J. Schalko

A suspended plate viscosity sensor featuring in-plane vibration and piezoresistive readout

Miniaturized viscosity sensors are often characterized by high-resonance frequencies and low-vibration amplitudes. The viscosity parameter obtained by such devices is therefore not always comparable to those probed by conventional laboratory equipment. We present a novel micromachined viscosity sensor with relatively low operating frequencies in the kHz range. The sensor utilizes Lorentz force excitation and piezoresistive readout. The resonating part consists of a rectangular plate suspended by four beam springs. The first mode of vibration is an in-plane mode. Thus, the contribution of the moving plate to the device damping is low, whereas the overall mass is high. This principle improves the quality factor and gives additional freedom to the device designer. This paper presents the device concept, the fabrication process and a prototype of the viscosity sensor. Measurement results demonstrate the feasibility of the device and show that the damping of the device is an appropriate measure for the viscosity.

Characterization of a roof tile-shaped out-of-plane vibrational mode in aluminum-nitride-actuated self-sensing micro-resonators for liquid monitoring purposes

This Letter reports on an advanced out-of-plane bending mode for aluminum-nitride (AlN)-actuated cantilevers. Devices of different thickness were fabricated and characterized by optical and electrical measurements in air and liquid media having viscosities up to 615 cP and compared to the classical out-of-plane bending and torsional modes. Finite element method eigenmode analyses were performed showing excellent agreement with the measured mode shapes and resonance frequencies. Quality factors (Q-factor) and the electrical behavior were evaluated as a function of the cantilever thickness. A very high Q-factor of about 197 was achieved in deionized water at a low resonance frequency of 336 kHz, being up to now, the highest quality factor reported for cantilever sensors in liquid media. Compared to the quality factor of the common fundamental out-of-plane bending mode, a 5 times higher Q-factor was achieved. Furthermore, the strain related conductance peak of the roof tile-shaped mode is superior. Compared ...

Robust Precision Position Detection With an Optical MEMS Hybrid Device

For vibration and displacement sensors, robustness is one of the key requirements. Optical measurement concepts are among the most promising possibilities to achieve it. The presented microoptoelectromechanical system sensor modulates a light flux by means of two congruently placed aperture gratings: one etched into a seismic mass and the other fixed to the sensor package. Commercially available LED and photodetector components at the top and bottom of the sandwich structure generate and detect this modulated light flux and allow for a cost-effective implementation. The prototype used for experimental verification is actuated by inertial forces and exhibits a high sensitivity of 0.85 mV/nm for displacements of the seismic mass and a corresponding noise level of about 14 pm/√Hz. This sensitivity and noise level can be further improved, paving the way for small, lightweight, robust, and high-precision displacement sensors for a large variety of applications.

An Optical In-Plane MEMS Vibration Sensor

This paper presents encouraging results of a novel optoelectronic conversion method for relative displacement. An optical modulator responding to acceleration and gravitation is used for characterization. The Si microelectromechanical system (MEMS) component comprise a spring suspended, in-plane oscillating mass carrying an array of optical apertures. Light flux modulation is achieved with a second array of complementary apertures that is fixed to the Si frame. The investigated device comprises a sandwich structure of an SMD LED, the MEMS aperture gratings, and a phototransistor. Relative displacements of the gratings generate a modulation of the LED light flux that is detected by the phototransistor. Depending on the aperture design, the relative displacement may extend over several tens of microns maintaining a sub-nm resolution. Thus, no closed-loop position control system is required, resulting in minimum complexity and energy consumption of the MEMS component. This setup simplifies the manufacturing process as much as possible, which is one of the significant advantages of the sensor principle. Furthermore, the presented prototype exhibits a promising high sensitivity of 60 nA/nm for displacement, featuring a noise level of about 8 pm/√Hz.

Arrays of open, independently tunable microcavities

Optical cavities are of central importance in numerous areas of physics, including precision measurement, cavity optomechanics and cavity quantum electrodynamics. The miniaturisation and scaling to large numbers of sites is of interest for many of these applications, in particular for quantum computation and simulation. Here we present the first scaled microcavity system which enables the creation of large numbers of highly uniform, tunable light-matter interfaces using ions, neutral atoms or solid-state qubits. The microcavities are created by means of silicon micro-fabrication, are coupled directly to optical fibres and can be independently tuned to the chosen frequency, paving the way for arbitrarily large networks of optical microcavities.

