822 research outputs found

    An Investigation of Micro and Nanomanufactured Polymer Substrates to Direct Stem Cell Response for Biomedical Applications

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    The rapidly advancing field of micro and nano-manufacturing is continuously offering novel advantages to existing technologies. Micro-injection molding provides a unique opportunity to create substrates capable of controlling the mechanical environment in stem cell culture in a high throughput industrially relevant manner. The modification of such polymer surfaces to match the target surface stiffness of relatively more compliant biological tissues necessitates the movement towards higher aspect ratio smaller dimension features. The requirements provide a significant manufacturing challenge which is approaching a solution. The development of high aspect ratio large feature density polymer microarrays requires the synergistic optimization of design, material, mold tooling, and processing. A conventional mold base with steel inserts and controllable resistance heating was assembled to incorporate interchangeable inserts with microfeatured silicon inlays. Ultraviolet (UV) lithography with dry etching was used to impart microfeatures into silicon wafers with a variety of different geometries containing aspect ratios ranging from 0.92 to 6. Multiple polymer resins, including polystyrene (HIPS, PS), low density polyethylene (LDPE), cyclic olefin copolymer (COC), and thermoplastic polyurethane (TPU), were used to test replication and cellular response to materials with different bulk stiffness and topography-modified surface stiffness. The maximum achieved microfeature aspect ratio was 9.3 (high impact polystyrene), owed to tensile stretching during part ejection. For non-stretched substrates, the maximum molded aspect ratio was 4.5 (LDPE) and highest replication quotient (RQ = feature height / tooling feature depth) was 0.97 (COC). The maximum aspect ratio molded with consistent features across the entire surface was 2.1 (TPU).Parameters shown to enhance replication were mold temperature (Tmold = Tg was a critical replication transition point), injection velocity at higher mold temperatures, holding time, holding pressure, and nozzle temperature. The importance of certain parameters was material dependent, but mold temperature consistently had a relatively large impact.A concern that was addressed for a high density array of microfeatures was the consistency of replication, which is vital for the intended application and seldom address in published literature. Increased consistency was attained through strategic placement of temperature control, modification of the main cavity design, and optimized silicon tooling with reduced microcavity nanoroughness.Silicon tooling was fabricated with the initial objective being to achieve high aspect ratio negative features. However, with the realization of molding and demolding limitations, the tooling microfeature profiles were altered to include a taper and reduction of sidewall scalloping. Sophisticated methods of dry etching were used, in which a novel etching technique known as passivation compensation, was utilized to manufacture microchannels containing low levels of roughness, a well-controlled tapered profile, and the prospect of high aspect ratios. With the new tooling, topography consistency was dramatically enhanced for both COC and TPU, with Taguchi orthogonal array optimization leading to RQs of 0.82 (aspect ratio = 2) and 0.85 (aspect ratio = 2.1), respectively.Water contact angle (WCA) measurements for both COC and TPU generally increased with an increase in surface roughness (dictated by microfeature dimensions), reaching WCA measurements of 139.8o and 141.1o, respectively. WCA hysteresis appeared to increase with roughness up to a critical value for COC while continuing to increase for TPU with a transition, which is thought to be the result of material properties. Moreover, hydrophobic surfaces containing high levels of hysteresis were attributed to the petal effect associated with hierarchical surface structures. Hydrophobicity has been shown to be related to biological cell behavior, and thus is an effective characterization technique to measure interfacial properties.Simulation of the injection molding process using conventional methods was used to describe general conditions present at microchannel inlets. The sprue gate and an increase in plate thickness gave the microfeatured region additional time to fill microfeatures prior to generation of a frozen layer. The delayed solidification is attributed to the low thermal conductivity associated with the polymer melt.A cell sensing model was developed based on the mechanical interaction between cell and substrate. The model provides a useful design map by which nanofeatured polymer geometry and material choice can be made to achieve a particular apparent surface stiffness. Bending mechanics were simulated for a few specific examples, providing an indication of the limitations associated with using higher aspect ratio nanostructures. A bending example was applied to a manufactured tapered pillar to note the stiffness reduction achieved through use of the substrates molded during the current study. Cell culture studies showed that the presence of topography had a dramatic effect on cellular morphology and on stress fiber thickness, causing an increase in thickness compared to flat controls. The cytoskeletal re-arrangements occurring may be indicative of a differentiation event, and future results will indicate whether that is the case. Unconventional morphology was observed in the presence of low aspect ratio COC microtopography, ranging from alignment with the micropattern to a circular conformation where adhesion is taking place exclusively in the middle of the cell. Micromolding of tensile bars was conducted to better understand the processing effects on mechanical and thermal properties of microscale molded components. Such results could provide useful general trends for the consideration of mechanical properties of molded microfeatures being exposed to cellular mechanical traction forces, especially considering the extreme processing conditions necessary to fill increasingly small and high aspect ratio features.Results revealed that the mechanical properties of COC is largely unaffected by a wide range processing conditions, but is reduced to approximately 41% of the value obtained from traditional tensile test results. TPU showed a dramatic dependence on molding properties, with higher injection velocities and lower mold temperatures resulting in reduced elastic modulus. Simulation was used to further elucidate the cause for varying properties. Microscale elastic modulus average values approximately 31% higher compared to traditional tensile test results. Trends in thermal properties were not apparent, and were difficult to detect from relatively weak melting peaks. The use of two different polymer lots elicited drastically different results, prompting the further investigation of the differences. Crystallinity, viscosity, and chemical bond structure was found to be very different from one lot to the other.The successful fabrication of uniform tapered microfeatures with middle range aspect ratios were manufactured, and the robust mold design and the tooling fabrication method provides a blueprint for achieving higher aspect ratios with a significant level of fidelity in the future. The enhanced macro and microscale mold design, combined with a deeper understanding of processing induced mechanical thermal microscale properties, can be used to tailor the substrate bio-interface properties to the desired mechanical structure for controllable hMSC behavior

