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Witness Wafers for Measuring Sedimented Particles

IP.com Disclosure Number: IPCOM000060541D
Original Publication Date: 1986-Apr-01
Included in the Prior Art Database: 2005-Mar-08
Document File: 1 page(s) / 12K

Publishing Venue

IBM

Related People

Poesch, W: AUTHOR

Abstract

Semiconductor wafers are manufactured in clean rooms with very low air pollution. The method described below allows dust particles of a size > 5 mm to be counted and measured by selectively collecting them on defined sedimentation or witness wafers. Known methods used for that purpose have a number of disadvantages. Larger dust particles or particles of material removed during manufacture, as well as fibers and generally polydisperse aerosols of > 5 mm, which, due to their size, are contained in clean air only for a limited time, cannot be reliably detected and analyzed by standard suction-type aerosol measuring devices. In clean rooms, the specific number of large particles is so low that, even when using an impactor, reliable samples can only be taken within several hours.

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Witness Wafers for Measuring Sedimented Particles

Semiconductor wafers are manufactured in clean rooms with very low air pollution. The method described below allows dust particles of a size > 5 mm to be counted and measured by selectively collecting them on defined sedimentation or witness wafers. Known methods used for that purpose have a number of disadvantages. Larger dust particles or particles of material removed during manufacture, as well as fibers and generally polydisperse aerosols of > 5 mm, which, due to their size, are contained in clean air only for a limited time, cannot be reliably detected and analyzed by standard suction-type aerosol measuring devices. In clean rooms, the specific number of large particles is so low that, even when using an impactor, reliable samples can only be taken within several hours. The reliability of electrostatic, oscillating crystal or diffusion measuring methods is also limited. Therefore, it is proposed that large aerosol particles be counted and measured by selectively collecting them on defined witness wafers. The witness wafers are used in class 10 000 and 100 000 clean rooms and inside or adjacent to machines and tools where particles are encountered. Reproducible results are obtained if the particles are allowed to sediment by gravitation without being unduly interfered with. The witness wafers are best shaped and dimensioned similar to semiconductor wafers, i.e., their diameter is 5". The wafer material is gree...