Combination Of Electron Beam Lithography Research Articles

Dc and high frequency magnetic properties of nanopatterned CoFe2O4 arrays fabricated using sol-gel precursors

Nanostructures of ferromagnetic oxides having Curie temperatures above room temperature have potential for applications in memory devices and future spin-based electronic applications. In this article, we report on the dc and high frequency magnetic properties of arrays of elliptical CoFe2O4 nanopillars, covering a large area, fabricated by combined electron beam lithography, and a sol-gel based chemical route. The nanopillars were successfully fabricated on insulating oxidized silicon substrates and on epitaxial thin films of ferroelectric BiFeO3. We performed magnetic force microscopy and ferromagnetic resonance spectroscopy on the arrays to probe their magnetic properties. Due to the possible existence of dominant pinning sites, the CoFe2O4 nanopillars are not single-domain even at nanometer size scales.

Extremely high aspect ratio GaAs and GaAs/AlGaAs nanowaveguides fabricated using chlorine ICP etchingwith N2-promoted passivation

Semiconductor nanowaveguides are the key structure for light-guiding nanophotonicsapplications. Efficient guiding and confinement of single-mode light in these waveguidesrequire high aspect ratio geometries. In these conditions, sidewall verticality becomescrucial. We fabricated such structures using a top-down process combining electronbeam lithography and inductively coupled plasma (ICP) etching of hard masksand GaAs/AlGaAs semiconductors with Al concentrations varying from 0 to100%. The GaAs/AlGaAs plasma etching was a single-step process using aCl2/BCl3/Ar gas mixture withvarious fractions of N2. Scanning electron microscope (SEM) observations showed that the presence of nitrogengenerated the deposition of a passivation layer, which had a significant effect on sidewallslope. Near-ideal vertical sidewalls were obtained over a very narrow range ofN2, allowing the production of extremely high aspect ratios (>32) for 80 nm wide nanowaveguides.

Pattern transfer optimization for the fabrication of arrays of silicon nanowires

The main challenges toward massive fabrication of silicon nanowires are the way to control the crystal orientation and size. In the present work, we report advances in the pattern transfer process to obtain arrays of holes on a Si substrate, aiming at the fabrication of ordered arrays of quantum wires. The wires obtained from this procedure can be uniform in terms of doping profile, crystal orientation and size, while enormously simplifying metallic contacting for applications where parallel biasing is needed. The samples have been fabricated using a combination of electron-beam lithography and reactive-ion etching. Electron-beam lithography is performed at 10 keV on a 100 nm thick 950 K PMMA. A specific recipe for deep reactive-ion etching was developed in order to minimize any widening or under-etching of the holes, as well as any type of wall roughness. Holes with diameters from 30 nm up to 900 nm, and pitch from 90 nm up to 1000 nm were fabricated, achieving no observable scalloping. The sample has been oxidized for further reduction of the size of interconnects (the site between two holes) and interstitials (the site between three holes). As a result, thin nanowires have been fabricated.

Diameter-dependent guided resonance of dielectric hole-array membrane

Silicon nitride photonic crystal slabs having various submicron lattice constants and hole diameters have been investigated, in which optical transmission measurements were utilized in the characterization of the guided resonance. Samples were fabricated by using standard electron beam lithography in combination with KOH wet etching and reactive ion etching dry etching through a silicon substrate and silicon nitride membrane, respectively. The transmittance data reveal the asymmetrical shape of absorption dips associated with Fano resonance, and the positions of the resonance were found to be in accordance with the lattice constant. In addition, multiabsorption dips were evolved from the main absorption dip and became discernable when increasing the hole diameter. The plane wave expansion method was used to identify the mode splitting in the band structure that is caused by a finite size hole diameter, giving rise to the multiabsorption dips. Furthermore, the finite element method was used to calculate the transmission spectra. The simulated representations were in positive agreement with the experimental results.

