Michael Stifter

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.

Planar Magnetostrictive Micromechanical Actuator

A magnetostrictive actuator approach based on micromechanical structures is presented. Planar geometries are designed, comprising V-shaped beams and lever transmissions. Several V-shaped beams are stacked in parallel and two of such stacks are placed facing each other. Both are connected to a lever beam. When a magnetic field is applied, the magnetostriction of the active material leads to an elongation/shortening of the V-shaped beams. The lever transmission converts the magnetically induced elastic energy into a reliable in-plane displacement. The overall dimensions of the devices are in the range of 4 mm × 2 mm × 0.4 mm and hence, way smaller compared with state-of-the-art magnetostrictive actuators. For a magnetic flux density of 14 mT, a stroke displacement of 10.2 μm was achieved. With a spring stiffness of 5.56 N/m obtained from simulations, a theoretical area specific work of 72 μJ/m2 was accomplished.

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.

Pressure dependence of the quality factor of a micromachined cantilever in rarefied gases

We present a study of the damping behavior of monocrystalline silicon cantilevers in different rarefied gas regimes. Mechanical quality factors Q were analyzed at controlled ambient pressures in the range of 0.01 Pa to 100 Pa. Emphasis was laid on the investigation of the fundamental vibration mode. Hence, the test structures were harmonically excited by the Lorentz force acting on the current carrying lead attached to the top surface of the cantilever. The micromachined clamped-free cantilevers featuring a length of 2 mm, a width of 1.5 mm and a thickness of 20 μm, were manufactured in SOI technology. The experimental results were compared with existing theories revealing an underestimate of the damping parameter for the Knudsen range Kn = 0.1 to 10. So far, squeeze-film damping by free molecular flow and kinetic damping were taken into account in damping models for the quasi-molecular regime. However, our measurements indicate that also the ongoing molecular flow around the test structures has to be considered. Hence the damping coefficient has to be calculated with methods of the free molecular aerodynamics. Thus, we used an algorithm based on the random walk model that allows the usage of already available knowledge in the field of Direct Simulation Monte Carlo. With this approach the quality factor of a squeezed-film damped cantilever in the quasi-molecular regime was derived. The results were compared with the most recent stochastic model, where the theoretical predictions and the experimental investigations indicate significant squeezing up to a Knudsen number of 10. In a superposition of both damping mechanisms, kinetic and squeeze-film damping, a satisfactory characterization of the damping behavior of an oscillating cantilever in the quasi-molecular regime with Knudsen numbers in the range of 10 down to 0.02 was achieved.

MEMS heterodyne AMF detection with capacitive sensing

A Lorentz-force actuated cantilever used as a magnetometer detecting alternating magnetic fields (AMF) is described. The device consists of a U-shaped single-crystal silicon cantilever manufactured in SOI technology. This micromachined cantilever features a length of 2mm, a base width of 90μm, and a thickness of 20μm, whereat the two 2mm cantilevers are hold together by a 1.5mm long bar at the free moving ends. The cantilever is placed in a vacuum chamber surrounded by a pair of coils configured as Helmholtz coil which generates the alternating magnetic field. The test structures are harmonically excited by the Lorentz force acting on the gold lead at the top surface of the cantilever carrying an alternating current. In the presence of a sinusoidal magnetic flux density, the resulting Lorentz force contains two alternating terms including the sum and difference of current and field frequencies. Therefore, the resonating cantilever is used as mixer in a heterodyne detector for alternating magnetic fields with variable frequency. Resonant excitation only occurs if one of these frequencies is close to a mechanical resonance that satisfies the selection rule imposed by the field configuration. In the experiments, emphasis is laid on the investigation of the first symmetric and first antisymmetric vibration mode, where the amplitude of the vibration is proportional to the exciting vector component of the magnetic field. For this work the harmonic deflection of the cantilever was measured with a capacitive readout system and additionally, with a laser-Doppler vibrometer. By changing the drive current, the operating range of the magnetometer can be varied from a few μT up to 1mT, whereas the sensitivity remains constant with an uncertainty of less than one percent, valid for both vibration modes. This operation principle of the prototype allows a further miniaturization leading to a spatial resolution of the magnetic field detection determined by the size of the cantilever.

