Atomic Lithography Research Articles

A Study on Length Traceability and Diffraction Efficiency of Chromium Gratings

Measurement traceability is a prerequisite for achieving accurate and reliable results as well as technical standardization. The period of Chromium (Cr) gratings fabricated by atomic lithography can be directly traced back to natural constants. Applying the Cr grating to grating interferometry can achieve nanometer measurement traceability. This research aims to analyze the diffraction efficiency characteristics of self-traceable Cr gratings to provide a theoretical basis for the fabrication and application of Cr gratings. In this regard, we establish the theoretical model of the laser beam incident angle and grating diffraction efficiency using the rigorous coupled-wave method. Then, we analyze the influence of the laser beam incident angle on grating diffraction efficiency by simulation, verify the accuracy of the theoretical model, and finally build a measurement system for grating diffraction efficiency. Through experiments, we find that the diffraction efficiency of the grating shows a rapid increase to reach a stable maximum value followed by a decrease, when a laser beam with a wavelength of 405 nm is incident on the surface of a self-traceable grating in Transverse Magnetic (TM) polarization and the incident angle changes within an effective range. The experimental results are consistent with the trend of theoretical calculation results.

Multi-scale alignment to buried atom-scale devices using Kelvin probe force microscopy

Abstract Fabrication of quantum devices by atomic-scale patterning with scanning tunneling microscopy (STM) has led to the development of single/few atom transistors, few-donor/quantum dot devices for spin manipulation, and arrayed few-donor devices for analog quantum simulation. We have developed atomic precision lithography, dopant incorporation, device encapsulation, ex situ device re-location, and contact processes to enable high-yield device fabrication. In this work, we describe a multiscale alignment strategy using Kelvin probe force microscopy to enable the alignment of buried device components to electronic support structures such as source/drain leads, in-plane and top gates, and waveguides while preserving flexibility in the placement of fabricated STM patterns. The required spatial accuracy to bridge the sub-micrometer scale central region of the device to millimeter scale large wire-bond pads is achieved through a multi-step alignment process at various stages of fabrication, including atom-scale device fabrication using STM, re-location and registration, and electron beam lithography for contact leads and pads. This alignment strategy allows imaging small device regions as well as large-scale fiducial marks, thereby bridging the gap from nanometer STM patterns to the millimeter-scale electrical contact fabrication with a 95% yield on more than 150 devices fabricated to date.

Simulation research on surface growth process of positive and negative frequency detuning chromium atom lithographic gratings

Chromium atom photolithography gratings are a promising technology for the development of nanoscale length standard substances due to their high accuracy, uniformity, and consistency. However, the inherent difference between the interaction of positive and negative frequency detuning standing wave field and the atoms can cause a difference in the adjacent peak-to-valley heights of the grating in positive and negative frequency detuning chromium atom lithography, which greatly reduces its accuracy. In this study, we performed a controlled variable growth simulation using the semi-classical theoretical model and Monte Carlo method with trajectory tracking and ballistic deposition methods to investigate the influence of key experimental parameters on the surface growth process of positive and negative frequency detuning atomic lithography gratings. We established a theoretical model based on simulation results and summarized empirical equations to guide the selection of experimental parameters. Our simulations achieved uniform positive and negative frequency detuning atomic lithography gratings with a period of 1/4 of the wavelength corresponding to the atomic transition frequency, and adjacent peak-to-valley heights differing by no more than 2 nm, providing an important theoretical reference for the controllable fabrication of these gratings.

Terahertz virus-sized gold nanogap sensor

Abstract We demonstrated an ultra-sensitive terahertz virus detection method combined with virus-sized gold nanogaps filled with Al2O3. Large-area high-density 20 nm-gap rectangular loop structures, containing a resonant frequency in the terahertz range, were fabricated on a 4-inch wafer using atomic layer lithography. When target viruses with a 60 nm diameter were located on the nanogaps, we observed a significant redshift of the resonant peak already with an average number of about 100 viruses per unit loop due to the strong field confinement and enhancement near the gap. Furthermore, when the virus was tightly attached to an etched gap like a bridge connecting metals, its sensitivity is doubled compared to the unetched gap, which resulted in 400% more resonance frequency shift per single virus particle than our previous work. Full-wave simulations and theoretical calculations based on modal expansions were in good agreement with the experiments, revealing that the resonant transmission spectrum was mostly determined by the change in refractive index in a two-dimensional-like optical hotspot near the nanogap. A further step could be taken to increase sensitivity by tuning nanogap-loops to the absorption frequencies associated with the intermolecular vibrational modes of the viruses and fingerprinting them as well.

