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BURN-IN, RELIABILITY TESTING, AND
MANUFACTURING OFSEMICONDUCTORS
Prepared By Cagatay Bozturk
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Index
What is BURN-IN?
Reliability of Semiconductors
What is Reliability&Life Testing
Semiconductor Manufacturing
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What is BURN-IN?
Burn-In is the application of thermal and
electrical stress for the purposes of inducing the
failure of "marginal (microelectronic) devices,
those with inherent defects or defects resultingfrom manufacturing aberrations which cause
time and stress dependant failures.
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Reliability of Semiconductors
To evaluate the reliability of an electronic system,
reliability information on the components used in that
system is important. Failure rates are often used as an
index for reliability. A failure rate indicates how often a
failure occurs per unit time, and failure-rate valuesgenerally change overtime as shown below
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Early Failure Stage
During this stage, failures occur at a high ratefollowing the initial operation of semiconductordevices. They occur very soon and thus the failurerate declines rapidly over time. This Is because thepotential' failures that could not be removed through
a selective process are included and surface in ashort time if a stress such as temperature or voltageis applied after use of the device is started. In thecase of semiconductors, these failures are usuallydue to defects that could not be removed duringproduction, such an micro dust collecting on the
wafer, or to material defects.
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Random Failure Stage
When early failures are eliminated, the
failure rate drops to an extremely low
value. However, there is always the
possibility of a potential failure accidentallyoccurring after a long time. Consequently
the failure rate never decreases to zero. It
is almost constant because failures occur
sporadically
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Wear-out Failure Stage
During this stage, failures occur with
increasing frequency over time and are
caused by age-related wear and fatigue. In
the case of a semiconductor device,electronic migration or oxide film
destruction (TDDB) may occur.
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What is Reliability & Life Testing?
The reliability of a semiconductor device is
determined by its ability to perform its
required functions under the stipulated
conditions for a finite period of time.Quantifiable yardsticks such as the
reliability rate, failure rate, and mean time
to failure (MTTF) are used to measure
reliability.
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Objective of Reliability Testing
The objective of reliability testing is to
confirm a semiconductor device's fault-free
operation and estimated useful life by
exposing the device to accelerated ormarginal stress, based on the amount of
stress (thermal stress, mechanical stress,
electrical stress etc) that the device is
estimated to undergo during manufacture,
shipping and normal use.
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Semiconductor Manufacturing
Semiconductor manufacturingconsists of
the following steps:
1) production of silicon wafers from verypure silicon ingots
2) fabrication of integrated circuits ontothese wafers
3) assembly of every integrated circuit onthe wafer into a finished product
4) testing and back-end processing of thefinished products.
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Wafer Fabrication
Wafer fabrication generally refers to the process of
building integrated circuits on silicon wafers. Priorto wafer fabrication, the raw silicon wafers to beused for this purpose are first produced from verypure silicon ingots, through either the Czochralski(CZ) or the Float Zone (FZ) method. The ingots areshaped then sliced into thin wafers through aprocess called wafering.
Wafer fab processes, allowing the device designerto optimize his design by selecting the best fab
process for his device.
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Assembly
The process of putting the integratedcircuit inside a package to make itreliable and convenient to use is knownas semiconductor package assembly, or
simply 'assembly'.In general, an assembly process wouldconsist of the following steps:
1. die preparation
2. die attach3. bonding
4. encapsulation
http://www.semiconfareast.com/dieprep.htmhttp://www.semiconfareast.com/dieattach.htmhttp://www.semiconfareast.com/wirebond.htmhttp://www.semiconfareast.com/molding.htmhttp://www.semiconfareast.com/molding.htmhttp://www.semiconfareast.com/wirebond.htmhttp://www.semiconfareast.com/dieattach.htmhttp://www.semiconfareast.com/dieprep.htm -
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Steps of Assembly
die preparation, which cuts the wafer intoindividual integrated circuits or dice;
die attach, which attaches the die to thesupport structure (e.g., the leadframe) ofthe package;
bonding,which connects the circuit to theelectrical extremities of the package,thereby allowing the circuit to beconnected to the outside world; and
encapsulation(usually by plastic molding),which provides 'body' to the package ofthe circuit for physical and chemicalprotection.
http://www.semiconfareast.com/dieprep.htmhttp://www.semiconfareast.com/dieattach.htmhttp://www.semiconfareast.com/wirebond.htmhttp://www.semiconfareast.com/molding.htmhttp://www.semiconfareast.com/molding.htmhttp://www.semiconfareast.com/wirebond.htmhttp://www.semiconfareast.com/dieattach.htmhttp://www.semiconfareast.com/dieprep.htm -
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Test
Once assembled, the integrated circuit is
ready to use. However, owing to the
imperfection of this world, assembled
devices don't always work. Many things
can go wrong to make a device fail, e.g.,
the die has wafer fab-related defects, or
the die cracked during assembly, or the
bonds were poorly connected or not
connected at all. Thus, prior to shipment tothe customer, assembled devices must first
be electrically tested.
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Electrical Testing
Electrical testingof devices in big volumesmust be done fast and inexpensively.
Mass-production electrical testing thereforerequires an automated system for doing
the test. Equipment used to test devicesare called, well, testers, and equipmentused to handle the devices whileundergoing testing are called, well,handlers. Tester/handler systems are alsoknown as automatic test equipment (ATE).
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BURN-IN
Burn-inis an electrical stress test thatemploys voltage and temperature toaccelerate the electrical failure of adevice. Burn-in essentially simulates the
operating life of the device, since theelectrical excitation applied during burn-inmay mirror the worst-case bias that thedevice will be subjected to in the course ofits useable life. Depending on the burn-in
duration used, the reliability informationobtained may pertain to the device's earlylife or its wear-out.
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BURN-IN
Burn-in is usually done at 125deg C, with electricalexcitation applied to thesamples. The burn-in processis facilitated by using burn-inboards (see Fig. 1) where the
samples are loaded. Theseburn-in boards are theninserted into the burn-in oven(see Fig. 2), which suppliesthe necessary voltages to thesamples while maintaining theoven temperature at 125 degC. The electrical bias appliedmay either be static ordynamic, depending on thefailure mechanism beingaccelerated.
Figure 1
Photo of Bare and Socket-populated Burn-in Boards
Figure 2
Two examples of burn-in
ovens
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Conclusion
Burn-in helps us to detect problem trends /determine critical components in a systemfailure (s), and, analyze the system forEffective reliability. Thanks to burn-in, we
can predict reliability performance and Life-cycle of the products. Itprovides valid fieldfailure data and timely corrected actions.
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References
http://www.semiconfareast.com
http://www.reltech.co.uk