Nanogap Device: Fabrication and Applications

A nanogap device as a platform for nanoscale electronic devices is presented. Integrated nanostructures on the platform have been used to functionalize the nanogap for biosensor and molecular electronics. Nanogap devices have great potential as a tool for investigating physical phenomena at the nanoscale in nanotechnology. In this dissertation, a laterally self-aligned nanogap device is presented and its feasibility is demonstrated with a nano ZnO dot light emitting diode (LED) and the growth of a metallic sharp tip forming a subnanometer gap suitable for single molecule attachment. For realizing a nanoscale device, a resolution of patterning is critical, and many studies have been performed to overcome this limitation. The creation of a sub nanoscale device is still a challenge. To surmount the challenge, novel processes including double layer etch mask and crystallographic axis alignment have been developed. The processes provide an effective way for making a suspended nanogap device consisting of two self-aligned sharp tips with conventional lithography and 3-D micromachining using anisotropic wet chemical Si etching. As conventional lithography is employed, the nanogap device is fabricated in a wafer scale and the processes assure the productivity and the repeatability. The anisotropic Si etching determines a final size of the nanogap, which is independent of the critical dimension of the lithography used. A nanoscale light emitting device is investigated. A nano ZnO dot is directly integrated on a silicon nanogap device by Zn thermal oxidation followed by Ni and Zn blanket evaporation instead of complex and time consuming processes for integrating nanostructure. The electrical properties of the fabricated LED device are analyzed for its current-voltage characteristic and metal-semiconductor-metal model. Furthermore, the electroluminescence spectrum of the emitted light is measured with a monochromator implemented with a CCD camera to understand the optical properties. The atomically sharp metallic tips are grown by metal ion migration induced by high electric field across a nanogap. To investigate the growth mechanism, in-situ TEM is conducted and the growing is monitored. The grown dendrite nanostructures show less than 1nm curvature of radius. These nanostructures may be compatible for studying the electrical properties of single molecule

Fabrication of ferroelectrics based MEMS structures for electronically switchable bulk acoustic wave resonators

The thesis describes the research carried out into fabrication of multilayer microwave capacitance structure with ferroelectric films in paraelectric state; and confirmation of the possibility to develop on their base an electronically switchable bulk acoustic wave (BAW) resonator. Different eigenmodes of acoustic resonances can be excited and switched electronically through the application to ferroelectric layers of the resonator unidirectional or oppositely directed dc biased electric fields.The resonator was fabricated out of a SrRuO3/SrTiO3/SrRuO3/YSZ multilayer structure deposited on top of Si substrate. Pulsed Laser Deposition, Magnetron Sputtering, Photolithography, Argon Ion Beam Milling, and Reactive Ion Etching were the fabrication methods used to make this resonator.This novel device is a demonstrator that will contribute to the telecommunications industry’s demand for flexibility in both microwave frequency switching and tuning. The Si MEMS concept of this resonator allows easy circuit board integration into many electronics products.Open Acces

Tribology of Microball Bearing MEMS

This dissertation explores the fundamental tribology of microfabricated rolling bearings for future micro-machines. It is hypothesized that adhesion, rather than elastic hysteresis, dominates the rolling friction and wear for these systems, a feature that is unique to the micro-scale. To test this hypothesis, specific studies in contact area and surface energy have been performed. Silicon microturbines supported on thrust bearings packed with 285 µm and 500 µm diameter stainless steel balls have undergone spin-down friction testing over a load and speed range of 10-100mN and 500-10,000 rpm, respectively. A positive correlation between calculated contact area and measured friction torque was observed, supporting the adhesion-dominated hysteresis hypothesis. Vapor phase lubrication has been integrated within the microturbine testing scheme in a controlled and characterized manner. Vapor-phase molecules allowed for specifically addressing adhesive energy without changing other system properties. A 61% reduction of friction torque was observed with the utilization of 18% relative humidity water vapor lubrication. Additionally, the relationship between friction torque and normal load was shown to follow an adhesion-based trend, highlighting the effect of adhesion and further confirming the adhesion-dominant hypothesis. The wear mechanisms have been studied for a microfabricated ball bearing platform that includes silicon and thin-film coated silicon raceway/steel ball materials systems. Adhesion of ball material, found to be the primary wear mechanism, is universally present in all tested materials systems. Volumetric adhesive wear rates are observed between 4x10^-4 µm^3/mN*rev and 4x10^-5 µm3/mN*rev were determined by surface mapping techniques and suggest a self-limiting process. This work also demonstrates the utilization of an Off-The-Shelf (OTS) MEMS accelerometer to confirm a hypothesized ball bearing instability regime which encouraged the design of new bearing geometries, as well as to perform in situ diagnostics of a high-performance rotary MEMS device. Finally, the development of a 3D fabrication technique with the potential of significantly improving the performance of micro-scale rotary structures is described. The process was used to create uniform, smooth, curved surfaces. Micro-scale ball bearings are then able to be utilized in high-speed regimes where load can be accommodated both axially and radially, allowing for new, high-speed applications. A comprehensive exploration of the fundamental tribology of microball bearing MEMS has been performed, including specific experiments on friction, wear, lubrication, dynamics, and geometrical optimization. Future devices utilizing microball bearings will be engineered and optimized based on the results of this dissertation

