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

• VT over O* ratio is proposed as a feedback signal for ZrO2 and HfO2 reactive sputtering process control.
• Ratio of VT and O* as a feedback signal allows operation in the ‘forbidden’ parts of the transition.
• Using ratio of VT and O* as the main feedback signal leads to high deposition rate.
• Low extinction coefficient and high refractive index of ZrO2 and HfO2 were obtained.

Target voltage (VT) 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 VT 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 VT 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 DT–S = 20 cm by pulse-DC sputtering). Such a process is able to deliver ZrO2 and HfO2 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.