Page 272 - An Introduction to Microelectromechanical Systems Engineering

Quality Control, Reliability, and Failure Analysis 251

their new and innovative MEMS product ideas had to face these types of reliability
questions as they transitioned from a prototyping phase to a manufacturing phase.
Analog Devices, Inc., invested tremendous time and resources to resolve the stiction
problem that plagued their early accelerometer designs. The following are major
observed failure modes that span both design and process control.

Cracks and Fractures
Cracks can occur in a number of locations in a microstructure and are the result of
a large stress that exceeds the fracture stress of the material or fatigue [35]. Observ-
ing and diagnosing a large fracture is readily achieved under an optical or scanning
electron microscope. Acoustic imaging is occasionally used, but its utility is limited
to detecting large embedded defects. However, hairline fractures can seldom be
seen. Instead, their existence is indirectly detected by measuring their effect on a
number of other parameters (e.g., by looking for anomalies in the frequency
response of a mechanical element). Often, a mechanical shock is the causing event
of fracture [36]. Naturally, the mass of the micromechanical structure must be rela-
tively large for the shock to pose any real risk. For instance, a 10-µm thick, 1 mm 2
membrane in a pressure sensor has very little mass (24 µg) and can sustain shocks
up to 100,000G [37]!
A mechanical shock can originate from several sources. It can be intentional, as
is the case for accelerometers or during the reliability testing of the product, or acci-
dental (e.g., from the saw during wafer dicing or rough handling during packaging).
A mechanical shock has a typical duration of tens to hundreds of microseconds,
which greatly exceeds the acoustic transit time defined as the time it takes an acoustic
wave to travel through the longest dimension of the micromechanical structure (typi-
cally on the order of a few microseconds). Therefore, the micromechanical structure
behaves as if the shock is static and wave analysis is not necessary [38]. This is not the
case with ultrasonic pulses. Such short pulses (of duration ≤1 µs) are common in
ultrasonic cleaning, wire bonding during packaging, or in micromachined resonators
that specifically utilize ultrasound in their operation. Waves will reflect back at geo-
metrical discontinuities and material boundaries due to mismatches in acoustic
impedances, causing standing waves and local amplification of stresses possibly
beyond the fracture stress. In such situations, a complete wave analysis using
computer-aided modeling is useful to identify the fragile boundaries.
Under shock, the displacement of a freestanding micromechanical structure
may exceed its maximum allowed design limit, thus causing excessive stress at
one or more particular locations (often corners) and consequently failure. Good
mechanical designs take such shocks into account. For example, some well-designed
accelerometers have travel limiters for the inertial mass (see Chapter 4) in order to
minimize the stress on the supporting spring in the presence of a large shock. Sharp
corners are also responsible for fracture under even small shocks because they con-
centrate mechanical stress. Designers have learned to round the profile of corners to
reduce this risk of failure. For example, the corner formed by the intersection of the
{111} planes of the thick frame and the thin membrane of a pressure sensor (see
Chapter 4) is characteristically sharp, and virtually all manufacturers of modern
commercial pressure sensors perform some type of corner rounding. While knowl-
edge about such failure modes is now common, details of the corresponding