Volume 28, Issue 6, November 2010
Index of content:
- PAPERS FROM THE 54th INTERNATIONAL CONFERENCE ON ELECTRON, ION, AND PHOTON BEAM TECHNOLOGY AND NANOFABRICATION
- Directed Assembly
28(2010); http://dx.doi.org/10.1116/1.3517643View Description Hide Description
The authors have successfully employed the charged particle nanopatterning (CHARPAN) technology for nanostructuring of a metal mold insert for a conventional injection molding machine. High-precision diamond-milled Ni–Cu mold inserts have been nanopatterned with 10 keV argon ion multibeam milling with feature sizes as small as 50 nm. A variety of structures such as circles, hexagons, and lines in different dimensions, with positive and negative shapes, have been fabricated in the metal mold. These structures have been successfully replicated in polymethylpentene samples by injection molding. To the authors’ best knowledge, the CHARPAN technology is one of the very few technologies that allow for resistless nanostructuring a field size of into a metal mold in a single shot. This is of high importance for the practical injection moldingfabrication of nanostructured polymer devices such as optical biosensors.
Directed assembly of solution processed single-walled carbon nanotubes via dielectrophoresis: From aligned array to individual nanotube devices28(2010); http://dx.doi.org/10.1116/1.3501347View Description Hide Description
The authors demonstrate directed assembly of high quality solution processed single-walled carbon nanotube(SWNT)devices via ac dielectrophoresis using commercially available SWNTsolutions. By controlling the shape of the electrodes, concentration of the solution, and assembly time, the authors are able to control the assembly of SWNTs from dense arrays down to individual SWNTdevices. Electronic transport studies of individual SWNTdevices show field effect mobilities of up to for semiconducting SWNTs and saturation currents of up to for metallic SWNTs. The field effect mobilities are more than an order of magnitude improvement over previous solution processed individual SWNTdevices and close to the theoretical limit. Field effect transistors(FET) fabricated from aligned two-dimensional arrays of SWNT show field effect mobility as high as , which is three orders of magnitude higher than the solution processed organic FETdevices. This study shows promise for commercially available SWNTsolution for the parallel fabrication of high quality nanoelectronic devices.
Mechanism and dynamics of block copolymer directed assembly with density multiplication on chemically patterned surfaces28(2010); http://dx.doi.org/10.1116/1.3518918View Description Hide Description
In this work, we used scanning electron microscopy(SEM),in situ coherent small angle x-ray scattering (SAXS), and Monte Carlo molecular simulation to gain insights into the dynamics of block copolymer directed assembly with density multiplication on chemically patternedsurfaces. During directed assembly, it was observed with SEM that poly(styrene-block-methyl methacrylate) initially formed discrete polystyrene domains that lacked long-range order at the free surface. After further annealing, the polystyrene domains gradually coalesced into linear domains that were not registered fully with the underlying chemical pattern. The linear domains could be trapped in metastable morphologies. Finally, the linear polystyrene domains formed perpendicular lamellae in full registration with the underlying chemical pattern. It was revealed with SAXS that scattering peaks characteristic of the period of the chemical pattern appeared and disappeared at the early stages of assembly. Finally, the morphological evolution of directed assembly of block copolymer on chemically patternedsurfaces was modeled by molecular simulations.
Nanostructure fabrication by self-assembly of block copolymers on three-dimensional diamondlike carbon structures28(2010); http://dx.doi.org/10.1116/1.3518462View Description Hide Description
The block copolymerself-assembly has the potential to form nanoscale periodic patterns on three-dimensional (3D) nanostructures and nanomaterials. However, block copolymerself-assembly processes have only been examined on two-dimensional flat plane substrates. Forming copolymer nanostructures onto 3D structures would extend the applicability of these nanoscale patterns to 3D wiring for electric circuits and 3D nanomechanical devices. Herein, we investigate the formation process of nanopatterns of block copolymers onto 3D structures. A “dip-coating process” enables the block copolymer to self-assemble onto 3D structures. In addition, treatment with materials possessing a large surface energy effectively forms regular and dense nanoscale patterns onto a 3D diamondlike carbon structure. Moreover, self-assembledpatterns are formed even if the angle of the slope of the 3D structure is changed.
Shape control and density multiplication of cylinder-forming ternary block copolymer-homopolymer blend thin films on chemical patterns28(2010); http://dx.doi.org/10.1116/1.3518910View Description Hide Description
The effect of the chemical pattern spot size, the spacing on the size, and the shape of the cylindrical domains in thin films of a ternary block copolymer/homopolymer/homopolymer blend was investigated over a range of homopolymer volume fractions. Cylinder-forming ternary blends were composed of polystyrene-block-poly(methyl methacrylate) , and the corresponding PS and PMMA homopolymers were directed to assemble on chemical patterns that had density multiplication ratios ranging from 1:1 to 4:1. By increasing the homopolymer fraction in the blends, the dimensions of the domains were expanded. When the size of the spots on the chemical pattern was not matched with the size of the domain of the blend in the bulk, the dimensions of the domains at the free surface of the assembled films differed from those at the interface with the chemical pattern.
