We demonstrate electronic cooling of a suspended AuPd island using
superconductor-insulator-normal metal tunnel junctions. This was achieved by
developing a simple fabrication method for reliably releasing narrow submicron
sized metal beams. The process is based on reactive ion etching and uses a
conducting substrate to avoid charge-up damage and is compatible with e.g.
conventional e-beam lithography, shadow-angle metal deposition and oxide tunnel
junctions. The devices function well and exhibit clear cooling; up to factor of
two at sub-kelvin temperatures.Comment: 4 pages, 3 figure

A. O. Niskanen

J. P. Pekola

J. S. Tsai

J. T. Muhonen

L. Sainiemi

M. Meschke

S. Franssila

Watanabe T.

Yu. A. Pashkin

English

arXiv

We demonstrate electronic cooling of a suspended AuPd island using superconductor-insulator-normal metal tunnel junctions. This was achieved by developing a simple fabrication method for reliably releasing narrow submicron-sized metal beams. The process is based on reactive ion etching and uses a conducting substrate to avoid charge-up damage and is compatible with, e.g., conventional e-beam lithography, shadow-angle metal deposition, and oxide tunnel junctions. The devices function well and exhibit clear cooling, up to a factor of 2 at sub-Kelvin temperatures

Muhonen, J.T.

Niskanen, Antti

Meschke, M.

Pashkin, Yu.A.

Tsai, J.S.

Sainiemi, L.

Franssila, S.

Pekola, J.P.

VTT Research System

Electronic cooling of a submicron-sized metallic beam

Muhonen, J. T.

Niskanen, A. O.

Pashkin, Yuri

Tsai, J. S.

Pekola, J. P.

Lancaster E-Prints

We demonstrate electronic cooling of a suspended AuPd island using superconductor-insulator-normal metaltunnel junctions. This was achieved by developing a simple fabrication method for reliably releasing narrow submicron-sized metal beams. The process is based on reactive ion etching and uses a conducting substrate to avoid charge-up damage and is compatible with, e.g., conventional e-beam lithography, shadow-angle metal deposition, and oxide tunnel junctions. The devices function well and exhibit clear cooling, up to a factor of 2 at sub-Kelvin temperatures.Peer reviewe

Pashkin, Yu. A.

Pekola, Jukka P.

Aaltodoc

This is an electronic reprint of the original article.
This reprint may differ from the original in pagination and typographic detail.
Author(s): Muhonen, J. T. & Niskanen, A. O. & Meschke, M. & Pashkin, Yu. A. &
Tsai, J. S. & Sainiemi, L. & Franssila, S. & Pekola, Jukka
Title: Electronic cooling of a submicron-sized metallic beam
Year: 2009
Version: Final published version
Please cite the original version:
Muhonen, J. T. & Niskanen, A. O. & Meschke, M. & Pashkin, Yu. A. & Tsai, J. S. &
Sainiemi, L. & Franssila, S. & Pekola, Jukka. 2009. Electronic cooling of a
submicron-sized metallic beam. Applied Physics Letters. Volume 94, Issue 7. P.
073101/1-3. ISSN 0003-6951 (printed). DOI: 10.1063/1.3080668.
Rights: © 2009 American Institute of Physics. This article may be downloaded for personal use only. Any other use
requires prior permission of the author and the American Institute of Physics. The following article appeared
in Applied Physics Letters and may be found at
http://scitation.aip.org/content/aip/journal/apl/94/7/10.1063/1.3080668
All material supplied via Aaltodoc is protected by copyright and other intellectual property rights, and
duplication or sale of all or part of any of the repository collections is not permitted, except that material may
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Electronic cooling of a submicron-sized metallic beam
J. T. Muhonen, A. O. Niskanen, M. Meschke, Yu. A. Pashkin, J. S. Tsai, L. Sainiemi, S. Franssila, and J. P.
Pekola 
 
Citation: Applied Physics Letters 94, 073101 (2009); doi: 10.1063/1.3080668 
View online: http://dx.doi.org/10.1063/1.3080668 
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/94/7?ver=pdfcov 
Published by the AIP Publishing 
 
