A Better Way to Manage Test Wafers
Laura Peters, Editor-in-Chief -- Semiconductor International, 5/8/2008 8:11:00 AM
Advanced Micro Devices (AMD, Sunnyvale, Calif.) is in the process of applying lean concepts throughout its organization — even to highly manual operations, such as the use of test wafers (TWs) in 300 mm fabs. Aziza Faruqi outlined a methodology to decrease the inventory of TWs, improve TW lifecycle and automate TW management in fab operations. She presented the new TW strategy and how it was optimized for tool qualification at the recent Advanced Semiconductor Manufacturing Conference (ASMC) in Cambridge, Mass., this week. The results included saved operator time (Fig. 1), reduced risk of human error and more effective utilization of sorter equipment so that about half of the existing sorting equipment was ultimately needed. “It frees up operators so that they can perform more value-added tasks, while significantly reducing the amount of wafer sorting equipment required,” Faruqi said.
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| 1. The greater the number of test wafers in the kit, the greater the savings in operator time. |
TWs are created as a group — a TW kit that tests different parameters over a range of specifications. In a typical litho module, Faruqi said there might be 2000 kits used over two months, most of which involve five wafers, but could contain between two and 30 wafers (Fig. 2). The complexity, time and manual effort involved in TW kitting is proportional to the number of wafers in the kit and the number of changes made to the kit specifications. The first step in the procedure was to automate the TW kit assembly so that the TW needs were predicted, TWs of correct grade were assembled for each equipment qualification, TWs would be automatically transported to/from the equipment, and the TW kits were disassembled after qualification — all with minimal manual intervention. “The kit should not be built when it is needed, but ahead of time to avoid delays,” she said. In some cases, the TWs needed already exist from a previous kit, and can be located in the computerized maintenance management system (CMMS) rather than recreated. The kit management software gets the kit order from the CMMS. If not, it is ordered by the CMMS and a fab technician reviews the CMMS report, then gets the TW specifications from AMD’s proprietary process control management system (PCMS). Complexity is introduced between the manufacturing execution system (MES), equipment interface and automated material handling system (AMHS), transferring FOUPs and sorting wafers until the appropriate grade of TWs is reached for the equipment qualification. This process can be repeated several times, so automation speeds the process.
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| 2. Distribution of test wafers in each of the kits. Complexity increases with the number of TWs and changes in kit specifications. |
Faruqi emphasized the improvements in wafer sorting efficiency, faster TW kit creation and reduced operator time. However, she noted that the project is not nearly completed. “We fixed our problem, but this is not the ultimate solution by any means,” she said. Next steps for the TW kitting project include further movement to a just-in-time rather than on-demand approach, improvement of the kit finding algorithms, development of self-correcting systems and alternative approaches to reducing AMHS delays.
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