Room temperature reactive sputtering deposition of titanium nitride with high sheet kinetic inductance

TL;DR

This work demonstrates room-temperature reactive sputtering of TiN to produce superconducting thin films with unusually high sheet kinetic inductance $L_{k,\square}$, enabling high-impedance, strongly nonlinear microwave elements suitable for quantum sensing and hybrid quantum systems. By fabricating lumped-element resonators on a single wafer and applying two independent estimation methods, the authors extract $L_{k,\square}\approx 394$–$423$ pH/□, with a sheet resistance of $R_{s,300}\approx 475\,\Omega/\square$ and a superconducting transition at $T_c\approx 3.8$ K. Structural analyses confirm a predominantly (111)-textured TiN with ~27 nm thickness and columnar grains, consistent with a high disorder state that can yield large kinetic inductance. The high $L_{k}$ and elevated impedance (roughly an order of magnitude higher than typical 50 Ω resonators) imply stronger microwave spin-photon coupling and Kerr nonlinearities, positioning room-temperature TiN as a promising platform for superconducting amplifiers and qubits operating at elevated temperatures. Future work will explore loss mechanisms, impedance control, and deposition parameter optimization to optimize device performance.

Abstract

Superconducting thin films with high intrinsic kinetic inductance $L_{k}$ are important for high-sensitivity detectors, enabling strong coupling in hybrid quantum systems, and enhancing nonlinearities in quantum devices. We report the room-temperature reactive sputtering of titanium nitride thin films with a critical temperature $T_{c}$ of \SI{3.8}{K} and a thickness of \SI{27}{nm}. Fabricated into resonators, these films exhibit a sheet kinetic inductance $L_{k, \square}$ of 394~$\textrm{pH}/\square$, as inferred from resonant frequency measurements. %from this film and measure quality factors of $4\times 10^{4}$; these quality factors are likely limited by the low resistivity wafer. X-ray diffraction analysis confirms the formation of stoichiometric TiN, with no residual unreacted titanium. The films also demonstrate a characteristic sheet resistivity of 475~$Ω/\square$, yielding an impedance an order of magnitude higher than conventional 50~$Ω$ resonators. This property could enhance microwave single\textendash photon coupling strength by an order of magnitude, offering transformative potential for hybrid quantum systems and quantum sensing. Furthermore, the high $L_{k}$ enables Kerr nonlinearities comparable to state\textendash of\textendash the\textendash art quantum devices. Combined with its relatively high $T_{c}$, this thin film presents a promising platform for superconducting devices, including amplifiers and qubits operating at higher temperatures.

Figures (8)

• Figure 1: (a), 'alpha' resonator design. Top three resonators are fabricated with one layer of Nb. They have inductor width of 18, 20 and 22µm and everything else same. Bottom three resonators are same with top three except the inductors are all fabricated with TiN instead of Nb. (b), microscope image of all Nb resonator with inductor width of 20µm indicated in upper black square in (a). (c), microscope image of TiN resonator with inductor width of 20µm indicated in lower black square in (a). (d) End of the Nb interdigital capacitor and Nb/TiN interface as indicated in black square in (c). (e) Amplitude plot of one of the resonators
• Figure 2: Resonator design for flux coupled lumped element LC resonators. (b), optical microscope image of the resonator with inductor width of 1.4µm marked by the black square in (a). Light yellow colored film is Nb for readout and interdigital capacitor. Grey colored film is TiN for two inductors in parallel. (c) Image of the inductor joining the capacitor end marked by the gray square in (b).
• Figure 3: Fitted $L_{k,\square}$ for all three group of resonators. LC 700 are the original three flux biased LC resonators. Alpha stands for the three resonators in the alpha design with TiN inductor. LC 100 are the three resonators by shorting 600µm lenght of the TiN inductor with Nb microstrip on top. The total inductor is the left 100µm TiN. Straight line is fitting a linear function to all the data points. Error bar in each data point is calculated from the uncertainty of fitting $S_{21}$ equation to the measured resonance curvescarter2017.
• Figure 4: Simulated resonant frequency as function of $L_k$ for alpha design resonators. Label '(Nb)' and '(TiN)' mark the resonance frequencies match with the all Nb resonators and the resonators with TiN inductor respectively.
• Figure 5: XRD image of unpatterned TiN (brown curve) compared to Ti (blue curve) films with 30nm thickness.