Direct Electron Beam Research Articles

Direct patterning of quantum dot nanostructures via electron beam lithography

Patterned quantum dot (QD) nanostructures were prepared by direct electron beam lithography on QD films. Time resolved photoluminescence measurements show that the optical properties of these QDs were retained after cross-linking.

Resist Free Patterning of Nonpreferential Buffer Layers for Block Copolymer Lithography

We report the design of a direct electron beam patternable buffer layer to spatially control the orientation of the microdomains in an overlaying polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) block copolymer (BCP) film. The buffer layer consists of a surface anchored low molecular weight PS-b-PMMA, with the PMMA segment anchored to the surface and a short PS block at the buffer layer/BCP interface. The block architecture of the buffer layer combines the essential features of "bottom up" and "top down" approaches as it functions as a nonpreferential layer to dictate perpendicular orientation of BCP domains from the substrate interface and as an e-beam resist to allow top-down lithographic process to spatially define the buffer layer on the substrate. The composition of the buffer layer can be tuned by changing the relative block lengths to create a nonpreferential surface which effectively induces perpendicular orientation of domains in an overlying BCP film. The grafted block copolymer can be locally shaved by e-beam lithography resulting in spatial control of domain orientation in the BCP film. The direct patterning approach reduces the number of steps involved in forming chemical patterns by conventional lithography.

Electron beam treatment of non-conducting materials by a fore-pump-pressure plasma-cathode electron beam source

In the irradiation of an insulated target by an electron beam produced by a plasma-cathode electron beam source operating in the fore-vacuum pressure range (5–15 Pa), the target potential is much lower than the electron beam energy, offering the possibility of direct electron treatment of insulating materials. It is found that in the electron beam irradiation of a non-conducting target in a moderately high pressure range, the electron charge on the target surface is neutralized mainly by ions from a volume discharge established between the negatively charged target surface and the grounded walls of the vacuum chamber. This allows the possibility of direct electron beam treatment (heating, melting, welding) of ceramics and other non-conducting and semiconductor materials.

Patterning of Polymer Brushes. A Direct Approach to Complex, Sub-Surface Structures

We report a unique method to directly fabricate complex polymer brush structures with nanometer scale features by means of electron beam lithography. Polymer brushes for direct patterning were grown from surface-anchored initiator sites using atom transfer radical polymerization. Selected monomers (poly(2-hydroxyethyl methacrylate) and poly(methyl methacrylate)) were used based on their ability to readily scission when exposed to radiation. Single step direct patterning of polymer brushes is attractive as this eliminates many process steps, reducing the possibility of contamination and possibly improving resolution. In addition, we report a method to form subsurface polymer brush channels with nanometer-scale features. With the chains tethered to a surface, a diblock copolymer brush with a negative tone upper layer (polystyrene) and a positive tone under layer (poly(methyl methacrylate)) or (poly(2-hydroxyethyl methacrylate) were patterned to create channels. In the work presented, the direct electron beam patterning behavior of the brushes was studied and fabrication of nanochannels was demonstrated. Imaging of the nanopatterned surfaces was carried out using atomic force microscopy and fluorescence microscopy.

Charge transport in nanocrystal wires created by direct electron beam writing

The authors report the fabrication and electrical characterisation of nanowires created via direct electron beam writing in films of passivated gold nanocrystals. Charge transport measurements yield room temperature resistances in the range 105–108 Ω. Variable temperature measurements yield two distinct sets of characteristics: activated conduction (high resistance) consistent with weakly coupled metal nanocrystals separated by alkyl–thiol tunnel barriers and quasi-localised behaviour (low resistance) consistent with stronger coupling between granular metallic islands in a carbonaceous matrix. The data indicate that electron beam writing is a promising method for local manipulation of inter-nanocrystal coupling in nanocrystal arrays.

Direct low-energy electron beam nanolithography

We describe here an alternative approach to direct low-energy electron beam nanolithography process with no conventional deposition of any resist or self-assembled monolayer. The method is based on direct formation of ultrathin dielectric layer on electron irradiated surface, without generation of structural defects. High-quality electron-induced patterns with lateral resolutions of about 10 nm are demonstrated on SiO 2 surface.

