The effects of nitrogen flow ratio, target area ratio of Cr, and substrate temperature on the structure of DC reactive sputtered Fe-Cr-N ternary films have been studied. X-ray diffraction measurements show that Fe-Cr-N films consist of alpha-Fe(Cr) and gamma'-(Fe,Cr)(4)N-x (x < 1) phases. The crystal grain of the alpha-Fe(Cr) phase becomes finer and a (200) texture of the gamma'-(Fe,Cr)(4)N-x phase becomes more marked with increasing the nitrogen flow ratio. X-ray photoelectron spectra of the films show that oxidation resistance of Fe-Cr-N films is superior to that of Fe-N films, and oxides are formed only in the film surface due to contacting with the ambient atmosphere and oxygen contamination is very small in the inner parts of these films. (C) 1999 Kluwer Academic Publishers

Konno, T. J.

Oku, M.

Peng, D. L.

Sumiyama, K.

Suzuki, K.

Wagatsuma, K.

彭栋梁

Journal of Materials Science

Xiamen University Institutional Repository

P1: FJL/FIX P2: FJL/FGD P3: FDB/FGD QC: FDB 29-KJ007-62-97 June 30, 1999 9:32JOURNAL OF MATERIALS SCIENCE34 (1999 )4623– 4628X-ray diffraction and X-ray photoelectronspectra of Fe-Cr-N films deposited by DCreactive sputteringD. L. PENG, K. SUMIYAMA, M. OKU, T. J. KONNO, K. WAGATSUMA, K. SUZUKIInstitute for Materials Research, Tohoku University, Sendai 980-8577, JapanE-mail: peng@snap8.imr.tohoku.ac.jpThe effects of nitrogen flow ratio, target area ratio of Cr, and substrate temperature on thestructure of DC reactive sputtered Fe-Cr-N ternary films have been studied. X-ray diffractionmeasurements show that Fe-Cr-N films consist of α-Fe(Cr) and γ ′-(Fe,Cr)4Nx (x <1)phases. The crystal grain of the α-Fe(Cr) phase becomes finer and a (200) texture of theγ ′-(Fe,Cr)4Nx phase becomes more marked with increasing the nitrogen flow ratio. X-rayphotoelectron spectra of the films show that oxidation resistance of Fe-Cr-N films issuperior to that of Fe-N films, and oxides are formed only in the film surface due tocontacting with the ambient atmosphere and oxygen contamination is very small in theinner parts of these films. C© 1999 Kluwer Academic Publishers1. IntroductionTo obtain ultra fine grains ofα-Fe sputtered films, theternary Fe-X-N (X=Zr, Hf, Nb, Ta) systems in whicheach elementX had strong affinity for nitrogen andability of amorphous formation were investigated ex-tensively [1–6]. Good soft magnetic properties and highsaturation flux density were obtained in each system.On the other hand, the addition of the thirdX el-ements has succeeded in solving high magnetostric-tion and poor thermal stability of binary Fe-N film.For each ternary system, as-prepared films consistedof an amorphous-like phase and annealed films werea homogeneous mixture of fine-grainedα-Fe andX(Zr, Hf, Nb, Ta) nitrides. In our previous studies on thestructure, magnetic properties and thermal stability ofthe ternary Fe-Ti-N films deposited by DC magnetronfacing target sputtering, Ti2N phase was formed in as-prepared and annealed films [7, 8].Recently, we reported a new ternary Fe-Cr-N filmwith the perpendicular magnetic anisotropy depositedby DC reactive sputtering [9]. Adjusting the chemi-cal composition and deposition parameters, saturationmagnetization of 0.37–0.5 Wb/m2 and perpendicularcoercivity of 6.4×104–8.8×104 A/m are observed forthe Fe-Cr-N films.In this paper, we report the detailed analyses of X-raydiffractions and photoelectron spectra of the Fe-Cr-Nternary films prepared at various sputter-depositionconditions.2. ExperimentalFe-Cr-N