Grayscale Electron Beam Lithography Research Articles

Fourier Surfaces Reaching Full-Color Diffraction Limits.

Optical Fourier surfaces (OFSs), characterized by sinusoidally profiled diffractive optical elements, can outperform traditional binary-type counterparts by minimizing optical noise through selectively driving diffraction at desired frequencies. While scanning probe lithography (SPL), gray-scale electron beam lithography (EBL), and holographic inscriptions are effective for fabricating OFSs, achieving full-color diffractions at fundamental efficiency limits is challenging. Here, an integrated manufacturing process is presented, validated theoretically and experimentally, for fully transparent OFSs reaching the fundamental limit of diffraction efficiency. Leveraging holographic inscriptions and soft nanoimprinting, this approach effectively addresses challenges in conventional OFS manufacturing, enabling scalable production of noise-free and maximally efficient OFSs with record-high throughput (1010-1012 µm2h-1), surpassing SPL and EBL by 1010 times. Toward this end, a wafer-scale OFSs array is demonstrated consisting of full-color diffractive gratings, color graphics, and microlenses by the one-step nanoimprinting, which is readily compatible with rapid prototyping of OFSs even on curved panels, demanding for transformative optical devices such as augmented and virtual reality displays.

Tuning Surface Adhesion Using Grayscale Electron-beam Lithography.

Surface texturing of manufactured products tailors their properties, such as friction, adhesion, biocompatibility, or fluid interactions. However, advancements in this area are largely the result of trial-and-effort testing and generally lack a science-guided framework for determining the surface topography that will optimize performance. The present investigation explores grayscale electron-beam lithography as a means to create multiscale surface patterns to control surface performance. Here, we created and characterized a set of surface textures on a silicon wafer; the textures were superpositions of sine waves of varying wavelengths and amplitudes. First, the multiscale topography of the patterned surface was characterized, using profilometry and atomic force microscopy, to understand its fidelity to the designed-in pattern. The results of this analysis demonstrated how grayscale lithography accurately controlled the lateral size of features but was less precise on the vertical height of the surface, and also introduced inherent roughness below the scale of patterning. Second, a micromechanical tester was used to characterize the adhesion of the surfaces with large-scale polished silicon spheres. The results showed that adhesion could be tailored, with significant contribution from all of the designed-in length scales of topography. The strength of adhesion did not correlate with conventional roughness parameters but could be accurately modeled using simple numerical integration. Taken together, this investigation demonstrates the promise and challenges of grayscale e-beam lithography with multiscale patterns as a method for the tailoring of surface performance.

Simulation study of three-dimensional grayscale ice lithography on amorphous solid water for blazed gratings

Electron beam lithography (EBL) on amorphous solid water (ASW), termed as ice lithography (IL), has demonstrated promising capability in pattern transfer with unique advantages such as reduced proximity effect. So far, ice lithography for binary patterning has been proved a great success, however, application for three-dimensional (3D) profiling in nanoscale has still not been addressed to the best of our knowledge. This paper reports, for the first time, our progress in simulating study of three-dimensional ice lithography on ASW for linear blazed gratings, aiming to overcome the difficulty in replicating high quality blazed gratings with high diffraction efficiency. Systematic simulation of grayscale ice lithography for 3-D blazed grating templates with desired surface quality as the task was carried out, using Monte Carlo algorithm based on the measured contrast curves of ASW. For comparison, grayscale electron beam lithography on PMMA was also performed. The resultant profiles of blazed wavelengths around 1550 nm by grayscale IL show less flaws and higher diffraction efficiencies than by EBL. The successful simulation of 3D grayscale IL provides us with instructive guide for the fabrication of 3D nanostructures as a whole through the grayscale ice lithography on ASW.

High-performance photodetector arrays for near-infrared spectral sensing

Spectral sensing is an emerging field driven by the need for fast and non-invasive methods for the chemical analysis of materials in agri-food, healthcare, and industrial applications. We demonstrate a near-infrared spectral sensor, based on a scalable fabrication process and combining high responsivity, narrow linewidth, and low noise. The sensor consists of 16 resonant-cavity-enhanced photodetectors, each showing a unique spectral response consisting of narrow peaks. The spectral sensor thereby covers the wavelength range between 890 and 1650 nm, where organic materials show relevant spectral features from first and second overtones. For the fabrication of the detector arrays, we propose a simple and scalable fabrication approach that yields largely improved device characteristics with respect to the grey-scale electron-beam lithography process reported earlier. Through a series of five optical lithography steps, tuning layers of silicon nitride are deposited stepwise to obtain 16 different thicknesses and reduced surface roughness. With this novel fabrication approach, the obtained photodetectors achieve an average peak linewidth of 55 nm, a maximum peak responsivity of 0.3 A/W, and high suppression of the non-resonant background. We also demonstrate the impact of these improvements on the sensing performance for two relevant problems through an experiment and a set of simulations. With lateral dimensions of ∼1.4 × 1.4 mm2, the proposed photodetector array can be the key to robust, portable, and low-cost sensing instrumentation for on-site material analysis in various application fields.

