Patent Publication Number: US-6705923-B2

Title: Chemical mechanical polisher equipped with chilled wafer holder and polishing pad and method of using

Description:
FIELD OF THE INVENTION 
     The present invention generally relates to a chemical mechanical polisher for polishing semiconductor wafers and a method of using and more particularly, relates to a chemical mechanical polisher that is equipped with a chilled wafer holder and polishing pad and a method for using the chemical mechanical polisher. 
     BACKGROUND OF THE INVENTION 
     Apparatus for polishing thin, flat semiconductor wafers is well-known in the art. Such apparatus normally includes a polishing head which carries a membrane for engaging and forcing a semiconductor wafer against a wetted polishing surface, such as a polishing pad. Either the pad, or the polishing head is rotated and oscillates the wafer over the polishing surface. The polishing head is forced downwardly onto the polishing surface by a pressurized air system or, similar arrangement. The downward force pressing the polishing head against the polishing surface can be adjusted as desired. The polishing head is typically mounted on an elongated pivoting carrier arm, which can move the pressure head between several operative positions. In one operative position, the carrier arm positions a wafer mounted on the pressure head in contact with the polishing pad. In order to remove the wafer from contact with the polishing surface, the carrier arm is first pivoted upwardly to lift the pressure head and wafer from the polishing surface. The carrier arm is then pivoted laterally to move the pressure head and wafer carried by the pressure head to an auxiliary wafer processing station. The auxiliary processing station may include, for example, a station for cleaning the wafer and/or polishing head; a wafer unload station; or, a wafer load station. 
     More recently, chemical-mechanical polishing (CMP) apparatus has been employed in combination with a pneumatically actuated polishing head. CMP apparatus is used primarily for polishing the front face or device side of a semiconductor wafer during the fabrication of semiconductor devices on the wafer. A wafer is “planarized” or smoothed one or more times during a fabrication process in order for the top surface of the wafer to be as flat as possible. A wafer is polished by being placed on a carrier and pressed face down onto a polishing pad covered with a slurry of colloidal silica or alumina in de-ionized water. 
     A schematic of a typical CMP apparatus is shown in FIGS. 1A and 1B. The apparatus  20  for chemical mechanical polishing consists of a rotating wafer holder  14  that holds the wafer  10 , the appropriate slurry  24 , and a polishing pad  12  which is normally mounted to a rotating table  26  by adhesive means. The polishing pad  12  is applied to the wafer surface  22  at a specific pressure. The chemical mechanical polishing method can be used to provide a planar surface on dielectric layers, on deep and shallow trenches that are filled with polysilicon or oxide, and on various metal films. CMP polishing results from a combination of chemical and mechanical effects. A possible mechanism for the CMP process involves the formation of a chemically altered layer at the surface of the material being polished. The layer is mechanically removed from the underlying bulk material. An altered layer is then regrown on the surface while the process is repeated again. For instance, in metal polishing, a metal oxide may be formed and removed repeatedly. 
     A polishing pad is typically constructed in two layers overlying a platen with the resilient layer as the outer layer of the pad. The layers are typically made of polyurethane and may include a filler for controlling the dimensional stability of the layers. The polishing pad is usually several times the diameter of a wafer and the wafer is kept off-center on the pad to prevent polishing a non-planar surface onto the wafer. The wafer is also rotated to prevent polishing a taper into the wafer. Although the axis of rotation of the wafer and the axis of rotation of the pad are not collinear, the axes must be parallel. It is known in the art that uniformity in wafer polishing is a function of pressure, velocity and the concentration of chemicals. Edge exclusion is caused, in part, by a non-uniform pressure applied on a wafer. The problem is reduced somewhat through the use of a retaining ring which engages the polishing pad. 
     Referring now to FIG. 1C, wherein an improved CMP head  20 , sometimes referred to as a Titan® head which differs from conventional CMP heads in two major respects is shown. First, the Titan® head employs a compliant wafer carrier and second, it utilizes a mechanical linkage (not shown) to constrain tilting of the head, thereby maintaining planarity relative to a polishing pad  12 , which in turn allows the head to achieve more uniform flatness of the wafer during polishing. The wafer  10  has one entire face thereof engaged by a flexible membrane  16 , which biases the opposite face of the wafer  10  into face-to-face engagement with the polishing pad  12 . The polishing head and/or pad  12  are moved relative to each other, in a motion to effect polishing of the wafer  10 . The polishing head includes an outer retaining ring  14  surrounding the membrane  16 , which also engages the polishing pad  12  and functions to hold the head in a steady, desired position during the polishing process. As shown in FIG. 1C, both the retaining ring  14  and the membrane  16  are urged downwardly toward the polishing pad  12  by a linear force indicated by the numeral  18  which is effected through a pneumatic system. 
