Search

The formation of conductive percolation path in high-κ (HK)/interfacial layer (IL) dielectric stack is accompanied by dynamic changes in the electrical and chemical properties at the nanometer length scale. It is therefore essential to study these breakdown (BD) events using high-precision nanoscale characterization tools to investigate the physical mechanisms of failure for advanced HK dielectric based devices. In this work, we carry out a new method for electrical nanoprobing of HfO2/SiOx (x

Moth's eye inspired multiscale ommatidial arrays offer multifunctional properties of great significance in optoelectronic devices. However, a major challenge remains in fabricating these arrays on large-area substrates using a simple and scalable technique. Here we present the fabrication of these multiscale ommatidial arrays over large areas by a distinct approach called sacrificial layer mediated nanoimprinting, which involves nanoimprinting aided by a sacrificial layer. The fabricated arrays exhibited excellent pattern uniformity over the entire patterned area. Optimum dimensions of the multiscale ommatidial arrays determined by the finite-difference time domain simulations served as the design parameters for replicating the arrays on glass. A broadband suppression of reflectance to a minimum of ∼1.4% and omnidirectional antireflection for highly oblique angles of incidence up to 70° were achieved. In addition, superhydrophobicity and superior antifogging characteristics enabled the retention of optical properties even in wet and humid conditions, suggesting reliable optical performance in practical outdoor conditions. We anticipate that these properties could potentially enhance the performance of optoelectronic devices and minimize the influence of in-service conditions. Additionally, as our technique is solely nanoimprinting-based, it may enable scalable and high-throughput fabrication of multiscale ommatidial arrays.

In this note, quasistatic solutions of a two-mass-two-spring Tomlinson model for atomic force microscopy are derived, and are compared with the corresponding results of the single-spring Tomlinson model. It is clarified that the solutions of the two models are equivalent provided that the effective spring constant is correctly defined. It is also shown that modeling the onset of stick-slip motion in terms of the criteria defined by explicit tip and cantilever stiffness is the most convenient to investigate the effects of the tip-flexibility on the stick-slip motion.

We propose a dose-finding weighted design for an early clinical trial which aims to determine the optimal dose, selected on the basis of both efficacy and toxicity, to be used in patients entering subsequent studies in a drug development process. The goal is to identify the optimal dose, while using a minimal number of subjects. For each dose under test, a decision table is defined with a utility value attached to each possible decision. The relationship between the utility and the target probability for each outcome is shown. A Dirichlet prior is used and we illustrate the process of maximizing the expected utility under the resulting posterior distribution to find the optimal decision at each stage of the trial. We show how this affects the eventual choice of optimal dose in various scenarios. Properties of our design are discussed and compared with a current standard design. Copyright © 2005 John Wiley & Sons, Ltd.

For the CMOS compatible micromechanical resonators in VHF transceiver applications, it is paramount to have a high quality factor (Q-factor) and high stiffness as it directly influences the dynamic range of the circuits. The resonator also needs to have a high natural frequency in order to reduce the effect of the external noise. A reduction in the size of the resonator may satisfy the above requirements. However, beyond a limit, this may increase its sensitivity to noise. In this paper, structural design improvements on an existing resonator have been carried out, and a prototype based on the new design has been fabricated and tested. The new design has demonstrated an improvement in both the natural frequency (typically 43%) and the Q-factor (typically 25%) as compared to the original design. The increase in natural frequency is due to the addition of reinforcements to the proof mass, and the improvement in Q-factor is due to a reduction in the squeeze film damping coefficient arising out of a higher driving frequency.

Fluorescence excitation enhancement by plasmonic nanostructures such as gold nanohole arrays has been a hot topic in biosensing and bioimaging in recent years. However, the high throughput and high yield fabrication of precisely designed metal nanostructures for optimized fluorescence excitation remains a challenge. Our work is the first report combining nanopattern nickel mould fabrication and UV imprinting for gold nanostructure mass fabrication in high yield. We report our successful gold nanohole array mass fabrication on a 4′′ glass wafer, by first fabricating a high fidelity nickel mould, then using the mould for UV nanoimprinting on a polymer coated on the glass, evaporating the gold film on the glass wafer, and lifting off the polymer to obtain a gold nanohole array on the glass. Our optimized process for wafer fabrication can achieve almost 100% yield from nanoimprinting to gold lift-off, while the fabricated nickel mould has >70% defect-free area with the rest having a few scattered defects. In our work, the size and pitch of the gold nanohole array are designed to enhance the fluorescent dye Alexa 647. When the fabricated gold nanohole array is used for prostate specific antigen (PSA) detection by establishing a sandwiched fluorescence assay on the gold surface, a detection limit of 100 pg ml−1 is achieved, while with a same thickness of gold film, only 1 ng ml−1 is detected. ASTAR (Agency for Sci., Tech. and Research, S’pore)

