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Electrochimica Acta 51 (2006) 1745–1751 EIS study of niobium films sputtered at different target–substrate angles S. Cattarin a,1 , M. Musiani a,,1 , V. Palmieri b , D. Tonini b,c a Istituto per l’Energetica e le Interfasi, CNR, C.so Stati Uniti 4, 35127 Padova, Italy b Laboratori Nazionali di Legnaro, INFN, Viale dell’Universit` a 2, 35020 Legnaro (PD), Italy c INFM, UdR Padova, Via Marzolo 8, 35131 Padova, Italy Received 23 June 2004; received in revised form 27 October 2004; accepted 9 February 2005 Available online 29 August 2005 Abstract Nb films have been magnetron sputtered onto quartz sheets oriented with respect to the target at angles varying between 0 and 90 , with 15 steps. Impedance plots have been obtained by contacting these films with aqueous Na 2 SO 4 , either at the open circuit potential or at a potential where Nb is covered by an anodic passive Nb 2 O 5 film. As the target–substrate angle θ increases, the shape of the impedance plots changes from that of a smooth electrode to that of a porous one, characterised in the high frequency range by a straight line forming a 45 angle with the real axis. The surface roughness of the Nb deposits, calculated from their double layer capacity, is low and constant at low θ, significantly increases at θ = 45 , goes through a maximum in the range 60–75 and drops at θ = 90 . AFM surface profiling confirms this trend, but estimates a lower surface roughness. Attempts to obtain Nb deposits with a surface roughness less strongly dependent on θ have been made by performing the depositions under pulsed conditions or by heating the substrates at 400–600 C. Heating at the higher temperature was a fairly effective method for decreasing the roughness of deposits formed at large θ. © 2005 Elsevier Ltd. All rights reserved. Keywords: Impedance; Magnetron sputtering; Porous electrodes; Radio-frequency resonators; Superconducting cavities; Surface roughness 1. Introduction A standard way to fabricate superconducting radio- frequency cavities for particle accelerators employs bulk niobium which is a superconductor below 9.25 K. The use of superconducting materials allows the generation of very large electric fields, with a limited electric power consumption. The energy balance is very favourable, even if superconduct- ing cavities must operate at the liquid He temperature. An approach alternative to the use of bulk Nb consists in sputter- ing Nb thin films onto oxygen-free high conductivity Cu, with both economical and technological advantages due to the low cost, high thermal conductivity and good mechanical strength of Cu [1]. In this technology, Nb is sputtered onto the inner Corresponding author. Tel.: +39 049 8295866. E-mail address: [email protected] (M. Musiani). 1 ISE member. cavity walls from a coaxial cylindrical target. Fig. 1 shows that, due to the shape of the cavities, Nb atoms impinge the Cu substrate surface under variable target–substrate angles θ (defined as the angle formed by the impinging atoms beam and the perpendicular to the surface), after travelling across variable distances. In fact, due to the non-negligible lateral dimension of the plasma, for each point of the cavity wall there is a certain distribution of the θ angles. There is some evidence that the superconducting prop- erties of the Nb deposit depend on θ, being the best when θ =0 and progressively deteriorating as θ increases from 0 to 90 ; a rather critical transition has been reported to occur at ca. 60 and tentatively ascribed to an increase in the deposit roughness and a corresponding decrease in its homogeneity [2]. However, no data on the roughness of Nb sputtered films as a function of θ have been reported and, in addition, no attempts have been made to separate the effect of θ on the film properties from that of the target–substrate distance. 0013-4686/$ – see front matter © 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2005.02.126

EIS study of niobium films sputtered at different target–substrate angles

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Page 1: EIS study of niobium films sputtered at different target–substrate angles

Electrochimica Acta 51 (2006) 1745–1751

EIS study of niobium films sputtered atdifferent target–substrate angles

S. Cattarina,1, M. Musiania,∗,1, V. Palmierib, D. Toninib,c

a Istituto per l’Energetica e le Interfasi, CNR, C.so Stati Uniti 4, 35127 Padova, Italyb Laboratori Nazionali di Legnaro, INFN, Viale dell’Universita 2, 35020 Legnaro (PD), Italy

c INFM, UdR Padova, Via Marzolo 8, 35131 Padova, Italy

Received 23 June 2004; received in revised form 27 October 2004; accepted 9 February 2005Available online 29 August 2005

