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Handling and Contamination Control for Critical Space Applications 291

13.3 HANDLING DURING DIE SINGULATION, RELEASE,
AND PACKAGING
Wafers will often be received, if coming from an external or an in-house foundry, in
a nonreleased condition in order to protect the MEMS devices during transportation.
In a typical process, the assembly house will have the responsibility to separate the
wafer into smaller dies and perform the release. In most cases, the fabrication of
surface micromachined devices involves layering, or intercalation steps, or both to
add mechanical protection. This protective material (often SiO 2 ) must be etched in
order to liberate or ‘‘release’’ the device from its carrier substrate. The drying
procedure is critical to minimize stiction for many structures.

13.3.1 DIE SINGULATION
Die singulation is a process in which a wafer is sawed into many single die
segments. Slicing and dicing down to the single die unit in the MEMS industry
has more intricate concerns than it does in the IC industry. Once released, active
movable components on the surface of the chips (front and back sides) are particu-
larly susceptible to damage from traditional microcircuit handling and cleaning
methods. The easiest way to get around this is to singulate the die before the MEMS
devices are released. This way the moving structures are protected and can hardly
be damaged by particulates and contamination such as saw slurry, particles gener-
ated by laser scribing and from scribe and break. In some cases, when the devices
are less sensitive to particulates, it can be advantageous to release the die before
singulation. It might be less labor intensive to release an entire wafer instead of
hundreds of small dies. Traditional techniques such as using forced inert gas to blow
particulates off the chip and other handling methods, such as vacuum pick-ups, may
compromise the devices. For skilled laborers in these areas, who are used to these
techniques and have used them successfully in the IC industry, retraining is required
to preclude damage when handling MEMS. Unlike standard ICs, MEMS devices
cannot be easily cleaned once they have been released. For this reason MEMS
wafers must be singulated (cut up into individual die) and assembled using very
specialized techniques.

13.3.2 HANDLING DURING RELEASE
Once the dies are singulated, they can be released. Special handling and process
controls will normally be put into place to reduce the possibility of stiction in the
drying process. The surface tension during drying can pull the moveable members
together increasing the likelihood of stiction. Several design options are available to
reduce the possibility of stiction, including the use of stand-off bumps, sacrificial
1
polymer, and polymer columns sustaining the released structure. Options for the
process include special mixes of methanol, hydrofluoric acid (HF) vapor, and
2
supercritical drying. The supercritical CO 2 drying method takes advantage of the
supercritical transition of a fluid, avoiding the formation of an interface between the

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