Background of the Invention Currently, various photolithographic optics-based processes are used in the manufacture of integrated circuits on semiconductor wafers. Because these optics-based processes generally require accurate focusing in order to produce a precise image, the surface planarity of the wafer is an important issue.

There are several techniques for planarizing the surface of a semiconductor wafer. One technique is chemical-mechanical polishing (CMP). A CM : P tool generally includes a carrier to mount a wafer and a polish pad. The CMP tool causes the polish pad and the wafer surface to come into contact, typically applying a specified pressure between the polish pad and the wafer surface. The CMP tool also imparts a relative motion between the wafer surface and the polish pad. In addition, the CMP tool typically introduces slurry at the interface between the polishing pad and the wafer surface. The slurry can have abrasive particles suspended in a chemical solution that reacts with selected materials on the wafer surface. The pressure, slurry and relative motion effectuate the polishing. After polishing, a secondary buff step is often used to remove microscratches and strongly adhering particles and to provide a final light polish. After buffing, the wafers go through a clean up and drying process

to remove residual slurry, metal particles, and other potential contaminants from its surface.

FIGURE 1 illustrates a known buff system 10 used to buff a wafer 12. In this example, buff system 10 is part of an AvantGaard@ 776 polisher, available from SpeedFam-IPEC Corp., Chandler, Arizona. Buff system 10 includes an arm 13; a buff head 14 with a polish pad mounted thereon, a platen 16 with polish pad mounted thereon, and roller supports 18. The roller supports 18 help hold the wafer 12 prior to buffing and also prevents lateral movement of the wafer 12 during buffing. During the buffing process, platen 16 moves upward to contact wafer 12, while buff head 14 moves downward to contact wafer 12 and apply a selected downforce. Buff head 14 and platen 16 then rotate, causing wafer 12 to rotate, thereby buffing the surface of wafer 12.

In this system, the buff head 14 has a diameter that is less than the diameter of wafer 12. The relative placement of buff head 14 and wafer 12 is illustrated in FIGURE 2. During the buff process, the center of buff head 14 is offset from the center of wafer 12 by a distance O (referred to as the offset), with buff head 14 "overlapping"the center of wafer 12 by a distance L referred to herein as the overlay or overlap.

In a standard configuration (adapted for wafers 200mm in diameter) of the aforementioned AvantGaard@ 776 buff system, the buff head is approximately 4.50 inches in diameter, with O being 1.828 inches and L being 0.420 inches. This configuration is illustrated in FIGURE 3. In this standard configuration, buff head 14 and lower platen 16 are rotated at approximately 300 rpm. The wafer 12 rotates at about 50 rpm due to friction and the offset of the system. With the conventional system, the removal rate profile illustrated in FIGURE 4 is produced. As can be seen in FIGURE 4, the removal rate profile obtained using this standard configuration is center fast meaning that more material is removed from the center of the wafer than from the edge. The relatively low uniformity of this removal profile indicates that there is a need for further improvement.

Summary of the Invention In accordance with aspects of the present invention, an improvement is provided to chemical-mechanical polishing machines. The improvement includes using a buffing pad having a geometrically optimized shape, along with an optimized configuration of the offset O and the overlay L (which is a function of the buff head diameter and the offset). In one embodiment, the pad is circular but mounted to the

buff head eccentrically. In another embodiment, the buffing pad has a generally square outer shape. In another embodiment, the buffing pad has at least three radially extending arms.

In another aspect of the present invention, the optimal configuration is determined iteratively for a selected process by changing the offset, overlay, buff head diameter and pad shape. For example, increasing the offset tends to increase the removal rate toward the edge of the wafer (in general). Increasing the overlap generally tends to increase the removal rate in the center of the wafer.

Besides changing the offset, overlap and buff head diameter, the shape of the pad itself can be modified to tailor the removal rate profile. Typically, the desired removal rate profile is as uniform as possible. In the embodiments of the present invention described herein, the removal characteristics of the system were optimized to balance the removal rate between the edge and the center. However, when optimizing only the overlap, offset and buff head diameter, the resulting removal rate profile was both edge and center fast. To further increase the uniformity of the removal rate profile, pad material was removed near the edge of the buff head so that the removal rate would decrease at both the edge and the center. This produced a significant improvement in uniformity. The best pad shape tested so far is a round pad smaller than the buff head (86% in diameter) and mounted to the buff head eccentrically and tangent at one point on its edge.

Brief Description of the Drawings The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same become better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein: FIGURE 1 is a simplified perspective view of a conventional buffing system.

FIGURE 2 is a plan view illustrating the parameters overlay and offset of FIGURE 1.

FIGURE 3 is a scaled plan view illustrating the standard buffing unit configuration.

FIGURE 4 is a diagram illustrating the removal rate profile obtained using the buffing unit configuration of FIGURE 3 and illustrates the inherent center fast property of a conventional configuration.

