Patent Publication Number: US-6336851-B1

Title: Substrate belt polisher

Description:
The present application is a continuation of U.S. application Ser. No. 08/568,188, filed Dec. 5, 1995. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to the field of chemical mechanical polishing. More particularly the present invention relates to apparatus and methods for chemical mechanical polishing of substrates used in the manufacture of integrated circuits. 
     BACKGROUND OF THE INVENTION 
     Chemical mechanical polishing is a method of planarizing or polishing semiconductor and other types of substrates. At certain stages in the fabrication of devices on a substrate, it may become necessary to polish the surface of the substrate before further processing may be performed. One polishing process, which passes a conformable polishing pad over the surface of the substrate to perform the polishing, is commonly referred to as mechanical polishing. Mechanical polishing may also be performed with a chemically active abrasive slurry, which typically provides a higher material removal rate and a higher chemical selectivity between films of the semiconductor substrate than are possible with mechanical polishing. When a chemical slurry is used in combination with mechanical polishing, the process is commonly referred to as chemical mechanical polishing, or CMP. 
     Prior art CMP process typically include a massive rotating platen containing colloidal particles in an alkaline slurry solution. The substrate to be polished is held against the polishing platen by a polishing head or carrier which can be moved in an x-y direction over the plane of the platen from a position near its outside diameter to a position close to its center. The platen is several times larger than the substrate to be polished. The substrate is rotated independently while pressure is maintained between the substrate and the polishing pad. 
     The rate of material removal from the substrate in CMP is dependent on several factors including, among others, the chemicals and abrasives used in the slurry, the surface pressure at the polishing pad/substrate interface and the net motion between the substrate and the polishing pad. Generally, the higher the surface pressure and net motion at the regions of the substrate which contact the polishing pad, the greater the rate of removal of material from the substrate. It should be appreciated that equipment capable of performing this process is relatively massive and difficult to control to the precision necessary to consistently remove an equal amount of material on all areas of the substrate. 
     Using a large polishing pad of CMP processing creates several additional processing limitations which lead to non-uniformities in the polished substrate. Because the entire substrate is rotated against the polishing pad, the entire surface of the substrate is polished to a high degree of flatness as measured across the diameter of the substrate. However, where the substrate is warped, the portions of the substrate which project upwardly due to warpage tend to have higher material removal rates than the remainder of the substrate surface. Furthermore, as the polishing pad polishes the substrate, material removed from the substrate forms particulates which may become trapped in the pad, as the polishing slurry dries on the pad. When the pad becomes filled with particulates and the slurry dries in the pad, the polishing surface of the pad glazes and its polishing characteristics change. Unless the user constantly monitors the removal rate of the polishing pad with each substrate, or group of substrates, and adjusts the slurry, load, position, and/or rotational speed of the polishing pad to maintain the desired material removal rate, the amount of material removed by the polishing pad from each substrate consecutively processed thereon will decrease. 
     SUMMARY OF THE INVENTION 
     The present invention provides methods and apparatus for polishing substrates where the polishing pad is a flexible membrane strip or belt (preferably continuous) which moves linearly between adjacent support rollers to provide uniform polishing of the substrate in contact with the moving membrane. In one embodiment a flexible polishing membrane has a substrate holder (polishing head), holding a substrate for polishing on a first side of the linearly moving membrane and a membrane backing member on a second side of the linearly moving membrane. The substrate holder and the membrane backing member are collectively configured to provide a set of clamping forces to urge the substrate and the first side of said membrane into contact with one another for polishing. 
     In one embodiment the membrane backing member is a flat surface having generally equally distributed fluid holes therein. The holes face the back of the flexible polishing membrane such that when the membrane backing member is brought into close proximity to the flexible membrane and fluid (liquid or gas) is flowing out from the holes a fluid layer is formed between the surface of the backing member and the second side of the flexible membrane (belt). Clamping forces urging the belt and backing member together are generally uniformly resisted by the intervening fluid layer which provides a nearly uniform pressure between the membrane and backing member. The uniform pressure on the backside (second side) of the membrane is substantially transferred through the membrane to provide uniform mechanical abrasion over the surface of the substrate being polished by rubbing against the first side of the membrane. The set of forces urging the substrate and membrane against one another can be varied in conjunction with, or independently of, any adjustment in the speed at which the membrane moves relative to the substrate being polished. 
     Preferably the substrate is fixed in the substrate holder at a location generally closely adjacent to the path of the freely moving membrane (belt). The backing member is supported by an urging member whose force can be adjusted. In one example, the force supplied by the urging member on the backing member is provided by a bellows assembly having bellows whose internal pressure is controlled to maintain a pre-set force on the back of the membrane regardless of dimensional variations in the surface of the substrate and in the thickness of the membrane belt and any liquids or slurries on its surface. 
     Alternately, the backing member can be held fixed while the substrate holder and substrate can be urged by an adjustable urging member whose force can be adjusted. Similar to the urging member discussed above for the backing member, the force supplied by the urging member on the substrate member is provided by a bellows assembly having bellows whose internal pressure is controlled to maintain a pre-set force on the membrane regardless of dimensional variations. 
     As a third alternative, adjustable urging forces can be provided to both the substrate holder and to the membrane backing member. However the balancing of such forces would have to be controlled carefully to assure that nearly central alignment of the flexible membrane between its adjacent rollers (pulleys) is maintained. 
     Polishing of wafers as described above is done by a belt which is generally wider and longer than the size of a single substrate (wafer). Polishing contact takes place over the whole surface of the wafer at once, as the belt is generally in contact with the full width and length of the substrate&#39;s surface at one time. If the wafer were held stationary relative to the belt, then anomalies or imperfections in the polishing membrane (belt) would be transferred to the wafers surface. To avoid or reduce the possibility that any such anomalies would form the substrate is slowly rotated and is also oscillated from side to side to distribute the effect of any such anomalies over a larger area. 
     To avoid excess polishing at the edges of the substrate from the natural bowing of the flexible membrane when it is subjected to pressure from one side, a perimeter or fence ring is provided around the substrate. The perimeter ring, made of a highly abrasion resistant material such as Delrin or Ultra High Molecular Weight plastics, such as polyethylene, provide an artificial extension of the edge of the substrate. The transition between the edge of the substrate and the inside diameter of the perimeter ring is flat. The edge effect which causes additional wear at locations where the membrane bends because it is displaced from its natural course by the action of either the membrane backing member or the substrate support head, occurs only at the outer edges of the perimeter ring. The edge of the substrate is therefore insulated from edge effects by the perimeter ring which acts as a buffer. 
     Polishing as described herein is preferably done in a horizontal plane, but can be performed in a vertical orientation, or at any other angle where the substrate can be held for engagement and disengagement with the flexible polishing membrane. 
     Polishing wafer can also be done by using flexible polishing membranes which provide coverage less than the full area of the wafer. One example of such a configuration provides for a flexible polishing membrane which has a width whose dimension is less than the diameter of a substrate to be polished. The substrate is mounted in a holding fixture which faces a narrow circulating belt. The belt is moved back and forth transversely across the substrate to provide polishing of the full width of the substrate. The substrate and/or the belt rotating mechanism can be slowly rotated to further avoid the localized effect of belt anomalies or imperfections from being detected in the final finish polished substrate. 
