Method for determining thickness of material layer and chemical mechanical polishing endpoint

A method for determining a thickness of a material layer, wherein the thickness of the material being decreased with time, the method comprising:

• continuously shining an incident light onto the material while performing a polishing step;
• measuring a reflected light intensity I of the incident light;
• integrating and differentiating the intensity of the reflected light with time;
• obtaining an I-Dt curve by dividing the integration of the intensity of the reflected light by a product of the differential of the intensity of the reflected light and time, the curve comprising a plurality of peaks which reflect characteristic thickness values of the material layer;
• obtaining each characteristic thickness value of the material layer using optical principle of reflection;
• selecting two of the peaks to calculate a thickness difference between these two peaks, as well as to record a time interval between these two peaks;
• and
• calculating a removal rate between these two peaks by a linear relationship;
• and
• obtaining a real time thickness at a time point between these two peaks by deducting the thickness value of an earlier peak between these two peaks by a product of the removing rate and a time interval between the earlier peak and this time point.