Investigation of ultra-thin layers by RF-GD-OES

The continuous scaling down of the dimensions of Cu interconnect structures in microelectronics requires ultra thin barriers against Cu diffusion. Typically 10 nm thin films of pure, binary and ternary nitrides of refractory metals as Ta, TaN, TaSiN, WN are investigated to prevent effectively the Cu diffusion into the substrates. X-ray diffraction, depth profile analysis by glow discharge optical emission spectrometry (GD-OES), Auger electron spectrometry (AES) and Rutherford backscattering (RBS) and electron microscopy are used to correlate results of microstructure and phase characterization with diffusion phenomena [1-3].
The degradation of the barrier performance after annealing was controlled by depth profile analysis using GD-OES [4]. For each barrier composition a critical temperature was found above which a significant Cu concentration in the substrate occurred in the GD-OES signal. Apart from the Cu diffusion, also the onset of barrier material diffusion towards the surface could be traced by GD-OES. At GD-OES depth profiling of laterally inhomogeneous layers containing conducting as well as non conducting parts it became clear that focusing and defocusing effects of the sputtering ions exist.
Essential properties of these barrier layers, as electrical and thermal conductivity and resistivity against oxidation, are optimized by variation of the chemical composition. Phase formation and transformation or diffusion also depends strongly on the initial distribution of elements in the layer system. Therefore, an accurate quantification of the element concentration is needed for reliable production and thorough investigation. However, all applied methods have special advantages, disadvantages and limitations. With RBS, e.g., it is impossible to determine N and the layers must be deposited on C, but RBS delivers absolute concentrations without calibration. AES has an excellent lateral and depth resolution, but suffers from preferential sputtering and matrix effects and needs, as GD-OES, standard material for calibration. Therefore, only the combination of all could solve this requirement successfully.
[1] N. Mattern, M. Hecker, D. Fischer, C. Wenzel, N. Schell, W. Matz, H. Engelmann, E. Zschech, Microelectronics Reliability 40, 1765 (2000)
[2] M. Hecker, D. Fischer, V. Hoffmann, H.-J. Engelmann, A. Voss, N. Mattern, C. Wenzel, C. Vogt, E. Zschech, Thin Solid Films, 414, 2, 184 (2002)
[3] R. Huebner, M. Hecker, N. Mattern, V. Hoffmann, K. Wetzig, C. Wenger, H.-J. Engelmann, C. Wenzel, E. Zschech, J.W. Bartha , Thin Solid Films, 437, 248 (2003)
[4] V. Hoffmann, R. Dorka, L. Wilken, V.D. Hodoroaba, K. Wetzig, Surface and Interface Analysis 35, 7, 575 (2003)