FIGURE 5 is a scaled plan view of a first test configuration.

FIGURE 6 is a scaled plan view of a second test configuration.

FIGURE 7 is a diagram illustrating the resulting removal rate profile obtained by using the configuration of FIGURE 5 and illustrates that the increased offset and pad size cause an edge fast removal rate profile.

FIGURE 8 is a diagram illustrating the resulting removal rate profile obtained by using the configuration of FIGURE 6 and illustrates the achieved balance between being edge fast and center fast.

FIGURE 9 is a plan view illustrating a first embodiment of a buffing pad with a generally square shape.

FIGURE 10 is a diagram illustrating the removal rate profile obtained by using the configuration of FIGURE 6 in conjunction with the pad shape of FIGURE 9.

FIGURES 11A-11H are plan views illustrating various embodiments of a buffing pad formed in accordance with the present invention.

Detailed Description of the Preferred Embodiment The present invention is an improvement to chemical-mechanical planarization equipment. The improvement can be applied to both buffing systems and primary polishing systems. The improvement includes using a buffing pad with a geometrically optimized shape and an optimized configuration of the offset O, overlay L, and buff head diameter.

The inventors herein have observed that the removal rate across a wafer (i. e., removal rate profile) is dependent on the diameter of the buff head, the shape of the buff pad, the offset O and the overlay L. The inventors have observed that changing the buff head diameter, offset O and the overlay L alters the removal rate profile.

These parameters affect the integrated relative velocity distribution (between a region of the wafer and the portion of the buff pad contacting that region of the wafer), as well as dwell time under the buff pad. Thus, by appropriately changing these parameters, the removal rate profile can be controlled. As used herein, the integrated relative velocity distribution refers to an area integration of the relative velocity due to the buff pad rotation with respect to a rotating coordinate system fixed to the wafer surface. The result of integrating the relative velocity is an integrated distribution that is a function of the distance from the center of the wafer. The integrated distribution is significant because the relative velocity distribution is one indicator of the relative material removal rate at a given point on the wafer. The integrated distribution does not take into account other factors such as the interaction of chemicals and liquid abrasives (slurries) and the distribution of these agents.

FIGURES 5 and 6 are schematic plan views illustrating different buffing unit configurations. The configuration of FIGURE 5 uses a larger buffing pad than the configuration of FIGURE 3, increases the offset, and decreases the overlap. The configuration of FIGURE 6 uses a larger buffing pad and offset than the configuration of FIGURE 3 but smaller than that of FIGURE 5, while decreasing the overlap compared to the configuration of FIGURE 3 but not as much as in the configuration of FIGURE 5. Table 1 summarizes the offsets, overlaps, buff head diameter, removal rate profile characteristics, pad shape and standard deviation of the removal rate over the wafer diameter. Table 1. Coniiguration Results Summary ............. :::::.. .. ::::::.::::::.. .: ::::::.:::::::::.::.: _::::::. ::: .:.:::. _::::.::::. ::::::. _:. _:::::.:::::. . :::.. :::::::::::::::::::::. ::::::: .:::::::::::::.::::::::::::::.::.:. _:::::::. : : : : : : : : : : : : : : : : : : :.. : : : : : : : : : : : : : : : : : : : : : : : : :..-. : : : : : : : : : : : : : : : : : : : : :..... : ;... :. : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : = : : : : : : : : : : : _ : : : : : : : : : : : : : : : : : < : : : : : : : : : : : : : : : : : < : : :...., : : : : > : : = : : : : : : : : : : : :..-. : : : : : : : : : : :... :. : : : : :... :.. : : :::.: _:::::::::::::::: :: :: v:: : ::::::::::: :::: =:::: °: v: :: :: ::: : :::: :::::: :: :: v: ::::.::::..::.,::.:::.:..:.:::::.:: t. : ...... :.. : -.... _.. -. : :......... : : : : : : : : : : : :... : : : : : : : : : : : : : :. w : : : : : : : : : : : : : : : : : : : : : :.. . an : : ............. .......... :. :::::::.::::::;;;::;:::.:...:::::::::::::::::::::::::::::::: ::::::::: .....:::::::::::: ::::.::::::::::::::::::::::::::::::::::::::::::::::::::::::: ::: ................. : : : : : : Original Fig 3 1.828.420 4.50 Round 52% Figure 5 2.917.084 6.00 Round 71% Figure 6 2.442.310 5.50 Round 24% Figure 6 2.442.310 5.50 Square (4. 06"side) 10. 2% Figure 6 2.442.310 5.50 4. 75" diameter eccentric 10. 1% The resulting removal rates profiles of the configurations of FIGURES 5 and 6 are shown in FIGURES 7 and 8, respectively. As can be seen, FIGURES 7 and 8 show that the removal rate profiles for the configurations of FIGURES 5 and 6 are "edge fast"and"center fast and edge fast", respectively.