Effect of electron beam writing parameters for ferroelectric domain structuring LiNbO 3:Nd 3+

We have successfully applied the versatile technique of direct electron beam writing to produce column shaped 180° micro-domains on a solid state laser crystal: Nd 3+ doped LiNbO 3. The effect of the different parameters used during the electron lithography process has been analyzed on the morphology and on the optical properties of the inverted domains. Namely, we have studied the effect of the value of the accelerating voltage of the electrons and the effect of the electronic charge deposited onto the sample. By means of optical micrography and computer simulation we show that the accelerating voltage has a major effect on the diameter of inverted domains. On the other hand, the value of the electronic charge not only affects the domain size but it has also an additional and interesting effect on the optical spectroscopy of Nd 3+ ions. Using low temperature site selective optical spectroscopy we demonstrate the possibility of acting selectively on the crystal field of the optically active Nd 3+ centers in LiNbO 3 by means of the different electronic doses used to reverse the polarization of the matrix. The work constitutes an approach towards the control of the properties of reversal ferroelectric domains structures prepared by direct electron beam lithography and towards understanding the role of optically active defects on the switching behaviour in ferroelectric crystals.

Encoded binary diffractive element to form hyper-geometric laser beams

A binary diffractive optical element (1500 pixels × 1500 pixels, diameter 4.5 mm) is designed by partial encoding and fabricated by direct electron beam writing in SiO2. Two conjugate hyper-geometric laser modes are generated using the fabricated element. The root-mean-square transverse intensity deviation of the experimental from theoretical diffraction pattern, determined within a circle of radius 1 mm, is below 13%.

Selective rearrangement ofNd3+centers inLiNbO3under ferroelectric domain inversion by electron beam writing

Different values of the electronic charge provided by a direct electron beam writing system have been used to produce polarization inverted domain regions in the micrometer range on Nd{sup 3+} optically activated LiNbO{sub 3}. The effect of the electronic charge on the Nd{sup 3+} center structure has been studied by means of low-temperature luminescence from Nd{sup 3+} ions. The axial crystal field acting on the Nd{sup 3+} centers has been analyzed through the {sup 4}F{sub 3/2} energy-level splitting of the Nd{sup 3+} ions. From there we have determined the position of Nd{sup 3+} ions into the Li{sup +} octahedra for the different unequivalent centers in domains reversed with different electronic doses. The results show that the axial crystal field acting on the unequivalent Nd{sup 3+} centers can be selectively modified by means of the different doses applied to produce the inversion of the polarization. Moreover, a control of the discrete shifts suffered by the Nd{sup 3+} ions into the Li{sup +} octahedra after the inversion process can be carried out in the range 0-0.02 A by selecting the type of Nd center to be shifted by means of the different electronic charge. The behavior of each Nd{sup 3+} center aftermore » the polarization inversion under different doses can be discriminated and the different nature, as well as the polar character of the Nd{sup 3+} centers, is clearly manifested.« less

Surface potential and magnetic properties of La0.7Sr0.3MnO3 periodic arrays fabricated by direct electron beam writing

It is demonstrated that magnetic periodic arrays can be easily fabricated from direct writing water developable La0.7Sr0.3MnO3 electron beam resist. Two unique features of our approach are (1) the patterned La0.7Sr0.3MnO3 resist film can be developed using nontoxic and environmentally friendly pure water and (2) either positive or negative patterns can be fabricated depending on the dosage of electron beam. The mechanism of the dual function characteristic of the resist was studied using Kelvin probe microscope. The surface potential of patterned La0.7Sr0.3MnO3 resist increases with increasing electron beam dosage due to the changes of resist composition. The formations of periodic magnetic arrays were confirmed by the studies of scanning electron microscope and magnetic force microscope. The magnetization of La0.7Sr0.3MnO3 can be enhanced by postsintering the sample at 900 °C after electron beam irradiation. We have therefore provided a one-step, simple, and convenient alternative technique for the fabrication of nanoscale magnetic patterns, which form the building blocks for the study of physical properties in periodic magnetic arrays.