Abstract

Nb films have been magnetron sputtered onto quartz sheets oriented with respect to the target at angles varying between 0◦ and 90◦, with15◦ steps. Impedance plots have been obtained by contacting these films with aqueous Na2SO4, either at the open circuit potential or at apotential where Nb is covered by an anodic passive NbO film. As the target–substrate angleθ increases, the shape of the impedance plotsc rming a 45a nt at lows d,b nm urew©

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hanges from that of a smooth electrode to that of a porous one, characterised in the high frequency range by a straight line fo◦

ngle with the real axis. The surface roughness of the Nb deposits, calculated from their double layer capacity, is low and constaθ,ignificantly increases atθ = 45◦, goes through a maximum in the range 60–75◦ and drops atθ = 90◦. AFM surface profiling confirms this trenut estimates a lower surface roughness. Attempts to obtain Nb deposits with a surface roughness less strongly dependent onθ have beeade by performing the depositions under pulsed conditions or by heating the substrates at 400–600◦C. Heating at the higher temperatas a fairly effective method for decreasing the roughness of deposits formed at largeθ.2005 Elsevier Ltd. All rights reserved.

eywords: Impedance; Magnetron sputtering; Porous electrodes; Radio-frequency resonators; Superconducting cavities; Surface roughness

. Introduction

A standard way to fabricate superconducting radio-requency cavities for particle accelerators employs bulkiobium which is a superconductor below 9.25 K. The use ofuperconducting materials allows the generation of very largelectric fields, with a limited electric power consumption.he energy balance is very favourable, even if superconduct-

ng cavities must operate at the liquid He temperature. Anpproach alternative to the use of bulk Nb consists in sputter-

ng Nb thin films onto oxygen-free high conductivity Cu, withoth economical and technological advantages due to the lowost, high thermal conductivity and good mechanical strengthf Cu [1]. In this technology, Nb is sputtered onto the inner

∗ Corresponding author. Tel.: +39 049 8295866.E-mail address: [email protected] (M. Musiani).

1 ISE member.

cavity walls from a coaxial cylindrical target.Fig. 1 showsthat, due to the shape of the cavities, Nb atoms impingCu substrate surface under variable target–substrate anθ(defined as the angle formed by the impinging atoms band the perpendicular to the surface), after travelling acvariable distances. In fact, due to the non-negligible ladimension of the plasma, for each point of the cavity wthere is a certain distribution of theθ angles.

There is some evidence that the superconducting perties of the Nb deposit depend onθ, being the best wheθ = 0◦ and progressively deteriorating asθ increases from 0◦to 90◦; a rather critical transition has been reported to occca. 60◦ and tentatively ascribed to an increase in the deroughness and a corresponding decrease in its homog[2]. However, no data on the roughness of Nb sputteredas a function ofθ have been reported and, in addition,attempts have been made to separate the effect ofθ on thefilm properties from that of the target–substrate distance

013-4686/$ – see front matter © 2005 Elsevier Ltd. All rights reserved.oi:10.1016/j.electacta.2005.02.126

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1746 S. Cattarin et al. / Electrochimica Acta 51 (2006) 1745–1751

Fig. 1. Schematic representation of the geometry of Nb deposition onto theinner wall of a resonating cavity by magnetron sputtering.

In the present study, aimed at quantifying the influence ofθ on the surface roughness of sputtered films, Nb depositswere obtained under different target–substrate angles and aquasi constant target–substrate distance, trying to avoid localvariations in the deposit thickness and properties along thedeposit. Then, these samples were submitted to EIS in an inertelectrolyte, and their roughness was estimated from the valueof their double layer capacity assumed to be proportional totheir effective area. The advantage of using EIS for measur-ing surface roughness is that the electrolyte deeply penetratesinto crevices and pores, so that the whole electrode surfaceis sampled, provided a sufficiently wide frequency range isexplored. Indeed, the lower the frequency, the deeper and lessopen are the pores that are probed. Instead, other techniqueslike surface profiling may be blind with respect to the innersurface of tiny, tortuous pores, thus providing unreliable sur-face roughness values, generally below the real ones[3].

