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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.
© 2006 by Taylor & Francis Group, LLC