Chemistry Reference
In-Depth Information
oxide film which may affect the transport inside the oxide as well as the etch rate of
the oxide.
Lehmann
122
proposed a model ascribing the anodic oscillation to the stress-
induced phase transformation of the surface oxide film from a dense state to a less dense
state which results in changes in the etch rate of the oxide film. As shown in Fig. 5.58,
in HF solutions the growth of oxide is accompanied by its dissolution. If the dissolu-
tion rate of the dense film is below its formation rate, the thickness of the oxide film
will increase. At a certain thickness (about 11 nm) the transition in the oxide morphol-
ogy to a state highly permeable to chemical species occurs. This leads to a sudden drop
in potential. As the etch rate of this less dense film is larger than the growth rate of the
dense film at the silicon oxide interface, the total film thickness decreases until the
dense slow etching film is exposed to the electrolyte. At a certain thickness the phase
transition occurs. Lehmann also postulated that the oscillation in fluoride-free solution
follows the same mechanism as in HF solutions except that there is little dissolution of
the oxide film and thus the total film increases all the time with periodic transforma-
tion of the dense film at the oxide/silicon interface. However, because of the monoto-
nic increase in the total oxide thickness the oscillation is damped after a few periods.
Similar to the phase transition mechanism Parkhutik
et al.
508
proposed that the oscilla-
tion is due to formation of a thin oxide film and it subsequent lift-off.
Chazalviel
et al.
950
proposed a model that differs from Lehmann's only in the
cause of the phase transformation. Their model is based on a defective nature of the
oxide and the change of oxide property is considered to be due to a sudden loss in
the ion and electron blocking properties of the oxide. Breakdown of oxide occurs
at the high field condition resulting in a large electronic current or ionic current.