A compound Kinoform/Fresnel zone plate lens with 15 nm resolution and high efficiency in soft x-ray

X-ray microscope as an important nanoprobing tool plays a prevailing role in nano-inspections of materials. Despite the fast advances of high resolution focusing/imaging reported, the efficiency of existing high-resolution zone plates is mostly around 5% in soft x-ray and rapidly goes down to 1%–2% when the resolution approaches 10 nm. It is well known that the rectangular zone shape, beamstop, limited height/width ratios, material absorption of light and structural defects are likely responsible for the limited efficiency. Although zone plates with Kinoform profile are supposed to be efficient, progress for achieving both high resolution (<30 nm) and high efficiency (>5%) have hardly been addressed in soft x-ray. In this work, we propose a compound Kinoform/Fresnel zone plate (CKZP) by combing a dielectric Kinoform zone plate with a 15 nm resolution zone plate. Greyscale electron beam lithography was applied to form the 3D Kinoform zone plate and atomic layer deposition was carried out to form the binary zone plate. Optical characterizations demonstrated 15 nm resolution focusing/imaging with over 7.8% efficiency in soft x-ray. The origin of the efficiency improvement behind the proposed compound lens is theoretically analyzed and discussed.

High-efficiency focusing and imaging by dielectric kinoform zone plate lenses with soft X-rays.

With fast advances in enhancing the focusing/imaging resolution of Fresnel zone plate lenses toward sub-10 nm, low diffraction efficiency in connection with their rectangular zone shape still remains a big issue in both soft and hard X-ray microscopy. In hard X-ray optics, encouraging progress has recently been reported in our earlier attempts of high focusing efficiency by 3D kinoform shaped metallic zone plates, formed by greyscale electron beam lithography. This paper addresses our efforts towards high focusing/imaging efficiency by developing a novel dielectric kinoform zone plate lens for soft X-rays. The effects of the zone materials and zone shapes on the focusing/imaging quality were first theoretically investigated by a modified thin-grating-approximation method, revealing superior efficiencies of dielectric kinoform zone plates over rectangular ones in metals. Optical characterizations of replicated dielectric kinoform zone plates by greyscale electron beam lithography demonstrate a focusing efficiency of 15.5% with a resolution of 110 nm in the water window of X-rays. Apart from high efficiency, the novel kinoform zone plate lenses developed in this work exhibit significant advantages over conventional zone plates, i.e. simplified process, low cost and no need for a beamstop.

Multiple material stack grayscale patterning using electron-beam lithography and a single plasma etching step

In this paper, we present a novel method to perform grayscale electron-beam lithography on multilayer stacks where the pattern transfer is done in a single plasma etching step. Due to the differences in material etch rates in the stack, the shape of the resist after development vs the shape of the multilayer stack after etching is significantly different. To be able to reach the desired shape in the multilayer stack, the final resist dose is defined by an etching calibration curve that describes the relationship between the electron-beam dose and the remaining materials thickness after plasma etching. With this method, a resistive memory crossbar array is fabricated with a height resolution of 10 nm and nanoscale dimension devices.

3D structured biochip for label free determinations at the point-of-need

A novel optical label-free biosensing approach is introduced aiming to remove the current limitations of optical biosensors in their application of on-site food analysis. The proposed methodology (GRADual thin film IntErferometry, GRADE) is based on light interferometry employing an illumination source of a few nm spectral width and a 3D micro-patterned sensor surface consisting of multilevel staircase structures, fabricated through grayscale electron beam lithography and appropriate pattern transfer on thick dielectric layer. The formation of the biomolecular adlayer on the 3D structured surface sensor, is monitored with a 2D camera, without the need for broadband illumination sources and spectral analysis instrumentation. Consequently, this novel sensing approach has the potential to enable the cost-efficient monitoring of bioreactions employing a simple optical setup, a low-cost photodetector and light source, thus being an ideal platform for use at the Point-of-Need. In the present study the system is demonstrated for the detection of aflatoxin B 1 (AFB1), a very potent carcinogen detected in various food categories. • A novel optical label-free biosensing approach is introduced aiming to remove the current limitations of optical biosensors. • Based on a sensing area with a gradually-varying-thickness dielectric, fabricated through grayscale e-beam lithography. • Cost-efficient monitoring of bioreactions, ideal platform for use at the Point-of-Need.

