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Fabrication and Characterization of Advanced Through Glass Via Interconnects
Keywords: 3D Interposer Through Glass Via, Cu/NiFe nano-superlattice, K-band
In an effort to further develop technologies to improve overall radio frequency signal integrity within the K-band (14-40GHz), this work expands upon characterization of Through Glass Via (TGV) structures by applying a structure presented in another work to the K-band and implementing thin-film conducting non-ferromagnetic/ferromagnetic layering techniques (Toepper 2010, Kim & Yoon 2013, Rahimi & Yoon 2016). The structure consists of two coplanar waveguides (CPW) on top of a glass substrate leading inward to two vias that are connected on the opposite side by a trace. Both vias are part of the same signal trace. This work seeks to experimentally determine the lumped element model parameters of a single, highly vertical and uniform copper conformal-coated TGV and compare to the same parameters for a multilayered copper/permalloy TGV in the K-band (Kim & Yoon 2013). Kuramochi et al. presented experimental results for the entire structure, but not the TGV itself (Kuramochi et al. 2015). In addition, we present a different metallization method and thinner structure. Obtaining characteristics for the via itself would allow for a model to be developed for layered conductor TGVs and applied to future TGV work, or compared with other methods of using TGVs (Kim et al. 2015). The multi-conductor layering consists of several layers of nanometer-thick films, making up what is called a superlattice conductor (Rahimi & Yoon 2016). The thickness of each film was chosen based on the desired operating frequency range and ease of fabrication. Only the solid copper variant of the proposed device was simulated; the superlattice variant required more resources than were available at the time to simulate and obtain physically accurate results. Fabrication involves standard photolithographic fabrication of the desired conductor pattern onto a glass substrate, followed by sputter or E-beam evaporator deposition of the metals. The devices are then put through a standard photoresist removal process (i.e. acetone followed by methanol and de-ionized water), and the metal not bonded to the glass is lifted off from the substrate through ultrasonic vibrations, leaving the desired pattern. The resulting devices are then analyzed using a network analyzer. Simulation results have shown that the copper TGV with dimensions h=115μm, d=100μm, and copper thickness of 1.75μm, has a series resistance of 7.5Ω, varying slightly with frequency. Simulation results also show that the TGV has a series inductance of 0.3nH, parallel capacitance of 0.151pF, and parallel conductance of 0.1mS, all at 20GHz. These values change with frequency rather significantly, hence no one value can be assigned for the whole band. The purpose behind the thin conductor is so that a comparison can be made more directly with the multilayered structure. The ABCD parameters of the TGV itself can be represented as a function of the ABCD parameters of the CPW and the two test structures (Kim & Yoon 2013). The results obtained by other works shows that there should be a decrease in series resistance in comparing the solid copper structure to the multilayered structure (Rahimi & Yoon 2016).
Timothy Clingenpeel, Graduate Student
University of Florida
Gainesville, Florida

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