Surface and Coatings Technology 177–178(2004) 434–440

0257-8972/04/$ - see front matter� 2003 Elsevier B.V. All rights reserved.doi:10.1016/j.surfcoat.2003.09.021

Deposition of NbN thin films by DC magnetron sputtering process

S.K. Kim*, B.C. Cha, J.S. Yoo

School of Materials Science and Engineering, University of Ulsan, Ulsan 680-749, South Korea

Abstract

Thin films of niobium nitride(NbN) were deposited on SKD11 tool steel substrate by a DC magnetron sputtering system. Theinfluence of the NyAr gas ratio of the inlet gases, the deposition temperature, the substrate bias potential on the mechanical and2

the structural properties of the films were investigated. X-ray diffraction data showed that cubic NbN was formed at a low Ny2Ar gas ratio. With the increase in the NyAr gas ratio, hexagonal NbN was formed, resulting in a mixture of cubic NbN and2

hexagonal NbN. The hardness of the films also increased with an increase in the deposition temperature up to 3008C. Adhesionof the films decreased, however, with an increase in the deposition temperature. The hardness of the films reached maximumlevel at the bias potential ofy200 V and decreased with a further increase of the bias potential. The films deposited at thesubstrate bias potential range fromy50 toy150 V exhibited relatively high adhesion.� 2003 Elsevier B.V. All rights reserved.

Keywords: DC magnetron sputtering; Niobium nitride(NbN); X-Ray diffraction

1. Introduction

Transition metal nitrides constitute one of the mostimportant families of materials for thin films and coat-ings next to the materials of the diamond family. Theyare considered for use as metallurgical coatingsw1x inmechanical applications, for protective coatings withfunctional optical propertiesw2x or for specific metalli-zation properties in microelectronicsw3x.Niobium nitride(NbN) thin films were initially inves-

tigated because of their superconducting properties rath-er than their mechanical properties. As such, earlyresearch was directed towards synthesizing NbN filmsto increase their superconducting transition temperaturesw4–6x. Thin films of NbN are attractive in wear appli-cations because of their good thermal expansion matchwith widely used tool steels. NbN thin films have beenprepared by various deposition techniques includingreactive magnetron sputteringw7,8x, ion beam assisteddepositionw9x, pulsed laser depositionw10x and filteredarc depositionw11–14x.Wong et al. w7x and Harvey et al.w8x studied the

effects of deposition parameters like nitrogen partialpressure, target power and substrate bias potential on

*Corresponding author. Tel.:q82-52-259-2228; fax:q82-52-259-1688.

E-mail address: skim@mail.ulsan.ac.kr(S.K. Kim).

the properties of magnetron sputtered NbN thin films.However, they did not investigate the effects of deposi-tion temperature. The substrate bias potential that theyinvestigated was only up toy150 V. The substrates thatthey used were M2 tool steel, glass and stainless steel.In this study, we report the effects of the NyAr gas2

ratio of inlet gases, the deposition temperature and thesubstrate bias potential on the mechanical and structuralproperties of magnetron sputtered NbN films. We usedSKD11 tool steel as a substrate and varied the substratebias potential up toy250 V.

2. Experimental procedures

NbN films were produced using unbalanced DCmagnetron sputtering equipment. A circular sputtersource was fixed to the lid of the chamber. A niobiumtarget(99.99% pure) with a diameter of 76.2 mm wasattached to the sputter source. A sample holder, whichcould be rotated to enable voltage bias, was located atthe center of the chamber. The substrate to targetdistance was 55 mm.After the chamber was evacuated to 1.3=10 Pay4

using a rotary pump and a diffusion pump, argon wasintroduced to maintain a working pressure. The SKD 11steel (1.5%C, 11.5%Cr, 0.8%Mo, 0.9%V) specimenswere polished and degreased ultrasonically in alcohol.

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Fig. 1. X-ray diffractograms of NbN films using various NyAr gas ratios of inlet gases.2

Fig. 2. Hardness of NbN films deposited at various temperatures.

Before deposition, the specimens were plasma etchedfor 10–60 min with 600 mA at a pressure of 1.3=102

Pa and 300 V.To determine the effects of nitrogen partial pressure,

the N yAr gas ratio of inlet gases was varied from 0.12

to 0.3. Target power was varied from 100 to 240 W.Before the deposition of the NbN, an interlayer of Nb

was deposited at 0.78 Pa for 20 min. The thickness ofthe Nb interlayer was approximately 0.5mm. Then anNbN layer was deposited at the same pressure for 60min.An X-ray diffractrometer (Rigaku, RAD-3C) was

used to determine phases of the films. A scanningelectron microscope(JEOL, JSM-820) was used to

436 S.K. Kim et al. / Surface and Coatings Technology 177 –178 (2004) 434–440

Fig. 3. SEM micrographs of surface morphology of NbN films depos-ited at various temperatures((a) 25 8C, (b) 200 8C, (c) 300 8C).

observe surface morphology. The hardness of the filmswas measured by a Vicker’s hardness tester using a 25g load. Adhesion was evaluated by a scratch tester(Revetest, CSEM).

