Visualized multiprobe electrical impedance measurements with STM tips using shear force feedback control

Here we devise a multiprobe electrical measurement system based on quartz tuning forks (QTFs) and metallic tips capable of having full 3D control over the position of the probes. The system is based on the use of bent tungsten tips that are placed in mechanical contact (glue-free solution) with a QTF sensor. Shear forces acting in the probe are measured to control the tip-sample distance in the Z direction. Moreover, the tilting of the tip allows the visualization of the experiment under the optical microscope, allowing the coordination of the probes in X and Y directions. Meanwhile, the metallic tips are connected to a current-voltage amplifier circuit to measure the currents and thus the impedance of the studied samples. We discuss here the different aspects that must be addressedwhenconductingthesemultiprobeexperiments,suchastheamplitudeofoscillation,shear force distance control, and wire tilting. Different results obtained in the measurement of calibration samples and microparticles are presented. They demonstrate the feasibility of the system to measure the impedance of the samples with a full 3D control on the position of the nanotips

ADVANCED MEMS RESONATOR FOR MASS DETECTION AND MICROMECHANICAL TRANSISTOR

2010/2011Cantilever sensors have been the subject of growing attention in the last decades and their use as mass detectors proved with attogram sensitivity. The rush towards the detection of mass of few molecules pushed the development of more sensitive devices, which have been pursued mainly through downscaling of the cantilever-based devices. In the field of mass sensing, the performance of microcantilever sensors could be increased by using an array of mechanically coupled micro cantilevers of identical size. In this thesis, we propose three mechanically coupled identical cantilevers, having three localized frequency modes with well-defined symmetry. We measure the oscillation amplitudes of all three cantilevers. We use finite element analysis to investigate the coupling effect on the performance of the system, in particular its mass response. We fabricated prototype micron-sized devices, showing that the mass sensitivity of a triple coupled cantilever (TCC) system is comparable to that of a single resonator. Coupled cantilevers offer several advantages over single cantilevers, including less stringent vacuum requirements for operation, mass localization, insensitivity to surface stress and to distributed a-specific adsorption. We measure the known masses of silica beads of 1µm and 4µm in diameter using TCC. As it is difficult to obtain one single bead at the free end of the cantilevers, we choose to use the Focused Ion Beam. By sequential removing mass from one cantilever in precise sequence, we proved that TCC is also unaffected from a-specific adsorption as is, on the contrary, the case of single resonator. Finally, we proposed shown the use of TCC can be as micromechanical transistor device. We implemented an actuation strategy based on dielectric gradient force which enabled a separate actuation and control of oscillation amplitude, thus realizing a gating effect suitable to be applied for logic operation.XXIV Ciclo198

Smart cantilever beams for nanomanipulation

A smart micro cantilever beam, consisting of an atomic force microscope probe bonded with a piezoelectric actuator, is proposed to enhance the ability of mechanical nanomanipulation. A precise three-section Euler-Bernoulli beam model is developed to describe the dynamics of the beam. The forced vibration solution of this model with respect to two independent inputs from the piezoelectric actuator and the base excitation is derived. Through the solution and the geometry relationship, the trajectory of the end of the tip is obtained from the motion of the free end of the AFM probe. Based on the resonant response from two harmonic inputs, nano-scale elliptical and linear tip trajectories are predicted at the second dynamic mode. Analytical and numerical studies show that the characteristics of the resulting trajectories are influenced by the magnitudes of the two inputs. The potential applications of the elliptical and linear trajectories for nanomanipulation are proposed

Multi-Eigenmode Control for Improved Tracking Speed in Multifrequency Atomic Force Microscopy

