Biomechanical Characterization at the Cell Scale: Present and Prospects

The rapidly growing field of mechanobiology demands for robust and reproducible characterization of cell mechanical properties. Recent achievements in understanding the mechanical regulation of cell fate largely rely on technological platforms capable of probing the mechanical response of living cells and their physico–chemical interaction with the microenvironment. Besides the established family of atomic force microscopy (AFM) based methods, other approaches include optical, magnetic, and acoustic tweezers, as well as sensing substrates that take advantage of biomaterials chemistry and microfabrication techniques. In this review, we introduce the available methods with an emphasis on the most recent advances, and we discuss the challenges associated with their implementation

Focused-Ion-Beam Growth of Nanomechanical Resonators

Nanoscale mechanical resonators exhibit excellent sensitivity and therefore potential advantages for application as ultrasensitive mass sensors by comparison with micromachined cantilevers. We fabricated three dimensional vertical C-W-nanorods on silicon substrates by focussed ion beam induced deposition (FIB-CVD) and investigated the factors which affected the growth rate and smoothness of the nanorod sidewall, including the heating temperature of precursor gas and the ion beam current. We also discussed the effects on reducing the thickness of the nanorod with FIB milling, including the ion beam current, ion beam energy and ion incident angle. We fabricated a doubly-clamped beam and a singly-clamped beam by felling a vertical nanorod over a trench with FIB milling. We investigated the static mechanical properties (i.e. Young’s modulus) of doubly-clamped and singly-clamped nanorods by atomic force microscopy (AFM) with force displacement measurement. Since the optical signal reflected from a cantilever whose dimensions are sub-wavelength is very weak, it is difficult to measure the absolute nanoscale displacement of such cantilevers with an optical technique. We describe an electron microscope technique for measuring the absolute oscillation amplitude and resonance of nanomechanical resonators with a model-independent method. A piezo-actuator mounted in a field-emission scanning-electron microscope (SEM) is used to excite the nanomechanical resonator to vibrate. The secondary electron signal is recorded as the primary electron beam is scanned linearly over the resonator. An absolute oscillation amplitude as low as 5 nm can be resolved, this being comparable to the size (~1.5 nm) of the primary electron beam. The Q-factor of nanomechanical resonators was measured ranging 300 to 600. The mass resolution of the resonators was also estimated to the level of 1E-15 g

Applications of Reflectometry Towards the Development of MEMS Gas Sensors

Reflectometry or reflectance spectroscopy is a relatively simple characterization technique based on the analysis of reflected light from a surface. Herein, reflectometry is used to attain significant insights towards the development of micro-electro-mechanical systems (MEMS) based gas sensors. Two main reflectometry applications are demonstrated which led to formation of unique MEMS gas sensor devices. The first application was the use of in-situ reflectance spectroscopy to characterize the growth behavior of metal oxide films grown by atmospheric pressure-spatial atomic layer deposition. The technique revealed an initial film nucleation period, where the length of the nucleation time was sensitive to the deposition process parameters. The in-situ reflectometry technique was then used to study and monitor the growth behavior of metal oxide films on various non-conventional surfaces. Doing so allowed for the accurate deposition of zinc oxide films on a variety of surfaces with desired thickness. This was instrumental, as it enabled the integration of the zinc oxide films into a novel MEMS resonant cantilever architecture for gas sensing. In this device, the zinc oxide layer serves as both the cantilever structural layer as well as the gas sensitive receptor layer. A key advantage of the approach was the reduction in overall mass of the cantilever which can lead to an enhancement in sensitivity to low quantities of analyte gases. The sensor had an outstanding sensitivity to low levels of relatively humidity(RH) when compared to other frequency shift-based humidity sensors. The zinc oxide cantilever demonstrated a sensitivity of 23649 ppm⁄%RH at 5.9 % RH and an average sensitivity of 1556 ppm⁄%RH in the range of 30-60% RH. The second application of reflectometry was its use as a screening technique to find suitable gas sensitive receptor materials for static deflection type MEMS gas sensors. The reflectance intensity of various materials exposed to gases, was monitored, where changes in the intensity indicated that the material was physically changing in the presence of the gas. This expansion behavior is ideal for static deflection type MEMS gas sensors such as the nanomechanical membrane type surface stress sensor (MSS) architecture. The reflectance screening technique identified that laser treated two dimension materials such as graphene oxide, molybdenum disulfide and tungsten disulfide were suitable candidates to be integrated as the receptor layer in the MSS platform. The sensing response of the coated devices was obtained for a select group of volatile organic compounds. The results showed that the laser treatment technique was advantageous to enhancing the sensor response and sensitivity, as it introduces defects, dopants and functional groups to the receptor materials for improved gas adsorption

