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4.4 MEMS Mechanical Sensor Packaging 77

the sensing elements protrude beyond the end of the collar and are in direct contact
with the pressurized media. This low-cost packaging approach provides a good
degree of mechanical isolation, but the drawbacks include the increased size of the
sensor chip and the fact that it is in direct contact with potentially corrosive media.

4.4.3.4 Use of Soft Adhesives
The die mount material and the method of attachment will also have an influence on
the mechanical isolation of the sensor and the level of in-built stresses trapped
within the assembly. The various methods of die attach used in the IC industry and
discussed in Section 4.3.2 are equally applicable to MEMS packaging. Typical
parameters of these processes are shown in Table 4.3. The TECs of the bond
materials, along with common packaging materials, are given in Table 4.4. The
TEC of silicon varies with temperature and is listed against different temperatures in
Table 4.5. The TEC of these materials is of fundamental importance to the MEMS
designer since the stresses arising from TEC mismatches account for the majority of
packaging-induced error.
The use of soft, ductile bond materials in the mounting of the die can provide a
high degree of isolation from undesirable mechanical stresses. These soft adhesives
absorb the stress in a manner similar to the mechanical decoupling structures
described previously. In addition, the lower temperature die attach processes associ-
ated with typical soft adhesives are advantageous since the magnitude of thermally
induced stresses trapped in the final assembly will be reduced. The drawbacks of
soft adhesive typically relate to their bond strength, which is very weak compared to
harder epoxies and especially solder and eutectic bonds. Soft adhesives are not
suited to applications that place the sensor die under shear of tensile stress. Where
harder, stronger bonds have to be used, trapped thermal stresses and the resulting
temperature cross-sensitivity can be minimized by keeping the adhesive film as thin
as possible.
Soft adhesives, such as RTV silicone, must be applied in a controlled thickness
to achieve maximum benefit. Experimental analysis showed that the thermal behav-
ior of the sensor shown in Figure 4.14 was improved by increasing the adhesive
thickness up to 50 µm, but no further improvement was observed beyond this [46].
Glass spheres can be used in the assembly of the sensor to control this thickness, as
shown in Figure 4.15.

Table 4.3 Typical Die Mounting Process and Material Parameters
Attachment Method Adhesion Material Process Temperature Thermal Young’s Modulus
9
2
(°C) Conductivity (10 N/m )
(W/m °C)
Eutectic AuSi (97/3) 400 27.2 87
Solder Pb/Sn 200 35 14
Glass Pb glass 450–800 0.25–2 60
Anodic Pyrex/ Borofloat 33 250–500 1.09 63
Epoxy Epoxy (Ag loaded) 150 1.2 0.2–27
Thermoplastic Thermoplastic 150 3 0.41
RTV silicone RTV silicone 25 0.1 6.9 10 –3
Source: [47].