Unlocking the potential of voltage control for high rate zirconium and hafnium oxide deposition by reactive magnetron sputtering

Target voltage (V_T) feedback-based closed-loop gas flow control is one of the most cost-effective and well-known methods to regulate reactive oxide thin film sputtering processes. However, it can be difficult to perform due to target voltage non-linearity, changes in signal direction and drifts in the case of Zr and Hf sputtering in an Ar/O₂ ambient. We show in this paper that the use of the ratio of V_T over the O* (oxygen plasma emission, or alternatively oxygen partial pressure) as a feedback signal in a closed-loop control, enables stable operation anywhere in the 'transition' region (the region between 'metal' and 'fully poisoned' sputtering target states) as well as extending into the 'forbidden' parts of it. The result of using V_T over the O* ratio as the main feedback signal is a stable process exhibiting constant sputter target state and partial pressure of oxygen as well as high deposition rate (e.g. 6.4 Å/s for ZrO₂ and 9.2 Å/s for HfO₂ at D_T-S = 20 cm by pulse-DC sputtering). Such a process is able to deliver ZrO₂ and HfO ₂ coatings that exhibit a low extinction coefficient in the near UV (0.0002 for HfO₂ and 0.001 for ZrO₂ at 350 nm) and a high refractive index (2.05 for HfO₂ and 2.16 for ZrO₂ at 1000 nm), without any ion/plasma assistance, at low substrate temperature (20-50 °C) and no nitrogen addition.