Abstract:
A chemical mechanical polishing (CMP) apparatus is provided. The CMP apparatus includes a first roller situated at a first point and a second roller situated at a second point. The first point is separate from the second point. Also included in the apparatus is a polishing pad strip having a first end secured to the first roller and a second end secured to the second roller. The first roller and the second roller are configured to reciprocate so that the polishing pad strip oscillates at least partially between the first point and the second point.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]    This Application is a continuation of application Ser. No. 09/608,513, filed Jun. 30, 2000, from which priority under 35 U.S.C. §120 is claimed. The disclosure of this Application is incorporated herein by reference. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates generally to chemical mechanical polishing (CMP) systems and techniques for improving the performance and effectiveness of CMP operations. Specifically, the present invention relates to CMP systems that use a fixed abrasive polishing pad arranged in a web handling system.  
           [0004]    2. Description of the Related Art  
           [0005]    In the fabrication of semiconductor devices, there is a need to perform CMP operations, including polishing, buffing and wafer cleaning. Typically, integrated circuit devices are in the form of multi-level structures. At the substrate level, transistor devices having diffusion regions are formed. In subsequent levels, interconnect metallization lines are patterned and electrically connected to the transistor devices to define the desired functional device. As is well known, patterned conductive layers are insulated from other conductive layers by dielectric materials, such as silicon dioxide. As more metallization levels and associated dielectric layers are formed, the need to planarize the dielectric material increases. Without planarization, fabrication of additional metallization layers becomes substantially more difficult due to the higher variations in the surface topography. In other applications, metallization line patterns are formed in the dielectric material, and then metal CMP operations are performed to remove excess metallization.  
           [0006]    In the prior art, CMP systems typically implement belt, orbital, or brush stations in which belts, pads, or brushes are used to scrub, buff, and polish one or both sides of a wafer. Slurry is used to facilitate and enhance the CMP operation. Slurry is most usually introduced onto a moving preparation surface, e.g., belt, pad, brush, and the like, and distributed over the preparation surface as well as the surface of the semiconductor wafer being buffed, polished, or otherwise prepared by the CMP process. The distribution is generally accomplished by a combination of the movement of the preparation surface, the movement of the semiconductor wafer and the friction created between the semiconductor wafer and the preparation surface.  
           [0007]    [0007]FIG. 1 illustrates an exemplary prior art CMP system  100 . The CMP system  100  in FIG. 1 is a belt-type system, so designated because the preparation surface is an endless belt  108  mounted on two drums  114  which drive the belt  108  in a rotational motion as indicated by belt rotation directional arrows  116 . A wafer  102  is mounted on a carrier  104 . The carrier  104  is rotated in direction  106 . The rotating wafer  102  is then applied against the rotating belt  108  with a force F to accomplish a CMP process. Some CMP processes require significant force F to be applied. A platen  112  is provided to stabilize the belt  108  and to provide a solid surface onto which to apply the wafer  102 . Slurry  118  composing of an aqueous solution such as NH 4 OH or DI water containing dispersed abrasive particles is introduced upstream of the wafer  102 . The process of scrubbing, buffing and polishing of the surface of the wafer is achieved by using an endless polishing pad glued to the belt  108 . Typically, the polishing pad is composed of porous or fibrous materials and lacks fixed abrasive particles.  
           [0008]    After the polishing pad polishes a limited number of wafers, the surface of the pad is conditioned and cleaned in order to remove the attached abrasive materials of the slurry and the particles removed from the wafer. Subsequent to cleaning and conditioning, the polishing pad will have a significant amount of particles that remain attached to the surface of the polishing pad causing the polishing pad to lose its effectiveness. The polishing pad also loses its effectiveness due to normal wear of the material itself. As a result, the polishing pad must be replaced in its entirety. The removal of the used polishing pad and its subsequent replacement with a new polishing pad is very time consuming and labor intensive. Additionally, the time needed to perform the replacement necessarily requires that the polishing system be taken off-line, which thus reduces throughput.  
