Spa-Leed Measurements on Etched and Polished Silicon (111) Surfaces

Abstract

ABSTRACTThe atomic structure of etched and polished Si (111) surfaces has been determined quantitatively on an atomic scale by SPA-LEED (spot profile analysis of low energy electron diffraction) in an ultrahigh vacuum. The energy dependant broadening of the diffracted electron beam yields the average size of step free terraces and the distribution of the atomic steps.Whereas acid etched Si (111) wafers exhibit surfaces with very high step atom densities, alkaline etched Si (111) wafers show atomically very flat surfaces besides macroscopic steps. In chemo-mechanical polishing of these silicon wafers bright LEED patterns exhibiting the atomic structure at the actual surface were obtained. No damage or defects could be observed with LEED during this process. This indicates that the removal of Si is mainly controlled by chemical reaction. In an intermediate step the wafers exhibit a slight surface roughness (“haze”) observable by optical Nomarski contrast microscopy or by light scattering. This is correlated qualitatively to a broadening of LEED spots for a phase relation, indicating that the atomic steps are distributed over many atomic layers creating a superimposed long range roughness. After polishing this long range roughness (haze) disappears and atomically smooth surfaces were obtained. Haze free polished Si (111) wafers exhibit on average less than one atomic step for a row of 30 atoms which represents the instrumental resolution. Definitely misoriented wafers show regular terraces. The regularity is increasing with proceeding polishing. The number of atoms per terrace is correlated to the angle of misorientation, demonstrating the high quality of the polished wafer surface.