Disordered superconductors play an important role in quantum devices such as high kinetic inductance amplifiers, where strong disorder provides large inductance but also leads to defects and increased losses. Here, we study the effect of thermal post-deposition annealing (TA) in reducing these losses. TA has been shown to improve superconducting properties, including increases in the energy gap and critical temperature. To investigate the origin of these improvements, we employ scanning tunneling microscopy (STM) to probe the superconducting order parameter in DC magnetron sputtered 11 nm and 20 nm NbTiN thin films, both as deposited and after annealing at 1200°C in N₂/H₂ atmosphere.

Our TA leads to an increase in the order parameter of up to 50 %, resulting in Δ=1.80 meV and 2.14 meV for the as deposited 11 nm and 20 nm film, and 2.76 meV and 2.90 meV after TA, respectively. Local maps of the order parameter reveal that the films consist of electronic islands that are correlated with the topography. Between the electronic islands the order parameter fluctuates with a standard deviation of 0.04 meV and 0.06 meV for the as deposited 11 and 20 nm films. In addition, we find that TA reduces magnetic impurities on the films surface that we observe as Yu-Shiba-Rusinov states in differential conductance measurements.