1. Field of the Invention
The present invention relates to the planarizing of a surface of wafer in the manufacturing of a semiconductor device. More particularly, the present invention relates to chemical mechanical polishing apparatus for polishing the surface of wafer.
2. Description of the Related Art
Generally, semiconductor devices are becoming more highly integrated and the distance between adjacent cell regions is constantly being reduced. Accordingly, the surface configuration of the semiconductor wafer is becoming more complicated such that height differences at the wafer surface are becoming more pronounced. Therefore, a broken wire or a short circuit can be produced more easily during manufacture, especially during a multi-stacked process. Therefore, planarization technology is required to reduce the height differences at the surface of the wafer and thereby make it less likely that defects will be produced during the manufacturing of wiring on the surface. In this respect, a chemical mechanical polishing (CMP) process is widely used as the planarization technique. The CMP process uses an abrasive solution (slurry) to eliminate the height difference(s) on the surface of the wafer.
More particularly, conventional CMP equipment comprises a head for supporting the wafer and a table onto which the slurry is dispensed. The wafer is adhered to the head, and a grinding pad is attached to the table. The slurry is supplied between the head and the table, and the head and the table are rotated independently of each other to grind the wafer.
Another type of CMP equipment that is widely used comprises an index table including a plurality of wafer chucks. The index table is divided into multiple areas, such as four areas. The wafer chucks hold respective wafers using a vacuum and rotate at a high speed. The four areas comprise a loading station, a first grinding station at which a rough grinding process is performed, a second grinding station at which a final grinding process is performed, and an unloading station, respectively. The index table is rotated in a predetermined direction to bring the wafer chuck into position with the respective stations in succession, whereby the wafer held by the chuck is subjected to a predetermined grinding process.
To this end, a first and a second grinding head are provided above the first and the second grinding stations of the index table, respectively. The first and second grinding heads can be moved up and down and rotated to perform a grinding process on each wafer disposed at the respective grinding station.
Furthermore, a cleaning and drying unit is disposed to one side of the index table to clean and dry wafers transferred thereto from the unloading station after the grinding process is completed. The cleaning and drying unit also includes a wafer chuck having an upper surface to which a wafer can be secured by a vacuum. This wafer chuck is also rotated in a predetermined direction.
However, recently, the wafers that are being subjected to the CMP process are thinner and larger in diameter than ever before. Accordingly, the wafers tend to warp, especially at the outer peripheral edge portions. A wafer that is warped may not be smoothly inserted into the slot of a cassette when the wafer is unloaded from the CMP equipment. That is, the wafer can be bumped into the cassette and become damaged or broken after the CMP process is completed.