Measuring the surface properties of polishing pads using ultrasonic reflectance

Abstract

The present invention provides a system and method for measuring the surface properties of polishing pads using non-contact ultrasonic reflectance. An ultrasonic probe is located over the polishing surface and configured to both transmit an ultrasonic signal to the polishing surface and receive a modified ultrasonic signal from the polishing surface without contacting the polishing surface. A subsystem coupled to the ultrasonic probe is configured to determine a surface property of the polishing pad from the modified signal.

What is claimed is:  
     
         1 . A system for measuring surface properties of a polishing pad comprising: 
 a polishing pad having a polishing surface associated therewith;    an ultrasonic probe located over said polishing surface and configured to both transmit an ultrasonic signal to said polishing surface and receive a modified ultrasonic signal from said polishing surface without contacting said polishing surface; and    a subsystem coupled to said ultrasonic probe and configured to determine a surface property of said polishing pad from said reflection.    
     
     
         2 . The system as recited in  claim 1 , wherein said ultrasonic probe comprises a single ultrasonic transducer.  
     
     
         3 . The system as recited in  claim 1 , wherein said ultrasonic probe comprises a first ultrasonic transducer configured for said transmitting and a second ultrasonic transducer configured for said receiving.  
     
     
         4 . The system as recited in  claim 2 , wherein said transmitted ultrasonic signal is between about 2 MHz and about 3 MHz and an air gap located between said probe and said polishing surface is between about 12 mm and about 25 mm.  
     
     
         5 . The system as recited in  claim 4 , wherein said transmitted ultrasonic signal is about 3 MHz and said air gap is about 12.5 mm.  
     
     
         6 . The system as recited in  claim 1 , wherein said polishing pad comprises: 
 a thermoplastic foam substrate; and    a polishing agent coating said polishing surface of said substrate.    
     
     
         7 . The system as recited in  claim 6 , wherein said thermoplastic foam substrate comprises a crosslinked polyethylene closed-cell foam.  
     
     
         8 . The system as recited in  claim 6 , wherein said polishing agent is selected from a group of ceramics consisting of: 
 Silicon Dioxide;    Titanium Dioxide;    Tetraethoxy Silane Polymer; and    Titanium Alkoxide Polymer.    
     
     
         9 . The system as recited in  claim 6 , wherein said polishing agent is selected from a group of polymers consisting of: 
 Polyalcohols; and    Polyamines.    
     
     
         10 . The system as recited in  claim 1 , wherein said surface property is selected from the group consisting of: 
 Density;    Surface Texture; and    Visco-Elasticity.    
     
     
         11 . A method for measuring the surface properties of a polishing pad, comprising: 
 situating an ultrasonic probe above a polishing surface of a polishing pad, without contacting said polishing surface;    transmitting an ultrasonic signal from said probe to said polishing surface, said ultrasonic signal being modified by said polishing surface; and    receiving said modified signal by said ultrasonic probe.    
     
     
         12 . The method as recited in  claim 11 , wherein said modified signal is a reflected signal received during coating of a polishing agent onto said polishing surface of a thermoplastic foam substrate.  
     
     
         13 . The method as recited in  claim 12 , wherein said probe and said polishing pad are located in a reaction chamber used for said coating.  
     
     
         14 . The method as recited in  claim 13 , wherein said coating comprises: 
 exposing a surface of said thermoplastic foam substrate to an initial plasma reactant to produce a modified surface thereon; and    exposing said modified surface to a secondary plasma reactant to create said polishing surface on said modified surface, said polishing surface comprised of said polishing agent.    
     
     
         15 . The method as recited in  claim 12 , wherein said coating includes exposing a plastic substrate to a polishing agent dissolved in a supercritical fluid to thereby produce a modified plastic.  
     
     
         16 . The method as recited in  claim 11 , wherein said polishing pad and said probe are coupled to a polishing apparatus comprised of a mechanically driven carrier head and a polishing platen.  
     
     
         17 . The method as recited in  claim 16 , wherein said polishing pad is attached to said polishing platen and said transmitting and said receiving are carried out while said carrier head holds a wafer and imparts a polishing force against said polishing pad to polish a wafer.  
     
     
         18 . The method as recited in  claim 11 , wherein said modified signal is a reflected signal used to calculate an acoustic reflectance spectrum of said polishing surface.  
     
     
         19 . The method as recited in  claim 18 , wherein said reflected signal is used to determine an acoustic reflectance image of said polishing surface.  
     
     
         20 . The method as recited in  claim 11 , wherein said modified signal is a transmittance signal used to determine a relative density of said polishing surface.  
     
     
         21 . The method as recited in  claim 20 , wherein said transmittance signal is used to determine a surface texture of said polishing surface.