Computational fluid dynamics modeling of a wafer etch temperature control system

Next-generation etching processes for semiconductor manufacturing exploit the potential of a variety of operating conditions, including cryogenic conditions at which high etch rates of silicon and very low etch rates of the photoresist are achieved. Thus, tight control of wafer temperature must be maintained. However, large and fast changes in the operating conditions make the wafer temperature control very challenging to be performed using typical etch cooling systems. The selection and evaluation of control tunings, material, and operating costs must be considered for next-generation etching processes under different operating strategies. These evaluations can be performed using digital twin environments (which we define in this paper to be a model that captures the major characteristics expected of a typical industrial process). Motivated by this, this project discusses the development of a computational fluid dynamics (CFD) model of a wafer temperature control (WTC) system that we will refer to as a “digital twin” due to its ability to capture major characteristics of typical wafer temperature control processes. The steps to develop the digital twin using the fluid simulation software ANSYS Fluent are described. Mesh and time independence tests are performed with a subsequent benchmark of the proposed ANSYS model with etch cooling system responses that meet expectations of a typical industrial cooling system. In addition, to quickly test different operating strategies, we propose a reduced-order model in Python based on ANSYS simulation data that is much faster to simulate than the ANSYS model itself. The reduced-order model captures the major features of the WTC system demonstrated in the CFD simulation results. Once the operating strategy is selected, this could be implemented in the digital twin using ANSYS to view flow and temperature profiles in depth

Characterization of an Ionization Readout Tile for nEXO

A new design for the anode of a time projection chamber, consisting of a charge-detecting "tile", is investigated for use in large scale liquid xenon detectors. The tile is produced by depositing 60 orthogonal metal charge-collecting strips, 3~mm wide, on a 10~\si{\cm} $\times$ 10~\si{\cm} fused-silica wafer. These charge tiles may be employed by large detectors, such as the proposed tonne-scale nEXO experiment to search for neutrinoless double-beta decay. Modular by design, an array of tiles can cover a sizable area. The width of each strip is small compared to the size of the tile, so a Frisch grid is not required. A grid-less, tiled anode design is beneficial for an experiment such as nEXO, where a wire tensioning support structure and Frisch grid might contribute radioactive backgrounds and would have to be designed to accommodate cycling to cryogenic temperatures. The segmented anode also reduces some degeneracies in signal reconstruction that arise in large-area crossed-wire time projection chambers. A prototype tile was tested in a cell containing liquid xenon. Very good agreement is achieved between the measured ionization spectrum of a $^{207}$Bi source and simulations that include the microphysics of recombination in xenon and a detailed modeling of the electrostatic field of the detector. An energy resolution $\sigma/E$=5.5\% is observed at 570~\si{keV}, comparable to the best intrinsic ionization-only resolution reported in literature for liquid xenon at 936~V/\si{cm}.Comment: 18 pages, 13 figures, as publishe

Graphene-based flexible sensors towards electronic wearables

Flexible electronics and wearable devices have attracted considerable attention because they produce mechanical liberty, in terms of flexibility and stretchability that can enable the possibility of a wide range of new applications. The term “wearable electronics” can be used to define devices that can be worn or mated with the sensed surface to continuously monitor signals without limitations on mechanical deformability of the devices and electronic performance of the functional materials. The use of polymeric substrates or other nonconventional substrates as base materials brings novel functionalities to sensors and other electronic devices in terms of being flexible and light weight. Conductive nanomaterials, such as carbon nanotubes and graphene have been utilized as functional materials for flexible electronics and wearable devices. Graphene has specifically been considered for producing next-generation sensors due to its impressive electrical and mechanical properties and a result, incorporation of flexible substrates and graphene-based nanomaterials has been widely utilized to form versatile flexible sensors and other wearable devices through use of different fabrication processes. Creation of a large-scale, simple, high-resolution and cost-effective technique that overcomes fabrication limitations and supports production of flexible graphene-based sensors with high flexibility and stretch ability is highly demanding. Soft lithography can be merged with a mechanical exfoliation process using adhesive tape followed by transfer printing to form a graphene sensor on a desired final substrate. In situ microfluidic casting of graphene into channels is another promising platform driving the rapid development of flexible graphene sensors and wearable devices with a wide dynamic detection range. Selective coating of graphene-based nanomaterials (e.g. graphene oxide (GO)) on flexible electrode tapes can, because of its flexibility and adhesive features, be used to track relative humidity (RH) variations at the surface of target surfaces. This thesis describes the design and development of flexible and wearable strain, pressure and humidity sensors based on a novel tape-based cost-effective patterning and transferring technique, an in situ microfluidic casting method, and a novel selective coating technique for graphene-based nanomaterials. First of all, we present a tape-based graphene patterning and transferring approach to production of graphene sensors on elastomeric substrates and adhesive tapes. The method utilizes the work of adhesion at the interface between two contacting materials as determined by their surface energies to pattern graphene on PDMS substrate and transfer it onto a target tape. We have achieved patterning and transferring method with the features of high pattern spatial resolution, thickness control, and process simplicity with respect to functional materials and pattern geometries. We have demonstrated the usage of flexible graphene sensors on tape to realize interaction with structures, humans, and plants for real-time monitoring of important signals. Secondly, we present a helical spring-like piezo resistive graphene sensor formed within a microfluidic channel using a unique and easy in situ microfluidic casting method. Because of its helical shape, the sensor exhibits a wide dynamic detection range as well as mechanical flexibility and stretch ability. Finally, we present a flexible GO-based RH sensor on an adhesive polyimide thin film realized by selectively coating and patterning GO at the surface of Au Interdigitated electrodes (IDEs) and subsequently peeling the device from a temporary PDMS film. Real-time monitoring of the water movement inside the plant has been demonstrated by installing GO-based RH sensor at the surfaces of different plant leaves

