Liquid-like behaviour of gold nanowire bridges

A combination of Focused Ion Beam (FIB) and Reactive Ion Etch (RIE) was used to fabricate free standing gold nanowire bridges with radii of 30 nm and below. These were subjected to point loading to failure at their mid-points using an Atomic Force Microscope (AFM), providing strength and deformation data. The results demonstrate a dimensionally dependent transition from conventional solid metallic properties to liquid-like behaviour including the unexpected reformation of a fractured bridge. The work reveals mechanical and materials properties of nanowires which could have significant impact on nanofabrication processes and nanotechnology devices such as Nano Electro Mechanical Systems (NEMS)

Temperature dependent electrical resistivity of a single strand of ferromagnetic single crystalline nanowire

We have measured the electrical resistivity of a single strand of a ferromagnetic Ni nanowire of diameter 55 nm using a 4-probe method in the temperature range 3 K-300 K. The wire used is chemically pure and is a high quality oriented single crystalline sample in which the temperature independent residual resistivity is determined predominantly by surface scattering. Precise evaluation of the temperature dependent resistivity ($\rho$) allowed us to identify quantitatively the electron-phonon contribution (characterized by a Debye temperature $\theta_R$) as well as the spin-wave contribution which is significantly suppressed upon size reduction

Effects of current on early stages of focused ion beam nano-machining

In this report we investigate the effects of focused ion beam machining at low doses in the range of 1015–1016 ions cm-2 for currents below 300 pA on Si(100) substrates. The effects of similar doses with currents in the range 10–300 pA were compared. The topography of resulting structures has been characterized using atomic force microscope, while crystallinity of the Si was assessed by means of Raman spectroscopy. These machining parameters allow a controllable preparation of structures either protruding from, or recessed into, the surface with nanometre precision

Pixel spacing effects for nanofabrication using focused ion beam

Focused ion beam (FIB) systems are widely used as a versatile tool for nanofabrication prototyping, device modification and ion beam lithography. However, there are still many unexplored effects due to the different methods that ion implantation could perform during FIB milling using Ga as a liquid metal ion source. In this report we studied the effects of pixel spacing when FIB is used for direct milling of a substrate at different milling currents for constant implantation doses. The experiment consists of FIB milling of a Si substrate at 30 keV using currents of 50 pA and 100 pA for a dose of 5×1016 ions/cm2. The dwell time was set to be 1 μs and the pixel spacing varied from 6.2 nm to 34.2 nm. The surface topographies of machined regions were examined using the atomic force microscope and the quality is described by comparing the intensities of a crystal to amorphous peak of the recorded trace from Raman spectroscopy measurements. This method was introduced by Wagner. In order to more accurately consider the sputtering yield the effect of second order deposition was neglected. It was observed that by increasing the pixel spacing the sputtering yield starts to increase and then gradually decreases for both currents. Ion implantation breaks the crystal structure and the process involves displacement of the atoms from the atomic rows which consequently increases the effect of de-channeling in ion implantation. The increase in sputtering yield could be because of the enhanced de-channeling which is due to the changes of the substrate structure which increases the collisions between implanted ions and the substrate atoms. After a threshold, the sputtering yield gradually decreases, which is due to having less implanted ions per unit of volume for each scan and therefore less applied damage. Pixel spacing at different currents and dose rates can yield different behavior due to the concentration of implanted ions per pixel dwell time. In our study the maximum concentration of implanted ions per pixel dwell time is about 5×1017 ions/cm3 and 1018 ions/cm3 for currents of 50 pA and 100 pA respectively; this concentration is lower than 1019 ions/cm3 which is the saturation point of Ga solubility in Si. It was observed that increasing the pixel spacing leads to rougher surfaces. It was also found that the quality of Si is at its highest when the pixel spacing is 14.8 nm. This is consistent with the topography results which were described by the de-channeling effect. As the de-channeling increases, the depth of implanted ions is decreased, and therefore fewer layers of substrate are damaged. In this study, we investigated the effect of FIB milling pixel spacing on substrate physical and structural changes at a dose of 5×1016 ions/cm2. We observed the sputtering yield is first increased and then decreased, which is mainly due to structural changes in substrate. The quality of substrate was also studied, revealing less damage when the pixel spacing is 14.8 nm for both currents

