Page 78 - MEMS Mechanical Sensors

4.4 MEMS Mechanical Sensor Packaging 67

The sensor packaging can be broken down into two distinct components. First
order packaging relates to the immediate mounting of the chip, and second order
packaging refers to the mechanical housing surrounding the mounted sensor. The
degree of engineering involved for each will depend upon the particular application.
It is certainly common for the first order package, and often the case for the second
order package, to perform an integral part of the device function.
The following sections present packaging solutions, both first and second order,
that address the key requirements described above. Section 4.4.1 details methods of
protecting the sensor die from its environment and includes a discussion of wafer
level packaging techniques. Section 4.4.2 describes packaging techniques used to
protect the environment from the presence of the sensor. Section 4.4.3 presents
stress-relieving techniques used to isolate sensors from undesirable external stresses.
It also includes an analysis of common packaging materials and bonding processes
and discusses their influence on the behavior and performance of a packaged
MEMS mechanical sensor. Finally, Section 4.4.4 discusses the latest developments
and looks towards future packaging trends.

4.4.1 Protection of the Sensor from Environmental Effects
MEMS mechanical sensors require careful packaging in order to protect the inher-
ently fragile mechanical components and to prevent undesirable external influences.
Damage to the sensor chip can arise from chemical exposure, particulate contami-
nation, mechanical shocks, and extremes of temperature [11]. Exposure to environ-
mental media, either gases or liquids, can adversely affect MEMS in several ways.
Corrosion of wire bonds, metal bond pads, or even the substrate material itself can
lead to premature failure and reliability problems [12]. Water molecules can
cause such effects. Another undesirable consequence is the occurrence of stiction,
whereby surface machined components can become stuck to the substrate. Simi-
larly, particle contamination will prevent mechanical components from functioning
correctly, as well as potentially shorting electrical contacts. Excessive mechanical
shocks can simply cause microstructures to fracture. Extremes of temperature will
maximize packaging stresses arising from TEC mismatches, which can affect both
performance and reliability, and possibly prohibit some forms materials and elec-
tronics. Finally, the electrical characteristics of interconnects and device electronics
must also be protected. Such protection must be provided by the package as a
whole, but packaging the device at wafer level provides the best level of protection.
This approach ensures a robust sensor chip with some level of protection in place
against the subsequent packaging processes.

4.4.1.1 Wafer Level Packaging
Wafer level packaging refers to any packaging step that can be performed using
wafer-processing techniques and that act on all the devices simultaneously across
the wafer. These packaging processes are carried out before dicing. Wafer level
packaging is commonly used to provide some level of sensor isolation or stress relief
(see Section 4.3.3) or to cap or seal part of or the whole die. The method of isolation
and sealing will depend upon the application. The advantages of wafer level packag-
ing compared to the normal packaging approach are: