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

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 (SiO 2 ) as a cavity layer and dielectric mirrors of multilayered magnesium fluoride (MgF 2 ) 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.