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INVAR Mask for Finely Spaced C4/Blm Deposition

IP.com Disclosure Number: IPCOM000038420D
Original Publication Date: 1987-Jan-01
Included in the Prior Art Database: 2005-Jan-31
Document File: 2 page(s) / 32K

Publishing Venue

IBM

Related People

Chance, DA: AUTHOR [+2]

Abstract

While it is desirable to decrease the dimensions and spacings of C4 pads on the Si wafer, it is difficult to locate fine holes, closely spaced (4 mils on 8 mils) in Mo having conventional 5-mil thickness. Thinner Mo is very difficult to manufacture and is likely to be too flexible to lie flat against a Si wafer. This latter difficulty would result in "halo-ing" of the evaporated solder during evaporation. A further deficiency in Mo as a mask material for Si is its relatively high coefficient of thermal expansion (4.9 x 10-6/ŒC) vs. Si (2.5 x 10-6/ŒC). Since alignment of the mask and the wafer occurs at room temperature and evaporation occurs at N 150ŒC, the Mo must have a compensated pattern and, furthermore, must be allowed to expand to match the underlying Si wafer.

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INVAR Mask for Finely Spaced C4/Blm Deposition

While it is desirable to decrease the dimensions and spacings of C4 pads on the Si wafer, it is difficult to locate fine holes, closely spaced (4 mils on 8 mils) in Mo having conventional 5-mil thickness. Thinner Mo is very difficult to manufacture and is likely to be too flexible to lie flat against a Si wafer. This latter difficulty would result in "halo-ing" of the evaporated solder during evaporation. A further deficiency in Mo as a mask material for Si is its relatively high coefficient of thermal expansion (4.9 x 10-6/OEC) vs. Si (2.5 x 10-6/OEC). Since alignment of the mask and the wafer occurs at room temperature and evaporation occurs at N 150OEC, the Mo must have a compensated pattern and, furthermore, must be allowed to expand to match the underlying Si wafer. Upon cooling, this mismatch in the coefficient of expansion is noted in the difficulty with which the Mo and wafer are separated. Proposed here is the use of a thin sheet of INVAR* magnetic material (Fig. 1) having a coefficient of expansion which matches that of Si to be used as a mask. The INVAR, which is currently available in thin sheet form, may be held in place by a magnet held behind the Si, the magnet being used also as a heat sink. The INVAR or other magnetic mask material may be made thinner than Mo, thereby allowing finer holes to be etched. The coefficient of expansion will match that of Si, thereby avoiding the compensation and alignment...