Tip Sharpness Research Articles

Active Control of the Oxidization of a Silicon Cantilever for the Characterization of Silicon-based Semiconductors

Abstract We propose a novel technique of oxidizing silicon cantilever tips to characterize silicon-based semiconductors by tip-enhanced Raman measurements. The technique uses thermal oxidization process under steam atmosphere. It is aimed for the suppression of Raman scattering originating from the silicon tip itself without degrading the tip sharpness. The thickness of the oxidized silicon on the silicon tip is controlled by the thermal oxidization time. We successfully obtained 250-nm thick silicon dioxide in 1100 °C temperature under steam at 10-min oxidization time. Using the oxidized tip, we experimentally verified that silicon Raman vibration was completely suppressed.

Self-aligned growth of single-walled carbon nanotubes using optical near-field effects

ABSTRACTBy applying optical near-field effects in a laser-assisted chemical vapor deposition (LCVD) process, self-aligned growth of ultra-short single-walled carbon nanotubes (SWNTs) was realized in a well controlled manner at a relatively low substrate temperature due to the nanoscale heating enhancement induced by the optical near-field effects. Bridge structures containing single suspending SWNT channels were successfully fabricated. Ultra-sharp tip-shaped metallic electrodes were used as optical antennas in localizing and enhancing the optical fields. Numerical simulations using High Frequency Structure Simulator (HFSS) reveal significant enhancement of electrical fields at the metallic electrode tips under laser irradiation, which induces localized heating at the tips. Numerical simulations were carried out to optimize SWNT growth conditions, such as electrode tip sharpness and film thickness, for maximal enhancement of electrical near field and localized heating.

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Guided plasmon polariton at 2D metal corners

We investigate the properties of plasmon polaritons guided by 2D monoangular metal corners with a vector finite-element method. Such corner waveguides in general include both V-channel- and Λ-wedge-type waveguides. The influences of both geometric parameters (i.e., corner angle, tip sharpness, etc.) and operating wavelength to the mode properties, such as effective mode index, loss, and mode field size, are inspected. It is noticed that both a smaller corner angle and a sharper corner tip help to better confine the mode field. The confinement of the V-channel waveguide is found to be especially sensitive to its angle, while the confinement of the Λ-wedge waveguide is affected about equally by its angle and tip sharpness. Almost all superior mode field confinement is realized at the expense of higher propagation loss for such waveguides. The chromatic dispersion of such waveguides is found to be adequate for applications in integrated optical circuits. The mode behaviors of realistic corner waveguides with finite sidewalls are also studied.

Nanoscale probe system for cell-organelle analysis

Abstract Planar and bended needle shaped probes with a dual electrode system in submicron size have been developed for electrochemical analyses of living single cells. The probe system is designed for local probing of the cytosolic cell environment and cell organelles using amperometric, potentiometric and impedance spectroscopic methods. Silicon nitride cantilevers with an electrode metal layer system are fabricated on 4-in. wafers using conventional micro-fabrication techniques. The probe needle structures with a tip in submicron scale are patterned using focus ion beam (FIB) technology. A focused ion beam is utilized to write the probe needle shape into the pre-shaped cantilever and, for a dual electrode system, the probe is divided into two parts to create two separate electrodes. Subsequently, the planar needle structures are released from the supporting bulk silicon during a wet etching step, and a silicon nitride layer is deposited to isolate and embed the electrode metal layer. Finally, FIB milling is used for a precise exposure of the buried metal layer by cutting the top of the tip. Bended nano-probes for simultaneous AFM analyses were fabricated by combining this process with pre-structured wafers. This way AFM probe systems with high aspect ratio silicon nitride tips with single and dual electrochemical transducers can be manufactured. Electrochemical and mechanical characterization of planar and bended probes showed full functionality. The sharpness of the probe tip with a radius of smaller than 50 nm and the mechanical robustness of the needle structure allow for a reliable scanning as well as penetration of cell membranes. Initial measurements of cell membrane potentials and cell membrane impedances of rat fibroblast cells using Ag/AgCl transducer probes demonstrate the analytical capability of these probes in biological environments.

