Abstract:
A CMP apparatus is provided with a replaceable polishing pad, a film thickness sensor, and a polishing control unit for switching polishing conditions in response to a fact that an output value from the film thickness sensor has exceeded a threshold value. The polishing control unit has a memory unit for storing a threshold value corresponding to the thickness of a new polishing pad when the polishing pad is replaced. The memory unit also store conversion information that shows the relationship between the output value of the film thickness sensor and the thickness of the polishing pad when the thickness of the film being polished is constant. The polishing control unit accesses the conversion information, obtain the output value of the film thickness sensor that corresponds to the thickness of the new polishing pad, and record the output value as the threshold value.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a CMP (chemical mechanical polishing) apparatus and a method of polishing a semiconductor wafer using the CMP, and more particularly relates to a method for detecting timing for switching polishing speed. 
       BACKGROUND OF THE INVENTION 
       [0002]    CMP is an important technique in manufacturing semiconductor devices. Semiconductor integrated circuit chips are manufactured by forming conductive layers, insulating layers, or other thin film layers in a prescribed order on a wafer, patterning the layers depending on need by photolithography and etching, and cutting and separating each chip on the wafer after forming all layers. When there are convexities and concavities or steps in the substrate when a film is formed, the thickness of the film formed on the substrate will be thin in parts and step coverage will worsen, causing lower yield and other problems. Also, problems occur in that focus during exposure is not fixed and precise patterns cannot be transferred because convexities and concavities appear in upper layers due to the effect of the lower patterned layers. For this reason, the surface of the film material must be planarized, and CMP is used for such a purpose. 
         [0003]    In CMP, efforts are made to increase throughput (the number of wafers that can be polished in a unit of time) by polishing at the maximum possible speed. However, when tungsten (W), copper (Cu), titanium nitride (TiN) or another metal film is polished at high speed, there is a problem in that erosion increases. Therefore, in metal-based CMP, high-speed polishing is performed first, and at the point when the thickness of the metal film reaches around several tens of nanometers, by switching from high-speed polishing to low-speed polishing. As a result, erosion is reduced. 
         [0004]    A method is known in which an eddy current sensor is used as one method for measuring the thickness of a metal film during high-speed polishing (Japanese Laid-open Patent Publication No. 2004-525521). The eddy current sensor measures the thickness of a metallic film by using a high-frequency magnetic field. Therefore, the metal film is polished at high speed while the film thickness is measured using the eddy current sensor, high-speed polishing is terminated when the eddy current sensor reaches a prescribed threshold value, and a switch is made to low-speed polishing. 
         [0005]    However, when there is variability in the thickness of the polishing pad, the remaining metal film will also have a thickness that corresponds to the variations in the polishing pad thickness. When the polishing pad thickness fluctuates due to polishing pad wear, the remaining metal film will also have a thickness that corresponds to the fluctuations in the polishing pad thickness. Since the distance to the metal film being polished increases when the polishing pad is thick, the thickness of the metal film becomes greater than the target thickness, even when high-speed polishing has ended at the point when the eddy current sensor has reached a prescribed output value. Also, since the distance to the metal film being polished is reduced when the polishing pad is thin, the thickness of the metal film is less than the target thickness, even when high-speed polishing has ended at the point when the eddy current sensor has reached a prescribed output value. Variations in the remaining metal film lead to variations in the polishing time in low-speed polishing (barrier clear polishing) that follows, and the remaining film also is thick when the polishing pad is thick. Therefore, there is a problem in that CMP throughput is reduced. When the polishing pad is thick, the remaining film will also be thick. Since the remaining film becomes thin when the polishing pad is thin, quality degradation is liable to occur due to erosion and the like. 
       SUMMARY OF THE INVENTION 
       [0006]    It is therefore an object of the present invention to provide a CMP apparatus and a wafer polishing method using CMP in which a film being polished on a wafer can be set to a constant thickness during high-speed polishing even if the thickness of the polishing pad fluctuates. 
         [0007]    The above and other objects of the present invention can be accomplished by a CMP apparatus for polishing a film on a wafer that comprises a replaceable polishing pad; a film thickness sensor for measuring a thickness of the film via the polishing pad; and a polishing control unit for switching polishing conditions in response to a fact that an output value from the film thickness sensor has exceeded a threshold value, wherein the polishing control unit has a memory unit for storing the threshold value corresponding to the thickness of a new polishing pad when the polishing pad is replaced. 
         [0008]    It is preferable in the present invention that the memory unit further stores conversion information that shows the relationship between the output value of the film thickness sensor and the thickness of the polishing pad when the thickness of the film being polished is constant, and the polishing control unit accesses the conversion information, obtains the output value of the film thickness sensor that corresponds to the thickness of the new polishing pad, and records the output value as the threshold value. 
