Annual report 2009 - Imec

RELIABILITy:
FORECASTINg
THE FUTURE
A
crucial requirement of micro- and nanoelectronic
systems is that they work perfectly, as they
were designed. But also that they remain doing
so during their lifetime, and under all circumstances,
such as with high and low temperatures, high humidity,
after voltage peaks, or during a rough mechanical treatment.
They have to be, in sum, reliable.
Until a few years ago, reliability research in microelectronics
was relatively simple: most components could
be tested as separate building blocks. And by testing
the reliability of the separate building blocks, you could
get a fairly good view on the reliability of the integrated
system. First test the silicon, then the package. If both
are ok, then there is a good chance that the packaged
IC will be reliable. There were almost no failures caused
by the influence of the package on the silicon chip. And
the tests of the separate chips and packages were fairly
straightforward, consisting of standardized test suites.
This has changed. The relentless miniaturization and the
use of new materials bring about new testing challenges.
And new technologies are more and more based on the
heterogeneous integration of components. These integrated
components can no longer be viewed as independent
building blocks that can be tested separately.
Examples are 3D stacks of thinned Si-chips, SiGe-MEMS
(micromechanical systems) integrated with CMOS, or
biosensors. The package can, for example, influence the
functionality of the MEMS, it can damage the fragile
thinned silicon, or it can change the characteristics of
the porous materials that are used in innovative processes.
And if the connections between the chips in a 3D
stack fail, then the complete 3D system fails.
Now everything is connected to everything. This calls
for an integrated vision on reliability. A 3D vision, in
which the influence of the components on each other
is taken into account. As a consequence, we have to
stop looking at individual components and study the
system as a whole. But if we do so, our tools and
standardized test recipes no longer serve us in the way
they used to.
To solve this, we increasingly work with models to predict
the behavior of the systems. Modeling allows us
to set up virtual reliability tests. These can pinpoint the
weakest spots in the system and show a way to optimize
the system’s design. Of course, these models still
have to be verified experimentally and the actual system
still has to be tested. Research into materials also
plays a major role with these new systems. How reliable
are the materials that you are using? It’s not because the
optimal process calls for a certain material, that this will
be the best choice from the point of view of reliability.
Worldwide, a lot of research is done on very innovative
MEMS. However, not many of these ideas ever become
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commercially available. One reason is that researchers
don’t always realize that companies will want to use
their R&D in real products. So they don’t take reliability
into account when they design and process the MEMS.
Their devices may show a first promising behavior, but
they often are not reliable. Turning them into a reliable
product may require a lot of extra funding and time. In
many cases this could have been avoided by including
reliability issues from the start, for example through a
better upfront choice of materials and designs.
The growing integration also results in a growing number
of failure mechanisms. But to predict if and when these
will cause failures, or how you can test for them, you
must know the underlying physics. Take for example
MEMS or NEMS, which have moving micro- or nanoparts.
These mechanical parts can suffer from failure
mechanisms such as creep, stiction, fatigue, cracking
etc. which are not an issue in conventional ICs. Moreover,
these systems must function under circumstances
that are not the same as for standard ICs. A MEMS resonator
for example must function in a vacuum. To study
the behavior under these circumstances, we have to set
up new optical, thermal, mechanical and electrical tests.
Reliability research is more and more application-
oriented. A failure of an accelerometer in a step counter
is not life-threatening. However, if the accelerometer is
used to decide when to deploy airbags in a car, it’s
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