Piezoelectric platen design for improving performance in CMP applications

An invention is disclosed for improved performance in a CMP process using piezoelectric elements as a replacement for a platen air bearing. In one embodiment, a platen for improving performance in CMP applications is disclosed. The platen includes a plurality of piezoelectric elements disposed above the platen. In operation, the piezoelectric elements are used to exert force on the polishing belt during a CMP process. In this manner, zonal control is provided during the CMP process.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An invention is disclosed for improved performance in a CMP process using piezoelectric elements as replacement for a platen air bearing. The present invention provides piezoelectric elements atop a platen, which provide zonal control during the CMP process. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be apparent, however, to one skilled in the art that the present invention may be practiced without some or all of these specific details. In other instances, well known process steps have not been described in detail in order not to unnecessarily obscure the present invention. FIGS. 1 - 2 have been described in terms of the prior art. FIG. 3 is a diagram showing a platen configuration 300 , in accordance with an embodiment of the present invention. Prior to describing a platen configuration having piezoelectric elements, another embodiment of the present invention, which utilizes annular bladders will be described. The platen configuration having piezoelectric elements will be described later with respect to FIGS. 7 - 9 . The platen configuration 300 of FIG. 3 includes a wafer head 302 having a retaining ring 304 and a wafer 306 positioned below the wafer head 302 . The platen configuration 300 also includes a platen 308 disposed below a polishing belt 310 . The platen 308 includes a pressurized membrane 312 pressurized via annular bladders 314 . During operation the platen 308 is placed against the polishing pad or belt 310 that polishes the surface of the wafer 306 . To promote polishing uniformity, each bladder 314 may be individually pressurized via an air source. Advantageously, the annular bladders 314 improve performance in the CMP process by providing increased zonal control to the pressurized membrane 312 . Unlike a conventional air bearing, the pressurized membrane 312 of the embodiments of the present invention greatly reduces the amount of air needed during the CMP process. Moreover, a CMP process using the pressurized membrane 312 of the present invention is not as sensitive to conditions as conventional CMP processes utilizing air bearings. Unlike air bearings, the pressure of the pressurized membrane 312 of the present invention does not experience as great a variance as experienced by air bearings when the gap between the polishing pad 310 and the platen 308 varies. Thus, if the polishing pad 310 is pushed toward the platen 308 in one area, the pressure in other areas of the pressurized membrane 312 are not as affected as other areas would be when utilizing an air bearing. FIG. 4 is a detailed diagram showing a platen configuration 400 , in accordance with an embodiment of the present invention. The platen configuration 400 shows a polishing belt 310 positioned above a platen 308 having a pressurized membrane 312 pressurized by annular bladders 314 . As shown in FIG. 4 , each annular bladder 314 comprises a thin tubular material 402 . In one embodiment, the tubular material 402 of each annular bladder 314 is pressurized via air. However, it should be noted that the tubular material 402 can be pressurized utilizing any other means capable of pressurizing an annular bladder 314 , such as a fluid, as will be apparent to those skilled in the art. The pressurized membrane 312 preferably comprises a smooth, flexible material. Suitable materials include; polyurethane, silicon, thin metals (e.g., stainless steel), peek, and Teflon. As previously mentioned, the annular bladders 314 provide increased zonal control during a CMP process. To further increase zonal control, the size of the annular bladders 314 within the pressurized membrane 312 can be varied, as described in greater detail subsequently. FIG. 5 is a diagram showing a platen configuration 500 having varied annular bladders, in accordance with an embodiment of the present invention. The platen configuration 500 includes a platen 308 having a pressurized membrane 312 pressurized via annular bladders 314 . As shown in FIG. 5 , the platen configuration 500 includes annular bladders 314 having varying sizes. More specifically, the annular bladders 314 decrease in size as the annular bladders 314 approach the edge of the platen 308 . Generally, during a CMP process, more difficulty occurs within about 10-15 mm of the wafer edge. For this reason, one embodiment of the present invention increases resolution near the wafer edge by decreasing the size of the annular bladders 314 near the edge of the platen 308 . Similarly, since the center of the wafer typically requires less resolution, the central annular bladders 314 often are larger than those at the edge of the platen 308 . FIG. 6A is a top view of an annular bladder configuration 600 a in accordance with an embodiment of the present invention. The annular bladder configuration 600 a includes concentric annular bladders 314 a . In one embodiment, each concentric annular bladder 314 a of the annular bladder configuration 600 a forms a complete circle about the center of the platen. In this manner each annular bladder 314 a can be individually pressurized to provide zonal control during the CMP process. To further increase zonal control during the CMP process, the length of each annular bladder can be reduced, as discussed next with reference to FIG. 6B . FIG. 6B is a top view showing an annular bladder configuration 600 b in accordance with an embodiment of the present invention. The annular bladder configuration 600 b includes concentric annular bladders 314 b . Unlike the embodiment of FIG. 6 A, each concentric annular bladder 314 b of the annular bladder configuration 600 b does not form a complete circle about the center of the platen. Each concentric annular bladder 314 b of the annular bladder configuration 600 b varies in size depending on a particular annular bladder's 314 proximity to the edge of the platen. As mentioned above, during a CMP process, more difficulty generally occurs within about 10-15 mm of the wafer edge. For this reason, one embodiment of the present invention increases resolution near the wafer edge by decreasing the size of the annular bladders 314 b near the edge of the platen. Similarly, since the center of the wafer typically requires less resolution, the central annular bladders 314 b often are larger than those at the edge of the platen. Advantageously, embodiments of the present invention improve performance in CMP applications by providing