High Resolution Electron Beam Research Articles

Novel Etch-Resistant Molecular Glass Photoresist Based on Pyrene Derivatives for Electron Beam Lithography.

Novel t-butyloxycarbonyl-protected molecular glass photoresists with pyrene as the core (Pyr-8Boc and Pyr-4Boc) were designed and synthesized. The thermal stability and film-forming ability were measured to assess their applicability for lithography. Pyr-Boc (Pyr-8Boc and Pyr-4Boc) photoresists were evaluated by high-resolution electron beam lithography (EBL), acting as chemically amplified resists. Pyr-4Boc showed a better lithography performance, achieving 25 nm line/space patterns at the dose of 50 μC/cm2. Under SF6/O2 plasma, the etch selectivity of the Pyr-4Boc photoresist to silicon was 12.3, which is twice that of the commercially available poly(methyl methacrylate) photoresist (950 k). The lithography mechanism of EBL was further investigated. Theoretical calculations of HOMO/LUMO orbital energies, cyclic voltammetry, and fluorescence quenching experiments were conducted to confirm the electron-transfer reactions between the Pyr-Boc and photoacid generator. The study provides an option of high sensitivity and etch-resistant photoresist for EBL.

Approaching Angstrom-Scale Resolution in Lithography Using Low-Molecular-Mass Resists (

Resists that enable high-throughput and high-resolution patterning are essential in driving the semiconductor technology forward. The ultimate patterning performance of a resist in lithography is limited because of the trade-off between resolution, line-width roughness, and sensitivity; improving one or two of these parameters typically leads to a loss in the third. As the patterned feature sizes approach angstrom scale, the trade-off between these three metrics becomes increasingly hard to resolve and calls for a fundamental rethinking of the resist chemistry. Low-molecular-mass monodispersed metal-containing resists of high atom economy can provide not only very high resolution but also very low line-width roughness without sacrificing sensitivity. Here we describe a modular metal-containing resist platform (molecular mass <500 Da) where a molecular resist consists of just two components: a metal and a radical initiator bonded to it. This simple system not only is amenable to high-resolution electron beam lithography (EBL) and extreme ultraviolet lithography (EUVL) but also unites them mechanistically, giving a consolidated perspective of molecular and chemical processes happening during exposure. Irradiation of the resist leads to the production of secondary electrons that generate radicals in the initiator bonded to metal. This brings about an intramolecular rearrangement and causes solubility switch in the exposed resist. We demonstrate record 1.9-2.0 nm isolated patterns and 7 nm half-pitch dense line-space features over a large area using EBL. With EUVL, 12 nm half-pitch line-space features are shown at a dose of 68 mJ/cm2. In both of these patterning techniques, the line-width roughness was found to be ≤2 nm, a record low value for any resist platform, also leading to a low-performance trade-off metric, Z factor, of 0.6 × 10-8 mJ·nm3. With the ultimate resolution limited by instrumental factors, potential patterning at the level of a unit cell can be envisaged, making low-molecular-mass resists best poised for angstrom-scale lithography.

Feasibility study of fabricating 20 nm resolution dielectric Fresnel zone plates with ultrahigh aspect ratio for EUV optics

EUV light optics are either reflective or diffractive due to the substantial absorption characteristics by almost all materials. Despite great successes in manufacturing integrated circuit chips, reflective EUV optics are still unfriendly to small-to-medium enterprise (SME) because of the enormous costs. Recently, diffractive EUV optics has come to the light in hopes to be able to establish manufacturing nanoscale products and inspecting nanoscale structures. Diffractive zone plates with high resolution in EUV wavelengths are urgently needed. This paper reports our latest success in developing 20 nm resolution zone plates for focusing and imaging in the EUV and soft X-ray regions. It firstly discusses the diffraction efficiency of FZPs tailored for 13.5 nm wavelength to decide the essential height of the zone plate. Then, Monte Carlo simulation method was used to figure out the achievable zone plate parameters by high-resolution electron beam lithography (EBL). Finally, this work systematically explored the viability of nanofabricating top-tier 20 nm resolution hydrogen silsesquioxane (HSQ) zone plates with the duty cycle ratio nearing 1:1 and the aspect ratio approaching 13:1 on 50 nm thick Si3N4 membranes.