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Preparation of bead metal single crystals by electron beam heating 
J. Vac. Sci. Technol. A 23, 1535 (2005); 10.1116/1.2101793 
 
Removable template route to metallic nanowires and nanogaps 
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Electronic cooling of a submicron-sized metallic beam
J. T. Muhonen,1,a A. O. Niskanen,2 M. Meschke,1 Yu. A. Pashkin,3,b J. S. Tsai,3
L. Sainiemi,4 S. Franssila,4 and J. P. Pekola1
1Low Temperature Laboratory, Helsinki University of Technology, P.O. Box 3500, 02015 TKK, Finland
2VTT Technical Research Centre of Finland, Sensors, P.O. Box 1000, 02044 VTT, Finland
3NEC Nano Electronics Research Laboratories, RIKEN Advance Science Institute, 34 Miyukigaoka,
Tsukuba, Ibaraki 305-8501, Japan
4Micro and Nanosciences Laboratory, Helsinki University of Technology, P.O. Box 3500,
02015 TKK, Finland
Received 1 July 2008; accepted 6 January 2009; published online 17 February 2009
We demonstrate electronic cooling of a suspended AuPd island using superconductor-
insulator-normal metal tunnel junctions. This was achieved by developing a simple fabrication
method for reliably releasing narrow submicron-sized metal beams. The process is based on reactive
ion etching and uses a conducting substrate to avoid charge-up damage and is compatible with, e.g.,
conventional e-beam lithography, shadow-angle metal deposition, and oxide tunnel junctions. The
devices function well and exhibit clear cooling, up to a factor of 2 at sub-Kelvin temperatures.
© 2009 American Institute of Physics. DOI: 10.1063/1.3080668
Nanomechanical devices1,2 at low temperatures are cur-
rently a topic of intense study.3 One motivation for this is the
possibility to drive these devices into their quantum limit by
cooling down their relevant mechanical modes below the
quantum of resonator energy.4 To actually perform nontrivial
quantum mechanical experiments, it is then necessary to
couple the resonator to some other elements. A large class of
interesting objects are metallic nanoelectronic devices, either
superconducting or normal metal. Many groups have ap-
proached the task at hand by separately fabricating a
nonconducting, possibly single crystal, mechanical
device, and then metallizing it.5–7 This has been done
also with superconductor-insulator-normal metal-insulator-
superconductor SINIS structures.8 Surprisingly, evaporated
metal beams can have almost as good mechanical
properties9,10 as their more obvious alternatives. Integrating
a metallic beam with a solid state refrigerator is therefore an
intriguing prospect. Suspended micrometer scale metallic
beams are in use in the field of bolometers11 and in micro-
electromechanical systems.12,13 However, only a few
examples14,15 of suspended metallic nanostructures with tun-
nel junctions have been fabricated so far.
With the motivation of studying the effect of suspension
on the properties of normal metal-insulator-superconductor
NIS electron coolers,16,17 we have developed a reliable and
straightforward fabrication process for fully metallic sus-
pended structures. The process is also useful for fabricating
other kinds of partly suspended metallic circuits which re-
quire tunnel junctions, e.g., qubit circuits. Our fabrication
process begins by e-beam patterning and angle evaporating a
circuit on a conducting substrate, in our case n-doped
3–5  cm unoxidized silicon. The normal metal used is
an alloy of Au three parts by mass and Pd one part by
mass and the superconducting material is Al. Al is deposited
first and then oxidized to form the tunnel barriers. After this,
AuPd is deposited from a different angle to form the island.
Film thicknesses are 65 nm for Al and 50 nm for AuPd and
the island width is about 90 nm. After lift-off, we release the
narrow parts of the pattern from the substrate by isotropic
etching using SF6 in a reactive ion etcher. Pressure of 100
mTorr, power of 80 W, SF6 flow of 30 SCCM SCCM de-
notes standard cubic centimeters per minute at STP and O2
flow of 5 SCCM were used. These parameters produced a
nicely isotropic etching profile, which allows us to selec-
tively suspend only the narrowest parts by using suitable
etching time, which was roughly 1 min in our case. With this
method, we fabricated SINIS refrigerators in which only the
normal metal island is released from the substrate, as de-
picted in Fig. 1. Naturally, the edges of wide structures are
also suspended.
High-resistivity Si and Si with a 300 nm thick SiO2 layer
on top of it were first tried as substrate materials but it turned
out that with these insulating materials always at least one of
aElectronic mail: juha.muhonen@tkk.fi.
bOn leave from Lebedev Physical Institute, Moscow 119991, Russia.
FIG. 1. Color online Suspended SINIS cooler and measurement schemat-
