Etching of Silicon - Electrochemistry of Silicon and Its Oxide - page 305

the OCP shown in Fig. 7.25 are much smaller than the peak values indicating that elec-
trochemical processes play a more important role than in KOH.
Etching Mechanism. Water is essential for anodic current and etching to occur
in EDP. 342,697 Water is the active etching species, as no etching occurs for either anhy-
drous amine or anhydrous amine-pyrocatechol mixtures. 334 Etching does not occur
without water even in the presence of fluoride ions. Measurement of etch rates indi-
cates that etching of silicon in EDP is mainly a chemical process with about 20% elec-
trochemical contribution, which is considerably higher than that in KOH solutions. 22
The stoichiometry of the evolved hydrogen and dissolved silicon is close to 2. This
indicates that evolution of hydrogen is the only reduction reaction accompanying the
oxidation of silicon. The dissolution product is identified to be

The etching activation energy is about 0.48 eV in a solution with an E: P: W ratio
of 28%: 3%: 69% suggesting that the etching process is probably not controlled by
diffusion. 334 Etching occurs via oxidation of the silicon by water and hydroxyl
ions resulting in evolution of hydrogen and hydrous silica at the silicon surface. Accord-
ing to Finne and Klein, 334 the hydrous silica is removed through the formation of the
amine-soluble silicon(IV). The overall reaction process was suggested to occur as
follows:

342

According to Finne and Klein, 334 an amine environment is required for the pyro-
catechol to dissolve (chelate) the hydrous silica. Different amines have different effect
on the etching due to their protophilic activity. For small-chain-length amines such as
diamine where the amine separation is small, the positive charge on the monocation
withdraws electrons from the second amine group, reducing its protophilic activity. In
ethylenediamine the charge separation is greater and the second amine retains its pro-
tophilic activity. The alkalinity of EDP medium is weak in comparison to KOH solu-
tions. The low alkalinity of EDP medium does not allow the formation of a soluble
silicate and the solvation of silicon is provided by chelating with pyrocatecholate which
exists as a partially dissociated monoanion but further dissociates on complexation with
Si(IV). 697
According to Campbell et , 697 the reaction process in EDP is the same as that

in KOH because of the essential role of water which acts not only as the oxidant but
also as a solvation agent. Etching does not occur in anhydrous ethylenediamine con-
taining strong organic oxidants, suggesting that water is not only the oxidant but is also
involved in the solvation steps of the oxidized silicon species. The only difference is
that the final step in the dissolution of Si(IV) corresponds to the formation of an organic
complex in EDP rather than a silicate in KOH.
Ethanolamine. Ethanolamine anisotropic etchants
have been developed so as to replace the toxic EDP etchant. 221,520 Linde and Austin 221
reported a functional formula of 100 g gallic acid + 305 ml ethanolamine + 140ml water

al.