Abstract:
Anisotropic wet chemical etching of silicon in alkaline solutions is
a key technology in the fabrication of sensors and actuators. In this technology
etching through masks is used for fast and reproducible shaping of micromechanical
structures. Experimentally it has been found that near the junction between
a slowly etching Si(111) surface and a mask, etching can be influenced
by etch pit nucleation at this junction. In this paper the influence of
the presence of such a junction on the etch rate and the surface topology
is investigated by means of Monte Carlo simulations of etching of the Kossel
(100) surface. To describe such a system only two parameters are needed:
one parameter that describes the interaction between two bulk atoms and
one parameter that describes the interaction between the mask and an adjacent
atom. If the latter interaction is significantly smaller than the first,
the nucleation rate at the mask junction is higher than throughout the
crystal surface, which induces the formation of a stepped facet at the
junction, which grows in time. An analytical expression for the misorientation
of this facet is derived that agrees with the Monte Carlo simulations.
The misorientation depends only on the interaction between the interface
and the mask. The Si(111) surface is more complicated than the Kossel (100)
surface. Underetching experiments have shown that a stepped facet is only
formed for an obtuse contact angle of the Si(I 11) surface with the mask.
This can be explained by comparing the topology of the mask junction for
an obtuse and an acute contact angle.