Reactive Ion Etching of Columnar Nanostructured ${\rm TiO}_{2}$ Thin Films for Modified Relative Humidity Sensor Response Time

A CF₄ dry etch recipe for TiO₂ was optimized for nanostructured thin films. The impact of our etching process and ultraviolet irradiation of nanostructured relative humidity (RH) sensors was studied. Reactive ion etching of titanium dioxide decreased device adsorption response time by opening high diffusivity channels while retaining the high surface area and high dynamic range of the interdigitated electrode device. The full electrical response (impedance and phase) and response time of our sensors was studied as a function of etch duration. Using a TiO₂ etch recipe consisting of CF₄ produced large changes to RH sensor electrical response and introduced a large hysteresis. As a result of significant microstructural change, the adsorption response time of the RH sensors is greatly improved from ap 150 ms to an instrument-limited 50 ms. The adsorption times are at least six times faster than previous, thinner sensors. However, the current sensors do not recover as well as previous sensors, possibly due to nodular defects observed here and which are absent in previous devices. Although sensor step desorption times improved from an unetched ap 130 ms to the instrument limit of 50 ms, full recovery times increased beyond ap 3 s. When the etch treatment was followed by a 48 h ultraviolet treatment, the hysteresis introduced by the CF₄ etch was significantly reduced, without reducing the improvement in response time.