Trapezoid-kinoform zone plate lens - a solution for efficient focusing in hard X-ray optics.

X-ray microscopes are powerful tools in the nano-inspection of materials owing to their ultra-high resolution at the molecular level. However, the focusing efficiency of binary zone plate lenses as key components in such probes is merely 5% in practice, hindering their application in advanced scientific research. Although kinoform zone plate lenses are in principle supposed to possess high efficiency beyond binary ones, little progress has been reported so far due to the shortage of both a theoretical calculation approach and greyscale lithography for generating fine three-dimensional (3D) kinoform zones of the lenses. This paper reports our theoretical work for a modified beam propagation method to compute the focusing performance and state-of-the-art 3D greyscale electron beam lithography for kinoform zone plate lenses. Three different zone shapes - binary, kinoform and top-flat kinoform (nicknamed the trapezoid-kinoform) - were compared both theoretically and experimentally. Theoretical calculations suggest, for the first time, that the trapezoid-kinoform zone plate gives rise to the highest focusing efficiency among the three lenses, which was proved by optical characterization of the fabricated lens with hard X-rays. As high as 40% of the focusing efficiency by Au trapezoid-kinoform lenses with resolution of 250 nm at 8 keV has been achieved, which is two times higher than that of binary zone plate lenses. The origin of the enhanced efficiency in the trapezoid-kinoform zone plate lens was explained by the joint contributions from both the refraction through the kinoform slope and the diffraction through the top flat part of the trapezoid-kinoform zone plate. Such a breakthrough in focusing efficiency sheds light on the further development of X-ray lenses with both high resolution and high efficiency.

Homeotropic liquid crystal alignments through periodically unidirectional nano-wedges patterned by nanoimprint lithography

This study demonstrates the fabrication of periodically unidirectional nano-wedges using nanoimprint lithography to achieve homeotropic alignments in sandwiched liquid crystal (LC) cells. When electrically operating LC cells for the purpose of electro-optical performance without the occurrence of disclination lines, the LC pretilt angles play an essential role in achieving uniform LC alignments and well-behaved device functions. The periodical nano-wedges composed of asymmetrically sawtooth-like units were just designed for providing LC pretilt angles. We used grayscale electron beam lithography to fabricate nanoimprint master templates with periodically unidirectional nano-wedges with a 1-μm pitch. The photoresist SU-8 was used as the nanoimprint polymer to form nano-wedge structures. An ultrathin poly(dimethylsiloxane) (PDMS) film was coated on the nano-wedges to assist homeotropic LC alignments. The fabricated nano-wedges were capable of better LC alignments than nano-rectangles in the cells, including fewer disclination lines and better optical performance, as supported by the data obtained. • Unidirectional nano-wedges coated with ultrathin polydimethylsiloxane to align liquid crystal cells homeotropically. • Nano-wedge was formed via nanoimprint, and the template was generated through grayscale electron beam lithography. • The achievement of better liquid crystal alignments via nano-wedges than via nano-rectangles.

A study of greyscale electron beam lithography for a 3D round shape Kinoform lens for hard X-ray optics

For high quality imaging of materials in nanoscale by hard X-ray optics, high resolution lenses with high focusing/imaging efficiency are needed. Refraction based conventional Kinoform lenses with 1D linear configuration in Si has been attempted for decades, but there has still been no sign of success due to the enormous difficulty in fabrication. This paper proposes a novel configuration of golden Kinoform lenses with round shape on a plate, in which each zone maintains the typical 3D parabolic profile as seen in the traditional 1D counterpart. The major focus in this work is on the generation of the 3D profile in e-beam resist through greyscale electron beam lithography (EBL) as the template for the golden Kinoform lens. Systematic study of the greyscale lithography conditions, including the comparisons of resists, developers and charge distributions, has been conducted by using Monte Carlo simulation as well as e-beam lithography. Initial results have been achieved for replicating the desired 3D profile of the golden Kinoform lens in the resist. Using the generated profile in resist as templates, the golden Kinoform lens with 200 μm diameter and 3.5 μm height has been successfully fabricated, for the first time, which will hopefully replace the existing Fresnel zone plates for hard X-ray imaging with high resolution and high efficiency.