increased zonal control via a membrane pressurized using internal annular bladders. Other embodiments of the present invention also improve performance in CMP applications by providing increased zonal control via piezoelectric transducers. Many polymers, ceramics, and molecules such as water are permanently polarized, having some parts of the molecule positively charged, while other parts of the molecule are negatively charged. When an electric field is applied to these materials, these polarized molecules align themselves with the electric field, resulting in induced dipoles within the molecular or crystal structure of the material. Furthermore, a permanently-polarized material such as quartz (SiO 2 ) or barium titanate (BaTiO 3 ) will produce an electric field when the material changes dimensions as a result of an imposed mechanical force. These materials are piezoelectric, and this phenomenon is known as the piezoelectric effect. Conversely, an applied electric field can cause a piezoelectric material to change dimensions. This phenomenon is known as electrostriction, or the reverse piezoelectric effect. Hence, one embodiment of the present invention utilizes piezoelectric materials to provide zonal control during a CMP process. FIG. 7 is a diagram showing a platen configuration 700 , in accordance with an embodiment of the present invention. The platen configuration 700 includes a wafer head 302 disposed above a wafer 306 , and having a retaining ring 304 . In addition, a platen 308 is positioned below the polishing belt 310 . The platen 308 of the platen configuration 700 includes a plurality of piezoelectric elements 702 disposed below the polishing belt 310 . During operation, the platen 308 is placed against the polishing pad or belt 310 that polishes the surface of the wafer 306 . To promote polishing uniformity, each piezoelectric element 702 may be individually activated to apply zonal force to the polishing pad. Advantageously, the piezoelectric elements 702 improve performance in the CMP process by providing increased zonal control to the polishing belt 310 . Unlike a conventional air bearing, the piezoelectric elements 702 of the embodiments of the present invention greatly reduce the amount of air needed during the CMP process. Moreover, as with the pressurized membrane, a CMP process using the piezoelectric elements 702 of the present invention is not as sensitive to conditions as conventional CMP processes utilizing air bearings. Unlike air bearings, the force exerted by the piezoelectric elements 702 of the present invention does not experience as great a variance as experienced by air bearings when the gap between the polishing pad 310 and the platen 308 varies. Thus, if the polishing pad 310 is pushed toward the platen 308 in one area, the force exerted on the polishing belt 310 by other piezoelectric elements 702 is not as affected as other areas would be when utilizing an air bearing. FIG. 8 is a top view of a piezoelectric element configuration 800 , in accordance with an embodiment of the present invention. The piezoelectric element 702 configuration 800 includes concentric piezoelectric elements 702 . Similar to the annular bladder configuration of FIG. 6 A, in one embodiment of the present invention, each concentric piezoelectric element 702 forms a complete circle about the center of the platen. However, to further increase zonal control during the CMP process, the length of each piezoelectric element 702 can be reduced, as shown FIG. 8 . Unlike the embodiment of FIG. 6 A, each concentric piezoelectric element 702 of the piezoelectric element configuration 800 does not form a complete circle about the center of the platen. Each concentric piezoelectric element 702 of the piezoelectric element configuration 800 varies in size depending on a particular piezoelectric element's 702 proximity to the edge of the platen. As mentioned previously, during a CMP process, more difficulty generally occurs within about 10-15 mm of the wafer edge. For this reason, one embodiment of the present invention increases resolution near the wafer edge by decreasing the size of the piezoelectric elements 702 near the edge of the platen. Similarly, since the center of the wafer typically requires less resolution, the central piezoelectric elements 702 often are larger than those at the edge of the platen. Unlike an air bearing, the embodiments of the present invention make physical contact with the polishing belt during the CMP process. As result, wear on the platen may be increased do to friction from the polishing belt. To provide additional protection from wear to the platen, a sacrificial material can be positioned between the platen and the polishing belt, as discussed next with reference to FIG. 9 . FIG. 9 is an illustration showing a CMP system 900 , in accordance with an embodiment of the present invention. The CMP system 900 in FIG. 9 is a belt-type system having an endless polishing belt 310 mounted on two drums 910 , which drive the polishing belt 310 in a rotational motion as indicated by belt rotation directional arrows 906 . A wafer 306 is mounted on the wafer head 302 , which is rotated in direction 908 . The rotating wafer 306 is then applied against the rotating polishing belt 310 with a force F to accomplish a CMP process. Some CMP processes require significant force F to be applied. A platen 308 , having piezoelectric elements 702 , is provided to stabilize the polishing belt 310 and to provide a solid surface onto which to apply the wafer 306 . Slurry 904 composing of an aqueous solution such as NH 4 OH or DI containing dispersed abrasive particles is introduced upstream of the wafer 306 . The process of scrubbing, buffing and polishing of the surface of the wafer is achieved by using an endless polishing pad glued to the polishing belt 310 . Typically, the polishing pad is composed of porous or fibrous materials and lacks fix abrasives. Disposed between platen 308 and the polishing belt 310 is a sacrificial material 914 fed roll-to-roll over the platen 308 via rollers 916 . During use, the sacrificial material 914 is fed slowly over the platen 308 to provide protection from wear. In an alternative embodiment, the sacrificial material 914 is indexed as the CMP process progresses. In this manner, the sacrificial material 914 is worn, rather than the material of the platen 308 . Hence, the piezoelectric elements 702 or the pressurized membrane are protected from wear caused by the friction of the rotating polishing belt 310 . Although the foregoing invention has been described in some detail for purposes of clarity of understanding, it will be apparent that certain changes and modifications may be practiced within the scope of the appended claims. Accordingly, the present embodiments are to be considered as illustrative and not restrictive, and the invention is not to be limited to the details given herein, but may be modified within the scope and equivalents of the appended claims.