Corrosion Inhibitor for Cu CMP Slurry

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

FIGURE 8.1 Potential-pH diagram for Cu-water system (from Ref 17). 

As far as passivating film is concerned, in principle as shown in Fig. 8.1, the native copper oxide structures formed in the presence of a strong oxidizer under certain pH conditions can serve the purpose for many applications. For CMP purposes, such a hard surface film does not usually lead to any meaningful material removal under the mechanical forces exerted by a polishing pad. Therefore, there is a need to build a soft passivating layer on a copper film softened by the complexing agent. The most commonly used 

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Corrosion inhibitors are an important chemical constituent of Cu CMP slurry. The critical performance of a slurry such as static etch rate, material removal rate, and post-CMP defect counts are greatly influenced by the concentration and the structure of the inhibitor used in the slurry. BTA is proven to be an effective corrosion inhibitor for Cu CMP especially in nonacidic medium. 

Corrosion inhibitors hold an important position in Cu CMP slurry design as they provide the necessary passivation for the low-lying regions. The most commonly used inhibitor in copper CMP slurries is BTA. The formation of the... 

Massive electrochemical attack known as galvanic corrosion [58,59] is the most severe form of copper corrosion. It can completely remove the copper from the structures (Figs. 17.25 and 17.26). It can occur when the wafers are exposed to a corrosive electrolyte for an extended period. It can also occur if the slurry does not contain enough or effective corrosion inhibitor. The source of such a galvanic potential on the patterned copper surface may be due to the fact that some copper structures connected to transistors have a different electrical potential than the rest of the wafer surface. Another possible cause of this type of galvanic potential is related to the barrier material induced metal metal battery effect. Most copper CMP slurries have been developed for Cu structures with Ta or TaN as a barrier material. In some cases, other metals may also be used in addition to the barrier metal. For example, a metal hard mask could contribute to the galvanic corrosion effects. It is also possible that some types of copper are more susceptible to corrosion that others. The grain... 

Historically, acidic chemicals were used extensively to remove oxide or trace metal contaminants. Nowadays, however, most of the post-Cu CMP clean chemicals for advanced technology nodes operate in the neutral to high pH regimes. The pH of post-Cu CMP clean chemicals can have dramatic impacts on Cu passivation and hence the formation of HM and DE. Cu tends to form Cu oxides in solution with pH > 7.5. Conversely, (2u is prone to corrosive attack in acidic environments. The addition of a corrosion inhibitor such as BTA in the slurries and clean chemicals is a standard practice to reduce HM/DE. However, just like other chelates, the thin layer of chemisorbed BTA on the Cu surface needs to be soluble and then removed in the clean chemical to... 

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