Self-wavefront interference using transverse splitting holography

Manufacturing diffractive lenses with a high Numerical Aperture (NA) is a challenging task due to limitations in lithography methods and the inverse relation between the width and the radius of the zones. With low-resolution lithography techniques such as photolithography, the zone width reaches the lithography limit within a short radius, resulting in low-NA diffractive lenses. With high-resolution electron beam lithography, it is possible to manufacture high-NA diffractive lenses by prolonged writing. However, in this case, the width of the outermost zones becomes subwavelength, inducing undesirable polarization effects. In this proof-of-concept study, a holography solution has been demonstrated to enhance the imaging resolution of low-NA diffractive lenses. The light from an object is partly modulated by the low-NA diffractive lens and interfered with the remaining unmodulated light outside the area of the diffractive lens. This self-interference hologram of the object is processed in the computer with the point spread hologram to reconstruct the object with a resolution corresponding to the NA of the image sensor. This new imaging technique is called Self-Wavefront Interference using Transverse Splitting Holography (SWITSH). A resolution enhancement of ∼10 times has been demonstrated using a low-NA diffractive lens and SWITSH compared to direct imaging with the same low-NA diffractive lens.

Development of 3D tomosynthesis technique for nondestructive technology application using vertically aligned carbon nanotube-based high-resolution cold cathode electron beam

Nondestructive technology (NDT) is the most popular method for detecting defects inside an object without harming it. A 3D tomography algorithm toolbox and an x-ray imaging system are the essential components of NDT. We fabricated a high-resolution cold cathode electron beam (HRC-beam) based on vertically aligned carbon nanotubes to accomplish this objective. Using an HRC-beam, multiple angle projection images were captured in an x-ray imaging system. This x-ray source's horizontal and vertical focal spot sizes were 0.57 and 0.49 mm, respectively. Using the homemade 3D tomography algorithm toolbox, all the multiple projection images were recreated in a 3D volume. This toolbox employs algorithms such as the radon transform and the inverse radon transform to generate the 3D volume. The matlab r2022b program was used to execute the algorithm. This study explains the development of a three-dimensional volume and the inspection of the 3D volume in a slice view using a high-resolution cold cathode electron beam and the 3D tomography algorithm toolbox. A large phantom cable was used to evaluate 3D reconstruction and void inspection inside the large phantom cable.

Direct Patterning of Optoelectronic Nanostructures Using Encapsulated Layered Transition Metal Dichalcogenides.

Direct top-down nanopatterning of semiconductors is a powerful tool for engineering properties of optoelectronic devices. Translating this approach to two-dimensional semiconductors such as monolayer transition metal dichalcogenides (TMDs) is challenging because of both the small scales required for confinement and the degradation of electronic and optical properties caused by high-energy and high-dose electron radiation used for high-resolution top-down direct electron beam patterning. We show that encapsulating a TMD monolayer with hexagonal boron nitride preserves the narrow exciton linewidths and emission intensity typical in such heterostructures after electron beam lithography, allowing direct patterning of functional optical monolayer nanostructures on scales of a few tens of nanometers. We leverage this fabrication method to study size-dependent effects on nanodot arrays of MoS2 and MoSe2 as well as laterally confined electrical transport devices, demonstrating the potential of top-down lithography for nanoscale TMD optoelectronics.

A high-resolution, variable-energy electron beam from a Penning\u2013Malmberg (Surko) buffer-gas trap

We describe the production of a high-resolution electron beam using a Penning–Malmberg buffer-gas trap, or Surko trap as they have become known. A high-flux beam with an energy width of ~ 30 meV (FWHM) is readily achieved and the efficiency of production is considerably higher than that for positrons in a similar trap configuration. The reasons for this become apparent when one considers the molecular collisions and the respective selection rules involved, for electrons and positrons. We demonstrate the production of the beam and the capacity that it realises for absolute scattering measurements and for high-resolution electron spectroscopy.Graphical abstract

High-resolution planar electron beam induced current in bulk diodes using high-energy electrons

