Investigating block mask lithography variation using finite-difference time-domain simulation

Simulation work has long been realized as a method for analyzing semiconductor processing expediently and cost-effectively. As technology advancements strive to meet increasingly stringent parameter constraints, difficult issues arise. In this paper, challenges in block mask lithography will be discussed with the aid of using simulation packages developed by Panoramic Technology®. Halo formation utilizes a 20-30° tilt-angle implantation [1]. The block mask defines the geometries of the resist opening to allow implantation of atoms to extend into the channel region. Due to designed resolution scaling and tolerance in conjunction with substrate topography, there can be undesired influence on the electrical device characteristics due to block variations. Although the block mask pattern definition is relatively simple, additional investigation is required to understand the sensitivities that drive the implant resist CD variation. In this study, block mask measurements processed using 248 nm and 193 nm illumination sources were used to calibrate the simulation work. Addition of optical proximity correction (OPC) and wafer topography geometry parameters have been shown to improve modeling capabilities. The modeling work was also able to show the benefits of a developable bottom anti-reflection coating (dBARC) process over a single layer resist (SLR) process in the resist intensity profiles as gate pitch is decreased. The goal of this work was to develop an accurate simulation model that characterizes the lithographic performance needed to support the transition into future technology nodes

Understanding Interfacial and Spin-Orbit Torque Effects in Thin Film Magnetic Multilayers.

The understanding of both the fundamentals of spin-orbit torques and the potential avenues for spin-orbit exploitation is crucial for the development of future spintronic devices. Accurate quantification of the relevant parameters, such as the spin-orbit effective field strengths, is vital for both performance comparisons and the understanding of the physical origins. In this work, a common effective field measurement technique is probed for its potential inaccuracies, a new method for achieving field-free magnetic reversal is demonstrated and both the structural and magnetic property variations in structurally inverted thin films are investigated. The design, construction and commissioning of a multi-functional MOKE instrument has been presented. The multi-functional nature of the instrument allowed for sample environments which included the simultaneous application of external fields and sample charge currents for spin-orbit torque effect measurements. Raster scanned 2D MOKE data were collected for multiple samples based upon a 20 µm-wide perpendicularly magnetised Pt/Co/Pt rectangular bar. Each rectangular bar had a differing lateral displacement with respect to the 60 µm wide electrical contacts, with positions varying from the centre of the electrical contacts to being parallel with the edge. It was shown that the series of samples exhibit deterministic bi-directional field-free magnetic reversal. After discussing the previously known methodologies for field-free magnetic reversal, a new methodology was required in order to explain the symmetry of the results, where the charge current Oersted field is the cause of both the initial domain nucleation and the overall bi-directional magnetic reversal pattern. The accuracy of a common methodology for quantifying the spin-orbit effective fields was studied by probing the effect of magnetic reversal. It was shown that if magnetic reversal occurs when otherwise unwanted, the resultant effective field values can be exaggerated by over an order of magnitude compared to their typical values. This inaccuracy can be undetectable, particularly in thin films with non unity magnetic remanence. Structural and magnetic property variations in Pt/CoFeTaB/Ir, together with its structural inverse, were probed with a variety of techniques including polarised neutron reflectometry. Both structural and magnetic variations were observed, with the most dominant change being a Curie temperature variation of over 100 K, where Ir/CoFeTaB/Pt is paramagnetic at room temperature whereas Pt/CoFeTaB/Ir is ferromagnetic

