Electron Beam Lithography Research Articles

Poly acrylic acid patterning by electron beam lithography

Poly acrylic acid (PAA) is a polymer and a derivative of acrylic acid that is a superabsorbent, being able to absorb and retain water, and swell many times beyond its original volume. This property classifies it into a group of polymers called hydrogels. Hydrogels are being investigated in emerging applications such as drug delivery, biosensors, tissue engineering, wound healing bandages, and more. The ability to lithographically pattern hydrogel materials to specific dimensions at the micro and nanoscales can be very useful in devices and sensors. Limited work has been done on characterizing PAA for lithographic purposes, and so, this work investigates the ability to pattern PAA by electron beam lithography (EBL). PAA is interesting in that its carrier solvent, developer, and remover are all water alone, which may make it attractive for processes or materials that cannot tolerate solvents, acids, and bases used with other common EBL resists. PAA behaves as a negative tone resist with a relatively low base dose of 75 μC/cm2 at 100 kV acceleration voltage. The resolution is limited to 1.8 μm due to a low contrast of 1.29. However, PAA may still have many uses at that resolution where the positioning and dimension control of a hydrogel could be useful. Furthermore, PAA can successfully be used for pattern transfer with either a metal liftoff process or a silicon plasma etch.

Photochemical Reaction Front Propagation and Control in Molecular Crystals.

4-Fluoro-9-anthracenecarboxylic acid (4F-9AC) undergoes a reversible [4 + 4] photodimerization reaction that can generate bending and twisting in microcrystals. Larger crystals resist photomechanical deformation, permitting direct visualization of the [4 + 4] photodimerization reaction dynamics under a variety of conditions. Both birefringence and fluorescence imaging show that the photodimerization reaction starts preferentially at a crystal edge and then sweeps across the crystal as a propagating reaction front. To explain these results, a theory is developed that postulates an exponentially decaying mechanical reaction field that extends from product domain into reactant domains to catalyze the photochemical reaction. Analyzing the data with this theory, we estimate a reaction field penetration distance of ∼20 nm for the 4F-9AC crystal. The propagation velocity can be controlled by varying the excitation intensity or by embedding amorphous barrier regions into the crystal using electron beam lithography. These barriers can channel the reaction flow to select areas of the crystal while other regions remain unreacted.

In Situ STEM Observation of Phase Transition Triggered by Mo Migration in MoTe2.

The heterostructure of transition metal dichalcogenides (TMDs), such as one-dimensional (1D) nanowires embedded in two-dimensional (2D) nanosheets, has drawn much research attention due to its unique electronic, spintronic, magnetic, and catalytic properties. The general approach for preparing such a heterostructure is through electron beam lithography or annealing on the 2D template, triggering direct formation of the 1D component within the 2D matrix. However, the thermodynamic mechanism behind the transition from 2D to 1D is still not well clarified. Here, by in situ scanning transmission electron microscopy (STEM), we present a direct observation of two metastable phases, M-Mo1+xTe2 and M-Mo6Te6, which enable a smooth transition from 2D 2H-MoTe2 nanosheets to 1D Mo6Te6 nanowires (NWs). As a result, an atomically sharp "NW-MoTe2-NW" heterojunction is formed, with a coherent interface between 2D 2H-MoTe2 and 1D Mo6Te6 NWs. The study provides a deep understanding of the growth of 1D Mo6Te6 NWs from 2D MoTe2 and a pathway for predictive and controlled atomic-level manipulation for the directed synthesis of the 1D/2D heterostructure.

Effects of lithographic and pattern parameters on stability of feature-edge location in electron beam lithography

As the feature size continues to decrease, a high level of accuracy in achieving the feature boundary (“edge”) as designed becomes more essential. One of the reasons for any deviation in the feature-edge location is the proximity effect due to electron scattering in the resist. However, even with a perfect proximity effect correction (PEC), the nonideal process of resist development can still lead to a deviation in the edge location. In general, a higher exposure contrast over the feature edge leads to a smaller deviation in the edge location. In this study, an analytic model is employed in understanding the effects of the lithographic and pattern parameters on the deviation in the edge location due to the uncertainty in the lithographic process. Specifically, the closed-form mathematical expression of the deviation is derived in terms of the parameters that determine the exposure contrast. The results reported in this paper should be helpful in understanding better the deviation in the edge location without time-consuming repetitive simulation and developing a PEC method that enhances the stability of the feature boundaries in the written pattern, improving the process latitude.

