PhD Dissertation Defense by Robert Grant Spurney
Robert Grant Spurney
Title: Advanced Materials and Processes for High-Density Capacitors in Next-Generation Integrated Voltage Regulators
Prof. Rao Tummala, Advisor, MSE
Prof. Dong Qin, MSE
Prof. Preet Singh, MSE
Prof. Meilin Liu, MSE
Prof. P.M. Raj, ECE, Florida International
Capacitors are key components for power conversion, delivery, and management. Along with inductors, they dictate the size and performance of voltage regulators in power distribution networks (PDNs), which convert and regulate the power that gets delivered to the increasingly power-hungry integrated circuits (ICs). When the voltage regulators are integrated into the package, referred to as integrated voltage regulators (IVRs), they provide many benefits over traditional voltage regulators, including higher power density, system miniaturization, and improved efficiency. To enable IVRs, capacitors must be integrated either on-chip or in the package, while providing high capacitance density, high frequency stability, low equivalent series resistance (ESR), and high temperature stability. Tantalum capacitors have an advantage over many other types of capacitor technologies due to their high capacitance density and high temperature stability they can provide. The tantalum nanoparticle-based anode provides an ultra-high surface area, so that high volumetric capacitance densities can be achieved. A Ta2O5 dielectric can be formed directly on the anode structure to provide a high-permittivity oxide that is incredibly stable with changes in temperature. However, the high-surface area electrodes result in long electrical pathways for charge and discharge current, which results in capacitors with high equivalent series resistance (ESR) and low frequency stability. Additionally, their bulky design limits their integration capability.
This research proposes, designs, and demonstrates a novel printed-tantalum thin-film capacitor design that solves many of the issues associated with traditional tantalum capacitors. The thin-film design results in capacitors with an ultra-thin form factor in thickness that can be integrated into the package. Additionally, the thin structure provides shorter pathways for the charge and discharge current, to dramatically improve the frequency stability and reduce the ESR of the capacitors, all while maintaining ultra-high capacitance density. In this thesis, a model is developed that is used to correlate the capacitor materials’ nanostructures to the bulk device properties, including capacitance density, frequency stability, and ESR. A process is then developed to fabricate the capacitors and integrate them directly on-package, while studying the relationships between process, structure, and performance. The integrated capacitors are shown to meet the performance objectives set out by this work. Finally, evaluation of the capacitor reliability is conducted. It is shown that the use of barrier layer materials can extend the high-temperature lifetime of the capacitors by limiting the diffusion of oxygen and moisture into the capacitor material system.