A post-processing sequence for liquid-crystal-on-silicon microdisplays

The backplanes of Liquid-Crystal-on-Silicon microdisplays are derived from a VLSI silicon chip that includes the active matrix as well as row and column drivers. One way to convert this silicon chip into a functional backplane is to planarize the silicon chip, then etch vias through the planarization layer and finally to pattern an array of flat, highly reflective electrodes, each of which is electrically connected to a corresponding cell of the active matrix underneath. Such a post-processing sequence can be carried out in different ways, using either Chemical-Mechanical Polishing or spin-on planarization. We have chosen spin-on planarization with Dow Chemical's Cyclotene(TM) resin followed by reactive ion etching of the vias. Finally, electrodes are patterned by aluminum sputtering and lift-off. This step also establishes the electrical connection to the underlying metalization. To demonstrate this sequence we have fabricated a two-level passive silicon backplane with aluminum stripe electrodes. We describe in detail the processing steps involved and report on the achieved degree of planarization, polymer and aluminum roughness.