Silicon microlenses for IR image sensors

We report a comprehensive approach to design, fabricate and characterize silicon microlenses for applications in IR image sensors. ZEMAX design tool was used to assist in the design of the lenses. Conventional photolithography and thermal reflow of melting resist technique were deployed to achieve spherical resists patterns. Reactive ion etching (RIE) was utilized for transfer of resist patterns to the substrate in O₂ and C₂F₆ environment. The shape of the fabricated lenses, i.e. radii of curvature and focal length were controlled within a wide range, by controlling the etch rates of resist and substrate, which were further controlled by varying the flow of gases, selectivity and power. Optical methods such as ray analysis and profilometry were used to obtain lens properties. Scanning electron microscopy was performed to analyze the morphology of the fabricated lenses.