Hybrid 300mm Wafer Handling

During the transition from 200mm to 300mm wafer production, the industry concentrated on the historic AMHS challenges of storage density, carrier weight, standard equipment interfaces, material identification, remote equipment control and some advanced dispatch and scheduling methodologies. As an industry, we focused on the well-understood existing delivery technologies and strived to enhance these technologies for use in the emerging 300mm applications. However, a few companies focused their efforts on innovation that was driven by the differences between 200mm and 300mm manufacturing philosophies.

For example, it was widely understood that direct tool loading was a fundamental requirement at 300mm, necessitating the development of purpose-built intrabay material movement and tool loading solutions. What was not foreseen, however, was just how much of a bottleneck this would present at the transition point between intrabay and interbay systems. The industry also underestimated the impact on system performance (as measured by throughput and tool utilisation) of pausing an overhead transport (OHT) vehicle on the track to load and unload tools. While these particular issues have been partially addressed by increasing the intrabay system's functionality to include limited point-to-point delivery and by merge and diverge vehicle technology - where moving vehicles can dynamically pass stationary vehicles - dozens of other such issues have surfaced without resolution. That is, until the recent emergence of new advanced 300mm AMHS technologies.

Conventional 300mm AMHS

Not diverging from the shared experiences of 200mm AMHS, operators of most of the early 300mm fabs elected to install conventional car-based AMHS. As a result, these early 300mm interbay solutions mimicked those of the 200mm era. This approach includes multiple kilometres of monorail traversing the fab with vehicles travelling up and down main corridors carrying one or two front-opening unified pods (FOUPs). Extending the same transport methodology to intrabay automation, the majority of fabs electing to install fully automated intrabay AMHS, selected vehicle-based overhead transport and tool loading technology.

While these OHT systems proved capable of handling the job during “normal” operation and typical capacity demand, shortcomings quickly emerged. For example, a fab's work in process (WIP) can accumulate due to a multitude of factors, including tool or system breakdowns and scheduling inefficiencies that result from the differing processing times and capacities of tools. As the WIP backlog increases, tool delivery times increase (Figure 3) and tool utilisation decreases as the AMHS labours to catch-up with the increasing supply of FOUPs. Typically this WIP backlog eventually is worked down, but the loss of productivity is extremely costly.

Complicating the issue further is the trend of decreasing lot size. One of the substantial benefits of the 300mm wafer is a higher population of die per wafer. This higher population allows for fewer wafer starts of a particular technology in order to meet demand. As a result, today's fabs (particularly foundry operations) are transitioning to extremely high-mix facilities in comparison to 200mm fabs. Lot sizes of fewer than 12 wafers are commonplace. Smaller lots result in shorter process times per FOUP, which in turn increases the moves per hour expected from the AMHS. Again, the result with traditional car-based AMHS is increased delivery times and further loss of tool productivity.

Given the astronomical cost of 300mm capital equipment, tool productivity is of paramount importance. Factories can ill afford to have process tools idle for extended periods of time while waiting for material to be transported.

Alternatives

For many of the reasons already discussed - chiefly bottleneck mitigation and scheduling flexibility (which in turn improve throughput and facility utilisation) - conveyor-based systems have emerged as the transport means of choice in other industries. Conveyor-based systems historically had not been successful in semiconductor manufacturing environments, mostly because of concerns about system cleanliness. But after many years of development, conveyor-based systems are now as clean or cleaner than most vehicle-based technologies, opening the door for conveyor-based systems to emerge as an alternate means of transport in the semiconductor industry.

Free from the necessity of reserving, calling and waiting for a vehicle to transport material, these conveyor systems inherently possess much greater transport volume and as such demonstrate the ability to better handle the occasional WIP backlog or bubble. With conveyor systems, the AMHS contribution to tool idle-time is greatly reduced because the tools spend less time on average waiting for material to be delivered.

During both normal and peak operations, conveyor-based AMHS demonstrates comparatively shorter delivery times with a much lower standard deviation than vehicle-based AMHS. Such performance enhancements prove invaluable when implementing advanced dispatching, handling priority lots and executing just in time manufacturing.

As a result of the 300mm trend towards smaller lot sizes (12 wafers or less), the demand for intrabay moves has nearly doubled, approaching 300 moves per hour in high volume production. In an effort to meet this need, new opportunities and developments have emerged in the form of local buffering. While it remains cost prohibitive to install WIP buffering for individual tools, a local buffering scheme, which services multiple tools, can be both cost effective and productive. The use of a telescopic mast Cartesian-style robot for direct tool loading also permits the use of the space directly above the loadport for WIP storage. A series of highly instrumented “shelves” arranged about the tool face provide ample storage locations for production, test and monitor wafers. Storage of these wafers in close proximity to the point of use greatly reduces the overhead on the intrabay delivery system, while providing safe, efficient storage through the use of active monitoring of the FOUPs, seismic lockdown restraints and optional N2 purging.

Again the result is increased capital equipment utilisation as a result of the material travelling distance being greatly reduced. Local buffering has tremendous advantages that we are just now beginning to exploit.

Hybrids

The continued search for the ultimate 300mm AMHS has led to what is being called hybrid AMHS solutions - the combined use of both conventional vehicle based AMHS and conveyor based AMHS in the same factory. Based on exhaustive simulations and field-based applications, a combination of vehicle and conveyor-based AMHS intertwined with local WIP buffering is proving to be an effective solution for today's high volume 300mm factories.

The industry has done a remarkable job of standardising communications protocols as well as the physical interface between the different brands and types of AMHS. AMHS suppliers around the world are being asked to work together to install the best combination of technologies for a particular application.

Those IC manufacturers that early recognise the benefits of such hybrid implementations will be well positioned to deal with the factory automation challenges they are sure to encounter on the road to advanced 300mm wafer processing.

Acknowledgements

AMHS Options Special Study, Kristin Rust, Silpa Sigireddy, Vinu Kurakose, International SEMATECH, Manufacturing Methods & Productivity, June 7, 2002

Evaluation and Comparison of a Car-based vs. a CFT Material Handling System for a 300mm Fab, Frederika Tausch, Asyst Technologies, Lawrence Hennessy, IDC