Patent Publication Number: US-6905397-B2

Title: Apparatus for enhanced rate chemical mechanical polishing with adjustable selectivity

Description:
This is a Divisional application of Ser. No. 09/746,470 filed Dec. 22, 2000, now U.S. Pat. No. 6,488,571. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to chemical mechanical polishing apparatus, which may be used to make semiconductor devices. 
     BACKGROUND OF THE INVENTION 
     To make a semiconductor device, several layers of different types of material are deposited on a substrate, e.g., a silicon wafer. After they are deposited, those layers are processed to create devices and interconnects that form the desired integrated circuits. Many of those layers must be planarized to ensure that subsequently deposited layers will be applied to a substantially flat surface. A widely adopted planarizing technique is chemical mechanical polishing (“CMP”). 
       FIG. 1   a  illustrates a cross-section of a structure that may be formed when making a semiconductor device. That structure includes conductive layer  100  upon which is formed dielectric layer  101 . Barrier layer  102  lines a via that has been etched into dielectric layer  101 , and copper layer  103  is formed on barrier layer  102 . A CMP step is applied to that structure to remove copper layer  103 , and the underlying barrier layer, from the surface of dielectric layer  101  generating the structure shown in  FIG. 1   b.    
     Current methods for controlling the CMP process rely on modifying slurry composition and polishing pad properties. Changes to the slurry composition and/or the polishing pad may not, however, enable the polish rate, or the selectivity of that rate across different layers, to be optimized. Taking the example illustrated in  FIGS. 1   a  and  1   b,  to produce the structure shown in  FIG. 1   b  requires polishing through copper layer  103 , then through barrier layer  102 . Because copper is a relatively soft metal, it will polish at a relatively high rate, when compared to the rate at which barrier layer  102  (typically made from a relatively hard material like tantalum or tantalum nitride) is polished. When continuing to polish the structure after breaking through copper layer  103  to barrier layer  102 , differences in selectivity of the polishing process to those two layers can cause significant dishing of wide features. 
     Such differences in selectivity may be a significant concern, when making damascene based structures. To make such structures, low selectivity between the primary metal (e.g., copper) and the underlying barrier layer (e.g., tantalum or tantalum nitride) is required; whereas, high selectivity must be maintained between those materials and the underlying dielectric layer to stop the CMP process on that layer. Optimally, the relative selectivity of the polishing process to the primary metal and the barrier layer is about 1:1; whereas, the relative selectivity to those materials, when compared to the dielectric layer, is about 100:1, or greater. Maintaining such a high degree of selectivity between the primary metal/barrier layer and the dielectric layer may be difficult, when such a layer is formed from polymer based, carbon based or porous low k dielectrics, as such materials are not as strong as silicon dioxide. 
     Accordingly, there is a need for an improved CMP apparatus that enables better control of the polishing rate and the selectivity of the polish rate across different layers. There is a need for such an apparatus that enables higher throughput for the CMP process. The present invention provides such an apparatus. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         FIGS. 1   a  and  1   b  illustrate cross-sections of structures that may be formed when making a semiconductor device. 
         FIG. 2  is a perspective view of an embodiment of the CMP apparatus of the present invention. 
         FIG. 3  is a cross-sectional view of the embodiment shown in  FIG. 2 , as taken along line  2 — 2  of FIG.  2 . 
     
    
    
