Abstract:
A method and apparatus are described for Chemical Mechanical Polishing of wafers which achieves a constant removal rate of material from the wafer over the entire surface of the wafer. The wafer is held in a wafer carrier rotating at a wafer carrier angular velocity and is polished using a platen rotating at a platen angular velocity. The pressure exerted on the wafer by the wafer carrier is the largest at the wafer edge and smallest at the center of the wafer. The wafer carrier is divided into a number of wafer carrier circular segments so that the temperature of each wafer carrier circular segment can be controlled. The platen is divided into a number of platen circular segments so that the temperature of each platen circular segment can be controlled. The temperatures of the wafer carrier circular segments and the platen circular segments are then adjusted to provide a removal rate of material from the wafer which is uniform across the surface of the wafer.

Description:
BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     This invention relates to removing material from the surface of a wafer using Chemical Mechanical Polishing and more particularly to the use of temperature compensation to achieve uniform removal rates across the entire surface of the wafer. 
     (2) Description of the Related Art 
     Chemical Mechanical Polishing, CMP, is frequently used to planarize wafers by removing material from the surface of a wafer. CMP uses a rotating wafer carrier to hold a wafer against a polishing pad held by a rotating platen. The polishing also uses a polishing slurry dispensed on the polishing pad. 
     U.S. Pat. No. 5,196,353 to Sandhu et al. describes a method and apparatus for thermally mapping the surface of a wafer during Chemical Mechanical Polishing. The thermal map information is then analyzed and used to modify the polishing process parameters. 
     U.S. Pat. No. 5,486,129 to Sandhu et al. describes a system and method for Chemical Mechanical Polishing wafers using a polish control subsystem and a polishing head with localized pressure applicators for controlling localized pressure at individual regions of the wafer. The polish control subsystem allows control of operational subsystems such as temperature at the wafer surface, platen velocity, and localized pressure. 
     U.S. Pat. No. 4,910,155 to Cote et al. describes a method for Chemical Mechanical Polishing insulators, such as silicon oxide and silicon nitride, formed on the surface of wafers. The method uses a slurry with particulates having a hardness commensurate to the hardness of the insulator to be polished at a temperature between 85° F. and 95° F. to maximize both polishing pad lifetime and planarization uniformity. 
     U.S. Pat. No. 5,422,316 to Desai et al. describes a Chemical Mechanical Polishing system for polishing wafers which uses a polishing limiter to limit the reduction in thickness of the wafer during polishing. 
     This invention describes a method and apparatus for achieving a uniform material removal rate across the entire surface of the wafer. The temperature of circular segments of the wafer carrier and circular segments of the platen are individually controlled. The temperature of the wafer carrier circular segments and platen circular segments are individually controlled to compensate for pressure variation across the wafer and variation of the relative velocity between the wafer and platen across the wafer to produce a uniform material removal rate across the entire surface of the wafer. 
     SUMMARY OF THE INVENTION 
     Chemical Mechanical Polishing, CMP, is becoming increasingly more important as a means to planarize wafers between steps of depositing metal or dielectric layers on the wafer. It is important that the removal rate of material from a wafer be uniform over the entire surface of the wafer. Chemical Mechanical Polishing is typically carried out using a platen 10 rotating at a first angular velocity, ω p , see FIG. 1. A wafer is held in a wafer carrier 12 rotating at a second angular velocity, ω c , and positioned above the rotating platen 10. Slurry is supplied from a slurry supply 34, through a slurry supply tube 32, and through a nozzle 30 onto the platen 10. The center of the wafer carrier 12 is a first distance 20 from the center of the platen. The radius 22 of the wafer carrier 12 is a second distance. The rate at which material is removed from a wafer element 14 is proportional to the temperature of the wafer element 14, the pressure exerted on the wafer element 14 by the wafer carrier 12, and the magnitude of the relative velocity between the wafer element 14 and the platen 10. The wafer element 14 has a radial distance r 18 from the center of the wafer carrier and a position angle θ 16 from a base line 24 connecting the center of the platen and the center of the wafer carrier. 
