As the physical size of devices and interconnects on
integrated circuits shrinks further and further, the
aspect ratio of the metal interconnects increases.
The aspect ratio is the via depth divided by the via
diameter. While anisotropic etching using
high-density plasmas is now commonly practiced to
create the via, refilling the deep, narrow
interconnect vias with barrier layers and metals
remains a substantial technical challenge.
approach to lining the vias with barrier layers and
refilling the vias with metals is Ionized Physical
Vapor Deposition (I-PVD), also known as Ionized
Metal Plasma (IMP). Metal atoms are ionized in an
intense plasma, then can be directed by electric
fields perpendicular to the wafer surface. This
directed flux of metal is necessary to ensure that
the metal species reach the bottom of deep vias.
The figure above is a cross-sectional sketch of
an I-PVD plasma reactor. The inside diameter of the
reactor is 450 mm. Metal atoms are introduced into
the plasma by sputtering from the target at the top
of the reactor. A high density plasma is generated
in the central volume of the reactor by an Inductively Coupled Plasma
(ICP) source. This electron density is
sufficient to ionize approximately 80% of the metal
atoms incident at the wafer surface. The ions from
the plasma are accelerated and collimated at the
surface of the wafer by the plasma sheath. The
sheath is a region of intense electric field which
is directed toward the wafer surface. The field
strength is controlled by applying a radio frequency
bias to the wafer chuck, as shown.
The evolution of the ionization process in I-PVD
has been studied in the Plasma Engineering Lab. In
the plot above, the density of aluminum atoms,
aluminum ions, and ion fraction are shown as a
function of distance from the source of aluminum
atoms. The degree of ionization has been determined
by optical emission spectroscopy and by a novel,
deposition-based technique which
takes advantage of enhanced ion transport through
the plasma presheath. The ionization fraction of the
flux incident on a wafer's surface is also shown at
the bottom of the figure. Here one may observe that
over 80% of the arriving aluminum is ionized.
The system has also been modeled by solving the
plasma diffusion equations for ions and neutrals.
These models are plotted as solid lines on the
figure. The agreement between the measured and
modeled ionization is seen to be quite good.
The conclusion of this study is that a high
degree of ionization is due to constant volume
generation of aluminum ions throughout the
inductively coupled plasma. The aluminum neutrals,
however, are generated only at the target
surface. The density of neutral species will decay
in the downstream region since their source is
confined to the top of the reactor. The result of a
more-or-less constant supply of ions and a
diminishing density of neutrals is a high ionization
fraction in the vicinity of the wafer.
A SEM image of a 1 um wide
trench that is lined with titanium using I-PVD
A more detailed description of these experiments
is published in the Journal of Vacuum Science and
Technology A, 15(4), 2307 (1997).
This research is funded by the National
Science Foundation and the Department of Energy
under Grant No. DMR-9712988