A Study on Electrochemical Machining Method in Ultrapure Water: Etching Process of Hydrogen-Terminated Si(001)Surface Atom by Hydroxyl Function

In order to reveal the mechanism of the electrochemical etching process in ultrapure water, first-principles molecular-dynamics simulations for the etching process of hydrogen-terminated Si(001) surfaces interacting with OH molecules were carried out on the basis of the Kohn-Sham local-density-functional formalism. A plane-wave basis set was used, and the cut-off energy is 327eV(24Ry). A norm-conserving pseudopotential was also used. The standard molecular-dynamics method for the optimization of the ionic system and the preconditioned conjugate-gradient (CG) method for the quenching procedure of the electronic degrees of freedom were adopted. The optimized ionic configurations, local density of states, and atomic and bond populations for OH chemisorbed Si(001) surfaces were determined and the electronic structures relevant to the etching mechanism were clarified. It was confirmed that the interaction between two OH molecules and the hydrogen-terminated surface silicon atom on the step edge breaks the Si-Si back-bond and initiates the etching process forming a SiH2(OH)2 molecule.