High-energy Electron Beam Lithography Research Articles

In Situ Study of the Impact of Aberration-Corrected Electron-Beam Lithography on the Electronic Transport of Suspended Graphene Devices.

The implementation of aberration-corrected electron beam lithography (AC-EBL) in a 200 keV scanning transmission electron microscope (STEM) is a novel technique that could be used for the fabrication of quantum devices based on 2D atomic crystals with single nanometer critical dimensions, allowing to observe more robust quantum effects. In this work we study electron beam sculpturing of nanostructures on suspended graphene field effect transistors using AC-EBL, focusing on the in situ characterization of the impact of electron beam exposure on device electronic transport quality. When AC-EBL is performed on a graphene channel (local exposure) or on the outside vicinity of a graphene channel (non-local exposure), the charge transport characteristics of graphene can be significantly affected due to charge doping and scattering. While the detrimental effect of non-local exposure can be largely removed by vigorous annealing, local-exposure induced damage is irreversible and cannot be fixed by annealing. We discuss the possible causes of the observed exposure effects. Our results provide guidance to the future development of high-energy electron beam lithography for nanomaterial device fabrication.

Fabrication of nanostructured transmissive optical devices on ITO-glass with UV1116 photoresist using high-energy electron beam lithography

High-energy electron beam lithography for patterning nanostructures on insulating substrates can be challenging. For high resolution, conventional resists require large exposure doses and for reasonable throughput, using typical beam currents leads to charge dissipation problems. Here, we use UV1116 photoresist (Dow Chemical Company), designed for photolithographic technologies, with a relatively low area dose at a standard operating current (80 kV, 40–50 μC cm−2, 1 nAs−1) to pattern over large areas on commercially coated ITO-glass cover slips. The minimum linewidth fabricated was ∼33 nm with 80 nm spacing; for isolated structures, ∼45 nm structural width with 50 nm separation. Due to the low beam dose, and nA current, throughput is high. This work highlights the use of UV1116 photoresist as an alternative to conventional e-beam resists on insulating substrates. To evaluate suitability, we fabricate a range of transmissive optical devices, that could find application for customized wire-grid polarisers and spectral filters for imaging, which operate based on the excitation of surface plasmon polaritons in nanosized geometries, with arrays encompassing areas ∼0.25 cm2.

A method to restrain the charging effect on an insulating substrate in high energy electron beam lithography

Pattern distortions caused by the charging effect should be reduced while using the electron beam lithography process on an insulating substrate. We have developed a novel process by using the SX AR-PC 5000/90.1 solution as a spin-coated conductive layer, to help to fabricate nanoscale patterns of poly-methyl-methacrylate polymer resist on glass for phased array device application. This method can restrain the influence of the charging effect on the insulating substrate effectively. Experimental results show that the novel process can solve the problems of the distortion of resist patterns and electron beam main field stitching error, thus ensuring the accuracy of the stitching and overlay of the electron beam lithography system. The main characteristic of the novel process is that it is compatible to the multi-layer semiconductor process inside a clean room, and is a green process, quite simple, fast, and low cost. It can also provide a broad scope in the device development on insulating the substrate, such as high density biochips, flexible electronics and liquid crystal display screens.

Influence of secondary electrons in high-energy electron beam lithography

The effect of secondary electrons in electron beam lithography is investigated based on a technique called point exposure distribution measurements. This technique involves printing a series of dots at different exposure doses using a high contrast electron beam resist. The experimental results indicate that the secondary electrons are the most likely cause for exposure events. The secondary electrons generated due to forward scattering also limit the highest achievable resolution. An analytical model is developed here based on the physics governing secondary electron generation and transport. The model is able to predict the dependence of dose on the observed diameter to within a reasonable accuracy. The experimental results were verified with the model at both 100 and 50 keV.

New method of optimizing writing parameters in electron beam lithography systems for throughput improvement considering patterning fidelity constraints

Low-energy electron beam lithography is one of the promising next-generation lithography technology solutions for the 21-nm half-pitch node and beyond because of fewer proximity effects, higher resist sensitivity, and less substrate damage compared with high-energy electron beam lithography. To achieve high-throughput manufacturing, low-energy electron beam lithography systems with writing parameters of larger beam size, larger grid size, and lower dosage are preferred. However, electron shot noise can significantly increase critical dimension deviation and line edge roughness. Its influence on patterning prediction accuracy becomes nonnegligible. To effectively maximize throughput while meeting patterning fidelity requirements according to the International Technology Roadmap for Semiconductors, a new method is proposed in this work that utilizes a new patterning prediction algorithm to rigorously characterize the patterning variability caused by the shot noise and a mathematical optimization algorithm to determine optimal writing parameters. The new patterning prediction algorithm can achieve a proper trade-off between computational effort and patterning prediction accuracy. Effectiveness of the new method is demonstrated on a static random-access memory circuit. The corresponding electrical performance is analyzed by using a gate-slicing technique and publicly available transistor models. Numerical results show that a significant improvement in the static noise margin can be achieved.

High-Energy Electron Beam Lithography of Octadecylphosphonic Acid Monolayers on Aluminum

Monolayers of octadecylphosphonic acid were self-assembled on silicon substrates sputter coated with aluminum. Patterning of the self-assembled monolayer was achieved by high-energy electron (50 kV) illumination using an electron beam lithography tool. The change in chemical composition of the exposed monolayer was investigated by time-of-flight secondary ion mass spectrometry over an area of 100 x 100 microm2. The electron dose required to fully expose the SAM was found to be about 6 mC/cm2. Gratings were exposed with line widths from 10 microm to 100 nm. The resulting patterns were imaged using friction force microscopy. It was found that the minimum line width is limited to ca. 100 nm by the patterning resolution. The pattern resolution achieved, ca. 40 nm, is equal to the grain size of the sputter-coated aluminum layer, and the possibility that the grain size limits the pattern resolution is discussed.

