Article Cite This: Chem. Mater. 2019, 31, 3624−3635 pubs.acs.org/cm SF4 as the Fluorination Reactant for Al2O3 and VO2 Thermal Atomic Layer Etching † † ‡ † Jonas C. Gertsch, Austin M. Cano, Victor M. Bright, and Steven M. George*, † ‡ Department of Chemistry and Department of Mechanical Engineering, University of Colorado, Boulder, Colorado 80309, United States ABSTRACT: Thermal atomic layer etching (ALE) is an important technique for the precise isotropic etching of nanostructures. Thermal ALE of many materials can be achieved using a two-step fluorination and ligand-exchange reaction mechanism. Most previous thermal ALE processes have used HF as the fluorination reactant. Alternative fluorination reactants may be needed because HF is a weak nucleophilic fluorination reactant. Stronger fluorination agents may be required for the fluorination of some materials. To demonstrate the usefulness of SF4 as an alternative to HF, thermal Al2O3 ALE was compared using SF4 or HF together with Sn(acac)2 as the metal precursor for ligand fl exchange. SF4 and HF were observed to behave similarly as uorination reactants during Al2O3 ALE. The mass gains during the initial SF4 and HF fi ° exposures on Al2O3 atomic layer deposition (ALD) lms at 200 C were comparable at 35 and 38 ng/cm2, respectively, using quartz crystal microbalance measurements. In addition, the etch rates were ° similar at 0.20 and 0.28 Å/cycle for Al2O3 ALE using SF4 and HF, respectively, at 200 C. Thermal VO2 ALE was also fi performed for the rst time using SF4 or HF and Sn(acac)2 as the reactants. There was evidence that SF4 is a stronger fl fl fi uorination reactant than HF for VO2 uorination. The mass gains during the initial SF4 and HF exposures on VO2 ALD lms 2 ° fl were 38 and 20 ng/cm , respectively, at 200 C. Thermal VO2 ALE also had a higher etch rate when uorinating with SF4 compared with HF. Etch rates of 0.30 and 0.11 Å/cycle were measured for VO2 ALE using SF4 and HF, respectively, together ° fl with Sn(acac)2 at 200 C. Fourier transform infrared experiments were also used to monitor uorination of the Al2O3 and VO2 fi ff − − ALD lms by SF4 or HF. FTIR di erence spectroscopy was used to observe the increase of Al F and V F stretching vibrations − and the loss of the Al O and V O/V O stretching vibrations for Al2O3 and VO2, respectively, versus SF4 or HF exposure at ° fl fi 200 C. Additional absorbance features after uorination of the Al2O3 ALD lms by SF4 were consistent with SFx surface fl species. SF4 is a useful uorination agent for thermal ALE processes and can be used as an alternative to HF. In addition, SF4 may be necessary when fluorination requires a stronger fluorination reactant than HF. from https://pubs.acs.org/doi/10.1021/acs.chemmater.8b05294. Downloaded by Steven George at 08:46:40:585 on June 14, 2019 8,9 I. INTRODUCTION and AlN ALE. Other metal precursors such as Al(CH3)3, ff AlCl(CH3)2, SiCl4, and TiCl4 have also been e ective for the Atomic layer etching (ALE) is possible using sequential and − 1 ligand-exchange reaction following fluorination with HF.10 13 self-limiting surface reactions. ALE is becoming increasingly − 2 10 13 important for advanced semiconductor manufacturing. ALE These reactants have led to Al2O3, HfO2, and ZrO2 ALE. can be viewed as the reverse of atomic layer deposition HF and Al(CH3)3 have also been used for SiO2 and ZnO ALE 3,4 “ ” (ALD). ALE can be accomplished using either plasma or using a conversion-etch mechanism, where Al(CH3)3 both is thermal methods.1,5 The first reaction during ALE usually involved in ligand-exchange after fluorination and also converts fi 14,15 involves surface modi cation by the adsorption of a reactive the surface of the remaining substrate to Al2O3. species that activates the surface. The second reaction during Although HF has been useful as a fluorination reactant ALE is a reaction that produces a volatile etch product. In during thermal ALE, HF does have some drawbacks. HF is a plasma ALE, the second reaction is the removal of the highly corrosive gas.16 Upon contact with moisture, HF forms activated layer by sputtering using an energetic ion or neutral fl 1 1 hydro uoric acid, which is also corrosive and toxic. Anhydrous species. Plasma ALE leads to anisotropic etching. In thermal HF is particularly problematic because its vapor pressure at ALE, the second reaction is a reaction between a gaseous room temperature exceeds atmospheric pressure. HF-pyridine reactant and the surface layer that produces stable and volatile 5 6 liquid sources of HF are safer