Michael Haub

Zusammenfassung

Die weitere Miniaturisierung von Beschleunigungssensoren stößt unter Verwendung herkömmlicher Wandlerprinzipien aufgrund der Skalierungsgesetze bei einer isometrischen Verkleinerung der Sensorstrukturen an ihre Grenzen. In dieser Arbeit wird die Miniaturisierung von Beschleunigungssensoren daher durch den Einsatz einer hochsensitiven quantenmechanischen Tunnelstrecke untersucht. Die theoretische Auslegung der Sensorstrukturen bedarf einer eingehenden Analyse der Zusammenhänge zwischen dem Tunneleffekt, der geometrischen Form und Größe der FederMasse-Systeme (FMS) sowie der Parameter der elektrostatischen Aktorik. Die Ergebnisse zeigen zwei Modelle mit einfachem (M1) und spiralförmigem (M2) Federbalken sowie zusätzlicher seismischer Masse und elektrostatischer Aktorik. Durch die geringere Steifigkeit kann mit M2 bei gleichbleibender Empfindlichkeit eine weitere Miniaturisierung erfolgen. Die Tunnelstrecke wird an der Position der größten Empfindlichkeit integriert. Die Steifigkeit der Strukturen ist durch die anziehenden Kräfte zwischen den Tunnelelektroden bei Abständen weniger nm begrenzt. Der Flächenbedarf der Sensorkernfläche (M1 & M2) beträgt einige 10x10 µm². Die Implementierung der Tunnelelektroden erfolgt, nach Herstellung der Sensorstrukturen durch den Foundry Service PolyMUMPs von MEMSCAP Inc., durch den Einsatz eines „Focused Ion Beam“ (FIB, Ga+) und die Abscheidung von metallorganischem Precursormaterial (MeCpPtMe3) mit einem „Gas Injection System“. Dies führt insbesondere zu zwei Herausforderungen: Zum einen muss ein Prozess entwickelt werden, um Elektrodenspitzen weniger nm im Radius zu generieren. Zum anderen bedarf es einer Untersuchung des metallorganischen Gefüges sowie geeigneter Parameter des FIB, um die Tauglichkeit der Elektroden für den Tunneleffekt sicherzustellen. Daher werden Materialanalysen des Gefüges (Transmissionselektronenmikrokopie und energiedispersive Röntgenspektroskopie) sowie der elektronischen Parameter (Widerstandsmessung und Tunnelmikroskopie) durchgeführt. Die Ergebnisse zeigen, dass die Tunnelelektroden mit hohen Blendenströmen des FIB (260 pA, 30 kV) hergestellt werden müssen. Dies führt zu einem erhöhten Platinanteil sowie einer homogenen Verteilung der Platinpartikel im Gefüge. Es werden Elektrodenspitzen mit Radien bis 5 nm hergestellt und initiale Elektrodenabstände von etwa 30 nm bis 300 nm erreicht. Die messtechnische Charakterisierung zeigt den Nachweis des Tunneleffekts bei Tunnelspannungen von 200 mV bis 1 V durch die exponentielle Abhängigkeit zwischen Tunnelstrom und Elektrodenabstand. Die Anregung der Sensorstrukturen mit einer äquivalenten Beschleunigung erfolgt durch die elektrostatische Aktorik. Durch die steigende Sensitivität des Tunneleffekts nimmt das Signalrauschen mit Verkürzung des Tunnelabstandes zu. Der Messbereich beträgt 20 g bei einer Empfindlichkeit des Messsignals von bis zu einigen 10 pA/g. Unter Verwendung der metallorganischen Elektroden lassen sich, je nach Tunnelspannung, Ströme bis 150 pA messen. Die Begrenzung des Tunnelstroms ist auf den hohen Materialwiderstand der Elektroden zurückzuführen. Aus den Ergebnissen folgt die Anforderung nach metallisch „reinen“ Tunnelelektroden, da der dem Tunnelstrom äquivalente Messbereich (einige 10 nA) durch die metallorganischen Materialien maßgeblich begrenzt wird. Im Bezug auf frühere Arbeiten zeigen die Ergebnisse dieser Arbeit, dass der hochempfindliche Tunneleffekt, anstatt zur Erhöhung der Sensorauflösung eines Beschleunigungssensors, auch zur deutlichen Miniaturisierung der Sensorfläche genutzt werden kann. The further miniaturization of accelerometers using conventional transducer principles reaches its limits due to the scaling laws at an isometrical reduction of the sensor structure’s size. In this work, the miniaturization potential of acceleration sensors using a highly sensitive tunneling section is investigated. The theoretical design of the sensor structures requires an in-depth analysis of the relationships between the tunneling effect, the geometric shape and the size of the spring-mass systems, as well as the parameters of the electrostatic actuator. The results show two models with a single (M1) and a spiral (M2) spring beam as well as the additional seismic mass and the electrostatic actuator. Due to the lower stiffness, further miniaturization can be achieved with M2 while maintaining the same sensitivity. The tunneling section is integrated at the position of the greatest sensitivity. The stiffness of the structures is limited by the attractive forces between the tunneling electrodes at distances of a few nm. The area requirement of the sensor core area (M1 & M2) amounts several 10x10 µm². After fabrication of the sensor structures by MEMSCAP Inc.’s foundry service PolyMUMPs, the tunneling electrodes are implemented using a “Focused Ion Beam“ (FIB, Ga+) and deposition of metal-organic precursor material (MeCpPtMe3) with a gas injection system. This presented significant challenges: First, a stable and reproducible process has to be developed to create electrode tips with radii of some nm. Second, an analysis of the metal-organic microstructure and suitable parameters of the FIB are required to ensure the suitability of the electrodes for the tunneling effect. Therefore, material analyses of the microstructure (transmission electron microscopy and energy dispersive X-ray spectroscopy) and electronic parameters (resistivity measurement and tunneling microscopy) are performed. The results show that high aperture currents (260 pA, 30 kV) are necessary to achieve an increased platinum content as well as a homogeneous distribution of the platinum particles in the microstructure. Electrode tips with radii down to 5 nm can be fabricated, and initial electrode spacing of about 30 nm to 300 nm are achieved. The tunneling effect can be demonstrated from tunneling voltages from 200 mV up to 1 V by the exponential dependence between the tunneling current and the electrode spacing. The excitation of the sensor structures with an equivalent acceleration is performed by the electrostatic actuator. Due to the increasing sensitivity of the tunneling effect, the signal noise increases with the shortening of the tunneling distance. The measuring range amounts 20 g with a sensitivity of the measuring signal of up to some 10 pA/g. Using the metal-organic electrodes, currents up to 150 pA can be measured reliably, depending on the tunneling voltage.The limitation is due to the high material resistance of the electrodes. The research results show the essential requirement for purer metallic materials since the measuring range equivalent to the tunneling current (some 10 nA) is significantly limited by the metal-organic materials. Regarding previous work, the results of this work show that the highly sensitive tunneling effect can also be used to significantly miniaturize the sensor area, instead of being used to increase the sensor resolution of an accelerometer.

