Materials and processes to enable polymeric waveguide integration on flexible substrates

Polymeric waveguide-on-flex has the potential to replace complex and costly light-turning devices in optoelectronic applications. As light signals are propagated and confined through the definition of core-cladding interface, the light guiding structure is required to adhere well and ensure long term interfacial stability. This thesis addresses the gap that has emerged in the fundamental material issues such as the polymeric optical waveguide materials deposited on the flexible substrates. In addition, this thesis investigates the feasibility of a new approach using electrostatic-induced lithography in micro-patterning of polymer, in optical waveguide fabrication. Plasma treatment is applied to enhance interfacial adhesion between flex substrates and optical cladding layers. The modified flex surfaces of polyimide KaptonHNTM and liquid crystal polymer VecstarTM materials are characterised. In addition, sonochemical surface treatment is evaluated on these flexible substrates. ToF-SIMS depth profiling has confirmed the interface reaction mechanisms where it has shown that plasma treatment increases the interfacial interpenetration. The larger interfacial width increases the possible entanglement mechanism between the polymer chains. These results, together with the double cantilever beam testing, indicate the strengthening of the polymeric interface upon plasma treatment, which is essential for long term optical and mechanical stability of waveguide-on-flex applications. A new method of micro-pattering of polymer material has been adopted for fabricating multimode waveguide-on-flex. The method, using an electrostatic-induced lithography, is developed to produce 50 μm x 50 μm arrays of polysiloxane LightlinkTM waveguide on flex. This thesis looks at various process recipes of the technique and reports the pattern formation of polymeric optical core. By adjusting the spin-coated liquid core thickness, pre-bake condition, UV exposure and applied voltage, the aspect ratio and profile of the optical core microstructure can be varied. As the electrostatic pressure overcoming the surface tension of spin-coated waveguide material induces the optical core formation, the core structure is smooth, making it ideal for low scattering loss waveguide. The propagation loss of fabricated waveguide is measured at 1.97 dB/cm at 850 nm wavelength. The result shows that the use of electrostatic-induced lithography in optical polymer is a promising approach for low cost and low temperature (<150 °C) processing at back end optical-electrical integrated circuitry assembly

Laser ablation of polymer waveguide and embedded mirror for optically-enabled printed circuit boards (OEPCB)

