Microfluidic thermal flow sensor with extended range via silicon sidewall heaters and a thermal shunt

This paper reports on a microfluidic thermal flow sensor with an extended calorimetric range via a thermal shunt and sidewall heaters. The thermal shunt between the up- and downstream parts of the sensor preheats the fluid upstream while cooling down the fluid downstream, which decreases the overall temperature difference, but maintains a monotonically increasing temperature difference for larger flow rates. The integration of highly-doped silicon heaters in the sidewalls of the microchannel allows for fully developed thermal boundary layers at higher mass flow rates, extending the flow range. The shunt and sidewall heaters are fabricated using refilled trenches of 1.2 μ[jls-end-space/]m and 3 μ[jls-end-space/]m wide. Due to aspect ratio dependent etching, different trench depths are obtained simultaneously. The sensor is thermally isolated from the substrate via a semi-isotropic cavity etch to reduce power consumption. The sensor is characterised using nitrogen, isopropanol, water, and binary mixtures of isopropanol in water at temperatures below 309 K to allow for biological lab-on-a-chip applications. In constant temperature actuation mode, a calorimetric flow range of 6 g/h and an anemometric flow range of at least 20 g/h of water is achieved. Furthermore, the fabrication technology is compatible with micro Coriolis mass flow sensors and inline microfluidic sensors for relative permittivity, thermal conductivity, and pressure.