The Study of Ultra-thin Diffusion Barrier in Copper Interconnect System

Abstract

Copper is used for the microchip backend interconnect system due to its excellent conductivity. However, it tends to diffuse into the surrounding dielectric materials, causing contamination of the junctions and electrical shortings. The use of a diffusion barrier in the metal lines is imperative for the successful implementation of copper into the system. Advancing technology requires robust ultra-thin barriers without sacrificing reliability. The present work investigates the various implications of implementing Ta based barriers with copper in the backend interconnect system. The effect of nitrogen flow rate on the phase formation of TaN formation by I-PVD deposition as well as its effect on the subsequent deposition of Cu seed and the bulk Cu layer was investigated. Quantification of the adhesion strength of Ta and Ta/TaN bilayer diffusion barrier films to FTEOS dielectric was carried out by Modified Edge Lift Off Test (MELT). Stress migration tests were then carried out on structures designed for qualifying the low-k interconnect systems reliability. Improvement steps in the damascene process (pre-cleaning of barrier and post-annealing of Cu) were tested for via structure reliability with corrective steps made to the depositing process to improve overall reliability of the structures. Development work was done at CIBA on the High Resolution Rutherford Backscattering Spectrometry system that would be used extensively to study the reliability of ultra-thin ALD TaN barriers. Due to the inherent complexity of equipment setup, data acquisition and processing in the HRBS system, several analyses on the data collection and processing methods were carried out, with addition improvement to the hardware for background reduction so as to ensure accuracy in the results. Finally, characterization of the ultra-thin ALD TaN and Ta/TaN bilayer via various surface analytical techniques was carried out, with quantitative studies of Cu diffusion by HRBS.