Electrochemical materials for integrated magnetics

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Smallwood, Daniel C.
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University College Cork
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Next generation microinductors with magnetically enhanced VIA technology hold great promise for power converter applications in broad technology domains such as automotive, space, high-end computing, mobile devices, radio frequency (RF), artificial intelligence (AI) and the internet of things (IoT). Microinductor VIAs enable monolithic 3D device topologies with reduced footprint, increased inductance density and minimal parasitics. These qualities are essential for emerging 2.5/3D packaging architectures that require granular point-of-load (PoL) power delivery to efficiently supply a multitude of heterogeneously integrated devices. This thesis addresses the challenges of 3D monolithic microinductor design and fabrication, inclusive of magnetically enhanced VIAs comprising a clad laminated soft magnetic core. The current state-of-the-art utilizes 2D microinductor topologies and 2D fabrication methods, therefore significant advancement is required to enable fabrication of a novel 3D monolithic microinductor device comprising vertically oriented integrated magnetics. The major challenges addressed in this thesis fall into two main categories: 1) predictive modeling with computational lithography and computational electrochemistry to enable optimization of the VIA formation process and 2) the design and fabrication of a novel magnetically enhanced monolithic 3D microinductor device. A major contribution from the computational lithography is the derivation of a novel polychromatic light attenuation equation that is used to produce a succinct formula comprising a complete coupling between resist photochemistry and light diffraction effects. Additionally, new photoresist exposure dose determination methods are presented that negate the need for time consuming and costly in-situ metrology. These equations and methods enable fast and accurate predictive modeling of 3D photoresist VIA latent images, which are verified by comparison to directly corresponding experimental work, with highly positive correlation. These formulas converge quickly on the average modern computer and can be readily integrated into lithography simulators. Photoresist development is then investigated, wherein spin development is identified as the optimal method for wet etching VIA latent images. With computational electrochemistry, the electroforming process of Cu VIAs is explored using the FEM in COMSOL Multiphysics to perform 2D and 3D time-dependent simulation studies. Simulations are then verified by comparison to experimental results, with highly positive correlation. Special attention is given to electroformed surface topographies, which is valuable for sensor and flip chip applications. The major contributions from the microinductor device design and fabrication first include designing a unique device that meets target specifications for reduced footprint, increased inductance density and minimized parasitics. A novel fabrication process flow is next engineered to enable a vertically meandering current path with a repeating unit cell comprising a bottom interconnect, a first Cu VIA, a top interconnect and a second Cu VIA. This process flow is compatible with conformal deposition of a soft magnetic laminate (e.g., CoZrTa) for formation of a vertically oriented magnetic core clad on the Cu VIAs. Next, a 5-tiered photomask stack is designed and the corresponding SOPs are engineered. This enables fully in-house microinductor device fabrication, after which vital metrology and characterization is performed. The measured inductance density of our prototype magnetically enhanced monolithic 3D microinductor devices is 16.85 nH/mm2, which is comparable to previously reported metrics for fabricated 3D microinductors. This metric could be significantly improved in future devices by increasing the magnetic core thickness and/or optimizing the magnetic anisotropy characteristic of the integrated magnetic material and/or reducing the pillar diameter, wherein the VIA fabrication research presented in this thesis will be essential. Therefore, this novel microinductor research holds great promise for applications in next generation power converters.
Physics , Engineering , Magnetics , Electrochemistry , Photolithography , Optics , Photonics , Simulation , Modelling , Microinductors , Packaging , MEMS , NEMS , Diffraction , FEM , Mathematics , COMSOL , Fabrication , Design
Smallwood, D. C. 2021. Electrochemical materials for integrated magnetics. PhD Thesis, University College Cork.