Pulsed laser deposition of hexagonal and cubic boron nitride films

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1 1 G. Reisse, S. Weissmantel: presented at the Fifth International Conference on Laser Ablation COLA 99, July , Göttingen, Germany, Applied Physics A 69 (1999) 749. Pulsed laser deposition of hexagonal and cubic boron nitride films Guenter Reisse and Steffen Weissmantel Hochschule Mittweida, University of Applied Sciences, Technikumplatz 17, Mittweida, Germany, Telephone , Telefax , steffen@htwm.de, greisse@htwm.de Hexagonal and cubic boron nitride films have been deposited by pulsed laser ablation from a boron nitride and a boron target using a KrF excimer laser. Hexagonal films are deposited in nitrogen as background gas or with nitrogen/argon ion bombardment at iontoarrivingtargetatom (I/A) ratios at the substrate below 0.5. Nucleation of the cubic phase takes place exclusively with ion bombardment at I/A ratios above 1.0, which may be reduced down to 0.6 after the completion of the nucleation process. The influence of the parameters of the laser and ion beams on the properties of the hexagonal films are presented. The Vickers microhardness and the intrinsic stress of those films vary in wide ranges of 5 to 25 GPa and 1 to 16 GPa, respectively. Pulsed laser deposited hexagonal boron nitride films show good adhesion to silicon and stainless steel, if they are deposited at I/A ratios below 0.5, and can be used as intermediate layers for improving the adhesion of cubic boron nitride films. So far, 0.5 µm thick nearly phase pure cubic boron nitride films with good adhesion have been deposited. The microstructural, mechanical and optical properties of those layer systems are presented and discussed. Keywords Cubic boron nitride, hexagonal boron nitride, ionassisted pulsed laser deposition, microstructure 1. Introduction Pulsed laser ablation has been used recently for the preparation of boron nitride films [15]. It was shown that the formation of the cubic phase requires additional ion bombardment of the growing films [2,5], even though the ablated species have high mean kinetic energies of several 10 ev. A general problem with the deposition of cubic boron nitride films (cbn) is their poor adhesion caused by the high intrinsic stresses in the films and the growth of a specific hexagonal nucleation layer between substrate and cbn film. For this reason there are only a few reports on the deposition of adherent cbn films with thickness greater than 100 to 200 nm in the literature. Litvinov et.al. [6] and Mirkarimi et.al. [7] achieved 2 µm and 0,5 µm, respectively by depositing c BN films at high substrate temperatures of some 1000 C, low ion energies below 100 ev and low growth rates below 1 nm/min. Intermediate layers have also been used to improve film adhesion. Examples are boron or graded BN x [8], B x N y Si 1xy [9] or turbostratic ibn [10] as intermediate layers between silicon or WCCo substrate and the cbn film. We have reported on the deposition of nearly phase pure cbn films on top of hbn nucleation layers with maximum growth rates of 16 nm/min using ionassisted pulsed laser deposition and their structural characterization [5,12]. Many of the films readily delaminated even at low thicknesses in the range of 100 nm due to the above mentioned problems. The use of pulsed laser deposited hexagonal boron nitride intermediate layers enabled us to improve significantly the adhesion of the cbn films and to deposit 400 nm thick adherent cbn films [11,13]. In our previous papers, we used the term lbn for the designation of those intermediate layers as they had some specific properties, such as high hardness, and in order to distinguish them from the nucleation layers. However, since those layers do not represent a new phase and to avoid misunderstandings we will not use the term any more. In this paper, we present the microstructural properties of hbn/hbn nucleation/cbn layer systems and the mechanical and optical properties of hexagonal boron nitride films.

