Abstract:
A polishing pad useful for chemical mechanical planarization has a polishing layer for planarizing substrates. The polishing layer comprises a radius that extends from a center of the polishing layer to an outer perimeter of the polishing layer; one or more continuous grooves formed in the polishing layer and extending inward from the outer perimeter of the polishing layer; and a circumference fraction grooved (CF). The CF occurs in the area extending from the outer perimeter of the polishing layer a majority distance to the center of the polishing layer; and CF is that portion of circumference at a given radius lying across the one or more continuous grooves divided by full circumference at the given radius. The CF remains within 25% of its average value as a function of the polishing layer radius.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to polishing pads for chemical mechanical polishing (CMP), and in particular relates to a polishing pad having optimized grooves. 
     In the fabrication of integrated circuits and other electronic devices, multiple layers of conducting, semiconducting, and dielectric materials are deposited on or removed from a surface of a semiconductor wafer. Thin layers of conducting, semiconducting, and dielectric materials may be deposited by a number of deposition techniques. Common deposition techniques in modern processing include physical vapor deposition (PVD), also known as sputtering, chemical vapor deposition (CVD), plasma-enhanced chemical vapor deposition (PECVD), and electrochemical plating (ECP). 
     As layers of materials are sequentially deposited and removed, the uppermost surface of the wafer becomes non-planar. Because subsequent semiconductor processing (e.g., metallization) requires the wafer to have a flat surface, the wafer needs to be planarized. Planarization is useful in removing undesired surface topography and surface defects, such as rough surfaces, agglomerated materials, crystal lattice damage, scratches, and contaminated layers or materials. 
     Chemical mechanical planarization, or chemical mechanical polishing (CMP), is a common technique used to planarize substrates such as semiconductor wafers. In conventional CMP, a wafer carrier or polishing head is mounted on a carrier assembly and positioned in contact with a polishing pad in a CMP apparatus. The carrier assembly provides a controllable pressure to the wafer, urging it against the polishing pad. The pad is moved (e.g., rotated) relative to the wafer by an external driving force. Simultaneously therewith, a chemical composition (“slurry”) or other fluid medium is flowed onto the polishing pad and into the gap between the wafer and the polishing pad. The wafer surface is thus polished and made planar by the chemical and mechanical action of the polishing layer and slurry. 
     In CMP, planarity and uniformity of the wafer surface are paramount. Accordingly, CMP systems are typically configured to provide orbital and/or oscillatory motion of the wafer to average out variations in instantaneous local polish rate. It is known that pad and wafer rotation speeds can be combined in a way that, over time, results in each point of the wafer surface being exposed to the same range and mean value of relative pad velocity. Such an arrangement is described in the article by D. A. Hansen et al, entitled “Characterization of a Multiple-Head Chemical Mechanical Polisher for Manufacturing Applications”, Proceedings of the 1 st International CMP-MIC, February 1996, which article is incorporated herein by reference. 
     The averaging mathematics for the wafer and pad rotations presume that the polishing layer is homogeneous with respect to radial position. However, where the polishing layer includes certain types of grooves (e.g., concentric circles, Cartesian grids, fixed-width radii, or combinations of these), the polishing surface area per unit pad area can vary as a function of pad radius. 
     FIG. 1A is plot of a standard prior art radial groove pattern, such as described in U.S. Pat. No. 5,177,908. FIG. 1B is a plot of the circumference fraction grooved CF as a function of pad radius R for the radial groove pattern of FIG.  1 A. For purposes of this application, the circumference fraction grooved CF is as follows: 
     
       
         (Portion of circumference at a given radius that lies across any groove)/CF=(Full circumference at the given radius) 
       
