Chemistry Reference
In-Depth Information
A 1:1 hydrazine solution at 110°C gives an etch rate of 360 Å/s and a (100)/(111)
ratio of about 6.
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Anodic polarization of silicon electrode in hydrazine reveals that
i-
curves are similar to those in other alkaline solutions as shown in Fig. 7.21.
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The
different behaviors on
p-
V
and
n
-Si are attributed to the effect of surface states on the
surface potential that is different for
p-
Si
Figure 7.22 shows the ternary
etching system of hydrazine, IPA, and water.
323,542
The etch rate is the highest when the
mixture contains no IPA, indicating that IPA acts only as a diluent and not as a com-
plexing agent. Some mixtures may result in stains on the surface, whereas other may
result in the formation of an oxide film which stops the etching. The slowest etching
planes are the (111) planes and the fastest planes are the (211) planes. The etched sur-
faces vary from being highly faceted to very flat. The faceted surfaces are made up of
square pyramidal hillocks, and the flat surfaces are marked by a cellular structure. The
quality of etched surfaces in terms of formation of hillocks and corner undercut strongly
varies with water content and temperature.
Si
and
n-
Si.
7.4.3. Organic Solutions
EDP Solutions.
EDP or EPW is a mixture of ethylenediamine (ED or E), pyro-
catechol (P), and water (W). This solution is usually employed at 110-120°C (about
boiling point).
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The typical compositions and etch rates can be found in Table 7.1.
Figure 7.23 shows the etch rate as a function of the water content.
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No silicon etching
occurs in solutions lacking water and the maximum etch rate is observed with a water
molar fraction of about 0.6. Figure 7.24 shows the effect of pyrocatechol on the etch
rate. The etch rate increases with increasing pyrocatechol content to about 5mol %
above which the etch rate becomes constant. There is a definite etch rate without addi-
tion of pyrocatechol, indicating that it is not an essential component of the etching
system but rather is acting as an catalyst. Addition of other agents such as pyrozine and
p
-benzoquinone has a similar effect.
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EPW etch solutions can be affected by many operational factors such as bath
design, contamination levels, silicon content in the solution, and oxygen pressure.
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