Electron Beam Lithography Technique Research Articles

Fabrication of two-dimensional hybrid photonic crystals utilizing electron beam lithography

A step-by-step description of a process for a reproducible fabrication of two-dimensional hybrid photonic crystals utilizing electron beam lithography and subsequent lift-off technique is presented. The devices consist of a periodic array of gold nanodots of (100 × 100) μm 2 up to (400 × 400) μm 2 in size on a dielectric layer as slab-waveguide. Arrays were fabricated on indium tin oxide, silicon oxynitride and tungsten oxide with dot diameters from 70 to 550 nm, respectively. Patterning is performed by electron beam lithography using the single pixel line writing mode. Dot sizes are achieved by overexposure and overdevelopment. For dots with diameters over 250 nm, a controlled defocused beam is used for exposure. Homogeneity of the dot shape over the array is achieved by refocusing prior to exposure and using a 10-μm aperture for larger depth of field. Metallization adhesion problems can be solved by introducing a plasma clean prior to deposition. Fabricated devices were characterized in extinction measurements, showing clear correlations between dot shape and particle-plasmon spectral width.

Patterning nanostructures to study magnetization processes

Lithography techniques such as electron-beam lithography and focused-ion-beam milling are widely used to fabricate structures with dimensions well below 1 µm. These techniques have been used to produce planar magnetic structures with sub-micrometer dimensions and well controlled geometry. This has allowed the study of basic magnetic behaviour and the development of structures with potential for applications in magnetic recording and magnetic logic devices. The techniques of electron beam lithography and focused-ion-beam milling for the fabrication of magnetic nanostructures are outlined here. These techniques have been used to fabricate ribbon-like planar nanowires to study the behaviour of the individual magnetic domain walls which mediate the reversal process in such elongated structures. These methods allow the production of structures in which the location of domain wall formation and position can be controlled, allowing separation and study of the domain wall nucleation and propagation processes. Domain wall injection and domain wall propagation behaviour are investigated and shown to be stochastic processes.

Fabrication of graphite nano-structures

Nano-sized graphite patterns were obtained by tailoring graphite sheets using different methods. With focused ion beam etching we are able to produce grooves of minimum width ～ 30 nm. However, at the edges of the grooves there deposit with Ga-C compound which prevents the easy cleavage of the graphite sheets. With electron beam lithography and reactive ion etching techniques we are able to produce graphite patterns of sizes down to 50 nm. Three different types of masks are tested, and the results are compared and discussed.

Switching behavior of nanoscale Co ferromagnetic elements using off-axis electron holography

The switching behavior of 20-nm-thick nanoscale Co ferromagnets has been investigated using the technique of off-axis electron holography. Elements having elliptical, dumbbell, ring and “Pacman” shapes were fabricated on electron-transparent silicon nitride membranes using standard electron-beam lithography and lift-off techniques. Ellipses generally showed linear switching but interactions between closely spaced elements occasionally caused inconsistent switching. Dumbbells, rings and disk-like Pacman shapes developed intermediate vortex states during switching, which caused broad switching fields. Ring-like Pacman shapes had negligible fringing field and switched very rapidly without the formation of any vortex state.

Block copolymer micelle nanolithography on non-conductive substrates

A new lithographic technique has been developed and applied to cell adhesion studies and electro-optical material development. Attachment of 6 nm Au particles, in periodic and non-periodic pattern, onto non-conductive substrates has been achieved. This was performed via a combination of diblock copolymer self-assembly and electron beam lithographic techniques. To optimize e-beam resolution on non-conductive materials, an additional carbon layer was thread-coated onto the substrates. This carbon coating and the diblock copolymer used in the self-assembly step were simultaneously removed by a final hydrogen plasma treatment to reveal Au nanodot patterns of unprecedented pattern quality. These optically transparent substrates (glass cover slips) were bio-functionalized via the Au-dot patterns to yield a platform for unique cell adhesion studies. The same Au-dot patterning technique was applied to sapphire substrates, which were subsequently employed to nucleate electro-optically active ZnO nanopost growth.

