Page 84 - MEMS Mechanical Sensors

4.4 MEMS Mechanical Sensor Packaging 73

Header cap

Wirebond
Pressure sensor die

TO header Die attach
Leadout

Pressure port
Figure 4.9 Basic packaging scheme.

4.4.3.1 Basic First Order Packaging Stage
A basic yet typical first order packaging arrangements is shown in Figure 4.10. The
support chip used in Figure 4.10 can be fabricated from either thermally matched
lead borosilicate glass, such as Pyrex 7740 or Schott Borofloat 33, or silicon itself.
The glass constraint is typically anodically bonded to the silicon chip, providing and
extremely strong molecular bond. This bond can be performed at wafer level, ena-
bling all devices to be simultaneously mounted. If the glass constraints are not
exactly matched to the silicon, some thermally induced stresses will occur because
of the TEC mismatch. This drawback is exaggerated by the anodic bond, which is
carried out at temperatures of around 400°C. As the bonded assembly cools, resid-
ual stresses will be inevitably introduced across the sensor chip. Thermal matching
between the sensors and constraint will naturally be improved if the constraint is
made from silicon [36, 37].
Another factor that should be considered in certain applications is that the
presence of the support chip can alter the sensitivity of the sensor to the measurand.
In the case of high-pressure sensors, for example, the pressure will not only be

TO header

Pressure sensor die

Glass or silicon Die attach
support chip
Leadout

Pressure port
Figure 4.10 Basic first order pressure sensor packaging.