Page 271 - An Introduction to Microelectromechanical Systems Engineering

250 Packaging and Reliability Considerations for MEMS

10 10

Extrapolated lifetime
10 8
(hours) Measurements at temperature

failure 10 6 elevated temperatures Slope = E

to a
Time operating

10 4
Normal

10 2
1/T (K −1 )
Figure 8.18 The Arrhenius model is a useful tool in accelerated life testing to extrapolate the
lifetime of a device at normal operating temperatures. Measurements of time to failure are made
at a few elevated temperatures, and an exponential curve fit is applied to the data to calculate the
activation energy. This extrapolation method assumes a constant failure rate and thus is specific to
the exponential distribution function that is graphically represented by the scatter in the measured
data at any particular temperature.

obtained or are difficult to enforce. This level of secrecy becomes even more
entrenched in packaging and assembly.
Much work remains to be completed before the reliability of relatively new
product entries, especially those with integrated optical, fluidic, RF, or acoustic
functions, reaches a level similar to that of widely deployed micromachined sensors.
Clearly, these new products have benefited from the existing body of knowledge on
reliability, but there remain failures specific to them. For instance, the reliability of
the junction between silicon (or glass) and a fluidic interconnect is constantly subject
to improvement. In another example, it is not clear whether outgassing from epoxies
or other adhesives used in the packaging of optical elements affects the reflectivity of
micromachined mirrors or interferes with the high voltages typically used in the elec-
trostatic actuators that drive those mirrors.
Failure modes stem either from weaknesses in the design itself (of both the
micromachined device and packaging) or from process variations that result in criti-
cal departures from the nominal design. Addressing the former usually follows a sys-
tematic course of tests and simulations to pinpoint the exact origin of the design
weakness. For example, computer-aided simulations are very useful in identifying
nodes of high stress that can result in fracture. Addressing the weaknesses stemming
from process control is a more tedious and time-consuming task, which necessitates
patience, experience, technical flair, and good organization. For example, the source
of an occasional electrical short or open in an electrostatic actuator may be quite dif-
ficult to diagnose. It may be the result of a variation in the thickness of a metal trace
over a topographical step, or it may be caused by defects in an insulating layer, or
there could be yet another plausible explanation. Countless companies proud of