Chemistry Reference
In-Depth Information
electrochemical dissolution depends only on the surface area at the pore bottom that is
independent of anodization time.
The values in Fig. 8.18 can be used to estimate the chemical dissolution rate
on the surface of pore walls. For a PS with a density of 50% and a pore diameter of
3 nm, the absolute dissolution rate of PS equals the dissolution rate of PS
divided by silicon density and by specific area The chemical
rate is estimated assuming the PS consists of
straight cylindrical pores of equal diameter. The order of magnitude is in agree-
ment with the planar etch rate of silicon in concentrated HF solutions (see Chapter
7). The real chemical dissolution rate of PS should be larger than that of a flat surface
due to the large amount of lattice defects associated with PS. The chemical disso-
lution during PS formation is responsible for the dissolution valence lower than 2
(Fig. 8.7
41
) and the change of PS density with depth, particularly for lowly doped
material.
dissolution
to be about
8.3. MORPHOLOGY
8.3.1. General
Morphology, which is determined by the distribution of materials in space, is the
least quantifiable aspect of a material. It is thus very difficult to characterize mor-
phology of PS, which has extremely rich details with respect to the range of variations
in pore size, shape, orientation, branch, interconnection, and distribution. Qualitatively,
the diverse morphological features of PS reported in the literature can be summa-
rized by Fig. 8.19 with respect to six different aspects: pore shape, pore orienta-
tion, shape of a pore bottom, fill of macropores, branching, and depth variation of a
PS layer.
The condition for PS formation is determined by current density and HF
concentration and is essentially independent of the condition of the silicon sub-
strate as described in Section 8.2.2. The morphology of PS, on the other hand, is
determined by all factors involved in anodization, particularly the factors related
to the substrate. For example, doping concentration, which does not affect the
nature of electrochemical reactions, is a principal factor determining the morphol-
ogy of PS.
Thus, the morphology of PS can be roughly grouped, according to the type and
concentration of doping, into four main categories: (1) moderately doped
p-
Si
(2) highly doped
p
-Si and
-Si and (4) lowly doped
. The PS formed on moderately doped
p-
Si has extremely small pores
ranging typically from 1 to 10 nm. The pores are highly interconnected as illustrated in
Fig. 8.19(5f). For heavily doped
p
and
n
types, the pores have diameters typically
ranging from 10 to l00nm. The pores show clear orientation and are less intercon-
nected as illustrated in Fig. 8.19(5e). For
n-
Si
,
the pores, with a wide range of diame-
ters from 10 nm to are generally straight and clearly separated as shown in Fig.
8.19(5a-d). For lowly doped
p
-Si, the PS can have two distinctive and continuous pore
diameter distributions: large pores on the order of micrometers and small pores on the
n
