Article

Intrinsic top-down unmanufacturability

IOP Publishing
Nanotechnology
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Abstract

Although small structures can be fabricated by deposition, lithography and etching, in some cases their intrinsic variability precludes their use as elements in useful arrays. Manufacture is a proper subset of fabrication. We show that structures with 3 nm design rules can be fabricated but not manufactured in a top-down approach—they do not have the reproducibility to give a satisfactory yield to a pre-ordained specification. It is also shown that the transition from manufacturability to intrinsic unmanufacturability takes place at nearer 7 nm design rules.

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... Due to the layered nature of fabrication processes, the top-down approach is mainly limited to 2D or 2.5D structures in manufacturing. Structures can be fabricated by repeated material deposition and removal processes, supporting very accurate manufacturing, but present manufacturability problems when the length scale is less than a few nanometers [216,217]. ...
... • All other figures in this chapter were originally published under a CC-BY license and appropriately cited, were original to this work, or were from official US Government documents and therefore not subject to copyright in the United States. 217 ...
Thesis
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Doctoral Dissertation: In the manufacturability-driven design (MDD) perspective, manufacturability of the product or system is the most important of the design requirements. In addition to being able to ensure that complex designs (e.g., topology optimization) are manufacturable with a given process or process family, MDD also helps mechanical designers to take advantage of unique process-material effects generated during manufacturing. One of the most recognizable examples of this comes from the scanning-type family of additive manufacturing (AM) processes; the most notable and familiar member of this family is the fused deposition modeling (FDM) or fused filament fabrication (FFF) process. This process works by selectively depositing uniform, approximately isotropic beads or elements of molten thermoplastic material (typically structural engineering plastics) in a series of pre-specified traces to build each layer of the part. There are many interesting 2-D and 3-D mechanical design problems that can be explored by designing the layout of these elements. The resulting structured, hierarchical material (which is both manufacturable and customized layer-by-layer within the limits of the process and material) can be defined as a manufacturing process-driven structured material (MPDSM). This dissertation explores several practical methods for designing these element layouts for 2-D and 3-D meso-scale mechanical problems, focusing ultimately on design-for-fracture. Three different fracture conditions are explored: (1) cases where a crack must be prevented or stopped, (2) cases where the crack must be encouraged or accelerated, and (3) cases where cracks must grow in a simple pre-determined pattern. Several new design tools, including a mapping method for the FDM manufacturability constraints, three major literature reviews, the collection, organization, and analysis of several large (qualitative and quantitative) multi-scale datasets on the fracture behavior of FDM-processed materials, some new experimental equipment, and the refinement of a fast and simple g-code generator based on commercially-available software, were developed and refined to support the design of MPDSMs under fracture conditions. The refined design method and rules were experimentally validated using a series of case studies (involving both design and physical testing of the designs) at the end of the dissertation. Finally, a simple design guide for practicing engineers who are not experts in advanced solid mechanics nor process-tailored materials was developed from the results of this project.
... In Figure 4 we summarize the constraints related to the operating frequency of Si exchangeonly qubits for realistic devices, including the technological limit of manufacturability at 3 nm (orange line). This limit was discussed and quantified by Kelly in 2011 by considering a number of arguments based on the intrinsic variability of top-down fabrication processes, on the unwanted electron tunneling and on the parasitic interferences, leading to a limit on the manufacturability at around 3 nm [2]. Furthermore, we highlight the physical bounds imposed by the requirements of adiabaticity, coherence and node-dependent minimum operation time, which hamper quantum computing in the blue, red and grey area respectively. ...
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... Due to the layered nature of fabrication processes, the top-down approach is mainly limited to 2D or 2.5D structures in manufacturability. Structures can be fabricated by repeated material deposition and removal processes, supporting very accurate manufactur-ing, but presenting manufacturability problems when the length scale is less than a few nanometers [164,165]. The bottom-up approach places material at the desired locations, similar to 3-D printing processes. ...
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... For arrays of nanometer half-pitch, ¼ 12%, highlighting a clearly intolerable variation for the electronic transport or optical properties of arrays made of elements with such intrinsic variability. Even if we could successfully fabricate such arrays by heroic processing, at very high cost, one must noteVatom by atom with a force microscope of sortsVthat there would never be such an array to write, store, or read information as the electrons and photons would leak extensively over extremely short timescales [48]. ...
