Friedemann Völklein

A Tunable Strain Sensor Using Nanogranular Metals

This paper introduces a new methodology for the fabrication of strain-sensor elements for MEMS and NEMS applications based on the tunneling effect in nano-granular metals. The strain-sensor elements are prepared by the maskless lithography technique of focused electron-beam-induced deposition (FEBID) employing the precursor trimethylmethylcyclopentadienyl platinum [MeCpPt(Me)3]. We use a cantilever-based deflection technique to determine the sensitivity (gauge factor) of the sensor element. We find that its sensitivity depends on the electrical conductivity and can be continuously tuned, either by the thickness of the deposit or by electron-beam irradiation leading to a distinct maximum in the sensitivity. This maximum finds a theoretical rationale in recent advances in the understanding of electronic charge transport in nano-granular metals.

Burnout current density of bismuth nanowires

Single bismuth nanowires with diameters ranging from 100nmto1μm were electrochemically deposited in ion track-etched single-pore polycarbonate membranes. The maximum current density the wires are able to carry was investigated by ramping up the current until failure occurred. It increases by three to four orders of magnitude for nanowires embedded in the template compared to bulk bismuth and rises with diminishing diameter. Simulations show that the wires are heated up electrically to the melting temperature. Since the surface-to-volume ratio rises with diminishing diameter, thinner wires dissipate the heat more efficiently to the surrounding polymer matrix and, thus, can tolerate larger current densities.

Thin film based thermoelectric energy conversion systems

Up to now thermoelectric materials used in commercial energy conversion devices like infrared sensors, Peltier-coolers or thermogenerators do not take advantage of the enormous potentials provided by low-dimensional structures. The scope of this presentation is the experimental verification of the predicted increase of the thermoelectric figure of merit (FOM) ZT in low-dimensional systems above values of bulk materials. Concepts for the realization of devices using low dimensional structures based on the classical thermoelectric materials (V-VI-compounds for temperatures around 300 K and IV-VI-materials for temperatures up to 600 K, silicides for high temperature applications) were made. We will present briefly the preparation and thermoelectric properties of IV-VI and V-VI superlattice (SL) structures grown by molecular beam epitaxy as well as on Si-Ge SL structures grown by magnetron sputter epitaxy. An important part is the theoretical modeling of these new thin film devices. The results were used to determine the best suited geometrical dimensions and to compare the calculated device performance with the experimental results. Further more, the results will be discussed with respect to the rising industrial interest in high performance thermoelectric thin and thick film devices.

Method for measuring thermal accommodation coefficients of gases on thin film surfaces using a MEMS sensor structure

A method for measuring the thermal accommodation coefficient α for surface-/gas interfaces is presented. It allows the determination of α for thin films produced by a variety of deposition technologies, such as chemical vapor deposition, physical vapor deposition, and atomic layer deposition (ALD). The setup is based on two microelectromechanical systems (MEMS) Pirani sensors facing each other in a defined positioning. Because these MEMS sensors show a very high sensitivity in their individual molecular flow regimes, it is possible to measure the accommodation coefficients of gases without the disturbing influence of the transition regime. This paper presents the analytical background and the actual measurement principle. The results for air and nitrogen molecules on sputtered Au and Pt surfaces are presented.

Advanced platform for the in-plane ZT measurement of thin films

The characterization of nanostructured samples with at least one restricted dimension like thin films or nanowires is challenging, but important to understand their structure and transport mechanism, and to improve current industrial products and production processes. We report on the 2nd generation of a measurement chip, which allows for a simplified sample preparation process, and the measurement of samples deposited from the liquid phase using techniques like spin coating and drop casting. The new design enables us to apply much higher temperature gradients for the Seebeck coefficient measurement in a shorter time, without influencing the sample holders temperature distribution. Furthermore, a two membrane correction method for the 3ω thermal conductivity measurement will be presented, which takes the heat loss due to radiation into account and increases the accuracy of the measurement results significantly. Errors caused by different sample compositions, varying sample geometries, and different heat profiles are avoided with the presented measurement method. As a showcase study displaying the validity and accuracy of our platform, we present temperature-dependent measurements of the thermoelectric properties of an 84 nm Bi87Sb13 thin film and a 15 μm PEDOT:PSS thin film.

Microelectromechanical sensors for measuring gas pressure

New prototypes and concepts of micro sensors for measuring gas pressure have been developed by using the fabrication technologies for Micro Electro Mechanical Systems (MEMS). The realization of such micro-structured sensors requires sofisticated fabrication processes such as thin film deposition, photolithography and etching techniques. This approach of MEMS sensors for gas pressure is demonstrated by few examples, such as micro-Pirani gauges, resonant vacuum micro gauges and micro spinning rotor gauges.

