Profile control during polishing of a stack of adjacent conductive layers

A method of controlling polishing, comprising:

• polishing a stack of adjacent conductive layers on a substrate, the stack of adjacent conductive layers including an outer conductive layer and an underlying conductive layer;
• generating with an in-situ eddy current monitoring system a signal that includes a sequence of signal values;
• calculate from the sequence of signal values a first sequence of thickness values for a first region of the substrate during polishing, the thickness values depending on thicknesses and conductivities of the conductive layers in the stack including a thickness of the outer conductive layer undergoing polishing in the first region, wherein calculation of the sequence of thickness values uses a correlation curve which receives as input each signal value from the sequence of signal values and, using a non-linear function of the signal values, returns a thickness value of the sequence of thickness values such that each thickness value represents a total thickness of the stack of adjacent conductive layers;
• calculating a first polishing rate from the first sequence of thickness values, the first polishing rate being a rate of change of thickness of the stack of adjacent conductive layers;
• calculating a first adjustment for a first polishing parameter based on the first polishing rate;
• detecting a first change in the polishing rate that meets at least one first predetermined criterion that indicates exposure of the underlying conductive layer;
• upon detecting the change in the polishing rate, calculate from a portion of the sequence of signal values generated after exposure of the outer conductive layer a second sequence of thickness values for the first region of the substrate during polishing, the second sequence of thickness values depending on thicknesses and conductivities of remaining conductive layers in the stack including a thickness of the underlying conductive layer undergoing polishing;
• calculating a second polishing rate from the second sequence of thickness values, the second polishing rate being a rate of change of thickness of the stack of the one or more remaining conductive layers;
• and
• calculating a second adjustment for the first polishing parameter based on the second polishing rate.

A polishing system, comprising:

• a platen to support a polishing pad;
• a carrier head to hold a substrate;
• a motor to generate relative motion between the platen and the carrier head;
• an in-situ eddy current monitoring system to monitor the substrate during polishing of the substrate;
• and
• a controller configured to
• receive from the in-situ eddy current monitoring system a signal including a sequence of signal values;
• calculate from the sequence of signal values a first sequence of thickness values for a first region of a substrate during polishing of the substrate, the first sequence of thickness values depending on thicknesses and conductivities of conductive layers in a stack of adjacent conductive layers on the substrate, wherein calculation of the sequence of thickness values uses a correlation curve which receives as input each signal value from the sequence of signal values and, using a non-linear function of the signal values, returns a thickness value of the sequence of thickness values such that each thickness value represents a total thickness of the stack of adjacent conductive layers;
• calculate a first polishing rate from the first sequence of thickness values, the first polishing rate being a rate of change of thickness of the stack of adjacent conductive layers;
• calculate a first adjustment for a first polishing parameter based on the first polishing rate;
• detect a first change in the polishing rate that meets at least one first predetermined criterion that indicates exposure of an underlying conductive layer in the stack;
• in response to detecting the first change in the polishing rate that indicates removal of the underlying conductive layer, calculate from a portion of the sequence of signal values generated after exposure of the underlying conductive layer a second sequence of thickness values for the first region of a substrate during polishing of the substrate, the second sequence of thickness values depending on thicknesses and conductivities of remaining conductive layers in the stack;
• calculate a second polishing rate from the second sequence of thickness values, the second polishing rate being a rate of change of thickness of the stack of the one or more remaining conductive layers;
• and
• calculate a second adjustment for the first polishing parameter based on the second polishing rate.