Superconducting Technology Assessment - nitrd

Fig. 4. Junction fabrication process showing key features: junction
anodization [Fig. 4(b)] is self-aligned to junction; anodization etch
[Fig. 4(c)] requires high selectivity to niobium; anodization ring
is contained entirely within the space between edges of junction
and base electrode [Fig. 4(d)]. (a) Deposit Nb/Al-AlO /Nb trilayer
and apply junction mask. (b) Etch counterelectrode and anodize.
(c) Apply second junction mask and etch anodization. (d) Remove
photoresist mask to complete junction fabrication.
. This requires junction sizes to decrease in order to
maintain approximately the same range. Since is
proportional to the square of the junction diameter, good
dimensional control becomes one of the critical issues that
affect targeting and spreads. For example, at 8 kA/cm
and minimum junction of 100 A, variations in junction
diameter must be controlled to less than 0.06 m
if variations in are to remain under 10%. In addition,
small junctions are also more sensitive to perimeter effects.
Improvements in photolithography and RIE processes (see
Section IV) have kept pace with the demand for improved
dimensional control with little additional investment in new
process tools.
In the NGST process, the junction is defined on the niobium
counterelectrode of the trilayer by a photoresist mask
shown in Fig. 4(a). Next, the niobium counterelectrode is
dry etched in SF [see Fig. 4(b)]. Since SF does not etch
aluminum or Al O , the etch stops on the tunnel barrier
protecting the niobium base electrode. The dry etch process
is performed in a simple parallel plate RIE tool. SF etch
chemistry in this tool produces clean, residue-free features
that have vertical edges and no undercutting [59]. Another
popular niobium dry etch gas is CF which has been shown
to produce residue free, submicrometer features in niobium
Table 5
Reactive Ion Etch Parameters
using an ECR plasma etch tool [60]. Immediately after
etching, the wafers are lightly anodized to passivate the
junctions, and then the photoresist is stripped. The anodization
process protects the perimeter of the junctions from
chemical attack during the photoresist strip and subsequent
processing steps.
Many junction anodization processes have been described
in the literature [48], [61]–[63], but only “light” anodization,
described first by Gurvitch [48] and recently refined by
Meng [64], offers protection from process damage and is
scalable to submicrometer dimensions. Postetch junction
passivation using “light” anodization has been a key development
in the 8-kA/cm process to minimize junction
damage from subsequent wet processing steps. For example,
AZ300T photoresist stripper [65] and deionized water rinse
in combination can attack or erode the exposed aluminum
and very thin ( 2 nm) AlO tunnel barrier along the edge or
perimeter of the junction. This can increase subgap leakage
and degrade spreads. The wafer is typically anodized
in a mixture of ammonium pentaborate, ethylene glycol,
and deionized water to 15 V. At 15 V the aluminum barrier
metal is anodized completely to Al O ( 15 nm thick),
and the exposed niobium layer is partially converted to
Nb O ( 22 nm thick) [18]. Nb O and Al O make good
passivation layers because of their resistance to attack by
standard process chemistries.
In the anodization process described by Meng [64],
Nb/Al-AlO /Nb junctions are formed with a self-aligned
annulus that is lightly anodized to form an insulating double
layer of Al O and Nb O on the bottom of the annulus
(base electrode) and a single layer of Nb O on the sidewalls
(counterelectrode). The anodization layer passivates
the junction and sidewalls of the annulus. For the 8-kA/cm
process, NGST developed a variation of Meng’s light anodization
process that uses a second junction-masking step.
In the NGST process, shown in Fig. 4(c), the bulk of the
anodization layer is removed everywhere on the counterelectrode
except for the region around the junction protected by
the second junction photoresist mask. A combination of wet
dips in a dilute HF-nitric acid mixture and buffered oxide
etch (BOE) is used to remove Al O layer and a dry etch in
CHF %O (see Table 5) is used to remove the Nb O
layer. This creates a self-aligned, anodization layer or passivation
ring around the junction as shown in Fig. 4(d). Fig. 5
is a scanning electron microscope (SEM) photograph of a
1522 PROCEEDINGS OF THE IEEE, VOL. 92, NO. 10, OCTOBER 2004