Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2003-01-14
2004-10-05
Thomas, Tom (Department: 2815)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S127000, C349S123000, C349S129000, C349S132000, C349S177000
Reexamination Certificate
active
06801285
ABSTRACT:
TECHNICAL FIELD
This invention relates to liquid crystal microdisplays. More particularly, this invention relates to microdisplays utilizing liquid crystal material disposed on a silicon substrate. Specifically, this invention relates to liquid crystal material captured between a silicon substrate and a clear substrate with a cell gap of less than 1.5 microns and vastly improved operating properties.
BACKGROUND ART
Microdisplays are generally classified as flat-panel displays that are under three inches in diagonal. These displays have a pixel density of about 800×600 and some are even as large as 4000×2000. Microdisplays are used in two different types of applications. One is a projection-type microdisplay, wherein optics magnify an image on the microdisplay for projection onto a screen. Such displays are used in front projection systems, high-definition television, data monitors, simulation systems, and the like. Another type of microdisplay application is a “virtual” display for near-eye use. In these types of systems, an image is magnified in a device so that it appears much larger than in reality. These displays are used as monitors for miniaturized personal computers, cell phones, eyepieces, personal digital assistant displays, and the like.
Either of the foregoing devices can be manufactured utilizing silicon substrates and a liquid crystal material, wherein the silicon substrate provides the control electronics for modulating the liquid crystal material to a desired state for producing an image. For reflective displays, external light sources such as red, green, and/or blue light emitting diodes or, in some instances, color lasers are projected onto the display which is concurrently modulated. As a result, a full-color image is generated.
There are several types of known reflective microdisplays, each of which has its advantages and drawbacks. These displays basically employ complex variations of a twisted nematic liquid crystal cell. As is well known in the art, a twisted nematic liquid crystal cell includes two opposed substrates, each of which has an electrode disposed thereon. Polarizers are typically used with these cells to obtain the desired optical effect. The glass surfaces of the opposed substrates are treated so that the liquid crystal molecules lie parallel to the surface, wherein one substrate aligns the liquid crystal molecules in one direction and the other substrate aligns the material in an orthogonal direction. Accordingly, the nematic liquid crystal molecules are forced to twist through an angle of 90° within the cell. This produces rotation of the polarized light as it propagates through the cell. The polarization direction of light is therefore rotated 90°, whereupon the light passes through a polarizer on the other substrate. Behind the second polarizer is typically a reflector that causes the light to traverse back through the cell. Accordingly, the light is rotated back 90° by the liquid crystal so that it passes through the top polarizer and then emerges from the cell. In this state, the cell adopts the color of the reflector which is usually silver. When a voltage is applied across the cell, the nematic liquid crystal material prefers to align parallel to the electric field. If the voltage is high enough, the liquid crystal molecules change from a twist configuration to a deformed state and the polarization direction of light traversing through the cell is rotated only slightly, meaning that almost all of the light that passes through the top polarizer cannot get through to the bottom polarizer. Since no light is reflected back out of the cell, areas with an applied voltage appear dark in contrast to areas without an applied voltage that appear silver. Removal of the voltage from any area causes relaxation of the material back to the twisted configuration and the display again appears silver.
In reflective microdisplays, the back substrate, the back polarizer, and the reflector are replaced with a silicon substrate and the other parameters of the cell are selected to coact with one another to provide optimum performance.
One variation of a liquid crystal microdisplay is called the 45° twist mode. This mode has a 45° twist angle, wherein the silicon substrate is provided with an alignment layer of 0°, while the top layer is provided with an alignment layer of about 45°. The advantages of this mode are its normally black mode, good contrast at achievable voltages, and relative ease of manufacturing. Unfortunately, this mode is relatively slow (10-20 milliseconds) and is very sensitive to cell gap distortions in the dark state. And separate liquid crystal cells are required to produce red, green, and blue colors. This tends to distort the overall appearance of a color image. The additional panels increase the cost of the display and the circuitry for modulating the panels is quite complex. This type of cell has a simulated contrast of about 270:1. Here, and elsewhere in this document, contrast ratio is defined in a telecentric optical system with light rays impinging on the display over a cone of angles ranging up to +/−22.5 degrees, in other words an F#/1.0 telecentric optical system. In addition, such a system uses full-spectrum visible illumination. In other words, white light, or light with wavelengths ranging from 400 nanometers to 700 nanometers.
Another type of display is referred to as a reflective twisted nematic which uses a negative 52° twist and only a single polarizer as opposed to crossed polarizers. This mode operates much the same as the 45° twist mode and so its operating characteristics are nearly the same as the previous mode. Both of these modes have a cell gap of about 2.6 microns.
One type of mode which has shown some promise is the Pi liquid crystal mode. Such a cell has a cell gap of about 1.5 microns and utilizes a polarizer with a 45° angle. The alignment on the liquid crystal on the top of the glass and bottom substrate is 0° and a compensation film with an effective retardance thickness of 35 nanometers and a 90° angle is utilized. The cell utilizes a critical voltage, above which the liquid crystal material is in a bending alignment state and below which the splay mode is more stable. Accordingly, a bias voltage is always required to keep the cell in a bending mode and the critical voltage for the cells is specified as being above 1.32 volts. A polarization conversion efficiency of about 98% can be obtained with this mode. Simulated and measured speeds of the turn-on times for such a cell is 0.6 milliseconds, whereas turn-off times are about 2.7 milliseconds, both of which are relatively good. Unfortunately, the viewing angle and contrast ratio achievable with this mode are limited. The simulated contrast is about 400:1 and it is below 100-to-1 outside the incidence angle of 10°. To increase the view angle, another compensation film called a wide view film must be applied.
Yet another mode is the electrically controlled birefringence mode. The cell construction of such a cell is similar to the Pi cell, the only difference being that the alignment direction on one substrate is 180°. A 1.5 micron cell gap is utilized, along with a 45° polarizer. Also a compensation film with an effective retardance thickness of 35 nanometers at 90° is utilized. The reflectance of this cell is very similar to a Pi cell and the light and dark state voltages are a little bit lower in the electrically controlled birefringence mode than those in the Pi cell. Driving voltages are between 1.89 volts and 6.03 volts. The measured contrast for such a cell is 384:1.
A thin, reflective mode using a 45° twist is known to be used on reflective substrates with a 0.2 millisecond switch to black time, but the achievable contrast is limited to 200:1 as measured photoptically from 400-700 nanometers. Other reflective modes involve vertically aligned nematic liquid crystals with 16 millisecond switch times. However, none of these modes can now provide optimal optical properties characterized in the critical areas of contrast, re
Jepsen Mary Lou
Shi Hongqin
Lee Eugene
Microdisplay Corporation
Thomas Tom
LandOfFree
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