M. Hasunuma et al., Electromigration induced aluminum atom migration retarding by grain boundary structure stabilization and copper doping, MICROEL REL, 39(11), 1999, pp. 1631-1645
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.