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Bonding processes for wafer-scale vacuum encapsulation

By Tony Rogers, Anna Draisey, Roger Dyer [Applied Microengineering Limited]
O ver the past few decades, Tab l e 1 . N o t e t h a t f o r C M O S With regard to electrical feedthroughs,

microelectromechanical
temperature needs to be <400ºC.
s ys t e m s ( M EM S ) compatibility, the wafer bonding process the use of vertical vias embedded in
the wafers is the most space-efficient
technologies have been implemented For long-term hermeticity, which is an method for enabling electrical signals
in the sensors and actuators industry to essential requirement for encapsulated in/out of the sealed cavities and it is
vastly improve the price:performance devices, it is necessary to use a metal- possible to keep development costs
ratio for a wide range of devices. based bonding f rame. No mat ter low by utilizing arrays of feedthroughs
For many of these MEMS devices, what type of bonding process is used, already embedded in bought-in wafers.
it has been necessary to include a however, it is essential to ensure that the Examples of these are Hermes wafers
wafer bonding step in the fabrication two wafers are bonded in such a way (tungsten pins embedded in glass wafers
sequence, in particular when there is the that a high-quality vacuum is established from Schott) [1], and polysilicon vias
need to incorporate a sealed cavity into in the sealed cavities. To achieve this, it embedded in silicon wafers from Silex
the device design. For some devices (e.g., is essential that the vacuum chamber can and Icemos [2].
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pressure sensors, bolometers), the sealed achieve <10 mbar, and most importantly,
cavity needs to contain a vacuum, that the wafers are thoroughly outgassed Examples of wafer-level
whereas for others (e.g., accelerometers prior to contacting. This requirement encapsulation
and atomic clocks) it is necessary to puts specific demands on the wafer Some of the building blocks discussed
encapsulate a gas at a defined pressure. bonding equipment, which must comply in the previous section are utilized in
Although the wafer bonding step is with the following: the following production examples of
a key technology in the design and, wafer-level encapsulation.
importantly, enables the concept of • Wide gap between the platens to Pressure sensor. In this application,
wafer-level packaging to be utilized, ensure fast and efficient pumping of an aligned, anodic-bonding, vacuum
there are many other necessary building the cavities; encapsulation process is performed at
blocks required in the wafer-scale • Capability of in situ oxide removal the wafer-level to realize an absolute
packaging of encapsulated devices. via a surface chemistry process; pressure sensor. The device works
These necessary technologies include: 1) • Ability to heat the platens to as an absolute pressure sensor by
Wafer bonding; 2) Provision of electrical different temperatures (e.g., for i ncor porat i ng vacuu m reference
feedthroughs; 3) Interconnection; and getter activation or outgassing); and cavities into each chip on the wafer.
4) Maintenance of vacuum (or gas • Ability to back-fill with a gas and The vacuum level inside each cavity is
composition) in sealed cavities. hold the pressure to a defined then maintained by the incorporation
For the wafer bonding step, there v a l u e (e . g ., f o r d a m p i n g of of a getter. Unfortunately, the 500ºC
are numerous available processes accelerometers). activation temperature of the getter was
and it is often necessary to define higher than the maximum temperature
the optimum technique based on the The longer-term maintenance of the limit for some aspects of the device
required device characteristics and the vacuum level in the sealed cavities is design. To overcome this restriction,
application limitations. A selection best dealt with by the inclusion of a the device was designed such that
of available techniques is listed in thin-film getter. the temperature-limiting components
were on one wafer, whereas the thin-
film getter was included on the other
wafer. A suitable aligner bonder (Model
AWB from Applied Microengineering
Limited) was then used to perform the
sequential getter activation followed by
wafer bonding steps. This aligner-bonder
system features differential heating and
a large platen separation. Additionally,
this capability allowed the getter
activation followed by wafer bonding
to be successfully performed – and
after the activation process, the getter
wafer was allowed to cool down to the
Table 1: A selection of wafer bonding techniques suitable for wafer-level encapsulation.

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42 Chip Scale Review January • February • 2020 [ChipScaleReview.com]