Page 77 - MEMS Mechanical Sensors

66 Mechanical Sensor Packaging

sealing process must be carefully considered to maximize the strength of the bond.
Lead-zinc-borate glasses are often used and these require a process temperature
below 420°C; and the TEC can be modified by the addition of suitable fillers to
reduce stresses in the seals. The actual sealing process typically involves heating the
package in a furnace to the required process temperature. The lid is normally pre-
glazed with the appropriate sealing glass. Furnace profiles, and especially cooling
rates, must be carefully controlled to reduce stresses and avoid reliability issues.

4.4 MEMS Mechanical Sensor Packaging

A MEMS sensor packaging must meet several requirements [7–9]:

• Protect the sensor from external influences and environmental effects. Since
MEMS inherently include some microscale mechanical components, the integ-
rity of the device must be protected against physical damage arising from
mechanical shocks, vibrations, temperature cycling, and particle contamina-
tion. The electrical aspects of the device, such as the bond wires and the electri-
cal properties of the interconnects, must also be protected against these
external influences and environmental effects.
• Protect the environment from the presence of the sensor. In addition to pro-
tecting the sensor, the package must prevent the presence of the MEMS from
reacting with or contaminating potentially sensitive environments [10]. The
classic examples of this are medical devices that contain packaged sensors that
can be implanted or used within the body; these must be biocompatible, non-
toxic, and able to withstand sterilization.
• Provide a controlled electrical, thermal, mechanical, and/or optical interface
between the sensor, its associated components, and its environment. Not only
must the package protect both the sensor and its environment, it must also
provide a reliable and repeatable interface for all the coupling requirements of
a particular application. In the case of mechanical sensors, the interface is of
fundamental importance since, by its nature, specific mechanical coupling is
essential but unwanted effects must be prevented. A simple example would be
a pressure sensor where the device must be coupled in some manner to the
pressure but isolated from, for example, thermally induced strains. The pack-
age must also provide reliable heat transfer to enable any heat generated to be
transmitted away from the MEMS device to its environment.

In the vast majority of cases, basic plastic, metal, or ceramic packages do not sat-
isfy these requirements. While the requirements for electrical connections and heat
transfers paths on sensor packages are typically much less than in the case of most
ICs, it is the mechanical interface that complicates the package design. The mechani-
cal interface must isolate the sensor from undesirable external stresses and provide
relief from residual stresses in the assembly while enabling the desired mechanical
effect arising from the measurand to be coupled to the sensor. In the vast majority of
practical sensor applications, each packaging solution will be developed specifically
for that particular application.