Micromechanical characterization of ALD thin films

Atomic layer deposited (ALD) films have become essential for various microelectromechanical systems (MEMS) due to their excellent properties: ALD films are conformal, uniform, dense, and pin-hole free. The main requirement for any film to be applied in MEMS is to exhibit good mechanical properties. Good mechanical properties mean that film has low residual stress, high fracture and interfacial strengths, and known elastic properties under applied mechanical load. MEMS devices are often subjected to the environmental stress. Therefore, it is important to evaluate mechanical properties also after environmental stress conditions. In this doctoral dissertation, the mechanical properties of ALD thin films are evaluated by means of bulge and MEMS shaft-loaded techniques (SLT). Both techniques are very valuable because mechanical properties of thin films are extracted without influence of underlying substrate. The bulge method is a non-contact method, in which overpressure is applied to load free-standing membrane until it fractures.In the MEMS SLT, the integrated shaft loads free-standing membrane facilitating the extraction of mechanical properties.The developed technique is attractive for characterization mechanical properties of variable thin films due to offered repeatability, precision, and non-piercing nature (the premature fracture by sharp indenter tip is avoided). In this doctoral dissertation, MEMS SLT was employed, in addition, for quantitative and qualitative evaluation of interfacial strength between two thin films. A new method to study adhesion between extra thin films and various substrates was developed (when conventional scratch testing is not appropriate: when substrates or coatings break before the coating is delaminated). The solution was to embed micro-spheres into the coating. These spheres were laterally detached using microrobotic set-up. This approach facilitated the extraction of interfacial mechanical properties, such as critical load and critical stress needed for removal of a coating. This doctoral dissertation describes the mechanical properties of ALD Al2O3, Al2O3/TiO2 nanolaminates, AlxTiyOz mixed oxide and graphene/ALD Al2O3 composites. These materials are promising for MEMS as suspended membranes in thermal devices like bolometers, in chemical sensors like microhotplates and as windows in X-ray optics. The adhesion properties between sputtered films and ALD Al2O3 were measured with MEMS SLT. A new method with the lateral displacement of microspheres led to extraction of interfacial properties between ALD TiO2 and glass substrate. This information is important to prevent debonding events when fabricating or using MEMS structures

Design, Fabrication, and Characterization of a MEMS Based Thermally Actuated Fabry-Pérot Interferometer.

MEMS devices have become ubiquitous in consumer devices and are also being used to conduct experiments on the nano and micro scale. There is a growing need to test the properties of materials at the micro and nanoscale. In order to test those materials, a reliable method of sensing displacement is needed. Another growing area of MEMS research is in creating micro optical cavities that allow for manipulation and control of atoms in QED research. This thesis describes a MEMS based thermally actuated Fabry-Pérot cavity interferometer that has potential as a displacement sensing mechanism for use in material testers and other devices which require motion feedback. Additionally the device has a potential application as a tunable cavity for use in cavity QED experiments. The theory behind the operation of the thermal actuator and the Fabry-Pérot cavity are shown. The design of the actuator and cavity is also discussed in detail as well as the fabrication of both structures. Experiments of the device were performed in a vacuum and in air. The data obtained from experiments are compared to FEA and MATLAB simulations to verify the performance of the device

Bi-stable buckled energy harvesters actuated via torque arms.

Vibrational energy harvesters (VEH) are one way to generate electricity. Though the energy quantities are not enough to run desktop computers, they can power remote devices such as temperature, pressure, and accelerometer sensors or power biological implants. New versions of the Bluetooth protocol can even be used with VEH technology to send wireless data. An important aspect of VEH devices is the power output, operating frequency, and bandwidth. This dissertation investigates a novel method of actuating the primary buckled energy harvesting structure using torque arms as a force amplification mechanism. Buckled structures can exhibit snap-through and has the potential to broaden the operating frequency for the VEH. Macro and MEMS size prototypes are fabricated and evaluated via a custom made shaker table. The effect of compliance arms, which pin the center beam with piezoelectric strips, are also evaluated along with damping ratios. ANSYS models evaluating generated power are created for use in future optimization studies. Lastly, high energy orbitals (HEO) are observed in the devices. Results show that buckling lowers and broadens the output power of the new devices. Reverse sweeps drastically increase the operating frequency during snap-through. Rectangular compliance arms made of poly-lactic acid (PLA) generated the most power of all compliance arms tested. HEO performance can be induced by perturbing the system while maintaining the same input force which increases power output