Superconducting MgB2 thin films nano-bridges for cryo-electronic application
M.Gregor, R. Micunek, T. Plecenik, T. Roch, A. Lugstein, E. Bertagnolli, I.Vavra, M. Stefecka, M. Kubinec, M.Leporis, V. Gasparik, P. Kus, A. Plecenik
11 Department of Experimental Physics, FMPI C U, Mlynská dolina, 842 48 Bratislava, Slovak Republic, Biont a.s., K arloveská 63, 842 29 Bratislava, Slovak Republic, Institute for Sol id State E lectronics, TU - W ien, F lorag. 7, A-1040 Vienna, Austr ia Instit ute of Elect rical Engineer ing, Slovak A cademy of Sciences, Dúbravská cesta 9, Bratislava, Slovak Republic________________________________________________________________________ *Corresponding author: Maroš Gregor, Karlovesk á 63, 84 2 29 Brat islava, Slova kia Tel:+421 - -206 70 748 E - mail: maros.gregor@gm ail.com Superconducting Mg B thin film’s nano-bridges for cryo-electronic application M.Gregor , R. Micunek , T. Plecenik ,T. Roch , A. Lugstein , E. Bertagnolli , I.Vávra , M. Štefecka , M. Kub inec , M.Leporis , V. Gašp arík , P. Kúš , and A. Plecenik , Precursor
MgB thin f ilms wer e prepared on sapphire substrates by magne tron sputtering. Influence of e x- situ annealing process on supercon ducting MgB thin films roughness isdiscussed. Optimized annealing process of MgB precursor thin films in vacuum resultsin smoot h supercon ducting MgB thin films with roughness below 10 nm, critical temperature T con = 31 K and transit ion width ?T c less than 1 K. Nano- bridges based on the superco nducting MgB th in films using optica l and Focuse d Ion Beam lithography were prepared. Critical current density j c (4.2 K) measured on 50 nm wide strip was x10 A/cm and no significant loss of superco nducting properties w as detected. R esistance vs. temperature and critical cur rent vs. temperature character istics were measured on these structures using standard DC four probe measurement s . , supercon ductivity, nano - bridge, F IB Abstract
PACS:Key words:
Cryoelectronic appl ications of MgB supercond ucting thin films still promise multi lateral availing. Good stability, high critical temperature and low anisotro py of this materia l are advantage s in comparison with conventional supercon ductors. Since 2001 when superco nducting properties of MgB were discover ed, many technique s to prepare thin films were used . Promis ing results were obtained by molecular beam epitaxy [1], by co- depo sition of boron and magnesium from two resistive sources [2], pulsed laser depo sition [3] , magnetron sputtering [4] and sequential depos ition of boron and magnesium . The best results wer e obtained by p ulsed laser deposit ion [5] .Extensive efforts to realize Josephs on junction fabricat ion technology have been made worldwide. Several exce llent reviews on the problems of the Josephs on junction fabricat ion have been published , including planar -type [ ] and edge -type [ . In spite of tunnel - junction domination through out the history of the Josephson - effect studies, appreciable attention has been attracted to the non - tunnel - type junctions like weak links with variable thickness and width [ . There have been a num ber of reports on the fabricat ion of various types of MgB weak links, including point - contact or break junctions [ ], nano - bridges, sandwich - type tunnel junctions [ ], planar junctions by loca lized ion damage in thin films [11] and ramp - type junctions [ In most cases the roughnes s of superco nducting MgB thin films and depressio n of superco nducting properties in tens nanometer scale de limit the ir applicat ion for preparation of sub - micro and na no - structures. Introduction
In this paper we focus our attention on preparation of MgB thin films with roughnes s below 10 nm and production of nano - bridges by optical lit h ography and Focus ed Ion Beam (FIB) technique . The ma in accentuation was put on the influence of dimensionality decreas ing on the superco nducting properties of nano -bridges. T he properties of MgB thin films and nano - bridges were inspected by Atomic Force Microscop y (AFM), Trans ition Electron Microscopy (TEM) and DC four probe measureme nts of resistance vs. temperature (R-T) and critical cur rent vs. temperature (I c - T) character istics. Preparat ion of smooth supercon ducting MgB thin films and nano-bridges with non - depressed superco nducting properties su itable for preparat ion of e.g. Superco nducting Quantu m Interferometer (SQUID), Superconducting Single Photon Detectors (SSPD), et c. are presented.
Superconducting MgB thin films were prepared by magnetro n sputtering and ex-situ annealing in argon atmos phere and in vacuum. B efore MgB thin film preparation , the sapp hire substrates were chemica lly clean ed in acetone , isopropylalcohol and dist illed water, and were inserted into vacuum chamber on a rotating holder. Then d eposition process was realized from two circular magnetrons with diameter 5 cm. The boron magnetron was fed by RF power supply and the magnesium magne tron was fed by DC power supply. The vacuum chamber was evacuated to overal l pressure - Pa and consecu tively filled with a rgon to working pressure 7 .4x10 - Pa. Boron was deposited at the power 250 W and magnesium was deposited at the power 20 W. The whole
Experiment al depo sition time was 40 minutes and the thickness of MgB precursor thin films was 300 nm. Two ex-situ anneal ing processes w ere realized. The first one was an ex - situ annealing in tubing vacuum furnace. MgB precursor samples were inserted to quartz tu be,and the quartz tube was evacuated to pressure 200 Pa and scavenged by a rgon (Ar) severa l times. After th is procedure the tube was filled with Ar to atmosp heric pressure and inserted into furnace. Optim ization of annealing process was done in temperatur e range from ° C to ° C and time r ange from m inutes to 2.5 minutes [ ]. The second ex - situ annealing process was performe d in high - vacuum chamber at pressure 1x10 - Pa.
