REVIEW ● OPEN ACCESSRead More
Atomic and close-to-atomic scale manufacturing (ACSM) aims not only to achieve atomic-level manufacturing precision and functional feature size, but also to realise material removal, migration, or addition at the atomic or close-to-atomic scale. Therefore, ACSM opens a new era in manufacturing engineering as one of three manufacturing paradigms, namely, Manufacturing III. The small structures enabled by ACSM endow materials with special features arising from quantum, electromagnetic, thermal effects and as a result, ACSM can potentially be applied to multiple fields. However, the challenges of realising ACSM are enormous, particularly when using traditional manufacturing tools. One fundamental difference from traditional manufacturing is that ACSM is governed by quantum theory, rather than classical theory. The uncertainty feature of quantum mechanics has caused two main challenges, i.e., deterministic manufacturing and product stability. Recently, Jian Gao and Prof. Xichun Luo from University of Strathclyde, Prof. Fengzhou Fang from Tianjin University and University College Dublin, Prof. Jining Sun from Dalian University of Technology, wrote a review "Fundamentals of atomic and close-to-atomic scale manufacturing: A review" on IJEM. This paper systematically introduces the quantum mechanics in ACSM, fundamentals of atomic and energy-matter interactions, mechanism studies in ACSM and perspectives on the future ACSM fundamental research. Figure 1 shows a length scale map of manufacturing systems, modelling methods, and dominant theories.
Fundamentals of atomic and close-to-atomic scale manufacturing are reviewed in this article:
● The theoretical boundary between ACSM and classical manufacturing is identified after a thorough discussion of quantum mechanics and their effects on manufacturing.
● The physical origins of atomic interactions and energy beams-matter interactions are revealed from the point view of quantum mechanics.
● The mechanisms that dominate several key ACSM processes are introduced, and a current numerical study on these processes is reviewed.
● A comparison of current ACSM processes is performed in terms of dominant interactions, representative processes, resolution and modelling methods.
● Future fundamental research is proposed for establishing new approaches for modelling ACSM, material selection or preparation and control of manufacturing tools and environments.
Figure 1. A length scale map of manufacturing systems, modelling methods, and dominant theories. (a) ACSM. Reprinted with permissions. CC BY 4.0. Copyright (2016) The Authors. (b) Nanomanufacturing. (c) Micromanufacturing. Reprinted with permission. CC BY 4.0. Copyright (2017) The Authors. (d) Conventional manufacturing.
The small structures enabled by ACSM endow materials with special features resulting from quantum, electromagnetic, and thermal effects. Thus, ACSM can potentially be applied to multiple fields. First, ACSM will provide manufacturing solutions for future-generation quantum, phonics or DNA chips that cannot be offered by extreme ultraviolet lithography (EUVL) which is currently used for the mass production of 5 nm integrated circuit (IC) chips. This is because EUVL is soon reaching its physical limit and will not be able to handle the higher structural complexity required by future-generation chips with functional features at atomic or close-to-atomic scale. These chips are predicted to replace IC chips in the next 50 years as IC chips will experience significant difficulties in building interconnections and reducing the current leaks when they are required for further miniaturisation. Second, ACSM is required for the manufacturing of single-electron transistors to realise single-electron transferability. Although single-atom transistors can be fabricated in a laboratory, achieving atomic manufacturing precision and stability are still the main problems for their mass production. Two-dimensional materials such as graphene and MoS2 are excellent substrates that enable transistors to sustain stability; however, a sophisticated manufacturing method is still required to modify the strong covalent bonds and form and stabilise the expected patterns. Furthermore, new devices, such as quantum bits, spin-based logic devices, atomically binary gates, and single-atom memory, that have emerged in the past decades will also certainly boost the demand for ACSM. Despite these challenges, considerable efforts and investments have been devoted to the research on ACSM, and some promising progress has been made recently. However, there is still no systematic analysis of the ACSM fundamentals to identify the intrinsic problems preventing its realisation; thus, this existing research gap inspired the present paper.
2. Research on mechanisms
This paper reveals the manufacturing mechanisms of several ACSM methods, including scanning probe microscope (SPM) tip-based processes, chemical self-limiting processes, light-based processes, particle beam-based processes and explains how the key interatomic or energy beam matter interactions influence the achievement of atomic-scale precision in ACSM. The modelling methods currently used to unveil the fundamental manufacturing mechanisms are also discussed.
SPM tip-based processes have a real atomic-scale patterning ability with angstrom-level control over the atomically sharp tips, as shown in figure 2. The chemical self-limiting processes rely on chemical self-limiting characteristics to obtain the atomic-layer resolution, as shown in figure 3. For atomic force microscope tip-based processes and chemical self-limiting processes, the key mechanisms that determine the atomic-scale patterning ability are the interactions, such as chemical bonding, van der Waals forces, and Pauli repulsion between atoms. For the scanning tunnelling microscope tip-based method, the dominant interactions also involve the interaction with the tunnelling current.