Fabrication of a dense array of tall nanostructures over a large sample area with sidewallprofile and tip sharpness control

We report a simple but efficient nanofabrication method to create a dense (nanoscale pitch)array of silicon nanostructures (post and grate) of varying height and shape over a largesample area. By coupling interference lithography with deep reactive ion etching (DRIE)in one process flow, we achieved silicon nanostructures of excellent regularity,currently with a pitch (i.e., period) of 230 nm, and uniform coverage, currently over2 × 2 cm2. The new nanofabrication practice of coupling interference lithography with DRIE notonly simplified the nanofabrication process but also produced high-aspect-ratio(higher than 10) nanostructures. By regulating etching parameters, the nanoscopicscalloping problem typical in Bosch DRIE was not only controlled but also utilized torealize sophisticated sidewall profiles, such as tips with a pointed or a re-entrantprofile. We showed the tips could be further sharpened by thermal oxidation andsubsequent removal of the oxide. Well-defined nanostructures over a large area withcontrollable sidewall profiles and tip shapes open new application possibilitiesin areas beyond nanoelectronics, such as microfluidics and tissue engineering.

Localized Surface Plasmon Resonance Spectroscopy of Single Silver Triangular Nanoprisms

The plasmonic properties of single silver triangular nanoprisms are investigated using dark-field optical microscopy and spectroscopy. Two distinct localized surface plasmon resonances (LSPR) are observed. These are assigned as in-plane dipolar and quadrupolar plasmon excitations using electrodynamic modeling based on the discrete dipole approximation (DDA). The dipole resonance is found to be very intense, and its peak wavelength is extremely sensitive to the height, edge length, and tip sharpness of the triangular nanoprism. In contrast, the intensity of the quadrupole resonance is much weaker relative to the dipole resonance in the single particle spectra than in the ensemble averaged spectrum. Several parameters relevant to the chemical sensing properties of these nanoprisms have been measured. The dependence of the dipole plasmon resonance on the refractive index of the external medium is found to be as high as 205 nm RIU(-1) and the plasmon line width as narrow as approximately 0.17 eV. These data lead to a sensing figure of merit (FOM), the slope of refractive index sensitivity in eV RIU(-1)/line width (eV), as high as 3.3. In addition, the LSPR shift response to alkanethiol chain length was found to be linear with a slope of 4.4 nm per CH2 unit. This is the highest short-range refractive index sensitivity yet measured for a nanoparticle.

Geometrical strengthening and tip-sharpening of a microneedle array fabricated by X-ray lithography

A novel fabrication method for LIGA (from the German “Lithographie”, “Galvanik”, and “Abformung”) microneedles with through holes is presented. Such microneedles are in demand by most bio-medical MEMS applications and in some fluidic MEMS applications. We propose a technique that combines conventional deep X-ray lithography, plane-pattern to cross-section transfer (PCT) process, and alignment X-ray lithography. The technique provides precise hole alignment with ± 3 μm tolerance. Finite-element simulations on various hole locations were performed to determine the optimum position. We previously fabricated a microneedle with a 100-μm base and a 300-μm height by a right-triangular mask. The resultant microneedle had a very sharp tip but was excessively steep, and thus resulted in a very low strength. Improved strength and tip sharpness was consequently achieved by changing the mask-pattern from a triangular pattern to a polygonal mask and changing the dimensions of the microneedle to have a 300-μm base with various heights between 350 and 800 μm. Using the proposed technique, we could produce a total of 100 hollow microneedles on a 5 × 5 mm2 chip. Moreover, we successfully fabricated sharpened microneedles that were stronger than that we have fabricated so far. The molding process or electroplating and the cost list of the LIGA microneedle will also be included.

Fabrication and Investigation of Nanostructures on Transition Metal Dichalcogenide Surfaces Using a Scanning Tunneling Microscope

Nanometer-scale holes have been fabricated on the surfaces of the semiconducting transition metal dichalcogenides (TMDCs) molybdenum ditelluride (MoTe2) and molybdenum disulfide (MoS2) by applying voltage pulses from the tip of a scanning tunneling microscope (STM) operating in ultrahigh vacuum (UHV). It was found that the tip geometry (tip shape and sharpness) influences the formation and structure of the atomic-scale nanostructures. Threshold voltage ranges for the surface modification of MoTe2 (3.0 +/- 0.3 V) and MoS2 (3.4 +/- 0.3 V) were determined. Negative sample voltage pulses applied to a p-type MoTe2 surface produced much larger and deeper nanometer-scale holes when compared with those produced by positive voltage pulses. The existence of threshold voltages and the pulse polarity dependence of nanostructure fabrication suggests that an electric field evaporation mechanism is applicable. Support for this mechanism was obtained by nanostructuring metallic TMDC NbSe2, where both the produced features and the threshold voltages (3.0 +/- 0.3 V) were similar for both positive and negative voltage pulses.

