Surface instabilities and interfacial phenomena for nanomanufacturing at the atomically-thin limit

Two-dimensional (2D) layered materials, exemplified by the prototypical graphene, have been intensively studied for their diverse material properties and superlative mechanical strength. Due to their atomically-thin nature, weak basal plane van der Waals interactions, and vanishing bending stiffness, 2D materials are extremely flexible and thus susceptible to mechanical instabilities that result in deformed out-of-plane morphologies. Such unique combination of material properties and mechanical anisotropy presents new scientific and practical challenges, but also enables novel opportunities in the nanomanufacture of 2D materials and of their derivative materials systems and devices. Surface instabilities (e.g. wrinkling and buckling) and interfacial phenomena (e.g. delamination) are typically deemed as engineering nuisances and failure modes. However, these universally ubiquitous phenomena can instead be harnessed to realize novel strategies and architectures for precise manipulation and assembly of 2D and other low-dimensional nanoscale materials, the combination of which contributes to an ever-growing toolset of capabilities towards layer-by-layer nanomanufacturing at the atomically-thin limit. This dissertation details new methods that have been developed to deterministically create hierarchical and deformed 2D materials via large-scale elastic strain engineering and controlled shape memory deformation. The emergent tunable 3D architectures arising from flat 2D materials exhibit large-scale, uniform, and well-organized patterns with characteristic length scales spanning from tens of nanometers to few microns without any a priori patterning or lithographic definition of the constituent sub-nanometer 2D thin films. By controlling bulk substrate deformation, this highly robust and scalable process imparts spatially heterogeneous strain gradients that perturb the intrinsic lattice structure and consequently the local optoelectronic properties of atomically-thin monolayer graphene analogs such as semiconducting transition metal chalcogenides, thus creating highly uniform and periodic lateral superlattice configurations. In addition, the generality of this self-patterning scheme allows for facile and scalable definition of nanoscale architectures for template guided nano-convective/capillary self-assembly of arbitrary 0D/1D nanoparticles onto deformed 2D substrates. Here, high quality colloidally prepared gold nanoparticles of diverse shapes and sizes readily self-assemble into various tunable structured mixed-dimensional metamaterials, opening the opportunity to investigate emergent phenomena such as those arising from coupling between metallic plasmonic nanostructures/nanoparticles with excitons and other quasiparticles in 2D materials. Finally, with the eventual goal towards large-scale nano-manufacturing of these 2D materials and devices, a new technique has been developed to cleanly and sustainably manufacture graphene and recycle the catalyst metal substrate using benign materials. By separating the 2D material from the growth substrate via electrochemical interfacial delamination, this method forgoes the harsh chemicals typically used in conventional processing of 2D materials while simultaneously avoiding expenditure of the expensive precursors, thus leading to scalable production of high quality, clean graphene with reduced negative externalities

Development and Packaging of Microsystems Using Foundry Services

Micro-electro-mechanical systems (MEMS) are a new and rapidly growing field of research. Several advances to the MEMS state of the art were achieved through design and characterization of novel devices. Empirical and theoretical model of polysilicon thermal actuators were developed to understand their behavior. The most extensive investigation of the Multi-User MEMS Processes (MUMPs) polysilicon resistivity was also performed. The first published value for the thermal coefficient of resistivity (TCR) of the MUMPs Poly 1 layer was determined as 1.25 x 10(exp -3)/K. The sheet resistance of the MUMPs polysilicon layers was found to be dependent on linewidth due to presence or absence of lateral phosphorus diffusion. The functional integration of MEMS with CMOS was demonstrated through the design of automated positioning and assembly systems, and a new power averaging scheme was devised. Packaging of MEMS using foundry multichip modules (MCMs) was shown to be a feasible approach to physical integration of MEMS with microelectronics. MEMS test die were packaged using Micro Module Systems MCM-D and General Electric High Density Intercounect and Chip-on-Flex MCM foundries. Xenon difluoride (XeF2) was found to be an excellent post-packaging etchant for bulk micromachined MEMS. For surface micromachining, hydrofluoric acid (HF) can be used

Fabrication of an Atom Chip for Rydberg Atom-Metal Surface Interaction Studies

This thesis outlines the fabrication of two atom chips for the study of interactions between ⁸⁷Rb Rydberg atoms and a Au surface. Atom chips yield tightly confined, cold samples of an atomic species by generating magnetic fields with high gradients using microfabricated current-carrying wires. These ground state atoms may in turn be excited to Rydberg states. The trapping wires of Chip 1 are fabricated using thermally evaporated Cr/Au and patterned using lift-off photolithography. Chip 2 uses a Ti/Pd/Au tri-layer, instead of Cr/Au, to minimize interdiffusion. The chip has a thermally evaporated Au surface layer for Rydberg atom-surface interactions, which is separated from the underlying trapping wires by a planarizing polyimide dielectric. The polyimide was patterned using reactive ion etching. Special attention was paid to the edge roughness and electrical properties of the trapping wires, the planarization of the polyimide, and the grain structure of the Au surface

