Passivation by coating with OCG285 (or equivalent) polyimide at critical steps of fabrication has been found to enhance the quality of tapered edges in high-critical-temperature (high-Tc) superconductor/normal-conductor/superconductor (SNS) Josephson junctions. In comparison with Josephson junctions that are nominally identical except for having been fabricated without such passivation, those fabricated with polyimide passivation exhibit cleaner and smoother edges and, consequently, smaller differences among the resistances and critical currents of individual junctions.

Figure 1. The Formation of a Tapered Edge on a high-temperature superconductor during the fabrication of an SNS Josephson junction is accomplished by ion milling through a patterned photoresist mask. The incorporation of the polyimide layer enhances the quality of the tapered edge formed in this process.

The formation of clean, smooth, tapered edges on films of high-Tc superconductors (primarily YBa2Cu3O7 -δ) and on related epitaxial insulating films (e.g., SrTiO3) is an essential part of the fabrication of SNS Josephson junctions and similar devices. The tapered edges are needed for the subsequent deposition of layers that are free of defects. In typical current practice, the edges are formed by ion milling through photoresist masks that have been patterned onto the high-Tc superconducting and overlying insulating films. This processing exposes the superconducting films to the photoresist patterning, which often leaves minute amounts of residues that degrade the quality of subsequently deposited high-Tc superconducting films. In addition, the exposure of the photoresist and superconducting films to the ion-milling process can lead to further degradation of the superconductor films and edges.

Figure 1 shows one aspect of the formation of a tapered edge in a process that incorporates the polyimide passivation step. Coating with the polyimide before patterning and ion milling protects the underlying high-temperature superconductor from the developer and photoresist. The polyimide layer also protects the photoresist against ion-milling-induced changes that degrade the superconducting film and edges.

Figure 2. The Spreads in Resistance and Critical Current for sample SNS Josephson junctions fabricated without and with polyimide were calculated from current-vs.-voltage measurements. The percent spread on each axis is defined as 100 × (the standard deviation ÷ the mean).

The polyimide-passivation technique has been tested in the fabrication of multiple SNS edge Josephson junctions on chips. YBa2Cu3O7 -δ followed by SrTiO3 was deposited on (100) LaAlO3wafers. The SrTiO3 layers were patterned with reflowed photoresist masks, then rotated during argon-ion milling without cooling. The Josephson junctions were then completed by the deposition of YBa2Cu2.79Co0.21O7 -δ as the normal conductor followed by YLa0.005Ba2Cu3O7 -δ superconducting counterelectrodes. The current-vs.-voltage characteristics of junctions fabricated with and without polyimide passivation were measured at a temperature of 60 K. Plotted in Figure 2 are the spreads in the critical current densities and quantities proportional to normal resistivities calculated from the measurements. The results show that in all cases except one, the variability among devices was decreased by use of polyimide passivation. In the one exceptional case in which a chip made with polyimide passivation exhibited a 74-percent spread in critical current, the SNS devices on the chip were just beginning to show a critical current as the temperature decreased to the measurement temperature; at a temperature of 50 K, the spread in critical current for this chip was 24 percent.

This work was done by Jeffrey Barner and Henry LeDuc of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.comunder the Electronic Components and Circuits category, or circle no. 106 on the TSP Order card in this issue to receive a copy by mail ($5 charge). NPO-20164