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Development of Superconducting Tunnel Junctions with an Aluminum-Oxide Insulation Layer for X-ray Detection

H. Sato, T. Ikeda, H. Kato, K. Kawai, H. Miyasaka, T. Oku, W. Ootani, C. Otani, H.M. Shimizu and H. Watanabe The Institute of Physical and Chemical Research (RIKEN), 2-1 Hirosawa, Wako, Saitama 351-0198, Japan

H. Nakagawa, H. Akoh and M. Aoyagi

Electrotechnical Laboratory (ETL), 1-1-4 Umezono, Tsukuba, Ibaraki 305-8568, Japan

T. Taino

Department of Nuclear Engineering, Kyushu University, Hakozaki, Fukuoka 812-8581, Japan

K. Inaba and Y. Kino

X-ray Research Laboratory, Rigaku corporation, 3-9-12 Matsubara-cho, Akishima, Tokyo 196, Japan

Abstract|Superconducting tunnel junctions (STJ), with a buer layer between the silicon substrate and junction, are being developed for use as highresolution x-ray detectors. Aluminum-oxide (Al2 O3 ) is employed as the buer layer in order to suppress the phonon mediated background from the Si substrate. The extent of phonon insulation was studied by measuring the x-ray spectra of STJs as a function of with buer layer thickness. The phonon insulation ability of Al2 O3 was compared to that of magnesiumoxide. The Al2 O3 layer should be a good phonon insulator, with the ability to suppress phonons with a single buer layer. I. Introduction

Fabrication techniques of Nb-based superconducting tunnel junctions (STJ) on Si substrates [1]{[2] have been applied to the development of STJ-based x-ray detectors. At present, high quality junctions showing the BCS0temperature dependence of the subgap current until 10 6 at 0.35 K (normalized by the value obtained at 4.2 K) have been developed [3]{[5] and an energy resolution of 93 eV for 5.9 keV x-rays has been achieved using them [6]. Since the Nb layer of the STJ was 150 nm thick, 95 % of the 5.9 keV incident x-rays transmitted through the Nb layers and reached the substrate. Large tails in the pulse height spectra were mediated by phonons induced by xrays absorbed in the substrate [6]{[7]. A magnesium-oxide (MgO) layer was tested as an etch-stop and a phonon insulation layer. The resultant insulation was not complete Manuscript received September 15, 1998. H. Sato, +81-48-467-9720, fax +81-48-467-9721, hsato@postman. riken.go.jp. This study was performed through Special Coordination Funds for promoting Science and Technology Agency of the Japanese Government. This work was supported in part by Grant-in-Aid for Encouragement of Young Scientists of the Ministry of Education, Science, Sports and Culture under the program numbers 10740136, 08554002 and 09490038.

[6]{[7]. More ecient insulation materials are necessary for a more detailed study of the response to x-rays. The phonon suppression becomes more important in the construction of an integrated x-ray detector system, such as when combining an STJ x-ray detector with the superconducting readout electronics such as preampli er, analog-to-digital converter and so on. The readout electronics would be required to process x-ray signals reliably in the presence of a radiation eld. It is therefore desirable that the phonon insulation between junctions and the substrate is as complete as achievable, while remaining consistent with the fabrication techniques and the thermal balance required to keep the junction at a suciently low temperature. Poelart et al. reported that a double buer layer of SiO2 and AlOx suppresses the phonon coupling between the substrate and junction, and that amorphous quartz is the best material for the intrinsic suppression of phonons produced in the substrate [8]. A single insulation layer on a Si substrate is a convenient simpli cation of the fabrication process. We have examined the characteristics of an aluminum-oxide (Al2 O3) layer on the Si substrate since the Al2O3 has stable properties and can be used as an etch-stop layer during the etching of the Nb layers. In this article, we introduce the fabrication process of Nb-based STJ using an Al2 O3 buer layer and discuss the phonon insulation ability of the Al2O3 buer layer. II. Fabrication of STJ A. Process

