Etching of Silicon - Electrochemistry of Silicon and Its Oxide - page 298

reaction because the etching behavior of silicon is essentially similar in different alkali
solutions. 207,984 In IPA-buffered solutions, the IPA does not alter the composition of the
solutions or participate in the etching reactions. 379,1144
With the identification of and as the reactants and and
as the reaction products, the overall reaction during etching in KOH
114,206,984

solutions has been established:

The details of this reaction are discussed in more detail in Chapter 5. As far as the
etching kinetics is concerned, according to Seidel et . 206 the dependence of etch rate

on KOH concentration in the concentration range of 10 to 60% can be best fitted by
the equation

al

Glembocki et al. 984 proposed a model based on the activities of and Accord-
ing to them, water exists in two forms, hydrated and free, and it is the free water and
free hydroxyl ions that are responsible for etching reactions. The corresponding rate
equation is

which has a form similar to Eq. (7.8). According to Glembocki et al. at low hydroxide
concentrations, is low resulting in a low etch rate, while at very high concen-
trations is low resulting in also low etch rates. In moderately concentrated
solutions, both and are high and etch rate exhibits a maximum as
shown in Fig. 7.13. The etch rate dependence on concentration in different alkaline
solutions is due to the different hydration numbers of the solutions. The model is,
however, not applicable for low concentrations from 0.1 to 2 M at which the etch rate
is relatively constant as a function of concentration. According to Glembocki et al. it
is perhaps due to a rate-limiting reaction associated only with free water, the concen-
tration of which is almost constant at low KOH concentrations.
The difference between n -Si and p -Si at the cathodic potentials indicates the effect
of carriers on the etching process. The hydrogen evolution may either obtain the elec-
tron directly from the dissolving surface silicon atom or from the semiconductor. For
n -Si, cathodic bias provides a high concentration of surface electrons for the hydrogen
reaction resulting in a decrease of the dissolution rate. However, for p -Si electrons
are the minority carriers, which are not available at a cathodic bias, and the etch rate
remains more or less constant at cathodic potentials.

7.4.2. Other Inorganic Solutions
The etching processes in all alkali hydroxides are similar to those in a KOH solu-
tion, namely, free and are the active agents. It has been suggested that the
difference among solutions such as NaOH, LiOH,

RbOH and CsOH is mainly