Etching of Silicon - Electrochemistry of Silicon and Its Oxide - page 336

smoothest surface appeared at the (100) plane whereas the (320) and (210) planes which
also have higher etch rates than other planes are very rough as shown in Fig. 7.56. The
relative roughness among the different planes can be quite different as reported by
different investigators.
The (100) surface tends to roughen quicker than the (111) surface and the rough-
ness tends to be permanent on the (100) surface whereas it is transient on the (111)
surface. 114 Such crystal orientation-dependent roughness can also be explained by the
anisotropic etching mechanism illustrated in Fig. 7.41. The preferential etching at the
(111) steps of the (111) terraces results in the removal of the terraces and reduction of
the (111) steps and a reduction of microroughness.

In particular, the hillocks formed on the (100) surface are crystallographic struc-
tures bounded by four (111) planes resulting from the anisotropic etching and their for-
mation directly contributes to the roughness of the surfaces. 557 Hillocks may, under
certain conditions, form in HF-based solutions, 349 but most commonly in alkaline
518,557,706 Because the (111) surface etches very slowly, the formation of hillocks
leads to a decreased etch rate. 557,706
The extent of hillock formation in alkaline solutions depends strongly on solu-
tion composition and operating conditions. Formation of hillocks in KOH solutions
occurs in certain concentration ranges. 206,557 Solution stirring and higher solution tem-
perature reduce the hillock-associated surface roughness. The density of hillocks is

etchants.