Medical Applications of Micro-Electro-Mechanical Systems (MEMS) Technology - Biomaterials Science: An Integrated Clinical and Engineering Approach - page 38

FIGuRES 3.3a And b
SEMs of a <100> oriented silicon substrate after immersion in an anisotropic wet etchant.
There are three basic types of anisotropic wet etchants that are commonly
used. The first and by far the most popular anisotropic etchants are aque-
ous alkaline solutions such as KOH, NH 4 OH, NaOH, CsOH, and TMAH
[9, 10]. These etchants have high etch rates and a relatively high etch-rate
ratio between the <100> and <111> planes. Also, TMAH is sometimes pre-
ferred for use on pre-processed microelectronic wafers, since this etchant
does not etch aluminum appreciably under certain conditions. The draw-
backs of these etchants are that they have a relatively high etch rate of sili-
con dioxide, which is frequently used as a masking material. There is also
the potential for alkali contamination of the wafer using these etchants,
although there are cleaning procedures that can be employed to minimize
these risks.
Another popular anisotropic wet etchant is ethylene-diamine and pyro-
catechol, or EDP. This etchant has a higher etch rate ratio of the <100> and
<111> planes and has a larger variety of masking materials that can be used
compared to the aqueous alkaline solutions. The drawbacks of EDP are that
it is a carcinogenic material, and when using this solution it can be difficult
to see the wafer etching. Also, EDP is a thick orange-yellow material and can
be hard to clean up.
The last type of anisotropic etchant is hydrazine and water (N 2 H 2 :H 2 O).
One advantage of this etchant is that it has a very low silicon dioxide etching
rate, yet it has a relatively poor etch rate ratio of the <100> and <111> planes.
The biggest disadvantage of hydrazine etching is that it is a very hazardous
material. Although it was used in the early days of micromachining, given
that its disadvantages outweigh its advantages, it is now rarely used.
Useful anisotropic wet etching requires the ability to successfully mask
certain areas of the substrate, and consequently an important criterion for
selecting an etchant is the availability of good masking materials. Silicon
nitride is a commonly used masking material for anisotropic wet etchants
since it has a very low etch rate in most etchant solutions. Some care must be
exercised in the type of silicon nitride used, since any pinhole defects will