A Cost Effective Direct Writing Laser System for Rapid Prototyping of Microfluidic Devices

This study is conducted to highlight the improvement in technology of manufacturing microstructures using maskless lithography technique. Direct laser writing technique was implemented, and a major section of this study is carried out on an experimental slant. Variables that were not covered experimentally were studied using lithography simulation software, GenISys – LAB. The aim of this study is to fabricate and analyze cost effective maskless lithography apparatus to ensure rapid prototyping and optimize the system to be used for at least two negative photoresist materials. A parametric study was carried out determining the best operating conditions from both perspectives of direct laser writing and material process parameters. All parameters were studied experimentally, but the impact of depth of focus was illustrated using lithography simulation. Using direct laser writing system, complex designs were manufactured. The developed system had a maximum writing speed of 0.834 mm/s. The minimum line width produced using optimized operating conditions was 3.94 μm. Experimentally, increasing laser intensity increased the line width and by increasing post bake timings, it was observed that less laser intensity was required. Simulation results showed that depth of focus plays a crucial role in manufacturing good quality 3D resist profile. We developed a cost effective direct laser writing system as a part of studying maskless lithography process for rapid manufacturing. The total cost associated to develop this system was AED 4800 ($ 1307). This system was optimized to be used with two negative photoresist materials. A significant contribution of our work is through cost effectiveness and performance to produce complex designs using a maskless lithographic process. This study will provide an opportunity for researchers to use their innovative designs with faster and cheaper methods of prototyping micro devices

Large area TMD-based van der Waals heterostructures featuring enhanced photoconversion in the flat optics regime

