Abstract:
A polishing pad, an apparatus for chemical mechanical polishing of semiconductor wafers and a method of making a device using the same are presented. The apparatus includes a first platform for mounting a semiconductor wafer; a second platform for mounting a polishing pad; a rotator for rotating the wafer against the polishing pad; and a diamond dresser for dressing the polishing pad. The polishing pad has a single groove of a width (w) surrounding the periphery of an undressed portion of the polishing pad thus eliminating contact of the undressed portion with the outer edge of the diamond dresser.

Description:
BACKGROUND 
     Chemical mechanical polishing (CMP) is used in semiconductor manufacturing processes for planarizing a wafer or other work piece. To maintain the stability and throughput of CMP, the polishing pads used in CMP should be dressed by a diamond dresser. 
     The function of the diamond dresser is to maintain or restore the polishing characteristics of the polishing pad to the maximum extent possible during the polishing process and in doing so, extend the useful life of the polishing pad. The dressing step may be performed simultaneously with the polishing of the wafer, or as a separate step after every wafer or an x number of wafers is polished. The diamond dresser performs this function by exerting pressure on the polishing pad thereby affecting the polishing characteristics of the polishing pad. 
     Diamond dressers are generally made with diamonds. The usage of diamonds and the designs of most diamond dressers are such that the last 2 mm to 3 mm towards the outer circumference of the dressers are not covered by diamonds. Therefore, when the polishing pad is dressed down, the travel of this 2 mm to 3 mm of flat outer circumference region would be obstructed by the undressed part of the pad material. Such undressed part of the polishing pad affect CMP removal rate stability and increases wafer range issues as the polishing pad ages, thus limiting any future pad consumable life extension. 
     From the foregoing, it is desirable to provide an improved polishing pad that addresses the issue caused by the undressed part of the pad material. 
     SUMMARY 
     Embodiments generally relate to a polishing pad, an apparatus for chemical mechanical polishing of semiconductor wafers and a method of making a device using the same. The apparatus includes a first platform for mounting a semiconductor wafer; a second platform polishing for mounting a polishing pad; a rotator for rotating the wafer against the polishing pad; and a diamond dresser for dressing the polishing pad. The polishing pad has a single groove of a width (w) surrounding the periphery of an undressed portion of the polishing pad thus eliminating contact of the undressed portion with the outer edge of the diamond dresser. 
     In another embodiment, a method of making a device comprises providing a wafer with a first surface; polishing the first surface of the wafer with a polishing pad, wherein the polishing pad comprises a dressed portion, an undressed portion and a groove separating the dressed and undressed portions; and dressing the polishing pad with a diamond dresser. The groove separating the dressed and undressed portion of the polishing pad eliminates contact of the undressed portion with the outer edge of the diamond dresser. 
     In yet another embodiment, a pad for polishing a wafer is disclosed. The pad includes a dressed portion, an undressed portion and a groove separating the dressed and undressed portions. The groove includes a width (w) surrounding the periphery of the undressed portion. The groove eliminates contact of the undressed portion with an outer edge of a diamond dresser when the pad is mounted to an apparatus for chemical mechanical polishing the wafer. 
     These advantages and features of the embodiments herein disclosed will become apparent through reference to the following description and the accompanying drawings. Furthermore, it is to be understood that the features of the various embodiments described herein are not mutually exclusive and can exist in various combinations and permutations. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings, like reference characters generally refer to the same parts throughout the different views. Also, the drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the invention. In the following description, various embodiments of the present invention are described with reference to the following drawings, in which: 
         FIG. 1  shows the ex-situ dressing of a polishing pad; 
         FIGS. 2 a - b    show top and side views of the ex-situ dressing in  FIG. 1 ; 
         FIGS. 3( a )-( c )  show various embodiments of a polishing pad with a groove including an embodiment with a cutout portion on the polishing pad; and 
         FIGS. 4 a - f    show experimental data comparing, among other factors, the removal rate of a convention polishing pad and an embodiment of a grooved polishing pad. 
     
    
    
     DESCRIPTION 
     Embodiments generally relate to a polishing pad for used in CMP. The polishing pad is dressed by a diamond dresser while planarizing a wafer. In one embodiment, the diamond dresser may be a Shinhan dresser made with blocky type diamonds. In other embodiments, the diamond dresser may be manufactured by other manufacturers and may be made with other types of diamonds, for example, irregular shaped diamonds. 
       FIG. 1  shows the ex-situ dressing of a polishing pad. As shown, the polishing pad  100  is being dressed by a diamond dresser  102  while a wafer (not shown) in the top ring  104  is being planarized by the polishing pad. Referring to  FIGS. 2 a  and 2 b   , which show top and side views of the ex-situ dressing in  FIG. 1 , respectively; an undressed portion  106  can be seen in both the top and side views. While the diagrams show an undressed circle, such undressed portion  106  may also be of another shape or be of irregular shape. 