Understanding the impact of high-energy electron radiation on device characteristics remains critical for the expanding use of semiconductor electronics in space-borne applications and other radiation harsh environments. Here, we report on in situ measurements of high-energy electron radiation effects on the hole diffusion length in low threading dislocation density homoepitaxial bulk n-GaN Schottky diodes using electron beam induced current (EBIC) in high-voltage scanning electron microscopy mode. Despite the large interaction volume in this system, quantitative EBIC imaging is possible due to the sustained collimation of the incident electron beam. This approach enables direct measurement of electron radiation effects without having to thin the specimen. Using a combination of experimental EBIC measurements and Monte Carlo simulations of electron trajectories, we determine a hole diffusion length of 264 ± 11 nm for n-GaN. Irradiation with 200 kV electron beam with an accumulated dose of 24 × 1016 electrons cm−2 led to an approximate 35% decrease in the minority carrier diffusion length.

Negative-tone molecular glass photoresist for high-resolution electron beam lithography.

A low molecular weight organic compound containing bis-phenol A backbone (BPA-6OH) is reported as a negative-tone photoresist. This material has a high glass transition temperature and excellent thermal stability. A good contrast, well-resolved line pattern around 73.4 nm and sensitivity of 52 µC cm−2 can be received for negative-tone molecular glass photoresist upon exposure in electron beam lithography system. It indicates that the negative-tone molecular glass photoresist is one of the promising candidates for use in electron beam lithography.

Nanopatterning of Weak Links in Superconducting Oxide Interfaces.

The interface between two wide band-gap insulators, LaAlO and SrTiO (LAO/STO), hosts a quasi-two-dimensional electron gas (q2DEG), two-dimensional superconductivity, ferromagnetism, and giant Rashba spin-orbit coupling. The co-existence of two-dimensional superconductivity with gate-tunable spin-orbit coupling and multiband occupation is of particular interest for the realization of unconventional superconducting pairing. To investigate the symmetry of the superconducting order parameter, phase sensitive measurements of the Josephson effect are required. We describe an approach for the fabrication of artificial superconducting weak links at the LAO/STO interface using direct high-resolution electron beam lithography and low-energy argon ion beam irradiation. The method does not require lift-off steps or sacrificial layers. Therefore, resolution is only limited by the electron beam lithography and pattern transfer. We have realized superconducting weak links with a barrier thickness of 30–100 nm. The barrier transparency of the weak links can be controlled by the irradiation dose and further tuned by a gate voltage. Our results open up new possibilities for the realization of quantum devices in oxide interfaces.

Computational proximity lithography with extreme ultraviolet radiation.

The potential of extreme ultraviolet (EUV) computational proximity lithography for fabrication of arbitrary nanoscale patterns is investigated. We propose to use a holographic mask (attenuating phase shifting mask) consisting of structures of two phase levels. This approach allows printing of arbitrary, non-periodic structures without using high-resolution imaging optics. The holographic mask is designed for a wavelength of 13.5 nm with a conventional high-resolution electron beam resist as the phase shifting medium (pixel size 50 nm). The imaging performance is evaluated by using EUV radiation with different degrees of spatial coherence. Therefore exposures on identical masks are carried out with both undulator radiation at a synchrotron facility and plasma-based radiation at a laboratory setup.

Lossless Monochromation for Electron Microscopy with Pulsed Photoemission Sources and Radio-Frequency Cavities

Resonant radiofrequency cavities enable exquisite time-energy control of electron beams when synchronized with laser driven photoemission. We present a lossless monochromator design that exploits this fine control in the one-electron-per-pulse regime. The theoretically achievable maximum beam current on target is orders of magnitude greater than state-of-the-art monochromators for the same spatial and energy resolution. This improvement is the result of monochromating in the time domain, unconstrained by the transverse brightness of the electron source. We show analytically and confirm numerically that cavity parameters chosen to minimize energy spread perform the additional function of undoing the appreciable effect of chromatic aberration in the upstream optics. We argue that our design has significant applications in both ultra-fast and non-time-resolved microscopy, provided photoelectron sources of sufficiently small size and laser sources of sufficiently high repetition rate. Our design achieves in simulations more than two orders of magnitude reduction in beam energy spread, down to single digit meV. Overcoming the minimum probe-size limit that chromatic aberration imposes, our design clears a path for high-current, high-resolution electron beam applications at primary energies from single to hundreds of keV.

