Reactive diffusion produced niobium nitride films for superconducting cavity applications

Nuclear Instruments and Methods m Physics Research A 336 (1993) 16-22 North-Holland

NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH Section A

Reactive diffusion produced niobium nitride films for superconducting cavity applications C. Benvenuti, P. Chiggiato, L. Parrini ' and R . Russo CERN, 1211 Geneva 23, Switzerland

Received 29 June 1993 In view of their potential interest for the construction of radiofrequency (RF) superconducting accelerating cavities, films of NbN have been produced by annealing Nb samples in a nitrogen atmosphere . These films have been characterized by critical temperature measurements performed with the magnetic permeation method, by metallographic examination and by X-ray diffraction. They exhibit critical temperatures up to 16 .6 K, and transition widths of about 0.4 K. The influence of nitriding conditions (temperature, nitrogen pressure and time) on the superconducting properties has been investigated, in order to obtain the optimal nitriding parameters . 1. Introduction The fabrication of superconducting RF cavities for nuclear particle accelerators are presently based on niobium, either in the form of a metal sheet or of a thin film deposited by sputtering on the inner surface of a copper cavity [1,2]. Nevertheless theoretical considerations predict that other superconducting materials, among which NbN, should be good candidates to replace Nb for this application, with better performance. On the other hand, their actual use has always been hindered by production difficulties which have not been overcome even for sputtered coatings . The first experimenters who discovered superconductivity in Nb-N system used the technology of powder metallurgy [3] which is not suitable for RF cavity applications. The sputter-coating technique has been widely studied [4-7] and also applied to RF coatings [8,9] but the results are not very satisfactory because of the high resistivity in the normal state and the high residual resistivity in RF operation. Similar conclusions have also been obtained at CERN showing that Nb films are to date far superior for RF cavity coatings [101 . Critical temperatures between 15 K and 16 K were obtained by heating Nb rods in a nitrogen atmosphere [11-13]. Two groups of authors [14,15] have thermally nitrided RF cavities made of massive Nb, obtaining Present address: Institut de Genie Atomique, EPFL, 1015 Lausanne, Switzerland.

reasonable radiofrequency performance, but still inferior to theoretical expectations . However, as testified by the critical temperatures obtained with cavities (14.6 K in ref. [14] and 11 .5 K in ref. [15] to be compared to 17 .3 K, the best T, reported for NbN so far [16]), the optimal nitriding conditions have not been reproduced . The investigation reported here concerns the production of NbN superconducting films via thermal diffusion, i.e . by heating Nb sheets in a nitrogen atmosphere . This study aims at : 1) Investigating both the thermodynamic aspects (phase transitions) and the kinetic aspects (nitriding temperature, pressure, time) of the nitriding process, to establish the conditions which allow the formation of a good superconductor . 2) Defining the practical relevance of the defined procedures for the production of superconducting cavities . The quality of the produced films with respect to superconducting properties has been evaluated on the ground of critical temperature Tc and superconducting transition width AT . 2. The Nb-N system The phase diagram of the binary system Nb-N up to N/Nb = 1 includes many different phases [17-19], characterized by different T. [20] . The a phase is the solid solution of nitrogen in niobium, the ß phase (hexagonal, of the Hdgg type) corresponds to Nb 2N, the y phase is tetragonal

0168-9002/93/$06 .00 © 1993 - Elsevier Science Publishers B.V . All rights reserved

C. Beneenuti et al. / Niobium nitride films

(Nb4 N3 ), the 8 phase has a cubic structure of NaCl type and it is stable only above 1300°C for a large range of Nb-N atomic ratios (0 .88-0.98) . Finally the e phase is the stoichiometric compound NbN which is hexagonal and unstable above 1380°C . The phase of interest in the present case is the 8 phase which has the highest T.. For the y phase different Ti's are reported in the literature, ranging from 8.7 K [20] and 10.7 K [21] to 15 K [14] . 3. Experimental set-up and results The Nb sheets are chemically polished in a HFHN03-H Z SO4 bath, degassed in vacuum at 900°C for 2 h, and placed in the nitriding system, where a vacuum of about 10 -8 Torr is generated by a turbomolecular pump . They are then ohmically heated, via suitable current leads, to 1300°C for about 15 min for surface activation . Finally high purity nitrogen (99.9999%) is injected at a given pressure (usually one bar) and nitriding is obtained by heating the sample at the desired temperature . The pressure in the system is monitored by a membrane capacitive gauge (baratron), whereas the temperature of the Nb sample is measured pyrometrically through a quartz window . All temperatures reported in this note, if not otherwise stated, are direct readings from the optical pyrometer. The produced specimens are characterized by critical temperature measurements performed by the magnetic permeation method : therefore from the measured superconducting transition widths AT information is obtained on the homogeneity of the superconducting phase over the whole surface of the sample . Furthermore, transverse optical and electronic micrographs of samples are usually taken to measure the

