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6-18 MEMS: Design and Fabrication

FIGURE 6.16 A three-pole Ka-band filter implemented in microcoaxial construction shown (top) with and (bottom)
without the upper layers, the latter to illustrate the center conductor. The whole filter fits in an 8-mm diameter package.

magnitude of the force does). The higher force can be used in a wide range of applications, from actuat-
ing electrical relays to powering micropumps or surgical tools.
These capacitive actuators also illustrate an example of sensor flexibility available with EFAB technology.
The capacitors that provide on the order of twenty times greater force provide similar increases in sensi-
tivity. It has been realized several times during an EFAB device design review that the capacitive output
change will swamp previously used circuitry, caused bytoomuch signal from the sensor!
For inertial sensors there are similar sets of advantages. The proof mass can be extremely large, often
larger than even bulk etched devices (caused by greater height and a material nearly four times denser
than silicon), and this can be combined with the high capacitances illustrated in the previous example. Both
of these elements can also coexist with springs of almost arbitrary shape and dimension, leaving it up to the
designer to decide where to place things and how to shape the various components. The EFAB process
offers the additional opportunity to build the supports, sense, drive, or other elements within the proof
mass if so desired, which can be useful for controlling mass distribution. Additionally it is usually feasi-
ble to build lateral and vertical accelerometers at the same time, which is often a challenge with other fab-
rication technologies. Figure 6.3 shows a multiple-axis EFAB accelerometer.
Pressure sensors and the like can also benefit from the mix of scales available. A simple membrane
pressure-sensing element can be coupled to finely detailed sensors to vastly increase sensitivity or, alternately,

© 2006 by Taylor & Francis Group, LLC