UCLA

A framework to study the thermodynamics of chemistries of the etching step in ALE processes is presented. The effects of two modifiers(O & N), two substrates(Cu & Ni) and two etchants(formic acid and formamidine) on etching energy are studied with two models. The bulk model uses the bulk formation energy to provide a quick estimate of the etching energy, while the layer model incorporates effects of surface stability and adsorbate configurations. The effect of etchants is independent of models used. Etching by formic acid is favored by 0.45 eV on Ni but slightly disfavored by 0.07 eV on Cu. The two models produce the same qualitative results: on the bare metal, all modifier/etchant pair is unfavorable; on activated substrates, etching is unfavorable for Ni but favorable for Cu. The bulk model shows that the effect of modifiers is insignificant on Cu, but on Ni the O activation is favored by 0.20 eV. With the layer model, a difference of ∼ 0.3 eV favoring the N activation is observed for Cu substrate. On Ni substrate this effect depends on coverage. Below ∼ 0.07 A^-2 , N is the favored modifier. The reverse is true above ∼ 0.07 A−2 . The layer model is applied to metastable structures on Ni/O/FAmd system formed by placing O atoms in sublayer interstitial sites, resulting in a favorable etching energy consistent with the experiments. The validity of the layer model is established, but the structures fed into the model must be carefully selected to match with the actual substrate.