2. Experimental

2.1. Nb film preparation

The sample holder shown inFig. 2A was designed anddifferent versions of it were realized with different materialsa sam-p that essc sitionr on ofa per-t e realc thep nglesθ as inF

Fig. 2. (A) Sample holder designed to carry seven quartz sheets, allowingsimultaneous deposition of Nb films with seven different orientations, i.e.θ

angles varying from 0◦ to 90◦ with 15◦ steps. (B) Schematic representationof the Nb deposition on quartz substrates. The target–substrate angleθ isdefined.

Nb deposits were formed onto quartz sheets ultrasoni-cally cleaned with an alkaline detergent, rinsed with waterand dried in a N2 stream, to ensure homogeneous depo-sition and good adhesion; the quartz dimensions variedfrom 9 mm× 9 mm to 25 mm× 75 mm. The target–substrateholder distance was 70 mm; different sheets were positionedon the holder in such a way that their distance from the targetwas as independent ofθ as possible, and never >75 mm.

The depositions were performed in a cylindrical stainlesssteel vacuum chamber 280 mm high and with a diameterof 120 mm, equipped with a home made planar magnetron

nd variable dimensions. Such a holder can carry sevenles at a time, with seven different orientations, i.e. wiθngles varying from 0◦ (parallel to the target) to 90◦ (normal

o the target) with 15◦ steps. Thus, exactly the same proconditions (e.g. residual pressure in the chamber, depoate and deposition time) may be used in the preparatiseries of samples, so that the film morphology and pro

ies depended only on the substrate orientation. As for thavities, the Nb atoms originating from different points oflasma reach the different points of the substrate with anarrowly distributed around an average value definedig. 2B.

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S. Cattarin et al. / Electrochimica Acta 51 (2006) 1745–1751 1747

source, designed and produced at the INFN laboratory, with a2 in. wide high purity niobium target (residual resistivity ratio300, purchased from Tokyo Denkai). The chamber, connectedto a turbomolecular and a rotary vane pumping system, wasevacuated and then baked for 13 h at 200◦C until a resid-ual pressure <2.0× 10−6 Pa was reached. Argon was usedas process gas (pressure = 2.2× 10−1 Pa). The Nb sputteringwas carried out either in a continuous or in a pulsed mode,with a current of 1 A and a potential difference between tar-get and substrate of 400 V; in the pulsed mode, current wasswitched on and off at a frequency of 70 kHz and a 10% dutycycle. For some deposition runs, a substrate heating systemwas employed, so that the substrate temperature could be con-trolled. Unless it is differently specified, the quartz substrateswere at room temperature.

The Nb deposit thickness was measured on partiallymasked quartz sheets, with a Tencor Instruments Alpha-step200 profilometer. AFM images were obtained with a ParkScientific Instruments model CP equipment in the constantstrength mode. In order to obtain roughness values by surfaceprofiling, the AFM images were submitted to line analysis;eight lines (four horizontal and four vertical) were consideredfor each sample and the surface roughness was obtained asan average.

2.2. EIS measurements

ronicc t orI plesi lassc iatei wasp inst iw ous0 plew t areass ele ed asr innerw artzs cha onec caseo ectedt tancef olytew dedm plesw n toc xper-i ncyR face,d andH

3. Results and discussion

Fig. 3 compares the impedance plots obtained with Nbfilms deposited under different orientations:θ = 0◦, 45◦ and75◦. Nyquist plots are shown in parts A (θ = 0◦ and 45◦) and B(45◦ and 75◦) and Bode plots in part C (impedance modulusonly). These samples belong to a series prepared in a sin-gle batch, with the same deposition time, on large substrates(25 mm× 75 mm) with the magnetron operating under con-tinuous current conditions, and were all tested at their open

Fig. 3. Plots of the impedance of Nb films deposited onto 25 mm× 75 mmquartz sheets with different orientations. (A) Nyquist plots for films obtainedatθ = 0◦ and 45◦, (B) Nyquist plots for films obtained atθ = 45◦ and 75◦ and(C) Bode plots. The impedance spectra were recorded at the open circuitpotential in 0.2 M Na2SO4. Working electrode area 1.54 cm2.