ics. AuPd light island is connected to Al gray leads through tunnel junc-
tions. The larger junctions are voltage biased for cooling and the smaller
ones current biased for thermometry. The gate electrode in the bottom was
not used in the present measurements.
APPLIED PHYSICS LETTERS 94, 073101 2009
0003-6951/2009/947/073101/3/$25.00 © 2009 American Institute of Physics94, 073101-1
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the junctions was shorted after etching. This might be due to
the so-called antenna effect that has been studied before18,19
in the context of gate oxide damage in semiconductors.
Metal films directly in contact with the plasma can act as
charge collecting antennas during etching; hence, causing
voltage to build up over the oxides separating them from
other layers. If we consider that in our samples the bonding
pads are acting as antennas and are connected to the Al leads,
assuming charge collection of 10 pA /m2 Ref. 19 could
lead to a voltage variation of some volts over the junctions.
Using conductive substrate, in our case n-doped Si, sup-
presses this effect as the junctions are effectively shunted
through the substrate.
The theoretical cooling power and electric current of
SINIS structures are quite well understood see, e.g., Ref. 20
for a review and formulas. Qualitatively, no current flows if
the voltage over each NIS junction is below  /e, i.e., 2 /e
for two junctions in series, where  is the superconducting
gap. When a voltage bias is applied across the junction so
that the Fermi level of the normal metal part is shifted up-
wards by energy eV current can start flowing. If the bias
voltage is just below the gap, the tunnel junctions serve as
energy filters by selectively allowing, depending on polarity,
either hot electrons to exit or cool ones to enter the normal
metal island. Since there are two such junctions in series,
both these processes coexist. This then leads to sharpening of
the Fermi distribution of the electron gas, i.e., cooling. When
the bias voltage is much above the gap value, the junction
will start to act as a normal resistor with linear current-
voltage I-V behavior and Joule heating appears. At elevated
temperatures, the I-V characteristics get increasingly rounded
at the gap edges. This effect is used for thermometry by
current biasing the smaller junctions with a small probe cur-
rent on the order of picoampere and measuring the induced
voltage, which approaches 2 /e when T /kB and zero at
high temperatures. Figure 1 illustrates the basic measurement
schematics. The resistances of the small junctions are ap-
proximately 65 k and the larger cooling junctions have
resistance of about 10 k.
To calibrate the thermometer, we sweep the bath tem-
perature of our dilution refrigerator while passing approxi-
mately a 12 pA probe current from a dc current source
through the probe junctions. We then record the probe volt-
age Vprobe at different bath temperatures, which is shown in
Fig. 2a. The cooler junctions are biased with a zero voltage
such that there is no intentional heating or cooling of the
suspended island electrons. This guarantees that the island
temperature follows the bath except for noise heating, which
can be seen as the saturation of the island electron tempera-
ture at low bath temperatures. From the theory,20 it is ex-
pected that at low temperatures, the relation between the
probe voltage and the electron temperature is linear, which
allows us to extrapolate the calibration to lowest tempera-
tures. The island temperature as a function of the bath tem-
perature at zero cooler bias voltage is shown in Fig. 2b.
Figure 3a illustrates some of the measured current-
voltage characteristics of the cooler junctions. The BCS gap
is extracted by fitting the theoretical I-V curves to the mea-
surement data, taking into account the charging energy,21
measured to be roughly 190 mK. The gap was determined to
be  /e1946 V. This is in accordance with  nor-
mally measured in aluminum films of such thickness. The
dependence of Tel on the cooler voltage Vcooler at different
bath temperatures is shown in Fig. 3b. The cooler functions
well. At best the cooled electron temperature Vcooler
400 V is a factor of 2 lower than the temperature at
zero bias Vcooler=0. From the lowest zero bias temperature
of 145 mK, the electron gas cools to roughly 80 mK.
In bulk samples at sub-Kelvin temperatures, the elec-
trons are effectively decoupled from the ambient phonons
and the power flowing from electrons to phonons is given by
P=VTel5 −Tph5 ,22 where V is the volume of the metal,  is
a material parameter, and Tel and Tph are the electron tem-
perature and phonon temperature, respectively. However, at
the lowest temperatures, our sample’s transverse dimensions