Method for improving the aspect ratio of ultrahigh-resolution structures in negative electron-beam resist

A method for improving the aspect ratio of ultrahigh-resolution structures in negative electron-beam resist is provided for enhanced pattern-transfer capabilities. The essence of the proposed method is to form a protective “cap” on top of the resist structure by means of electron-beam-induced deposition (EBID) in a self-aligned approach. This is implemented by a combination of electron-beam lithography and EBID during exposure of the resist material in the presence of a precursor gas. The results of the proposed method using hydrogen silsesquioxane resist material are presented and discussed, including various attempts to further optimize this method.

Organization of silicon nanocrystals by localized electrochemical etching

An approach to form a monolayer of organized silicon nanocrystals on a monocrystalline Si wafer is reported. Ordered arrays of nanoholes in a silicon nitride layer were obtained by combining electron beam lithography and plasma etching. Then, a short electrochemical etching current pulse led to formation of a single Si nanocrystal per each nanohole. As a result, high quality silicon nanocrystal arrays were formed with well controlled and reproducible morphologies. In future, this approach can be used to fabricate single electron devices.

Freestanding GaN Resonant Gratings at Telecommunication Range

We theoretically and experimentally demonstrate a freestanding gallium nitride (GaN) resonant grating at telecommunication range. The optical responses of the freestanding GaN resonant gratings are analyzed by the rigorous coupled-wave analysis method. The freestanding GaN resonant gratings are validated on 850-nm freestanding membrane by a combination of electron beam lithography, fast atom beam, etching, and deep reactive ion etching. The polarization properties of such freestanding GaN resonant gratings are demonstrated in reflectance measurements, and the experimental results correspond well to the theoretical model. The strong resonant peaks show a clear dependence on the duty ratio under transverse magnetic polarization, and a promising resonant peak of the fabricated freestanding GaN resonant grating, in which the grating period <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">P</i> is 1500 nm, the grating height <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">h</i> is 230 nm, and the grating width is 280 nm, is observed at 1516.4 nm with a full-width at half-maximum of 4 nm.

Fabrication and Characterization of Graphitic Carbon Nanostructures with Controllable Size, Shape, and Position

The incorporation of carbon materials in micro- and nanoscale devices is being widely investigated due to the promise of enhanced functionality. Challenges in the positioning and addressability of carbon nanotubes provide the motivation for the development of new processes to produce nanoscale carbon materials. Here, the fabrication of conducting, nanometer-sized carbon structures using a combination of electron beam lithography (EBL) and carbonisation is reported. EBL is used to directly write predefined nanometer-sized patterns in a thin layer of negative resist in controllable locations. Careful heat treatment results in carbon nanostructures with the size, shape, and location originally defined by EBL. The pyrolysis process results in significant shrinkage of the structures in the vertical direction and minimal loss in the horizontal direction. Characterization of the carbonized material indicates a structure consisting of both amorphous and graphitized carbon with low levels of oxygen. The resistivity of the material is similar to other disordered carbon materials and the resistivity is maintained from the bulk to the nanoscale. This is demonstrated by fabricating a nanoscale structure with predictable resistance. The ability to fabricate these conductive structures with known dimensions and in predefined locations can be exploited for a number of applications. Their use as nanoband electrodes is also demonstrated.

Patterned Growth of Horizontal ZnO Nanowire Arrays

We report an approach to fabricating patterned horizontal ZnO nanowire arrays with a high degree of control over their dimensionality, orientation, and uniformity. Our method combines electron beam lithography and a low temperature hydrothermal decomposition. This approach opens up possibilities to fabricate ZnO NW array based strain and force sensors, two-dimensional photonic crystals, integrated circuit interconnects, and alternative current nanogenerators.

Fabrication and characterization of nanoscale resonant gratings on thin silicon membrane

We report the design and fabrication of nanoscale resonant gratings which is of interest for narrow bandwidth filtering application. The linear/circular grating structures, of which the grating width is 200nm and the grating height is 260nm, are generated on silicon-on-insulator wafer. Nanoscale gratings are fabricated on the silicon device layer by a combination of electron beam lithography and fast atom beam etching. The silicon handle layer under grating region is removed by deep reactive ion etching, and the buried oxide layer is kept. The reflectance measurements are performed to characterize the optical response of fabricated freestanding nanoscale gratings. The resonant behavior of linear gratings agrees with the theoretical predication, and the polarization-independent responses of circular gratings are also experimentally demonstrated.

