Integrated endpoint detection system with optical and eddy current monitoring
A chemical mechanical polishing apparatus and method can use an eddy current monitoring system and an optical monitoring system. Signals from the monitoring systems can be combined on an output line and extracted by a computer. A thickness of a polishing pad can be calculated. The eddy current monitoring system and optical monitoring system can measure substantially the same location on the substrate.
1. A method of determining polishing pad thickness, the method comprising:
positioning a substrate having a conductive layer disposed thereon in contact with a polishing surface of a polishing pad;
generating an alternating magnetic field from an inductor to induce eddy currents in the conductive layer;
measuring a strength of the magnetic field; and
calculating a thickness of the polishing pad from at least the strength of the magnetic field, wherein the thickness calculation is based on at least one coefficient that relates the thickness of the polishing pad to the strength of the magnetic field, and wherein the at least one coefficient is determined by a magnetic-field-strength calibration that is based on at least two known pad thicknesses.
2. The method of claim 1 , wherein generating the alternating magnetic field includes driving the inductor with a drive signal, and further comprising measuring a phase difference between the magnetic field and the drive signal.
3. The method of claim 2 , wherein the thickness of the polishing pad is calculated from at least the strength of the magnetic field and the phase difference.
4. The method of claim 1 , further comprising alerting a user if the thickness of the polishing pad falls below a predetermined thickness.
5. The method of claim 1 , wherein the two known pad thicknesses are obtained from different polishing pads.
6. The method of claim 1 , wherein the two known pad thicknesses are obtained from a same polishing pad.