Roland Müller-Fiedler

Reliability aspects of microsensors and micromechatronic actuators for automotive applications

Abstract In modern cars microsensors and micromechatronic actuators play an essential and still increasingly important role as the interface between the vehicle with its complex functions of motor management, chassis systems, safety as well as comfort and convenience on the one hand and the respective electronic control units on the other hand. They have to fulfill their task in a harsh environment over the entire lifetime of an automobile. Therefore reliability aspects have moved more and more into the focus of engineering and research activities in microsystem technology. A methodical procedure is described that allows reliability issues to be approached efficiently. The implementation of the methodology is illustrated with real-life examples of reliability aspects of hot film mass air flow sensors, inertial sensors as well as piezoelectric actuators.

Experimental Study of In-Plane and Out-of-Plane Adhesions in Microelectromechanical Systems

Two different but similar microdevices were used for studying in-plane (IP) and out-of-plane (OP) adhesions in microelectromechanical systems (MEMS). With these devices, the combined influence of temperature, surface chemistry, contact geometry, and applied load on the adhesion force was investigated. The adhesion force is the maximum force required to separate two contacting surfaces. Distinguished between IP and OP contacts, the adhesion mechanisms for hydrophilic and hydrophobic surfaces were identified. The microdevices, fabricated within the same wafer, may exhibit different IP and OP adhesion mechanisms, dependent upon the wafer surface chemistry. For the hydrophilic surface and IP contact, the dominant adhesion mechanism is hydrogen bonding, while for OP contact, the electrostatic force is dominant. The dominant adhesion mechanism for the hydrophobic surface is the Van-der-Waals force for both IP and OP contacts. Furthermore, the influence of elastic-plastic deformation of interacting asperities on the adhesion force is addressed by experiments at different loads. At constant applied load in a cycling contact-release test, a decreasing and asymptotic behavior of the adhesion force is observed, suggesting strain hardening and reverse plastic deformation of the contacting asperities.

Vertical Silicon K-Band CPW Through-Wafer Interconnects

With the increase in production volume of RF devices (e.g. for automotive applications), packaging and interconnection become more and more important. Furthermore, new system concepts such as chip-on-chip or RF-MEMS demand new packaging strategies. This paper presents a vertical silicon micromachined RF CPW through-wafer feedthrough with excellent performance in the K-band. In particular, the feed through demonstrates an insertion loss of 0.16dB and a return loss of 20dB at 25GHz. Alumped element model was developed and was evaluated with measurements.

Mechanical reliability of epipoly MEMS structures under shock load

The design, fabrication and characterization of epi-polysilicon test structures for dynamic tensile tests under shock loads ranging from 2,500 g to 30,000 g are reported. In accordance with published data for static fracture strength, their dynamic failure rate obeys a Weibull distribution model. The fracture surfaces suggest that failure criteria based on maximum principal stress are effective. Stresses resulting from shock load were obtained using finite element simulations. It was shown that even slight deviations from optimum loading conditions (i.e., out-of-plane components) can considerably increase the maximum experienced stress. Therefore, the usage of simplistic models to estimate maximum stress levels from shock loads is greatly discouraged. An out-of-plane component was identified in the experimental setup used. Based on an assessment of the magnitude of deviation, the failure stresses are estimated to fall below the quasi-static fracture strength by ca. 50%.

A four-point-bending-test for the stability assessment of glass frit bonded molded microsensors

For increasingly miniaturized micromechanical sensors the silicon package is thinned and therefore more sensitive to thermomechanical stresses caused by the production stages. We present a four-point-bending-test that enables the investigation of reliability relevant loading like warping. The initiation of crack growth at the glass frit bonding frame is observed by an infrared camera and critical fracture mechanical parameters are determined for different tensile and shear mode contributions. Based on this crucial fracture mechanical data the stability of silicon packages can be assessed.

Hermeticity of eutectic bond layers for sensor packages on wafer-level

In MEMS industry there is a high demand for reliable hermetic wafer-level encapsulation. This paper presents a methodology to monitor the pressure inside a package after the wafer-level bonding process. Therefore, novel micro-electro-mechanical resonators designed as double ended tuning forks (DETFs) were built to measure the quality factor Q as a function of the inside cavity pressure. Obtained experimental results clearly verified the feasibility of the developed MEMS structures, providing a simple method to evaluate the hermeticity of wafer bonding technologies.

Reliability of micromachined membranes under particle impact

We report experimental and numerical results on the mechanical reliability of silicon nitride membranes under particle impact. Over 1000 membranes of various geometries were subjected to controlled particle bombardment leading to their failure. Based on the statistical analysis of the characteristic times to failure /spl tau/ and their dependence on geometry, on the optical failure analysis, and on numerical simulations of impact events, the following conclusions are drawn: failure is due to a singular event as opposed to fatigue, failure is caused by impacts close to the membrane edge, the absolute distance of the impact point to the nearest edge is the dominant factor, the characteristic lifetime /spl tau/ is inversely proportional to the perimeter length, reinforcing the membrane perimeter secures an up to tenfold increase in lifetime.

Scalability of Capacitive RF MEMS Switches

MEMS capacitive shunt switches have attracted large interest in various microwave or mm-wave applications, such as low power communication systems or phased arrayed antennas [1,2,3]. Two types of RF MEMS switches are discussed with respect to their impedance matching, scalability in frequency and related mechanical limitations. As a result, a W-band MEMS switch with a measured insertion loss of 33 dB at 94 GHz is presented.

Novel Test Structures for Hermeticity Testing of Wafer Bonding Technologies

Introduction Packaging of microsensors fulfills the important task to protect the microstructure from environmental influences and mechanical impact. In the case of resonantly driven sensors, a very low pressure has to be maintained within an enclosed cavity in the package in order to ensure the required quality factor. Eutectic bonding is a promising technology for the hermetical encapsulation of surface micro-machined structures. In order to monitor the pressure inside a package after the bonding process novel test structures have been developed.

Mikromechanische (MEMS) Schalter für Millimeterwellen-Systeme in KFZ-Rundumsichtanwendungen

In recent years, great advances have been made in the field of silicon millimeter-wave integrated circuits (SIMMWIC) as well as in microsystems technology. These developments jointly allow the preparation of micromechanical capacitive switches, which exhibit excellent RF properties far into the millimeter-wave frequency range. This paper presents coplanar bridge and longitudinal switches and discusses their applicability for automotive surround sensing system. It comprises a detailed review of RF properties in addition to an introduction to the systems background. Further, aspects are treated which are important for the application, as mechanical properties, reliability, and packaging and mounting technologies. Finally, the potential for further systems integration will be shown. Für die Dokumentation Mikromechanik / Schalter / Millimeterwelle / Radar / Integration