A. Kainz

A miniaturized linear shaker system for MEMS sensor characterization

A miniaturised, piezoelectrically driven shaker system is presented which is suitable for MEMS characterisation in vacuum. It offers a broad frequency and amplitude range. The fully vacuum compatible shaker is constructed out of one single peace of aluminium with a piezo-stack-actuator working in-plane against four beam springs. It can easily be fabricated at low costs using a hand operated milling machine. The systems characteristics are easily tuned to different applications as the first resonance frequency is given by the stiffness of the beam springs and the mass of the moving shaker table. The utilised piezoelectric stack determines the maximum reachable amplitude for a given spring stiffness. Finite Element simulations have been carried out to design a at transfer characteristic of the shaker up to 10 kHz and amplitudes in the range from sub nanometres up to 1μm. The simulations were evaluated by laser vibrometer measurements of the shaker which also show a good linearity between electrical excitation signal and output deection amplitude. To account for other resonance frequencies introduced by a preexisting MEMS mounting device, the resulting vibration amplitude on the MEMS structure can be normalised by adjusting the electrical excitation amplitude with the help of a Polytec laser vibrometer.

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.

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.

Air damping model for laterally oscillating MOEMS vibration sensors

This paper presents a comprehensive model for the damping coefficient of a laterally moving micro-opto-electro-mechanical system. While viscous forces acting onto large areas of laterally oscillating microstructures are well understood, there are contributions to the air damping that receive less attention. These include pressure forces, the effects of regularly placed holes perforating the large surfaces as well as forces exerted onto the typically much smaller side faces. The results of our analytic models are backed up with very good agreement by finite volume method simulations with the open source software OpenFOAM® as well as measurements. Since the read-out of our MOEMS sensor is optical, it does not involve highly complicated comb-drive geometries that are hard to model. Thus, our semi-numerical model describes the damping behavior of the MOEMS very accurately.

Extremely low resonance frequency MOEMS vibration sensors with sub-pm resolution

The majority of MEMS vibration sensors requires relatively high resonance frequencies of several kHz to avoid mechanical contact of moving parts such as plates or electrodes. In contrast to that, the presented hybrid micro-opto-electro-mechanical system (MOEMS) principle enables displacement sensors applicable at low frequencies. This work describes a distinct MOEMS featuring extremely low resonance frequencies of below 200 Hz. The discussed sensor operates in ambient air without closed loop feedback, extensive electronics, or cooling. Due to the soft suspension of the micro-mechanical sub-system, the fundamental limit, i.e., the Brownian noise floor, is reached for frequencies below the resonance frequency. The related noise equivalent displacement is 1.9 pm/√Hz. Above resonance, the measured noise of 0.86 pm/√Hz, which is equivalent to 0.29 μg/√Hz is dominated by the noise of the electrical components. Also a discussion about feasible improvements regarding the sensitivity and pushing the mechanical resonance frequency below 100 Hz is given.

Novel high resolution MOEMS inclination sensor

Inclination sensors are essential elements in many different fields of application such as navigation, metrology, geodetics, geoscience, as well as in the consumer market. In general, for high precision navigation. The sensors resolution is one of the key parameters to improve the performance and to open up new areas of applications. The implementation of a miniaturized optical readout is one of the most promising ways to accomplish this goal. This paper discusses the principle of a novel inclination sensor along with obtained measurements of two prototypes with different mechanical parameters. The achieved sensitivity is 0.044 V/° resulting in an angular resolution of 0.0051°.

Exploiting infrared transparency of silicon for the construction of advanced MOEMS vibration sensors

The motion of the seismic mass that is induced by thermal noise limits the resolution of typical micromachined vibration sensors. Its value can be adjusted by the size of the proof mass which is also a quantity for the inertial actuation input. Owing to a novel transduction concept, micro-opto-electro-mechanical vibration sensors featuring approximately twice as much mass per chip area are feasible, while decreasing the technological efforts during fabrication. The essence of the devised sensor principle is the modulation of the intensity of a light flux propagating perpendicularly through a pair of micromachined apertures. One aperture is fixed to the encapsulation and the second one is deffected by inertial forces. Earlier attempts have employed opto-electrical transmitters and receivers operating at a wavelength where silicon is intransparent. Thus, openings in the silicon mass were necessary. The presented evaluation technique utilizes the transparency of silicon in the infrared region at wavelengths well above 1.1 μm. In contrast to the previously used optoelectronic components, an InGaAs LED and an InGaAs pin-diode were integrated. This all enables of thin-film metal apertures deposited on top of the silicon seismic mass instead of etched silicon windows. Beside the increase in mass, this approach offers larger scope for design and implies a reduced damping coefficient yielding an improved quality factor. A structure for the proof of concept was fabricated and characterized together with a sensor based on the preceding principle. The results are in good agreement with the predicted behavior and the parameters tested by FEM analysis considering the fabrication related underetching as well.

Passive optomechanical electric field strength sensor with built-in vibration suppression

Methods for measuring low-frequency and static electric field strength are of great use in many areas ranging from meteorology to high-voltage infrastructure or safety. Nevertheless, all state-of-the-art methods have grave intrinsic drawbacks such as severe inherent field distortions or overpronounced temperature behavior. Recently, a method has been developed which allows for distortion-free and temperature-stable measurement. In this work, a micromechanical sensor based on this method is presented which features suspensions that suppress cross-sensitivities to vibrations. Two such types of suspensions were evaluated and compared in terms of their mechanical modes and susceptibility to electric fields and vibrations. It is shown that these suspensions indeed suppress the cross-sensitivities. The sensors exhibit field strength resolutions down to 737 V / m / Hz with a theoretical limit as low as 59.3 V / m / Hz.Methods for measuring low-frequency and static electric field strength are of great use in many areas ranging from meteorology to high-voltage infrastructure or safety. Nevertheless, all state-of-the-art methods have grave intrinsic drawbacks such as severe inherent field distortions or overpronounced temperature behavior. Recently, a method has been developed which allows for distortion-free and temperature-stable measurement. In this work, a micromechanical sensor based on this method is presented which features suspensions that suppress cross-sensitivities to vibrations. Two such types of suspensions were evaluated and compared in terms of their mechanical modes and susceptibility to electric fields and vibrations. It is shown that these suspensions indeed suppress the cross-sensitivities. The sensors exhibit field strength resolutions down to 737 V / m / Hz with a theoretical limit as low as 59.3 V / m / Hz.