     The ideal situation is to have the removal rate of material from the wafer be constant for all values of the radial distance 18 and position angle 16. However, The pressure exerted on the wafer by the wafer carrier 12 is not constant but is smallest at the center of the wafer carrier and increases to a maximum at the edge of the wafer carrier. FIG. 2 shows the normalized pressure exerted on the wafer by the wafer carrier as a function of distance from the center of the wafer carrier. The magnitude of the relative velocity between the wafer element 14 at a radial distance 18, r, from the center of the wafer carrier and a position angle 16, θ, from the base line 24 connecting the center of the platen and the center of the wafer carrier and the platen 10 is not constant over all positions of the wafer carrier but varies with both the radial distance 18 and the position angle 16. 
     This positional variation of both the magnitude of the relative velocity between the platen and the wafer element and the pressure exerted on the wafer by the wafer carrier makes it very difficult to achieve the ideal case of a constant removal rate of material from the surface of the wafer for all positions on the wafer carrier. 
     It is a principle objective of this invention to provide a method of Chemical Mechanical Polishing wafers which achieves a constant removal rate of material from the surface of the wafer for all positions on the wafer carrier. 
     It is another principle objective of this invention to provide an apparatus for Chemical Mechanical Polishing wafers which achieves a constant removal rate of material from the surface of the wafer for all positions on the wafer carrier. 
     These objectives are achieved by controlling the temperature of circular segments of the wafer carrier forming annular rings concentric with the center of the wafer carrier and circular segments of the platen forming annular rings concentric with the center of the platen. The temperature of the wafer carrier circular segments and the temperature of the platen circular segments are adjusted to provide a constant removal rate of material from the surface of the wafer for all positions on the wafer carrier. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a top view of a conventional Chemical Mechanical Polishing apparatus. 
     FIG. 2 shows a curve of the variation of the normalized pressure exerted on the wafer by the wafer carrier as a function of distance from the center of the wafer carrier. 
     FIG. 3 shows curves of normalized removal rate as a function of position angle for the case where the platen angular velocity is the same as the wafer carrier angular velocity. 
     FIG. 4 shows a top view of a Chemical Mechanical Polishing apparatus showing the wafer carrier circular segments with separate temperature control. 
     FIG. 5 shows a top view of a wafer carrier with separate temperature control to the wafer carrier circular segments provided by electric power supplied to electrical heating elements or heated or cooled liquid supplied to heating or cooling tubes. 
     FIG. 6A shows curves of normalized removal rate as a function of position angle for the case where the platen angular velocity is greater than the wafer carrier angular velocity, the wafer carrier circular segments are at the same temperature, and the platen circular segments are at the same temperature. 
     FIG. 6B shows curves of normalized removal rate as a function of position angle for the case where the platen angular velocity is greater than the wafer carrier angular velocity, the temperatures of wafer carrier circular segments are controlled, and the temperatures of the platen circular segments are controlled. 
     FIG. 7A shows curves of normalized removal rate as a function of position angle for the case where the platen angular velocity is less than the wafer carrier angular velocity, the wafer carrier circular segments are at the same temperature, and the platen circular segments are at the same temperature. 
     FIG. 7B shows curves of normalized removal rate as a function of position angle for the case where the platen angular velocity is less than the wafer carrier angular velocity, the temperatures of wafer carrier circular segments are controlled, and the temperatures of the platen circular segments are controlled. 
     FIG. 8 shows a top view of a Chemical Mechanical Polishing apparatus showing temperature controlled wafer carrier circular segments and temperature controlled platen circular segments. 
     FIG. 9 shows a bottom view of a platen with separate temperature control to the platen circular segments provided by electric power supplied to electrical heating elements or heated or cooled liquid supplied to heating or cooling tubes. 
     FIG. 10 shows a top view of a platen with separate temperature control to the platen circular segments provided by supplying temperature controlled slurry from separate slurry nozzles to a polishing pad having grooves concentric with the center of the platen. 
     FIG. 11 shows a cross section of the platen and polishing pad of FIG. 5 showing the grooves in the polishing pad. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The preferred embodiments of the Chemical Mechanical Polishing method and apparatus for achieving a constant removal rate of material from a wafer over the entire surface of the wafer will now be described with reference to FIGS. 1-11. 
     FIG. 1 shows a top view of a Chemical Mechanical Polishing apparatus showing a platen 10 rotating at a platen angular velocity, a wafer carrier 12 rotating at a wafer carrier angular velocity, and a slurry dispensing apparatus comprising a slurry supply 34, a supply tube 32, and a nozzle 30. The diameter 22 of the wafer carrier 12 will be designated R and in this example is between about 3 and 7 inches but can be any diameter appropriate for the wafer being polished. The distance 20 between the center of the platen 10 and the center of the wafer carrier 12 will be designated a and, in this example, is between about 6 inches and 7 inches. 
