Etching of Silicon - Electrochemistry of Silicon and Its Oxide - page 318

(111) silicon at the OCP but not on other planes. Faust and Palik 378 found that the pas-
sivation peak current of the (111) surface is only a small fraction of that on the (100)
surface and postulated that a prepassive layer existing on the (111) surface at poten-
tials cathodic of the passivation potential is responsible for the slow etch rate of the
(111) surface. Smith et al . 291 also suggested that an oxide film may be present at the
OCP which passivates the surface.
In a slightly different angle, Kendall 478 reasoned that because the {111} surface
is oxidized thermally more rapidly than other low-index surfaces, the silicon surface
can be covered with a silicon oxide (or a hydrated silicon oxide) during etching in
aqueous solution, much faster than other planes. The formation of the oxide film pas-
sivates the {111} plane and blocks the dissolution reactions. This model implies that
the etch rate of a {111} surface should be similar to that of silicon oxide in KOH solu-
tion. Alternatively, Kendall et al . 984,1008 proposed that the (111) surface is blocked by
inactive hydration complexes of and According to this hypothesis, in concen-
trated KOH solutions the (111) surface is almost completely passivated by
hydration complexes attaching to the individual surface atoms. The fact that the rela-
tive etch rates of the different planes vary with the type of solution is attributed to the
orientation-dependent adsorption of solvation complexes on the surface.
The passivation models are not in agreement with a number of experimental
observations. First, it has been found that the (111) surface in KOH at the OCP is free
of oxide but terminated by hydrogen and formation of an oxide film occurs only at
more anodic potentials. 227,234 Also, the passivation potential for the (100) surface in
KOH solutions is more negative than that of the (111) surface indicating that it is easier
to passivate the (100) surface than the (111) surface. 192,697 Furthermore, the etching of
silicon at the OCP in alkaline solution is mainly a chemical reaction; etch rate changes
very little with potential in the vicinity of the OC

22,109,697 The difference in passiva-
tion currents between (100) and (111) surfaces may be due not to a preexisting subox-
ide layer on the (111) surface but rather to the reaction kinetics. The effect of passivation
at an anodic potential, which reflects only the electrochemical portion of the reactions,
may not be related to the etching reactions occurring at the OCP. Because the passiva-
tion potential is several hundred millivolts from the OCP, the contribution of the elec-
trochemical reaction in the electrode behavior at the passivation potential must be very
different from that at the OCP.
The etch rate of the (111) surface, although much smaller than those of the (100)
and (110) planes, shows definite values, in the range of 2-10 Å/s in KOH solutions. It
is still much larger than the dissolution current density on a passivated surface in KOH
(a dissolution rate of 2-10 Å/s is equivalent to a current density of several milliamperes
per square centimeter). In alkaline solutions, the dissolution rate of silicon oxide is less
than 0.01 Å/s (see Chapter 4), which is several orders of magnitude smaller than the
etch rates of a (111) surface. Thus, it is unlikely that the silicon surface of any orien-
tation is covered by during etching.
Surface Reaction Kinetics-Based Models. The basic consideration in reaction
kinetics models is that the reaction rate is determined by the lattice structure on the
surface. The difference in the lattice structures of various crystal planes gives rise to
differences in surface bond density, electron density, surface free energy, and so on,
which then determine the dissolution rate of the surface silicon atoms. All etching

P.