Fabrication of silicon nanowires with controlled nano-scale shapes using wet anisotropic etching

Silicon nanowires can enable important applications in energy and healthcare such as biochemical sensors, thermoelectric devices, and ultra-capacitors. In the energy sector, for example, as the need for more efficient energy storage continues to grow for enabling applications such as electric vehicles, high energy storage density capacitors are being explored as a potential replacement to traditional batteries that lack fast charge/discharge rates as well as have shorter life cycles. Silicon nanowire based ultra-capacitors offer increased energy storage density by increasing the surface area per unit projected area of the electrode, thereby allowing more surface “charge” to reside. The motivation behind this dissertation is the study of low-cost techniques for fabrication of high aspect ratio silicon nanowires with controlled geometry with an exemplar application in ultra-capacitors. Controlled transfer of high aspect ratio, nano-scale features into functional device layers requires anisotropic etch techniques. Dry reactive ion etch techniques are commonly used since most solution-based wet etch processes lack anisotropic pattern transfer capability. However, in silicon, anisotropic wet etch processes are available for the fabrication of nano-scale features, but have some constraints in the range of geometry of patterns that they can address. While this lack of geometric and material versatility precludes the use of these processes in applications like integrated circuits, they can be potentially realized for fabricating nanoscale pillars. This dissertation explores the geometric limitations of such inexpensive wet anisotropic etching processes and develops additional methods and geometries for fabrication of controlled nano-scale, high aspect ratio features. Jet and Flash Imprint Lithography (J-FIL™) has been used as the preferred pre-etch patterning process as it enables patterning of sub-50 nm high density features with versatile geometries over large areas. Exemplary anisotropic wet etch processes studied include Crystalline Orientation Dependent Etch (CODE) using potassium hydroxide (KOH) etching of silicon and Metal Assisted Chemical Etching (MACE) using gold as a catalyst to etch silicon. Experiments with CODE indicate that the geometric limitations of the etch process prevent the fabrication of high aspect ratio nanowires without adding a prohibitive number of steps to protect the pillar geometry. On the other hand, MACE offers a relatively simple process for fabricating high aspect ratio pillars with unique cross sections, and has thus been pursued to fabricate fully functional electrostatic capacitors featuring both circular and diamond-shaped nano-pillar electrodes. The capacitance of the diamond-shaped nano-pillar capacitor has been shown to be ~77.9% larger than that of the circular cross section due to the increase in surface area per unit projected area. This increase in capacitance approximately matches the increase calculated using analytical models. Thus, this dissertation provides a framework for the ability to create unique sharp cornered nanowires that can be explored further for a wider variety of cross sections.Mechanical Engineerin

Fabrication of lightweight Si/SiC LIDAR mirrors

A new, chemical vapor deposition (CVD) process was developed for fabricating lightweight, polycrystalline silicon/silicon-carbide (Si/SiC) mirrors. The process involves three CVD steps: (1) to produce the mirror faceplate; (2) to form the lightweight backstructure, which is deposited integral to the faceplate; and (3) to deposit a layer of optical-grade material, e.g., Si, onto the front surface of the faceplate. The mirror figure and finish are fabricated into the faceplate

A Comprehensive Review on Convex and Concave Corners in Silicon Bulk Micromachining based on Anisotropic Wet Chemical Etching

