FLCs Introduced for Miniature Displays

Displaytech (Longmont, Colo.) recently introduced its ferroelectric liquid crystal (FLC) technology for projection displays. Most flat panel displays are made by fabricating thin film transistors (TFTs) on a piece of glass that is the size of the display. The FLC displays are made by sandwiching a layer of ferroelectric material between a regular CMOS chip and a piece of cover glass (Fig. 1).

   One advantage of this design is that an actual silicon device is used to drive the display, rather than polysilicon circuitry that is deposited onto the display. The backplane can be made using well-understood 0.8 µm CMOS technology. In addition to making it more reliable, it also uses standard CMOS voltages. Driving it is just like driving a write-only memory with 0 V and 5 V. The cover glass is coated with a transparent electrode that is held at 2.5 V. The field applied to the FLC layer is ±2.5 V/µm.

Fig. 1. The LightCaster display has a 1 µm thick layer of ferroelectric material sandwiched between the CMOS backplane and the cover glass.

Fig. 2. The reflective display is illuminated by three different colors, eliminating the need for subpixels in the display.

The display works as a switchable quarter-wave plate. The FLC material reflects polarized light in one of two optical states. In one state, the material reflects the incoming light with no change in the polarization. In the other state, the light is re-flected with a 90° rotation in its polarization. External polarizing optics are used to change the reflected light into light or dark pixels (Fig. 2).

The display is illuminated using light emitting diodes (LEDs) of three different colors, rather than a white light source. The LEDs are lit in succession, and they are synchronized to the display drivers. The image is actually a succession of red, green and blue images that time average into a full image. The switching speed of the FLC material makes this possible.

Since each LED is on only when needed, no color filters are re-quired, and power efficiency is minimized. The whitepoint of the display can be adjusted over a wide range by adjusting the relative brightnesses of the LEDs. Greyscale images are accomplished using pulse width modulation; pixels are held in the on state for a fraction of the cycle that corresponds to the greyscale value.

Every pixel in the display produces all the primary colors. The subpixels, or color triads, required in typical LCD displays are not needed. This can produce a sharper image, because the colors are not separated within a pixel (Fig. 3).

FLC/VLSI		AMLCD
Fig. 3. Time sharing on one pixel produces a sharper line than having subpixels.

The absence of subpixels allows for a high fill factor without the use of a black matrix. The displays are small; a 640 x 480 pixel VGA display has a 10.4 mm diagonal. The spaces between pixels are on the order of the wavelength of visible light. Light going between the pixel mirrors gets refracted at large angles, and very little of it becomes part of the image.

The small size of the display is also considered an advantage from a yield and cost perspective. A smaller die leads to a smaller probability of having a killer defect as well as more die per wafer.

The FLC display can be used in magnified displays, such as cellular phones, games, personal TVs or head-mounted displays. They can also be used for projection displays, such as projectors and large screen TVs. Its use in an application is determined by lighting and optics, rather than by the display element itself. VGA format FLC displays are now available. XGA and SXGA displays will be available later this year.