8 MEMS Wet-Etch Processes and Procedures 491
The etch rate tables throughout this chapter emphasize recipes mixed from
standard integrated circuit laboratory chemicals, although many other fine etchant
recipes exist. The etch tables are separated into dielectrics then metals, first for
materials compatible with clean IC processing and later for nonstandard materi-
als of interest to the MEMS community. The etched material name is generally
followed by its chemical symbol in parentheses for ease in searching. The partic-
ular deposition technique may also be indicated. Within each table, the etchants
and etch processes are organized alphabetically first by etched material, then by
etchant components, and then by etch rate for similar etchants (e.g., with different
temperatures or citations). Within each etch mixture, the chemical components are
generally organized alphabetically, with hydrogen peroxide, diluents such as acetic
acid, and water positioned last in the component sequence. The chemical concentra-
tion in weight percent is generally listed in parentheses after the chemical symbol,
except for those with 100% concentrations and solid forms. Acetic acid (>99%) is
denoted simply as acetic. Organic solvents and etchants are generally denoted with
an acronym such as IPA for isopropyl alcohol, EtOH for ethyl alcohol, MeOH for
methyl alcohol, and KOH for potassium hydroxide. Semiconductor or reagent grade
chemicals are available from a number of fine suppliers [44].
The composition ratios for chemical mixtures follow the etchant components,
with numbers separated by colons to indicate the relative volumes (not weight)
of each component unless otherwise indicated. The sequence of the etchant com-
ponents does not imply the order in which the components should be added. For
example, it is not to be inferred that water is added last (remember to add acids to
water, not the other way around). Commercial etchants of particular interest with
published etch rates are included, although the components of the etchant may not
be fully known.
The etch rates are presented uniformly in angstroms per second (Å/s), in part to
keep the etch rates of interest in the single-digit range and above. Etch rates of 1–
10 Å/s are generally suitable for film thicknesses up to several thousand angstroms;
etch rates of 100–1000 Å/s or more are generally suitable for extensive lateral etch-
ing or removal of significant portions of the substrate such as a backside anisotropic
etch. Etch rates less than 1 Å/s are generally too s low for common use, although the
information is valuable to determine the selectivity of the etchant to masking layers
and other exposed layers during the etch sequence or to layers that become exposed
during the overetch portion of the etch step. Notes of interest and references are
included for each etchant, including etch rates and selectivities to other materials
when available. The recipes are believed to be accurate, although many have not
been used or confirmed by the author. Some etch rates have been rounded slightly
from the literature values, and some etchant components have been tweaked to align
them with commercial concentrations for standard electronics-grade reagents. The
recipes and etch rates are intended to serve as a guide to wet chemical etching for a
wide variety of MEMS-related materials, and a selected etchant should be verified
by the user with samples under representative etching conditions prior to actual use.
Wet etching of other dielectric films such as silicon nitride have characteris-
tics and concerns similar to those of oxide etching. Silicon nitride films, being