Abstract
Ruthenium is one of the most promising materials for the realization of barrier/seed layers in low-k copper interconnects technology. It is a possible candidate for the substitution of materials like Ta/TaN or TiN. This metal presents interesting properties like a reasonably low resistivity (7.1 µΩcm in bulk), a high melting point and high mechanical properties. Moreover, it presents almost no solubility with copper, preventing thus interdiffusion [1]. Finally, ruthenium can be directly plated with copper [2], avoiding the deposition of seed layers. Ruthenium can be easily PVD deposited on Si, obtaining ultrathin barrier layers suitable for the most advanced patterning techniques. However, PVD deposition presents some major disadvantages like high cost, limited productivity and a significant difficulty in uniformly plating vias and trenches having high aspect ratio (this effect is known as shadowing). Alternative techniques, including wet metallization, can overcome these limitations. Electrodeposition in particular proved to be able to provide uniform metallization even in very high aspect ratio features, like in the case of copper superfilling [3]. Ruthenium tends to oxidize in aqueous solution, and for this reason electrodeposition from standard electrolytes can present technological problems. Alternative solvents have been proposed in the past to solve this issue, the most notable being ionic liquids like BMIPF6 [4]. Another possible alternative are deep eutectic solvents, which are characterized by interesting properties like water stability, high conductivity and the absence of harmful chemicals (which make them suitable also for employment in the green chemistry field). The present work describes the electrodeposition of ruthenium thin films for microelectronic applications from a deep eutectic solvent. Metallic layers are deposited and characterized from the electrical and morphological point of view to evaluate their possible use as barrier layers in microelectronics industry. [1] R. Chan, T. N. Arunagiri, Y. Zhang, O. Chyan, R. M. Wallace, M. J. Kim and T. Q. Hurd, Electrochem. Solid-State Lett., 7, G154 (2004) [2] Y. Zhang, L. Huang, T. N. Arunagiri, O. Oieda, S. Flores, O. Chyan, and R. M. Wallace, Electrochem. Solid-State Lett., 7, C107 (2004) [3] D. Josell, D. Wheeler and T. P. Moffat, Electrochem. Solid-State Lett., 5, C49 (2002) [4] O. Raz, G. Cohn,W. Freyland, O. Mann, Y. Ein-Eli, Electrochim. Acta, 59, 6042 (2009)
Highlights
Ruthenium, a transition metal belonging to the same group of platinum,[1] is used nowadays in many different industrial applications: decorative and corrosion resistant coatings, wear protection for electrical contacts[2,3] and interdiffusion barrier layers.[4,5] interesting catalytic properties have been demonstrated for this bright white metal both in its metallic form[6] and as oxide.[7]
Ru deposition optimization.—Initially, galvanostatic ruthenium electrodeposition was attempted from a mixture of Choline chloride (ChCl)/ethylene glycol (EG) and 40 mM RuCl3
Copper was used as substrate and current density was varied over a broad range, but no reproducible ruthenium plating was achieved
Summary
A transition metal belonging to the same group of platinum,[1] is used nowadays in many different industrial applications: decorative and corrosion resistant coatings, wear protection for electrical contacts[2,3] and interdiffusion barrier layers.[4,5] interesting catalytic properties have been demonstrated for this bright white metal both in its metallic form[6] and as oxide.[7]. Ruthenium electrodeposition typically takes place from aqueous sulfamate based electrolytes containing the metal in the form of a bridged complex known as μ-nitridobisaquatetrachlororuthenate [Ru2(μ-N)(H2O)2Cl8].14–16 Deposition from these baths is characterized by a good cathodic efficiency (exceeding 90%) and by crack-free deposits up to 1–2 μm thickness. The aim of the present paper is the investigation of ruthenium electrodeposition from a DES obtained mixing choline chloride and ethylene glycol in 1:2 molar ratio This DES, belonging to the class III and commercially known as ethaline, is characterized by interesting physical and chemical properties and it constitutes a good electrolyte for the deposition of many metals. RuCl3 in different amounts (20 or 40 mM) was added to form the plating solution, which
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