    DIRECTED SELF-ASSEMBLY OF NANOSTRUCTURES AND THE OBSERVATIONS OF SELF-LIMITING GROWTH OF MOUNDS ON PATTERNED CRYSTAL SURFACE DURING EPITAXIAL GROWTH

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    In this thesis I describe an approach toward investigating moving interfaces, surface stabilities and directing self assembly of nanostructures, using lithographic patterning to perturb a flat crystalline surface over a range of spatial frequencies, followed by epitaxial growth. GaAs(001) shows a transient instability toward topographical perturbations. We model this behavior using an Ehrlich-Schwoebel (ES) barrier which impedes the diffusion of atoms across steps from above. We show via both kinetic Monte Carlo (kMC) simulations and molecular beam epitaxial (MBE) growth experiments that patterning in the presence of an ES barrier can be used to direct the self assembly of mounds. Second, as we track the time evolution of mound formation, we find the evidence of "Self-Limiting Growth" on surfaces - we find that in the initial stage of growth, the pattern directs the spontaneous formation of multilayer islands at 2-fold bridge sites between neighboring nanopits along [110] crystal orientation, seemingly due to the presence of an Ehrlich-Schwoebel barrier and the effect of heterogeneous nucleation sites on the surfaces. However, as growth continues, the height of mounds at 2-fold bridge sites "self-limits": the mounds cease to grow. Beyond this point an initially less favored 4-fold bridge sites dominate, and a different pattern of self assembled mounds begins. The observation of self-limiting behavior brings us new understanding of mechanism for crystal growth. We also find that the transient amplification of pattern corrugation during growth is correlated with self-limiting behavior of mounds. We propose that a minimum, `critical terrace size' at the top of each mound is responsible for the observed self-limiting growth behavior. Finally, the observation of the sequence of the mounds forming on the patterned surfaces gives us rather direct evidence that the formation of growth mounds on the surface is a nucleated process, rather than an instability