Die Sensoren von Rasterkraftmikroskopen sind mechanische Schwinger, die zur zeitgleichen Aufnahmevon Topographie und Materialeigenschaften genutzt werden können. Besonders wichtig sinddie Bildrastergeschwindigkeit und Kraftsensitivität, die oft einen Kompromiss benötigen. In dieserArbeit wird ein neuartiger Multi-Eigenmode Kompensator basierend auf einem Zustandsschätzervorgestellt, der die dynamischen Eigenschaften jeder Cantilever-Resonanz unabhängig voneinandermodifizieren kann. Dargelegt wird die Modellierung, Kompensator-Design und Implementierungsstrategiein ein digitales System. Als Erstes wird der Kompensator zur Modifikation desQ Faktors einzelner Eigenmoden genutzt. Somit kann die Abbildungsrate um das 20-fache erhöhtwerden. Die Modifikation der natürlichen Frequenz erlaubt die Abbildung von Proben mitvollständig verschobenen Resonanzen. Moderne Mehrfachfrequenz-Abbildungsverfahren nutzenhöheren Eigenmoden, um bessere Abbildungsraten und Materialsensitivitäten zu erreichen. Beieiner Methode werden die angeregten höheren Harmonischen extrahiert, die beim Rastern einerOberfläche im Fourier-Spektrum entstehen. Eine andere Methode regt die erste und höhere Eigenmodengleichzeitig an. In Experimenten wird der Kompensator in Kombination mit beiden Abbildungsverfahrengenutzt, um speziell den Q Faktor der ersten beiden transversalen Eigenmoden gleichzeitigzu beeinflussen. Experimente zeigen, dass beste Abbildungsraten und Materialkontrastemit geringen Q Faktoren in der ersten und hohen Q Faktoren in der zweiten Eigenmode erreichtwerden. Eine Erweiterung des Kompensators erlaubt die Hochgeschwindigkeits-Demodulationvon Cantilever-Amplituden ohne Einsatz eines Lock-in Verstärkers, was anhand von Abbildungenmit der ersten Eigenmode gezeigt wird. Eine weitere Möglichkeit zur Verbesserung des Materialkontrastesbasiert auf der strukturellen Modifikation des Cantilevers. Mit Hilfe einer Ionenfeinstrahlanlagewird Material an bestimmten Bereichen des Cantilevers entfernt, so dass die erste undhöheren Eigenmoden aufeinander abgestimmt werden. Die Bestimmung von Form und Ort derMaterialentfernung wird entweder durch Simulationen im Voraus oder mit einem in situ Ansatzerreicht. Die extrahierten höheren harmonischen Signale des harmonischen Cantilevers zeigen eindeutlich verstärktes Signal von bis zu 10 % im Vergleich zur ersten Resonanz.Atomic Force Microscope probes are mechanical beams that can be used to simultaneously maptopography and material properties. In particular the imaging speed and force sensitivity aremajor concerns that often require a trade-off approach. In this work, a novel estimator basedmulti-eigenmode compensator is introduced to modify the dynamics of each resonance independently.Modeling, compensator design, implementation strategy in a digital system and validationin experiments will be presented. A single-eigenmode version of the compensator is used to modifythe Q factor of the first three eigenmodes separately. Using higher eigenmodes in combinationwith a modified Q factor leads to a 20-fold increase in image acquisition rates. The modificationof the natural frequency (F control) allows imaging at resonance frequencies that are not naturalto the cantilever. The emerging multifrequency Atomic Force Microscopy utilizes higher eigenmodesto improve imaging speed and force sensitivity concurrently. One method actuates the firsteigenmode for topography imaging and records the excited higher harmonics to map a sample’snanomechanical properties. To enhance the higher frequencies’ response two or more eigenmodesare actuated simultaneously, where the higher eigenmodes are used to quantify the nanomechanics.In experiments, the compensator is used to specifically modify the Q factors of the cantilever’sfirst two transversal eigenmodes concurrently in both imaging schemes. The experiments indicatemost enhanced material contrast and imaging rate with low Q factors in the first eigenmode andhigh Q factors in the higher eigenmode. An extension of the compensator allows for a high speedLock-in amplifier free amplitude demodulation, which is used for topography imaging with the firstresonance. A different technique for improving material property sensitivity is presented basedon structural modifications of the cantilever. Focused Ion Beam milling is used to remove massfrom specific areas in the cantilever such that the first and higher eigenmodes are tuned towardseach other. The shape and location of mass removal is determined either by simulation beforehandor through an in-situ approach. Higher harmonics of the harmonic active cantilevers indicate asignificant response of up to 10% in respect to the first resonance/harmonic

Cooling and sensing using whispering gallery mode resonators

This thesis reports on a detailed exploration of the optomechanical interaction between a tapered optical fibre and a silica microsphere mounted on a cantilever. The amount of light evanescently coupled from the fibre into the optical whispering gallery mode of the sphere is exquisitely sensitive to their separation allowing fast measurement of picometre displacements of both the microsphere-cantilever and the fibre. By exploiting this enhanced transduction, strong active feedback damping/cooling of the thermal motion of both the fibre and microsphere-cantilever have been demonstrated to the noise limit of the system. The cavity enhanced optical dipole force between the fibre and the sphere was used to damp multiple mechanical modes of the tapered fibre, while a piezo-stack at the clamped end of the microsphere-cantilever allowed for cooling of its centre-of-mass motion and the second mechanical eigenmode. The effect of noise within the feedback loop was shown to invert the measured mechanical mode spectrum at high feedback gain as the noise itself is fed into the resonator. A rich variety of feedback induced spring stiffening and softening of the mode is measured when time delays are introduced. Cooling of the mechanical modes of the taper, which are ubiquitous to many whispering gallery mode experiments and are considered as unwanted noise, has not been achieved previously. Simultaneous operation of both feedback schemes was demonstrated for the first time, providing stabilization of the system. By using the microsphere-cantilever as an inertial test mass, measurement of its displacement induced by acceleration can resolve micro-g accelerations at high bandwidth

