Development and Application of an Abrasive-Free Polishing Solution for Copper
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Development and
Application of an Abrasive-Free Polishing Solution for Copper
Masanobu Hanazono, Jin Amanokura, and Yasuo Kamigata Abstract An abrasive-free polishing (AFP) solution for chemical–mechanical planarization (CMP) of copper films on semiconductor wafers has been developed to overcome such disadvantages of conventional CMP as dishing, erosion, Cu and oxide loss, and microscratching. Electrochemical methods are an effective way of understanding the role of each chemical component in the AFP solution in order to optimize its performance. Analysis of the reaction layer of Cu elucidates the reasons for the excellent results that have been obtained. By applying the AFP solution for Cu CMP in combination with a slurry for CMP of the metal barrier layer, seven-level multilayer Cu interconnections can be successfully fabricated. Keywords: abrasive-free polishing, chemical–mechanical planarization, chemical–mechanical polishing, CMP, chemical reactivity, electronic materials, semiconductor materials.
Introduction Since the introduction of copper interconnections for high-speed ultralarge-scale integration (ULSI) applications in 1997,1 many semiconductor manufacturers have begun to develop them. To do so, chemical– mechanical planarization (CMP) methods2,3 are required, as the subtractive schemes that are widely applied to polishing conventional aluminum interconnections will not work on Cu. In developing conventional slurries for Cu CMP, the H2O-Cu potential pH diagram (Pourbaix diagram) has been a guide.4 Based on this diagram, the composition of the reaction layer of Cu was identified as Cu oxide. This layer is known to be hard and stiff. In order to remove this reaction layer by means of the CMP process, ceramic abrasive particles (SiO2 or Al2O3) have typically been added to the slurry. How-
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ever, the use of these abrasives has many disadvantages with respect to the topography of the Cu pattern obtained, such as dishing, erosion, Cu and oxide loss, microscratching, and particle residue. These problems are summarized in Figure 1. Cu is widely known as one of the metals that typically forms a corrosion-resistant complex. The Cu complex is softer than Cu oxide. From this viewpoint, we carried out research to obtain a softer Cu complex layer that could be removed with the use of a CMP pad only; thus, we developed a new abrasive-free polishing solution5–8 that avoids the problems of conventional slurries. The advantages of applying an AFP solution for CMP of Cu are shown in Figure 1. A schematic diagram comparing the CMP process using a conventional slurry and using the AFP solution is shown in Figure 2.
The main composition of the AFP solution for Cu consists of Cu complex formers, oxidizers, pH controllers, and accelerators. The effects of each component on the Cu removal rate were studied. Through the development of the AFP solution for Cu, it became clear that an electrochemical method (using a Tafel plot, which shows the corrosion current density versus the corrosion potential at the surface of an el
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