Conventional Electron Beam Lithography Research Articles

Fabrication of mesoscopic superconducting Nb wires using conventional electron-beam lithographic techniques

Conventional electron-beam lithography has been used to fabricate mesoscopic Nb wires. Nb was deposited in an ultrahigh vacuum evaporation chamber using electron gun heating. The typical linewidth and the thickness were 200 and 50 nm, respectively. The transition temperatures were above 7 K. They increased with thickness and linewidth. To demonstrate the feasibility of two angle evaporation techniques, we also fabricated Nb/(Al–)AlOx/Nb tunnel junctions showing superconducting single-electron transistor characteristics.

Electron induced chemical nanolithography with self-assembled monolayers

We demonstrate a simple scheme to generate chemical surface nanostructures. Electron-beam writing is used to locally modify the terminal nitro functionality in self-assembled monolayers of 4′-nitro-1,1′-biphenyl-4-thiol to amino groups, while the underlying aromatic layer is dehydrogenated and cross linked. Using low energy electron proximity printing and conventional electron-beam lithography with a beam energy of 2.5 keV and doses from 2500 to 50 000 μC/cm2, templates of reactive amino sites with lateral dimensions down to ∼20 nm could be fabricated. The templates were used for the surface immobilization of fluorinated carboxylic acid anhydrides and rhodamine dyes. The molecular structures were then imaged and analyzed by atomic force and scanning confocal fluorescence microscopy.

Controlled deposition of carbon nanotubes on a patterned substrate

We describe a new technique for the preparation of reactive templates of 1,2-aminopropyltriethoxysilane on silica surfaces, based on the chemical vapour deposition of silane molecules through a PMMA mask patterned by conventional electron-beam lithography. The template thickness and width are controlled down to the monolayer and 50 nm ranges respectively. These templates are successfully used for the selective deposition of sodium dodecylsulphate-covered single walled nanotubes at controlled locations on the surface. We demonstrate that this technique allows one to contact nanotubes with metallic electrodes deposited on top of the tubes and opens the way towards the formation of controlled crossings of nanotubes.

Direct electron beam writing of nanostructures using passivated gold clusters

Direct electron beam writing of metallic nanostructures using metallorganic compounds as precursors offers a method of fabricating nanostructures with a reduced number of processing steps compared with conventional electron beam lithography (EBL). We have used passivated gold nanoclusters, prepared as a thin layer on highly orientated pyrolitic graphite (HOPG), to produce gold nanostrures directly. The passivating organic ligands were removed by an electron beam in a scanning electron microscope (SEM), thus allowing the gold to aggregate and form nanostructures. The resulting structures have a width as small as 26nm with a separation as low as 30nm. The nanostructures are stable and remain intact after removal from the SEM. Energy dispersive X-ray analysis confirms that the structures do indeed contain gold.

Josephson persistent-current qubit

A qubit was designed that can be fabricated with conventional electron beam lithography and is suited for integration into a large quantum computer. The qubit consists of a micrometer-sized loop with three or four Josephson junctions; the two qubit states have persistent currents of opposite direction. Quantum superpositions of these states are obtained by pulsed microwave modulation of the enclosed magnetic flux by currents in control lines. A superconducting flux transporter allows for controlled transfer between qubits of the flux that is generated by the persistent currents, leading to entanglement of qubit information.

Nano-lithography by electron exposure using an Atomic Force Microscope

We have used a conductive Atomic Force Microscope (AFM) tip to expose a very thin resist film. An exposing current of low energy electrons was induced from the tip to the substrate by applying a small bias voltage. Uniform resist films as thin as 10 nm were fabricated using the Langmuir–Blodgett technique. To orient the defined pattern and to make electrical connections a special larger scale alignment structure was first defined by conventional electron beam lithography, either directly in the Langmuir–Blodgett resist film or in a separate first lift-off process with a thicker resist. The results from the one resist process gave conducting 50 nm lines with a 60 Å thick vacuum deposited aluminium film after the pattern transfer. The two step process, which is aiming towards definition of ultra small tunnel junctions, has produced similar conductive lines.

Superconducting nanobridges prepared by a self-limiting wet chemical etching process

An optimized patterning process of thin films resulting in superconducting nanobridges is reported. The process is based on conventional electron beam lithography (EBL, 20 keV) combined with wet chemical etching using dilute phosphoric acid. This etching is found to be self-limiting and, thus, the minimum width of the bridge becomes independent of the etching time and is determined only by the electron dose applied for EBL. For the self-limiting to be effective, the dilution of the acid is essential. With the above procedure, nanobridges of a well-defined geometry and lateral dimensions down to 60 nm could be reproducibly fabricated exhibiting superconducting transition temperatures K.

Fabrication and characterization of small tunnel junctions through a thin dielectric membrane

We show that a small tapered hole through a thin silicon nitride membrane provides a mask for tunnel junction structures. Our experiments imply, unlike in the conventional planar electron beam lithography, that tunnel junctions are well voltage biased in this structure with vanishingly small on-chip impedance. Our technique allows fabrication of double junctions, and even multijunction linear arrays, with small metallic islands in between.

