Improved critical-current-density uniformity by using anodization

Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
IEEE Transactions on Applied Superconductivity (Impact Factor: 1.24). 07/2003; 13(2):111 - 114. DOI: 10.1109/TASC.2003.813658
Source: IEEE Xplore


We discuss an anodization technique for a Nb superconductive-electronics-fabrication process that results in an improvement in critical-current-density Jc uniformity across a 150-mm-diameter wafer. We outline the anodization process and describe the metrology techniques used to determine the NbOx thickness grown. In the work described, we performed critical current Ic measurements on Josephson junctions distributed across a wafer. We then compared the Jc uniformity of pairs of wafers, fabricated together, differing only in the presence or absence of the anodization step. The cross-wafer standard deviation of Jc was typically ∼5% for anodized wafers but >15% for unanodized wafers. This difference in Jc uniformity is suggestive of an in-process modification from an unknown cause that is blocked by the anodic oxide. It is interesting that small junctions do not see an improvement in Ic uniformity - apparently the anodization improves only the Jc uniformity and not the variation in junction size. Control of Jc is important for all applications of superconductive electronics including quantum computation and rapid single-flux quantum (RSFQ) circuitry.

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Available from: Karl K. Berggren, Mar 28, 2013
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    • "To improve the quality of Nb/Al–AlOx/Nb junctions, anodization process was widely used for covering the side-leakage current. Since Kroger et al. [7] developed the selective niobium anodization process (SNAP), a number of modifications and variations have been introduced [8] [9] [10] [11]. However, most of the reports only focused on application or experiment changing of Nb anodization process, which is not suitable for understanding how it works in details. "
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    ABSTRACT: The profile and interface characteristics of anodized Nb (Nb-oxide) layer were investigated using atomic force microscopy (AFM) and transmission electron microscopy (TEM). The surface morphology of Nb-oxide layer shows smoother as well as the Nb grain gradually vanished with increasing anodization depth. The root mean square (RMS) roughness of Nb-oxide layer was decreased to be 0.35 nm with increasing applied voltage of anodization to 100V. An amorphous NbOx layer in the interface between Nb layer and Nb2O5 layer was confirmed by X-ray reflectometry (XRR) and transmission electron microscopy (TEM) analysis. The thickness of NbOx layer decreases to be 1.5 nm with the increasing anodization depth for 45 nm depth Nb-oxide layers, which is comparable to the value observed on the surface of Nb films.
    Physica C Superconductivity 04/2014; 499. DOI:10.1016/j.physc.2014.02.009 · 0.94 Impact Factor
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    • "The device was fabricated with a planarized Nb trilayer process at MIT Lincoln Laboratory [9]. The PC qubit is a superconducting loop interrupted by three Josephson junctions. "
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    ABSTRACT: We have implemented a resonant circuit that uses a SQUID as a flux-sensitive Josephson inductor for qubit readout. In contrast to the conventional switching current measurement that generates undesired quasiparticles when the SQUID switches to the voltage state, our approach keeps the readout SQUID biased along the supercurrent branch during the measurement. By incorporating the SQUID inductor in a high-Q resonant circuit, we can distinguish the two flux states of a niobium persistent-current (PC) qubit by observing a shift in the resonant frequency of both the magnitude and the phase spectra. The readout circuit was also characterized in the nonlinear regime to investigate its potential use as a nonlinear amplifier.
    IEEE Transactions on Applied Superconductivity 07/2005; DOI:10.1109/TASC.2005.850077 · 1.24 Impact Factor
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    ABSTRACT: Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2004. Includes bibliographical references (p. 192-201). Recent successes with superconducting Josephson junction qubits make them prime candidates for the implementation of quantum computing. This doctoral thesis details the study of a niobium Josephson junction circuit for quantum computing applications. The thesis covers two main areas: 1) the fabrication of sub-micron niobium Josephson junction devices using a Nb/Al/A1Ox/Nb trilayer process and 2) measurements of unique quantum properties of a superconducting device proposed as a quantum bit--the Persistent Current (PC) qubit. The thesis discusses the fabrication of niobium Josephson junction devices which is integral to the design and measurement of the circuit. The devices were fabricated at MIT Lincoln Laboratory using optical projection lithography to define features. A technique to produce more uniform critical-current densities across a wafer is developed within the scope of the thesis. We also introduce experimental work on the PC qubit performed at dilution refrigerator temperatures (T [approximately] 12mK). Microwave spectroscopy was used to map the energy level separation between macroscopic quantum states of the qubit system. We measured the intrawell energy relaxation time [tau]d between quantum levels in this particular device. The intrawell relaxation measurements are important in determining whether a promising decoherence time can be achieved in Nb-based Josephson devices, which has a more mature fabrication process compared to other superconducting fabrication processes. by Daniel Yuki Nakada. Ph.D.
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