One of the most urgent technological needs of this century concerns the research of innovative nanomaterials for applications in optoelectronics, nanophotonics and photovoltaics for renewable energy conversion. In fact, downscaling of the silicon-based devices (e.g. field-effect transistors) has come to an end due to intrinsic physical limitations such as size, inadequate carrier mobility, short channel effects, atomic-scale interactions, heat generation, and energy consumption at atomic scale thickness, requiring innovative materials to overcome such difficulties. Graphene discovery in 2004 by Novoselov and Geim arose a tremendous interest for the two-dimensional (2D) materials, in which the reduced dimensionality brings new properties with respect to their bulk counterparts. Despite the extraordinary properties exhibited by graphene, its gapless nature strongly limits the fabrication of graphene-based optoelectronic devices. For this reason, the interest of researchers shifted towards the class of 2D semiconductors. Among these materials, Transition Metal Dichalcogenides (TMDs) represent the most important family due to suitable bandgap energy values which match the Schockley-Queisser efficiency criterion for solar photoconversion, and to their extraordinary optical absorption coefficient. Additionally, 2D materials can be vertically stacked to form the so-called van der Waals heterostructures that are endowed by pristine interface thanks to the relatively weak van der Waals interactions that keep the stack together. This offers the opportunity to fabricate heterostructures of arbitrary 2D materials, independently by their crystal structure, with no limitations on the engineering of the optoelectronic and photonic properties of the new stacked metamaterial. In particular, the possibility to realize van der Waals p-n junctions by coupling 2D-TMDs layers is very intriguing for photoconversion and photovoltaic applications. So far, TMDs employment has been mostly limited to the fabrication of prototypical devices such as field-effect transistors and photodetectors, most of which were realized via mechanical exfoliation from single crystal of flakes with thickness ranging from one to few atomic layers. Despite their intriguing properties, these materials do not represent an industrially relevant alternative to traditional semiconductors, being the exfoliation a randomic process with very low yields and limited areas in the micrometer range. Consequently, one of the key frontiers of TMDs research is the large area growth of homogeneous ultra-thin layers with controlled thickness over macroscopic areas. To meet this requirement, several large area techniques have been studied to synthesize TMDs layers extending over cm² areas. However, in contrast with the exfoliation process, these techniques result in polycrystalline layers where the high concentration of grain boundaries degrades the macroscopic conduction. The second crucial issue to be solved for ultra-thin TMD-based devices to be used in photoconductive and photodetection applications is the maximization of the optical absorption. Despite the excellent optical absorption coefficient, an ultra-thin TMD film indeed cannot absorb efficiently the incoming light due to the ultimately reduced thickness, which means a nanometric optical path. It is thus evident that new strategies for the optical absorption amplification in ultra-thin 2D semiconductors need to be developed to allow proper performances in TMD-based photodetection and photovoltaics applications. Because of the atomic thickness of the 2D layer, traditional solutions developed for the light harvesting amplification in conventional silicon-based photovoltaic devices, based on pyramidal microstructuring or on the addition of antireflective coatings, cannot be transferred to these materials. Recently, the research group where I carried out my activity worked on large area ultra-thin MoS₂ films conformally grown on self-organized rippled substrate fabricated by ion beam sputtering. Their results clearly showed a modification of the TMD optical and electronic properties grown on the nanostructured substrate with respect to a flat one. This observation was explained with the stress induced by the substrate morphology in the MoS₂ layer in correspondence to the high curvature regions given by the crests and valleys of the ripples, meaning that by control of the substrate morphology it is possible to engineer the material intrinsic properties. Additionally, the nanostructures anisotropy adds a polarization-dependent optical response, offering a way to engineer the optical absorption of the 2D material in view of photoconversion applications. However, self-organized nanostructures suffer from a relevant size dispersion and long range disorder, whereas more interesting optical effects are expected for periodic nanogratings in which the subwavelength TMD layers reshaping provide them the functionality of flat optic diffractive elements. Starting from here, I devoted the most of my research activity to face the two main challenges described above: developing a growth process for large area ultra-thin TMD films, and studying an efficient light harvesting strategy to maximize the optical absorption in few-layer semiconductor films. An overview of the state-of-the-art regarding 2D materials and nanophotonics approaches for light harvesting in ultra-thin films is given in Chapter 1, while the growth synthesis techniques are postponed in Chapter 2. At the beginning of my PhD, MoS₂ films were grown by external collaborators of my group, thus limiting the activities. In the first phase of my research activity, I developed a novel large area growth process based on the physical deposition of solid precursor films and sulfurization, as I will describe in Chapter 2. This novel technique enabled not only the in-house growth of ultra-thin MoS₂ films for the first time, but also allowed to extend the process to ultra-thin WS₂ layers. Having now the capability to control the deposition of two different TMDs layers, I moved to the growth of large area van der Waals heterostructures. Due to the type-II heterojunction formed by the band structure coupling of MoS₂ and WS₂, such heterostructures are expected to have a high potential in photoconversion applications. In Chapter 3 I will show the nanofabrication of planar MoS₂/WS₂ heterostacks, and their application in photocatalytic experiments and in a prototype of photonic device, featuring first evidence of photovoltage and photocurrent under illumination. This latter application also required me to develop large area transparent electrodes, so that I will dedicate a part of the chapter to indium tin oxide thin films and large area graphene. In the second phase of my research activity, I focused on light harvesting in ultra-thin TMDs layers. Thanks to the conformality achieved by the physical deposition process, I explored a nanophotonic approach based on the optical anomalies arising from periodic modulation of the TMD layer at the subwavelength scale, obtained by conformal growth of the TMD layers onto nanostructured substrates. To this end, periodic nanogratings have been used as a template for the growth of ultra-thin MoS₂ layers. Differently from the self-organized nanostructures mentioned before, the periodicity induces diffractive effects that are exploited to steer the light parallel to the active 2D material enhancing the optical absorption, as demonstrated in Chapter 4 both directly by absorption measurements and indirectly by a photo-to-chemical energy conversion experiment where we detected enhanced photocatalytic performances. In the final part of my project, I started preliminary studies on the elastic scattering properties of subwavelength periodical lattices based on nanostructured tilted TMD layers. By defocused ion beam sputtering, I was able to reshape the morphology of the initial nanograting templates to further engineer the TMD optical response. In particular, by off-normal incidence sputtering it is possible to tailor a specific slope of the tilted nanofacets, on top of which I deposited laterally disconnected MoS₂ nanostripes by grazing angle physical deposition. By developing a custom-made scatterometer, optical characterization of ultra-thin MoS₂ nanostripes and thicker MoS₂ films was performed, giving interesting preliminary results on the directional light scattering properties of these reshaped layers, as reported in Chapter 5. Finally, I adopted a similar deposition approach for the nanofabrication of large area heterostructures nanoarrays based on few-layer WS₂ nanostripes coated by a conformal MoS₂ layer, demonstrating further engineering of the optical response of few-layer TMD films with impact in photoconversion

