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Use of Supramolecular Assemblies as Lithographic Resists
Abstract
A new resist material for electron beam lithography has been created that is based on a supramolecular assembly. Initial studies revealed that with this supramolecular approach, high-resolution structures can be written that show unprecedented selectivity when exposed to etching conditions involving plasmas.
... Upon exposure, SEs and AEs break carbon bonds in the resist, liberating some C and O while permitting other O and Cr to react to form a chromium-oxide hard mask that is particularly resistant to the ICP-RIE chemistry used to etch both silicon and tungsten. 14 Prior to the spin-on application of Cr 8 F 8 (Pivalate) 16 resist, atomic force microscopy (AFM) was used to evaluate the surface morphology of silicon and tungsten substrates (Figure 2a-b). The root mean square (RMS) roughness was measured to be 0.29 nm for silicon. ...
Field emission devices are promising candidates to replace silicon FinFETs as next-generation nanoelectronic components. For these devices to be adopted, nanoscale field emitters with nanoscale gaps between them need to be fabricated, requiring the transfer of e.g. sub-10 nm patterns with sub-20 nm pitch into substrates like silicon and tungsten. New resist materials must therefore be developed that exhibit the properties of sub-10 nm resolution and high dry etch resistance. A negative tone, metal–organic resist is presented here. It can be patterned to produce sub-10 nm features when exposed with helium ion beam lithography at line doses on the order of 10s of pC/cm. The resist was used to create 5 nm wide, continuous, discrete lines spaced on a 16 nm pitch in silicon, and 6 nm wide lines on 18 nm pitch in tungsten, with line edge roughness of 3 nm. After the lithographic exposure, the resist demonstrates high resistance to silicon and tungsten dry etch conditions (SF6 and C4F8 plasma), allowing the pattern to be transferred into the underlying substrates. The resist’s etch selectivity for silicon and tungsten was measured to be 6.2:1 and 5.6:1, respectively; this allowed 3-4 nm thick resist films to yield structures that were 21 and 19 nm tall, respectively, while both maintained sub-10 nm width on sub-20 nm pitch.
... The sample with resist/solvent was then soft baked at a temperature of 100 °C for 2 minutes, resulting in an 100 nm thick resist evenly coated on the sample. Further details are given elsewhere 52 . ...
Coherent diffraction imaging (CDI) or lensless X-ray microscopy has become of great interest for high spatial resolution imaging of, e.g., nanostructures and biological specimens. There is no optics required in between an object and a detector, because the object can be fully recovered from its far-field diffraction pattern with an iterative phase retrieval algorithm. Hence, in principle, a sub-wavelength spatial resolution could be achieved in a high-numerical aperture configuration. With the advances of ultrafast laser technology, high photon flux tabletop Extreme Ultraviolet (EUV) sources based on the high-order harmonic generation (HHG) have become available to small-scale laboratories. In this study, we report on a newly established high photon flux and highly monochromatic 30 nm HHG beamline. Furthermore, we applied ptychography, a scanning CDI version, to probe a nearly periodic nanopattern with the tabletop EUV source. A wide-field view of about 15 × 15 μm was probed with a 2.5 μm−diameter illumination beam at 30 nm. From a set of hundreds of far-field diffraction patterns recorded for different adjacent positions of the object, both the object and the illumination beams were successfully reconstructed with the extended ptychographical iterative engine. By investigating the phase retrieval transfer function, a diffraction-limited resolution of reconstruction of about 32 nm is obtained.
Understanding the (dis)assembly mechanisms of large metallosupramolecules is critical in their design, stability and application. The inherent complexity of these structures leads to many potential pathways for combining (or separating) the constituent building blocks, which makes this task difficult. Here we use collision-induced dissociation mass spectrometry to study the disassembly of heterometallic complexes. Collisional activation leads to the formation of a series of previously unknown smaller ring products and we characterize their geometry using ion mobility. The disassembly of both { Cr x Cu 2 } hourglass structures ( x = 10, 12) and of a { Cr 12 Gd 4 } cluster shows the formation of rare closed, heptametallic species { Cr 6 Cu }, { Cr 5 Cu 2 } and { Cr 5 Gd 2 } as dominant products, as well as other closed ions such as { Cr 5 Cu }, { Cr 10 Cu }, { Cr 12 Cu }, { Cr 10 }, { Cr 12 } and { Cr 6 Gd 2 }. The collision cross-section of cyclic products and precursors has a linear correlation with ion mass—a relationship that does not hold for acyclic systems. As these rings are non-trivial to synthesize individually in solution, we propose the presented workflow to identify and characterize feasible molecules for bulk phase synthesis.
