Seeking a better view

What do the first HDTV 50-in. screen television, a 25-in. rear-projection SXGA computer monitor, and forthcoming mobile Internet personal display units have in common? The answer is liquid-crystal-on-silicon (LCoS) microdisplays, an emerging technology that promises high performance at low cost.

"We call it liquid crystal on silicon because a liquid-crystal layer sits on top of a silicon chip, which serves as an active matrix drive circuit," says seminal LCoS researcher Robert Melcher, CTO at Three Five Systems, Inc. (Tempe, AZ). Typically 19 mm on a diagonal with 7- to 20-µm pixels, the devices operate in reflective mode and have resolutions ranging from 320 X 240 (QVGA) to 2048 X 2048.

LCoS applications are divided into two areas: projection (front and rear) and near-eye personal display systems. "LCoS technology offers a new alternative that in the long term could allow for much lower display prices and the integration of more complex electronic functionality right into the underlying silicon," says Chris Chinnock from Insight Media (Norwalk, CT). "In addition, the number of products that could benefit from LCoS technology is quite impressive."

Melcher agrees. "I believe LCoS will eventually take over much of the large-screen display market, especially in the television industry. In terms of resolution, brightness, and crispness, there is a noticeable difference," he says. "In most measures, LCoS is simply better, and several TV manufacturers are coming to that conclusion as well."

The optical system required to make an LCoS display is fairly complex. Using complementary-metal-oxide-semiconductor (CMOS) technology, manufacturers fabricate the active matrix onto a silicon chip. According to Melcher, it is designed very much like a dynamic random access memory or memory chip except that at each memory site (pixel) there is a 12-µm aluminum-coated electrode that also acts as a static optical mirror (see figure 1). The active matrix circuit applies a voltage between the pixel electrode and an optically transparent common electrode that is separated from the pixel electrodes by a thin layer of liquid crystal. To reach the aluminum mirror of the pixel electrode, light must first pass through the liquid-crystal layer, which, depending on the applied voltage, imposes a polarization on the beam. Once the light reflects from the mirror and passes back through the liquid crystal, it propagates through to a polarization beamsplitter to yield a bright, dark, or gray-scale pixel.

Figure 1. In an LCoS switch, liquid crystal rotates the polarization of input light by a varying degree to control whether all or part of the incident light at each pixel passes through a polarization beamsplitter to reach the imaging plane.

To form an image, the chip is illuminated with linearly polarized white light from an arc lamp. The active matrix circuitry drives the transistor at each mirror/pixel site to apply a specific voltage to the liquid-crystal material above the pixel. The light passes through the liquid crystal, reflects off of the mirrors, then passes back through the liquid crystal and into a polarizing beamsplitter designed to pass linearly polarized light of a specific orientation, for example, oriented to the y-axis. When the light passes through the liquid crystal, the material rotates the polarization of the incident light by an amount determined by the applied voltage. Light linearly polarized at an angle can be considered as a combination of a certain percent of x-polarized light and a certain percent of y-polarized light. The beamsplitter in this example passes only the y-component of the light.

By applying the appropriate voltage to the liquid crystal, one can rotate the polarization the amount necessary to achieve the desired gray scale in a pixel. "Depending on the brightness we want that pixel to have on the screen, this is a very precisely defined voltage in a range between zero and about 6 V," Melcher says. The polarization beamsplitter optically separates the incident light from the transmitted image, which is magnified for projection onto a screen or onto the retina of the viewer.

So far so good for a monochrome display, but most applications require color capabilities. "Producing a color image is a critical aspect of this system," says Melcher. Using a prism, the system separates the light into red, green, and blue beams that are each modulated by a separate, dedicated LCoS chip to create red, green, and blue images, respectively. These image components are then recombined into a full-color image by a projection objective.

A discussion of RGB displays brings up the question of using diode laser sources rather than white light. "People have built prototypes with lasers instead of arc lamps," Melcher acknowledges. "You get deeply saturated colors, and the collimated beam makes all the optical components work better. Lasers have enormous advantages at this level." So why not make the switch? "Three reasons," says Melcher. "The cost of the red laser, the cost of the green laser, and the cost of the blue laser." Until diode lasers become cost-competitive with $200 arc lamps, lasers are unlikely to find a home in commercial systems.

