Page 300 - MEMS and Microstructures in Aerospace Applications

Osiander / MEMS and microstructures in Aerospace applications DK3181_c013 Final Proof page 293 1.9.2005 12:45pm

Handling and Contamination Control for Critical Space Applications 293

An approach to address the four issues is delineated below: 6

. Direct contamination. The active device is placed in a hermetic cavity at
wafer scale as an integral part of the MEMS foundry flow before any
postfoundry operations occur. Hermeticity is established at the earliest pos-
sible step of the manufacturing process.
. Leakage. The leakage standard set by the MIL-STD-883 fine leak testing
protocol may need to be reevaluated.
. Permeation. No materials such as polymers or epoxies known to allow long-
term permeation should be used. In fact, typically, no new materials should be
added. The cap used is normally a combination of Si and SiO 2 . It is applied to
the MEMS wafer without adhesives or other bonding materials. Covalent
bonding between prepared surfaces of conventional semiconductor process
materials occurs without applied pressure, temperature, or electric field.
Prepared materials are aligned, and they simply bond on contact. The menu
of bondable materials is large and includes materials common to IC and
MEMS processing, such as Si, SiO 2 , and Si 3 N 4 .
. Outgassing. Because the process takes place at wafer scale, the cavity
formed can be arranged to include only the active MEMS device. Other
materials used for die attach, bump preparation, or packaging, are not in-
cluded in the hermetic cavity. This is a large change from what is convention
today when all of these materials are in the package. With this approach,
materials known to create outgassing effects are simply excluded from the
hermetic cavity.

Minimizing MEMS packaging contamination sometimes requires ‘‘thinking outside
the box.’’ Capping the devices before dicing provides advantages that could be
gained in both contamination control and in cost. Ziptronix has developed a process
whereby a cap-wafer is placed over the production MEMS wafer before any
postfoundry operation. The cap-wafer has cavities etched on the surface to provide
the headspace for the MEMS devices. The bonding operation of the cap-wafer uses
no glue, solder, or elevated temperatures. But most importantly, the headspace is
designed to include only the MEMS device. 7
Because of this precapping, the postprocessing operations can proceed along
more conventional lines with only minimal customized MEMS postprocessing
needed. With precapping, the dicing operation results in separation of devices
which are already totally sealed. If conventional wire bonding is required, the
cap-wafer leaves these leads available to be bonded without exposing the MEMS
device.

13.4 IN-PROCESS HANDLING AND STORAGE REQUIREMENTS

The following conditions are recommended for the proper storage of MEMS
devices once released. Any deviation from the following conditions should be
avoided. MEMS devices should be stored in cabinets with an atmosphere of

© 2006 by Taylor & Francis Group, LLC