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thickness of the produced nitride film . In this case, to avoid edge thinning, the samples are sputtered with gold, coated with 100 win electrolytic nickel and embedded in a hard resin before mechanical machining. For the optical micrographs etching is then performed with a solution containing HF, HN03 and lactic acid . The etching time is five seconds. Additional information on film structure is obtained by an X-ray diffractometer in the Bragg-Brentano configuration (Cu-K« radiation) . The experimental results are summarized in table 1. In the first column the reference number of the samples is reported . In the other columns are given the nitriding temperature T , the nitriding time t n , the nitriding pressure P, the obtained critical temperature T, the transition width AT, the thickness D of the nitride phases, the Nb-N phases detected by X-ray diffraction . The T, values reported in this table represent the onset of the superconductive transition, i.e . the critical temperature of the best part of the sample . The reported values of AT represent the difference between T, and the temperature at which 15% of the magnetic field is transmitted by the sample . When not otherwise specified, the thermal cycle implies tempering obtained by switching off abruptly the electrical heating of the Nb sample . In this way we have tried, by fast cooling (about 100°C/s), to avoid the transition 8 ~* 'Y + e of NbN, retaining a metastable 8 phase down to room temperature . The analysis of the obtained Ti's demonstrates the presence of the 8 phase on all the samples except 4 and 10 . Also X-ray diffraction analysis confirms the existence of the 8 phase (see fig . 1) . The presence of the typical peaks of the y and ß phases is attributed to the formation of these phases underlying the 8 phase which has been produced on the surface.

Table 1 Summary of the characteristics of the nitrided samples Ref. num. 1 2 3 4 5

6

7 8° 9b

10

T [ICI

tn

[min]

P [bar]

Te [KI

1480 1380 1340 1270 1380 1380 1380 1380 1380 1500

120

1

15 .5 15 .8 15 .6 13 .35 16 .0 16 .2 160 16 .6 16 .3 8 .75

120 120 120 25 6 1 6 6 120

1 1 1 1 1 1 1 1 0 .01

AT [K] 5 .1 2 .5 1 .6 0 .55 0 .4 0 .2 0 .2 0 .4 0 .3 0 .04

D [[Lm] R 17 .4 8 .5 5 .5 2.8 4 3 .2 1 5 .0 6 .5 47

8+ y 48 23 19.5 9 11 .4 6 .5 3 .1 8 .5 11 .3 -

X-rays 8, -y

6 S e, 8, y

8, 8, 8, 8, 8, (3

y y y, y

R

1'

7 min at 1200°C, followed by tempering. b Slow cooling (about 9°C/mm) down to the transition S - y + e of NbN at 1140°C . After transition the temperature is raised again to 1380°C, lowered at 1240°C for 7 min and finally quickly reduced by tempering. a

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C. Benuenutc et al. /Niobium nitride films

Examination of the transverse section of samples shows the presence of two distinct nitride layers (see fig. 2) of which the deeper can be attributed to the ß phase and the outer to y + S. The identification of the ß phase is made easy by its typical strongly oriented columnar structure. The y and 8 phases are more difficult to separate because at our production temperatures they are intermixed [18] .

4. Discussion Since the main goal of this study is to obtain good superconductors with economically and technologically acceptable production conditions, the correlation between nitriding conditions (T , P, t, thermal cycle) and obtained T, is of fundamental importance . Poor superconductive properties may be ascribed to

2-Theta - Scale

mm m mN N

N a U

85

90

95

100

105

110

115

120

12S

130

Fig . 1 . X-ray diffraction spectra of sample 2 of table 1. The Cu-K . doublet is not resolved . Vertical lines correspond to 8-NbN

peaks of the JCPDS data base (file nr . 11-3855) .

C. Benvenuti et al. / Niobium nitride films

two distinct causes namely the formation of undesired NbN phases with lower T,, and/or high concentration of N vacancies in the nitrogen sublattice of the 8 phase . These vacancies can be either thermodynamical (i .e . in thermodynamical equilibrium with the gaseous nitrogen during nitriding), or kinetic vacancies due to the lack of thermodynamic equilibrium consequent to short nitriding time or low nitriding temperature . The concentration of thermodynamical vacancies C is expressed as a function of temperature Tn and NZ pressure Pn by the formula: Ca

1 Pn

e - U/kT

where U is the formation energy of a N vacancy and k is the Boltzmann constant . Since N vacancies have a negative influence on the Tc of the 8 phase [20], the above formula suggests that best results could be ob-

