Page 243 - An Introduction to Microelectromechanical Systems Engineering

222 Packaging and Reliability Considerations for MEMS

Another serious effect of packaging on stress-sensitive sensors is long-term drift
resulting from slow creep in the adhesive or epoxy that attaches the silicon die to
the package housing. Modeling of such effects is extremely difficult, leaving
engineers with the task of constant experimentation to find appropriate solu-
tions. This illustrates the type of “black art” that exists in the packaging of
sensors and actuators, and it’s a reason companies do not disclose their packaging
secrets.

Protective Coatings and Media Isolation
Sensors and actuators coming into intimate contact with external media must be
protected against adverse environmental effects, especially if the devices are subject
to long-term reliability concerns. This is often the case in pressure or flow sensing,
where the medium in contact is other than dry air. For example, sensors for automo-
tive applications must be able to withstand salt water and acid rain pollutants (e.g.,
SO ,NO ). In home appliances (white goods), sensors may be exposed to alkali envi-
x x
ronments due to added detergents in water. Even humidity can cause severe corro-
sion of sensor metallization, especially aluminum.
In many instances of mildly aggressive environments, a thin conformal coating
layer is sufficient protection. A common material for coating pressure sensors is
parylene (poly(p-xylylene) polymers) [2, 3] (see Table 8.1). It is normally deposited
using a near-room-temperature chemical vapor deposition process. The deposited
film is conformal covering the sensor element and exposed electrical wires. It is resis-
tant to automotive exhaust gases, fuel, salt spray, water, alcohol, and many organic
solvents. However, extended exposure to highly acidic or alkali solutions ultimately
results in the failure of the coating.
Recent studies suggest that silicon carbide may prove to be an adequate coating
material to protect MEMS in very harsh environments [4]. Silicon carbide deposited
in a plasma-enhanced chemical vapor deposition (PECVD) system by the pyrolysis
of silane (SiH ) and methane (CH ) at 300ºC proved to be an effective barrier for
4 4
protecting a silicon pressure sensor in a hot potassium hydroxide solution, which is
a highly corrosive chemical and a known etchant of silicon. However, much

Table 8.1 Material Properties for Three Types of Parylene Coatings*
Property Parylene-N Parylene-C Parylene-D
−3
Density (g/cm ) 1.110 1.289 1.418
Tensile modulus (GPa) 2.4 3.2 2.8
Permittivity 2.65 3.15 2.84
Volume resistivity (Ω•cm) at 23ºC, 50% RH 1.2 × 10 17 8.8 × 10 16 1.2 × 10 17
Refractive index 1.661 1.639 1.669
Melting point (ºC) 410 290 380
Coefficient of expansion (10 −6 /K) 69 35 <80
Thermal conductivity (W/m•K) 0.12 0.082 —
Maximum water absorption (%) 0.01 0.06 <0.1
Gas permeability (amol/Pa•s•m)
N 2 15.4 2.1 9.0
CO 2 429.0 15.4 26.0
SO 2 3,790.0 22.0 9.53
*They are stable at cryogenic temperatures to over 125ºC [2].