"When we started this work seven years ago, one of the big issues was the complexity of the optics," says Melcher. "Originally, it was believed this process would be unbelievably expensive and not at all practical for commercial use."

The fundamental optical problem is that the system must illuminate the device, then separate the reflected light that has passed through the liquid crystal from the incident light. A polarizing beamsplitter can perform the task well, but such components consist of fairly expensive glass with sophisticated coatings that made it expensive and difficult to get the properties required.

As the concept of LCoS caught on, however, companies put a lot of work into building the support infrastructure. "This brought the cost into a range that we now believe will enable competitive consumer products. In the beginning, the optics were considered extremely costly and difficult," Melcher says. "Now they are just considered difficult."

"Smallness is goodness in this business," says Melcher. "Silicon technology is amazing. The limitation on the resolution and the size of the microdisplay is not due to the silicon but because of other issues."

In the case of LCoS technology, the primary limiting factor is the illumination source. In an arc lamp, voltage applied across a pair of electrodes ignites and ionizes a gas, which emits light similar to a spotlight. If the electrodes are too far apart, the resultant beam cannot be efficiently focused down to a beam small enough for use with a microdisplay. "There are theorems on just how small you can focus the light from an arc lamp," says Melcher. "Below some microdisplay sizes, it becomes difficult to achieve efficient illumination with arc lamps." The efficiency and screen brightness provided by arc lamps are important properties, however. "The lamp industry understands this, and [companies] are working hard at making lamps that have a smaller gap between the two electrodes."

Three Five System's MD1280 SXGA microdisplay with a format of 1280 * 1024 pixels, is about as small as arc-lamp technology can go at this moment. "As the arc lamps get better, we will continue to make smaller microdisplays," says Melcher.

The next steps in development will be to continue to increase device performance--resolution, contrast ratio, reflectivity, and so on--by material and structural innovations, while reducing cost by achieving higher levels of integration and total system optimization. "Right now we achieve an on-screen contrast ratio in the range of 600:1. We plan to do better while making the display higher resolution and more highly reflective," says Melcher. Reflectivity is particularly important as the brightness of the image on the screen depends on how effectively the display reflects light.

Today, most computer monitors and televisions use cathode-ray tube technology. To compensate for a larger screen, the glass tube must become bigger and thicker to withstand the vacuum pressure. This makes the device much heavier. "A 25-in.-cathode-ray-tube computer monitor, for example, would be too big and too heavy for your desk, so it isn't even an option," says Melcher.

Figure 2. Microdisplay technology combined with a folded optical path yields a large-screen monitor significantly thinner than the CRT versions.

A 25-in. LCoS monitor, on the other hand, while it is not a flat panel, is much shallower than a traditional monitor. Melcher and colleagues accomplished this by folding the optical path a couple of times using mirrors to produce a unit 10 to 12 in. deep (see figure 2).

Insight Media's Chinnock expects LCoS technology will be a prominent player for digital cinema, viewfinders for digital cameras and camcorders, video headsets for DVD, wearable computers, laptop computers, and hybrid products that combine Internet, wireless phone, imaging, and computing capabilities.

"LCoS has the capacity to be a disruptive technology because it could help enable many new unfore- seen products," he says. "Among them could be a host of new convergence products that blend wireless connectivity, web access, cell phones, GPS, PDA, digital imaging, etc., into hybrid products. Eventually there will be the desire to have high-resolution screens, which is where microdisplays come in. The big unknown is if the implementations of the viewers can be improved so that they are user friendly, lightweight, and unobtrusive."

One of the major benefits of LCoS-based products will be the cost. "They will likely be cost competitive to begin with," says Chinnock, "but if and when volumes increase and prices decline, they could become much cheaper." In addition, there is the ability to add more functionality into the display by integrating it with the silicon. "And this adds another interesting element."