  The superconducting critical temperature (Tc > 15K) of niobium titanium
nitride (NbTiN) thin films allows for low-loss circuits up to 1.1 THz, enabling
on-chip spectroscopy and multi-pixel imaging with advanced detectors. The drive
for large scale detector microchips is demanding NbTiN films with uniform
properties over an increasingly larger area. This article provides an
experimental comparison between two reactive d.c. sputter systems with
different target sizes: a small target (100mm diameter) and a large target (127
mm x 444.5 mm). This article focuses on maximizing the Tc of the films and the
accompanying I-V characteristics of the sputter plasma, and we find that both
systems are capable of depositing films with Tc > 15 K. The resulting film
uniformity is presented in a second manuscript in this volume. We find that
these films are deposited within the transition from metallic to compound
sputtering, at the point where target nitridation most strongly depends on
nitrogen flow. Key in the deposition optimization is to increase the system's
pumping speed and gas flows to counteract the hysteretic effects induced by the
target size. Using the I-V characteristics as a guide proves to be an effective
way to optimize a reactive sputter system, for it can show whether the optimal
deposition regime is hysteresis-free and accessible.