Tabletop tools for micron and sub-micron scale functional rapid prototyping

Thesis (S.M.)--Massachusetts Institute of Technology, School of Architecture and Planning, Program in Media Arts and Sciences, 2001.Includes bibliographical references (leaves 78-81).Three tools for the rapid prototyping of micron and sub-micron scale devices are presented. These tools represent methods for the manufacture of PEMS, or Printed micro Electro Mechanical Systems, and are enabled because they exploit the novel properties of nanocrystalline materials and their interactions with energetic beams. UV contact mask lithography was used to directly pattern metallic nanocrystals on glass and polyimide surfaces without vacuum or etching processes or the use of photoresist layers. Direct electron beam lithography of nanocrystalline metals was used to pattern multiple layer, multiple material, structures with minimum feature sizes of 100nm. Finally a micro-mirror array based selective laser sintering apparatus was built for the rapid, maskless patterning of PEMS. This tool was used to directly pattern metal structures, and for the rapid manufacture of elastomeric stamps for "nano embossing". Minimum feature sizes under 10 microns were achieved and routes to 2 micron features described. Processing time was reduced to hours from the weeks for traditional photomask / photolithography based systems. These tools are examined in the greater context of rapid prototyping technologies.Saul Griffith.S.M

Roadmap on structured light

Structured light refers to the generation and application of custom light fields. As the tools and technology to create and detect structured light have evolved, steadily the applications have begun to emerge. This roadmap touches on the key fields within structured light from the perspective of experts in those areas, providing insight into the current state and the challenges their respective fields face. Collectively the roadmap outlines the venerable nature of structured light research and the exciting prospects for the future that are yet to be realized.Peer ReviewedPostprint (published version

Additive nanomanufacturing: a review

Additive manufacturing has provided a pathway for inexpensive and flexible manufacturing of specialized components and one-off parts. At the nanoscale, such techniques are less ubiquitous. Manufacturing at the nanoscale is dominated by lithography tools that are too expensive for small- and medium-sized enterprises (SMEs) to invest in. Additive nanomanufacturing (ANM) empowers smaller facilities to design, create, and manufacture on their own while providing a wider material selection and flexible design. This is especially important as nanomanufacturing thus far is largely constrained to 2-dimensional patterning techniques and being able to manufacture in 3-dimensions could open up new concepts. In this review, we outline the state-of-the-art within ANM technologies such as electrohydrodynamic jet printing, dip-pen lithography, direct laser writing, and several single particle placement methods such as optical tweezers and electrokinetic nanomanipulation. The ANM technologies are compared in terms of deposition speed, resolution, and material selection and finally the future prospects of ANM are discussed. This review is up-to-date until April 2014

