The controlled fabrication of nanopores by focused electron-beam-induced etching

Morphology around Nanopores Fabricated by Laser-Assisted Dielectric Breakdown and Its Impact on Ion and DNA Transport and Sensing

Laser-assisted controlled dielectric breakdown (LaCBD) has emerged as an alternative to conventional CBD-based nanopore fabrication due to its localization capability, facilitated by the photothermal-induced thinning down in the hot spot. Here, we reported the potential impact of the laser on forming debris around the nanopore region in LaCBD. The debris was clearly observable by scanning electron microscopy (SEM) and photoluminescence (PL) spectroscopy. We found that debris formation is a unique phenomenon in LaCBD that is not observable in the conventional CBD approach. We also found that the LaCBD-induced debris is more evident when the laser power and voltage stress are higher. Moreover, the debris is asymmetrically distributed on the top and bottom sides of the membrane. We also found unexpected rectified ionic and molecular transport in those LaCBD nanopores with debris. Based on these observations, we developed and validated a model describing the debris formation kinetics in LaCBD by considering the generation, diffusion, drift, and gravity in viscous mediums. These findings indicate that while laser aids in nanopore localization, precautions should be taken due to the potential formation of debris and rectification of molecular transport. This study provides valuable insights into the kinetics of LaCBD and the characteristics of the LaCBD nanopore.

Combined Focused Electron Beam-Induced Deposition and Etching for the Patterning of Dense Lines without Interconnecting Material

High resolution dense lines patterned by focused electron beam-induced deposition (FEBID) have been demonstrated to be promising for lithography. One of the challenges is the presence of interconnecting material, which is often carbonaceous, between the lines as a result of the Gaussian line profile. We demonstrate the use of focused electron beam-induced etching (FEBIE) as a scanning electron microscope (SEM)-based direct-write technique for the removal of this interconnecting material, which can be implemented without removing the sample from the SEM for post processing. Secondary electron (SE) imaging has been used to monitor the FEBIE process, and atomic force microscopy (AFM) measurements confirm the fabrication of well separated FEBID lines. We further demonstrate the application of this technique for removing interconnecting material in high resolution dense lines using backscattered electron (BSE) imaging to monitor the process.

Confocal scanning photoluminescence for mapping electron and photon beam-induced microscopic changes in SiN x during nanopore fabrication

Focused electron and laser beams have shown the ability to form nanoscale pores in SiN _x membranes. During the fabrication process, areas beyond the final nanopore location will inevitably be exposed to the electron beams or the laser beams due to the need for localization, alignment and focus. It remains unclear how these unintended exposures affect the integrity of the membrane. In this work, we demonstrate the use of confocal scanning photoluminescence (PL) for mapping the microscopic changes in SiN _x nanopores when exposed to electron and laser beams. We developed and validated a model for the quantitative interpretation of the scanned PL result. The model shows that the scanning PL result is insensitive to the nanopore size. Instead, it is dominated by the product of two microscopic material factors: quantum yield profile (i.e. variations in electronic structure) and thickness profile (i.e. thinning of the membrane). We experimentally demonstrated that the electron and laser beams could alter the material electronic structures (i.e. quantum yield) even when no thinning of the membrane occurs. Our results suggest that minimizing the unintended e-beam or laser beam to the SiN _x during the fabrication is crucial if one desires the microscopic integrity of the membrane.

The influence of electrolyte concentration on nanofractures fabricated in a 3D-printed microfluidic device by controlled dielectric breakdown

A three-dimensional-printed microfluidic device made of a thermoplastic material was used to study the creation of molecular filters by controlled dielectric breakdown. The device was made from acrylonitrile butadiene styrene by a fused deposition modeling three-dimensional printer and consisted of two V-shaped sample compartments separated by 750 µm of extruded plastic gap. Nanofractures were formed in the thin piece of acrylonitrile butadiene styrene by controlled dielectric breakdown by application voltage of 15-20 kV with the voltage terminated when reaching a defined current threshold. Variation of the size of the nanofractures was achieved by both variation of the current threshold and by variation of the ionic strength of the electrolyte used for breakdown. Electrophoretic transport of two proteins, R-phycoerythrin (RPE; <10 nm in size) and fluorescamine-labeled BSA (f-BSA; 2-4 nm), was used to monitor the size and transport properties of the nanofractures. Using 1 mM phosphate buffer, both RPE and f-BSA passed through the nanofractures when the current threshold was set to 25 µA. However, when the threshold was lowered to 10 µA or lower, RPE was restricted from moving through the nanofractures. When we increased the electrolyte concentration during breakdown from 1 to 10 mM phosphate buffer, BSA passed but RPE was blocked when the threshold was equal to, or lower than, 25 µA. This demonstrates that nanofracture size (pore area) is directly related to the breakdown current threshold but inversely related to the concentration of the electrolyte used for the breakdown process.

