Hans Goosen

On the size-dependent elasticity of silicon nanocantilevers: impact of defects

Recent measurements have indicated that the elastic behaviour of silicon nanocantilevers and nanowires is size-dependent. Several theoretical models have been proposed to explain this phenomenon, mainly focused on surface stress effects. However, discrepancies are found between experiments and theories, indicating that there could be other influences in addition to surface effects. One of the important issues, which was experimentally confirmed and has not been considered, is accounting for the fact that experimentally tested nanocantilevers and nanowires are not defect free. In this paper molecular dynamics (MD) is utilized to study the effects of defects on the elasticity of silicon. The effective Young’s modulus ˜ E of [100] and [110] oriented silicon nanoplates is extracted in the presence of defects, showing that such defects significantly influence the size-dependent behaviour in ˜ E. The MD results are compared with the results of continuum theory, showing that continuum theory holds, even for very small defects. Taking into account the surface effects, native oxide layers together with fabrication-induced defects, the experimental measurements can be explained. The studied example involved nanocantilevers, but can be extended to nanowires. (Some figures in this article are in colour only in the electronic version)

Pressure and flow sensor for use in catheters

The small size and possible low cost of micromachined sensors make them attractive for some medical applications. Minimally invasive therapy aims to reduce the damage done to healthy tissue by reaching the affected area through existing pathways through the body. However, information is scarce as direct view or touch is lacking. Small sensors are needed on catheters inside the blood vessels to gather the data such as blood pressure and flow. To this end a combined pressure and flow sensor is fabricated in an epi-poly process that uses a 4 micrometer thick polysilicon membrane grown during epitaxial growth, to form the diaphragm of the pressure sensor and the thermal insulation of the thermal flow sensor. Using RIE etching of holes through the membrane, sacrificial etching and closing of the etch holes by oxide depositions, a closed reference chamber is formed for an absolute pressure sensor. The process is compatible with standard bipolar electronics to enable integration of signal conditioning, multiplexing, etc. Measurements of the two sensors show that fabrication of flow and pressure sensors using epi-micromachining is possible and that the sensors have the required measurement range, but drift necessitates calibration before use.

Surface contamination-induced resonance frequency shift of cantilevers

Nanoelectromechanical cantilevers have achieved unprecedented sensitivity in the detection of displacement, mass, force and charge. Although resonating cantilevers are demonstrated to be excellent mass sensors, environmental effects like humidity, ambient gases and contamination result in uncertainty in the calculation of either adsorbed mass or mechanical properties due to shifts in resonance frequencies. In this work the resonance frequency shift due to surface contamination has been studied. Single crystalline silicon (SCS) cantilevers of various dimensions (1025, 340, and 93 nm-thick) were fabricated and their resonance frequencies due to thermal noise were measured in air and in low vacuum. A resonance frequency shift was seen in air after keeping the samples in vacuum. Our measurement shows that this shift comes from surface contamination. The thinner cantilever showed more sensitive behavior to the air conditions. These results can be used to decrease the errors in the calculation of adsorbed mass and mechanical properties of nanostructures. The calculated equivalent mass-induced resonance frequency shifts of our experiments were measured to be 183×10−15 gram (183 femtogram) and a mass sensitivity of about 6.5×10−18 g/Hz (6.5 ag/Hz) was obtained. Our results and analysis indicate that mass sensing of individual molecules will be realizable when taking into account the surface contamination

Optimization of Capacitive Membrane Sensors for Surface-Stress-Based Measurements

Surface stress-based measurement is a relatively new mechanism in biological and chemical sensing. The viability of this mechanism depends on the maximum sensitivity, accuracy, and precision that can be achieved with these sensors. In this paper, an analytical approximate solution and a finite-element model are employed to describe the electromechanical behavior of a surface stress-based sensor with capacitive measurements. In the proposed model, a circular membrane is assumed as the sensing component, while only a smaller concentric circular area of its surface is subjected to a change in surface stress. The presented approximate analytical solution has a good correspondence with the finite-element model and is computationally fast and accurate enough to be an effective design tool. Based on this modeling study, we can determine the optimum design of the sensor to obtain the maximum capacitive sensitivity. Moreover, we study the effect of this optimization on the precision of the system in surface stress sensing. This paper shows that the ratio of sensing area to the whole membrane plays a key role in the overall performance of such a sensor.

Static and Dynamic Pull-In of Electrically Actuated Circular Micro-Membranes

Micro-Electro-Mechanical devices have shown enormous popularity in engineering devices as sensors and actuators. In this paper, the instability, i.e. the dynamic pull-in behavior, of an electrically actuated circular micro-membrane is studied. In order to investigate the periodic solutions, detect bifurcations and follow branches of the solution, the non-linear equation of motion is derived using an energy approach, and, is solved by using a pseudo arc-length continuation and collocation technique. It has been shown that, both hardening and/or softening nonlinear responses could emerge depending on the applied DC voltage. The results indicate that the critical load parameters, namely DC and AC voltages and the excitation frequency, have a major influence on the pull-in characteristics of the micro-membrane. The results reveal different dynamic pull-in mechanisms. In addition, they accurately show the decrease of the pull-in voltage due to dynamic loading.The proposed approximate solution is very fast and robust for detecting the pull-in instability. It allows observation of both global and local softening behavior even close to dynamic pullin, where the resonance frequency is almost equal to zero.Copyright