     Still other polishing configurations reduce the contact area between the flexible polishing membrane and the surface of the substrate to a small fraction of the area of the surface of the wafer. A set of two or more small rollers cause a narrow belt to rotate in a belt carrier unit. The unit is then manipulated to move relative to the surface of the substrate to evenly polish each unit of area on the surface. For example when the substrate is rotating independently from the movement of the belt carrier unit, the higher surface velocity of the substrate near its circumference must be taken into account by providing a lower dwell time at the perimeter while compensating for the lower surface velocity near the center of the substrate by providing a longer dwell time for the belt carrier unit. 
     In another embodiment, the apparatus includes a rotating plate on which the substrate is held, and polishing arm which is located adjacent the plate and is moved across the surface of the substrate as the substrate rotates on the rotating plate. The polishing arm includes a polishing pad on the end thereof, which is preferably variably loadable against the surface of the substrate as different areas of the substrate are polished thereby. The speed of rotation of the substrate may be varied, in conjunction with, or independently of, any adjustment of the polishing pad against to control the rate of material removed by the polishing pad as it crosses the substrate. The polishing arm includes a cartridge of polishing pad material in tape form, a discrete length of which is exposed over the lower tip of the of the polishing arm to contact the substrate for polishing. The tape of polishing pad material may be moved over the polishing arm tip to continuously provide a new polishing pad surface as the substrate is processed, or may be moved to provide a discrete new section of polishing pad tape to polish each new substrate or allow the movement of the tape to move together with the arm to provide polishing. In another arm based configuration, the polishing pad may be offset from the polishing arm, and the polishing arm may be rotated over the rotating substrate to cause the polishing pad to contact the rotating substrate as the polishing pad also rotates about the axis of the polishing arm. 
     The mechanical abrading of the surface of a substrate being polished is performed by placing a slurry of colloidal particles on the surface of the polishing membrane to act as the agent for polishing. This slurry is messy and must be kept wet to remain fluid to avoid excessive build up of particles and the polishing anomalies that such buildups may create. Deionized water is therefore run onto the belt along with the slurry to maintain its fluid state and replenish the abrasive colloidal members. An option to a stream of de-ionized water is to run the belt (continuous flexible membrane) through a bath of fluid and/or to condition the surface of the belt by winding the path of the belt over a conditioning/idler pulley. The surface of the pulley would include a grooved surface pattern such as knurling to allow a nonuniform build-up of caked on slurry to be knocked off or distributed by the pattern (usually regular) on the surface of the conditioning idler pulley. While not presently available, a dry belt which would provide the same or a very similar abrading action would be preferred to eliminate the mess and complications associated with the use of slurry. As far as is known no dry-type continuous belts for CMP are presently available. 
     In CMP the chemical part of the activity is performed by providing typically an alkali (reducing) solution such as NaOH to the surface of the substrate during processing. The alkali solution causes softening of the surface of the substrate. The softened layer can then be more easily removed by the mechanically abrasive colloidal particles in the slurry. The depth of softening of the surface by the alkali solution is dependent on the time of contact between the solution and the surface. The introduction and removal of alkali solution must be carefully controlled to avoid over or under polishing the surface of the substrate. The chemical treatment provides for removal of the surface layer of the substrate to a uniform depth, rather than a strictly mechanical planarization which when planarizing substrates with high and low points takes more from high points and less from low points thereby increasing the possibility that layers of material which have been uniformly deposited over underlying undulating layers will be breached and the substrate features damaged or rendered less reliable as a result of the build up of manufacturing tolerances. 
     A method according to the present invention includes the nearly theoretically ideal arrangement where the surface of the substrate being processed is uniformly exposed to an abrasive agent with a uniform force between the membrane carrying the abrasive and the substrate. The method includes the method steps of: holding a substrate to be processed in close proximity to a linearly moving membrane 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of an embodiment according to the invention showing a continuous flexible polishing membrane (belt) wrapped on three rollers with a polishing head holding the substrate being polished on top of the membrane, a membrane backing assembly opposite the polishing head below the polishing membrane; 
     FIG. 2 is a cross section of FIG. 1 taken at  2 — 2  showing the internal configuration of the polishing head and the polishing membrane backing assembly; 
     FIG. 3 is a close-up view of FIG. 2 taken at  3 — 3 ; 
     FIG. 4 shows an exploded view of the polishing head assembly and the polishing membrane backing assembly, according to the invention, in relation to the polishing membrane; 
     FIG. 5 shows a schematic top view of the polishing membrane at its interface with the polishing membrane as shown in FIGS. 1-4; 
     FIG. 6 shows a top view of FIG. 1; 
     FIG. 7 is an elevation view of a configuration according to the invention showing the substrate being polished at a polishing location between two rollers on top of the polishing membrane, the polishing head not being shown and the flexible membrane circulating through a vessel partially filled with a rinse solution to assist in conditioning the polishing surface of the membrane being polished; 
     FIG. 8 shows a configuration according to the invention showing the polishing location at the bottom side of a set of three membrane rollers with the substrate on the inner surface of the polishing membrane, the polishing head not being shown; 
     FIG. 9 shows a configuration according to the invention showing the polishing location at the bottom side of a set of three membrane rollers with the substrate on bottom of the polishing membrane, the polishing head not being shown; 
     FIG. 10 shows a configuration according to the invention showing the polishing location on the top side of a set of two membrane rollers with the substrate on top of the polishing membrane, the polishing head not being shown; 
     FIG. 11 shows a configuration according to the invention showing the polishing location on the top side of a set of four membrane rollers with the substrate on top of the polishing membrane, and an alternate arrangement with the polishing location on a vertical leg of travel, the polishing heads not being shown; 
     FIG. 12 shows a configuration according to the invention showing two polishing locations on a polishing membrane having a width so that the processing of a substrate at one polishing location generally does not affect the polishing of a second substrate at a second polishing location, the polishing heads not being shown; 
     FIG. 13 shows a cut away perspective view of a partial width polishing membrane and its movement across a substrate being polished, the return side of the polishing membrane loop is cut away for clarity, the polishing head away from the substrate not being shown; 
     FIG. 14 shows a cross sectional view of the polishing membrane backing faceplate assembly used in FIG. 13 taken at  14 — 14 ; 
     FIG. 15 is a perspective view of a belt polishing head/carrier according to the invention for use in a relative motion which sweeps over the surface of the wafer in a predetermined pattern for uniform polishing of the surface of the wafer; 
     FIG. 16 shows a close-up view of the polishing membrane carrier assembly shown in FIG. 15; 
     FIG. 17 shows a two roller generally vertical orientation for a polishing head/carrier of the type shown in FIG. 15; 
     FIGS. 18,  19 ,  20 ,  21 ,  22 ,  23 ,  24  and  25  show a variety of schematic arrangements of the polishing head, the substrate, and the polishing membrane backing assembly (faceplate), according to the invention; 
     FIG. 26 is a perspective view partially cutaway of another embodiment of the chemical mechanical polishing apparatus according to the present invention; 
     FIG. 27 is a partial side view of the apparatus of FIG. 26 with the side of the base removed 
     FIG. 28 is a partial side view of an alternative embodiment of the apparatus of FIG. 27; 
     FIG. 29 is a side view of the polishing arm of the apparatus of FIG. 28; 
     FIG. 30 is perspective view of a further embodiment according to the present invention; and 
     FIG. 31 is a schematic view of the control system used with a chemical mechanical polishing apparatus of the present invention. 
