Patent ID: 6924641

Claim:
A chemical mechanical polishing apparatus, comprising: a polishing pad; a carrier to hold a substrate against a polishing surface of the polishing pad; a sensor for monitoring a conductive film in a substrate, the sensor including a core positionable in proximity to the substrate, a first coil wound around a first portion of the core, an oscillator electrically coupled to the first coil to induce an alternating current in the first coil and generate an alternating magnetic field in proximity to the substrate, a second coil wound around a second portion of the core, a capacitor electrically coupled to the second coil, and an amplifier electrically coupled to the second coil and the capacitor to generate an output signal, wherein the oscillator induces an alternating current with a substantially constant frequency selected to provide a resonant frequency when the substrate is not in proximity to the core; a motor coupled to at least one of the polishing pad and carrier head for generating relative motion therebetween; and a computer to receive and monitor the output signal from the sensor while the substrate contacts the polishing pad and the oscillator induces the alternating current with the substantially constant frequency.