Chemistry Reference
In-Depth Information
mechanisms involve an attack on a subsurface Si-Si bond. The rate of this reaction is
determined by the bonding condition of the surface silicon atoms.
237,697
The {111} plane
has only one Si-OH surface bond whereas the {100} surface has two. The surface
lattice configuration for (111) and (100) surfaces can be simplistically represented by
the scheme in Fig. 7.39.
109,237
The atoms on the (100) surface have weaker back bonds
to the underneath silicon atoms compared with those on the (111) surface due to the
large electronegativity of As a result, the silicon atoms on the (100) surface are
etched at a faster rate than those on the (111) surface. This model appears to offer a
reasonable explanation for the anisotropic etching in alkaline solutions although it is
unable to explain why such effect does not occur in HF solutions.
The difference in etch rate between (111) and (100) surfaces was related to the
bond densities on the two surfaces in the early surface kinetics models.
109
According
to Hesketh
et al
.,
235
the etch rate difference between (100) and (111) planes is due to
the difference in the surface free energy of the crystal planes which is proportional to
the number of bonds on the surface. The (111) plane, which has the lowest surface free
energy measured in vacuum, has the lowest bond density and thus has the lowest etch
rate. They postulated that the etch rate of crystal planes is a function of the total number
of bonds at the surface, that is, the sum of the in-plane, lateral bonds between atoms
in the plane of the surface, and surface bonds, dangling bonds. It was recognized
however, that this effect alone will not cause etch rate differences of more than a factor
of two.
109
Jakob and Chabal
895
reasoned that the silicon atoms, such as those at the steps
and kink sites, that are most physically accessible are preferentially attacked. The reac-
tants and the dissolved complex have certain physical dimension and orientation so that
certain pathways may be forbidden due to steric constraints. While this argument is
intuitively sound, its verification requires information on the solvation structure of the
involved species and their interaction with the surface atomic structures.
Seidel
et al
.
206
proposed that the rate of silicon atom dissolution depends on
the density of energetically favorable surface states based on the assumption that the
etching of silicon in alkaline solutions is of electrochemical nature. They attributed
the anisotropic etching to the different energy levels of surface states on (111) and
According to Seidel
et al
(100) surfaces arising from binding surface atoms to
,
the difference in the activation energy of various crystal planes corresponds to the
difference in the energy states of the back bonds of differently oriented surfaces. A
difference of 0.12eV between the activation energies of two different planes was
considered to be sufficient to cause an etch rate ratio of 100:1 for (100) and (111)
planes.
.