           [0009]    In view of the foregoing, a need therefore exists in the art for a chemical mechanical polishing system that will enable polishing surface layers of a wafer using a polishing pad that is less expensive to maintain and is more effectively serviced after its use degrades the effectiveness of the polishing.  
         SUMMARY OF THE INVENTION  
         [0010]    Broadly speaking, the present invention fills these needs by providing an apparatus and related methods for efficiently polishing layer surfaces of a semiconductor wafer. Preferably, the CMP system is designed to implement a polishing pad strip that is less expensive to maintain and is more efficiently serviced after it loses its effectiveness to polish. In preferred embodiments, the polishing pad is a fixed abrasive polishing pad strip that is connected between a feed roll and a take-up. It should be appreciated that the present invention can be implemented in numerous ways, including as a process, an apparatus, a system, a device, or a method. Several inventive embodiments of the present invention are described below.  
           [0011]    In one embodiment, a chemical mechanical polishing (CMP) apparatus is provided. The CMP apparatus includes a polishing pad strip, a feed roll, and a take-up roll. The polishing pad strip is defined between a first point and a second point wherein the first point is separate from the second point. The feed roll defines the first point and has a supply of the polishing pad strip. The take-up roll defines the second point and is configured to collect at least a linear portion of the polishing pad strip. The feed roll and the take-up roll are configured to reciprocate so that the polishing pad strip oscillates at least partially between the first point and the second point.  
           [0012]    In another embodiment, a chemical mechanical polishing (CMP) apparatus is provided. The CMP apparatus includes a polishing pad strip defined between a first point and a second point wherein the first point being separate from the second point. A feed roll that defines the first point and has a supply of the polishing pad strip. A take-up roll that defines the second point and collects at least a linear portion of the polishing pad strip. The feed roll and the take-up roll are configured to reciprocate so that the polishing pad strip oscillates at least partially between the first point and the second point at a programmable rate at least partially between the first point and the second point. The programmable rate defines a linear velocity for the polishing pad strip in a direction between the first point and the second point as well as between the second point and the first point.  
           [0013]    In still a further embodiment, a chemical mechanical polishing (CMP) apparatus is provided. The CMP apparatus includes a first roller situated at a first point and a second roller situated at a second point. The first point is separate from the second point. Also included in the apparatus is a polishing pad strip having a first end secured to the first roller and a second end secured to the second roller. The first roller and the second roller are configured to reciprocate so that the polishing pad strip oscillates at least partially between the first point and the second point.  
           [0014]    The advantages of the present invention are numerous. Most notably, instead of a continuous belt polishing pad, a supply of polishing pad strip is provided between a feed roll and a take-up roll in a web handling arrangement. Thus, replacing used portions of the polishing pad strip with fresh portions of the polishing pad strip can be accomplished utilizing minimal effort and in significantly less amount of time. Furthermore, the re-supplying of the polishing pad strip can be achieved easily and expeditiously thereby minimizing the length of time needed to take the polishing system off-line thus having minimal effect on the throughput. Accordingly, the apparatus and the methods of the present invention provide for polishing surface layers of a wafer using a polishing pad that is less expensive to maintain and is more effectively serviced after its use degrades the effectiveness of the polishing.  
           [0015]    Other aspects and advantages of the invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    The present invention will be readily understood by the following detailed description in conjunction with the accompanying drawings, and like reference numerals designate like structural elements.  
         [0017]    [0017]FIG. 1 illustrates an exemplary prior art CMP system.  
         [0018]    [0018]FIG. 2A is a cross-sectional view of an oscillating CMP system, in accordance with one embodiment of the present invention.  
         [0019]    [0019]FIG. 2B is a cross-sectional view of an oscillating CMP system, illustrating the system&#39;s tension setting mechanism and velocity control mechanism, in accordance with another embodiment of the present invention.  
         [0020]    [0020]FIG. 2C is a cross-sectional view of an oscillating CMP system, illustrating the feed roll&#39;s design to hold an ample supply of the polishing pad strip, in accordance with yet another embodiment of the present invention.  