become smaller than the thermal wavelength of the phonons,
defined as hcl / kBT, where cl is the speed of sound; hence,
affecting the phonon dimensionality and presumably this
power law.23–25 In our geometry, this criterion means that at
temperatures below roughly 450 mK, the phonon system
should be fully one dimensional, assuming that the speed of
FIG. 2. Color online a Calibration of the integrated electron thermom-
eter. The measured data Vcooler=0 is shown by dots. The dashed line is a
linear fit to the center region. The solid line is the calibration used in this
paper. Note that the exact normal metal temperature values below 100 mK
or so may not be fully accurate in b and Fig. 3 due to extrapolation. The
asymptotic linear behavior and intersection at about 2 /e are quantitatively
correct though. The thermometer loses sensitivity at high temperatures. b
Dependence of island temperature Tel on the bath temperature Tbath at zero
cooler bias. The normal metal island does not cool below 145 mK even
when the cryostat phonon temperature is reduced below this. The solid line
is a fit assuming conventional bulk electron-phonon coupling. The inset is a
close up on the low temperature end showing a fit with the T3 electron-
phonon coupling as a dashed line.
FIG. 3. Color online a Measured current-voltage curves of the cooler
junctions at different bath temperatures. b Suspended island electron tem-
peratures as function of the cooler bias voltage at same temperatures as in
a. The cooling curves and I-V curves are measured simultaneously using
the configuration shown in Fig. 1. The bath temperatures are from bottom
to top 103, 211, 295, 373, and 460 mK. As the bath temperature is lowered,
the I-V curves get sharper.
073101-2 Muhonen et al. Appl. Phys. Lett. 94, 073101 2009
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sound in AuPd is close to that of Au and Pd.
In the present experiments, we see no clear evidence of
modified electron-phonon coupling but the data are still in-
conclusive. In Fig. 2b, the saturation of the electron tem-
perature due to noise heating is illustrated. The solid line is a
fit assuming the conventional bulk electron-phonon coupling
and that the island phonon temperature follows the bath tem-
perature. The dashed line in the inset shows similar fit with
T3 coupling, which is expected for purely one-dimensional
phonons.25 The conventional coupling gives a better fit and
using literature value of =2109 W K−5 m−3 and volume
of V=10−20 m3, we get a heating power of 1 fW, which is in
accordance to our previous experiences with the measure-
ment apparatus.26 We also calculated the theoretical cooling/
heating power for the whole bias range with the parameters
extracted from the fits to the I-V curves. However, it turned
out that the input impedance of our current amplifier is com-
parable with the dynamical resistance of our sample at the
gap edges. This resistance covers four orders of magnitude
and makes a reliable analysis of the data over the whole bias
range very difficult. At low bias voltages, i.e., at the high-
impedance regime, we see hints of a T3 power law but the
data are still inconclusive as we cannot extend this fit to the
heating regime.
In conclusion, we have fabricated a suspended SINIS
cooler and demonstrated electronic cooling of a nanoscale
metallic beam. For this purpose, we developed a simple
single lithography step process for fabricating released me-
tallic nanostructures with tunnel junctions. In the future,
these devices might be of interest as bolometric detectors or
in the studies of the quantum limit of nanomechanics. Also
the effects of reduced phonon dimensionality in these struc-
tures are a topic of considerable interest for future experi-
ments.
This work is supported by the Academy of Finland and
the NanoSciERA project “NanoFridge” of the EU. J.S.T. ac-
knowledges partial support of Japan Science and Technology
Agency through the CREST Project. We thank H. Im and
T.F. Li for their assistance in fabrication of the devices.
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073101-3 Muhonen et al. Appl. Phys. Lett. 94, 073101 2009
 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
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We demonstrate electronic cooling of a suspended AuPd island using superconductor-insulator-normal metal tunnel junctions. This was achieved by developing a simple fabrication method for reliably releasing narrow submicron-sized metal beams. The process is based on reactive ion etching and uses a conducting substrate to avoid charge-up damage and is compatible with, e.g., conventional e-beam lithography, shadow-angle metal deposition, and oxide tunnel junctions. The devices function well and exhibit clear cooling, up to a factor of 2 at sub-Kelvin temperatures.Peer reviewe

