CAD modelling and fabrication of planar thin film airflow sensors

As MEMS devices have evolved, the micromachined hot wire anemometer has provided a substitute to the conventional mechanical wind speed sensor. These micromachined flow sensors are commonly implemented with thermal isolation of the sensor from the bulk substrate mass using methods such as reverse side etching or sacrificial layers; however this paper will primarily present a sensor relying on thermal insulation only. Insulation may be provided by layers of material exhibiting relatively poor thermal conduction characteristics such as silicon dioxide or polyimide. A number of advantages are realised such as removing the process of reverse side etching. Limiting fabrication to use of simple processes such as photolithography and sputtering/evaporative deposition also simplifies this design and assists in greatly increasing the compatibility with standard CMOS fabrication processes and materials. Computer modelling and analysis, physical fabrication and testing are presented. Preliminary testing of a design has demonstrated small yet measurable temperature gradients across the device surface during steady state operation. Transient (time of flight) measurements are under investigation.