Planarization strategy of Cu CMP: Interaction between plated copper thickness and removal rate

The simple analytical model of Boning and co-workers is extended by taking variable removal rate (RR) into account. The advantage of rapid process characterization is retained and model parameters can be obtained from experimental data, and more importantly, analytical solutions are available to provide necessary analysis. The result shows that tradeoff between electrochemically plated copper thickness and throughput exists when the conventional constant RR strategy is applied. At the low RR extreme, minimal Cu thickness is obtained while low throughput is also experienced as a result of low polish rate. At the high RR end, however, highest throughput can be achieved but extremely thick Cu layer should be plated in order to meet planarization specifications. This results in additional operating cost for post-CMP waste water treatment. Therefore, an optimal control problem is formulated for CMP planarization and an analytical solution for optimal RR trajectory is derived. The optimal trajectory achieves the highest throughput with minimal amount of copper thickness (47% less than the conventional practice). In addition to the variable RR strategy to carry out optimal process control, stepwise implementation is also proposed and it can readily be practiced on present CMP stations with little extra engineering effort.