Thermodynamics of Atomic Layer Etching Chemistry on Copper and Nickel Surfaces from First Principles

Abstract

Plasma-thermal atomic layer etching is a promising technique to enable selective and directional etching on metals. It involves a plasma activation step and a thermal step where etchant molecules remove a fraction of the surface. To accelerate process development, a computational model for the thermodynamics of the thermal removal step is highly desirable. An energy expression is developed here to calculate the removal step energy for an activated slab structure. The approach samples the configurations of the activated metal surfaces and determines the thermodynamic balance of the removal step for each obtained configuration. The heterogeneity of the surface terminations is treated by the equilibrium crystal shape method. The models are put to test with combinations of two modifiers (O and N), two substrates (Cu and Ni), and two etchants (formic acid and formamidine). It is found that higher coverages of modifiers lead to more favorable etching. In addition, our results show that removal step energies vary among different terminations, with differences on the order of 0.5 eV. This suggests that etching can preferentially occur over certain crystal terminations. Qualitative agreement with the experiment on the Ni/O/formic acid system is obtained with the layer model at high coverages of oxygen atoms.