Grain-boundary voiding in self-passivated Cu-1 at.% Al alloy films on Si substrates
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new generation of semiconductor metallization is based on Cu-film technology. In this paper we report on detrimental changes in surface morphology of self-passivated Cu–1 at.% Al films which were magnetron-sputtered on Si substrates. Large voids appeared at grain boundaries after annealing, oxidation at temperatures at or above 500 °C, and cooling to room temperature under vacuum. Grain-orientation imaging with electron backscatter diffraction revealed that preferentially high-energy grain boundaries were voided. Contrary to reports in the literature, void growth was not prevented by the extremely clean ultrahigh-vacuum conditions during film fabrication nor by the strong and very sharp grain texture. Instead, it was clearly correlated with the stable surface oxide on the self-passivated films. Void growth was not found after annealing passivation-free films such as pure Cu or unoxidized Cu–1 at.% Al films.
I. INTRODUCTION
The traditional interconnect materials in ultralargescale integrated manufacturing, aluminum and aluminum alloys, have been replaced since 1997 by copper because of its lower resistivity, better electromigration resistance, and the lower production costs due to the doubledamascene technique. As a significant drawback, however, the lack of self-terminating oxidation has for a long time restrained the use of Cu. Li et al.1 suggested an elegant method to “self-passivate” Cu films by alloying with less noble metals. A thin surface layer of an oxide of the alloying element notably reduced oxidation rates. This idea was further pursued by Ding et al., who carried out a detailed study of the oxidation behavior of dilute alloys with Al and Mg.2,3 These authors found that alloying furthermore decreased surface roughness upon annealing and significantly improved film adhesion to SiO2. It was further shown for Cu–Al that the passivation layer was formed even if the Al was deposited as a thin interlayer between a pure Cu film and the substrate.4,5 Recently, the mechanical properties of self-passivated Cu–Al alloy films were investigated by Weiss et al.6 An increased high-temperature strength was found, strongly suggesting the suppression of diffusional relaxation mechanisms in these films. Thermal-stress-induced voiding has been a threat to pure-Cu conductor lines ever since Cu has been investigated as a potential interconnect material.7–12 As for a)
Present address: Massachusetts Institute of Technology, Room 18-0801, 77 Massachusetts Ave, Cambridge, MA 02139. J. Mater. Res., Vol. 17, No. 6, Jun 2002
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aluminum lines, where stress voiding had been a problem for many years,13 this phenomenon was interpreted as a reaction to high triaxial tensile stresses within the lines under a passivation during cooling. On the other hand, in blanket films, where two-dimensional in-plane stresses are found, only few cases of stress-induced voiding have been reported (see Ref. 14 for Al and Refs. 15 and 16 for Cu films). In this paper we report for the first time on s
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