M. Fischer

Black silicon—new functionalities in microsystems

Black silicon and its application as a new assembly method for silicon wafers at room temperature is presented. Needle-like structures on the surface after deep reactive ion etching with a length of 15–25 µm and 300–500 nm in diameter interlock with each other to form a bonding interface. After compression of two wafers at room temperature they generate retention forces up to 380 N cm−2 (3.8 MPa). If low contact forces are applied with partially interlocking of the needles, it is possible to generate a reversible Velcro®-like assembly. This new bonding process can be used for applications in the area of microfluidics with catalysts, microoptical or mechanical mountings or carrier wafer bonding in microelectronics.

Microcavity array (MCA)-based biosensor chip for functional drug screening of 3D tissue models.

Multicellular tumour spheroids that mimic a native cellular environment are widely used as model systems for drug testing. To study drug effects on three-dimensional cultures in real-time we designed and fabricated a novel type of sensor chip for fast, non-destructive impedance spectroscopy and extracellular recording. Precultured spheroids are trapped between four gold electrodes. Fifteen individual 100microm deep square microcavities with sizes from 200 to 400microm allow an optimised positioning during the measurement. Although apoptosis was induced in human melanoma spheroids by Camptothecin (CTT), treated cultures did not show disintegration but displayed increased impedance magnitudes compared to controls after 8h resulting from an altered morphology of the outer cells. Contractions in cardiomyocyte spheroids were monitored when the innovative chip was used for recording of extracellular potentials. The silicon-based electrode array is used as an acute test system for the monitoring of any kind of 3D cell cultures. Since no adherence of cells or labelling is necessary the multifunctional sensor chip provides a basis for improved drug development by high content screenings with reduced costs and assay times. Additional improvements for parallel testing of different substances on one chip are presented.

LTCC-based Fluidic Components for Chemical Applications

The material properties of low-temperature co-fired ceramics (LTCCs) enable new application areas beyond the use as basic material for high-density circuits. Therefore, LTCCs are increasingly used for microfluidic applications. A modular LTCC-based microreaction system is presented in this article. One focal point of our work was the development of a connecting system for these ceramic fluidic modules. Importance was attached to solid mechanical design, good temperature resistance of fluidic and electrical connections, and minimized dead volume. In contrast to LTCC-based substrates with only electrical circuits, integration of fluidic components makes great demands, especially on the mechanical processing of the green tapes. Mechanical processing of LTCC was made by laser cutting or laser ablation. Single modules and a complete system are the subject of chemical characterization. LTCC-based mixer structures are characterized using the Villermaux-Dushman method. The functionality of the whole system is shown with an example reaction.

Silicon on Ceramics―A New Integration Concept for Silicon Devices to LTCC

A new integration concept for MEMS-devices to ceramic substrates based on a new bonding technique between nano-scaled black silicon (BSi) and an adapted LTCC substrate is presented. The novel technique allows combining advantages of silicon and ceramic technology whereby a new wafer compound material (silicon on ceramics) and innovative ceramic carrier as well as chip packages become available. The new compound is fabricated by the use of standard technologies (reactive ion etching, lamination, and pressure assisted sintering) without additional materials and devices. A bonding strength up to 1750 N/cm2 and gas tightness are remarkable features of the bond interface. Simultaneously, electrical interconnects between silicon and LTCC can be manufactured during lamination and sintering.

Suspended nanowire web

A complex three-dimensional, nanowire based nanoarchitecture is presented, which can be processed by high-throughput bottom-up procedures without any high-resolution lithography. It combines the benefits of three self-organization mechanisms to produce nanostructures, i.e., the formation of nanoneedles, the droplet formation out of a thin metal film, and the vapor-liquid-solid growth of nanowires. The principle is demonstrated for a silicon based suspended nanowire web. Cell adherence on this assembly was found to be superior to other nanostructures. The possibility of fluid transport beneath the nanowire web enables improved microcatalyst principles and the realization of novel interfaces for biosensing or bioelectronics.

A novel 384-multiwell microelectrode array for the impedimetric monitoring of Tau protein induced neurodegenerative processes

Over the last decades, countless bioelectronic monitoring systems were developed for the analysis of cells as well as complex tissues. Most studies addressed the sensitivity and specificity of the bioelectronic detection method in comparison to classical molecular biological assays. In contrast, the up scaling as a prerequisite for the practical application of these novel bioelectronic monitoring systems is mostly only discussed theoretically. In this context, we developed a novel 384-multiwell microelectrode array (MMEA) based measurement system for the sensitive label-free real-time monitoring of neurodegenerative processes by impedance spectroscopy. With respect to the needs of productive screening systems for robust and reproducible measurements on high numbers of plates, we focused on reducing the critical contacting of more than 400 electrodes for a 384-MMEA. Therefore, we introduced an on top array of immersive counter electrodes that are individually addressed by a multiplexer and connected all measurement electrodes on the 384-MMEA to a single contact point. More strikingly, our novel approach provided a comparable signal stability and sensitivity similar to an array with integrated counter electrodes. Next, we optimized a SH-SY5Y cell based tauopathy model by introducing a novel 5-fold Tau mutation eliminating the need of artificial tauopathy induction. In combination with our novel 384-MMEA based measurement system, the concentration and time dependent neuroregenerative effect of the kinase inhibitor SRN-003-556 could be quantitatively monitored. Thus, our novel screening system could be a useful tool to identify and develop potential novel therapeutics in the field of Tau-related neurodegenerative diseases.