MgB precursor thin films were inserted to annealing vacuum chamber with Boraletric heater. Optim ization of the annealing temperature w as done at temperature range of 700 – ° C and anneal ing time between 2 -
4 min. In order to prepare well def ined nanostruct ures, it is necessary to obtain very low surface roughness of prepared t hin films. From this point of view, post - depo sition anneal ing process seems to be the most critical one . Thus, an optimiz ing process was done to determine the smallest roughness a nd the highest crit ical temperature T c . Atomic Force Microscope ( NT - MD T So lver P47 ) has been used for surface roughnes s analysis. AFM scanning was operat ed in semi - contact mode using conventional Si tip with high reflectivity Au coat ing on back side of canti lever for better reflection of laser control system. The maximum latera l scan range of the piezo- tube is 50 × 50 µm , the maximum vertical range is 2 µm. Typica l curvature radius of the tip is 10 nm, and the resona nt frequency is 255 kHz. On MgB thin films prepared using in vacuum anneal ing process ,
5 and 10 µm wide strips we prepared by optical litography. For determination of the micro structures the Ar + ion etch ing with ener gy of ions 6 00 eV and ion current density about 130 µA/cm generated by ion gun PLA TAR Klan 53M was used . For FIB machin ing, the MgB micro-structructur es were coat ed with 20 nm Au film in vacuum chamber , and on such samples the FIB lithography was applied.FIB experiments were carried out using the Micrion twin lens FIB system (model 2500) equipped with a Ga liquid meta l ion source . The system was opera ted at an accele ration voltage of 50 kV with selectable 25 µm -beam- limiting aperture correspon ding to a beam current of 20 pA. For pattern ing, the ion beam was raster scanned in a digital scan mode over a defined milling box. The scanning strategy wa s enco ded by the choice of the distance between the pixe ls, as we ll as the t ime each p ixel is expo sed to t he ion beam.After FIB milling addit ional Ar + ion beam etching was applied, in order to remove the Au coating from the strip. On these structures, resistance vs. temperature (R-T) and cr itical current vs. temperature ( I c - T ) character istics were measured using standard DC four probe mea sureme nts in transport He Dewar container.The structure of MgB films was invest igated by TEM (JEO L 200FX) on cross -section al spec imens prepared by both side ion (Ar) beam (5keV) m illing in the Gattan (PIPS equipment.
The m icrostructure of the films was inspected at acce lerating voltage 200 kV.
MgB precursor th in films with th ickness 300 nm were prepared by co - deposition from two magnetrons. Prepared thin films exhibit smooth surface with good adhesion.The ex- situ anneal ing process of MgB precursor thin films at temperature 680 ° C for 1.5 min in Ar atmosphere results in superco nducting MgB thin films with T con about 33 .7 K and zero resistance c ritical temperature T c0 about 32 K (Fig. 1a). The other way of ex- situ annealing process at temperature 850 ° C for 3 min in vacuum results in supercond ucting
MgB thin films with T con about 31. K and zero resistance critical temperature T c0 about K (Fig. ). Difference s in transit ion temperatures to superco nducting state are not substantia l, howeve r , the main d ifference is in the quality of the surface. MgB thin films annealed in argon atmos phere exhibit grainy surface with peak - to- peak value of z coordinate up to 260 nm (Fig. 2a, Tab. 1). Even if critical temperature T c0 is high, such MgB thin films are not suitable for preparat ion of nanostruct ures because of their high roughness. MgB thin films annealed in vacuum exhibit three t imes lower roughness than thatones annealed in argon (Fig. b). For additiona l decreas ing of roughness of such thin films addit ional anneal ing in Ar + ion beam was appl ied. More than twice addit ional decreasing of the roughness was achieved. For comparison of the surface roughness of the MgB thin films anneal ed in Aratmos phere and in vacuum , two different methods w ere used. The first one describe s maximal peak -to- peak posit ion of z coordinates ( Tab. 1). Becau se of existence of random failure on the surface , the peak-to- peak method do es no t describe adequate ly the real surface roughness. Fo r this reason , a secon d statistica l processing of the image surface Result s and di scussion was applied (using “GRAIN ANAL YSIS” software included in AFM menu). From experimental data the value of the standard deviat ion for z- coordinate (Root Mean Square) on the sample surface with in the area was calcu lated as: ),(1 Results of the surfaces annealed in Ar atmosp here, in vacuum and in vacuum with additional A r + etching during 30 min a re shown in T able 1.As one can see, conventional annealing in argon atmos phere is highly unsuitable for sub - micrometer scale manufacturing