Hollow MEMS: An Integrated Sensor for Combined Density, Viscosity, Buoyant Mass and IR Absorption Spectrometry

Miniaturization of electro mechanical sensor systems to the micro range and beyond has shown impressive sensitivities measuring sample properties like mass, viscosity, acceleration, pressure and force just to name a few applications. In order to enable these kinds of measurements on liquid samples a hollow MEMS sensor has been designed, fabricated and tested. Combined density, viscosity, buoyant mass spectrometry and IR absorption spectroscopy are possible on liquid samples and micron sized suspended particles (e.g. single cells). Measurements are based on changes in the resonant behavior of these sensors.
Optimization of the microfabrication process has led to a process yield of almost 100% .This is achieved despite the fact, that the process still offers a high degree of flexibility. By simple modifications the Sensor shape can be optimized for different size ranges and sensitivities.
Microfluidic interfacing has been realized using high throughput and low cost technologies such as injection molding and ultra-sonic welding. Standard fluidic LUER connections were used that are widely applied in other micro fluidic projects at DTU Nanotech to enable future interfacing of the system with other technologies and pre-concentration approaches.
A thorough theoretical analysis of the expected sensor responsivity and sensitivity is performed. Predictions made are confirmed by finite element simulations. Using these tools the sensor geometry is optimized for ideal performance in both mass density and IR spectroscopy measurements of samples, the size of single yeast cells (≈ 5 μm). A relative frequency shift of 69 ppm/single cell buoyant mass in case of the mass spectroscopy measurements and 40 ppm/μW in case of the IR absorption spectroscopy measurements are calculated and confirmed by FE simulations for the sensor geometry fabricated.
In order to verify sufficient frequency stability, Allan Deviation measurements are performed on the fabricated sensors. In combination with the calculated responsivities these measurements confirm that the sensor sensitivity will enable single cell measurements.
Initial experiments confirming the calculated responsivities are performed. Experiments filling the sensor with liquids of different densities confirmed the predicted mass responsivity. The resonance frequency shifts 29% when filled with water compared to air.
By irradiating the sensor with a tunable IR laser source and tracking the resonance frequency the capability of the sensor to perform spectroscopic measurements is tested. Experiments with both an empty and a paraffin wax filled channel confirm the predicted heating responsivity. A resonance shift
of >8000 ppm at the absorption peak of paraffin is observed. Individual absorption peaks can be resolved with a wavenumber resolution below 1 cm^-1.