     Referring to  FIG. 2 b   , it can be seen that the gap of about, for example, 2-3 mm in the blocky diamond  108  of the diamond dresser  102  results in physical contact with an undressed portion  106 , which would result in obstruction of the dressing action. As the pad is progressively dressed, the obstruction of the dressing action by the undressed portion becomes more severe, limiting downward travel of the diamond dresser, altering the polishing pad cross section profile and the polishing pad cut rate. This could cause increase polishing time, thus increasing slurry usage, thereby causing wastage. 
       FIGS. 3( a )-( c )  show various embodiments of a polishing pad with a groove including an embodiment with a cutout portion on the polishing pad. Referring to  FIG. 3( a ) , an embodiment of a polishing pad  300  with cross-sectional view and an enlarged diagram of a groove  302  on the polishing pad  300  is being shown. As can be seen, groove  302  is a single groove surrounding the periphery of an undressed portion  304 . As groove  302  is at the polishing pad region where the dresser edge makes contact with the undressed portion  304 , groove  302  helps to eliminate contact of the dressed portions of polishing pad  300  with undressed portion  304  of polishing pad  300 . Given that the undressed portion  304  is shown as a circle, the groove is therefore also circular in shape as it surrounds the circumference of the circle. 
     Referring to the enlarged diagram of groove  302  as shown to the left of  FIG. 3( a ) , given that undressed portion  304  is shown as a circle, it can be seen that groove  302  may have, for example, an inner radius r I , which is the length extending from the center of polishing pad  300  to the inner circumference of groove  302 . Groove  302  may further have, for example, an outer radius r O , which is the length extending from the center of polishing pad  300  to the outer circumference of groove  302 . In one embodiment, r I  may be about 48 mm and r O  may be about 56 mm, in which case, groove  302  may have a center radius r C  (which is a radius from the center of polishing pad  300  that is in equidistance apart from r I  and r O ) of about 52 mm. Other values for r I , r O  and r C  may also be helpful. For example, r I  may be about 23 mm to 58 mm, r O  may be about 31 mm to 66 mm and r C  may be about 27 mm to 62 mm. 
     As shown in  FIG. 3( a ) , groove  302  may include a gap or width (w). The width of the groove, for example, may be about 8 mm. In order embodiments, the width could be larger or smaller as long as it is wide enough to ensure the outer part of undressed portion  304  could be removed beforehand thus eliminating contact of undressed portion  304  with the flat outer edge of a dresser. The width (w) may also have a minimum and maximum dimension that is expressed in relation to the region of the dresser that is not covered by diamonds (D) and the groove center radius r C  as follows: D&lt;=w&lt;=r C +w/2. 
     In addition, the groove may have a depth (d), and an angle (θ) as shown in the right most drawing in  FIG. 3( a ) . Angle θ may have a range of 0°&lt;=θ&lt;=100°. The depth (d) of the groove, for example, may be about 0.7 mm. It should be understood that in other embodiments, the groove may have a shallower or deeper depth. The groove depth (d) in relation to the thickness (T) of polishing pad  300  may be 0&lt;d/T&lt;=1. The radius r C  in relation to the radius (R) of polishing pad  300  may be 0&lt;r C /R&lt;=1. 
       FIG. 3( b )  shows several alternative embodiments of the groove bottom cross sectional design. As can be seen, groove  302 , instead of having the rectangular bottom as shown in  FIG. 3( a ) , may instead have a polygonal bottom or a circular elliptical bottom as shown in  FIG. 3( b ) . In other embodiments, the groove may also have a bottom that is of another shape or of an irregular shape. Where the groove bottom is polygonal, various variations are also possible by varying the value of the width (w), the angle (θ) and the depth (d). As shown, in some of the embodiments, the width (w) may have a wider first part (w O ) and a narrower second part (w I ); alternatively, the polygonal bottom may comprise more than one angle (θ). Similarly, variations of the circular elliptical bottoms may also be obtained by varying the value of the width (w) and the depth (d). 
