Chemistry Reference
In-Depth Information
passive oxide film on the
n-
Si, occurs when the
pn
interface is reached. This current
peak signals the onset of the etch stop. Extra control can be obtained by applying dif-
ferent potentials on the
p-
Si and
n-
Si regions with
p-
Si maintained at a value near the
OCP and
n
-Si at a value positive of the passivation potential.
536
This prevents the
unwanted passivation of the
p-
Si which can result from short-circuiting due to point
defects and current leakage. Etching of
p-
Si can also be realized by selective passiva-
tion using an accumulation layer for etch stop instead of a
pn
junction.
463
Based on the
same principle as in-depth selective etching, laterally selective etching can be obtained
by biasing a laterally positioned
pn
junction because
p-
and
n-
Si have different passi-
vation potentials.
521
Another group of methods for selective etching utilizes the high reactivity of
porous silicon.
502
Porous silicon can selectively be formed in a patterned area. The
porous silicon can then be easily dissolved in a KOH solution leading to the selective
dissolution of the patterned area. Also, straight hole array with high aspect ratios can
be generated through formation of macropores by back illumination on
n
-Si.
12,763
Locally confined etchant can also be used to preferentially etch small holes on
830,1018,1098
In this technique an active etchant is generated through a reduction
reaction at the tip of a fine electrode which is positioned near the silicon surface. In sit-
uations where diffusion-controlled process limits the in-depth etch rate, deep etching
can obtained using a centrifugal force.
82
Many different structures can be micromachined on silicon through etching.
Whereas IC technology usually employs planar processes, in micromechanics, sensors,
and actuators, selective etching must be used to fabricate three-dimensional and mobile
elements.
285,517
Two aspects are of particular important in the fabrication of structures
by etching: etch stop and etch feature definition, both of which rely on the selective
etching of materials on a patterned silicon surface.
851
Virtually all microstructures utilize
at least one etch stop technique during the course of their fabrication. Etch feature def-
inition, on the other hand, is mainly achieved through masking and anisotropic etching
which makes anisotropic etching one of the key processes for the fabrication of three-
dimensional micromechanical devices.
517,918
Table 7.8 lists some of the silicon structures that may be fabricated through
etching. The many etching characteristics of silicon and the numerous etching systems
provide a large range of variation in etch rate. This range of rate variation, in com-
bination with various etching techniques, provides many methods for selective
removal of materials on silicon as illustrated in Fig. 7.62, allowing the fabrication of
diverse structures on silicon. Also, process simulation models and software tools
have been developed to assist design and fabrication of structures of specific
geometries.
515,517,524,1006,1080
silicon.
