Investigating Microwave-Synthesized Nanoferrites as Core Materials for RF On-Chip Inductors

Abstract

On-chip inductors play a major role in radio frequency integrated circuits (RFICs), finding applications in radio frequency microelectromechanical systems (RF MEMS), power-electronics, and biomedical MEMS. Despite advancements in transistor technology enabling more transistors on a single chip, the lag in miniaturizing passive components, especially on-chip inductors, poses a bottleneck in meeting the demands of 5G and beyond. Significant progress has been made by leveraging advanced 3D MEMS fabrication techniques to enhance the inductance density of on-chip inductors. Nevertheless, when traditional fabrication methods like planar processing are applied to 3D structures, challenges such as mechanical stability, conformity, alignment, and cost arise. Researchers have looked at magnetic core materials that can be integrated into the CMOS technology to improve the inductance performance at GHz frequencies. The Microwave-assisted solvothermal (MAS) process offers a backend-of-the-line (BEOL) compatible, low-temperature, rapid, and single-step method for depositing conformal ferrite thin films. Work by Sai et al. with MAS-based zinc ferrite and manganese zinc ferrite has shown significant improvements, approximately 13% in inductance and about 25% in the Q factor, when integrated into on-chip inductors operating at GHz frequencies. This suggests that the ferromagnetic resonance frequency (f_FMR) of MAS-based ferrites is well in the GHz range. However, the reason behind MAS-based ferrites exhibiting high-frequency response, unlike bulk ferrites, remains unknown. It is crucial to gain a deeper understanding of the physics governing the high-frequency response and its connection to the material structure to leverage these unique properties. This dissertation takes a comprehensive approach, from examining the formation of ferrites in the MAS process to analyzing the resulting structural properties that lead to distinctive high-frequency magnetic properties and their integration into on-chip inductors to enhance performance. In this regard, this thesis investigates the chemical pathways within the MAS process, leading to iron oxides forming from the reactions between iron(III) acetylacetonate in solvents with varying microwave activity levels. This detailed analysis of the reaction products enables us to elucidate prenucleation reactions and growth mechanisms, shedding light on some unique properties of MAS-based ferrites, as discussed in the subsequent part of the presentation. Then, we will focus on the synthesis and characterization of spinel ferrite nanoparticles and thin films, which are relevant as magnetic cores for on-chip inductors. A comprehensive Raman analysis unveils the far-from-equilibrium cation distribution of MAS-based superparamagnetic ferrites. Additionally, two distinct methods are introduced—a post-synthesis process and an in-situ process—to fine-tune the cation distribution of ferrites. These methods provide valuable insights into how cation distribution impacts the static and high-frequency magnetic properties of these materials. In-depth studies of the frequency-dependent complex permeability were conducted to determine the operational frequency of the materials. The results illuminate the coexistence of superparamagnetic and ferromagnetic resonance in thin films characterized by well-formed, ultra-fine crystallites of spinel ferrite. Notably, the superparamagnetic resonance frequency extends the cut-off frequency of nickel-zinc ferrites up to 25 GHz, breaching Snoek’s limit. Subsequently, this film is integrated as the core of an on-chip inductor, increasing inductance and Q factor up to 30% and 17%, respectively. During the deposition process, we encountered two significant challenges. Firstly, with the increase in film thickness, the film delaminates from the substrate surface due to the variations in the thermal expansion coefficients between the substrate and the film. Secondly, the ferrite material was needed to effectively etch above the contact pads to enable accurate measurements. To overcome these hurdles, a multi-layer deposition approach was adopted to alleviate stress during film growth. Additionally, a chlorine-based wet etching method was also devised to pattern the ferrite film precisely. Finally, we explain the random distribution of cations in the materials using the hotspot theory. Furthermore, we explore the potential utilization of high-temperature hotspots in developing entropy-stabilized oxide materials