Etching of Silicon - Electrochemistry of Silicon and Its Oxide - page 292

This reaction is considered to involve several intermediate steps. is produced
and acts as an active oxidizing agent because addition of trace amounts of
increases the etch rate in the solutions. It is an autocatalytic process because for many
compositions the etching, which started on a work-damaged surface, fails to continue
after the specimen is taken out and reintroduced in the solution, indicating that the cat-
alytic agent is a surface bond intermediate. A steady-state condition can be achieved
only after a concentration of in excess of a threshold value is built up in the
vicinity of the surface either through the reduction of at a sufficiently high etch
rate or by addition of small amounts of in the solution.
Turner 111 found that the OCP varies with type of doping and concentration
as shown in Fig. 2.37. The OCP as a function of the etch rate in the solutions is shown
in Fig. 7.10. Because the OCP varies with doping concentration and etch rate, Turner
concluded that the etching process of silicon in solutions is an electro-
chemical process in which the dissolution of silicon takes place at local anodic sites
while the oxidizing agent is reduced at local cathodic sites. The etching process
may also depend on the surface carrier concentration determined by doping because
lowly doped materials etch slower than highly doped materials. 1024 This indicates that
the reduction of HNO 3 , in addition to injecting holes into the semiconductor, has an
effect of changing the OCP, which in turn changes the surface concentration of the car-
riers. The variation of the OCP due to the oxidation and reduction reactions through

the coupling effect is illustrated in Fig. 1.26. The electrochemical nature of the etching

has been further studied recently by Kooij et al. 969,981 The etching, with
a rate about the same for both n- Si and p- Si, proceeds mainly through capturing the
holes, which are injected into the valence band due to the reduction of

in

The

details of the reaction processes involving the reduction of are described in

Chapter 6.