Formation and stabilization of pyramidal etch hillocks on silicon {100} in anisotropic etchants: Experiments and Monte Carlo simulation

On Si{100} surfaces etched in anisotropic etchants such as aqueous solutions KOH and TMAH, pyramidal etch hillocks are frequently found. Besides these hillocks, we have investigated hillocks that have partially disappeared using scanning electron microscopy (SEM). During re-etching numerous additional pyramidal etch hillocks are formed on the exact spots where SEM pictures were made earlier. These observations suggest that semipermeable particles adhering to the surface are responsible for the development of the pyramidal etch hillocks. In order to investigate the influence of such nanometer scale particles on the etch rate and the surface morphology, Monte Carlo simulations were performed of etching of Si{100} surfaces on which small semimasks are present. The presence of the microscopic semimasks is shown to cause the formation of macroscopic hillocks, which closely resemble experimentally observed hillocks. Removal of the semimask on top of a hillock leads to a vanishing pyramidal etch hillock. In the Monte Carlo model, however, the etch rate as a function of surface orientation has a maximum for {100}, while in reality {100} corresponds to a local minimum. This implies that for typical experimental conditions an etch hillock should not be stable despite a semipermeable particle on top, because of underetching starting from < 110 > ridges of the hillock. This paradox can be resolved by assuming that the ridges act as sinks of tiny particles. This gives a reduction in etch rate of the ridges, next to the top, which is necessary for the hillock to remain stable. The exact nature of these masking particles is unknown, but silicate particles are a likely candidate. (C) 2001 American Institute of Physics.