Scanning Electron Beam Lithography Research Articles

Memory efficient constrained optimization of scanning-beam lithography.

This article describes a memory efficient method for solving large-scale optimization problems that arise when planning scanning-beam lithography processes. These processes require the identification of an exposure pattern that minimizes the difference between a desired and predicted output image, subject to constraints. The number of free variables is equal to the number of pixels, which can be on the order of millions or billions in practical applications. The proposed method splits the problem domain into a number of smaller overlapping subdomains with constrained boundary conditions, which are then solved sequentially using a constrained gradient search method (L-BFGS-B). Computational time is reduced by exploiting natural sparsity in the problem and employing the fast Fourier transform for efficient gradient calculation. When it comes to the trade-off between memory usage and computational time we can make a different trade-off compared to previous methods, where the required memory is reduced by approximately the number of subdomains at the cost of more computations. In an example problem with 30 million variables, the proposed method reduces memory requirements by 67% but increases computation time by 27%. Variations of the proposed method are expected to find applications in the planning of processes such as scanning laser lithography, scanning electron beam lithography, and focused ion beam deposition, for example.

Comparison of technologies for nano device prototyping with a special focus on ion beams: A review

Nano device prototyping (NDP) is essential for realizing and assessing ideas as well as theories in the form of nano devices, before they can be made available in or as commercial products. In this review, application results patterned similarly to those in the semiconductor industry (for cell phone, computer processors, or memory) will be presented. For NDP, some requirements are different: thus, other technologies are employed. Currently, in NDP, for many applications direct write Gaussian vector scan electron beam lithography (EBL) is used to define the required features in organic resists on this scale. We will take a look at many application results carried out by EBL, self-organized 3D epitaxy, atomic probe microscopy (scanning tunneling microscope/atomic force microscope), and in more detail ion beam techniques. For ion beam techniques, there is a special focus on those based upon liquid metal (alloy) ion sources, as recent developments have significantly increased their applicability for NDP.

Patterning via optical saturable transitions--fabrication and characterization.

This protocol describes the fabrication and characterization of nanostructures using a novel nanolithographic technique called Patterning via Optical Saturable Transitions (POST). In this technique the chemical properties of organic photochromic molecules that undergo single-photon reactions are exploited, enabling rapid top-down nanopatterning over large areas at low light intensities, thereby, allowing for the circumvention of the far-field diffraction barrier.(4) Simple, cost-effective, high throughput and resolution alternatives to nanopatterning are being explored, such as, two-photon polymerization(5,6), beam pen lithography (BPL)(7), scanning electron beam lithography (SEBL), and focused ion beam (FIB) patterning. However, multi-photon approaches require high light intensities, which limit their potential for high throughput and offer low image contrast. Although, electron and ion beam lithographic processes offer increased resolution, the serial nature of the process is limited to slow writing speeds, which also prevents patterning of features over large areas. Beam-pen lithography is an approach towards parallel near-field optical lithography. However, the gap between the source of the beam and the surface of the photoresist needs to be controlled extremely precisely for good pattern uniformity and this is very challenging to accomplish for large arrays of beams. Patterning via Optical Saturable Transitions (POST) is an alternative optical nanopatterning technique for patterning sub-wavelength features(1-3). Since this technique uses single photons instead of electrons, it is extremely fast and does not require high light intensities(1-3), opening the door to massive parallelization.

Patterning via optical-saturable transformations: A review and simple simulation model