Muhonen, Juha

Meschke, Matthias

Pashkin, Yury

Pekola, Jukka

Aaltodoc Publication Archive

Applied Physics Letters

Electronic cooling a submicron-sized metallic beam

This is an electronic reprint of the original article.This reprint may differ from the original in pagination and typographic detail.Author(s): Muhonen, J. T. & Niskanen, A. O. & Meschke, M. & Pashkin, Yu. A. &Tsai, J. S. & Sainiemi, L. & Franssila, S. & Pekola, JukkaTitle: Electronic cooling of a submicron-sized metallic beamYear: 2009Version: Final published versionPlease cite the original version:Muhonen, J. T. & Niskanen, A. O. & Meschke, M. & Pashkin, Yu. A. & Tsai, J. S. &Sainiemi, L. & Franssila, S. & Pekola, Jukka. 2009. Electronic cooling of asubmicron-sized metallic beam. Applied Physics Letters. Volume 94, Issue 7. P.073101/1-3. ISSN 0003-6951 (printed). DOI: 10.1063/1.3080668.Rights: © 2009 American Institute of Physics. This article may be downloaded for personal use only. Any other userequires prior permission of the author and the American Institute of Physics. The following article appearedin Applied Physics Letters and may be found athttp://scitation.aip.org/content/aip/journal/apl/94/7/10.1063/1.3080668All material supplied via Aaltodoc is protected by copyright and other intellectual property rights, andduplication or sale of all or part of any of the repository collections is not permitted, except that material maybe duplicated by you for your research use or educational purposes in electronic or print form. You mustobtain permission for any other use. Electronic or print copies may not be offered, whether for sale orotherwise to anyone who is not an authorised user.Powered by TCPDF (www.tcpdf.org)Electronic cooling of a submicron-sized metallic beamJ. T. Muhonen, A. O. Niskanen, M. Meschke, Yu. A. Pashkin, J. S. Tsai, L. Sainiemi, S. Franssila, and J. P.Pekola  Citation: Applied Physics Letters 94, 073101 (2009); doi: 10.1063/1.3080668 View online: http://dx.doi.org/10.1063/1.3080668 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/94/7?ver=pdfcov Published by the AIP Publishing  Articles you may be interested in Effect of relaxation state on nucleation and grain growth of nanoscale quasicrystal in Zr-based bulk metallicglasses prepared under various cooling rates Appl. Phys. Lett. 99, 061903 (2011); 10.1063/1.3622117  Preparation of bead metal single crystals by electron beam heating J. Vac. Sci. Technol. A 23, 1535 (2005); 10.1116/1.2101793  Removable template route to metallic nanowires and nanogaps Appl. Phys. Lett. 79, 2073 (2001); 10.1063/1.1405813  Fabrication of extremely narrow metal wires Appl. Phys. Lett. 77, 1991 (2000); 10.1063/1.1312256  Nanolithography of metal films using scanning force microscope patterned carbon masks J. Vac. Sci. Technol. B 16, 3879 (1998); 10.1116/1.590435    This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:130.233.216.27 On: Tue, 19 May 2015 09:45:53Electronic cooling of a submicron-sized metallic beamJ. T. Muhonen,1,a A. O. Niskanen,2 M. Meschke,1 Yu. A. Pashkin,3,b J. S. Tsai,3L. Sainiemi,4 S. Franssila,4 and J. P. Pekola11Low Temperature Laboratory, Helsinki University of Technology, P.O. Box 3500, 02015 TKK, Finland2VTT Technical Research Centre of Finland, Sensors, P.O. Box 1000, 02044 VTT, Finland3NEC Nano Electronics Research Laboratories, RIKEN Advance Science Institute, 34 Miyukigaoka,Tsukuba, Ibaraki 305-8501, Japan4Micro and Nanosciences Laboratory, Helsinki University of Technology, P.O. Box 3500,02015 TKK, FinlandReceived 1 July 2008; accepted 6 January 2009; published online 17 February 2009We demonstrate electronic cooling of a suspended AuPd island using superconductor-insulator-normal metal tunnel junctions. This was achieved by developing a simple fabricationmethod for reliably releasing narrow submicron-sized metal beams. The process is based on reactiveion etching and uses a conducting substrate to avoid charge-up damage and is compatible with, e.g.,conventional e-beam lithography, shadow-angle metal deposition, and oxide tunnel junctions. Thedevices function well and exhibit clear cooling, up to a factor of 2 at sub-Kelvin temperatures.