R. Hübner

Structure and thermal stability of graded Ta–TaN diffusion barriers between Cu and SiO2

Abstract Sputter deposited Ta and TaN single layers of 10 nm thickness as well as graded TaN/Ta and Ta/TaN/Ta layer stacks that act as diffusion barriers for Cu metallization were investigated after annealing at temperatures between T an =300 and 700 °C. By means of glancing angle X-ray diffraction, glow discharge optical emission spectroscopy and transmission electron microscopy, results of microstructure and phase characterization were correlated with diffusion phenomena. For the pure Ta barrier, Ta diffusion through the Cu cap layer to the sample surface is observed at T an =500 °C, and the transformation of initially grown metastable β-Ta into the equilibrium α-Ta phase occurs at T an =600 °C. In contrast, a fcc TaN layer remains stable at least up to T an =700 °C. In the case of the graded layer stacks, first signs of N diffusion out of the TaN film into the adjacent Ta layers are observed after annealing at T an =300 °C, and formation of hexagonal Ta 2 N starts at T an =500 °C. Whereas in the course of thermal treatments for the threefold graded Ta/TaN/Ta barrier all TaN reacts with Ta to form Ta 2 N, some fcc TaN remains in the twofold graded TaN/Ta barrier.

Effect of annealing on the microstructure of ultrathin tungsten nitride diffusion barriers for copper metallization

Microstructure, phase composition and interface properties of 10- and 50-mm thick CVD-deposited W-N layers covered with Cu were investigated. Phase formation and structural changes of the layer stacks occurring at elevated temperatures (450-600 °C) were correlated. The initially amorphous barriers undergo an abrupt crystallization between 550 °C/1 h and 600 °C/1 h anneals in vacuum. Further annealing at 600 °C up to 16 h leads to changes in the layer configuration such as N redistribution and Cu agglomeration. No signs of significant Cu diffusion through the barriers were observed for the performed anneals up to 600 °C/16 h.

Thermal Behavior of Graded Ta‐Si/Ta‐Si‐N Diffusion Barriers for Cu Metallization

Using glancing angle X‐ray diffraction, glow discharge optical emission spectroscopy, and transmission electron microscopy, the crystallization behavior and thermal stability of graded Ta‐Si/Ta‐Si‐N diffusion barriers was analyzed after annealing at various temperatures. For a Ta30Si18N52/Ta73Si27 bilayer and a Ta73Si27/Ta30Si18N52/Ta73Si27 trilayer, nitrogen redistribution within the whole barrier is observed at Tan ⩾ 500 °C. Further heat supply leads to barrier crystallization into Ta2N. Depositing the layer stacks directly onto silicon, a critical temperature of Cu silicide formation was determined. For graded Ta‐Si/Ta‐Si‐N diffusion barriers, this temperature turned out to be between the corresponding values for Ta73Si27 and Ta30Si18N52 single layers.

Advanced Barriers for Copper Interconnects

Both Ta-Si-N and W-Si-N films have promising properties as diffusion barriers for copper metallization. For Ta-Si-N films with a silicon content of ∼ 20 at%, as an optimised system regarding thermal stability and high conductivity a composition containing ∼ 20 at% nitrogen was obtained. If the PVD techniques can be developed further to remain applicable for high aspect ratio structures, the PVD Ta-Si-N films are promising candidates for future technology nodes in semiconductor industry. Their integration behaviour for CVD and, in particular, ALD techniques has still to be investigated. The PE-CVD W-N barriers form amorphous barrier layers, which are expected to remain stable films for thicknesses down to 5 nm. Their thermal stability is slightly below that of PVD Ta-Si-N films, but can be improved by incorporation of silicon. Further optimisation of the ternary W-Si-N system is still ongoing.