MEMS Thermal Flow Sensor With Smart Electronic Interface Circuit

A smart system for flow measurement is presented, consisting of a micromachined thermal flow sensor combined with a smart front-end electronic interface. The flow sensor is based on a novel thermal transduction method, which combines the hot-film and calorimetric sensing principles. The sensor consists of four germanium thermistors embedded in a thin membrane and connected to form a Wheatstone bridge supplied with a constant DC current. In this configuration, both the bridge unbalance voltage and the voltage at the bridge supply terminals are functions of the flow offering high initial sensitivity, i.e., near zero flow and wide measurement range, respectively. The front-end interface is based on a CMOS relaxation oscillator circuit where the frequency and the duty cycle of a rectangular-wave output signal are related to the bridge unbalance voltage and the voltage at the bridge supply terminals, respectively. Furthermore, the amplitude of the output signal is a linear function of the operating temperature. In this way, a single output signal advantageously carries two pieces of information related to the flow velocity and provides an additional measurement of the sensor operating temperature, which enables the correction of the temperature dependence of the sensor readouts. The system has been experimentally characterized for the measurement of nitrogen gas flow velocity at different sensor temperatures. The initial sensitivities at room temperature result 13.7 kHz/(m/s) and 23.5%/(m/s), in agreement with FEM simulations, for frequency and duty cycle readouts, respectively, with an equivalent velocity resolution of about 0.5 and 1.3 cm/s.

A novel thermal transduction method for sub-mW flow sensors

We present a novel approach to micromachined thermal flow sensors embedding four thin-film germanium thermistors in a silicon-nitride membrane. The appropriately arranged thermistors act as heat sources and as temperature sensors simultaneously. They were connected to form a Wheatstone bridge supplied with a constant electric current. Both, the bridge voltage and the voltage at the bridge supply terminals, are immediately available output signals offering high initial sensitivity or wide measurement range, respectively. The self-heating based flow transduction mechanism combine advantages of the hot-film and the electrocalorimetric flow sensors. With respect to comparable electrocalorimetric flow sensors, the power requirement is reduced by more than an order of magnitude. This feature is beneficial for remote sensing applications and crucial for measuring the flow of fluids that endure only slight temperature elevations.

A Novel calorimetric flow sensor implementation based on thermal sigma-delta modulation

Miniaturized calorimetric flow sensors suffer from a limited measurement range if operated with constant heating power. To overcome this drawback, we utilize a thermal sigma-delta modulator to achieve a constant average excess temperature of the sensor membrane. The duty cycle of the modulator output fully covers the flow information. This novel method exhibits the advantage that the slope of the output characteristic and hence the flow sensitivity can be optimized for a certain flow range by choosing an appropriate amplitude of the heating voltage. Finally, the influence of the clock frequency on the system accuracy was investigated by extensive SPICE modeling of the thermal system.

Highly Efficient Passive Thermal Micro-Actuator

A passive thermal micro-actuator with large area specific work and large displacement, fabricated of electroplated nickel on a silicon substrate is presented. The actuation relies on the thermal expansion of beams in a V-shaped geometry. Two V-shaped beam stacks are aligned opposite to each other and are coupled to a lever transmission. The actuator exhibits low energy losses due to the deformation of the structure and can efficiently convert the thermally induced elastic energy into mechanical work. An analytical model considers these thermally induced mechanical energies and the energy losses caused by the deformation of the material. The calculated deflections are compared with the measured ones and results of finite-element method simulations. The presented actuator operates completely passive, relies only on temperature changes of the surrounding environment, and exhibits a measured temperature-dependent linear deflection coefficient of 1.48 μm/K with a simulated blocking force of 57 μN/K. The structure occupies an area of 2135 × 1831 μm2 and the area specific work is calculated to be 21.7 μJ/K2/m2, beating state of the art thermal actuators. As proof-of-concept, a passive micro-electro-mechanical systems temperature threshold sensor is fabricated, featuring the actuator and a bistable beam that switches between two stable positions when a specific threshold temperature is exceeded.

Effect of oxygen plasma on nanomechanical silicon nitride resonators

Precise control of tensile stress and intrinsic damping is crucial for the optimal design of nanomechanical systems for sensor applications and quantum optomechanics in particular. In this letter, we study the influence of oxygen plasma on the tensile stress and intrinsic damping of nanomechanical silicon nitride resonators. Oxygen plasma treatments are common steps in micro and nanofabrication. We show that oxygen plasma for only a few minutes oxidizes the silicon nitride surface, creating several nanometer thick silicon dioxide layers with a compressive stress of 1.30(16) GPa. Such oxide layers can cause a reduction in the effective tensile stress of a 50 nm thick stoichiometric silicon nitride membrane by almost 50%. Additionally, intrinsic damping linearly increases with the silicon dioxide film thickness. An oxide layer of 1.5 nm grown in just 10 s in a 50 W oxygen plasma almost doubled the intrinsic damping. The oxide surface layer can be efficiently removed in buffered hydrofluoric acid.