Both the cluster chemistry of tin and lanthanides have attracted extensive research interest, showing wide applications in catalysis, magnetism, luminescence, and lithography. However, their fusion into heterometallic Sn-Ln oxo clusters is still to be explored. In this study, through the stabilization of alkenyl-type cis-5-norbornene-endo-2,3-dicarboxylic acid (H2NE) ligands, a series of atomically precise Sn-Ln oxo nanoclusters have been successfully constructed from the assembly of heterometallic tetranuclear SnxLn(4−x) building blocks. Thereinto, Sn12Eu8 and Sn13Er6 with the highest nuclearities are built from multiple assembly of 8 {Sn2Eu2} units and 6 {Sn3Er} and {Sn2Er2} units, respectively. ESI-MS analysis indicates that Sn13Er6 has high solution stability, allowing their packing into thin films for lithography applications. As a result of electron beam lithography (EBL) studies, the condensation of Sn13Er6 can be triggered by low energy radiation of 10 µC/cm2, and 50 nm lines have been fabricated at expose energy of 50 µC/cm2, confirming the satisfying sensitivity and resolution of Sn13Er6. Hence, the success of this study develops the chemistry of heterometallic tin-lanthanide clusters that can be applied as novel negative photoresist materials.
Metal-organic materials such as [NH2(CH2-CH=CH2)2][Cr7NiF8(Pivalate)16] can act as negative tone resists for electron beam lithography (EBL) with high-resolution patterning of sub-40 nanometer pitch while exhibiting ultrahigh dry etch selectivities >100:1 and giving line dose exposures >11,000 pC/cm. It is clear that the resist sensitivity is too low to be used to manufacture the latest nanoscale photomasks that are suitable for extreme ultraviolet lithography. Therefore, the focus of this work here is to improve the sensitivity of this resist while maintaining its resolution and dry etch selectivity. Using our latest Monte Carlo simulation called Excalibur, we predict that the sensitivity would increase by a factor of 1.4 when the nickel atom is substituted by a cadmium atom. EBL studies showed an excellent agreement with the simulation, and plasma etching studies demonstrated that this did not affect the dry etch performance of the resist which remains very good with a selectively of ca. 99:1 for the etching of silicon at these resolutions with a low sensitivity of 7995 pC/cm.
A new class of electron bean negative tone resist materials has been developed based on heterometallic rings. The initial resist performance demonstrates a resolution of 15 nm half‐pitch but at the expense of a low sensitivity. To improve sensitivity a 3D Monte Carlo simulation is used that utilizes a secondary and Auger electron generation model. The simulation suggests that the sensitivity can be dramatically improved while maintaining high resolution by incorporating appropriate chemical functionality around the metal–organic core. The new resists designs based on the simulation have the increased sensitivity expected and illustrate the value of the simulation approach.
In this paper, the utilization of lithographic materials for semiconductor patterning applications based on optical and EUV radiation sources is reviewed. Photoresist platforms including novolac, chemically amplified, chain scission, molecular and inorganic materials are discussed in relation to their chemistry, design, processing, and performance. If the focus of Moore's Law 1.0 was on laterally scaling the number of components on a single chip, Moore's Law 2.0 can be understood as the coming era of 3D scaling, where the improved performance from 2D shrinking is replaced by integrated AI, 3D Packaging, 3D transistors and new types of memory, among other enabling enablers. As the historical patterning challenges faced by the lithographic industry to scale down semiconductor devices over multiple technology nodes are reviewed from a materials perspective, insight is given regarding future patterning materials utilization and the importance of back-implementing leading-edge materials for Moore's Law 2.0 patterning applications.
