Stéphane Kühne

Influence of air humidity on polymeric microresonators

The influence of air humidity on polymeric microresonators is investigated by means of three different resonator types. SU-8 microbeams, SU-8 microstrings and a silicon micromirror with SU-8 hinges are exposed to relative humidities between 3% and 60%. The shifts of the resonant frequencies as a function of the relative humidity (RH) are explained based on mechanical models which are extended with water absorption models in polymer materials. The dominant effect causing the resonant frequency change is evaluated for each structure type. The eigenfrequency of the microstrings and the micromirror in the out-of-plane mode, which both mainly are defined by the pre-stress of the polymeric structures, are found to be highly sensitive to changes of air humidity. The humidity-induced (hygrometric) volume expansion reversibly reduces the pre-stress which results in relative frequency changes of up to 0.78%/%RH for the microstrings. A maximum coefficient of humidity-induced volume expansion for SU-8 of ?hyg = 52.3 ppm/%RH is evaluated by fitting the data with the analytical model. It was found that microstrings that were stored at 150 ?C over 150 h are more moisture sensitive compared to structures that were stored at room temperature. For the SU-8 microbeams and the micromirror in the tilt mode, the eigenfrequency is mainly defined by the modulus of the polymer material. The measured relative resonant frequency changes were below 1% for the given RH range. For low RH values, antiplasticization is observed (the modulus increases) followed by a plasticization for increasing RH values.

Wafer-level flame-spray-pyrolysis deposition of gas-sensitive layers on microsensors

This paper presents a CMOS-compatible wafer-level fabrication process for monolithic CMOS/MEMS sensor systems coated with sensitive layers directly deposited by means of flame spray pyrolysis (FSP). Microhotplate (?HP)-based devices, featuring an FSP directly deposited SnO2/Pt layer, have successfully been realized on a wafer level. The thermal characterization evidenced a thermal resistance of 10.6 ?C mW?1; moreover, gas test measurements with ethanol have been performed. Microhotplate membrane deformations during device operation have been investigated and have been reduced by adjustment of the intrinsic stress of a deposited silicon nitride layer.

Large Stroke Staggered Vertical Comb-Drive Actuator for the Application of a Millimeter-Wave Tunable Phase Shifter

This paper presents the design, fabrication, and characterization of a MEMS actuator with large static deflection as a waveguide-mounted variable millimeter-wave phase shifter. The actuator is composed of a pair of interdigitated microplates actuated by vertical comb-drives and suspended by SU-8 torsional springs. The SU-8 spring possesses a thin metallization top layer and a reverse-T-shaped cross-section enabling low torsional stiffness and high in-plane stability. A maximum mechanical deflection of 10.3° is obtained under a dc actuation voltage of 35 V. The dynamic characterization of the device shows that the resonance frequency of the torsional mode is well separated from the other three bending modes, confirming the designed low torsional stiffness and high in-plane stability. The torsional viscoelastic creeping is measured as a function of time at different loads and shows a maximum of 0.5° for an applied voltage of 27.5 V. A high operation cycle test is conducted and the metalized SU-8 spring withstands 800 million cycles without showing fatigue. RF measurements show that a variable mechanical deflection angle between 0° and 8.2° results in a variable transmission phase shift up to 58.0°. The measured insertion loss is always below 5.1 dB at 98 GHz, corresponding to a figure of merit of 11.5°/dB.

MEMS scanning mirror supported by soft polymeric springs and actuated by electrostatic charge separation

We present a novel MEMS scanning mirror featuring soft polymeric suspensions of the mirror plate. The combination of the polymeric suspension and a stiff silicon mirror plate enables large scan angles with a negligible distortion of the mirror plate itself at frequencies up to 1 kHz. Thus, the scan angle per applied actuation voltage is high compared to single material solutions by H. Miyajima et al. (2004). Furthermore, the mirror is actuated by electrostatic charge separation. Therefore, no electrical connection of the mirror is required.

A microdevice with large deflection for variable-ratio RF MEMS power divider applications

In this paper, we present the design and fabrication of a large deflection MEMS actuator directly integrated as a variable-ratio radio frequency (RF) power divider in a waveguide. The device is based on a tilting microplate suspended by SU-8 springs and driven by a vertical comb drive actuator. The fabricated device is characterized by laser Doppler vibrometer and white light interferometer measurements. The dynamic measurement confirms the resonance frequency of the torsional mode being around 370 Hz, from which the spring stiffness has been extracted and used for static modeling of the device. The fabricated microplate devices achieved deflection angles of 5.9° with a dc actuation voltage of 30 V. First RF transmission measurements show good agreement with results from electromagnetic field simulations. A variable power split ranging from equal division up to a ratio of more than 1:2 is measured at 82.5 GHz whilst keeping the amount of dissipated power below 25%. This is the first reported actuated RF MEMS device integrated in a metal waveguide operating at such high frequency.

Low Temperature Fabrication Process for High-Aspect-Ratio and Multi-Compliant MEMS

The presented process technology enables the fabrication of 3-dimensional high-aspect-ratio microstructures featuring the integration of multiple materials. The technology offers the possibility to benefit from unique material properties for optimized MEMS functionality and performance. The fabrication process relies on wafer stacking by low temperature plasma activated direct bonding, SOI layer transfer and SU-8 structuring. The proposed process flow is validated by the fabrication of micro mirrors featuring a soft polymeric suspension and a high-aspect-ratio vertical comb-drive actuator. The devices are characterized and confirm the expected performance with dynamic resonant optical deflection angles of 59° at 50V.

Power enhancement of micro thermoelectric generators by micro fluidic heat transfer packaging

This paper reports the design, fabrication and proof of concept of a multilayer fluidic packaging system enabling an increase in the output power performance of micro thermoelectric generators (μTEGs). The complete integration of the fluidic heat transfer system (HTS) with a μTEG is successfully demonstrated. The fabricated μHTS prototypes were tested with respect to their heat transfer resistance and consumed pumping power. A heat transfer resistance of 0.48cm²K/W was achieved with a small pumping power of 3.7mW/cm². This results in a potential μTEG power output enhancement of 38 times at a heat source temperature of 320K, cold side temperature of 300K and a thermoelectric figure of merit (Z) of 1.1×10⁻³ K⁻¹.

Fabrication and characterization of a tethered rotational planar variable capacitance micro drive

In this paper, we present the design, fabrication and characterization of a planar variable capacitance micro drive. The rotational micro drive is developed for high-speed applications with contactless active electrostatic micro bearings. The associated low-temperature process allows the fabrication of devices with narrow stator–rotor gaps. The drive performance is characterized by means of tethered functional prototype devices. The multicompliant devices have silicon rotors and a soft polymer suspension, which allows the validated modeling of the drive capacitance and accurate measurement of the static drive torques. The devices achieve up to 2.6 nNm static drive torque per phase at an actuation voltage of 12 V. These results demonstrate the highest torque generation of a planar variable capacitance drive at low actuation voltages.