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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.
              3  4
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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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               18
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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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