There is an increasing need for the development of superior, safe, and more sophisticated implants, especially as our society historically has been moving towards an increasingly aging population. Currently, most research is being focused on the next generation of advanced medical implants, that are not only biocompatible but have modified surfaces that direct specific immunomodulation at cellular level. While there is a plethora of information on cell-surface interaction and how surfaces can be nanofabricated at research level, less is known about how the academic knowledge has been translated into clinical trials and commercial technologies. In this review, we provide a clinical translational perspective on the use of controlled physical surface modification of medical implants, presenting an analysis of data acquired from clinical trials and commercial products. We also evaluate the state-of-the-art of nanofabrication techniques that are being applied for implant surface modification at a clinical level. Finally, we identify some current challenges in the field, including the need of more advanced nanopatterning techniques, the comparatively small number of clinical trials and comment on future avenues to be explored for a successful clinical translation.
A new class of resist materials has been developed that is based on a family of heterometallic rings. The work is founded on a Monte Carlo simulation that utilizes a secondary and Auger electron generation model to design resist materials for high resolution electron beam lithography. The resist reduces the scattering of incident electrons to obtain line structures that have a width of 15 nm on a 40 nm pitch. This comes at the expense of lowering the sensitivity of the resist, which results in the need for large exposure doses. Low sensitivity can be dramatically improved by incorporating appropriate functional alkene groups around the metal-organic core, for example by replacing the pivalate component with a methacrylate molecule. This increases the resist sensitivity by a factor of 22.6 and demonstrates strong agreement between the Monte Carlo simulation and the experimental results. After the exposure and development processes, what remains of the resist material is a metal-oxide that is extremely resistant to silicon dry etch conditions; the etch selectivity has been measured to be 61:1.
Polymetallic complexes can be assembled using a wide array of polydentate ligands that give an almost unlimited toolbox to prepare new molecular architectures with fascinating structures and interesting magnetic properties. Bis-tris propane is one such a polydentate ligand that has been used to prepare homo- (3d or 4f) and heterometallic (3d/3d′ or 3d/4f) complexes, ranging from simple complexes such as {Ni4} to spectacular 3d/3d′ {Cu8Zn8} or {Mn18Cu6} complexes. It shows a flexibility in binding mode, utilizing up to six of its potential ligand donor atoms and displaying multiple levels of deprotonation, able to bridge up to six metal ions. The ligand has a particular affinity for binding 3d ions such as Cu(II) or Co(III) in heterometallic syntheses and this can provide a flexible structure-directing effect. This concept has been exploited to prepare new heterometallic 3d/3d′ complexes that display interesting levels of complexity; 3d/4f complexes such as {Cu3Tb2} that show single-molecule magnet behavior where superexchange interactions quench quantum tunneling of the magnetization, or {Co3Gd3} where the magnetocaloric properties arise by using Bis-tris propane to separate the Gd(III) ions and weaken Gd(III)…Gd(III) interactions.
Quantum information processing (QIP) would require that the individual units involved - qubits - communicate to other qubits while retaining their identity. In many ways this resembles the way supramolecular chemistry brings together individual molecules into interlocked structures, where the assembly has one identity but where the individual components are still recognizable. Here a fully modular supramolecular strategy has been to link hybrid organic-inorganic [2]- and [3]-rotaxanes into still larger [4]-, [5]- and [7]-rotaxanes. The ring components are heterometallic octanuclear [Cr 7 NiF 8 (O 2 C t Bu) 16 ] - coordination cages and the thread components template the formation of the ring about the organic axle, and are further functionalized to act as a ligand, which leads to large supramolecular arrays of these heterometallic rings. As the rings have been proposed as qubits for QIP, the strategy provides a possible route towards scalable molecular electron spin devices for QIP. Double electron-electron resonance experiments demonstrate inter-qubit interactions suitable for mediating two-qubit quantum logic gates.