Figure 2. Schematic of SPM tip-based atom manipulation processes. (a) vertical manipulation, (b) lateral manipulation, (c) vertical interchange, (d) lateral interchange.
Figure 3. An illustration of chemical self-limiting processes. One cycle of a typical (a) atomic layer etching (ALE) process and (b) atomic layer deposition (ALD) process.
The patterning resolutions for energy beam-based methods are currently around several nanometres when the energy beam-matter interaction region is well controlled; particularly, the scanning tunnelling electron microscope-based electron beam is a special tool that can manipulate atoms like SPM tip-based methods and realise the atom manipulation. The interatom interactions still play important roles for the energy beam-based processes, but the patterning resolution is normally directly determined by energy beam-matter interactions, such as photo-chemical reactions in figure 4, focused electron beam etching and deposition in figure 5 and helium ion beam sputtering in figure 6.
Figure 4. Photolysis of a DNQ molecule. Reprinted with permission. Copyright (1992) American Chemical Society.
Figure 5. Schematics of (a) FEBE and (b) FEBD processes. Reprinted with permission Copyright (2015) American Vacuum Society.
Figure 6. Schematic of defects created by helium ion sputtering on single-layer MoS2 on graphene/SiC. The defects include Sulphur vacancies, a molybdenum vacancy, and defects in the graphene layer. Reprinted with permission. Copyright (2020) American Chemical Society.
As manufacturing approaches atomic and close-to-atomic scale, physical difficulties become more critical in comparison with the problems met in nanofabrication. A reliable physical and mathematical modelling method is in demand to address these difficulties. To reveal the underlying mechanisms of ACSM processes, we need to resort to the quantum mechanics-based first-principles method which provides a more reliable description for the ACSM processes than the classical Newtonian mechanics-based methods. As reviewed in this paper, the density functional theory (DFT) /density functional-based tight-binding (DFTB) methods are represented tools contributing to the mechanism study and process optimisation for different ACSM processes. Through descriptions of atomic structures, interaction energy, minimum energy path, and the possibility for atom transfer, the DFT/DFTB methods can reveal the underlying mechanisms of SPM tip-based processes, chemical self-limiting processes, and interatom interactions for energy beam-based processes. The time dependent-density functional theory (TD-DFT) methods can describe the time-dependent features of light-matter and particle beam-matter interactions and provide an unbiased insight into light absorption, photochemical reactions, light excitations for light-based processes, and the electronic stopping and excitations for particle beam-based processes.
Research on advanced manufacturing processes have demonstrated the manufacturing ability for atomic-scale patterns based on different work principles for various materials, with or without external energy sources. However, ACSM is still in its infancy and enormous challenges exist in its fundamental study to achieve deterministic manufacturing and product stability. To overcome these challenges, future research should focus on the following aspects. First, a time- and cost-effective modelling approach and framework are required to provide reasonable fidelity in the simulation of ACSM process while maintaining sufficient computational efficiency. Second, an intelligent material selection or preparation approach is required to improve the probability of forming expected patterns and stabilise atomic-scale patterns. Third, a cost-effective modelling approach to reveal the relationship between tip size or energy distribution with determinism of ASCM is necessary to control and optimise the size of the manufactured patterns and increase the probability of expected processing results. Furthermore, a fidelity modelling approach to reflect the influence of environment on ACSM will be helpful to overcome environmental restrictions and develop a cost-effective ACSM process that can operate under a ‘normal’ manufacturing environment.
5. About the Authors
Xichun Luo is a Professor in ultra precision manufacturing, Director of Research of Department of Design, Manufacturing and Engineering Management and technical director of Centre for Precision Manufacturing at the University of Strathclyde, UK. He is a Fellow of the International Society for Nanomanufacturing, the International Academy of Engineering and Technology and the International Association of Advanced Materials and UK Higher Education Academy. He is an associate editor for Proceeding of IMechE Part C: Journal of Mechanical Engineering Science, Journal of Micromanufacturing and Mechanical Sciences. He also sits in the editorial board for Micromachines and Nanomanufacturing and Metrology. Prof Luo’s interests include ultra precision machining brittle materials, freeform machining, precision motion control, digital manufacturing, hybrid micromachining, nanomanufacturing, atomic and close-to-atomic scale manufacturing. He chaired two IEEE International Conferences in Automation and Computing in 2014 and 2015. He won UK Institution of Mechanical Engineers (IMechE) 2015 Ludwig Mond Prize for his work in the application of digital technology in micro and nanomanufacturing.