Design and fabrication of blazed gratings for a waveguide-type head mounted display.

In a waveguide-type display for augmented reality, the image is injected in the waveguide and extracted in front of the eye appearing superimposed on the real-world scene. An elegant and compact way of coupling these images in and out is by using blazed gratings, which can achieve high diffraction efficiencies. We report the design of blazed gratings for green light (λ = 543 nm) and a diffraction angle of 43°. The blazed gratings with a pitch of 508 nm and a fill factor of 0.66 are fabricated using grayscale electron beam lithography. We outline the subsequent replication in a polymer waveguide material with ultraviolet nanoimprint lithography and confirm a throughput efficiency of 17.4%. We finally show the in- and outcoupling of an image through two blazed gratings appearing sharp and non-distorted in the environment.

Extreme Ultraviolet and Soft X-Ray Diffraction Efficiency of a Blazed Reflection Grating Fabricated by Thermally Activated Selective Topography Equilibration

Abstract Future observatories utilizing reflection grating spectrometers for extreme ultraviolet (EUV) and soft X-ray (SXR) spectroscopy require high-fidelity gratings with both blazed groove facets and custom groove layouts that are often fanned or feature a slight curvature. While fabrication procedures centering on wet anisotropic etching in monocrystalline silicon produce highly efficient blazed gratings, the precision of a nonparallel groove layout is limited by the cubic structure of the silicon crystal. This motivates the pursuit of alternative techniques to grating manufacture, namely thermally activated selective topography equilibration (TASTE), which uses gray-scale electron-beam lithography to pattern multilevel structures in resist followed by an optimized polymer thermal reflow to smooth the 3D patterns into continuous surface relief profiles. Using TASTE, a mold for a reflection grating with a periodicity of 400 nm and grooves resembling an asymmetric sawtooth was patterned in 130 nm thick poly(methyl methacrylate) resist on a silicon substrate over a 50 mm by 7.5 mm area. This structure was coated with 15 nm of gold by electron-beam physical vapor deposition using titanium as an adhesion layer and then tested for EUV and SXR diffraction efficiency at beamline 6.3.2 of the Advanced Light Source synchrotron facility. Results demonstrate a quasi-blaze response characteristic of a 27◦ blaze angle with groove facets smooth to 1.5 nm rms. Absolute peak-order efficiency ranges from 75% to 25%, while total relative efficiency measures ⪆90% across the measured bandpass of 15.5 nm &gt; λ &gt; 1.55 nm.

Grayscale e-beam lithography: Effects of a delayed development for well-controlled 3D patterning

Grayscale electron beam lithography (g-EBL) is a fabrication technique that allows for tunable control of resist topography. In most cases, the height of the structures is in the submicron regime. Here, we present an extensive experimental characterization of the post electron beam exposure behavior of poly(methyl methacrylate) (PMMA) 950 K for grayscale structuring with several micrometers in height. The obtained results show that the development depth for the same electron dose is dependent on the time between exposure and development. This dependence becomes more prominent at higher exposure doses. Additionally, it was found that a post-exposure bake influences the dose-response behavior of the resist material and, therefore, also the obtained three-dimensional (3D) structure. This work paves the way for well-controlled 3D micrometer structuring via g-EBL.

Single-digit 6-nm multilevel patterns by electron beam grayscale lithography

Abstract We report on the fabrication of very high-resolution discrete four-resist-level grayscale patterns in poly(methyl methacrylate) with just 6-nm step height and down to 32-nm step width using dose-modulated, grayscale electron beam lithography and a low-contrast resist-developer system. This direct pattern writing is important for replication in high-volume manufacturing of diffractive optics. An innovative concept of unexposed auxiliary spacers helped to enhance the discrete character of the multi-level patterns. For pattern step widths between 100 and 32 nm, a transformation toward blazed gratings with increasingly continuous-slope character was obtained. All high-resolution patterns were prepared in a single exposure and development process from an initially about 30-nm thin film. The pattern roughness due to a relatively large polymer molecular weight was reduced using selective thermal annealing with only minimally affecting the global pattern shape by reflow. The results will enable further approaches toward single-digit vertical and prospectively single-digit lateral resolution grayscale patterns.

