Liquid Crystal Meets Silicon in Projection Displays

Various types of high-resolution digital displays, including front-projection active-matrix LCDs, are rapidly replacing rear-projection televisions based on CRTs. However, relatively high cost still prohibits many consumers from accessing this technology, which must meet high standards with respect to size, weight, spatial resolution and response speed.

Liquid crystal on silicon (LCOS), a relatively new microdisplay technology (Fig. 1), is proving promising for large-scale applications such as high-definition televisions (HDTVs), and home theater and cinema systems. LCOS delivers superior resolution, speed and quality advantages over its closest competitors — transmissive LCD, digital micromirror devices (DMDs) and CRTs.

Most importantly, it is lower in cost than competing digital technologies. The active substrate, which integrates drivers and other functions on the chip, is manufactured using existing semiconductor lines. Whole silicon wafers are mated with glass substrates and then diced into individual display chips, using standard semiconductor manufacturing equipment.

Using silicon as the active-matrix substrate also allows for significant integration. The LCOS platform is a completely digital solution with fully integrated row and column drivers. Digital signals are converted internally, essentially at the pixel, into analog driving voltages to eliminate any artifacts common to bit sequential techniques used to obtain individual gray levels.

1. State-of-the art LCOS microdisplay technology enables a 44-in. high-resolution rear-projection television image with million-pixel resolution and high contrast (1000:1).

In today's LCDs — used in cellular phones, notebook PCs, watches and calculators — the liquid crystal and electrodes are sandwiched between polarized glass plates. In contrast, LCOS aluminum pixels are patterned on a silicon active substrate. The liquid crystal is sandwiched between this Si substrate and a glass plate (Fig. 2).

Because LCOS technology utilizes single-crystal silicon rather than polysilicon thin-film transistors (TFTs), LCOS-based systems benefit from the high-speed switching capability of single-crystal silicon. Furthermore, the reflective makeup of the LCOS system enables thin cell-gap liquid-crystal modes, allowing very rapid light modulation (switching), which improves the eye's ability to perceive a smooth, unified, color image, without flicker.

2. Unlike LCDs, the LCOS technology incorporates driver circuits into the silicon substrate.

LCOS uses some dedicated components, such as a shield, to prevent incident light from reaching the transistors and deteriorating their properties. The active area is covered by a passivation layer, which is planarized using CMP. Vias are made in this passivation layer to allow contact from the transistors to the pixel electrodes. These reflective aluminum electrodes are deposited and patterned on top of the passivation layer. Typical dimensions are 10-20 square pixels and 0.7-1.0 µm interpixel gaps.

In this way, all circuitry is hidden under the pixel electrodes and >80% of the active area can be used for displaying information. This would not be possible with transmissive displays of this size and resolution, where rows, columns and transistors would take up a considerable part of the active area (<50% usable display area).

An LCD modulates the intensity of the transmitted or reflected light by changing the polarization of the light passing through the LC layer. The effect of the LCD on the polarization depends on the LC effect used, i.e. twist angle, angle of the incident polarization, retardation ("optical thickness") and the use of additional retarder foils. Moreover, the polarization change depends on the voltage applied to the LC layer, creating white, black and intermediate gray states (Fig. 4).

4. The principle of black and white switching in reflective LCOS displays. The red arrows indicate the direction of the light, while the black arrows show the polarization direction.

In case a normally white effect is used, the LC effect is designed to rotate the polarization 90° after passing the LC layer, reflecting and passing the LCD layer in the opposite direction. This light is now transmitted by the PBS and then projected on the screen, thereby obtaining a white state.

When the maximum voltage is applied to the LC layer, the polarization of light is not affected. As a consequence, the light is reflected by the PBS and does not reach the screen (black state). At intermediate voltages, gray levels are obtained, as the polarization is partly rotated and elliptically polarized. The light not projected on the screen is absorbed or recycled in the optical system.

A full-color image can be obtained in two ways. A three-panel configuration uses one display for each of the primary colors and spatial separation of colors in the optical system. Alternatively, a single-panel configuration uses one display for red, green and blue; colors are incident on the display sequentially.

Many LCOS panel manufacturers employ a three-panel design. Key challenges include:

• Cost: The three-panel system requires three times the amount of materials and components, including three imagers and recombination optics.
• Image convergence: The panels must be overlaid precisely, which requires a full complement of recombination elements, including multiple polarizers.
• Resolution issues: Producing three separate images, combining them, and projecting them onto a screen creates a perceptible loss in picture resolution.

The culmination of seven years of intensive research by Philips Research (Briarcliff Manor, N.Y.) resulted in a single-panel LCOS display called engaze. The LCOS display consists of a thin layer of LC material between two substrates. The substrates are coated with a thin polyimide alignment layer, which is rubbed in a specific direction that defines the surface alignment of the LC.

Philips' engaze single-panel LCOS approach also resolves some of the problems that have arisen with other manufacturers' attempts to produce a single-panel LCOS solution. One challenge involves achieving simultaneous projection and perception of the red/green/blue color fields using a single panel.

In the field-sequential color systems used by some panel makers, the imager produces a red, green or blue image individually and sequentially. This approach allows only a third of the available light to be transmitted to the viewer at a time, because when one color is on the panel the others are, essentially, discarded. Second, if the images are not scanned fast enough, the viewer will perceive the individual color fields, called color breakup. Instead of perceiving a single, unified full-color image, the human eyeball moves or vibrates to see each color image.

The engaze technology uses a technique that scans the three colors across the panel simultaneously so that they illuminate different areas on the panel at the same time. In addition, because none of the colors are ever blocked from the panel, the system can theoretically achieve brightness that is on par with a three-panel system.

With this single-panel LCOS approach, the degree of resolution that can be achieved is virtually limitless. The company has designed an LCOS panel with pixels as small as 10 µm, a resolution that common projection lens technology cannot resolve, and that is far below the resolution of near-eye optics.

The latest implementation of LCOS technology, Philips' DD720 single-panel microdisplay, is a 1.18 in. diagonal active panel for next-generation large-screen television. It features unprecedented contrast capabilities (1000:1), high refresh rate (180 Hz), high resolution (1280 × 768) and significant cost benefits.

This latest solution demonstrates the manufacturability of the LCOS technology. As a result, consumer electronics manufacturers will be able to offer high-resolution TVs, with extremely thin form factors, at compelling prices.