1. Field of the Invention
The present invention relates to a polishing apparatus that includes a dressing device to maintain the surface quality of a polishing tool while minimizing polishing defects on a workpiece such as a semiconductor wafer.
2. Description of the Related Art
As the density of circuit integration in semiconductor devices becomes ever higher, circuit patterns are becoming finer and interline spacing narrower. In this type of technology, the depth of focus becomes very shallow in photolithographic reproduction of circuit patterns, and it requires that the surface of the wafer placed at the focal plane of a stepper must be microscopically flat to produce the required degree of image sharpness. As illustrated in FIGS. 4 and 5, a method of obtaining a flat surface is to polish the workpiece in a polishing apparatus comprised by a turntable 12 having a polishing tool 10 such as a polishing cloth or a grinding stone thereon, and a top ring (wafer holding member) 14 known as chemical mechanical polishing (CMP).
In the conventional CMP apparatus, a wafer W is held on the bottom surface of the top ring 14, and the wafer is pressed against the polishing cloth 10 mounted on the rotating polishing table 12 by means of a pressing cylinder. During the polishing process, a polishing solution Q is supplied from a nozzle 16 so that polishing action takes place while the polishing solution Q is retained between the wafer surface to be polished and the polishing cloth 10.
The ability of the polishing cloth 10 to provide efficient surface material removal becomes degraded as polishing is continued, and to regenerate the polishing ability of the polishing cloth 10, a dressing device 18 is used at suitable intervals, such as when exchanging wafers. The dressing device 18 is disposed opposite to the top ring 14 across the center of the turntable 12. The dressing device 18, similar to the top ring 14, may be attached to a swing arm 20 so that it can be swung back and forth between the dressing position above the polishing cloth 10 on the turntable 12 and a standby position outside of the turntable 12. Dressing tool 24 is attached to a shaft 22 at the bottom end of the arm 20, so that it can be driven by an elevating device and a rotation device. A dressing operation is carried out by rotating the turntable 12 and the dressing tool 24 and pressing the dressing tool 24 onto the work surface of the polishing cloth 10.
After dressing is finished, the dressing tool 24 returns to the standby position, but because the tool 24 has been contaminated with polishing debris and polishing solution adhering to the polishing cloth 10, it is necessary that the tool 24 be cleaned before the next cycle of dressing. For this purpose, a cleaning device 28 having a container 26 filled with a cleaning solution L is placed in the standby location, and the dressing surface of the dressing tool 24 is immersed in the cleaning solution L, as shown in FIG. 6. A similar device has also been used to clean the top ring.
However, in such a conventional cleaning device, even if a fresh solution L is used each time, there is always a chance of contamination of the fresh solution L by spent polishing solution or polishing debris that were removed from the dressing tool 24 during the previous cleaning step and are adhering to the inner surface of the container 26. Also, polishing debris adhering to the bottom surface of the dressing tool 24 float into the cleaning solution L, and when the dressing tool 24 is immersed in the cleaning solution L, the solution level rises, the debris will adhere to upper surfaces 24a or side surfaces 24b of the tool 24 (see FIG. 1). Furthermore, mist produced during the normal polishing operation of the polishing solution can sometimes adhere to the upper surfaces 24a or side surfaces 24b of the tool 24, and such a contaminant cannot be removed simply by immersing the tool 24 in the cleaning solution L. such contaminants can sometimes fall on the polishing cloth 10 during the dressing operation. Such dried and solidified contaminants that have fallen on the polishing cloth 10 have been known to cause scratching on the polished surface of the wafer.