Chemistry Reference
In-Depth Information
surrounding area. Sharp etch pits are found to develop in etchants with a sensitivity
factor of
Not all etch pits developed in etching solutions are related to crystal defects. For
example, random and flat-bottomed etch pits are found to develop on dislocation-free
silicon wafer in solutions.
168
Similar well-defined square and
randomly distributed pits of several micrometers in size are found after immersion of
in a solution.
727
Large etch pits
the surface of which is covered with a porous silicon layer, have been found to form
on
p
-Si in quiescent concentrated HF solutions.
1007
The number of pits is greatly reduced
in diluted solutions. This phenomenon can be attributed to the combined effect of
porous silicon formation and pitting corrosion.
7.8.3. Material Removal
Etching is widely used as a process for material removal in silicon device fabri-
cations. The important aspect of any etching process is the ability to control the amount
of materials with sufficient spatial accuracy. The process and structural diversity in
device fabrication necessitates diverse etching techniques—chemical or electrochemi-
cal, junction, anisotropicity, masking, and illumination—to provide uniform as well as
selective etching, with lateral selectivity as well as in-depth selectivity. Figure 7.62
highlights the basic uniform and selective etching techniques for removing materials.
Uniform Material Removal.
In the category of uniform etching the simplest
method is immersing silicon samples in etchants such as
solutions
103,149,331,450
as shown in Fig. 7.62(1). It is normally used in applications such as damaged
layer removal
450,490
and polishing
33,306
and in-depth doing profiling.
18,852
The etching is
most commonly operated at the OCP but can be done under an anodic potential and/or
illumination to add extra control. Etching with potential control requires an ohmic
contact at the back side of the wafer in order to apply an anodic potential on the silicon