Correlated nanoelectronics and the second quantum revolution

The growing field of correlated nanoelectronics exists at the intersection of two established fields: correlated oxide electronics and semiconductor nanoelectronics. The development of quantum technologies that exploit quantum coherence and entanglement for the purposes of computation, simulation, and sensing will require complex material properties to be controlled at nanoscale dimensions. Heterostructures and nanostructures formed at the interface between LaAlO3 and SrTiO3 exhibit striking behavior that arises from the ability to program the conductive behavior at extreme nanoscale dimensions. The active electronic layer, SrTiO3, exhibits a wide range of gate-tunable phenomena such as ferroelectricity, ferroelasticity, magnetism, superconductivity, and spin–orbit coupling, all of which can be controlled at the nanoscale using two reversible methods: conductive atomic force microscope lithography and ultra-low-voltage electron beam lithography. Mesoscopic devices such as single-electron transistors and quasi-one-dimensional electron waveguides can be “sketched” using these techniques, and the properties of these devices differ significantly from those created from traditional semiconductors, such as Si or GaAs. The strongly correlated nature of the SrTiO3 system is evident from superconducting behavior as well as a state in which electrons are paired outside the superconducting state. A highly exotic phase was discovered in which a degenerate quantum liquid is formed from bound states of n = 2, 3, 4, … electrons. Further development of correlated nanoelectronics based on the LaAlO3/SrTiO3 system can potentially lead to a general platform for quantum simulation as well as a pathway for the development of highly entangled states of multiple photons.

Local ultra-densification of single-walled carbon nanotube films: Experiment and mesoscopic modeling

Fabrication of nanostructured metasurfaces poses a significant technological and fundamental challenge. Despite developing novel material systems that support reversible elongation and distortion, their nanoscale patterning and control of optical properties remain an open problem. Herein we report the atomic force microscope lithography application for nanoscale patterning of single-walled carbon nanotube films and the associated optical reflection coefficient tuning. Large scale mesoscopic distinct element method atomic force nanoindentation simulations of single-walled carbon nanotube samples comprising entangled dendritic nanotube bundles with branches extending down to individual tubes explain the mesoscale mechanism of local irreversible densification and further predict its impact on mechanical properties. All observed and calculated phenomena support each other and present a platform for developing patterned optical devices using nanofibrous matter.

Interaction between Graphene Nanoribbon and an Array of QDs: Introducing Nano Grating

In this work, the interaction between an array of QDs and Graphene nanoribbon is modeled using dipole–dipole interaction. Then, based on the presented model, we study the linear optical properties of the considered system and find that by changing the size, number, and type of quantum dots as well as how they are arranged, the optical properties can be controlled and the controllable grating plasmonic waveguides can be implemented. Therefore, we introduce different structures, compare them together and find that each of them can be useful based on their application in optical integrated circuits. The quantum dot arrays are located on a graphene nanoribbon with dimensions of 775 × 40 nm2. Applying electromagnetic waves with a wavelength of 1.55 µm causes polarization in the quantum dots and induces surface polarization on graphene. It is shown that, considering the large radius of the quantum dot, the induced polarization is increased, and ultimately the interaction with other quantum dots and graphene nanoribbon is stronger. Similarly, the distance between quantum dots and the number of QDs on Graphene nanoribbon are basic factors that affect the interaction between QDs and nanoribbon. Due to the polarization effect of these elements between each other, we see the creation of the effective grating refractive index in the plasmonic waveguide. This has many applications in quantum optical integrated circuits, nano-scale atomic lithography for nano-scale production, the adjustment coupling coefficient between waveguides, and the implementation of optical gates, reflectors, detectors, modulators, and others.

Nanoindentation-enhanced tip-enhanced Raman spectroscopy

We combine nanoindentation, herein achieved using atomic force microscopy-based pulsed-force lithography, with tip-enhanced Raman spectroscopy (TERS) and imaging. Our approach entails indentation and multimodal characterization of otherwise flat Au substrates, followed by chemical functionalization and TERS spectral imaging of the indented nanostructures. We find that the resulting structures, which vary in shape and size depending on the tip used to produce them, may sustain nano-confined and significantly enhanced local fields. We take advantage of the latter and illustrate TERS-based ultrasensitive detection/chemical fingerprinting as well as chemical reaction imaging-all using a single platform for nano-lithography, topographic imaging, hyperspectral dark field optical microscopy, and TERS.