Comparison of Al- and Hf-based hybrid photoresists grown by molecular layer deposition for extreme ultraviolet lithography

The continued downscaling of electronic device dimensions requires the development of high-performance resist materials for advanced lithographic patterning. In this study, we examine Al-based hybrid “alucone” thin films grown by molecular layer deposition (MLD) for application to extreme ultraviolet (EUV) lithography and compare their resist properties with those of previously studied Hf-based MLD hybrid “hafnicone.” Both alucone films presented here—standard alucone and oxygen-rich alucone—are deposited using the precursors trimethylaluminum and ethylene glycol. Using electron-beam lithography as a proxy for EUV, we demonstrate that alucone behaves as a negative-tone resist capable of resolving line widths down to ∼20 nm. It is found that the sensitivity of oxygen-rich alucone is 4800 μC/cm2 using 0.125M HCl as the developer, whereas standard alucone is somewhat less sensitive. The resolution of alucone is higher than that of hafnicone, although the sensitivity is poorer. By performing x-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy, we investigate the degradation mechanism of standard alucone and compare this mechanism to that of hafnicone. We rationalize the observed differences in resist behavior between hafnicone and alucone by considering the oxophilicity of the metal atom in the respective thin films. This study provides a deeper understanding of the relationship between MLD film chemistry and irradiation responsiveness, which can help advance the optimization of resist materials for microelectronics fabrication.

Room-temperature infrared photoluminescence and broadband photodetection characteristics of Ge/GeSi islands on silicon-on-insulator

In this study, molecular beam epitaxial growth of strain-driven three-dimensional self-assembled Ge/GeSi islands on silicon-on-insulator (SOI) substrates, along with their optical and photodetection characteristics, have been demonstrated. The as-grown islands exhibit a bimodal size distribution, consisting of both Ge and GeSi alloy islands, and show significant photoluminescence (PL) emission at room temperature, specifically near optical communication wavelengths. Additionally, these samples were used to fabricate a Ge/GeSi islands/Si nanowire based phototransistor using a typical e-beam lithography process. The fabricated device exhibited broadband photoresponse characteristics, spanning a wide wavelength range (300-1600 nm) coupled with superior photodetection characteristics and relatively low dark current (∼ tens of pA). The remarkable photoresponsivity of the fabricated device, with a peak value of ∼11.4 A W-1(λ∼ 900 nm) in the near-infrared region and ∼1.36 A W-1(λ∼ 1500 nm) in the short-wave infrared (SWIR) region, is a direct result of the photoconductive gain exceeding unity. The room-temperature optical emission and outstanding photodetection performance, covering a wide spectral range from the visible to the SWIR region, showcased by the single layer of Ge/GeSi islands on SOI substrate, highlight their potential towards advanced applications in broadband infrared Si-photonics and imaging. These capabilities make them highly promising for cutting-edge applications compatible with complementary metal-oxide-semiconductor technology.

A Fabrication Method for Realizing Vertically Aligned Silicon Nanowires Featuring Precise Dimension Control

Silicon nanowires (SiNWs) have garnered considerable attention in the last few decades owing to their versatile applications. One extremely desirable aspect of fabricating SiNWs is controlling their dimensions and alignment. In addition, strict control of surface roughness or diameter modulation is another key parameter for enhanced performance in applications such as photovoltaics, thermoelectric devices, etc. This study investigates a method of fabricating silicon nanowires using electron beam lithography (EBL) and the deep reactive ion etching (DRIE) Bosch process to achieve precisely controlled fabrication. The fabricated nanowires had a pitch error within 2% of the pitch of the direct writing mask. The maximum error in the average diameter was close to 25%. The simplified two-step method with tight control of the dimensions and surface tunability presents a reliable technique to fabricate vertically aligned SiNWs for some targeted applications.

