Capacity Planning in the Face of Product-Mix Uncertainty

Capacity Planning in the Face of Product-Mix Uncertainty -...

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© 1999 IEEE. Reprinted, with permission, from Proceedings of the 1999 IEEE International Symposium on Semiconductor Manufacturing Conference, Santa Clara, CA, October 11-13, 1999, 73-76. 1 Capacity Planning in the Face of Product-Mix Uncertainty Robert Kotcher Headway Technologies, Inc. Milpitas, CA, U.S.A. Frank Chance FabTime Inc. Menlo Park, CA, U.S.A. This paper describes a straightforward method of quantifying the sensitivity of production capacity to product mix. In capacity planning over horizons with unpredictable product mix, this methodology quantifies the risk of under- investment in capacity. Actions that can be taken to prevent mix-driven capacity shortfalls are described. The implementation of this methodology at Headway Technologies is also presented. INTRODUCTION In capacity planning for wafer fabrication facilities, the need for additional production equipment must be identified and the equipment ordered far in advance of its actual use. If product mix is highly predictable, or if all products use each piece of production equipment equally, an overall production forecast is all that is needed to determine equipment requirements. But when a wafer of one product loads a tool to a greater degree than does a wafer of another product, any deviation from the expected product mix could leave the facility with insufficient capacity, even if total production volume is right on forecast. To demonstrate, let’s look at a simple case of a single-tool factory, with no downtime or rework. Product A requires 1 hour per wafer on the tool; Product B requires 2 hours per wafer. With a 50/50 mix and a total of 16 wafers per day, total required production time is (8)(1) + (8)(2) = 24 hours—so we have just enough capacity. But if product mix shifts to 25% Product A and 75% Product B, total required production time is (4)(1) + (12)(2) = 28 hours, and we have insufficient capacity, even though total production volume has remained unchanged. Expand this example to include a dozen or more constantly changing products and hundreds of tools, and we have the real world…and a threat to capacity that is immensely complicated to answer. This is a concern for Headway Technologies, a manufacturer of leading-edge read-write heads. A read- write head is a tiny integrated circuit, about the size of a grain of pepper, that magnetically reads and writes data onto and off of a disk in a disk drive. Heads are manufactured in the form of wafers, each consisting of 15,000-20,000 identical heads, each containing many layers of microscopic circuitry. Each wafer makes 300 to 400 visits to different process equipment during its manufacture. At any time, Headway has ten to twenty products in production, each with its own unique process flow. Each product is typically produced for only a few months before being replaced by a new, advanced version. New products typically start out as low-volume R&D projects but sometimes ramp quickly into mass production when they are qualified and approved by a customer. In this environment, although total production volume can be
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