Nagoya University achieves world first anisotropic atomic-layer etching of HfO₂ through halogen-free plasma process

A research group led by Designated Professor Shih-Nan Hsiao and Designated Professor Masaru Hori from the Center for Low-temperature Plasma Sciences at Nagoya University, carrying out joint research with Ming Chi University of Technology in Taiwan, achieved the world's first success in anisotropic atomic-layer etching (ALE) of hafnium (IV) oxide (HfO₂) used in ferroelectric memory and gate insulators of advanced semiconductor devices through a halogen-free plasma process. The results were published in Small Science.

The miniaturization of circuit patterns has been progressing to enable higher integration and enhanced performance of semiconductor devices. Consequently, this has driven the advancement of new materials and device structures. In recent years, research and development of ALE technology for HfO₂ has become essential in the fields of advanced gate insulators and ferroelectric memory for ultrathin transistors. However, in plasma etching using halogen-based gases, HfO₂ is considered a hard-to-etch material due to the low volatility of the generated hafnium halides and the high bonding energy between Hf and O.

The research group achieved anisotropic ALE of HfO₂ at room temperature by alternately performing nitrogen plasma exposure and oxygen plasma exposure using a low-pressure, high-density plasma generation apparatus.

In the method developed by the group, the nitrogen plasma exposure causes nitrogen ions to replace oxygen atoms on the HfO₂ film surface with nitrogen atoms, forming a surface layer of hafnium nitride (HfN₄). Subsequently, oxygen plasma exposure converts this surface layer into highly volatile reaction products at room temperature, which are then removed. These reactions were observed using real-time infrared absorption spectroscopy surface analysis equipment, and the structure of reaction products was elucidated and reaction models for each step were constructed. Furthermore, the results obtained by controlling the nitrogen ion energy at low levels showed surface roughness reduction compared with before the ALE process treatment, confirming that low-damage ALE could be achieved.

The establishment of a practical ALE process for HfO₂, which has previously been considered hard to etch, is expected to be an extremely important milestone in the development of manufacturing technology for next-generation semiconductor devices. Furthermore, since it does not use environmentally harmful etching gases, it can be said to be an environmentally friendly method.

Journal Information
Publication: Small Science
Title: Halogen-Free Anisotropic Atomic-Layer Etching of HfO₂ at Room Temperature
DOI: 10.1002/smsc.202500251