Titanium nitride films for ultrasensitive microresonator detectors

Titanium nitride (TiNx) films are ideal for use in superconducting microresonator detectors for the following reasons: (a) the critical temperature varies with composition (0<Tc<5 K); (b) the normal-state resistivity is large, ρn∼100 μΩ cm, facilitating efficient photon absorption and providing a large kinetic inductance and detector responsivity; and (c) TiN films are very hard and mechanically robust. Resonators using reactively sputtered TiN films show remarkably low loss (Qi>107) and have noise properties similar to resonators made using other materials, while the quasiparticle lifetimes are reasonably long, 10–200 μs. TiN microresonators should therefore reach sensitivities well below 10−19 W Hz−1/2.