Tunable infrared filter based on elastic polymer springs

Abstract

This paper describes the design, fabrication and testing of tunable Fabry-Perot filters. The goal of this research is to develop novel tunable filter with an area of 5x5 mm² that will be used in infrared gas sensors. This exploits the fact that most gases have unique infrared absorption signatures in the 2-14 µm wavelength region. The filter consists of two thin silicon wafers coated with quarter wave dielectric layers to form wavelength dependent high reflection mirrors and separated by air gaps with an average height of 8, 5.1 and 3.5 μm. The mirrors are supported by four elastic polymer posts (springs) each with an area of 100×100 μm² made by using photo definable polydimethylsilxane (PDMS). An electrostatic voltage is used to compress the springs, change the airgap height and hence shift the transmission peaks to a shorter wavelength. A finesse of 12 with full width at half maximum (FWHM) of 70 nm, and a peak transmission of 63% were achieved by applying 100 volts on a device with 8 µm post height and wafer thickness of 125 µm. In addition, the measured tunability before and after hard baking of the device was 210 nm and 130 nm respectively. The tunability stayed constant after hard baking the devices and did not show any changes with time. The tunability was also measured on a thinner silicon mirror with 3.5 µm post height. In this case, the filter was tuned 180 nm by applying 10 volts. However, the filter finesse was 3, transmission peak was 40% and FWHM over 200 nm. An antireflection coating was deposited on one side of silicon wafers and a Fabry-Perot filter to study transmission enhancement and satisfactory results were achieved.