Electromigration induced aluminum atom migration retarding by grain boundary structure stabilization and copper doping

In order to clarify the relationship between Al line reliability and film m icrostructure, 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 elect romigration (EM), compared with conventional Al lines deposited on TiN/Ti a nd SiO2. The improvement of Al line reliability is attributable to the foll owing factors; firstly, homogeneous microstructure and high activation ener gy, 1.28 eV, of the single-crystal Al line (omega = 0.18 degrees); secondly , subgrain boundaries, consisting of dislocation arrays found in the quasi single-crystal Al line (omega = 0.26 degrees), have turned out to be no mon effective mass transport paths because dislocation lines are perpendicular to the direction of electron wind; finally, the decrease of the (1 1 1) fu ll width at half maximum (FWHM) value promotes the formation of subgrain bo undaries and low-angle boundaries, which have small grain boundary diffusiv ity, as revealed by the detailed orientation analysis of individual grains in the hyper-textured line (FWHM = 0.5 degrees) formed by using an amorphou s 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 d iffusivity reduction and the uniformity of atomic Aux result in the suppres sion of void/hillock pair in the Al lines. It has been clarified that a FWH M value is a useful criterion of reliability for an interconnection. Also, the Cu doping effect against Ehl endurance by using Cu implantation of the single-crystal Al lines has been examined. It has been clarified that EM li fetime is lengthened by about one order of magnitude for the Cu concentrati on 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 ca se 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 dop ing. These results have confirmed that control of texture and/or grain boun dary structure so as to suppress EM induced metal atom migration is a promi sing approach for the development of Al lines and Cu lines capable of withs tanding the higher current densities required in future ULSIs. (C) 1999 Els evier Science Ltd. All rights reserved.