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ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 18 NO. 2
OBSERVATION OF ALUMINUM
STRESS CRACKS
Very small cracks in aluminum are not detectable by
optical or scanning electron microscopy while the pas-
sivation is intact. Figures 1 and 2 show typical aluminum
lines after passivation removal. Figure 1 is anoptical photo
showing wedgelike voids at the metal edge. Wedgelike
voids do not cause failure, but they are characteristic
of stress voiding. Material that was previously in the
void has been pulled into the remaining aluminum on
either side. Stress in the aluminum is therefore lessened.
Figure 2 shows a cracklike void totally across ametal line.
Obviously, such cracks cause the line to be electrically
open. Cracks may occur most frequently at oxide steps,
where stress is enhanced. Cracks may also be where the
strength of a line is compromised by amaterial weakness
such as a silicon nodule. Such precipitates effectively
reduce the cross section of aluminum, therefore reducing
its strength. However, cracks can occur anywhere along
a line.
[5]
SUMMARY OF STRESS VOID
SYMPTOMS
Stress voiding is most prevalent in aluminum lines
less than 3 µmwide. Voids may be coincident with silicon
nodules or oxide steps. However, voids can occur any-
where without discontinuities that concentrate stress or
weaken line strength. Longer lines also tend to be more
“SILICON NITRIDE PASSIVATION IS
HARDER THAN PHOSPHORUS-DOPED
GLASS AND THEREFORE CREATES MORE
STRESS IN UNDERLYING ALUMINUM. ”
susceptible to stress voiding, but this is not a necessary
condition.
Void failures may occur immediately after fabrication
andcontinue tooccurwith time. Failure ratesmay increase
withmoderate temperature increase. However, failure rate
decreases for very high temperature, because the driving
tensile stress is relieved asmetal temperature approaches
deposition temperature. Note that this is a countercondi-
tion to most other reliability mechanisms where heating
is used as an accelerating factor. Failures are also found
in high-temperature operating life, but nominal current
and voltage conditions are not a factor.
Although aluminum is a soft metal, it is also a brittle
metal. Additives to aluminumsuch as silicon and nitrogen
make aluminum films harder and more brittle. Severity
of stress voiding is increased for the more brittle, less
ductile metal.
Passivation is as important as the metal itself in the
stress voidmechanism. Greater compressive stress in the
passivation increases the severity of stress voiding. Silicon
nitridepassivation is harder thanphosphorus-dopedglass
and therefore createsmore stress inunderlyingaluminum.
FAILURE ANALYSIS
For the open failures described, defect sites were elec-
trically isolated. Cracks associated with failure became
Fig. 1
Wedgelike voids. Source: Ref 3
Fig. 2
Cracklike void. Source: Ref 4