Powing Cheng

P-151: Color Filter Liquid-Crystal-on-Silicon Microdisplays

a color liquid-crystal-on-silicon microdisplay that integrates color filters on silicon for colors. Fabrication parameters, color purity and light stability of this breed of microdisplays were characterized. A single-panel WXGA color projector based on this color filter microdisplay technology was demonstrated. 1. Introductionn-silicon (LCOS) microdisplay is an advanced display that integrates silicon very large-scale integration (VLSI) circuits with liquid crystal displays (LCD). The LCOS microdisplay has very high resolution and yet maintains a large aperture ratio or optical efficiency. Ancillary VLSI circuits such as display drivers or digital signal processors can also be integrated into the display for the goal of system on a chip. The LCOS microdisplay is monochrome and requires a color technique to produce colors. Conventional three-panel designs, which use three panels for three primary colors, are expensive and very difficult in manufacturing (1, 2). Another method that uses time sequential color has limitations in bandwidth and response time of the display (3, 4). In short, the LCOS microdisplay is promising, but lacks of a simple color method to make it prosperous. In this paper, we present a simple color method that integrates color filters on LCOS microdisplays. Color filters are used in LCD for colors. The color filters are coated on a separate glass plate and aligned to pixels on a thin-film-transistor (TFT) plate. These color dots have a typical size of 100µm by 300µm and a thickness of 1.5µm. The alignment accuracy of these two plates is typically 10µm. To apply this color filter technology to LCOS microdisplays, several modifications have to be done. Firstly, the size of color dots shall be reduced to less than 10µm for matching fine pixels on silicon. Secondly, the thickness of color filters shall be reduced from 1.5 to 0.75µm, since the light transverse through the color filters twice in reflective LCOS microdisplays. Thirdly, these micro color filters shall be coated directly on silicon to achieve sub-micron alignment accuracy. Finally and most importantly, a very flat surface of this micro-color-filter array should be obtained for good liquid crystal alignment.

Trichroic prism assembly for separating and recombining colors in a compact projection display

A trichroic prism assembly design, believed to be new, is proposed and demonstrated. This new design has the advantages of low s- and p-polarization dependence in the reflectance spectra of the optical coatings. Hence it can be used for both color separation and color recombination with polarization change. This new trichroic prism assembly is especially useful in a compact color projector employing reflective liquid-crystal light valves.

Processes, characterizations, and system applications of color-filter liquid-crystal-on-silicon microdisplays

— A color-filter liquid-crystal-on-silicon (CF-LCOS) microdisplay that integrates color filters on silicon for color will be presented. The color-filter process on silicon was optimized to achieve fine resolution and precise alignment of the color filters on the pixel array, good adhesion to the silicon suface, and a flat surface for the liquid-crystal cell assembly. Important optical and electrical parameters of the color filters were extracted to establish an electro-optical model of the CF-LCOS microdisplays for device simulation. Thermal, chemical, and light-stability characterizations were performed to ensure the stabilty of the color filters and CF-LCOS microdisplays. With color CF-LCOS microdisplays already available, the projection or viewing optics is greatly simplified. This CF-LCOS microdisplay is ideal for near-to-eye displays because of its low-power consumption and compactness. The CF-LCOS microdisplay could also withstand medium light illumination for medium-sized projectors. A single-panel projector based on one CF-LCOS microdisplay of 1280 × 768 × RGB resolution was demonstrated.

Characterization and minimization of flicker in silicon light valves

We have performed systematic characterizations of flicker in silicon light valves. It was found that there were four conduction mechanisms accounting for the flicker. These four mechanisms were residual dc charge on the silicon surface, voltage holding capability of the liquid crystal cell, voltage holding capability of the silicon panel, and parasitic capacitor coupling of the pixel. Major causes of these four mechanisms were identified. Solutions of flicker minimization were obtained for each mechanism. Among these solutions, offset of common voltage was found very useful to compensate for residual dc charge and parasitic capacitor coupling. Frame rate multiplication was found very useful for the minimization of flicker due to low voltage holding capabilities of the liquid crystal cell and silicon panel.