Nb coated quartz sheets were provided with an electontact by soldering a Cu wire on their edge, with Ag painn. EIS measurements were made by mounting the samn a three-electrode cell consisting of a double-wall gylinder, open at both ends. A Viton O-ring of an approprnner diameter was fit to the bottom opening, the celllaced onto the sample to be tested and pressed agaith four screws. Once the cell was filled with aque.2 M Na2SO4, a reproducible circular area of the samas wet and acted as working electrode. The Nb deposiampled in the EIS experiments was 1.54 cm2 for large quartzubstrates, 0.28 cm2 for small ones. A saturated calomlectrode, inserted through the upper opening was useference electrode and a platinum wire, fastened to theall of the cell, as counterelectrode. Large Nb coated quheets (25 mm× 75 mm) were mounted in the cell in suway that the area exposed to the electrolyte was the

loser to the target during Nb sputtering because, in thef these large substrates, the deposit thickness is exp

o decrease significantly along the substrate as the disrom the target increases. The temperature of the electras controlled to 25◦C. Impedance spectra were recorostly at the open circuit potential. Some series of samere also tested at 4 V (SCE), i.e. at a potential knowause oxidation and passivation of Nb. Impedance ements were carried out with a Solartron 1254 Frequeesponse Analyser and a 1286 Electrochemical Interriven by a commercial software (Fracom by P. Bernard. Takenouti).

t

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1748 S. Cattarin et al. / Electrochimica Acta 51 (2006) 1745–1751

circuit potential (around−0.3 V versus SCE). The averageNb deposit thickness monotonically varied from ca. 1200 toca. 200 nm, asθ changed from 0◦ to 90◦.

In Fig. 3A, at sufficiently low frequency, both samplesshow an essentially capacitive response, but only the oneobtained atθ = 45◦ clearly shows at high frequency a straightline forming an angleα ∼= 45◦ with the real axis, as it isexpected for a rough (porous) electrode on the basis of boththeoretical and experimental evidences. Indeed, it was calcu-lated by de Levie[4–7] that the Nyquist plot of the impedanceof an electrode having semi-infinite cylindrical pores all equalin diameter, and polarized at a potential allowing no faradaicprocesses, consisted of a vertical line at low frequency and aline forming a 45◦ angle with the real axis at high frequency.The low frequency capacitive behaviour is observed whenthe frequency is low enough that the penetration depth ofthe ac signal is equal to the pore length, and corresponds tothe double layer charging of the whole inner pore/electrolyteinterface. According to Keddam and co-workers[8,9], deLevie’s calculation implies that the impedance of a cylindricalpore (or an assembly of identical cylindrical pores) is equalto that of a flat electrode of the same area as the developedpore(s) surface. Later, it was shown by various groups thatde Levie’s model for cylindrical pores satisfactorily accountsfor pores of complex shape too[8–10], that the pores may bedescribed as one-dimensional as long as their diameter/lengthr onc encyc 0o encyd rys yoe

h or as gh-n d bya uet andt essi sn lm isd acityw yf peo heri encyr um oft nde-p ront tacts,i com-p and,f thee

Fig. 4. Influence of the target–substrate angleθ on the high frequency resis-tance (A) and double layer capacity (B) of Nb films deposited with differentorientations. Two series of samples are compared, deposited with either aconstant deposition time (on 25 mm× 75 mm quartz sheets, electrode area1.54 cm2—full symbols) or a constant thickness (on 9 mm× 9 mm, electrodearea 0.28 cm2—empty symbols).