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tained by decreasing Tn and increasing P as much as possible . Thus, in most nitriding operations, a temperature of about 1350°C has been maintained (a lower temperature, i.e . 1270°C, did not result in the formation of the 8 phase) whereas the pressure has been generally stabilised at about 1 bar for practical reasons. It should be noticed that the minimum temperature at which the 8 phase is in equilibrium with the nitrogen gas at one bar is reported to be about 1380°C in the literature [18] . This points out that the temperatures reported in this paper (direct pyrometer readings) are probably lower than the real temperatures by about 100°C. The Tc values measured on samples 1 to 3, and 5 to 9 are all close to 16 K showing that the selected production conditions have led to the formation of films of the required 8 phase. The low values of AT, in some cases less than 0.4 K, show that this phase covers homogeneously the whole surface of the samples.

Fig. 2. Cross section of the sample 2 of table 1 observed by electron microscopy. In this case chemical etching has not been applied. The 0 phase is easily identified because of the strongly oriented columnar structure.

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C. Bencenuu et al. /Niobium mtnde films

4.1 . The influence of the nitriding temperature

Fig. 3 shows the effect of T. on the obtained T., for samples nitrided at 1 bar for 2 h. The X-ray analysis of the sample 4, produced at 1270°C (see table 1), indicates the dominance of the e and y phases near the surface, whereas the 8 phase is present at a much lower percentage . Progressive surface removal by mechanical polishing or acid etching resulted in a fast disappearance of the e phase, which was therefore confined to the uppermost surface layer. The elimination of the surface e phase resulted also in a change of color from gray to yellowlike, a colour which is common to both the y and S phases . Since the critical temperature of sample 4 was only 13 .35 K (with AT= 0.55 K), too low for the 8 phase, it seems justified to conclude that in this case the T, was representative of the y phase (the e phase is not superconducting) . The small decrease of the T, of these samples consequent to the increase of nitriding temperature is ascribed to the increase of N thermodynamical vacancies, which take place according to eq . (1). This inconvenience may be overcome by annealing the samples at the lowest possible temperature compatible with the conservation of the 6 phase . The thermal cycle applied to sample 9 indicates that the transition 6 - y + e takes place at about 1140°C when decreasing the temperature, while the reverse transition y + e - 6 takes place at about 1300°C . Therefore, annealing at 1200°C has been applied, resulting in a Tc increase of about 0.4 K. After this

17 1615 _, U

E.

1413 12 1 l

lo

1250

--ob--a-

Tc Tc-àT 1300

1350

1400

1450

1500

r ['cl Fig. 3. Variation of the critical temperatures of samples 1-4 of table 1 as a function of the nitriding temperature . The squares correspond to the onset of the transition, the dots represent the temperature at which 15% of the magnetic field is transmitted by sample .

t  [min]

Fig 4 Variation of critical temperature as a function of nitriding time . The minding temperature is 1380°C (pyrometric reading) . The characteristics of the samples are given in table 1 . The squares and the dots have the same meaning as m fig. 3. The upper sample at 6 mm has been annealed at 1200°C (7 min)

operation, tempering retains the (metastable) 6 phase down to room temperature . The benefit of this procedure is shown in fig. 4 for sample 8 . 4 .2. The influence of annealing time t 

Fig. 4 shows the effect of t  on T, for samples heated at 1380°C in N, at atmospheric pressure . The maximum T, value is obtained for t  = 6 min. For shorter nitriding times, a small reduction of T, is noticeable which may be ascribed to N kinetic vacancies : the heating time is too short to allow reaching thermodynamical equilibrium. On the other hand, the Tc decrease and the related increase of AT, which are noticed when increasing the nitriding time, cannot be easily described in terms of thermodynamical or kinetic arguments. According to the literature [22], all phases presenting a higher nitrogen content than the 6 one are not stable at the applied nitriding temperature . The most plausible cause for this decrease of T. might be found in a progressive accumulation of impurities in the samples (mainly OZ present in the N2 gas and H Z O adsorbed on the system walls). It may be noticed that the dependence of the nitride layer thickness on time follows a parabolic law as expected (see fig. 5), and that after about 6 min the y + 8 phases layer is about 5 ~Lm thick, i .e. sufficient

C. Benvenutc et al. / Niobium nitride films

21

120 [min .]