Preparation of Carboxylic Acid Functionalized Glycopolymers through RAFT and Post-Polymerization Modification for Biomedical Application

The primary theme of this dissertation involves the synthesis of well-defined primary amine functionalized polymers, subsequent modification of the polymers to produce novel carboxylic acid functionalized glycopolymers and surface polymerization of these systems utilizing controlled polymerization techniques. Additionally, the synthesis of new water-based allylic copolymer latexes is described. Carbohydrates are natural polymers which possess unlimited structural variations. They carry a huge density of information, and play major roles in recognition events and complex biological operations. For example, hyaluronic acid (HA), an anionic glycosaminoglycan, provides lubricating and cushioning properties in the extracellular matrix and has been found to be involved in the regulation of many cellular and biological processes. In industry, HA is used in a wide range of biomedical applications, including post surgical adhesion prevention, rheology modification in orthopedics, ophthalmic procedures, tissue engineering, hydrogels and implants. Limitations of current systems include cost, allergy induction and reduced performance capabilities in comparison to native HA. Therefore, it is of interest to prepare synthetic glycopolymer analogues to specifically target performance capabilities for biomedical applications. Reversible addition-fragmentation chain transfer polymerization (RAFT) is arguably the most versatile living radical polymerization technique in terms of the reaction conditions and monomer selection. Since the introduction of RAFT in 1998, researchers have employed the RAFT process to synthesize a wide range of water soluble (co)polymers with predetermined molecular weights, low polydispersities, and advanced architectures. However the RAFT polymerization of primary amine containing monomers such as 2-(aminoethyl metharylate) (AEMA) and ./V-(3-aminopropyl methacrylamide) (APMA) directly in water has yet to be reported. Since primary amine groups are amenable to a wide range of post-polymerization chemistries, primary amine functionalized polymers enable developments in the synthesis of controlled architecture glycopolymers. In addition, click chemistry can provide us an easy route to modify solid substrates with these polymers due to its simple reaction conditions and high reaction yield properties. The overall goal of this research is to prepare well-defined synthetic anionic glycosaminoglycan polymers by combining well-defined primary amine functionalized polymers with carboxylic acid functionalized sugars through a one-step reductive amination reaction. To achieve these goals, first, primary amine functionalized polymers were prepared through aqueous RAFT polymerization of AEMA and APMA. Second, Dglucuronic acid sodium salt was attached to reactive polymer precursors via reductive amination reactions in alkaline medium. Finally, the surface modification capabilities of primary amine functionalized polymers were investigated using click chemistry to create reactive surfaces allowing post-polymerization reactions. In this thesis, the first chapter concerns the first successful RAFT polymerization of unprotected AEMA directly in water and its successful block copolymerization with iV-2-hydroxypropylmethacrylamide (HPMA). The controlled living polymerization of AEMA was carried out directly in aqueous buffer using 4-cyanopentanoic acid dithiobenzoate (CTP) as the chain transfer agent (CTA), and 2,2\u27-Azobis(2- imidazolinylpropane) dihydrochloride (VA-044) as the initiator at 50 °C. The living character of the polymerization was verified with pseudo first order kinetic plots, a linear increase of the molecular weight with conversion, and low polydispersities (PDIs) (\u3c1.2). In addition, well-defined copolymers of poly(2aminoethyl methacrylate-6-./V-2- hydroxypropylmethacrylamide) (PAEMA-6-PHPMA) have been prepared through chain extension of poly(2-aminoethyl methacrylate) (PAEMA) macroCTA with HPMA in water. It is shown that the macroCTA can be extended in a controlled fashion resulting in near monodisperse block copolymers. The second chapter demonstrates the synthesis of novel carboxylic acid functionalized glycopolymers prepared via one step post-polymerization modification of poly(JV-[3-aminopropyl] methacrylamide) (PAPMA), a water soluble primary amine methacrylamide, in aqueous medium. PAPMA was first polymerized via aqueous RAFT polymerization using CTP as CTA, and 4,4\u27-Azobis(4-cyanovaleric acid) (V-501) as the initiator at 70 °C. The resulting well-defined PAPMA was then conjugated with Dglucuronic acid sodium salt through reductive amination in alkaline medium (pH 8.5) at 45 °C. The successful bioconjugation was proven through proton (^H) and carbon (13C) Nuclear Magnetic Resonance (NMR) spectroscopy and Matrix Assisted Laser Desorption Ionization Time of Flight (MALDI-TOF) mass spectroscopy analysis, which indicated near quantitative conversion. A similar bioconjugation reaction was conducted with PAEMA and PAEMA-6-PHPMA. For the PAEMA homo and block copolymers, however, poor conversion was obtained, most likely due to degradation reactions of PAEMA in alkaline medium. The third chapter details the direct preparation of a-alkynyl-functionalized PAEMA via RAFT polymerization. The controlled living polymerization of AEMA was carried out directly in dimethylsulfoxide (DMSO) using a-alkynyl functionalized CTP as CTA, and 2,2\u27-azobis(2,4-dimethyl-4-methoxyvaleronitrile) (V-70) as the initiator at 45 °C. The resulting polymers display low PDIs (\u3c1.2). In addition, the a-alkynylfuntionalized PAEMA was attached to an azide functionalized silicon wafer via click chemistry. Various characterization techniques including ellipsometry, contact angle measurements, attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-IR), and atomic force microscopy (AFM) were used to characterize the polymer modified silicon wafers. It was shown that a non-uniform surface with a thickness of 11.1 nm was obtained. The last chapter (an additional chapter) details the copolymerization behavior of styrene with sec-butenyl acetate, whose copolymerization properties have not been reported. Copolymers were produced via semicontinuous emulsion polymerization and characterized via NMR, gel permeation chromatography, differential scanning calorimetry, dynamic light scattering, and atomic force microscopy. A high degree of chain termination due to allylic hydrogen abstraction was observed, as expected, with resultant decreases in molecular weight and in monomer conversion. How percentages of the ever, high conversions were achieved, and it was possible to incorporate high allylic acetate comonomer into the polymer chain. Copolymer thermal properties are reported