The development of magnetic tunnel junction fabrication techniques

The discovery of large, room temperature magnetoresistance (MR) in magnetic tunnel junctions in 1995 sparked great interest in these devices. Their potential applications include hard disk read head sensors and magnetic random access memory (MRAM). However, the fabrication of repeatable, high quality magnetic tunnel junctions is still problematic. This thesis investigates methods to improve and quantify the quality of tunnel junction fabrication. Superconductor-insulator-superconductor (SIS) and superconductor-insulatorferromagnet (SIF) tunnel junctions were used to develop the fabrication route, due to the ease of identifying their faults. The effect on SIF device quality of interchanging the top and bottom electrodes was monitored. The relationship between the superconducting and normal state characteristics of SIS junctions was investigated. Criteria were formulated to identify devices in which tunneling is not the principal conduction mechanism in normal metal-insulator-normal metal junctions. Magnetic tunnel junctions (MTJs) were produced on the basis of the fabrication route developed with SIS and SIF devices. MTJs in which tunneling is the principal conduction mechanism do not necessarily demonstrate high MR, due to effects such as magnetic coupling between the electrodes and spin scattering. Transmission electron microscope images were used to study magnetic tunnel junction structure, revealing an amorphous barrier and crystalline electrodes. The decoration of pinholes and weak-links by copper electrodeposition was investigated. A new technique is presented to identify the number of copper deposits present in a thin insulating film. The effect of roughness, aluminium thickness and voltage on the number of pinholes and weak-links per unit area was studied. High frequency testing of read heads at wafer level was performed with a network analyser. Design implications for read head geometry were investigated, independent of magnetic performance. This technique has great potential to aid the rapid development of read and write heads whilst improving understanding of the system.EPSRC Seagate Technolog

Novel Materials and Devices for Terahertz Detection and Emission for Sensing, Imaging and Communication

Technical advancement is required to attain a high data transmission rate, which entails expanding beyond the currently available bandwidth and establishing a new standard for the highest data rates, which mandates a higher frequency range and larger bandwidth. The THz spectrum (0.1-10 THz) has been considered as an emerging next frontier for the future 5G and beyond technology. THz frequencies also offer unique characteristics, such as penetrating most dielectric materials like fabric, plastic, and leather, making them appealing for imaging and sensing applications. Therefore, employing a high-power room temperature, tunable THz emitters, and a high responsivity THz detector is essential. Dyakonov-theory Shur\u27s was applied in this dissertation to achieve tunable THz detection and emission by plasma waves in high carrier density channels of field-effect devices. The first major contribution of this dissertation is developing graphene-based THz plasmonics detector with high responsivity. An upside-down free-standing graphene in a field effect transistor based resonant room temperature THz detector device with significantly improved mobility and gate control has been presented. The highest achieved responsivity is ~3.1kV/W, which is more than 10 times higher than any THz detector reported till now. The active region is predominantly single-layer graphene with multi-grains, even though the fabricated graphene THz detector has the highest responsivity. The challenges encountered during the fabrication and measurement of the graphene-based detector have been described, along with a strategy to overcome them while preserving high graphene mobility. In our new design, a monolayer of hBN underneath the graphene layer has been deposited to increase the mobility and electron concentration rate further. We also investigated the diamond-based FETs for their potential characteristics as a THz emitters and detectors. Diamond\u27s wide bandgap, high breakdown field, and high thermal conductivity attributes make it a potential semiconductor material for high voltage, high power, and high-temperature operation. Diamond is a good choice for THz and sub-THz applications because of its high optical phonon scattering and high momentum relaxation time. Numerical and analytical studies of diamond materials, including p-diamond and n-diamond materials, are presented, indicating their effectiveness as a prospective contender for high temperature and high power-based terahertz applications These detectors are expected to be a strong competitor for future THz on-chip applications due to their high sensitivity, low noise, tunability, compact size, mobility, faster response time, room temperature operation, and lower cost. Furthermore, when plasma wave instabilities are induced with the proper biasing, the same devices can be employed as THz emitters, which are expected to have a higher emission power. Another key contribution is developing a method for detecting counterfeit, damaged, forged, or defective ICs has been devised utilizing a new non-destructive and unobtrusive terahertz testing approach to address the crucial point of hardware cybersecurity and system reliability. The response of MMICs, VLSI, and ULSIC to incident terahertz and sub-terahertz radiation at the circuit pins are measured and analyzed using deep learning. More sophisticated terahertz response profiles and signatures of specific ICs can be created by measuring a more significant number of pins under different frequencies, polarizations, and depth of focus. The proposed method has no effect on ICs operation and could provide precise ICs signatures. The classification process between the secure and unsecure ICs images has been explained using data augmentation and transfer learning-based convolution neural network with ~98% accuracy. A planar nanomatryoshka type core-shell resonator with hybrid toroidal moments is shown both experimentally and analytically, allowing unique characteristics to be explored. This resonator may be utilized for accurate sensing, immunobiosensing, quick switching, narrow-band filters, and other applications

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