1. Field of the Invention
This invention relates to an elastic foamed sheet which is particularly suitable for backing pads to be used for retaining a semiconductor wafer on a rotary attaching disc of a polishing device in the process of mirror polishing of the semiconductor wafer and a wafer-polishing jig using the elastic foamed sheet.
2. Description of the Prior Art
The semiconductor wafers to be used for IC's and LSI's require at least one of the opposite surfaces thereof to be given a mirror finish by polishing. Generally, this polishing is effected by keeping a given wafer securely on the rotary attaching disc of the polishing device and pressing this wafer against an abrasive cloth laid on a stationary disc similarly kept in rotation while supplying an abrasive liquid to the interface of abrasion.
As means to retain the wafers on the polishing carrier plates in this case, the wax method which attains fast retention of the wax on the carrier plates by applying wax to one surface of the wafer and fastening the wafer to the carrier plates through the medium of the wax. This method enjoys the advantage of enabling the wafer surface to be polished with high planar accuracy. Owing to the use of the wax for fastening the wafer to the polishing device, however, this method suffers from numerous disadvantages that the work of attaching or detaching the wafer to and from the polishing device consumes much time and labor, that the work of cleaning the polishing device after each use thereof calls for an enormous toil, that the remaining wax defiles the wafer being handled, and that the special solvent to be used in the process of cleaning goes to jeopardize the work environment.
As means to eliminate these problems, the waxless method has been developed which effects the fast retention of a wafer on the rotary attaching disc of the polishing device not through the medium of wax but through the medium of a laminate of sheets each obtained by impregnating an artificial leather sheet or a non-woven fabric of polyester fibers with a polyurethane resin and imparting a finely foamed structure to the surface of the impregnated sheet. At present, this meshod is in popular use.
The conventional laminate mentioned above is generally constructed as illustrated in FIG.8. To be more specific, a retaining backing 51 constructed to have a wafer held fast against the lower surface thereof, a reinforcing member 52, a carrier 53, and a peel paper 54 are superposed sequentially in the order mentioned and adhesive agents 55, 56, and 57 are interposed between the adjoining layers so as to join them fast. The peel paper 54 can be peeled from the layer of the adhesives 57 when the laminate is attached to the rotary attaching disc of the polishing device.
The waxless method which used the laminate described above has the advantage that the laminate permits the wafer to be attached thereto and detached therefrom so easily as to enhance the efficiency of quantity production of wafer. It has been pointed out, however, that wafers polished by the waxless method are inferior in planar accuracy to those produced by the wax method. When wafers are to be polished by the use of the conventional laminate discribed above, the highest attainable flatness of the polished surfaces expressed by TTV (total thickness variation) is on the order of 5 .mu.m. This polishing cannot decrease this magnitude any further. This limited flatness may be ascribed to the use of the peel paper 54 in the conventional laminate and to the numerosity of the component layers of the laminate. The term "TTV" mentioned above refers to the difference between the highest point and the lowest point of thickness of a polished wafer expressed in .mu.m.
The reason for the aforementioned inability to lower the magnitude of flatness below about 5 .mu.m may be logically explained as follows.
Since the peel paper 54 itself contains fairly large undulations in the surface thereof and further since the peel paper 54 engulfs air while a tackiness agent or adhesive agent 57 is applied to the surface of the carrier 53 and the peel paper 54 is superposed on the applied layer of the tackiness agent or adhesive agent and the peel paper 54 is then wound up, the layer of the tackiness agent or adhesive agent 57 fails to assume a uniform thickness. Thus, the surface of the retaining backing 51 does not become flat when the laminate is attached to the rotary attaching disc.
Further, owing to the fact that the conventional laminate has a large number of component layers (seven layers inclusive of the peel paper 54 in the illustrated example), the rises and falls or undulations formed on the surface of the retaining backing 51 are suffered to become large because the ununiformities of thickness in the component layers of the laminate are accumulated while they are superposed even if these component layer are produced each with the highest possible uniformity.
In the internal structure of the conventional laminate, the bubbles occluded therein have a random size distribution and the reinforcing fibers incorporated therein have a random density and direction arrangement. Owing to this internal structure, when the laminate is pressed and polished in conjunction with the wafer, the compression deformation of the laminate is locally deprived of uniformity on the rear surface of each of a plurality of wafers retained on the carrier plates or on the rear surface of one and the same wafer. As a result, the amount of polishing to be attained is locally deprived of uniformity. This local ununiformity may well be considered to form one of the factors responsible for the limited flatness mentioned above.