MEMS ̄ow sensors for nano- ̄uidic applications

This paper presents micromachined thermal sensors for measuring liquid ̄ow rates in the nanoliter-per-minute range. The sensors use a boron-doped polysilicon thin-®lm heater that is embedded in the silicon nitride wall of a microchannel. The boron doping is chosen to increase the heater's temperature coef®cient of resistance within tolerable noise limits, and the microchannel is suspended from the substrate to improve thermal isolation. The sensors have demonstrated a ̄ow rate resolution of 0.4 nl/min, as well as the capability for detecting micro bubbles in the liquid. Heat transfer simulation has also been performed to explain the sensor operation and yielded excellent agreement with experimental data. # 2001 Elsevier Science B.V. All rights reserved.