In-Situ TEM Study on the Mechanical Behavior of Nanowires

One-dimensional nanomaterials have qualitatively different mechanical behavior comparing with their bulk form owing to their small length scale and huge surface area1. To predict the mechanical properties and deformation behaviors of material in nanometer length scale and disclose the deformation mechanisms of them, plenty of computational simulations have been conducted. However, due to the sample mounting difficulty and their quite small volume, it is very tough to perform high-quality mechanical testing and validate the predictions from computational simulations. Using the unique Nanofactory probing systems, in-situ mechanical tests combined with observations by transmission electron microscopy (TEM) with atomic resolution have been performed successfully on one-dimensional nanomaterials such as silver (Ag) nanowries, silica (SiO2) nanowires, nanoscale Al90Fe5Ce5 metallic glass and sodium chloride (NaCl) nanowires. 19.3% strain was achieved in the bicystalline Ag nanowires. Stacking faults formed on the (111) plane and interestingly, the stacking fault (local hexagonal close-packed (hcp) structure) was not induced by partial dislocations movement, but by the Frank loops formation and expansion. SiO2 glass at room temperature is usually brittle due to fracture instability. However, showered by electron beam, silica nanowires with big diameters (>100 nm) can flow superplastically more than 670%. But once beam is blanked more than 2 minutes, the mechanical response can recover back to brittle failure if silica nanowire’s diameter is large than 20 nm. However, unrecovered beam damage will trigger the brittle to ductile transition if silica nanowire’s diameter is less than 20 nm. Al90Fe5Ce5 metallic glass with size less than 20 nm can be super plastic deformed with elongation ~200%. Necking occurred without shear bands in the nanoscale sample with an area reduction nearly 100%. Surprisingly, it is first time to see atomic chain formation in metallic glasses. Fast diffusion of surface atom and no chance to form shear band is thought to attributed to such extraordinary ductility. The mechanical test on common salt shows very interesting results. NaCl nanowires can be formed by touching sharp probe with NaCl surface in the transmission electron microscope and deform superplastically. The nanowires can be stretched to 280% and be very flexible under compression(can be bent over 90°) under the electron beam irradiation. During the elongation process, there were no dislocations observed due to the fast diffusion

Focused electron- and ion-beam induced processes:in situ monitoring, analysis and modeling

Focused electron- and ion-beam induced processing are well established techniques for local deposition and etching that rely on decomposition of precursor molecules by irradiation. These high-resolution nanostructuring techniques have various applications in nanoscience including attach-and-release procedures in nanomanipulation and fabrication of sensors (magnetic, optical and thermal) for scanning probe microscopy. However, a complete physical and chemical understanding of the process is hampered by the lack of suitable means to monitor and to access the numerous interrelated and time-varying process parameters (deposition and etch rate, yield, molecule flux and adsorption/desorption). This thesis is a first attempt to fill this gap. It is based on experimental and simulative approaches for the determination of process conditions and mechanical properties of deposited materials: Mass and force sensors: The use of tools merging micromechanical cantilever sensors and scanning electron microscopy was demonstrated for in situ monitoring and analysis. A cantilever-based resonant mass sensing setup was developed and used for real-time mass measurements. A noise level at the femtogram-scale was achieved by tracking the resonance frequency of a temperature stabilized piezoresistive cantilever using phase-locking. With this technique the surface coverage and residence time of (CH3)3PtCpCH3 molecules, the mass deposition rate, the yield, and the material density of corresponding deposits were measured. In situ cantilever-based static force sensing and mechanical modal vibration analysis were employed to investigate the Young's modulus and density of individual high aspect ratio deposits from the precursor Cu(hfac)2. Precursor supply simulations and experiments: A prerequisite to understand and quantify irradiative precursor chemistry is the knowledge of the local flux of molecules impinging on the substrate. Therefore, Monte Carlo simulations of flux distributions were developed and gas flows injected into a vacuum chamber were analyzed experimentally for the precursors Co2(CO)8, (hfac)CuVTMS, and [(PF3)2RhCl]2. The process parameters extracted from the mentioned approaches are valuable input for numerical focused electron- and ion-beam induced process models (Monte Carlo, continuum). We evaluated the precursor surface diffusion coefficient and the electron impact dissociation cross-section by relating deposit shapes to a continuum model

