Improving magnetic-field resilience of NbTiN planar resonators using a hard-mask fabrication technique

High-quality factor microwave resonators operating in a magnetic field are a necessity for some quantum sensing applications and hybrid platforms. Losses in microwave superconducting resonators can have several origins, including microscopic defects, usually known as two-level-systems (TLS). Here, we characterize the magnetic field response of NbTiN resonators patterned on sapphire and observe clear absorption lines occurring at specific magnetic fields. We identify the spin systems responsible for these features, including a yet unreported spin with $g=1.85$ that we attribute to defects in the NbTiN thin film. We develop mitigation strategies involving namely an aluminum etch mask, resulting in maintaining quality factors above $Q>2 \times 10^5$ in the range $0$-$0.3$ T.