Resolution Electron Beam Lithography Research Articles

A Self-Assembled Graphene Ribbon Device on SiC

Graphene ribbons, which may be fabricated by a wide variety of experimental techniques such as chemical processing, unzipping or etching of carbon nanotubes, molecular precursors, ion implantation, and so on, can find promising applications in interconnects, terahertz sensors, and plasmonic devices. Here we report measurements on self-assembled graphene ribbons that are prepared by a controlled high-temperature sublimation technique. The epitaxial graphene ribbons on SiC can be readily and efficiently located by confocal laser scanning microscopy for device fabrication using a removable metal protection layer to avoid contamination of the graphene and hexagonal boron nitride to serve as a top-gate dielectric spacer. These self-assembled graphene ribbons have smooth edges, and the observation of a magnetoresistance side peak in such a structure is consistent with diffusive boundary scattering in the quasi-ballistic regime. In contrast, graphene ribbons defined by electron-beam lithography and subsequent conventional reactive ion etching on the same SiC wafer only show pronounced negative magnetoresistance due to strong disorder in the edge structures (chemical dopants, the resolution of electron-beam lithography, etc.). Our experimental approaches are applicable to wafer-scale, graphene-based integrated circuits.

Ultra-violet nanoimprint lithography-compatible positive-tone electron beam resist for 3D hybrid nanostructures

A fabrication technique to produce a 3D hybrid structure having different scale complex patterns is studied. A fabrication method is proposed and an ultra-violet nanoimprint lithography (UV-NIL) compatible positive-tone electron beam (EB) resist is prepared. The resist can be shown to have microstructures and nanostructures on the same surface by UV-NIL and positive-tone EB lithography (EBL). The resist can behave as a positive-tone EB resist after the UV-NIL process, exhibits a high UV-NIL and EBL resolution, and a sensitivity sufficient to fabricate 3D hybrid nanopatterns with a high aspect ratio. The fabrication of a micro lens array with nano anti-reflection structure is then performed. The resist can accurately duplicate a microscale concave lens array with a diameter of 3.8 μm. Nano anti-reflection structures are then directly fabricated by EBL on the lens array. The positive-tone dot pattern, as anti-reflection structures with diameters of 220 nm, are fabricated with an EB dose of 1000 μC/cm2. UV-NIL is used to duplicate the fabricated sample. The resist products can successfully be used as the master mold for UV-NIL. A convex micro lens array, with a 200 nm anti-reflection structure, is obtained. These methods using the resist can provide complex 3D hybrid nanostructures. The usefulness of this method and resist are discussed.

An Application of Chalcogenide Alloy Other than Storage Memory Field

The necessity to handle mechanical functionality at nanoscale has recently motivated the prosperity of the nanoelectromechanical systems (NEMs). The fabrication of NEMS strongly depends on the so-called "topdown" techniques that are however limited by the resolution of electronbeam lithography. Meanwhile, the size of the NEMS needs to be shrunk continuously in order to further enhance the system performance. As a result, current research interest has been dedicated to "bottomup" techniques or even a hybridization of two aforementioned approaches, leading to the presence of the nanowire-based NEMs. Here, we presented some recent patent for nanowire-based NEMS. We investigate the resonant frequency and the frequency tuneability of the nanowire-based nanoelectromechanical system using Ge2Sb2Te5 media. By varying the nanowire dimensions, corresponding resonant frequencies and frequency tuneability are calculated using an established mechanical model. We theoretically study the frequency tuneability of the nanowire-based NEMs using GST media. The resonant frequencies and the corresponding frequency tuneabilities for different nanowire dimensions are investigated using a developed mechanical model, and a previously established electrothermal model is performed to imitate the frequency tuning behavior of the system along with the phase-change phenomenon. By carefully controlling the amorphous fraction of the active region, a very high resonant frequency can be tuned within an ultra-high adjustable bandwidth. In addition, the merits of the phase-change memories including great scalability, low power consumption, fast transition time, and non-volatility can be also found on the proposed system. These results will open up a route for designing the next generation NEMs, and also pioneer a new application field for the GST media. Today phase-change materials have received a wide range of applications from nonvolatile memories to neuromorphic networks due to its unique combinations of structural, electrical, and thermal properties. However, as the mechanical properties of phase-change materials exhibits a remarkable difference between the amorphous and crystalline phases, the feasibility of continuously changing the resonant frequency of the nanowires based on phase-change materials becomes viable.

