Improved critical-current-density uniformity by using anodization
ABSTRACT 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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ABSTRACT: A fabrication process for Nb/Al-AlOx/Nb Josephson junctions (JJs) with sizes down to 200 nm has been developed on a 200-mm-wafer tool set typical for a CMOS foundry. This process is a core of several nodes of fully-planarized fabrication processes developed at MIT Lincoln Laboratory for superconductor integrated circuits with 4, 8, and 10 niobium layers. The process utilizes 248-nm photolithography, anodization, high density plasma etching, and chemical mechanical polishing (CMP) for planarization of SiO2 interlayer dielectric. JJ electric properties and statistics such as on-chip and wafer spreads of critical current, Ic, normal-state conductance, GN, and run-to-run reproducibility have been measured on 200-mm wafers in a broad range of JJ diameters from 200 nm to 1500 nm and critical current densities, Jc, from 10 kA/cm^2 to 50 kA/cm^2 where the JJs become self-shunted. Diffraction-limited photolithography of JJs is discussed. A relationship between JJ mask size, JJ size on wafer, and the minimum printable size for coherent and partially coherent illumination has been worked out. The GN and Ic spreads obtained have been found to be mainly caused by variations of the JJ areas and agree with the model accounting for an enhancement of mask errors near the diffraction-limited minimum printable size of JJs. Ic and GN spreads from 0.8% to 3% have been obtained for JJs with sizes from 1500 nm down to 500 nm to be utilized in Single-Flux-Quantum circuits with Jc from 10 kA/cm^2 to 50 kA/cm^2. The spreads increase to about 8% for 200-nm JJs. Prospects for circuit densities > 10^6 JJ/cm2 and 193-nm photolithography for JJ definition are discussed.IEEE Transactions on Applied Superconductivity 08/2014; 25(3). DOI:10.1109/TASC.2014.2374836 · 1.32 Impact Factor
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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 · 1.11 Impact Factor