Micromachining porous silicon thin films for thermal sensing applications

This work reports a novel CMOS-compatible micromachining process to fabricate large porous silicon membranes which are electrically isolated from the silicon substrate. The process developed successfully addresses the challenges of photoresist seepage into pores and the instability of the films in alkaline developers. These fabricated membranes can be potentially used as a temperature sensing membrane in uncooled thermal detectors operating in long wavelength infrared region (LWIR). Porous Silicon (PS) films fabricated on low resistivity Si substrates have very high resistivity, potentially up to 10 orders of magnitude higher than the substrate, depending on the porosity of the films. The challenge with these films is ensuring that the electric current path is primarily through the high resistivity porous films between the lateral electrical contacts through metal-PS-metal path and not through parasitic/leakage paths to the lower resistivity silicon substrate in the transverse (top-down from metal-PS-Si-metal) direction. From these studies the electrical characterization of the fabricated structures indicates the dominant current path was through the suspended porous membrane, largely eliminating effects from the porous silicon/silicon interface. The temperature coefficient of resistance of membranes measured using such an isolated structure was found to be 4 %/K at room temperature, higher than commercially used amorphous silicon films in uncooled thermal detectors.