Estimation of pattern resolution using NaCl high-contrast developer by Monte Carlo simulation of electron beam lithography

The influences of different concentrations of NaCl in tetramethyl ammonium hydroxide (TMAH) developer on the electron beam lithography resolution of hydrogen silsesquioxane (HSQ) resist were studied, using original development model based on the three-dimensional (3D) energy deposition distributions (EDDs) and the experimental developing behaviors. The development behaviors of HSQ resist developed in 2.3wt.% TMAH, 2.3wt.% TMAH with 2wt.% NaCl and 2.3wt.% TMAH with 4wt.% NaCl developer were measured, respectively, to provide the solubility rates at different exposure dosages for the simulation. On the basis of the solubility rates and 3D-EDDs, the optimal EDD regions were determined for achieving sharp nanodot patterns. The optimal resist profiles of the three different developers were compared, showing that the 2.3wt.% TMAH with 4wt.% NaCl developer is suitable to form 7-nm-sized, 15-nm-pitched nanodots with sufficient height. The simulations were demonstrated to be useful for estimating the pattern resolutions with different solubility rates of developers, especially for sub-10nm nanodot fabrication.

Graphene as discharge layer for electron beam lithography on insulating substrate

Charging of insulating substrates is a common problem during Electron Beam lithography (EBL), which deflects the beam and distorts the pattern. A homogeneous, electrically conductive, and transparent graphene layer is used as a discharge layer for EBL processes on insulating substrates. The EBL resolution is improved compared with the metal discharge layer. Dense arrays of holes with diameters of 50 nm and gratings with line/space of 50/30 nm are obtained on quartz substrate. The pattern placement errors and proximity effect are suppressed over a large area and high quality complex nanostructures are fabricated using graphene as a conductive layer.

Direct Surface Relief Formation in Polymer Films

Due to active development of nanoelectronics, the studies of methods of nanorelief surface formation in different materials, in particular polymers are very important. Organic polymer films in consequence of their dielectric and optical properties have been used as basis of these devices. In this paper, the possibility of UV optical record and electron beam lithography in different type of polymeric films was studied. Mechanisms of molecular structure changes: photoisomerization, destruction, cross-linking and oxidation have been discussed. The results of UV illumination of polyurethanes, polyacrylates, and some block-copolymers were described. The element analysis of polybutadiene block copolymer was performed before and after UV illumination, and the changes in optical transmission spectra of the polymer film were measured. The resolution of electron beam lithography on polymeric films also was studied.

UV optical record and electron beam lithography in polymer films

Possibility of UV optical record and electron beam lithography in different type of organic polymer films was studied. Mechanisms of molecular structure changes: photoisomerization, destruction, cross-linking and oxidation have been discussed. The results of UV illumination of polyurethanes, polyacrilates, and some block-copolymers are described. The element analysis of polyisoprene block copolymer was performed before and after UV illumination, and the changes in transmission spectra of the polymer film were measured. The resolution of electron beam lithography on polymer films was studied. In the polyisoprene block copolymer film the oxidation polymerization was ascertain at UV-illumination.

High-Density Electron-Beam Recording of Circumferentially Aligned Dots by Using Substrates with Low Atomic Numbers

Reducing proximity effects is a key factor for achieving a higher resolution in electron-beam lithography and realizing the mastering of patterned media. The effect of substrate materials on backscattering electrons was investigated by simulation and experiment, and resolution enhancement was demonstrated. In Monte Carlo simulations with 100 keV incident electrons, the intensity of backscattering electrons decreased with decreasing atomic number of substrates. On the other hand, both the density of substrates and the existence of 10 nm thin films had negligible effects on the intensity of backscattering electrons. The measured exposure distributions from line-scanned electron beams supported the results of simulations. The intensity of backscattering electrons was reduced by using a carbon substrate, and circumferentially aligned high-density patterns of 878 Gbit/in.2 were resolved.

Technology platform for the fabrication of titanium nanostructures

This paper presents two approaches for the fabrication of top-down titanium nanostructures. The first approach involves electron beam lithography followed by a tailored titanium plasma etching. The two main challenges of this process lie in the optimization of the negative tone Ma–N electroresist resolution and in the definition of a controlled titanium etching process for titanium patterns less than 20 nm thick and wide. The second proposed approach is a damascene process where the titanium nanostructures are buried in the oxide. Very shallow and narrow (20 nm × 20 nm) trenches are first patterned in the oxide and nanostructures are obtained by planarization of an evaporated titanium film. The dimensions of the structures are defined by the electron beam lithography resolution and the etching recipe. The third dimension is given by the titanium or any other metal thickness and can be controlled down to few nanometers thanks to the planarization step.