Toughening polymeric composites using nanoclay: Crack tip scale effects on fracture toughness

Fracture behavior of vinyl ester resin and the methods that can be used to toughen vinyl ester resin were studied. Neat resin, 5% by weight nanoclay, 5% by weight core shell rubber (CSR) and hybrid system (3% nanoclay and 2% CSR by weight) were the material systems considered for comparing fracture toughness. Three types of cracks were used to determine the stress intensity factors at failure, viz., sharp crack, blunt crack and notch. The critical stress intensity factor in the case of sharp cracks improved significantly when compared to neat resin. In the case of notched and blunt cracked specimens, a reduction in stress intensity factors (at failure) was observed for reinforced systems. However, for notched and blunt cracked specimens, it was shown from the morphology of the fracture surface that the stress intensity factor calculated by assuming a notch or a blunt crack as an ideal crack was not the controlling parameter for fracture. A method for quantifying the crack tip sharpness using fracture surface roughness has been proposed.

Nanoscale electrochemical probes for single cell analysis

Needle shaped probes with a dual electrode system in submicron size have been developed for electrochemical analyses of living single cells. The probe system is designed for local probing of the cytosolic cell environment and cell organelles using amperometric, potentiometric and impedance spectroscopic methods. Silicon nitride cantilevers with an electrode metal layer system are fabricated on 4 in. wafers using conventional micro fabrication techniques. The probe needle structures with a tip in sub micron scale are patterned using focus ion beam (FIB) technology. A focused ion beam is utilized to write the probe needle shape into the pre-shaped cantilever and, for a dual electrode system, the probe is divided into two parts to create two separate electrodes. Subsequently, the needle structures are released from the supporting bulk silicon during a wet etching step, and a silicon nitride layer is deposited to isolate and embed the electrode metal layer. Finally, FIB milling is used for a precise exposure of the buried metal layer by cutting the top of the tip. Electrochemical characterization of nano-probes showed full functionality of Ag/AgCl as well as of platinum transducer systems. The sharpness of the probe tip with a radius of less than 50 nm and the mechanical robustness of the needle structure allow for a reliable penetration of cell membranes. Initial measurements of cell membrane potentials and cell membrane impedances of rat fibroblast cells using Ag/AgCl transducer probes demonstrate the analytical capability of these probes in biological environments.

Surface Potential Measurement by Atomic Force Microscopy Using Quartz Resonator

Scanning Kelvin probe microscopy (SKPM) is demonstrated using a 1 MHz quartz length extension resonator as a force sensor. A tungsten probe tip glued onto the end of the quartz rod is electrically connected to the electrode of the resonator for the detection of Coulomb force between the tip and the surface. To detect Coulomb force, a frequency shift signal is used instead of the oscillation amplitude of the resonator. Surface potential mapping is measured on a Au(111) single crystal with nanoscale carbon dots. A potential difference of 100 mV is observed between the Au surface and the carbon dots. The resolution of surface potential mapping is discussed in terms of sharpness of tip, tip–sample separation and Kelvin probe measurement technique.

Rf microelectromechanical system device with a lateral field-emission detector

We propose a micromachined device that utilizes the field-emission (FE) phenomenon as a mean to modulate signal for radio-frequency microelectromechanical system applications. In this article, we present the stationary reference (SR) device and the resonator-embedded (RE) device and compare their field-emission performances. The SR device contains no moving part and is used to examine the conditions to excite field emission. The RE device has an embedded microresonator of bandpass filter characteristic. Due to enhanced tip sharpness and closer gap, initial results show that compared to the SR device, the FE current of the RE device has been increased by 192 times under the same anode-cathode potential difference of 240V and 2×10−8Torr vacuum level.

Study of Polymeric Microneedle Arrays for Drug Delivery

ABSTRACTTwo-photon polymerization (2PP) is a novel technology for the fabrication of complex three-dimensional (3D) microstructures. The number of applications employing this technology is rapidly increasing, and includes the fabrication of three-dimensional photonic crystals [1-4], medical devices, and scaffolds for tissue engineering [5, 6]. We have used 2PP to fabricate microneedle arrays with various geometries. These devices provide a unique approach for transdermal delivery of nucleic acid- and protein-based pharmacologic agents. Many of issues associated with conventional intravenous drug administration, including pain to the patient, trauma at the injection site, and difficulty in providing sustained release of a pharmacological agent, may be eliminated by applying the microneedles. The effect of microneedle geometry (e.g., tip sharpness and aspect ratio) on skin penetration was examined. Our results indicate that microneedles created using 2PP technique are suitable for in vivo use, and integration with next generation MEMS- and NEMS-based drug delivery devices.

