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.

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.

The thermal atomic layer etching (ALE) of SiO was performed using sequential reactions of trimethylaluminum (TMA) and hydrogen fluoride (HF) at 300 °C. Ex situ X-ray reflectivity (XRR) measurements revealed that the etch rate during SiO ALE was dependent on reactant pressure. SiO etch rates of 0.027, 0.15, 0.20, and 0.31 Å/cycle were observed at static reactant pressures of 0.1, 0.5, 1.0, and 4.0 Torr, respectively. Ex situ spectroscopic ellipsometry (SE) measurements were in agreement with these etch rates versus reactant pressure. In situ Fourier transform infrared (FTIR) spectroscopy investigations also observed SiO etching that was dependent on the static reactant pressures. The FTIR studies showed that the TMA and HF reactions displayed self-limiting behavior at the various reactant pressures. In addition, the FTIR spectra revealed that an AlO/aluminosilicate intermediate was present after the TMA exposures. The AlO/aluminosilicate intermediate is consistent with a "conversion-etch" mechanism where SiO is converted by TMA to AlO, aluminosilicates, and reduced silicon species following a family of reactions represented by 3SiO + 4Al(CH) → 2AlO + 3Si(CH). Ex situ X-ray photoelectron spectroscopy (XPS) studies confirmed the reduction of silicon species after TMA exposures. Following the conversion reactions, HF can fluorinate the AlO and aluminosilicates to species such as AlF and SiOF. Subsequently, TMA can remove the AlF and SiOF species by ligand-exchange transmetalation reactions and then convert additional SiO to AlO. The pressure-dependent conversion reaction of SiO to AlO and aluminosilicates by TMA is critical for thermal SiO ALE. The "conversion-etch" mechanism may also provide pathways for additional materials to be etched using thermal ALE.

Thermal Al 2 O 3 atomic layer etching (ALE) can be performed using sequential, self-limiting reactions with tin(II) acetylacetonate (Sn(acac) 2 ) and HF as the reactants.To understand the reaction mechanism, in situ quartz crystal microbalance (QCM) and Fourier transform infrared (FTIR) measurements were conducted versus temperature. 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. Arrhenius analysis of the temperature-dependent MCPC values yielded an activation barrier for Al 2 O 3 ALE of E = 6.6 ± 0.4 kcal/mol. The mass changes after the individual Sn(acac) 2 and HF exposures also varied with temperature. The mass changes after the Sn(acac) 2 exposures were consistent with more Sn(acac) 2 surface reaction products remaining at lower temperatures. The mass changes after the HF exposures were consistent with more AlF 3 species remaining at higher temperatures. The FTIR spectroscopic analysis observed Al 2 O 3 etching by measuring the loss of absorbance of Al−O stretching vibrations in the Al 2 O 3 film. The infrared absorbance of the acetylacetonate vibrational features from Sn(acac) 2 surface reaction products was also smaller at higher temperatures. The correlation between the MCPC values and the acetylacetonate infrared absorbance suggested that the Al 2 O 3 ALE rate is inversely dependent on the acetylacetonate surface coverage. In addition, the QCM and FTIR measurements explored the nucleation of the Al 2 O 3 ALE. A large mass gain and loss of infrared absorbance of Al−O stretching vibrations after the initial HF exposure on the Al 2 O 3 film was consistent with the conversion of Al 2 O 3 to AlF 3 . FTIR experiments also observed the formation of AlF 3 after the initial HF exposure and the presence of AlF 3 on the surface after each HF exposure during Al 2 O 3 ALE. In the proposed reaction mechanism, AlF 3 is the key reaction intermediate during Al 2 O 3 ALE. HF converts Al 2 O 3 to AlF 3 prior to removal of AlF 3 by Sn(acac) 2 .

The atomic layer etching (ALEt) of HfO 2 was performed using sequential, self-limiting thermal reactions with tin(II) acetylacetonate (Sn(acac) 2 ) and HF as the reactants. The HF source was a HF-pyridine solution. The etching of HfO 2 was linear with atomic level control versus number of Sn(acac) 2 and HF reaction cycles. The HfO 2 ALEt was measured at temperatures from 150-250 • C. Quartz crystal microbalance (QCM) measurements determined that the mass change per cycle (MCPC) increased with temperature from −6.7 ng/(cm 2 cycle) at 150 • C to −11.2 ng/(cm 2 cycle) at 250 • C. These MCPC values correspond to etch rates from 0.070 Å/cycle at 150 • C to 0.117 Å/cycle at 250 • C. X-ray reflectivity analysis confirmed the linear removal of HfO 2 and measured an HfO 2 ALEt etch rate of 0.11 Å/cycle at 200 • C. Fourier transform infrared (FTIR) spectroscopy measurements also observed HfO 2 ALEt using the infrared absorbance of the Hf-O stretching vibration. FTIR analysis also revealed absorbance features consistent with HfF 4 or HfF x surface species as a reaction intermediate. The HfO 2 etching is believed to follow the reaction: HfO 2 + 4Sn(acac) 2 + 4HF → Hf(acac) 4 + 4SnF(acac) + 2H 2 O. In the proposed reaction mechanism, Sn(acac) 2 donates acac to the substrate to produce Hf(acac) 4 . HF allows SnF(acac) and H 2 O to leave as reaction products. The thermal ALEt of many other metal oxides, as well as metal nitrides, phosphides, sulfides and arsenides, should be possible by a similar mechanism. Atomic layer etching (ALEt) is a thin film removal technique based on sequential, self-limiting surface reactions.1-3 ALEt can be viewed as the reverse of atomic layer deposition (ALD). 4 ALEt is able to remove thin films with atomic layer control. ALD and ALEt are able to provide the necessary processing techniques for surface engineering at the atomic level. 5,6 This atomic level control is needed for the nanofabrication of a wide range of nanoscale devices. 7Until recently, ALEt processes have been reported using only ionenhanced or energetic noble gas atom-enhanced surface reactions. [1][2][3] In these ALEt processes, a halogen is adsorbed on the surface of the material. Subsequently, ion or noble gas atom bombardment is used to desorb halogen compounds that etch the material. Using this approach, ALEt has been reported for Si,2,3,[8][9][10][11][12] Ge, 6,13 and compound semiconductors.14-17 ALEt has also been demonstrated for a variety of metal oxides. 7,[18][19][20] Additional ALEt studies have been conducted on various carbon substrates. 21-23The ALEt of Al 2 O 3 was recently reported using sequential, selflimiting thermal reactions with Sn(acac) 2 and HF as the reactants. 24The linear removal of Al 2 O 3 was observed at temperatures from 150-250• C without the use of ion or noble gas atom bombardment. Al 2 O 3 ALEt etch rates varied with temperature from 0.14 Å/cycle at 150• C to 0.61 Å/cycle at 250• C. 24 The Sn(acac) 2 and HF thermal reactions were both self-limiting versus reactant exposure. In addition, the Al 2...