         [0021]    FIGS.  2 D- 1  is a plan-view of an abrasive polishing pad strip, in accordance with yet another embodiment of the present invention.  
         [0022]    FIGS.  2 D- 2  is a cross-sectional view of an abrasive polishing pad strip, revealing the plurality of posts containing a plurality of abrasive particles, in accordance with yet another embodiment of the present invention.  
         [0023]    [0023]FIG. 3A is a cross-sectional view of the CMP system in which the tension actuators are positioned to the right and to the left of the feed roll and the take-up roll, respectively, in accordance with yet another embodiment of the present invention.  
         [0024]    [0024]FIG. 3B is a cross-sectional view of the CMP system, depicting the system&#39;s tension setting and velocity control mechanisms, in accordance with yet another embodiment of the invention.  
         [0025]    [0025]FIG. 4A is a cross-sectional view of the CMP system in which the tension actuators are connected to the idler rollers, in accordance with yet another embodiment of the present invention.  
         [0026]    [0026]FIG. 4B is a cross-sectional view of the CMP system, depicting the system&#39;s tension setting mechanism as well as velocity control mechanism, in accordance with yet another embodiment of the invention.  
         [0027]    [0027]FIG. 5A is a cross-sectional view of the CMP system in which the feed roll and take-up roll maintain and control both the tension exerted on the polishing pad strip as well as the linear velocity of the polishing pad strip, in accordance with yet another embodiment of the invention.  
         [0028]    [0028]FIG. 5B is a cross-sectional view of the CMP system, depicting the system&#39;s tension and velocity control mechanism, in accordance with yet another embodiment of the invention.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0029]    An invention for a CMP system, which enables efficient polishing of layer surfaces of a wafer is described. The CMP system preferably implements a polishing pad that is less expensive to maintain and is more efficiently serviced after it loses its effectiveness to polish. In preferred embodiments, the polishing pad is a fixed abrasive polishing pad. The fixed abrasive polishing pad is preferably provided as a polishing pad strip that is connected between a feed roll and a take-up. This configuration is referred to herein as a web handling arrangement. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be understood, 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 operations have not been described in detail in order not to unnecessarily obscure the present invention.  
         [0030]    [0030]FIG. 2A is a cross-sectional view of an oscillating CMP system  200 , in accordance with one embodiment of the present invention. The CMP system  200  in FIG. 2A includes a feed roll  212   a  positioned at a first point  211   a . The feed roll  212   a  is configured to hold a roll of a polishing pad strip  202 . A take-up roll  212   b  is positioned at a second point  211   b , and is placed, in this embodiment, symmetrically across from the feed roll  212   a  and is configured to receive the polishing pad strip  202 . The direct distance between the feed roll  212   a  and take-up roll  212   b  is estimated to be about 20 inches. Of course, the distance between the feed roll  212   a  and take-up roll  212   b  may vary depending on the specific implementation. In this embodiment, each of the feed roll  212   a  and the take-up roll  212   b  is designed to contain an internal motor. Preferably, the internal motor is a servo drive, such as a direct drive servo. The internal motors are designed to allow the feed roll  212   a  and take-up roll  212   b  to reciprocate. The reciprocating motions of the feed roll  212   a  and take-up roll  212   b  cause the polishing pad strip to oscillate at a linear velocity ranging from about 140 feet per second to about 350 feet per second. The actual linear velocity selected for a polishing operation will also depend on the force at which a polishing head holding a wafer is applied to the polishing pad strip and the platen. The limits of the linear velocity and the force are generally calibrated using the well known Preston&#39;s Equation. According to Preston&#39;s Equation, Removal Rate=KpPV, where the removal rate of material is a function of Downforce (P) and Linear Velocity (y), with Kp being the Preston Coefficient, a constant determined by the chemical composition of the slurry (or fixed abrasive material and chemicals), the process temperature, and the pad surface, among other variables.  
