Page 244 - An Introduction to Microelectromechanical Systems Engineering

Key Design and Packaging Considerations 223

development remains to be done to fully characterize the properties of silicon car-
bide as a coating material.
For extreme environments such as in applications involving heavy industries,
aerospace, or oil drilling, special packaging is necessary to provide adequate
protection to the silicon microstructures. If the silicon parts need not be in direct
contact with the surrounding environment, then a metal or ceramic hermetic
package may be sufficient. This is adequate for accelerometers, for example, but
inappropriate for pressure or flow sensors. Such devices must be isolated from
direct exposure to their surrounding media and yet continue to measure pressure or
flow rate. Clever media-isolation schemes for pressure sensors involve immers-
ing the silicon microstructure in special silicone oil with the entire assembly
contained within a heavy-duty stainless-steel package. A flexible stainless-steel
membrane allows the transmission of pressure through the oil to the sensor’s
membrane. Media-isolated pressure sensors are discussed in further detail later in
this chapter.
Media-isolation can be more difficult to achieve in certain applications. For
instance, there are numerous demonstrations of optical microspectrometers capable
of detecting SO and NO , two components of smog pollution. But incorporating
x x
these sensors into the tail pipe of an automobile has proven to be of great difficulty
because the sensor must be isolated from the harsh surrounding environment, yet
light must reach the sensor. A transparent glass window is not adequate because of
the long-term accumulation of soot and other carbon deposits.

Hermetic Packaging
A hermetic package is theoretically defined as one that prevents the diffusion of
−3
helium. For small-volume packages (<0.40 cm ), the leak rate of helium must be
3
−8
lower than 5 × 10 atm•cm /s. In practice, it is always understood that a hermetic
package prevents the diffusion of moisture and water vapor through its walls. A
hermetic package must be made of metal, ceramic, or millimeter-thick glass. Silicon
also qualifies as a hermetic material. Plastic and organic-compound packages, on
the other hand, may pass the strict helium leak rate test, but they allow mois-
ture into the package interior over time; hence, they are not considered her-
metic. Electrical interconnections through the package must also conform to
hermetic sealing. In ceramic packages, metal pins are embedded and brazed
within the ceramic laminates. For metal packages, glass firing yields a hermetic
glass-metal seal.
A hermetic package significantly increases the long-term reliability of electrical
and electronic components. By shielding against moisture and other contaminants,
many common failure mechanisms including corrosion are simply eliminated. For
example, even deionized water can leach out phosphorous from low-temperature
oxide (LTO) passivation layers to form phosphoric acid that, in turn, etches and
corrodes aluminum wiring and bond pads. The interior of a hermetic package is
typically evacuated or filled with an inert gas such as nitrogen, argon, or helium.
The DMD from Texas Instruments and the infrared imager from Honeywell, both
discussed in a previous chapter, utilize vacuum hermetic packages with transparent
optical windows. The package for the DMD even includes a getter to absorb any
residual moisture.