Top-down Lithographic Techniques Research Articles

Synthetic Electronics

Building electronic circuitry with atomic precision is beyond the capability of current, as well as in the foreseeable future, top-down lithographic techniques. In recent years, the US Office of Naval Research (ONR) has championed several research initiatives that sought to address this key challenge. One of the most exciting development and initial success that resulted from these initiatives is a bottom-up chemical synthesis technique that has proven highly effective at assembling two-dimensional (2D) graphene nanostructures with spatial resolution down to single atom and single bond level. In this presentation I will briefly review, from a government program manager perspective, the current status of this exciting new research field which I call Synthetic Electronics. I will highlight some recent technical and scientific advances, such as various on-surface synthesis techniques, that enable and help define future directions of synthetic electronics. I will share some of my own personal perspectives and speculate on possible future directions, and also draw attention to some of the major challenges that are facing synthetic electronics. The motivation behind the discussion is to socialize the basic ideas and capabilities with, and seek feedback from, the relevant research communities, in order to inform and further refine research agenda in the future.

Self-Aligning Nanojunctions for Integrated Single-Molecule Circuits.

Robust, high-yield integration of nanoscale components such as graphene nanoribbons, nanoparticles, or single-molecules with conventional electronic circuits has proven to be challenging. This difficulty arises because the contacts to these nanoscale devices must be precisely fabricated with angstrom-level resolution to make reliable connections, and at manufacturing scales this cannot be achieved with even the highest-resolution lithographic tools. Here we introduce an approach that circumvents this issue by precisely creating nanometer-scale gaps between metallic carbon electrodes by using a self-aligning, solution-phase process, which allows facile integration with conventional electronic systems with yields approaching 50%. The electrode separation is controlled by covalently binding metallic single-walled carbon nanotube (mCNT) electrodes to individual DNA duplexes to create mCNT-DNA-mCNT nanojunctions, where the gap is precisely matched to the DNA length. These junctions are then integrated with top-down lithographic techniques to create single-molecule circuits that have electronic properties dominated by the DNA in the junction, have reproducible conductance values with low dispersion, and are stable and robust enough to be utilized as active, high-specificity electronic biosensors for dynamic single-molecule detection of specific oligonucleotides, such as those related to the SARS-CoV-2 genome. This scalable approach for high-yield integration of nanometer-scale devices will enable opportunities for manufacturing of hybrid electronic systems for a wide range of applications.

Scalable, Lithography-Free Plasmonic Metasurfaces by Nano-Patterned/Sculpted Thin Films for Biosensing

Scientific research in plasmonic metasurfaces has been widely widespread in the last years, motivated by the recent advances in the nanofabrication field and the increasing demand for high throughput sensing platforms. The recent advances in electronics, microfluidics, and signal processing have enabled the complete development of highly integrated devices with broad application potential. However, the progress observed from a fabrication point of view has been remarkable, led by the potential benefits metamaterials can offer in plasmonic sensing: sensor miniaturization, multiplexing opportunities, and extreme sensitivity biodetection. Although conventional top-down approaches, i.e., electron-beam lithography, have been extensively employed to develop plasmonic metasurfaces for biosensing, lithography-free bottom-up nanofabrication strategies based on nano-patterned/sculpted thin-films are candidates to surpass the limitations of top-down lithographic techniques with large-scale and high-throughput fabrication processes for 2D and 3D plasmonic metasurfaces over a broad material set. This perspective paper focuses on the challenges and opportunities to achieve lithography-free plasmonic metasurfaces by nano-patterned/sculpted thin films to conduct scalable and high-throughput plasmonic metamaterials for sensitive biosensing platforms.

Micro/Nanostructure Engineering of Epitaxial Piezoelectric α-Quartz Thin Films on Silicon.

The monolithic integration of sub-micron quartz structures on silicon substrates is a key issue for the future development of piezoelectric devices as prospective sensors with applications based on the operation in the high-frequency range. However, to date, it has not been possible to make existing quartz manufacturing methods compatible with integration on silicon and structuration by top-down lithographic techniques. Here, we report an unprecedented large-scale fabrication of ordered arrays of piezoelectric epitaxial quartz nanostructures on silicon substrates by the combination of soft-chemistry and three lithographic techniques: (i)laser interference lithography, (ii) soft nanoimprint lithography on Sr-doped SiO2 sol-gel thin films, and (iii) self-assembled SrCO3 nanoparticle reactive nanomasks. Epitaxial α-quartz nanopillars with different diameters (from 1 μm down to 50 nm) and heights (up to 2 μm) were obtained. This work demonstrates the complementarity of soft-chemistry and top-down lithographic techniques for the patterning of epitaxial quartz thin films on silicon while preserving its epitaxial crystallinity and piezoelectric properties. These results open up the opportunity to develop a cost-effective on-chip integration of nanostructured piezoelectric α-quartz MEMS with enhanced sensing properties of relevance in different fields of application.

