3 - Electrochemical techniques and their applications for chemical mechanical planarization (CMP) of metal films

Abstract

The introduction of mechanically fragile low-k dielectrics in copper interconnects has severely limited the usable down pressure of chemical mechanical planarization (CMP) for processing such structures. The CMP strategies emerging from this development in recent years have consequently become more reliant on the slurry chemistry to achieve planarization, while the mechanical function of CMP has assumed a comparatively reduced role. At the same time, the novel (and often chemically complex) barrier materials, as well as the multilevel copper lines (of highly variable widths) included in the new interconnect architectures have presented several additional challenges in the chemical design of metal CMP. The strict requirements of controlling surface defects while maintaining adequate material selectivity are particularly notable in this regard. Electrochemistry plays a vital role in these processing issues, because the chemical steps of CMP are largely governed by electrochemical reactions. Thus, electrochemical techniques are now recognized as essential tools for designing and evaluating the chemical protocols necessary to tackle the new challenges of metal CMP. This chapter focuses on these electrochemical aspects of metal CMP, and discusses the primary utilities as well as the phenomenological backgrounds of a number of related experimental techniques. The electrochemical basis of metal CMP is discussed to illustrate the corrosion–erosion characteristics of these systems. The experimental techniques considered in this discussion include potentiodynamic polarization, potentiostatic perturbation, open circuit voltage and galvanic measurements, impedance spectroscopy, electrochemical noise analysis, as well as pulsed voltage and triboelectrochemical methods. Illustrative experimental results of these techniques are presented to address various CMP aspects of Ta, TaN, Ru, Co, Mn, Al, and Cu.