Manipulation and in situ transmission electron microscope characterization of sub-100 nm nanostructures using a microfabricated nanogripper

We present here a polysilicon electrothermal microfabricated nanogripper capable of manipulating nanowires and nanotubes in the sub-100 nm range. The nanogripper was fabricated with a mix and match microfabrication process, combining high throughput of photolithography with 10 nm resolution of electron beam lithography. Vertically grown III–V nanowires with a diameter of 70 nm were picked up using the nanogripper, allowing direct transfer of the nanogripper-nanowire ensemble into a transmission electron microscope (TEM) for structural characterization. By refining the end-effectors with focused ion beam milling and subsequently coating these with Au, the nanogripper could lift up laterally aligned single-walled carbon nanotubes from a 1 µm wide trench, while immediately making good electrical contact. One such carbon nanotube was structurally and electrically characterized real-time in TEM, showing a breakdown current density of approximately 0.5 × 1012Am−2. The nanogripper is the smallest microfabricated gripper to date and is the first tool showing repeatable, 3D nanomanipulation of sub-100 nm structures.

Influence of the development process on ultimate resolution electron beam lithography, using ultrathin hydrogen silsesquioxane resist layers

The influence of the development process on the ultimate resolution of electron beam lithography using ultrathin HSQ layers was studied. Different developers, of different types and strengths, were used to develop lines exposed at a variety of doses. Optimum exposures could be found for lines of widths between 7 and 12nm, at a pitch of 20nm. Lines smaller than 5nm could not be fabricated using any of these developers. Changing the development time had no influence on this result. Width versus dose curves are presented for all three developers and a 60s development time. A scaling is presented to enable an easy calculation of the exposure latitude for each linewidth. Using 100keV electron beam lithography, we achieved 6nm isolated features in a 10nm thick HSQ layer on a silicon substrate. We also showed that dense structures (5nm wide at a pitch of 20nm) could be obtained using a 1:5 developer solution of Microposit 351:H2O.

Characterization of hydrogen silsesquioxane as a Cl[sub 2]∕BCl[sub 3] inductively coupled plasma etch mask for air-clad InP-based quantum well waveguide fabrication

Air-clad InGaAsP∕InGaAs quantum well waveguides have been fabricated by inductively coupled plasma (ICP) etching using hydrogen silsesquioxane (HSQ) as an etch mask. First, HSQ has been studied for its contrast and resolution in electron beam lithography by varying e-beam exposure conditions and developer concentrations. Second, its etch resistance has been investigated in a chlorine-based ICP along with UVN30, a commercially available negative tone e-beam resist. Then, the optimum conditions for the exposure and development of HSQ for a chlorine-based ICP etching of InP-based material have been explored in terms of etch resistance. InGaAsP∕InGaAs quantum well material was patterned with HSQ by electron beam lithography. The waveguide was formed by the Cl2∕BCl3 ICP etching of InGaAsP∕InGaAs quantum well material and subsequent HCl-based wet etching of InAlAs sacrificial layer. The optical properties of the released waveguides were investigated and the initial optical measurements show low waveguide loss.

Matching the Resolution of Electron Beam Lithography by Scanning Near-Field Photolithography

Molecular features with widths of only 20 nm have been fabricated in self-assembled monolayers of alkanethiols on gold using a new lithographic tool, scanning near-field photolithography, based upon the use of a near-field scanning optical microscope (NSOM) coupled to a UV laser. Quite unexpectedly it has proved possible to routinely fabricate structures significantly smaller than the aperture in the NSOM probe. This exceptional performance is strongly correlated with the morphology of the gold film. In particular, the best results are achieved on films with comparatively small grain sizes. In contrast, the use of atomically flat, epitaxially deposited gold films leads to a minimum feature size comparable to the aperture diameter (ca 50 nm). It is concluded that nonradiative interactions (possibly the excitation of surface plasmons) between the gold substrate and the fiber lead to a pronounced focusing of the electric field beneath the aperture.

Negative-tone chemically-amplified resist development for high resolution hybrid lithography

Experimental negative-tone chemically-amplified resist formulations have been designed to reach sub-30 nm resolution in electron-beam lithography. With these formulations based on Sumitomo NEB-33 commercial platform resist, we achieved 20 nm isolated line resolution thanks to process optimization. Optical 248 nm exposures, were performed as well, leading to patterns with high side-walls roughness.

