High Quality NH₂SAM (Self Assembled Monolayer) Diffusion Barrier for Advanced Copper Interconnects

Abstract

The trend for future integrated circuits (IC) is decreasing in size beyond the conventional limits. The recent transition from aluminum to copper as the interconnect material for IC is due to copper's higher resistance to electromigration and its lower resistivity. Unfortunately, copper has high mobility in Si and SiO₂ and may cause destruction of electrical connections on the chip. Hence, there is a significant necessity in finding ultra thin, thermally stable, high quality and good adhered diffusion barriers. The most widely used barrier is pure Ta films or layer stacks consisting of Ta and TaN. These have excellent conformality, very good uniformity and high thermal stability. But The continuous scaling down of the interconnect dimensions lead to an essential decrease in the barrier layer effective thickness to less than 5nm; coupled with the replacement of silicon oxide by advanced low-k dielectrics it demand further improvements of the diffusion barrier performance. For that reason Self-assembled monolayers (SAMs), with thicknesses of 2nm or less, have been propose for copper diffusion barrier application. By tailoring the structure of these monomolecular organic films, atomic scale properties can be controlled and selective surfaces and interfaces can be engine as desired for a specific application. In the presented work, the quality of an amino-terminated SAM barrier (NH₂SAM) is tested. A high density and the absence of pinholes in the barrier layer are essential for a good barrier performance. First, the macroscopic quality of the NH2SAM barrier has been characterized by Water contact angle (CA) and High resolution AFM (HR-AFM). Secondly, the density and the presence and/or absence of pinholes have been tested by Ellipsometry and Cylic Voltametry (CV). Finally, the intrinsic barrier performance in form of Time- dependent dielectric breakdown (TDDB) lifetime has been extracted from planar capacitor structures that permitted to measure the leakage/Cu diffusion through barrier in the vertical direction. The Contact angle of layers formed at different deposition times show a variation of the hydrophilic SiO2 substrate to hydrophobic already with 1min deposited NH2SAM layer. A 15min deposited NH2SAM (~1nm), results in a continuous and pinhole free layer observed by HR-AFM. The refraction index (η) calculated by ellipsometry, indicates an increase in the density of the layer with the deposition time. On the other hand, cyclic voltametry shows inhibition of the electrochemical reduction of Fe³⁺ specimen to Fe²⁺ when NH2SAM are formed on ~2nmSiO2/Si electrodes. A decrease in the capacitive current is observed by increasing the layer thickness and density. The intrinsic barrier performance of the NH₂SAM barrier by TDDB is demonstrated with an increase of 10 times the capacitor lifetime by comparing with no barrier system.