Micro-fabrication of Carbon Structures by Pattern Miniaturization in Resorcinol-Formaldehyde Gel

A simple and novel method to fabricate and miniaturize surface and sub-surface micro-structures and micro-patterns in glassy carbon is proposed and demonstrated. An aqueous resorcinol-formaldehyde (RF) sol is employed for micro-molding of the master-pattern to be replicated, followed by controlled drying and pyrolysis of the gel to reproduce an isotropically shrunk replica in carbon. The miniaturized version of the master-pattern thus replicated in carbon is about one order of magnitude smaller than original master by repeating three times the above cycle of molding and drying. The micro-fabrication method proposed will greatly enhance the toolbox for a facile fabrication of a variety of Carbon-MEMS and C-microfluidic devices.Comment: 16 pages, 5 figure

Production of carbonized micro-patterns by photolithography and pyrolysis

The preparation of carbon micro-patterns is reported in this paper. Different carbon micro-patterns were created using photolithography of the epoxy-based negative photoresist SU-8. Photoresist patterns were optimized in terms of resolution and aspect ratio and subsequently subjected to pyrolysis to obtain carbonized and conductive 3D structures. The latter step requires the optimization of the resist cross-linking time as well as the temperature and time of the resist post-bake. This step is crucial in order to avoid any severe modification of the geometry of the patterns produced during the actual pyrolysis. By observing optical and scanning electron microscope images, the morphology of the structures before and after pyrolysis was studied and the same patterns were also characterized by a laser probe profilometer. Finally, the thus obtained carbon patterns on Si wafers were used to carry out cell culture tests with Neural Stem Cells (NSC). The adhesion and the arrangement of the stem cells were analyzed to verify the ability of the patterned substrates to guide the orientation and, therefore, the differentiation of the cells

INTEGRATION AND CHARACTERIZATION OF TOBACCO MOSAIC VIRUS BASED NANOSTRUCTURED MATERIALS IN THREE-DIMENSIONAL MICROBATTERY ARCHITECTURES

The realization of next-generation portable electronics, medical implants and miniaturized, autonomous microsystems is directly linked with the development of compact and efficient power sources and energy storage devices with high energy and power density. As the components of these devices are continuously scaled down in size, there is a growing demand for decreasing the size of their power supply as well, while maintaining performance comparable to larger assemblies. This dissertation presents a novel approach for the development of microbattery electrodes that is based on integrating both micro and nano structured components for the formation of hierarchical electrodes. These electrodes combine both high energy density (enabled by the high surface area and mass loading) with high power density (due to the small thickness of the active battery materials). The key building block technologies in this work are the bottom-up self-assembly and metallization of a biological template and the top-down microfabrication processes enabled by Microelectromechanical Systems (MEMS) technology. The biotemplate used is the Tobacco mosaic virus (TMV), a rod-like particle that can be genetically modified to express functional groups with enhanced metal binding properties. In this project, this feature is combined with standard microfabrication techniques for the synthesis of nanostructured energy-related materials as well as their hierarchical patterning in device architectures. Specifically, synthesis of anode (TiO2) and cathode (V2O5) materials for Li-ion batteries in a core/shell configuration is presented, where the TMV biomineralization is combined with atomic layer deposition of the active material. These nanostructured electrodes demonstrate high energy storage capacities, high rate capabilities and superior performance to electrodes with planar geometries. In addition, a toolbox of biofabrication processes for the defined patterning of virus-templated structures has been developed. Finally, the nanocomposite electrodes are integrated with three-dimensional micropillars to form hierarchical electrodes that maintain the high rate performance capabilities of nanomaterials while exhibiting an increase in energy density compared to nanostructures alone. This is in accordance with the increase in surface area added by the microstructures. Investigation of capacity scaling for varying active material thickness reveals underlying limitations in nanostructured electrodes and highlights the importance of this method in controlling both energy and power density with structural hierarchy. These results present a paradigm-shifting technology for the fabrication of next-generation microbatteries for MEMS and microsystems applications

