Chemistry Reference
In-Depth Information
Dissolution of PS.
The dissolution of PS during PS formation may be due to
two processes: a process in the dark and a process under illumination. Both are essen-
tially corrosion processes by which the silicon in the PS is oxidized and dissolved with
simultaneous reduction of the oxidizing species in the solution. The corrosion process
is responsible for the formation of micro PS of certain thickness (stain film) as well as
the dissolution of the existing PS. The material in the PS which is at a certain distance
from the pore tips is little affected by the external bias due to the high resistivity of PS
and is essentially at an open-circuit condition (OCP). This dissolution process, which
is often referred to as chemical dissolution, is an electrochemical process because it
involves charge transfer across the interface. The anodic and cathodic reactions in the
microscopic corrosion cells depend on factors such as surface potential and carrier con-
centration on the surface which can be affected by illumination and the presence of oxi-
dants in the solution.
Because the dissolution of silicon in HF solution requires holes according to reac-
tion II in Fig. 5.68, the corrosion rate of silicon in HF solutions is very low due to the
unavailability of holes at the OCP (see Chapter 7). But due to the large surface area of
PS the amount of dissolution still has a significant effect on the density of PS (e.g., PS
density decreases with increasing PS thickness as shown in Fig. 8.51). The presence of
oxidants in the solution can greatly increase the corrosion rate of PS.
Illumination generates holes within the material of the PS and causes photocor-
rosion of the PS. Depending on the illumination intensity and time, the PS can be
thinned to various extents by the photoinduced corrosion. This corrosion process is
responsible for the etched crater between the initial surface and the surface of PS as
shown in Fig. 8.45. It is also responsible for the fractal structure of the micro PS formed
under illumination due to the different widths of the surface charge layer at which the
holes are generated.
Potential Drops in Different Phases of the Current Path.
There are five pos-
sible physical phases in the current path in which significant potential drops may occur
as illustrated in Fig. 8.71. They are the substrate, the space charge layer, the Helmholtz
layer, the surface oxide film, and the electrolyte. The overall change in the applied