Fast nanopatterning of two-dimensional photonic crystals by electron beam lithography

A non-conventional low energy electron beam lithography technique for fast patterning of PMMA electron resist is presented for the fabrication of two-dimensional photonic crystal (2D-PC) devices, such as micro-cavity lasers and waveguides. Our technique consists of a single exposure to a large defocused Gaussian beam instead of the conventional pixel-by-pixel mode of approach, in which each feature is composed of an array of smaller focused spots. Experimental results are provided, which show the accurate control of the hole size with the exposure time, for different values of defocusing length. The advantages of our approach include higher (5 to 10 times) exposure speed and reusability of the same scan description for the definition of photonic crystals with different filling factors and the same lattice period. The resulting 2D-PC patterns, having the lattice period of 300 nm and radii in the range of 30–140 nm, have been morphologically characterized by scanning electron microscopy. Monte Carlo simulations of electron scattering in the resist have been carefully performed for both kinds of exposure techniques using a low energy electron beam.

Precise Control of Nanowire Formation Based on Polysilane for Photoelectronic Device Application

A High-energy single ion trajectory in a Si-based polymer thin film produces a cross-linked polymer nanowire with the length of 10–1000 nm and the thickness of 4.0–19.4 nm (radius of the cross section). The energy density deposited by incident ions and the molecular weight of the target polymer materials principally determine the thickness of the nanowires at any discrete radius in this range. Surface treatment of a substrate completely controls the adhesion of the nanowires, which makes it possible to pattern the nanowire distribution on the substrate. Nanogap platinum electrodes with 100 nm gap width are fabricated on the nanowire dispersed SiO2 substrate by a lift-off method with electron beam lithography techniques. Current-voltage analysis of the nanowires in the gap clearly indicates intrinsic semiconductive features based on crosslinked polysilanes.

Gold Nanoparticle Patterning of Silicon Wafers Using Chemical e-Beam Lithography

This paper demonstrates a novel facile method for fabrication of patterned arrays of gold nanoparticles on Si/SiO2 by combining electron beam lithography and self-assembly techniques. Our strategy is to use direct-write electron beam patterning to convert nitro functionality in self-assembled monolayers of 3-(4-nitrophenoxy)-propyltrimethoxysilane to amino functionality, forming chemically well-defined surface architectures on the 100 nm scale. These nanopatterns are employed to guide the assembly of citrate-passivated gold nanoparticles according to their different affinities for amino and nitro groups. This kind of nanoparticle assembly offers an attractive new option for nanoparticle patterning a silicon surface, as relevant, for example, to biosensors, electronics, and optical devices.

Multi-walled carbon nanotube-based gas sensors for NH 3 detection

Multi-walled carbon nanotube (MWNT) based gas sensors for detecting gaseous molecules of ammonia (NH 3) were developed. MWNTs were prepared using microwave plasma-enhanced chemical vapor deposition. The gas sensors, in which the nanotubes laid under the electrodes, were fabricated using electron-beam lithography and lift-off techniques. It is found that the conductance of MWNTs decreases when the sensors successively exposed to NH 3 gas at room temperature of 25 °C. The MWNT-based sensors are sensitive to NH 3 gas. This suggests that the MWNTs could be a good candidate material for NH 3 detection at room temperature.

