Clean breakthrough

An increase in the size of semiconductor devices has prompted fab designers to take a bold new approach to higher specification handling. Charles Masi explains

A revolution in cleanroom technology  for wafers up to 200mm diameter was achieved thanks to the standard mechanical interface (SMIF) which not only increased automation, but also improved cleanliness at lower cost.

But mechanical issues, such as size, shape and sensitivity, make it impossible to scale SMIF pods up to handle the new 300mm wafers. Instead, the latest generation of semiconductor fab facilities are installing an entirely new automated material handling system (AMHS) standard using front-opening universal pods (FOUPs).

First developed and commercialised in the 1980s, SMIF pods have long been the technology of choice for controlling contaminates in semiconductor fabrication facilities. They allow fab designers to enclose all the areas through which wafers pass in so-called ‘micro-environments’ having extremely high cleanliness characteristics, while the rest of the facility operates at a much lower cleanroom classification.

As semiconductor device manufacturers move to larger 300mm wafer processes, however, their equipment suppliers have found that it is not practical to scale SMIF pods up to handle them.

The wafers’ increased flexibility and weight overburden the SMIF-style internal support system. In addition, the SMIF handling system incorporates considerable manual intervention, which is anathema to fab designers striving for more and more automation.

SMIF pods are small enclosures in which wafer lots can be stored between processing steps, and transported between micro-environment-equipped semiconductor processing tools. The pods maintain the ultra-clean microenvironment around the wafers, while they are away from the processing tools. These pods support the wafers they enclose with an internal ‘cassette’ support structure rising from the pod’s base.

To transfer wafers in the pod to a semiconductor tool for further processing, the SMIF pod is placed on top of a ‘load port’ to which it mates. A door forming the pod’s base mates with a similar door in the tool’s load-port, trapping any contamination between the doors.

This load-port mechanism then draws the mated doors along with the cassette down into the tool’s micro-environment, where the tool’s transport machinery can remove wafers one at a time for processing.

‘Theoretically, any contaminants on the outside of the carrier are trapped between the surfaces of the base plate and the base that opens,’ explained Steven Fulton, project manufacturer for e-manufacturing at International Sematech Manufacturing Initiative (ISMI).

This subsidiary of Sematech Inc is a 12-member consortium including companies such as Texas Instruments, IBM, Hewlett-Packard, Panasonic and Philips.

SMIF works well for wafers smaller than 200mm, but above that size and the wafers become increasingly difficult for the movable structure to support without undue flexure. Making the structure more robust to reduce flexure would also make it much heavier, and therefore more difficult for the load-port mechanism to handle.

The solution has been to completely rethink the pod and load-port. Instead of opening at the bottom, as SMIF pods do, the FOUPs open at the front. They still mate with a load-port door built into the tool’s micro-environmental enclosure, but now the load-port is in a side wall, rather than the top.

This FOUP plan allows vertically stacked wafers to be supported by their edges on shelves moulded into the container’s side walls, making the support structure much stiffer. Having the door open on the front, rather than the base, allows the tool’s robotic handler to reach into the pod and lift wafers out one at a time.

Another advantage of this door position also facilitates automating movement of FOUPs between tools. The AMHS system manufactured by US company Asyst Technologies is one example.

FOUPs can be stored in one or more storage facilities — called ‘stockers’ — where they can be hung while awaiting their next turn on a process tool.

When a FOUP’s turn comes, a lift picks it from its storage location and places it on a track along which it travels to the load-port of the tool that is ready to process its wafers. Once there, a mechanism picks the FOUP off the track, aligns it using kinematic coupling pins, then locks it on to the load port and opens the door.

‘Now we have a FOUP that has an RFID tag on it so we know which one it is, and we can be sure exactly which wafers are in exactly which slots,’ said Fulton.

‘That information goes up to the factory manufacturing execution (MES) system where it is associated with some process flow that defines what equipment and what process the wafers in that FOUP need to go through.’

Semiconductor fabs haven’t yet realised the productivity enhancement they had hoped for, largely because, while the hardware systems are operating as intended, there is still much work to be done automating the overarching control software.

Some ISMI members actually refer to the current system as ‘semiautomated’ because of the level of human interaction with the software needed to move the material around.

For example, with the current system software it is possible for a FOUP to become ‘lost.’

This means that because it may end up sitting idly for an indefinite period while the software processes other wafer lots, overseers are still necessary. And, while engineers work to take humans out of the loop, they also have to look ahead to future requirements.

‘As we contemplate the future challenges of handling even bigger wafers at even higher levels of factory integration,’said Fulton, ‘we’ve got the advantage of the learning that we’ve done so far for even higher performance in our material delivery and handling systems.

‘We expect that as we go forward into larger wafer sizes we will need another quantum shift in material carriers and a new vision of what constitutes automated material handling and delivery.’