    Miniaturized Transistors

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    What is the future of CMOS? Sustaining increased transistor densities along the path of Moore's Law has become increasingly challenging with limited power budgets, interconnect bandwidths, and fabrication capabilities. In the last decade alone, transistors have undergone significant design makeovers; from planar transistors of ten years ago, technological advancements have accelerated to today's FinFETs, which hardly resemble their bulky ancestors. FinFETs could potentially take us to the 5-nm node, but what comes after it? From gate-all-around devices to single electron transistors and two-dimensional semiconductors, a torrent of research is being carried out in order to design the next transistor generation, engineer the optimal materials, improve the fabrication technology, and properly model future devices. We invite insight from investigators and scientists in the field to showcase their work in this Special Issue with research papers, short communications, and review articles that focus on trends in micro- and nanotechnology from fundamental research to applications

    Advanced design of periodical structures by laser interference metallurgy in the micro/nano scale on macroscopic areas

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    Methods for micro- and nanofabrication of structures are essential for many applications in different scientific areas like physics, chemistry, and materials science. In particular, interference lithography is a widely used method to produce periodic patterns over large areas. However, this method normally requires multiple steps to obtain the final structured surface. The Laser interference Metallurgy method is similar to the interference lithography technique in that an interference pattern is used. However, when using the interference metallurgy method the only step of processing is the irradiation of the surface of the sample with an interference pattern of a high-power pulsed laser without any subsequent steps like development of a photo resist and etching. This results in a direct, periodical, and local heating of the metal based on photo-thermal nature mechanisms with a welldefined long-range order. In this work, different aspects of the "Laser Interference Metallurgy\u27; method were studied. By means of the interference theory it was demonstrated that many different periodical patterns can be explored. Moreover, the design of advanced patterns has been verified by calculating the solution of the inverse problem. This means that, for a desired periodical pattern, it is possible to establish a configuration of electromagnetic waves that reproduces the pattern. Several metallic systems were irradiated with the laser interference patterns. In the case of thin metallic film systems, the changes in the topographic types that can be obtained can be explained in terms of the laser fluence which is required to melt or vaporize one or more of the layers of the film. Moreover, it was demonstrated that the local and periodical heat of the interference pattern can successfully serve to create a phase array in a microstructural scale. Bulk metals are structured by the flow of molten material along the surface tension gradient resulting from the temperature gradient. In several cases, the results were compared to thermal simulations. Laser fluences necessary to produce different topography regimes are consistent with the thermal simulations. The consistency of the thermal simulations with the experiments was further verified by means of in-situ electrical measurements. As examples of potential applications of metallic surfaces structured by laser interference metallurgy, the modulation of optical and tribological properties is discussed.Methoden zur Mikro- und Nanostrukturierung sind unabdingbar für viele Anwendungen in unterschiedlichen Wissenschaftszweigen wie z.B. der Physik, Chemie oder in den Materialwissenschaften. Insbesondere die Interferenz-Lithographie ist eine weit verbreitete Methode, um periodische großflächige Mikrostrukturen zu erzeugen. Allerdings beinhaltet die Anwendung dieser Methode mehrere Prozeßschritte um die gewünschte Strukturierung zu realisieren. Bei der Laser Interferenz Metallurgie wird ähnlich wie bei der Interferenz Lithographie die Probe mit einem Interferenzmuster belichtet. Dieser Belichtungsschritt ist im Gegensatz zur Lithographie der einzige Bearbeitungsschritt. Weitere Schritte wie Entwicklung oder Ätzen entfallen. Bei der Laser-Interferenzmetallurgie erfolgt die Belichtung mit einem gepulsten Hochleistungslaser. Dabei werden einzelne kohärente Lichtstrahlen an der Oberfläche zur Interferenz gebracht, woraus eine direkte, ferngeordnet periodische und lokale Aufheizung des Metalls aufgrund photothermischer Wechselwirkungen erfolgt. In dieser Arbeit werden verschiedene Aspekte der Laser-Interferenzmetallurgie untersucht. Durch die Anwendung der entsprechenden Interferenz-Theorie wird gezeigt, daß verschiedenste periodische Muster bzw. Strukturen verwirklicht werden können. Eine bestimmte Form eines Interferenzmusters kann mittels Lösung des inversen Problems eingestellt werden. Daher kann für beliebige Interferenzmuster die entsprechende Konfiguration von elektromagnetischen Strahlen berechnet werden, die bei Interferenz dieses Muster reproduzieren. Mehrere metallische Systeme wurden mit Laser Interferenzmustern bestrahlt. Im Fall von metallischen Dünnfilmen können die Änderungen in der Topographie mit der Laserfluenz erklärt werden, die nötig ist, um ein oder mehrere Schichten zu schmelzen oder zu verdampfen. Desweiteren wurde demonstriert, daß der lokale und periodische Wärmeeintrag durch das Interferenzmuster erfolgreich zur Bildung neuer intermetallischer Phasen führen kann. Die Strukturierung von Bulk-Metallen erfolgt durch Materialfluss entlang des Oberflächenspannungsgradienten, der aus dem Temperaturgradienten resultiert. In verschiedenen Fällen wurden die Ergebnisse mit thermischen Simulationen verglichen. Die Laserfluenz, die nötig ist, um ein bestimmtes Topographie-Regime zu verwirklichen, ist konsistent mit den thermischen Simulationen. Die Konsistenz der thermischen Simulationen mit den Experimenten wurde weiterhin durch elektrische in-situ Messungen verifiziert. Als Beispiele für potentielle Anwendungen der mittels Laserinterfernzmetallurgie strukturierten Oberflächen wird die Modulation von optischen und tribologischen Eigenschaften diskutiert

