Page 24 - Principles and Applications of NanoMEMS Physics

10 Chapter 1

It is seen in this figure that the fidelity of the pattern transferred is function
of how precisely the resulting width of the etched layer resembles that of the
PR pattern, as quantified by the selectivity and aspect ratio. Accordingly,
four scenarios may be envisioned, Figure 1-6, which reflect the relative
strength with which the etchant attacks the PR, the etched material, and the
etch stop. In particular, it may be surmised from Figure 1-6(d) that the
minimum width of a pattern, i.e., how narrow it may be, is limited by the
lithography process to define the pertinent width in the PR and the resulting
degree of undercut of the PR mask. Thus, etchants producing isotropic
profiles (ones in which the vertical and horizontal etching rates are equal),
are not amenable to pattern the narrowest features. In general, the results
depend on a number of factors controlling the etching chemical reaction,
such as temperature and mixing conditions, whether or not the etching agent
employed is in the liquid or gaseous state, how well the PR adhered to the
wafer during spin-casting. In the next section we address two of the most
important factors, namely, the state of the etchant.

(a
(b
(a ) ) (b ) )

(c ) ) ( d ) )
( d
(c

Figure 1-6. Etching characterization. (a) Over Etch<<Etch DepthÆSelective. (b) Over
Etch~Etch DepthÆNon-selective. (c) Side Etch<<Etch Depth. (d) Side Etch~Etch Depth.
(After [25].)

1.2.1.3.1 Wet Etching

In this approach to dissolve the weakened PR, the patterned wafers are
immersed in a liquid chemical etchant, Figure 1-7. The etched profile may
be isotropic or anisotropic depending of the wafer orientation. If this is
amorphous, an isotropic profile will result, i.e., the horizontal and vertical
etching rates are similar. Otherwise, if it is single-crystal, an anisotropic
profile may result. A number of chemicals employed to effect anisotropic
etching in silicon are in use. These include tetramethylammonium hydroxide
(TMHA), potassium hydroxide (KOH), and ethylene diamine pyrochatecol