Universal multimode waveguide crossing based on transformation optics

Mode-division multiplexing is a promising and cost-effective way to increase the communication capability of integrated photonic circuits, for both classical and quantum information processing. To construct large-scale on-chip multimode routing systems, the multimode waveguide crossing is one of the key components. However, there have been only a few dual-mode waveguide crossings reported, which can support merely two waveguide modes or two crossing channels. This severely limits the density and capacity of multimode routing systems. Here we demonstrate for the first time, to the best of our knowledge, a universal multimode waveguide star crossing based on transformation optics, which can handle, in principle, any number of waveguide modes and any number of crossing channels as well. The structure is transformed from a Maxwell fisheye and can realize aberration-free imaging for each waveguide mode. A gray-scale electron-beam lithography is adopted to fabricate it on a commercial silicon-on-insulator wafer. The proposed multimode waveguide star crossing has little loss and low crosstalk throughout an ultra-broad wavelength range of ∼400 nm. Our study paves the way for realizing highly integrated and large capacity on-chip multimode routing and communication systems.

Fabrication of astronomical x-ray reflection gratings using thermally activated selective topography equilibration

Thermally activated selective topography equilibration (TASTE) enables the creation of 3D structures in resist using grayscale electron-beam lithography followed by a thermal treatment to induce a selective polymer reflow. A blazed grating topography can be created by reflowing repeating staircase patterns in resist into wedgelike structures. Motivated by astronomical applications, such patterns with periodicities 840 and 400 nm have been fabricated in 130 nm-thick poly(methyl methacrylate) using TASTE to provide a base for x-ray reflection gratings. A path forward to integrate this alternative blazing technique into grating fabrication recipes is discussed.

An effective approach for reducing surface roughness of PMMA by thermal radiation induced local reflow

Roughness including surface roughness and line edge roughness (LER) of lithographically generated patterns in resists has been one of the most important issues in semiconductor industry because it limits the further reduction of the minimum feature size in lithography. Despite enormous amount of attempts have been made to reduce the roughness by changing developing condition and developers, ultrasonic agitation in the development, heat reflow, etc., the progress is still far away from satisfaction. In this paper, we report, for the first time, a novel and effective approach to reduce the surface roughness on PMMA surface by a process termed as thermal radiation induced local reflow (TRILR). The basic working principle of the process is first comprehensively described. PMMA surfaces with various scales of roughness are deliberately created by a greyscale electron beam lithography (EBL). Reductions of the roughness by the TRILR process are quantitatively characterized by an atomic force microscope (AFM). Optical spectra taken on the PMMA surface with different processing conditions also demonstrate the significant improvement of treated surfaces. It is believed that this novel method should be applicable to all the positive tone resists which can undergo a glass transition process at high temperatures.

Nanofabrication of 80 nm asymmetric T shape gates for GaN HEMTs

This paper tackles the issue of current collapse frequently seen in AlGaN/GaN based high electron mobility transistors, caused by peak electric field on the device surface around the gate edges, which are worsened when shrinking the gate length down to sub-100nm for high frequency operation. To improve the device performance, a new configuration of T shape gates with asymmetric arms is proposed, based on our device simulation results. 3D grayscale electron beam lithography was developed to fabricate various shapes of the asymmetric T shape gates, indicating that such kind of gate configuration is technically feasible. The asymmetric T shape gates developed in this work to some extent can suppress the electric field on the gate edge efficiently as well as reduce the parasitic capacitance between the gate and the drain, leading to higher operation frequency. Applications of such a gate onto the GaN based HEMTs are hopefully to achieve high breakdown voltage for high frequency operation.

Multi-Functional CMOS Compatible Nano-Electro-Mechanical Relays for Vapor Phase Explosive Detection

Nano-electro-mechanical (NEM) relays play a very important role in ultra-low standby power and ultra-low power vapor phase gas sensor for Internet-of-Things sensor applications. In this paper, for the first time, we demonstrate a multi-functional fully-CMOS compatible titanium-based NEM relay with 1.8V actuation voltage, ~100Ω R ON , 50ns turn ON delay, and 10 9 ON/OFF current ratio in the ambient conditions. The innovative feature of this paper lies on the novel combination of grayscale electron beam lithography and a bi-layer lift-off process at room temperature (RT) to realize an NEM relay. This novel combination facilitates easy release of structures with an air gap of 25nm and a beam size of 1μm × 1.5μm. As an example, using the proposed self-purging NEM relay, a vapor phase gas sensing application is successfully demonstrated for explosives detection in an ambient air at RT.