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To: Petz who wrote (4929)	3/16/1998 1:36:00 AM
From: Yousef	Respond to of 6843

John, Re: " I'm a little surprised that there is such a large queue of wafers ready for processing at each stage of production, apparently due to lack of equipment." Generally, there will be maybe one to two lots at each piece of equipment. But just like on a highway, if a piece of equipment goes down (like an accident on a highway), lots start to back up and equipment needs to have adequate "over capacity" to work off the few large queues. A "Rocket Lot" generally requires that upcoming equipment be held open for the "Rocket Lot". For example, if one had a deposition process that took 6 hours to complete and you had a "Rocket Lot" scheduled to be at that equipment within that 6 hour window, then you wouldn't process any "regular" production material. You would keep that Dep piece of equipment available for the "Rocket Lot". So there is a large production/capacity penalty for running these high priority lots through the Fab. Re: " But doubling the # of machines might only increase production 50%, because then you will sometimes be in the situation where a machine is waiting for wafers." As you increase the number of pieces of equipment, you would also increase the amount of WIP (Work/Wafers In Process). So if I double my capacity, then I would double the WIP to keep the same cycle time while approximately doubling my output. The other interesting thing to remember is that you only have to double the capacity of the limiting steps to double overall capacity. Obviously not all pieces of equipment have the same capacity. (In fact this varies greatly) Factory planning/modeling is very important. A typical process might have 120 distinct operations. The key manufacturing measure is the "Turn Rate" (TR). This measures how many operations did each wafer move today. TR's of 2 - 3 are common for normal priority manufacturing. Cycle time can be calculated: Cycle time = (# Operations)/(Turn Rate) = (40 - 60 Calendar Days) Make It So, Yousef