Optimization of Silicon Nitride Films For Use in Phase Qubits

The lifetime (coherence time) of superconducting phase qubits is currently severely limited by lossy materials used in standard fabrication techniques. In particular, the insulator material - typically Silicon Nitride - used to isolate and physically separate different layers of the qubit is of interest. We have conducted a fractional factorial design experiment to optimize SiNx loss properties with respect to several deposition parameters in an Electron Cyclotron Resonance (ECR) Plasma-Enhanced Chemical Vapor Deposition (PECVD) reactor. Our experimental design included a three-level, four-parameter matrix with N2/SiH4 ratio, microwave power, rf power, and pressure as the parameters. The test-bed for these films is a low temperature microwave LC resonator circuit in which the various insulator films are used as the dielectric between a parallel plate capacitor and the Q (Quality Factor) of the circuit gives the relevant loss information for qubit operations.