Plots of the high frequency resistance and double layercapacity as a function ofθ are shown inFig. 4, full symbols.The hf resistance remains essentially unchanged forθ

varying from 0◦ to 45◦, so that one may assume that it isentirely due to the electrolyte, then monotonically increasesup to 100 times. Such an increase is much larger than theone expected by calculating the Nb film resistance by thesecond Ohm’s law, assuming the value of 1.25× 10−6 � cmfor Nb resistivity and considering that film thicknessis homogeneous. Therefore, to explain the observed hfresistance–θ dependence, it may be speculated that eitherthe deposit formed by magnetron sputtering at largeθ has apeculiar structure causing a marked increase in its resistivityor the deposit thickness is very heterogeneous on the largesubstrates, becoming very low in positions remote from thetarget. One should also consider that atmospheric oxidationof Nb to its oxides may convert the outer part of the depositto a less conductive material, with an especially markedeffect on the resistance of very porous and thin films.

The double layer capacity is almost constant forθ ≤ 30◦,increases sharply to attain a maximum forθ = 60◦ and thendecreases again (Fig. 4B); the Nb deposit obtained at 90◦ hasa roughness comparable to that of the deposits obtained at low

atio is smaller than 1[11,12]and that a pore size distributiauses a decrease of the angle formed by the low frequapacitive line with the real axis to values lower than 9◦,wing to the fact that the penetration depth at each frequepends on the pore radius[13]. Experimental diagrams veimilar to the one shown inFig. 3A, θ = 45◦, were recentlbtained in our laboratory with porous PbO2 [14] and Ni[15]lectrodeposits.

Fig. 3A also shows that the sample deposited atθ = 45◦as a larger capacity (testified by a lower frequency fimilar value of Im(Z)) and therefore a larger surface rouess. Nb is known to be covered by an oxide film formetmospheric oxidation[16–18]. The observed capacity is d

o the series combination of the capacity of this oxidehe double layer capacity. As long as the oxide film thickns ≤1 nm, its capacity is≥40�F cm−2, i.e. its contribution iot dominating. For this reason, unless an anodic oxide fieliberately formed on Nb (see below), the electrode capill be called double layer capacity.Fig. 3B shows that b

urther increasingθ to 75◦, no significant change in the shaf the Nyquist plot is observed, but in addition to a furt

ncrease in capacity, a large increase in the high frequesistance occurs. This high frequency resistance is the swo components: the electrolyte resistance (obviously iendent ofθ) and the resistance of the Nb film to the elect

ransport between the electrolytic and the electronic conn a direction parallel to the substrate surface. The latteronent is inversely proportional to the deposit thickness

or very thin films, may become much higher than that oflectrolyte.

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S. Cattarin et al. / Electrochimica Acta 51 (2006) 1745–1751 1749

target–substrate angle. As a general trend, upon increasingθ

from 0◦ to 90◦, thinner and rougher Nb films are obtained.However, the low roughness of the deposit obtained at thelargestθ is striking and we considered the idea that it mightbe an artefact linked to its low thickness.

The following series of samples were all prepared onsmall substrates (9 mm× 9 mm), to minimise the problemsassociated with inhomogeneous thickness. Furthermore, inorder to eliminate the dependence of the deposit thicknesson θ, a series of samples was prepared on small substrates(9 mm× 9 mm) by simultaneously varyingθ and thedeposition time in such a way that the film thickness wasessentially the same (ca. 500 nm) at allθ. Of course, sucha procedure does not represent what happens when Nb issputtered onto the walls of a real cavity, in which case thedeposition time is identical for allθ. The Nyquist plotsobtained with these samples showed again the shape typicalof porous electrodes when and only whenθ was 60◦ or 75◦.The high frequency resistance and double layer capacitymeasured by EIS with the samples of this new series areplotted inFig. 4, empty symbols. As expected, a resistanceindependent ofθ is observed (Fig. 4A). The capacity plotshows that, in this second series too, the electrode roughnessgoes through a maximum whenθ is in the range 60–75◦, anda very low roughness is measured atθ = 90◦ (Fig. 4B). Rathersurprisingly, the maximum roughness obtained in this seriesi efi

s hasb thatN atedb tht o( easei e oft angeb ari V( nmt d at4ls penc -i e isc sim-i estst ursw . Ino filmi lyteit weentfi en-t .