A new type of nanoscale reference grating manufactured by combined laser-focused atomic deposition and x-ray interference lithography and its use for calibrating a scanning electron microscope

Calibration of magnification and nonlinearity of scanning electron microscopy (SEM) is an essential task. In this paper, we proposed a new type of 1D grating sample fabricated by combining laser-focused atomic deposition and x-ray interference lithography as a lateral standard for calibrating SEMs. The calibrations of the grating pattern by a metrological large-range atomic force microscope indicate that the grating sample exhibits outstanding pattern uniformity that surpasses conventional samples fabricated by e-beam lithography: (1) the nonlinear deviation of the grating structures is below +/- 0.5 nm over a measurement range of 5 µm; (2) the maximal variation of the calibrated mean pitch values is lower than 0.01 nm at different locations randomly selected all over the pattern area. The proposed new sample is applied for accurately calibrating the magnification and nonlinearity of a commercial SEM, showing its advantages of easy-of-use and high accuracy. The influence of the defocus level of SEM on the calibration result is also demonstrated. This research offers a feasible solution for highly accurate SEM calibration needed for 3D nanometrology and hybrid metrology demanded in metrology of modern nanoelectronics devices and systems.

Ultraflat Sub-10 Nanometer Gap Electrodes for Two-Dimensional Optoelectronic Devices.

Two-dimensional (2D) materials are promising candidates for building ultrashort-channel devices because their thickness can be reduced down to a single atomic layer. Here, we demonstrate an ultraflat nanogap platform based on atomic layer deposition (ALD) and utilize the structure to fabricate 2D material-based optical and electronic devices. In our method, ultraflat metal surfaces, template-stripped from a Si wafer mold, are separated by an Al2O3 ALD layer down to a gap width of 10 nm. Surfaces of both electrodes are vertically aligned without a height difference, and each electrode is ultraflat with a measured root-mean-square roughness as low as 0.315 nm, smaller than the thickness of monolayer graphene. Simply by placing 2D material flakes on top of the platform, short-channel field-effect transistors based on black phosphorus and MoS2 are fabricated, exhibiting their typical transistor characteristics. Furthermore, we use the same platform to demonstrate photodetectors with a nanoscale photosensitive channel, exhibiting higher photosensitivity compared to microscale gap channels. Our wafer-scale atomic layer lithography method can benefit a diverse range of 2D optical and electronic applications.

Engineering ferromagnetic lines in graphene by local oxidation and hydrogenation using nanoscale lithography

Graphene-based spintronics has attracted much attention owing to the weak spin–orbit interaction and long spin relaxation length in graphene. For implementation of practical and high-density graphene-based spintronic devices, we need to define nanoscale areas with room-temperature ferromagnetism on graphene. Here we report the room-temperature ferromagnetism observed in nanoscale functionalized (oxidized and hydrogenated) graphene using atomic force microscope lithography without involving potential surface contamination and chemical agents. By performing magnetic force microscope (MFM) measurements, we can clearly distinguish the local ferromagnetic signal of selectively functionalized graphene from that of surrounding non-magnetic pristine graphene. The nanoscale functionalized graphene shows experimental evidence of room-temperature ferromagnetism: (1) larger MFM signal than that of graphene; (2) repulsive and attractive interaction with an MFM tip the magnetization of which points into and out of the graphene, respectively; and (3) MFM signal reversal after applying a high magnetic field at an elevated temperature of 400 K. Our first-principles calculations reveal that unpaired spins are present at non-passivated dangling bonds of vacancies on functionalized graphene and the stable ferromagnetic exchange interactions between them are favored. Therefore, nanoscale functionalized graphene is a good candidate for use as the spin injector or detector of high-density graphene-based spintronic devices.