Parametric optimization of two-photon direct laser writing process for manufacturing polymeric microneedles

Laser-based additive manufacturing methods have been rapidly developed in recent years and it is anticipated that this new generation of microfabrication tools will dominate the market in the near future. Three-dimensional (3D) microprinting based on two-photon polymerization allows manufacturing of complex microstructures with a resolution down to hundreds of nanometers. In recent years, the technology has been used for the manufacturing of a variety of nano- and micro-sized features such as microfluidic devices, photonics, micro-optics, and microneedle arrays. Microneedles have been mainly studied for transdermal drug delivery of therapeutic agents across the skin and withdrawing bio samples for point-of-care (POC) diagnostics. With significant development being made, it is now possible to 3D print complex microneedle structures directly from computer-aided design (CAD) models by the two-photon direct laser writing technique. However, selecting the optimal parameters and investigating the possible arrangement for the print process often involves intensive optimization process and testing. Herein we report on fabrication of highly detailed microneedles using the two-photon direct laser writing process with a discussion of optimization and parameter selection for microprinting tall microneedles with side-channels and other complex designs. Due to the long printing time, this manufacturing process is currently best suited to print master microneedles. Therefore, we further demonstrate the replication capabilities of the master microneedles by the soft embossing process. Additionally, the mechanical characteristics and insertion force of the microneedle replicas are investigated

Microneedle Arrays for Drug Delivery and Diagnostics: Toward an Optimized Design, Reliable Insertion, and Penetration

In the past two decades, microneedles have been extensively studied for transdermal delivery of drug or vaccines and for bio-fluid extraction. Microneedle-based devices consist of micron-size needles that are arranged on a small patch. Despite ongoing advances in microneedle technology, one of the major problems associated with transdermal delivery or sampling is that many of the microneedle patches are not able to fully penetrate the skin surface and reach the epidermis layer of skin. In this regard, for an optimal and effective penetration—painless and safe—it is crucial to consider complex skin mechanics and how it impacts microneedle design and insertion practice. Various types of skin modelling and insertion simulations are considered, followed by discussion of ways of improving microneedle robustness, reduction of insertion force, and enhancement of penetration efficiency

Fabrication and testing of polymer microneedles for transdermal drug delivery

Microneedle (MN) patches have considerable potential for medical applications such as transdermal drug delivery, point-of-care diagnostics, and vaccination. These miniature microdevices should successfully pierce the skin tissues while having enough stiffness to withstand the forces imposed by penetration. Developing low-cost and simple manufacturing processes for MNs is of considerable interest. This study reports a simple fabrication process for thermoplastic MNs from cycloolefin polymers (COP) using hot embossing on polydimethylsiloxane (PDMS) soft molds. COP has gained interest due to its high molding performance and low cost. The resin master MN arrays (9 × 9) were fabricated using two-photon polymerization (TPP). A previous gap in the detailed characterization of the embossing process was investigated, showing an average of 4.99 ± 0.35% longitudinal shrinkage and 2.15 ± 0.96% lateral enlargement in the molded MN replicas. The effects of bending, buckling, and tip blunting were then examined using compression tests and also theoretically. MN array insertion performance was studied in vitro on porcine back skin using both a prototype custom-made applicator and a commercial device. An adjustable skin stretcher mechanism was designed and manufactured to address current limitations for mimicking skin in vivo conditions. Finite element analysis (FEA) was developed to simulate single MN insertion into a multilayered skin model and validated experimentally using a commercial Pen Needle as a model for the thermoplastic MNs. Margins of safety for the current MN design demonstrated its potential for transdermal drug delivery and fluid sampling. Experimental results indicated significant penetration improvements using the prototype applicator, which produced array penetration efficiencies as high as >92%, depending on the impact velocity setting

FIB direct write of ALD Al₂O₃ mask for silicon DRIE

The focussed ion beam mill rate of ALD AlOx has been measured and found to be 0.147µm3/nC. Standard FIB milling of the AlOx is conjectured to result in re-deposition of the AlOx material on the milled surfaces hindering the subsequent DRIE etching of the underlying silicon. The use of the enhanced etch gas iodine in the FIB system has been shown to remove this re-deposition and allows the subsequent etch to progress unhindered. Optimisation of the mill gas pressure and beam current density has also been shown to be essential for consistent pattern transfer into the underlying silicon