1. Field of Invention
The present invention relates to a chemical-mechanical polishing machine. More particularly, the present invention relates to a method for increasing the working life of a retainer ring used in a chemical-mechanical polishing machine.
2. Description of Related Art
In the fabrication of semiconductor devices, planarizing a wafer surface is an important process before high-density microlithographic techniques can be carried out. Planarization is necessary because only when surface topographic variations are removed can diffraction of light be avoided, and, consequently, transfer of a highly accurate pattern be achieved. Currently, the two main techniques for planarizing a wafer surface include spin-on-glass (SOG) and chemical-mechanical polishing (CMP). However, ever since semiconductor fabrication has reached the sub-half micron stage, spin-on-glass technique is insufficient for providing the degree of planarity necessary for a wafer. This leaves chemical-mechanical polishing as the only means for global planarization of very large scale integration (VILSI) or even ultra large scale integration (LJLSI) circuits. Since chemical-mechanical polishing is such an important planarization techniques, various semiconductor manufacturers and research organizations are participating in the development of CN/tp techniques in order to get a head start in the field.
FIGS. 1A and 1B are respective top-view and side-view of the components of a conventional chemical-mechanical polishing machine. Components of a chemical-mechanical polishing machine include: a polishing table 10; a polishing pad 13 on top of the polishing table 10; a carrier 21 and a retaining ring 20 for grabbing a wafer 12; a spinning axle 11 for rotating the carrier 21, a transporting pipe 14 for transporting slurry 19 to the polishing pad 13; and a pump 15 for pumping the slurry 19 through the transporting pipe 14. When chemical-mechanical polishing starts, the polishing table 10 and the axle 11 will both rotate separately in a predefined direction, as shown by arrows 18a and 18b in FIGS. 1A and 1B. Carrier 21 grabs the back of the wafer 12, which is retained in place by the retaining ring 20. The wafer 12 is pressed with its front face 17 onto the polishing pad 13. Slurry 19 running through the transporting pipe 14 driven by the pump 15 is constantly dropping onto the polishing pad 13. Polishing is achieved through chemical reaction between the chemical reagent inside the slurry 19 and the silicon on the front face 17 of the wafer 12. The chemical reaction produces an easy-to-polish layer on the front face 17. Moreover, abrasive particles inside the slurry 19 also offer some assistance in removing the protruding parts in the easy-to-polish layer. Therefore, by repeating the above chemical reaction and mechanical polishing action, a surface of high planarity can be obtained. In general, chemical-mechanical polishing is a process that uses mechanical polishing together with chemical reaction through special chemical reagents to smooth out and planarize a highly irregular wafer surface.
FIG. 2 is a cross-sectional view showing the location of pocket depth in a wafer holding assembly. As shown in FIG. 2, one end of a retaining ring 20 is connected to the periphery of the lower surface of a carrier 21. A wafer is fixed in position by the carrier 21 and the retaining ring 20. In FIG. 2, the pocket depth 22 refers to the distance from the front face 17 of the wafer 12 to the other end of the retaining ring 20. During chemical-mechanical polishing operation, a pocket depth 22 of about 0.3 mm must be maintained so that the slurry on the polishing pad 13 can execute the best possible polishing action.
However, the retaining ring 20 is also be worn away under repeated cycles of chemical reaction and mechanical polishing action. In general, a newly installed retaining ring 20 has a thickness of about 6.35 mm. When the thickness of the retaining ring 20 has worn down to about 5.6 mm, pocket depth 22 can no longer be adjusted to about 0.3 mm. Under such circumstances, the old retaining ring 20 has to be replaced. Normally, a new retaining ring 20 is able to polish roughly between 1500 to 2000 wafers before its working life is finished. Because the retaining ring is made from a rather expensive ceramic material, any method that can increase the working life of the retaining ring can save considerable operating costs.
In light of the foregoing, there is a need to develop a method for increasing the working life of a retaining ring.