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300 MEMS and Microstructures in Aerospace Applications

(CARAMEL) for analyzing the impact of particles on the structural and material
properties of surface-micromachined MEMS. CARAMEL accepts as input a micro-
electromechanical design represented as a layout in Caltech Interchange Format
(CIF), a particulate description, and a process (fabrication) recipe. It performs
process simulation that includes the foreign particle and creates a three-dimensional
representation of the resulting defective microelectromechanical structure. CARA-
MEL then extracts a mesh netlist representation of the defective structure whose
form is compatible with finite-element analysis (FEA) tools. Performing FEA of the
CARAMEL mesh output correlates the contamination of concern to a defective
structure and a faulty behavior. CARAMEL has been used to investigate the impact
13
of particles on electrostatic comb-drive actuated microresonator. This technique is
demonstrated on a resonator as shown in Figure 13.1. Interestingly enough, experi-
ments through CARAMEL reveal that the resonator is susceptible to a variety of
misbehaviors as a result of a single particle contamination. Figure 13.2 shows two
representative defects caused by particles.
Protection of MEMS devices from the environment is an important concern as a
hermetic package significantly increases the long-term reliability of the devices.
Traditional hermetic IC packaging techniques, when applicable, offer protection
from contamination; however, only a subset of devices can be packaged in this
manner. This subset includes accelerometers, which may be packaged with the
hermetic schemes used for ICs. Numerous devices however require interaction with
the environment such as gas detectors, optical switches (requiring optical windows)
and lab-on-chip systems. In this case, while functionality must be maintained,
vulnerabilities must be reduced. MEMS devices, which require free space to
function, may be at particular risk. There are few standardized solutions to this
problem and for the low quantities required by the space industry most solutions
will be customized.

spring fixed
outer beam
beam finger

shuttle mass
inner
beam
anchors
movable
finger

finger gap
FIGURE 13.1 Top view of a surface-micromachined, electrostatic comb-drive actuated
structure that is suspended over the die substrate and is anchored only at the shuttle
movement to a capacitance change between the moveable and fixed potential difference
between the shuttle and fixed fingers, or from an inertial force caused by external acceler-
ation. (Courtesy: CMU S. Blanton.)

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