Sub-100 nm lithography and high aspect-ratio masks for fabrication of Josephson devices by ion implantation

Freely positionable Josephson junctions in high-temperature superconductor thin films are fabricated by a combination of conventional photolithography and electron-beam lithography. In order to create weak-links, point-defects in selected regions of YBa 2Cu 3O 7− x thin films were generated by 200 keV oxygen ion irradiation. In a temperature range just below the transition temperature T c , all junctions can be described by the resistively shunted junction model. The magnetic modulation of the critical current on temperature clearly demonstrates SNS-behaviour. Under microwave irradiation, Shapiro steps appear, and the dependence on microwave power reveals Josephson contributions over the entire temperature range.

Nanometer-scale lithography in thin carbon layers using electric field assisted scanning force microscopy

Resist patterning by scanning probe microscopy is a promising method to create structures in the nanometer range beyond the resolution of conventional electron beam or photo lithography. In conventional resist processing one has to remove either the exposed or unexposed resist in a solvent by an additional step. In this article we demonstrate the possibility of directly writing nano-scaled patterns in a thin amorphous carbon layer, which can be used as an etching mask, by a scanning force microscope. Above a threshold voltage between tip and sample small trenches can be created, whereby the carbon is completely removed from the exposed areas. Evidence is given that the mechanism responsible for the trench formation is a local field-induced oxidation of the carbon layer underneath the tip.

Nanometer biodevice fabrication by electron beam lithography

A conventional electron beam lithography machine operated at 50 kV is used in this work to fabricate devices and structures for biophysical and molecular electronics applications featuring critical dimensions down to the nanometer region. Such nanostructures are used for deposition and manipulation of organic molecules; fabricated devices include fine pitch self-standing meshes for laser deposition of molecules, a bimetallic miniaturized glucose sensor and nanogaps for molecular trapping and probing. The developed process is described in detail and by means of Monte Carlo simulation, the various electron scattering processes are modelled. Critical issues, such as fabrication of nanogaps with dimension down to 5 nm, are also addressed.

Nanoscale silicon field effect transistors fabricated using imprint lithography

We report the fabrication and characterization of nanoscale silicon field effect transistors using nanoimprint lithography. With this lithographic technique and dry etching, we have patterned a variety of nanoscale transistor features in silicon, including 100 nm wire channels, 250-nm-diam quantum dots, and ring structures with 100 nm ring width, over a 2×2 cm lithography field with good uniformity. Compared with devices fabricated by the conventional electron-beam lithography, we did not observe any degradation in the device characteristics. The successful fabrication of the semiconductor nanodevices represents a step forward to make nanoimprint lithography a viable technique for the mass production of semiconductor devices.

Fabrication and characterization of III–V compound semiconductor Bragg-Fresnel lenses for hard x-ray microfocusing

Hard x-ray phase Bragg-Fresnel lenses (BFLs) have been made in III–V semiconductors of (111) GaAs and InP, and in Si for comparison purposes. Diffractive linear and circular patterns were defined with conventional electron beam lithography. Pattern transfer was accomplished with reactive ion etching (RIE) using gases of BCl3/Cl2/SiCl4, CH4/H2/Ar, and BCl3Cl2, respectively. In addition, selective etching of heteroepitaxial GaAs devices for better process depth control was demonstrated by incorporating an epitaxial AlGaAs etch stop layer between the GaAs top layer and substrate and then etching with Cl2O2. Preliminary tests on focusing have been made at the Stanford Synchrotron Radiation Laboratory (SSRL) showing the ability to focus a 50 micron x-ray beam to ∼5 microns for Si structures. This is an essential requirement for x-ray microprobe technology development which has wide applications in x-ray imaging, deep field x-ray lithography, and in probing nanometer scale complex fluids such as boundary lubrication layers.

Nanoscale colloidal particles: Monolayer organization and patterning

The monolayer deposition of nanoscale colloidal particles (Au citrate sols) was demonstrated by employing an aminofunctional silane [3-(2-Aminoethlyamino) propyltrimethoxysilane] as a coupling agent. The compatibility of this colloidal Au deposition method with conventional electron beam lithography techniques was examined, and the two-dimensional patterning of the Au colloidal monolayer was demonstrated. Using this fabrication method, a proposal for a single electron device structure based on nanoscale (2 nm diameter) gold colloidal particles was made.

