Perhaps under emphasized relative to the red brick wall associated with scaling CMOS, as early as 2004 “manufacturable solutions are not known” looms for many 2001 ITRS facets of factory ramps, equipment lead time, factory cycle time, throughput, maintenance, material handling, and the intertwining importance of factory information and control hardware and software.
Specifically for material handling, the ITRS requirements are that automated material handling systems (AMHS) must interface directly with all 300mm tools used in a normal production process flow, while providing acceptable ROI. The roadmap states, “Solutions to provide higher wafer storage densities, short lead and install times, and better utilization of floor space through integration of process and metrology equipment must be developed.” Further, the ITRS specifically states the need to combine interbay and intrabay AMHS into one integrated capability (i.e., a “unified” tool-to-tool transport system that replaces conventional “segregated” interbay and intrabay approaches), all within the context of lower failure rates and faster repairs, substantially increased throughput, and designs that accommodate extendibility, flexibility, and scalability demands on the factory.
Using discrete event simulation, engineers at PRI Automation, Billerica, MA, have evaluated 300mm fabs with fully unified AMHS (a single OHT system throughout the fab), partially unified (still using interbay transport, but with several bays of OHT “linked” together), and a combination of both, all compared to conventional segregated AMHS on several criteria — equipment set, lot delivery times for normal lots and hot lots, relative reliability, relative cost, and implementation considerations. These simulations here done for a bay-chase and ballroom fab, with similar results for both.
The data for the bay-chase fab, for example, clearly showed that the total number of material handling moves was less for the unified configuration, simply because every tool-to-tool unified move requires only two transports, as opposed to three in a segregated configuration. Although, the total required storage space for WIP remains between each configuration (since WIP level is driven by manufacturing factors, not by AMHS design), the number of stocker cycles required to accomplish lot moves decreases for a fully unified system. In the modeled segregated system, 22 stockers were required because at least one stocker was needed in each bay (bay with high intrabay activity and lot move frequency require more stockers). But the unified system required only 15 stockers, since less stocker work is required and adjacent bays can share stockers.
As a result of requiring fewer stockers, the system costs of the linked and unified configurations are lower. It is worth mentioning that more OHVs are required in the unified than any other configuration. However, fewer overall vehicles are needed with a unified system. This number could further decrease as more intelligence is added to vehicle dispatch system and as real time scheduled data is integrated with the material control system.
From this work, the PRI engineers concluded that unified AMHS had the highest MTBF and lowest system cost, the former correlated with fewer system components and the latter directly related to the need for fewer stockers (i.e., conventional segregated AMHS requires more stockers to support individual intrabay loops and handle more total lot moves). A unified approach also enabled more flexible tool placement with a saving of fab floor space; for example, a unified configuration takes advantage of the chase of repositioning of metrology tools so they can be shared between two bays. In addition, unified AMHS provided shorter tool-to-tool average delivery times — 32% lower for normal lots, 66% lower for hot lots. Lower delivery times for unified AMHS result because there are fewer lot hand-offs; this shortens the total transport and handling times and eliminates queue times for these unneeded hand offs.
According to Joe Reiss, director of strategic marketing for PRI Automation, it is generally accepted that the primary challenge to implementing unified AMHS is the sophisticated control software required. However, Reiss notes that PRI’s simulations were run using the same control algorithms found in PRI’s existing AMHS transport control and MCS software.
“In addition to showing the performance benefits of unified AMHS, our simulations demonstrate the ability of the AMHS software to manager the complex move-routing task of a single, large-scale transport network,” he says.
In ongoing work, these engineers are investigating the effect of AMHS configurations on fab performance using full-fab simulation. One fab configuration under evaluation is one used in a SEMATECH study that looked at various tool layouts considered for a 300mm fab. They are currently examining affects of Unified AMHS on wafer cycle time and equipment utilization.