Page 274 - An Introduction to Microelectromechanical Systems Engineering

Quality Control, Reliability, and Failure Analysis 253

wafers that were ground and polished may exhibit large intrinsic stresses and exces-
sive bowing, which weaken the bond. Experience has shown that the bond will ulti-
mately fail unless the stress is not annealed at an elevated temperature (>500ºC).
Stresses play an equally important role in the failure of thin-film stacks, especially
those incorporating platinum, palladium, and gold. In such cases, it is standard
practice to include an intermediate layer (often chromium or a titanium alloy) to
promote better adhesion. A primitive yet effective test for gauging sensitivity to
delamination is the tape peel test: an adhesive tape is attached to the layers or sub-
strates in question and manually peeled back.
In some circumstances, the area of adhesion between two layers or wafers may
be too small to sustain large forces in the event of a shock. This is the case with poly-
silicon hinges that are anchored to the substrate using small polysilicon staples (see
Figure 4.6). A reliability analysis at the design stage should uncover such a weakness
and increase the bonding or adhesion area.

Stiction
Stiction is the failure mode that describes the situation when surface adhesion forces
are larger than the mechanical restoring force of a suspended micromechanical
element. Surface adhesion forces include capillary forces, electrostatic attraction,
and van der Waals forces.
Stiction is a serious problem in surface-micromachined devices that
occurs immediately after removing the die from the aqueous solution used to etch
the sacrificial layer. A liquid meniscus formed on hydrophilic surfaces inside the
suspension gap pulls the microstructure towards the substrate, causing the two
surfaces to come into contact and stick (see Figure 3.20). Stiction can also occur
after deployment in the field if water condenses inside the gap. The humidity tests
outlined in the Telcordia and MIL specifications are useful in accelerating failures
due to stiction. There are currently two leading methods that can alleviate the
impact of stiction induced by liquid capillary forces, both described in great detail in
Chapter 3: the use of antistiction coatings and supercritical drying when pulling
the structures out of liquid solutions. Coatings remain the topic of much-needed
research.
A different type of sticking happens in electrical relays and RF capacitive
switches. In relays operating at dc currents with ohmic contact between the poles
[39], repeated impacting between the two metal surfaces of the poles degrades the
contacts, ultimately leading to stiction. This process is akin to microwelding, due to
localized heating in the contact area. In RF capacitive switches, electrical charging
within the dielectric layers permanently pulls the switch membrane to the substrate.
The electrical charges are normally trapped within the dielectric with no available
conduction path. The origin of the charging is not well known, though tunneling
into the dielectric is a leading hypothesis [40].

Electrical and Thermal Failure Modes
MEMS share with integrated circuits many electrical and thermal failure modes,
such as burnout of electrical interconnects and diode junctions. Metal traces on the
silicon die are subject to the same rules of electrical reliability found in the
integrated circuit industry, including electrical crowding, electromigration, current