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Materials for Microelectromechanical Systems 2-3
Anisotropic Si etchants attack the (100) and (110) crystal planes significantly faster than the (111) crys-
tal planes. For example, the (100)–to–(111) etch-rate ratio is about 400:1 for a typical KOH/water etch
solution. Silicon dioxide (SiO ), silicon nitride (Si N ), and some metallic thin films (e.g., Cr, Au) provide
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good etch masks for most Si anisotropic etchants. In structures requiring long etching times in KOH,
Si N is the preferred masking material due to its chemical durability.
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In terms of etch stops, heavily B-doped Si ( 7 l0 /cm ), commonly referred to as a p etch stop,
is effective for some etch chemistries. Fundamentally, etching is a charge transfer process, with etch rates
dependent on dopant type and concentration. Highly doped material might be expected to exhibit higher
etch rates because of the greater availability of mobile carriers. This is true for isotropic etchants such as
HNA, where typical etch rates are 1 to 3 mm/min for p- or n-type dopant concentrations greater than
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10 /cm and essentially zero for concentrations less than 10 /cm . On the other hand, anisotropic
etchants such as EDP and KOH exhibit a much different preferential etching behavior. Si that is heavily
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doped with B ( 7 10 /cm ) etches at a rate that is about 5 to 100 times slower than undoped Si when
etched in KOH and 250 times slower when etched in EDP. Etch stops formed by the p technique are
often less than 10µm thick, as the B doping is often done by diffusion. Using high diffusion temperatures
(e.g., 1175°C) and long diffusion times (e.g., 15 to 20 hours), thick ( 20µm) p etch stop layers can be
created. It is also possible to create a p etch stop below the Si surface using ion implantation; however,
the implant depth is limited to a few microns and a high-energy/high-current ion accelerator is required
for implantation. While techniques are available to grow a B-doped Si epitaxial layer on top of a p etch
stop to increase the thickness of the final structure, this is seldom utilized due to the expense of the
epitaxial process step.
Due to the high concentration of B, p Si has a high density of defects. These defects are generated as
a result of stresses created in the Si lattice because B is a smaller atom than Si. Studies of p Si report that
stress in the resultant films can either be tensile [Ding et al., 1990] or compressive [Maseeh and Senturia,
1990]. These variations may be due to postprocessing steps. For instance, thermal oxidation can signifi-
cantly modify the residual stress distribution in the near-surface region of p Si films, thereby changing
the overall stress in the film. In addition to the generation of crystalline defects, the high concentration
of dopants in the p etch stops prevents the fabrication of electronic devices in these layers. Despite some
of these shortcomings, the p etch-stop technique is widely used in Si bulk micromachining due to its
effectiveness and simplicity.
A large number of dry etch processes are available to pattern single-crystal Si. The process spectrum
ranges from physical etching via sputtering and ion milling to chemical plasma etching. Two processes,
reactive ion etching (RIE) and reactive ion beam etching (RIBE), combine aspects of both physical and
chemical etching. In general, dry etch processes utilize a plasma of ionized gases along with neutral par-
ticles to remove material from the etch surface. Details regarding the physical processes involved in dry
etching can be found elsewhere [Wolfe and Tauber, 1999].
Reactive ion etching is the most commonly used dry etch process to pattern Si. In general, fluorinated
compounds such as CF , SF , and NF or chlorinated compounds such as CCl or Cl sometimes mixed
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with He, O or H are used. The RIE process is highly directional, thereby enabling direct pattern trans-
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fer from the masking material to the etched Si surface. The selection of masking material is dependent on
the etch chemistry and the desired etch depth. For MEMS applications, photoresist and SiO thin films
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are often used. Si etch rates in RIE processes are typically less than 1mm/min, so dry etching is mostly
used to pattern layers on the order of several microns in thickness. The plasmas selectively etch Si relative
to Si N , or SiO , so these materials can be used as etch masks or etch-stop layers. Development of deep
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reactive ion etching processes has extended Si etch depths well beyond several hundred microns, thereby
enabling a multitude of new designs for bulk micromachined structures.
2.3 Polysilicon
Without doubt the most common material system for the fabrication of surface micromachined MEMS
devices utilizes polycrystalline Si (polysilicon) as the primary structural material, SiO as the sacrificial
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© 2006 by Taylor & Francis Group, LLC