    
    
     DETAILED DESCRIPTION 
     Chemical mechanical polishing (CMP) involves polishing a substrate surface by using a chemical (e.g. an alkaline solution) to react with the surface to be polished and then abrading the surface by mechanical means. A uniform distribution of the chemical and a uniform application of the abrading agent will result in a generally smooth, but not necessarily planar surface which is compatible with subsequent substrate processing steps. 
     A continuous belt sanding device can contact the substrate with a spatially uniform pressure to uniformly abrade the surface to be polished. A continuous belt, subject to variations in properties across its width, provides uniform abrasion (wear pattern) across the substrate surface. Uniform abrasion is achieved when an equal net length of a polishing membrane (or belt) travels past each unit of surface area on the surface of the substrate and the abrasive media is evenly distributed on the polishing membrane. If a large width of the substrate is being swept by a single pass of the belt, then it is possible that some variation in abrasion might be detected when an abrasive track (assuming parallel imaginary tracks on a continuous belt) moves over a longer length of the substrate (for instance between its leading and trailing edges near the centerline of a circular wafer) when compared to a similar track moving over a shorter length of substrate (for instance near the edge of a circular wafer). This potentially very slight variation is explained by the fact that colloidal abrasive particles present in the slurry and become contaminated with removed material as they move across the substrate so that the belt&#39;s abrasive efficiency decreases with a longer contact surface. 
     A configuration according to the invention executing the principle of uniform pressure over the surface of the substrate with a uniform belt contact distance across the wafer is shown in FIG.  1 . The perspective view of FIG. 1 shows a configuration including a flexible membrane (polishing belt)  60  (usually an unimpregnated polyester material to which abrasive particles are added in use) routed around three rollers  68 ,  70 ,  72 . A substrate (wafer) holder (polishing head) assembly  30  includes a fixed support  32  connected to a cantilevered arm  34 . The cantilevered arm  34 , as shown in FIG. 1, rigidly supports a polishing head shaft  38  which can be rotated by a rotation mechanism (not shown) and whose vertical motion can be adjusted by a vertical adjustment mechanism (not shown). Alternatively, the fixed support  32  can include hinged or pivoting features to raise or pivot the polishing head assembly  30  so that the substrate  50  being polished (not shown in FIG. 1 as it is on the underside of the polishing head assembly  30 ) can be loaded and unloaded to access polishing operations on the belt  60 . 
     The flexible polishing membrane  60  moves in a right to left longitudinal direction between the top two rollers, i.e. from roller  70  to roller  72 . As the flexible membrane (belt)  60  moves, an abrasive slurry containing colloidal abrasive particles of SiO 2  is distributed over the width of the belt  60  by a slurry distribution manifold  74 . Abrasive slurry is thereby placed on the flexible membrane  60  as it moves towards the polishing head  30 . As the abrasive slurry on the polishing membrane  30  contacts the substrate held by the polishing head  30 , mechanical abrasion polishing of the substrate occurs. The chemical, e.g., NaOH, used to control the polishing rate can be part of the slurry or can be applied to the polishing membrane and substrate at another location in the cycle of the belt, e.g., by using spray nozzles (not shown). 
     It is important to provide an uniform belt pressure across the surface area of the substrate being polished. It is generally not sufficient to place the polishing head  30  against a belt  60  and rely only on the tension of the belt  60  between rollers  70  and  72  to assure uniform polishing of the substrate surface. Instead, a flexible membrane backing assembly  62  (shown in dashed lines in FIG. 1) is provided at a location adjacent to the belt  60  directly opposite to the polishing head  30  on top of the belt. The moving belt is sandwiched between the head  30  and the membrane backing assembly  62 . The backing assembly  62 , when in contact with the belt, assists in providing a uniform contact pressure between the belt  60  and the substrate  50 . 
     The membrane backing assembly  62  includes a fixed support member (membrane backing support bridge)  64  and a generally flat-topped membrane backing faceplate assembly  66 . The membrane backing faceplate assembly  66  provides a uniform pressure to the underside of the moving belt  60  so that a uniform abrading pressure is applied over the surface of the substrate by uniformly pressing the polishing belt  60  upwards, with a small or negligible displacement, toward the fixed polishing head  30  which is located immediately adjacent to the path of the continuous belt  60 . 
     A cross section of the substrate polishing location as shown in FIG. 1 is shown in FIGS. 2 and 3. FIG. 3 is a closeup view of the configuration around one side of the polishing membrane  30 . FIG. 4 shows a perspective exploded view of the details of the polishing head  30  and the membrane backing assembly  62 . The polishing head  30  is supported by a lateral cantilever support  34 . A continuous upper bridge support  36  shown in FIG. 2 presents an example of an alternate support scheme for the polishing head (also shown for example by the bridge support  186  in FIG.  15 ). In either of these configurations, although not shown in the Figures, the substrate  50  and polishing head  30  may be rotated by a rotating mechanism. The substrate  50  and polishing head  30  can also be oscillated laterally (up and down as shown in FIG. 5) across the width of the belt  60 . Such rotation and oscillatory movement prevents any surface defect or anomaly in the polishing belt  60  from creating a corresponding anomaly the surface of the substrate  50  being polished. Slow rotation of the polishing head  30  (providing a diametral speed which is less than {fraction (1/100)}th of the translational speed of the belt  60 ) distributes the action of a defect on the surface of the belt over the surface of the substrate to help minimize its effect. If the polishing head moves at a rate of 100 ft/min then the rotation of the polishing head for an eight inch wafer should be about 1 rpm or provide a 100:1 ratio between the movement of the belt versus the movement related to the rotation of the substrate. Under these conditions, belt or backing assembly defects located far from the center of a stationary rotating substrate are well distributed, while those which are closer to the center of the substrate are less well distributed. If a defect were to be located at the center of the substrate, rotation alone would cause no distribution of the defect. Therefore, to avoid the deleterious effects of such defects, the polishing head  30  is oscillated from side to side in an oscillatory motion. To prevent the polishing head  30  from coming off the belt  60  during such sideways oscillation, the belt  60  is wider than the polishing head  30  by a dimension at least equal to the full amplitude of the oscillation. This necessitates that the membrane backing assembly  62  also be wide enough or move together with the polishing head  30  to maintain uniform pressure on the bottom of the belt  60  opposite the polishing head throughout the extremes of sideways oscillatory travel. In the configuration as shown in FIGS. 1-6, the polishing belt  60  and membrane backing assembly are wider than the substrate  50 . 
     Increased abrasion at the edge of the substrate (edge effects) can result from bowing of the flexible membrane outside the area clamped between the polishing head  30  and the membrane backing assembly  62 . Edge effects can also result from the perimeter (edge) having to ride over or break down (cause distribution of) areas where slurry and/or the colloidal abrasive particles have built up and are not evenly distributed. It is preferable to eliminate the possibility of such edge effects. The configurations of FIGS. 1-6 include a retaining (edge—surface conditioning) ring  52 . The retaining ring  52  surrounds the substrate  50  and prevents it from sliding out from under the polishing head  30 . The retaining ring  52  and substrate are collectively held (or in other configurations pressed) against the moving belt  60 . The thickness of the retaining ring  52  is generally equal to the thickness of the substrate being polished  50  together with any backing pad (e.g., item  46  in FIGS.  2 - 4 ). The retaining ring  52  is attached to the bottom of a main polishing head member  40  so that pressure on the polishing head  50  is uniformly distributed to both the substrate  50  and the retaining ring  52 . The presence of a retaining ring  52  requires that a larger diameter polishing head  30  be used. This in turn requires that the width of the polishing membrane  60  also be increased to prevent any part of the head  30  from coming off the polishing belt  60  during sideways oscillatory motion. The substrate retaining ring  52  is attached to the holding assembly backing plate by screws or generally mechanical holding mechanisms. The ring  52  can be released and replaced when the wear is excessive. 