Fabrication of an Atom Chip for Rydberg Atom-Metal Surface Interaction Studies

This thesis outlines the fabrication of two atom chips for the study of interactions between ⁸⁷Rb Rydberg atoms and a Au surface. Atom chips yield tightly confined, cold samples of an atomic species by generating magnetic fields with high gradients using microfabricated current-carrying wires. These ground state atoms may in turn be excited to Rydberg states. The trapping wires of Chip 1 are fabricated using thermally evaporated Cr/Au and patterned using lift-off photolithography. Chip 2 uses a Ti/Pd/Au tri-layer, instead of Cr/Au, to minimize interdiffusion. The chip has a thermally evaporated Au surface layer for Rydberg atom-surface interactions, which is separated from the underlying trapping wires by a planarizing polyimide dielectric. The polyimide was patterned using reactive ion etching. Special attention was paid to the edge roughness and electrical properties of the trapping wires, the planarization of the polyimide, and the grain structure of the Au surface

New organic-inorganic sol-gel resists for micro and nanoimprinting

In this study new hybrid organic-inorganic TiO2 sol-gel materials with high TiO2 content (more than 50% respect silicon dioxide (SiO2) content) are synthesized, characterized and micro-/nano-patterned using thermal nanoimprint lithography

Design and micro-fabrication of tantalum silicide cantilever beam threshold accelerometer

Microfabricated threshold accelerometers were successfully designed and fabricated following a careful analysis of the electrical, mechanical, and fabrication issues inherent to micron-sized accelerometers. A uniform cantilever beam was chosen because of the simplicity of design and fabrication. New models for the electrostatic force exerted on the cantilever beam were developed and calculations were made that accurately predicted the electrical characteristics of the accelerometer. The calculations also provided design guidelines for optimizing the accelerometer dimensions. Computer simulation demonstrated that the error of the electrostatic force, calculated using the most accurate model, was within 2% of the actual force which was obtained by integrating the closed formula, through the bent beam curvature, for device parameters designed to detect an acceleration of 50 g. Conversely, it was shown that the widely used conventional parallel plate model had an error of approximately 90%. Novel surface micromachining process steps were successfully developed to fabricate the cantilever beam accelerometers. Sputter deposited tantalum silicide and commercially available spin-on-glass were used as a structural layer and a sacrificial layer, respectively. The dependence of resistivity, crystalline structure, Young\u27s modulus, and hardness of the tantalum silicide films on the annealing temperatures were measured. These results were employed to design accelerometers that were successfully operated. Excluding the metallization steps, only two masks and four photolithography steps were required. However, both positive and negative photoresists had to be utilized. NJIT\u27s standard photolithography steps were used for positive photoresist; however for the negative photoresist a specially developed multi-puddle process was used to obtain 4 micron resolution. Electrostatic attraction tests, of accelerometers, were performed using the Keithley current-voltage measurement system. These tests used deflection voltages ranging from 2.2 to 37.0 volts, corresponding to threshold acceleration levels from 580 to 18,500 g. Nearly 70 percent of the threshold voltage results fell within the expected error limits set by the accuracy of the device dimensions when processing tolerances were taken into account including the thickness variation caused by 8% uncertainty in the buffered HF etch rate of tantalum silicide. Some accelerometers were closed and opened 3 times without failure. The accelerometers tended to break after 3 times of operation and this was attributed to the welding of contacts. Centrifuge acceleration tests of accelerometers were carried out in a specially designed centrifuge in an acceleration range of 282 to 11,200 g. Nearly 80 percent of the threshold acceleration results fell within the expected error limits set by the accuracy of the device dimensions when processing tolerances were taken into account

The Journal of Microelectronic Research 2005

https://scholarworks.rit.edu/meec_archive/1014/thumbnail.jp

Phosphide-based optical emitters for monolithic integration with GaAs MESFETs

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Civil and Environmental Engineering, 1995.Includes bibliographical references (p. 137-144).by Joseph F. Ahadian.M.S

Nanoscale mechanical and electrical properties of low-dimensional structures

In this thesis, we mainly study the mechanical, electrical and electromechanical properties of low-dimensional structures of advanced materials, in particular two-dimensional (2D) materials and compound semiconductor (CS) structures and devices. Given the scarcity of methods for direct nano-mapping of physical properties of complex three-dimensional (3D) multilayer CS and 2D materials heterostructures, we adapted and developed suitable optical methods and functional scanning probe microscopies (SPM) approaches based in atomic force microscopy (AFM). These allowed us to successfully investigate the behaviour of one- and two dimensional (1D and 2D) free oscillating structures, such as AFM cantilevers, tuning forks (TF), Si3N4 membranes and graphene drums using the optical laser Doppler vibrometry (LDV) and dynamic AFM modes, finding governing relations of the dynamic behaviour in real-life systems and comparing these with modelling. In addition to the existing ultrasonic SPM, such as force modulation and ultrasonic force microscopy (FMM and UFM), we developed a new method called modulation ultrasonic force microscopy (M-UFM), which allows for nonlinear local excitation and the probing of membrane vibrations. Furthermore, we probe mechanical, electrical and thermal properties of supported layers and heterostructures of diverse transition metal dichalcogenides (TMDCs) and franckeite, understanding their intrinsic surface and subsurface nanostructure. In the final part of this thesis, we explored the feasibility of combining nano-sectioning via Beam Exit Cross-sectional Polishing (BEXP) and the material sensitive SPM analysis for the investigation of defects in CS structures, such as multiple quantum wells (MQW) and nanowires (NWs), and 2D material heterostructures. We applied this methodology to investigate the propagation of material defects, such as antiphase domains in CS, and their effects on the morphology, nanomechanics and electric properties in MQW structures, and to directly observe reverse piezoelectric domains inside individual GaN NWs