         [0031]    In this embodiment, tension actuators  214   a  and  214   b  are positioned directly below the feed roll  212   a  and take-up roll  212   b , respectively. The tension actuators  214   a  and  214   b  are configured to controllably pull on the feed roll  212   a  and take-up roll  212   b  thereby causing the feed roll  212   a  and take-up roll  212   b  to exert tension on the polishing pad strip  202 . It should be understood that each of the tension actuators can be any type of linear actuator. For instance, each tension actuator can be replaced with cylinders, coils, screws or linear motors.  
         [0032]    Positioned above the feed roll  212   a  is a load cell roller  208   a  defined by a roller that measures the tension exerted on the polishing pad strip  202  on the side closest to intermediate point  207   a  (e.g., left side). The load cell roller  208   b  is also defined by a roller that measures the tension exerted on the polishing pad strip  202  on the side closest to the intermediate point  207   b  (e.g., right side). In this example, the load cell roller  208   b  is positioned symmetrically across from the load cell roller  208   a  and directly above the take-up roll  211   b . Therefore, the polishing pad strip  202  is located on top of the load cell rollers  208   a  and  208   b , and the load cell rollers  208   a  and  208   b  are configured to provide a location where the polishing pad strip  202  is caused to change angular orientation. For instance, the angular orientation may be about 90 degrees so that only the horizontal components of the forces applied on the load cell rollers  208   a  and  208   b  are measured. An idler roller  210   a  defined by a roller fixed to a point is positioned between feed roll  212   a  and load cell roller  208   a . Across from the idler roller  210   a , is positioned an idler roller  210   b . The idler rollers  210   a  and  210   b  are designed to support the polishing pad strip along a path that will ensure the 90-degree angle described above. Thus, the idler rollers  210   a  and  210   b  are further designed to allow the load cell rollers  208   a  and  208   b  to measure only the horizontal components of the forces applied on the load cell rollers  208   a  and  208   b . The horizontal components of the applied forces are equivalent to the tension exerted on the polishing pad strip  202  on the left side and the right side of the polishing head  204 .  
         [0033]    A polishing head  204  is designed to carry a wafer (not shown in the figure) and rotates in a rotation direction  205 . A platen  206  is positioned horizontally between load cell rollers  208   a  and  208   b . Platen  206  is configured to stabilize the polishing pad strip  202  and to provide a solid surface onto which to apply the polishing head  204 . In some cases, it is possible to control the surface between the platen  206  and the polishing pad strip  202  to control the removal rate in different locations on the wafer. In one embodiment, the polishing pad strip  202  is a fixed abrasive polishing pad, which has a polishing layer containing abrasive particles extended throughout the surface and the material thickness. As the polishing head  204  applies the wafer (not shown in the figure) against the polishing pad strip  202 , the abrasive particles of the polishing pad strip  202  become loose thereby eliminating the necessity to use a slurry containing abrasive materials. Although a slurry containing abrasive particles is not required, a liquid solution (e.g., NH 4 OH or DI water) is preferably used to facilitate the polishing process.  
         [0034]    As depicted in the embodiment of FIG. 2B, a certain portion of the supplied polishing pad strip  202  held in the feed roll  212   a  is fed around the load cell rollers  208   a  and  208   b  to the take-up roll  211   b . After polishing a given number of wafers, the portion of the polishing pad strip  202  which came into contact with the wafers loses its effectiveness and must be replaced. The used portion of the polishing pad strip  202  is replaced by an unused portion of the polishing pad strip  202  by way of the feed roll  212   a  indexing the polishing pad strip  202 , utilizing a programmable amount (e.g., enough to place a fresh portion of the polishing pad strip  202  over the platen  206 ). The indexing causes the used portions of the polishing pad strip  202  to be pushed farther and farther away from the polishing area. The used portions of the polishing pad strip  202  are collected by the take-up roll  212   b  and will ultimately be discarded. Once the supply of the polishing pad strip  202  held in feed roll  212   a  is completely consumed, it can easily be replaced with a new roll of the polishing pad strip  202 . The process of re-supplying the feed roll  212   a  with the polishing pad strip  202  is neither labor intensive nor time consuming. More importantly, the CMP machine will be off-line, if necessary, less frequently and for a significantly less amount of time thereby causing minimal effect on the throughput of the machine.  
