Planarization of semiconductor substrates is becoming more important as the number of layers used to form a semiconductor device increases. Nonplanar semiconductor substrates have many problems including difficulty in patterning a photoresist layer, formation of a void within a film during the film deposition, and incomplete removal of a layer during an etch process leaving residual portions of the layer, which are sometimes called "stringers." A number of planarization processes have been developed and include chemical-mechanical polishing.
FIGS. 1 and 2 include illustrations of a part of one type of a chemical-mechanical polisher that is used to polish semiconductor substrates. FIG. 1 is a cross-sectional view of a chemical-mechanical polisher 10. The polisher 10 has a platen 14 and a polishing pad 11 attached to the platen 14 with an adhesive compound (not shown). Above the polishing pad 11 are substrate holders 12, and each substrate holder 12 has a semiconductor substrate 13. The polisher 10 also includes a polishing slurry and a slurry feed, both of which are not shown. The polishing pad 11 may be made of a porous polyurethane material that has a relatively uniform thickness of about 1-2 millimeters. FIG. 2 includes a top view illustrating the relationships of motion between the polishing pad 11 and the substrates 13. During polishing, the polishing pad 11 rotates counterclockwise or clockwise, but the substrates 13 typically rotate in the same direction as the polishing pad 11. While the substrates 13 and polishing pad 11 are rotating, the substrates 13 are being oscillated back and forth across the polishing pad. The oscillating motion covers a distance called an oscillating range and is performed at an oscillating velocity. While the polishing is being performed, the polishing slurry may be recycled.
In actual use, chemical-mechanical polishing typically has nonuniform polishing rates across a substrate surface. In many cases, the polishing rate near the edge of the substrate is higher than the polishing rate near the center of the substrate because the relative velocity between polishing pad and the substrate is higher near the edge of the substrate compared to the center of the substrate. Therefore, some area of the substrate near the center may be underpolished, some area of the substrate near the edge may be overpolished, or both.
The polishing pad may contribute to the nonuniformity. A brief overview of the formation of polyurethane polishing pads is now presented. Polyurethane polishing pads are typically formed by reacting the chemicals that form polyurethane within a cylindrical container. After forming a cylindrical-shaped piece of polyurethane, the piece is cut into slices that are subsequently used as polishing pad. The polishing pad typically has pores that have a size of about 100-200 microns. Although the pores may vary in size, the average pore size for any region of the polishing pad is typically about the same as any other region of the polishing pad. As used hereinafter, this type of prior art polishing pad is referred to as a conventional polishing pad. The nonuniformity occurs because the edge of the substrate is moving faster relative to the polishing pad compared to the center of the substrate and the conventional polishing pad does not have a feature to compensate of the polishing nonuniformity.
The prior art has addressed the problem of nonuniformity polishing by modifying a conventional polishing pad by forming a pattern within the polishing pad. These polishing pads include forming a variety of geometric patterns including openings. It should be kept in mind that polishing pads are typically porous, and the pores are formed during the reaction to form the polishing pad material. As used in this specification, openings are distinguished from pores because openings are formed within the pad after the reaction to form the polishing pad material has occurred. A conventional polishing pad has pores but does not have any openings. The prior art polishing pad with openings typically have a width on the order of centimeters, or the prior art polishing pad has a density of openings that decreases with the distance from edge of the polishing pad.