As feature sizes have diminished the need for extremely thin photoresist films has grown. Given the poor selectivity of typical resists with respect to silicon during plasma etching, it has become common to use an intermediate hardmask to transfer the pattern. Furthermore the use of trilayer etch stacks to amplify the achievable etch aspect ratio is becoming increasingly popular for critical layers. Here we introduce a new fullerene based spin-on-carbon layer for use in a multilayer etch stack. Carbon films of between 20 and 1270nm were prepared by spin coating. Thin silicon films were deposited on the carbon layer and patterned using a thin photoresist. Patterns were transferred to the carbon layer with high anisotropy at resolutions down to 40nm using an oxygen plasma, and then subsequently etched into the silicon substrate using an SF"6/C"4F"8 etch with high aspect ratio.
Ribosomal Rotaxane?
The ribosome is an extraordinarily sophisticated molecular machine, assembling amino acids into proteins based on the precise sequence dictated by messenger RNA. Lewandowski et al. (p. 189 ) have now taken a step toward the preparation of a stripped-down synthetic ribosome analog. Their machine comprises a rotaxane—a ring threaded on a rod—in which the ring bears a pendant thiol that can pluck amino acids off the rod; the terminal nitrogen then wraps around to form a peptide bond and liberate the thiol for further reaction. The system was able to link three amino acids in order from the preassembled rod.
Magnetic head fabrication for 100 Gbit/in. 2 areal density requires minimum lithographic feature size 0.15 m, with aspect ratios of 8:1–10:1. Electron-beam lithography can provide adequate resolution for research and development of magnetic heads, and at 100 kV can provide greater than 10:1 aspect ratios in 1–3 m thick single-layer resist polymethylmethacrylate. Poly methylmethacrylate PMMA is well known for withstanding the rigors of plating baths, but at these thicknesses requires a nonswelling, low-stress developer such as the LIGA mixture also known as ''GG Developer U.S. Patent No. 4,393,129''. In this work we present the results of isopropyl alcohol:water development for thick PMMA, and describe the dependence of resist contrast on the temperature of the developer. We also demonstrate the advantage of ultrasonic agitation during development. These development techniques have brought resist profiles in PMMA to the theoretical limit predicted by Monte Carlo simulations. © 2002 American Vacuum Society.
Magnetic molecules are potential building blocks for the design of spintronic devices. Moreover, molecular materials enable the combination of bottom-up processing techniques, for example with conventional top-down nanofabrication. The development of solid-state spintronic devices based on the giant magnetoresistance, tunnel magnetoresistance and spin-valve effects has revolutionized magnetic memory applications. Recently, a significant improvement of the spin-relaxation time has been observed in organic semiconductor tunnel junctions, single non-magnetic molecules coupled to magnetic electrodes have shown giant magnetoresistance and hybrid devices exploiting the quantum tunnelling properties of single-molecule magnets have been proposed. Herein, we present an original spin-valve device in which a non-magnetic molecular quantum dot, made of a single-walled carbon nanotube contacted with non-magnetic electrodes, is laterally coupled through supramolecular interactions to TbPc(2) single-molecule magnets (Pc=phthalocyanine). Their localized magnetic moments lead to a magnetic field dependence of the electrical transport through the single-walled carbon nanotube, resulting in magnetoresistance ratios up to 300% at temperatures less than 1 K. We thus demonstrate the functionality of a supramolecular spin valve without magnetic leads. Our results open up prospects of new spintronic devices with quantum properties.
The past decade has witnessed intensive research efforts related to the design and fabrication of photonic crystals. These periodically structured dielectric materials can represent the optical analogue of semiconductor crystals, and provide a novel platform for the realization of integrated photonics. Despite intensive efforts, inexpensive fabrication techniques for large-scale three-dimensional photonic crystals of high enough quality, with photonic bandgaps at near-infrared frequencies, and built-in functional elements for telecommunication applications, have been elusive. Direct laser writing by multiphoton polymerization of a photoresist has emerged as a technique for the rapid, cheap and flexible fabrication of nanostructures for photonics. In 1999, so-called layer-by-layer or woodpile photonic crystals were fabricated with a fundamental stop band at 3.9 microm wavelength. In 2002, a corresponding 1.9 microm was achieved, but the important face-centred-cubic (f.c.c.) symmetry was abandoned. Importantly, fundamental stop bands or photonic bandgaps at telecommunication wavelengths have not been demonstrated. In this letter, we report the fabrication--through direct laser writing--and detailed characterization of high-quality large-scale f.c.c. layer-by-layer structures, with fundamental stop bands ranging from 1.3 to 1.7 microm.
