Method of measuring changes in impedance of a variable impedance load by disposing an impedance connected coil within the air gap of a magnetic core

TLDR: In this paper, the end point of polishing for removing a surface layer during the processing of semiconductor substrates is detected by measuring the induced voltage in the polishing wheel.

Abstract: The present invention relates to a method and apparatus for remotely detecting impedance. It is specifically adapted for use on a polishing machine wherein the end point of polishing for removing a surface layer during the processing of semiconductor substrates is detected. A first, or stationary coil having a high permeability core is wound having an air gap and an AC voltage is applied to the stationary coil to provide a magnetic flux in the air gap. A second coil is mounted for rotation on the polishing table, in a position to periodically pass through the air gap of the stationary coil as the table rotates. The second coil is connected at its opposite ends to contacts which are embedded in the surface of the polishing wheel. The contacts are positioned to engage the surface of the substrate which is being polished and provide a load on the second or rotating coil. The rotating coil, when it is in the air gap of the stationary coil, will perturb the flux field therein as a function of the resistance of the load caused by the contacts contacting either a conducting surface or a non-conducting surface. This perturbance of the flux field is measured as a change in the induced voltage in the stationary coil which in turn is converted to a signal which is processed to indicate the end point of polishing, the end point being when a metallic layer has been removed to expose a dielectric layer therebeneath or, conversely, when a dielectric layer has been removed to expose a metallic layer therebeneath.