28(2010); http://dx.doi.org/10.1116/1.3501348View Description Hide Description
An integration scheme of block copolymer directed assembly with 193 nm immersion lithography is presented. It is experimentally shown that a thin silicon nitride film can be used as an antireflective coating (ARC). With such an ARC, directed assembly of a block copolymer (BCP) to triple the feature density of a chemical pattern was demonstrated. A high quality of assembly was obtained over a large area, and pattern transfer feasibility was illustrated. The integration of feature density multiplication via directed assembly of a BCP with 193 nm immersion lithography provided a pattern quality that was comparable with existing double patterning techniques, suggesting that the process could be a promising candidate for extending the use of current 193 immersion lithography tools to higher pattern densities.
- Electron Beams
High-current electron optical design for reflective electron beam lithography direct write lithography28(2010); http://dx.doi.org/10.1116/1.3505130View Description Hide Description
The authors present the latest design and results for the second generation column used in the reflective electron beamlithography program. The previous magnetic prism based concept to separate the illumination and projection beams has been replaced with a Wien filter that allows the column to be shrunk in size by a factor of 3, resulting in reduced Coulomb blur and energy spread. Experimental data from the column are presented and compared to simulation. The authors also discuss design considerations for this and future columns, including beam voltage, numerical aperture selection, and cathode optimization.
Reflective electron beam lithography: A maskless ebeam direct write lithography approach using the reflective electron beam lithography concept28(2010); http://dx.doi.org/10.1116/1.3511436View Description Hide Description
Reflective electron beam litography (REBL) utilizes several novel technologies to generate and expose lithographic patterns at throughputs that could make ebeam maskless lithography feasible for high volume manufacturing. The REBL program was described in a previous article [P. Petric et al., J. Vac. Sci. Technol. B27, 161 (2009)] 2 years ago. This article will review the system architecture and the progress of REBL in the past 2 years. The main technologies making REBL unique are the reflective electron optics, the rotary stage, and the dynamic pattern generator (DPG). Changes in how these concepts have been implemented in a new design will be discussed. The main disadvantage of today’s electron beam direct write is low throughput; it takes many tens of hours to expose a 300 mm wafer today using ebeam lithography. The projected system throughput performance with the integrated technology of the reflective optics, DPG, and a multiple wafer rotary stage will be shown incorporating the performance data for the new column design.
28(2010); http://dx.doi.org/10.1116/1.3517664View Description Hide Description
A multielectron beam tool from MAPPER lithography was installed in LETI premises in July 2009. It is based on low voltage lithography. In order to prepare acceptance tests, a preliminary study was carried out with a Leica VB6 HR at 5 kV in order to define 5 kV suitable resist processes. Results obtained at higher voltages are compared, since this tool has the capability to accelerate electrons up to 50 kV. The dependence of the deposition of backscatteredenergy on voltage is also evaluated. The 5 kV results are compared with those obtained on the MAPPER tool. Its spot size is measured, while a 32 nm half pitch resolution is reached.
Excitation and imaging of resonant optical modes of Au triangular nanoantennas using cathodoluminescence spectroscopy28(2010); http://dx.doi.org/10.1116/1.3504566View Description Hide Description
Cathodoluminescence(CL) imaging spectroscopy is an important technique to understand the resonant behavior of optical nanoantennas. The authors report high-resolution CL spectroscopy of triangular gold nanoantennas designed with near-vacuum effective index and very small metal-substrate interface. This design helped in addressing issues related to background luminescence and shifting of dipole modes beyond visible spectrum. Spatial and spectral investigations of various plasmonic modes are reported. Out-of-plane dipole modes excited with a vertically illuminated electron beam showed high-contrast tip illumination in panchromatic imaging. By tilting the nanostructures during fabrication, in-plane dipole modes of antennas were excited. Finite-difference time-domain simulations for electron and optical excitations of different modes showed excellent agreement with experimental results. Their approach of efficiently exciting antenna modes by using low index substrates is confirmed both with experiments and numerical simulations. This should provide further insights into a better understanding of optical antennas for various applications.
