Inductive and Capacitive Coupling Noise in Superconductive VLSI Circuits

The increasing complexity of modern superconductive circuits, and single flux quantum (SFQ) circuits in particular, has made the issue of coupling noise of growing importance. Limited metal resources within superconductive circuits have exacerbated this issue. In this article, the different sources of coupling noise within SFQ circuits are described. Coupling noise among inductors, routing striplines, and bias microstriplines within SFQ circuits degrade performance while decreasing margins. In this article, inductive and capacitive coupling between the different layers are characterized. Inductive coupling models between different layers in the MIT LL SFQ5ee process match experimental data within 3%. The dependence of inductive coupling on the thickness of the oxide and metal layer is also discussed. An understanding of inductive and capacitive coupling can determine the minimum physical distance between lines. In addition, tradeoffs exist among inductive coupling, capacitive coupling, layout complexity, and the vias between ground layers. The different coupling sources are characterized, and guidelines are provided to enhance the automated routing process.