Most of the nanoscale fabrication in the semiconductor industry is based on patterning with scanning-electron beam lithography (SEBL). Although this approach is very versatile and has very high resolution, it is intrinsically a serial writing process, and therefore, relatively slow. Our group has been investigating alternative nano-fabrication techniques, adapted from ideas of saturating optical transitions such as those used in stimulated emission-depletion microscopy and related methods, and optical interference lithography. Linewidths and resolutions on the scale of a few tens of nanometers and below are highly desirable for various applications in nanotechnology. However, the spatial resolution of optical lithography is restricted by diffraction. In the past, we developed absorbance modulation to overcome this limit. This approach utilizes photochromic molecules that can be optically switched between two thermally stable states, one opaque and the other transparent. However, absorbance modulation is limited to surface (2-D) patterning. Here, we report on an alternative approach that exploits unique combinations of spectrally selective reversible and irreversible photochemical transitions to achieve deep subwavelength resolution with potential extension to 3-dimensions. This approach, which we refer to as patterning via optical-saturable transformations have the potential for massive parallelism, enabling the creation of nanostructures and devices at a speed far surpassing what is possible with SEBL. The aim of our research is to translate the success in circumventing Abbe's diffraction limit in optical microscopy to optical lithography.

Electron-beam-induced deposition of 3-nm-half-pitch patterns on bulk Si

This paper demonstrates electron-beam-induced deposition of few-nm-width dense features on bulk samples by using a scanning electron-beam lithography system. To optimize the resultant features, three steps were taken: (1) features were exposed in a repetitive sequence, so as to build up the deposited features gradually across the entire pattern, and thus avoid proximity effects; (2) an additional delay was added between exposures to permit diffusion of reactants into the exposed area; and (3) the exposures were phase-synchronized to the dominant noise source (the 50-Hz line voltage) to minimize the effect of noise. The reasons these steps led to significant improvements in patterning resolution are discussed.

Tuning of Graphene Properties via Controlled Exposure to Electron Beams

Controlled modification of graphene properties is essential for its proposed electronic applications. Here we describe a possibility of tuning electrical properties of graphene via electron beam irradiation. We show that by controlling the irradiation dose one can change the carrier mobility and increase the resistance at the minimum conduction point in the single layer graphene. The bilayer graphene is less susceptible to the electron beam irradiation. The modification of graphene properties via irradiation can be monitored and quantified by the changes in the disorder D peak in Raman spectrum of graphene. The obtained results may lead to a new method of defect engineering of graphene physical properties, and to the procedure of "writing" graphene circuits via e-beam irradiation. The results also have implications for fabrication of graphene nanodevices, which involve scanning electron microscopy and electron beam lithography.

Device Architecture and Precision Nanofabrication of Microring-Resonator Filter Banks for Integrated Photonic Systems

To achieve the maximum benefit of electronic-photonic integrated circuits wavelength-division multiplexing must be used. This requires the design and fabrication of a highly integratable photonic device, capable of performing multiplexing/demultiplexing operations with low loss and minimal crosstalk. A filter bank consisting of high-index-contrast microring-resonator filters, with accurately spaced resonant frequencies can meet these requirements. This paper describes the basic architecture of microring-resonator filter banks, and how to maximize performance while keeping fabrication challenges reasonable. The greatest challenge in fabricating such devices is achieving the dimensional precision, on the scale of tens of picometers, needed to attain accurately spaced resonant frequencies. To do this, a fabrication method based on varying the electron-beam dose during scanning-electron beam lithography is used. This approach is used to create a dual twenty-channel filter bank, comprised of second-order silicon-rich silicon nitride microring resonators. The average resonant frequency spacing is off from the target spacing by only 3 GHz, corresponding to a dimensional precision of 75 pm. This approach is also shown to be compatible with the fabrication process for silicon microring resonators. Furthermore, it is shown that any remaining resonant frequency errors can be corrected with postfabrication thermal tuning. Also, a method of using the contra-propagating mode of a microring-resonator filter is demonstrated, enabling a single filter bank to multiplex/demultiplex two signals at the same time.

Limiting factors in sub-10nm scanning-electron-beam lithography

Achieving the highest possible resolution using scanning-electron-beam lithography (SEBL) has become an increasingly urgent problem in recent years, as advances in various nanotechnology applications [F. S. Bates and G. H. Fredrickson, Annu. Rev. Phys. Chem. 41, 525 (1990); Black et al., IBM J. Res. Dev. 51, 605 (2007); Yang et al., J. Chem. Phys. 116, 5892 (2002)] have driven demand for feature sizes well into the sub-10nm domain, close to the resolution limit of the current generation of SEBL processes. In this work, the authors have used a combination of calculation, modeling, and experiment to investigate the relative effects of resist contrast, beam scattering, secondary electron generation, system spot size, and metrology limitations on SEBL process resolution. In the process of investigating all of these effects, they have also successfully yielded dense structures with a pitch of 12nm at voltages as low as 10keV.

