Prediction and Validation of the Process Window for Atomic Layer Etching: HF Exposure on TiO₂

Abstract

A combined computational and experimental study is employed to understand the competition between self-limiting (SL) and chemical vapor etch (CVE) reactions to design an atomic layer etch (ALE) process. The pulses in an ALE process have to be self-limiting; i.e., the reactions should reach saturation after sufficient pulse time. By comparing the reaction free energies of corresponding SL and CVE reactions using density functional theory (DFT), the temperature and pressure conditions can be predicted that favor the SL or CVE reactions. The etching of TiO2 when exposed to HF gas is utilized as a test case. Simulations reveal that when TiO2 is exposed to reactant HF at a pressure of 0.2 Torr, the SL reaction removing H2O at 0.01 Torr and fluorinating the surface is preferred up to 87 °C (360 K). At higher temperatures, continuous removal of TiO2 by CVE occurs according to the reaction TiO2 + HF → TiF4 + H2O subject to kinetic activation barriers. Experimental results from in situ Fourier transform infrared (FTIR) spectroscopy and quadrupole mass spectrometry (QMS) are compared with the theoretical predictions. In good agreement with theory, the FTIR spectroscopy studies revealed an onset of spontaneous etching (CVE) at temperatures around 80–90 °C. In addition, the QMS analysis observed TiF4 and H2O as the etch products, further validating the calculations. The calculations also predicted that an increase in the reactant gas pressure would enhance etching at high temperatures. The low computational cost of this theoretical approach allows for rapid screening of etch reagents and prediction of the temperature/pressure windows where the reactions will be in the SL or CVE regimes.

Origin of enhanced thermal atomic layer etching of amorphous HfO2

“…Of course, it is not possible to control the product pressure in an etch reactor. 16 It is, however, typically much lower than the reactant pressure, so a value of 0.01 Torr for our calculations is consistent with this and previous work of DFT investigation in thermal etching of TiN, 16 W, 22 TiO 2 , 9 and Co. 48 A. Bulk and slab models Bulk amorphous HfO 2 (henceforth aHfO 2 ) was prepared using classical molecular dynamics (MD) simulations with the LAMMPS package.…”

“…The reactant gas pressure is adjustable in the reactor whereas the product pressure is not an experimentally adjustable parameter. 9 Examining the spontaneous etch reaction and the self-limiting reactions at different reactant pressures at a constant product pressure was beyond the scope of this study. A previous study of the thermal ALE of TiO 2 using HF by Natarajan et al examined the influence of reactant and product pressures on the FEPs of the spontaneous etch and self-limiting reactions that are similar to this study.…”

“…A previous study of the thermal ALE of TiO 2 using HF by Natarajan et al examined the influence of reactant and product pressures on the FEPs of the spontaneous etch and self-limiting reactions that are similar to this study. 9 It was found that the temperature at which the FEPs of the spontaneous etch and selflimiting reactions cross over is constant at 360 K with respect to an increase in the reactant pressure with a constant product pressure. 9 HF coverages on aHfO 2 ranging from 1.1 ± 0.3 to 18.0 ± 0.3 HF/nm 2 resulted in complete dissociation or mixed dissociated and molecular HF adsorption.…”

“…9 It was found that the temperature at which the FEPs of the spontaneous etch and selflimiting reactions cross over is constant at 360 K with respect to an increase in the reactant pressure with a constant product pressure. 9 HF coverages on aHfO 2 ranging from 1.1 ± 0.3 to 18.0 ± 0.3 HF/nm 2 resulted in complete dissociation or mixed dissociated and molecular HF adsorption. Water and hydrogen peroxide form spontaneously after relaxation and their computed desorption energies are low enough to be overcome under process conditions.…”

Chemical Mechanism of AlF3 Etching during AlMe3 Exposure: A Thermodynamic and DFT Study

“…Based on quartz crystal microbalance and Fourier transform infrared measurements, it was suggested that the HF exposure fluorinates Al 2 O 3 to form an AlF 3 or AlO x F y layer and H 2 O as the reaction products. Theoretically, Natarajan and Elliott − introduced a thermodynamic methodology (N–E analysis) to compare the self-limiting and continuous etching processes occurring during the HF exposure. The self-limiting reactions were found energetically more favorable than the continuous etching reactions, confirming that the fluorination of Al 2 O 3 is self-limiting in nature .…”