         [0035]    Also clearly shown in FIG. 2B are the tension actuators  214   a  and  214   b  which are configured to controllably pull on the feed roll  212   a  and take-up roll  212   b  causing the feed roll  212   a  and take-up roll  212   b  to apply pressure to the polishing pad strip  202  at the first intermediate point  207   a  and the second intermediate point  207   b , respectively. Due to normal wear, the polishing pad strip  202  can stretch, thereby causing the amount of tension exerted on the polishing pad strip  202  to reduce. This system is designed to maintain a desired tension by way of changing the amount of force the tension actuators  214   a  and  214   b  apply on the feed roll  212   a  and take-up roll  212   b , respectively.  
         [0036]    This task is achieved by the load cell roller  208   a  sending a tension feedback signal to an amplifier  222   a , which is a part of a first tension-velocity controller  220   a . Subsequently, a tension setting command, either supplied manually or automatically through a computerized device, is fed to the amplifier  222   a . Thereafter, the amplifier  222   a  sends a tension output signal to a tension control device  226   a , which is also a part of the tension-velocity controller  220   a . Finally, the tension control device  226   a  sends a tension (TN) signal to the tension actuator  214   a.    
         [0037]    In a like manner, an amplifier  222   b , which is a part of a tension-velocity controller  220   b  receives a tension feedback (FB) signal from load cell roller  208   b . Subsequently, a tension setting command, either supplied manually or automatically through a computerized device, is fed to the amplifier  222   b . Thereafter, the amplifier  222   b  sends a tension (TN) output signal to a tension control device  226   b , which is also a part of the tension-velocity controller  220   b . Finally, the tension control device  226   b  sends a tension signal to the tension actuator  214   a . Depending on the tension signals received from the tension-velocity controllers  220   a  and  220   b , the tension actuators  214   a  and  214   b  may or may not exert additional force on the feed roll  212   a  and take-up roll  212   b  so as to achieve a desired tension (e.g., either higher or lower).  
         [0038]    Once the desired tension is exerted on the polishing pad strip  202 , the internal motors located inside the feed roll  212   a  and take-up roll  212   b  will cause the feed roll  212   a  and take-up roll  212   b  to reciprocate, synchronously, thereby causing the polishing pad strip  202  to oscillate at a linear velocity. In one embodiment, to achieve optimum performance, the linear velocity of the polishing pad strip  202  should be maintained within the range of about 140 ft/sec and about 350 ft/sec. Thus, the linear velocity of the polishing pad strip  202  should be measured frequently by the feed roll  212   a  and take-up roll  212   b . Besides measuring the velocity of the polishing pad strip  202 , the feed roll  212   a  and take-up roll  212   b  control and change, if necessary, the velocity of the polishing pad  202  so as to maintain a desired velocity.  
         [0039]    As an example, the feed roll  212   a  initially sends out a velocity feedback to a Proportional, Integral and Derivative (PID)  224   a , which is a part of the tension-velocity controller  220   a . Then, a velocity setting command, either supplied manually or automatically using a computerized device, is fed to the PID  224   a . Finally, the PID  224   a  sends out a velocity signal to the feed roll  212   a.    
         [0040]    Similarly, the take-up roll  212   b  sends out a velocity feedback to a Proportional, Integral and Derivative (PID)  224   b , which is a part of the tension-velocity controller  220   b . Then, a velocity setting command, either supplied manually or by way of a programmable machine, is fed to the PID  224   b . Finally, the PID  224   b  sends out a velocity signal to the take-up roll  212   b . The velocity signals received by the feed roll  212   a  and the take-up roll  212   b  are the determinative factors as to whether the feed roll  212   a  and take-up roll  212   b  must maintain or change the rate of reciprocating. Although the tension-velocity controllers  220   a  and  220   b  have been illustrated using exemplary electronics, it should be understood that the electronics and control signals can be processed using any other suitable well known processing techniques (e.g., software/hardware combinations). For instance, the PID electronics can be substituted with other circuitry that can process and control the signals as may be desired.  
