Page 21 - MEMS Mechanical Sensors

10 Materials and Fabrication Techniques

hydrogen ions, is then executed through the oxide layer by a standard high-current
ion implanter to form the Smart Cut layer. The implanted hydrogen ions alter the
crystallinity of the silicon, creating a plane of weakness in the wafer. After the wafers
are bonded together, the implanted wafer can be cleaved along this plane to leave a
thin layer of silicon on top of the oxide layer. The wafer is then annealed at 1,100°C
to strengthen the bond, and the surface of the silicon is polished to reduce the defect
level to a level approaching that of bulk silicon. The buried oxide layer is pinhole
free. SOI layers in the range from 0.1 to 1.5 µm and BOX layers from 200 nm to 3
µm can be fabricated by this method.
Other substrates, however, should not be ignored. Among those that have been
used in micromachining are glasses, quartz, ceramics, plastics, polymers, and met-
als. Quartz and glass are often used in MEMS mechanical sensors; therefore, a short
description of these materials is given here.

2.2.1.2 Quartz and Glasses
Quartz is mined naturally but is more commonly produced synthetically in large,
long faceted crystals. It has a trigonal trapezohedral crystal structure and is similar
to silicon in that it can be etched anisotropically by selectively etching some of the
crystal planes in etchants such as ammonium bifluoride or hydrofluoric acid. Unlike
silicon, however, this has not been extensively used as an advantage but has been
identified more as a disadvantage due to the development of unwanted facets and
poor edge definition after etching. Since the fastest etch rate is along the z-axis [1],
most crystalline quartz is cut with the z-axis perpendicular to the plane of the wafer.
The property of quartz that makes it useful in MEMS mechanical sensors is that it is
piezoelectrical. Quartz has been used to fabricate resonators, gyroscopes, and accel-
erometers. Another form of quartz is fused quartz, but be careful not to confuse this
material with crystalline quartz, as fused quartz is used to denote the glassy noncrys-
talline, and, therefore, isotropic form better known as silica. It is tough and hard and
has a very low expansion coefficient.
Glass can be etched in hydrofluoric acid solutions and is often electrostatically
bonded to silicon to make more complicated structures. Both phosphosilicate
and borosilicate glasses can be used. One of the more favored glasses is Pyrex,
which is a borosilicate glass composition with a coefficient of thermal expansion of
–6
3.25 × 10 /°C, which is close to that of silicon, an essential property for structures
to be used in thermally unstable environments. Some of the properties of quartz and
Pyrex are shown in Table 2.2. The substrate is sometimes used purely as a

Table 2.2 Selected Properties of Quartz and Pyrex
Property Quartz Pyrex
Young’s modulus (GPa) 107 64
Poisson’s ratio, (100) orientation 0.16 0.20
–3
Density (gcm ) 2.65 2.33
Dielectric constant 3.75 4.6
–6
–1
Thermal expansion coefficient (10 K ) 0.55 3.25
–1
–1
Thermal conductivity (Wm K ) 1.38 1.13
–1
–1
Specific heat (Jg K ) 0.787 0.726
Refractive index 1.54 1.474