Wind Turbine

semiconductor
Optimization of Vapor Purging
in Wafer Isolation Pods
by Keyvan Keyhani and Sameer Abu-Zaid, Asyst Technologies, Inc., Fremont, CA
Asyst Technologies, Inc. is the
world’s leading provider of
environmental and automated
work-in-progress material
management systems for the semiconductor
manufacturing industry.
For the fabrication of integrated circuits,
Asyst’s automated wafer isolation
solutions enable the safe and
rapid transfer of wafers between
process equipment and the fabrication
line, thus increasing production
yield and reducing operating
expenses. At Asyst, CFD analysis is
used for design optimization during
product development, performance
verification of existing
systems, and troubleshooting of contamination
problems. CFD has
proven to be a valuable tool in the
design and analysis of a broad range
of environmental isolation systems.
As an example of the value of
CFD analysis at Asyst, FLUENT has
been used to optimize nitrogen purg-
Geometry of the CFD model. The front surfaces of the FOUP
have been removed to display the wafers. The inlet and
outlet ports are shown on the bottom in blue and green,
respectively.
ing of a 300mm front opening unified
pod (FOUP). Vapors inside FOUPs
can damage wafers during transport,
storage, and queuing between
processes. For example, moisture
can cause native oxide growth, corrosion,
and cracking of films, and
contamination by various organic
compounds can degrade the electrical
properties of integrated circuits.
Purging with an inert gas, such
as nitrogen, is a method of removing
harmful vapors from FOUPs.
To determine optimal purging
methods, a CFD model of the system
was developed. A FOUP
geometry filled with 25 wafers was
first constructed using Pro/E, and
the model was imported into
GAMBIT for meshing. The FOUP was
initially set to contain air (with 20.7%
oxygen). Pure nitrogen was then
injected through inlet ports on the
bottom of the FOUP, and the transient
change in vapor concentration
was computed. Various injection
and exhaust methods were simulated
using the same total amount
of nitrogen for all cases, to determine
the fastest rate of oxygen
removal in all regions of the FOUP.
Examination of a series of oxygen
contour plots on the center
plane of the FOUP show that the
average concentration of oxygen
drops rapidly over time, and that
regions between the wafers can be
effectively purged within an acceptable
time period. Plots of oxygen
concentration vs. time between two
wafers show good agreement
between FLUENT predictions and
experiment. Using CFD as a predictive
tool for purging optimization
is less expensive than
experimentation and provides
Contours of mass percent of oxygen on a plane through the
wafer centers after 40s of purging (not the optimal purge
results). The FOUP initially has 20.7% oxygen (red) in every
region.
% oxygen
100
10
1
0.1
0.01
0 40 80
FLUENT
Measurement
120 160 200 240
time (s)
Comparison of FLUENT results and data at the center point
between the top two wafers (not the optimal purge results)
detailed concentration results in every
location within the FOUP.
Work is ongoing at Asyst to further
improve the purging of FOUPs
using FLUENT simulations. Asyst is
also presently using FLUENT for
design optimization of the next generation
of ultra-clean mini-environments
for automated 300 mm
wafer handling. ■
materials processing
Fluent NEWS spring 2002
S7