Fabrication of superconducting devices and circuits

Abstract

The insertion of superconducting circuits as a technology has been only as successful as the capabilities developed to fabricate these circuits. Clearly, the greatest strides in circuit demonstration have been taken using niobium-based integrated circuit (IC) fabrication where IC technology has matured to support large-scale integration levels (∼10K gates/cm2). To successfully fabricate large-area and dense IC, stable and reproducible processes are needed. Moreover, IC fabrication involves a number of subprocess steps. Each step has its own inherent variability, the combination of which contributes to the overall degree of circuit functionality, or yield. Fabrication groups face challenges in reducing variability or spreads in critical device parameters such as junction critical currents, values of inductance, and layer-to-layer alignment. In addition, there are design-driven needs to reduce device dimensions and parasitic inductances that put further constraints on practical circuit fabrication. These challenges are multiplied in IC fabrication using ceramic high-temperature superconductors (HTS). Also there are inherent material issues in this family of superconductors such as maintaining film epitaxy and environmental sensitivity. As a result, the aforementioned fabrication issues are even more daunting. Current HTS circuit fabrication requires an increased understanding of the relative importance of topology and film morphology in vital circuit elements. This is the case in multilayer process elements such as interconnect crossovers, layer-to-layer vias and Josephson junctions, where trade-offs often exist between ease of fabrication and requirements for applications. Despite the increased challenge, if these material and processing issues can be successfully implemented, it would allow more compact, lower-power systems, making them increasingly attractive for applications. In this chapter, the emphasis will be on the practical aspects of active-device superconductor fabrication, especially with respect to their relationship to system application. Present limitations will be noted along with the future direction of the field necessary to allow insertion of superconducting IC technology into commercial systems.