On-chip metallization layers for reflective light valves
by E. G. Colgan, M. Uda
The metallization layers required for a silicon-based reflective liquid crystal light valve have been developed and integrated with a medium-voltage CMOS process using standard microelectronic manufacturing tools. Unique requirements include the following: 1) shielding the Si devices from incident light so that electron-hole pairs are not formed; 2) high optical throughput and contrast, which are dependent on the mirror fill factor, reflectivity, and flatness; 3) pixel storage capacitance to maintain the voltage across the liquid crystal cell with sufficient accuracy to select the desired gray level until the data are updated; and 4) precise control of the liquid crystal cell thickness without spacers obscuring the mirrors. Wafers have been successfully fabricated to support a technology demonstration of a 2048 × 2048-pixel ("four-million-pixel") projection display. The process is based on a medium-voltage CMOS process and uses six masks (for three metal levels, one via level, and two insulator levels) after Si device processing has been completed.