Challenges Of 300-mm Wafer Fabs

Cleanroom engineers are now tasked to uncover more innovative products and design solutions to meet the increasingly fragile and precise nature of the newer 300-mm wafer fabs.

While some companies are maintaining a cautious approach and continuing to use the gel seal grid systems with 100% filtration coverage there has been a surge in the use of heavy duty gasket grid systems using a mixed flow design.

• The traditional gel grid approach which is usually associated with a pressurized plenum system or fan filter modules minimizes design change and is therefore a relatively low risk solution. The cost however is prohibiting due to the extensive air delivery system (full coverage filters, AHU’s, chilled water, etc.).
  
• The conventional gasket grid approach to air flow in the newer 300-mm fabs utilizes less filter coverage (25-30%) with a concentration of filtration in some of the more critical areas. By introducing turbulence inducing devices down stream of the filter media you can improve the classification by mixing the air to create a “Turbulent Flow”.

With a handful of 300-mm production lines up and running, contamination control data is starting to emerge, pointing out sources of contamination within the tools, the infrastructure and the silicon wafer itself.

To address this concern, several media types have been developed to help minimize the risk of Boron or other AMC (air born molecular contamination) from being released into the cleanroom. Some of the most prevalent for 300-mm applications are the “Low Boron” wet resistant micro fiberglass medias or the “Non Boron” PTFE (Polytetrafluororethylene) medias, which possess a far greater resistance to a range of inorganic and organic chemicals. Because of their resilience, the life span of the filter media is greatly increased offering a lower cost of ownership.

When selecting the right filtration system, cost of ownership has to be a consideration. The recent introduction of (EEF) electrically enhanced filtration systems is one of the approaches being considered to reduce operational costs of running a 300-mm fab. The EEF when used as a primary filter in a series of terminal end filters can significantly lower the pressure drop through the media and thus lower the overall energy consumption of the room. This may also be placed down stream of the AC unit for use with fan-powered filter modules in a membrane type ceiling to accomplish better airflow distribution.

In addition to higher end filtration systems, the control of ionization, pressure, temperature, and vibration isolation are also becoming a driving force behind the new Cleanroom architectural systems. Some of the different criteria driving the acceptance of these products include:

• Gasketed grid systems that can support higher imposed loads and that can be reconfigurable to accommodate integral AMHS hanging systems.
  
• Walls that require a portion of the panel to remain fixed to accommodate permanent utility penetrations while maintaining its non-progressive, demountable capabilities to support tool relocations.
  
• Flooring pedestals that permit greater air-pad loads (8,000 lbs. on a 24” diameter bearing) and panels that are less susceptible to impact loading failures.

Contamination Concerns

Two of the major contamination control issues’ facing the next generation facility are oxidation, which occurs at the interface layers between wafer process steps, and contamination of wafers as they sit in the stockers in the cleanroom. To prevent this, oxidation–sensitive processes need to be maintained within an extremely dry-air environment to eliminate all moisture and hydrocarbon formation. Then the automated material handling systems can transport the wafers to and from the ultraclean process stations within a standard cleanroom environment. To eliminate contamination in the stockers, they need to be washed continuously with “fresh’ dry, highly-filtered air, separate from the highly moist and hydrocarbon filled air inside the cleanroom. As the next level of contamination control requirements draw nearer, people will not even be able to survive in the actual clean environment because of the need to remove all of the moisture and potentially all oxygen from the area.

Mini-Environments

As the need for tighter environmental controls comes into place, it has become essential for the Tool Builder and the Mini-Environment supplier to form a partnership. Mini-environments are in fact the inevitable technological approach of the future. The space around the process equipment has already shrunk down to front end access “bulkhead” methodology. The mini-environment is designed to reduce particle deposition by incorporating the following:

• Highly efficient filtration methods; ULPA or GIGA filters.
• Calculated airflow patterns to prevent particle generation upstream of the wafer.
• Pressurization to meet a minimum +0.003 inches w.c. relative to the ambient environment
• Static-dissipative material properties and air ionization to reduce static build-up and prevent static discharge.

In addition, the use of mini-environments makes it possible to cut back on the air velocity in the cleanroom, therefore reducing the possibility of ESD build-up. Because of the instability of the cleanroom fab environment, it is hard to maintain consistency from site to site. Overall, the integration of mini-environments with 300-mm equipment will improve the process by providing better control of the environmental parameters and by providing repeatability in the process results.

Transportation Concerns

One area where contamination control data is emerging is in the transportation devices designed to carry the wafers, such as the AMHS and the FOUP. The FOUP although designed to protect the wafers from contamination are themselves a vehicle of concern because they are usually made up of polycarbonate plastic which outgases. Coupled with the fact that the concentration of AMC is greatly magnified due to a decrease in air volume, the potential for wafer molecular contamination is greatly increased.

In summary, this article touches on various challenges facing the process engineers and contamination control experts of the microelectronics industry today. Overall, I believe the entire manufacturing process will be less dependent on the cleanroom environment due to the increased use of mini-environments around critical processes. Although several full-blown 300-mm production lines are already in place, the contamination control data is still limited. With geometries getting even smaller, it will be even more important to use qualified cleanroom engineers in the planning stages of the next generation fabs.

References:

Anderson, Chris, “Targeting AMC”, Cleanrooms, August 2002.
Jaisinghani, R. “New Ways of Thinking About Air Handling” CleanRooms, January 2001.
Hogan, H., “300-mm Theory and Practice”, CleanRooms, March 2000.
Tannous, A.G. and K.H. Compton, “Studies Conclude Low Air Velocity Increases Effectiveness of Minienvironment Design,” Cleanrooms, March 1998.
Liu, B.Y.H. and Yoo, S.H. “Isolation Ratio and Particle Performance Measurement of SMIF System”.
MacGibbon, B. And Cleary, T. “Cleanliness Performance in a Dry-in/Dry-out CMP Tool”. Proc. Symposium on Contamination Free Manufacturing for Semiconductor Processing, SEMI, 1998.
Haystead, John, “Japan – Tackling the Challenges of Today and Tomorrow”, CleanRooms, August 1996.

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