Mikro-Nano-Integration für metallische Mikrosysteme mit vertikal integrierten Federelementen

Mikro-Nano-Integration (MNI) ist ein skalenübergreifender Ansatz, um Nanomaterialien in Mikrosystemen zur Anwendung zu bringen. Die Nanotechnologie bietet vielfältige, vollständig neuartige Effekte sowie wesentlich verstärkt auftretende Effekte und stellt so eine Bereicherung für die Funktionalität von Mikrosystemen dar. Gleichzeitig liefert die Mikrotechnik eine sehr gezielte Anbindung der Nanomaterialien an die Systemtechnik, sodass sich aus geringen Mengen Nanomaterial große Effekte im MNI-System erzielen lassen. Daher ist zu erwarten, dass der Einsatz von Nanomaterialien in Mikrosystemen zukünftig stark anwachsen wird. Das Anwendungsspektrum der MNI-Systeme erstreckt sich bereits heute von einem sehr starken Sektor der Mikrosensorik, über Mikroaktorik, Mikroelektronik und Optik bis hin zu Chemie, Energie und biotechnischen Systemen. Eine umfangreiche Analyse zum Stand der Technik und zum Stand der Standardisierung verdeutlicht die Relevanz des Themenfelds. Die Technologie zur Integration von Nanomaterialien weist eine Reihe an Herausforderungen auf, da die Integrationsschritte erheblichen Einfluss auf die Nanomaterialeigenschaften haben. In dieser Arbeit werden Verfahren zur Vor-Ort-Synthese hochgeordneter 1-D Nanomaterialien betrachtet, insbesondere galvanisch abgeschiedener metallischer Nanodrähte. Sind diese Nanodrähte senkrecht stehend auf einem Trägersubstrat verankert, können sie als einseitig eingespannte Biegestäbe betrachtet und in alle lateralen Richtungen flexibel federnd gebogen werden. Diese Eigenschaft macht sich der hier untersuchte Ansatz zum Aufbau eines Inertialsensors zunutze. Fixiert man eine Inertialmasse am freien Ende des Biegestabs, ist diese in erster Näherung mit zwei lateralen translatorischen und zwei lateralen rotatorischen Freiheitsgraden aufgehängt. Somit lässt sich mit einer einzigen Inertialmasse die Beschleunigung in zwei lateralen Raumrichtungen bzw. die Drehrate aus der Ebene hinaus in Richtung der Biegestab-Hauptachse messen. Die Besonderheit dieses Ansatzes liegt in den geringen Abmessungen sowie der Skalierbarkeit des Konzepts. Im Gegensatz zum Stand der Technik bei Silizium-Inertialsensoren wird für Federelement und Masseelement deutlich weniger Chipfläche benötigt. Die Arbeit beschreibt die statische und dynamische Auslegung des Beschleunigungs- und des Drehratensensors einschließlich Stabilitätsbetrachtung des Biegestabs, der Übertragungsfunktionen und der Dimensionierung von der Mikroaktorik. Ein weiterer Schwerpunkt liegt auf der Fertigung des Technologie-Demonstrators basierend auf den Verfahren UV-Lithographie mit anschließender Galvanoformung (UV LIGA) und Röntgen-Synchrotron-Lithographie mit anschließender Galvanoformung (Röntgen LIGA). Diese ermöglichen die Fertigung senkrecht stehender dünner Stäbe aus Metall, die als Federelemente dienen, in direkter Umgebung von Metallquadern, die als Inertialmassen fungieren. Mit Hilfe tiefenlithographischer Verfahren auf Basis von UV-Strahlung bzw. von Röntgen-Synchrotron-Strahlung lassen sich Photoresiste so mikrostrukturieren, dass Öffnungen mit Länge-zu-Durchmesser-Verhältnissen (Aspektverhältnissen) von bis zu 14,5 für UV-Strahlung und von bis zu 70 für Röntgen-Synchrotron-Strahlung entstehen. Die Kombination von Lithographieschritten in mehreren aufeinander folgenden Ebenen mit Metallabscheideschritten erlaubt die Vor-Ort-Synthese der Inertialsensor-Funktionselemente. Im Rahmen dieser Arbeit entstehen so Technologie-Demonstatoren für einachsige, differentiell kapazitiv auswertbaren Beschleunigungssensoren mit Federelementen und Inertialmassen aus galvanisch abgeschiedenem Kupfer. Ihr Aufbau zu Sensor-Demonstratoren mündet in der Charakterisierung des statischen und dynamischen Übertragungsverhaltens. Der Übertragungsfaktor eines Sensor-Demonstrators beträgt 26,46 fF/g. Die Durchmesser der als Federelemente eingesetzten Stäbe lassen sich entsprechend der Auslegung gezielt zwischen 1,5 µm und 75 µm bei Längen zwischen 94 µm und 409 µm einstellen. Die Skalierbarkeit des Konzepts stellt jedoch in Aussicht, auch Submikro- und Nanodrähte mit Durchmessern kleiner als 1 µm einzusetzen. Diese Arbeit stellt den internationalen Stand der Technik zur Mikro-Nano-Integration in einem neuen Umfang dar. Beispielhaft geht sie intensiv auf die Auslegung eines Multi-Inertialsensor-Technologie-Demonstrators mit nur einer Probemasse und nur einem Federelement ein und stellt so einen wegweisenden Ansatz für neuartige hochminiaturisierte Inertialsensoren vor. Auf technologischer Ebene geht die Arbeit auf neuartige Ansätze zur Optimierung der galvanischen Multiskalenfertigung ein und gibt detaillierte Parameter zur Reproduktion der gesamten Prozesskette an. Erstmals wird die Funktion eines Inertialsensors mit nur einem vor Ort synthetisierten Biegestab aus Metall als Federelement experimentell nachgewiesen

