Fabrication of CMOS-compatible optical filter arrays using gray-scale lithography

Abstract

An integrated optical filter array is demonstrated using simple gray-scale lithography and a subsequent reactive ion etching process. Gray-scale lithography allows three-dimensional structure patterning to form controllable cavity thickness in a Fabry–Perot resonance structure. This approach avoids repeated photolithography and etching processes in conventional filter array fabrications. The filter array is formed by single gray-scale lithography and does not require a repeated alignment process of each filter. The demonstrated filter array is fabricated with silicon dioxide (SiO2) as a cavity layer and dielectric mirrors of multilayered magnesium fluoride (MgF2) and zinc selenide (ZnSe). The smallest demonstrated filter size is 10 µm which can be fitted into the size of current CMOS-based photodetectors. However, its ultimate size will be determined by the minimum resolution of gray-scale lithography. This will allow an optical filter array with high resolution and small size which can be directly integrated onto a detector array or CCD for miniaturized spectrometers.