Advances in High-Speed Atomic Force Microscopy

High-speed atomic force microscopy (HS-AFM) is a scanning probe technique capable of recording processes at the nanometre scale in real time. By sequentially increasing the speed of individual microscope components, images of surfaces can be recorded at up to several images per second. We present a HS-AFM platform composed of custom¿built measurement head, controller and software, scanners and amplifiers that is shared with the community in an open¿hardware fashion. A new scanner design combined with an advanced control system is shown. The simple addition of a secondary actuator to widely available tube scanners increases the scan speed by over an order of magnitude while allowing for a 130 ¿m × 130 ¿m wide field of view, which is not possible with traditional high¿speed scanner designs. Controllers beyond standard proportional-integral controllers are capable of significantly increasing imaging speed by anticipating resonances. Such filters are cumbersome to design with conventional methods. It is shown how convex optimization can be used to design optimal controllers with guaranteed stability for atomic force microscopy in an automated fashion. By integrating two lasers into the small spot¿size optics of an AFM readout head we are able to use the first laser for detecting the deflection of the smallest, and thus fastest currently available high¿speed cantilevers, while using the second for photo¿thermal actuation. Using this instrument, we demonstrate multi¿frequency atomic force microscopy (MF-AFM) at previously not accessible frequencies of more than 20 MHz. By employing the driving laser not for resonant excitation as is usual in dynamic AFM, a new imaging mode, photothermal off-resonance tapping (PORT) is presented. By repeatedly thermally bending the cantilever below it¿s resonant frequency, the surface is probed at a rapid rate. The resulting force is extracted from the deflection of the cantilever in time¿ domain at real time and used for feedback and image generation. The dynamic and static force contributions in both PORT and state of the art high-speed amplitude modulation atomic force microscopy (AM-AFM) are measured and analyzed in detail. It is shown that by decoupling the driving frequency from the resonant frequency the dynamic tip¿sample impact forces can be drastically reduced when compared to resonance based AFM modes. SAS-6 is a centriolar scaffolding protein with a crucial role in the duplication of centrioles, which are the main microtubule organizing organelle of eukaryotic cells. Defects in centriole duplication are associated with cancer and microencephaly. To understand these defects, is therefore important to understand the kinetics of SAS-6. In¿vitro, SAS-6 polymerizes into rings of between eight and ten monomers. Using the new PORT mode we are able to study the dynamic assembly of SAS-6. It is shown how SAS-6 rings can not only assemble by canonical one-by-one addition, but can form as a fusion of larger, already assembled fragments. Finally, it is shown how PORT can be used to observe fast processes of and on living cells. The adhesion and detachment of thrombocyte cells is studied. Membrane disruptive effects are shown on gram¿negative as well as gram¿positive bacteria

Statics and dynamics of electrothermal micromirrors

Adaptive and smart systems are growing in popularity as we shift toward personalization as a culture. With progressive demands on energy efficiency, it is increasingly important to focus on the utilization of energy in a novel way. This thesis investigates a microelectromechanical system (MEMS) mirror with the express intent to provide flexibility in solid state lighting (SSL). By coupling the micromirror to an optical source, the reflected light may be reshaped and directed so as to optimize the overall illumination profile. In addition, the light may be redirected in order to provide improved signal strength in visible light communications (VLC) with negligible impact on energy demands. With flexibility and full analog control in mind, the design of a fully integrated tip-tilt-piston micromirror with an additional variable focus degree of freedom is outlined. Electrothermal actuators are used to both steer the light and tune the focal length. A detailed discussion of the underlying physics behind composite beams and thermal actuators is addressed. This leads directly into an overview of the two main mirror components, namely the segmented mirror and the deflection actuators. An in-depth characterization of the dynamics of the mirror is discussed including the linearity of the thermal response. Frequency domain analysis of such a system provides insight into tunable mechanical properties such as the resonant frequency and quality factor. The degenerate resonant modes can be separated significantly. It is shown that the frequency response may be tuned by straining specific actuators and that it follows a predictable pattern. As a result, the system can be scanned at increasingly large angles. In other words, coupled mechanical modes allow variable damping and amplification. A means to determine the level of coupling is examined and the mode shape variations are tracked as a function of the tuning parameters. Finally, the applications of such a device are explored and tested. Such applications include reliable signal-to-noise ratio (SNR) enhancements in VLC of 30 dB and color tunable steerable lights using laser diodes. A brief discussion of the implications of dynamic illumination and tunable systems is juxtaposed with an explanation behind the integration of an electrothermal micromirror and an all digital driver