Fig. 5. (A) Influence of the target–substrate angleθ on the capacity ofNb films deposited with different orientations and a constant thickness (on9 mm× 9 mm quartz sheets). Full symbols represent double layer capacitymeasured at open circuit, empty symbols represent the dielectric capacity ofthe oxide film formed at 4 V (SCE). (B) Comparison of the relative roughnessfactors measured by either EIS [at the open circuit potential (full squares)or at 4 V (SCE) (empty squares)] or AFM surface profiling (stars)[23].

The relative surface roughness values obtained by EIS maybe compared with those measured by AFM surface profil-ing [24] and reported inFig. 5B (stars). It may be seen that(i) the general shape of the curves obtained by both tech-niques is similar, (ii) AFM data confirm that the roughnessgoes through a maximum whenθ is in the range 60–75◦,then markedly decreases and (iii) for the samples of higherroughness, AFM provides lower roughness values than EIS,as expected, due to its lower ability to probe narrow, tortuouspores.

The shape of the capacity–θ curves may be explained asfollows. The mean free path of Nb atoms (λ), defined as thedistance a particle travels between two successive collisions,is given byλ = 1

πd2nv, whered is the atom diameter and

nv is the particle density. Since, according to the ideal gaslaw, nv = NAP

RT(whereNA is Avogadro’s number), the mean

free path isλ = RTπd2NAP

. Since Ar atomic diameter is about

0.35 nm andP ≈ 2× 10−1 Pa, under the adopted experimen-tal conditions,λ is of the order of 5 cm. However, light bumpsdo not noticeably deflect the trajectory of the particles whichcan normally travel over distances about 10 times larger themean free path before undergoing a significant change in

s significantly lower, for the sameθ values, than that of thrst series.

The impedance of the samples of this second serieeen measured also at a potential of 4 V. It is well knownb is a valve metal which, upon oxidation, becomes coy a Nb2O5 film the thickness of which varies linearly wi

he applied potential[16–22]. The Nb2O5 anodisation ratialso called formation ratio, and defined as the incrn the oxide film thickness caused by a 1 V increashe polarization potential) has been measured in the retween 3 and 4 nm V−1; taking into account that the line

ncrease in Nb2O5 thickness with potential starts around 0SCE)[23], it may be estimated that an oxide layer ca. 15hick is present at the surface of a Nb sample polarizeV. At this potential, the capacity of the dielectric Nb2O5

ayer dominates the impedance response.Fig. 5A shows aemi-logarithmic plot of the capacity measured either at oircuit or at 4 V, as a function ofθ. Clearly, oxidation diminshes significantly the capacity (the capacity of the oxida. 10 times lower than the double-layer capacity), but alar trend is shown by both curves. This similarity sugghat oxide formation at the Nb/electrolyte interface occithout major modifications of the deposit morphologyther words, the developed area of the 15 nm thick oxide

s proportional to the developed area of the Nb/electronterface. This conclusion is strengthened byFig. 5B in whichhe relative surface roughness (defined as the ratio bethe capacity of an Nb film sputtered at a genericθ and that of alm obtained atθ = 0◦) measured at either open circuit potial or 4 V is compared and found to follow similar trends

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1750 S. Cattarin et al. / Electrochimica Acta 51 (2006) 1745–1751