Influence of sub-100 nm scattering on high-energy electron beam lithography

Electron beam lithography tools have evolved in the direction of higher beam energy in order to achieve high-resolution, fine feature definition. As the beam energy is increased, the “forward” scattering is reduced and the “backscatter” range is increased. Over the years, tools became available data 20 then 50 kV, and now 100 kV operation is common. Operation at higher voltages has several advantages, such as better resolution and process latitude due to reduced forward scattering, and a few disadvantages such as higher dose requirements, substrate heating, and lower contrast for backscatter electron alignment and calibration signals (due to reduced primary electron backscattering generation in thin film). The backscatter range for 100 kV on silicon is about 27 μm compared to 8 μm at 50 kV resulting in different strategies for efficient proximity correction. However, even at 100 kV, scattering in an intermediate range is observed and must be taken into account in order to achieve good linewidth control at the highest resolution. Measurements of the scattering range for both 50 and 100 kV have been made using the point exposure distribution measurement technique [S. A. Rishton and D. P. Kern, J. Vac. Sci. Technol. B 5, 135 (1987)]. For comparison, measurements taken on the same wafer at different voltages show that 50 and 100 kV scattering range functions overlap, after normalizing for the different resist sensitivity, at length scales below 0.5 μm, suggesting a common mechanism, which is independent of the initial electron energy. For thin resists, this suggests that the significant resolution difference between 50 and 100 kV lithography is limited to the “forward” scattering effect as the incident electrons traverse the resist. Extrapolating the scattering function to the approximate beam diameter of 10 nm allows an impulse response function to be numerically determined. The convolution of this function gives reasonably good agreement with dose versus linewidth measurements.

Surface Silylation: Chemistry and Process Issues.

Three different silylation chemistries and processes are presented. The highly sensitive epoxy resist (EPR) can be silylated with chlorosilanes while the hydroxyl functionalized SAL601 and AZPN114 are silylated with the standard silyl amine chemistry (positive tone, single layer, top surface imaging (TSI), liquid or gas phase silylation processes). The process developed for epoxy functionalized resists is applied effectively to 193 nm and high energy (50KeV) electron-beam lithography. Process issues are discussed. In addition, a novel bilayer silylation process for an experimental 157nm chemically amplified positive (CAP) resist is presented in which the silylation reaction takes place after the wet development following the standard silyl amine chemistry. Alternatively, the acrylate moieties can be silylated with diamino siloxanes in alcoholic solutions.

Precision of micromilled x-ray masks and exposures

Micromechanical milling has been shown to be a rapid and direct method for the fabrication of structures with the geometry and size suitable for use as x-ray mask absorbers. While the micromilling process can not duplicate the size and resolution of absorber patterns created by high energy electron beam or optical lithography methods, micromilling can repeatedly create absorber line widths down to 10 micrometers, or less, with a one-sigma tolerance of 0.5 micrometers. A method for easily characterizing milling tool run out has been adapted so tool change out can be more routine. The milling process leaves some absorber burrs and the absorber is apparently tapered at the machined wall which introduce process biases, both of which add to exposure degradation. Nevertheless, based on work to date, it appears both of these effects can be reduced to acceptable limits.

100 kV electron beam lithography using a Schottky field emission source

We have investigated the usage of high energy (100 kV) electron beam lithography for various applications. Exposures with high energy electrons are less susceptible to resist thickness variations. Additional exposures from adjacent pixels and shapes can be corrected relatively easily compared to medium energy exposures (20–50 kV). The 100 kV performance of the Leica Cambridge EBPG5-FE system is described. A beam stability of better than 10 nm and a drift of less than 20 nm/h are combined with a Schottky field emission source for high current density. Spot sizes of 7–165 nm with corresponding currents between 0.7 and 120 nA can be selected to enable both very high resolution and accurate mask making.

Fabrication of quantum semiconductor structures by high energy electron beam lithography of an inorganic epitaxial resist on GaAs

Epitaxially grown calcium strontium fluoride on gallium arsenide has been patterned by high energy electron beam lithography and wet development. Fabrication of lines and dots of 8nm size is reported. Electron diffraction experiments indicate that fluorine is removed under electron bombardment with the formation of a calcium and strontium oxide. The end product of the radiolysis can be selectively etched in an acetic acid solution. A dose requirement of around 2×10 4C/cm 2 was necessary to form these features but a large dose latitude was observed. Possible applications of this epitaxial inorganic resist for the fabrication of quantum semiconductor structures are discussed.

Dry etching studies of YBa2Cu3O7-δ nm structures at cryogenic temperatures

Abstract We report a systematic study about the influence of substrate temperature during dry etching on the superconducting properties in YBa2Cu3O7-δ nanobridge structures. The substrate temperature has been varied between 300 K and 20 K. High energy electron-beam lithography has been used to fabricate submicron bridges between large area contact pads. The linewidth has been varied from 1.5 μm to 200 nm whereas the length has been kept constant at 10 μm. It is shown that LN2 cooling of the substrate is an efficient method to minimize ion beam induced degradation in YBa2Cu3O7-δ nanostructures. Compared to 300 K substrate temperature, an increase in the critical current density jc at 77 K by an order of magnitude is reported for 400 nm wide structures. As a result of further reduced substrate temperature (20 K) during the dry etching process, 200 nm wide YBa2Cu3O7-δ structures have been fabricated with 64.5 K zero resistance temperature.

The proximity effect in electron beam nanolithography

We have investigated the mechanisms which governs the increase in the width of structures produced by high energy electron beam lithography taking into account the effects of secondary electrons arising from exposure of neighbouring structures.