28(2010); http://dx.doi.org/10.1116/1.3501362View Description Hide Description
The origin of the frequently observed long and short term current drifts after a change in the extractor voltage for the ZrO/W(100) Schottkycathode has been investigated. It has been found that a reversible, field dependent change in the equilibrium work function and shape of the cathode end form occurs at the typical operating temperature of 1800 K. The shape change results in three distinct geometric end form shapes. Although these end forms have been observed previously [L. W. Swanson and G. A. Schwind, in Handbook of Charged Particle Optics, 2nd ed., edited by J. Orloff (CRC, New York, 2008), Chap. 1, p. 1; S. Fujita, T. R. Wells, W. Ushio, H. Sato, and M. M. El-Gomati, J. Microsc.235, 215 (2010); M. S. Bronsgeest and P. Kruit, J. Vac. Sci. Technol. B27, 2524 (2009)], the purpose of this study is to elucidate the operating conditions under which these changes occur and the associated changes in emission properties. Emitters in the radii range 200–900 nm were investigated over the typical current density operating range employed in most electron probe forming systems. A correlation between work function and electric field has been found, which can be explained by a reversible, field induced change in the ZrO equilibrium coverage in the high field region of the emitter, thereby causing a change in the work function. It is believed that these field induced changes in the equilibrium work function and emitter shape at 1800 K explain the short and long term drifts observed after a change in extractor voltage.
28(2010); http://dx.doi.org/10.1116/1.3517721View Description Hide Description
As scaling continues, the need for reliable sub-10-nm electron beamlithography is apparent. Throughput is a major drawback and complex test structure fabrication would be constrained by practical limits on writing time. A major challenge for sub-10-nm patterning with electron beamlithography is tool and process efficiency especially for high sensitivity resists. This article presents current work done at the College of Nanoscale Science and Engineering where the authors investigated three different commercially available resist systems, namely, SU-8, NEB-31, and HSQ, which have a range of sensitivity from close to the shot noise limit to slow material with high resolution. The authors present the results obtained from these resists with their respective critical dimension, line edge roughness (LER), and line width roughness (LWR) values that correlate with sensitivity and are consistent with the well known resolution, line edge roughness, sensitivity trade-off. Due to the inability of tools to deliver low doses at step sizes close to grid size limit of the tool, the ultimate resolution limit of SU-8 and NEB-31 with acceptable LER and LWR is yet to be determined.
Simulation of scanning electron microscope images taking into account local and global electromagnetic fields28(2010); http://dx.doi.org/10.1116/1.3518917View Description Hide Description
The authors report the development of a simulation tool with unique capabilities to comprehensively model a scanning electron microscope(SEM) signal. This includes electron scattering,charging, and detector settings, as well as modeling of the local and global electromagnetic fields and electron trajectories in these fields. Experimental and simulated results were compared for SEM imaging of carbon nanofibers embedded into bulk material in the presence of significant charging as well as for samples with applied potential on metal electrodes. The effect of the potentials applied to electrodes on the secondary emission was studied; the resulting SEM images were simulated. The image contrast depends strongly on the sign and the value of the potential. SEM imaging of nanofibers embedded into silicon dioxide resulted in the considerable change in the apparent dimensions of the fibers as well as tone reversal when the beam voltage was varied. The results of the simulations are in agreement with experimental results.
28(2010); http://dx.doi.org/10.1116/1.3497019View Description Hide Description
The authors report a numeric simulation tool that they developed for the modeling and analysis of electron beam lithography (EBL) of nanostructures employing a popular positive tone resist polymethylmethacrylate (PMMA). Modeling and process design for EBL fabrication of 5–50 nm PMMA structures on solid substrates is the target purpose of the simulator. The simulator is functional for exposure energies from 1 to 50 keV with arbitrary writing geometries. The authors employ a suite of kinetic models for the traveling of primary, secondary, and backscattered electrons in the resist, compute three-dimensional (3D) distributions of the yield of main-chain scission in PMMA, and convert these into the local volume fractions of fragments of various sizes. The kinetic process of development is described by the movement of the resist-developer interface with the rate derived from the mean-fieldtheory of polymerdiffusion. The EBL simulator allows the computation of detailed 3D distributions of the yield of main-chain scission in PMMA for various conditions of exposure, the corresponding volume fractions of small fragments, and the clearance profiles as functions of the development in time and temperature. This article describes the models employed to simulate the EBL exposure and development, reports examples of the computations, and presents comparisons of the predicted development profiles with experimental cross-sectional resist profiles in dense gratings.