AFM tip gauge by nuclear tracks methodology

Atomic force microscopy (AFM) instrumentation in the different modes of operation is a metrologic system for evaluating some surface properties of solid and semisolid materials. The resolution of this instrument depends strongly on the tip sharpness, which can be changed by contamination of the AFM tip apex due to wearing and/or breaking. In order to assess new and old tips, scanning electron microscopy (SEM) inspection is often used, which is not very convenient due to the availability and demand of SEM services. In the market there are some expensive devices for verification of the tip geometry, and for the particular case of AFM in the tapping mode, a simple proposal has been published based on fiber-like samples. In this work, we present an AFM tip gauge device based on the use of a pattern of etched tracks on CR-39 material. For the preparation of the device, the requirements are a radioactive alpha particle source with specific energy, controlled temperature bath and KOH solution, with all these parameters optimized for the tip evaluation, based on the AFM profilometry image. This work shows another interesting and a very useful application of nuclear tracks methodology (NTM).

Fabrication of nanometer-sized ferromagnetic probe

A sharp needlelike metal probe can be used in a scanning tunneling microscopy (STM) for surface analysis. The probe requires a small-apex tip to maintain a tunneling current from the sample surface. These conditions give STM an atomic resolution. Such a sharp probe also can be applied in the scanning-electron microscope (SEM) for the electron-beam source from electric-field-induced tunneling-electrons. This field-induced current comprises a high brightness and coherent e-beam which gives the SEM a better resolution. Regardless of the different applications, the sharpness of the metal tip dominates the performance of these applications. Tungsten is a common material that can be etched electrochemically to an ultrasharp needlelike probe for these applications. Soft magnetic materials such as iron and nickel also can be fabricated into a similar shape but it is difficult to generate an ultrasharp probe from a simple electrochemical-etch process due to less anisotropic etching. We report a method to fabricate iron and nickel probes within 40nm of the tip’s radius. The iron and nickel wires were first electrochemically etched at the interface of an air bubble in the electrolyte. Because very high surface tension of the micron sized bubble gives a higher anisotropic etching to the wire which in results in the wire etched into needle like sharp probe. By additional sharpen process, the small air bubble acts as a sharpener, the tip was then “sharpened” into nanometer size by periodic stripping the apex of the tip surface.

In Vitro and In Vivo Characterization of MEMS Microneedles

Transdermal drug delivery TDD systems have many advantages but are conventionally limited by the low permeability of skin. The idea of using microneedles to painlessly penetrate the topmost impermeable stratum corneum has previously been put forward. In this paper, the fabrication of solid and hollow silicon microneedles with straight side-walls and with the following dimensions: 20-100 microm in diameter and 100-150 microm in length is described. In vitro tests demonstrate that with prior solid microneedle application, transdermal drug transport is significantly increased by 10-20 times, with the degree of enhancement being related to needle diameter. In vivo tests in diabetic animals, however, were unable to demonstrate any delivery of insulin through the hollow microneedles. It is proposed that two factors, microneedle length and tip sharpness, have to be improved for systemic drug delivery to be seen in vivo.

Molecular dynamics study of dislocation nucleation from a crack tip

We have performed a systematic molecular dynamics study of the competition between crack growth and dislocation emission from a crack tip. Two types of boundary conditions are adopted: either planar extension or boundary displacements according to the anisotropic mode-I asymptotic continuum solution. The effects of temperature, loading rate, crystal orientation, sharpness of the crack tip, atomic potential, and system size are investigated. Depending on the crystal orientation, dislocation nucleation can be driven either by the strain or by concerted fluctuations at the crack tip. In the latter case, crystal orientation and temperature have the largest influence on the process of dislocation nucleation.

Bulk micromachined tunneling tips integrated with positioning actuators

We have successfully developed an integrated micromechanical system for controlling tunneling current. A pair of nanoscale tunneling tips have been integrated with a silicon micromachined electrostatic actuator of high-aspect ratio. The tip sharpness has been observed to be as sharp as commercial tips by scanning over surface of carbon graphite as an atom scale. We have also succeeded to observe the tunneling current in the air and in the vacuum condition (in TEM).

LaB6 nanowires and their field emission properties

AbstractFor field-induced electron emission, the two factors that enable a high emission current density at low applied voltages are (a) low work function of the emitter and (b) sharpness of the emitter tip. We have developed and applied a chemical vapor deposition method to synthesize single-crystalline LaB6 nanowires for applications as point electron emitters. The crystallographic orientation of the grown nanowires can be controlled by the catalysts used in synthesis and their typical diameter is ranged from below 20 nm to over 100 nm. The nanowires’ tip is either hemispherical or flat top with rectangular cross-section depending on the catalyst being utilized. The field emission properties have also been measured from the single nanowire emitters and the results are discussed for applications as point electron sources used in high performance electron optical instruments such as the transmission and scanning electron microscopes.