    Functional surface microstructures inspired by nature – From adhesion and wetting principles to sustainable new devices

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    In the course of evolution nature has arrived at startling materials solutions to ensure survival. Investigations into biological surfaces, ranging from plants, insects and geckos to aquatic animals, have inspired the design of intricate surface patterns to create useful functionalities. This paper reviews the fundamental interaction mechanisms of such micropatterns with liquids, solids, and soft matter such as skin for control of wetting, self-cleaning, anti-fouling, adhesion, skin adherence, and sensing. Compared to conventional chemical strategies, the paradigm of micropatterning enables solutions with superior resource efficiency and sustainability. Associated applications range from water management and robotics to future health monitoring devices. We finally provide an overview of the relevant patterning methods as an appendix

    Functional surface microstructures inspired by nature : From adhesion and wetting principles to sustainable new devices

    Get PDF
    In the course of evolution nature has arrived at startling materials solutions to ensure survival. Investigations into biological surfaces, ranging from plants, insects and geckos to aquatic animals, have inspired the design of intricate surface patterns to create useful functionalities. This paper reviews the fundamental interaction mechanisms of such micropatterns with liquids, solids, and soft matter such as skin for control of wetting, self-cleaning, anti-fouling, adhesion, skin adherence, and sensing. Compared to conventional chemical strategies, the paradigm of micropatterning enables solutions with superior resource efficiency and sustainability. Associated applications range from water management and robotics to future health monitoring devices. We finally provide an overview of the relevant patterning methods as an appendix

    Investigating block mask lithography variation using finite-difference time-domain simulation

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    Simulation work has long been realized as a method for analyzing semiconductor processing expediently and cost-effectively. As technology advancements strive to meet increasingly stringent parameter constraints, difficult issues arise. In this paper, challenges in block mask lithography will be discussed with the aid of using simulation packages developed by Panoramic Technology®. Halo formation utilizes a 20-30° tilt-angle implantation [1]. The block mask defines the geometries of the resist opening to allow implantation of atoms to extend into the channel region. Due to designed resolution scaling and tolerance in conjunction with substrate topography, there can be undesired influence on the electrical device characteristics due to block variations. Although the block mask pattern definition is relatively simple, additional investigation is required to understand the sensitivities that drive the implant resist CD variation. In this study, block mask measurements processed using 248 nm and 193 nm illumination sources were used to calibrate the simulation work. Addition of optical proximity correction (OPC) and wafer topography geometry parameters have been shown to improve modeling capabilities. The modeling work was also able to show the benefits of a developable bottom anti-reflection coating (dBARC) process over a single layer resist (SLR) process in the resist intensity profiles as gate pitch is decreased. The goal of this work was to develop an accurate simulation model that characterizes the lithographic performance needed to support the transition into future technology nodes

    Nanolithography using an AFM

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    DEVELOPMENT OF NANO/MICROELECTROMECHANICAL SYSTEM (N/MEMS) SWITCHES

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    Ph.DDOCTOR OF PHILOSOPH
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