2 2 2. Experimental details The schematic experimental arrangement used for the preparation of boron nitride films by ionassisted pulsed laser deposition is shown in Fig. 1. The film forming species are produced by laser ablation from % pure pyrolytic hexagonal boron nitride targets using a KrF excimer laser (248 nm wavelength, 30 ns pulse duration), where laser pulse energy densities in the range of 7 to 20 J/cm 2 have been used. Those species are supplied to the surface of the growing films in form of pulsed highdensity and highenergy particle fluxes with durations in the range of a few microseconds. The films were deposited either with continuous bombardment with nitrogen/argon ions from an r.f. ion source with space charge neutralization or with only the nitrogen/argon plasma kept on in the ion source without ion acceleration, in order to deposit stoichiometric boron nitride films and to activate the film growth. In both cases the working pressure was Pa (with Pa base pressure), where the ratio of partial pressure of nitrogen and argon was 2:1. The ion energies were 400 to 700 ev yielding ion current densities of 190 to 570 µa/cm 2, respectively. The iontoarrivingtargetatom (I/A) ratio at constant ion beam parameters was varied by varying the laser pulse repetition rate. Additionally, the influence of photons on film growth can be investigated by using the substrate laser beam [5]. The laser beam was focused onto the target surface and motioned in spirals with constant vector velocity (12.5 mm/s) across a target area 10 mm in diameter. Films were deposited on (100) oriented silicon or stainless steel substrates. Prior to deposition the substrates were cleaned chemically and strongly argon ion bombarded in order to remove macroscopic and microscopic impurities. A continuous transition to film deposition was realized by adding nitrogen to the working gas of the ion source, at first, and switching on the target laser beam afterwards. The cleaning procedure and the film growth was controlled by means of an insitu ellipsometer operating at nm wavelength and 70 angle of incidence. The refractive indices of boron nitride films were obtained by simulation of the measured insitu ellipsometric PsiDelta curves. The microstructure of the films was investigated by means of transmission electron microscopy (TEM) using a 200 kev highresolution electron microscope Philips CM 20. The Vickers microhardness were measured dynamically on 1 µm thick films on silicon substrates using a maximum load of 7.5 mn, where the maximum indentation depth was 150 nm. Film stress was determined from substrate bending induced by the deposited films using also silicon substrates. The substrate bending curves were measured by using a DEKTAK profilometer. 3. Results and discussion Pulsed laser deposition has been used for the preparation of hexagonal and cubic boron nitride films. Phase pure hexagonal films are obtained without ion bombardment or with ion bombardment of the growing films at relatively low iontoarrivingtargetatom (I/A) ratios. The microstructure of those films is generally characterized by a strong preferred orientation of the crystallites with the c axis parallel to the substrate surface and by a random orientation about the substrate normal. This specific preferred orientation is always observed if the film growth takes place under the influence of energetic particles. In our case it is attributed to the ion bombardment and/or the high mean kinetic energies of the pulsed laser ablated species, which were estimated from kalorimetric measurements to be in the range of 25 to 75 ev/atom depending on the laser pulse energy densities used. Nucleation of the cubic phase takes place upon that hexagonal phase exclusively at I/A ratios above 1.0. The I/A ratio required for the further growth of nearly phase pure cbn films after nucleation is only 0.6 allowing increasing growth rates at constant ion beam parameters. The cbn crystallites show a strong <110> preferred orientation perpendicular to the substrate and are also randomly oriented about the substrate normal. In earlier investigations by HRTEM [5,12], we observed in the nucleation regions that both (111) and (001) lattice planes of cbn grow parallel to the (0002) hbn planes. The lattice matching for the transition from hbn to cbn is 2 : 3.2 and 1 : 0.92, respectively. We also observe the commonly known poor adhesion of cbn films, if the nucleation parameters are adjusted from the beginning of film deposition. One reason for that poor adhesion is the constant formation of a 15 to 20 nm thin hexagonal nucleation layer prior to the nucleation of the cubic phase.