films were deposited on glass and aluminumfoil substrates by a facing-target-type DC magnetronsputtering in a mixed Ar+N2 plasma, using a com-posite target consisting of pure Fe (99.9%) and Cr(99.9%) plates. The alloy composition was adjustedby changing the surface area ratio of Cr plates,ACr,defined asACr= (an area of Cr)/(a total area of Feand Cr). In order to obtain Fe-Cr-N films with variousnitrogen contents, a N2 flow ratio, R(N2)= (N2-flowrate)/(Ar-flow rate+N2-flow rate), was changed byfine control of mass flow controllers. The substratetemperature,TS, during the sputter-deposition was keptat about 50◦C by water-cooling and at 150, 250 and350◦C by indirect resistive heating, respectively. Thesputtering conditions were described elsewhere [9].The chemical composition of deposited films wasdetermined by inductively coupled plasma (ICP)optical emission spectrometry and helium carrierfusion-thermal conductivity methods.X-ray diffraction (XRD) analysis of the films wasperformed on a conventional diffractometer with CuKαradiation using a graphite monochromator and a film X-ray diffractometer designed according to the Seemann-Bohlin focusing principle for small incident angles.X-ray photoelectron spectra (XPS) were measured bySurface Science Laboratories model SSX-100 spec-trometer with a monochromatic AlKα source having a300×450µm spot size. The base pressure in the spec-trometer was 1.3×10−7 Pa or better. Magnetic proper-ties of the films were measured by a vibrating samplemagnetometer (VSM) in a magnetic field up to 16 kOeapplied parallel and perpendicular to the film plane.3. Results3.1. X-ray diffraction analysesThe perpendicular magnetic anisotropy of Fe-Cr-Nfilms depends on sputter-deposition parameters [9],0022–2461 C© 1999 Kluwer Academic Publishers 4623P1: FJL/FIX P2: FJL/FGD P3: FDB/FGD QC: FDB 29-KJ007-62-97 June 30, 1999 9:32Figure 1 Magnetization curves at room temperature in magnetic fieldsparallel and perpendicular to the Fe71Cr20N9 (a) and Fe68.5Cr19N12.5 (b)film planes.in particular, on R(N2), ACr and TS. Fig. 1 showsthe magnetization curves of (a) Fe71Cr20N9 and (b)Fe68.5Cr19N12.5 films of about 7µm thickness.Hc⊥ ismore than twice ofHc//, displaying large perpendicularanisotropy.Fig. 2 shows the evolution of X-ray diffraction pat-terns of the films deposited at (a)ACr=33% and(b) 35% as a function ofR(N2). The X-ray diffrac-tion patterns show similarR(N2) dependences for bothfilms. The films consist ofα-Fe(Cr) andγ ′-(Fe,Cr)4Nx(x<1) phases. For theα-Fe(Cr) phase, the (110) peaksbroaden and their intensities become smaller asR(N2)increases. This shows a tendency of refinement of thecrystal grain and piling of strain asR(N2) increases. Fortheγ ′-(Fe,Cr)4Nx phase, the intensity of (200) peak be-comes larger asR(N2) increases and the (200) texturegrowth becomes more remarkable. Moreover, the (200)peak of theγ ′-(Fe,Cr)4Nx phase shifts to a lower 2θ sideasR(N2) increases. Fig. 3a and b show the lattice con-stants,a, calculated from the interplanar distance,d200,of theγ ′-(Fe,Cr)4Nx(200) peak and the nitrogen con-tent,CN, as a function ofR(N2). CN increases asR(N2)increases and is 18 at % atR(N2)=40% forACr=33%,being smaller than the nitrogen content (20 at %) ofpureγ ′-Fe4N. Thea values of these films are alwayslarger than that of pure fcc Fe, smaller than that ofγ ′-Fe4N and increase with increasingR(N2). Namely,the a values are close to that of fcc Fe (a=3.60 A◦ )for lower R(N2) and that of γ ′-Fe4N (fcc, a=3.795 A◦) for higherR(N2), respectively. Therefore, thepeak near 2θ =50◦ of the X-ray diffraction pattern inFig. 