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and future developments constitute the fourth phase. A few commonly used MEMS
materials are described next.
Single-crystal silicon is commercially available in the form of circular wafers of diam-
eters ranging from 2in to 12in and thicknesses in the range of a few hundred microns.
Selective chemical etching is used to carve out features in silicon wafers. The wafers are
available in a number of crystallographic orientations, which are important when chemi-
cally etching patterns on them. Polycrystalline silicon, often called polysilicon or simply
poly in the MEMS literature, is also used extensively as a structural material for MEMS
devices. Polysilicon is deposited as a thin film on top of a substrate (e.g., the silicon wafer)
using chemical vapor deposition or other techniques. In addition to being used as a struc-
tural material, polysilicon is also used to form electronic elements such as resistors, con-
ductors, and ohmic contacts to crystalline silicon structures.
Several compounds of silicon are useful in the fabrication and operation of MEMS
devices. Silicon dioxide is used as a dielectric material for electrical insulation. It is also
useful in the fabrication of MEMS devices as a sacrificial layer in creating multilayered
structures with movable parts. It can be either grown by thermal oxidation or deposited
using a variety of methods. Silicon nitride is another useful dielectric material. It is also
used as a structural material in some applications. It is usually deposited using chemical
vapor deposition. Both silicon dioxide and silicon nitride are used as masks for etching
silicon because many etchants are available that selectively etch only silicon but not the
oxide and nitride. Silicon carbide is useful as both a dielectric material and structural mate-
rial. Due to its extreme hardness, its ability to operate in high temperature, high-power,
and high-radiation environments, and its resistance to chemical attack, silicon carbide, is
becoming a very important material for MEMS applications in the automobile, aerospace,
and power generation industries.
In addition to silicon and its compounds, metals, ceramics, and polymers are also used
in MEMS. Many metals (e.g., aluminum, nickel, copper, gold) have been in use at the
microscale for a long time, whereas ceramics and polymers are more recent additions to the
repertoire of MEMS materials. Several active materials, such as piezoelectric, ferroelectric,
conducting polymers, shape-memory alloys, etc., are also making their way into MEMS
devices. These new materials call for new manufacturing techniques to make MEMS
economically viable and functionally better than previous devices. Some established micro-
machining techniques as well as some new ones are briefly described in the next section.

15.4 MANUFACTURING OF MEMS

As in any field, it is important to know how the devices are manufactured in order to model
and design them effectively or to conceive new device concepts. In a developing field like
MEMS, manufacturing techniques are numerous and varied. However, they can be broadly
classified into four categories: surface micromachining, bulk micromachining, micro-
molding techniques, and miscellaneous micromanufacturing techniques. The first two
categories are especially applicable to silicon and its compounds. Thin-film deposition,
photolithography, and selective etching are the primary tools for micromachining. In
photolithography, optically sensitive polymers called photoresists are exposed through a
photomask consisting of transparent and opaque regions. Exposed or unexposed regions
in the photoresist can be selectively washed away, leaving a photoresist mask through
which a material underneath the mask can be selectively etched. Film deposition and
etching by means of masks make micromachining techniques particularly suitable for
batch production, as of IC chips. Many new techniques do not use photolithography and
are not always amenable for batch production.