The study of scanning tunneling microscopy (STM) induced localized degradation and polarity dependent breakdown (BD) of HfO 2 /SiO x dielectric stacks is presented in this work, together with a correlated investigation of the BD locations by transmission electron microscopy (TEM). The localized dielectric BD events are also analysed using conductive-atomic force microscopy. The analysis of the degradation and breakdown phenomenon has been performed from a macroscopic (device) level to a localized nanometer scale BD location. A new technique is adopted to induce the degradation and BD of the HfO 2 /SiO x dielectric stacks locally using a combined STM/scanning electron microscopy nano-probing system. The BD locations were identified on blanket wafers and gate electrode area of the dielectric, and the sample containing these regions was prepared using focused ion beam for the physical analysis using TEM. This method of analysis is very useful in studying the nature of the BD events in dielectrics with and without the gate electrode, elucidating the role of the gate electrode in dielectric BD events.

A method to create impacting jets at the micrometer length scale by means of a collapsing cavitation bubble is presented. A focused shock wave from a lithotripter leads to the nucleation of a cavitation bubble below a hole of 25 $\ensuremath{\mu}$m diameter etched in a silicon plate. The plate is placed at an air-water interface. The expansion and collapse of the bubble leads to two separate jets---an initial slow jet of velocity $\ensuremath{\sim}$10 m/s and a later faster jet of velocity $\ensuremath{\sim}$50 m/s. The jets subsequently impact coaxially, resulting in a circular sheet of liquid in the plane perpendicular to their axis. The sheet is characterized by a ring of droplets at its rim and breaks up into a spray as the shock pressure is increased. The results demonstrate an approach to create a high-speed jet and fine spray on demand at the micrometer scale.

Control of photonic crystal resonances in conjunction with large spectral shifting is critical in achieving reconfigurable photonic crystal devices. We propose a simple approach to achieve nano-mechanical control of photonic crystal resonances within a compact integrated on-chip approach. Three different tip designs utilizing an in-plane nano-mechanical tuning approach are shown to achieve reversible and low-loss resonance control on a one-dimensional photonic crystal nanocavity. The proposed nano-mechanical approach driven by a sub-micron micro-electromechanical system integrated on low loss suspended feeding nanowire waveguide, achieved relatively large resonance spectral shifts of up to 18 nm at a driving voltage of 25 V. Such designs may potentially be used as tunable optical filters or switches.

We present dynamic tuning of optical resonance using microelectromechanical systems (MEMS)-driven coupled photonic crystal (PhC) nanocavities. The device consists of an air-suspended one-dimensional PhC nanocavity coupled to input and output waveguides and a perturbing nanocavity attached to a submicrometer MEMS comb drive. Resonance tuning is achieved through varying the gap between the two coupled cavities. We demonstrate experimentally that resonance can be tuned up to 8nm with no significant deterioration in the Q factor. The proposed mechanism potentially enables a new platform of on-chip photonic devices that can achieve a large tuning range with low power and small footprint and may find useful applications in tunable optical/photonic devices.

We report a MEMS-integrated on-chip tunable resonator consisting of two coupled one-dimensional (1D) Photonic Crystal nanocavities capable of achieving a relatively large tuning range with no significant deterioration of quality factor and sufficiently large transmissions.

We report in this paper a sub-wavelength 1D photonic crystal resonator tuned using a nanomechanical approach. This device demonstrated a compact and low power approach to achieve tunability of photonic crystal resonances. The device is tuned utilizing coupled cavity approach on an air suspended silicon waveguide with tapered etched holes. The etched holes are fine tuned to achieve a reasonably high-Q that is suitable for optical communications. Effects of wavelength detuning by longitudinal shifting and lateral shifting of the perturbing cavity are investigated. Finally a nano-electromechanical actuators are designed and optimized for low power consumption and yet achieving large displacement range for effective de-coupling of both cavities. Large resonance shift can be used in highly sensitive sensors, optical filters or an optical switch.