     
     Note: If CF is constant as a function of radius, then the fractional area of the pad that is grooved (or ungrooved) at a given radius is also constant as a function of radius. 
     With continuing reference to FIG. 1A, since the number and width of the grooves is fixed, the total grooved length along a circumference is the same regardless of radius. Thus, as shown in FIG. 1B, CF decreases as the distance from the center increases, with the value of CF near the outer edge of the pad being many times smaller than that near the center. 
     FIG. 2A is a plot of a standard prior art concentric circular groove pattern. FIG. 2B is a plot of the circumference fraction grooved CF as a function of pad radius R for the concentric circular groove pattern of FIG.  2 A. In this case, CF is unity at any radius that falls within a groove, and zero at any radius that does not. The area fraction grooved is thus a sharply changing function of radius. 
     FIG. 3A is a plot of a standard prior art Cartesian grid groove pattern with equal pitch in both coordinate directions. FIG. 3B is a plot of CF as a function of pad radius R for the Cartesian grid groove pattern of FIG.  3 A. Note that CF decreases with increasing radius until a new set of grid lines is crossed, at which point the fraction sharply increases. At larger values of radius, even small increments in radial distance cross additional grid lines, so that CF is a highly irregular function. At large radius values where CF begins to asymptote, there is significant (i.e., over 50%) variation in the polishing area per unit pad area. 
     FIG. 4A is a plot of a standard prior art spiral groove pattern, such as disclosed in U.S. Pat. Nos. 5,921,855 and 5,690,540 (the &#39;540 Patent). FIG. 4B is a plot of CF as a function of pad radius R for the spiral groove pattern of FIG.  4 A. Note that CF decreases with increasing radius because the spiral curve does not grow in exact proportion to the radius. 
     Accordingly, there is a need for a polishing pad with grooves that properly account for the mutual rotations of the wafer and polishing pad. 
     Statement of the Invention 
     An aspect of the invention is a polishing pad useful for chemical mechanical planarization, the polishing pad having a polishing layer for planarizing substrates, the polishing layer comprising: a radius that extends from a center of the polishing layer to an outer perimeter of the polishing layer; one or more continuous grooves formed in the polishing layer and extending inward from the outer perimeter of the polishing layer; and a circumference fraction grooved (CF) in an area extending from the outer perimeter of the polishing layer a majority distance to the center of the polishing layer, CF being that portion of circumference at a given radius lying across the one or more continuous grooves divided by full circumference at the given radius, and wherein CF remains within 25% of its average value as a function of the polishing layer radius in the area extending from the outer perimeter of the polishing layer the majority distance to the center of the polishing layer. 
     In another aspect of the invention, the one or more continuous grooves start at a base radius and extend to an outer perimeter of the pad. Alternatively, the one or more continuous grooves start at a starting radius between the base radius and the outer perimeter, and extend to the outer perimeter. 
     Another aspect of the invention is a method of planarizing a wafer surface. The method of chemical mechanical planarizing a substrate comprises the steps of: introducing a polishing solution to a wafer; rotating the wafer with respect to a polishing pad, the polishing pad having a polishing layer, and the polishing layer comprising: i) a radius that extends from a center of the polishing layer to an outer perimeter of the polishing layer; ii) one or more continuous grooves formed in the polishing layer and extending inward from the outer perimeter of the polishing layer; and iii) a circumference fraction grooved (CF) in an area extending from the outer perimeter of the polishing layer a majority distance to the center of the polishing layer, CF being that portion of circumference at a given radius lying across the one or more continuous grooves divided by full circumference at the given radius, and wherein CF remains within 25% of its average value as a function of the polishing layer radius in the area extending from the outer perimeter of the polishing layer the majority distance to the center of the polishing layer; and planarizing the wafer with the polishing pad and the polishing solution. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A is plot of an example prior art polishing pad radial groove pattern having 60 grooves on a 24 inch outer pad radius and a 2 inch base radius, with each groove being 0.093 inches wide; 
     FIG. 1B is a plot of the circumference fraction grooved CF as a function of pad radius R for the radial groove pattern of FIG. 1A; 
     FIG. 2A is a plot of a standard prior art concentric circular groove pattern having 11 grooves on a 24 inch outer pad radius, with each groove being 0.093 inches wide; 
     FIG. 2B is a plot of the circumference fraction grooved CF as a function of pad radius R for the concentric circular groove pattern of FIG. 2A; 
