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M. Mellier, T. Berger, R. Duru, O. Hinsinger & G. Wyborn, STMicroelectronics, France; M. Rivoire, CEA–LETI, France & K-L. Chang, Y. Wang, V. Ripoche, S. Tsai & M. Thothadri, Applied Materials, USA


ABSTRACT

With the most advanced generation of integrated circuits using the integration of copper and fragile low-k or ultra low-k (ULK) dielectrics in Cu interconnects, the constraints on Cu chemical mechanical polishing (CMP) have become critical. There has been a great effort made to develop Cu CMP processes at lower pressures with improved topography behaviors to reduce sheet resistance (Rs) variations and to meet the stringent designs rules and compatibility with the lithography budget for depth of focus (DOF).

The classical slurry approach at lower pressures results in slow polish rates. Therefore, the industry has to introduce more complex slurry systems, which are harder to stabilize in order to achieve a total planarization after Cu clearing with dishing and erosion values below 200Å.

A new approach is Electro-Chemical Mechanical Polishing (ECMP) that allows achievement of these performances at much reduced pressures, using an electrolyte simpler in its conception compared to slurry systems. The ECMP approach presented replaces both the copper bulk and copper clearing steps of conventional CMP, providing polishing directly onto the diffusion barrier.

In this article, the full sequence of Cu ECMP performance with ULK dielectric is shown. A comparison with today’s slurry systems is made in terms of topography and electrical behavior over a wide range of feature sizes and densities. The ECMP approach achieves the topography targets, whereas significant development is still required to achieve similar results with conventional CMP. Moreover, it is shown that these topography gains directly transfer into sheet resistance distribution gains. Lithographic Depth of Focus (DOF) measurements indicate a 60% reduction in within-die depth of focus variation is achieved. These CMP-induced topography improvements translate directly into lithography process window gains.