The primary metric for gauging progress in the various semiconductor integrated circuit technologies is the spacing, or pitch, between the most closely spaced wires within a dynamic random access memory (DRAM) circuit. Modern DRAM circuits have 140nm pitch wires and a memory cell size of 0.0408 μm^2. Improving integrated circuit technology will require that these dimensions decrease over time. However, at present a large fraction of the patterning and materials requirements that we expect
to need for the construction of new integrated circuit technologies in 2013 have ‘no known solution’. Promising ingredients for advances in integrated circuit technology are nanowires, molecular electronics and defect-tolerant architectures, as demonstrated by reports of single devices and small circuits. Methods of extending these approaches to large-scale, high-density circuitry are largely undeveloped. Here we describe a 160,000-bit molecular electronic memory circuit, fabricated at a density of 10^(11) bits cm^(-2) (pitch 33 nm; memory cell size 0.0011 mm^2), that is, roughly analogous to the dimensions of a DRAM circuit projected to be available by 2020. A monolayer of bistable, [2]rotaxane molecules 10 served as the data storage elements. Although the circuit has large numbers of defects, those defects could be readily identified through electronic testing and isolated using software coding. The working bits were then configured to form a fully functional random access memory circuit for storing and retrieving information.
In this chapter, the focus was on investigating a suitable technology to fabricate the next generation optical photomasks with a one-step process. This was achieved by a fabricating a novel nanocomposite electron beam resist that incorporated azo dyes into polymethylmethacrylate (PMMA). The azo dyes were introduced to attenuate the ultraviolet radiation propagating through the electron beam resist as the PMMA was found to be transparent at the wavelength of the incident radiation, thus, a metallic blocking layer is not required. When the nanocomposite resist was patterned by the electron beam, the pattern was transferred to a photoresist via contact printing using the conventional photolithography technique. To push the resolution limits, the SML resist films were spun to 600-nm and 300-nm thicknesses. The resulting pattern had linewidths of 55 and 33 nm, respectively. Therefore, an aspect ratio of 10:1 and 9:1 has been demonstrated in resist.
Helicity switching in biological and artificial systems is a fundamental process that allows for the dynamic control of structures and their functions. In contrast to chemical approaches to responsive behaviour in helicates, the use of light as an external stimulus offers unique opportunities to invert the chirality of helical structures in a non-invasive manner with high spatiotemporal precision. Here, we report that unidirectional rotary motors with connecting oligobipyridyl ligands, which can dynamically change their chirality upon irradiation, assemble into metal helicates that are responsive to light. The motor function controls the self-assembly process as well as the helical chirality, allowing switching between oligomers and double-stranded helicates with distinct handedness. The unidirectionality of the light-induced motion governs the sequence of programmable steps, enabling the highly regulated self-assembly of fully responsive helical structures. This discovery paves the way for the future development of new chirality-dependent photoresponsive systems including smart materials, enantioselective catalysts and light-driven molecular machines.
This is a unique book, combining chemistry and physics with technology and history in a way that is both enlightening and lively. No other book in the field of lithography has as much breadth. Highly recommended for anyone interested in the broad application of chemistry to lithography. --Chris Mack, Gentleman Scientist. This book provides a comprehensive treatment of the chemical phenomena in lithography in a manner that is accessible to a wide readership. The book presents topics on the optical and charged particle physics practiced in lithography, with a broader view of how the marriage between chemistry and optics has made possible the print and electronic revolutions of the digital age. The related aspects of lithography are thematically presented to convey a unified view of the developments in the field over time, from the very first recorded reflections on the nature of matter to the latest developments at the frontiers of lithography science and technology. Part I presents several important chemical and physical principles involved in the invention and evolution of lithography. Part II covers the processes for the synthesis, manufacture, usage, and handling of lithographic chemicals and materials. Part III investigates several important chemical and physical principles involved in the practice of lithography. Chemistry and Lithography is a useful reference for anyone working in the semiconductor industry. © 2010 Society of Photo-Optical Instrumentation Engineers. All rights reserved.