     FIG. 3A is a plot of a standard prior art Cartesian grid groove pattern for a 24 inch outer pad radius, with equal-pitch grooves extending in both coordinate directions with a 20 mm groove pitch and a 0.093 inch groove width; 
     FIG. 3B is a plot of the circumference fraction grooved CF as a function of pad radius R for the Cartesian grid groove pattern of FIG. 3A; 
     FIG. 4A is a plot of a standard prior art spiral groove pattern consistent with that disclosed in the &#39;540 Patent; 
     FIG. 4B is a plot of the circumference fraction grooved CF as a function of pad radius R for the spiral groove pattern of FIG. 4A; 
     FIG. 5A is plan view of a polishing pad and groove pattern formed therein; 
     FIG. 5B is a close-up view of a groove segment of the groove of FIG. 5A; 
     FIG. 5C is a close-up view of a point P at the base radius R B  of the polishing pad of FIG. 5A, illustrating incremental changes in groove angle θ as a function of radius R; 
     FIG. 6A is a plot of a groove pattern according to the present invention, with a 24 inch pad outer radius R O  and a 10 inch base radius R B ; 
     FIG. 6B is a plot of the curved groove pattern according to the present invention, with a 24 inch pad outer radius R O , a 6 inch base radius R B , and 8 curved grooves; 
     FIG. 6C is a plot of the curved groove pattern according to the present invention similar to FIG. 6B, but with a 2 inch base radius R B ; 
     FIG. 6D is a plot of the curved groove pattern according to the present invention similar to FIG. 6C, but with the pattern starting at a starting radius R S =10 inches; 
     FIG. 6E is a plot of the circumference fraction grooved CF as a function of pad radius R for the curved groove pattern of the present invention, illustrating the invariance of CF as a function of pad radius R; and 
     FIG. 7 is a schematic side view of a CMP system employing a grooved polishing pad formed in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 5A is a plan view of a polishing pad  100  having an outer radius R O  and a surface  102  with a groove  104  formed therein. In example embodiments, one or more continuous (i.e., unbroken and elongate) grooves  104  are formed in surface  102 . The pad radius R is measured from an origin O. A circle C R  (dashed line) with a circumference 2πR is also shown. The outer radius of pad  100  is R 0 . The one or more grooves  104  extend out to outer radius R 0  (i.e., to the edge of the pad). The outer radius R O  of pad  100  defines the outer perimeter  106 . 
     On orbital polishers, there is often a region surrounding the origin O that is not contacted by the wafer. This region typically extends a few inches from the origin O. Accordingly, groove  104  need not necessarily start at the origin O. Alternatively, one or more grooves  104  may start at or near the origin O, but the constraint of CF ratio may be relaxed within the region that does not contact the semiconductor wafer. For example, the polishing pad may contain no grooves, a single grooved region or random grooves near the origin. Although polishing may occur near the origin O, most advantageously the polishing occurs only within the area extending from the outer perimeter of the polishing layer the majority distance to the center or origin O of the polishing layer. This embodiment maintains the wafer within a “wafer track” having the controlled CF. 
     In example embodiments, a base radius R B  is chosen to obtain a desirable groove curvature without compromising uniform polishing. In example embodiments where the workpiece tends to polish slower at the edge than near the center, the base radius R B  is chosen somewhat larger than the radius of the uncontacted central region. While this increases the material removal at the edge of the workpiece, it does not guarantee uniform polishing. 
     Thus, in an example embodiment, one or more grooves  104  start from a base radius R B , as shown. In another example embodiment, one or more grooves  104  start from origin O. In another example embodiment, grooves  104  start from a starting radius R S  that is larger than the base radius R B  (see FIG. 6D, discussed below). 
     FIG. 5B is a close-up view of polishing layer  102  of FIG. 5A, showing a small differential segment  110  of groove  104 . At a given radius R, groove  104  has a given width W and a central axis A that forms an angle θ (“groove angle”) with respect to a radial line L connecting the origin O to the given radial position R. 
     In order for the pad to have the same fractional grooved area at any radius, each circumference C R  needs to traverse an amount of grooved polishing layer that is a fixed fraction of the circumference C R . As discussed above, the ratio of the grooved to total polishing layer at a given circumference C R  is referred to herein as the “circumference fraction grooved,” or “CF.” 
     For CF to be constant as a function of radius, each differential groove segment  110  must have an increasingly large groove angle θ as the radius increases so that the groove width taken along a circumference increases to keep up with the increasing length of the circumference. The locus of the segments  110  constitutes a continuous curve corresponding to one groove connecting the base radius R B  to the outer radius R 0 . 
     In mathematical terms, if N represents the number of grooves (groove count) N, then: 
     