3. Results and discussion

X-ray diffractrograms of NbN films that were depos-ited using various NyAr gas ratios of inlet gases are2

shown in Fig. 1. Other deposition parameters keptconstant had a deposition pressure of 0.8 Pa and a targetpower of 240 W. At a low NyAr gas ratio, cubic NbN2

was formed. With an increase in the NyAr gas ratio,2

hexagonal NbN was formed resulting in a mixture ofcubic NbN and hexagonal NbN. This agrees with theresults of Wong et al.w7x. The hardness of each thinfilm deposited using various NyAr gas ratios was also2

measured. Since thin films deposited with an NyAr gas2

ratio of 0.2 exhibited the highest hardness levels, sub-sequent deposition was carried out using this NyAr gas2

ratio.Deposition of NbN films were done at 0.52, 0.78 and

1.04 Pa. Since the films deposited at 1.04 Pa had thehighest hardness levels, subsequent experiments wereperformed at a deposition pressure of 1.04 Pa. Etchingwas done for 10, 20, 40 and 60 min and the criticalload of the NbN layer deposited with respect to eachetching time was 6.8 N, 6 N, 7.6 N, 6.8 N, respectively.Since an etching time of 40 min yielded the highestcritical load, subsequent experiments were performedusing this etching time. The hardness levels of NbNlayer obtained at a target power of 120 W, 180 W and240 W were 600 Hv , 1600 Hv , 2800 Hv0.025 0.025 0.025,

respectively. The target power of 240 W was used insubsequent experiments.The hardness levels of the NbN films deposited at

various temperatures without applying a substrate biaspotential are shown in Fig. 2. The highest hardnesslevels were observed at the deposition temperature of300 8C. SEM micrographs of surface morphology foreach of the films are shown in Fig. 3. As the depositiontemperature increased, a dome structure in the filmdeveloped. This accounts for the increase of the filmhardness. Similar trends in the development of domestructures in the films deposited with increasing temper-atures were previously observed in the TiCN filmsdeposited by MO-PACVDw15x. By increasing the sub-strate temperature, the mobility of the atoms is alsoincreased leading to a more perfect structure(i.e. apromoted grain growth).Fig. 4 shows the critical load of the films obtained at

various deposition temperatures. With an increase in thedeposition temperature, the adhesion of the filmsdecreased. Ordinarily, one might expect an increasedadhesion of the films with an increase of the depositiontemperature due to the diffusion of the film and the

437S.K. Kim et al. / Surface and Coatings Technology 177 –178 (2004) 434–440

Fig. 4. Critical load of NbN films deposited at various temperatures.

Fig. 5. Hardness of NbN films deposited at various substrate bias potentials.

substrate. XRD data of these films showed a develop-ment of NbN O phasewJCPDS 25–1360x. Inclusion0.9 0.1

of this oxide phase could be a reason for the observeddecrease in adhesion.

Fig. 5 shows the hardness levels of NbN filmsdeposited at various substrate bias potentials. As thebias potential was increased, the hardness of the filmsincreased reaching a maximum value aty200 V and

438 S.K. Kim et al. / Surface and Coatings Technology 177 –178 (2004) 434–440

Fig. 6. SEM micrographs of surface morphology of NbN films deposited at various substrate bias potentials((a) 0 V, (b) y50 V, (c) y100 V,(d) y150 V, (e) y200 V, (f) y250 V).

then decreased. SEM micrographs of the surface mor-phology of each film are shown in Fig. 6. At low biaspotentials, dome structures were not developed. As bias

potential increased toy150 V, a little coarse domestructure developed. Fine dome structures were observedat a y200 V bias potential. This accounts for the

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Fig. 7. Critical load of NbN films deposited at various substrate bias potentials.

highest hardness levels of the films. Further increases inthe bias potential toy250 V made a loose domestructure. This resulted in low hardness levels of thefilms. The thickness of the films that were deposited atzero bias,y50, y100, y150 andy250 V were 4.5,3.75, 3.5 and 2.5mm. An increased substrate biasvoltage raises the kinetic energy of the Ar ions andq

niobium particles. A bombardment of the growing filmwith highly energized niobium particles and Ar ionsq

caused a dense structure. As a result, the hardness ofthe film increased. However, a too high bias potentialcaused a collapse of the structure. This accounted fordecreasing hardness levels aty250 V.Fig. 7 shows the critical load of the films obtained at

various bias potentials. Films deposited at a substratebias potential range fromy50 to y150 V exhibitedrelatively high critical loads. Further increases in thebias potential decreased adhesion

4. Conclusion

NbN thin films were deposited using a DC magnetronsputtering method. There was a transition from a cubicNbN phase to a hexagonal NbN phase with an increasein the nitrogen partial pressure of the inlet gas. Hardnesslevels of the films increased with an increase of thedeposition temperature within the temperature intervalinvestigated. Adhesion levels of the film, however,decreased with an increase in the deposition temperature.

As the substrate bias potential was increased, hardnesslevels of the film also increased reaching a maximumvalue at y200 V and then they decreased. Filmsdeposited at a substrate bias potential range fromy50to y150 V exhibited relatively high critical loads.

Acknowledgments

This work was supported by the Korea Science andEngineering Foundation through the Research Centerfor Machine Parts and Materials Processing at theUniversity of Ulsan(Project No. R12-1998-016-09007-0).

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