Analysis of the effect of abrasive particle size on the tribological mechanisms arising in the chemical mechanical polishing of copper-film wafers

In the chemical mechanical polishing (CMP) of a copper-film silicon wafer, the average Reynolds equation developed for a cylindrical coordinate system is applied to study the mixed lubrication. The pad's elastic deformations are considered in the evaluation of the contact pressure arising at the interface of a pad's asperity and the wafer. The normal force acting on the wafer by an abrasive particle with different size is thus obtained in order to calculate the elastic and plastic deformations of a copper film having a thin passivation layer. A theoretical abrasive wear model is developed to evaluate the removal rate of the copper film. The increase in the real contact area of an abrasive, due to the frictional force produced at the interface by adhesive wear, is also taken into account. A nano tester was applied to measure the composite hardness and Young's modulus of the copper-film wafer having a passivation layer which has different thickness due to the uses of different particle sizes. These two material properties are of importance in the calculation of a wafer's theoretical removal rate. Experimental results for the removal rates of the copper film are exhibited to compare with that predicted by the present theoretical model. The removal rate of a copper-film wafer is theoretically independent of the particle size itself of abrasive particles; on the other hand, it is dependent upon the passivation layer thickness formed and retained during the CMP process due to the use of different particle sizes.