     The polishing head  30  includes a vacuum manifold  42  to distribute vacuum to vacuum holes  44  in the bottom of the main head member  40 . The vacuum supply to the vacuum manifold  42  is through the polishing head shaft  38  to a rotatable coupling at the top of the shaft (not shown). The pattern of vacuum holes  44  on the bottom side of the main head member  40  partially or fully matches (a partial match utilizes some of the holes to retain the elastomer pad against the main head member) a pattern of holes  48  in the substrate backing pad  46  (preferably an elastomeric pad) to provide a conformable surface which can help to seal the vacuum passages against the substrate  50  during substrate loading and unloading operations and against which the substrate  50  can be pressed for polishing. Other arrangements for holding the wafer utilizing an elastomeric pad may be provided. They include placing an elastomer without holes across larger holes in the main head member  40 . Pulling a vacuum partially pulls the elastomer into the larger holes and creates inverted craters in the elastomer, which when in contact with a wafer, act as suction cups to hold the wafer. When vacuum is pulled in the vacuum manifold  42 , the substrate is held to the bottom surface of the polishing head  30  inside a cavity formed by the retaining ring  52 . Vacuum pressure to the vacuum manifold  42  is controlled to allow loading and unloading of the substrate from the polishing head when the polishing head  30  is shifted to the loading or unloading position (for example as shown by dashed lines  30   a  and  34   a  in FIG.  6 ). These vacuum passages can also be pressurized to assist in release of the substrate  50  from the polishing head  30  or in other configurations to assist in pressing the substrate uniformly toward the moving belt. 
     The membrane backing assembly  62  faces the underside of the polishing membrane  60 . The top surface of the assembly  62  is generally square or rectangular and is located to oppose the polishing head  30 , so that the moving polishing belt is clamped between the two. The membrane backing assembly  62  includes the horizontally extending fixed support member (bridge)  64  supporting a vertically extending fixed support frame (a perimeter wall—forming an open box) consisting of a series of sidewalls, e.g.  96 ,  98 , over which a generally horizontally extending faceplate  76  floats. The faceplate  76  is allowed to float vertically, but is retained horizontally, by the fixed sidewalls, e.g.,  96 , 98 . The sidewalls, e.g.,  96 , 98  can be seen in FIGS. 2 and 4. An extendible bellows  100  flexibly connects the membrane backing support  64  to the floating faceplate  76 . The bellows  100  can be pressurized to a fixed pressure or the pressure within the bellows can be controlled to provide a pre-set variable or pre-set constant vertical force (as seen in FIGS. 2 and 3) on the bottom of the moving flexible membrane (belt)  60 . 
     A rubbing plate (not shown), commonly used in belt sanders, can be molded over the top of the floating faceplate  76  to provide a flat surface against which generally uniform rubbing can take place. The faceplate  76  with a top surface in contact and rubbing against the bottom of the flexible polishing membrane  60  wears both elements over time and either the membrane or the top of the backing plate would have to be replaced periodically. Many defects in the surface of the backing plate present at installation or which form later would tend to displace the flexible membrane unevenly and tend to cause uneven wear on the surface of the substrate being polished. To eliminate this wear between the bottom of the flexible membrane  60  and the top of the face  78  of the floating faceplate  76 , a pressurized fluid of either gas or liquid is provided through the holes  80  of the faceplate  76  and provides a uniform fluid bed or film of gas or liquid which acts as a nearly friction free buffer between the back of the flexible membrane  60  and the upper surface of the floating backing faceplate  76 . The passage of fluid at the surface holes of the floating backing plate member provide a generally uniformly pressurized fluid layer between the back of the membrane and top of the backing plate assembly which therefore evenly pressurizes the back of the moving flexible membrane  60 . The fluid or gas creating this layer is continuously replenished so that the thickness of the layer remains generally constant as the liquid or gas escapes sideways. 
     A set of small fluid holes  80  in the top of the faceplate membrane surface  78  provide for fluid (gas or liquid) passage from the faceplate fluid manifold cavity  82  to its surface  78  in contact with the moving belt  60 . The fluid layer (illustrated by arrows  108  showing fluid flow) is thereby created between the moving polishing belt  60  and top surface  78  of the faceplate  76 . The fluid can be either a gas or a liquid. The need to re-capture expended liquid weighs in favor of using a compressible gas. However, the containment used to capture the slurry could also be used to capture a liquid used in producing the fluid layer on the faceplate. 
     Fluid, either gas or liquid, is provided to the faceplate manifold  82  through a flexible hose  102  which is routed through the bellows  101  (or could be routed outside the bellows) such that fluid reaching the manifold enters a fluid feed opening  86  and is distributed within the manifold  82  as shown by the arrows  110 . The bellows top flange  101   a  (FIG. 4) is fixed to and sealed against the faceplate back surface  84 . Faceplate side surfaces  88 ,  90  face adjacent fixed sidewalls  96 ,  98  to prevent the faceplate  76  from being displaced sideways. 
     Since liquid slurry is present on the top of the flexible membrane (belt), it is important that the area around the bellows does not become plugged. Therefore, a labyrinth-type vertically moving skirt seal  92 ,  93 ,  94  is provided around the edge of the floating faceplate  76  to prevent any liquid, such as the slurry or pressurized liquid flowing from faceplate fluid holes  80 , from flowing into the box-like container inside the sidewalls  96 ,  98  and restricting the vertical motion of the bellows  100 . 
     The sidewalls of the box-shaped member enclosing the bellows also act as a guide to prevent sideways motion of the floating member backing plate. The friction generated when the floating piece rubs against the stationary piece can adversely affect the uniformity of polishing. The two surfaces can be coated with a friction reducing coating (such as PTFE). Alternately, the two surfaces may be separated by using a fluid passing nozzle configuration which interposes a fluid layer between the floating and stationary pieces. These configurations easily accommodate variations in the thickness of the slurry or the thickness of the belt  60  as the belt moves over the substrate being polished to enhance the ability of the membrane backing assembly  62  to move very rapidly according to the instantaneously encountered dimension. 
     Since the floating faceplate  76  is facing the moving belt  60 , the belt  60  tends to pull the floating faceplate  76  in the direction that the belt is moving. The moving belt  60  will also have a hydrodynamic (aerodynamic) effect in that the fluid at the leading edge of the floating membrane backing plate will tend to be sucked away and cause the belt  60  to touch the faceplate  76  at its leading edge. The hydrodynamic effect can be compensated for by adding fluid holes at the leading edge of this interface. Alternately, a curved transition could be provided so that the belt  60  sucks enough air towards the fluid layer that undesirable touching does not occur. 
     The leading edge of the floating faceplate  76  can also be slightly rounded to avoid excessive wear that might be experienced as a result of the membrane catching on a sharp corner of such a leading edge. 