Heterometallische Ringe wurden in großer Zahl hergestellt, angefangen von den ersten Cr7M-Ringen mit M=NiII, ZnII und MnII bis hin zu Ringen mit vierzehn Metallen in der cyclischen Struktur. Bekannte Beispiele entweder außen durch Carboxylate oder innen durch Fluoride oder ein fünffach deprotoniertes Polyol verbrückt. Die Ringgröße wird durch Template gesteuert, z. B. durch Ammonium- oder Imidazoliumionen, Alkalimetalle oder Koordinationsverbindungen. Die Ringe können funktionalisiert werden und wirken dadurch als Liganden, sie lassen sich außerdem zum Aufbau von organisch-anorganischen Rotaxanen oder von Makromolekülen mit bis zu 200 Metallzentren verwenden. Eine Reihe von physikalischen Studien wurde an heterometallischen Ringen durchgeführt, darunter magnetische Messungen, inelastische Neutronenstreuung (einschließlich Einkristallmessungen), paramagnetische Elektronenresonanzspektroskopie (einschließlich Messungen der Phasengedächtniszeiten), NMR-Spektroskopie (Lösung und Festkörper) und polarisierte Neutronenstreuung. Die Ringe sind somit ideale Modellsystem, um Einblicke in den Magnetismus bei austauschgekoppelten Systemen zu gewinnen.
An enormous family of heterometallic rings has been made. The first were Cr7 M rings where M=Ni(II) , Zn(II) , Mn(II) , and rings have been made with as many as fourteen metal centers in the cyclic structure. They are bridged externally by carboxylates, and internally by fluorides or a penta-deprotonated polyol. The size of the rings is controlled through templates which have included a range of ammonium or imidazolium ions, alkali metals and coordination compounds. The rings can be functionalized to act as ligands, and incorporated into hybrid organic-inorganic rotaxanes and into molecules containing up to 200 metal centers. Physical studies reported include: magnetic measurements, inelastic neutron scattering (including single crystal measurements), electron paramagnetic resonance spectroscopy (including measurements of phase memory times), NMR spectroscopy (both solution and solid state), and polarized neutron diffraction. The rings are hence ideal for understanding magnetism in elegant exchange-coupled systems.
The fabrication of an integrated circuit requires a variety of physical and chemical processes to be performed on a semiconductor substrate. In general, these processes fall into three categories: film deposition, patterning, and semiconductor doping. Films of both conductors and insulators are used to connect and isolate transistors and their components. By creating structures of these various components millions of transistors can be built and wired together to form the complex circuitry of modern microelectronic devices. Fundamental to all of these processes is lithography, ie, the formation of three-dimensional relief images on the substrate for subsequent transfer of the pattern to the substrate. This book presents a complete theoretical and practical treatment of the topic of lithography for both students and researchers. It comprises ten detailed chapters plus three appendices with problems provided at the end of each chapter. Additional Information: Visiting http://www.lithoguru.com/textbook/index.html enhances the reader's understanding as the website supplies information on how you can download a free laboratory manual, Optical Lithography Modelling with MATLAB®, to accompany the textbook. You can also contact the author and find help for instructors.
Here we show an elegant and general route to the assembly of a giant cage {M12C24} from 12 palladium ions (M) and 24 heterometallic octanuclear coordination cages (C = {Cr7Ni-Py2}). The molecule is 8 nm in size, and the methods for synthesis and characterisation provide a basis for future developments at this scale.
Ring a ring of roses: Spectacular nanoscale molecular assemblies have been created by design of individual polymetallic components that are then linked together through simple reactions. These include an assembly where six octametallic rings surround a dodecametallic central ring.