Due to their inherent BW capacity, optical interconnect (OI) offers a means of replacement to BW limited copper as bottlenecks begin to appear within the various interconnect levels of electronics systems. Low-cost optically enabled printed circuit boards are a key milestone on many electronics roadmaps, e.g. iNEMI. Current OI solutions found in industry are based upon optical fibres and are capable of providing a suitable platform for inter-board applications especially on the backplane. However, to allow component assembly onto high BW interconnects, an integral requirement for intra-board applications, optically enabled printed circuit boards containing waveguides are essential. Major barriers to the deployment of optical printed circuit boards include the compatibility of the technique, the cost of acquiring OI and the optical power budget. The purpose of this PhD research programme is to explore suitable techniques to address these barriers, primarily by means of laser material processing using UV and IR source lasers namely 248 nm KrF Excimer, 355 nm UV Nd:YAG and 10.6 µm IR CO2. The use of these three main lasers, the trio of which dominates most PCB production assembly, provides underpinning drive for the deployment of this technology into the industry at a very low cost without the need for any additional system or system modification. It further provides trade-offs among the suitable candidates in terms of processing speed, cost and quality of waveguides that could be achieved. This thesis presents the context of the research and the underlying governing science, i.e. theoretical analysis, involving laser-matter interactions. Experimental investigation of thermal (or pyrolitic) and bond-breaking (or photolytic) nature of laser ablation was studied in relation to each of the chosen lasers with regression analysis used to explain the experimental results. Optimal parameters necessary for achieving minimum Heat Affected Zone (HAZ) and surface/wall roughness were explored, both of which are key to achieving low loss waveguides. While photochemical dominance – a function of wavelength and pulse duration – is desired in laser ablation of photopolymers, the author has been able to find out that photothermallyprocessed materials, for example at 10.6 µm, can also provide desirable waveguides. Although there are literature information detailing the effect of certain parameters such as fluence, pulse repetition rate, pulse duration and wavelength among others, in relation to the etch rate of different materials, the machining of new materials requires new data to be obtained. In fact various models are available to try to explain the laser-matter interaction in a mathematical way, but these cannot be taken universally as they are deficient to general applications. For this reason, experimental optimisation appears to be the logical way forward at this stage of the research and thus requiring material-system characterisation to be conducted for each case thereby forming an integral achievement of this research. In this work, laser ablation of a single-layer optical polymer (Truemode™) multimode waveguides were successfully demonstrated using the aforementioned chosen lasers, thus providing opportunities for rapid deployment of OI to the PCB manufacturing industry. Truemode™ was chosen as it provides a very low absorption loss value < 0.04 dB/cm at 850 nm datacom wavelength used for VSR interconnections – a key to optical power budget – and its compatibility with current PCB fabrication processes. A wet-Truemode™ formulation was used which required that optical polymer layer on an FR4 substrate be formed using spin coating and then UV-cured in a nitrogen oxygen-free chamber. Layer thickness, chiefly influenced by spinning speed and duration, was studied in order to meet the optical layer thickness requirement for multimode (typically > 9 µm) waveguides. Two alternative polymers, namely polysiloxane-based photopolymer (OE4140 and OE 4141) from Dow Corning and PMMA, were sparingly utilized at some point in the research, mainly during laser machining using UV Nd:YAG and CO2 lasers. While Excimer laser was widely considered for polymer waveguide due to its high quality potential, the successful fabrication at 10.6 µm IR and 355 nm UV wavelengths and at relatively low propagation loss at datacom wavelength of 850 nm (estimated to be < 1.5 dB/cm) were unprecedented. The author considered further reduction in the optical loss by looking at the effect of fluence, power, pulse repetition rate, speed and optical density on the achievable propagation but found no direct relationship between these parameters; it is therefore concluded that process optimisation is the best practice. In addition, a novel in-plane 45-degree coupling mirror fabrication using Excimer laser ablation was demonstrated for the first time, which was considered to be vital for communication between chips (or other suitable components) at board-level

Laser Ablation for Polymer Waveguide Fabrication

An increase in interconnection density, a reduction in packaging sizes and the quest for lowcost product development strategy are some of the key challenges facing micro-optoelectronics design and manufacture. The influence of high-density, small-sized products has placed significant constraints on conventional electrical connections prompting various fabrication methods, e.g. photolithography, being introduced to meet these challenges and ameliorate the rapidly changing demand from consumers. While high-power solid state lasers are fundamental to large scale industrial production, excimer laser on the other hand has revolutionised the manufacturing industry with high precision, easy 3D structuring and less stringent production requirements. Micro-structuring using excimer laser, best known as laser ablation, is a non-contact micro- and nano-machining based on the projection of high-energy pulsed UV masked beam on to a material of interest such that pattern(s) on the mask is transferred to the substrate, often at a demagnified dimension with high resolution and precision. The use of mask with desired patterns and beam delivery system makes the fabrication in this case accurate, precise and easily controllable. The first part of this chapter introduces the fundamentals of laser technology and material processing. In the second part, optical interconnects as a solution to ‘bottlenecked’ conventional copper interconnections is introduced with emphasis on excimer laser ablation of polymer waveguides and integrated mirrors. Key research findings in the area of optical circuit boards using other techniques are also briefly covered

Synthesis, Characterization and Surface Modification of Polydimethylsiloxane and Its Composites