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Developments in nanotechnology are making increasing demands on our ability to manipulate materials to designed patterns and structures. Patterning at increasingly diminished scales becomes very challenging, particularly with a view to manufacturing reproducible structures en masse. In fact Kelly recently argued that 'There are strict limits for which one-off fabrication is possible, but manufacture is not' [1]. Kelly was referring to top-down manufacturing processes and while activity in the field remains high and fruitful, there is at least as much research into bottom-up processes including self-assembly. In this issue, researchers at SASTRA University in India report programmable biomimetic self-assembly of carbon nanotubes (CNTs) using proteins [2]. The process uses changes in the surface charge and conformation of an unfolding protein, showing promise for biomedical applications and nanobiotechnology. Since the discovery of carbon nanotubes in 1990, their exceptional properties have inspired a cornucopia of applications, from the realms of science fiction with space elevators [3, 4], to more commercial merchandise such as tennis rackets [5]. A lot of applications based on CNTs use composites, and the properties of these composites can vary significantly depending on the manner in which the nanotubes are assembled. Researchers in the US recently reported experimental and simulation studies of thermal transport in carbon nanotubes [6]. Their work dealt with the discrepancy between the exceedingly high conductivity predicted and observed in single nanotubes and that reported for large assemblies. The work highlights some of the factors which diminish thermal conductivity in carbon nanotube assemblies, as well as identifying some optimal structures for maximizing thermal properties. The possible biomedical properties of carbon nanotubes have also attracted interest. However there has been debate over the potential for carbon nanotubes to support cell growth versus their potential toxicity. Researchers in New Jersey in the US have tackled these issues with a study of primary calvaria osteoblastic cell growth on single-walled carbon nanotube thin film substrates [7]. They describe a mechanism through which carbon nanotubes not only induce toxicity but also promote bone cell differentiation, leading to the formation of bone nodules. Bioapplications of nanomaterials often require bio-sympathetic methods for their production. The self-assembly achieved by viruses attracted the attention of Bancroft and his colleagues in the late 1960s [8, 9]. Since then a number of biomimetic approaches inspired by nature have been developed that allow fabrication of controlled complex structures with the potential for mass production [10, 11]. The tobacco mosaic virus has been demonstrated as one example of a useful biological template for simple and versatile nanopatterning [12]. Structures patterned using this biological toolbox may have applications in fields as broad as gas sensing and Li-battery electrodes. Researchers in Finland have also demonstrated the fabrication of complex protein structures using a DNA origami technique [13]. As the authors point out, controlled assembly of proteins has great potential for a range of biological applications and is also important for understanding fundamental biomolecular interactions. Many approaches to CNT assembly taken so far have required pre-patterning of a suitable substrate with a structure-directing agent to promote assembly of the nanotubes. However, as Nithiyasri et al in India point out in this issue, 'Assembly of CNTs in the dispersion medium would be more advantageous in terms of overcoming patterning hardships and cost as well as being suitable for a wide range of applications like biosensors, tissue engineering scaffolds and devices requiring high-density aligned domains of CNTs'. Their biomimetic approach uses thermally reversible denaturation of bovine serum albumin without the need for pre-patterning. In 1959 in his famous lecture 'There's plenty of room at the bottom', Feynman said 'What I want to talk about is the problem of manipulating and controlling things on a small scale' [14]. Fifty years later people are still talking about this, and as the work reported in this issue demonstrates so well, research in the field is still brimming with exciting developments. However, the paper by Nithiyasri et al does more than report an interesting phenomenon; it also proposes a mechanism for explaining how it happens. Feynman was also famous for his ability to explain things at an elementary level, and perhaps that was largely attributable to his genius for in-depth understanding of complex subjects. In fact he once remarked 'I couldn't do it', after an attempt to prepare a first-year student lecture on why spin one-half particles obey Fermi-Dirac statistics. He added, 'I couldn't reduce it to the freshman level. That means we don't really understand it.' What makes the developments in nanotechnology today so exciting is not just the advances in our ability to manipulate very small things, but our increasingly deepening understanding of how these processes operate. References [1] Kelly M J 2011 Intrinsic top-down unmanufacturability Nanotechnology 22 245303 [2] Nithiyasri P, Balaji K, Brindha P and Parthasarathy M 2012 Programmable self-assembly of carbon nanotubes assisted by reversible denaturation of a protein Nanotechnology 23 465603 [3] Clarke A C 1979 The Fountains of Paradise (London: Victor Gollancz) [4] Audacious & Outrageous: Space Elevators http://science.nasa.gov/science-news/science-at-nasa/2000/ast07sep_1/ [5] Babolat® NS™ Drive Tennis Racket http://www.nanotechproject.org/inventories/consumer/ browse/products/5130/ [6]Aliev A E, Lima M H, Silverman E M and Baughman R H 2010 Thermal conductivity of multi-walled carbon nanotube sheets: radiation losses and quenching of phonon modes Nanotechnology 21 035709 [7] Tutak W, Park K H, Vasilov A, Starovoytov V, Fanchini G, Cai S-Q, Partridge N C and Chhowalla M 2009 Toxicity induced enhanced extracellular matrix production in osteoblastic cells cultured on single-walled carbon nanotube networks Nanotechnology 20 255101 [8] Bancroft J B, Hills G J and Markham R 1967 A study of the self-assembly process in a small spherical virus formation of organized structures from protein subunits in vitro Virology 31 354-79 [9] Bancroft J B 1970 The self-assembly of spherical plant viruses Adv. Virus Res. 16 99-134 [10] Sarikaya M, Tamerler C, Jen A K-Y, Schulten K and Baneyx F 2003 Molecular biomimetics: nanotechnology through biology Nature Mater. 2 577-85 [11] Dickerson M B, Sandhage K H and Naik R R 2008 Protein- and peptide-directed syntheses of inorganic materials Chem. Rev 108 4935-78 [12] Gerasopoulos K, McCarthy M, Banerjee P, Fan X, Culver J N and Ghodssi R 2010 Biofabrication methods for the patterned assembly and synthesis of viral nanotemplates Nanotechnology 21 055304 [13] Kuzyk A, Laitinen K T and T¨orm¨a P 2009 DNA origami as a nanoscale template for protein assembly Nanotechnology 20 235305 [14] Feynman R P 1959 Plenty of room at the bottom www.its.caltech.edu/ ~feynman/plenty.html.
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