A novel nanocomposite resist system was developed for sub-100 nm resolution e-beam lithography by dispersing surface-treated silica nanoparticles in a commercial ZEP520® resist. At 4.0 wt.% loading of silica nanoparticles, the system exhibited a much higher resolution than ZEP520® without sacrificing the intrinsic sensitivity and contrast of the starting polymer. The first major result is that 46 nm-wide isolated lines were obtained in the nanocomposite system (∼250 nm-thick layer), whereas comparatively 130 nm-wide lines were obtained in ZEP520® under the same experimental conditions. Contrary to standard e-beam resists, this important reduction of line broadening already occurred at 20 keV while higher energy e-beams (up to 100 keV) did not lead to further line broadening reduction. Moreover, it was shown that the addition of silica nanoparticles resulted in a higher resistance of the nanocomposite to plasma etching with O2 gas. Subjecting this nanocomposite resist to 75-keV Xe+ ion irradiation showed that it is also particularly suitable for ion projection lithography as a preliminary resolution of 114 nm (l/s) was obtained while the sensitivity increased by a factor of 40 compared to 30-keV electrons. Extending the nanocomposite approach to KRS-XE®, a chemically amplified resist, led to both enhanced resolution and mechanical stability for electron beam lithography. The major resolution and etch resistance improvements in both resist systems indicate that nanocomposite systems are promising candidates not only for sub-100 nm resolution e-beam lithography but also for ion projection lithography. Supported by preliminary Monte Carlo simulations a tentative mechanism highlighting the electron–nanocomposite interactions as the explanation for line broadening reduction is proposed.
A variety of host compounds have been used as molecular-scale reaction vessels, protecting guests from their environment or restricting the space available around them, thus favouring particular reactions. Such molecular 'flasks' can endow guest molecules with reactivities that differ from those in bulk solvents. Here, we extend this concept to crystalline molecular flasks, solid-state crystalline networks with pores within which pseudo-solution-state reactions can take place. As the guest molecules can spontaneously align along the walls and channels of the hosts, structural changes in the substrates can be directly observed by in situ X-ray crystallography during reaction. Recently, this has enabled observation of the molecular structures of transient intermediates and other labile species, in the form of sequential structural snapshots of the chemical transformation. Here, we describe the principles, development and applications of crystalline molecular flasks.
This article presents an overview of the essential aspects in the fabrication of silicon and some silicon/germanium nanostructures by metal-assisted chemical etching. First, the basic process and mechanism of metal-assisted chemical etching is introduced. Then, the various influences of the noble metal, the etchant, temperature, illumination, and intrinsic properties of the silicon substrate (e.g., orientation, doping type, doping level) are presented. The anisotropic and the isotropic etching behaviors of silicon under various conditions are presented. Template-based metal-assisted chemical etching methods are introduced, including templates based on nanosphere lithography, anodic aluminum oxide masks, interference lithography, and block-copolymer masks. The metal-assisted chemical etching of other semiconductors is also introduced. A brief introduction to the application of Si nanostructures obtained by metal-assisted chemical etching is given, demonstrating the promising potential applications of metal-assisted chemical etching. Finally, some open questions in the understanding of metal-assisted chemical etching are compiled.