Quantized patterning using nanoimprinted blanks

Quantum lithography (QL) is a revolutionary approach, increasing the throughput andlowering the cost of scanning electron beam lithography (EBL). But it has not beenpursued since its inception 17 years ago, due to the lack of a viable method formaking the blanks needed. Here we propose and demonstrate a new general viableapproach to QL blank fabrication, that is based on (a) nanoimprinting and (b) a newwafer-scale nanoimprint mold fabrication that uses not EBL but a unique combinationof interference lithography, self-perfection, multiple nanoimprinting, and othernovel nanopatterning. We fabricated QL blanks (a 2D Cr square tile array of200 nm pitch, 9 nm gap, and sub-10 nm corners, corresponding to a 50 nm node4 × photomask) and demonstrated that QL can greatly relax the requirements for the EBLtool, increase the throughput and reduce the cost of EBL by orders of magnitude, and isscalable to the 22 nm node.

Enhanced fluorescence and near-field intensity for Ag nanowire/nanocolumn arrays: evidence for the role of surface plasmon standing waves

We use scanning fluorescence microscopy and electron beam lithography to probe the mechanism of fluorescence enhancement by periodic arrays of silver nanostructures, determining the optimum size and spacing of both Ag nanowires and Ag nanocolumns for incident light of different wavelengths and polarizations. Finite difference time domain (FDTD) calculations show a systematic variation with spatial period and incident polarization of the local electric field above the surface of the arrays which correlate well with that of the measured fluorescence enhancement, but a lack of a simple proportionality indicates that the dependence of the radiative and nonradiative decay rates on array geometry must be included in models for this effect. The dependence of the enhancement on spatial period and polarization indicates the importance of surface plasmon standing waves in this effect.

Accurate frequency alignment in fabrication of high-order microring-resonator filters

Frequency mismatch in high-order microring-resonator filters is investigated. We demonstrate that this frequency mismatch is caused mainly by the intrafield distortion of scanning-electron-beam-lithography (SEBL) used in fabrication. The intrafield distortion of an SEBL system is measured, and a simple method is also proposed to correct this distortion. By applying this correction method, the average frequency mismatch in second-order microring-resonator filters was reduced from -8.6 GHz to 0.28 GHz.

Fabrication of miniaturized Schottky emitter by wire electrical discharge machining (WEDM)

The Schottky emitter is extensively used in scanning electron beam lithography machines because of its high brightness and stable current. For its use in parallel electron beam lithography to incre...

The Fabrication of 2-D Global Fiducial Grid with High Resolution for Spatial Phase Locked e-Beam Lithography

The spatial phase locked scanning electron beam lithography systems (SPLEBL) is a new lithography technique with a pattern placement precision of about 1 nm. The SPLEBL technique can solve the major problem of poor placement accuracy existed in the conventional scanning electron beam lithography for that it uses a Fourier technique to detect the beam position in real time during exposure. The fiducial grid plays a key role in SPLEBL. The two-dimensional global fiducial grid with a grid period of 250 nm placed on top of the e-beam resist used in SPLEBL with high contrast, high brightness, long-range spatial-phase coherence, large area and a pattern placement precision of about 1 nm is fabricated using optical interference lithography in this article. The detail fabrication process is described and the SEM images of the fabricated grid are also presented in this paper. Only one evaporation step and several spin-coating steps are required in the fabrication process, so it is simple and user friendly.

