Planarization Technologies Involving Electrochemical Reactions

CMP was first developed at IBM East Fishkill in 1983 and is being used for copper planarization since the late 1990s. An important characteristic of CMP is that it planarizes Cu across the wafer with least dependence on pattern densities compared to other methods. An inadequate planarization will lead to 

Microelectronic Applications of Chemical Mechanical Planarization, Edited by Yushuo Li Copyright 2008 John Wiley Sons, Inc. 

FIGURE 11.1 Plated Cu topography across pattern densities. 

The chemical component of CMP slurry creates porous unstable oxides or soluble surface complexes. The slurries are designed to have additives that initiate the above reactions. The mechanical component of the process removes the above-formed films by abrasion. In most planarization systems the mechanical component is the rate-limiting step. As soon as the formed porous film is removed, a new one is formed and planarization proceeds. Therefore, the removal rate is directly proportional to the applied pressure. To achieve practical copper removal rates, pressures greater than 3 psi are often required. These pressures should not create delamination, material deformation, or cracking on dense or relatively dense dielectrics used in silicon microfabrication on conventional dielectrics. However, the introduction of porous ultra-low-fc (low dielectric constant) materials will require a low downpressure ( 1 psi) polishing to maintain the structural integrity of the device [7-9]. It is expected that dielectrics with k value less than 2.4 will require a planarization process of 1 psi downpressure or less when they are introduced to production. It is expected that this process requirement will become even more important for the 45-nm technology node [10]. 

The principal steps of Cu polishing are shown in Fig. 11.3 as (1) chemical reaction Cu oxidized by oxidizer, Cu Cu (2) chemical reaction oxidized Cu becomes Cu complex, Cu Cu (complex) and (3) mechanical reaction Cu complex is removed by slurry and pad. Removal rate is governed by Preston s law RR = ki Pp P, where, RR is the removal rate (nm/min) ki is the coefficient of Preston s Law Pp is the polishing pressure (kPa) and P is the polishing speed (m/s). 

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