Hierarchical structures of magnetic nanoparticles for controlling magnetic interactions on three different length scales

By combining top-down lithographic techniques with the meniscus-force deposition method, hierarchical structures consisting of defined regular elements on length scales from a few tens of nanometers to millimeters can be assembled out of magnetic nanoparticles. Varying the size and shape of the regular elements and distance between them offers the possibility to study magnetic coupling phenomena on three different length scales. As an example, we study hierarchical arrangements of magnetite nanoparticles (Fe3O4) with a diameter of d = 20 nm by ferromagnetic resonance measurements and demonstrate that the macroscopic properties of the structures are dominated by the assemblies of densely packed nanoparticles on the sub μm scale rather than by the interactions between these assemblies which are arranged on a grid with μm spacings or than by the macroscopic outer shape of the grid on the mm scale.

Multiresonant layered plasmonic films

Multiresonant nanoplasmonic films have numerous applications in areas such as nonlinear optics and sensing and as spectral tags. Although techniques such as focused ion beam milling and electron beam lithography can produce high-quality multiresonant films, these techniques are expensive, serial processes that are difficult to scale at the manufacturing level. Here, we present the fabrication of multiresonant nanoplasmonic films using a layered stacking technique. Periodically spaced gold nanocup substrates were fabricated using self-assembled polystyrene nanospheres followed by oxygen plasma etching and metal deposition via magnetron sputter coating. By adjusting etch parameters and initial nanosphere size, it was possible to achieve an optical response ranging from the visible to the near-infrared. Singly resonant, flexible films were first made by performing peel-off using an adhesive-coated polyolefin film. Through stacking layers of the nanofilm, we demonstrate fabrication of multiresonant films at a fraction of the cost and effort compared with top-down lithographic techniques.

Large-scale self-organization of reconfigurable topological defect networks in nematic liquid crystals.

Topological defects in nematic liquid crystals are ubiquitous. The defects are important in understanding the fundamental properties of the systems, as well as in practical applications, such as colloidal self-assembly, optical vortex generation and templates for molecular self-assembly. Usually, spatially and temporally stable defects require geometrical frustration imposed by surfaces; otherwise, the system relaxes because of the high cost of the elastic energy. So far, multiple defects are kept in bulk nematic liquid crystals by top-down lithographic techniques. In this work, we stabilize a large number of umbilical defects by doping with an ionic impurity. This method does not require pre-patterned surfaces. We demonstrate that molecular reorientation controlled by an AC voltage induces periodic density modulation of ions accumulated at an electrically insulating polymer interface, resulting in self-organization of a two-dimensional square array of umbilical defects that is reconfigurable and tunable.

Plasmonic Metasurfaces for Nonlinear Optics and Quantitative SERS

Plasmonic metasurfaces consist of two-dimensional arrays of metallic nanoresonators (plasmonic “meta-atoms”), which exhibit collective and tunable resonance properties controlled by electromagnetic near-field coupling. These man-made surfaces can produce a range of unique optical properties unattainable with natural materials. In this review, we focus on the emerging applications of metasurfaces with precisely engineered plasmonic properties for nonlinear optics and surface-enhanced Raman spectroscopy (SERS). In practice, these applications are quite susceptible to material losses and structural imperfections, such as variations in size, shape, periodicity of meta-atoms, and their material states (crystallinity, impurity, and oxidation, etc.). In these aspects, conventional top-down lithographic techniques are facing major challenges due to inherent limitations in intrinsic material properties and material quality introduced during growth, synthesis, and fabrication processes, as well as achievable lithog...

Modeling the Optical Properties of Bowtie Antenna Generated By Self-Assembled Ag Triangular Nanoprisms

Self-organized metal nanoparticles often possess assembly defects that can have a profound impact on the optical properties of the resulting nanoparticle assembly. Modeling these defects and evaluating their optical outcomes can provide a better understanding of how to design the assembly process and can evaluate the quality of the resulting materials. Here, we use finite element methods to examine the fabrication of bowtie nanoantenna, a commonly sought-after plasmonic structure with resonances in the visible and near-infrared wavelengths, through the self-assembly of colloidal triangular Ag nanoprisms. We model perfect and defective antenna structures and examine the effects of commonly observed assembly defects such as imperfect nanoprism shapes, off-axis antenna structures, and trimer or tetramer formation. We also evaluate the ability to fabricate antenna structures that possess comparable structural parameters (e.g., thickness, gap distance) to top-down lithographic techniques. We find that structural defects in self-assembled bowties can shift the resonant wavelength of the antenna by as much as 200 nm. Our models also indicate that self-assembled bowties possess high defect tolerances with respect to near-field enhancement, suggesting that they are viable structures for nanophotonic and nanoplasmonic applications.