Zusammenfassung

The use of focused ion and focused electron beam (FIB/FEB) technology permits the fabrication of micro- and nanometer scale geometries. Therefore, FIB/FEB technology is a favorable technique for preparing TEM lamellae, nanocontacts, or nanowires and repairing electronic circuits. This work investigates FIB/FEB technology as a tool for nanotip fabrication and quantum mechanical tunneling applications at a low tunneling voltage. Using a gas injection system (GIS), the Ga-FIB and FEB technology allows both additive and subtractive fabrication of arbitrary structures. Using energy dispersive X-ray spectroscopy (EDX), resistance measurement (RM), and scanning tunneling microscope (STM)/spectroscopy (STS) methods, the tunneling suitability of the utilized metal–organic material–platinum carbon (PtC) is investigated. Thus, to create electrode tips with radii down to 15 nm, a stable and reproducible process has to be developed. The metal–organic microstructure analysis shows suitable FIB parameters for the tunneling effect at high aperture currents (260 pA, 30 kV). These are required to ensure the suitability of the electrodes for the tunneling effect by an increased platinum content (EDX), a low resistivity (RM), and a small band gap (STM). The STM application allows the imaging of highly oriented pyrolytic graphite (HOPG) layers and demonstrates the tunneling suitability of PtC electrodes based on high FIB aperture currents and a low tunneling voltage.

Zusammenfassung

Most accelerometers today are based on the capacitive principle. However, further miniaturization for micro integration of those sensors leads to a poorer signal-to-noise ratio due to a small total area of the capacitor plates. Thus, other transducer principles should be taken into account to develop smaller sensors. This paper presents the development and realization of a miniaturized accelerometer based on the tunneling effect, whereas its highly sensitive effect regarding the tunneling distance is used to detect small deflections in the range of sub-nm. The spring-mass-system is manufactured by a surface micro-machining foundry process. The area of the shown polysilicon (PolySi) sensor structures has a size smaller than 100 µm × 50 µm (L × W). The tunneling electrodes are placed and patterned by a focused ion beam (FIB) and gas injection system (GIS) with MeCpPtMe3 as a precursor. A dual-beam system enables maximum flexibility for post-processing of the spring-mass-system and patterning of sharp tips with radii in the range of a few nm and initial distances between the electrodes of about 30–300 nm. The use of metal–organic precursor material platinum carbon (PtC) limits the tunneling currents to about 150 pA due to the high inherent resistance. The measuring range is set to 20 g. The sensitivity of the sensor signal, which depends exponentially on the electrode distance due to the tunneling effect, ranges from 0.4 pA/g at 0 g in the sensor operational point up to 20.9 pA/g at 20 g. The acceleration-equivalent thermal noise amplitude is calculated to be 2.4–3.4 mg/Hz. Electrostatic actuators are used to lead the electrodes in distances where direct quantum tunneling occurs.

Zusammenfassung

To realize quantum tunneling applications with movable electrodes, sharp tips with radii down to several tens of nanometers are necessary. The use of a focused ion beam (FIB) and focused electron beam (FEB) with a gas injection system (GIS) allows the integration of geometries in the nanoscale directly into micro and nano systems. However, the implementation of the tunneling effect clearly depends on the material. In this work, a metal-organic precursor is used. The investigation of the prepared tunneling electrodes enables an insight into FIB/FEB parameters for the realization of quantum tunneling applications. For this purpose, a high-resolution transmission electron microscopy (HRTEM) analysis is performed. The results show a dependence of the material nanostructure regarding platinum (Pt) grain size and distribution in an amorphous carbon matrix from the used beam and the FIB currents. The integration of the tips into a polysilicon (PolySi) beam and measuring the current signal by approaching the tips show significant differences in the results. Moreover, the approach of FEB tips shows a non-contact behavior even when the tips are squeezed together. The contact behavior depends on the grain size, proportion of platinum, and the amount of amorphous carbon in the microstructure, especially at the edge area of the tips. This study shows significant differences in the nanostructure between FIB and FEB tips, particularly for the FIB tips: The higher the ion current, the greater the platinum content, the finer the grain size, and the higher the probability of a tunneling current by approaching the tips.