         [0009]    It is preferable in the present invention that the polishing control unit measures the thickness of the film being polished with aid of the film thickness sensor after a reference wafer provided with the film being polished that has a prescribed thickness is set, and records as the threshold value the output value of the film thickness sensor. 
         [0010]    It is preferable in the present invention that the film thickness sensor includes an eddy current sensor. It is also preferable that the polishing target is a metal film or a metal compound film. 
         [0011]    The above and other objects of the present invention can also be accomplished by a wafer polishing method using CMP, comprising a polishing pad setting step for setting a new polishing pad; a threshold value recording step for recording a threshold value that has been corrected based on a thickness of the new polishing pad; a high-speed polishing step for performing high-speed polishing of the wafer while a thickness of a film being polished is monitored using a film thickness sensor; a polishing condition switching step for switching from high-speed polishing to low-speed polishing when an output of the film thickness sensor reaches the threshold value; and a low-speed polishing step for performing low-speed polishing of the wafer as far as a polishing terminal point. 
         [0012]    It is preferable in the present invention that the threshold value recording step includes a step for accessing conversion information that shows a relationship between the output value of the film thickness sensor and the thickness of the polishing pad when the thickness of the film being polished is constant, and obtaining the output value of the film thickness sensor that corresponds to the thickness of the new polishing pad; and a step for recording the output value as the threshold value. 
         [0013]    It is preferable in the present invention that the threshold value recording step includes a step for setting a reference wafer provided with a film being polished that has a prescribed thickness, a step for measuring a thickness of the film being polished using the film thickness sensor, and a step for recording the output value of the film thickness sensor as the threshold value. 
         [0014]    In this way, according to the present invention, the timing for switching the polishing conditions in metal-based CMP can be precisely measured, and throughput improvement and erosion prevention can be assured because a threshold value of an eddy current sensor can be corrected in accordance with the thickness of the polishing pad. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    The above and other objects, features and advantages of this invention will become more apparent by reference to the following detailed description of the invention taken in conjunction with the accompanying drawings, wherein: 
           [0016]      FIG. 1  is a schematic view showing a configuration of a CMP apparatus according to a preferred embodiment of the present invention; 
           [0017]      FIG. 2  is a flowchart showing a method for polishing a wafer using a CMP apparatus; 
           [0018]      FIG. 3  is a flowchart showing an example of procedures to reset the threshold value; 
           [0019]      FIG. 4  is a flowchart showing an example of procedures to reset the threshold value; and 
           [0020]      FIG. 5  is a schematic partial sectional view showing the structure of a reference wafer. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0021]    Preferred embodiments of the present invention will be described hereinafter with reference to the accompanying diagrams. 
         [0022]      FIG. 1  is a schematic view showing a configuration of a CMP apparatus according to a preferred embodiment of the present invention. 
         [0023]    As shown in  FIG. 1 , the CMP apparatus  100  is provided with a polishing head  12  for holding a wafer  11 , a rotary surface plate  14  on which a polishing pad  13  is mounted, a slurry supply unit  15  for supplying a slurry that contains silica (SiO 2 ) microparticles or another abrasive, a pad probe  16  for measuring the state of a polishing pad  13 , a dresser  17  for dressing the polishing pad  13 , an eddy current sensor  18  for measuring the thickness of a tungsten film, which is the metal film being polished on the wafer  11 , and a polishing control unit  19  that controls these components. 
         [0024]    The polishing head  12  is provided with a spindle mechanism for rotating the wafer  11 , and a pressing mechanism for pressing the wafer  11  against the polishing pad  13  using an optimal pressure. A guide ring  12   a  is provided at the external periphery of the wafer  11  that is set in the polishing head  12 , whereby the wafer  11  can be reliably held in place. The rotary surface plate  14  is also provided with a spindle mechanism for rotating the polishing pad  13 . The wafer  11  and the polishing pad  13  can thereby be moved relative to each other, and uniform polishing can be efficiently performed. 
         [0025]    The polishing pad  13  is attached to the main surface of the rotary surface plate  14 . The polishing pad  13  is composed of a two-layer structure of a cushion sheet and a polishing sheet having a microporous structure. Rigid polyurethane foam is used as the polishing sheet. The polishing pad  13  is a consumable article. The polishing surface of the polishing pad  13  is restored by performing periodic dressing using a dresser  17 , but a polishing pad  13  that has been entirely worn is removed from the rotary surface plate  14  and replaced with a new polishing pad. 
         [0026]    The pad probe  16  detects the service life of the polishing pad  13 , the terminal point of the dressing, and processing abnormalities by monitoring the friction coefficient of the surface of the polishing pad  13 . The pad probe  16  measures the surface of the polishing pad  13 , and when the friction coefficient is lower than the reference level, the dressing of the polishing pad  13  is performed. 
