| |
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 semi-conductor
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.
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.
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. 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.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 detailed concentration results in every
location within the FOUP.
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.
Comparison of FLUENT results and data at the center point between the
top two wafers (not the optimal purge results)
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.
|
|
|
