|
| Fig. 1 – Adkins
representation of STJ. When an incident photon breaks a Cooper pair, it
excites quasiparticles (a), which then relax through phonon
interactions (b), and diffuse into the trapping layer (c and d) where
they tunnel across either directly (e) or through a quasiparticle
recycling process known as back tunneling (f ) and then recombine in
the base electrode. [O. Drury, PhD Thesis UC Davis, 2007]. |
Why tantalum?
|
Current
state-of-the
art STJs use niobium (Z = 41) with an aluminum trap to increase
tunneling probability. Unfortunately, these detectors have an energy
threshold above which the top Nb electrode does not completely absorb
all the incoming photons; the remainder are absorbed in the bottom
electrode. Since the Al traps are not perfectly symmetric, the
resulting current pulse creates a line-splitting artifact in the
resulting spectrum at an apparent energy roughly 5% above that of the
actual photon (Fig. 2).
Tantalum
(Z = 73) provides more x-ray absorption at higher energies than niobium
and is also more opaque in the low energies. Additionally, the energy
gap in a superconducting metal is proportional to the critical
temperature. Since bulk Ta has a Tc that is roughly half of the value
for bulk Nb, the energy required to break a Cooper pair in Ta will be
about half that required for a Nb device; this will ultimately mean an
increase in intrinsic energy resolution of nearly a factor of square
root of 2.
|
|
Fig. 2 –
Fluorescence
spectrum taken with Nb STJs. Notice the line splitting due to the
incomplete absorption in the top electrode [S. Friedrich et. al., J.
Electron Spectroscopy, 101, 891-896, (1999)].
|
Body centered cubic (bcc) vs. tetragonal Ta
|
Tantalum offers
many
fabrication challenges over niobium when it comes to constructing STJs.
The most vexing is that Ta comes in two phases, and the bcc phase that
is useful for STJs is not the natural tetragonal phase for thin-film
Ta. Coaxing Ta into the correct phase may be accomplished a number of
ways. Growing Ta on a heated sapphire substrate results in the correct
phase, but this method is not possible for depositing the counter
electrode or wiring layer. A thin (5 nm) niobium seed layer has been
shown to nucleate the desired phase in Ta. We have chosen to utilize
this method of nucleating Ta as we have considerable experience with Nb
thin films. Figure 3 shows experimental data confirming our ability to
reliably construct thin films of bcc-Ta.
|
|
Fig. 3 – Critical
temperature plot for two phases of Ta (left). XRD plot showing phases
of Ta in test films (right).
|
Fabrication Process
|
We have
developed a
full-wafer fabrication process for constructing Ta STJs. The tunneling
structure is created without removing the wafer from the process
chamber which means the process is well controlled and nearly immune to
contamination. The mask set produces four single, square, STJ pixels of
different sizes; 10, 20, 40, and 80 microns on a side. Additionally
there are a number of test structures for measuring Tc, pinhole
density, wiring layer interface, and field oxide step coverage.
|
|
Fig. 4 – Cross
sectional (left) and birds-eye (right) view of fabrication process. Nb
layers have been omitted for clarity, and dimensions are not to scale.
|
Starting with a
bare
SiOx on Si wafer, the Nb/Ta/Al base electrode is DC sputtered. The Al
surface is oxidized in situ to form the tunnel barrier, and then the
Al/Nb/Ta counter electrode is sputtered on. The junction region is then
defined and the junction walls are anodized. Field oxide is sputtered
and windows are etched over the junction. Then the Nb/Ta wiring layer
is sputtered and etched. Finally, vias are etched down to the base
electrode.
|
Photos of Ta STJ pixels
|
|
Fig. 5 –
Microscope
photos of: 10 μm2 and 20 μm2 junctions (left), and 80 μm2 and 40 μm2
junctions (right).
|
Preliminary DC characteristics
|
The devices are mounted in
an
adiabatic demagnetization fridge which cools them down to about 70 mK.
A custom IV curve tracer is used to current-bias the STJ and the
voltages are read out with a standard 4-terminal measurement and a
Tektronix digital oscilloscope.
Initial results from the Ta STJ indicate a large amount of sub-gap
current (Fig. 6-left) and an unexpectedly low Josephson current.
However, the superconducting energy gap (2Δ/e) was measured at 0.7 mV.
This is consistent with our expectations for bcc-Ta. As a test of our
fabrication process, a Nb STJ was constructed using the same mask set.
The IV curve from the Nb STJ (Fig. 6-right) displays a much lower
sub-gap current, but also exhibits a troubling lack of Josephson
current. One possibility is that the interface between the wiring layer
and counter electrode is not pure and is acting as a second tunnel
junction in series with the actual device. This could account for the
lack of Josephson current and could also account for some of the
apparent leakage. Figure 7-right shows a close up view of the pair
tunneling region for a 10 micron bcc-Ta STJ. The cusp is a possible
indicator of S1-I-S2 tunneling, which is likely, as there are three
superconducting metals and an unknown number of superconducting oxides
and alloys present in the STJ.
|
|
Fig. 6 – IV curve
from
first bcc-Ta STJ (left). IV curve from Nb STJ fabricated with same mask
set (right). Both curves were taken without any magnetic field
suppression of the Josephson currents.
|
|
Fig. 7 – IV curve
from first bcc-Ta STJ (left). Close up of same STJ (right).
|
Acknowledgements
|
Thanks to: O. Drury and M.
Velazquez for wirebonding and good advice, J. Hamilton for last minute
dicing, D. Danielson for beautiful fab drawings, P. Verdone, A. Der, A.
Kelly, and F. Lipschultz for keeping the lab running, and S. Burt and
F. Ponce for lending a hand when needed.
|
|
|
