Breakthrough in atomic-level etching of hafnium oxide, a promising material for advanced semiconductors

Hafnium oxide (HfO₂) has attracted attention as a promising material for ultrathin semiconductors and other microelectronic devices. The strong ionic bond between hafnium and oxygen atoms in HfO₂ gives it a high dielectric constant, superior thermal stability, and a wide band gap.

Notably, these properties can be maintained even at the atomic scale. Meanwhile, these properties also pose challenges in achieving highly precise and smooth etching of HfO₂ films.

Now, a group of researchers from Japan and Taiwan has successfully etched HfO₂ films with atomic-level precision, smoothness, and uniformity without the use of halogen-based gases.

Halogen-based gases, containing fluorine and/or chlorine, are commonly used in plasma-enhanced atomic-layer etching (ALE) methods for HfO₂ and most other materials. However, these gases can be highly toxic and may act as greenhouse gases. Therefore, eliminating their use in the etching methods could also contribute to sustainable manufacturing. This achievement was published in the journal Small Science.

As semiconductor devices advance, the critical dimensions of their circuits are required to shrink to just a few nanometers. HfO₂ is a strong candidate for applications in such next-generation semiconductor devices, including ultrathin gate insulators in 2D material-based field-emission transistors and advanced nonvolatile memory devices.

The plasma-enhanced ALE method is often used for the anisotropic etching of HfO₂. This method utilizes energetic species, usually low-energy ions, to provide the energy needed to remove surface atoms from materials by forming volatile products.

"Conventional plasma-enhanced ALE methods for HfO₂ typically rely on a combination of physical and chemical etching via halogen-based gases and high-energy ion bombardment to facilitate the removal of nonvolatile halides," explained Shih-Nan Hsiao, a professor at Nagoya University and the study's lead author. "However, the byproducts generated through physical sputtering often have low volatility, causing them to adhere to the chamber walls and feature sidewalls. This could impair the performance of electronic devices."

To overcome this drawback, a research group led by Professors Hsiao and Masaru Hori from the Center for Low-temperature Plasma Sciences at Nagoya University in Japan collaborated with researchers from Ming Chi University of Technology in Taiwan. They aimed to develop a new method for etching HfO₂ films that would produce smooth and uniform surfaces, along with consistent etched depths through anisotropic etching.

Researchers used a low-pressure, high-density plasma generation device to irradiate HfO₂ films with N₂ and O₂ plasmas alternately. During the first half-cycle, N⁺ ions bombarded the HfO₂ surface with an applied bias voltage, leading to the bonding of nitrogen with the HfO₂. Subsequently, the HfO₂ films were treated with an O₂ plasma without an applied bias voltage. This procedure effectively eliminated the nitrogen-bonded surface layer through a self-limiting reaction.

Applying radio-frequency power to the bottom electrode adjusted the energy of the N⁺ ion. This adjustment led to an etch depth per cycle between 0.023 and 0.107 nm/cycle.

The researchers also analyzed the underlying surface reaction mechanism using in situ techniques, specifically attenuated total reflection Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy.

This analysis revealed a consistent formation of Hf-N bonds through a ligand exchange mechanism. During this process, nitrogen atoms replaced surface oxygen atoms when the sample was exposed to N₂ plasma. During the following half-cycle with O₂ plasma, these bonds decomposed into volatile byproducts.

Furthermore, this cyclic etching technique effectively smoothed the HfO₂ surface. After 20 cycles, the surface roughness was reduced by 60%.

Hsiao concluded, "We have successfully achieved halogen-free atomic-layer etching of HfO₂ film at room temperature for the first time in the world. Eliminating the use of halogen gases helps reduce environmental impacts. Performing the etching process at room temperature saves energy and simplifies the procedure, leading to lower manufacturing costs. Additionally, this process is clean and eliminates reaction byproducts. Our work could therefore contribute to sustainable manufacturing."