Deep learning-based detection, classification, and localization of defects in semiconductor processes

Defects in semiconductor processes can limit yield, increase overall production cost, and also lead to time-dependent critical component failures. Current state-of-the-art optical and electron beam (EB) inspection systems rely on rule-based techniques for defect detection and classification, which are usually rigid in their comparative processes. This rigidity limits overall capability and increases relative engineering time to classify nuisance defects. This is further challenged due to shrinkage of pattern dimensions for advanced nodes. We propose a deep learning-based workflow that circumvents these challenges and enables accurate defect detection, classification, and localization in a unified framework. In particular, we train convolutional neural network-based models using high-resolution EB images of wafers patterned with various types of intentional defects and achieve robust defect detection and classification performance. Furthermore, we generate class activation maps to demonstrate defect localization capability of the model “without” explicitly training it with defect location information. To understand the underlying decision-making process of these deep models, we analyze the learned filters in pixel space and Fourier space and interpret the various operations at different layers. We achieve high sensitivity (97%) and specificity (100%) along with rapid and accurate defect localization. We also test performance of the proposed workflow on images from two distinct patterns and find that in order to retain high accuracy a modest level of retraining is necessary.

A study of nano-structural effect on the polarization characteristics of metallic sub-wavelength grating polarizers in visible wavelengths

The structural effect of subwavelength gratings as the polarizer on the polarization characteristics in visible wavelengths (400–700 nm) is systematically investigated through both numerical simulation with Finite Difference Time Doman algorithm and experiments. The grating parameters such as the material, the pitch, the duty cycle, the height and the cross-sectional shape of the grating are addressed in this work. The optimized range for those parameters is worked out, based on the desired performance margin at the level of 70% transmittance and 50:1 for the extinction ratio. Polarization performance such as the transmittance over 70% and the extinction ratio above 50:1 has been experimentally demonstrated by the 200 nm pitched grating in Al with the height of 150 nm, which was fabricated by high resolution electron beam lithography in this work, guided by the simulation result. Au grating as the polarizer was also compared but ruled out in this work because of the poor performance in polarization. This work is essential in developing grating based polarizers for polarimetric detection and imaging in visible wavelengths.

Holographic Silicon Metasurfaces for Total Angular Momentum Demultiplexing Applications in Telecom

The simultaneous processing of orbital angular momentum (OAM) and polarization has recently acquired particular importance and interest in a wide range of fields ranging from telecommunications to high-dimensional quantum cryptography. Due to their inherently polarization-sensitive optical behavior, Pancharatnam–Berry optical elements (PBOEs), acting on the geometric phase, have proven to be useful for the manipulation of complex light beams with orthogonal polarization states using a single optical element. In this work, different PBOEs have been computed, realized, and optically analyzed for the sorting of beams with orthogonal OAM and polarization states at the telecom wavelength of 1310 nm. The geometric-phase control is obtained by inducing a spatially-dependent form birefringence on a silicon substrate, patterned with properly-oriented subwavelength gratings. The digital grating structure is generated with high-resolution electron beam lithography on a resist mask and transferred to the silicon substrate using inductively coupled plasma-reactive ion etching. The optical characterization of the fabricated samples confirms the expected capability to detect circularly-polarized optical vortices with different handedness and orbital angular momentum.

Pancharatnam–Berry Optical Elements for Spin and Orbital Angular Momentum Division Demultiplexing

A Pancharatnam–Berry optical element is designed, fabricated, and optically characterized for the demultiplexing of beams with different polarization and orbital angular momentum states at the telecom wavelength of 1310 nm. The geometric phase control is achieved by fabricating properly-oriented subwavelength gratings on a silicon substrate, inducing a spatially-variant form birefringence. The digital grating pattern is transferred to the silicon substrate with a two-step nanofabrication protocol, using inductively coupled plasma reactive ion etching to transfer the resist pattern generated with high-resolution electron beam lithography. The optical characterization of the sample confirms the expected capability to sort circularly polarized optical beams with different handedness and orbital angular momentum. Encompassing optical element design and silicon photonics, the designed silicon metasurface paves the way to innovative devices for total angular momentum mode division multiplexing with unprecedented levels of integration.

