Bidirectional micromachined flow sensor featuring a hot film made of amorphous germanium

We report on simulation-based sensor design and measurement results of a bidirectional micromachined hot-film anemometer. The device is based on a thin-film thermistor made of amorphous germanium embedded in a silicon nitride membrane. The germanium structure is divided by metal-strip electrodes into four thermistor segments which are connected to a Wheatstone bridge. The sensor combines hot-film and calorimetric transduction principles. The flow dependent cooling of the hot film results in a unipolar, strictly monotonic transduction characteristic which is utilized for flow velocity measurements. Moreover, convective heat transfer between the hot-film segments causes thermal asymmetry yielding a bridge detuning voltage with a bipolar characteristic where its sign corresponds to the flow direction. Consequently, the sensor features a strictly monotonic transduction characteristic over a wide flow velocity range combined with simultaneous detection of the flow direction. The device was characterized in constant-current and constant-temperature operating modes for stationary flows as well as step-like changes of the flow velocity. The sensor behaviour as a result of ambient temperature variations was studied in detail.