Trimethylaluminum as the Metal Precursor for the Atomic Layer Etching of Al₂O₃ Using Sequential, Self-Limiting Thermal Reactions

Abstract

Trimethylaluminum (TMA, Al­(CH3)3) was used as the metal precursor, together with HF, for the atomic layer etching (ALE) of Al2O3 using sequential, self-limiting thermal reactions. Al2O3 ALE using TMA demonstrates that other metal precursors, in addition to Sn­(acac)2, can be employed for Al2O3 ALE. The use of TMA for Al2O3 ALE is especially interesting because TMA can also be used for Al2O3 atomic layer deposition (ALD). Quartz crystal microbalance (QCM) experiments monitored Al2O3 ALE at temperatures from 250 to 325 °C. The Al2O3 ALE was linear versus the number of HF and TMA reaction cycles. The QCM studies showed that the sequential HF and TMA reactions were self-limiting versus reactant exposure. The Al2O3 etching rates increased at higher temperatures. The QCM analysis measured mass change per cycle (MCPC) values that varied from −4.2 ng/(cm2 cycle) at 250 °C to −23.3 ng/(cm2 cycle) at 325 °C. These MCPCs correspond to Al2O3 etch rates from 0.14 Å/cycle at 250 °C to 0.75 Å/cycle at 325 °C. X-ray reflectivity and spectroscopic ellipsometry analyses confirmed the linear removal of Al2O3 and etching rates. Fourier transform infrared spectroscopy measurements monitored Al2O3 ALE by observing the loss of infrared absorbance from Al–O stretching vibrations. Surface intermediates were also identified after the HF and TMA exposures. Al2O3 ALE with TMA is believed to occur by the reaction Al2O3 + 4Al­(CH3)3 + 6HF → 6AlF­(CH3)2 + 3H2O. The proposed mechanism involves fluorination and ligand-exchange reactions. The HF exposure fluorinates the Al2O3 and forms an AlF3 surface layer and H2O as a volatile reaction product. During the ligand-exchange transmetalation reaction, TMA accepts F from the AlF3 surface layer and donates CH3 to produce volatile AlF­(CH3)2 reaction products. The QCM measurements were consistent with an AlF3 surface layer thickness of 3.0 Å on Al2O3 after the HF exposures. The larger etch rates at higher temperatures were attributed to the removal of a larger fraction of the AlF3 surface layer by TMA exposures at higher temperatures.

Competition between Al2O3 atomic layer etching and AlF3 atomic layer deposition using sequential exposures of trimethylaluminum and hydrogen fluoride

“…4 In contrast, thermal processes have been developed only recently for ALE. [5][6][7][8][9][10] The recently developed thermal ALE processes are based on sequential fluorination and ligand-exchange reactions. 5,9,11 Fluorination converts the metal compound, such as a metal oxide, to a metal fluoride.…”

“…5,9,11 HF has been a successful fluorine precursor for Al 2 O 3 , HfO 2 , AlF 3 , and AlN ALE. [5][6][7][8][9][10] HF is also an effective fluorine precursor for the ALD of a variety of metal fluoridess such as AlF 3 , ZrF 4 , HfF 4 , MnF 2 , MgF 2 , ZnF 2 , and LiF. 12 Trimethylaluminum (TMA) is the most common precursor for Al 2 O 3 ALD and AlF 3 ALD.…”

“…13,14 TMA can also serve as a metal precursor for Al 2 O 3 ALE. 8 The ability of the TMA and HF precursors to produce either Al 2 O 3 ALE or AlF 3 ALD indicates that there is competition between the etching and growth processes.…”

“…5,9,11 During the TMA exposures, TMA then accepts fluorine from the newly formed AlF 3 layer in a ligand-exchange process. 8 TMA also donates CH 3 ligands to the surface to form volatile AlF(CH 3 ) 2 species. The desorption of these species results in an overall loss of the original Al 2 O 3 film.…”

Enhanced atomic layer etching of native aluminum oxide for ultraviolet optical applications

“…One example utilizes alternating exposure to hydrogen fluoride (HF) and tin(II) acetylacetonate, 27 and another implements a similar approach with trimethylaluminum (TMA) and HF. 28 In this work, we investigate the use of a modified version of this TMA-based thermal ALE approach and demonstrate the efficacy of this method in improving the UV performance of evaporated Al thin films. The reduction in surface native Al2O3 is characterized by near ultraviolet (NUV, 200 < λ < 400 nm) and FUV reflectance measurements, and the prospects for applications related to future astronomical mirrors are discussed.…”