Page 143 - An Introduction to Microelectromechanical Systems Engineering

122 MEM Structures and Systems in Industrial and Automotive Applications

diaphragm in the middle of the silicon wafer. The top glass wafer is processed
separately to form a sputtered thin-film metal heater. Ultrasonic drilling opens a fill
hole through the top Pyrex glass substrate, as well as the inlet and outlet ports in the
lower Pyrex glass substrate. Both glass substrates are sequentially bonded to the sili-
con wafer using anodic bonding. In the final step, the Fluorinert liquid fills the cav-
ity. Special silicone compounds dispensed over the fill hole permanently seal the
Fluorinert inside the cavity.

Micromachined Valve from TiNi Alloy Company
TiNi Alloy Company of San Leandro, California, is another small company with the
objective of commercializing micromachined valves. Its design approach, however, is
very different than that of Redwood Microsystems. The actuation mechanism relies
on titanium-nickel (TiNi) [45], a shape-memory alloy—hence the name of the com-
pany. The rationale is that shape-memory alloys are very efficient actuators and can
produce a large volumetric energy density, approximately five to 10 times higher
than competing actuation methods. It is, however, the integration of TiNi processing
with mainstream silicon manufacturing that remains an important hurdle.
The complete valve assembly consists of three silicon wafers and one beryllium-
copper spring to maintain a closing force on the valve poppet (plug) (see
Figure 4.34). One silicon wafer incorporates an orifice. A second wafer is simply a
spacer defining the stroke of the poppet as it actuates. A third silicon wafer contains
the valve poppet suspended from a spring structure made of a thin-film titanium-
nickel alloy. A sapphire ball between a beryllium-copper spring and the third silicon
wafer pushes the poppet out of the plane of the third wafer through the spacer of the
second wafer to close the orifice in the first wafer. Current flow through the
titanium-nickel alloy heats the spring above its transition temperature (~ 100ºC),

Flow orifice
Orifice die
Spacer
Actuator die

TiNi spring and actuator
Sapphire ball

Silicon
Poppet

Beryllium-copper

Bias spring
Figure 4.34 Assembly of the micromachined, normally closed valve from TiNi Alloy Company.
The beryllium-copper spring pushes a sapphire ball against the silicon poppet to close the flow ori-
fice. Resistive heating of the TiNi spring above its transition temperature causes it to recover its
original flat (undeflected) shape. The actuation pulls the poppet away from the orifice, hence per-
mitting fluid flow. (After: A. D. Johnson, TiNi Alloy Company of San Leandro, California.)