Thermal Atomic Layer Etching of Gallium Oxide Using Sequential Exposures of HF and Various Metal Precursors

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Gallium oxide (Ga 2 O 3 ) is a transparent semiconducting oxide with a large band gap that has applications for power electronics and optoelectronics. Ga 2 O 3 device fabrication requires etching for many processing steps. In this work, the thermal atomic layer etching (ALE) of Ga 2 O 3 was performed using hydrofluoric acid (HF) and a wide range of different metal precursors including BCl 3, AlCl(CH 3 ) 2, Al(CH 3 ) 3, TiCl 4, and Ga(N(CH 3 ) 2 ) 3 . Because Ga 2 O 3 is not a particularly stable oxide, the B-, Al-, or Ti-containing metal precursors can possibly convert the surface of Ga 2 O 3 to B 2 O 3, Al 2 O 3, or TiO 2 . These metal precursors can also provide Cl, CH 3, and N(CH 3 ) 2 ligands for ligand-exchange reactions. Consequently, the thermal ALE of Ga 2 O 3 can occur via “conversion-etch” or fluorination and ligand-exchange reaction pathways. Using sequential HF and BCl 3 exposures and in situ spectroscopic ellipsometry techniques, Ga 2 O 3 etch rates were observed to vary from 0.59 to 1.35 Å/cycle at temperatures from 150 to 200 °C, respectively. The Ga 2 O 3 etch rates were also self-limiting versus HF and BCl 3 exposure. The lack of BCl 3 pressure dependence for the etch rates argued against the conversion-etch mechanism and in favor of a fluorination and ligand-exchange reaction pathway. In situ quartz crystal microbalance techniques also revealed that Ga 2 O 3 could be etched using sequential exposures of HF and various other metal precursors. Ga 2 O 3 etch rates at 250 °C were 1.2, 0.82, 0.85, and 0.23 Å/cycle for AlCl(CH 3 ) 2, Al(CH 3 ) 3, TiCl 4, and Ga(N(CH 3 ) 2 ) 3 as the metal precursors, respectively. The mass changes during the individual exposures of HF and the AlCl(CH 3 ) 2 and Al(CH 3 ) 3 metal precursors argued for a fluorination and ligand-exchange mechanism. The AlCl(CH 3 ) 2 and Al(CH 3 ) 3 exposures may also lead to some conversion of Ga 2 O 3 to Al 2 O 3 . In contrast, the mass changes during the HF and TiCl 4 exposures were consistent with the conversion of the surface of Ga 2 O 3 to TiO 2 and then the spontaneous removal of the TiO 2 surface layer by HF. Distinctly different behavior was observed during the HF and Ga(N(CH 3 ) 2 ) 3 exposures. The large mass gain during the Ga(N(CH 3 ) 2 ) 3 exposures suggested that Ga(N(CH 3 ) 2 ) 3 can adsorb on the fluorinated Ga 2 O 3 surface prior to the ligand-exchange reaction. The wide range of metal precursors that can etch Ga 2 O 3 argues that the ability of these precursors to convert Ga 2 O 3 or to undergo ligand-exchange reactions provides multiple pathways for effective thermal Ga 2 O 3 ALE.