     Polishing is accomplished by relative motion between a wafer element 14 and the platen 10. The wafer element is a distance r 18 from the center of the wafer carrier and at a position angle θ 16 from a base line 24 between the center of the wafer carrier and the center of the platen. The magnitude of the relative velocity, |V 12  |, between the wafer element 14 and the platen 10 is given by the following equation: 
     
         |v.sub.12 |= r.sup.2 (ω.sub.p -ω.sub.c).sup.2 +(ω.sub.p a).sup.2 -2arω.sub.p (ω.sub.p -ω.sub.c)cosθ!.sup.1/2. 
    
     In the above equation r is the distance 18 from the wafer element 14 to the center of the wafer carrier, a is the distance 20 between the center of the platen and the center of the wafer carrier, θ is the position angle 16 of the wafer element 14, ω p  is the angular velocity of the platen, ω c  is the angular velocity of the wafer carrier, and |V 12  | is the magnitude of the relative velocity between the wafer element 14 and the platen 10. 
     It can be seen from the above equation that for the case where the platen angular velocity is equal to the wafer carrier angular velocity the magnitude of the relative velocity between the wafer element 14 and the platen 10 is equal to the platen angular velocity multiplied by the distance between the distance 20 between the center of the platen 10 and the center of the wafer carrier 12 and does not depend on either the position angle 16 or the distance 18 between the wafer element 14 and the center of the wafer carrier 12. 
     FIG. 2 shows a curve of the normalized pressure between the wafer element and the platen as a function of the distance from the center of the wafer carrier. The normalized pressure is lowest at the center of the wafer carrier, greatest at the edge of the wafer carrier and increases with increasing distance from the center of the wafer carrier. 
     FIG. 3 shows the normalized removal rate of wafer material as a function of position angle for the case where the platen angular velocity is equal to the wafer carrier angular velocity. The removal rate of material from the wafer is proportional to the temperature, the pressure exerted on the wafer by the wafer carrier, and the magnitude of the relative velocity between the wafer element and the platen. Curves 70, 71, and 72 show the normalized removal rates of material from the wafer as a function of the position angle with no adjustments to the temperature of either the wafer carrier or the platen. The bottom curve 70 shows the normalized removal rate at the center of the wafer. The second curve from the bottom 71 shows the normalized removal rate at a distance between the center and the edge of the wafer. The third curve from the bottom 72 shows the normalized removal rate at the edge of the wafer. 
     As shown in FIG. 4 a number of concentric heating elements 40 are placed in the wafer carrier so that the temperature of circular regions of the wafer carrier, concentric with the center of the wafer carrier, can be independently controlled. The temperatures are adjusted so that the temperature at the center of the wafer carrier is the greatest and decreases with increasing distance from the center of the wafer carrier to the edge of the wafer carrier, which is at the lowest temperature. With this temperature variation the same wafer material removal rate, independent of either distance from the center of the wafer or position angle, can be achieved over the entire wafer surface of the wafer as shown by the top curve 74 in FIG. 3. 
     FIG. 5 shows methods and apparatus of heating the concentric heating elements 40 in greater detail. The concentric heating elements 40 can be electric heating elements in the wafer carrier 12 supplied by independent electrical power sources 50. The concentric heating elements 40 can be tubes in the wafer carrier 12 through which heated or cooled liquid from independent sources 50 flows. 
     FIG. 6A shows the normalized removal rate of wafer material as a function of position angle for the case where the platen angular velocity is greater than the wafer carrier angular velocity. Curves 80, 81, and 82 show the normalized removal rates of material from the wafer as a function of the position angle with no adjustments to the temperature of either the wafer carrier or the platen. The bottom curve 80 shows the normalized removal rate at the center of the wafer. The middle curve 81 shows the normalized removal rate at a distance between the center and the edge of the wafer. The top curve 82 shows the normalized removal rate at the edge of the wafer. 
     As shown in FIGS. 4 and 5 and described above the temperature of circular segments of the wafer carrier can be adjusted. The temperatures are adjusted so that the temperature at the center of the wafer carrier is the greatest and decreases with increasing distance from the center of the wafer carrier to the edge of the wafer carrier, which is at the lowest temperature. The result of this temperature adjustment is that the removal rates at different distances from the center of the wafer are nearly the same but still show a dependence on position angle, as shown in curves 80, 81, and 82 of FIG. 6B. 