1 is allotted toγ ′-(Fe,Cr)4Nx(200) (withx<1). OnFigure 2 X-ray diffraction patterns of the Fe-Cr-N films deposited onwater cooled substrates under several N2 flow ratio (R(N2)) from a com-posite target with area ratio: (a)ACr=33% and (b) 35%.the other hand, as can been seen from Fig. 2, the fullwidth of the half maximum of theγ ′-(Fe,Cr)4Nx(200)peak is much wider than that ofα-Fe(Cr)(110) in Fe-Crfilms, being about 1.25◦ for R(N2)=40% although theintensity of theγ ′-(Fe,Cr)4Nx(200) peak is large. Thisindicates that large distortion exists in the nonequilib-rium γ ′-(Fe,Cr)4Nx phase due to insufficient nitrogencontent (less than 20 at %). In addition, the largerACr ofthe Fe-Cr composite target is, the smaller the lattice con-stant at the sameR(N2) values. This also suggests thatthere is tendence of formation of the fcc-type Fe(Cr)phase with the increase of Cr content.Fig. 4 shows the XRD patterns of the films depositedat differentTS at R(N2)=30 and 35%, respectively.The patterns of the films deposited at bothR(N2) in-dicate the sameTS dependences. The crystal structure4624P1: FJL/FIX P2: FJL/FGD P3: FDB/FGD QC: FDB 29-KJ007-62-97 June 30, 1999 9:32Figure 3 R(N2) dependence of lattice constant of theγ ′-(Fe,Cr)4Nxphase and nitrogen content (CN) in the Fe-Cr-N films deposited atACr=33 and 35% on water cooled substrates.change of these films is not obvious with increasingTSup to 250◦C; the films consist of a fine-grainα-Fe(Cr)phase and aγ ′-(Fe,Cr)4Nx phase with a strong (200)texture. AtTS=350◦C, the (110) peak of theα-Fe(Cr)phase becomes higher and sharper and the (200) peak oftheγ ′-(Fe,Cr)4Nx phase lower. This indicates that theamount and grain size of theα-Fe(Cr) phase increases,comparing with those prepared atTS=250◦C. In addi-tion, the (200) peak of theγ ′-(Fe,Cr)4Nx phase shifts tothe larger 2θ side asTS increases. The lattice constants,a, are plotted versusTS in Fig. 5. They decrease grad-ually with increasingTS and are close to that of purefcc Fe atTS=350◦C. Moreover, thea values of thefilms deposited atR(N2)=35% are always larger thanthose of the films atR(N2)=30% for the sameTS, be-ing in agreement withR(N2) dependence atTS=50◦C(Fig. 3).3.2. X-ray photoelectron spectraFig. 6 shows Fe2p3/2 and Fe2p1/2 XPS spectra ofFe64Cr18N18, Fe87N13 and Fe74Cr26 films. The samplesare deposited on water-cooled substrates and then ex-posed to air. The sample surfaces were not cleaned byAr+ sputtering before the XPS measurements. As canbe seen, Fe2p3/2 and Fe2p1/2 peaks of the iron oxidesare detected for all of these films. However, Fe2p3/2 andFe2p1/2 peaks of metallic iron are clearly observed in Fe-Cr and Fe-Cr-N films, while they are slightly observedin Fe-N films. This indicates that oxidation resistanceFigure 4 X-ray diffraction patterns of the Fe-Cr-N films deposited atACr=33% at (a)R(N2)=30% and (b) 35% at different substrate tem-perature,TS.Figure 5 Substrate temperature dependence of lattice constanta of theγ ′-(Fe,Cr)4Nx phase in the Fe-Cr-N films deposited atACr=33% andR(N2)=30 and 35%.of Fe-Cr-N and Fe-Cr films are superior to that of Fe-Nfilms.Fig. 7 shows Fe2p XPS spectra of the Fe64Cr18N18film as a function of Ar+ sputter-etching time at 1 kVand 1 mA. Large amount of iron oxides are observedin the film surface of the as-deposited film, while theFe2p XPS peak of iron oxides disappear almost com-pletely in the film