     The size and number of fluid holes  80  ideally should provide a bed or film of fluid behind the polishing membrane so that the substrate  50  is evenly and uniformly polished. The pattern of holes  80  in the rectangular floating faceplate  76  covers nearly the full width of the belt. However, when unopposed by a polishing head  30  the moving belt  60  tends to bow up as shown by the dashed lines  61  in FIG.  3 . 
     The floating faceplate  76  as shown in FIGS. 2 and 3 can either have a labyrinth skirt seal extension (e.g.,  91 ,  93 ) whose top surface is planar with the top surface  78  of the faceplate  76  or can be offset slightly (e.g.  91   a ) as shown in FIG.  7 . 
     FIG. 4 shows an exploded view of the items discussed above for FIGS. 1-3. The polishing head main member  40  has a series of holes  44  on its lower surface. A retaining ring  52 , preferably made of Delrin, surrounds the bottom edge of the polishing head main member  40 . A flexible elastomer backing pad  46  has holes  48  whose locations correspond to the holes  44  in the polishing pad main member. The backing pad  46  is placed in the cavity at the bottom of the polishing head and acts as a compliant member to the extreme local pressures that would be present if a hard metal surface pressed a silicon substrate against an abrasive medium. The substrate  50  is then sandwiched between the flexible membrane  60  and the bottom of the polishing head assembly  30  (including, but not limited to items  40 ,  52 ,  46  and  48 ). On the bottom of the moving flexible membrane  60 , the faceplate  76  is supported by bellows  100  attached by flanges  101   a ,  101   b  and held in a particular alignment with the bottom of the moving polishing belt  60  by a perimeter wall including sidewalls  96 ,  98 . The perimeter wall sits on support member  64 . 
     A schematic top view of the substrate  50  and its retaining ring  52  are shown in FIG.  5 . Arrows  58  show the direction of travel of the moving belt  60 . The wave pattern  56  around the centerline  60   a  of the moving membrane  60  shows the oscillating action of the center  54  of the substrate retaining ring assembly (which also correlates to the centerlines of the polishing head assembly). 
     A top view of the configuration of FIGS. 1-4 is shown in FIG.  6 . While the polishing head  30  and the cantilevered arm  34  appear to show a fixed orientation in FIGS. 1-4, loading and unloading of the polishing head must generally take place by moving the belt  60  relative the polishing head  30 . The dashed lines  30   a ,  34   a  in FIG. 6 show one example of such a location for loading and unloading of a substrate from the polishing head  30 . While not shown in the drawings, as discussed above, the polishing head  30  can be configured to rotate about its own axis  30   b  and the cantilevered arm  34  may oscillate across the polishing belt  30 . 
     FIG. 7 is a configuration according to the invention showing in which the polishing head  30  would be positioned against a substrate  50 . A three roller  68 ,  70 ,  72  arrangement is provided around which the flexible membrane  60  is wound. A tensioning roller  114  is provided which can also act as a surface conditioner for the polishing surface of the flexible polishing membrane  60 . The tensioning/conditioning roller  114  (for example, made of a ceramic or a hard plastic material to avoid contaminating the substrate  50  being polished by introducing conductive or abrasive contaminants) may have a knurled pattern in its surface to actively displace and distribute colloidal particles of slurry which have become aggregated on and attached themselves to the flexible moving membrane  60 . As shown in FIG. 7, a slurry introduced by droplets  75  is distributed over the width of the moving belt  60  by a manifold  74  situated upstream from the substrate  50  being polished. The membrane backing faceplate assembly  66  is situated opposite the substrate  50  being polished. The polishing membrane  60  is routed through a bath  117  of liquid having a liquid level  118 , such as de-ionized water or an alkaline solution, to assist in maintaining moisture on the belt. The small arrows  104 ,  106  (also seen in FIGS. 2 and 3) show fluid (such as slurry) escaping from the surface of the belt  60 . The take-up roller  70  and drive roller  72  (identified by the drive arrow  73 ) include surface linings  70   a  and  72   a , respectively, on their surface. These linings are made of elastomers such as neoprene and rubber or other material generally used in the art. 
     FIG. 8 shows another orientation according to the invention. The location of the substrate  50  alone represents the location of the polishing head  30  (which is not shown) on the inside of the belt  60 . In this configuration the substrate is shown and polishing occurs on the inside surface of the moving belt  60   a . The three rollers  120 ,  124 , and  126  and a tensioning roller  122  are located so that the actual drive  120  and guide rollers  124 ,  126  condition the surface of the belt  60   a  which is the polishing the wafer while new colloidal particles to abrade the substrate are added by the manifold  74 . The membrane backing faceplate assembly  66  in this configuration is located below the belt  60   a.    
     FIG. 9 shows the orientation of rollers as shown in FIG. 8, but the membrane backing assembly  66  pressurizing the belt is shown above the belt and the tensioning roller  122  acts as conditioning roller in this instance. New droplets of colloidal slurry are added in this configuration to the surface of the moving belt  60   b  as the moves down the right hand path between rollers  124  and  126   
     FIG. 10 shows an alternative arrangement in which a moving belt  60   c  circulates around two rollers  130 ,  134 . The substrate polishing position is shown by the location of substrate  50 . The membrane backing faceplate assembly  66  is shown with variable tensioning  136  of the belt  60   c  between the two rollers  130 ,  134  relative to the fixed support  132 . 
     The tension of the belt  60 ,  60   a ,  60   b ,  60   c  in any of these configurations should be great enough to provide the motive force (frictional force) between the rollers and the belt to drive the belt even at the most aggressive abrasion conditions. The force attempting to restore the belt to its natural path tends to wear the retaining ring  52  and tends to over-polish the edge of the substrate. Therefore, the tension should not be so great as to excessively wear the belt or to provide rapid wear of the edge of the retaining ring if the substrate being polished is slightly displaced from the line directly between adjacent belt rollers. 
     FIG. 11 shows a configuration according to the invention including four rollers  138 ,  140 ,  144 ,  146 . The drive roller  146  is tensioned by a tensioning roller  142 . The polishing location is on the belt  60   d  between the top two rollers  140 ,  148 . Gravity influences the membrane polishing belt if it is on a horizontal plane. In an alternate configuration, shown by a dashed line  150  a substrate may be polished on a side of the arrangement. This configuration would eliminate the effect of gravity on the polishing belt  60   d . A spray nozzle  152  can spray chemical solutions and/or slurry onto the belt as it approaches the substrate  50  being polished. 
     FIG. 12 shows a wide flexible polishing membrane  60   e  having two polishing positions identified by substrates  50   a  and  50   b.  The locations of membrane backing assemblies  62   a ,  62   b  (shown in dashed lines) are opposite the positions  50   a ,  50   b  at which polishing can take place. In this configuration each substrate  50   a ,  50   b  being polished has its own separate track on the surface of the belt  60   e . Another configuration with a reliable belt membrane could have the tracks on which polishing takes place overlaps or coincide, so long as polishing performance specifications are maintained. 
