MATERIALS CHARACTERIZATION FOR SUB-MICRON SUPERCONDUCTING INTERCONNECTS IN RECIPROCAL QUANTUM LOGIC CIRCUITS
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Today's server based computing consumes a considerable amount of energy.Reciprocal Quantum Logic (RQL) is a classical logic family within superconducting electronics, and is a candidate for energy efficient computing technologies. Similar to the current complimentary metal-oxide semiconductor technologies, RQL interconnects are responsible for dissipating the majority of the energy. The energy dissipated in RQL interconnects comes from finite resistive losses in the superconducting wires and embedded dielectrics at radio frequencies. Therefore, material properties, processing, and performance are critical to understanding the mechanisms of loss and mitigation of power dissipation in RQL interconnects.
This dissertation presents work on three aspects of materials characterization of RQL interconnects: implementing a method to deconvolve superconducting and dielectric losses, evaluating losses in three generations of RQL fabrication, and understanding the microscopic physics that determines the performance of RQL interconnects in a temperature and frequency range from 1.5-6 K and 3-12 GHz, respectively.
A novel method to accurately deconvolve superconducting and dielectric losses by exploiting their frequency dependence is described. Furthermore, a finite element modeler is used to accurately extract the losses. This method is termed Dispersive Loss Deconvolution. The designed microstrip transmission line resonators are fabricated in a 0.25 $\mu m$ RQL fabrication process composed of Nb wires embedded in Tetraethyl orthosilicate (TEOS) dielectric. The Nb and TEOS losses as a function of microstrip width down to 0.25 $\mu m$ are modeled and measured.
The electrical and physical material properties for 3 RQL processes over 5 wafers are evaluated.The electrical properties were evaluated by characterizing resonators in cryogenic dip probes and a dry system with $\pm 10 : mK$ temperature control. The physical properties were evaluated using Transmission Electron Microscopy and Energy-Dispersive Spectroscopy. Two of the processes use chemical mechanical polishing (CMP) to planarize the Nb wires, and the other using reactive ion etching (RIE) to define Nb wires.
At 4.2 K, the Nb loss in the 0.25 $\mu m$ resonators between the 3 processes were surprisingly distinct. The two CMP processes yield Nb losses up to 2 times higher relative to the RIE process, and have a discernible increase in loss by as much as 20% going from 4 to 0.25 $\mu m$ microstrip widths. For the RIE process, there is no detectable upturn in Nb losses for microstrip widths down to 0.25 $\mu m$. Most notably, the RIE process produced 0.25 $\mu m$ Nb wires with loss reaching the theoretical lower limit of intrinsic surface resistance $R_s = 17 : \mu \Omega$ at 4.2 K and 10 GHz. The superior RIE process may be linked to the incorporation of thin metal passivation layers protecting the Nb, which prevented Nb oxide from participating in additional loss mechanisms. %The CMP processes had detectable Ar concentrations in the Nb up to 1 $at%$ most likely due to the trench filling process. For all 3 processes and microstrip widths from 0.25-4 $\mu m$, the TEOS losses had negligible width dependence and varied by as much as $\pm 20%$.
From the electrical characterization at 4.2 K, it was found that the Nb wires are the limiting loss mechanisms in RQL interconnects. As temperature is decreased below 4.2 K, it is well known that Nb loss will exponentially decrease and amorphous dielectrics like TEOS can have loss with a non-monotonic temperature dependence depending on the input power. This offered the opportunity to explore a possible optimum operating temperature to minimize power dissipation by the RQL interconnects. At relatively low input powers, TEOS became the limiting loss mechanism for temperatures below 3 K, and I conclude this can be attributed to losses coming from two-level system tunneling relaxation and resonant absorption processes. %Estimates of peak currents and voltages are used to %A loss spectroscopy method is presented as a tool to
The work in this dissertation describes the development of methods to aid in characterization, design, and fabrication of RQL interconnects, and can be extended to potentially other Single Flux Quantum and Quantum Computing technologies.