ECMP

Electric chemical polish (ECP) and electric chemical mechanical polish (ECMP) [53] have been developed as promising methods for global planarization of LSI fabrication and abrasive-free polish. 

The formation of an anodic film (ion-concentrated diffusion layer or salt film) depends on electrolyte and processing conditions. The characteristics of the film are critical to the planarization efficiency of ECP and ECMP. Hence, the analysis, especially in situ analysis of anodic films, is highly valuable. A few techniques can be used for the purpose. 

FIGURE 10.12 Influence of electrolyte on ECMP planarization efficiency. 

Electrochemical impedance measurement is convenient and usually no extra instrument is needed since contemporary potentiostat, which is normally used in an ECP or ECMP system, tends to possess EIS function. However, the thickness of an anodic film cannot be easily determined from the measured impedance or impedance spectra because the electrical resistivity of the film is usually unknown. 

To achieve better planarization efficiency than ECP, different modified ECP approaches have been explored. These approaches, including ECP-DI water technique [27], sectorial cathode ECP [28], membrane-mediated ECP [29], and contact ECP (i.e., ECMP) [30,31], work by different mechanisms and exhibit different performances. 

These conditions mainly depend on the electrolyte. However, the thickness and surface profile of the anodic film, which strongly affect planarization efficiency, also depend on the ECMP pad. 

Therefore, a good electrolyte should produce an anodic film that gives low limiting current iL and low polarization resistance tan 4> (thus high ip) for good planarization ability [32]. Additionally, an electrolyte should also meet the requirements for other performances such as corrosion and post-ECMP cleaning. 

Compare three different ECP approaches with ECMP. What are the planarization mechanism, advantages, and disadvantages of each technique ... 

FIGURE 11.7 An illustration of ECMP polishing mechanism in relation to a conventional electropolishing process (from Ref. 25). 

The removal amount on the wafer is proportional to the charge flowing through the polishing cell, in accordance with Faraday s law of electrolysis. The linear relationship is also in agreement with the understanding of the ECMP electrochemical reaction Copper is oxidized into Cu " ions and the reaction releases two electrons for each atom removed from the wafer surface. 

ECMP, end point is based on the amount of charges delivered to each zone, which follows Faraday s law. There is a linear relationship between the charge required by each zone and the amount of copper removed. By modulating the charge on each zone as shown in Fig. 11.12, a desirable removal profile can be obtained. 

ECMP process is typically non-Prestonian therefore, the control knobs of conventional CMP, such as downpressure and platen speeds, cannot be directly used to modify the polishing profile. The control knobs of ECMP are mainly the charges applied to each zone. Different voltages are used for simultaneous completion of the applied charge by each zone. 

In most ECMP applications a perforated polishing pad with holes through it is used. Thus, electrical contact between the Cu surface that is contacted to the anode and the cathode located under the polishing pad is achieved via the electrolyte [29]. In the applied materials ECMP reflexion platform, the wafer is contacted to the anode located at the center of the pad as shown in Fig. 11.14. 

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