Improved Critical-Current-Density Uniformity of Nb Superconducting Fabrication Process Using Anodization Personnel

cross-wafer standard deviation of Jc was typically ~ 5% for anodized wafers but was ≥15% for unanodized wafers (Figure 6). A low variation in Jc results in a higher yield of device chips per wafer with the desired current density. As a result of the improved cross-wafer distribution, the cross-chip uniformity is greatly improved as well; typically < 1% for anodized chips. Low cross-chip Jc variation is for fast digital superconducting electronics. We studied an anodization technique for a Nb/Al/AlOx/Nb trilayer process which provides a significant improvement in critical-current-density Jc uniformity across a 150-mm-diameter wafer. Anodization is an electrolytic process in which the surface of a metal is converted to its oxide form, this metal oxide layer serves as a protective barrier to further ionic or electron flow. The superconducting Josephson junctions were fabricated in a class10 cleanroom facility at MIT Lincoln Laboratory. The Nb superconducting process uses optical projection lithography, chemical mechanical planarization of two oxide layers, a self-aligned via process and dry Reactive Ion Etching (RIE) of the Nb and oxide layers. The most critical step in the fabrication process however is the definition of the tunnel junction. The junction consists of two Nb layers, the Base Electrode (B.E.) and Counter-Electrode (C.E.) separated by a thin AlOx barrier. Figure 5a shows a crosssection of the Josephson junction region after RIE is performed on the counter-electrode. After RIE, the junction perimeter is exposed to the atmosphere and is therefore vulnerable to chemical and/or plasma damage. Anodization is useful in minimizing damage to the junction region (Figure 5b). Critical current Ic measurements of Josephson junctions were performed at room temperature using specially designed test structures. We used an automatic probing station to determine the Ic values of junctions across the entire wafer. The Jc uniformity of anodized/unanodized wafer pairs, fabricated together, were compared. The