Transient tip-sample interactions in high-speed AFM imaging of 3D nano structures

The maximum amount of repulsive force applied to the surface plays a very important role in damage of tip or sample in Atomic Force Microscopy(AFM). So far, many investigations have focused on peak repulsive forces in tapping mode AFM in steady state conditions. However, it is known that AFM could be more damaging in transient conditions. In high-speed scanning, and in presence of 3D nano structures (such as FinFET), the changes in topography appear in time intervals shorter than the response time of the cantilever. In this case, the tip may crush into the sample by exerting much higher forces than for the same cantilever-sample distance in steady state situations. In this study the effects of steep upward steps in topography on the tip-sample interactions have been investigated, and it has been found that the order(s) of magnitude higher forces can be applied. The information on the worst case scenario obtained by this method can be used for selection of operation parameters and probe design to minimize damage in high-speed imaging. The numerically obtained results have been verified with the previous works in steady state regime. Based on this investigation the maximum safe scanning speed has been obtained for a case study.

Mechanics of Nanoelectromechanical Systems: Bridging the Gap Between Experiment and Theory

Nowadays, mechanical designers and engineers of elastic structures at ultra-small scales face an interesting challenge; traditional flexures with several components, mechanical joints/ welds and linkages are almost impossible to manufacture with existing microsystem fabrication technologies. Therefore, the majority of mechanical components, elements and building blocks are based on micro/nano machined elastic flexures [1]. Despite the complexity of modeling and analyzing these systems, their design uses the averaged relation between stress and strain, which requires a relatively accurate knowledge of their effective elastic properties, specifically the effective elastic modulus. Experimental results show that the elastic properties are constant at length scales of meters down to micrometers [2]. However, in order to increase the performance, i.e. sensitivity and dynamic range, the dimensions of mechanical devices have been scaled down towards a few nanometers. Consequently, high performances such as single-electron tunneling [3], sub-attonewton force sensing [4], and sub-femtometer displacement sensing [5] have been successfully achieved. Unlike at micron and higher length scales, at sub-micron and nanometer length scales the effective elastic behavior shows strong scaledependent behavior, meaning that the elastic properties are no longer constant, but a function of length scale. For all the application examples above, the high performance was achieved due to the mechanical response of a nanosystem, which strongly depends on the effective elastic properties. Therefore, a clear understanding of the scale-dependent behavior is important for the design and performance of nanosystems.

Silicon sensors for catheters and guide wires

One area that can make use of the miniature size of present day micro electromechanical systems (MEMS) is that of the medical field of minimally invasive interventions. These procedures, used for both diagnosis and treatment, use catheters that are advanced through the blood vessels deep into the body, without the need for surgery. However, once inside the body, the doctor performing the procedure is completely reliant on the information the catheter(s) can provide in addition to the projection imaging of a fluoroscope. A good range of sensors for catheters is required for a proper diagnosis. To this end, miniature sensors are being developed to be fitted to catheters and guide wires. As the accurate positioning of these instruments is problematic, it is necessary to combine several sensors on the same guide wire or catheter to measure several parameters in the same location. This however, brings many special problems to the design of the sensors, such as small size, low power consumption, bio-compatibility of materials, robust design for patient safety, a limited number of connections, packaging, etc. This paper will go into both the advantages and design problems of micromachined sensors and actuators in catheters and guide wires. As an example, a multi parameter blood sensor, measuring flow velocity, pressure and oxygen saturation, will be discussed.

On the origin of amplitude reduction mechanism in tapping mode atomic force microscopy

The origin of amplitude reduction in Tapping Mode Atomic Force Microscopy (TM-AFM) is typically attributed to the shift in resonance frequency of the cantilever due to the nonlinear tip-sample interactions. In this paper, we present a different insight into the same problem which, besides explaining the amplitude reduction mechanism, provides a simple reasoning for the relationship between tip-sample interactions and operation parameters (amplitude and frequency). The proposed formulation, which attributes the amplitude reduction to an interference between the tip-sample and dither force, only deals with the linear part of the system; however, it fully agrees with experimental results and numerical solutions of the full nonlinear model of TM-AFM.

Active Control of the Hinge of a Flapping Wing with Electrostatic Sticking to Modify the Passive Pitching Motion

Wing designs for Flapping Wing Micro Air Vehicles (FWMAVs) might use a properly tuned elastic hinge at the wing root to obtain the required passive pitching motion to achieve enough lift production to stay aloft. Practical use of this type of FWMAVs requires some form of control which can be achieved by actively adjusting the elastic hinge stiffness and, thus, the pitching motion and lift production of the wing. This paper studies an elastic hinge design consisting of stacked layers which can be sticked together using electrostatics. This sticking changes the bending stiffness of the hinge. The voltage-dependent behavior of this elastic hinge during the large pitching motion are described in detail. The passive pitching motion is governed by the equation of motion which is a function of the elastic hinge stiffness and the applied control voltage. The lift generated by the passive pitching wings is predicted by a quasi-steady aerodynamic model. Numerical simulations show significant changes of the passive pitching motion and, consequently, of the lift production, if slipping stacked layers stick together. Experiments are conducted to study the practical applicability of this method on FWMAVs. The experiments show similar trends as the numerical simulations in modifying the pitching motion although the effect is less significant which is mainly due to manufacturing difficulties. This approach is, in conclusion, promising to control FWMAV flight.