© 2009 American Institute of Physics. DOI: 10.1063/1.3080668Nanomechanical devices1,2 at low temperatures are cur-rently a topic of intense study.3 One motivation for this is thepossibility to drive these devices into their quantum limit bycooling down their relevant mechanical modes below thequantum of resonator energy.4 To actually perform nontrivialquantum mechanical experiments, it is then necessary tocouple the resonator to some other elements. A large class ofinteresting objects are metallic nanoelectronic devices, eithersuperconducting or normal metal. Many groups have ap-proached the task at hand by separately fabricating anonconducting, possibly single crystal, mechanicaldevice, and then metallizing it.5–7 This has been donealso with superconductor-insulator-normal metal-insulator-superconductor SINIS structures.8 Surprisingly, evaporatedmetal beams can have almost as good mechanicalproperties9,10 as their more obvious alternatives. Integratinga metallic beam with a solid state refrigerator is therefore anintriguing prospect. Suspended micrometer scale metallicbeams are in use in the field of bolometers11 and in micro-electromechanical systems.12,13 However, only a fewexamples14,15 of suspended metallic nanostructures with tun-nel junctions have been fabricated so far.With the motivation of studying the effect of suspensionon the properties of normal metal-insulator-superconductorNIS electron coolers,16,17 we have developed a reliable andstraightforward fabrication process for fully metallic sus-pended structures. The process is also useful for fabricatingother kinds of partly suspended metallic circuits which re-quire tunnel junctions, e.g., qubit circuits. Our fabricationprocess begins by e-beam patterning and angle evaporating acircuit on a conducting substrate, in our case n-doped3–5  cm unoxidized silicon. The normal metal used isan alloy of Au three parts by mass and Pd one part bymass and the superconducting material is Al. Al is depositedfirst and then oxidized to form the tunnel barriers. After this,AuPd is deposited from a different angle to form the island.Film thicknesses are 65 nm for Al and 50 nm for AuPd andthe island width is about 90 nm. After lift-off, we release thenarrow parts of the pattern from the substrate by isotropicetching using SF6 in a reactive ion etcher. Pressure of 100mTorr, power of 80 W, SF6 flow of 30 SCCM SCCM de-notes standard cubic centimeters per minute at STP and O2flow of 5 SCCM were used. These parameters produced anicely isotropic etching profile, which allows us to selec-tively suspend only the narrowest parts by using suitableetching time, which was roughly 1 min in our case. With thismethod, we fabricated SINIS refrigerators in which only thenormal metal island is released from the substrate, as de-picted in Fig. 1. Naturally, the edges of wide structures arealso suspended.High-resistivity Si and Si with a 300 nm thick SiO2 layeron top of it were first tried as substrate materials but it turnedout that with these insulating materials always at least one ofaElectronic mail: juha.muhonen@tkk.fi.bOn leave from Lebedev Physical Institute, Moscow 119991, Russia.FIG. 1. Color online Suspended SINIS cooler and measurement schemat-ics. AuPd light island is connected to Al gray leads through tunnel junc-tions. The larger junctions are voltage biased for cooling and the smallerones current biased for thermometry. The gate electrode in the bottom wasnot used in the present measurements.APPLIED PHYSICS LETTERS 94, 073101 20090003-6951/2009/947/073101/3/$25.00 © 2009 American Institute of Physics94, 073101-1 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:130.233.216.27 On: Tue, 19 May 2015 09:45:53the junctions was shorted after etching. This might be due tothe so-called antenna effect that has been studied before18,19in the context of gate oxide damage in semiconductors.Metal films directly in contact with the plasma can act ascharge collecting antennas during etching; hence, causingvoltage to build up over the oxides separating them fromother layers. If we consider that in our samples the bondingpads are acting as antennas and are