     FIG. 13 shows an alternate arrangement according to the invention. The substrate  50   c  in FIG. 13 is held in a generally fixed position, either stationary or rotating slowly, in a faceup orientation with respect to the polishing belt  60   f  and its carrier (items including rollers  160 ,  162 , and narrow belt backing assembly  164 ). A set of two rollers  160 ,  162  (as shown in FIG. 13, although more are possible) move polishing belt  60   f . Polishing belt  60   f  is narrower than the substrate  50   c  surrounded by a retaining ring  52   a . The belt carrier mechanism includes a backing assembly  164  which moves with the rollers as the rollers move from side to side. While a single linear side to side movement is shown in FIG. 13 by arrows  166 , it is possible the that the membrane polishing assembly (carrier) will rotate as well as translate, instead of or in addition to the substrate rotating providing a similar polishing effect as when the substrate alone rotates. Alternatively, the substrate could move laterally with respect to the belt. 
     FIG. 14 is a closeup view of the membrane backing assembly showing a series of bellows  174 ,  176  which are equally pressurized to provide a generally uniform pressure to the backside of the moving flexible membrane  60   f  so that polishing across the width of the substrate is generally uniform. 
     FIG. 15 shows another embodiment according to the invention. A substrate  50   d  is retained within a retaining ring  52   b  and a flexible polishing membrane  60   g  is wound around a series of rollers which provide a belt polishing contact area much smaller than the area of the substrate  50   d . Examples of alternate roller carriers are illustrated in FIGS. 16 and 17. Such carriers are attached and guided by a carrier linkage (or mechanism)  184  connected to, for example, a bridge support  186 . Carrier linkage  184  causes the roller carrier to move across the surface of the substrate  50   d  in a pre-programmed pattern, possibly rotary, to provide uniform polishing of the substrate  50   d  surface. The retaining ring  52   b , similar to the retaining rings discussed above, minimizes edge effects which cause differential polishing at the perimeter. 
     An urging linkage, as provided, for example, in the linkage  184 , can be provided to attempt to provide uniform polishing pressure as the pre-programmed polishing path is carried out by the carrier assemblies. 
     A series of three rollers and a carrier are shown in FIGS. 15 and 16. A centralized pivoting frame  188  equalizes the pressure on the substrate between the two rollers so that generally equal polishing occurs within the region covered by the belt between the rollers. Because the distance between the rollers  194  and  196  is small, the polishing belt path  192  generally maintains contact with the surface of the substrate  50   d  as long as the each of the rollers  194 ,  196  also do. A backing plate assembly may be placed between the rollers  194 ,  196  to provide uniform pressure the polishing belt path  192 . 
     When a carrier according to FIG. 17 is used, a very small area (almost a line contact) is made between the roller  202  and belt  60   h  at the location  200  in contact with the substrate  50   d . The carrier  190  moves in a preprogrammed manner over the surface of the substrate as guided by the carrier links  198  to the support bridge  186 . The configuration of FIG. 17 is more like the stylus or cutter tool of a lathe. If there is relative rotation between the substrate and the carrier, the polishing program directing the movement of the carrier takes into account the fact that surface speed of a rotating substrate is greater the larger the distance from the center of rotation. The polishing program makes accommodations so that the center of the substrate is not polished any more or less than any of the regions away from the center. Alkaline solution and colloidal particles can be introduced by mounting a slurry and/or alkaline solution drip to the carriers so that fluid is introduced ahead of the locations where the polishing roller carrier is about to travel. 
     FIGS. 18,  19 ,  20 ,  21 ,  22 ,  23 ,  24  and  25  schematically show a variety of arrangements of the polishing head, the substrate, and the polishing membrane backing assembly (faceplate), according to the invention. In each configuration the substrate  210  to be polished is located above the polishing belt  212  and a fixed support is provided both above and below the belt, but there are variations in the assemblies in the supports and the belt. 
     FIG. 18 shows a vertically fixed gimbaled  216  polishing head  214 , and the backing faceplate  218  is supported by a set of fixed or variable spring members  222 , 223  from a lower fixed support  220 . Only rubbing contact is provided between the backing faceplate  218  and the bottom of the belt  212 . 
     FIG. 19 shows a configuration like FIG. 18, except that a backing faceplate  244  provides a fluid layer contact between the bottom of the belt  212  and the top of the faceplate  224 . 
     FIG. 20 inverts the fixed and spring elements of FIG.  18 . The polishing head  214  in this configuration is urged by fixed or adjustable spring members  226 ,  227  toward the polishing belt  212 . A bottom faceplate  218  which rubs the belt  212  is vertically fixed by the gimbaled support  228 . 
     FIG. 21 is a variation of the configuration of FIG. 20 in which a two piece polishing head  230 ,  232  having a fluid layer interface assures a uniform pressure across the head on the belt  212 . 
     FIG. 22 is a variation of the configuration of FIG. 21 in which a bellows  224  replaces the spring members of FIG.  21 . The bellows pressure may be controlled, or the bellows may be closed and provide a reduced force at greater extensions and a greater force on compression. 
     FIG. 23 is variation of the configuration of FIG. 22 in which a polishing head  236  provides fluid force directly to one side of the wafer being polished without any intervening elements. This arrangement provides uniform pressure over each unit of substrate area urging the substrate toward to belt  212  for polishing. 
     FIG. 24 shows a configuration similar to that shown in FIG. 19 with the addition of sidewalls  238 ,  240 , sidewalls  238 ,  240  each have friction reducing inserts  242 ,  244 , respectively, to reduce the friction caused by any vertical motion between the backing faceplate  224  and the sidewalls  238 ,  240 . 
     FIG. 25 shows a configuration according to the invention similar to that shown in FIG. 24. A bellows element, as explained for FIG. 22 above, is interposed between the backing faceplate  218  and the fixed support  220 . Fluid nozzles  246 ,  248  are provided to separate the backing faceplate from the side walls. 
     Use of the configurations as described above includes a method according to the invention including the steps of: holding a substrate  50  in contact with linearly moving flexible polishing membrane  60  and providing a generally uniform pressure to the substrate  50  to accomplish generally uniform polishing across the area of the substrate  50 . The step of applying uniform pressure is accomplished by pressurizing a bellows  234  (FIG.  22 ). Bellows  234  can be positioned between a substrate holder fixed support  32  and the substrate holder  30 . The pressure within the bellows  234  is controlled to be generally uniform. 
     Bellows  100  can also be positioned between which is used as a member intermediate the membrane backing support bridge  64  and the side of the polishing membrane  60  opposite the substrate  50  being polished. The backing faceplate  78  includes a series of holes  80  in its surface through which pressurized fluid flows to create a fluid layer.  108  separating the polishing membrane  60  from the surface of the backing faceplate  78 . 
     The substrate  50  can be rotated during polishing and can be moved in an oscillatory motion generally perpendicular to the relative motion between the belt  60  and the substrate  50 . 
     An alternate method according to the invention includes the steps of: holding a substrate  50  in contact with the flexible polishing membrane  60  opposite a backing faceplate position (corresponding to the membrane backing assembly  62 ) behind the flexible membrane  60  and moving the polishing membrane  60  in a generally linear path past the substrate  50  to polish the substrate  50 . A further additional steps may include: providing a clamping force to urge the substrate  50  and the backing faceplate  78  toward the other and in contact with the flexible membrane  60 , and or reconditioning the flexible membrane  60  (e.g., by the rollers  114 ,  122 ) as it is moved toward the polishing location where the substrate  50  is polished. 
