Kinetics of HfO2 etching and impurity elimination with atomic hydrogen beams

Modification of the surface composition, stoichiometry, morphology, and structure of HfO2 ultra-thin layers upon exposure to atomic hydrogen beams has been investigated by a combination of time-of-flight secondary ion mass spectrometry (ToF-SIMS) depth profile analysis, angle-resolved x-ray photoelectron spectroscopy (ARXPS), atomic force microscopy (AFM) imaging, and transmission electron microscopy (TEM). The etching reaction has been found to be thermally activated (Ea = 96 kJ/mol) and governed by the expression Retch(Å/h) = 3.03 × 108 e-1.15x104T within the 350–400 °C temperature range. The rate law determined experimentally, i.e., Retch(Å/h) = 1.07 × 10−1[H2]1/2, is consistent with a reaction mechanism that involves the formation of a volatile hafnium dihydroxide, Hf(OH)2 in two successive elementary steps. Precise control of the etching conditions allows to reach atomically smooth HfO2 surfaces having r.m.s. roughness below 0.2 nm, with no evidence of Hf metal formation derived from the reaction with atomic H. Elimination of C, Cl, F, and S contaminants from the HfO2 surface through the formation of the corresponding volatile hydrides has proved to be generally most effective at high temperatures and H2 flow rates.