Tantalum on sapphire and silicon substrates for superconducting quantum circuits

Materials science of superconducting circuits is considered with increasing importance, particularly as it directly affects qubit coherence. Appropriate nanofabrication and film growth techniques need to be developed to incorporate quality-factor engineered components. One emerging structure for superconductor ground planes and feedlines is tantalum (Ta) on a sapphire substrate, for which high coherence times were achieved for transmon qubits. The oxide formation and stoichiometry of α-phase Ta films leads to fewer sources of noise for the qubit to incoherently exchange energy with. In this presentation, we demonstrate growth techniques for deposition of Ta on heated sapphire substrates, and deposition of Ta on Si substrates using a Nb seed layer. We will also present different recipes that were used to dry etch Ta films into resonator structures, and discuss the extracted internal quality factors from these film. We discuss our investigations into fabricating Ta resonators on Si at room temperature which opens up a way to fabricate highly coherent circuits on systems without heating capabilities, and avoids thermally induced diffusion of pre-deposited materials. Finally, we detail the different dry etch chemistries that can be used and which one we have found to be optimal.