sputter-etched with Ar+ for 5 min.This indicates that the oxides are formed only in the filmsurface due to contacting with the ambient atmosphere4625P1: FJL/FIX P2: FJL/FGD P3: FDB/FGD QC: FDB 29-KJ007-62-97 June 30, 1999 9:32Figure 6 Fe2p3/2 and Fe2p1/2 XPS spectra of the Fe74Cr26, Fe87N13and Fe64Cr18N18 films whose surface were not cleaned by Ar+ sputter-etching.Figure 7 Time dependence of Fe2p XPS spectra of Fe64Cr18N18 filmwith Ar+ sputter-etching at 1 kV and 1 mA.and oxygen contamination is very small in the innerparts of these nitride films.4. DiscussionFig. 8 shows (a) a rocking curve of theγ ′-(Fe,Cr)4Nx(200) peak for Fe-Cr-N film deposited atR(N2)=40% on a water-cooled substrate; (b) an XRDpattern of the crushed powder made from the sputter-deposited film; (c) an XRD pattern measured by the filmX-ray diffractometer. In Fig. 8a, the full width of thehalf maximum is about 10◦. Therefore, the (200) peakis hardly observed, when the incident angle is fixed at 2◦(Fig. 8c). Because few peaks appear in X-ray diffractionpatterns in Fig. 2 and the estimated lattice constant isdifferent from those of pure fcc Fe and Fe4N, it is verydifficult to determine the structure of Fe-Cr-N films.However, this problem is solved by the measurementsof X-ray diffraction shown in Fig. 8b and c, displayingseveral peaks of theγ ′-(Fe,Cr)4Nx phase.Moreover, the film surfaces were cleaned by Ar+sputtering for 10 min and the XPS measurements wereFigure 8 X-ray diffraction patterns of the Fe-Cr-N film deposited atR(N2)=40% on water-cooled substrate: (a) the rocking curve of theγ ′-(Fe,Cr)4Nx(200) peak; (b) the result for powder made from the film;(c) the result observed by a film X-ray diffractometer.carried out. Fig. 9 shows Fe2p, Cr2p and N1s XPS peaksof the Fe74Cr26, Fe87N13 and Fe64Cr18N18 films, whoseXRD patterns are indicated in Fig. 10. The sputter-deposited Fe74Cr26 film is composed of a singleα-Fe(Cr) phase, while the Fe87N13 film consists ofα-Feandγ ′-Fe4N phases, and the Fe64Cr18N18 film consistsof α-Fe(Cr) andγ ′-(Fe,Cr)4Nx phases, respectively.As can be seen from Fig. 9a, the binding energies ofFe2p3/2 and Fe2p1/2 are 707.1 and 720.1 eV for thesethree films, being in agreement with those of metalliciron [10]. The binding energies of Cr2p3/2 and Cr2p1/2(574.5 and 583.8 eV) in the Fe-Cr and Fe-Cr-N filmsare also the same to those of pure metallic chromium,being different from that of Cr2p3/2 and Cr2p1/2 (576.1and 585.4 eV) in chromium nitrides [10]. The bindingenergy of N1s (397.6 eV) in the Fe-Cr-N film is also inagreement with that of the Fe-N film (Fig. 8c), lowerthan that of the free nitrogen atoms (398.5 eV). Fromthese results, we can propose the following two expla-nations. The first one is that the electronic states of Featoms in the iron nitride (γ ′-Fe4N) and iron-chromiumnitride (γ ′-(Fe,Cr)4Nx) and the electron state of Cratoms in theγ ′-(Fe,Cr)4Nx are similar to those of puremetallic iron and chromium, respectively. The other isthat as the film surface is sputter-etched by Ar+ dur-ing XPS measurement, theγ ′-Fe4N andγ ′-(Fe,Cr)4Nx4626P1: FJL/FIX P2: FJL/FGD P3: FDB/FGD QC: FDB 29-KJ007-62-97 June 30, 1999 9:32Figure 9 Fe2p, Cr2p and N1s XPS peaks of the Fe74Cr26, Fe87N13 andFe64Cr18N18 films whose surface were cleaned by Ar+ sputter-etchingfor 10 min before the XPS measurements.phases are destroyed to become metallic Fe and Crstates in which nitrogen atoms dissolve interstitiallly.It is worth noting that though Cr content is large(about 