       
           CF =( NW  sec θ)/(2 π R )  EQ. 1 
       
     
     Note that at the base radius R B , θ=0 so that 
     
       
           CF =( NW )/(2 π R ).  EQ.2 
       
     
     Equating CF at R B  to CF at any radius R requires that the groove angle θ be: 
     
       
         θ( R )=sec −1 ( R/R   B ).  EQ. 3 
       
     
     The global equation for, the precise form of the one or more grooves  104  is obtained by taking incremental radial steps directed at the corresponding local groove angle θ(R). This is illustrated in FIG. 5C, which is a close-up view of a point P at the base radius R B  of the polishing pad of  5 A. From FIG. 5C, the circumferential segment dS of circumference C R  is related to the radial segment dR as given by: 
     
       
         dS=dR tan θ.  EQ. 4 
       
     
     From equations 3 and 4, it follows that:              dS   =       dR                 tan                 θ     =             (     R   Rs     )     2     -   1          dR               EQ   .              5                                
     Central angle φ(R) is given by:                ϕ        (   R   )       =         ∫   Rs   R          dS   R       =       ∫   Ra   R                  (     R   Rw     )     2     -   1            dR   R                   EQ   .              6                                
     Therefore,                ϕ        (   R   )       =               (     R   Rn     )     2     -   1       ÷       sin     -   1            (     Rs   R     )         -     π   2               EQ   .              7                                
     The one or more grooves  104  are thus formed based on the equations: 
     
       
         X=R cos φ(R) and  EQ. 8 
       
     
     
