Page 38 - An Introduction to Microelectromechanical Systems Engineering

Silicon-Compatible Material System 17

Secondary flat No secondary flat

(100) n-type flat (100) p-type (111) n-type (111) p-type
Secondary 90° 45°

Primary flat Secondary flat Primary flat
Primary flat
Primary flat
(a)
(100) plane (010) plane
(110) plane
z, [001] (001) plane
y, [010]
x, [100]
[110] direction
45 º

(110)
(b)

[001] (111)
[010] Surface
is (001)
(111) (111)
(111)
[100]
Flat is along [110] direction
(c)
Figure 2.2 (a) Illustration showing the primary and secondary flats of {100} and {111} wafers for
both n-type and p-type doping (SEMI standard); (b) illustration identifying various planes in a
wafer of {100} orientation (the wafer thickness is exaggerated); and (c) perspective view of a {100}
wafer and a KOH-etched pit bounded by {111} planes.

of impurity doping, but stresses tend to rise when dopant concentrations reach high
levels (~ 10 cm ).
20
−3
Polysilicon is an important material in the integrated circuit industry and has
been extensively studied. A detailed description of its electrical properties is found
in [2]. Polysilicon is an equally important and attractive material for MEMS. It
has been successfully used to make micromechanical structures and to integrate
electrical interconnects, thermocouples, p-n junction diodes, and many other elec-
trical devices with micromechanical structures. The most notable example is the
acceleration sensor available from Analog Devices, Inc., of Norwood, Massachu-
setts, for automotive airbag safety systems. Surface micromachining based on poly-
silicon is today a well-established technology for forming thin (a few micrometers)
and planar devices.
The mechanical properties of polycrystalline and amorphous silicon vary with
deposition conditions, but, by and large, they are similar to that of single crystal sili-
con [3]. Both normally have relatively high levels of intrinsic stress (hundreds of
MPa) after deposition, which requires annealing at elevated temperatures (>900ºC).