Atom Lithography Research Articles

Self-traceable angle standards with simplified traceability chain for dimensional metrology

Traceability is a fundamental issue for ensuring accuracy of nanometrology. A shortened traceability chain is advantageous for reducing calibration steps, thus reducing errors and raising application efficiency. A novel kind of two-dimensional grating is manufactured by atom lithography, whose pitch and (orthogonal) angle values are directly determined by natural constants, offering a feasible approach for effectively shortening the traceability chain. The PTB’s calibration results show that the two-dimensional grating has excellent orthogonality with a deviation of only 0.001°. The application of two-dimensional grating is demonstrated for the characterization of the angular distortion of a SEM, showing its great application potential.

Grating pitch comparator traceable to the Cr atom transition frequency

Calibration of the grating pitch typically requires metrological instruments or diffraction techniques. The traceability and stability demands for laser wavelength in these techniques present substantial challenges for on-site grating calibration. This paper introduces a novel traceability approach to grating pitch calibration using a grating pitch comparator that utilizes the transition frequency of the chromium. The chromium transition frequency is converted into a chromium grating through atom lithography, establishing a self-traceable length standard dCr = 212.7787 ± 0.0049 nm. Calibration of the test grating’s pitch is achieved by comparing the phase shift caused by the simultaneous movement of the chromium grating and the test grating. By using the dCr, this approach combines the high-accuracy of fundamental physical constants with the environmental stability of grating interferometers, eliminating the need for measuring laser wavelength, air refraction, diffraction angles. It significantly enhances the precision and makes more straightforward and portable, crucial for facilitating traceable on-site grating measurements.

Shortened and simplified traceability chain for dimensional metrology based on self-traceable standards

Abstract Nanoscale measurement is an essential task of nanomanufacturing, and measurement traceability is a fundamental aspect of nanoscale measurement. High-precision nanoscale measurement instruments (e.g. atomic force microscopes (AFM) and scanning electron microscopes (SEM)) need to be calibrated by traceable standards to ensure their accuracy and reliability. However, due to the suboptimal accuracy, uniformity, and consistency of existing standards, they need to be calibrated by metrological instruments traceable to primary length standards (e.g. physical wavelength standards) before use. This results in a long traceability chain that leads to error accumulation and significantly reduces calibration efficiency. This paper proposes a novel shortened and simplified traceability chain, where the physical wavelength standard corresponding to the 7S3 → 7P4° transition frequency of chromium atoms is materialized into self-traceable gratings using the atom lithography technology. The self-traceable gratings can then be directly applied for calibrating measurement instruments. To verify this approach, the self-traceable gratings are calibrated using a metrological AFM of the Physikalisch-Technische Bundesanstalt. Measurement results confirmed the feasibility of the approach. Particularly, our results show that the self-traceable gratings have excellent uniformity over different measurement areas and consistency over different samples, both at 0.001 nm level. Finally, the application of the self-traceable gratings for the calibrations of a commercial AFM and SEM is demonstrated. The new traceability chain significantly simplifies the calibration process, providing a more reliable and higher efficient calibration approach for advanced nanomanufacturing than that of the state-of-the-art.

Linewidth characterization of a self-traceable grating by SEM

To achieve high-precision nanometrology, a self-traceable grating reference material has been reported and prepared using atom lithography and soft x-ray interference techniques (Liu et al 2021 Nanotechnology 32 175 301). In this work, we employ a Monte Carlo simulation method to investigate the scanning electron microscopy (SEM) image contrast and linewidth characterization of the grating linewidth. The 3D structure of mushroom-shaped grating lines made of multilayers (Pt, SiO2 and Si) is modeled according to transmission electron microscopy (TEM) images, enabling the SEM linescan profiles of secondary electron signals to be obtained for different values of structural linewidth parameters from Monte Carlo simulations. Using the principle of the model-based library method, a model database of Monte Carlo-simulated SEM linescan profiles is thus constructed by varying the incident electron beam conditions and the grating linewidths; then, the grating linewidth is successfully characterized using experimental SEM images. The comparison with the TEM measurement reveals that the measurement accuracy is verified to within 0.3% for the linewidth of ∼25 nm.