2$

10 Nftn

11

12

[min O .5 [

Fig. 5. Thickness D of the nitride layer as a function of the nitriding time at 1380°C (pyrometrical reading) . The displaced variation closely follows the expected parabolic law. The negative crossing point is consequent to the temperature rising time which has not been taken into account. for the purpose of shielding the RF field in a superconducting cavity . The practical consequences of these results is therefore that the duration of the nitriding process should not exceed 6-7 min. 4.3 . The effect of the nitriding pressure P

It is known from the litterature [16] that the 8 phase cannot be obtained at NZ pressures lower than about 0.1 bar. Sample 10, produced under the same conditions as sample 1 but at only 0.01 bar Nz pressure, confirms this fact by showing only the presence of the ß phase and a T, value of 8.75 K with a very narrow transition . This value probably corresponds to the T, of the underlying Nb, reduced by the presence of NZ (solid solution) in equilibrium with the R phase [23,24] which has a T. value of 8.6 K [25] . 5. Conclusions Good quality films of NbN a few win thick were obtained reproducibly by heating Nb samples in a NZ atmosphere .

The optimization of the process parameters leading to Tc values higher than 16 K, resulted in the definition of the following production guide-lines . - Nitriding should be carried out at temperatures higher than 1380°C (pyrometric reading about 1300°C) for a time not exceeding 6 min; - Higher temperatures and/or longer process times result in NbN films of poorer quality; - A further T, increase may be obtained by annealing for a few minutes after nitriding, at a temperature of about 1200°C . - Annealing followed by fast cooling does not destroy the (metastable) 8 phase, which may be preserved down to room temperature . After the completion of this preliminary programme it is intended to proceed with the nitriding of Nb cavities in the near future . The advantages of thermal diffusion, with respect to sputter-coating, consist in providing a larger grain size and in its applicability to cavities of small dimensions (e .g . of frequencies higher than 3 GHz), for which sputtering may be difficult or even impossible . The larger grain size (up to 10 win instead of a few tens of nm as obtained by sputtering) should result in better cavity performance (higher Q-values) due to the reduced presence of grain boundaries which are sup-

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C. Benvenuti et al. / Niobium nitride films

posed to be the main cause of the observed surface

resistance [8].

The main inconvenience of thermal nitriding consist

is the risk of embrittlement consequent to fast diffu-

sion of N along the grain boundaries of niobium.

However, this risk could be minimized by applying a short thermal treatment.

References [1] H. Padamsee, Proc . 1989/1990 US Particle Accelerator School, Brookhaven Nat. Lab., AIP Conf. Proc. nr . 249, part 2 (AIP, 1992) p 1402-82. [21 C. Benvenuti, Proc . 5th Workshop on RF Superconductivity, ed . D. Proch, DESY, Hamburg (1991) p. 189. [3] G. Ashermann, E. Friedrich, E. Justi and J. Kramer, Z. Physik 42 (1941) 349. [4] T.H . Geballe et al ., Physics (NY) 2 (1966) 293. [5] Y.M . Shy, L.E. Toth and R. Somasundaram, J. Appl . Phys ., 44 (1973) 5539 . [6] J.R . Gavaler et al ., IEEE Trans . on Mag. TM-17 (1981) 573. [7] J.R . Gavaler et al ., J. Vac. Sci. Technol. 6 (1969) 177. [8] C. Attanasio et al , Phys . Rev. B 43 (1991) 6128.

[91 S. Isagawa, J. Appl . Phys. 52 (1981) 921 . [101 Benvenuti et al ., Proc . 5th Workshop on RF Superconductivity, ed . D. Proch, DESY, Hamburg (1991) p. 518. [11] H. Rogener, Z. Physik 132 (1952) 446. [12] K. Hechler and E. Saur, Z Physik 205 (1967) 392. [131 H. Bauer, J . Low Temp . Phys . 24 (1976) 219. [l4] P. Fabbricatore et al ., J. Appl. Phys . 66 (1989) 5944. [151 M. Pham Tu et a] ., J. Appl . Phys . 63 (1988) 4586 . [161 M.W. Williams, K.M . Rails and M.R . Pickus, J. Phys . Chem. Solids 28 (1967) 333. [17] G. Brauer, J. Less-Common Metals 2 (1960) 131 [l8] R.W . Guard, J.W . Savage and D .G . Swarthout, Trans. Metall . Soc. ATME 239 (1967) 643 [19] H. Hollek, Binare und Ternare Carbid- und Nitrid- Systeme der Ubergangsmetalle (Gebruder Borntraeger, Berlin, 1984) p. 41-45. [20] L. Toth, Transition Metal Carbides and Nitrides (Academic Press, New York, 1971), chap . 7 [21] R. Kaiser, W. Spengler, S. Schicktonz and C Politis, Phys . Stat . Sol. (b) 87 (1978) 565 [22] C. Politfis and G. Relman, KfK-Ext 6/78-1, July 1978 . [231 W. DeSorbo, Phys . Rev 132 (1963) 107. [241 H.W. Weber et al., Phys Rev. B 44 (1991) 7585 . [25] B.L . Chamberland, in : Chemistry of superconductor materials, ed . T.A. Vanderah (Noyes Publications, Park Ridge, NJ, USA, 1992) p.16.