     The enlarged cross-sectional representation of the polishing action which results form a combination of chemical and mechanical effects is shown in FIG.  1 B. The CMP method can be used to provide a planner surface on dielectric layers, on deep and shallow trenches that are filled with polysilicon or oxide, and on various metal films. A possible mechanism for the CMP process involves the formation of a chemically altered layer at the surface of the material being polished. The layer is mechanically removed from the underlying bulk material. An outer layer is then regrown on the surface while the process is repeated again. For instance, in metal polishing, a metal oxide layer can be formed and removed repeatedly. 
     During a CMP process, a large volume of a slurry composition is dispensed. The slurry composition and the pressure applied between the wafer surface and the polishing pad determine the rate of polishing or material removal from the wafer surface. The chemistry of the slurry composition plays an important role in the polishing rate of the CMP process. For instance, when polishing oxide films, the rate of removal is twice as fast in a slurry that has a pH of 11 than with a slurry that has a pH of 7. The hardness of the polishing particles contained in the slurry composition should be about the same as the hardness of the film to be removed to avoid damaging the film. A slurry composition typically consists of an abrasive component, i.e, hard particles and components that chemically react with the surface of the substrate. 
     For instance, a typical oxide polishing slurry composition consists of a colloidal suspension of oxide particles with an average size of 30 nm suspended in an alkali solution at a pH larger than 10. A polishing rate of about 120 nm/min can be achieved by using this slurry composition. Other abrasive components such as ceria suspensions may also be used for glass polishing where large amounts of silicon oxide must be removed. Ceria suspensions act as both the mechanical and the chemical agent in the slurry for achieving high polishing rates, i.e, larger than 500 nm/min. While ceria particles in the slurry composition remove silicon oxide at a higher rate than do silica, silica is still preferred because smoother surfaces can be produced. Other abrasive components, such as alumina (Al 3 O 2 ) may also be used in the slurry composition. 
     The polishing pad  28  is a consumable item used in a semiconductor wafer fabrication process. Under normal wafer fabrication conditions, the polishing pad is replaced after about 12 hours of usage. Polishing pads may be hard, incompressible pads or soft pads. For oxide polishing, hard and stiffer pads are generally used to achieve planarity. Softer pads are generally used in other polishing processes to achieve improved uniformity and smooth surface. The hard pads and the soft pads may also be combined in an arrangement of stacked pads for customized applications. 
     Referring now to FIG. 1D, wherein a perspective view of a CMP polishing station  42  is shown. The polishing station  42  consists of a conditioning head  52 , a polishing pad  28 , and a slurry delivery arm  54  positioned over the polishing pad. The conditioning head  28  is mounted on a conditioning arm  58  which is extended over the top of the polishing pad  28  for making sweeping motions across the entire surface of the pad. The slurry delivery arm  54  is equipped with a single slurry dispensing nozzle  62  which is used for dispensing a slurry solution on the top surface  60  of the polishing pad  56 . Surface grooves  64  are further provided in the top surface  60  to facilitate even distribution of the slurry solution and to help entrapping undesirable particles that are generated by coagulated slurry solution or any other foreign particles which have fallen on top of the polishing pad during a polishing process. The surface grooves  64  while serving an important function of distributing the slurry also presents a processing problem when the pad surface  60  gradually worn out after successive use. 
     In a typical polishing slurry composition of a colloidal suspension of particles, a dispersion agent is added to facilitate the distribution of the particles in the suspension. During a chemical mechanical polishing process, the temperature of the slurry gradually increases by to the mechanical heat generated between the polishing pad, the wafer surface and the slurry particles. The increased temperature of the slurry affects the efficiency of the dispersion agent and as a result, large agglomerates of particles are formed in the slurry suspension. These agglomerates have sizes substantially larger than the surface grooves provided in the polishing pad and thus, cannot be trapped by the surface grooves. The agglomerates of particles can thus become a serious source of contamination leading to significant scratches on the wafer surface. A major scratch on the wafer surface may cause the scrap of the entire wafer. 
