Elastic foamed sheet and wafer-polishing jig using the sheet

Claim 1

A backing pad for retaining a semiconductor wafer during polishing, possessing at least a foamed layer, characterized by the fact that a plurality of bubbles in said foamed layer meets the following conditions: (1) that said bubbles are ellender discrete bubbles erected parallel to one another and dispersed at a substantially equal pitch in the direction of width of said foamed layer and said bubbles are substantially equal in size, shape, and position of formation in the direction of thickness of said foamed layer, (2) that the center lines of said bubbles in the direction of length thereof are parallel to the direction of thickness of said foamed layer, (3) that the diameters of said bubbles are minimized in the terminal part of the foamed layer on one surface side thereof and gradually increase in the direction from said one surface side to the other surface side of said foamed layer until said bubbles form openings thereof in the surface of said foamed layer, (4) that the diameters of said openings of bubbles are from 40 to 200 μm, (5) that the thickness of said foamed layer exceeds 20 μm and does not exceed 250 μm, (6) that the surface void ratio of said foamed layer (total sum of the areas of said openings of bubbles divided by the area of the wafer-supporting surface of said foamed layer (inclusive of the areas of openings of bubbles) and multiplied by 100) is from 90 to 98%, (7) that the softness of said foamed layer (difference, D 1 -D 2 , wherein D 1 stands for the thickness of said foamed layer assumed under a load of 300 gf/cm 2 ×10 seconds and D 2 for the thickness of said foamed layer assumed under a load of 1,800 gf/cm 2 ×10 seconds respectively exerted on the wafer-retaining surface of said foamed layer) is from 50 to 100 μm, (8) that the recovery ratio of said foamed layer defined by the following formula is from 50 to 80%: Recovery Ratio=(D.sub.3 -D.sub.2)/(D.sub.1 -D.sub.2)×100(%) (wherein D 1 and D 2 have the same meanings as defined above and D 3 stands for the thickness of said foamed layer assumed under a load of 300 gf/cm 2 ×10 seconds exerted on the wafer-supporting surface of said foamed layer subsequently to the sequential exertion of a load of 300 gf/cm 2 ×10 seconds and a load of 1,800 gf/cm 2 ×10 seconds in the order mentioned), and (9) that the compression ratio of said foamed layer defined by the following formula is from 30 to 50%: Compression Ratio=(D.sub.1 -D.sub.2)/D.sub.1 ×100(%) (wherein D 1 and D 2 have the same meanings as defined above).