Semi-transparent CMOS backplane for advanced near-to-eye microdisplays

Emissive OLED-on-silicon microdisplays have been considered being opaque only so far. However, modern and advanced silicon CMOS process nodes are increasingly made on silicon-on-insulator (SOI) substrates. By separating the SOI handle wafer from the buried oxide (BOX) layer (that has the active silicon on top) and applying space-cautious layout design of the CMOS active devices as well as wiring layers it is possible to achieve semitransparent, high-resolution CMOS backplanes for microdisplays. Similar to regular OLED-on-silicon the emissive frontplane becomes embedded by wafer-level OLED post-processing. Yet, depending on pixel density and array layout a microdisplay transparency of >20% can be achieved now. Consequently, the semi-transparent microdisplay becomes the optical combiner itself, eliminating the exit pupil expander (EPE), which drastically improves the optical efficiency from the light source into the eye box.