Silicone elastomers are widely employed in composite, coatings, biomedical, sealant and electronics applications. This dissertation highlights new methodologies and formulation approaches to improve the performance of siloxane polymers, including the mechanical properties, surface properties, optical properties and transport properties. It also provides an in-depth understanding about dip-coating deposition on chemically patterned surfaces. The first chapter presents the process of transforming sand into siloxane polymers of excellent mechanical strength. History, properties and applications of siloxane polymers are systematically discussed. Another focus of this chapter is wetting, including the concepts of contact angles and capillary bridge ruptures. Five interconnected projects are discussed following the introduction. Base-catalyzed siloxane equilibration is fully utilized in the first and second projects. First, extremely stretchable silicone elastomers are prepared by copolymerizing silica particles and octamethylcyclotetrasiloxane with siloxane equilibration. It is demonstrated that Stöber silica particles can actively participate in the chemical cross-linking of linear PDMS, serving as SiO4 linking group reservoirs. Second, permanently hydrophilic silicones are obtained by grafting PEO-PDMS copolymers to cross-linked silicones via siloxane equilibration. High power light-emitting diodes require a transparent encapsulant of high refractive index and low water permeability. The third project concerns the uniform incorporation of high refractive index inorganic fillers into siloxane polymers. Silicone composites with tailored refractive indices, different molecular structures, tunable mechanical properties and excellent transparency are achieved via sol-gel chemistry. In the fourth project, the high water transport rate in pure silicone is clarified, for both linear and cross-linked silicones. This high water transport rate results from the special structural features of silicone. Incorporation of water-resistant coating layers, silanized fillers and polyimide blocks are shown to be several effective approaches to decrease the water permeability of silicones. The fifth project deals with dip-coating deposition on chemically patterned surfaces. The mechanisms of capillary bridge deformation and rupture for dip-coating deposition on chemically and topographically patterned surfaces are compared. Ordered inorganic and organic individual crystal arrays are successfully patterned on smooth and chemically patterned surfaces prepared by photolithography

Novel Organosilicone Materials and Patterning Techniques for Nanoimprint Lithography.

Nanoimprint Lithography (NIL) is a high-throughput patterning technique that allows the fabrication of nanostructures with great precision. It has been listed on the International Technology Roadmap for Semiconductors (ITRS) as a candidate technology for future generation Si chip manufacturing. In nanoimprint Lithography a resist material, e.g. a thermoplastic polymer, is placed in contact with a mold and then mechanically deformed under an applied load to transfer the nano-features on the mold surface into the resist. The success of NIL relies heavily in the capability of fabricating nanostructures on different types of materials. Thus, a key factor for NIL implementation in industrial settings is the development of advanced materials suitable as the nanoimprint resist. This dissertation focuses on the engineering of new polymer materials suitable as NIL resist. A variety of silicone-based polymer precursors were synthesized and formulated for NIL applications. High throughput and high yield nanopatterning was successfully achieved. Furthermore, additional capabilities of the developed materials were explored for a range of NIL applications such as their use as flexible, UV-transparent stamps and silicon compatible etching layers. Finally, new strategies were investigated to expand the NIL potentiality. High throughput, non-residual layer imprinting was achieved with the newly developed resist materials. In addition, several strategies were designed for the precise control of nanoscale size patterned structures with multifunctional resist systems by post-imprinting modification of the pattern size. These developments provide NIL with a new set of tools for a variety of additional important applications.Ph.D.Macromolecular Science & EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/64783/1/pinac_1.pd

Preparation And Characterization Of Self-Assembled Monolayers And Mesoscale Protein Patterning

Bottom-up approach was used to develop self-assembled monolayers of octadecyltrichlorosilane (OTS) and undecenyltrichlorosilane(UTS) on Si(100) wafer. Undecenyltrichlorosilane monolayer was oxidized at the vinyl terminal to generate a carboxylic acid group. Lysozyme protein was immobilized on the polar carboxylic acid group. The developed protein patterns were investigated using fluorescence microscopy. Lysozyme has an isoelectronic point of 11.35. At a pH below this value the protein is positively charged making it a good candidate for electrostatic adsorption on the negatively charge -COO- group. Fluorescence images confirm formation of lysozyme across the silicon wafer. The patterned Si(100) wafer can be used as a biosensor against lysozyme antibodies. Another approach to develop varied surface properties was used to grow OTS on oxidized UTSox via chemical phase deposition (CVD). In this case we used polystyrene and silicon nanospheres as masking agents on the already developed and oxidized UTS. Fluorescence images revealed that OTS layers were formed on the interstitial spaces of the nanosphere masks. Varied protein can be immobilized on this surface due to different terminal groups on the surface