Challenges in flexible microsystem manufacturing : fabrication, robotic assembly, control, and packaging.

Microsystems have been investigated with renewed interest for the last three decades because of the emerging development of microelectromechanical system (MEMS) technology and the advancement of nanotechnology. The applications of microrobots and distributed sensors have the potential to revolutionize micro and nano manufacturing and have other important health applications for drug delivery and minimal invasive surgery. A class of microrobots studied in this thesis, such as the Solid Articulated Four Axis Microrobot (sAFAM) are driven by MEMS actuators, transmissions, and end-effectors realized by 3-Dimensional MEMS assembly. Another class of microrobots studied here, like those competing in the annual IEEE Mobile Microrobot Challenge event (MMC) are untethered and driven by external fields, such as magnetic fields generated by a focused permanent magnet. A third class of microsystems studied in this thesis includes distributed MEMS pressure sensors for robotic skin applications that are manufactured in the cleanroom and packaged in our lab. In this thesis, we discuss typical challenges associated with the fabrication, robotic assembly and packaging of these microsystems. For sAFAM we discuss challenges arising from pick and place manipulation under microscopic closed-loop control, as well as bonding and attachment of silicon MEMS microparts. For MMC, we discuss challenges arising from cooperative manipulation of microparts that advance the capabilities of magnetic micro-agents. Custom microrobotic hardware configured and demonstrated during this research (such as the NeXus microassembly station) include micro-positioners, microscopes, and controllers driven via LabVIEW. Finally, we also discuss challenges arising in distributed sensor manufacturing. We describe sensor fabrication steps using clean-room techniques on Kapton flexible substrates, and present results of lamination, interconnection and testing of such sensors are presented