In the past decade, the feature size in ultra large-scale integration (ULSI) has been continuously decreasing, leading to nanostructure fabrication. Nowadays, various lithographic techniques ranging from conventional methods (e.g. photolithography, x-rays) to unconventional ones (e.g. nanoimprint lithography, self-assembled monolayers) are used to create small features. Among all these, resist-based electron beam lithography (EBL) seems to be the most suitable technique when nanostructures are desired. The achievement of sub-20-nm structures using EBL is a very sensitive process determined by various factors, starting with the choice of resist material and ending with the development process. After a short introduction to nanolithography, a framework for the nanofabrication process is presented. To obtain finer patterns, improvements of the material properties of the resist are very important. The present review gives an overview of the best resolution obtained with several types of both organic and inorganic resists. For each resist, the advantages and disadvantages are presented. Although very small features (2-5 nm) have been obtained with PMMA and inorganic metal halides, for the former resist the low etch resistance and instability of the pattern, and for the latter the delicate handling of the samples and the difficulties encountered in the spinning session, prevent the wider use of these e-beam resists in nanostructure fabrication. A relatively new e-beam resist, hydrogen silsesquioxane (HSQ), is very suitable when aiming for sub-20-nm resolution. The changes that this resist undergoes before, during and after electron beam exposure are discussed and the influence of various parameters (e.g. pre-baking, exposure dose, writing strategy, development process) on the resolution is presented. In general, high resolution can be obtained using ultrathin resist layers and when the exposure is performed at high acceleration voltages. Usually, one of the properties of the resist material is improved to the detriment of another. It has been demonstrated that aging, baking at low temperature, immediate exposure after spin coating, the use of a weak developer and development at a low temperature increase the sensitivity but decrease the contrast. The surface roughness is more pronounced at low exposure doses (high sensitivity) and high baking temperatures. A delay between exposure and development seems to increase both contrast and the sensitivity of samples which are stored in a vacuum after exposure, compared to those stored in air. Due to its relative novelty, the capabilities of HSQ have not been completely explored, hence there is still room for improvement. Applications of this electron beam resist in lithographic techniques other than EBL are also discussed. Finally, conclusions and an outlook are presented.
We introduce using sputtered aluminum oxide (alumina) as a resilient etch mask for fluorinated
silicon reactive ion etches. Achieving selectivity of 5000:1 for cryogenic silicon etching and 68:1 for
SF6/C4F8
silicon etching, we employ this mask for fabrication of high-aspect-ratio silicon
micropillars and nanopillars. Nanopillars with diameters ranging from below
50 nm up to several hundred nanometers are etched to heights greater than
2 µm. Micropillars of 5, 10,
20, and 50 µm diameters are
etched to heights of over 150 µm
with aspect ratios greater than 25. Processing and characterization of the sputtered
alumina is also discussed.
To prepare tumor antigen peptides from HL-60 cells and to induce specific immune response.
HL-60 antigen peptides were obtained using techniques including freezing and thawing, heat precipitation and acid precipitation. The stimulating effect of the in vitro Hsp70 binding HL-60 peptides on PBMC and the proliferation of stimulated PBMC were observed by T cell activation test. The cytotoxicity of proliferated PBMC is detected by incubating HL-60 cells or K562 cells with PBMC respectively.
The obtained tumor antigen peptides were a peptides mixture. The mixed peptides could activate PBMC and cause PBMC proliferation in vitro after presented by Hsp70. The proliferated PBMC showed specific cytotoxicity to HL-60 cells but not to K562 cells.
The method for preparing of human leukemia tumor antigen peptides used in this paper is simple and easy; the obtained antigen peptides can induce specific immune response in vitro.
Selective non-covalent interactions have been widely exploited in solution-based chemistry to direct the assembly of molecules into nanometre-sized functional structures such as capsules, switches and prototype machines. More recently, the concepts of supramolecular organization have also been applied to two-dimensional assemblies on surfaces stabilized by hydrogen bonding, dipolar coupling or metal co-ordination. Structures realized to date include isolated rows, clusters and extended networks, as well as more complex multi-component arrangements. Another approach to controlling surface structures uses adsorbed molecular monolayers to create preferential binding sites that accommodate individual target molecules. Here we combine these approaches, by using hydrogen bonding to guide the assembly of two types of molecules into a two-dimensional open honeycomb network that then controls and templates new surface phases formed by subsequently deposited fullerene molecules. We find that the open network acts as a two-dimensional array of large pores of sufficient capacity to accommodate several large guest molecules, with the network itself also serving as a template for the formation of a fullerene layer.
We present a systematic study of metal-organic honeycomb lattices assembled from simple ditopic molecular bricks and Co atoms on Ag(111). This approach enables us to fabricate size- and shape-controlled open nanomeshes with pore dimensions up to 5.7 nm. The networks are thermally robust while extending over microm2 large areas as single domains. They are shape resistant in the presence of further deposited materials and represent templates to organize guest species and realize molecular rotary systems.
Monte Carlo Modeling for Electron Microscopy and Microanalysis
- D. C. Joy