Etching of Oxides - Electrochemistry of Silicon and Its Oxide - page 144

bath with added gives terraces and pits, and a bath with added gives no ter-

races or pits at 25 °C.
The dissolution rate of silicon oxides in nonfluoride acidic or neutral aqueous
solution is extremely low as shown in Fig. 4.13, which is responsible for the slow
changes in geological systems. 898 In alkaline solutions, the etch rate of fused silica
depends on the type of alkli as shown in Fig. 4.15. 527 The etch rate is not affected by
solution stirring, indicating that the rate-limiting step is a surface reaction, in agree-
ment with the high activation energy (about 0.9 eV). In NaOH and KOH solutions, there
is a maximum etch rate at about 8 M. The dissolution rate tends to decrease with increas-
ing dissolved silica in the solution at relatively low alkaline concentrations as shown
in Fig. 4.16. Figure 4.17 shows the dissolution of different types of natural silicon
oxides as a function of time in a diluted sodium hydroxide. In organic solutions, the
dissolution requires the presence of water. The dissolution rate increases with increas-
ing water content in a methanol solution, in which the etch rate is almost zero.
In NaCl solutions, Fig. 4.18 shows that the etch rate of silica increases sharply
with increasing pH in the pH range of 5-11. 897 The concentration of NaCl also has an
effect on the etch rate.