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4.4 MEMS Mechanical Sensor Packaging 71

4.4.2 Protecting the Environment from the Sensor

The MEMS package must also protect the environment from the presence and func-
tion of the sensor. Application areas where this may be of particular concern include
healthcare, food, beverage, and bioprocessing. These typically require the microsen-
sor to be isolated from the chemical or biological media by a mechanical interface or
sensor housing made from a suitable material. Types of interface materials include
polymer membranes [30], ceramics, glass ceramics, and some metals. The duration
of contact with the environment is a fundamental factor in choosing the material,
and it must possess the following characteristics in the typical applications men-
tioned earlier:

• Biocompatible;
• Nontoxic;
• Able to withstand sterilization.

This is particularly important in biomedical applications, where the small size
and performance characteristics of MEMS sensors make them highly attractive.
Examples of such devices include catheter blood pressure sensors and chemical
monitoring systems (e.g., glucose). These have been successfully used in both in vivo
and in vitro applications.
Many of the applications discussed also impose space constraints upon the final
packaged solution—catheter pressure sensors being the obvious example. Tech-
niques such as flip chip assembly and wafer level packaging can be employed to
reduce the packages’ volumes. In extreme cases where more than one sensing die or
separate ICs are to be incorporated, chip stacking can be employed to further reduce
package size [31, 32]. Chip stacking introduces many potential difficulties including
electrical interconnects, thermal issues, and packaging stresses [33]. Electrical
interconnects have been realized using through-wafer techniques discussed above,
purpose-made intermediate chips with a suitable track layout, and also by forming
metal tracks on the outside of the stack [34]. A basic stacking approach can be used
to reduce packaging stresses, and this is discussed in the following section.
Another interesting development that may suit some applications is the devel-
opment of spherical semiconductors. A 1-mm-diameter spherical semiconductor
has three times the surface area of a 1-mm-square chip [35]. Many sensing applica-
tions have been suggested for this form of device including medical sensors and
accelerometers.

4.4.3 Mechanical Isolation of Sensor Chips
The mechanical isolation of the sensor chip is vital in avoiding unwanted cross sen-
sitivities and the transmission of external stresses through the packaging to the sen-
sor. Indeed, the package design itself must avoid introducing such undesirable
effects and should provide relief from residual stresses trapped in the assembly dur-
ing the packaging process. Factors such as the long-term stability of packaging
materials and methods are of fundamental importance. A well-isolated sensor chip
mounted on a carefully designed package will be less affected by changes in its envi-
ronment, leading to improved long-term stability, resolution, and sensor accuracy.