Mitigating interface damping of metal adhesion layers of nanostructures through bright-dark plasmonic mode coupling

Metal (such as Cr, Ti, etc.) adhesion layers, which are generally used to prevent nanostructures from falling off during electron beam lithography processes, will introduce interface damping, decrease the near-field enhancement, and shorten the dephasing time of localized surface plasmons (LSP). Maintaining metal adhesion layers while alleviating the induced interface damping in nanostructures is crucial for high-performance sensing, surface-enhanced Raman scattering elements, plasmon-based photocathodes, and plasmon-mediated catalysis. Here, we experimentally demonstrated that the mitigation of interface damping of metal adhesion layers can be achieved through the coupling between the bright and dark plasmonic modes of gold nanorods. We attribute the mitigation to stronger confinement across the plasmon energy, which effectively reduces the proportion of plasmon energy injected into the Cr adhesive layers. Compared to weak coupling, the non-radiative damping of plasmonic modes 1 and 2 is reduced by approximately 74% and 85%, respectively, under strong coupling conditions. The experimental results are supported by finite-difference time-domain simulations and are well explained by the calculated interaction potential for different gap sizes. This research will further benefit applications where low interface damping is required, such as the construction of low-threshold nanolasers and ultrasensitive sensing systems.

Quantitative evaluation of residual resist in electron beam lithography based on scanning electron microscopy imaging and thresholding segmentation algorithm

Electron beam lithography is a critical technology for achieving high-precision nanoscale patterning. The presence of resist residues in the structures can significantly affect subsequent processes such as etching and lift-off. However, the evaluation and optimization of resist residues currently relies on qualitative observations like scanning electron microscopy (SEM), necessitating multiple experiments to iteratively optimize exposure parameters, which is not only labor-intensive but also costly. Here, we propose a quantitative method to evaluate resist residues. By processing the obtained SEM images using a threshold segmentation algorithm, we segmented the resist structure region and the residual resist region in the images. The grayscale values of these two regions are identified, and the residues are quantified based on the ratio of these values. Furthermore, a relationship curve between the residue amount and the exposure dose is plotted to predict the optimal exposure dose. To validate this method, we fabricated hydrogen silsesquioxane annular grating structures with 30 nm linewidth and analyzed the residue levels over an exposure dose range of 2000-2500μC cm-2, predicting the optimal dose to be 1800μC cm-2and confirming this through experiments. Additionally, we applied the method to polymethyl methacrylate and ZEP-520A structures, achieving similarly accurate results, further confirming the method's general applicability. This method has the potential to reduce experimental costs and improve yield and production efficiency in nano fabrication.

Research on Chip Manufacturing and Fault Detection Technology

In the wave of the digital era, the chip plays a key role in promoting the whole information technology revolution with its core position in the development of world science and technology. As the cornerstone of computing and data processing, the development of chips has promoted the progress of the entire technology field. The chip’s manufacturing is the key to determining the performance and computing power of the whole chip, and fault detection technology in the production process is an important part of determining the product’s quality and cost. Therefore, under the background of Moore’s law approaching the physical limit, how to ensure the yield of chip production while continuing to improve chip performance has become a major challenge for the chip manufacturing industry. Based on this, this paper summarizes the key manufacturing steps of the chip. The chip manufacturing technology is comprehensively sorted from design to silicon wafer manufacturing, photolithography, etching, ion implantation, and other processes. At the same time, the research direction of monocrystalline silicon manufacturing technology and the existing lithography technology are introduced, including optical lithography, extreme ultraviolet lithography, electron beam lithography, and so on. Fault detection technology in chip production has also been introduced. In addition, the chip manufacturing and fault detection technology introduced in this paper is summarized and its future development is prospected, which provides reference and inspiration for promoting the continuous progress of chip technology.

Application of Graphene in Acoustoelectronics.

