1983–1998: Spatial Light Modulators

As passive-matrix and thin-film-transistor addressing of liquid crystal displays became widespread, interest in LCOS devices shifted from displays to spatial light modulators (SLMs). In an interesting twist on their "liquid crystal light valve" Hughes Research Laboratories reported in 1983 a silicon-based SLM that was addressed by a charge-coupled device (CCD) (Efron, J. Appl. Phys. vol. 57). Unlike the structure of an LCOS device, the Hughes SLM had the liquid crystal on the back side of the silicon, with the electrically-addressed CCD on the front.

In 1988, the first LCOS SLM, a 50 × 50 pixel using a 45°-twist nematic liquid crystal was reported by a group at the University of Edinburgh (McKnight Appl. Opt. vol. 28). Figure 9 shows (a) the schematic of the 8-transistor (8-T) XNOR pixel that provided AC drive to the liquid crystal, (b) an SEM of the 44 μm × 45 μm mirror electrode within the 74-μm-pitch pixel, as fabricated in a 1.5 μm nMOS process implemented at the University of Edinburgh, and (c) a pattern displayed on the SLM.

In 1989 Displaytech (Cotter, Opt. Lett. Vol. 15) reported a 64 × 64 FLC SLM implemented in a 3 μm CMOS process available through MOSIS. Figure 10 shows an early SLM with a detail of a pattern on the 60-μm-pitch pixel array. This SLM marked the first LCOS device utilizing foundry silicon as well as the first use of ferroelectric liquid crystals. The proliferation in the early 1990's of foundries providing chemical-mechanical planarization (CMP) solved the problem of SLM pixel mirror roughness caused by the topography of underlying circuitry. This enabled Displaytech to demonstrate SLMs with 15 μm, 7.5 μm (Figure 11), and 5.7 μm pixel pitches (Banas, Proc. SPIE 2650).

In addition to light modulators, the LCOS SLM methodology also offers simple integration of light detectors. The second LCOS SLM designed at Displaytech, in 1990, incorporated a small array of optically addressed pixels (Drabik, Appl. Opt. vol. 29). Separation of the write-light from the read-light can be effected by the addition of a pixel-matched lenslet array, as illustrated in Figure 12(a) (Chase, Opt. Lett. Vol. 20), which also shows how the apparent modulator fill-factor can be increased to 100% (c) when the SLM+lenslet array is viewed on the modulator axis.

Next Page: 1992–present: Microdisplays »