Page 147 - An Introduction to Microelectromechanical Systems Engineering

126 MEM Structures and Systems in Industrial and Automotive Applications

This actuator design avoids rubbing parts, greatly improving the reliability of
the valve. The force provided by the ribs can be raised by increasing both the silicon
thickness and the number of ribs and can be on the order of one Newton, which is
considered to be a very large force in micromachined structures. As the slider moves
to the right, it reduces the areas of the upper and lower controlling orifices and thus
the flow. Eventually, the slider closes off the orifices, and the flow drops to a
negligible amount. A small amount of leakage occurs through the thin recess that is
required to allow motion. In many applications, the leakage is considered small and
is acceptable.
Because the ribs and the frame that constrains their ends are made of the same
material (single-crystal silicon), the actuation force depends on the temperature gra-
dient between them. Any changes in temperature that are uniform to the entire
valve, such as fluctuations in the ambient temperature, cause both the ribs and the
frame to expand and contract at the same rate, resulting in no actuation. This
enables this valve design to operate over a very wide temperature range. The penalty
for the use of an all-silicon valve is a much lower power efficiency, as silicon is a
good thermal conductor (see Table 2.1) and heat is rapidly conducted out the ends
of the ribs. A design advantage of using silicon is that the resistivity of the middle
wafer can be specified by the designer over a range of several orders of magnitude,
allowing the actuator resistance to be designed independently of the actuator
dimensions.
To fabricate the valves, shallow recess cavities are etched in the top and bottom
wafers for the clearances required for actuator motion. Etching in KOH creates the
ports, deep recess, and through hole for electrical contacts (see Figure 4.36). These
might also be formed using DRIE, but KOH etching is an inexpensive option and
works well for this application. The actuator in the middle wafer is etched using
DRIE. Aligned silicon fusion bonding combines the wafer stack. Metal is applied to
the electrical contact areas of the middle wafer. Finally, the ports are protected with
dicing tape to keep them clean, and the wafer is diced (described in Chapter 8). In
use, the chips are held to the surface of a ceramic or metal package with an adhesive
or solder and wire bonded.
A typical design may include ten or more rib pairs, where each pair is formed by
two ribs connected in the middle to the pushrod. Each rib is approximately 100 µm
wide, 2,000 µm long, and 400 µm thick, and is inclined at an angle of a few degrees.
For an average temperature rise of 100°C, each rib pair contributes a force at the
pushrod (and center of rib pair) of about 0.15N. The force falls nearly linearly to
zero at the end of the stroke (about 5 to 10 µm). The lever structure formed by the
fixed hinge and slider transform this large force and small displacement at the actua-
tor to a moderate force and large displacement (>100 µm) at the tip of the slider
near the fluid ports. The prototype valve initially demonstrated at GE NovaSensor
[47] controlled water at pressures reaching 1.3 MPa (190 psig) and flows of 300
ml/min. Further design and fabrication improvements can increase these values to
match the requirements of the automotive and industrial applications.

Micropumps
Micropumps are conspicuously missing from the limelight in the United States. By
contrast, they receive much attention in Europe and Japan, where the bulk of the