Assaf Ya'akobovitz

Integration of suspended carbon nanotubes into micro-fabricated devices

The integration of suspended carbon nanotubes into micron-scale silicon-based devices offers many exciting advantages in the realm of nano-scale sensing and micro- and nano-electromechanical systems (MEMS and NEMS). To realize such devices, simple fabrication schemes are needed. Here we present a new method to integrate carbon nanotubes into silicon-based devices by applying conventional micro-fabrication methods combined with a guided chemical vapor deposition growth of single-wall carbon nanotubes. The described procedure yields clean, long, taut and well-positioned tubes in electrical contact to conducting electrodes. The positioning, alignment and tautness of the tubes are all controlled by the structural and chemical features of the micro-fabricated substrate. As the approach described consists of common micro-fabrication and chemical vapor deposition growth procedures, it offers a viable route toward MEMS–NEMS integration and commercial utilization of carbon nanotubes as nano-electromechanical transducers.

Toward Sensitivity Enhancement of MEMS Accelerometers Using Mechanical Amplification Mechanism

We report on the novel architecture and operational principle of microelectromechanical accelerometers, which may lead to enhanced sensitivity achieved through mechanical amplification of a proof mass displacements. The integrated amplification mechanism serving also as a linear-to-angular motion transformer is realized as an eccentric elastic torsion link that transforms small out-of-plane motion of the proof mass into significantly larger motion of a tilting element whose displacements are sensed to extract acceleration. The design parameters as well as the amplification ratio were elaborated using a lumped model and numerical finite element simulations. The device was fabricated from silicon-on-insulator wafer and is distinguished by a robust single-layer architecture and simple fabrication process. The devices were operated using electrostatic and inertial actuation of the proof mass combined with the optical sensing. Theoretical and experimental results, which are in a good agreement with each other, indicate that the motion amplification scheme realized in the framework of the suggested architecture results in larger detectable displacements and could be efficiently used for sensitivity improvement of microaccelerometers.

The influence of perforation on electrostatic and damping forces in thick SOI MEMS structures

The influence of perforation on the electrostatic force for thick micro-electromechanical (MEMS) structures is analyzed theoretically and experimentally. A three-dimensional numerical model is provided in order to evaluate the influence of the fringe capacitive field on the electrostatic force. Several configurations of perforated MEMS structures were characterized under ambient air conditions and experimental results demonstrate good consistency with the model prediction. Moreover, a comparative study on the effect of perforation on damping (quality factor) was performed. A quality factor was experimentally determined by analyzing frequency response under electrostatic excitation and time response under pulse loading, and was compared to a few analytical models, which demonstrate reasonable agreement with the measured results. Our study demonstrates that perforation has a significant effect on the quality factor, while its contribution of the electrostatic fringe capacitive field ranges between additional few to few tens of per cents.

Carbon nanotube self-assembeled high frequency resonator

We present a single wall carbon nanotube (SWCNT) based high frequency resonator fabricated using a novel process suitable for mass fabrication. The integration of the electrostatically actuated SWCNT into the silicon structure was achieved by a specially tailored fabrication process which is characterized by simplicity and compatibility with commonly used micro-machining processes. The fabrication process enables the control of the SWCNT positioning and its length and results in a taut and clean suspended SWCNT. Electro-mechanical testing of these devices demonstrates a resonance frequency of 13.4 MHz. Resonators with various resonance frequencies can be readily fabricated by varying the SWCNT length.

Large deflections mechanical analysis of a suspended single-wall carbon nanotube under thermoelectrical loading

Following the recent progress in integrating single-wall carbon nanotubes (SWCNTs) into silicon-based microelectromechanical systems (MEMS), new modeling tools are needed to predict their behavior under different loads, including thermal, electrical and mechanical. In the present study, the mechanical behavior of SWCNTs under thermoelectrical loading is analyzed using a large deflection geometrically nonlinear string model. The effect of the resistive heating was found to have a substantial influence on the SWCNTs behavior, including significant enhancement of the strain (up to the millistrains range) and buckling due to the thermal expansion. The effect of local buckling sites was also studied and was found to enhance the local strain. The theoretical and numerical results obtained in the present study demonstrate the importance of resistive heating in the analysis of SWCNTs and provide an additional insight into the unique mechanics of suspended SWCNTs.

