Etching of Silicon - Electrochemistry of Silicon and Its Oxide - page 296

IPA to KOH results in different decreases of etch rates for the three major orienta-
tions. 1026 As a result, at certain alcohol concentrations, the order of the etch rates is
changed for (100) and (110) as in the case shown in Fig. 7.14. The effect of various
factors on anisotropic etching is discussed in more detail in Section 7.6.

For a given type and orientation the etch rate is essentially independent of doping
above which the etch rate dras-
tically decreases with further increasing doping concentration, particularly for boron-
doped materials. 207,269 The reduction in the etch rate for heavily doped materials is
commonly referred to as etch stop and will be discussed later.

concentration up to a concentration of about

The etch rate of all silicon materials depends, to a varying extent, on poten-
109,192,541 Figure 7.15 109 shows the effect of potential on the etching rate in a 2 M
KOH solution at room temperature. The etch rate is the highest at the OCP and for the
(100) surface the etch rate is similar for p -Si and n -Si but on the (111) surface it is
much higher for p -Si than

tial.

-Si. At potentials positive of the passivation potential the
etching stops due to the formation of oxide film. At cathodic potentials the effect is
different for n -Si and p -Si. For n -Si the etch rate decreases with increasing cathodic
bias and etching stops at certain potentials. For p -Si the etching maintains relatively
high rates comparable to those at the OCP. Formation of a sluggish layer, which was
observed on n -Si at cathodic potentials but much less on p -Si, was suggested by Palik
et

n

114 to be responsible for the etching stop of the n -Si. The surface of the samples
etched at potentials more negative than the passivation potential is hydrophobic, indi-
cating a high degree of hydrogen termination, whereas the surface after polarization at
potentials positive of the passivation potential is hydrophilic. 541
The morphology of the etched surface depends on the composition of the solu-
tion. In general, due to the anisotropic nature of the etching of the crystal confined in
a given area, perfectly smooth lateral (111) sidewalls form on both (100) and (110)
wafers. 54,531 During etching of silicon in KOH, formation of a white residual may occur,
the tendency of which increases with increasing concentration below 15%. 206 Accord-
ing to Seidel et al ., 206 the high etch rate at these concentrations results in an accumu-
lation of

al.

at the surface leading to the formation of Also, at low KOH