A Chip-Scale, Low Cost PVD System

Standard physical vapor deposition systems are large, expensive, and slow. As part of an on-going effort to build a fab-on-a-chip, we have developed a chip-scale, low cost, fast physical vapor deposition system designed to be used with atomic calligraphy or dynamic stencil lithography to direct write nanostructures. The system comprises two MEMS devices: a chip-scale thermal evaporator and a mass sensor that serves as a film thickness monitor. Here, we demonstrate the functionality of both devices by depositing Pb thin-films. The thermal evaporator was made by fabless manufacturing using the SOIMUMPs processs (MEMSCAP, inc). It turns on in 1:46 s and reaches deposition rates as high as 7.2Å s <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> with ~1 mm separation from the target. The mass sensor is a re-purposed quartz oscillator (JTX210) that is commercially available for less than one dollar. Its resolution was measured to be 2.65 fg or 7.79E-5 monolayers of Pb.

Deep learning-guided surface characterization for autonomous hydrogen lithography

As the development of atom scale devices transitions from novel, proof-of-concept demonstrations to state-of-the-art commercial applications, automated assembly of such devices must be implemented. Here we present an automation method for the identification of defects prior to atomic fabrication via hydrogen lithography using deep learning. We trained a convolutional neural network to locate and differentiate between surface features of the technologically relevant hydrogen-terminated silicon surface imaged using a scanning tunneling microscope. Once the positions and types of surface features are determined, the predefined atomic structures are patterned in a defect-free area. By training the network to differentiate between common defects we are able to avoid charged defects as well as edges of the patterning terraces. Augmentation with previously developed autonomous tip shaping and patterning modules allows for atomic scale lithography with minimal user intervention.

Fabrication of SiNWs-FET Nanostructure Via Atomic Force Microscopy Lithography

The unique electrical properties of silicon nanowires (SiNWs) is one of the reasons it become an attractive transducer for biosensor nowadays. Positive (holes) and negative (electron) charge carriers from SiNWs can simply interact with either positive or negative charge of sensing target. In this paper, we have studied the fabrication of silicon nanowires field effect transistor (SiNWs-FET) nanostructure patterned on 15 Ω resistivity of p-type silicon on insulator (SOI) wafer fabricated via atomic force microscopy lithography technique. To fabricate SiNWs-FET nanostructure, a conductive AFM tip, Cr/Pt cantilever tip, was used then various value of applied voltage, writing speed and relative humidity were studied. Subsequent, followed by wet etching processes, admixture of tetramethylammonium hydroxide (TMAH) and isopropyl alcohol (IPA) were used to remove the undesired of silicon layer and diluted hydrofluoric acid (HF) was used to remove the oxide layer. From the results, it shows that, cantilever tip at 9 V with 0.4 μm/s writing speed and relative humidity between 55% - 60% gives the best formation of silicon oxide to fabricate SiNWs-FET nanostructure.

Nanopatterning of Cu-Ligated Mercaptoalkanoic Acid Multilayers on Si Substrates via Atomic Force Lithography

Chemical self-assembly has garnered tremendous interest as a tool for generating nanometer-scale structures and devices. Organosilane self-assembled monolayers (SAMs) are of particular interest because of their ability to assemble on a wide range of substrates with varied chemical functionalities. Nanoshaving, an atomic force lithographic technique, has been demonstrated as a method to generate nanopatterns of organosilane SAMs. However, this method requires extremely high force setpoints, which rapidly dulls atomic force microscopy tips and degrades the resolution of the resulting nanopattern. In this work, we utilize Cu-ligated mercaptohexadecanoic acid (MHDA) multilayers to circumvent this limitation. Initially, a 10-undecenyltrichlorosilane (UTS) SAM is assembled onto a Si substrate, and the terminal olefin groups of the UTS SAM are oxidized to carboxyl groups. Subsequently, a Cu-ligated MHDA multilayer is assembled via the sequential deposition of Cu2+ ions and MHDA molecules. The interface between the oxidized UTS SAM and Cu-ligated MHDA multilayer serves as a natural low force breakpoint for nanoshaving. We demonstrate that the resulting nanopatterns can function as a chemical resist to fabricate metal nanostructures.

Modeling and observation of mid-infrared nonlocality in effective epsilon-near-zero ultranarrow coaxial apertures

With advances in nanofabrication techniques, extreme-scale nanophotonic devices with critical gap dimensions of just 1–2 nm have been realized. Plasmons in such ultranarrow gaps can exhibit nonlocal response, which was previously shown to limit the field enhancement and cause optical properties to deviate from the local description. Using atomic layer lithography, we create mid-infrared-resonant coaxial apertures with gap sizes as small as 1 nm and observe strong evidence of nonlocality, including spectral shifts and boosted transmittance of the cutoff epsilon-near-zero mode. Experiments are supported by full-wave 3-D nonlocal simulations performed with the hybridizable discontinuous Galerkin method. This numerical method captures atomic-scale variations of the electromagnetic fields while efficiently handling extreme-scale size mismatch. Combining atomic-layer-based fabrication techniques with fast and accurate numerical simulations provides practical routes to design and fabricate highly-efficient large-area mid-infrared sensors, antennas, and metasurfaces.