     It is therefore an object of the present invention to provide a chemical mechanical polisher that does not have the scratching defect caused by the formation of large slurry particle agglomerates. 
     It is another object of the present invention to provide a chemical mechanical polisher that is equipped with a chilled wafer holder and a chilled polishing pad. 
     It is a further object of the present invention to provide a chemical mechanical polisher wherein any increase in temperature of a slurry solution is controlled. 
     It is another further object of the present invention to provide a chemical mechanical polishing method without scratching defects caused by the formation of large slurry particle agglomerates. 
     It is still another object of the present invention to provide a chemical mechanical polishing method by flowing a cooling fluid into a wafer holder and a polishing pad such that the temperature increase of the slurry solution can be controlled. 
     It is yet another object of the present invention to provide a chemical mechanical polishing method by flowing a high heat capacity fluid into a wafer holder and a polishing pad for controlling the temperature rise of the holder and the pad. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, a chemical mechanical polisher that is equipped with a chilled wafer holder and polishing pad and a method of using the polisher are provided. 
     In a preferred embodiment, a chemical mechanical polisher that is equipped with a chilled wafer holder and polishing pad is provided which includes a polishing head that has a retaining ring chamber for pressing a retaining ring downwardly, the retaining ring defines a membrane chamber therein for contacting and pressing a wafer downwardly onto a polishing pad; the membrane chamber receives a first heat exchanging fluid for removing heat from the wafer during a polishing process; and a pedestal for mounting a polishing pad on a top surface, the polishing pad has a bottom surface provided with fluid channels for circulating a second heat exchanging fluid therein, the second heat exchanging fluid being fed into the fluid channels through a fluid passageway in the pedestal for removing heat from the polishing pad. 
     In the chemical mechanical polisher that is equipped with a chilled wafer holder and polishing pad, the membrane chamber may further include an inlet and an outlet for the first heat exchanging fluid, the polisher may include at least one fluid reservoir for holding the first and the second heat exchanging fluid, the polisher may further include a temperature controller for controlling the temperature of the first and second heat exchanging fluid. The first heat exchanging fluid may be water. The polisher may further include a temperature controller for controlling the temperature of the first heat exchanging fluid to not higher than 23° C., and preferably to not higher than 18° C. The first and the second heat exchanging fluids may be the same, or may be different. 
     The present invention is further directed to a chemical mechanical polishing method that is without scratching defect caused by large slurry particle agglomerates which can be carried out by the operating steps of first providing a polishing head equipped with a retaining ring chamber and a membrane chamber situated inside the retaining ring chamber; mounting a wafer on the membrane chamber with a surface to be polished exposed; flowing a first heat exchanging fluid into the membrane chamber for pressing the wafer in a downward direction onto a polishing pad and for removing heat during a polishing operation; providing a pedestal for holding a polishing pad thereon; forming a plurality of flow channels on a bottom side of the polishing pad for intimately contacting a top surface of the pedestal; and flowing a second heat exchanging fluid into the plurality of flow channels and removing heat from the polishing pad during a polishing process such that the generation of large slurry particle agglomerates is avoided. 
     The chemical mechanical polishing method that is without scratching defect caused by large slurry particle agglomerates may further include the step of providing fluid passageways in the pedestal for flowing the second heat exchanging fluid into the plurality of flow channels. The method may further include the step of selecting the first heat exchanging fluid from one that has a heat capacity of at least that of water. The method may further include the step of flowing the first heat exchanging fluid of water at a temperature not higher than 23° C. into the membrane chamber, or flowing the first heat exchanging fluid of water at a temperature preferably not higher than 18° C. into the membrane chamber. The method may further include the step of selecting the second heat exchanging fluid the same as the first heat exchanging fluid, or selecting the second heat exchanging fluid different than the first heat exchanging fluid. The method may further include the step of forming the plurality of flow channels on the bottom side of the polishing pad to a depth that is not larger than ¾ of the thickness of the pad. The method may further include the step of providing a fluid inlet and a fluid outlet on the membrane chamber. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects, features and advantages of the present invention will become apparent from the following detailed description and the appended drawings in which: 
     FIG. 1A is a cross-sectional view of a conventional chemical mechanical polishing apparatus. 
     FIG. 1B is an enlarged, cross-sectional view illustrating the interaction between the wafer surface, the polishing pad and the slurry solution. 
     FIG. 1C is a cross-sectional view of a conventional membrane-pressured wafer holder for the CMP apparatus. 