Wet anisotropic etching based silicon micromachining is an important technique to fabricate freestanding (e.g. cantilever) and fixed (e.g. cavity) structures on different orientation silicon wafers for various applications in microelectromechanical systems (MEMS). {111} planes are the slowest etch rate plane in all kinds of anisotropic etchants and therefore, a prolonged etching always leads to the appearance of {111} facets at the sidewalls of the fabricated structures. In wet anisotropic etching, undercutting occurs at the extruded corners and the curved edges of the mask patterns on the wafer surface. The rate of undercutting depends upon the type of etchant and the shape of mask edges and corners. Furthermore, the undercutting takes place at the straight edges if they do not contain {111} planes. {100} and {110} silicon wafers are most widely used in MEMS as well as microelectronics fabrication. This paper reviews the fabrication techniques of convex corner on {100} and {110} silicon wafers using anisotropic wet chemical etching. Fabrication methods are classified mainly into two major categories: corner compensation method and two-steps etching technique . In corner compensation method, extra mask pattern is added at the corner. Due to extra geometry, etching is delayed at the convex corner and hence the technique relies on time delayed etching. The shape and size of the compensating design strongly depends on the type of etchant, etching depth and the orientation of wafer surface. In this paper, various kinds of compensating designs published so far are discussed. Two-step etching method is employed for the fabrication of perfect convex corners. Since the perfectly sharp convex corner is formed by the intersection of {111} planes, each step of etching defines one of the facets of convex corners. In this method, two different ways are employed to perform the etching process and therefore can be subdivided into two parts. In one case, lithography step is performed after the first step of etching, while in the second case, all lithography steps are carried out before the etching process, but local oxidation of silicon (LOCOS) process is done after the first step of etching. The pros and cons of all techniques are discussed

Determination of precise crystallographic directions for mask alignment in wet bulk micromachining for MEMS

In wet bulk micromachining, the etching characteristics are orientation dependent. As a result, prolonged etching of mask openings of any geometric shape on both Si{100} and Si{110} wafers results in a structure defined by the slowest etching planes. In order to fabricate microstructures with high dimensional accuracy, it is vital to align the mask edges along the crystal directions comprising of these slowest etching planes. Thus, precise alignment of mask edges is important in micro/nano fabrication. As a result, the determination of accurate crystal directions is of utmost importance and is in fact the first step to ensure dimensionally accurate microstructures for improved performance. In this review article, we have presented a comprehensive analysis of different techniques to precisely determine the crystallographic directions. We have covered various techniques proposed in the span of more than two decades to determine the crystallographic directions on both Si{100} and Si{110} wafers. Apart from a detailed discussion of each technique along with their design and implementation, we have provided a critical analysis of the associated constraints, benefits and shortcomings. We have also summed up the critical aspects of each technique and presented in a tabular format for easy reference for readers. This review article comprises of an exhaustive discussion and is a handy reference for researchers who are new in the field of wet anisotropic etching or who want to get abreast with the techniques of determination of crystal directions

CMOS Compatible Wet Bulk Micromachining for MEMS Applications

Wet bulk micromachining of silicon is a convenient and economical method for realizing various silicon-based microsensors and actuators. Tetramethylammonium hydroxide (TMAH) based anisotropic wet etching is popular due to it being less toxic and CMOS compatible. The etch rate of TMAH depends on the wafer’s crystal plane orientation and temperature/concentration of solution. While using TMAH to realize a pressure sensor diaphragm, the etching of {111} planes causes underetching, causing a deviation in the intended size of the diaphragm, inducing variation in the designed characteristics of the device. It is necessary to estimate and minimize these deviations. Experiments were designed and the rate of etching for (100) and (111) planes using 25 wt.% TMAH have been determined at different temperatures. Linear fit equations are obtained from experimental data to relate the underetch per unit depth to the solution temperature. These findings are extremely useful in the fabrication of silicon diaphragms with precise dimensions. While using anisotropic wet etchants to realize proof mass for accelerometers, the etchants attack the convex corners. This necessitates a suitable design of compensating structure while realizing microstructures with sharp convex corners. Experimental studies are carried out to protect convex corners from undercutting and the results are reported

