Moisture measurement using porous aluminum oxide coated microcantilevers

Two types of aluminum oxide (Al2O3) modified microcantilevers (MCLs) were tested for their sensitivity and reproducibility for detection of low levels of moisture. Aluminum was sputtered on the MCLs and oxidized to Al2O3 through thermal oxidation (Method I) and anodization method (Method II). Both deflection and frequency change of the MCLs were investigated. Thermally treated MCLs showed a 16 ± 2 nm deflection and a 25 Hz change in resonant frequency, i.e., a mass change of 1.85 × 10−11 g, to a 200 ppm level of moisture. MCLs from Method II showed a 95 ± 5 nm deflection and a 340 Hz change in resonant frequency, i.e., a mass change of 41.76 × 10−11 g, to the 200 ppm level of moisture. The MCLs’ response time to moisture was less than 3 min using the deflection mode, and less than 25 s using the frequency method. The MCLs prepared by Method II demonstrated a higher sensitivity toward moisture measurement due to the larger surface area of the porous Al2O3 coating. The comparison also suggests that the frequency responses of microcantilever sensors to moistures are much faster than the deflection responses. These sensors were stable for months stored under ambient conditions. The bending amplitudes and frequency change were proportional to the moisture level, and the detection of moisture was not affected by alcohols in the environment.