High frequency, micromechanical bandpass filter, with tunable frequency and bandwidth are demonstrated in a polysilicon surface micromachining technology. These filter utilized a parallel-resonator architecture, in which properly phased outputs from two or more micromechanical resonators are combined to yield a desired filter spectrum. Damping effect was shown to have a significance factor in affecting the flat passband of the filter and will be further analyzed in this paper.© (2001) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

High frequency, micromechanical bandpass filter, with tunable frequency and bandwidth are demonstrated in a polysilicon surface micromachining technology. These filter utilized a parallel-resonator architecture, in which properly phased outputs from two or more micromechanical resonators are combined to yield a desired filter spectrum. Damping effect was shown to have a significant factor in affecting the flat passband of the filter and will be further analyzed in this paper.© (2000) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

In this paper, it presents an improvement over a simple free-free resonator via structural modification. Two modified designs have been considered and named as an Inverse-Pi and Inverse-T free-free beam micromechanical resonator, utilizing torsional-mode support springs. The operating range, according to the simulation is 90-130 MHz. This is a great improvement over its predecessor2 whose operating frequency is 30-90 MHz. Three main design aspect of this resonator are the modified structural design, the support beam structure and the operation parameter. In the former, the relationship between the natural frequency and resonator beam thickness, width and length are found to be interrelated. For the supporting structure design, the length of the supporting beam is determined by (lambda) /4 based on torsional vibration studies. Finally, in the operation parameter, the pull down electrostatic force voltage of the transducer is determined. The experimental testing was done on an inverse-T structure and found that it conforms with the simulation results.© (2000) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Although vibrating microgyroscopes has been an active area of research in the last few years, the resolution requirements for high performance applications have not yet been met. This paper reports the development of a high performance comb-driven vibratory angular rate gyroscope. The gyroscope uses a rotary electrostatic actuation and differential capacitive sensing. The calculations show a resolution of about 8 deg/hr. The gyroscope is fabricated using SOI wafer with a 40 (mu) m thick layer of single-crystal silicon. The overall size of the gyroscope is about 2.8 mm diameter. The high sensitivity of the present design is achieved by employing a large sensing area couled with a higher layer thickness. Simply increasing the sensor area without increasing the layer thickness may yield a higher sensitivity, but lead to problems such as stiction and lower pull-in voltages. Increasing the suspension stiffness increases the pull-in voltage. However, an increase in suspension stiffness may also also increase the natural frequencies which will lower the sensitivity and resolution. The design of the gyroscope involves complex and often conflicting requirements, and some of these important aspects are discussed in this paper.

A method is introduced to measure friction on small, free standing objects, specifically microfabricated silicon plates, based on atomic force microscopy (AFM). An AFM tip is brought into contact with the plate resting on a substrate. The substrate is displaced laterally and, provided the AFM tip does not slide over the plate, the twisting of the AFM cantilever is used to measure the friction of the underlying plate-substrate interface. The method can measure nano-Newton to micro-Newton forces (both friction and applied load) and provides a means to measure friction of macroscopic structures at low load.

A wide-range split-ladder photonic crystal cavity which is tuned by changing its intrinsic gap width is designed and experimentally verified. Different from the coupled cavities that feature resonance splitting into symmetric and anti-symmetric modes, the single split-ladder cavity has only the symmetric modes of fundamental resonance and second-order resonance in its band gap. Finite-difference time-domain simulations demonstrate that bipolar resonance tuning (red shift and blue shift respectively) can be achieved by shrinking and expanding the cavity's gap, and that there is a linear relationship between the resonance shifts and changes in gap width. Simulations also show that the split-ladder cavity can possess a high Q-factor when the total number of air holes in the cavity is increased. Experimentally, comb drive actuator is used to control the extent of the cavity's gap and the variation of its displacements with applied voltage is calibrated with a scanning electron microscope. The measured wavelength of the second-order resonance shifts linearly towards blue with increase in gap width. The maximum blue shift is 17 nm, corresponding to a cavity gap increase of 26 nm with no obvious degradation of Q-factor.

A cantilever has been microfabricated for use in non-contact Atomic Force Microscopy (AFM) using a very thick magnetic film to actuate the cantilever motion. The thick magnetic block is deposited electrochemically over a defined area of the cantilever. This cantilever is particularly suitable for driving stiff AFM cantilevers in a liquid environment. Clean mechanical resonances are easily observed. Examples are given of a hard (CoPt) magnet of dimension 29 × 21 × 6 μm(3) electroplated on Silicon cantilevers of stiffness ~22 N/m, giving a static displacement of ~0.2 nm in an applied field of 10(-3) T.

We use conducting atomic force microscopy (AFM) in ultra high vacuum to measure the thermoelectric power of Au, Pt, and 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) films. Tips coated with thick (1200 nm) Pt films or highly doped diamond film give reproducible data. The thermoelectric power of metal junctions formed with diamond tips is high but dominated by the diamond material thus making diamond tips of limited applicability in thermovoltage AFM. Pt coated tips on Au or Pt films gives small thermovoltage signal, making quantitative analysis of the thermopower on metal sample problematic. The thermovoltage AFM technique appears best suited to study organic thin films and the thermoelectric power of 1.5 nm and 2 nm thick PTCDA deposited on Au measured with Pt tips is −342 and −372 μV/K, respectively. The negative sign indicates that the lowest unoccupied molecular orbital level dominates electrical transport.