Thermal Atomic Layer Etching of Al₂O₃ Using Sequential HF and BCl₃ Exposures: Evidence for Combined Ligand-Exchange and Conversion Mechanisms

Abstract

The atomic layer etching (ALE) of Al2O3 was demonstrated using sequential HF and BCl3 exposures. BCl3 is a new precursor for thermal Al2O3 ALE that can provide pathways for both ligand-exchange and conversion etching mechanisms. Fourier transfer infrared (FTIR) spectroscopy was utilized to observe the growth of Al2O3 ALD films using Al­(CH3)3 (trimethylaluminum) and H2O and the subsequent etching of the Al2O3 ALD films using HF and BCl3. To confirm the conversion reaction, FTIR difference spectra revealed that initial BCl3 exposures on the Al2O3 ALD film converted the Al2O3 surface to a B2O3 layer. Surprisingly, larger BCl3 exposures on the B2O3 layer could also etch the B2O3 layer. Quadrupole mass spectrometry (QMS) measurements revealed that BCl3 produced ion intensities for AlCl3 + from AlCl3 during the conversion of the Al2O3 surface to a B2O3 layer. Concurrently, the BCl3 also etched the converted B2O3 layer and ion intensities for B3O3Cl3 + were observed from B3O3Cl3 boroxine rings. After the conversion of the Al2O3 surface to a B2O3 layer, the initial HF exposure then removed the B2O3 layer and fluorinated the underlying Al2O3 film. Following the initial BCl3 and HF exposures, the FTIR difference spectra showed that Al2O3 ALE proceeded primarily by a reaction pathway where HF fluorinates the Al2O3 and then BCl3 removes the surface fluoride layer by a ligand-exchange reaction. However, there was still evidence for some conversion of Al2O3 to a B2O3 layer during the subsequent BCl3 exposures and then removal of the B2O3 layer by the HF exposures. Spectroscopic ellipsometry measurements determined the etch rates during thermal Al2O3 ALE during sequential HF and BCl3 exposures. The etch rates were 0.03, 0.31, 0.65, and 0.92 Å/cycle at temperatures of 230, 255, 280, and 290 °C, respectively. QMS analysis also investigated the volatile etch products during the sequential HF and BCl3 exposures on Al2O3 at 270 °C. During the BCl3 exposures after the initial cycle, the QMS measurements observed ion intensities for BFCl2 + and AlCl2 +. BFCl2 was the major ligand-exchange product, and AlCl3 was the main metal chloride etching product. In addition, small ion intensities for B3O3Cl3 + were also present from the conversion of Al2O3 to B2O3 and subsequent etching of B2O3 by BCl3 to yield boroxine ring products. These results indicate that thermal Al2O3 ALE using sequential HF and BCl3 exposures occurs by combined ligand-exchange and conversion mechanisms.

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“…Conversion of the initial material to a different material can also play a role in the thermal ALE of SiO 2 , ZnO, VO 2 , and Al 2 O 3 . Oxidation of metal nitrides to metal oxides can also facilitate the etching of nitrides such as TiN .…”

“…17,18 Conversion of the initial material to a different material can also play a role in the thermal ALE of SiO 2 , 19 ZnO, 20 VO 2 , 21 and Al 2 O 3 . 22 Oxidation of metal nitrides to metal oxides can also facilitate the etching of nitrides such as TiN. 23 Semiconductor materials, such as Si, 24 SiGe, 25 and Si 3 N 4 , 26 can also undergo thermal ALE through oxidation, conversion, and fluorination and ligand exchange mechanisms.…”