3 3 We found that the adhesion of cubic boron nitride films on both silicon and stainless steel substrates can be improved significantly by depositing a hexagonal boron nitride film without ion bombardment or at I/A ratios below 0.5 prior to switching to the nucleation parameters of the cubic phase (cbn). In this way, we were able to prepare 0.5 µm thick adherent cbn films. In order to optimize the process for the deposition of welladherent cbn films, the variation of the properties of the hexagonal boron nitride films with deposition parameters has been investigated. The effective refractive indices of the hexagonal boron nitride films deposited without ion bombardment at laser pulse energy densities above 10 J/cm 2 were found to be in the range of 1.94 to 1.98 (see Table 1). The extinction coefficients of those films, determined from the ellipsometer curves of 1 µm thick films, are below provided the growth rates are below 30 nm/min, indicating to good stoichiometry. In consequence of nitrogen/argon ion bombardment of the growing hexagonal films, a decrease in the effective refractive indices is observed (see Fig. 2.). At I/A ratios above 1.0, that is at the nucleation parameters, the refractive indices of nucleation layers deposited directly on the substrate are only 1.70 to Higher effective refractive indices should be related to higher packing densities of the corresponding films in comparison with the hbn layers deposited at the nucleation parameters (Note that the measurements were performed at the same angle of incidence of 70 to the substrate normal and that all the hexagonal films have a similar microstructure with the same preferred orientations.). The hexagonal films deposited without ion bombardment show a high Vickers microhardness depending on the laser pulse energy density as shown in Fig. 3a, indicating to a strong crosslinking of the hexagonal lattice planes. Increasing nitrogen/argon ion bombardment of the growing films results in significantly lower Vickers microhardness down to values of only 5 GPa (see sample 15). Apparently, the degree of crosslinking and the packing density decrease under those conditions in agreement with the corresponding variation of the effective refractive index. The intrinsic stresses of hbn films deposited without ion bombardment also show a distinct dependency on laser pulse energy density (see Fig. 3b). Films deposited at room temperature have compressive stresses up to 8 GPa decreasing down to 2 GPa at laser pulse energy densities below 8 J/cm 2 and above 40 J/cm 2. Deposition of hbn films at elevated substrate temperature, as used during the deposition of cbn films, results in strongly increasing stresses of 10 to 16 GPa. The stresses of those hbn films decrease strongly with growing ion bombardment (see Fig. 2.). Though the reduced stresses are accompanied by a decreasing hardness, the hbn films deposited at elevated substrate temperature and I/A ratios of 0.4 might be particularly suited as adhesion improving interlayers for cbn films, as they, too, show excellent adhesion and, moreover, as the nucleation of cbn apparently takes place direct on top of them without formation of the mechanically less stable hbn nucleation layer, which has been deduced from the in situ ellipsometric curves [13]. An example for an hbn/hbn nucleation/cbn layer system is shown in Fig. 4. The selected area diffraction pattern taken from the near substrate region shows reflections from the (100) oriented silicon substrate and from hexagonal as well as cubic boron nitride. The form of the inner hbn (0002) reflections, coming from the hexagonal layers, shows the preferred caxis orientation of the crystallites of both layers parallel to the substrate surface, which can also be seen in the corresponding HRTEM image. The form of the outer cbn reflections shows that the cbn crystallites are strongly oriented, too. This orientation can be observed over the entire thickness of the nearly phase pure cbn film, where no crystalline hexagonal BN can be observed (see the selected area diffraction pattern taken from the upper region of the cbn film). The two dark field images, both taken from the same crosssectional region but one formed with an hbn (002) reflection and the other with a cbn (111) reflection of the nearsubstrate region diffraction pattern, also show that the hexagonal phase is present exclusively in the hexagonal layers. The evaluation of the cbn diffraction pattern yielded a <110> preferred orientation of the crystallites perpendicular to the substrate. The indexing of the diffraction pattern as shown in Fig. 4e can be visualized by rotating the cbn lattice about an axis parallel to the [110] direction, being perpendicular to the substrate surface and the electron beam. In this specific orientation the (001), the (110) and the (111) lattice planes are always perpendicular to the substrate surface, that is

4 4 parallel to the growth direction. The reflections appear successively during the rotation, which corresponds to the random orientation of the crystallites about the substrate normal. Thereby, the reflections marked red appear simultaneously if the (001) lattice planes are parallel to the electron beam. 4. Conclusions Ion assisted pulsed laser deposition is a suitable method for the preparation of boron nitride films. Depending on the I/A ratio at the substrate hexagonal or nearly phase pure cubic films can be deposited. The adhesion of cubic films can significantly be improved by using hexagonal interlayers deposited at I/A ratios below 0.5. Reasons for this are the high adhesive strength of those hexagonal films to silicon and stainless steel substrates as well as a reduction of the thickness or the complete avoidance of the hbn nucleation layer. Acknowledgements The authors gratefully acknowledge financial support of the present work by the Deutsche Forschungsgemeinschaft (Project No. RE883/32) carried out under the auspices of the trinational DACH German, Austrian and Swiss cooperation on the Synthesis of Superhard Materials and by the Sächsisches Staatsministerium für Wissenschaft und Kunst (Project No /3). The authors thank Mrs. G. Baumann, Dr. S. Schulze and Mr. T. Chudoba from the Technical University Chemnitz for the crosssectional preparations, the TEM investigations and the hardness measurements, respectively. References 1. G.L. Doll, J.A. Sell, C.A. Taylor II, R. Clarke, Phys. Rev. B 43 (1991). 2. D.L. Medlin, T.A. Friedmann, P.B. Mirkarimi, P. Rez, M.J.Mills, K.F. McCarty, J. Appl. Phys. 76, 1 (1994) B. Angleraud, C. Girault, C. Champeaux, F. Garrelie, C. Germain, A. Catherinot, Appl. Surf. Sci (1996) W. Pfleging, T. Klotzbüchner, D.A. Wesner, E.W. Kreutz, Diamond Relat. Mater. 4 (1995) S. Weissmantel, G. Reisse, B. Keiper, A. Weber, U. Falke, M. Röder, Appl. Surf. Sci (1998) D. Litvinov, R.Clarke, Appl. Phys. Lett. 74, 7 (1999) P.B. Mirkarimi, D.L. Medlin, K.F. McCarty, D.C. Dibble, W.M. Clift, J.A. Knapp, J.C. Barbour, J. Appl. Phys. 82, 4 (1997) M. Okamoto, H. Yokoyama, Y. Osaka, Jpn. J. Appl. Phys. 29 (1990) K. Inagawa, K. Watanabe, K. Saitoh, Y. Yuchi, A. Itoh, Surf. Coat. Technol. 39/40 (1989) T. Ikeda, Y. Kawate, Y. Hirai, J. Vac. Sci. Technol. A8 (1990) S. Weissmantel, G. Reisse, B. Keiper, S.Schulze, Diamond Relat. Mater. 8 (1999) S. Weissmantel, G. Reisse, Characterization of ionassisted pulsed laser deposited cubic boron nitride films, presented at the ICMCTF99, San Diego 1999, to appear in the Proceedings and in Thin Solid Films. 13. G. Reisse, S. Weissmantel, The use of pulsed laser ablated boron nitride interlayers for improving the adhesion of cubic boron nitride films, presented at the ICMCTF99, San Diego 1999, to appear in the Proceedings and in Thin Solid Films.