Design of linear and nonlinear hybrid optical MEMS displacement sensors

Although capacitive and piezoresistive readouts are commonly used for microelectromechanical structures, they suffer from serious drawbacks like limited range of displacement, inherent nonlinearity, insucient sensitivity, and technological complexity. Our complementary readout approach relies on a novel hybrid optomechanical device, where the displacement range is not limited by typical constraints of capacitive or piezoresistive conversion principles. Furthermore, no electrical connections are required on the micromechanical part. Moreover, this principle enables custom linear or nolinear output characteristics at the same technological effort.

Distortion-free measurement of electric field strength with a MEMS sensor

Small-scale and distortion-free measurement of electric fields is crucial for applications such as surveying atmospheric electrostatic fields, lightning research and safeguarding areas close to high-voltage power lines. A variety of measurement systems exist, the most common of which are field mills, which work by picking up the differential voltage of the measurement electrodes while periodically shielding them with a grounded electrode. However, all current approaches are bulky, suffer from a strong temperature dependency or severely distort the electric field, and thus require a well-defined surrounding and complex calibration procedures. Here we show that microelectromechanical system (MEMS) devices can be used to measure electric field strength without significant field distortion. The purely passive MEMS devices exploit the effect of electrostatic induction, which is used to generate internal forces that are converted into an optically tracked mechanical displacement of a spring-suspended seismic mass. The devices exhibit resolutions on the order of 100 V m−1 Hz−1/2 with a measurement range of up to tens of kilovolts per metre in the quasi-static regime ≲300 Hz). We also show that it should be possible to achieve resolutions of around 1 V m−1 Hz−1/2 by fine-tuning the sensor embodiment. These MEMS devices are compact and could be mass produced easily for wide application.The detection of force-induced displacements within compact MEMS (microelectromechanical system) devices can be used to measure electric field strength without significant field distortion.

Making optical MEMS sensors more compact using organic light sources and detectors

Vibration and displacement sensors need to be compact for many applications in automation or consumer electronics, and microelectromechanical structures are a convenient way to implement such sensors. For these MEMS devices, optical readout methods have proven to be superior to capacitive or piezoresistive strategies in terms of sensitivity as well as noise and interference immunity, however the integration of light sources and detectors is not easily possible. This paper presents an approach to use organic optoelectronic devices for the readout. OLED and OPD (organic photo detector) are structured on the glass substrate and cover encapsulating the MEMS devices, allowing for a tightly integrated sensor based on vertical light flux modulation by a horizontally moving proof mass. The paper describes the principle sensor structure as well as the fabrication of suitable organic devices. First test results show that the approach is feasible.

A Lorentz force actuated magnetic field sensor with capacitive read-out

We present a novel design of a resonant magnetic field sensor with capacitive read-out permitting wafer level production. The device consists of a single-crystal silicon cantilever manufactured from the device layer of an SOI wafer. Cantilevers represent a very simple structure with respect to manufacturing and function. On the top of the structure, a gold lead carries AC currents that generate alternating Lorentz forces in an external magnetic field. The free end oscillation of the actuated cantilever depends on the eigenfrequencies of the structure. Particularly, the specific design of a U-shaped structure provides a larger force-to-stiffness-ratio than standard cantilevers. The electrodes for detecting cantilever deflections are separately fabricated on a Pyrex glass-wafer. They form the counterpart to the lead on the freely vibrating planar structure. Both wafers are mounted on top of each other. A custom SU-8 bonding process on wafer level creates a gap which defines the equilibrium distance between sensing electrodes and the vibrating structure. Additionally to the capacitive read-out, the cantilever oscillation was simultaneously measured with laser Doppler vibrometry through proper windows in the SOI handle wafer. Advantages and disadvantages of the asynchronous capacitive measurement configuration are discussed quantitatively and presented by a comprehensive experimental characterization of the device under test.