A novel shaped-controlled fabrication of nanopore and its applications in quantum electronics

High-intensity (10⁷–10⁸ A m⁻²) electron beams can be used to fabricate nanoscale pores. This approach enables real-time observation of nanopore drilling and precise control of the diameter of the nanopore. Nevertheless, it is not suitable for tuning the nanopore’s sidewall shape. In this study, we demonstrate the use of low-intensity electron beams to fabricate nanopores on a silicon nitride (SiN_x) membrane. This technique allows the precise adjustment of the nanopore dimension and the shaping of its three-dimensional (3D) nanostructure. The 3D structures of the nanopore were evaluated by electron tomography, and series of oblique images were used in reconstructing the 3D images of nanopores using a weighted back-projection method. The sidewall shape of the nanopore was observed at different electron-beam conditions, and the formation mechanism was elucidated based on these results. The nanopore fabricated with this technique can be used as a template to develop electronics at the nanoscale based on which a quantum-dot device can be prepared with a simple evaporation process. The measured results show that the device can resolve well-defined electronic states that are characteristic for the behaviors of the quantum-dot device.

Functional Micro–Nano Structure with Variable Colour: Applications for Anti-Counterfeiting

Colour patterns based on micro-nano structure have attracted enormous research interests due to unique optical switches and smart surface applications in photonic crystal, superhydrophobic surface modification, controlled adhesion, inkjet printing, biological detection, supramolecular self-assembly, anti-counterfeiting, optical device and other fields. In traditional methods, many patterns of micro-nano structure are derived from changes of refractive index or lattice parameters. Generally, the refractive index and lattice parameters of photonic crystals are processed by common solvents, salts or reactive monomers under specific electric, magnetic and stress conditions. This review focuses on the recent developments in the fabrication of micro-nano structures for patterns including styles, materials, methods and characteristics. It summarized the advantages and disadvantages of inkjet printing, angle-independent photonic crystal, self-assembled photonic crystals by magnetic field force, gravity, electric field, inverse opal photonic crystal, electron beam etching, ion beam etching, laser holographic lithography, imprinting technology and surface wrinkle technology, etc. This review will provide a summary on designing micro-nano patterns and details on patterns composed of photonic crystals by surface wrinkles technology and plasmonic micro-nano technology. In addition, colour patterns as switches are fabricated with good stability and reproducibility in anti-counterfeiting application. Finally, there will be a conclusion and an outlook on future perspectives.

Charged particle single nanometre manufacturing

Following a brief historical summary of the way in which electron beam lithography developed out of the scanning electron microscope, three state-of-the-art charged-particle beam nanopatterning technologies are considered. All three have been the subject of a recently completed European Union Project entitled "Single Nanometre Manufacturing: Beyond CMOS". Scanning helium ion beam lithography has the advantages of virtually zero proximity effect, nanoscale patterning capability and high sensitivity in combination with a novel fullerene resist based on the sub-nanometre C₆₀ molecule. The shot noise-limited minimum linewidth achieved to date is 6 nm. The second technology, focused electron induced processing (FEBIP), uses a nozzle-dispensed precursor gas either to etch or to deposit patterns on the nanometre scale without the need for resist. The process has potential for high throughput enhancement using multiple electron beams and a system employing up to 196 beams is under development based on a commercial SEM platform. Among its potential applications is the manufacture of templates for nanoimprint lithography, NIL. This is also a target application for the third and final charged particle technology, viz. field emission electron scanning probe lithography, FE-eSPL. This has been developed out of scanning tunneling microscopy using lower-energy electrons (tens of electronvolts rather than the tens of kiloelectronvolts of the other techniques). It has the considerable advantage of being employed without the need for a vacuum system, in ambient air and is capable of sub-10 nm patterning using either developable resists or a self-developing mode applicable for many polymeric resists, which is preferred. Like FEBIP it is potentially capable of massive parallelization for applications requiring high throughput.