Optimization and fabrication of chromium grating in self-traceable interferometer

Metrological methods rooted in natural constants present a robust strategy for ensuring traceability in nanometric measurements. A Chromium (Cr) grating, meticulously constructed through atom lithography and possessing a pitch of d = 212.7705 ± 0.0049 nm intrinsically traceable to the Cr transition frequency 7S3→7P40, exhibits promising potential as a self-traceable length standard in nano-length metrology via a grating interferometer. The objective of this research is to elucidate and optimize the diffraction characteristics that augment the Cr grating's functionality as a self-traceable length standard within the length traceability chain predicated on the Cr transition frequency. To this end, we explore the geometric morphology and diffraction characteristics of the Cr grating. This investigation also dissects the influence of the grating's polarization-sensitive traits on the Littrow configuration grating interferometer and establishes the criteria for Cr grating fabrication. Empirically, we fabricate an enlarged Cr grating through scanning atom lithography, delineate its diffraction performance, and perform an initial verification of length measurement in a self-traceable grating interferometer. This investigation aligns with the global progression towards a more streamlined metrology development, offering a valuable benchmark for propelling future advancements in metrological technologies within the new traceability chain.

Tunable atom lithography scheme with high resolution in atomic system

We theoretically propose a scheme that utilizes the excited state of a Λ-type three-level atom system for atom lithography. A standing-wave coupling field and a probe field provide localized probability distribution of the excited atoms. Unlike the previous studies in atom localization, we focus on splitting the single-peak structure localized on the node of the standing-wave field to a double-peak structure. The shift distance of double-peak related to the parameters of external laser fields is investigated. The period of lithography pattern could reach λ/4, while the resolution of lithography is promising to be smaller than 0.02λ by adjusting system parameters. This scheme can be extended to form a two-dimensional pattern for lithography.

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.

Realistic mask generation for matter-wave lithography via machine learning

Fast production of large-area patterns is crucial for the established semiconductor industry and enables industrial-scale production of next-generation quantum devices. Metastable atom lithography with binary holography masks has been suggested as a higher resolution/low-cost alternative to the current state of the art: extreme ultraviolet lithography. However, it was recently shown that the interaction of the metastable atoms with the mask material (SiN) leads to a strong perturbation of the wavefront, not included in the existing mask generation theory, which is based on classical scalar waves. This means that the inverse problem (creating a mask based on the desired pattern) cannot be solved analytically, even in 1D. Here we present a machine-learning approach to mask generation targeted for metastable atoms. Our algorithm uses a combination of genetic optimisation and deep learning to obtain the mask. A novel deep neural architecture is trained to produce an initial approximation of the mask. This approximation is then used to generate the initial population of the genetic optimisation algorithm that can converge to arbitrary precision. We demonstrate the generation of arbitrary 1D patterns for system dimensions within the Fraunhofer approximation limit.

Ultraprecise Off-Axis Atom Localization With Hybrid Fields

Atom localization enables a high-precision imaging of the atomic position, which has provided vast applications in fundamental and applied science. In the present work, we propose a scheme for realizing two-dimensional off-axis atom localization in a three-level Λ system. Benefiting from the use of a hybrid coupling field, which consists of one Gaussian beam and one Laguerre–Gaussian beam, our scheme shows that the atoms can be localized at arbitrary position with high spatial resolution. Considering realistic experimental parameters, our numerical simulation predicts that the atoms can be precisely localized with a spatial resolution of ∼200 nm in the range of a radial distance of a few micrometers to the beam core. Our results provide a more flexible way to localize atoms in a two-dimensional system, possibly paving one-step closer to the nanometer scale atom lithography and ultraprecise microscopy.

Scanning and Splicing Atom Lithography for Self-traceable Nanograting Fabrication

Atom lithography is a unique method to fabricate self-traceable pitch standards and angle standards, but extending its structure area to millimeter-level for application is challenging. In this paper, on the one hand, we put forward a new approach to fabricate a full-covered self-traceable Cr nanograting by inserting and scanning a Dove prism in the Gaussian beam direction of atom lithography. On the other hand, we extend the structure area along the standing-wave direction by splicing two-step atom deposition. Both nanostructures manufactured via scanning atom lithography and splicing atom lithography demonstrate good pitch accuracy, parallelism, continuity, and homogeneity, which opens a new way to fabricate centimeter-level full-covered self-traceable nanograting and lays the basis for the application of square ruler and optical encoders at the nanoscale.

