Page 149 - An Introduction to Microelectromechanical Systems Engineering

128 MEM Structures and Systems in Industrial and Automotive Applications

scenario is impossible because of the geometry of the two check valves. This is true
as long as the pump diaphragm displaces liquid at a frequency lower than the natu-
ral frequencies of the two check valve flaps. But at higher actuation frequencies—
above the natural frequencies of the flap—the response of the two flaps lags the
actuation drive. In other words, when the pump diaphragm actuates to draw liquid
into the chamber, the inlet valve flap cannot respond instantaneously to this action
and remains closed for a moment longer. The outlet check valve is still open from
the previous cycle and does not respond quickly to closing. In this instance, the
outlet check valve is open and the inlet check valve is closed, which draws liquid
into the chamber through the outlet rather than the inlet. Hence, the pump reverses
its direction. Clearly, for this to happen, the response of the check valves must lag
the actuation by at least half a cycle—the phase difference between the check valves
and the actuation must exceed 180º. This occurs at frequencies above the natu-
ral frequency of the flap. If the drive frequency is further increased, then the
displacement of theflaps becomes sufficiently small that the check valves do not
respond to actuation.
The pump rate initially rises with frequency and reaches a peak flow rate of 800
µl/min at 1 kHz. As the frequency continues to increase, the time lag between the
actuation and the check valve becomes noticeable. At exactly the natural frequency
of the flaps (1.6 kHz), the pump rate precipitously drops to zero. At this frequency,
the phase difference is precisely 180º, meaning that both check valves are simultane-
ously open—hence no flow. The pump then reverses direction with further increase
in frequency, reaching a peak backwards flow rate of –200 µl/min at 2.5 kHz. At
about 10 kHz, the actuation is much faster than the response of the check valves,
and the flow rate is zero. For this particular device, the separation between the
diaphragm and the fixed electrode is 5 µm, the peak actuation voltage is 200V, and
the power dissipation is less than 1 mW. The peak hydrostatic back pressure devel-
oped by the pump at zero flow is 31 kPa (4.5 psi) in the forward direction and 7 kPa
(1 psi) in the reverse direction.
The fabrication is rather complex, involving etching many cavities separately in
each wafer and then bonding the individual substrates together to form the stack
(see Figure 4.38). Etching using any of the alkali hydroxides is sufficient to define the
cavities. The final bonding can be done by either gluing the different parts or using
silicon fusion bonding.

Summary

This chapter presented a set of representative MEM structures and systems
used in industrial and automotive applications, including a number of
micromachined sensors, actuators, and a few passive devices. The basic sensing and
actuation methods vary considerably from one design to the other, with significant
consequences to the control electronics. Design considerations are many; they
include the specifications of the end application, functionality, process feasibility,
and economic justification.