because R q is about 29.3 nm . Afte r anneal ing in high vacuum R q significant ly decrease s on the value lower than nm , and lower than 1.5 nm after additional Ar + ion etching (Tab. 1).For a complex description of the th in film microstructure the TEM ana lysis was performed in cross- sectional TEM specimens. Mg B thin films prepared by both meth ods exhibit nanocrystall ine MgB phase embeded in amorp hous phase. The MgB crystal lite size is varied from severa l nanometers to severa l tens of nanometers. The 011 prefe rentia l orientation of MgB in the normal to the film plane direction is observed. Electron diffract ion exhibits also lines indicat ing existence of randomly oriented MgO inside the film (Fig. 3). However in MgB thin films annealed in vacuum, the MgO - diffract ion spots are suff iciently weaker and exhibit hom ogenou s crystall ine structure with the size of crystall ites up to ten nano meters. The cross - sectional TEM micrographs of MgB thin film annealed in vacuum are shown on Fig.4a,b,c. Because no significant differences of superco nducting parameters (T con and ?T c ) between both types of MgB thin films were ( ) (cid:229) (cid:229) = = -= x y N i Nj mean y xq zj i zNNR observed, the secon d type was chosen for preparation of nanostruct ures, because of its lower roughness. 20 - µm- wide strips were prepared by optica l lithography on MgB thin films. Critical current density j c (4.2 K) of th ese strip s was higher than 5x A/cm . On these microstrips the FIB machin ing was appl ied. FIB cuts comprise two 10 µm long and . focused ion beam in a mult iscan raster mode with a p ixel spacing of 10 nm, dwel l time of 5 µs, and an ion dose of 0.8 nC/µm . The spac ing between the trenches and the over lap were varied , in order to ach ieve nano -bridges rang ing from 50x50 nm up to 15
150 nm. The patterns were generated outside the field of view to avo id any unintentional ion expos ure by imaging. All experiments were carried out under ambient conditions in a clean room environment. R- T, V-I and I c -T character istics of nano - bridges were measured using standa rd DC four probe measureme nts in transport He Dewar container. Figure 6a show s R -T dependence of 50 nm MgB bridge. No cruc ial changes of critical temperature of MgB were observed before and after machining by FIB. Critical current of 50 nm bridge was 0.73 mA (Fig. and a critical cur rent density j c (4.2 K) was 7. A/cm . This value of critical current density is a little higher than in microbr idge. One can expla in th is phen omenon by decreasing of para llel and seri es wea k link connections between n anocry stallites inside the nano -bridge . From above mentioned results we can conclude that the surface roughne ss of
MgB thin films strongly depend s on annealing condition s . MgB thin films annealed in vacuum exhibit roughness below 10 nm and supercon ducting properties of these thin films were not significantly dec reased in the bridge of 50 nm scale. The great difference in the surface roughness was observed when using twodifferent ways of annealing. MgB thin films anneal ed in argon atmosphere exhibit surface roughness higher than 100 nm and they are unsuitable for sub - micrometer scale manufacturing. Anneal ing of M gB thin films in vacuum results in superconducting MgB thin films with surface roughness below 10 nm and critical temperatu re T c ~ 31 K, ? T c < 1 K and critical current density j c (4.2 K) A/cm . On such type of thin films , the testing nano - bridges were fabr icated by Focus Ion Beam. Critical curr ent density j c (4.2 K) measured on 50 nm wide strip was x10 A/cm , critical current T con = 25 K and ?Tc = 1.5 K. This work w as supp orted b y the APVT projects No. APV T- -016604 , APV T- -011804 and project aAV/112 6/2004 . [1] Y.Harada, M.Uduka, Y.Nakan ishi, N.Yashimoto, and M.Yoshizawa, Synthesi s of as -grown superco nducting Mg B thin f ilms by molecular beam epitaxy in UHV conditions, Phy sica C 4 12 -
414 (2004) 1383.
Conclusion
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Mg O (ove rlapping re flex) dark field imag e reveals the Mg B and MgO crysta llites distr ibution in the f ilmc) 101
MgB2 dark field image revea ls the M gB crysta llites form and their distr ibution in film. Fig.5 AF M image of the 50 nm Mg B nanobridge prepared by FIB mi lling. Figure captions Fig.6 a) Resistance vs. t emperature and b) crit ical current vs. temperature dependence s of the 50 nm wide str ip prepared by FIB ( T con = 25 K, ? T c = 1.5 K).Table1 Comparison of the surface quality of MgB annealed in Ar atmosphere and in vacuum. Table c aptions Fig.1aGregor et. al Fig.1bGregor et. al Fig.2 a Gregor et. al Fig.2 b Gregor et. al Fig.2 c Gregor et. al Fig.3Gregor et. al Fig.4aGregor et. al Fig.4bGregor et. al Fig.4cGregor et. al Fig.5Gregor et. al Fig.6 a Gregor et. al Fig.6 b Gregor et. a l6Sample
Ar atmosp here
Vacuum Vaccum +Ar etchPeak-to- peak, R max qq