MEMS Thermal Flow Sensors - An Accuracy Investigation

The geometric structure of a low-power thermal micro-hot film sensor has been investigated and optimized, using both computational and experimental methods. The response and accuracy of eight CMOS designs with different heater and membrane sizes were studied and found to vary considerably with geometry. It is found that reducing the heater length causes an improved electro-thermal efficiency and that a large reduction in accuracy was seen when reducing the membrane size. Our simulations suggest that this effect is due to higher temperature gradients causing localized stronger natural convective flows above the measuring resistor. However, the reduced accuracy disappears as flow rate increases due to a higher proportion of forced convection compared with natural convection. We believe that this paper will help in the design of a new generation of high accuracy MEMS thermal flow sensors for low-cost, low-power application