Exciton dynamics in quantum nano‐structures of II–VI diluted magnetic semiconductors fabricated by electron‐beam lithography

Exciton dynamics in quantum dots (QDs) of II–VI diluted magnetic semiconductors (DMSs) has been studied by using an electron-beam lithography technique. The peak energy of excitonic photoluminescence in the QDs shows blue shifts up to 3.5 meV toward the dot diameter of 20 nm, indicating a lateral confinement effect for the exciton. The time-dependent energy shift of the exciton due to the localization is small as 2.9 meV, which originates from the suppression of the exciton diffusion due to the finite dot structure. The coupled QDs composed of a DMS magnetic well (MW) and a non-magnetic well (NW) were sucessfully fabricated, where the exciton energy in the NW was designed to be lower than that in the MW. The spin-polarized excitons migrate from the MW to the NW in magnetic fields and the exciton spin injection is demonstrated in the coupled QDs. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Inverse tunnel magnetoresistance associated with Coulomb staircases in micro-fabricated granular systems

Abstract We have investigated spin-dependent single-electron tunneling (SET) in micro-fabricated magnetic granular systems with current-perpendicular-to-plane geometry. Electron beam lithography technique and Ar ion etching were employed for the micro-fabrication process to form a Co/Al-O/Co 31 Al 24 O 45 /Co/Pt pillar with the cross section of 0.4×0.4 μm 2 . Inverse tunnel magnetoresistance is observed around the step-up points of Coulomb staircases. The observed results are beyond the orthodox theory of SET for a double junction model without spin-accumulation effects.

Fabrication of FePt nanoparticles for self-organized magnetic array

A self-organized array of magnetic nanoparticles can be potentially used to increase the storage density in magnetic recording. One challenge in this approach is to obtain long-range order assemblies of the nanoparticles. One method to solve this problem is to pattern a substrate having circumferential patterns, whose dimensions are within the coherent length of the self-organized array. By patterning the disk into topographically confined circumferential patterns with such dimensions, thermally stable magnetic nanoparticles may be used to fabricate magnetic recording disks. The circumferential patterns in this case are formed on the disk substrate with dimensions of 100–500 nm and depths of 5–20 nm, prepared by electron beam lithography and reactive ion etching techniques. The monodispersed FePt nanoparticles were synthesized by thermal decomposition of iron pentacarbonyl and reduction of platinum salt simultaneously in the presence of surfactant molecules, achieving a size distribution of 3.15±0.20 nm. Our initial experimental results showed that the monodispersed FePt nanoparticles were successfully deposited into the circumferential patterns and they self-organized into a superlattice. The self-assembly of the FePt nanoparticles in the circumferential pattern strongly relied on the line edge roughness (LER) of the circumferential pattern. This suggests that we need to fabricate the circumferential pattern with LER at the length scales of approximate an individual nanoparticle size 3–4 nm in order to improve the assembly quality further. This is another challenge for today’s advanced lithography and etching processes.

Structuring of self-assembled three-dimensional photonic crystals by direct electron-beam lithography

An electron-beam lithography technique is described capable of structuring three-dimensional self-assembled photonic crystals. It is shown that the control of the writing depth can be achieved by varying the electron acceleration voltage. Microscopic structures with a depth from 0.4 up to 2 μm are fabricated with a typical lateral resolution of 0.4 μm. The relevance of this technique for the fabrication of deterministic defects sites in opal photonic crystals is discussed and its extension towards buried structures is suggested.

Quantum interference in a one-dimensional silicon nanowire

We study electronic transport within a lithographically defined silicon nanowire for zero and finite bias. The 10-nm wide and 500-nm long nanowire is fabricated by advanced electron-beam lithographic techniques. Transport experiments reveal clear quantum size effects in the conduction through the wire. Energy quantization within the wire leads to a shift in conduction threshold. Quantum interference effects cause an oscillatory pattern in the conductance. At low source-drain bias, transport is dominated by shallow tunneling barriers. At higher bias, additional nanowire modes are found to contribute to the conductance.

