Self-aligned coplanar waveguides with crossovers, for resonators, couplers, and amplifiers

We implement a self-aligned fabrication technology for low-loss, superconducting coplanar waveguides (CPWs). In this talk I describe our fabrication process and present loss measurements of resonators fabricated with this technique. We form the CPW center lines and ground planes using a common hard mask. Using this approach, we can also connect separated ground planes by forming crossover bridges within the ground plane layer. This obviates the need to make crossovers in separate lithography steps or use flip-chip geometries. The narrow gaps (down to 200 nm) yielded by this process give characteristic impedances approaching 50 ohms even when using high-kinetic inductance superconductors such as NbTiN. More importantly, our process is compatible with modern planarization techniques to enable large-scale integration of 2D quantum circuits. Initial measurements for 20-nm-thick NbTiN on Si(100) yielded Q_i = 4.63e5 at 7.9756 GHz with a 1 micrometer gap.