Generation of metastable krypton using a 124-nm laser

We have realized the optical excitation of krypton to a metastable level with an efficiency as high as $23\phantom{\rule{0.16em}{0ex}}%$ using near-resonant 124-nm light produced by four-wave mixing in mercury vapor. Self-absorption in mercury is circumvented by adjusting the detuning and phase matching according to experimental and theoretical characterizations. Density-matrix calculations of the metastable krypton generation agree with the measured dependencies of the excitation efficiency, indicating pathways towards a further improvement. The obtained excitation efficiency, being one order of magnitude higher than that of previously demonstrated techniques, enables an extension of the metastable density regime for atomic physics applications, including magnetometry, atom lithography, and radioisotope dating.

Experimental analysis of submicrometer optical intensity distributions after an opaque disk.

Generation of subwavelength beam sizes is a fascinating challenge with several implications. The observation of a 120 nm laser spot in the visible part of the spectrum is reported here. It has a size variation of less than 10% in a distance of $ 50\;\unicode{x00B5}{\rm m} $50µm along the axis of propagation. This so-called Arago spot results from the diffraction of the light from a laser diode by the edges of an absorbing disk. Applications are discussed and hollow beams carrying orbital angular momentum with a 400 nm diameter dark spot in the center are evidenced. This paves the way toward atom lithography via atom guiding or new spectroscopy on forbidden transitions.

Optimization of laser focused atomic deposition by channeling**Project supported by the National Key Research and Development Program of China (Grant No. 2016YFA0200902), Science and Technology Commission of Shanghai Municipality, China (Grant No. 17JC1400801), and Young Scientists Fund of the National Natural Science Foundation of China (Grant No. 51705369).

Laser focused atomic deposition is a unique and effective way to fabricate highly accurate pitch standards in nanometrology. However, the stability and repeatability of the atom lithography fabrication process remains a challenging problem for massive production. Based on the atom–light interaction theory, channeling is utilized to improve the stability and repeatability. From the comparison of three kinds of atom–light interaction models, the optimal parameters for channeling are obtained based on simulation. According to the experimental observations, the peak to valley height of Cr nano-gratings keeps stable when the cutting proportion changes from 15% to 50%, which means that the channeling shows up under this condition. The channeling proves to be an effective method to optimize the stability and repeatability of laser focused Cr atomic deposition.

Deposition of Cr Atoms Using Switching-Detuning Light Mask for Direct Atom Lithography

Background: As progress on the nanofabrication has made semiconductor developed rapidly, there is an increasing need in precise pitch standards to calibrate the structure of devices at nanoscale. Nano-gratings fabricated by atom lithography are unique and suitable to act as precise pitch standard because its pitch distance is directly traceable to a natural constant. As the scaling down of nano-devices, it is very challenging to double the spatial frequency of nano-grating while keeping the self-traceability in atom lithography. Methods: In this study, the switching-detuning light mask is utilized for Cr atom lithography. During a single deposition process, the standing wave frequency is switching from positive detuning to negative detuning alternatively. Results: Nano-gratings fabricated using switching-detuning light mask is successfully replicated with double spatial frequency and self-traceability. Non-uniformity between neighboring Cr lines shows up with a corrected pitch of 107.15 Conclusion: Non-uniformity is mainly caused by the dipole force discrepancy between positive and negative detuning light mask. Therefore, to increase the high uniformity of nano-gratings, the deposition time of negative detuning should be at least twice as the positive detuning. On the other hand, to reduce the pitch uncertainty, it is necessary to reduce the distance between the atom beam and reflection mirror as close as possible. These two significant optimization designs are promising to increase the spatial frequency doubling performance with high uniformity and accuracy.

