Chemistry Reference
In-Depth Information
of current will be affected by the surface curvatures that have a radius close or smaller
than the width of the space charge layer.
9.5. SENSITIVITY TO LATTICE STRUCTURE
The surface of a single crystalline silicon can have different reactivities depend-
ing on the orientation of the surface. This orientation dependent reactivity is determined
by the bonding condition of the surface silicon atoms in the surface lattice structure
which is different for surfaces of different orientations. Characteristically, for example,
the atoms on (111) surface have three bonds connecting to the substrate lattice while
those on (100) surface have only two. Also, the bonding of the surface silicon atoms
change the bond strength of the atoms to the
substrate, and thus their reactivity, to varying extent. In addition, a real surface has
atomic structures associated with roughness and defects, which deviate from the lattice
structure of determined by the orientation. The atoms at these structures have different
bonding conditions and different reactivity.
The reactivity of the surface can be described by the density of the active surface
atoms as illustrated in Fig. 7.40. The density and distribution of surface active atoms
determines the difference in the reactivity of different surfaces. Thus, the etching of
silicon in KOH is anisotropic because the difference in the density of surface active
to the species in the solution such as
atoms on (100) and (111) surfaces is large. On the other hand, the etching of silicon in
is largely isotropic because the density of surface active atoms is similar for
surfaces of different orientations. In HF solutions, the surface has an intrinsic tendency
to roughen due to the sensitivity of the electrochemical reactions to surface curvature.
As a result, the surface which is rough at the atomic scale has no distinct crystallo-
graphic character and can be viewed as an amorphous surface, as illustrated in Fig.
9.3a. On the other hand, the surface shown in Fig. 9.3b is less rough and has a clear
crystallographic character. It can be generalized that when the number atoms at kink
sites, steps, and other defects is close to the density of surface atoms, the surface loses
its crystallographic nature. Similarly, the reaction is also essentially isotropic in the
case of electropolishing in HF solutions because the dissolution is through the forma-
tion and dissolution of silicon oxide which is amorphous in structure. The formation
