Continuous flow manufacturing for adaptive fab management
11/01/1997
Continuous flow manufacturing for adaptive fab management
As yield-based improvements in productivity become more difficult to achieve, fab managers have looked to capital utilization and cycle time reduction for additional gains. Unfortunately, these two goals are contradictory, according to Olivier Ruelle, of IBM France, speaking at the recent IEEE/SEMI Advanced Semiconductor Manufacturing Conference in Cambridge, MA. Full equipment utilization (lowest cost of ownership) creates bottlenecks behind low throughput or high maintenance tools, increasing cycle time. Conversely, the lowest cycle time (shortest time to market) is achieved by running batch processing equipment like furnaces at less than full capacity.
Operations management systems try to balance utilization and cycle time by varying wafer starts to prevent excess WIP accumulation around bottlenecks. Without any operations management at all, upstream tools "push" material through the line at a given rate. If a tool breaks down, WIP accumulates behind it until it is repaired. Downstream tools may be idled for lack of material, wasting potential throughput. Furthermore, the WIP accumulation behind the tool is permanent, since upstream tools replenish the backlog before the repaired tool can eliminate it (see part a) of figure). More sophisticated systems may reduce wafer starts to avoid buildup behind known bottlenecks, but are less successful in dealing with the constant process evolutions in a fab.
Ruelle discussed an alternative, continuous flow manufacturing (CFM), which manages starts while using continuous improvement techniques to locate and eliminate bottlenecks. The CFM implementation he discussed divides the production line into 75 ranges, each representing slightly more than one day of cycle time. Thus, each range should be able to deliver all wafers to the next within 24 hr.
Each range, Ruelle said, is treated as a small factory, receiving WIP from an upstream supplier and delivering it to a downstream customer. A daily order/commit process maintains smooth flow in the line: each range orders WIP from its supplier and commits to deliver a certain amount of product to its customer. If the range is experiencing problems, it reduces its commitments accordingly, while reducing orders to maintain one-day cycle time. Once the problems are solved, the range requests the maximum output possible from its supplier in order to meet its customer`s needs as quickly as possible. Instead of developing a permanent WIP bubble at the bottleneck, the line gradually returns to a steady state (see part b) of figure).
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a) Accumulation of WIP behind a bottleneck in a push mode manufacturing line; b) gradual absorption of excess WIP under a CFM system.
This range management process, Ruelle said, is an efficient way to balance cycle time and utilization around bottlenecks, but the tradeoff between the two still exists. To improve overall line productivity, bottlenecks must be eliminated altogether. Each day, the fab MES system generates a report of missed commitments, showing which ranges were unable to fill customer orders. Ranges with problems receive immediate attention from process and maintenance personnel. Meanwhile, upstream, supplier, ranges, having lower than expected demand, can schedule cleaning, setups, and other movable activities in order to be ready to run when demand returns.
Weekly and monthly cumulative reports show ranges with ongoing problems or erratic behavior. Production engineers analyze these to determine which specific operation and which "partner" - WIP, operators, process, or equipment availability - is limiting output, and take appropriate action. A total CFM strategy, Ruelle said, combines range management with continuous improvement to increase overall productivity. - K.D.