Fabricating Suspended Carbon Structures Using SU-8 Photolithography

The phenomenon of T-topping has been deemed as an imperfection of the SU-8 photolithography process, due to light diffraction, overexposure of the SU-8, and other process parameters. The first objective of this work is to demonstrate that T-topping can be used as a microfabrication resource to produce suspended structures between photo-patterned high aspect ratio SU-8 posts, as precursors for carbon wires (width\u3e1µm) and bridges (width\u3e1µm). Such carbon structures could be used in a number of applications, such as the fabrication of nanowire based biosensors for the medical and food industry. The second objective is to develop a model able to predict what type of structures will be featured by an array of SU-8 posts, and in case of suspended structures, their length and width, in function of the particular choice of process parameters. The parameters examined are: type of contact, exposure time, type of gap, nominal size and nominal gap. A variety of suspended structures are obtained, and repeatable carbon wires of diameter as low as 800nm can be fabricated with the right choice of parameters. Given a choice of the parameters, the model proposed succeeds into predicting the presence and length of posts of hexagonal, squared and circular shape, but fails in calculating their width. The model needs future work to reliably calculate the width of the suspended structures, and needs to be calibrated for triangles and diamonds. Also, the SU-8 thickness will have to be integrated in the model

Fabrication and characterization of suspended pyrolytic carbon microstructures in various pyrolysis temperatures

The aim of this Master’s thesis is to fabricate and study the issues related to the fabrication of suspended C-MEMS microstructures, as well as to investigate the properties of unpatterned pyrolytic carbon films in relation to the pyrolysis temperatures. In recent years, suspended pyrolytic carbon microstructures have started to emerge as part of the next generation C-MEMS devices. Although the use of such structures can greatly improve the quality and expands the application of C-MEMS devices, suspended pyrolytic carbon microstructures are far more susceptible to fabrication issues than substrate-bound structures. So, in order to further advance the C-MEMS process we must first understand the underlying fabrication issues that these structures face. Suspended SU-8 microstructures with varying shapes and sizes were prepared with the use of sacrificial layers and pyrolyzed in an inert atmosphere, in order to obtain suspended pyrolytic carbon microstructures. The structures were then analyzed in terms of their structural stability (optical microscope, SEM) and contraction (profilometer). The pyrolytic carbon films were prepared by pyrolyzing unpatterned SU-8 films at four different pyrolysis temperatures between 800 and 1100 °C. The films were characterized in terms of their electrical resistivity (4-point probe), crystallinity (Raman spectroscopy) and surface roughness (AFM). During the fabrication process various issues were observed. This allowed us to determine a correlation between the shape and size of the microstructures with the specific fabrication issue, a potential reasoning as to why these issues would occur and how they can be avoided in the future. Based on the obtained results, a new analysis of the pyrolysis process was performed from a structural standpoint of SU-8 microstructures. Novel microstructures were also presented in the form of pyrolytic carbon cups, which show great promise as structures used for the trapping of micro and nanoparticles. Analysis of the pyrolytic carbon films show an increase in the electrical conductivity, surface roughness and crystallinity of the material with higher pyrolysis temperatures. The electrical resistivity drops from 1.29·10-4 to 2.92·10-5 Ωm as the pyrolysis temperature is increased from 800 to 1100 °C. At the same time, the surface roughness of the pyrolytic carbon films increases from 0.33 to 1.27 nm. The Raman spectra indicate a very high level of structural disorder and small crystallinity of the material. The crystallite size was calculated to increase from 6.45 to 9.15 nm with higher pyrolysis temperatures. Furthermore, detailed analysis of the Raman spectra also indicates a buildup of intrinsic stress at temperatures up to 1000 °C. Upon increasing the pyrolysis temperature further, the stress is gradually reduced from the material as the structure begins to anneal

Investigating the Use of Streaming and Robotic Dielectrophoresis to Enable Continuous Cell Sorting and Automatic Cell Transfer in Sample Preparation