because the vapor pressure of etching products. Thermal ALE leads to isotropic etching. − During thermal ALE, the main reaction that activates the HF above the HF-pyridine solution is 90 100 Torr at room fl temperature.9 However, alternative fluorination reactants to surface has been uorination. For example, thermal Al2O3 ALE was initially demonstrated using HF and Sn(acac)2 as the 5,7 reactants. The mechanism for thermal Al2O3 ALE is based Received: December 22, 2018 on fluorination and ligand-exchange reactions.5,7 HF and Revised: April 11, 2019 Sn(acac)2 have also been employed as the reactants for HfO2 Published: April 12, 2019 © 2019 American Chemical Society 3624 DOI: 10.1021/acs.chemmater.8b05294 Chem. Mater. 2019, 31, 3624−3635 Chemistry of Materials Article fl fl replace HF are desirable because HF is a weak uorination SF4 was used as the uorination reactant for the thermal reactant. ALE of Al2O3 and VO2. Sn(acac)2 was used as the metal fl fl HF is a nucleophilic uorination reactant where the uoride precursor for the ligand-exchange reaction. Al2O3 ALE was anion serves as the active reaction species. HF is a convenient studied to compare SF4 with the previous results using HF as fl fl fl 5,7,10,23 uorination reactant that can uorinate most metal oxides or the uorination reactant for thermal Al2O3 ALE. VO2 fl metal nitrides. HF produces metal uorides and H2OorNH3 ALE was also examined using HF or SF4 and Sn(acac)2 as the as the reaction products. However, HF is a relatively weak reactants. SF may be needed as the fluorination agent for fl 4 uorination agent compared with other inorganic nucleophilic thermal VO2 ALE. Thermochemical calculations indicate that fl Δ ° fl uorination reactants, such as SF4. Many electrophilic the predicted G for the uorination of VO2 to VF4 is slightly fl fi ° 17 fl uorinating agents also exist in which an electron-de cient positive for HF at 200 C. In contrast, uorination of VO2 to fl Δ ° ° 17 uorine serves as the active reaction species. The most widely VF4 has a large negative G using SF4 at 200 C. fl fi used inorganic electrophilic uorination reactant is F2. Another Thermal VO2 ALE is reported for the rst time in this paper. fl − common inorganic electrophilic uorination reactant is XeF2. VO2 is a semiconductor at room temperature and has a metal The standard free energy changes, ΔG°, for the fluorination insulator transition around 68 °C.24,25 This transition is of a variety of materials using HF, SF4,F2, and XeF2 are given accompanied by a large change in resistivity and optical 17 Δ ° ° in Table 1. The G values are all reported at 200 C. Most transmittance. Consequently, VO2 is a useful material for thermochromic films,25,26 bolometers,27,28 and switching 29,30 Table 1. Fluorination Reactions for Various Metal devices. Thermal VO2 ALE may be useful to produce a fi fi Compounds Using HF, SF4,F2, and XeF2 thin VO2 lms with low thermal mass and high lm stability. A low thermal mass is needed for larger thermal transients when Al O 2 3 using VO as a thermal sensor. Thin films employed for their → Δ ° − 2 Al2O3 + 6HF 2AlF3 +3H2O G = 58 kcal − → Δ ° − metal insulator transition are also less susceptible to fracture Al2O3 + 3SF4 2AlF3 + 3SOF2 G = 199 kcal → Δ ° − from stress that can result from temperature cycling and Al2O3 +3F2 2AlF3 + 3/2O2 G = 297 kcal − → Δ ° − structural changes around the metal insulator transition. Al2O3 + 3XeF2 2AlF3 + 3Xe + 3/2O2 G = 258 kcal HfO2 → Δ ° − II. EXPERIMENTAL SECTION HfO2 + 4HF HfF4 +2H2O G = 19 kcal HfO + 2SF → HfF + 2SOF ΔG° = −113 kcal II.A. Growth and Etching of Al2O3 and VO2 Films. The initial 2 4 4 2 fi HfO +2F → HfF +O ΔG° = −178 kcal Al2O3 and VO2 lms were prepared using Al2O3 and VO2 ALD. The 2 2 4 2 fi → Δ ° − Al2O3 ALD lms were grown using Al(CH3)3 (trimethylaluminum HfO2 + 4XeF2 HfF4 + 4Xe + 3/2O2 G = 152 kcal (TMA)) (97%, Sigma-Aldrich) and DI H2O as the reactants at GaN ° deposition temperatures between 130 and 200 C. TMA and H2O are GaN + 3HF → GaF +NH ΔG° = −40 kcal fi 31 3 3 known to yield amorphous Al2O3 ALD lms at these temperatures. → Δ ° − fi GaN + 3/4SF4 GaF3 + 1/2N2 + 3/16S8 G = 114 kcal The VO2 ALD lms were deposited using tetrakis(ethylmethylamino) → Δ ° − GaN+ 3/2F2 GaF3 + 1/2 N2 G = 239 kcal vanadium (TEMAV) (Air Liquide) and DI H2O as the reactants at → Δ ° − ° 32 GaN+ 3/2XeF2 GaF3 + 3/2Xe + 1/2N2 G = 219 kcal 150 C. TEMAV is a useful vanadium source for VO2 ALD because ZnS vanadium is already in the +4 oxidation state. TEMAV and H2O are fi 33 → Δ ° known to yield vanadium(IV) oxide lms.