Sub-10-nm half-pitch electron-beam lithography by using poly(methyl methacrylate) as a negative resist28(2010); http://dx.doi.org/10.1116/1.3501353View Description Hide Description
Developing high-resolution resists and processes for electron-beam lithography is of great importance for high-density magnetic storage, integrated circuits, and nanoelectronic and nanophotonic devices. Until now, hydrogen silsesquioxane (HSQ) and calixarene were the only two reported negative resists that could approach sub-10-nm half-pitch resolution for electron-beam lithography. Here, the authors report that 10-nm half-pitch dense nanostructures can also be readily fabricated using the well known poly(methyl methacrylate) (PMMA) resist operating in negative tone, even at exposure energies as low as 2 keV. In addition to scanning electron microscopymetrology,transmission electron microscopymetrology was done to confirm the high-resolution capability of negative-tone PMMA. This process was compared to HSQ with salty development and showed similar ultimate resolution, so it could be used as an alternative for applications incompatible with HSQ.
Analysis of surface electromagnetic wave resonant structures for potential application in an array of compact photoelectron sources28(2010); http://dx.doi.org/10.1116/1.3504590View Description Hide Description
Two different surface electromagnetic wave assisted light concentrators are analyzed that can, in principle, be integrated into an array with each concentrator delivering optical power to a photoelectron emitting metalnanodot. Such arrays could be used as a source for multiple electron-beam lithography and inspection systems. The optical power concentrators, composed of metalstructures embedded onto the surface of a quartz substrate, can, in principle, be tuned to any wavelength from UV to IR, capturing and enhancing broad area optical excitation and focusing it to subwavelength dimensions. Results of the finite difference time domain computer simulations of the structures are presented, and the optical power density enhancement factors at the focal spot are calculated to be over an order of magnitude for both structures. General methods for manipulating the spectral response are proposed and analyzed.
Inspection of open defects in a thin film transistor-liquid crystal display panel by using a low-energy electron microcolumn28(2010); http://dx.doi.org/10.1116/1.3502658View Description Hide Description
The demand on the electron beam (e-beam) for the inspection of semiconductor devices or display panel is rapidly increasing since e-beam cannot only monitor the small structures but also has the potential of detecting electrical troubles or repairing the defects. However, the merit of e-beam is limited because of the high cost, low throughput, and the possible damage due to the high e-beam energy. A microcolumn is a strong candidate to solve these limitations as its size is extremely miniaturized (both column diameter and height can be reduced down to a few millimeters) and the output e-beam energy is as low as 100–1000 eV. In this work, the authors tried to test the inspection of defects by applying a low voltage microcolumn to liquid crystal display panel. In order to demonstrate the authors’ inspection method, they extracted a 7’’ thin film transistor-liquid crystal display (TFT-LCD) panel from the production line just after completing the pixel structures and used this panel as a test sample. On the selected panel, the authors intentionally made some defects such as open data or gate lines by cutting some points using a laser beam. They operated their microcolumn with a beam energy of 300 eV and obtained the scanning images of the panel while operating the panel with specific operation conditions. The operation parameters for the test TFT-LCD panel such as the voltages applied to the gate lines, data lines, and storage capacitors was fixed at two specific sets of values. Then, the image obtained by the secondary electron reflects the information on the electrical state of the pixels as well as the geometrical ones. By combining the two sets of data, the authors could explain the correlation between the irregular behavior in the image contrast and the open defects and the detailed results will be discussed.
28(2010); http://dx.doi.org/10.1116/1.3502642View Description Hide Description
Emission theory predicts that high brightness cold field emitters can enhance imaging in the electron microscope. This (neglecting chromatic aberration) is because of the large (coherent) probe current available from a high brightness source and is based on theoretically determined values of reduced brightnesses up to . However, in their analysis, the authors find that statistical Coulomb interactions limit the reduced brightness of even atomically sharp cold field emitters to and regular tungsten cold field emitters to around . The authors also find that for tip radii in the range from 5 nm to , cold field emitters do not outperform larger Schottky (thermal field) emitters. Although this is applied to only one geometry, they expect that similar results will occur for most other cases due to a distinct difference in the behavior of different beam regimes.
- Emerging Technologies
Direct transformation of a resist pattern into a graphene field effect transistor through interfacial graphitization of liquid gallium28(2010); http://dx.doi.org/10.1116/1.3511511View Description Hide Description
The authors found that an extremely thin resist pattern on a silicon dioxide can be directly transformed into a graphene channel through interfacial graphitization of liquid gallium. These patterned graphenefield effect transistors show -type field effect conductance characteristics and a maximum conductance modulation of 100% against an applied gate voltage range from −50 to at room temperature, which is almost identical to the on/off ratio of 2. These conductance modulation ratios improved with decreasing the initial resist thickness below 2 nm; however, the absolute value of the channel conductance also deteriorated with decreasing the resist thickness, suggesting that electron scattering at the domain boundary dominates the channel conductance.