Page 120 - MEMS Mechanical Sensors

5.7 Smart Sensors 109

It will retain this shape until the temperature exceeds the phase transition tempera-
ture, at which point the material reverts to its parent structure known as Austenite.
One of the most widely used SMA materials is an alloy of nickel and titanium called
Nitinol. This has excellent electrical and mechanical properties and a long fatigue
life. In its bulk form, it is capable of producing up to 5% strain. The transition tem-
perature of Nitinol can be tailored between –100°C and +100°C by controlling the
impurity concentration. The material has been used in MEMS by sputter depositing
TiNi thin-film layers [13].

5.6.4 Magnetic
If a current-carrying element is placed within a magnetic field, an electromagnetic
force (Lorentz force) will occur in a direction perpendicular to the current and mag-
netic field. The magnitude of the force is proportional to the current, length of the
element, and the magnetic field. The availability of permanent magnetic materials,
which are compatible with MEMS processing, is very limited, and thus it is common
for the magnetic field to be generated externally. Discrete magnetic actuators often
comprise coils, but such structures are not currently achievable with conventional
MEMS processing and planar coils must be used.
Another approach that can be used as the basis of a magnetic actuator is the
magnetostrictive effect. Magnetostriction is defined as the dimensional change of a
magnetic material caused by a change in its magnetic state. Like the piezoelectric
effect, it is reversible, and an applied stress results in a change of magnetic state. All
magnetic materials exhibit varying degrees of magnetostriction. J. P. Joule discov-
ered the effect in 1847 by observing the change in length of an iron bar when it was
magnetized. A popular modern-day magnetostrictive material is Terfenol-D, an
alloy of terbium, dysprosium, and iron. The magnetostriction of Terfenol-D is sev-
eral orders of magnitude greater than that of iron, nickel, or cobalt and gives rise to
–3
strains in the region of 2 ×10 . Bulk Terfenol-D produces much larger strains than
those achievable with piezoelectric materials. Research has been undertaken to
investigate the feasibility of depositing thin and thick-films of magnetostrictive
material onto substrates such as silicon, glass, and alumina; the magnetostriction
achievable, however, is inferior to that of the bulk material.
Figure 5.19 shows an example of a magnetic actuator as described by Judy et al.
[14]. The device comprises a 7-µm-thick layer of Permalloy, which was electro-
plated onto a polysilicon cantilever. The root of the beam is thin and narrow and
acts as a spring, thereby allowing the tip to deflect over a wide angular range. The
magnetic field is applied externally to the device, and this causes a deflection of the
actuator in the direction of the plane of the substrate. The device is made using
polysilicon surface micromachining techniques. Deflections exceeding 90° were
achieved with this configuration.

5.7 Smart Sensors

Advances in the area of microelectronics in recent years have had a major effect on
many aspects of measurement science. In particular, the distinction between the