STJs were fabricated following a procedure modi ed from the method reported by Joosse et al. [4]{[5]. An Al2 O3 layer of 20 to 100 nm thickness was deposited by rf magnetron sputtering on a sputter cleaned Si substrate. Next, a 150 nm thick Nb underlayer (UL) was deposited by dc magnetron sputtering, and structured to the base electrode (BE) dimensions by reactive ion etching (RIE) in a CF4 plasma. At this time, the Al2 O3 buer layer acted as an etch-stop layer. The main role

of the UL is to release the stress occurring in the Nb(50 nm)/ Al(10 nm)-AlOx / Nb(150 nm) trilayer [1], which was subsequently produced in a whole-wafer process using dc magnetron sputtering for all the layers. The AlOx barrier was formed by room-temperature oxidation of the Al layer immediately after the deposition and without removing the wafer from the vacuum chamber. The O2 pressure was 133 Pa and the oxidation time was 1 hour. These parameters were selected 0to2 give a critical current density (Jc ) of around 150 A1cm . In the early stages of development, in order to de ne and isolate the STJs, the BE was structured rst and the counter electrode (CE) was de ned second [4]{[5]. Recently, we have modi ed the process follows; the CE is de ned by RIE in CF4 using the AlOx barrier as the etch-stop layer, and next, the BE is structured by RIE. The advantage of this process is that the Al2 O3 buer layer isn't damaged by developer during the photoresist development for the CE patterning, and also the edge of the barrier is well de ned as the exposure time to the etch gas is reduced. The entire STJ was covered with a 350 nm thick electrically-insulating SiO2 layer, formed by sputter deposition. Next, holes for the electrical contact to the BE and CE were etched into the SiO2 by CF4 RIE and by electron cyclotron resonance (ECR) plasma etching in a mixture of H2 and CF4 . The latter process provides highly selective etching of SiO2 compared to Nb (the ratio of etching rate of SiO2 to Nb was 7:1). Finally, a 600 nm thick Nb wiring layer was deposited by sputtering and etched by RIE into current leads of 2 m width. Throughout this process, an i-line stepper was used for photolithographic patterning, which enabled the de nition of ne patterns in the photoresist. The schematic cross section of an STJ is shown in Fig. 1. B. Properties of the STJ

The properties of the STJs were measured at temperatures 4of 4.2 K and 1.5 K using liquid 4He and pumped liquid He, respectively. From an analysis of the current-voltage characteristics at 4.2 K, most of the STJs have energy subgap voltages (Vg ) of  2.9 mV and critical current densities (Jc ) of 150 A1cm02 which indicates that Jc was well controlled by the oxidation procedure. The quality parameter Vm

(the product of Ic and the subgap resistance at 2 mV) was around 80 mV. The decrease in the temperature-dependent subgap current, Isg , at 0.5 mV was used as a measure of the quality of the STJs.pAccording to the BCS theory, the Isg is proportional to T exp (01=kT ) where T is temperature, 1 the energy gap of the superconductor and k the Boltzmann constant. Deviation from the theoretical value indicates that physical defects or microshorts exist in the barrier of the junction. The values of Isg measured at 1.5 K were several times 1004 relative to those measured at 4.2 K,and were therefore consistent with the BCS theory. From these characteristics, we regard the fabrication process as ideal for the production of STJs to be used as X-ray detectors. III. Experiments

The extent of phonon suppression by the Al2 O3 buer layer was studied by measuring the x-ray spectra obtained from STJs with Al2 O3 layers of various thickness. A. Experimental Setup

A 5 mm 2 5 mm STJ chip consisting a number of STJs was mounted on a gilded copper plate. The plate was attached to a copper cold stage of a cryostat (Infrared HDL-8) and refrigerated to 0.35 K by using liquid 3 He. Two coils, placed by the cryostat, were used to apply a magnetic eld of about 10 mT55to suppress the dc Josephson current. X-rays from a Fe source irradiated the STJ through a Be window in the cryostat. The output of the x-ray detector was passed to a multi channel analyzer (SEIKO EG&G MCA7700) via a charge sensitive preampli er (Canberra 2003BT), a low pass lter, a pulse transformer and a shaping ampli er (ORTEC 142C), all operating at room temperature. The setup is shown in Fig. 2. For these experiments, four types of STJs were prepared; three STJs with Al2O3 buer layers of 20 nm, 50 nm, and 100 nm thickness, and one STJ having 20 nm thick MgO buer layer deposited by thermal evaporation L4He heat switch

LN 2

(a)

(b) (c)

(e)

(g)

preamp.