     DETAILED DESCRIPTION OF THE PRESENT INVENTION 
     The present invention relates to an improved chemical mechanical polishing apparatus. That device includes a platen, a polishing pad that is attached to the platen, and means for adjusting the temperature of the polishing pad. In the following description, a number of details are set forth to provide a thorough understanding of the present invention. It will be apparent to those skilled in the art, however, that the invention may be practiced in many ways other than those expressly described here. The invention is thus not limited by the specific details disclosed below. 
       FIG. 2  provides a perspective view of an embodiment of the CMP apparatus of the present invention. In that device, polishing pad  200  is attached to platen  201 . Platen  201  may be rotated by rotating shaft  202 , upon which platen  201  is fixed. A substrate  203 , such as a silicon wafer, is mounted to substrate carrier  204 , which may be rotated through shaft  205 . Substrate  203  may be held in carrier  204  by friction or by vacuum. A pad (not shown) may be inserted between carrier  204  and substrate  203  to provide a cushion between the substrate and the carrier. The surface of substrate  203 , which is to be polished, faces polishing pad  200  and is pressed against polishing pad  200  as platen  201  rotates. 
     A polishing slurry may be deposited on polishing pad  200 . Such a slurry may initiate the polishing process by chemically reacting with the layer being polished. That slurry may be fed through tube  206  onto the surface of polishing pad  200 . Alternatively, the slurry may be deposited onto that pad by forcing it upward through the pad. 
     Each of the features described above may be implemented with conventional components that are used to make CMP devices. In addition to those features, however, the CMP apparatus of the present invention includes heating element  207 , which may be coupled to (or integrated into) polishing pad  200 . Heating element  207  enables the polishing pad temperature to be varied as substrate  203  is polished. That capability adds another variable for controlling the polishing process. 
       FIG. 3  is a cross-sectional view of the embodiment shown in  FIG. 2 , as taken along line  2 — 2  of FIG.  2 . Substrate  203  is placed face down on polishing pad  200 , which is attached to rotatable platen  201 . Carrier  204  is used to press substrate  203  against polishing pad  200 , while a slurry is deposited onto polishing pad  200  from nozzle  206  during polishing. Shaft  205  may be used to apply a downward force and to rotate substrate  203 . Retaining ring  324  prevents substrate  203  from slipping laterally during polishing and pad  326  provides a cushion between wafer  203  and carrier  204 . 
     Integrated into polishing pad  200  is heating element  207 . Heating element  207  may comprise an electrically resistive plate, or other component (e.g., a coil or mesh), that may be used to vary the temperature of the polishing pad. Heating element  207  may be coupled to a power source using wires or cables (not shown.) Although the embodiment shown in  FIGS. 2 and 3  uses an electrically resistive heating element to vary the temperature of polishing pad  200 , other means may be used to perform that function. For example, a lamp module may be integrated into the CMP apparatus of the present invention for that purpose. Such a module may comprise a single lamp positioned above polishing pad  200 , or alternatively, a ring configuration that includes several lamps that are spaced along the perimeter of the pad. In other embodiments, the temperature of polishing pad  200  may be varied through convection. In those embodiments, a reservoir that contains a gas or liquid may be positioned around the perimeter of pad  200 . In such an apparatus, the pad temperature may be varied by heating or cooling that gas or liquid. 
     In embodiments of the present invention that include an under pad between the polishing pad and the platen, a heating element may be integrated into the under pad instead of the polishing pad. Those skilled in the art will recognize that components other than those described above may be added to the CMP apparatus to vary the temperature of the polishing pad and/or the surface of the substrate held by the substrate carrier. In that respect, any CMP apparatus that provides for such a temperature varying function falls within the spirit and scope of the present invention. 
     The polish rate may be correlated with temperature, i.e., the rate may increase with increasing temperature and decrease with decreasing temperature. Because the CMP apparatus of the present invention can vary the temperature of the polishing pad, it can be used to tailor the polishing rate and/or selectivity of that rate across different layers. In addition, varying the temperature at which the substrate is polished may control the relative degree to which chemical processes and mechanical abrasion remove material from the substrate. 
     Returning to the  FIG. 1   a/   1   b  example, a first layer (e.g., copper layer  103 ) may be polished while maintaining its temperature within a first temperature range, and a second layer (e.g., barrier layer  102 ) may be polished while maintaining its temperature within a second temperature range. When the first layer comprises copper and the second layer comprising tantalum, tantalum nitride, or a tantalum/tantalum nitride composite, that first temperature range may be 25° C. to 50° C. while the second temperature range may be 150° C. to 200° C. If that second layer is made from a different type of barrier material, e.g., titanium nitride or tungsten nitride, the optimum polishing pad temperature may fall within, or slightly outside of, the temperature range specified here. 
     By increasing the temperature, after copper layer  103  has been removed, the polishing rate for barrier layer  102  can be increased. As a result, the selectivity of the polish rate between the copper layer and the barrier layer may be substantially reduced, when compared to the selectivity that applies when those layers are polished at the same temperature. Because increasing the temperature increases the polishing rate, the same slurry can be used to polish both the copper layer and the barrier layer while reducing differences in the selectivity of the polishing rate to those layers. Optimally, temperatures are selected for polishing those layers that cause the selectivity to be close to 1:1. 
     When the barrier layer removal step is almost complete, the temperature may be reduced to decrease the rate at which the remainder of that layer is removed, making it easier to stop the polishing process at dielectric layer  101 . Such a practice may be particularly useful when dielectric layer  101  comprises a polymer based, carbon based or porous low k insulating material. 
     The CMP apparatus of the present invention may be used in many other contexts. It may be used to polish various types of metal layers (including those made from materials other than copper), various types of barrier layers (including those made from materials other than the ones mentioned above), and various types of insulating layers. This apparatus may, in essence, be used to polish any of the wide variety of materials that are used to form layers that must be planarized, when making a semiconductor device. This apparatus may be used to increase the rate at which such materials are removed—even when polishing silicon dioxide or another insulating material. Alternatively, when it is desirable to reduce that polishing rate, or increase selectivity, this apparatus may be used to decrease the polishing rate. A chamber that contains a coolant may, for example, supply a means for lowering the polishing pad temperature to decrease the polishing rate. 
     The CMP apparatus of the present invention thus enables polishing rates and selectivity to be adjusted in a relatively simple and controllable way for any type of material to which a CMP process may be applied. That, in turn, should afford better control and higher throughput of CMP processing. By providing another means for controlling polishing performance—in addition to slurry and polishing pad composition—the CMP apparatus of the present invention should render the CMP process more robust. 
     Features shown in the above figures are not intended to be drawn to scale, nor are they intended to be shown in precise positional relationship. Additional components that may be used to make the CMP apparatus of the present invention have been omitted when not useful to describe aspects of the present invention. Although the foregoing description has specified certain features that may be included in such an apparatus, those skilled in the art will appreciate that many modifications and substitutions may be made. Accordingly, it is intended that all such modifications, alterations, substitutions and additions be considered to fall within the spirit and scope of the invention as defined by the appended claims.