M. Dijkstra

Modeling, design, fabrication and characterization of a micro Coriolis mass flow sensor

This paper discusses the modeling, design and realization of micromachined Coriolis mass flow sensors. A lumped element model is used to analyze and predict the sensor performance. The model is used to design a sensor for a flow range of 0–1.2 g h−1 with a maximum pressure drop of 1 bar. The sensor was realized using semi-circular channels just beneath the surface of a silicon wafer. The channels have thin silicon nitride walls to minimize the channel mass with respect to the mass of the moving fluid. Special comb-shaped electrodes are integrated on the channels for capacitive readout of the extremely small Coriolis displacements. The comb-shaped electrode design eliminates the need for multiple metal layers and sacrificial layer etching methods. Furthermore, it prevents squeezed film damping due to a thin layer of air between the capacitor electrodes. As a result, the sensor operates at atmospheric pressure with a quality factor in the order of 40 and does not require vacuum packaging like other micro Coriolis flow sensors. Measurement results using water, ethanol, white gas and argon are presented, showing that the sensor measures true mass flow. The measurement error is currently in the order of 1% of the full scale of 1.2 g h−1.

A versatile surface channel concept for microfluidic applications

MEMS fluidic devices often require the integration of transducer structures with freely suspended microchannels. In this paper a versatile microchannel fabrication concept is presented, allowing for easy fluidic interfacing and integration of transducer structures in close proximity to the fluid. This is achieved by the reliable fabrication of completely sealed microchannels directly below the substrate surface. The resulting planar substrate surface allows for the deposition of transducer material and pattern transfer by lithography. The microchannels are subsequently released and fluidic entrance holes are created, while the transducer structures can be protected by photoresist. Several monolithic microfluidic device structures have been fabricated, demonstrating the versatility of the concept. Fabricated surface microchannel devices can optionally be vacuum sealed by anodic bonding.

Highly sensitive micro coriolis mass flow sensor

We have realized a micromachined micro Coriolis mass flow sensor consisting of a silicon nitride resonant tube of 40 mum diameter and 1.2 mum wall thickness. Actuation of the sensor in resonance mode is achieved by Lorentz forces. First measurements with both gas and liquid flow have demonstrated a resolution in the order of 10 milligram per hour. The sensor can simultaneously be used as a density sensor.

Read-Out of Cantilever Bending With a Grated Waveguide Optical Cavity

We present results related to the fabrication of a novel and potentially highly sensitive mechano-optical sensor for hydrogen gas, based on microcantilevers, provided with a selectively gas-absorbing palladium layer, suspended above a Si3N4 grated waveguide (GWG). Integrated microcantilever-GWG devices have been fabricated successfully using microelectromechanical systems (MEMS) techniques. Several technical problems encountered during the preparation of such integrated devices (i.e., grating production, surface roughness, facet quality) will be discussed, and solutions to address these issues will be given as well. We also present preliminary experimental results, showing the sensing of cantilever nano-displacements, and so the feasibility of proposed read-out principle.

Low-drift flow sensor with zero-offset thermopile-based power feedback

A thermal flow sensor has been realised consisting of freely-suspended silicon-rich silicon-nitride microchannels with an integrated Al/poly-Si++ thermopile in combination with up- and downstream Al heater resistors. The inherently zero offset of the thermopile is exploited in a feedback loop controlling the dissipated power in the heater resistors, eliminating inevitable influences of resistance drift and mismatch of the thin-film metal resistors. The control system cancels the flow-induced temperature difference across the thermopile by controlling a power difference between both heater resistors, thereby giving a measure for the flow rate. The flow sensor was characterised for power difference versus water flow rates up to 1.5 mul-min-1, being in good agreement with a thermal model of the sensor, and the correct low-drift operation of the temperature-balancing control system has been verified.

