Integrated throughflow mechanical microfluidic sensors
Dennis is a PhD-student in the research group of Integrated Devices and Systems (IDS). His supervisor is Joost Lötters from the of Electrical Engineering, Mathematics and Computer Science.
Measurement of flow is essential for continuous dosing of fluids. Accurate dosing is important for medical applications, e.g. drug delivery using intravenous therapy, of which the settling time of the flow as a result of tubing and needles is significant. In industrial applications, e.g. controlling gas concentrations in reaction chambers for the production of semiconductors, measurement of flow is also crucial. By measuring flow, pressure and fluid properties, such as density and viscosity, the composition of mixtures can be measured, provided that the ingredients are known. Miniaturization of these sensors using microtechnology offers advantages in terms of resolution, mass production, channel wall material and internal volumes. This dissertation describes novel designs, fabrication methods and experiments of microfluidic sensors that use a mechanical transduction principle and that can be integrated throughflow with other sensors on a single chip.
Especially microfabricated Coriolis mass flow sensors are suitable for this purpose. A Coriolis mass flow meter consists of a suspended channel that is actuated at one of its resonance frequencies. A fluid flow through the channel causes a secondary vibration mode at the same frequency. A family of methods to fabricate such microfluidic sensors has been described and is called surface channel technology. These methods enable the fabrication of suspended silicon nitride channels of 10 μm to 100 μm. A metal layer can be deposited and patterned to realize wiring and electrodes. A number of improvements have been presented, enabling the fabrication of silicon electrodes at the sides of the channels, piezoelectric material on top of the channels and multiple layers of channels. For characterization, sensors need fluidic and electrical connections. Therefore, a universal and modular platform has been developed. The assembly process of the sensor for characterization with this platform requires only two steps: the sensor has to be glued on a printed circuit board and wirebonded. The printed circuit board can then be clamped into the main setup, which leads directly to 8 fluid connections and 72 electrical connections. Electronic modules can be connected to the main setup for actuation and reading.
Since Coriolis mass flow sensors use a mechanical transduction principle, the resolution is fundamentally limited by thermomechanical noise. A thermomechanical noise model has therefore been developed. The Coriolis mass flow sensor is modeled as a second order system. The excitation by noise is derived from the equipartition principle. RMS amplitudes are measured with laser Doppler vibrometry and correspond to theory. A noise equivalent mass flow of 0.3 ng/s has been derived for currently most accurate Coriolis mass flow sensor. The resolution of the latter is not yet limited by thermomechanical noise and can be improved by at least a factor of 10. One way to improve the resolution is by increasing the sensitivity to mass flow. This can be achieved by decreasing the influence of the actuation mode on the output signal and thus increasing the sensitivity of the Coriolis induced mode. This can be realized for capacitive Coriolis mass flow sensors by the addition of two readout electrodes that are crosswise connected to the original electrodes. The thermomechanical noise has only been measured for a single point of the channel of a Coriolis mass flow sensor. For vibration mode analysis, the magnitudes and phases of several points need to be known. Laser Doppler vibrometers can measure the magnitudes of the velocities of different points by scanning, the phase information is then obtained by triggering on the actuation signal. The actuation signal must therefore be known. The phase information can still be retrieved from unknown signals using a two stage measurement and the presented post processing.
Surface channel technology is also suitable for the fabrication of throughflow pressure sensors. Two designs have been presented. One design consists of a channel with a deforming ceiling dependent on pressure. Resistive readout structures in a Wheatstone bridge detect this deformation with a sensitivity of 4·10⁵ /bar. The other pressure gauge consists of a suspended U-shaped channel. Due to the non-circular cross section of the channel, it deforms in its entirety. This displacement is capacitively detected with a sensitivity of 1 fF/bar. Coriolis mass flow sensors share this phenomenon: it has been validated that Coriolis mass flow meters can also measure pressure simultaneously with mass flow. This can be achieved by measuring the static deflection of the capacitive readout structures in addition to phase shift. If pressure and mass flow are known, the viscosity can be derived using a fluid modeling. The Hagen-Poiseuille law is sufficient for liquids. A more complex model has been derived for gases, since gases are compressible. Density can also be measured, since the resonance frequency of a Coriolis mass flow sensor is dependent on the density of the liquid in the channels. This resonance frequency is also dependent on the pressure, but this can be compensated for with the pressure sensors. Surface channel technology has also been used to realize a relative permittivity sensor. This sensor consists of two silicon electrodes at both sides of the channel and measures the capacity through the fluid.
