Tantalum-Nitride Diffusion Barrier Studies Using the Transient-Ion-Drift Technique for Copper Detection
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Tantalum-Nitride diffusion barrier studies using the transient-ion-drift technique for copper detection T. Heiser, C. Brochard, M. Swaanen1 University Louis Pasteur, Laboratoire de Physique et Applications des Semiconducteurs, CNRS, BP20 F67037 Strasbourg Cedex 2, France 1 at ST Microelectronics, 850, rue Jean Monnet B.P. 16, F-38921 Crolles cedex, on leave of absence from Philips Electronics, Eindhoven, Netherlands. ABSTRACT The permeability of 5 nm thick TaN, Ta and TiN diffusion barriers has been studied by monitoring the bulk copper concentration in silicon after isothermal and isochronal annealing experiments with a transient-ion-drift (TID) technique. The method estimates quantitatively the bulk copper concentration in silicon from capacitance transients of a Schottky barrier which arise when copper ions drift out of the depletion region towards the quasi-neutral region. The correlation between the copper lateral distribution and the position of the copper metal is used to distinguish between background contamination and copper originating from barrier leakage. The TID detection limit is found lower than 1012at/cm3, which makes this technique particularly well adapted for quantitative diffusion barrier studies. Isothermal and isochronal annealing experiments show that TaN fails at a higher temperature than Ta barriers. The copper concentration does not exceed the solubility limit, indicating that copper precipitates nucleate rapidly at the interface. The opposite is found in TiN covered samples where a large copper supersaturation is obtained even after short annealing times. INTRODUCTION Although diffusion barriers are a key part of the copper interconnect technology, a quantitative method to evaluate barrier efficiency against copper diffusion in terms of metal contamination is still missing. Standard analytical tools, such as Secondary Ion Mass Spectroscopy (SIMS) or Auger Electron Spectroscopy (AES) are limited by a detection limit for copper higher than 1015at/cm3.1 The presently most sensitive techniques measure electrical parameters of the barrier (i.e. barrier resistivity) or of a test component (oxide breakdown of a MOS capacitor) which do not allow a quantitative estimation of the copper flux through the barrier.2 The copper concentration needed to affect these electrical parameters is not well established yet. On the other hand, diffusion barriers, less than a few nanometers thick, should allow to keep copper contamination of silicon oxide and bulk silicon below 1010at/cm2.3 A quantitative copper detection tool with a low enough unambiguous detection limit would thus be of considerable interest to study diffusion barrier quality versus barrier composition, following mechanical and thermal stress. Validation and calibration of existing techniques, which may be better suited for on-line investigations, could be achieved by such a tool as well. In the present study we show that the recently developed transient-ion-drift (TID) technique,4,5 which combines both, high sensitivity of electrical measurements
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