Page 68 - MEMS Mechanical Sensors

CHAPTER 4
Mechanical Sensor Packaging

4.1 Introduction

As with micromachining processes, many MEMS sensor-packaging techniques are
the same as, or derived from, those used in the semiconductor industry. However,
the mechanical requirements for a sensor package are typically much more stringent
than for purely microelectronic devices. Microelectronic packages are often generic
with plastic, ceramic, or metal packages being suitable for the vast majority of IC
applications. For example, small stresses and strains transmitted to a microelectron-
ics die will be tolerable as long as they stay within acceptable limits and do not affect
reliability. In the case of a MEMS physical sensor, however, such stresses and strains
and other undesirable influences must be carefully controlled in order for the device
to function correctly. Failure to do so, even when employing electronic compensa-
tion techniques, will reduce both the sensor performance and long-term stability.
The need to control such external stresses is complicated by the simple fact that
all MEMS sensors designed for physical sensing applications have to interact with
their environment in order to function. The physical measurand must therefore be
coupled to the sensor in a controlled manor that excludes, where possible, other
undesirable influences and cross-sensitivities. In order to achieve this, the design of
the sensor packaging is as important as the design of the sensor itself. The sensor
packaging has a major influence on the performance of the device, especially with
respect to factors such as long-term drift and stability. It is very important that the
packaging of the sensor is considered at the outset and that the package design is
developed in parallel with that of the sensor die itself. This is especially true when
you consider that the cost of the package and its development can often be many
times that of the sensor die.
The packaging of MEMS devices will often be specific to the application being
addressed. Such a packaging solution will therefore involve a design, as well as the
selection of materials and processes suitable for that particular application. Generic
solutions suitable for a range of applications, such as is the case of microelectronic
devices, are limited to simple, low-cost, high-volume MEMS applications. This
chapter briefly describes the technologies developed for the packaging of inte-
grated circuits before discussing the design considerations relating to the packaging
of mechanical sensors. Typical problems encountered, and their potential solu-
tions, are discussed in more detail. Example MEMS packaging solutions are given
throughout the chapter in order to highlight some of the principles involved.

57