direction, so that the so-called mean deflection path is ca.50 cm, i.e. much larger than the target–substrate distance.Therefore, most Nb atoms impinge the substrate withoutundergoing preliminary deflections, and only a minority aftereffectively colliding with other atoms. While the former travelalong a straight line, the latter may be assumed, in a firstapproximation, to reach the substrate with random orienta-tion. It may be also assumed that all impinging atoms stickon the substrate position which they hit, without significantsurface diffusion. If one considers the atoms undergoing nodeflections, it is rather obvious that, upon increasingθ, the“shadow” produced by protruding atom clusters adhering tothe substrate extends over larger areas, thus preventing fur-ther deposition and inducing the formation of less compact,i.e. more porous structures[25,26]. Such a shadowing effectis not important for atoms reaching the substrate at randomorientation. For geometric reasons, the number of atoms perunit area and per unit time impinging the surface after trav-elling along a straight line must decrease whenθ increases,while those reaching the substrate after undergoing one ormore deflections is expected to depend onθ less markedly.Therefore, it is possible that most of the Nb deposit formedon substrates oriented at 90◦ is due to atoms impinging witha random orientation, which should produce compact struc-tures of low porosity.

Attempts to achieve Nb films of properties less stronglyd ag-n Botha on oft r bya singt essoθ f Nbs larityo ta e Nbfi idea

F c-i mptys

Fig. 7. Influence of the target–substrate angleθ on the double layer capacity(full symbols) and high frequency resistance (empty symbols) measured withNb deposits produced by heating the quartz substrates at 400 or 600◦C, asindicated on the figure. Working electrode area 0.28 cm2.

that the Nb atoms tend to remain fixed in the positions wherethey hit the substrate.

Other series of samples were obtained by depositing Nbfilms onto small quartz substrates heated at 400 or 600◦C.The impedance response of samples prepared at 400◦C wasthat of smooth electrodes forθ = 0–45◦ and that of porouselectrodes forθ = 60–90◦. At 600◦C, the porous electrodebehaviour was detected only forθ = 75◦C.Fig. 7summarisesthe dependence of double layer capacity and high frequencyresistance onθ. The formation of porous deposits at largetarget–substrate angles is observed in these series too; thecapacity values of the 400◦C series are comparable to thoseobtained on substrates of the same size maintained at roomtemperature (Fig. 4B, empty symbols), but heating at 600◦Cis effective in reducing the tendency to form porous deposits.A roughness decrease is still observed atθ = 90◦, thoughless evident than in other series. In spite of the fact that thedeposits were produced with a constant deposition time, andtherefore are expected to have a thickness slightly variablewith θ, only a moderate variation of the high frequencyresistance is observed (detected also inFig. 4A, emptysymbols).

4. Conclusions

t Nbfi rouswm theN ed toa t theo ositsh erents ono oge-n t alsow s them hness

ependent onθ were made either by operating the metron in pulsed conditions or by heating the substrates.pproaches were expected to favour the surface diffusi

he impinging Nb atoms to sites of lower energy, eithellowing a longer time (pulsed deposition) or by increa

he Nb atom mobility (high temperature). The effectivenf these approaches was verified by EIS.Fig. 6 shows thedependence of the double layer capacity of a series oamples obtained in the pulsed mode. The close simif this curve with that inFig. 4B (full symbols) is evidennd means that no improvement of the morphology of thlms is achieved by pulsed deposition, supporting the

ig. 6. Influence of the target–substrate angleθ on the double layer capaty of Nb films deposited under continuous (full squares) or pulsed (equares) operation of the magnetron (on 9 mm× 9 mm quartz sheets).

The EIS study reported in this paper has shown thalms deposited by magnetron sputtering become quite pohen the target–substrate angleθ exceeds 45◦, with a markedaximum at 60–75◦. This fact is testified by the shape ofyquist plots obtained when these deposits are exposn inert electrolyte and their impedance is measured apen circuit potential. The surface roughness of the depas been quantified. The comparison of samples of diffize has shown that the dependence of the roughnessθ isbserved not only when the deposit thickness is inhomeous (due to a variable target–substrate distance) buhen such inhomogeneities are avoided. Shadowing iost probable reason for the increase in surface roug

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with increasingθ. Two methods have been used with the aimof limiting the formation of rough deposits: deposition underpulsed conditions and deposition on substrates heated at ahigh temperature. The latter method has been partially suc-cessful, provided the temperature was as high as 600◦C. Thistemperature may represent a technological limit for Nb depo-sition on real resonator cavities, owing to possible softeningof Cu and Cu–Nb interdiffusion.

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