Multi-technology design of an integrated MEMS-based RF oscillator using a novel silicon-ceramic compound substrate

In this paper, an approach towards the realization of a hybrid MEMS-CMOS RF oscillator module using the novel silicon-ceramic (SiCer) compound substrate technology is described. Piezoelectric aluminium-nitride MEMS resonators with quality factors Q up to 2,200 and resonant frequencies of 240, 400 and 600 MHz have been investigated as frequency-selective elements. For RF-compatible hybrid-integrated assembly and packaging, the SiCer compound substrate has been adapted, promising an efficient integration of both, microelectronic and micromechanical devices, on a single carrier substrate. Multiphysical circuit design and simulations using parametrized behavioural MEMS models have been carried out, indicating stable oscillator operation at the design frequency. As one prospective application, such an oscillator module could form part of a compact and power-efficient reconfigurable RF transceiver frontend in SiCer technology, e.g., for mobile communications.

RF-MEMS-platform based on silicon-ceramic-composite-substrates

In the last few years, several Low Temperature Co-fired Ceramics (LTCC) materials with a silicon adapted Coefficient of Thermal Expansion (CTE) have been developed for direct wafer bonding to silicon. BGK (special type designation of Fraunhofer IKTS), a sodium containing LTCC was originally developed for anodic bonding of the sintered LTCC whereas BCT (Bondable Ceramic Tape) tailored for direct silicon bonding of green LTCC tapes to fabricate a quasi-monolithic silicon ceramic compound substrate. This so-called SiCer technique is based on homogeneous nano-structuring of a silicon substrate, a lamination step of BCT and silicon and a subsequent pressure assisted sintering. We present a new approach for an integrated RF-platform-setup combining passive, active and mechanical elements on one SiCer substrate. In this context RF parameters of the silicon adapted LTCC tapes are investigated. We show first technological results of creating cavities at the silicon ceramic interface for SiCer-specific contacting options as well as windows in the ceramic layer of the SiCer substrate for additional silicon processing. A further investigated platform technology is deep reactive ion etching of the silicon-ceramic-composite-substrate. The etching behavior of silicon on BCT will be demonstrated and discussed. With the SiCer technique it is possible to reduce the silicon content at the setup of RF MEMS to a minimum (low signal damping).

Electrostatic parallel-plate MEMS switch on silicon-ceramic-composite-substrates

In this work we will present the capabilities of this monolithic SiCer (silicon on ceramics) compound by producing a parallel-plate RF-MEMS switch with flexible electrodes and integrated coplanar waveguides. The series switch is one part of a LTE demonstrator and is developed in a heterogeneous process design. Here, the aim is to create a low-voltage switch for mobile use with a simple layout. The modelling and simulation of the parallel-plate switch with flexible electrodes is carried out using ANSYS (electro-mechanical simulation) and CADENCE (circuit simulation). To demonstrate the advantages of the composite substrate, the coplanar waveguides for the RF-signal and the control lines for the actuation of the electrostatic parallel plates of the switch are processed by screen printing them on the LTCC tapes before sintering the composite. The relocation of the waveguides into the LTCC avoids damping influences on RF signals by the silicon. An optimal process flowchart for modifying the silicon surface is shown through which bond areas with a homogeneous bond strength between silicon and LTCC are achieved and certain areas with cavities at the bond interface can be produced.

Hybrid-integrated RF MEMS-based reference oscillator using a silicon-ceramic composite substrate

In this paper, the design of a RF MEMS oscillator on a silicon-ceramic composite substrate using a high-Q Lamb-wave resonator as frequency-selective device is described. The MEMS resonator is designed on a 1.8 μm thick piezoelectric AlN layer, deposited on silicon using thin-film processes. The finite-element simulation results of the resonator structure are presented, and the derivation of the electrical equivalent-circuit is described. The active part of the MEMS oscillator, which was laid out in a Pierce topology, has been integrated in an application-specific integrated circuit fabricated in CMOS technology. Both, amplifying and frequency-selective parts are hybrid-integrated on a unique silicon-ceramic composite substrate, which enables a very compact high-quality module design with minimal parasitics. The MEMS oscillator serves as a technology demonstrator combining the advantages of microelectronic and microelectromechanical components towards a compact and power-efficient hybrid technology, e.g. for mobile communications or wireless sensors.