The atomic layer etching (ALE) of Al2O3 was demonstrated using sequential, self-limiting thermal reactions with tin(II) acetylacetonate (Sn(acac)2) and hydrogen fluoride (HF) as the reactants. The Al2O3 samples were Al2O3 atomic layer deposition (ALD) films grown using trimethylaluminum and H2O. The HF source was HF-pyridine. Al2O3 was etched linearly with atomic level precision versus number of reactant cycles. The Al2O3 ALE was monitored at temperatures from 150 to 250 °C. Quartz crystal microbalance (QCM) studies revealed that the sequential Sn(acac)2 and HF reactions were self-limiting versus reactant exposure. QCM measurements also determined that the mass change per cycle (MCPC) increased with temperature from -4.1 ng/(cm(2) cycle) at 150 °C to -18.3 ng/(cm(2) cycle) at 250 °C. These MCPC values correspond to etch rates from 0.14 Å/cycle at 150 °C to 0.61 Å/cycle at 250 °C based on the Al2O3 ALD film density of 3.0 g/cm(3). X-ray reflectivity (XRR) analysis confirmed the linear removal of Al2O3 and measured an Al2O3 ALE etch rate of 0.27 Å/cycle at 200 °C. The XRR measurements also indicated that the Al2O3 films were smoothed by Al2O3 ALE. The overall etching reaction is believed to follow the reaction Al2O3 + 6Sn(acac)2 + 6HF → 2Al(acac)3 + 6SnF(acac) + 3H2O. In the proposed reaction mechanism, the Sn(acac)2 reactant donates acac to the substrate to produce Al(acac)3. The HF reactant allows SnF(acac) and H2O to leave as reaction products. The thermal ALE of many other metal oxides using Sn(acac)2 or other metal β-diketonates, together with HF, should be possible by a similar mechanism. This thermal ALE mechanism may also be applicable to other materials such as metal nitrides, metal phosphides, metal sulfides and metal arsenides.

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.

Atomic layer etching (ALE) can result from sequential, selflimiting thermal reactions. The reactions during thermal ALE are defined by fluorination followed by ligand exchange using metal precursors. The metal precursors introduce various ligands that may transfer during ligand exchange. If the transferred ligands produce stable and volatile metal products, then the metal products may leave the surface and produce etching. In this work, selectivity in thermal ALE was examined by exploring tin(II) acetylacetonate (Sn(acac) 2 ), trimethylaluminum (TMA), dimethylaluminum chloride (DMAC), and SiCl 4 as the metal precursors. These metal precursors provide acac, methyl, and chloride ligands for ligand exchange. HF-pyridine was employed as the fluorination reagent. Spectroscopic ellipsometry was used to measure the etch rates of Al 2 O 3 , HfO 2 , ZrO 2 , SiO 2 , Si 3 N 4 , and TiN thin films on silicon wafers. The spectroscopic ellipsometry measurements revealed that HfO 2 was etched by all of the metal precursors. Al 2 O 3 was etched by all of the metal precursors except SiCl 4 . ZrO 2 was etched by all of the metal precursors except TMA. In contrast, SiO 2 , Si 3 N 4 , and TiN were not etched by any of the metal precursors. These results can be explained by the stability and volatility of the possible reaction products. Temperature can also be used to obtain selective thermal ALE. The temperature dependence of ZrO 2 , HfO 2 , and Al 2 O 3 ALE was examined using SiCl 4 as the metal precursor. Higher temperatures can discriminate between the etching of ZrO 2 , HfO 2 , and Al 2 O 3 . The temperature dependence of Al 2 O 3 ALE was also examined using Sn(acac) 2 , TMA, and DMAC as the metal precursors. Sn(acac) 2 etched Al 2 O 3 at temperatures ≥150 °C. DMAC etched Al 2 O 3 at higher temperatures ≥225 °C. TMA etched Al 2 O 3 at even higher temperatures ≥250 °C. The combination of different metal precursors with various ligands and different temperatures can provide multiple pathways for selective thermal ALE.

The atomic layer deposition (ALD) of AlF 3 was demonstrated using trimethylaluminum (TMA) and hydrogen fluoride (HF). The HF source was HF-pyridine. In situ quartz crystal microbalance (QCM), quadrupole mass spectrometer (QMS), and Fourier transform infrared (FTIR) spectroscopy measurements were used to study AlF 3 ALD. The AlF 3 ALD film growth was examined at temperatures from 75 to 300°C. Both the TMA and HF reactions displayed self-limiting behavior. The maximum mass gain per cycle (MGPC) of 44 ng/(cm 2 cycle) for AlF 3 ALD occurred at 100°C. The MGPC values decreased at higher temperatures. The MGPC values were negative at T > 250°C when TMA and HF were able to etch the AlF 3 films. Film thicknesses were also determined using ex situ X-ray reflectivity (XRR) and spectroscopic ellipsometry (SE) measurements. The AlF 3 ALD growth rate determined by the ex situ analysis was 1.43 Å/cycle at 100°C. These ex situ measurements were in excellent agreement with the in situ QCM measurements. FTIR analysis monitored the growth of infrared absorbance from Al−F stretching vibrations at 500−900 cm −1 during AlF 3 ALD. In addition, absorption peaks were observed that were consistent with AlF(CH 3 ) 2 and HF species on the surface after the TMA and HF exposures, respectively. X-ray photoelectron spectroscopy (XPS) and Rutherford backscattering spectrometry (RBS) measurements revealed that the deposited films were nearly stoichiometric AlF 3 with an oxygen impurity of only ∼2 at %. AlF 3 ALD may be useful for a number of applications such as ultraviolet optical films, protective coatings for the electrodes of Li ion batteries, and Lewis acid catalytic films.

Thermal atomic layer etching (ALE) of Al2O3 and HfO2 using sequential, self-limiting fluorination and ligand-exchange reactions was recently demonstrated using HF and tin acetylacetonate (Sn(acac)2) as the reactants. This new thermal pathway for ALE represents the reverse of atomic layer deposition (ALD) and should lead to isotropic etching. Atomic layer deposition and ALE can together define the atomic layer growth and removal steps required for advanced semiconductor fabrication. The thermal ALE of many materials should be possible using fluorination and ligand-exchange reactions. The chemical details of ligand-exchange can lead to selective ALE between various materials. Thermal ALE could produce conformal etching in high-aspect-ratio structures. Thermal ALE could also yield ultrasmooth thin films based on deposit/etch-back methods. Enhancement of ALE rates and possible anisotropic ALE could be achieved using radicals or ions together with thermal ALE.