     Referring to FIG. 26, another chemical mechanical polishing apparatus according to the present invention generally includes a base  310  for rotatably supporting a rotating plate  312  therein, and a moveable tubular polishing arm  314  suspended over the rotating plate  312  and supported in position on a cross arm  316 . Cross arm  316  is maintained on the base  310 , and over the plate  312 , by opposed uprights  315 ,  315   a  which extend upwardly from the base  310 . The rotating plate  312  preferably includes a conformable pad  334  fixed to its upper surface. A substrate  318  having an upper surface  319  to be polished, is placed on the conformable pad  334  with its upper surface  319  exposed opposite the plate  312 . The conformable pad  334  is wetted, so that surface tension will adhere the substrate  318  to the conformable pad  334  to maintain the substrate in position on the conformable pad  334  as the substrate  318  is polished. The tubular polishing arm  314 , with a polishing pad  320  located over the lower open end  328  thereof, is moved generally radially across the upper surface  319  of the substrate  318  to perform the polishing. The polishing pad  320  is preferably continuously moved linearly across the rotating upper surface  319  of the substrate  318 , from the edge to center thereof, until the polishing end point is reached. The polishing pad  320  is preferably five to fifty millimeters wide. Therefore, when a five, six, seven or eight inch (125-200 mm) substrate is located on the plate  312  the surface area of the polishing pad  320  is substantially smaller than the overall substrate area to be polished, generally at least three times smaller, and preferably at least 10 times smaller. The polishing pad  320  material is preferably a polyurethane impregnated polyester felt such as IC 1000, or Suba IV, both of which are available from Rodel, Inc. of Newark, Pa. To provide controllable substrate surface material removal rate across the entire substrate  318 , the polishing arm  314  and cross arm  316  are provided with apparatus to control the positioning, and load, of the polishing arm  314  and polishing pad  320  with respect to substrate upper surface  319 . 
     The positioning of the polishing arm  314 , with respect to the substrate  318 , is provided by a linear positioning mechanism  322  formed as an integral part of the cross arm  316 . In one embodiment, as shown in FIG. 26, the linear positioning assembly  322  includes an internally-threaded slide member  323 , and cross bar  316  includes mating threads to receive slide member  323  thereon. A secondary cross bar  317  is attached to uprights  315 ,  315   a  generally parallel to cross bar  316 . Slide member  323  is received on cross bar  316 , and secondary cross bar  317  projects through slide member  323  to prevent its rotation with respect to cross bar  316 . A stepper motor  321  is coupled to the cross bar  316  at upright  315  to rotate the cross bar  316  in discrete angular steps. In this configuration, the slide member  323 , and polishing arm  314  with the polishing pad  320  attached to the lower open end  328  thereof, may be moved axially across the substrate  318  in increments as small as 0.01 mm by rotating the cross bar  316  in discrete small arcuate steps by stepper motor  321 . Other drive means, such as a linear actuator, a geared tape pulley, or other precision positioning mechanism may be easily substituted for this polishing arm  314  drive system. 
     Referring still to FIG. 26, linear positioning assembly  322  precisely aligns the cross arm  316  over the substrate  318  to move the polishing arm  314  from the edge to the center of the substrate  318 . As polishing pad  320  moves from the edge to the center of the substrate  318 , the substrate  318  rotates on plate  312 , and thus the polishing pad  320  contacts and polishes all areas of the substrate  318 . To polish the center of the substrate  318  where the relative motion between the polishing pad  320  and the substrate  318  is at its minimum, the polishing arm may vibrate or rotate to create motion between the polishing pad  320  and the substrate  318  center. 
     To rotate the polishing arm  314 , a servo motor  325  is coupled to slide member  323 , and a drive shaft  327  extends from motor  325  into slide member  323  to engage the upper end of polishing arm  314 . The upper end of polishing arm  314  is received in a rotary union at the base of slide member  323 , which allows polishing arm  314  to rotate and also permits the transfer of liquids or gasses from slide member  323  into the hollow interior of the polishing arm  314 . To provide vibratory motion, an offset weight may be coupled to the motor drive shaft  327 . As the motor rotates, this offset weight causes the motor  325 , and thus slide member and polishing arm attached thereto, to vibrate. 
     To partially control material removal rate of polishing pad  320 , the load applied at the interface of the polishing pad  320  and substrate upper surface  319  is also variably maintained with load mechanism  324  which is preferably an air cylinder, diaphragm or bellows. Load mechanism  324  and is preferably located integrally with polishing arm  314  between cross arm  316  and substrate  318 . The load mechanism  324  provides a variable force to load the polishing pad  320  against the substrate  318 , preferably on the order of 0.3 to 0.7 Kg/cm 2 . A load cell  326 , preferably a pressure transducer with an electric output, is provided integrally with polishing arm  314 , and it detects the load applied by the polishing pad  320  on substrate upper surface  319 . The output of the load cell  326  is preferably coupled to the load mechanism  324  to control the load of the polishing pad  320  on the substrate upper surface  319  as the polishing pad  320  actuates across the substrate  318 . 
     To provide the slurry to the polishing pad  320 , the slurry is preferably passed through the polishing arm  314  and out the open end  328  of polishing arm  314  to pass through the polishing pad  320  and onto the substrate. To supply slurry to the polishing arm, a slurry supply tube  332  is connected to slide member  323 , and passages within the slide member  323  direct the slurry from the supply tube  332  through the rotary union and into the hollow interior of polishing arm  314 . During polishing operations, a discrete quantity of chemical slurry, selected to provide polishing selectivity or polishing enhancement for the specific substrate upper surface  319  being polished, is injected through tube  332 , slide member  323  and arm  314 , to exit through polishing pad  320  to contact the substrate upper surface  319  at the location where polishing is occurring. Alternatively, the slurry may be metered to the center of the substrate  318 , where it will flow radially out to the edge of the rotating substrate  318 . 
     Referring now to FIG. 27, to rotate the plate  312  and the substrate  318  located thereon, a motor  336  is coupled to the underside of the plate  312  with a drive shaft. Motor  336  rotates the plate  312 , and is preferably a variably speed direct current motor, such as a servo-motor, which may selectively provide variably substrate  318  rotation speeds during polishing operations. 
     Referring again to FIG. 26, to polish a substrate  318  with the CMP apparatus of the present invention, the substrate  318  is loaded onto pad  334 , and the plate  312  is rotated to the proper polishing speed by the motor  336 . The slide member  323  of the linear positioning mechanism  322  moves polishing arm  314  from a position beyond the substrate radial edge to a position adjacent the substrate edge to begin polishing the substrate upper surface  319 . As the polishing arm  314  is moved to contact the substrate edge, the polishing pad  320  is passed over a reconditioning blade  338  maintained on base  310  to remove any particulates which may have collected in polishing pad  320  during previous polishing with the polishing pad  320 . Blade  338  is preferably a sharp blade, and as polishing pad  320  is brought across it, the fibers of the pad are raised and particulates trapped therein are removed. Other reconditioning apparatus, such as diamond wheels or stainless wire brushed may also be used to recondition the polishing pad. Once polishing pad  320  is brought into contact with the outer edge of the substrate  318 , chemical slurry is pumped through the tube  332  and out through polishing pad  320 , and polishing arm  314  is rotated and/or vibrated. As the substrate  318  rotates under the polishing pad  320 , slide member  323  moves the polishing arm  314  and polishing pad  320  from the substrate edge and across the substrate upper surface  319  to the center of the substrate  318 . As the polishing pad  320  is controllably varied by load mechanism  324  to compensate for the decrease in net motion between the polishing pad  320  and substrate upper surface  319  which occurs as the polishing pad  320  approaches the center of the substrate  318 . Further, the speed of rotation of plate  312 , and thus the net motion between polishing pad  320  and the substrate  318 , may be varied in conjunction with, or independently of, the relative radial position of polishing pad  320  on substrate  318  by varying the motor  336  speed. Once the polishing end point is reached, the chemical slurry stops flowing, the rotation and/or vibration stops, and the slide member  323  moves polishing arm  314  across reconditioning blade  338  and back to its original position adjacent the upright  315 . To properly position polishing arm  314  for the next substrate  318  to be polished, a zero position stops  342  extends from upright  315 , generally parallel to cross arm  316 , and slide member  323  stops moving when it engages zero position stop  342 . When the next substrate  318  is positioned on the plate  312 , and the next polishing cycle begins, the polishing pad  320  will again cross the reconditioning blade  338  to raise fibers in the polishing pad  320  and remove particulates which may have collected in polishing pad  320  as a result of accumulated substrate polishing. Alternatively, the polishing pad  320  may be replaced after each polishing cycle. 
