1. Field of the Invention
This invention relates to chemical mechanical polishing (CMP) used in semiconductor manufacturing. More particularly, it relates to a chemical mechanical polishing tool and to its use.
2. Discussion of the Related Art
Modern semiconductor manufacturing is a highly competitive industry that requires the ability to fabricate complex semiconductor devices at high speed, with high yields, and at low cost.
Semiconductor devices are fabricated on semiconductor wafers. Such wafers are made by carefully growing a large, high purity semiconductor crystal, which is then sliced into individual semiconductor wafers. For storage and protection the sliced semiconductor wafers are usually loaded into wafer cassettes. A wafer cassette individually stacks the sliced semiconductor wafers in slots. Wafer cassettes are beneficial in that the large numbers of semiconductor wafers can be stored and transported in a protected environment.
Unfortunately, immediately after slicing a semiconductor wafer is unsuitable for semiconductor device fabrication because the slicing leaves rough surfaces on the semiconductor wafers. Surface roughness is a serious problem because modern fabrication processes require accurate focusing of photolithographic circuit patterns onto the semiconductor wafer. As the density of the circuit patterns increases, focus tolerances better than 0.1 μmeters can be required. Focusing with such small tolerances is not practical if the surface of a semiconductor wafer not highly smooth and planar.
A number of techniques for reducing semiconductor wafer surface roughness exist. A semiconductor wafer can be mechanically worked by an abrasive pad to produce a fairly smooth surface. However, as indicated above, modern semiconductor wafer surfaces must be exceptionally smooth and planar.
One technique that can suitably finish the surface of a semiconductor is Chemical-Mechanical Polishing (“CMP”). In CMP, a semiconductor wafer is mechanically and chemically worked under carefully controlled conditions. Such work is performed using a special abrasive substance that is rubbed over the surface of the semiconductor wafer. The special abrasive substance is typically a slurry that contains minute particles that abrade, and chemicals that etch, dissolve, and/or oxidize, the surface of the semiconductor wafer.
CMP is a well-known and commonly used process. As shown in FIG. 1, a conventional chemical mechanical polishing apparatus includes a mount 3 for holding and rotating a semiconductor substrate 4. That apparatus also includes a rotating disk 1 that retains a polishing pad 2. As shown, that pad has a diameter that is much larger than that of the semiconductor substrate 4. Furthermore, a nozzle 6 applies a polishing slurry 7 to the polishing pad 2.
The semiconductor substrate 4 is polished by the applied polishing slurry, by rotating the mount 3 in the direction B, by moving the mount 3 in directions C while pressing the substrate 4 against the polishing pad 2, and by rotating the polishing pad 2 in the direction A.
While the chemical mechanical polishing apparatus illustrated in FIG. 1 has been generally successful, in practice using a polishing pad 2 with a larger diameter than that of the semiconductor substrate 4 may not be optimal. For example, vibration, which can be detrimental to precise polishing, is a significant problem if a large polishing pad is rotated too fast. Thus, when using a chemical mechanical polishing apparatus similar to that illustrated in FIG. 1, the achievable polishing rate is limited. Another problem with using a large polishing pad is that since the semiconductor substrate 4 is polished over its entire surface, it is difficult to efficiently remove localized defects.
Another approach to chemical mechanical polishing is provided in U.S. patent application Ser. No. 6,179,695 B1. Referring now to FIG. 2, that patent discloses a chemical mechanical polishing apparatus having a polishing station E1 that holds a semiconductor substrate W. The polishing station E1 further includes a slider 104 that both rotates and horizontally moves a table 105 on a support 106. The semiconductor substrate W is placed on and held by the table 105. The slider 104 itself is on a guide table 103 on a base 101.
Also included in the chemical mechanical polishing apparatus of FIG. 2 is a polishing head E2 having a plurality of polishing-tools 110. Referring now to FIGS. 2 and 3, the polishing-tools 110 are circumferentially disposed above the polishing station E1. The polishing-tools 110 are mounted such that they can rotate.
Still referring to FIGS. 2 and 3, the polishing head E2 also includes a revolution table 108 that is rotatably supported on a lower yoke 102a, which extends from a supporting member 102 that mounts on the base 101. The revolution table 108 is attached to an output shaft of a driving mechanism 107, which is supported on an upper yoke 102b, which extends from the supporting member 102. The driving mechanism 107 revolves the revolution table 108 at a predetermined rate, which causes the polishing-tools 110 to revolve.
The three polishing-tools 110 are interchangeable. Turning now to FIGS. 3 and 4, each polishing-tool 110 includes a plurality of ring-shaped polishing pads 111a and 111b on the end of shafts 113a and 113b. Beneficially, the polishing pads are made of a nonwoven fabric, foamed polyurethane or the like.
Referring now to FIG. 5, the outer cylindrical shaft 113a is bearing 115a mounted and rotatable with respect to a lower supporting member 108a (also shown in FIG. 2). The inner cylindrical shaft 113b is co-axially disposed within the outer cylindrical shaft 113a. The inner cylindrical shaft is also bearing 115b mounted and rotatable. The ring-shaped polishing pads 111a and 111b, which are held in position by holding members 112a and 112b, have surface areas centered at radiuses r1 and r2.
Referring now to FIGS. 2 and 5, drive mechanisms 114a and 114b (which are on the revolution table 108) connect to the cylindrical shafts 113a and 113b, respectively. Thus, the ring-shaped polishing pads 111a and 111b can be independently rotated at high speeds. The drive mechanisms 114a and 114b are controlled such that the linear velocity of the polishing pads are the same. That is, the rotational velocity of the ring-shaped polishing pads 111a and 111b are used to compensate for the different radiuses r1 and r2.
To polish a semiconductor substrate W, the ring-shaped polishing pads 111a and 111b are moved into contact at a predetermined pressure with the surface of the semiconductor substrate W. Then, the slider 104 is moved such that the semiconductor substrate W is at a polishing position. Then, the driving mechanisms 114a and 114b rotate the ring-shaped polishing pads 111a and 111b while a polishing slurry is applied to the surface of the semiconductor substrate W. At the same time, the rotating table 105 is rotated and is moved radially (with short strokes).
Since the surface being polished is polished using multiple, small diameter ring-shaped polishing pads it is possible to rotate the polishing pads at high speeds while very precisely polishing the surface irrespective of local defects. Additionally, the ring-shapes reduce vibration over that of a continuous polishing pad. It should also be noted that it is possible to use only one of the ring-shaped polishing pads when polishing.
Beneficially, the inner and outer ring-shaped polishing pads 111a and 111b can move axially with respect to each other. This makes it possible to adjust the relative heights of the polishing pads 111a and 111b, and to independently set the polishing pad pressures against the surface of the semiconductor substrate W. In turn, this enables pressure control such that the optimum processing pressures can be used.
While the apparatus illustrated in FIGS. 2-5 is beneficial, it also may not be optimal. For example, the polishing area is relatively small, even when both polishing pads contact the semiconductor wafer W. This increases the required polishing time. Furthermore, while the apparatus illustrated in FIGS. 2-5 is believed to be effective in reducing the detrimental effects of vibration, vibration is primarily only a problem after polishing has been performed for some time. Finally, the apparatus illustrated in FIGS. 2-5 may not be the best for localized polishing as the radiuses of the polishing pads causes relatively widely separated areas to be polished.
Therefore, a new semiconductor wafer polishing apparatus, and a method of using such an apparatus, that can reduce the detrimental effects of vibration, that can polish both broad and localized areas, and that can rapidly remove material from a semiconductor wafer would be beneficial.