Integrated digital input driver for active matrix liquid-crystal-on-silicon display

A digital input driver was designed and integrated with an active matrix nematic liquid-crystal-on-silicon (LCOS) display. Fabricated by conventional 2-/spl mu/m CMOS technology, the driver in the periphery of the active matrix can easily incorporate versatile electronic functions into the silicon backplane. This integration makes the LCOS display easier to be interfaced with different image sources of various formats and, hence, leads to a versatile display system.

Empirical model of flicker in silicon light valves

We have performed systematic characterizations of flicker in silicon light valves. It was found that temperature was the most important factor associated with flicker. Temperature could assist conduction mechanisms in silicon light valves and enhance the flicker accordingly. The major conduction mechanisms of flicker in silicon light valves were residual DC charge on the silicon surface, the voltage holding capability of a liquid crystal cell, the voltage holding capability of a silicon panel, light leakage and parasitic capacitor coupling. Major causes of these flicker mechanisms were identified through systematic characterizations. An empirical model of flicker was proposed with quantitative experimental data and theories. Among these conduction mechanisms, temperature could boost the voltage holding capabilities of a silicon panel to become the most dominant cause of flicker when the temperature was above 40°C. At temperatures below 30°C, the other four conduction mechanisms contributed to the flicker in different ways. Minimization of flicker could be achieved through materials and device optimization.

Generalized mixed mode reflective liquid crystal displays for three‐panel color projection applications

The general parameter space was introduced for generalized mixed mode reflective liquid crystal displays (RLCDs). A contrast ratio parameter space was also derived to provide a better picture of both the voltage-off and voltage-on states. A guideline for the optimization of RLCD modes in terms of high light efficiency and high contrast at low operating voltage is given. Such features are important for reflective displays, particularly for crystalline silicon based RLCDs. As a result, we find a number of new RLCD modes with high contrast and high brightness, which are quite suitable for the three-panel color projection system.

18.1: On the Minimization of Flicker and Image Retention in Silicon Light Valves

The Hong Kong University of Science and Technology, Clear Water Bay, Hong Kong We have performed a systematic characterization of flicker and image retention in silicon light valves. It was found that there were four mechanisms accounting for the flicker and image retention. These four mechanisms were residual DC charge on silicon surface, voltage holding ratio of the LC cell, voltage holding ratio of the silicon panel, and parasitic capacitor coupling. Causes of these mechanisms were identified and solutions for the minimization of flicker were found for each mechanism. Among these solutions, the frame rate multiplication was the most useful for all these four flicker mechanisms.

A compact and high-resolution video projector based on silicon light valves

We have developed a highly integrated liquid-crystal-on-silicon light valve for three-panel color projector. The silicon panel was designed and fabricated by a custom 0.5 /spl mu/m, 3-metal CMOS technology with a spatial resolution of 1024/spl times/768 pixels. The pixel pitch was 13.8 /spl mu/m and the fill factor was 91%. Six-bit digital data drivers and gamma-correction circuitry were integrated within the silicon panel for true gray scale and full color representation. The display panel was assembled with a mixed twisted nematic and birefringence liquid crystal cell for high contract ratio at CMOS compatible voltage. The contrast ratio was 100:1 at 3.5 Vrms. The optical system utilized a trichroic prism assembly for both the color separation and recombination. With this trichroic prism assembly incorporating three silicon light valves, a compact and high-resolution video projector was demonstrated.

Frame sequential miniature silicon display using mixed-mode twisted nematic liquid crystal

We present a mixed-mode twisted nematic (MTN) silicon display integrated with 4-bit digital data drivers. With high bandwidth of the digital data driver, pixel access time of less than 10 ns was achieved. Digital gray-scale addressing technique, which utilizes multiple fields per frame, synchronous field voltages and weighted field time, was applied to increase gray scale from 4 to 8 bits. Chromatic characterization of the display using 3-color-in-1 LED as light source was performed. Contrast ratios on pixel array were 49, 32 and 21, respectively, for red, green and blue colors at 3 V root-mean-squared voltage. It was observed that frame inversion gave rise to higher contrast ratio, while column inversion was less color dispersive. Using color sequential technique, we have demonstrated 4 bits per color for this highly integrated MTN display.