Chemistry Reference
In-Depth Information
completely dissolved before formation of new oxide. Because the growth rate of the
oxide film is limited by the film thickness, at a fixed potential, the growth rate of the
oxide film will greatly increase with thinning of the film, which prevents the film from
being completely dissolved by a chemical reaction process.
Advances in the Understanding of Electrochemical Reactions.
Starting at the
same time as the discovery of PS and particularly during the period from the late 1970s
to the 1990s, a number of systematic studies have been made on the fundamental
aspects of the electrochemistry of silicon in HF solutions as highlighted in Fig. 8.3.
These studies revealed many important details regarding the electronic structure, nature
of reactions and the carrier transfer kinetics of the silicon/electrolyte interface as dis-
cussed in detail in Chapters 2-5. Some of the key findings, which are relevant to the
formation of PS in HF solutions, can be summarized in the following:
In aqueous solutions silicon reacts spontaneously with water to form an oxide
film which passivates the surface.
The presence of HF results in the dissolution of silicon oxide and activates the
surface to a variable extent depending on HF concentration and potential.
The fluoride species such as HF and
also react directly with the bare silicon
surface, and thus in aqueous HF solution, the dissolution of silicon atoms has
two principal competing paths, one via the reaction with HF and the other with
H
2
O.
Silicon, having four valence electrons, dissolves in multiple steps, each of
which may occur at different energy levels and may proceed via the conduc-
tion band as well as the valence band depending on the condition of the Si/elec-
trolyte interface.
The surface of silicon during anodic dissolution is dynamically terminated by
hydrogen and thus the dissolution of silicon atoms proceeds by first forming a
Si-H bond.
The replacement of the hydrogen by fluoride polarizes and weakens the Si-Si
back bond and facilitates the subsequent attack on this bond by water or HF.
The quantum efficiency of photoelectrochemical reactions may vary from 2 to
4, effective dissolution valence from 2 to 4, and efficiency of hydrogen evolu-
tion from 1 to near zero depending on light intensity and potential.
Anodic oxide films formed under different kinetic conditions have drastically
different structures, compositions, and properties (e.g., etch rate) and they
change with time during the anodization.
The applied anodic potential may mostly or partially drop in the space charge
layer or in the Helmholtz layer depending on doping type and concentration as
well as on the potential range.
The rate of removal of surface silicon atoms by the electrochemical reactions
is orientation dependent, lowest on (111) and higher on other orientations.
These reaction features must be involved in the formation of PS on silicon in HF.
However, because of the great complexity of the anodic reactions on silicon in HF solu-
tions on the one hand and of the extremely rich PS morphology on the other hand, the
results from the two research domains, fundamental electrochemistry and PS formation
mechanisms, have not been well integrated.
