Sensor for in-situ pad wear during CMP

Abstract

Wear of a CMP polishing pad is detected in-situ by monitoring a change in the height of an identification mark positioned on a rotatable spindle supporting a wafer in contact with the chemical mechanical polishing pad. As the pad experiences wear, its thickness decreases and the relative height of the identification mark declines. This change in height can be detected by optical interrogation of the identification mark, and then calibrated to reveal a precise amount of pad wear. Alternatively, changes in thickness of the polishing pad can be correlated to a drop in dielectric properties of the pad as monitored by an electrical sensor measuring capacitance. Apparatuses for sensing pad wear in accordance with the present invention can also be utilized to evaluate the quality of new polishing pads, to indicate the optimum time for replacing worn polishing pads, and to identify the point at which a worn polishing pad begins to damage a wafer being polished.

What is claimed is:  
     
         1 . A method for detecting wear of a chemical mechanical polishing pad comprising: 
 detecting in-situ a change in thickness of a chemical mechanical polishing pad by sensing a change in position of a spindle supporting a head and a wafer into contact with the chemical mechanical polishing pad; and    correlating the change in pad thickness with an amount of pad wear.    
     
     
         2 . The method of  claim 1  wherein the change in position is detected by monitoring a change in an intensity of light reflected from a visual identification mark positioned on the spindle.  
     
     
         3 . The method of  claim 1  further comprising correlating the amount of pad wear with a rate of pad wear of polishing pads of known quality in order to identify a quality of the polishing pad.  
     
     
         4 . A method for detecting wear of a chemical mechanical polishing pad comprising: 
 detecting in-situ a change in thickness of a chemical mechanical polishing pad by sensing an increase in capacitance resulting from application of a potential difference across the worn polishing pad; and    correlating the change in pad thickness with an amount of pad wear.    
     
     
         5 . The method of  claim 4  further comprising correlating the amount of pad wear with a rate of pad wear of polishing pads of known quality in order to identify a quality of the polishing pad.  
     
     
         6 . A method of determining in situ a quality of a chemical mechanical polishing pad material, the method comprising: 
 applying a compression force to a spindle supporting a wafer into contact with the polishing pad;    detecting a distance of displacement of the spindle toward the polishing pad in response to the compression force; and    correlating the displacement distance with displacement distances of prior polishing pads of known quality.    
     
     
         7 . The method of  claim 6  wherein the displacement distance is sensed from a change in a height of a visual identification mark located on the spindle.  
     
     
         8 . The method of  claim 7  wherein the displacement distance is sensed from a change in intensity of light from a light source that is reflected from the visual identification mark.  
     
     
         9 . The method of  claim 6  wherein the displacement distance is detected from an increase in a capacitance resulting from application of a potential difference across the polishing pad.  
     
     
         10 . An apparatus for detecting pad wear in-situ comprising: 
 a platen supporting a chemical mechanical polishing pad;    a head supporting a wafer into contact with the chemical mechanical polishing pad;    a spindle in contact with the head;    a visual identification mark positioned on the spindle;    a light source configured to illuminate the visual identification mark; and    a detector configured to receive light reflected from the visual identification mark, such that a reduction in thickness of the pad causes a change in position of the visual identification mark that may be correlated with an amount of pad wear.    
     
     
         11 . An apparatus for detecting wear of a chemical mechanical polishing pad in-situ comprising: 
 a platen supporting a chemical mechanical polishing pad in contact with a wafer;    a power supply having a first terminal in electrical communication with the platen and the second terminal in electrical communication with the wafer; and    a detector configured to sense a change in capacitance between the first and second terminals, such that a reduction in thickness of the pad causes a change in capacitance that may be correlated with an amount of pad wear.