     FIG. 8 shows a diagram of the platen 10 and the wafer carrier 12 showing circular segments, 61, 63, and 65 of the platen and circular segments, 41, 43, and 45 of the wafer carrier. It can be seen from FIG. 8 that the temperature of each circular segment of the platen affects the removal rate for a position angle 16 of θ and 360°-θ. Next the temperature of circular segments of the platen, 61, 63, and 65 are adjusted so that the temperature of the inner circular segment 61 of the platen corresponding to position angle of 0° or 360° is the highest, the temperature of the outer circular segment 65 of the platen is the lowest, and the temperature of the circular segments progress between these extremes in a regular progression. 
     The temperature adjustment of the platen circular segments described above superimposed on the temperature adjustment of the wafer carrier circular segments described above produces a removal rate which is nearly constant over the entire surface of the wafer, as shown by curve 84 in FIG. 6B. The actual variation of the normalized removal rate of material from the wafer is less than about 4% from the highest removal rate to the lowest removal rate. 
     FIG. 7A shows the normalized removal rate of wafer material as a function of position angle for the case where the platen angular velocity is less than the wafer carrier angular velocity. Curves 90, 91, and 92 show the normalized removal rates of material from the wafer as a function of the position angle with no adjustments to the temperature of either the wafer carrier or the platen. The bottom curve 90 shows the normalized removal rate at the center of the wafer. The middle curve 91 shows the normalized removal rate at a distance between the center and the edge of the wafer. The top curve 92 shows the normalized removal rate at the edge of the wafer. 
     As shown in FIGS. 4 and 5 and described above the temperature of circular segments of the wafer carrier can be adjusted. The temperatures are adjusted so that the temperature at the center of the wafer carrier is the greatest and decreases with increasing distance from the center of the wafer carrier to the edge of the wafer carrier, which is at the lowest temperature. The result of this temperature adjustment is that the removal rates at different distances from the center of the wafer are nearly the same but still show a dependence on position angle, as shown in curves 90, 91, and 92 of FIG. 7B. 
     FIG. 8 shows a diagram of the platen 10 and the wafer carrier 12 showing circular segments, 61, 63, and 65 of the platen and circular segments, 41, 43, and 45 of the wafer carrier. It can be seen from FIG. 8 that the temperature of each circular segment of the platen affects the removal rate for a position angle 16 of θ and 360°-θ. Next the temperature of circular segments of the platen, 61, 63, and 65 are adjusted so that the temperature of the inner circular segment 61 of the platen corresponding to position angle of 0° or 360° is the lowest, the temperature of the outer circular segment 65 of the platen is the highest, and the temperature of the circular segments progress between these extremes in a regular progression. 
     The temperature adjustment of the platen circular segments described above superimposed on the temperature adjustment of the wafer carrier circular segments described above produces a removal rate which is nearly constant over the entire surface of the wafer, as shown by curve 94 in FIG. 7B. The actual variation of the normalized removal rate of material from the wafer is less than about 4% from the highest removal rate to the lowest removal rate. 
     FIG. 9 shows an embodiment of methods and apparatus for adjusting the temperatures of the platen circular segments using circular heating elements 60 in the platen concentric with the center of the platen. FIG. 9 shows a bottom view of the platen showing the concentric heating elements 60 which can be electric heating elements in the platen 10 supplied by independent electrical power sources 52 or tubes in the platen 10 through which heated or cooled liquid from independent sources 52 flows. 
     Another embodiment of methods and apparatus for adjusting the temperatures of the platen circular segments is shown in FIGS. 10 and 11. FIG. 10 shows a top view of the platen showing a polishing pad with small concentric grooves, having a depth of between about 0.01 and 0.02 inches and a width of between about 0.01 and 0.015 inches. FIG. 11 shows a cross section of the platen and polishing pad along the line 11-11&#39; of FIG. 10 showing the grooves in the polishing pad. Polishing slurry is deposited from a slurry source 48 onto the polishing pads using a series of nozzles 46 so that slurry deposited at different distances from the center of the platen can have different temperatures. The grooves in the polishing pad keeps the slurry at the same distance from the center of the platen at which it was deposited so that temperature control of the circular segments of the platen can be accomplished. 
     While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the invention.