Study of assembly processes for liquid crystal on silicon (LCoS) microdisplays

This paper involves the development of two critical assembly processes for liquid crystal on silicon (LCoS) microdisplays. The processes investigated include the lamination of cover glass on silicon and a LC filling process. For the lamination process, a tool and process were designed and developed with the objective to attach the cover glass on a silicon die and achieve a cell gap of 3-μm between them. The gap was achieved and verified with various measurement techniques. The lamination process involved the deposition of an adhesive ring and subsequently squeezing the two parallel plates until the desired cell gap was achieved. An analytical model was developed to estimate the time required to achieve a 3-μm cell gap by applying different pressures on the cover glass. The analytical model proved to be a guideline in developing the lamination process. The second process in the assembly is filling the cavity between the cover glass and silicon with LC material. For LC filling, a tool was designed and process was developed. The process works on the principle of flow of fluid in a capillary. The flow rate was enhanced by applying a vacuum to the cavity. An analytical model was developed to estimate the time needed to fill the 3-μm wide cavity. Fill time is a function of gap width between glass and silicon, pressure applied within the cavity, cavity length and the viscosity of the LC material.