Gas-assisted focused electron beam and ion beam processing and fabrication

Beams of electrons and ions are now fairly routinely focused to dimensions in the nanometer range. Since the beams can be used to locally alter material at the point where they are incident on a surface, they represent direct nanofabrication tools. The authors will focus here on direct fabrication rather than lithography, which is indirect in that it uses the intermediary of resist. In the case of both ions and electrons, material addition or removal can be achieved using precursor gases. In addition ions can also alter material by sputtering (milling), by damage, or by implantation. Many material removal and deposition processes employing precursor gases have been developed for numerous practical applications, such as mask repair, circuit restructuring and repair, and sample sectioning. The authors will also discuss structures that are made for research purposes or for demonstration of the processing capabilities. In many cases the minimum dimensions at which these processes can be realized are considerably larger than the beam diameters. The atomic level mechanisms responsible for the precursor gas activation have not been studied in detail in many cases. The authors will review the state of the art and level of understanding of direct ion and electron beam fabrication and point out some of the unsolved problems.

Evolution of chemical states within the HfO 2/Si(1 0 0) interface upon annealing, prepared by direct electron beam evaporation

We demonstrate the preparation of the system HfO 2/Si(1 0 0) by direct evaporation of hafnium dioxide from a specially prepared electron beam evaporator. Investigating the system by means of photoelectron spectroscopy in the soft X-ray regime, we show the system’s interface being free of silicon dioxide and hafnium silicide after evaporation. Upon annealing the formation of SiO 2 at the interface is investigated by using high resolution photoemission spectra. Their separation into different sub-oxides at the interface is presented. The structural order at the crystalline/amorphous interface is investigated by X-ray photoelectron diffraction. The system’s degradation is observed at above 700 °C.

Polarization selective electro-optic polymer waveguide devices by direct electron beam writing

A novel technique for the fabrication of polarization selective electro-optic polymer waveguide devices with direct electron beam writing was described. Birefringence induced by the electric field poling in the electro-optic polymer film was erased in the electron beam exposed regions. The formed waveguides had stronger confinement for the light polarized along the poling direction. High fabrication resolution on the 100 nm scale or smaller could be achieved. Fabrication of polymer polarizer and polarization selective microring resonators with this technique was reported. The highest polarization extinction ratio was measured to be 21.4 dB.

Microring resonators fabricated by electron beam bleaching of chromophore doped polymers

Decomposition of chromophore molecules under direct electron beam irradiation reduces the refractive index of chromophore containing polymers. The induced refractive index contrast between the exposed and unexposed regions is high enough for waveguide bends of small radius and thus microring resonator devices. This electron beam bleaching of chromophore-containing polymers provides a fabrication approach for nonlinear polymer optical waveguide devices. Fabrication of high quality microring resonators with critical feature size on the order of 100nm was demonstrated with this technique in an electro-optic polymer that contains YL124 chromophores.

Direct electron beam patterning and molecular beam epitaxy growth of InAs: Site definition of quantum dots

An electron beam (EB) lithography technique to form patterns directly on a semiconductor crystal surface, without using a resist or an oxide film, combined with molecular beam epitaxy (MBE) growth is described for potential applications to site definition of quantum dots. Periodically arranged submicrometer protrusions on a GaAs wafer surface were observed by EB irradiation from a scanning electron microscope with moderately high vacuum. This protrusion has a caldera shape with a concave on the top. On the other hand, when the GaAs was irradiated with an EB, although not well focused, from reflection high energy electron diffraction gun in an ultrahigh vacuum annealing chamber of MBE, a pit with concave shape was obtained. Clear photoluminescence emission was observed from an InAs ultrathin layer overgrown on the pit area.

Direct electron beam writing of electro-optic polymer microring resonators

Electro-optic polymer waveguides in electron beam sensitive polymethyl methacrylate (PMMA) polymer matrix doped with organic nonlinear chromophores could be directly patterned by electron beam exposure with high resolution and smoothness. The polymer in the exposed regions was removed with standard electron beam resist developer and without damaging the chromophore containing polymer waveguides. Feature sizes on the order of 100 nm could be clearly resolved. High quality microring resonators made of YL124/PMMA electro-optic polymer were successfully fabricated with this technique. The measured resonance extinction ratios were more than 16 dB and quality factors were in the range of 10(3) approximately 10(4).

