Thermal stability enhancement in nanostructured Cu films containing insoluble tungsten carbides for metallization
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We report enhanced thermal stabilities of nanostructured Cu films containing insoluble tungsten carbides prepared by sputter deposition. Tungsten carbides in the form of W2C are present in the supersaturated solid solution of Cu, as confirmed by x-ray photoelectron spectroscopy, scanning electron microscopy, and x-ray diffraction analyses. Focused ion beam analysis revealed that the films are thermally stable during annealing when they are in contact with Si without a diffusion barrier, and the copper silicide was not formed up to an annealing temperature of 400 °C. Leakage current characteristic evaluation on SiO2/Si metal oxide semiconductor (MOS) structure also revealed the superior reliability of Cu with a dilute amount of tungsten carbides, indicating their usefulness in advanced barrierless metallization applications. The films with higher amount of tungsten carbides exhibited good thermal stability at high temperatures and could be rationalized as a consequence of a refined grain structure together with the strengthening effect of W2C.
Cu is a promising material for metallization in ultralargescale integration devices on account of its low resistivity and high resistance to electromigration.1 However, the fact that Cu reacts with Si at relatively low temperatures (∼200 °C), and its low mechanical strength at high temperatures (>500 °C) makes Cu and its alloys unsuitable for advanced applications. For instance, diffusion barriers including carbides and nitrides (such as tungsten carbide2,3 and tantalum nitride) are commonly used for Cu metallization. As the device features continue to shrink, fabrication of thin and effective diffusion barriers becomes a challenging issue. Also, when the device dimension decreases, the electron diffuse scattering due to the barrier becomes the most detrimental effect on resistivity.4,5 Alternatively, a new class of Cu material with enhanced thermal stability appears to be a solution for the future barrierless metallization. For the thermal stability enhancement, the addition of solute elements into the Cu films has been the subject of many studies.6–13 Our prior studies show that sputter-deposited Cu films with insoluble elements (e.g., C, W, and Mo) on Si substrates are stable during annealing up to 400 °C without the formation of copper silicides.12,13 These films exhibit resistivity values close to that of pure Cu a)
Address all correspondence to this author. e-mail: [email protected] b) Permanent address: Department of Physics, Alagappa University, Karai Kudi-630 003, India. DOI: 10.1557/JMR.2005.0179 J. Mater. Res., Vol. 20, No. 6, Jun 2005
while maintaining reasonable mechanical properties after annealing. For this paper, we studied the thermal properties of the Cu films with insoluble tungsten carbide. Tungsten carbide (WC) was selected because the performance of W is often improved by incorporating a second element such as C. The incorporation of high-melting-point (2785 °C) WC in copper yields very slow lattice diffusion, which, in turn, acts as a beneficial
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