     FIGS. 28 and 29 show a second embodiment of the polishing arm  314  useful with the chemical mechanical polishing apparatus of the present invention. In this embodiment, the polishing arm  314  includes a tubular roller support arm  346  which extends downwardly from the load member  324 , and a roller member  348  which is attached to the lower terminus of roller support arm  346 , by bearing plates  350 . The plates  350  are located on opposite sides of the roller support arm  346  and extend downwardly therefrom to receive rotatable roller axle  352  extending from either end of the roller member  348 . The roller member  348  preferably freewheels within the plates  350 , although it may be coupled to a drive system to be positively rotated. To provide the polishing pad surface to polish the substrate  18 , a cassette  354  is loaded on the upper end of the roller support arm  346  and a tape  356  of polishing pad material is looped over the roller  348  such that the ends thereof are wound between spools  358  in the cassette  354 . The tape  356  of polishing material is preferably aligned on the substrate by aligning the axles  352  parallel to the radius of the substrate  318 . The cassette  354  preferably includes an integral drive motor which rotates the spools  358  to provide a clean polishing pad surface at roller  348  as required. It also optionally includes a pair of reconditioning blades  360  which contact the polishing tape  356  surface to clean it of particulates which accumulate therein from substrate polishing. The tape  356  may be incrementally moved, to provide a clean polishing pad surface on roller  348  after each polishing cycle, or may be continuously or incrementally moved to provide a fresh, clean polishing pad surface at the polishing pad/substrate interface while each individual substrate  318  is being polished. To provide the fresh polishing pad material against the substrate  318 , the roller  348  may alternatively by positively driven by a drive mechanism to move the tape  356  over the roller  348  and the substrate upper surface  319 , and the reconditioning blade my be located adjacent roller  348 . Polishing slurry may be provided, in metered fashion, through the hollow interior of the roller support arm  346  to supply the polishing slurry directly at the polishing pad/substrate interface. 
     Referring now to FIG. 30, an additional alternative embodiment according to the invention is shown. In this embodiment, polishing arm  314  extends downwardly from load mechanism  324  and terminates on secondary plate  380  located above, and generally parallel to, the rotating plate  312 . A pair of secondary polishing arms  384 , each having a polishing pad  320  on the end thereof, extend downwardly from intermediate plate  380  to position the polishing pads  320  in position to engage the substrate upper surface  319 . Secondary polishing arms  384  are preferably located adjacent the edge of intermediate plate  380 ,  180  degrees apart, and polishing arm  314  is preferably connected to the center of secondary plate  380 . Thus, a polishing arm  314  is rotated by motor  325 , secondary polishing arms  384  traverse a circular path having a mean diameter equal to the linear distance between the centers of secondary polishing arms  384 . As linear positioning assembly  322  moves polishing arm  314  over the substrate  318 , and the secondary polishing arms  384  rotate about the longitudinal axis of the polishing arm  314 , net movement will occur between the pads  320  and all areas of the substrate upper surface  319 . 
     To ensure even net relative motion between the polishing pads  320  and the substrate upper surface  19 , the length of the span between the secondary polishing arms  384  on intermediate plate  380 , in combination with the length of travel of the slide member to position the pads  320  from the edge to center of the substrate, should not exceed the radius of the substrate, and the rate in rpm, and direction, of rotation of both plate  312  and polishing  314  must be equal. Preferably, the span between the centers of the two polishing pads  320  on the ends of secondary polishing arms  384  is 3 to 4 cm. Additionally, although two secondary polishing arms  384  are shown, one, or more than two, polishing arms, or an annular ring of polishing pad material may be connected to the underside of the intermediate plate  80  without deviating from the scope of the invention. 
     Referring now to FIG. 31, a schematic of the control system  370  for controlling the chemical mechanical polishing apparatus of the present invention is shown. The control system  370  includes a controller  372  which is coupled, by electrical cables, to load mechanism  324 , load cell  326 , plate drive motor  336 , cross bar stepper motor  321  and motor  325 . When the chemical mechanical polishing apparatus is first used, the controller  372  signals the stepper motor  321  of the linear positioning mechanism  322  to rotate the threaded cross bar  316 , and thus move the slide member  323  and polishing arm  314  attached thereto to the fully-retracted position adjacent to upright  15 . As slide member  323  positions the polishing arm  314  in the fully-retracted position, a signal member thereon, preferably a signal pin, touches the zero position stop  342  which sends a signal to the controller  372  indicating that the polishing arm  314  is in the fully retracted position. Controller  372  then actuates the stepper motor  321  to move polishing arm  314  to the edge of substrate upper surface  319 . As polishing pad  320  is moving into position to engage the edge of substrate  318 , the controller  37  starts motor  336  to rotate substrate  318  at the desired speed. 
     Once polishing pad  30  engages the edge of substrate  318 , the controller  372  further signals the load member  324  to create a bias force, or load, at the interface of the polishing pad  320  and the substrate upper surface  319 , signals motor  325  to vibrate and/or rotate polishing arm  314 , and simultaneously starts the flow of the polishing slurry into polishing pad  320 . The controller  372  monitors and selectively varies the location, duration, pressure and linear and rotational relative velocity of the polishing pad  320  at each radial location on the substrate upper surface  319  through the linear position mechanism  322 , load member  324 , motor  325  and motor  336  until the polishing end point is detected. An end point detector, such as an ellipsometer capable of determining the depth of polishing at any location on the substrate  318 , is coupled to the controller  372 . The controller  372  may stop the movement of the linear position apparatus  322  in response to end point detection at a specific substrate radius being polished, or may cycle the linear position apparatus  322  to move polishing pad  320  back and forth over the substrate  318  until the polishing end point is reached and detected at multiple points on substrate upper surface  319 . In the event of a system breakdown, a stop  340  projects from upright  315   a  generally parallel to cross bar  316  to prevent slide member  323  from travelling completely over the substrate  318 . Once polishing end point is reached, the controller  372  signals the load cell of lift polishing arm  314  off the substrate  318 , stop delivery of the polishing slurry, and move slide member  323  back into engagement with zero position stop  342 . The polished substrate  318  is then removed, and a new substrate  318  may be placed on plate  312  for polishing. 
     While the invention has been described with regards to specific embodiments, those skilled in the art will recognize that changes can be made in form and detail without departing from the spirit and scope of the invention.