Gap and channelled plasmons in tapered grooves: a review

Tapered metallic grooves have been shown to support plasmons -- electromagnetically coupled oscillations of free electrons at metal-dielectric interfaces -- across a variety of configurations and V-like profiles. Such plasmons may be divided into two categories: gap-surface plasmons (GSPs) that are confined laterally between the tapered groove sidewalls and propagate either along the groove axis or normal to the planar surface, and channelled plasmon polaritons (CPPs) that occupy the tapered groove profile and propagate exclusively along the groove axis. Both GSPs and CPPs exhibit an assortment of unique properties that are highly suited to a broad range of cutting-edge nanoplasmonic technologies, including ultracompact photonic circuits, quantum-optics components, enhanced lab-on-a-chip devices, efficient light-absorbing surfaces and advanced optical filters, while additionally affording a niche platform to explore the fundamental science of plasmon excitations and their interactions. In this Review, we provide a research status update of plasmons in tapered grooves, starting with a presentation of the theory and important features of GSPs and CPPs, and follow with an overview of the broad range of applications they enable or improve. We cover the techniques that can fabricate tapered groove structures, in particular highlighting wafer-scale production methods, and outline the various photon- and electron-based approaches that can be used to launch and study GSPs and CPPs. We conclude with a discussion of the challenges that remain for further developing plasmonic tapered-groove devices, and consider the future directions offered by this select yet potentially far-reaching topic area.Comment: 32 pages, 34 figure

Fabrication and electrical integration of robust carbon nanotube micropillars by self-directed elastocapillary densification

Vertically-aligned carbon nanotube (CNT) "forest" microstructures fabricated by chemical vapor deposition (CVD) using patterned catalyst films typically have a low CNT density per unit area. As a result, CNT forests have poor bulk properties and are too fragile for integration with microfabrication processing. We introduce a new self-directed capillary densification method where a liquid is controllably condensed onto and evaporated from CNT forests. Compared to prior approaches, where the substrate with CNTs is immersed in a liquid, our condensation approach gives significantly more uniform structures and enables precise control of the CNT packing density and pillar cross-sectional shape. We present a set of design rules and parametric studies of CNT micropillar densification by this method, and show that self-directed capillary densification enhances the Young's modulus and electrical conductivity of CNT micropillars by more than three orders of magnitude. Owing to the outstanding properties of CNTs, this scalable process will be useful for the integration of CNTs as functional material in microfabricated devices for mechanical, electrical, thermal, and biomedical applications

3D integration of micro- and nanostructures into bio-analytical devices

This study aims to develop a process which allows 3D integration of micro and nanostructures in microchannels. A fabrication process was established for the large area integration of hierarchical micro and nanostructures in microchannels. This novel process, which is called 3D molding, takes advantage of an intermediate thin flexible stamp such as PDMS from soft lithography and a hard mold such as brass from hot embossing process. However, the use of a thin intermediate polydimethylsiloxane (PDMS) stamp inevitably causes dimensional changes in the 3D molded channel, with respect to those in the brass mold protrusion and the intermediate PDMS stamp structures. We have investigated the deformation behavior of the 3D molded poly(methyl methacrylate) (PMMA) substrate and the intermediate PDMS stamp in 3D molding through both experimentation and numerical simulation. It was found that for high aspect ratio brass mold protrusion, the maximum strain of the intermediate layer occurs in the bottom center of the 3D channels. However, with decreasing the aspect ratio of brass mold protrusion the highest elongation occurs at the bottom corners of the channel causing less elongation of the intermediate PDMS stamp and imprinted structures on the bottom surface of the 3D channel. A modified 3D molding process which is called 3D nanomolding is developed which allows nanopatterning the surface of small microfeatures. Using 3D nanomolding process and solvent assisted bonding microdevices with no side, one side, three sides and four sides patterned were fabricated. To characterize 3D flow patterns induced by the surface structures on microdevices, confocal microscopy was used as dyed water and undyed water injected from separate inlets of micromixer were mixed along the microchannel at flow rates of 10 and 40 μL/min. The standard deviation of the normalized intensity measured in the confocal image of the cross section of the channel was used for quantifying the degree of mixing and evaluating the mixing performance of all four different microdevices. Experimental and simulation results show that by patterning the surface of the micromixer, flow patterns can be manipulated, which can improve mixing through stretching and folding of fluid elements and therefore increasing the interfacial area between fluids and cutting down the diffusion length. The effect of increasing velocity on increasing standard deviation (decreasing mixing) was also found to be less for the micromixers whose surfaces are patterned compared to the plain channel