Three - University of Arkansas Physics Department

Sputter-deposited yttrium-barium-copper-oxide multilayer structures
incorporating a thick interlayer dielectric material
R. G. Florence, S. S. Ang, W. D. Brown, G. Salamo, L. W. Schaper, and R. K. Ulrich
High Densiw Electmaics Center Universiw of Arkansas, Fayettevile, Arkansas 72701
(Received 12 June 1995; accepted for publication 14 November 1995)
Physical vapor deposition technologies have been developed which allow the fabrication of
multilayer structures consisting of two yttrium-barium-copper-oxide (YBa2Cu307-x or YBCO)
layers separated by a thick (-4 pm), low dielectric constant material. The YBCO is buffered from
both the substrate and the other films with ion-beam assisted deposited (IBAD) films of
yttria-stabilized zirconia (YSZ). The YSZ layer provides both the texture necessary to deposit
high-quality YBCO films and protection from the insulating layer material. Using these deposition
processes, a variety of materials, such as Pyrex and Haynes alloy, may be used for the substrate. The
critical temperature and current values obtained for the two YBCO layers of the completed structure
were on the order of 85 K and 2X 10' ~/cm~, respectively. O 1996 Americar~ Institute of Plzysics.
[SO02 1 -8979(96)06904-XI
I. INTRODUCTION ide has been used as a buffer material on these and other
substrate materials.'
One of the potential applications of yttrium-barium-
copper-oxide (YBa,Cu30,-, or YBCO) films is as the electronic
interconnects on a multichip module (MCM) substrate.
By using the interconnected mesh power system
(IMPS) MCM topology, a superconducting MCM can be realized
with two levels of superconducting interconnection.'
This is possible because the use of superconductor material
for the interconnects allows for very high wiring densities.
Thus, in order for this application to be realized, a large-area
film of YBCO must be deposited onto a substrate, followed
by a fairly thick layer of dielectric material, and finally, a
second layer of YBCO. The thick layer of dielectric material
1s required to provide impedance control of the interconnect
lines. As an additional constraint, the dielectric must have a
relatively low dielectric constant in order to minimize the
thickness of this layer.
Most efforts directed at the fabrication of multilayer
YBCO structures have required each of the layers to maintain
epitaxy of the substrate throughout the thickness of the
st~ucture.~ This requirement greatly constrains the choice of
substrate and dielectric materials that can be used. The materials
must be chemically compatible with YBCO, and their
lattice constants and coefficients of thermal expansion (CTE)
must closely match that of YBCO.
Ion-beam assisted deposition (IBAD) is a technique
which eliminates the constraint of maintaining epitaxy
throughout the entire structure. This is due to a combination
of the mechanisms of preferential sputtering, ion channeling,
and shadowing of misoriented grains by the ion beam. The
formation of a textured microstructure results from the
higher sputtering yields of all orientations other than that of
the channeling direction. Thus, a textured microstructure can
be formed by carefully balancing the deposition rate with the
sputtering due to the ion beam. Using IBAD, epitaxial films
have been grown on top of amorphous layers. IBAD has
been used to deposit textured yttria-stabilized zirconia (YSZ)
on ~ ~rex~,' and Haynes aIloy4-7 substrates, and cerium diox-
II. EXPERIMENT
In this article, YSZ was first deposited onto Pyrex substrates
using IBAD under the deposition conditions outlined
in Ref. 4. Briefly, the ion gun was positioned at a distance of
80 mm from the substrate and at an angle of 55' from the
substrate normal. Argon gas was flowed through the gun at a
rate of 20 sccm and an acceleration potential of 300 eV was
used, with a beam current of 7 mA and a deposition pressure
of 0.7 mTorr. The thickness of the YSZ layer was about 5000
A. Next, YBCO was sputter-deposited to a th~ckness of 3000
A using an off-axis configuration. The substrate was heated
to 735 "C at a deposition pressure of 200 mTorr in an 80%
argon/20% oxygen gas mixture. The YBCO was then capped
with a 2000 A thick layer of IBADIYSZ to prevent interdiffusion.
Next, a 4 pm thick layer of silicon dioxide (SiO)
was deposited on top of the YSZ layer by reactive sputtering
of a 3 in. diameter pure silicon target. The conditions were as
I r ,
20 30 40 50 60 70
28, degrees
FIG. 1. Theta-two theta scan for a typical YSZ film depos~ted on Pyrex.
J. Appl. Phys. 79 (4), 15 February 1996 0021 -8979/96/79(4)/2003/3/$10.00 O 1996American Institute of Physics 2003