Medical Applications of Micro-Electro-Mechanical Systems (MEMS) Technology - Biomaterials Science: An Integrated Clinical and Engineering Approach - page 58

large deflections (at least compared to electrostatic actuation), a linear deflec-
tion versus power relationship, and sensitivity to environmental conditions.
A simple bimetallic microactuator can be made by depositing a thin-film
of aluminum onto a thin compliant silicon cantilever (Figure 3.24) while
passing current through the aluminum layer. As the aluminum layer heats
up through Joule heating, the cantilever will bend because of the different
expansion coefficients of the two materials, with the aluminum metal having
a larger expansion under heating than the silicon semiconductor material.
Obviously, the choice of materials used in a bimetallic microactuator will
depend on the application.
Yet another popular material for implementing microactuators in MEMS
is shape-memory alloys [40-42]. A shape-memory alloy is a material that
undergoes a martensite to austensite phase change upon heating. During
this phase change, the material will return to its unstrained shape (i.e., the
material has memory). Figure 3.25 is an illustration of the shape memory
effect. Shape memory alloys can be sputter deposited in thin film form on
silicon wafers. Heating of the film is usually achieved by Joule heating. The
shape memory effect is a reversible effect and can be repeated many times.
The attributes of shape memory in MEMS actuators include the following:

Aluminum thin film

Silicon

FIGuRE 3.24
A bimetallic microactuator made from a thin film aluminum layer on a silicon cantilever. When
heated, the aluminum has a higher thermal expansion coefficient than silicon and causes the
cantilever to deflect downward.

Martensite Phase

L

Martensite Phase with applied force
and resultant strain

F F

T

Heated in Austensite phase with
strain recovered

FIGuRE 3.25
The shape memory effect. The SMA sample in the martensite phase (top) is at room temperature
and unstrained. In the middle, the sample is strained at room temperature. At bottom, the SMA
sample is heated and the sample undergoes a phase change from the martensite to the aus-
tensite phase, and the strain is completely recovered. This cycle can be repeated over and over.