Rapid fabrication of solid-state nanopores with high reproducibility over a large area using a helium ion microscope

The fabrication of solid-state nanopores in an insulating membrane has attracted much attention for biomolecule analysis such as DNA sequencing and detection in recent years. For practical applications and device integration, the challenges include precise size control for sub 10 nm nanopores, excellent repeatability and rapid fabrication over a large area to reduce the cost for mass production. A helium ion beam could provide an effective fabrication approach to produce such solid-state nanopores. It is easy to control the nanopore size and reach sub 10 nm pore size with a simple change in the milling time with an appropriate ion beam current. Here we report new results in a set of experiments demonstrating that with a small range of stage automatized motions and equal mill times one can obtain good fabrication reproducibility in nanopore sizes (<10% variation in size). The automation in the stage motion and milling time opens a door for the rapid mass production of nanopore chips over a wafer size of several inches.

Fabrication of multiple nanopores in a SiNx membrane via controlled breakdown

This paper reports a controlled breakdown (CBD) method to fabricate multiple nanopores in a silicon nitride (SiN_x) membrane with control over both nanopore count and nanopore diameter. Despite the stochastic process of the breakdown, we found that the nanopores created via CBD, tend to be of the same diameter. We propose a membrane resistance model to explain and control the multiple nanopores forming in the membrane. We prove that the membrane resistance can reflect the number of nanopores in the membrane and that the diameter of the nanopores is controlled by the exposure time and strength of the electric field. This controllable multiple nanopore formation via CBD avoids the utilization of complicated instruments and time-intensive manufacturing. We anticipate CBD has the potential to become a nanopore fabrication technique which, integrated into an optical setup, could be used as a high-throughput and multichannel characterization technique.

Electron beam induced etching of carbon nanotubes enhanced by secondary electrons in oxygen

Multi-walled carbon nanotubes (CNTs) are subjected to electron-beam-induced etching (EBIE) in oxygen. The EBIE process is observed in situ by environmental transmission electron microscopy. The partial pressure of oxygen (10 and 100 Pa), energy of the primary electrons (80 and 200 keV), and environment of the CNTs (suspended or supported on a silicon nitride membrane) are investigated as factors affecting the etching rate. The EBIE rate of CNTs was markedly promoted by the effects of secondary electrons that were emitted from a silicon nitride membrane under irradiation by primary electrons. Membrane supported CNTs can be cut by EBIE with a spatial accuracy better than 3 nm, and a nanogap of 2 nm can be successfully achieved between the ends of two suspended CNTs.

Focused electron beam induced etching of copper in sulfuric acid solutions

We show here that copper can be locally etched by an electron-beam induced reaction in a liquid. Aqueous sulfuric acid (H2SO4) is utilized as the etchant and all experiments are conducted in an environmental scanning electron microscope. The extent of etch increases with liquid thickness and dose, and etch resolution improves with H2SO4 concentration. This approach shows the feasibility of liquid phase etching for material selectivity and has the potential for circuit editing.

Fabrication of Solid State Nanopore in Thin Silicon Membrane Using Low Cost Multistep Chemical Etching

Nanopore-based analysis is currently an area of great interest in many disciplines with the potential for exceptionally versatile applications in medicine. This work presents a novel step towards fabrication of a single solid-state nanopore (SSSN) in a thin silicon membrane. Silicon nanopores are realized using multistep processes on both sides of n-type silicon-on-insulator (SOI) <100> wafer with resistivity 1-4 Ω·cm. An electrochemical HF etch with low current density (0.47 mA/cm²) is employed to produce SSSN. Blue LED is considered to emit light in a narrow band region which facilitates the etching procedure in a unilateral direction. This helps in production of straight nanopores in n-type Si. Additionally, a variety of pore diameters are demonstrated using different HF concentrations. Atomic force microscopy is used to demonstrate the surface morphology of the fabricated pores in non-contact mode. Pore edges exhibit a pronounced rounded shape and can offer high stability to fluidic artificial lipid bilayer to study membrane proteins. Electrochemically-fabricated SSSN has excellent smoothness and potential applications in diagnostics and pharmaceutical research on transmembrane proteins and label free detection.

