Atomic layer etch (ALE) is rapidly becoming the key enabling technology to bring the critical dimensions (CD) of semiconductor devices below 10 nm. In 2017, IBM had unveiled its latest gate-all-around field effect transistor (GAAFET) with a CD of 5 nm which required an isotropic ALE processing prior to atomic layer deposition (ALD). Historically, etching is performed with the help of plasma that removed target material continuously from pulse to purge. Therefore, it did not allow for finer control over the layer thickness and surface roughness. ALE consists of sequential, self-limiting reactions that would enable the removal of material up to one atomic layer at a time. ALE can be directional or isotropic. While there is a plethora of information available on directional ALE, which uses energetic ion bombardment to make volatile products, only a little is known about isotropic ALE. This talk will be focused on thermal ALE, which uses gas phase thermal reactions to isotropically etch the material under study. A typical thermal ALE cycle consists of two precursor pulses, a 'modification' pulse and a 'ligand-exchange' pulse. The 'modification' pulse alters the surface composition of the material through chemical reactions and the 'ligand-exchange' pulse volatilize the atoms of the modified layer.
First principles based calculations can be of great help in understanding the critical mechanisms involved in thermal ALE processes. Recently, a thermal atomic layer etch (ALE) process for alumina has been reported where HF and Sn(acac)2 / tri-methyl aluminium (TMA) are used as precursors. In this talk, insights into the mechanism of HF adsorption on alumina will be provided with the help of first principles calculations using density functional theory. On adsorption, an HF molecule dissociated spontaneously at the surface of alumina and formed surface bound Al-F and O-H species. At higher coverages, a hydrogen-bonded network of dissociated and intact HF molecules was observed at the alumina surface. Finally, based on the saturated coverage of dissociated F atoms (13.7 ± 0.3 F/nm2), the theoretical maximum of the etch rate was estimated to be -1.28 ± 0.03 Å/cycle.