Volker Biefeld

Laterally driven accelerometer fabricated in single crystalline silicon

Abstract The request for wireless sensor operations grows in medical and automotive applications. These sensors receive their energy and send their data by a telemetric unit. The wireless transferred energy restricts the power consumption of the sensor and signal processing to less than 3 mW. Therefore, the sensor has to be operated in open-loop. Furthermore, a main focus is directed to increase the sensitivity of the mechanical–electrical transducer. Considering both open-loop and sensitivity, the sensor has to be optimized by referring to the structure height. The way for realizing high structures, as described in this paper, is the micromachining of silicon wafers with a specified thickness. The superior mechanical properties of single crystalline silicon compared to electroplated metals or surface-micromachined devices confirm the use of silicon as sensor material. A laterally driven accelerometer is simulated, designed and fabricated comprising the technologies of deep reactive ion etching (DRIE) of silicon, silicon direct bonding (SDB) and chemical mechanical polishing (CMP). Characterization results confirm the performance of this new technology. The open-loop sensor, which was characterized, had a height of 50 μm with damping constant greater than 0.1.

A Multi-Channel, Flex-Rigid ECoG Microelectrode Array for Visual Cortical Interfacing

High-density electrocortical (ECoG) microelectrode arrays are promising signal-acquisition platforms for brain-computer interfaces envisioned, e.g., as high-performance communication solutions for paralyzed persons. We propose a multi-channel microelectrode array capable of recording ECoG field potentials with high spatial resolution. The proposed array is of a 150 mm2 total recording area; it has 124 circular electrodes (100, 300 and 500 μm in diameter) situated on the edges of concentric hexagons (min. 0.8 mm interdistance) and a skull-facing reference electrode (2.5 mm2 surface area). The array is processed as a free-standing device to enable monolithic integration of a rigid interposer, designed for soldering of fine-pitch SMD-connectors on a minimal assembly area. Electrochemical characterization revealed distinct impedance spectral bands for the 100, 300 and 500 μm-type electrodes, and for the arrays own reference. Epidural recordings from the primary visual cortex (V1) of an awake Rhesus macaque showed natural electrophysiological signals and clear responses to standard visual stimulation. The ECoG electrodes of larger surface area recorded signals with greater spectral power in the gamma band, while the skull-facing reference electrode provided higher average gamma power spectral density (γPSD) than the common average referencing technique.

Design and fabrication of multi-contact flexible silicon probes for intracortical floating implantation

This paper reports on a novel design and process flow development for the fabrication of multi-contact silicon probes with monolithically integrated highly flexible ribbon cables on wafer level, based on the biocompatible polymer parylene-C. Compared to the state-of-the-art silicon probes, this novel development allows for the first time a floating implantation of these neural probes in the cortex with reduced destructive forces applied to the brain tissue. In-vitro electrical impedance spectroscopy measurements and first in-vivo measurements in the cortex of a rat demonstrate the functionality of these probes.

Overcritical damped laterally moving microstructures by ADRIE using SOI-substrates for automotive applications

A fabrication process for laterally moving single crystal silicon microstructures on SOI substrates is presented. Due to an ADRIE process high aspect ratio structures are realized. The underlying silicon dioxide layer of the SOI substrate serves as sacrificial layer. A HF vapor etching system is used for the sacrificial layer etching to avoid sticking effects of the structures. For the fabrication of an acceleration threshold switch a metallized contact area is necessary. The switching contact is realized using a sidewall metalization of the laterally moving structures. The sensor structure is that of a spring mass system. To avoid uncontrollable switchings of the device, an overcritical damping of the sensor structure is needed. The high aspect ratio of the structures makes these high damping coefficients possible. The dynamic behavior of the device is achieved by squeeze-film damping of the high aspect ratio structures. Using optical measurement equipment for the device characterization, overcritical damping coefficients can be verified for the fabricated structures. The mechanical properties and the dynamic behavior of the structures are ideal for the construction of acceleration threshold switches for automotive applications.