Pulsed laser-assisted focused electron-beam-induced etching of titanium with XeF2: enhanced reaction rate and precursor transport

In order to enhance the etch rate of electron-beam-induced etching, we introduce a laser-assisted focused electron-beam-induced etching (LA-FEBIE) process which is a versatile, direct write nanofabrication method that allows nanoscale patterning and editing. The results demonstrate that the titanium electron stimulated etch rate via the XeF2 precursor can be enhanced up to a factor of 6 times with an intermittent pulsed laser assist. The evolution of the etching process is correlated to in situ stage current measurements and scanning electron micrographs as a function of time. The increased etch rate is attributed to photothermally enhanced Ti-F reaction and TiF4 desorption and in some regimes enhanced XeF2 surface diffusion to the reaction zone.

DNA motion induced by electrokinetic flow near an Au coated nanopore surface as voltage controlled gate

We used fluorescence microscopy to investigate the diffusion and drift motion of λ DNA molecules on an Au-coated membrane surface near nanopores, prior to their translocation through solid-state nanopores. With the capability of controlling electric potential at the Au surface as a gate voltage, Vgate, the motions of DNA molecules, which are presumably generated by electrokinetic flow, vary dramatically near the nanopores in our observations. We carefully investigate these DNA motions with different values of Vgate in order to alter the densities and polarities of the counterions, which are expected to change the flow speed or direction, respectively. Depending on Vgate, our observations have revealed the critical distance from a nanopore for DNA molecules to be attracted or repelled-DNA's anisotropic and unsteady drifting motions and accumulations of DNA molecules near the nanopore entrance. Further finite element method (FEM) numerical simulations indicate that the electrokinetic flow could qualitatively explain these unusual DNA motions near metal-collated gated nanopores. Finally, we demonstrate the possibility of controlling the speed and direction of DNA motion near or through a nanopore, as in the case of recapturing a single DNA molecule multiple times with alternating current voltages on the Vgate.

Directly observing the motion of DNA molecules near solid-state nanopores

We investigate the diffusion and the drift motion of λ DNA molecules near solid-state nanopores prior to their translocation through the nanopores using fluorescence microscopy. The radial dependence of the electric field near a nanopore generated by an applied voltage in ionic solution can be estimated quantitatively in 3D by analyzing the motion of negatively charged DNA molecules. We find that the electric field is approximately spherically symmetric around the nanopore under the conditions investigated. In addition, DNA clogging at the nanopore was directly observed. Surprisingly, the probability of the clogging event increases with increasing external bias voltage. We also find that DNA molecules clogging the nanopore reduce the electric field amplitude at the nanopore membrane surface. To better understand these experimental results, analytical method with Ohm's law and computer simulation with Poisson and Nernst-Planck (PNP) equations are used to calculate the electric field near the nanopore. These results are of great interest in both experimental and theoretical considerations of the motion of DNA molecules near voltage-biased nanopores. These findings will also contribute to the development of solid-state nanopore-based DNA sensing devices.

3D nanofluidic channels shaped by electron-beam-induced etching

In-plane nanofluidic channels with 3D topography are fabricated. Nanochannel masters are written by electron beam lithography in SU-8 resist and shaped by electron-beam-induced etching (EBIE) with water as the precursor gas. Nanofunnel replicas cast from unmodified and EBIE-modified masters show that the funnel tip dimensions decrease from a 150-nm depth and 80-nm width to a 70-nm depth and 40-nm width.

Synthesis and electroplating of high resolution insulated carbon nanotube scanning probes for imaging in liquid solutions

High resolution and isolated scanning probe microscopy (SPM) is in demand for continued development of energy storage and conversion systems involving chemical reactions at the nanoscale as well as an improved understanding of biological systems. Carbon nanotubes (CNTs) have large aspect ratios and, if leveraged properly, can be used to develop high resolution SPM probes. Isolation of SPM probes can be achieved by depositing a dielectric film and selectively etching at the apex of the probe. In this paper the fabrication of a high resolution and isolated SPM tip is demonstrated using electron beam induced etching of a dielectric film deposited onto an SPM tip with an attached CNT at the apex.

