Aerospace & Defense Technology - February 2021 - 38

Tech Briefs

cuit ground-signal-ground layouts used
for radio frequency (RF) electronics. The
active region was modified to act as a
tunneling junction. As for the tunneling junction, structures were produced

(a)

that have a " closed " but constricted gap
at or near the coherence length for niobium to behave as a weak-link and
where functional materials can readily
be integrated in the tunneling region.

(b)

Nb
Nb

siliconoxide

G
S

G
Microwave
Layout
(c)

Nb

siliconoxide

Tunneling
(closed)
Junction

(d)

Nb

Nb

Nb

siliconoxide

siliconoxide

90 nm
Tunneling
(gap)
Junction

Tunneling
(gap)
Junction

Nb

Nb

Figure 1. High resolution scanning electron microscopy of a full device in microwave ground-signal-ground
layout (a), the case of a closed tunneling junction region (b), open (c), and open tunneling junction (d).

(a)

(b)

Nb

Nb
ITO Gate

Tunneling
Junction
Nb

Tunneling
Junction

ITO Gate

silicon- oxide
(d)

(c)
Nb

Nb

Nb

Nb
silicon- ITO
oxide
Gate
Nb

silicon- oxide

Nb

ITO
Tunneling
Junction

Nb

siliconoxide

Figure 2. High resolution scanning electron microscope images of gate electrode in the vicinity of the tunneling junction (a), close ups of a gate electrode over a closed gap (b), gate electrode over an open gap (c)
and gate electrode in an open gap (d).

38

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Cov

ToC

The starting material was 4-inch silicon wafers with 300 nm of thermal
SiO2. One hundred nm of niobium was
DC sputtered in the presence of argon
at 200 watts. The samples were pattered
and developed with ma-N 2405 electron
beam negative resist and exposure with
a Vistec Electron Beam lithography system. Dose tests measured optimal exposure conditions.
After dose testing, resist development
and bake SF6 etching was done to define
the lateral niobium nanoscale patterns,
followed by two cycles of O2 plasma etching and SF6 etching to clean the surface.
Layouts were developed in a complete
microwave layout as shown in Figure 1a
where the device is arranged in a groundsignal-ground configuration. The layout
was made where the gap region of the active tunneling junction was varied from
closed configurations in Figure 1b to
opens in Figure 1c and 1d with varying
gap length. The open junctions were
formed specifically to enable deposition
of novel functional materials in the tunneling gap region, such as a S-B-I-B-S
structure previously examined, or other
structures as desired by the application.
After patterning of the niobium, a second exposure was completed to define
indium-tin-oxide gate electrodes (ITO)
by a lift-off process. The aligned exposure was assisted by the formation of
alignment markers during the previous
lithography step. Polymethyl methacrylate (PMMA) positive resist was used as
the resist and dose tests were completed
to optimize the exposure conditions.
After the resist development and bake,
a 2-second descum oxygen-plasma exposure was performed, followed by RF sputtering at 100 watts in argon of indiumtin-oxide (ITO) and subsequent lift-off in
acetone using standard lift-off procedures. Figure 2 shows completed structures with nanowire gate widths down to
sub-100 nm dimensions that are well positioned over the tunneling junction.
This work was done by Osama Nayfeh
and Dave Rees of the Naval Information
Warfare Systems Command (formerly
SPAWAR). For more information, download the Technical Support Package (free
white paper) at www.aerodefensetech.
com/tsp under the Manufacturing and
Materials category. SPAWAR-0007

Aerospace & Defense Technology, February 2021


http://www.aerodefensetech.com/tsp http://www.aerodefensetech.com http://www.abpi.net/ntbpdfclicks/l.php?202102ADTNAV

Aerospace & Defense Technology - February 2021

Table of Contents for the Digital Edition of Aerospace & Defense Technology - February 2021

Aerospace & Defense Technology - February 2021 - Intro
Aerospace & Defense Technology - February 2021 - Sponsor
Aerospace & Defense Technology - February 2021 - Cov I
Aerospace & Defense Technology - February 2021 - Cov II
Aerospace & Defense Technology - February 2021 - 1
Aerospace & Defense Technology - February 2021 - 2
Aerospace & Defense Technology - February 2021 - 3
Aerospace & Defense Technology - February 2021 - 4
Aerospace & Defense Technology - February 2021 - 5
Aerospace & Defense Technology - February 2021 - 6
Aerospace & Defense Technology - February 2021 - 7
Aerospace & Defense Technology - February 2021 - 8
Aerospace & Defense Technology - February 2021 - 9
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Aerospace & Defense Technology - February 2021 - 11
Aerospace & Defense Technology - February 2021 - 12
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Aerospace & Defense Technology - February 2021 - 38
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Aerospace & Defense Technology - February 2021 - 48
Aerospace & Defense Technology - February 2021 - Cov III
Aerospace & Defense Technology - February 2021 - Cov IV
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