Matthias Sachse

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.

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.

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.

Optical MEMS vibration sensor

In this work a competitive OMEMS (optical micro electromechanical system) readout for displacement is presented. Congruent positioned gratings modulate the light flux caused by a relative in-plane displacement. As demonstrator, an inertial sensor is used. Consisting of a Si-chip bearing a perforated and spring suspended MEMS structure (seismic mass), which is bonded with SU8 to a glass chip featuring vapor deposited Cr-pattern. Both form a partially transparent sandwich structure where the transmittance depends on the position of the suspended Si mass. The modulated light flux is generated and detected by an SMD-LED and a phototransistor at the top and bottom side of the sandwich structure, respectively. First results show a high sensitivity of 21 mV/nm displacement of the seismic mass featuring a noise level of about 200 pm/√Hz.

Design of an implantable seismic sensor placed on the ossicular chain

This paper presents a design guideline for matching a fully implantable middle ear microphone with the physiology of human hearing. The guideline defines the first natural frequency of a seismic sensor placed at the tip of the manubrium mallei with respect to the frequency-dependence hearing of the human ear as well as the deflection of the ossicular chain. A transducer designed in compliance with the guideline presented reduces the range of the output signal while preserving all information obtained by the ossicular chain. On top of a output signal compression, static deflections, which can mask the tiny motions of the ossicles, are reduced. For guideline verification, a microelectromechanical system (MEMS) based on silicon on insulator technology was produced and tested. This prototype is capable of resolving 0.4 pm/Hz with a custom made read-out circuit. For a bandwidth of 0.1 kHz, this deflection is comparable with the lower threshold of speech (≈ 40 phon).

Noise considerations on hybrid optical MEMS displacement sensors

Light flux modulators based on micro-opto-electromechanical systems (MOEMS) enable high resolution displacement based inertial sensors. To unveil the resolution limiting mechanisms, we investigated the performance of modulators with a rectangular aperture design. The two major noise sources for such devices are the Brownian noise originating from the surrounding air and the noise emerging from the opto-electronic photodetector. The experimental data suggests that the noise equivalent displacement is determined by the transmitive aspect ratio of the aperture.

Experimental setup for the coating of chlorosilane based self assembling monolayers to reduce stiction in MEMS devices

An often reported problem during production and operation of silicon MEMS is stiction. It describes the sticking of movable MEMS parts to surrounding structures. The probability of the occurrence of stiction is linked to the surface energy of the MEMS. Self assembling monolayers can be used to reduce the surface energy and therefore the probability of stiction. These monolayers have to resist high temperatures up to 400°C to be compatible with various micro-production processes, e.g., eutectic bonding. Several groups tried to coat such monolayers with different success and results. One problem is the instability of the coating method due to water contaminations of the coating solution. To circumvent this error source, an experimental setup was designed and built up to minimize the water content of the monolayer solvent and ensures reproducible conditions during the coating process. The required set of liquids is piped through a system of valves and tubes to rinse a trench with a silicon die. To avoid contamination of the liquids with water, the setup is partly placed in a box flushed with nitrogen. With this experimental setup, the surface energy γs of the MEMS structures had been reduced from 18.1 mJ/m2 to 33.1 μJ/m2 and 36.6 μJ/m2 for FDTS and DDMS, respectively.