Nano-lithography using a resist, patterned by electron exposure in an AFM configuration

We have used a metallised force microscope tip to apply a voltage and thereby expose a very thin resist film. It is possible to image the film surface before, during and after the exposure, without interference with the process. Uniform resist films as thin as 10 nm are fabricated using the Langmuir-Blodgett technique. To orient the defined pattern and to make electrical connections a special larger scale alignment structure is first defined by conventional electron beam lithography, either directly in the Langmuir-Blodgett resist film or in a complete first process with a separate resist system. The results from the one resist process gave conducting 50 nm lines in a 60 A thick aluminium film after pattern transfer. The two step process, which is aiming towards definition of extremely small tunnel junctions for low temperature measurements, have produced similar conductive lines; still though, without tunnelling barriers between the two metal layers.

First order gain and index coupled distributed feedback lasers in ZnSe-based structures with finely tunable emission wavelengths

First order gain and index coupled distributed feedback (DFB) gratings were realized in ZnSe-based laser structures using direct implantation with a focused ion beam for gain-modulated structures and conventional electron beam lithography for index modulation. With both technologies, gratings with periods below 90 nm could be achieved, permitting DFB emission in the blue spectral range. Fine tuning of the emission wavelength in steps of 0.14 nm is demonstrated by sampled DFB gratings based on a periodic modulation of the resonator period.

Miniature low voltage beam systems producable by combined lithographies

The project of a miniaturized vacuum microelectronic 100 GHz switch is described. It implies the development of a field emission electron gun as well as the investigation of miniaturized lenses and deflectors. Electrostatic elements are designed and developed for this application. Connector pads and wiring pattern are created by conventional electron beam lithography and a lift-off or etching process. Wire and other 3-dimensional structures are grown using electron beam induced deposition. This additive lithography allows to form electrodes and resistors of a preset conductivity. The scanning electron microscope features positioning the structures with nm precision. An unconventional lithography system is used that is capable of controlling the pixel dwell time within a shape with different time functions. With this special function 3-dimensional structures can be generated like free standing square shaped electrodes. The switch is built by computer controlled additive lithography avoiding assembly from parts. Lenses of micrometer dimensions were investigated with numerical electron optics programs computing the 3-dimensional potential and field distribution. From the extracted axial field distribution the electron optic characteristic parameters, like focal length, chromatic and spherical aberration, were calculated for various lens excitations. The analysis reveals that miniaturized optics for low energy electrons, as low as 30 eV, are diffraction limited. For a lens with 2 μm focal length, a chromatic aberration disc of 1 nm contributes to 12 nm diffraction disc. The spherical aberration blurs the probe by 0.02 nm, assuming an aperture of 0.01 rad. Employing hydrogen ions at 100 V, a probe diameter of 0.3 nm generated by chromatic aberration is possible. Miniaturized electron optical probe forming systems and imaging systems can be constructed with those lenses. Its application as lithography systems with massive parallel beams can be forseen.

Patterning of an electron beam resist with a scanning tunnelling microscope operating in air

A scanning tunnelling microscope operating in air is employed to expose a 50 nm thick electron beam sensitive resist with low energetic electrons. We demonstrate that it is possible to expose the resist under ambient conditions with voltages of ~50 V without observable modification of the resist surface after exposure prior to development. A resolution of 150 nm has been achieved. The dose for complete exposure has been determined as 10 mC cm −2. This is about 1000 times higher than the value for conventional high-voltage electron beam lithography.

Improvements of Nanostructure Patterning in X-Ray Mask Making

In order to evaluate the resolution limits and to achieve a high resolution X-ray lithography under proximity condition, masks compatible with Karl Suss XRS200 stepper have been fabricated with a conventional electron beam lithography at 50 keV energy and a reactive ion etching (RIE) technique for the pattern transfer. It was found that with a relative thin (300 nm) tungsten absorber 50 nm linewidth and high density gratings of period down to 120 nm can be fabricated without proximity correction. Experimental results with gratings of different linewidths and different periods are analyzed to clarify the process parameter influences on the dose correction. In addition, the fabricated masks were tested by proximity replication with Karl Suss stepper at a gap of 5 µ m using synchrotron radiation. Finally the replication results were discussed in terms of the resolution limit as a function of proximity gap.

Electron beam lithography for fabrication of 0.1μm scale structures in thick single level resist

The issues involved in high-resolution pattern fabrication with high aspect ratio by means of conventional electron beam lithography (EBL), in a thick single-level resist have been investigated. A commercial EBL machine which operates at medium accelerating voltage was used to expose 1 μm thick single-level PMMA resist on bulk and thin substrates. Monte Carlo simulation was used extensively in this study for better understanding of electron scattering behavior in order to quantify the achievable ultimate resolution and experimental latitudes in a thick single-level resist on bulk silicon and thin substrate by means of conventional EBL techniques. Our experimental and theoretical (Monte Carlo simulation) studies based on the effects of various process variables, including the effect of beam voltage and exposure dosage (for latitude-related experiments), show that closely spaced lines with dimensions down to 0.12μm can be fabricated in a thick single-level resist, even on bulk silicon at 40 kV accelerating voltage. Finally, issues concerning resist profile angle, varying beam voltage and structure dimensions for exposure on bulk silicon are discussed.