Optimisation of wirebond interconnects by automated parameter variation

A numerical optimisation strategy for interconnections in electronic packaging is demonstrated. The method is based on a toolbox for the parametric generation of finite- element models of package types such as Chip Scale Package (CSP), Micro Lead Package (MLP) or Ball Grid Array (BGA). The novelty of this work is the combination of this modeling toolbox with an optimisation software for automatic parameter variation. Resulting in a convenient tool to investigate the influence of geometry on the relevant quality characteristics of the device. Users can set the parameters to be varied, the ranges of parameter variation and the number of iterations. The optimisation software automatically generates the parameter sets depending on the number of iterations. The generation of a finite-element model for each parameter set, the meshing and the implementation of the required material properties are also automated by the toolbox. Thereafter, the simulation of the desired load conditions results in quality characteristics such as the maximum mechanical stress for each set. After completion of all iterations, the optimisation software provides a user interface for statistical analysis and graphic visualisation of the results. The wirebond geometry is also included in the toolbox. Influence on maximum mechanical stress and fatigue properties under thermal loads is examined during this study. As an example, the effect of the bonding tool geometry on the locations and the value of the maximum mechanical stress in the wirebond material during thermal shocking is determined. This combination of parametric finite-element model generation and automatic parameter variation represents a powerful tool for design automation in packaging technology and product development. The effects of several geometrical parameters on the thermal and mechanical behaviour of packaging interconnects can be predicted. In a virtual product-development process, time- and cost-intensive prototyping and testing can thus be reduced.

Grundstrukturen und Anwendungen

Erste mikromechanische Funktionselemente und Komponenten wurden unter Anwendung der Silizium-Halbleitertechnologie bereits Anfang der 60er Jahre entwickelt und gefertigt: 1962 wurden Siliziumwafer als Verformungskorper mit integrierten Piezowiderstanden realisiert [Tufte62]. Als kommerziell erfolgreiche Produkte kamen seit Beginn der 70er Jahre Drucksensoren auf den Markt, bei denen Silizium-Membranen mit integrierten Piezowiderstanden als mikromechanische Sensorelemente mit bipolaren integrierten Schaltungen verknupft waren. Breite Anwendung finden inzwischen Beschleunigungssensoren (z.B. in Airbag-Systemen), bei denen die Tragheitskraft einer „seismischen Mikro-Masse“ zur Auslenkung eines mikromechanischen Verformungskorpers fuhrt. Diese Auslenkung ist eine Funktion der Beschleunigung und wird meist kapazitiv oder piezoresistiv detektiert.

Basistechnologien der Mikrosystemtechnik

Die typische Umgebung fur die Entwicklung und Fertigung von mikrotechnischen Produkten ist der Reinraum. Er gewahrleistet saubere Umgebungsbedingungen in Form von gefilterter Luft in dem Bereich, in dem Substrate partikelarm mit geeigneten Prozesmedien und -anlagen prozessiert und gehandhabt werden Auserdem erfolgt in peripheren Einheiten die Bereitstellung aller erforderlichen Medien (z.B. Prozesgase, Druckluft, DI-Wasser, Kuhlwasser, Stromversorgung, Vakuum) sowie die Entsorgung (toxische Abluft, Abwasserneutralisation). Der eigentliche Reinraum, die erforderliche Klimatechnik und die peripheren Einheiten bilden einen komplex organisierten, zusammenhangenden und mit Hilfe von Sensoren uberwachten Bereich. Seine detaillierte Auslegung, geometrische Anordnung und Eigenschaften werden durch die Anwendung, d.h. die in dieser Fertigungsumgebung herzustellenden Produkte, definiert. Die Grose eines Reinraums kann dabei wenige Quadratmeter (z.B. fir einen isolierten Mikromontageplatz) oder mehrere tausend Quadratmeter fir eine komplette Prozeslinie (z.B. eine Speicherchipfertigung) betragen.

Beispiele komplexer Mikrosysteme

Die Rastersondenmikroskopie ist die empfmdlichste Art der Charakterisierung von Oberflachen. Seit der Entwicklung der Rastertunnelmikroskopie [Binnig82], die auf elektrisch leitende Proben beschrankt war, haben sich eine Reihe weiterer Rastersondentechniken als eigenstandige oder kombinierte Verfahren entwickelt. Sie liefern neben Informationen uber elektrische (STM, Scanning Tunneling Microscopy), magnetische (MFM, Magnetic Force Microscopy) und optische Eigenschaften (SNOM, Scanning Near Field Optical Microscopy) insbesondere auch Informationen zur Topologie (AFM, Atomic Force Microscopy) einer Probenoberflache, z.T. auf atomarer Skala [Binnig86, Sarid9l, Minne98]. Fur derartige Mikroskopieverfahren geeignete Sonden konnen insbesondere durch mikrotechnische Herstellungsprozesse erzeugt werden und besitzen gegenuber konventionell hergestellten Sonden die Vorteile der Massenfertigung und Integrierbarkeit unterschiedlicher funktioneller Elemente.