Method, device and system for semiconductor wafer transfer

Details Method, device and system for semiconductor wafer transfer Method, device and system for semiconductor wafer transfer

2002-09-10

2004-11-16

Bidwell, James R. (Department: 3651)

Material or article handling

Apparatus for moving material between zones having different...

C414S416080, C414S938000

Reexamination Certificate

active

06817823

ABSTRACT:

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not Applicable
SEQUENCE LISTING OR PROGRAM
Not Applicable
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to a method, device and system for semiconductor wafer transfer in an automated wafer transfer system (WTS) for wafer processing.
2. Description of the Related Art
The semiconductor industry continues a predictable trend toward higher densities of the features within integrated circuits. From the 10-micron range in 1970, feature dimensions of 0.13-micron are now common. These extremely high densities have driven the need for extremely clean processes within semiconductor fabrication facilities. The concern for contamination has been only one force driving the automation of processes and the removal of human operators from the processing area.
As the features within devices have become smaller, the devices themselves have grown larger to accommodate greater functional complexity. With that has come a growth in the size of wafers. Larger wafers allow for more devices with a smaller percentage of devices being lost at the circumference of the wafer. As more devices are produced with fewer wafers being handled, the manufacturing cost of each device can be reduced.
When the first generation of microcomputer chips reached production volumes, they were being fabricated on 2-inch diameter wafers. In 1994 sales of 200-mm (8-inch) diameter wafers began and those involved in industry standards agreed that 300-mm (12-inch) would be the next step in size. In 1998, the first robots for handling 300-mm wafers came into being, although hardware standards for wafer carriers, load ports and other now common component sub-assemblies of Automated Material Handling Systems would not be published for another two years. The industry has now settled on transport carriers containing 25 of the 300-mm wafers. This means that a full carrier weighs significantly more than humans can carry safely and reliably. Also, considering that a single finished wafer may be valued at more than $200,000, extreme care is required in the handling of wafers to avoid even the slightest damage, furthering the need for robotic assistance. Not many human operators can be trusted to repetitiously lift a heavy carrier containing $5-million of product at the end of a long work shift, but this is of lesser concern for a tireless robot.
Given these and other considerations, a high level of automation is employed for handling semiconductor wafers during fabrication. The processing environment is commonly arranged as a wafer transfer system (WTS) that supplies wafers to a process section, often referred to as a bench, having its own separate process automation and wafer handling.
As the semiconductor industry has developed into a commodity market with high volumes and low margins, it has become extremely important to create and adopt standards wherever possible. One such standard, developed specifically for use with 300-mm wafers, is a transport carrier and storage device known as a Front Opening Unified Pod, or FOUP. Containing as many as twenty-five 300-mm wafers, a FOUP is a sealed case with a locked panel. The panel can be removed by automation at the WTS for extraction of the wafers while supporting the wafers in an ultra clean environment.
Previous generations of wafer carriers for the smaller (four, six and eight inch) wafers are known as Standard Mechanical Interface (SMIF) pods. In a SMIF pod, the wafers are transported on edge, in a wafer-vertical orientation. This facilitates transfer of wafers between tools since processing applications require that the WTS position the wafers vertically for hand-off to a process bench. Though cassettes of 300-mm wafers are preferably transferred to the bench automation in a wafer-vertical configuration, a FOUP holds wafers in a horizontal orientation, requiring that the wafers be tilted before being presented to the process bench. Once inside a clean chamber, wafers have historically been transferred in open containers, such as cassettes, using various transfer systems.
Because of the high capital cost associated with a wafer fabrication facility, two other considerations come into play. One is the cost per square foot, particularly of the floorspace that lies within cleanrooms. The other is the productivity of that floorspace. Productivity alone is usually measured as throughput in units of wafers per hour (wph), while together the two issues can be quantified as wafers per hour per square-foot. Typically, semiconductor device manufacturers prohibit increases in equipment footprint unless there is a proportional increase in wafer processing throughput.
Considerations for Improvement over the Related Art
Much of the prior art related to the movement of semiconductor wafers focuses on transfer of the wafers from one processing station to the next within the process bench. In many such systems, wafers are loaded into the system one at a time, through an air-locked slit to reduce the chance of introducing contaminants to the bench. Other systems accept a transport carrier, such as a SMIF or FOUP, mounted to an air-locked panel, from which wafers are removed, one or more at a time, for processing.
It is important that the WTS automation be fast and reliable so that the bench has a constant supply of wafers readily available for processing. Typically, the industry has seen WTS throughput in the range of 200-300 wph. This means that the WTS in such a scenario can supply the bench with eight to twelve 25-wafer cassettes every hour.
In order to increase throughput rates, equipment manufacturers have added a buffer to the wafer handling tool. The buffer serves as a storage area for wafers until they are ready to be processed or removed from the tool. This local stock of wafers reduces the risk of the processing system running idle for lack of wafers. U.S. Pat. No. 6,079,927 “Automated wafer buffer for use with wafer processing equipment” shows one approach for stocking wafers within their over-sized protective FOUPs at the front-end (operator side) of the WTS. Other systems may provide buffered storage in more space-efficient open cassettes at the back-end of the WTS, nearer the process bench. Separate buffering equipment, due to the physical space it requires, increases the footprint of the entire processing system. It also introduces additional interfacing requirements. However, buffers can significantly increase throughput by being an ever-ready source of wafers for the bench. This trade-off is evaluated by individual chip manufacturers based on their specific process and application requirements. Without regard to the presence or absence of a buffer, or where such optional buffer might reside, it is the task of a WTS to transfer wafers from the FOUP to the process bench and back while presenting each wafer in the proper orientation according to the interface requirements of each port.
As buffer systems have grown in capacity, they have also grown in physical dimensions. The resulting distances involved in transporting wafers and their cassettes have been traversed using mechanisms appropriate to the dimensions. These mechanisms have included trolleys, and gantry and crane systems. Even robotic carts have been used to cover longer distances guided by wire or tape on floors. Much of the prior art describes such mechanisms as they are used not only for external buffer storage but also within the process automation itself for transferring wafers from one process station to the next.
While high production throughput rates are important to chip manufacturers, so is product quality. Process analysis is a key element in determining the quality of the product that is output from the system. In order to gather data for the process side of the system, it is required that all wafers be subjected to the chemicals in the same manner. This may be accomplished in three ways.
In a first means, the wafers are introduced to the chemicals in a vertical position as mentioned above. For wet processing appl

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