystal material. A glass substrate; and an alignment layer disposed between the glass substrate and the first liquid crystal layer and coupled to the glass substrate and the first liquid crystal layer, the alignment layer and the first liquid crystal layer The compensator according to claim 1, wherein the interface with the liquid crystal layer is the first interface,
The first, second and third interfaces are parallel to each other, and the first, second and third tilt angles are associated with the first, second and third interfaces, respectively;
The thickness of the first liquid crystal layer exceeds 100 nm;
The thickness of the second liquid crystal layer exceeds 1.0 microns and is thicker than the first liquid crystal layer;
The first tilt angle ranges from 5 to 20 degrees, the second and third tilt angles range from 20 to 70 degrees;
The first liquid crystal layer and the second liquid crystal layer are rotationally coated layers formed from the same liquid crystal polymer;
The compensator, wherein the liquid crystal polymer has a birefringence of 0.05 to 0.25 and includes a polymerizable monomer compound. A method of manufacturing an O-plate compensator, comprising:
(A) providing a substrate;
(B) providing a layer of material capable of aligning the liquid crystal material;
(C) providing a polymerizable liquid crystal monomer high pretilt film on the alignment film;
(D) adjusting the temperature of the polymerizable liquid crystal monomer high pretilt film to a temperature between the isotropic clearing point temperature and the freezing point temperature of the liquid crystal monomer to produce an aligned liquid crystal film;
(E) irradiating the polymerizable liquid crystal monomer high pretilt film with chemical radiation to polymerize the high pretilt film;
(F) providing a polymerizable liquid crystal monomer O-plate film on the polymerized high pretilt film;
(G) adjusting the temperature of the polymerizable liquid crystal monomer O plate film to a temperature between the isotropic clearing point temperature and the freezing point temperature of the liquid crystal monomer to form an alignment liquid crystal film;
(H) irradiating the polymerizable liquid crystal monomer O-plate film with chemical radiation to polymerize the O-plate film. The step (c) of providing a polymerizable liquid crystal monomer high pretilt film on the alignment film comprises:
(1) dissolving the polymerizable liquid crystal monomer material in a solvent to form a solution;
(2) providing the solution to the alignment layer;
(3) the solvent is evaporated and forming the polymerizable liquid crystal monomer and high pretilt film, The method of claim 12.

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