In-situ endpoint detection method and apparatus for chemical-mechanical polishing using low amplitude input voltage

A polishing apparatus having real-time in-situ thickness monitoring and being especially well suited for reducing the thickness of a dielectric film layer on a surface of a workpiece during polishing with a conductive polishing slurry, said polishing apparatus comprising:

• a) an electrically grounded rotatable polishing table, said polishing table receiving the conductive polishing slurry thereon;
• b) means for holding the workpiece above and in pressure contact with said polishing table, the surface of the workpiece to be polished being in contact with said polishing table;
• c) a center electrode embedded within and electrically isolated from said polishing table, said center electrode electrically contacting the workpiece via the conductive polishing slurry;
• d) at least one guard electrode embedded within and electrically isolated from said polishing table, said guard electrode being concentric with and surrounding said center electrode and further electrically contacting the workpiece via the conductive polishing slurry;
• e) means for generating a sinusoidal voltage signal, said signal generating means providing an output signal on an output thereof, the output signal having a frequency in the range of 500 to 50,000 hertz and an amplitude of less than one-half volt.;
• f) a voltage follower having an input and an output, the voltage follower input being electrically connected to the output of said signal generating means and the voltage follower output being electrically connected to said guard electrode;
• g) a current-to-voltage converter having first and second inputs and an output, the first input being electrically connected to said center electrode and the second input being electrically connected to the output of said signal generating means, said current-to-voltage converter for converting a current passing through the center electrode into a voltage signal;
• h) a differential amplifier having first and second inputs, the first input being electrically connected to the output of said current-to-voltage converter and the second input being electrically connected to the output of said signal generating means, said differential amplifier further having an output for providing an output voltage representative of the current passing through said center electrode, whereby the amplitude of the differential amplifier output voltage is inversely proportional to the thickness of the film layer of the workpiece being polished during movement of said center electrode and said at least one guard electrode through a detection region;
• and
• i) means for controlling polishing of the workpiece in response to a monitored thickness of the film layer of the workpiece being polished, the monitored thickness being derived from the differential amplifier output voltage.