Silicon surface processing techniques for micro-systems fabrication

Micromachining of silicon surface and near-surface regions by electron-beam lithography (EBL) and reactive ion dry-etching (RIE), as well as by laser microfabrication techniques is described. Combination of EBL and RIE techniques is used to fabricate quasi two-dimensional (2D) photonic crystal (PhC) structures with aspect ratios at approximately 15. Structural and optical properties of PhC samples with 2D honeycomb lattice are reported, and a complete photonic band gap at wavelengths approximately 2.16 μm is identified from the optical transmission data. Next, laser microfabrication of silicon by femtosecond laser pulses is reported. Processing of silicon under low air pressure conditions (≃5 Torr) is demonstrated to be essentially debris-free process, highly suitable for cutting and scribing of wafers as well for hole drilling. Removal of thin films from the silicon surface by a single-shot laser ablation using mask projection is also demonstrated.

Single InGaAs quantum dot flash memory

Modulation doped GaAs/AlGaAs heterostructures have been grown by molecular beam epitaxy with a layer of self-assembled InGaAs quantum dots located in the center of the undoped AlGaAs spacer. Electron beam lithography and etching techniques were used to define 70 nm wide current channels in combination with lateral side gates. Depending on the direction of gate voltage sweeps the onset of conductance differs by up to 1.7 V, which is associated to charging and discharging of the quantum dots in the spacer. From Coulomb blockade oscillation single electron effects are revealed.

Spectral characteristics of single InAs/InGaAs quantum dots in a quantum well

Self assembled InAs quantum dots embedded in an InGaAs quantum well were grown by molecular beam epitaxy. At room temperature these dots in a well (DWELLS) have an emission wavelength of 1.3�μm. Small mesas with lateral sizes down to 200 × 200 nm2 were fabricated by electron beam lithography and etching techniques. By photoluminescence spectroscopy at low temperatures we observe narrow lines, which we attribute to excitons and excitonic molecules. Biexciton binding energies ranging between 3.5–5�meV are found.

Fabrication of microbowtie structures for optical probing of nanoscale objects

A microbowtie array critical to a newly proposed optical probe is studied. The optical probe is used for high-speed detection of nanoscale objects. Based on the concept of Wave Interrogated Near Field Array (WINFA), an array of microbowtie structures acts as a detector in amplifying the incident light source to overcome the difficulties of the conventional optical probe that are limited by the diffraction limits of the optical light. In the present study, the micro bowties are fabricated by an electron beam lithography technique. The paper begins with an introduction of the probe concept and then the design considerations are given followed by the fabrication procedure and results. Two different substrates are considered to study the charge accumulation problem caused by an insulating substrate, required by the probe. Results indicate that the bowtie structure with an indium-tin-oxide coated glass substrate can minimize the charge accumulation problem, making it a suitable substrate for the bowtie structures considered. The paper concludes with recommendations for making even smaller nano-sized bowtie structures.

A probe-positioning method with two-dimensional calibration pattern for micro-multi-point probes

A probe-positioning method for micro-multi-point probes that can be independently driven is proposed. By the electron-beam lithography technique, we fabricated a 0.5 mm×0.5 mm-sized probe-positioning pattern matrix consisting of 750 nm×750 nm-sized cells. Each cell has 150 nm×150 nm-sized pits that represent a bit array, which specifies its address. Reading the address information on the pattern with the microprobes allowed us not only to determine the probe positions, but also to calibrate the orientation and dimension of each scan.

Metastable states during magnetization reversal in square permalloy rings

The magnetic reversal process in a two-dimensional array of permalloy square rings is presented. Rings of thickness of 25 nm, of lateral size of 2.1 \ensuremath{\mu}m, and with ring width of 240 nm were microfabricated using electron-beam lithography and lift-off techniques. Analysis of the diffracted magneto-optical Kerr effect hysteresis loops, magnetic force microscopy images, and micromagnetic simulations show that the magnetization reversal path depends on the direction of the in-plane applied magnetic field. On reducing the field from saturation, for fields along an edge or a diagonal of the square, the ``onion'' state is the stable state at remanence. In a narrow field range around reversal we find that the reversal occurs via a metastable intermediate state. For fields along the diagonal this intermediate state is a magnetic vortex. When the field is applied along an edge direction the intermediate state is a ``horseshoe'' state.