Liquid cooled micro-scale gradient system for magnetic resonance

Schaltbare magnetische Feldgradientspulen sind ein geeignetes Werkzeug für die Modulation der Kernspinpräzession in der gepulsten Kernspinresonanzspektroskopie und Bildgebung. Die Magnetresonanztomographie von mikroskopischen Proben benötigt starke, schnell schaltbare Magnetfeldgradienten, um diffusionsbedingte Artefakte zu unterdrücken, Suszeptibilitätseffekte abzuschwächen und um die Messzeit zu verkürzen. Verschiedene Techniken können eingesetzt werden, um eine hohe Gradientenintensität zu erreichen, wie zum Beispiel die Erhöhung der Stromstärke oder die Steigerung der Windungsdichte der Feldspule. Ein weiterer, geeigneter technischer Ansatz besteht darin, die Gradientenspulen näher an der Probe zu platzieren. Als Konsequenz wird aber die durch die Joule-Erwärmung verursachte Wärmeentwicklung zu einem zentralen Problem. In dieser Arbeit wird ein neuartiges Design, ein Mikroherstellungsprozess und eine Kernspin-Evaluierung eines Feldgradientenchips präsentiert. Die Gradientenspulen wurden besonders hoch miniaturisiert und durch den Einsatz von verbesserten und neuartigen Strukturierungsverfahren entwickelt. Zuerst wird ein Fertigungsverfahren zur Herstellung einer kompakten Hochfrequenzspule vorgestellt. Durch den Einsatz einer maskenlosen Rückseitenlithographie konnte die Prozesskomplexität reduziert werden. Dieses Verfahren wurde durch Tintenstrahldruck mit Nanopartikeln realisiert, wobei die gedruckten Strukturen selbst als lithographische Maske für die Herstellung einer galvanischen Form dienen. Somit werden die Seitenwände der galvanischen Form durch die gedruckte Seed-Schicht optimal selbst ausgerichtet. Dies ermöglichte eine anisotrope Galvanisierung, um eine höhere elektrische Leitfähigkeit der gedruckten Leiterbahnen zu erzielen. Aus den Erkenntnissen der ausgearbeiteten Herstellungsprozesse wurde ein optimiertes Spulendesign für ein-axiale sowie drei-axiale linearen Gradientenchips entwickelt. Die einachsige lineare $z$-Gradientenspule wurde mit der Stream-Function-Methode berechnet, wobei die Optimierung darauf abgestimmt wurde, eine minimale Verlustleistung zu erzielen. Die Gradientenspulen wurden auf zwei Doppellagen implementiert, die mittels Cu-Galvanik in Kombination mit fotodefinierbaren Trockenfilm-Laminaten aufgebracht wurden. Bei dem hier vorgestellten Herstellungsverfahren diente die erste Metallisierungschicht gleichzeitig dazu, Widerstands-Temperaturdetektoren zu integrieren. Um niederohmige Spulen zu realisieren wurde der Galvanisierungsprozess soweit angepasst, um eine hohe Schichtdicke zu erzielen. Die Chipstruktur beinhaltet ein aktives Kühlsystem, um dem Aufheizen der Spulen entgegenzuwirken. Thermographische Aufnahmen in Kombination mit den eingebetteten Temperatursensoren ermöglichen es, die Erhitzung der Spule zu analysieren, um die Strombelastbarkeit zu ermitteln. Die Gradientenspule wurde mit einer Hochfrequenz-Mikrospule in einer Flip-Chip-Konfiguration zusammengebaut, und mit diesem Aufbau wurde ein eindimensionales Kernspinexperiment durchgeführt. Es wurde eine Gradienteneffizienz von 3.15 $T\,m^{−1}\,A^{−1}$ bei einer Profillänge von 1.2 $mm$ erreicht

Integration of Ferroelectric HfO2 onto a III-V Nanowire Platform

The discovery of ferroelectricity in CMOS-compatible oxides, such as doped hafnium oxide, has opened new possibilities for electronics by reviving the use of ferroelectric implementations on modern technology platforms. This thesis presents the ground-up integration of ferroelectric HfO2 on a thermally sensitive III-V nanowire platform leading to the successful implementation of ferroelectric transistors (FeFETs), tunnel junctions (FTJs), and varactors for mm-wave applications. As ferroelectric HfO2 on III-V semiconductors is a nascent technology, a special emphasis is put on the fundamental integration issues and the various engineering challenges facing the technology.The fabrication of metal-oxide-semiconductor (MOS) capacitors is treated as well as the measurement methods developed to investigate the interfacial quality to the narrow bandgap III-V materials using both electrical and operando synchrotron light source techniques. After optimizing both the films and the top electrode, the gate stack is integrated onto vertical InAs nanowires on Si in order to successfully implement FeFETs. Their performance and reliability can be explained from the deeper physical understanding obtained from the capacitor structures.By introducing an InAs/(In)GaAsSb/GaSb heterostructure in the nanowire, a ferroelectric tunnel field effect transistor (ferro-TFET) is fabricated. Based on the ultra-short effective channel created by the band-to-band tunneling process, the localized potential variations induced by single ultra-scaled ferroelectric domains and individual defects are sensed and investigated. By intentionally introducing a gate-source overlap in the ferro-TFET, a non-volatile reconfigurable single-transistor solution for modulating an input signal with diverse modes including signal transmission, phase shift, frequency doubling, and mixing is implemented.Finally, by fabricating scaled ferroelectric MOS capacitors in the front-end with a dedicated and adopted RF and mm-wave backend-of-line (BEOL) implementation, the ferroelectric behavior is captured at RF and mm-wave frequencies