Wafer scale fabrication of Josephson junctions

Abstract

In this thesis variations in the Josephson junction conductance across a 100 mm wafer are studied. For this study junctions fabricated with a Dolan bridge and with the Manhattan layout are used. These junctions are fabricated on a planar and a non-planar NbTiN base layer to characterise the uniformity. On a planar
base junctions fabricated with a Dolan bridge showed higher uniformity and yield than junctions with the Manhattan layout. On a non-planer base junctions fabricated with a Manhattan layout showed a higher uniformity
than the Dolan bridge junctions. This can be attributed to the through silicon vias, which affect the resist spinning required for the Dolan bridge.
Manhattan style junctions are fabricated on a Si base layer in an attempt to improve the uniformity across a 100 mm wafer. By fabricating junctions on Si overall fabrication complexity is reduced, while also reducing the fabrication time. These junctions are used to investigate the role of oxygen ashing prior to the junction deposition. We find that the ashing increase the uniformity of the junctions, however, further attempts to improve the uniformity using different ashing steps did not how any additional improvements. Finally, to explain the variations in Manhattan style junctions a geometric model is introduced. This model is based on the thickness of the top resist stack, which could potentially affect the width of the junction electrodes. The model is compared with SEM images and with the coefficient of variation. The model shows correspondence with the SEM data, but overestimates the variations when compared with the coefficient of variation.