Chemistry Reference
In-Depth Information
In a similar approach, Campbell
et al.
697
postulated that because the work func-
tions of (111) and (100) planes differ by about0.25eV, the flatband potentials may differ
by a similar extent. This difference in the flatband potential is then responsible for the
different etching rates of the two surfaces. They recognized, however, that the electro-
chemical dissolution contributes only a very small fraction of the total amount of
etching and suggested that the etching of silicon in alkaline solutions is not solely due
to the supply of carriers. Some chemical reactions must also play an important role and
the origin of anisotropic etching is related to the differences in the rates of the chemi-
cal reactions as well as in the surface potentials of the different silicon surfaces.
Allongue
et al
.
22,234
found with STM imaging, the silicon surface during etching
in NaOH consists of hydrogen-terminated (111) terraces and dissolution principally
occurs at the terrace edges. The dissolution rate of the terrace surface is negligible com-
pared with that at the terrace edge. Based on this observation and the knowledge that
the etching at the OCP is mainly of a chemical nature, they proposed that the anisotropic
etching is due to the chemical substitution of Si-H bonds by Si-OH bonds preferen-
tially at the kink-site atoms at the edges of terraces.
A similar atomic step-dependent etching mechanism has also been proposed by
Elwenspoek
17
who postulated that anisotropy of etch rate is related to anisotropy of
step-free energy which is the free energy required to generate steps on a flat surface.
The kinetics of crystal dissolution in solutions is governed by the kinetics of step
formation and removal. If the crystal surface is flat, the rate-determining step in the
dissolution process is the step generation and not the removal of atoms from the steps.
Thus, the etch mechanism of a smooth face is characterized by a nucleation
barrier associated with the step-free energy. The difference between the etch rates of
the major crystal planes (111), (110), and (100) is due to the difference in step-free
energy, which is the largest for the (111) surface. For a rough face with many steps,
e.g., a misoriented crystal plane, there is no nucleation barrier as the step-free energy
is near zero. Atoms may be removed from the surface without changing the number
of steps.
Mechanism of Anisotropic Etching.
The rich details as to anisotropic etch rates,
nature of reactions, and surface topography indicate that a complex mechanism is
involved in silicon etching in alkaline solutions. A coherent mechanistic model has to
address three basic aspects: (1) the physical cause of the difference in the removal rates
of atoms from the surface of different crystal orientations, (2) the kinetic processes that
actualize such physical cause, and (3) the surface condition that determines the global
removal rate of the surface atoms.
The physical cause in the first aspect is, most convincingly, associated with the
back-bond strength theory. That is, atoms on the (111) surface have three bonds con-
nected to the substrate lattice while those on the (100) surface have only two as shown
in Fig. 7.39. Bonding of the surface silicon atoms to solution species such as OH
-
reduces the strength of the back bonds. The number of back bonds and the strength of
surface bonds to the adsorbed species are then responsible for the different reactivity
of different crystal surfaces. This can be regarded as the fundamental physical cause of
anisotropic etching.
Regarding the second aspect, generally, as shown in Fig. 7.40, three species may
be involved in the etching reaction: species in the form of charge carrier, i.e., electrons