       
         Y=R sin φ(R)  EQ. 9 
       
     
     A groove formed consistent with the above analysis results in a constant CF, which translates into constant polishing layer area as a function of radius, which in turn translates into more uniform CMP performance than a polishing pad having grooves with a non-constant CF. 
     Alternative embodiments of the present invention include forming one or more radial grooves  104  to have widths that increase with radius at a rate that maintains a constant CF. For large diameter pads, however, this embodiment is less advantageous than a continuous curve. 
     Thus, one example embodiment of the present invention is a polishing pad comprising one or more continuous grooves  104  formed in a manner such that CF is constant (i.e., non-varying) as a function of pad radius. CF can have almost any constant value. However, in a preferred embodiment, the value of CF is in the range from 10% to 25%. 
     In addition, the present invention applies to forming grooves having a wide range of curvatures. However, in a preferred embodiment, the one or more grooves  104  make anywhere from {fraction (1/60)}th to ½ of a revolution. That is, any individual groove occupies a wedge of the polishing pad forming a central angle of 6 to 180 degrees. 
     In another example embodiment, the value of CF is non-constant, but remains within 25% of its average value as a function of pad radius, and more preferably remains within 10% of its average value as a function of radius. These limits on CF allow for, among other things, variations from ideal groove formation (e.g., relaxing the groove design tolerance to make the process of groove formation less expensive and less time consuming), and for compensating any polishing effects that are a function of radius (e.g., material removal as a function of slurry distribution). 
     Grooves  104  formed according to the present invention may be oriented in either direction relative to the pad rotation direction. 
     FIGS. 6A-6D show a variety of example embodiments of groove patterns formed in accordance with the present invention. FIG. 6A is a plot of the curved groove pattern formed according to the present invention, wherein the polishing pad  150  has eight grooves  154  formed in its polishing layer  152 . This polishing pad  150  has an outer radius R O =24 inches defining the outer perimeter  156  and a base radius R B =10 inches. 
     FIG. 6B is the same as FIG. 6A, but with a base radius R B =6 inches. 
     FIG. 6C is the same as FIG. 6A, but with a base radius R B =2 inches. 
     FIG. 6D is the same as FIG. 6C, but with R B =2 inches and the grooves starting at a 10 inch starting radius R S . 
     FIG. 6E is a plot of the circumference fraction grooved CF as a function of pad radius R for the curved groove patterns of FIGS. 6A-6D. As can be seen from FIG. 6E, CF is invariant as function of pad radius R. 
     CMP System and Method of Operation 
     FIG. 7 shows a CMP system  200  that employs an embodiment of a polishing pad  202  of the present invention as described in detail above. Polishing pad  202  has a polishing layer  204 . System  200  includes a polishing platen  210  rotatable about an axis A 1 . Platen  210  has an upper surface  212  upon which pad  202  is mounted. A wafer carrier  220  rotatable about an axis A 2  is supported above polishing layer  204 . Wafer carrier  220  has a lower surface  222  parallel to polishing layer  204 . Wafer  226  is mounted to lower surface  222 . Wafer  226  has a surface  228  that faces polishing layer  204 . Wafer carrier  220  is adapted to provide a downward force F so that wafer surface  228  is pressed against polishing layer  204 . 
     System  200  also includes a slurry supply system  240  with a reservoir  242  (e.g., temperature controlled) that holds a slurry  244 . 
     Slurry supply system  240  includes a conduit  246  connected to the reservoir and in fluid communication with polishing layer  204  for dispensing slurry  244  onto the pad. 
     System  200  also includes a controller  270  coupled to slurry supply system  240  via a connection  274 , to wafer carrier  220  via a connection  276 , and to polishing platen  210  via a connection  278 . Controller  270  controls these system elements during the polishing operation. In an example embodiment, controller  270  includes a processor (e.g., a CPU)  280 , a memory  282  connected to the processor, and support circuitry  284  for supporting the operation of the processor, memory and other elements in the controller. 
     With continuing reference to FIG. 7, in operation controller  270  activates slurry supply system  240  to dispense slurry  244  onto the rotating polishing layer  204 . The slurry spreads out over the polishing pad upper surface, including the portion of the surface beneath wafer  226 . Controller  270  also activates wafer carrier  220  to rotate at a select speed (e.g., 0 to 150 revolutions-per-minute or “rpm.”) so that the wafer surface moves relative to the polishing surface. Wafer carrier  220  also provides a select downward force F (e.g., 0-15 psi) so that the wafer is pressed against the polishing pad. Controller  270  further controls the rotation speed of the polishing platen, which speed is typically between 0-150 rpm. 
     Because polishing layer  204  has a groove structure formed using the methods described above to have a constant CF, the planarization efficiency is higher than that for grooves having a non-constant CF. The benefits to planarization efficiency are realized regardless of the direction of rotation of polishing layer  204 . Increased planarization efficiency results in planarization with less material being removed from the wafer, faster processing of the wafer, and less chance of damaging the wafer surface. 
     Because of the more uniform polishing area per unit pad area of polishing pad  202  in contact with the wafer, in an example embodiment the downward force provided by the wafer carrier may be less than that required with conventional polishing pads to achieve material removal at all desired points on the wafer.