Low-drift U-shaped thermopile flow sensor

A thermal flow sensor has been realised consisting of a freely-suspended U-shaped microchannel. The structure is symmetrically heated by a heater at the top of the U-shape. The thermal imbalance caused by liquid flow is sensed by an integrated Al/poly-Si++ thermopile. The U-shape microchannel facilitates the integration of a large number of thermocouple junctions, resulting in a highly-sensitive calorimetric flow sensor (40 mV/mulldrmin-1 at 2 mW heating power). The heating power is controlled accurately by forcing a current, while measuring the voltage over the heater resistor. Influences of thermal gradients across the chip are minimised by the freely-suspended microchannel ends being fixed to the substrate over a small distance. The inherently zero-offset of the thermopile can furthermore be exploited in a control system cancelling temperature imbalance by liquid flow using additional heaters. This makes the flow sensor independent of heater resistor values and thermopile output characteristics. Accurate measurements up to 400 nlldrmin-1 water flow have been obtained applying a temperature-balancing control system.

Monolithically integrated cantilevers with self-aligned tips for wavelength tuning in a photonic crystal cavity-based channel-drop filter

A technology to monolithically integrate micro-bimorph cantilevers equipped with tips that are self-aligned with respect to the holes of a 2D photonic crystal cavity-based channel-drop filter is presented. On electrostatic actuation, the tips move into the holes and provide electromechano-optical modulation of light. The technology allows the fabrication of tips on specific photonic crystal holes by controlling the hole diameter and the sacrificial layer thickness. The integrated device is both mechanically and optically characterized. A 180 pm wavelength shift at the first band edge of the photonic crystal cavity-based channel-drop filter is measured on the application of a 2 V dc voltage to the cantilever. This CMOS-compatible device is designed to operate in the C-band of the telecommunication wavelengths and constitutes a promising candidate for future integrated all-optical devices.

High quality factor Al2O3 microring resonators for on-chip sensing applications

Microring resonators find many applications for on-chip integrated optical sensors. Their spectral response contains resonance dips that shift due to variations of the optical path length of the microring probed. Numerous examples of such microring resonator sensors in the SOI, Si3N4 and SiON waveguide technologies have been reported for the detection of bulk refractive index variations and the label-free detection of biomarkers. Al2O3 is an alternative waveguide technology that exhibits low optical propagation losses, is transparent over a large spectral range extending from the visible to the mid-IR and permits co-doping with active rare-earth ions, which enables the co-integration of active devices on the chip. In this work an Al2O3 microring resonator sensor was developed for the label-free detection of protein biomarkers. The uncladded microring with a radius of 200 μm had a measured quality factor of 3.2 × 105 at 1550 nm. Submerging the devices in water decreased the quality factor to 45 × 103. This corresponds with propagation losses in the rings of 0.6 dB/cm and 5.7 dB/cm respectively. The bulk refractive index sensitivity of the sensor was determined by flowing NaCl dissolved in water in different concentrations. A sensitivity of 102.3 ± 0.5 nm/RIU with a corresponding limit of detection of 1.6 × 10-6 RIU was demonstrated for TM polarized light. High affinity human monoclonal antibodies mAb S100A4 were immobilized on the sensor to detect the S100A4 protein biomarker down to 12 nM concentrations. These results demonstrate the feasibility of this material for label-free optical biosensors.

Thermal-wave balancing flow sensor with low-drift power feedback

A control system using a low-drift power-feedback signal was implemented applying thermal waves, giving a sensor output independent of resistance drift and thermo-electric offset voltages on interface wires. Kelvin-contact sensing and power control is used on heater resistors, thereby inhibiting the influence of heater resistance drift. The thermal waves are detected with a sensing resistor using a lock-in amplifier and are mutually cancelled by a thermal-wave balancing controller. Offset due to thermal gradient across the chip and resistor drift are eliminated by the lock-in amplifier and power controller, and therefore do not influence the sensor output signal. A microchannel thermal-wave balancing flow sensor with integrated Al resistors has successfully been fabricated. The thermal flow sensor is capable of measuring water flow rates with nl· min−1 precision, up to about 500 nl· min−1 full scale. Measurement results are in good agreement with a dynamic model of the flow sensor. Drift measurements show the sensor output signal to be ompensated for resistance drift and thermal gradient across the chip.

Lable-free Enzyme Sensing with a Si3N4 Grated Waveguide Optical Cavity

We report the label-free, sensitive detection of PepN enzyme using a Si3N4 grated waveguide optical cavity covered with an immobilized, selective (antibody) receptor layer. The receptor-enzyme reaction was monitored in real-time.