Silver Nanoassemblies Constructed from Boranephosphonate DNA

Spatially selective deposition of metal onto complex DNA assemblies is a promising approach for the preparation of metallic nanostructures with features that are smaller than what can be produced by top-down lithographic techniques. We have recently reported the ability of 2'-deoxyoligonucleotides containing boranephosphonate linkages (bpDNA) to reduce AuCl4(-), Ag(+), and PtCl4(2-) ions to the corresponding nanoparticles. Here we demonstrate incorporation of bpDNA oligomers into a two-dimensional DNA array comprised of tiles containing double crossover junctions. We further demonstrate the site-specific deposition of metallic silver onto this DNA structure which generates well-defined and preprogrammed arrays of silver nanoparticles. With this approach the size of the metallic features that can be produced is limited only by the underlying DNA template. These advances were enabled due to a new method for synthesizing bpDNA that uses a silyl protecting group on the DNA nucleobases during the solid-phase 2'-deoxyoligonucleotide synthesis.

Remediation of Line Edge Roughness in Chemical Nanopatterns by the Directed Assembly of Overlying Block Copolymer Films

Block copolymer structures have been directed to assemble on chemically patterned surfaces with the domain interfaces oriented perpendicular to the substrate. Such methods have been pursued for lithographic applications to achieve long-range order in the assembled structures and, potentially more important, provide nanometer-level control over the interfaces between structures. The chemically striped surfaces used for the directed assembly of lamellae are patterned by top-down lithographic techniques and thus often have rough edges between the regions of different chemistry. Here we quantitatively characterize, using experiments and molecular-level simulations, the propagation of line edge roughness from the chemically patterned surfaces into the interfaces between domains of block copolymer lamellae as a function of the wavelength, amplitude, and geometry of the roughness. Two geometries of surface pattern roughness are considered with oscillatory neighboring interfaces that are either in-phase or out-of-phase. Block copolymer lamellae of poly(styrene-block-methyl methacrylate) effectively self-corrected surface patterns with small wavelength in-phase and out-of-phase roughness such that little or no memory of the substrate pattern roughness could be observed at the top surface of a 40 nm thin film. Larger wavelength in-phase roughness, and to a lesser extent larger wavelength out-of-phase roughness, propagated farther from the surface pattern such that the domain interfaces between block copolymer lamellae maintained the roughness throughout the film. These self-healing capabilities of block copolymers will be essential for lithographic applications with tight tolerances on line edge roughness and line width control, e.g., in patterning transistor gates.

Localized CVD growth of oriented and individual carbon nanotubes from nanoscaled dotsprepared by lithographic sequences

Using a combination of top-down lithographic techniques, isolated, individual and orientedmulti-wall carbon nanotubes (MWNTs) were grown on nickel or iron nanoscaled dots. Inthe first step of the process, micron-sized catalytic metallic dots (either iron or nickel) wereprepared using UV lithography. MWNTs were then synthesized from these catalysts usinga direct current plasma-assistance and hot-filament-enhanced chemical vapor deposition(CVD) reactor. Samples were characterized by means of scanning electron microscopy. Itturns out that the splitting up of the micron-sized dot is favored in the iron case andthat the surface diffusion of the metal is enhanced using ammonia in the gaseousmixture during the CVD process. The results are discussed giving arguments for theunderstanding of the MWNT growth mechanism. In a second step, a focused ion beam(FIB) procedure is carried out in order to reduce the initial dot size down tosubmicronic scale and subsequently to grow one single MWNT per dot. It isfound that nickel is most appropriate to control the size of the dot. Dots of size200 nm ± 40 nm are then required to grow individual MWNTs.

Synthesis, assembly and physical properties of magnetic nanoparticles

Compared with the top-down lithographic techniques, bottom-up chemical synthesis and self-assembly approaches offer much more flexibilities in creating magnetic nanostructures with controlled size, shape, composition and physical properties. This review summarizes some of the latest developments in this field, with emphasis mainly on transition metals, their alloys and metal oxide nanoparticles. The focus is directed towards the conditions of individual particles as well as large assemblies of particles through colloidal chemistry. Furthermore, some of the future directions in nanomagnetism from the perspective of physical chemists is also presented.