5 Figure captions 5 Table 1 Deposition parameters and properties of various pulsed laser deposited boron nitride films on silicon substrates (T S substrate surface temperature, H 0T laser pulse energy density at the target, A laser spot size on the target, f T laser pulse repetition rate, E I ion energy, j S ion current density at the substrate, rgrowth rate, I/Aratio of ions to target atoms arriving at the substrate, p I /BNion supplied momentum per deposited BN, E/Vion supplied energy per film volume, d f film thickness, nrefractive index, VHVickers microhardness, σfilm stress). The number of target atoms arriving at the substrate was determined from the growth rates without ion bombardment. Densities used for the calculations were 2.28 g/cm 3 for hbn and 3.49 g/cm 3 for cbn. a measured on the whole sample 1, b measured on a 50 nm as well as a 150 nm thick film deposited at the same parameters, c n=1.85 (4 nm), n=1.88 (5 nm), d extinction coefficient k=0.02 probably due to a slight nitrogen deficiency. Fig. 1. Schematics of the experimental arrangement. Fig. 2. Variation of refractive index and film stress of hbn films with ion beam parameters ( C substrate temperature in dependence of ion bombardment, 15 J/cm 2 laser pulse energy density). Fig. 3. Variation of Vickers microhardness (a) and film stress (b) of hbn films deposited without ion bombardment with laser pulse energy density. Fig. 4. Crosssectional TEM micrographs and diffraction patterns of a hbn/hbn nucleation/cbn layer system. Deposition parameters: 10 J/cm 2 laser pulse energy density, 360 C substrate temperature; hbn: no ion bombardment; cbn: 500 ev ion energy, 310 µa/cm 2 ion current density; cbn nucleation: I/A=1.06; cbn growth: I/A=0.60. a) General view including selected area diffraction patterns from the Sisubstrate/hBN/h BN nucleation layer/cbn region and from the cbn layer itself. b) Dark field image formed with a hbn (002) reflection from the lower diffraction pattern. c) Dark field image formed with a cbn (111) reflection from the lower diffraction pattern. d) HRTEM image of the section of the hbn layer shown in a). e) Comparison of the diffraction pattern of the cbnlayer with the theoretical pattern considering preferred orientation of the cbn crystallites with the (110) lattice planes lying parallel to the substrate surface and random orientation about the substrate normal.

6 6 Table 1. Sample Deposition parameters Film properties T S H 0T A f T E I j S r I/A p I /BN E/V d f n VH σ [ C] [J/cm 2 ] [mm 2 ] [Hz] [ev] [µa/cm 2 ] [nm/min] [(ev. amu) 1/2 ] [ev/nm 3 ] [nm] [GPa] [GPa] 1 hbn hbn 385 cbn c a 2 hbn hbn cbn 3 hbn cbn cbn a 5.3 a 4 hbn hbn hbn hbn hbn hbn d hbn hbn hbn b 13 hbn hbn hbn

7 7 Fig. 1. Vacuum chamber Target laser beam Substrate Ellipsometer / Transmitter Ion beam Particle flux Ellipsometer / Receiver Ion source Ion beam neutralizer Target Plasma Substrate Laser beam

8 Fig Film stress [GPa] Ion current density [µa/cm 2 ] Stress Refractive Index Ion energy [ev] 2 1,9 1,8 1,7 1,6 1,5 Refractive Index Fig. 3. a Vickers microhardness [GPa] T < 50 C T = 350 C Laser pulse energy density [J/cm 2 ] Fig. 3. b Film stress [GPa] T < 50 C T = 350 C Laser pulse energy density [J/cm 2 ]

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Substrate surface effect on the structure of cubic BN thin films from synchrotron-based X-ray diffraction and reflection

$Substrate surface effect on the structure of cubic BN thin films from synchrotron-based X-ray diffraction and reflection$ Substrate surface effect on the structure of cubic BN thin films from synchrotron-based X-ray diffraction and reflection X.M. Zhang, W. Wen, X.L.Li, X.T. Zhou published on Dec 2012 PHYS 570 Instructor