An interdigital transducer structure was fabricated from multilayer graphene on the surface of the YZ-cut of a LiNbO3 ferroelectric crystal. The multilayer graphene was prepared by CVD method and transferred onto the surface of the LiNbO3 substrate. The properties of the multilayer graphene film were studied by Raman spectroscopy. A multilayer graphene (MLG) interdigital transducer (IDT) structure for surface acoustic wave (SAW) excitation with a wavelength of Λ=60 μm was fabricated on the surface of the LiNbO3 crystal using electron beam lithography (EBL) and plasma chemical etching. The amplitude-frequency response of the SAW delay time line was measured. The process of SAW excitation by graphene IDT was visualized by scanning electron microscopy. It was demonstrated that the increase in the SAW velocity using graphene was related to the minimization of the IDT mass.

Xenon plasma-focused ion beam milling for fabrication of high-purity, bright single-photon sources operating in the C-band

Electron beam lithography is a standard method for fabricating photonic micro and nanostructures around semiconductor quantum dots (QDs), which are crucial for efficient single and indistinguishable photon sources in quantum information processing. However, this technique is difficult for direct 3D control of the structure shape, complicating the design and enlarging the 2D footprint to suppress in-plane photon leakage while directing photons into the collecting lens aperture. Here, we present an alternative approach to employ xenon plasma-focused ion beam (Xe-PFIB) technology as a reliable method for the 3D shaping of photonic structures containing low-density self-assembled InAs/InP quantum dots emitting in the C-band range of the 3rd telecommunication window. The method is optimized to minimize the possible ion-beam-induced material degradation, which allows exploration of both non-deterministic and deterministic fabrication approaches, resulting in photonic structures naturally shaped as truncated cones. As a demonstration, we fabricate mesas using a heterogeneously integrated structure with a QD membrane atop an aluminum mirror and silicon substrate. Finite-difference time-domain simulations show that the angled sidewalls significantly increase the emission collection efficiency to approx. 0.9 for NA = 0.65. We demonstrate experimentally a high purity of pulsed single-photon emission (∼99%) and a superior extraction efficiency value reported in the C-band of η = 24 ± 4%.

High-efficiency and easy-processing thin-film lithium niobate edge coupler

Fiber-to-chip coupling with ultralow loss and broadband operation wavelength range is essential in the practical applications of thin-film lithium niobate (TFLN) integrated photonic devices. However, the existing edge couplers often require electron beam lithography overlaying and multiple etching processes, which are expensive and complex. In this Letter, we demonstrate an edge coupler that includes only a vertically tapered TFLN waveguide and silicon dioxide cladding. The coupling efficiency between a lensed fiber and an on-chip LN waveguide is down to 1.43 dB/facet, while the 3-dB bandwidth exceeds the range from 1510 to 1630 nm. These edge couplers also show reliable fiber misalignment tolerance. Furthermore, the fabrication complexity is greatly reduced since only a single etching step for the TFLN waveguide is needed. The minimum waveguide width of 1.3 μm guarantees compatibility with i-line photolithography, providing a potential for massive production of TFLN devices.

Resistive Switching Characteristics of NiO Thin Films Influenced by Changes in the Diameter of Nanometer-Scale Top Electrodes.

We investigated the forming, set, and reset voltages affected by the area of the top electrodes of a resistive random access memory (RRAM) capacitor fabricated by epitaxial NiO thin films. NiO RRAM capacitors with Au top electrode with a diameter of 100 μm showed typical unipolar switching characteristics. Au top electrodes with diameters of 10, 20, and 30 nm were formed on the surface of epitaxial NiO thin films by e-beam lithography. The forming, set, and reset voltages tended to decrease as the diameter of NiO RRAM capacitors with nanosized Au top electrodes decreased. As the area of the Au top electrodes increases, the volume of space in which the conductive filaments formed within the NiO RRAM capacitors can be formed becomes larger. This causes the conductive filament to be formed at relatively low energy, giving low forming voltages, while increasing the diversity of paths, causing a wide forming voltage distribution.