     FIG. 1D is a perspective view of a conventional CMP apparatus with a slurry dispenser and a conditioning arm positioned on top of the polishing pad. 
     FIG. 2 is a cross-sectional view of the present invention chemical mechanical polishing apparatus equipped with a chilled wafer holder and a chilled polishing pad. 
     FIG. 2A is a bottom view of the polishing pad shown in FIG. 2 illustrating the flow channels on the backside of the pad. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention discloses a chemical mechanical polisher that is equipped with a chilled wafer holder and a chilled polishing pad which can be used to carry out a chemical mechanical polishing process without scratching defect caused by the large slurry particle agglomerates generated when the slurry temperature is not controlled. The invention further discloses a chemical mechanical polishing method that can be carried out without scratching defect by large slurry particle agglomerates which would otherwise be produced by heated slurry solution during the polishing process. 
     The present invention novel method and apparatus utilizes a unique backside flow channel design in a polishing pad to increase heat removal, while simultaneously, flowing a high heat capacity fluid, i.e. water, into the membrane chamber of a wafer holder to increase heat removal from the wafer. Any temperature increase in the slurry suspension during the polishing process can thus be controlled such that the formation of large slurry particle agglomerates can either be significantly reduced or can be completely eliminated. 
     Referring now to FIG. 2, wherein a present invention wafer holder  40  and a polishing pad  12  are shown in a cross-sectional view. Two separate pressure chambers of a retaining ring chamber  30  and a membrane chamber  32  are used during a polishing process. A retaining ring pressure  34  exerts on the retaining ring  14 , while a membrane pressure  18  translates into wafer backside pressure. Generally, the wafer retaining pressure in the wafer holder  40  is a function of both the membrane pressure  18  and the retaining ring pressure  34 . 
     The polishing pad  12  is generally fabricated of a rigid polymeric material with surface grooves (not shown in FIG. 2) provided in a top surface. In the present invention polishing pad  12 , a plurality of fluid channels  38  is formed in a bottom surface  36  that is in contact with a pedestal (not shown in FIG.  2 ). The plurality of fluid channels  38  allows the circulation of a heat exchanging fluid therethrough when in fluid communication with fluid passageways provided in the pedestal. A fluid inlet  44  and a fluid outlet  46  are further provided for flowing the heat exchanging fluid into and out of the plurality of surface grooves  38 . A suitable heat exchanging fluid should have a sufficiently high heat capacity, i.e. at least that of water, such that heat can be efficiently carried away from the otherwise low thermal conductivity polymeric material that is frequently used to fabricate the polishing pad  12 . The cooled or chilled polishing pad  12  can therefore efficiently reduce the temperature of the slurry suspension deposited thereon during a chemical mechanical polishing process. 
     A second important aspect of the present invention is the chilled wafer holder  40  achieved by flowing into the membrane chamber  32  a first heat exchanging fluid through an inlet  48  and a fluid passageway  50 , and out of the membrane chamber  32  through an outlet  56  and a fluid passageway  58 . A suitable first heat exchanging fluid may be one that has a sufficiently high heat capacity, i.e. at least that of water. The first heat exchanging fluid not only provides the heat exchanging function by carrying away heat from the wafer backside  70 , but also applying membrane pressure  18  onto the wafer  10  such that the active surface  90  of the wafer  10  intimately engages the top surface  78  of the polishing pad  12  during a chemical mechanical polishing process. 
     By combining the cooling, or chilling functions of the membrane chamber and the polishing pad, the present invention novel chemical mechanical polisher effectively reduces the temperature of the slurry suspension by at least 10° C. such that the temperature of the slurry suspension maintains at close to room temperature, i.e. at about 23° C. By sufficiently cooling the slurry suspension, any formation of large slurry particle agglomerates which are the major contamination source, can be avoided. 
     The present invention chemical mechanical polisher that is equipped with a chilled wafer holder and a chilled polishing pad and a method for operating the chemical mechanical polisher have therefore been amply described in the above description and in the appended drawings of FIGS. 2 and 2A. 
     While the present invention has been described in an illustrative manner, it should be understood that the terminology used is intended to be in a nature of words of description rather than of limitation. 
     Furthermore, while the present invention has been described in terms of a preferred embodiment, it is to be appreciated that those skilled in the art will readily apply these teachings to other possible variations of the inventions. 
     The embodiment of the invention in which an exclusive property or privilege is claimed are defined as follows.