Jahresbericht 2005 - IPHT Jena

MAGNETIK & QUANTENELEKTRONIK / MAGNETICS & QUANTUM ELECTRONICS
1.2 Scientific results
1.2.1 Micro- and nanofabrication
(Uwe Hübner, Ludwig Fritzsch,
Solveig Anders, Jürgen Kunert)
The microfabrication group is responsible for the
maintenance of the existing micro- and nanotechnological
fabrication processes for quantum
electronic devices (SQUID sensors, voltage standard
chips, Qubits) and other applications such
as nanoscale calibration standards, photonic
crystals and micro optical components, and for
the development of technologies appropriate for
new device requirements. In 2005, 210 chromium
masks and 240 electron beam direct writing
exposure jobs were made using the ZBA 23H.
120 masks were prepared by using the optical
pattern generator MANN 3600. About 80 high
resolution exposures were made using the
e-beam-tool LION. For the further improvement
of the electron beam lithography the software
tool SCELETON was installed as a proximity
function calculator.
In 2005 a new DFG-project, “Electrooptically Tunable
Photonic Crystals”, was started. The European
project PLATON (PLAnar Technology for
Optical Networks) and the R&D project “KALI II”
were successfully finished. In the “KALI II” project
a new type of nanoscale CD-standard for AFM
and a “Nanoscale Linewidth/Pitch Standard”
were realized. These standards consist of different
grating structures etched in nanocrystalline
silicon on a quartz substrate. They contain
patterns on nanometer scale for the calibration
and resolution-check of high-resolution optical
microscopy techniques, such as deep ultraviolet
microscopy and laser scanning microscopy.
One important task of the year 2005 was to
develop a process to reduce the standard 3.5 µm 2
Nb/Al process to junction dimensions of 1 µm 2 .At
the start of 2005 a chemical-mechanical polishing
(CMP) machine was installed and the process
of SiO 2-planarization on 4” wafers developed. In
parallel, optical lithography using a g-line stepper
(AÜR, Zeiss Jena) and the RIE process for sub-
µm Nb structures were optimized. A first wafer
run with test structures of SQUIDs with Josephson
junctions of different dimensions down to 0.8
µm showed in principal the functionality of the
planarization process. However, there was a
large parameter spread and a small yield, caused
by an insufficient reliability and overlap accuracy
of the stepper and the strong dependence of the
polishing rate on topology and structure dimensions.
Therefore, for the ongoing test runs new
technological concepts were developed including
direct e-beam exposure and the CALDERA
process for the CMP step in order to overcome
the dimensional effects when polishing. Fig. 1.1
shows a cross section of planarized SiO 2 isolated
Nb lines. The standard 3.5 µm 2 Nb/Al process
for SQUIDs was upgraded by integrating
Nb-oxide capacitors with specific capacitances of
app. 4 fF/µm 2 for areas of up to 25 mm 2 . They are
used in highly balanced gradiometers for mobile
SQUID system applications. The development of
large area Josephson junctions for applications
as x-ray detectors in synchrotron radiation experiments
was started with special emphasize on
the minimization of the subgap leakage currents.
The first results are promising and future work will
be concentrated on the preparation of sensor
arrays and the implementation of different
absorber materials.
Fig. 1.1: Cross section of Nb lines with planarized
SiO 2 isolation.
1.2.2 SQUID sensors and systems
(Volkmar Schultze, Ronny Stolz,
Viatcheslav Zakosarenko,
Andreas Chwala, Sven Linzen,
Nilolay Ukhanski, Torsten May)
Except for SQIFs (Superconducting Interference
Filters – a special combination of number of various
SQUIDs) which are developed and produced
within a long-standing cooperation with the University
of Tübingen and the company QEST, all
SQUID projects are based now upon the use of
low temperature superconductors.
In the fourth phase of the development of the
LTS SQUID gradiometer system for mobile applications
the second generation prototype for measuring
the full tensor of the Earth’s magnetic field
gradient with extremely high sensitivity was built.
It features several improvements compared to its
antecessors.
With integrated low pass filters the frequency gap
for disturbances between the system bandwidth
of approximately 4 MHz and 500 MHz of the RFI
screen around the cryostat could be closed.
Here, the main task was the development of a
technology for integrated capacitances up to 80
nF. After several aborts in the past few years the
successful implementation of intrinsic capacitors
is a highlight in the SQUID development in 2005.
Now, due to these filters the new gradiometer
sensors provide higher stability in environments
with high frequency radiation and their intrinsic
noise could be decreased down to 20 fT/(m·√Hz).
The second task was to reduce the motion noise
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