The sorting of targeted cells or particles from a sample is a crucial step in the sample preparation process used in medical diagnosis, environmental monitoring, bio-analysis and personalized medicine. Current cell sorting techniques can be broadly classified as label based or label-free. Label-based techniques mostly rely on fluorophores or magnetic nanoparticles functionalized to bind with targeted cells. Although highly specific, this approach can be expensive and suffers from limitations in the availability of suitable markers. Label-free techniques exploit properties inherent to the cell, such as density and size, to simplify the sorting protocol and reduce cost by eliminating the need to incubate samples with labels. However, the specificity of separation is low due to minor differences between the density and size of many cells of interest. In this work, the use of dielectrophoresis (DEP) is emphasized as a label-free technique that exploits the combination of size and membrane capacitance of a cell as a marker. DEP is the movement of dielectric particles in the presence of a non-uniform electric field, which can be towards the electrode (positive DEP) or away from electrode (negative). The cell membrane capacitance used in this project can distinguish between cells based on their type, age, fate, and circadian rhythm to provide higher specificity than other label free sorting techniques. DEP has been demonstrated to separate various bio particles including viruses, plant and animal cells, biomolecules as well as stem cells. The work presented here integrates fabrication, numerical simulations, analysis and experimentation to focus on three main objectives 1) addressing the gaps of knowledge in electrode fabrication 2) developing analytical system for rapid cell sorting of cell population 3) demonstrating feasibility of automated single cell sorting. These are addressed in the following paragraphs in that order. Carbon electrodes are excellent alternatives for DEP because of the ease of fabrication of 3D electrode geometries and low voltages involved. Previous works have used these electrodes for applications like DEP, electrochemical bio sensing applications, fuel cells and micro-capacitors. The fabrication process involves photo patterning of SU-8 posts followed by carbonization in an inert atmosphere. During pyrolysis, the structures retain their shape, but show shrinkage. Though the fabrication process is reproducible, limited knowledge is available about the shrinkage process. Shrinkage affects the design of devices where these structures are used because the electrode dimensions after pyrolysis vary from the design and resulting electric field in the domain is affected. Previous works observed dependence of shrinkage on structure height and width, but a defining relation between shrinkage and the geometry was lacking. In this work, shrinkage is studied as an effect of degassing through the lateral and top surface area of the electrodes. Empirical relations are to enable prediction of shrinkage in the design stage StreamingDEP refers to focusing particles into narrow streams with a proper play of positive DEP and drag force. This is important in continuous sorting because of the high throughput, limited exposure of cells to electric field and ability of integration to further analysis steps. Though streamingDEP has been demonstrated previously, the dependence of the particle focusing on system parameters has not been studied. In this work, an analytical model is built to study the effect of electrode geometry, flow and electric field parameters as well as cell properties. The analytical expression developed here is validated by experiments and simulations. Robotic transfer is required for efficient handling of cells and integration with analysis steps. Liquid handling robots are currently used in laboratories to transfer cells between different steps. Though they have precise control over the transfer of cells, the sorting ability is limited. To address these limitations, a proof-of-concept of roboticDEP device was innovated to enable transfer of targeted cells. The development of this system required studying the influence of DEP parameters in pick up and transfer of cells. The device was studied for elimination of contamination by using flow and electric field. The robotic DEP platform demonstrates a novel and unique approach to automated cell sorting with potential applications for single cell analysis, cell sorting and cell patterning

Suspended 1D metal oxide nanostructure-based gas sensor

Department of Materials Science and EngineeringWe developed a novel batch fabrication technology for the ultralow-power-consumption metal oxide gas sensing platform consisting of a suspended glassy carbon heating nanostructure and hierarchical metal oxide nanostructures forests fabricated by the carbon-micro electromechanical systems (carbon-MEMS) and selective nanowire growth process. We have developed a new manufacturing process for suspended glass carbon nanostructures such as single nanowire, nano-mesh and nano-membranes fabricated using carbon-MEMS consisting of the UV-lithography and the polymer pyrolysis processes. We designed a gas sensing platform consisting of suspended glassy carbon heating nanostructures and suspended hierarchical metal oxide nanostructure forests for the sensing part. Glassy carbon structure produced by the carbon-MEMS has many advantages such as high thermal & chemical stabilities, good hardness, and good thermal & electrical characteristics. The electrical conductivity of glassy carbon nanostructures has been increased more than three times by using rapid thermal annealing (RTA) process owing to the inferior heating property of glassy carbon nano-heater in the electrical conductivity. In order to divide the suspended glassy carbon nano-heater and the suspended hierarchical metal oxide nanostructures forests, the insulating layer of HfO2 materials is a high dielectric constant and is deposited uniformly using a atomic layer deposition (ALD) process on a suspended glassy carbon nano-heater. Suspended hierarchical metal oxide nanostructures forests were grown circumferentially on the suspended HfO2/glassy carbon nano-heater using a hydrothermal method consisting of the seed deposition and the growth processes. For selective metal oxide seed layer deposition process, a short-time exposed polymer patterning process was performed using the positive photoresist. After the polymer patterning process, a metal oxide seed layer is deposited using the rf-sputtering system, followed by a metal oxide nanostructure growth process. The distinguishing architecture of a suspended hierarchical metal oxide nanostructures forests/HfO2/glassy carbon nanostructure ensures efficient mass transport to the metal oxide nanostructure detection point of the gas analyte, resulting in highly sensitive gas detection. In the absence of an external heating system, the ultralow-power-consumption gas sensing platform of a suspended hierarchical metal oxide nanostructures forests/HfO2/glassy carbon nanostructure has excellent the gas sensing characteristics.ope