One of the reasons that the configuration of FIGURE 5 is edge fast is that the edge of the wafer has a relatively large dwell time under the buffing pad because the edge of the wafer is aligned near the center of the buffing pad. As a result, the wafer edge is in contact with the buffing pad for a longer duration (dwell time). In contrast, in the standard configuration (FIGURE 3), the edge of the wafer is aligned near the edge of the buffing pad resulting in the wafer edge having a relatively low dwell time.

Further, the wafer edge moves in more nearly the same direction as the buffing pad, thereby reducing the relative velocity difference, thereby reducing removal rate. The removal rate near the center of the wafer is relatively high for the conventional configuration due to the 100% dwell time and the high integrated velocity distribution. This creates the undesirable property of a highly non-uniform removal rate profile.

On the other hand, the configuration of FIGURE 6 is both edge fast and center fast (see the associated removal rate profile of FIGURE 8). The offset in this configuration causes the wafer edge to be aligned fairly near the center of the buffing pad, which increases the dwell time of the wafer edge. However, because the offset and the buff pad are smaller in this configuration compared to the configuration of FIGURE 5, the removal rate at the wafer edge is slightly lower. Thus, this configuration demonstrates a balance between being edge fast and center fast.

Once an optimal offset and overlap configuration is achieved, the inventors appreciated that further improvements in the uniformity of the removal rate profile can be realized by changing the shape of the pad on the polish head. In one embodiment, pad material is removed from the outer portion of the polish pad to decrease the removal rate at both the edge and the center of the wafer, thereby improving the removal rate uniformity of the system.

Although the removal rate profiles of FIGURES 4,7 and 8 may be desirable in certain applications, in general, a uniform removal rate profile is desired. The inventors appreciated that appropriate shaping of the buffing pad can improve the uniformity of the removal rate profile.

FIGURE 9 illustrates a first embodiment of a buffing pad 18 formed in accordance with the present invention. The shape is essentially that of a square with portions of the squares corners clipped or rounded. In the embodiment shown in FIGURE 9, the pad has a side length, S, in the range of about 9.9 cm to about 10.6 cm, with one particular embodiment having a length of about 10.3 cm (-4. 06 inches) with the corners rounded to match the buff head diameter. In this case, the square is not completely inscribed within the circular buff head, but instead has the rounded corners as shown in FIGURE 9. In light of the present disclosure, those skilled in the art will appreciate that the size of the square will vary depending on the diameter of the wafer being processed.

FIGURE 10 is a diagram illustrating the removal rate profile obtained using the buffing pad of FIGURE 9. As can be seen in FIGURE 10, the removal rate profile is significantly more uniform. In tests, the standard deviation of the removal rate across the wafer diameter was reduced to approximately 10.2%.

In addition to the embodiments described with reference to FIGURES 3-11, other pad shapes are possible that will also reduce the amount of pad material along the pad outer edge, e. g., star shapes, wheel spoke shapes, triangles, etc. Example shapes are shown in FIGURES 11A-11H. FIGURE 11A shows a"concentric square"

buffing pad (i. e., concentric with the buff head). FIGURE 11B shows a"concentric square with clipped comers"buffing pad similar to that shown in FIGURE 9.

FIGURE 11C shows a"offset square"buffing pad (i. e., offset with respect to the axis of rotation of the buff head). FIGURE 11D shows an"offset circular"buffing pad where the buffing pad is smaller than the buff head and is tangent at one point so that pure rotation of the buff head produces an orbital type of motion of the pad.

FIGURE l lE shows a"cross"buffing pad. FIGURE 11F shows a"scalloped cross" buffing pad. FIGURE 11G shows a"modified cross"buffing pad. FIGURE 11H shows a concentric circular buffing pad with a diameter smaller than that of the buff head. The actual removal rate profile for these configurations tend to vary. These different removal rate profiles can be optimal for a particular application. For example, the polishing process performed before the buffing process may result in a center fast or edge fast removal rate profile. An appropriate buffing unit configuration, including buffing pad shape, can be determined to compensate for the resulting topography after the primary polishing process.

While the preferred embodiment of the invention has been illustrated and described, it will be appreciated that various changes can be made therein without departing from the spirit and scope of the invention. For example, the buff pad may include grooves or slurry holes. Further, the present invention may be applied to other, more primary, processes in which a rotating head is pressed to a circular wafer for the purpose of removing portions of the wafer surface. In this regard, the term "buff'pad and"buffing"as used herein are generically defined to mean a material removing article including a primary polish step.

While the preferred embodiment of the invention has been illustrated and described, it will be appreciated that various changes can be made therein without departing from the spirit and scope of the invention.