Fabrication of free-standing Pt nanowires for use as thermal anemometry probes in turbulence measurements

We report a robust fabrication method for patterning free-standing Pt nanowires for the use as thermal anemometry probes for small-scale turbulence measurements. Using e-beam lithography, high aspect ratio Pt nanowires (~300 nm width, ~70 $\mu$m length, ~100 nm thickness) were patterned on the surface of oxidized silicon (Si) wafers. Combining precise wet etching processes with dry etching processes, these Pt nanowires have been successfully released free-standing between two silicon dioxide (SiO2) beams supported on Si cantilevers. Moreover, the unique design of the bridge holding the device allowed to release the device gently without damaging the Pt nanowires. The total fabrication time was minimized by restricting the use of e-beam lithography to the patterning of the Pt nanowires while standard photolithography was employed for other parts of the devices. We demonstrate that the fabricated sensors are suitable for turbulence measurements when operated in a constant-current mode. A robust calibration between output voltage and fluid velocity was established over the velocity range from 0.5 m s-1 to 5 m s-1 in an SF6 atmosphere at a pressure of 2 bar and a temperature of 21{\deg}C. The sensing signal from the nanowires showed negligible drift over a period of several hours. Moreover, we confirmed that the nanowires are able to withstand high dynamic pressures by testing them in air at room temperature velocities up to 55 m/s.