Chemistry Reference
In-Depth Information
7.4. ALKALINE SOLUTIONS
Alkaline solutions are another major group of etchants widely used for etching
of silicon materials. The most commonly used alkaline solutions have been KOH and
EDP (or EPW) which is a mixture of ethylenediamine (ED or E), pyrocatechol (P), and
water (W).
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Other solutions such as hydrazine, ethanolamine, and tetram-
ethyl ammonium hydroxide (TMAH) are also used. The development and application
of these etching solutions are dictated by a number of factors such as etch rate,
anisotropic selectivity, corrosiveness to masking materials, surface quality, processing
controllability, safety, and more recently environmental impact.
The advantages of KOH solutions are its high (100)/(111) etch rate ratio and non-
toxic nature.
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The disadvantages are the significant etch rate of undesirable
etching of metallic film, and potential potassium contamination.
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KOH solutions
also lack the controlled uniformity and boron specificity as do the EDP solutions.
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On
the other hand, EDP has three properties that make it very useful for micromachining:
It is anisotropic, it is highly selective and can be masked by a variety of materials such
as
silicon.
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and it exhibits near-zero etch rates on heavily boron doped
However, EDP solutions have much lower anisotropic etch rate ratios than
do KOH. Also, like KOH, EDP etches aluminum rapidly and EDP solutions have to be
controlled at a specific temperature range to avoid precipitation. In addition, EDP is
carcinogenic.
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Because of the disadvantages of these two etching solutions, other
systems of varying etching characteristics have been developed.
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Silicon etching in KOH solutions has been extensively investigated, resulting in
a body of information that shapes the current understanding of the etching behavior of
silicon in alkaline solutions. It appears that the major characteristics and the principal
reaction processes involved in all alkaline solutions are similar to those in the KOH
system although details vary from system to system. Most notably, all alkaline solu-
tions show the sensitivity of etch rate to crystal orientation, which is the basis for
anisotropic etching. Also, all of these etchants show an etch rate reduction for highly