Nanomechanical Characterization of Vertical Nanopillars Using an MEMS-SPM Nano-Bending Testing Platform

Nanomechanical characterization of vertically aligned micro- and nanopillars plays an important role in quality control of pillar-based sensors and devices. A microelectromechanical system based scanning probe microscope (MEMS-SPM) has been developed for quantitative measurement of the bending stiffness of micro- and nanopillars with high aspect ratios. The MEMS-SPM exhibits large in-plane displacement with subnanometric resolution and medium probing force beyond 100 micro-Newtons. A proof-of-principle experimental setup using an MEMS-SPM prototype has been built to experimentally determine the in-plane bending stiffness of silicon nanopillars with an aspect ratio higher than 10. Comparison between the experimental results and the analytical and FEM evaluation has been demonstrated. Measurement uncertainty analysis indicates that this nano-bending system is able to determine the pillar bending stiffness with an uncertainty better than 5%, provided that the pillars’ stiffness is close to the suspending stiffness of the MEMS-SPM. The MEMS-SPM measurement setup is capable of on-chip quantitative nanomechanical characterization of pillar-like nano-objects fabricated out of different materials

Nanoscale Self-Assembly: Nanopatterning and Metrology

The self-assembly process underlies a plethora of natural phenomena from the macro to the nano scale. Often, technological development has found great inspiration in the natural world, as evidenced by numerous fabrication techniques based on self-assembly (SA). One striking example is given by epitaxial growths, in which atoms represent the building blocks. In lithography, the use of self-assembling materials is considered an extremely promising patterning option to overcome the size scale limitations imposed by the conventional photolithographic methods. To this purpose, in the last two decades several supramolecular self-assembling materials have been investigated and successfully applied to create patterns at a nanometric scale. Although considerable progress has been made so far in the control of self-assembly processes applied to nanolithography, a number of unresolved problems related to the reproducibility and metrology of the self-assembled features are still open. Addressing these issues is mandatory in order to allow the widespread diffusion of SA materials for applications such as microelectronics, photonics, or biology. In this context, the aim of the present Special Issue is to gather original research papers and comprehensive reviews covering various aspects of the self-assembly processes applied to nanopatterning. Topics include the development of novel SA methods, the realization of nanometric structures and devices, and the improvement of their long-range order. Moreover, metrology issues related to the nanoscale characterization of self-assembled structures are addressed