Superconducting Technology Assessment - nitrd

Fig. 2. Current density @t A versus oxidation pressure-time product for t of 2–20 kA/cm . The
symbols indicate NGST data, and rectangular boxes outline all data from other processes [58].
Typical niobium base electrode thickness is 150 nm, and
the aluminum thickness is 8 nm. The wafer is transferred to
the oxidation chamber to partially oxidize the exposed aluminum
layer to form a thin ( 1 nm thick) aluminum oxide
(AlO ) tunnel barrier and transferred back to the deposition
chamber to deposit the niobium top or counterelectrode layer
to complete the in situ formation of the Nb/Al-AlO /Nb
trilayer. The niobium counterelectrode thickness is typically
100 nm. In order to produce junctions of high electrical
quality, it is important to optimize the argon sputter gas
pressure to produce near zero stress in both niobium base
and counterelectrode films [53]–[56]. Small junctions and
submicrometer-sized junctions are particularly sensitive to
film stress, which tends to increase subgap leakage current
and decrease uniformity [57]. Table 4 lists the optimized
NGST deposition parameters.
Since varies exponentially with barrier thickness,
junctions require precise control over oxidation pressure,
time, and temperature. The NGST oxidation chamber uses
a mass flow controller to provide constant flow of high
purity oxygen (99.999%) and a capacitance manometer
and variable throttle valve connected in a feedback loop
to dynamically control pressure. The feedback loop controls
pressure to better than 0.1 mtorr. Typical oxidation
pressure and time for the NGST 8-kA/cm process are
9.0 mtorr and 30 min. For current densities greater than
about 10 kA/cm , a dilute 10% O , 90% Ar mixture is used
in order to maintain a more favorable pressure range for
feedback control.
The dynamic oxidation process used at NGST has excellent
stability over time and good run-to-run repeatability.
Fig. 2 shows the dependence on oxidation pressure-time
product for the NGST process and for several processes [58].
All data are in good agreement up to a of 20 kA/cm .
To minimize run-to-run variations in and to achieve low
subgap leakage, it is important to keep the wafer temperature
Fig. 3. Trend chart of current density @t A. Mean aVXQ 6
HXQU kA/cm or 64.4% @1'A. Target specification a
VXH kA/cm 6 10% @1'A.
constant and near room temperature, or colder if possible,
either by active temperature control or by passive cooling.
Active temperature control during deposition and barrier oxidation
is highly desirable, but it is often not practical due to
limitations of existing deposition tools. In the NGST trilayer
deposition tool, the wafer and carrier are clamped to a large
heat sink using indium foil backing, which minimizes the
temperature rise and reduces temperature gradients across
the wafer. The temperature rise during deposition is limited
to a few degrees above room temperature and remains nearly
constant during oxidation. For the 8-kA/cm process, this
method of passive cooling is sufficient to keep the average,
run-to-run variations in below 5% 1 as shown by the
trend chart for in Fig. 3. is calculated from the slope
of a least-squares fit of the square root of junction versus
junction diameter for five junction sizes. Error bars are determined
from measurements of five chips distributed across
the wafer. Across-wafer spreads are as low as 1.5% 1 .
B. Josephson Junction Fabrication
To improve circuit speed and performance, each
new-generation process is based on increased junction
ABELSON AND KERBER: SUPERCONDUCTOR INTEGRATED CIRCUIT FABRICATION TECHNOLOGY 1521