Microdisplays are a fairly recent outgrowth of the flat panel and liquid crystal display (LCD) industries. As their name suggests, microdisplays are small LCDs, typically with a diagonal measurement of one inch or less. Microdisplays can be either transmissive or reflective. In transmissive microdisplays, a layer of liquid crystal is sandwiched between two glass plates and incident light is transmitted through the display. Thin film transistors (TFTs) are built into one of the glass plates, similar to the method used for large flat panel displays. There are two categories of reflective microdisplays: microelectromechanical systems (MEMS) and LCoS.LCoS microdisplays consist of a liquid crystal sandwiched between a silicon chip and a glass plate. The silicon chip is manufactured using standard complementary metal oxide semiconductor (CMOS) technology, which provides a much higher degree of circuit integration than TFT microdisplays. Incident light passes through the glass and liquid crystal, and is then reflected off an array of micro-mirrors that are fabricated in the top metal layer of the silicon. The reflected image is then processed through an optical system and projected.
Packaging of liquid crystal on silicon microdisplays
For those accustomed to packaging chips for use on printed circuit boards (PCBs), microdisplay technology is something new. Liquid crystal on silicon (LCoS) microdisplay imagers represent one of the newest technologies in the field of liquid crystal displays, and the manufacturing process combines both familiar and novel packaging concepts. Manufacturing of an LCoS device begins with standard semiconductor processing, but the path to a finished imager requires innovative thinking from the packaging industry. The key difference is that the liquid crystal material cannot withstand the elevated temperatures common to semiconductor packaging. Standard processing temperatures used during die attach and encapsulation would ruin the device, so alternatives are needed.LCoS microdisplays are still a fairly new technology, but the technology has matured to the point where commercial production is a reality. Technical issues have been resolved through the appropriate choice of materials, process flow and processing conditions. Exact manufacturing specifications and qualification requirements are still evolving, and it will be interesting to see what happens in LCoS microdisplay technology over the next few years.