Synthesis and electrical transport properties of superconducting platinum silicide thin films and devices

We evaluate the material characteristics of superconducting platinum silicide (PtSi) thin films as candidate materials for superconducting quantum information devices compatible with silicon technology. These films were synthesized using magnetron sputtering under ultrahigh vacuum conditions, followed by rapid thermal annealing. Polycrystalline PtSi films synthesized by this method have the favorable properties of superconducting critical temperature of 0.95 K and relatively long zero-temperature Ginzburg-Landau coherence length of 76 nm. We further studied coplanar microbridge devices fabricated by electron beam lithography and chlorine-free reactive ion etching, finding that the temperature-dependent critical current density follows the Ginzburg Landau depairing mechanism.

Reliable fabrication of 3D freestanding nanostructures via all dry stacking of incompatible photoresist

Three-dimensional (3D) free-standing nanostructures based on electron-beam lithography (EBL) have potential applications in many fields with extremely high patterning resolution and design flexibility with direct writing. In numerous EBL processes designed for the creation of 3D structures, the multilayer resist system is pivotal due to its adaptability in design. Nevertheless, the compatibility of solvents between different layers of resists often restricts the variety of feasible multilayer combinations. This paper introduces an innovative approach to address the bottleneck issue by presenting a novel concept of multilayer resist dry stacking, which is facilitated by a near-zero adhesion strategy. The poly(methyl methacrylate) (PMMA) film is stacked onto the hydrogen silsesquioxane (HSQ) resist using a dry peel and release technique, effectively circumventing the issue of HSQ solubilization by PMMA solvents typically encountered during conventional spin-coating procedures. Simultaneously, a dry lift-off technique can be implemented by eschewing the use of organic solvents during the wet process. This pioneering method enables the fabrication of high-resolution 3D free-standing plasmonic nanostructures and intricate 3D free-standing nanostructures. Finally, this study presents a compelling proof of concept, showcasing the integration of 3D free-standing nanostructures, fabricated via the described technique, into the realm of Fabry-Perot cavity resonators, thereby highlighting their potential for practical applications. This approach is a promising candidate for arbitrary 3D free-standing nanostructure fabrication, which has potential applications in nanoplasmonics, nanoelectronics, and nanophotonics.

Constant period line gratings as a metric for patterning fidelity in electron beam lithography

As smaller critical dimensions of devices fabricated via electron-beam lithography (EBL) are achieved over large areas, the need for new metrology techniques follows. Large (cm × cm) substrates have traditionally been both time and labor intensive to measure using traditional techniques such as scanning electron microscopy (SEM) or atomic force microscopy. When optimizing an EBL process over large areas, stitch error must be eliminated to maximize feature placement fidelity. Simultaneously, traditional EBL requirements, such as low line-edge roughness, proximity effect correction, and appropriate write times, must be maintained. With this plethora of requirements, we propose a technique to characterize placement errors over large areas using interferometric measurements. This method, when combined with traditional techniques like SEM and optical microscopy, assesses the full domain of potential errors over large areas in a time- and cost-efficient manner. As a proof of concept, a set of five small format (5 × 10 mm2) gratings with an 855 nm period were written twice, each set produced under two different tool error conditions. We report on the efficacy of interferometric metrology to accurately assess feature placement errors and report measured groove displacement across all ten gratings.

Fabrication of thick Cr masks for reactive ion substrate etching by electron beam lithography and lift-off techniques

Fabrication of tall Cr nanostructures of different shapes by lithography and lift-off processes is demonstrated. By varying resist thickness, metal layer thickness, and diameter of holes in the resist mask, it is demonstrated that metal structures with shapes ranging from sharp-tipped conical over flat-top cones to nearly cylindrical can be fabricated. A comparison of resist layer dissolution in acetone, covered by Ag and Cr films, reveals that Cr films grow with an open structure of particles that allow rapid solvent diffusion through Cr layers that are several hundred nanometers thick. On the other hand, the 2D growth of Ag on the resist forms a barrier against acetone diffusion. The open structure of Cr enables the lift-off process to fabricate several-μm-high nanostructures using a single resist layer. As an example, high-aspect-ratio Si structures are demonstrated by reactive ion etching using thick Cr layers as a mask, fabricating nanopillars with 3 μm height at room temperature.

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.

Efficient Calculation of Charging Effects in Electron Beam Lithography Using the SA-AMG

Efficient Calculation of Charging Effects in Electron Beam Lithography Using the SA-AMG