Design and fabrication of ripple-free CMOS-compatible stacked membranes for airgap optical filters for UV-visible spectrum

CMOS-compatible fabrication of thin dielectric membranes for the ultraviolet and visible spectrum is presented for use in airgap/SiO2 -based interference filter design. A typical optical design consists of multiple membranes of 50 - 100 nm thickness. Maintaining flatness over a large area, as required by the optical application, is challenging. In such a free-standing membrane, the residual stress is the main force acting on the structure. Although an overall tensile residual stress can effectively stretch the membrane, too much stress would exceed the yield strength of the material and results in fracturing. Furthermore, the presence of a residual stress gradient causes the membrane to deform. In this work, the effect of a stress profile in the thin film has is investigated. Although PECVD SiO2 layers with an average tensile stress level of 178 MPa are used for the fabrication of the membranes, the presence of a stress gradient of about 0.67 MPa/nm results in a deformation in the membrane. A simple straining method is applied to reduce flatness. The preliminary results and discusses the challenges in the fabrication of stacked membranes for optical filters are presented.