         [0041]    As clearly evident from the embodiment of FIG. 2C, the feed roll  212   a  is designed to hold an ample supply of the polishing pad strip  202 . Utilizing minimal effort, the feed roll  212   a  can be re-supplied with the fresh polishing pad strip  202  thereby having minimum effect on the throughput of the CMP machine.  
         [0042]    FIGS.  2 D- 1  depicts one of many types of the polishing pad strip  202 , which has a fixed abrasive polishing layer. The approximate thickness of this type of polishing pad strip  202  ranges from about 0.004 inch to about 0.010 inch. Embedded and extended through out the surface of this type of polishing pad strip  202  are several three-dimensional protrusions, which are defined as posts  202 ′. The cross-sectional view of the polishing pad strip  202 , as shown in FIGS.  2 D- 2 , reveals that each post  202 ′ contains a plurality of abrasive particles having an approximate size in the range from about 40 micrometer and about 200 micrometer.  
         [0043]    Another embodiment of the present invention is shown in FIG. 3A wherein the tension actuator  314   a  is positioned to the right of the feed roll  212   a . In a like manner, the tension actuator  314   b  is situated to the left of the take-up roll  212   b . In this embodiment, by respectively pulling on the feed roll  212   a  and take-up roll  212   b , the tension actuators  314   a  and  314   b  will cause the feed roll  212   a  and take-up roll  212   b  to controllably exert tension on the polishing pad strip  202 .  
         [0044]    For example, in the embodiment of FIG. 3B, the tension actuators  314   a  and  314   b  control the amount of tension exerted on the polishing pad strip  202 . This is achieved by the load cell roller  208   a  sending out a tension feedback to the tension/velocity controller  220   a , which in turn, after internally processing the tension feedback, sends a tension signal to the tension actuator  314   a . Similarly, the load cell roller  208   b  sends out a tension feedback to the tension/velocity controller  220   b . Once the tension/velocity controller  220   b  processes the tension feedback, internally, it sends a tension signal to the tension actuator  314   b . Depending on the tension signals received, if necessary, the tension actuators  314   a  and  314   b , may change the amount of force each of them exerts on the feed roll  212   a  and take-up roll  212   b  so as to achieve a desired tension.  
         [0045]    Once the desired tension is set for the polishing pad strip  202 , the synchronous reciprocation of the feed roll  212   a  and take-up roll  212   b  start thereby causing the polishing pad strip  202  to oscillate at a linear velocity. In one embodiment, the linear velocity of the polishing pad strip  202  may be measured frequently or at set times. Depending upon the measurements, adjustments can be made to the tension that is controlled by the feed roll  212   a  and take-up roll  212   b . The feed roll  212   a  and take-up roll  212   b  each send out a velocity feedback to the tension/velocity controllers  220   a  and  220   b , respectively. Then, after internally processing the velocity feedbacks, the tension/velocity controllers  220   a  and  220   b , each sends out a velocity signal to the feed roll  212   a  and take-up roll  212   b . Depending on the velocity signals received, if necessary, the feed roll  212   a  and take-up roll  212   b  may change the rate of reciprocating, thus fixing a new linear velocity for the polishing pad strip  202 .  
         [0046]    The embodiment of FIG. 4A depicts an oscillating CMP system  200   b  that is similar to the embodiment of FIG. 2A, with the exception that the tension actuators  414   a  and  414   b  are positioned outside the idler rollers  210   a  and  210   b . In this embodiment, the tension actuators are configured to pull on the idler rollers  210   a  and  210   b  so as to cause the idler rollers  210   a  and  210   b  to exert tension on the polishing pad strip  202 .  
         [0047]    In this case, there will be points in time when the vertical portions of the polishing pad strip  202  will not be at a 90 degree angle relative to the polishing region (e.g., where the platen  206  is located) of the polishing pad strip  202 . Nevertheless, the tension can be controllably adjusted to a correct desired level. It should therefore be understood that it is not necessary to have the vertical and horizontal portions of the polishing pad strip  202  at a 90 degree angle at all times so long as the polishing pad strip  202  provides the desired optimum polishing condition at the location where polishing is to be performed on the wafer surfaces.  