Individual Pd nanowire hydrogen sensors fabricated by electron-beam lithography

We report on the hydrogen gas(H2)sensing performance of lithographically patterned Pd nanowires as a function of the nanowire thicknessand H2 concentration. A combination of electron-beam lithography and a lift-off process has beenutilized to fabricate four-terminal devices based on individual Pd nanowires with widthw = 300 nm, lengthl = 10 µm, andthickness t = 20–400 nm from continuous Pd films. The variation of the resistance and sensitivity at 20 000 ppmH2 of Pd nanowires was found to be much lager than at 10 000 ppmH2, which can beexplained by an α–β phase transitionoccurring at 20 000 ppm H2. This is confirmed by the observation of hysteresis behavior in the resistance versusH2 concentration for Pd thin films. The response time was found to decrease with decreasing thickness regardlessof H2 concentration due to a higher surface-to-volume ratio and a higher clamping effect. A single Pd nanowire witht = 100 nm was found tosuccessfully detect H2 at a detection limit of 20 ppm. Our results suggest that lithographically patternedPd nanowires can be used as hydrogen gas sensors to quantitatively detectH2 over a wide range of concentrations.

“Controlling” internal microstructure of nanopatterned oxides via soft electron beam lithography (soft-eBL)

A nanoscale patterning approach for ceramic oxides, based on the combination of electron beam lithography and liquid precursor (so-called soft-electron beam lithography, soft-eBL), is elaborated in the context of control over “internal microstructure”. Soft-eBL synergistically combines the traditional top-down approach with the emerging bottom-up method. Depending on the thermal treatment, a wide variety of “tailored” microstructures can be obtained, ranging from nanoscale porosity to single-crystal epitaxy. Such exquisite control over the size, shape, materials diversity and the internal microstructure of nanopatterns provides an excellent platform for further detailed and quantitative understanding of the role of spatial and dimensional constraints on fundamentals of nucleation, growth and external shape evolution of oxides.

Directed Self-Assembly of Diblock Copolymer Thin Films on Chemically-Patterned Substrates for Defect-Free Nano-Patterning

We demonstrate that polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) can self-assemble in a well-aligned, long-range ordered nanopattern over arbitrarily large areas, commensurate with chemically prepatterned templates prepared by electron beam (EB) lithography. The epitaxially grown cylindrical microdomain formed a defect-free hexagonal lattice although the Chemically-Patterned substrate had some defects in its pattern and errors in pattern position. Furthermore, we demonstrated that the self-assembly process can interpolate points in between the EB generated pattern, thus multiplying the pattern density. The result suggests that the self-assembly of PS-b-PMMA can repair the defects of the patterned substrate, while the patterned substrate can align the domain structures of block copolymer into a long-range ordered single array. The developed process, which combines EB lithography and self-assembly of diblock copolymer provides a promising fabrication method for extension of top down-type lithogra...

Tunneling Junctions as Molecular Sensors

Electron tunneling between nanospaced electrodes provides a mechanism for directly transducing the presence of molecular analytes into electrical signals. Crossbar junctions with vertical separations on the order of a few nanometers were fabricated using a combination of electron-beam lithography and selective chemical etching. The current-voltage properties of the nanojunctions are highly sensitive to the chemical environment. The tunneling currents increase over one order of magnitude in response to water vapor diluted with a background of pure nitrogen. The resistance of the junctions is also dependent on the concentration of the analyte. These results demonstrate that tunneling can be used to detect changes in the chemical environment.

