Medical Applications of Micro-Electro-Mechanical Systems (MEMS) Technology - Biomaterials Science: An Integrated Clinical and Engineering Approach - page 39

result in the attack of the underlying silicon. Also, some low-stress silicon-
rich nitrides can etch at much higher rates compared to stoichiometric silicon
nitride formulations. Thermally grown SiO 2 is frequently used as a mask-
ing material, but some care must be exercised to ensure a sufficiently thick
masking layer when using KOH etchants, since the etch rates of oxide can
be high. Photoresists are unusable in any anisotropic etchant. Many metals,
including Ta, Au, Cr, Ag, and Cu, hold up well in EDP, and Al holds up in
TMAH under certain conditions.
In general, the etch rate, etch rate ratios <100>/<111>, and etch selectivities
of anisotropic etchants are strongly dependent on the chemical composi-
tion and temperature of the etchant solution. The etch rate [R] obeys the
Arrhenius law given by
[R] = R o exp (-E a /kT) (micron/hour)
where R o is a constant, E a is the activation energy, k is Boltzman's constant,
and T is temperature in degrees Kelvin. Both R o and E a will vary with the
type of etchant, etchant composition, and crystallographic orientation of the
material being etched. Fortunately, there is a wealth of published literature
characterizing many of the commonly used anisotropic etchants, and read-
ers are referred to [9, 10] for more information.
Frequently, when using bulk micromachining it is desirable to make thin
membranes of silicon or control the etch depths very precisely. As with any
chemical process, the uniformity of the etching can vary across the substrate,
making this difficult. Timed etches whereby the etch depth is determined
by multiplying the etch rate by the etch time are difficult to control, and etch
depth is very dependent on sample thickness uniformity, etchant species
diffusion effects, loading effects, etchant aging, surface preparation, etc. To
allow a higher level of precision in anisotropic etching, the MEMS field has
developed solutions to this problem, namely etch stops. Etch stops are very
useful in controlling the etching process and providing uniform etch depths
across the wafer, from wafer to wafer, and from wafer lot to wafer lot. There
are two basic types of etch stop methods that are used in micromachining:
dopant etch stops and electrochemical etch stops.
Etch stops in silicon are commonly made by the introduction of dopants
into the silicon material. The most popular etch stop is heavy p-type dop-
ing of silicon with boron (>5 x 10 19 cm -3 ) to create an etch stop. The lightly
doped region of the wafer will etch at the normal rate and the highly doped
region of the silicon will have a very slow etch rate. The dopant is introduced
into the silicon using the standard techniques of diffusion or ion implanta-
tion followed by an anneal, providing for a controlled depth and reasonable
uniformity of the dopants in the substrate. Figure 3.4 is a graph of the nor-
malized etch rate of <100> oriented silicon wafer in KOH at various concen-
trations as a function of the boron dopant concentration [13, 14]. As can be
seen, the etch rate falls off very quickly at dopant concentrations above 10 19