Effects of electrons on the shape of nanopores prepared by focused electron beam induced etching

The fabrication of nanometric pores with controlled size is important for applications such as single molecule detection. We have recently suggested the use of focused electron beam induced etching (FEBIE) for the preparation of such nanopores in silicon nitride membranes. The use of a scanning probe microscope as the electron beam source makes this technique comparably accessible, opening the way to widespread fabrication of nanopores. Since the shape of the nanopores is critically important for their performance, in this work we focus on its analysis and study the dependence of the nanopore shape on the electron beam acceleration voltage. We show that the nanopore adopts a funnel-like shape, with a central pore penetrating the entire membrane, surrounded by an extended shallow-etched region at the top of the membrane. While the internal nanopore size was found to depend on the electron acceleration voltage, the nanopore edges extended beyond the primary electron beam spot size due to long-range effects, such as radiolysis and diffusion. Moreover, the size of the peripheral-etched region was found to be less dependent on the acceleration voltage. We also found that chemical etching is the rate-limiting step of the process and is only slightly dependent on the acceleration voltage. Furthermore, due to the chemical etch process the chemical composition of the nanopore rims was found to maintain the bulk membrane composition.

Focused electron beam induced etching of titanium with XeF2

Titanium is a relevant technological material due to its extraordinary mechanical and biocompatible properties, its nanopatterning being an increasingly important requirement in many applications. We report the successful nanopatterning of titanium by means of focused electron beam induced etching using XeF(2) as a precursor gas. Etch rates up to 1.25 × 10(-3) µm(3) s(-1) and minimum pattern sizes of 80 nm were obtained. Different etching parameters such as beam current, beam energy, dwell time and pixel spacing are systematically investigated, the etching process being optimized by decreasing both the beam current and the beam energy. The etching mechanism is investigated by transmission electron microscopy. Potential applications in nanotechnology are discussed.

XeF(2) gas-assisted focused-electron-beam-induced etching of GaAs with 30 nm resolution

We demonstrate the gas-assisted focused-electron-beam (FEB)-induced etching of GaAs with a resolution of 30 nm at room temperature. We use a scanning electron microscope (SEM) in a dual beam focused ion beam together with xenon difluoride (XeF(2)) that can be injected by a needle directly onto the sample surface. We show that the FEB-induced etching with XeF(2) as a precursor gas results in isotropic and smooth etching of GaAs, while the etch rate depends strongly on the beam current and the electron energy. The natural oxide of GaAs at the sample surface inhibits the etching process; hence, oxide removal in combination with chemical surface passivation is necessary as a strategy to enable this high-resolution etching alternative for GaAs.

Nanopore formation by low-energy focused electron beam machining

The fabrication of nanopores in thin silicon nitride and aluminum oxide membranes by water vapor assisted, low-energy (0.2-20 kV) electron beam machining using a scanning electron microscope (SEM) is described. Using this technique, pores with diameters ranging in size from < 5 to 20 nm are easily formed. The nanopores are characterized by SEM, transmission electron microscopy (TEM) and atomic force microscopy (AFM). The mechanism of etching is briefly discussed.

De novo designed coiled-coil proteins with variable conformations as components of molecular electronic devices

Conformational changes of proteins are widely used in nature for controlling cellular functions, including ligand binding, oligomerization, and catalysis. Despite the fact that different proteins and artificial peptides have been utilized as electron-transfer mediators in electronic devices, the unique propensity of proteins to switch between different conformations has not been used as a mechanism to control device properties and performance. Toward this aim, we have designed and prepared new dimeric coiled-coil proteins that adopt different conformations due to parallel or antiparallel relative orientations of their monomers. We show here that controlling the conformation of these proteins attached as monolayers to gold, which dictates the direction and magnitude of the molecular dipole relative to the surface, results in quantitative modulation of the gold work function. Furthermore, charge transport through the proteins as molecular bridges is controlled by the different protein conformations, producing either rectifying or ohmic-like behavior.