A Quantitative Study of the Adhesion Between Copper, Barrier and Organic Low-K Materials
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A QUANTITATIVE STUDY OF THE ADHESION BETWEEN COPPER, BARRIER AND ORGANIC LOW-K MATERIALS F. Lanckmans*, S. H. Brongersma, I. Varga, H. Bender, E. Beyne, K. Maex*, IMEC Kapeldreef 75, B-3001 Leuven, BELGIUM,*also at E.E. Dept., K.U.-Leuven, BELGIUM Abstract The adhesion between several materials implemented in Cu/low-k integration is studied. Adhesion issues at different interfaces are important with regard to the reliability of back-end processing. Layered test structures are processed to study different interfaces. A tangential shear tester allows quantifying the adhesion force at the interface and provides a relative measurement to compare various materials. Failed interfaces are analyzed using auger electron spectroscopy (AES) and scanning electron microscopy (SEM). Among all studied structures, the strongest interface is seen between a barrier (Ti(N), Ta(N), WxN) and Cu. A weaker interface proves to be between a low-k dielectric and Cu. However, the presence of a barrier increases the adhesion. The weakest interface occurs between an oxide cap and the low-k material, with a lower adhesion when the low-k material is fluorinated. The low-k/cap oxide interface forms a critical issue with regard to Cu/low-k integration processing such as chemical mechanical polishing (CMP). All test structures show no significant degradation of the adhesion after a thermal cycle up to 400°C. Introduction The integration of Cu and low dielectric constant (k) materials in back-end processing will replace aluminium and silicon dioxide by offering reduced signal propagation delay, crosstalk, and power dissipation [1]. The structures used in a Damascene approach consist of a multitude of layers (fig.1.a). The adhesion between these layers in multilevel metallization is crucial for the reliability of integrated circuits. Adhesion refers to the ability of the interface between two materials to resist mechanical separation [2]. Interfaces to be investigated are Cu/barrier, metal/low-k, and metal/cap oxide/low-k (fig.1.a). Other topics of concern are: the role of ambient gases, adhesion degradation during manufacturing and stress effects due to different mechanical properties of the materials [3]. In this work, layered test samples, which reflect interfaces in Cu Damascene technology, are quantitatively evaluated by shear testing to determine the adhesion force of the weakest interface. This provides a relative measurement to compare various materials. The failed interfaces are analyzed using AES or SEM. Experimental The test structure, on which the adhesion measurements are performed, was chosen taking into consideration the materials and interfaces occurring in the Cu Damascene technology (fig.1.a) and the size limits imposed by the shear tester. Both the test structure and the tip of the shear tester are shown in figure 1.b. The shear tester is able to apply a shearing force to a sample situated on a flat horizontal surface. The force is transferred to the sample through a metallic tip. During the measurement, the tip approaches the surfac
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