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2-2                                                              MEMS: Design and Fabrication


             serving a critical function. It is in this light that this chapter is constructed. The chapter does not attempt
             to present a comprehensive review of all materials used in MEMS because the list of materials is just too
             long. It does, however, detail a selection of material systems that illustrate the importance of viewing
             MEMS in terms of material systems as opposed to individual materials.

             2.2 Single-Crystal Silicon


             Use of silicon (Si) as a material for microfabricated sensors can be traced to 1954, when the first paper
             describing the piezoresistive effect in germanium (Ge) and Si was published [Smith, 1954]. The results of
             this study suggested that strain gauges made from these materials could be 10 to 20 times larger than
             those for conventional metal strain gauges, which eventually led to the commercial development of Si
             strain gauges in the late 1950s. Throughout the 1960s and early 1970s, techniques to mechanically and
             chemically micromachine Si substrates into miniature, flexible mechanical structures on which the strain
             gauges could be fabricated were developed and ultimately led to commercially viable high-volume pro-
             duction of Si-based pressure sensors in the mid 1970s. These lesser known developments in Si microfab-
             rication technology happened concurrently with more popular developments in the areas of Si-based
             solid-state devices and integrated-circuit (IC) technologies that have revolutionized modern life. The
             conjoining of Si IC processing with Si micromachining techniques during the 1980s marked the advent
             of MEMS and positioned Si as the primary material for MEMS.
               There is little question that Si is the most widely known semiconducting material in use today. Single-
             crystal Si has a diamond (cubic) crystal structure. It has an electronic band gap of 1.1 eV, and like many
             semiconducting materials, it can be doped with impurities to alter its conductivity. Phosphorus (P) is a
             common dopant for n-type Si and boron (B) is commonly used to produce p-type Si. A solid-phase oxide
             (SiO ) that is chemically stable under most conditions can readily be grown on Si surfaces. Mechanically,
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             Si is a brittle material with a Young’s modulus of about 190GPa, a value that is comparable to steel
             (210GPa). Being among the most abundant elements on earth, Si can be refined readily from sand to pro-
             duce electronic-grade material. Mature industrial processes exist for the low-cost production of single-
             crystal Si wafered substrates that have large surface areas ( 8 in diameter) and very low defect densities.
               For MEMS applications, single-crystal Si serves several key functions. Single-crystal Si is perhaps the
             most versatile material for bulk micromachining, owing to the availability of well-characterized anisotropic
             etches and etch-mask materials. For surface micromachining applications, single-crystal Si substrates are
             used as mechanical platforms on which device structures are fabricated, whether they are made from Si
             or other  materials. In  the case  of Si-based integrated MEMS  devices, single-crystal Si is the primary
             electronic material from which the IC devices are fabricated.
               Bulk micromachining of Si uses wet and dry etching techniques in conjunction with etch masks and
             etch stops to sculpt micromechanical devices from the Si substrate. From the materials perspective, two
             key  capabilities  make  bulk  micromachining  a  viable  technology: (1)  the  availability  of anisotropic
             etchants such as ethylene–diamine pyrocatecol (EDP) and potassium hydroxide (KOH), which preferen-
             tially etch single-crystal Si along select crystal planes, and (2) the availability of Si-compatible etch-mask
             and etch-stop materials that can be used in conjunction with the etch chemistries to protect select regions
             of the substrate from removal.
               One of the most important characteristics of etching is the directionality (or profile) of the etching
             process. If the etch rate in all directions is equal, the process is said to be isotropic. By comparison, etch
             processes that are anisotropic generally have etch rates perpendicular to the wafer surface that are much
             larger than the lateral etch rates. It should be noted that an anisotropic sidewall profile could also be
             produced in virtually any Si substrate by deep reactive ion etching, ion beam milling, or laser drilling.
               Isotropic  etching  of a  semiconductor  in  liquid  reagents  is  commonly  used  for  removal  of work-
             damaged surfaces, creation of structures in single-crystal slices, and patterning single-crystal or polycrys-
             talline semiconductor films. For isotropic etching of Si, the most commonly used etchants are mixtures
             of hydrofluoric (HF) and nitric (HNO ) acid in water or acetic acid (CH COOH), usually called the HNA
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             etching system.



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