Andreas Vogl

Fabrication and characterization of a wideband MEMS energy harvester utilizing nonlinear springs

This paper presents the fabrication, characterization and modeling of a wideband MEMS electrostatic energy harvester utilizing nonlinear springs. The experimental results show that the vibration energy harvester displays a strong softening spring effect. For narrow band vibration, the energy harvester exhibits a widening bandwidth during frequency down-sweeps. For increasing levels of broadband random noise vibration, the energy harvester displays a broadening bandwidth response. Furthermore, the vibration energy harvester with softening springs not only increases the bandwidth, but also harvests more output power than a linear energy harvester at a sufficient level of broadband random vibration. At a broadband random vibration of 7.0 × 10−4 g2 Hz−1, we found that the bandwidth increases by more than 13 times and the average harvesting output power increases by 68% compared to that of a linear vibration energy harvester. Numerical analysis confirmed that the softening springs are responsible for the band broadening.

A novel ultra-planar, long-stroke and low-voltage piezoelectric micromirror

A novel piston-type micromirror with a stroke of up to 20 µm at 20 V formed out of a silicon-on-insulator wafer with integrated piezoelectric actuators was designed, fabricated and characterized. The peak-to-valley planarity of a 2 mm diameter mirror was better than 15 nm, and tip-to-tip tilt upon actuation less than 30 nm. A resonance frequency of 9.8 kHz was measured. Analytical and finite element models were developed and compared to measurements. The design is based on a silicon-on-insulator wafer where the circular mirror is formed out of the handle silicon, thus forming a thick, highly rigid and ultra-planar mirror surface. The mirror plate is connected to a supporting frame through a membrane formed out of the device silicon layer. A piezoelectric actuator made of lead–zirconate–titanate (PZT) thin film is structured on top of the membrane, providing mirror deflection by deformation of the membrane. Two actuator designs were tested: one with a single ring and the other with a double ring providing bidirectional movement of the mirror. The fabricated mirrors were characterized by white light interferometry to determine the static and temporal response as well as mirror planarity.

A miniaturized pressure sensor with inherent biofouling protection designed for in vivo applications

The design, fabrication, and measurement results for a diaphragm-based single crystal silicon sensor element of size 820 μm × 820 μm × 500 μm are presented. The sensor element is designed for in vivo applications with respect to size and measurement range. Moreover, it is optimized for longtime operation in the human body through a built-in protection preventing biofouling on the piezoresistors. The sensitivity is about 20 mV/V for a change from 500 to 1500 mbar absolute pressure. This result is comparable to conventional sized micromachined pressure sensors. The output signal is not found to be influenced by exposure to 60 °C for three hours, a normal temperature load for a typical sterilization process for medical devices (Ethylene Oxide Sterilization). The hysteresis is low; < 0.25% of full scale output signal. The sensor element withstands an overload pressure of 3000 mbar absolute pressure. Observed decrease in the output signal with temperatures and observed nonlinearity can easily be handled by traditional electronic compensation techniques.

Design and processing of a cost-effective piezoresistive MEMS cantilever sensor for medical and biomedical use

In this special section article, cost-effective methods for fabrication of a piezoresistive cantilever sensor for industrial use are focused on. The intended use of the presented cantilever is a medical application. A closer description of the cantilever design is given. The low-cost processing sequence is presented and each processing step is explained in detail. The processing sequence is also compared to other low-cost fabrication techniques. Results from the electrical probing and mechanical strength test are given. The results demonstrate that the chosen low-cost processing route results in high yield and a mechanical robust device.

Cost-Efficient Wafer-Level Capping for MEMS and Imaging Sensors by Adhesive Wafer Bonding

Device encapsulation and packaging often constitutes a substantial part of the fabrication cost of micro electro-mechanical systems (MEMS) transducers and imaging sensor devices. In this paper, we propose a simple and cost-effective wafer-level capping method that utilizes a limited number of highly standardized process steps as well as low-cost materials. The proposed capping process is based on low-temperature adhesive wafer bonding, which ensures full complementary metal-oxide-semiconductor (CMOS) compatibility. All necessary fabrication steps for the wafer bonding, such as cavity formation and deposition of the adhesive, are performed on the capping substrate. The polymer adhesive is deposited by spray-coating on the capping wafer containing the cavities. Thus, no lithographic patterning of the polymer adhesive is needed, and material waste is minimized. Furthermore, this process does not require any additional fabrication steps on the device wafer, which lowers the process complexity and fabrication costs. We demonstrate the proposed capping method by packaging two different MEMS devices. The two MEMS devices include a vibration sensor and an acceleration switch, which employ two different electrical interconnection schemes. The experimental results show wafer-level capping with excellent bond quality due to the re-flow behavior of the polymer adhesive. No impediment to the functionality of the MEMS devices was observed, which indicates that the encapsulation does not introduce significant tensile nor compressive stresses. Thus, we present a highly versatile, robust, and cost-efficient capping method for components such as MEMS and imaging sensors.

The effect of true human synovial fluid on the functionality of an in vivo pressure sensor element

This paper presents a study on the feasibility of packaging a sensor element by a thin biocompatible coating. The goal of the work was twofold; Firstly to investigate the possible impact of the coating on sensor element performance; Secondly to examine the sensor element functionality after soaking into true human synovial fluid for more than 30 days. Sensor elements with two different structures of TiO2, the amorphous and the anatase, were examined and compared to uncoated elements. The device under test was a piezoresistive pressure sensor element designed for in vivo applications. Pressure characteristics were measured before and after Atomic Layer Deposition of the TiO2 coatings. Sensor signals were examined and visual inspection of the sensor element surfaces were done after more than 30 days soaking in true human synovial fluid. Throughout the soaking period the shift in output signal was higher and varied more for uncoated elements than for coated ones. Our results indicate that a 20 nm thick TiO2 coating can provide good protection towards the harsh synovial fluid.

pMUT for high intensity focused ultrasound

Capacitive ultrasonic tranducers, cMUTs rely on the electrostatic field between the membrane and a back plate for sensing andactuation. This is an excellent solution for small amplitudes. But the movement of the membrane is physically limited by the bottom plate (risk of collapse). Furthermore, pull-in and linearity considerations restrict the available range to about one percent of thegap. Piezoelectric micromachined ultrasonic transducers, pMUTs, on the other hand have no such restrictions. The excitation is basedon lateral contraction of a thin film of Lead Zirconate Titanate, PZT, deposited on top of the membrane. Then there is no need for abackplate, and the linear range is greatly increased. Therefore, pMUTs are ideally suited for applications demanding large excitationamplitude, such as high intensity focused ultrasound, HIFU. In this work, we present pMUTs designed for HIFU operation around 1MHz.

3D Modelling of MEMS Components for Coupled Elastic &#8212; Electrostatic Characterization

This paper presents a method for modelling MEMS components whereby the designer can perform precise electrical, mechanical, and coupled electrical and mechanical simulations. Both surface and bulk micromachined components can be modelled at a geometrical level by defining simplified pseudo processes for a commercial CAD tool such as CovertorWare. The procedure for this approach is described with emphasis on the bulk case. A motivating example of a simple comb-finger structure in an accelerometer shows that this method is required to obtain adequate precision. A long thin bulk micromachined mirror array, which is not easily described analytically, was modelled by using a pseudo process and the coupled field problem of its elastic-electrostatic behaviour has been investigated.