Page 275 - An Introduction to Microelectromechanical Systems Engineering

254 Packaging and Reliability Considerations for MEMS

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density limitation (typically <100 kA/cm ), and localized resistive heating in the wir-
ing. Metal patterns over rising topographical features are particularly susceptible to
failure because of the potentially thin metal at the corners. Failure of electrical wir-
ing is not limited to on-chip metal traces. Wire bonds are also subject to failure if
they are not matched to the current requirements of the device. Additionally, wire
bonds can fail under mechanical shock and vibration because they are often made of
gold (a dense material) and have long lengths exceeding many millimeters.
Thermal failures occur when there is an excessive temperature rise due to local-
ized heating (e.g., from a high electrical resistance or an accidental current surge) or
when there is poor conduction of heat generated within the device (e.g., in thermal
actuators that can dissipate several watts). In all cases, proper thermal management
at the die and package level is important to mitigate the risk of thermal failures.

A Reliability Case Study: The DMD
The DMD, described in detail in Chapter 5, is an excellent example of a complex
microsystem that merges electronic, mechanical, optical, and chemical attributes,
thus making its reliability a highly interactive relationship between many diverse
operational factors and environmental parameters. The historical evolution of the
DMD and its reliability over nearly two decades highlights the depth and breadth of
the development effort that has yielded this commercial success.
Early in the 1990s, the lifetime of the DMD was only 100 hours at 65ºC,
whereas the target application (primarily printing) required a minimum of 5,000
hours. Some parts worked well, but others did not. The design was marginal, and
the fabrication processes were not under control. The origins of the failures were
largely unknown, which made improving the reliability a daunting task. Texas
Instruments undertook a program of extensive testing, characterization, and analy-
sis of the failure modes that gradually increased the understanding of the underlying
physics and resulted in new designs that were more robust and reliable.
With the novelty of the DMD design and the emerging nature of the MEMS
industry, Texas Instruments had to develop many specialized tests and build the
corresponding equipment in house. These tests varied many operational parameters,
including temperature, voltage and timing waveform, the number of mirror land-
ings, mirror duty cycle, and light intensity. It then sought to identify statistical
relationships that would lead to the location of design parameters that yielded a
more reliable performance. They also performed a number of environmental tests,
many similar to those defined under the Telcordia and MIL standards, to verify the
product’s environmental robustness (see Table 8.9).
One important characterization test is the bias/adhesion mirror mapping
(BAMM) [41]. It involved the statistical analysis of the number of mirrors that
land with increasing applied bias while holding the other operational parameters
constant. The result is a distribution curve for mirror landing whose tightness (i.e.,
spread in voltage) is a measure of process variability that led the engineers to further
optimization of the hinge design and voltage drive waveform. The BAMM test also
uncovered another weakness: the bias voltage at landing decreased from 16V to
about 14V after 2,000 hours of operation. The finding and the data became use-
ful in implementing additional improvements and developing models for lifetime
prediction [42].