connected to the Al leads,assuming charge collection of 10 pA /m2 Ref. 19 couldlead to a voltage variation of some volts over the junctions.Using conductive substrate, in our case n-doped Si, sup-presses this effect as the junctions are effectively shuntedthrough the substrate.The theoretical cooling power and electric current ofSINIS structures are quite well understood see, e.g., Ref. 20for a review and formulas. Qualitatively, no current flows ifthe voltage over each NIS junction is below  /e, i.e., 2 /efor two junctions in series, where  is the superconductinggap. When a voltage bias is applied across the junction sothat the Fermi level of the normal metal part is shifted up-wards by energy eV current can start flowing. If the biasvoltage is just below the gap, the tunnel junctions serve asenergy filters by selectively allowing, depending on polarity,either hot electrons to exit or cool ones to enter the normalmetal island. Since there are two such junctions in series,both these processes coexist. This then leads to sharpening ofthe Fermi distribution of the electron gas, i.e., cooling. Whenthe bias voltage is much above the gap value, the junctionwill start to act as a normal resistor with linear current-voltage I-V behavior and Joule heating appears. At elevatedtemperatures, the I-V characteristics get increasingly roundedat the gap edges. This effect is used for thermometry bycurrent biasing the smaller junctions with a small probe cur-rent on the order of picoampere and measuring the inducedvoltage, which approaches 2 /e when T /kB and zero athigh temperatures. Figure 1 illustrates the basic measurementschematics. The resistances of the small junctions are ap-proximately 65 k and the larger cooling junctions haveresistance of about 10 k.To calibrate the thermometer, we sweep the bath tem-perature of our dilution refrigerator while passing approxi-mately a 12 pA probe current from a dc current sourcethrough the probe junctions. We then record the probe volt-age Vprobe at different bath temperatures, which is shown inFig. 2a. The cooler junctions are biased with a zero voltagesuch that there is no intentional heating or cooling of thesuspended island electrons. This guarantees that the islandtemperature follows the bath except for noise heating, whichcan be seen as the saturation of the island electron tempera-ture at low bath temperatures. From the theory,20 it is ex-pected that at low temperatures, the relation between theprobe voltage and the electron temperature is linear, whichallows us to extrapolate the calibration to lowest tempera-tures. The island temperature as a function of the bath tem-perature at zero cooler bias voltage is shown in Fig. 2b.Figure 3a illustrates some of the measured current-voltage characteristics of the cooler junctions. The BCS gapis extracted by fitting the theoretical I-V curves to the mea-surement data, taking into account the charging energy,21measured to be roughly 190 mK. The gap was determined tobe  /e1946 V. This is in accordance with  nor-mally measured in aluminum films of such thickness. Thedependence of Tel on the cooler voltage Vcooler at differentbath temperatures is shown in Fig. 3b. The cooler functionswell. At best the cooled electron temperature Vcooler400 V is a factor of 2 lower than the temperature atzero bias Vcooler=0. From the lowest zero bias temperatureof 145 mK, the electron gas cools to roughly 80 mK.In bulk samples at sub-Kelvin temperatures, the elec-trons are effectively decoupled from the ambient phononsand the power flowing from electrons to phonons is given byP=VTel5 −Tph5 ,22 where V is the volume of the metal,  isa material parameter, and Tel and Tph are the electron tem-perature and phonon temperature, respectively. However, atthe lowest temperatures, our sample’s transverse dimensionsbecome smaller than the thermal wavelength of the phonons,defined as hcl / kBT, where cl is the speed of sound; hence,affecting the phonon dimensionality and presumably thispower law.23–25 In our geometry, this criterion means that attemperatures below roughly 450 mK, the phonon systemshould be fully one dimensional, assuming that the speed ofFIG. 