The present invention relates to a wafer polishing apparatus employing CMP (Chemical-Mechanical Polishing) and a backing pad for wafer polishing.
In a semiconductor device manufacturing process, a planarizing process for planarizing the surface of a semiconductor wafer is performed to improve the precision of photolithography and the like. A typical example of this planarizing process is SOG (Spin On Glass). As the micropatterning degree of semiconductor devices increases, higher-degree planarization is required. In order to satisfy this demand, in recent years, CMP using polishing cloth impregnated with a chemical etching solution attracts attention, and is becoming the mainstream technique including the Damascene method for planarization.
FIG. 4A shows a wafer polishing apparatus using conventional CMP.
Referring to FIG. 4A, a wafer polishing apparatus entirely denoted by reference numeral 1 is constituted by a circular polishing table 2, a columnar carrier head 3, and a slurry supply pipe 4. The carrier head 3 opposes the polishing table 2 and has a smaller diameter than that of the polishing table 2. The slurry supply pipe 4 drops slurry 5. A spindle 6 rotatably supporting the polishing table 2 is fixed to the center of the lower portion of the polishing table 2. A soft lower pad 7 and a hard upper pad 8 stacked on it are sequentially fixed to the upper surface of the spindle 6.
The carrier head 3 is constituted by an annular retainer ring 10, a stainless steel base plate 11, and a spindle 12. The base plate 11 is fixed to be buried in the upper half of the retainer ring 10. The spindle 12 is fixed to the center of the upper portion of the base plate 11 to rotatably support the carrier head 3. A backing pad 15 is fixed to the lower surface of the base plate 11. Six air pipes 13, circularly arranged, as shown in FIG. 4B, are formed to extend through the base plate 11 and backing pad 15. Compressed air is supplied from an air supply source (not shown) provided above the base plate 11 to the air pipes 13.
In this arrangement, the carrier head 3 is moved downward to press a wafer 16 against the upper pad 8 with the lower surface of the backing pad 15. After that, air is supplied from the air supply source (not shown) to the air pipes 13 to form an air layer between the backing pad 15 and wafer 16. In this state, the polishing table 2 is rotated in the direction of an arrow while the carrier head 3 is rotated in a direction opposite to the rotational direction of the polishing table 2, and the slurry 5 is dropped from the slurry supply pipe 4, so that the lower surface of the wafer 16 is polished by the upper pad 8.
FIG. 5A shows a conventional head using a backing pad for wafer polishing which is formed of a foamed body having open cells. FIG. 6A shows a conventional head using a backing pad for wafer polishing which is formed of a foamed body having closed cells.
When a backing pad 25 is formed of only a foamed body having open cells, as shown in FIG. 5A, it is entirely polished substantially uniformly, as shown in FIG. 5B. However, the polishing amount is small, and even if the air pressure is increased, a decrease in polishing amount cannot be improved. A drop in polishing amount is large near the outer peripheral edge portion of a wafer 16. A decrease in polishing amount is found to typically occur when the outer diameter of the wafer 16 exceeds 8 inches.
This is supposed to be based on the following reason. A foamed body having open cells has a specific nature of high air transmission in the lateral direction (along the surface of the wafer 16). Hence, an air layer 17 is formed uniformly between the backing pad 25 and wafer 16, and accordingly the wafer 16 can be easily polished flat. However, since the air layer 17 spreads over the entire surface of the backing pad 25, pressing of the backing pad 25 against the wafer 16 depends on the air layer 17. The wafer 16 thus becomes less influenced by pressing the backing pad 25, resulting in a small polishing amount.
The reason why the polishing amount of the wafer 16 drops largely at the outer peripheral edge portion is as follows. As shown in FIG. 5A, the air layer 17 tends to be released from the outer peripheral portion of the backing pad 25 to the outside, and the backing pad 25 cannot press the outer peripheral edge portion of the wafer 16 supposed to be pressed.
When a backing pad 35 is formed of only a foamed body having closed cells, as shown in FIG. 6A, the polishing amounts at two measurement points separated from a center C of a wafer 16 by 1 inch to 8 inches are larger than those at other measurement points. This leads to variations in polishing amount as a whole. However, the overall polishing amount is improved by increasing the air pressure.
This may be based on the following reason. A foamed body having closed cells has such a specific nature that, although its air transmission is low in the lateral direction (along the surface of the wafer 16), it can locally apply a pressure to the wafer. As a result, as shown in FIG. 6A, a portion where an air layer 17 is formed and a portion where it is not formed are produced between the backing pad 35 and wafer 16. Hence, the entire surface of the wafer 16 is not uniformly pressed by the air layer 17, so that the surface of the wafer 16 cannot be polished flat.
FIGS. 5B and 6B show graphs of the polishing amounts of the wafer 16 at measurement points separated from the center C of the wafer 16 having a diameter of 8 inches in the radial direction by 1 inch, which are obtained by using the backing pads shown in FIGS. 5A and 6B.