Page 278 - MEMS and Microstructures in Aerospace Applications

Osiander / MEMS and microstructures in Aerospace applications DK3181_c012 Final Proof page 270 1.9.2005 9:13pm

270 MEMS and Microstructures in Aerospace Applications

(MEMS) package: mechanical support, protection from the environment, and elec-
trical connection to other system components.
In addition to providing mechanical support, electrical connections, and thermal
management, MEMS packaging for space applications must meet operational
environmental requirements such as high temperature operation, thermal cycling,
humidity, vibration, shock, radiation, outgassing, and depressurization, to name a
few. Radiation, for example, can impact the on-board analog and digital microelec-
tronic components of MEMS devices, the transduction mechanism of the sensor,
and mechanical components of MEMS. 1

12.1.1 MECHANICAL SUPPORT
Due to the very nature of MEMS being mechanical, the requirement to support and
protect the device from thermal and mechanical shock, vibration, high acceleration,
particles, and other physical damage (possibly radiation) during storage and oper-
ation of the part becomes critical. The mechanical stress endured depends on the
mission or application. For example, landing a spacecraft on a planet’s surface creates
greater mechanical shock than experienced by a communication satellite operating in
space. There is also a difference between space and terrestrial applications.
The coefficient of thermal expansion (CTE) of the package should be equal to
or slightly greater than the CTE of silicon for reliability, since thermal shock or
thermal cycling may cause die-cracking and delamination if the materials are
unmatched or if the silicon is subject to tensile stress. Other important parameters
are thermal resistance of the carrier, the material’s electrical properties, its chemical
properties, and resistance to corrosion.
Once the MEMS device is supported on a carrier (chip) and the wire bonds or
other electrical connections are made, the assembly must be protected from scratches,
particulates, and other physical damage. This is accomplished either by adding walls
and a cover to the base or by encapsulating the assembly in plastic or other material.
Since the electrical connections to the package are usually made through the walls,
the walls are typically made from glass or ceramic. The glass or ceramic can also be
used to provide electrical insulation of the leads as they exit through a conducting
package wall (metal or composite materials). Although the CTE of the package walls
and lid do not have to match the CTE of silicon-based MEMS as they are not in
intimate contact (unless an encapsulating material is used), it should match the CTE
of the carrier or the base to which the walls are connected.

12.1.2 PROTECTION FROM ENVIRONMENT
12.1.2.1 Simple — Mechanical Only
Many MEMS devices are designed to measure something in the immediate sur-
rounding environment. These devices range from biological ‘‘sniffers’’ to chemical
MEMS that measure concentrations of certain types of liquids. So the traditional
‘‘hermeticity’’ that is generally thought of for protecting microelectronic devices
may not apply to all MEMS devices. These devices might be directly mounted on a

© 2006 by Taylor & Francis Group, LLC