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By Roland Bernard and Hisanori Kambara, Alcatel Vacuum Technology, Annecy, France; and Arnaud Favre, INOPRO, Grenoble, France
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The front-opening unified pod (FOUP)
is a 300mm wafer handling and contamination
control device, used in the
semiconductor manufacturing industry. The
pods are used to transport wafers between
processes, and care must be taken when
using the pods to avoid wafer contamination.
For example, the presence of humidity in
the enclosed environment of the pods can
cause a variety of phenomena, such as native
oxide growth, corrosion, and film cracking.
The presence of emitted organic compounds
can lead to degradation of the electrical properties
of integrated circuits fabricated
from the wafer material. Simple intuitive
purging with an inert gas such as nitrogen,
a technique used to keep the pods free of
contamination, does not always maintain
wafer cleanliness compatible with high yields.
At Alcatel, rigorous numerical simulation with
CFD has been used to show how pod purging
can be improved.
The potential inefficiency of purging a
FOUP with an inlet and outlet located on
the bottom of the pod – the classical purge
configuration – has been examined using
FLUENT. The FOUP was initially filled with
ambient air with 40% relative humidity, a
typical value for a wafer fabrication cleanroom.
A transient analysis was done as a
steady flow of nitrogen was introduced
through the inlet. The calculation was continued
until the air mass fraction reached
1% (with 0.4% relative humidity). To test
the effect of the nitrogen gas flow rate, a
range of inlet velocities was considered.
The original humid air mass fraction after
about 5 minutes of purging has taken place
For each case studied, the time required
to reach the above conditions was recorded
and compared to a computed value for
the ideal purge time, defined as the ratio
of the FOUP volume to the volumetric flow
rate. For the base case, using a 2L/min gas
flow rate, results showed that classical purging
requires a factor of three more time than
ideal purging to reach the same condition.
While classical and ideal purging begin with
comparable discharge rates for air, the classical
method drops off considerably after
the initial 25% of air is removed from the
device. The FLUENT results also showed that
with a fully loaded FOUP, air flow between
the wafers was significantly less than that
along the wafer edges. This was attributed
to the fact that the classical inlet position
directs nitrogen upwards toward the base
of the bottom wafer, where it is diverted
to flow around the entire stack. Furthermore,
the narrow spacing between wafers discourages
circulation, and the slightly
denser humid air causes the nitrogen to rise
above, rather than mix with the air.
Comparison of different FOUP nitrogen purge times
To improve the purging performance,
a number of modifications were tested. A
plenum purging system was introduced that
injects purge gas from the sides of the FOUP.
Different purge gases were tested, such as
argon and dry air, and the temperature of
the purging gas was altered to promote natural convection between the wafers. These
and other modifications have reduced the
purge time to a point that is closer to the
ideal case.
Mass fraction of the residual gas using plenum injector purging after a) 1.5 minutes, b) 5 minutes, and c) 22 minutes
This work was supported by the European
MEDEA+ T301 project and by the MINEFI
(French Ministry of Finances & Industry).
The authors also wish to thank ST
Microelectronics and Alcatel Vacuum
Technology for providing the experimental
data and organization.
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