Liquid Crystal On Silicon (LCOS). David Vettese.
nature photonics.| VOL 4 | NOVEMBER 2010 | www.nature.com/naturephotonics, page 752
Microdisplays based on liquid-crystal-on-silicon technology may soon gain wider recognition as they penetrate an increasing number of markets, ranging from electronic viewfinders to miniature data projectors and head-up displays.
When it comes to display technologies, the concepts of both liquid-crystal displays (LCDs) and cathode ray tubes (CRTs) are widely known. The same cannot be said of liquid crystal on silicon (LCOS), even though this technology is used extensively in many industries that require high quality microdisplays.
Most LCOS devices comprise a layer of liquid crystal sandwiched between a top sheet of glass coated with a transparent electrode, and a pixelated silicon substrate (or backplane) made by the complementary metal–oxide–semiconductor (CMOS) fabrication process. Because the silicon is reflective, it serves as a mirror element for each pixel, with the strength of the reflection electronically controlled by the amount of light transmitted through the liquid crystal above. Separate channels of red, green and blue light illuminate the LCOS chip in turn, with electronic drive signals controlling the transmission state of each pixel to form an active-matrix colour image. In some cases, a dedicated LCOS chip is used for each colour channel.
This is a very simple and elegant design concept: the backplane is formed from a standard CMOS processed wafer and the glass substrate has no patterning. Indeed, the CMOS industry benefits from the heavy investments made by other industries, which allows for increases in pixel density (and, thus, higher resolution) without the need for LCOS manufacturers’ investment.