Etching of Silicon - Electrochemistry of Silicon and Its Oxide - page 337

found to decrease with addition of a reducing agent, such as ferricyanide, to the solu-
tion. 541 In NH 4 OH solutions it occurs for all concentrations, temperatures, stirring con-
ditions, and sample preparations. 706,711 It can be suppressed by the addition of
TMAH solutions at concentrations lower than 25%, small hillocks start to develop and
the size and number increase with decreasing concentration. 1002,1005 Addition of IPA in
TMAH increases the size of hillocks but does not change hillock density. Electrode
potential can also affect hillock formation. It is found that the p (100) surface etched in
KOH at potentials negative of the OCP is rough covered with pyramidal hillocks
whereas that etched at potentials anodic of the OCP remains mirrorlike. 541 Increasing
the pressure above the etchant bath is found to enhance the formation of hillocks in
KOH solution. 696

In

Hillocks tend to readily form on poorly cleaned surfaces and in etchants con-
taining excessive amounts of dissolved silicon. 349,434,518,577 Hillocks may result from local
masking by contaminants or deposits of reaction products. The pyramidal hillocks have
been found to be associated with a microsubstance, a few nanometers in size, at the
apex of the pyramidal hillocks. 349 The development of hillocks is similar to the forma-
tion of a pyramid on a masked surface as shown in Fig. 7.45. The anisotropic dissolu-
tion of the material around the masked area results in the formation of pyramids.
Initially the pyramids at the surface are tiny but as etching continues new pyramids are
formed while the existing ones grow. Once the surface is fully covered with hillocks,
the etch rate in the (100) direction will drop as the surface is covered by planes of
slower etch rate. The masking substances causing the initiation of hillocks can be
surface contaminants, precipitates in silicon material, surface deposits, and formation
of gas bubbles. Formation of hydrogen bubbles, which is a basic reaction in the etching
of silicon at the OCP in aqueous solutions, and precipitation of silicates have been con-
sidered to be most important for hillock formation and associated surface rough-
518,557

ness. The four (111) planes of hillocks are flat, as the {111} planes resulting from

cavity etching of both (100) and (110) wafers are smooth at the macroroughness level
in all of the anisotropic etchants. 54,206,458