Freescale Semiconductor, Inc.
PIEZORESISTIVE
TRANSDUCER
DIAPHRAGM
SILICON DIE
UNIBODY PACKAGE
DIE ATTACH
LEAD FRAME
WIREBOND
NITRIC
Figure 3. Examples of uniform corrosion of a gold leadframe in nitric acid at 5 Vdc and galvanic corrosion on an
unbiased device at the gold wire/aluminum bondpad interface in commercial detergent.
Part of figure 3 shows an example of what we have
described as electrolytic corrosion (i.e., corrosion of similar
metallic surfaces in an electrolytic solution caused by a
sufficient difference in potential between the two surfaces).
This appears to be uniform corrosion of the gold leadframe
surface. It should be noted that this type of failure is observed
evenon‘noble’metalslikegold. Appliedpotentialisthedriving
force for the reaction. All metals can corrode in this fashion
depending on the solution concentration (pH) and the applied
potential. Pourbaix diagrams describe these thermodynamic
relationships [18].
inverse square of the silicon thickness. Moreover, it can lead
to loss in bond integrity between wafers (Fig. 4). Silicon
etching [19–20], like corrosion reactions, is a chemical
reaction, so the contributing factors include the silicon
material, its crystal orientation and its doping level, the
solution type, concentration and pH, and the appliedpotential.
Temperature, concentration (i.e., pH), and voltage all act to
accelerate this process. Figure 5 shows an example of
modeling results that illustrates two of these variables.
Figure 3 shows an example of galvanic corrosion. The
figure illustrates that corrosion can also occur because of
dissimilar metals that are connected electrically and are
immersed in an electrolytic solutions. A difference in the
corrosion potential between the two metals is the driving force
for the reaction. Localized corrosion examples are prevalent
as well. Often they may be the precursor to what appears on
a macro scale to be uniform or galvanic corrosion. In situ
monitoring of devices in electrolytic media will allow better
diagnosis of this failure mechanism. Typical ex situ or interval
reliability testing may not allow diagnosis of the root cause to
the failure, thus limiting the predictive power of any resulting
reliability models.
Silicon Etching
Figure 4. Photograph of silicon etching after
exposure to an aqueous detergent solution at
elevated temperature for an extended time. A frit layer,
horizontally in the middle, adheres to silicon on
either side. The amount of etching is evident by
referencing the glass frit edge on the far left.
These two silicon edges were aligned to the frit
edge when the die was sawn.
Figure 4 shows the result of an accelerated test of a
pressure sensor die to a high temperature detergent solution.
Thedetergentusedwasamajorconsumerbrandandresulted
in dramatic etching of the silicon. Alkaline solutions that
undergo a hydrolysis reaction may result in etching of the
silicon similar to a bulk micromaching operation. This failure
mechanism can cause a permanent change in the sensitivity
of the device because the sensitivity is proportional to the
Motorola Sensor Device Data
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