Large Angle Silicon-on-Insulator Tilting Actuator with Kinematic Excitation and Simple Integrated Parallel-Plate Electrostatic Transducer

We report on the novel architecture and operational principle of an electrostatically actuated large angle tilting microelectromechanical (MEMS) actuator with kinematic excitation. The device transforms and amplifies small linear out-of-plane motion of a parallel-plate transducer into a large angular motion of a tilting element attached to the transducer with a certain offset using an elastic torsion axis. The actuator, which is distinguished by the single layer robust architecture and incorporating the simple integrated electrostatic transducer, was fabricated from a silicon-on-insulator (SOI) substrate using a common deep reactive ion etching (DRIE) based process. Detailed modeling of the device was followed by the experimental characterization of the actuator. Large tilting angles were observed in experiments, consistenty with the theoretical prediction. An optical peak to peak angle of 37.5° was measured under relatively low actuation voltages. Our theoretical and experimental results collectively demonstate the feasibility of the suggested apporach and show that simple electrostatic actuation combined with the dynamic amplification results in efficient large amplitude operation of tilting devices.

Electromechanical behavior of suspended taut single-walled carbon nanotubes

In this work we present an experimental study of the electromechanical behavior of suspended, taut, single walled carbon nanotubes (SWCNTs). A novel top-down fabrication process was developed in order to integrate the suspended SWCNTs into silicon MEMS structures fabricated using conventional micro-machining techniques. The resonant response of suspended SWCNTs under a time-varying electric field was analyzed and resonant frequencies in MHz range were registered. In addition, the electromechanical characterization of metallic-like, small band-gap-like and semiconductor-like SWCNTs under steady electric fields of varying strength was carried out and high sensitivity of SWCNTs to the gate voltage was observed. The experimental results demonstrate feasibility of the adopted fabrication framework and provide an additional insight into the complex behavior of taut, suspended SWCNT.

Carbon nanotube forest devices with negative poisson's ratio

We demonstrate a negative Poissons ratio in carbon nanotube forest devices subjected to extension motion obtained by means of electrostatic actuation. Actuated devices were optically monitored, while electrostatic force was applied and the axial (parallel to the carbon nanotubes) and lateral (perpendicular to the carbon nanotubes) motions were extracted. Poissons ratios were then calculated for the top, middle and bottom portions of the carbon nanotube forest devices. Since extension motion is associated with morphology change of enhancement of carbon nanotube alignment, negative Poissons ratios were obtained. Large negative Poissons ratios were obtained in the top portion (where morphology change is most significant), while other portions (where morphology change is less significant) demonstrated smaller value of negative Poissons ratio. This property makes carbon nanotube forest attractive material for building of micro-electromechanical devices with versatile motion transformation.

A MEMS device with sub-nanometer displacement sensing using carbon nanotubes

In this work we present, for the first time, an on-chip sub-nanometer displacement sensing scheme. The sensing is achieved using integrated single wall carbon nanotubes (SWCNTs) as the sensing element. A novel fabrication process was used to suspend SWCNTs in a micro electromechanical (MEMS) device and integrate them onto special MEMS stretching devices, which was used to apply controlled nanoscale stretching to the SWCNTs while monitoring their electrical resistivity. Experimental results show that the SWCNTs resistance significantly changed under sub-nanometer stretching, thus demonstrating the realization of an ultra-sensitive MEMS displacement sensor.

Large angle SOI tilting actuator with integrated motion transformer and amplifier

In this work we report on the novel architecture and operational principle of a tilting actuator fabricated using a single structural layer of silicon on insulator (SOI) wafer and demonstrate the functionality of the device both theoretically and experimentally. The device incorporates an integrated compliant motion amplifier realized as an eccentric elastic torsion link that transforms small out-of-plane motion of the parallel plate electrostatic transducer into large amplitude angular motion of the tilting element. A feasibility study was performed using a lumped model of the device and verified by a coupled three-dimensional simulation. Experimental and model results indicate that this generic architecture, combining simple fabrication process with robustness of SOI based devices, is efficient for static and resonant operation of various tilting micro devices.