Page 242 - An Introduction to Microelectromechanical Systems Engineering

Key Design and Packaging Considerations 221

an area of several square millimeters, the role of thermal management is to ensure
long-term thermal stability of the piezoresistive sense elements by verifying that no
thermal gradients arise within the membrane. The situation becomes more compli-
cated if any heat-dissipating elements are positioned on very thin membranes,
increasing the effective thermal resistance to the substrate and the corresponding
likelihood of temperature fluctuations. Under some circumstances, maintaining an
element at a constant temperature above ambient brings performance benefits. One
example is the mass-flow sensor from Honeywell (see Chapter 4).
Thermal management at the package level must take into account all of the ther-
mal considerations of the die level. In the case of the mass-flow sensor, it is impera-
tive that the packaging does not interfere with the die-level thermal isolation
scheme. In the example of the infrared imager also from Honeywell (see Chapter 5),
the package housing needs to hold a permanent vacuum to eliminate convective
heat loss from the suspended sensing pixels.
Thermal actuators can dissipate significant power. It can take a few watts for a
thermal actuator to deliver a force of 100 mN with a displacement of 100 µm. With
efficiencies typically below 0.1%, most of the power is dissipated as heat that must
be removed through the substrate and package housing. In this case, thermal
management shares many similarities with the thermal management of electronic
integrated circuits. This is a topic that is thoroughly studied and discussed in the
literature [1].
Metals and some ceramics make excellent candidate materials for the package
housing because of their high thermal conductivity. To ensure unimpeded heat
flow from the die to the housing, it is necessary to select a die-attach material
that does not exhibit a low thermal conductivity. This may exclude silicones
and epoxies and instead favor solder-attach methods or silver-filled epoxies,
polyimides, or glasses. A subsequent section in this chapter explores various
die-attach techniques. Naturally, a comprehensive thermal analysis should take
into account all mechanisms of heat loss, including loss to fluid in direct contact
with the actuator.

Stress Isolation
The previous chapters described the usefulness of piezoresistivity and piezo-
electricity to micromachined sensors. By definition, such devices rely on converting
mechanical stress to electrical energy. It is then imperative that the piezoresistive or
piezoelectric elements are not subject to mechanical stress of undesirable origin and
extrinsic to the parameter that needs to be sensed. For example, a piezoresistive
pressure sensor gives an incorrect pressure measurement if the package housing sub-
jects the silicon die to stresses. These stresses need only be minute to have a cata-
strophic effect because the piezoresistive elements are extremely sensitive to stress.
Consequently, sensor manufacturers take extreme precautions in the design and
implementation of packaging. The manufacture of silicon pressure sensors, espe-
cially those designed to sense low pressures (<100 kPa), includes the anodic bond-
ing of a thick (>1 mm) Pyrex glass substrate with a coefficient of thermal expansion
matched to that of silicon. The glass improves the sensor’s mechanical rigidity and
ensures that any stresses between the sensor and the package housing are isolated
from the silicon piezoresistors.