2. Color online a Calibration of the integrated electron thermom-eter. The measured data Vcooler=0 is shown by dots. The dashed line is alinear fit to the center region. The solid line is the calibration used in thispaper. Note that the exact normal metal temperature values below 100 mKor so may not be fully accurate in b and Fig. 3 due to extrapolation. Theasymptotic linear behavior and intersection at about 2 /e are quantitativelycorrect though. The thermometer loses sensitivity at high temperatures. bDependence of island temperature Tel on the bath temperature Tbath at zerocooler bias. The normal metal island does not cool below 145 mK evenwhen the cryostat phonon temperature is reduced below this. The solid lineis a fit assuming conventional bulk electron-phonon coupling. The inset is aclose up on the low temperature end showing a fit with the T3 electron-phonon coupling as a dashed line.FIG. 3. Color online a Measured current-voltage curves of the coolerjunctions at different bath temperatures. b Suspended island electron tem-peratures as function of the cooler bias voltage at same temperatures as ina. The cooling curves and I-V curves are measured simultaneously usingthe configuration shown in Fig. 1. The bath temperatures are from bottomto top 103, 211, 295, 373, and 460 mK. As the bath temperature is lowered,the I-V curves get sharper.073101-2 Muhonen et al. Appl. Phys. Lett. 94, 073101 2009 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:130.233.216.27 On: Tue, 19 May 2015 09:45:53sound in AuPd is close to that of Au and Pd.In the present experiments, we see no clear evidence ofmodified electron-phonon coupling but the data are still in-conclusive. In Fig. 2b, the saturation of the electron tem-perature due to noise heating is illustrated. The solid line is afit assuming the conventional bulk electron-phonon couplingand that the island phonon temperature follows the bath tem-perature. The dashed line in the inset shows similar fit withT3 coupling, which is expected for purely one-dimensionalphonons.25 The conventional coupling gives a better fit andusing literature value of =2109 W K−5 m−3 and volumeof V=10−20 m3, we get a heating power of 1 fW, which is inaccordance to our previous experiences with the measure-ment apparatus.26 We also calculated the theoretical cooling/heating power for the whole bias range with the parametersextracted from the fits to the I-V curves. However, it turnedout that the input impedance of our current amplifier is com-parable with the dynamical resistance of our sample at thegap edges. This resistance covers four orders of magnitudeand makes a reliable analysis of the data over the whole biasrange very difficult. At low bias voltages, i.e., at the high-impedance regime, we see hints of a T3 power law but thedata are still inconclusive as we cannot extend this fit to theheating regime.In conclusion, we have fabricated a suspended SINIScooler and demonstrated electronic cooling of a nanoscalemetallic beam. For this purpose, we developed a simplesingle lithography step process for fabricating released me-tallic nanostructures with tunnel junctions. In the future,these devices might be of interest as bolometric detectors orin the studies of the quantum limit of nanomechanics. Alsothe effects of reduced phonon dimensionality in these struc-tures are a topic of considerable interest for future experi-ments.This work is supported by the Academy of Finland andthe NanoSciERA project “NanoFridge” of the EU. J.S.T. ac-knowledges partial support of Japan Science and TechnologyAgency through the CREST Project. We thank H. Im andT.F. Li for their assistance in fabrication of the devices.1A. N. 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B 77,033401 2008.26M. Meschke, W. Guichard, and J. P. Pekola, Nature London 444, 1872006.073101-3 Muhonen et al. Appl. Phys. Lett. 94, 073101 2009 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:130.233.216.27 On: Tue, 19 May 2015 09:45:53

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Electronic cooling of a submicron-sized metallic beam

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