PANalytical Tests TaN Barrier

ALMELO, The Netherlands -- Work done at PANalytical has demonstrated a repeatability of within +/-0.3 Å for characterizing tantalum nitride barrier/adhesion films buried under copper interconnect. The measurements were taken on the company’s RQ Jade/PW2830 wafer analyzer – an instrument for R&D but with small face/footprint, and easy conversion to production environments by adding a load port and automated handler. The instrument can detect defects in the TaN film during device manufacture that would later allow copper to diffuse into active silicon areas and cause early device failures.

Copper migration – a device killer

Copper’s better conductivity is the main driver leading to its widespread replacement of aluminium in leading-edge ICs. However left without a barrier layer, the metal would diffuse through silicon dioxide into any underlying silicon crystal, killing the device. This is prevented by a barrier layer: a conductive layer deposited between the substrate and copper to prevent copper migration into the silicon. Tantalum nitride forms an excellent barrier, with low resistivity and excellent adhesion to provide good contact to the copper. It can be deposited by physical vapour deposition (PVD) or sputtering in very thin films, and retains a low sheet resistance even at annealing temperatures of 750 ˚C.

Characterizing TaN films is still an issue, however. Copper interconnects are mainly found on high speed devices, and holes in the TaN film will allow copper migration into silicon areas. Even if immediate device failures do not occur, copper migration greatly reduces carrier lifetimes. If undetected, faults in the barrier layer would mean expensive failures once the devices are built into equipment. Because the TaN films are buried below copper, it is difficult to get meaningful results from techniques like 4-point probes.

XRF takes all three measurements simultaneously

Excellent selectivity and precision make X-ray fluorescence spectrometry well suited to this type of measurement. Results were obtained with a 4 kW Rh anode Super Sharp tube, set at 32 kV/125 mA, and with fixed measuring channels for N, Cu and Ta (a specially designed fixed channel to measure Cu was used to avoid interference). The Ta and Cu signals gave the TaN and Cu layer thicknesses respectively, while the N signal determined the layer stoichiometry.

XRF analyzers normally find it difficult to produce reliable results for the stoichiometry of the TaN under copper films that are relatively thick (above about 500 Å), since the copper absorbs almost all the N radiation. Philips’ FP-Multi fundamental parameter software instead analyzed the TaN film stoichiometry and thickness by calculating the absorption in the different layers.

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