Abstract In order to clarify the relationship between Al line reliability and film microstructure, most notably grain boundary structure, we have tested three kinds of highly textured Al lines, namely a single-crystal Al line, a quasi single-crystal Al line and a hyper-textured Al line. Consequently, it has been shown that these kinds of lines have excellent endurance against electromigration (EM), compared with conventional Al lines deposited on TiN/Ti and SiO 2. The improvement of Al line reliability is attributable to the following factors; firstly, homogeneous microstructure and high activation energy, 1.28 eV, of the single-crystal Al line ( ω=0.18°); secondly, subgrain boundaries, consisting of dislocation arrays found in the quasi single-crystal Al line ( ω=0.26°), have turned out to be no more effective mass transport paths because dislocation lines are perpendicular to the direction of electron wind; finally, the decrease of the (1 1 1) full width at half maximum (FWHM) value promotes the formation of subgrain boundaries and low-angle boundaries, which have small grain boundary diffusivity, as revealed by the detailed orientation analysis of individual grains in the hyper-textured line (FWHM=0.5°) formed by using an amorphous Ta–Al underlayer (Toyoda H, Kawanoue T, Hasunuma M, Kaneko H, Miyauchi M. Proc. 32nd Ann. Int. Reliab. Phys. Symp., IEEE, 1994;178). Moreover, the diffusivity reduction and the uniformity of atomic flux result in the suppression of void/hillock pair in the Al lines. It has been clarified that a FWHM value is a useful criterion of reliability for an interconnection. Also, the Cu doping effect against EM endurance by using Cu implantation of the single-crystal Al lines has been examined. It has been clarified that EM lifetime is lengthened by about one order of magnitude for the Cu concentration of 0.1 at% in spite of almost the same diffusion coefficients. Moreover, the incubation time for a void nucleation has been observed even in the case of a pure-Al line. Thus, in accordance with the stress evolution model, it is concluded that the mechanism of lifetime improvement by Cu doping is such that critical stress for EM void nucleation is increased by the Cu doping. These results have confirmed that control of texture and/or grain boundary structure so as to suppress EM induced metal atom migration is a promising approach for the development of Al lines and Cu lines capable of withstanding the higher current densities required in future ULSIs.