Surface micromachined optical low-cost all-air-gap filters based on stress-optimized Si₃N₄layers

A new surface micromachining approach based on a multiple Si 3 N 4 - and silicon-layer stack is presented. The fabrication process is implemented by plasma-enhanced chemical vapour deposition of stress-optimized films, reactive ion etching using SF 6 /CHF 3 /Ar, wet chemical etching of the sacrificial silicon layers by KOH and critical point drying. Using this approach, the fabrication of an optical all-air-gap vertical-cavity Fabry–Pérot filter is demonstrated. The surface micromachined filter consists of two DBR mirrors, each having five 590 nm thick Si 3 N 4 membranes separated by 390 nm wide air gaps. The distance between the mirrors (cavity) is 710 nm. The optical characterization and a white light interferometer measurement document the accuracy of the layer positioning and the performance of this low-cost approach. The filter shows the designed filter dip at 1490 nm, the full width at half maximum (FWHM) of the filter is 1.5 nm and the insertion loss is just 1.3 dB. The process is compatible with a variety of materials, e.g. III–V compounds, silicon, as well as organic materials, facilitating a huge application spectrum for sensors.