Silicon-on-insulator based nanopore cavity arrays for lipid membrane investigation

We present the fabrication and characterization of nanopore microcavities for theinvestigation of transport processes in suspended lipid membranes. The cavitiesare situated below the surface of silicon-on-insulator (SOI) substrates. Singlecavities and large area arrays were prepared using high resolution electron-beamlithography in combination with reactive ion etching (RIE) and wet chemical sacrificialunderetching. The locally separated compartments have a circular shape andallow the enclosure of picoliter volume aqueous solutions. They are sealed attheir top by a 250 nm thin Si membrane featuring pores with diameters from2 µm down to 220 nm. The Si surface exhibits excellent smoothness and homogeneity as verifiedby AFM analysis. As biophysical test system we deposited lipid membranes by vesiclefusion, and demonstrated their fluid-like properties by fluorescence recovery afterphotobleaching. As clearly indicated by AFM measurements in aqueous buffer solution,intact lipid membranes successfully spanned the pores. The nanopore cavity arrays havepotential applications in diagnostics and pharmaceutical research on transmembraneproteins.

Studies of optical emission in the high intensity pumping regime of top‐down ZnO nanostructures and thin films grown on c‐sapphire substrates by pulsed laser deposition

AbstractWe report on the emission of Zinc Oxide (ZnO) thin films obtained by Pulsed Laser Deposition (PLD) under high intensity excitation. In order to clarify the origin of the emission bands, we compared results for high quality thin films (75 nm) before and after “top‐down” nanopatterning. A nanopattering technique was developed for this purpose. The technique combined Electron Beam Lithography (EBL) and lift‐off techniques and Inductively Coupled Plasma Reactive Ion Etching (ICP RIE). The emission spectra of the two types of samples were found to have a difference in their fine structure that was attributed, in part, to the existence of guided emission in the thin films and exciton weak confinement effects in the nanostructures. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Ordering of silver flowerlike nanosheet structures on an aluminium substrate

We present a facile method for ordering flowerlike structures of silver nanosheets on an aluminium covered surface. It is shown that using a combination of electron-beam lithography and standard silver mirror reaction, structures of silver nanosheets can be grown at predefined positions as single or multiple “flowers”, depending on the underling bare aluminium pattern. Arbitrary shaped areas, defined by electron-beam lithography, can be covered with closely packed silver nanosheets.

Periodic flux jump in superconducting Pb networks as consequence of the extended Little–Parks effect

The extended Little–Parks effect of superconducting network is known as a periodic Tc variation as a function of magnetic field. Superconducting Pb honeycomb networks of matching field 0.106G and triangular microhole lattice of Pb of matching field 0.425G have been fabricated by the combination of electron-beam lithography and a lift-off process of evaporated Pb films. The application of magnetic field corresponds to the vortex filling into superconducting networks. We measured the magnetization of the networks systematically by using a SQUID magnetometer. We found that flux jump appears rather periodically as a function of magnetic field. Flux jumps may be induced by a periodic decrease of the critical current density of the network. To the authors’ knowledge, this is for the first time to observe the regular flux jumps due to the critical current modification coming from the extended Little–Parks effect of the superconducting networks.

Ultrahigh electron mobility in suspended graphene

We have achieved mobilities in excess of 200,000 cm 2 V −1 s −1 at electron densities of ∼2 ×10 11 cm −2 by suspending single layer graphene. Suspension ∼150 nm above a Si/SiO 2 gate electrode and electrical contacts to the graphene was achieved by a combination of electron beam lithography and etching. The specimens were cleaned in situ by employing current-induced heating, directly resulting in a significant improvement of electrical transport. Concomitant with large mobility enhancement, the widths of the characteristic Dirac peaks are reduced by a factor of 10 compared to traditional, nonsuspended devices. This advance should allow for accessing the intrinsic transport properties of graphene.

High aspect ratio GaAs nanowires made by ICP-RIE etching using Cl2/N2 chemistry

We report an experimental study of GaAs etching by ICP-RIE based on Cl2:N2 chemistry. The influence of the RF source power, the chlorine dilution, the RF platen power, and the process pressure at several substrate temperatures are investigated. An optimized process is proposed to get routinely GaAs nanowires of 1μm high and about 30nm in diameter with a full top-down approach combining electron beam lithography and Ni lift-off steps.