Novel humidity sensing method based on the transient response of a micro-heater

This paper introduces a novel method for sensing humidity. It is fundamentally based on the measurement of the heating time of a thermally actuated micro-wire surrounded by humid air, which is dependent upon the thermophysical properties of humid air. It is an improvement of related methods based on the steady state response of such a sensor and benefits of a lower heating temperature, increased sensitivity, as well as improved resolution since it strongly reduces low frequency noise due to bulk motion convective heat transfer. The sensing part is a 1 mm long, 4 kΩ resistive micro-wire made of polysilicon and coated with silicon nitride, a well known water resistant barrier. Its fabrication relies on a fully standardized CMOS-like industrial process as well as on a single-step FSBM in post-process by means of TMAH anisotropic wet etching in order to release the micro-wire from the silicon bulk, which is well known to be a very competitive fabrication strategy. The measurement itself relies on a 1-bit ADC (i.e. a single voltage comparator), and the digitalization of the sensor's response is achieved by a digital clock, which is very effective in terms of extra-power consumption, silicon area and design time, if monolithically integrated with the sensor. These features add up to already established benefits of related methods, which are CMOS compatible, absence of water reacting material, fastest response time, lowest long term drift, very low hysteresis, fastest design time and production cost. In its prototyping form, the here presented CMOS-compatible sensor features, at 20 °C, 0.497 μs/%RH sensitivity, with a maximal nonlinearity of 3%RH at 60%RH, with a ±1.5%RH resolution in the 10%RH–80%RH range as well as 1.58 mW power consumption integrated within less than 2 ms for a single measurement. These results are very encouraging given the proof of concept nature of this work.