MIL-PRF-38535K
APPENDIX H
H.3.2.2.2.1 TCV certification. For initial reliability assessment, sufficient TCV test structures for each wear-out mechanism
should be subjected to accelerated aging experiments. The TCV test structures should be randomly chosen from and evenly
distributed from three homogeneous wafer lots in the technology to be certified in the fabrication facility to be certified. These
wafers shall have passed the wafer or wafer lot acceptance requirements. The accelerated aging experiments should produce
an estimate of the mean time to failure (MTTF) and a distribution of the failure times under worst case operating conditions and
circuit layout consistent with the design rules for each wear-out mechanism. From the MTTF and distribution of failures a worst
case operating lifetime or a worst case failure rate can be predicted. Test structures should be from completed wafers which
have been passivated/ glassivated. A summary of the accelerated aging data and analysis should be available for review by
the qualifying activity. The initial certification MTTF, failure distribution and acceleration factors should be used as benchmarks
for the technology to which subsequent TCV results shall be compared. This includes evaluation of activation energy,
acceleration factors based on voltage and temperature for the technology, long term reliability and known failure mechanism
(FMEA) and mitigation strategies. The current density and temperature acceleration factors for electromigration should be
determined and a MTTF and failure distribution determined for the worst case current, temperature, and layout geometry
allowed in the technology. From the MTTF and failure distribution, a failure rate for electromigration in the technology should be
calculated.
All of the TCV test structures shall use the same packaging materials and assembly procedures as standard circuits in the
technology. The TCV structures need not use a fully qualified package since qualified packages shall tend to have lead counts
far in excess of those needed for intrinsic reliability studies. The packaging requirement for the TCV may be waived by the
qualifying activity if the manufacturer can supply documentation showing the equivalence of wafer level and packaged
accelerated aging results.
NOTE: In those cases where this may not be possible, the TCV should use a suitable package to allow for the evaluation of the
chip technology to be qualified, without adversely affecting the outcome of the test.
An example of the requirements of packages for TCV test structure concerns the hydrogen content of a ceramic package and
its effect on hot carrier aging, and can differ substantially for packaged and non-packaged devices. The minimum requirements
to be addressed for the TCV structures for specific mechanisms are given below.
a. Hot carrier aging. The TCV should use structures that monitor hot carrier aging applicable to the technology to be
used in QML microcircuits. Device degradation is to be characterized in terms of both linear transconductance (gm)
and threshold voltage (VTH) and the resistance to hot carrier aging is to be based on whichever parameter
experiences the manufacturers' specified degradation limit for the minimum channel length and width allowed in the
technology. A wafer level fast-test screen should be established for technologies that are susceptible to hot carrier
aging. This test should be part of the wafer acceptance criteria.
(1) Metal oxide semiconductor (MOS). The TCV should have structures to characterize the effects of hot carrier
aging as a function of channel length for MOS transistors for each of the nominal threshold voltages used in the
technology. Degradation should be characterized in terms of gm and VTH
.
(2) Bipolar. The TCV should contain structures for characterizing hot carrier aging of diodes in bipolar technologies.
b. Electromigration. The TCV should contain structures for the worst case characterization of metal electromigration
over:
(1) Flat surfaces.
(2) Worst case noncontact topography.
(3) Through contacts between conductive layers.
(4) Contacts to the substrate.
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ACTEL [ Actel Corporation ]