Resist Requirements and Limitations for Nanoscale Electron-Beam Patterning

ABSTRACTElectron beam lithography still represents the most effective way to pattern materials at the nanoscale, especially in the case of structures, which are not indefinitely repeating a simple motif. The success of e-beam lithography depends on the availability of suitable resists. There is a substantial variety of resist materials, from PMMA to calixarenes, to choose from to achieve high resolution in electron-beam lithography. However, these materials suffer from the limitation of poor sensitivity and poor contrast.In both direct-write and projection e-beam systems the maximum beam current for a given resolution is limited by space-charge effects. In order to make the most efficient use of the available current, the resist must be as sensitive as possible. This leads, naturally, to the use of chemically amplified (CA) systems. Unfortunately, in the quest for ever smaller feature sizes and higher throughputs, even chemically amplified materials are limited: ultimately, sensitivity and resolution are not independent. Current resists already operate in the regime of < 1 electron/nm2. In this situation detailed models are the only way to understand material performance and limits.Resist requirements, including sensitivity, etch selectivity, environmental stability, outgassing, and line-edge roughness as they pertain to, high-voltage (100 kV) direct write and projection electron-beam exposure systems are described. Experimental results obtained on CA resists in the SCALPEL® exposure system are presented and the fundamental sensitivity limits of CA and conventional materials in terms of shot-noise and resolution limits in terms of electron-beam solid interactions are discussed.

Fabrication technique for nanometer-scale InAs quantum devices: Observation of quantum interference in Aharonov–Bohm rings and Coulomb blockade in quantum dots

We report an approach to the lateral confinement of electrons in InAs/AlSb single quantum wells. Using electron-beam lithography and reactive ion etching, we have fabricated conducting wires, rings, and dots with lateral dimensions ⩾50 nm. Characterization on narrow wires and rings indicates that the electron transport is in the quasiballistic regime at 4.2 K. The current–voltage characteristics of 70-nm-diam dots in single-electron transistor structures show the Coulomb gap and the Coulomb staircase features. These artificially patterned devices have an ultimate lateral dimension of a few nanometers, limited by the resolution of electron-beam lithography.

Resolution limiting mechanism in electron beamlithography

The actual mechanism limiting resolution in electron beam lithography is studied experimentally. A higher resolution can be obtained through a smoother pattern edge, which is derived from a steeper beam profile slope at the threshold level. Reducing the beam blur improves the resolution limit because it increases the slope and smooths the pattern edge.

Bilayer resist used in e-beam lithography for deep narrow structures

A bilayer reist system developed by Olin Microelectronic Materials, consisting of a UV curable polystyrene bottom coat and a silicon loaded UV sensitive positive tone top coat ('bamboo resist'), has been investigated for its sensitivity and resolution in electron beam lithography, and its etch selectivity during subsequent use in Reactive Ion Etching of silicon. The ultimate resolution obtainable with the top coat appears to be around 80 nm, with a Dose-to-Clear of 28 @mC/cm^2 and a contrast value above 25. The mechanical and chemical stability of the bottom coat and its adhesion on silicon are excellent, allowing the fabrication of 30 nm thick free standing resist 'fences'. The best resolution obtained here in e-beam lithography, indicates that the resolution of this system will most probably not be limited by the resist performance down to at least 80 nm in optical lithography as well.

Silicon single-electron devices

Silicon single-electron transistors (SETs) were fabricated by using very flat silicon-on-insulator (SOI) substrates, high resolution electron beam (EB) lithography and etching techniques. In addition, we developed a special fabrication method called pattern-dependent oxidation with which a one-dimensional Si wire can be converted into a small Si island with a tunnelling barrier at each end. Since the Si island is around 10 nm, we could observe the conductance oscillation in the SET even at room temperature. The controllability and reproducibility of this method were confirmed through analysing the effects of size on electrical characteristics. We were also able to observe single-electron memory effects by using these novel techniques.

Electron beam lithography—Resolution limits

Electron beam lithography is generally accepted to have the highest practical resolution capability. In this paper the state of the art in terms of resolution is reviewed. This covers conventional resists such as PMMA, contamination resist, inorganic resists and damage processes. Some insights into the resolution limiting factors are given although the precise limitations still remain unclear. Consideration is also given as to the best measure of resolution and it is suggested that the minimum line spacing is a more appropriate and less subjective measure than minimum achievable feature size.