         [0027]    The dresser  17  is used for dressing the polishing pad  13  when the friction coefficient has been reduced due to clogging and the like. Diamond grains are embedded in a contact surface with the polishing pad  13 , and the surface of the polishing pad  13  is cut by the diamond grains. 
         [0028]    The eddy current sensor  18  measures the thickness of a metal film by using a high frequency magnetic field, and is disposed in the vicinity of the main surface of the rotary surface plate  14 . An output signal of the eddy current sensor  18  is supplied to a polishing control unit  19 , and is used in determining the timing for switching the polishing conditions. 
         [0029]    The polishing control unit  19  controls the polishing head  12 , the rotary surface plate  14 , the slurry supply unit  15 , and the like, and more specifically controls the position and rotating speed of the polishing head  12 , the rotating speed of the rotary surface plate  14 , the amount of slurry supplied from the slurry supply unit  15 , and other parameters. 
         [0030]    A data table  19   b  is recorded in a memory  19   a  inside the polishing control unit  19 . The data table  19   b  shows the relationship between the thickness of the polishing pad  13  when the thickness of a tungsten film is constant (e.g., 20 nm) and the output value of the eddy current sensor  18 . The eddy current sensor  18  faces the wafer  11  via the polishing pad  13 . The output of the eddy current sensor  18  changes depending on the thickness of the polishing pad  13  because the distance from the eddy current sensor  18  to the polishing surface of the wafer  11  also changes in accordance with the thickness of the polishing pad  13 . For example, when variability in the thickness of the polishing pad  13  according to the specifications is ±0.25 mm, a variability of ±15 nm will also occur in the thickness of the tungsten film. Ordinarily, the thickness of the remaining tungsten film that is used in determining the switching timing is set to 10 to 30 nm, and when the thickness of the remaining tungsten film is set to 20 nm, the thickness of the remaining tungsten film will be 20±15 nm, i.e., the thickness of the film will be a maximum of 35 nm and a minimum of 5 nm depending on the variability in the thickness of the polishing pad  13 . 
         [0031]    In this way, the thickness of the tungsten film after polishing has been stopped will vary depending on whether the polishing pad  13  is relatively thick or relatively thin, even when polishing terminates at a point when the output of the eddy current sensor  18  reaches a prescribed value. However, it is possible to make the tungsten film thickness constant by accessing the data table  19   b  and correcting the threshold value of the film thickness without being affected by fluctuations in the thickness of the polishing pad. Accordingly, a switching timing from high-speed polishing to low-speed polishing can be determined with a high degree of precision. In this way, the threshold value  19   c  of the tungsten film thickness thus obtained is also recorded in memory  19   a.    
         [0032]      FIG. 2  is a flowchart showing a method for polishing a wafer  11  using a CMP apparatus  100 . 
         [0033]    As shown in  FIG. 2 , in the polishing of the wafer  11  according to the present embodiment, first, the processing surface of the wafer  11  is made to face downward and is set in the polishing head  12 , the wafer  11  is pressed against the polishing pad  13  while slurry is provided, and high-speed polishing of the wafer  11  is performed (S 11 ) by rotating the wafer  11  and the polishing head  13  at high speed (first speed) Afterward, high-speed polishing ends (S 12 Y) when the thickness of the tungsten film has reached around several tens of nanometers, and a switch is made to low-speed (second speed) polishing (S 13 ). When the polishing terminal point is detected, the polishing of wafer  11  is terminated (S 14 Y). 
         [0034]    As described above, the thickness of the tungsten film is measured using the eddy current sensor  18 , but fluctuations in the thickness of the polishing pad  13  due to dressing produces variability in the thickness of the tungsten film when high-speed polishing terminates. Accordingly, when the old polishing pad  13  is replaced with a new polishing pad, the threshold value used for determining switching timing of polishing conditions is reset. 
         [0035]      FIG. 3  is a flowchart showing an example of procedures to reset the threshold value. 
         [0036]    As shown in  FIG. 3 , in resetting the threshold value, a new polishing pad  13  is first set on the rotary surface plate  14  (S 21 ). As used herein, a new polishing pad  13  refers to a polishing pad  13  having variability in thickness in terms of specification. Therefore, it is not necessary for the polishing pad  13  to be an unused pad, and a polishing pad  13  having variability in thickness becomes subject to all threshold value settings. 
         [0037]    Next, the thickness of the new polishing pad  13  is measured (S 22 ). The thickness of the polishing pad  13  can be measured by, e.g., calculating the output voltage value when a wafer  11  using a specific tungsten film is water polished, and subtracting the output voltage value of the wafer  11  from the result. The measurement results are inputted to the polishing control unit  19 . 
         [0038]    Next, the polishing control unit  19  accesses the data table  19   b,  and determines the threshold value corresponding to the thickness of the new polishing pad  13  (S 23 ). As described above, the relationship between the output value of the eddy current sensor  18  and the thickness of the polishing pad  13  when the thickness of the tungsten film is set to a threshold value for the switching timing of polishing conditions is recorded in data table  19   b.    