Over 100-THz bandwidth selective difference frequency generation at LaAlO3/SrTiO3 nanojunctions

The ability to combine continuously tunable narrow-band terahertz (THz) generation that can access both the far-infrared and mid-infrared regimes with nanometer-scale spatial resolution is highly promising for identifying underlying light-matter interactions and realizing selective control of rotational or vibrational resonances in nanoparticles or molecules. Here, we report selective difference frequency generation with over 100 THz bandwidth via femtosecond optical pulse shaping. The THz emission is generated at nanoscale junctions at the interface of LaAlO3/SrTiO3 (LAO/STO) that is defined by conductive atomic force microscope lithography, with the potential to perform THz spectroscopy on individual nanoparticles or molecules. Numerical simulation of the time-domain signal facilitates the identification of components that contribute to the THz generation. This ultra-wide-bandwidth tunable nanoscale coherent THz source transforms the LAO/STO interface into a promising platform for integrated lab-on-chip optoelectronic devices with various functionalities.

Integrated Nanogap Platform for Sub-Volt Dielectrophoretic Trapping and Real-Time Raman Imaging of Biological Nanoparticles.

A rapid, label-free, and broadly applicable chemical analysis platform for nanovesicles and subcellular components is highly desirable for diagnostic assays. We demonstrate an integrated nanogap plasmonic sensing platform that combines subvolt dielectrophoresis (DEP) trapping, gold nanoparticles (AuNPs), and a lineated illumination scheme for real-time, surface-enhanced Raman spectroscopy (SERS) imaging of biological nanoparticles. Our system is capable of isolating suspended sub-100 nm vesicles and imaging the Raman spectra of their cargo within seconds, 100times faster than conventional point-scan Raman systems. Bare AuNPs are spiked into solution and simultaneously trapped with the nanovesicles along the gap to boost local optical fields. In addition, our platform offers simultaneous and delay-free spatial and temporal multiplexing functionality. These nanogap devices can be mass-produced via atomic layer lithography and provide a practical platform for high-speed SERS analysis of biological nanoparticles.

Addressable electron spin resonance using donors and donor molecules in silicon.

Phosphorus donor impurities in silicon are a promising candidate for solid-state quantum computing due to their exceptionally long coherence times and high fidelities. However, individual addressability of exchange coupled donors with separations ~15 nm is challenging. We show that by using atomic precision lithography, we can place a single P donor next to a 2P molecule 16 ± 1 nm apart and use their distinctive hyperfine coupling strengths to address qubits at vastly different resonance frequencies. In particular, the single donor yields two hyperfine peaks separated by 97 ± 2.5 MHz, in contrast to the donor molecule that exhibits three peaks separated by 262 ± 10 MHz. Atomistic tight-binding simulations confirm the large hyperfine interaction strength in the 2P molecule with an interdonor separation of ~0.7 nm, consistent with lithographic scanning tunneling microscopy images of the 2P site during device fabrication. We discuss the viability of using donor molecules for built-in addressability of electron spin qubits in silicon.

Low-Power Optical Trapping of Nanoparticles and Proteins with Resonant Coaxial Nanoaperture Using 10 nm Gap.

We present optical trapping with a 10 nm gap resonant coaxial nanoaperture in a gold film. Large arrays of 600 resonant plasmonic coaxial nanoaperture traps are produced on a single chip via atomic layer lithography with each aperture tuned to match a 785 nm laser source. We show that these single coaxial apertures can act as efficient nanotweezers with a sharp potential well, capable of trapping 30 nm polystyrene nanoparticles and streptavidin molecules with a laser power as low as 4.7 mW. Furthermore, the resonant coaxial nanoaperture enables real-time label-free detection of the trapping events via simple transmission measurements. Our fabrication technique is scalable and reproducible, since the critical nanogap dimension is defined by atomic layer deposition. Thus our platform shows significant potential to push the limit of optical trapping technologies.