An important challenge in achieving
successful commercial MEMS products is associated with MEMS reliability.
Reliability and qualification can be much more complex than for Integrated
Circuits. Many of the MEMS failure mechanisms are not well understood.
This lack of understanding presents a challenge in developing practical
qualification techniques for MEMS products.
For the world
of integrated circuits, there are industry standard tools and techniques for
understanding and quantifying reliability. For the world of MEMS, this
knowledge base is much more limited. In many cases, companies that do
have a firm grip on techniques for quantifying reliability view that
knowledge as a competitive advantage, and are hesitant to share it.
In order to develop reliable MEMS devices, reliability
must be considered at the earliest stages of product development. Decisions
made in the design stage can result in devices that will never be reliable.
Reliability must be understood at a fundamental physical and statistical
There is often a perspective that by there very nature
MEMS will be unreliable because they have moving parts. The truth is
that it is not moving parts that kill reliability, but rubbing surfaces.
MEMS can be designed with moving surfaces, but no rubbing parts, and can be
Avoiding rubbing surfaces is one of the key elements in
achieving reliable MEMS devices. A second primary issue affecting
reliability is the issue of packaging. Again, it is useful to consider
the case of integrated circuits. Integrated circuits are known for
their reliability. It should be noted, that integrated circuits are
packaged in such a way as to protect the sensitive transistors on the
surface of the chip from the environment. The chips are typically
packaged in a hermetic environment, or are potted to protect the devices.
For the case of MEMS, some devices by their very nature require them to be
exposed to the environment, creating a reliability challenge.
The types of MEMS devices that are the most reliable, and
the easiest to qualify are devices that can be packaged in such a way as to
protect them from the environment. An excellent example of this is the
case of optical MEMS devices. These devices can be packaged in a
traditional ceramic package, with a glass lid. The glass lid,
hermetically attached to the ceramic package creates a "safe" environment
for the sensitive MEMS chip, while still allowing photons to interact with
the MEMS device.
The data presented below demonstrates the high level of
reliability that can be achieved when MEMS are designed with no rubbing
surfaces, and are packaged properly. The
This chart represents 300 MEMS mirrors under test at 70 degrees C.
It can be seen that there have been no failures with the total
accumulated device cycles in excess of a half trillion.