Page 91 - MEMS Mechanical Sensors

80 Mechanical Sensor Packaging

Table 4.6 Summary of Techniques for Mechanically Isolating the Sensor Chip
Technique Advantages Disadvantages

Pyrex intermediate Simplest first order package Pyrex not exactly matched with
(Figure 4.10) Low cost silicon
Bonded at wafer level Limited machining of Pyrex possible
Suitable for a wide range of Relatively large first order assembly
applications
Soft bond Simple Lower bond strength—reduced
(Figures 4.15 and 4.17) Low cost applications
No modifications to sensor chip Bond material unsuitable for certain
required applications
Can negate the need for, or be used,
with first order packaging
Etched silicon intermediate Exact thermal match Reduced bond area to second order
(Figure 4.12) Bonded at wafer level packaging
Machining of intermediate possible Critical alignment required over
High degree of isolation pressure port
Smaller assembly size
On-chip decoupling Can negate the need for, or be used, Increased chip area—fewer sensors
(Figure 4.11) with first order packaging per wafer
Complicates sensor chip fabrication
Not suitable for many applications
Vertical displacement–glass High level of isolation Labor-intensive assembly
tube (Figure 4.13) Allows simple evacuation of resonator Sensors individually packaged–high
surrounding cost
Large assembly
Lateral displacement Simple Limited applications
(Figure 4.14) Low cost Relatively poor degree of isolation
Can negate the need for, or be used, Increased chip area—fewer sensors
with first order packaging per wafer

4.5 Conclusions

It is clear that the packaging of the sensor is as important as the design of the sensor
in determining the overall performance of the device. This is emphasized further by
the fact that the packaging operation is likely to be more costly than the fabrication
of the sensor itself. Many techniques for packaging microsensors can be taken from
IC packaging techniques. However, microsensor packaging also requires that the
sensor die remains well isolated from any undesirable stresses transmitted through,
or arising from, the packaging while still transmitting the measurand to the sensor.
In order to minimize the total cost of the transducer, the simplest isolation tech-
niques, utilizing wafer level processing, are preferable where possible. Of the basic
isolation techniques, the use of soft adhesives is most promising, especially when
combined with a glass or silicon constraint. It is interesting to note that the more
complex, and more costly, isolating techniques involving machining stress-relieving
structures into the silicon support or the sensor die have not been used in commer-
cially available physical microsensors. The suitability of these isolation techniques
will depend upon particular applications and the various design considerations
involved. Important considerations include the temperature required by the various