Self-Aligned Sidewall Heaters Applied in a Microfluidic Thermal Flow Sensor

We present a reproducible fabrication technology to realise self-aligned silicon sidewall heaters for microfluidic flow channels by KOH etching from the back side of the wafer. The semi-circular channel shape results in self-aligned sidewall heaters. The KOH etch is self-limited and yields a uniform etch profile over the wafer. The sidewall heaters can be created at the sides of a single channel and between two parallel channels. The electrical resistance of the heaters has a relative standard deviation of 8.7 %, which corresponds to the wafer-scale non-uniformity of the channel etch. The technology is applied in a thermal flow sensor to realise two heaters and a passive thermal shunt. Flow measurements with water show a measurable range of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$0.75 \mathrm{g} / \mathrm{h}$</tex>. The measurements with three other fluids show the theoretically expected scaling of the sensor response with the specific heat capacity of the fluid. The sidewall elements can also be exploited for capacitive sensing and electrostatic actuation.