Electronic imaging of subcritical defect accumulation in single crystal silicon under fatigue loading

High resolution electron beam induced current (EBIC) imaging technique successfully visualized defect accumulation process in single crystal silicon under compression fatigue loading at 50 °C with the relative humidity of 75%. Dark line-like defect contrast emanating from deep-etched stress concentration notch appeared in EBIC images in a scanning electron microscope, after a certain number of fatigue cycles, but they were mostly invisible with secondary electron. This line-like defect contrast eventually extended along the direction which was expected in correspondence to one of the crystal slip systems of silicon with the largest resolved shear stress. Hence it is strongly suggested that crystal slips take place in bulk silicon under repeated loading, which presumably leads to local stress concentration and further to fatigue fracture especially in case tensile stress is applied in fatigue cycles.

Electrically activated spin-controlled orbital angular momentum multiplexer

We present and test the integration of a static orbital angular momentum mode multiplexer with a dynamical geometric-phase optical element, enabling on-demand spin-controlled angular momentum multiplexing. A diffractive optic multiplexer fabricated by 3D high-resolution electron beam lithography performs a conformal mapping for the conversion from linear to azimuthal phase gradients. The latter is functionalized by a dynamic spin-orbit add-on that consists of a self-engineered electrically activated liquid crystal optical vortex generator having large clear-aperture and high-resolution. By combining several functionalities based on the optical angular momentum of light in a compact manner, the proposed hybrid device could find applications in next-generation high-dimensional mode switchers and routers based on orbital angular momentum.

Configurable Integration of On‐Chip Quantum Dot Lasers and Subwavelength Plasmonic Waveguides

The integration of on-chip dielectric lasers and subwavelength plasmonic waveguides has attracted enormous attention because of the combination of both the advantages of the high performances of the small dielectric lasers and the subwavelength plasmonic waveguides. However, the configurable integration is still a challenge owing to the complexity of the hybrid structures and the damageability of the gain media in the multistep micro/nanofabrications. By employing the dark-field optical imaging technique with a position uncertainty of about 21 nm and combining the high-resolution electron beam lithography, the small colloidal quantum dot (CQD) lasers without any damages are accurately aligned with the silver nanowires. As a result, the integration of the CQD lasers and the silver nanowires can be flexibly configured on chips. In the experiment, the tangential coupling, radial coupling, and complex coupling between the high-performance CQD lasers and the subwavelength silver nanowires are demonstrated. Because of the subwavelength field confinements of the silver nanowires, the deep-subwavelength coherent sources (multimode, one-color single-mode, or two-color single-mode) with a mode area of only 0.008λ2 are output from these hybrid structures. This configurable on-chip integration with high flexibility and controllability will greatly facilitate the developments of the complex functional hybrid photonic-plasmonic circuits.

Electron beam induced changes in optical properties of glassy As35S65 chalcogenide thin films studied by imaging ellipsometry

Spectroscopic imaging ellipsometry was used to study 50 μm × 50 μm squares recorded into glassy As35S65 thin film by electron beam with different exposure doses. Optical constants (refractive index n and extinction coefficient k) of electron exposed areas were determined from imaging ellipsometric data recorded over a spectral range 390–800 nm. Complex dielectric permittivity of each of electron exposed square (glassy As35S65 with different electron exposure dose) was parametrized by a Tauc-Lorentz oscillator. Gradient optical model with the refractive index changing linearly within the layer thickness was used.Evaluation of parameters of Tauc-Lorentz oscillator revealed decrease of energy bandgap, increase of the refractive index in whole recorded spectra (more pronounced at the top of the film) and increase of extinction coefficient in the part of spectra where the material absorbs with the increasing electron exposure dose used for exposure of the squares.The decrease in local disorder with increasing electron exposure dose is quantified from the short wavelength absorption edge onset using a Mott-Davis model. The dependence of the parameters of Wemple-DiDomenico model for the transparent part of the optical constants spectra on electron exposure dose is shown.Thin film with recorded image was subsequently etched in amine based solution in aprotic solvent. While native film was fully etched off the remaining height of the exposed areas is increasing with the increasing electron exposure dose used.Knowledge of the changes of optical properties and etching rates induced by electron beam of different exposure doses can be used for fabricating of optical elements by high resolution electron beam lithography.