Nanometer-Resolution Mask Lithography with Matter Waves: Near-Field Binary Holography

Mask-based pattern generation is a crucial step in microchip production. The next-generation extreme-ultraviolet- (EUV) lithography instruments with a wavelength of \SI{13.5}{\nano\meter} is currently under development. In principle, this should allow patterning down to a resolution of a few nanometers in a single exposure. However, there are many technical challenges, including those due to the very high energy of the photons. Lithography with metastable atoms has been suggested as a cost-effective, less-complex alternative to EUV lithography. The great advantage of atom lithography is that the kinetic energy of an atom is much smaller than that of a photon for a given wavelength. However, up till now no method has been available for making masks for atom lithography that can produce arbitrary, high resolution patterns. Here we present a solution to this problem. First, traditional binary holography is extended to near-field binary holography, based on Fresnel diffraction. By this technique, we demonstrate that it is possible to make masks that can generate arbitrary patterns in a plane in the near field (from the mask) with a resolution down to the nanometer range using a state of the art metastable helium source. We compare the flux of this source to that of an established EUV source (ASML, NXE:3100) and show that patterns can potentially be produced at comparable speeds. Finally, we present an extension of the grid-based holography method for a grid of hexagonally shaped subcells. Our method can be used with any beam that can be modeled as a scalar wave, including other matter-wave beams such as helium ions, electrons or acoustic waves.

Suppression of quantum noises in coherent atom lithography through squeezing

The Abbe's diffraction limit restricts the resolution of an optical imaging and lithography system. Coherent Rabi oscillation is shown to be able to overcome the diffraction limit in both optical and atom lithography. In previous studies, semiclassical theory is applied where the driving field is treated as a classical light and quantum fluctuation is neglected. Here, we show that the quantum fluctuation may reduce the visibility of the superresolution pattern. However, by squeezing the photon number fluctuation we are able to significantly increase its visibility.

原子光刻中原子通量的优化研究

原子光刻中原子通量的优化研究

Optimization of Nano-Grating Pitch Evaluation Method Based on Line Edge Roughness Analysis

Abstract Pitch uncertainty and line edge roughness are among the critical quality attributes of a pitch standard and normally the analyses of these two parameters are separate. The analysis of self-traceable Cr atom lithography nano-gratings shows a positive relevance and sensitivity between LER and evaluated standard deviation of pitch. Therefore, LER can be used as an aided pre-evaluation parameter for the pitch calculation method, such as the gravity center method or the zero-crossing points method. The optimization of the nano-grating evaluation method helps to obtain the accurate pitch value with fewer measurements and provide a comprehensive characterization of pitch standards.

Tuning the velocity and flux of a low-velocity intense source of cold atomic beam**Project supported by the National Natural Science Foundation of China (Grant Nos. 61473166 and 41404146).

We investigate experimentally and numerically the quantitative dependence of characteristics of a low-velocity intensity source (LVIS) of atomic beam on light parameters, especially the polarization of cooling laser along the atomic beam axis (pushing beam). By changing the polarization of the pushing beam, the longitudinal mean velocity of a rubidium atomic beam can be tuned continuously from 10 to 20 m/s and the flux can range from 3 × 108 to 1 × 109 atoms/s, corresponding to the maximum sensitivity of the velocity with respect to the polarization angle of 20 (m/s)/rad and the mean sensitivity of flux of 1.2 × 109 (atoms/s)/rad. The mechanism is explained with a Monte-Carlo based numerical simulation method, which shows a qualitative agreement with the experimental result. This is also a demonstration of a method enabling the fast and continuous modulation of a low-velocity intense source of cold atomic beam on the velocity or flux, which can be used in many fields, like the development of a cold atomic beam interferometer and atom lithography.

Compact acceleration of energetic neutral atoms using high intensity laser-solid interaction

Recent advances in high-intensity laser-produced plasmas have demonstrated their potential as compact charge particle accelerators. Unlike conventional accelerators, transient quasi-static charge separation acceleration fields in laser produced plasmas are highly localized and orders of magnitude larger. Manipulating these ion accelerators, to convert the fast ions to neutral atoms with little change in momentum, transform these to a bright source of MeV atoms. The emittance of the neutral atom beam would be similar to that expected for an ion beam. Since intense laser-produced plasmas have been demonstrated to produce high-brightness-low-emittance beams, it is possible to envisage generation of high-flux, low-emittance, high energy neutral atom beams in length scales of less than a millimeter. Here, we show a scheme where more than 80% of the fast ions are reduced to energetic neutral atoms and demonstrate the feasibility of a high energy neutral atom accelerator that could significantly impact applications in neutral atom lithography and diagnostics.