Design of hybrid sol–gel films for direct x-ray and electron beam nanopatterning

New epoxy based sol–gel organic inorganic materials, showing lithographic resist-likeproperties without the addition of any photocatalysts, are presented. To obtain a materialsensitive to radiation, specific sol–gel syntheses based on an organically modified alkoxidecontaining an epoxy ring, 3-glycidoxypropyltrimethoxysilane (GPTMS), have beendeveloped. The synthesis and the patternability of hybrid materials have been obtainedcontrolling the inorganic crosslinking degree and with an almost total absence of organicpolymerization. Two examples of directly patternable hybrid films, called GB and GGe,have been synthesized using acidic (GGe) and basic (GB) conditions and obtainingdifferent compositions.After electron beam lithography (EBL) or x-ray synchrotron radiation lithography (XRL)the polymerization of the organic component of the sol–gel film occurs, generating ahardening of the structure after post-exposure baking. The exposed polymerized materialbecomes insoluble, determining a negative resist-like behaviour of the film: the lithographicprocess of nanopatterning results from the dissolution of the unexposed areas in propersolvents (developers). Spatial resolution of the order of 200 nm is reported and a contrast of2.2 is achieved.The novelty of this work is that epoxy based materials, which have enhancedthermomechanical stability with respect to the more usual acrylic based resins, are directlynanopatterned for the first time by electron beam (EB) and/or x-ray beam radiationexposure without the aid of catalysts for polymerization. In contrast to common resiststhat are sacrificial layers of the fabrication process, direct patternable sol–gel hybridsconstitute the final material of the devices. In fact, an example of doping with a lightemitting dye is reported together with the achievement of directly patterned structures byEBL and XRL.

DNA Nanopositioning and Alignment by Electron-Beam-Induced Surface Chemical Patterning

Nanopositioning and alignment of arrays of DNA molecules on a surface by combination of high-resolution prepatterning and standard macroscopic deposition is presented. Direct electron beam exposure of a graphite substrate activated by amino groups neutralizes locally the surface charge, preventing the DNA adhesion during the consequent deposition. Because of the high resolution of the electron beam writing process, precise active patches can be created directly on the functionalized surface. Narrow (50 nm) stripe patterns produce both positioning and alignment acting as electrostatic traps for the DNA molecules. The approach is demonstrated using triple- and double-stranded DNA of medium length (350 nm). High yield of alignment and regular arrangement of the deposited molecules are achieved in a simple way within large areas.

Simple fabrication of UV nanoimprint templates using critical energy electron beam lithography

Ultraviolet nanoimprint lithography (UV-NIL) is a promising nanoscale patterning method, but the template fabrication processes are expensive and time consuming because a chrome layer is necessary both as a charge dissipation layer to minimize pattern distortion during electron beam patterning and as a physical mask during subtractive quartz etching. In this paper, the authors propose a simple UV-NIL template fabrication scheme based on the direct electron beam patterning of hydrogen silsesquioxane (HSQ) on quartz substrates using critical energy electron beam lithography (CE-EBL). By operating at the critical energy (E2) where the charge balance between incoming and outgoing electrons leaves the surface neutral, charge induced pattern distortion typically seen on quartz is practically eliminated. This template fabrication process eliminates conventional deposition and etching of charge dissipation layers. Quartz with sub-100-nm HSQ structures was used as a UV-NIL template, and SU-8 polymer structures were successfully replicated by a UV nanoimprint process. This simple template fabrication approach may lead to the development of new biological devices, nanoelectronics, and optoelectronics.

Nd 3 + ion shift under domain inversion by electron beam writing in LiNbO3

Ferroelectric domain inversion has been obtained in Nd3+ doped lithium niobate by means of direct electron beam writing. The local effects of the polarization inversion on the optical transitions of Nd3+ ions have been studied by low temperature high resolution site selective spectroscopy. Inverted regions present different axial crystal field acting over Nd3+ ions compared with noninverted (original) regions. The results can be interpreted in terms of slight shifts of Nd3+ ions along the ferroelectric c axis within the Li+ octahedrons, as a result of the lattice rearrangement after the domain inversion processes.