Mechanical Hydrogen Sensing

This project aims to analyze, fabricate and characterize a mechanical hydrogen sensor. The potential of hydrogen gas detection using beam resonators is investigated. The beams consist mainly of Si3N4, providing a tensile stress to prevent buckling. Pd expands in the presence of hydrogen gas, but as it is constrained in the longitudinal direction, a compressive stress is generated. Similar to a guitar string being tuned, the stress change will change the resonance frequency of the beam. Pt layer beams are also fabricated to enable differential readout for increased resolution. For actuation and readout, a capacitor is formed between a bottom layer of doped poly-Si, patterned by photolithography (fig. 1), and a metal layer on top of a Si3N4 beam. The bottom electrode is protected by a thin Si3N4 layer. SiO2 is deposited, defining the capacitor gap (fig. 2). Poly-Si is deposited and then chemo-mechanically polished (fig. 3), enabling a well-defined release mechanism. After Si3N4 is deposited, lift-off processes define beams of either Pd or Pt (fig. 4) and the Si3N4 is defined by dry etching. After deposition and patterning of the electrodes, the beams are released through KOH etching. Ideally the result resembles figure 7. A cantilever design is fabricated in parallel (fig. 6), enabling hydrogen detection by deflection measurement.