Novel Photostructurable Polymer for On-Board Optical Interconnects Enabled by Femtosecond Direct Laser Writing

Die integrierte Optik hat sich als vielversprechende Lösung für elektronische Verbindungen erwiesen, die eine hohe Bandbreitendichte und einen geringen Stromverbrauch ermöglicht. Seit kurzem ist es möglich photochemische und physikalische Reaktionen auf ein Mikrovolumen zu begrenzen. Dies hat der optischen Verbindungstechnik unter Verwendung von Glas oder Polymer eine zusätzliche Dimension verliehen. Dreidimensionale Wellenleiter können das optische Signal zwischen Blöcken aller Dimensionen verbinden, kombinieren oder aufteilen. Die Erhöhung des Brechungsindex ist jedoch immer noch eine Herausforderung für die Herstellung stabiler Freiform- und monomodaler Wellenleiter mit dreidimensionaler Ausdehnung, welche sich innerhalb der Platine befinden. Diese Dissertation stellt ein neues Konzept vor, um dieser Herausforderung zu begegnen, indem direktes Femtosekunden-Laserschreiben in Polymer und externe Diffusion eines gasförmigen Monomers verwendet wird. Direktes Laserschreiben mit Zwei-Photonen-Absorption wurde verwendet, um die Vernetzung entlang eines vorher definierten Pfades zur Bildung des Wellenleiterkerns zu initiieren. Es wurde ein ausreichender Brechungsindexkontrast erzeugt, um gaußförmige Strahlen mit einem Modus zu führen. Feature-Größen konnten durch Variieren der Scangeschwindigkeit und der Laserintensität linear angepasst werden. Dieses Herstellungsverfahren erfordert nur eine Schicht eines einzelnen Materials ohne Masken-, Kontakt- oder Nassbearbeitung. Durch Verwendung dieser neuartigen Methode wurden dreidimensionale optische Wellenleiter-Arrays, Fan-in/Fan-out- und Splitter-Strukturen hergestellt. Dreidimensionale freiforme Wellenleiter haben ein hohes Potential zur Verbesserung der Packungsdichte und Flexibilität optischer Verbindungen auf Platinenebene

A microfabricated microconcentrator for sensors and chromatography

The detection and quantitative measurement of trace components is a challenging task. The key component in such an instrument is the concentration step where the analytes are accumulated before the analysis. In this research, simple and inexpensive processes for the microfabrication of microconcentrator that can be used with sensors and as an injector in GC were developed. Analytes are selectively concentrated in the microconcentrator. Rapid electrical heating of the microconcentrator releases the adsorbed species as a 66 concentration pulse , which serves as an injection for the detection system. The relatively small size of the microconcentrator allows it to be heated and cooled rapidly. The microconcentrator serves the dual purposes of sample concentration and injection. The devices were fabricated on 6-inch silicon substrate using standard photolithographic processes. First, a microheater embedded in silicon wafer was fabricated. The channels were lined with a conductive layer by sputtering metal film through which an electric current could be passed causing Ohmic heating. The preconcentration was done on thin-film polymeric layer deposited in the channel. Rapid heating of the conductive layer caused the desorption pulse to be injected into the sensor/detector. Several channel configurations were fabricated with a width between 50 to 456 μ-m depth between 35 and 350 μ-m and length between 6 and 19 cm. The separation distance between the channels was varied such that the entire microheater fitted in a 1cm 2 area. Due to their small size, the microconcentrators could be fabricated more than 50 at a time on a 6-inch silicon wafer. In the first part of this research, the heating characteristics of the microheaters are studied. Deposition of metals to form a resistive heating element in microchannels was demonstrated. It was found that temperature as high as 360°C could be attained in a ten seconds. The microconcentrator was effective as a concentrator plus injector. It exhibited high signal enhancement and precision