(d)

(f)

low pass filter heat switch

pulse transformer shaping amp.

L3He signal

55

Fe

pad

junction Si substrate

MCA

pad Be window

Fig. 1. Schematic cross section of STJ. (a) Al2 O3 , (b) Nb UL, (c) Nb BE, (d) Al/AlOx , (e) Nb CE, (f) SiO2 , (g) Nb wire.

STJ on chip

charcoal pump

Fig. 2. Experimental setup of the x-ray spectrometer. A magnetic eld was applied perpendicular to the page by two coils (not shown).

[4]{[5]. All of the STJs were 100 m 2 100 m in size. The characteristics of the STJs are listed in Table I.

0.05 1 (MgO 20 nm)

0.04

The x-ray spectra were summed over 5 channels and normalized to yields from the junction. The corrected spectra are shown in Fig. 3 in witch the phonon signals appear as a rising edge on the left of each spectrum. From spectra 1 (MgO 20 nm) and 2 (Al2 O3 20 nm), it is clear that Al2 O3 is better than MgO as a buer layer, which justi es the selection of Al2 O3 as a buer layer in our new fabrication process. Spectra 2, 3 and 4 shows that the rising edge shifts to the left with increasing Al2 O3 buer layer thickness. This result shows that the Al2 O3 buer layer suppress as the phonon signals. In the case of spectrum 4 (Al2 O3 100 nm), the start of the rising edge corresponds to an energy of 0.56 keV, which indicates clear measurement of x-rays over 0.56 keV is possible. The thickness dependence of the phonon suppression gives the phonon attenuation depth  of Al2O3. The rising points in the spectra were plotted as a function of the Al2O3 thickness and tted using an exponential curve. As a result, =80 nm is obtained. This value indicates that the Al2O3 layer has a shorter mean free path than that of AlOx, but longer than that of SiO2 calculated in [8]. Although the phonon suppression in Al2O3 is not as large as that of SiO2 , Al2 O3 is suitable as a single buer layer as it can be used as an etch-stop layer, whereas SiO2 requires another layer above it in order to protect it during etching. The single buer layer is advantageous for fabricating STJs not only for simplifying the fabrication process but also for obtaining a reliable tunnel barrier to avoid additional accumulation of surface roughness of the layers. The technique for phonon suppression using an Al2 O3 buer layer on the Si substrate is also applicable to the fabrication of electric circuits requiring stable operation in a radiation eld. IV. Summary

High quality Nb-based STJs, with an Al2O3 buer layer, have been fabricated. The degree of phonon insulation from the substrate has been tested using STJs with Al2O3 buer layers of dierent thickness. Analysis of x-ray spectra from the STJs indicates that Al2 O3 works TABLE I CHARACTERISTICS OF THE STJS STJ Buer Thickness Jc Isg a No. layer (nm) (A cm02 ) at 0.35 K 1 MgO 20 196 1.4 1004 2 Al2 O3 20 154 6.8 1005 3 Al2 O3 50 121 3.4 1004 4 Al2 O3 100 203 6.2 1004 a Normalized by the value obtained at 4.2 K. b Dynamic resistance measured at 0.35 K. c Normal resistance measured at 0.35 K. 1

2

2

2

2

Rd b (k ) 15 39 6.4 2.3

Rn c ( ) 0.12 0.12 0.15 0.09

Counts (a.u.)

B. Results and Discussions

2 (Al2O3 20 nm)

0.03

55

Fe Kα (5.9 keV)

3 (Al2O3 50 nm)

0.02

4 (Al2O3 100 nm)

0.01

55

Fe Kβ (6.5 keV)

0 0

0.2

0.4

0.6 0.8 1 Pulse Height (a.u.)

1.2

1.4

Fig. 3. X-ray spectra obtained from four STJs. The numbers in the gure indicate the STJ No. in Table I. The arrows show the tail of the phonon signals.

well as a phonon insulation layer. To summarize, Al2O3 is a good material to reduce the phonon induced signal from a Si substrate, and it should be noted that a single Al2 O3 layer combines the role of a phonon insulation layer with that of an etch-stop layer.

Acknowledgment

H.S., T.I., and W.O. are grateful to Special Postdoctoral Researchers Program for support of this research.

References

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