Improvements to the alignment process in a commercial vector scan electron beam lithography tool

This paper examines the desirable properties of marker patterns for use in correlation-based alignment systems and demonstrates alignment accuracies of better than 1nm. A framework for evaluating different classes of marker patterns has been developed and a figure of merit for marker patterns used in correlation-based alignment has been defined. We show that Penrose tilings have many desirable properties for correlation-based alignment. An alignment system based on correlation and using marker patterns derived from Penrose tilings has been developed and implemented on a commercial Vistec VB6 UHR EWF electron beam lithography tool. A new method of measuring alignment at the sub-nm level using overlaid gratings and a Fourier Transform based analysis scheme is introduced.

Fabrication of miniaturized Schottky emitter by wire electrical discharge machining (WEDM)

The Schottky emitter is extensively used in scanning electron beam lithography machines because of its high brightness and stable current. For its use in parallel electron beam lithography to increase the throughput, we are investigating the possibility of creating an array of miniaturized Schottky emitters. This paper discusses a novel method of fabricating a miniaturized Schottky emitter of 1mm diameter by wire-electro discharge machining (WEDM). Various WEDM process parameters were optimized for better reproducibility and surface finish of the fabricated emitter. The fabricated samples match well with the intended input design, with no visible stress or bending. Joule heating in vacuum shows good operational feasibility and temperature stability. After heating, the tip becomes as sharp as conventional Schottky emitters. The miniaturized emitter operates at a relatively low power.

Optimal temperature for development of poly(methylmethacrylate)

The authors have investigated a range of poly(methylmethacrylate) (PMMA) development temperatures as low as −70°C and characterized their effect on the resolution of PMMA as an electron resist. The results show that cooling, in addition to reducing the sensitivity of the commonly used positive-tone mode of PMMA, also increases the sensitivity of its less commonly used negative-tone mode. They have shown that the resolution-enhancing properties of cold development peak at approximately −15°C as a result of these competing sensitivity changes. At lower temperatures, the high doses required to expose the resist produce significant cross-linking of the polymer, altering its solubility properties and sharply degrading the contrast. If the correct development temperature is used, however, sub-10nm features are readily achievable in PMMA-based scanning electron-beam lithography.

Robust shadow-mask evaporation via lithographically controlled undercut

Suspended shadow-mask evaporation is a simple, robust technique for fabricating Josephson-junction structures using scanning electron-beam lithography. The basic process entails the fabrication of an undercut structure in a resist bilayer to form a suspended “bridge,” followed by two angle evaporations of superconducting material with a brief oxidation step in between. The result is two overlapping wires separated by a thin layer of oxide. Josephson junctions with sub-50-nm diameters are of particular interest in quantum computing research. Unfortunately, standard shadow-mask fabrication techniques are highly variable at linewidths below 100nm, due to the difficulty of simultaneously fabricating a narrow line and a large undercut region. While most previous processes used poly(methylmethacrylate) (PMMA) for the top (imaging)layer and either lower-molecular-weight PMMA or a PMMA/methacrylic acid copolymer for the bottom (support) layer, the authors’ process uses a PMMA/poly(methylglutarimide) (PMGI) bilayer. The advantage of using PMGI as the support layer is that it develops in aqueous base solutions, while PMMA is insensitive to aqueous solutions and only develops in certain organic solvents. This allows the two layers to be developed independently, ensuring that the imaging layer is not biased during the development of the support layer and allowing the process to achieve the full resolution of the PMMA imaging layer, which can be extremely high. Additionally, the extent of the undercut in the support layer can be precisely controlled by defining it lithographically, rather than simply varying the PMGI development time as in previous processes. Although PMGI is sold as a “liftoff resist” and widely assumed to be electron insensitive, their experiments have shown that this is not the case. Instead, when dilute developer and low electron doses are used, PMGI behaves very much like a conventional photoresist. By exploiting this behavior, as well as its high electron sensitivity with respect to PMMA, the authors were able to define undercuts by defining low-dose regions adjacent to their features, exposing the underlying PMGI separately. In this manner, it is possible to create well-controlled undercut regions as large as 600nm. Extensive modeling of both the exposure and development processes was used to verify their results. By using a Monte Carlo simulation of electron scattering to simulate the electron exposure and mass-transfer relationships to simulate the process of developing the undercut region, the authors were able to produce a model that closely matches experimental results. With the process fully characterized, it is possible to produce nearly any linewidth/undercut combination, limited only by PMMA resolution and the mechanical stability of large overhang structures. This robustness, combined with the high resolution of the PMMA imaging layer, will allow the reliable fabrication of many interesting devices and circuits based on nanoscale Josephson junctions.