20 at %) and the maximum nitrogen content isabout 18 at % (from Fig. 3b), a chromium nitride is notformed in the sputtered Fe-Cr-N films, as can be seenfrom the X-ray diffraction and XPS results. This is amarked contrast to the Fe-Ti-N films deposited undersame deposition conditions in which the titanium ni-tride, Ti2N, is formed [7, 8]. This also suggests that Tiatoms react more preferentially with N in comparisonto Cr atoms.5. ConclusionsFe-Cr-N ternary films with large perpendicular mag-netic anisotropy have been obtained using a facingtarget sputtering system with the reactive atmosphere(Ar+N2 mixture). From our X-ray diffraction and XPSmeasurements of the Fe-Cr-N, Fe-Cr and Fe-N films,We can conclude:(1) Fe-Cr-N films consist of a mixture of theα-Fe(Cr) andγ ′-(Fe,Cr)4Nx (x<1) phases, whose vol-ume fraction depends on the deposition parametersR(N2), ACr andTS. The incorporation of N into Fe-CrFigure 10 Comparison of X-ray diffraction patterns of the Fe74Cr26,Fe87N13 and Fe64Cr18N18 films.films causes the refinement of crystal grain of theα-Fe(Cr) phase and the growth of theγ ′-(Fe,Cr)4Nx(200)texture.(2) The lattice constants of theγ ′-(Fe,Cr)4Nx phasein the Fe-Cr-N films are always smaller than that ofpureγ ′-Fe4N, and increase with increasingR(N2) anddecreasingACr, and decrease with increasingTS.(3) The oxidation resistance of Fe-Cr-N films are su-perior to that of Fe-N films, and no obvious differencein the binding energies of Fe, Cr and N is detected inthe XPS spectra of the Fe-Cr-N, Fe-Cr and Fe-N films.AcknowledgementsThe authors express their appreciation to Dr. T. Takadaand Mr. Y Murakami for their support of the chemicalanalysis. One of the author (D. L. Peng) appreciatesthe financial support from the Ministry of Education,Science, Culture and Sport, Japan. This work was alsopartially supported by Grant-in-Aid for General Scien-tific Research (No. 08405043) given by the Ministry ofEducation, Science, Culture and Sport, Japan.References1. Y . I I T A K E andY . S H I M A D A , J. Mag. Soc. Jpn.15(1991) 361.2. K . N A K A N I S H I , O. S H I M I Z I and S. Y O S H I D A, ibid. 15(1991) 371.3. K . N A G O, H. S A K A K I M A andK . I H A R A , ibid. 15 (1991)365.4. J. C. L I N , L . J. C H E N andC. J. C H E N, IEEE Trans. Magn.30 (1994) 3912.4627P1: FJL/FIX P2: FJL/FGD P3: FDB/FGD QC: FDB 29-KJ007-62-97 June 30, 1999 9:325. G. Q I U , E. H A F T E K andJ. A . B A R N A R D, J. Appl. Phys.73(1993) 6573.6. T . S H I M A T S U, H. U W A Z U M I , M . T A K A H A S H I and T.W A K I Y A M A , J. Magn. Soc. Jpn.15 (1991) 375.7. D. L . P E N G, K . S U M I Y A M A , M . O K U, D. X . L I andK .S U Z U K I , Phys. Stat. Sol. (a)157(1996) 139.8. D. L . P E N G, K . S U M I Y A M A and K . S U Z U K I , J. AlloysCompd.255(1996) 50.9. D. L . P E N G, K . S U M I Y A M A , T. J. K O N N O and K .S U Z I K I , Jpn. J. Appl. Phys.36 (1997) L479.10. J. F. M O U L D E R,W. F. S T I C K L E,P. E. S O B O L andK . D.B O M B E N, in “Handbook of X-ray Photoelectron Spectroscopy,”edited by J. Chastain (Perkin-Elmer Corp., Physical Electronics Di-vision, Minnesota, 1992).Received 27 January 1997and accepted 9 March 19994628

X-ray diffraction and X-ray photoelectron spectra of Fe-Cr-N films deposited by DC reactive sputtering

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X-ray diffraction and X-ray photoelectron spectra of Fe-Cr-N films deposited by DC reactive sputtering

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