       FIG. 3( c )  shows two alternative embodiments of polishing pad  300 . Referring to  FIG. 3( c )( i ) , the undressed portion  304  may be, for example, an octagonal shape instead of being circular. While an octagonal shape is shown, it should be understood that the undressed portion  304  may also be of irregular shape. As can be seen, groove  302  surrounding the periphery of undressed portion  304  may still be circular in shape. However, in other embodiments, groove  302  in surrounding the periphery of undressed portion  304  may mimic the shape of undressed portion  304 . Referring to  FIG. 3( c ) ( ii ), it can be seen that, instead of a groove  302  surrounding the periphery of undressed portion  304 , the groove  302  and the undressed portion  304  may be merged and molded to form a depressed region that is below the surface of the polishing pad by a depth (d) to avoid physical contact with the diamond dresser. Alternatively, the entire undressed portion  304  may be cut out to avoid contact with the diamond dresser. 
       FIGS. 4 a - f    show experimental data comparing, among other factors, the removal rate of a convention polishing pad and an embodiment of a grooved polishing pad. Referring to  FIG. 4 a   , the removal rate of an extrapolated process on record (“POR”) polishing pad is compared to a single grooved polishing pad as described above. As can be seen, the removal rate of the single grooved polishing pad remains steady throughout the pad life and even increases slightly as time passes. On the other hand, the extrapolated POR polishing pad removal rate increases for a spell but ultimately decreases as time passes. 
       FIG. 4 b    shows the standard deviation of the within wafer non-uniformity for an extrapolated POR polishing pad as compared to a single grooved polishing pad. As shown, the single grooved polishing pad has standard deviation data that fluctuates more closely around the reference line, which indicates that the single grooved polishing pad has a lower within wafer non-uniformity. The boxplot comparison shown to the right, which is a compression of the standard deviation data, also confirms this as the spread of the standard deviation data for the single grooved polishing pad is much tighter as compared to the extrapolated POR polishing pad. Furthermore, the fact that the single grooved polishing pad data does not overlap with the extrapolated POR polishing pad data shows that the difference between the 2 pads is significant. 
       FIG. 4 c    shows the removal rate profile plot for an extrapolated POR polishing pad as compared to a single grooved polishing pad. As can be seen, profile wise, the single grooved polishing pad achieves higher edge removal as time passes as compared to the extrapolated POR polishing pad.  FIG. 4 d    shows the pad profile of an extrapolated POR polishing pad as compared to a single grooved polishing pad. As can be seen, the single grooved polishing pad has a pad profile that is flatter, and has a gentler slope as compared with the pad profile of the extrapolated POR polishing pad. 
       FIG. 4 e    shows a measurement of the particle performance of an extrapolated POR polishing pad as compared to a single grooved polishing pad. The measurement of the particle performance indicates how clean the wafer surface is after polishing. As can be seen, the single grooved polishing pad has comparable or better particle performance as compared with the extrapolated POR polishing pad. 
     Finally,  FIG. 4 f    shows the inspection result of an extrapolated POR polishing pad and the single grooved polishing pad after both pads have been used. As can be seen, the extrapolated POR polishing pad has a reflective mirror like sliver of pad glazing at the outer edge of an undressed portion, whereas the single grooved polishing pad shows no glazing at the outer edge of the undressed portion. While  FIG. 4 f    shows a perforated polishing pad, in other embodiments, a non-perforated polishing may also be used. 
     To ensure consistency in the results, the above experiments were all done on test wafer from the same pod. The wafers are further randomized to minimize any possible Chemical Vapor Deposition influence. All the runs are done on the same table, with the same topring and dresser consumable set. The removal rate data is sampled when the pad life reaches designated life regions using manual dressing to approximate productions conditions. For both the extrapolated POR polishing pad and the single grooved polishing pad, 5 dummy wafers were run to season the polishing pads before 2 test wafers are used to sample the removal rate of the polishing pads. 
     As shown by the above discussion, the use of a single grooved polishing pad would result in improved removal rate stability, improved within wafer non-uniformity, as well as improved cut rate of the polishing pad. The pad profile has improved due to the fact that an acting dressing action downwards was not impeded by any undressed pad material. The particle performance of the single grooved polishing pad is also comparable to the extrapolated POR polishing pad. Use of the single grooved polishing pad also prevents the formation of glazing, which is characteristic of impeded dressing travel at the outer edge of the undress circle. 
     In yet other embodiments, the single grooved polishing pad may be used in a method for making a device. The method includes providing a wafer and polishing a first surface of the wafer with the single grooved polishing pad as described above. The method further includes dressing the single grooved polishing pad with a diamond dresser while the single grooved polishing pad is polishing the wafer; wherein the singular groove separating the dressed and undressed portion on the single grooved polishing pad is sufficiently wide to eliminate obstruction of the diamond dresser during dressing. 
     The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The foregoing embodiments, therefore, are to be considered in all respects illustrative rather than limiting the invention described herein. Scope of the invention is thus indicated by the appended claims, rather than by the foregoing description, and all changes that come within the meaning and range of the equivalency of the claims are intended to be embraced therein.