         [0039]    Next, the threshold value obtained in this way is recorded (S 24 ). Afterward, the wafer  11 , which is the polishing target, is polished using the threshold value. In other words, the wafer  11  undergoes high-speed polishing until the output of the eddy current sensor reaches the threshold value. A switch is made from high-speed polishing to low-speed polishing when the threshold value is reached, and polishing is terminated when a polishing termination point is detected. Such polishing of the wafer  11  is repeatedly performed until the polishing pad  13  is worn, and the polishing pad  13  is dressed by the dresser  17  as necessary. Thus, the threshold value is calibrated by obtaining information related to the thickness of the new polishing pad. 
         [0040]    As described above, in accordance with the present embodiment, the thickness of the tungsten film can be measured with greater accuracy and the switching timing of polishing conditions can be determined with a high degree of precision because the threshold value for determining the switching timing of polishing conditions is varied in accordance with the thickness of the polishing pad  13 . Therefore, the thickness of the remaining tungsten film can be kept constant even if the thickness of the polishing pad  13  has varied, and the throughput of the CMP steps, and erosion and other quality concerns can be made consistent. 
         [0041]    Next, a second embodiment of the present invention will be described. The present embodiment is characterized in that a reference wafer having a prescribed thickness is prepared, the reference wafer is measured using an eddy current sensor when the polishing pad  13  is set, and the measurement result at this time is recorded as a threshold value. The CMP apparatus used in the present embodiment is substantially the same as the CMP apparatus  100  shown in the first embodiment, except that a data table is not required. 
         [0042]      FIG. 4  is a flowchart showing another example of the procedure for resetting the threshold value. 
         [0043]    As shown in  FIG. 4 , in resetting the threshold value, a new polishing pad  13  is first set on the rotary surface plate  14  (S 31 ). Also, a reference wafer provided with a tungsten film having a prescribed thickness is set in place (S 32 ). 
         [0044]      FIG. 5  is a schematic partial sectional view showing the structure of a reference wafer. 
         [0045]    As shown in  FIG. 5 , a tungsten film  51  is formed on a reference wafer  50 . The tungsten film  51  has already been polished and adjusted to a target film thickness to at which a switch is made from high-speed polishing to low-speed polishing. 
         [0046]    Next, the eddy current sensor  18  measures the thickness of the tungsten film  51  of the reference wafer  50  (S 33 ). Since the tungsten film  51  of the reference wafer  50  already has an ideal film thickness, the output of the eddy current sensor  18  obtained when the thickness of the film is measured at this time is a correct output value showing the switching timing for polishing conditions, and is also a value for which consideration has been given to the variability in the thickness of the polishing pad  13 . 
         [0047]    Next, the output value of the eddy current sensor  18  obtained in this manner is recorded as the threshold value that is used when the wafer is actually polished (S 34 ). Afterward, the wafer, which is the polishing target, is polished using the threshold value. In other words, the wafer undergoes high-speed polishing until the output of the eddy current sensor reaches the threshold value. A switch is made from high-speed polishing to low-speed polishing when the threshold value is reached, and polishing is terminated when a polishing termination point is detected. The polishing of wafers as polishing targets is repeatedly performed until the polishing pad  13  is worn, and the polishing pad  13  is dressed by the dresser  17  as necessary. 
         [0048]    As described above, in accordance with the present embodiment, the thickness of a tungsten film being polished can be measured with greater accuracy, and the switching timing of polishing conditions can be determined with a high degree of precision because the threshold value for determining the switching timing of polishing conditions is varied in accordance with the thickness of the polishing pad  13 . Therefore, the thickness of the remaining tungsten film can be kept constant even if the thickness of the polishing pad  13  varies, and the throughput of the CMP steps, and erosion and other quality concerns can be made consistent. 
         [0049]    The present invention has thus been shown and described with reference to specific embodiments. However, it should be noted that the present invention is in no way limited to the details of the described arrangements but changes and modifications may be made without departing from the scope of the appended claims. 
         [0050]    For example, in the above embodiments, the eddy current sensor is used as a sensor for measuring the thickness of the film being polished on the wafer, but the present invention is not limited to an eddy current sensor, and various other sensors can be used. 
         [0051]    Also, the film being polished is not limited to tungsten, and copper (Cu), titanium nitride (TiN), and various other metals and metal compounds can be used as the target. 
         [0052]    In the above embodiments, a data table is used as conversion information for obtaining the thickness of the polishing pad  13  from the output value of the eddy current sensor  18 , but the present invention is not limited to a data table, and the thickness of the polishing pad  13  can be obtained from the output value of an eddy current sensor  18  using a computational equation.