Development and characterization of surface micromachined, out-of-plane hot-wire anemometer

In this paper, we report the development of a new type of hot-wire anemometer (HWA) realized by using a micro-fabrication process that combines surface micromachining and an efficient three-dimensional assembly technique. The HWA uses a thermal element (hot wire) that is made of Pt/Ni/Pt film with a measured temperature coefficient of resistance (TCR) of 2400 ppm/degreesC. The thermal element is elevated out of plane by using support beams made of polyimide. In our current design, the support beam is 2.7 mum thick and up to 1 mm tall, and the length of the thermal element varies from 50 mum to 200 mum. Steady-state response to air velocity has been experimentally obtained up to 20 m/s under both constant current (CC) and constant temperature (CT) modes. The transient-state response has been examined using square wave and sinusoidal wave excitation signals in CT modes with the maximum cutoff frequency found to be approximately 10 kHz. This new HWA offers a number of unique materials and performance characteristics. The sensor does not require the use of silicon as either substrate or sensing materials. Using this process, it is possible to form large arrays of HWA on a variety of substrate materials.