MEMS Packaging
As introduced in section 1.1.1, wafer-level MEMS packaging is desired since it allows low cost with batch production, small package size, and improved reliability. This section will
introduce the development of MEMS packaging techniques and discuss the associated problems
with several examples.
A common practice for wafer-level MEMS packaging is to use a silicon/glass cap wafer
to bond with the MEMS device wafer to achieve hermetic/vacuum sealing as shown in Figure 1.
Different bonding techniques have been utilized such as anodic bonding, eutectic bonding, fusion
bonding, soldering, and glass frit bonding [2], [5], [10]. A common problem for these bonding
techniques is high temperature (from several hundreds to thousand oC) to ensure good bonding
quality with short bonding time. It can be difficult to achieve hermetic/vacuum sealing of a high
percentage of devices on a wafer. Polymer adhesives, such as epoxy and BCB [33-38], were also
used to make the bonding ring to lower the bonding temperature. But, polymer is non-hermetic
and usually used for the applications where the requirement for hermeticity and vacuum is low.
To reduce the thermal load of bonding process to the device, localized heating has been
developed to confine the heating region around the bonding ring [5], [6]. The localized heating is
achieved by using a micro-heater fabricating on top of the device substrate and patterned in the
shape of bonding ring. An input current to the micro-heater and the joule heating helps to form a
strong bond to a silicon or glass cap. As an example, a group from University of Michigan
demonstrated localized aluminum/silicon to glass bonding [6]. Figure 1.4 shows the schematic
diagram of the localized heating for bonding and the SEM image of the packaged μ-resonator
after the glass cap was forcefully broken away. Using this approach, they reported the heating
region was confined locally within 15μm of the heating source of above 1000 oC. This technique,
however, is difficult to be applied to wafer-level packaging, since it has to put resistive
micro-heater around every device and the total power consumption should be high and therefore