Fabrication of high-secondary-electron-yield grids for spatial-phase-locked electron-beam lithography

Poor placement accuracy is a major issue and inhibitor in all forms of electron-beam lithography (EBL), a direct result of the open-loop nature of EBL. In scanning-electron-beam lithography (SEBL), the beam position is not continuously monitored during exposure. Instead, one depends upon a laser-interferometer-controlled stage and secondary referencing. In SEBL, the beam can deviate from its intended position causing pattern-placement errors that are often worse than the resolution. Spatial-phase-locked electron-beam lithography (SPLEBL) is being developed at MIT to achieve nanometer-level absolute placement accuracy. In its global-fiducial-grid mode, SPLEBL directly references the beam location to an electron-transparent grid, placed on top of the e-beam resist, during exposure. Recently pattern-placement precision of ∼1nm was demonstrated with secondary electrons as the reference signal [J. T. Hastings, F. Zhang, and H. I. Smith, J. Vac. Sci. Technol. B 21, 2650 (2003)]. However, the fabrication process used for the reference grid was complex and would not be acceptable in manufacturing. In this article, we describe a process for putting down the grid that is user friendly and preserves its long-range spatial-phase coherence. We also investigated a variety of candidate grid materials (various low-Z metals and C60) for their secondary-electron yields and processibility.

Partial blanking of an electron beam using a quadrupole lens

Inadequate pattern-placement accuracy is becoming a limiting factor for conventional scanning-electron-beam lithography (SEBL) in many applications, such as mask writing and fabrication of integrated optical devices. Spatial-phase-locked electron-beam lithography (SPLEBL) with continuous feedback promises sub-1-nm placement accuracy by monitoring and correcting the beam position in real-time during exposure [J. T. Hastings, F. Zhang, and H. I. Smith, J. Vac. Sci. Technol. B 21, 2650 (2003)]. Ideally, SPLEBL provides a continuous reference signal to infer and correct beam position. Hence, the beam current must be reduced by a factor of 10 or more when traversing areas that are not to be exposed. To accomplish this we have investigated a partial-beam-blanker which consists of a quadrupole lens and a current-limiting aperture. This design has several desirable features. First, it does not shift the center of mass of the beam, so that spurious deflection can be avoided. Second, the operating voltage of the device is very low, which makes it suitable for high modulation rate. Third, this device can be easily incorporated into existing SEBL system design. In this article, the theoretical background will be provided, and the effect of the quadrupole lens on the beam probe size evaluated.

Nanometer-level stitching in raster-scanning electron-beam lithography using spatial-phase locking

Pattern-placement inaccuracy is a persistent problem in scanning-electron-beam lithography (SEBL) despite the high-resolution obtained in SEBL systems. Pattern-placement errors stem from a variety of environmental and system variations; however, the fundamental issue is the open-loop nature of the system, i.e., the beam location on the substrate is not monitored during exposure. In contrast, spatial-phase-locked electron-beam lithography (SPLEBL) provides closed-loop control of the beam position by monitoring the signal from a fiducial grid on the substrate. By detecting the spatial phase of the grid signal one can estimate the beam position within a small fraction of the grid period. We have implemented SPLEBL by adding real-time signal processing, feedback control, and raster-scan exposure capability to an inexpensive SEBL system. Using a 246 nm period, electron-transparent fiducial grid that covers the entire substrate, we have exposed patterns that exhibit global placement accuracy with respect to the grid and field-stitching precision better than 1.3 nm (one standard deviation).