Etching of HfO2, deposited on LPCVD Si3N4, and cleaning of Hf residues

In FEOL silicon nitride is often deposited on the backside of the wafer before the high-k deposition, because it offers some advantages, such as avoiding damage of the Si surface during removal of the gate material from the backside of the wafer. In order to prevent cross contamination in shared tools the backside has to be cleaned from remaining high-k contamination. This work indicates that the etching of HfO₂ is affected by the Post Deposition Anneal condition and also by the underlying silicon nitride. Furthermore the etching behavior of silicon nitride is different from the usual etch characteristics of a LPCVD Si₃N₄. This, and additionally Hf contamination inside the Si₃N₄ layer, even down to the Si-Si₃N₄ interface, and the difficulty to clean Hf contamination at the Si-Si₃N₄ interface lead to the conclusion that the backside clean of Hf contamination is not a straightforward process.