An improved barrier stack for inhibiting diffusion of atoms or molecules, such
as O2 is disclosed. The barrier stack is particularly useful in capacitor
over plug structures to prevent plug oxidation which can adversely impact the reliability
of the structures. The barrier stack includes first and second barrier layers.
In one embodiment, the first barrier layer comprises first and second sub-barrier
layers having mismatched grain boundaries. The sub-barrier layers are selected
from, for example, Ir, Ru, Pd, Rh, or alloys thereof. By providing mismatched grain
boundaries, the interface of the sub-barrier layers block the diffusion path of
oxygen. To further enhance the barrier properties, the first barrier layer is passivated
with O2 using, for example, a rapid thermal oxidation. The RTO forms
a thin oxide layer on the surface of the first barrier layer. The oxide layer can
advantageously promote mismatching of the grain boundaries of the first and second
sub-barrier layer. The second barrier layer comprises a conductive oxide.