Chemistry Reference
In-Depth Information
Fig. 5.43.
109,925
In the passive region, the amount of dissolution is more than the current
can account for, indicating that the dissolution in the passive region is partially chem-
ical in nature due to the small amount of hydrogen evolution in the passive region.
5.9.3. Passive Films
Passive films formed on a silicon surface in aqueous solutions are in general oxide
films. There is rather limited systematic information on the structure and properties of
thin silicon anodic oxide films, particularly those formed in solutions of high silicon
solubilities. On the other hand, the thicker oxide films formed at large potentials have
been better characterized (see Chapter 3) and the information associated can be used
for understanding the thin oxide films formed at relatively low potentials.
The studies on anodic oxide films reveal the following characteristics: (1) anodic
oxides of silicon are amorphous and nonstoichiometric; containing varying contents of
water, hydroxyl ions, and other foreign species depending on formation conditions; (2)
the properties of anodic oxides formed on
n-
and
p-
type materials are similar; (3) the
structures and properties of anodic films are not stable and tend to change with time;
(4) when the oxide is a continuous phase of one to a few monolayers, it acts as a barrier
to the interaction between the silicon surface and the electrolyte, resulting in a drastic
reduction of the electrode reactions; (5) the formation rate of anodic oxide films is a
function of potential, electrolyte composition, substrate condition, and oxide film thick-
ness; (6) the dissolution rate of passive film depends sensitively on formation condi-
tions; (7) the solubility of silicon oxides is high in both HF and alkaline solutions, but
the dissolution rate is high in HF solutions and low in alkaline solutions; (8) the com-
position and structure of passive film varies in thickness from the silicon/oxide inter-
face to the oxide/electrolyte interface; (9) some silicon atoms which enter into the oxide
phase are only partially oxidized and tend to locate near the silicon/oxide interface