         [0048]    As shown in the embodiment of FIG. 4B, the load cell roller  208   a  sends out a tension feedback to the tension/velocity controller  220   a . After internally processing the tension feedback, the tension/velocity controller  220   a  sends out a tension signal to the tension actuator  414   a . Similar signals are also exchanged between the load cell roller  208   b , tension/velocity controller  220   b  and tension actuator  414   b.    
         [0049]    Once each of the tension actuators  414   a  and  414   b  respectively receive a tension signal from  220   a  and  220   b , depending on the tension signals received, tension actuators may, if necessary, change the force by which they exert tension on the polishing pad strip  202 . After achieving the desired tension, the feed roll  212   a  and take-up roll  212   b  start reciprocating, preferably synchronously, causing the polishing pad strip to oscillate at a desired linear velocity. Similar to the embodiments of FIGS. 2B and 3B, the feed roll  212   a  and take-up roll  212   b  maintain and if necessary, change the velocity of the oscillation of the polishing pad strip  202 .  
         [0050]    [0050]FIG. 5A depicts an oscillating CMP system  200   c  wherein the feed roll  212   a  and take-up roll  212   b  maintain and control both the tension exerted on the polishing pad strip  202  as well as the linear velocity of the polishing pad  202 . Accordingly, the tension actuators have completely been eliminated from the CMP system  200   c.    
         [0051]    As illustrated in FIG. 5B, in a CMP system  200   c ′, the load cell roller  208   a  sends a tension feedback to an amplifier  322   a  that is part of the tension-and-velocity controller  320   a . Thereafter, a tension setting command, supplied either manually or automatically through a computerized device, is fed to the amplifier  322   a . Then, the amplifier  322   a  sends a tension output signal to a tension and velocity control device  326   a.    
         [0052]    Thereafter, a velocity feedback is sent from feed roll  212   a  to a PID  324   a  also positioned within the tension-and-velocity controller  320   a . In a subsequent operation, a velocity setting command, supplied either manually or by way of a programmable machine, is fed to the PID  324   a . Then, the PID  324   a  sends a velocity output signal to the tension and velocity control  326   a . After receiving the tension output signal and the velocity output signal, the tension and velocity control  326   a  sends out a tension and velocity signal to the feed roll  212   a.    
         [0053]    Similarly, a tension feedback and a velocity feedback are respectively fed to an amplifier  322   b  and a PID  324   b , which are part of the tension-and-velocity controller  320   b . Then, a tension setting command is fed to the amplifier  322   b , which in turn, sends out a tension output signal to a tension and velocity control  326   b , which is also a part of the tension-and-velocity controller  320   b . Next, a velocity setting command is fed to the PID  324   b , which subsequently sends out a velocity command signal to the tension and velocity control  326   b . After receiving the tension output signal and the velocity output signal, the tension and velocity control  326   b  sends out a tension and velocity signal to the take-up roll  212   b.    
         [0054]    Depending on the tension and velocity signals received by the feed roll  212   a  and take-up roll  212   b , the feed roll  212   a  and take-up roll  212   b  may, if necessary, each rotate inwardly in the direction (TA) so as to adjust the tension exerted on the polishing pad strip  202  to a desired level. Once the tension applied to the polishing pad strip  202  is set to a desired level, the feed roll  212   a  and take-up roll  212   b  start, preferably, a synchronous reciprocation thereby causing the polishing pad to oscillate at a linear velocity under the polishing head  204 . Thus, in this embodiment, similar to some of the embodiments, the feed roll  212   a  and take-up roll  212   b  can change, if necessary, the velocity of the polishing pad  202  so as to maintain a desired velocity for optimum polishing performance.  
         [0055]    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. For example, embodiments described herein have been primarily directed toward wafer polishing, however, it should be understood that the polishing operations are well suited for precision polishing of any type of substrate. 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.