Etching of Oxides - Electrochemistry of Silicon and Its Oxide - page 155

increasing molarity. The inhibitive effect of dissolved silica in the solution also agrees
with the reaction scheme. The inhibitive effect of dissolved silica is the greatest at low
molarity when reaction 4.5 is the rate-limiting step and should disappear for concen-
trated solutions when reaction 4.4 is the rate-limiting step.
In HF-Based Solutions. The overall reaction involving the dissolution of silicon
oxide in HF solution can be expressed as

The elemental reactions are, however, rather complicated and remain the subject of
debate. For many studies on the kinetics of silicon oxide etching, identifying the fluo-
ride species in the solution responsible for the etching reactions has been a major
concern. This is because HF is a weak acid, which ionizes only partially to fluoride
ions, and its aqueous solutions may contain a significant quantity of fluorine in the form
of un-ionized HF and partially ionized bifluoride
In an earlier study, Judge 57 found that in solutions of

and close to

neutral pH, essentially all fluoride is present in the unprotonated state and the solutions
dissolution. Judge thus suggested that the free fluoride
ion is not one of the species responsible for the dissolution. This conclusion was further
supported by Kikuyama et al ., 239 who found that the etch rate of in KF solutions,
in which all the fluorine is in the form of free fluoride ions, is very small as shown in
Fig. 4.33. The etch rate in 1M KF is 0.001 Å/s, which is threefold smaller than that in
a HF solution of the same concentration.
According to Judge, 57 HF and which are the dominant species in acidic solu-
tion of HF, HCl, and are responsible for the etching reactions involved.
Clear correlation between etch rate and

exhibit a near zero rate of

concentration is also observed in

is a major species as shown in Fig. 4.34. 239 solutions in which is also found

solutions. 512 According to

to be the rate determining species in

Kikuyama et al ., 239 because the dependence of etch rate on

is different in HF and