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A simplified mechanism for the chemical reaction in reported in Figure 3 [10].
Figure 3: mechanism of silicon oxidation during the formation of porous silicon.
Application of anodic current oxidizes a surface silicon atom, which is then attacked
by fluoride. The net process is a 4 electron oxidation, but only two equivalents are
supplied by the current source. The other two equivalents come from reduction of
protons in the solution by surface SiF2 species. Pore formation occurs as Si atoms
are removed in the form of SiF4, which reacts with two equivalents of F − in solution to
form SiF6 2− .
The porosity and the thickness of the PSi layers are among the most important
parameters which characterize porous silicon. The porosity is defined as the fraction
of void within the PSi layer and can be determined easily by weight measurements.
The virgin wafer is first weighted before anodization (m1), then just after anodization
(m2) and finally after dissolution of the whole porous layer in a molar NaOH aqueous
solution (m3). Uniform and rapid stripping in the NaOH solution is obtained when the
PSi layer is covered with a small amount of ethanol which improves the infiltration of
aqueous NaOH in the pores. The porosity is given simply by the following equation:
%
1 2
1 3
From these measured masses, it is also possible to determine the thickness of the
layer according to the following formula:
1 3
where d is the density of bulk silicon and S the wafer area exposed to HF during
anodization.
The porosity of a growing porous Si layer is proportional to the current density being
applied, and it typically ranges between 40 and 80%. Pores form only at the
Si/porous Si interface, and once formed, the morphology of the pores does not
change significantly for the remainder of the etching process.
However, the porosity of a growing layer can be altered by changing the applied
current. The film will continue to grow with this new porosity until the current
changes. This feature allows the construction of layered nanostructures simply by