Microstructure of BN:C films deposited on Si substrates by reactive sputtering from a B4C target

ELSEVIER

Thin SolidFilms 287 (1996) 193-201

Microstructure of BN:C films deposited on Si substrates by reactive sputtering from a B a C target M.P. Johansson a, L. Hultman a, S. Daaud b, K. Bewilogua b, H. Ltithje b, A. Schiitze b, S. Kouptsidis b, G.S.A.M Theunissen c a Thin Film Physics Division, Department of Physics, LinkOping University, S-581 83 Link@ing. Sweden b Fraunhofer-lnstitutefor Surface Engineering and Thin Films, Bienroder Weg 54 E, D-38108 Braunschweig, Germany c Philips Research, Prof. Holstlaan 4, 5656 AA Eindhoven, Netherlands

Received 22 September 1995;accepted29 January 1996

Abstract

The micmstmcture and composition of boron nitride:carbon (BN:C) thin films prepared by reactive r.f. diode sputtering of a B4C target in mixed Ar and N2 discharges were determined by high-resolution electron microscopy, electron diffraction, infrared spectroscopy, electron probe microanalysis, and X-ray photoelectron spectroscopy. Films were prepared with three characteristic phase compositions; cubic BN:C (c-BN), turbostratic BN:C (t-BN), and phase mixture of c-BN and t-BN on Si (001 ) substrates. While keeping the B/N ratio close to unity, phase structures were mainly correlated to the energy and flux of impinging (Ar + + Nf ) ions towards the negatively d.c. biased substrate. At a constant ion flux, substrate biases of 500 V yielded c-BN films while biases lower than 300 V resulted in t-BN. Films prepared with the same ion flux and with biases between 300 and 500 V consisted of c-BN and t-BN phase mixtures. The film phase evolution in c-BN films was from an initial amorphous BN:C (a-BN) layer, over a highly oriented t-BN layer with the c axis parallel to the film surface, to a c-BN layer exhibiting (110) preferred orientation. Films consisting of c-BN and t-BN phase mixtures were non-textured nano- to sub-microcrystalline. The c-BN layers/grains showed twinning on the c-BN( 111 ) lattice planes. As-deposited films contained as much as 5-15 at.% of C with mainly C-.C and B-C bonds. The film C content decreased with increasing volume fraction of c.BN. Keywords: Boronnitride; Carbon; Depositionprocess; Sputtering;Boroncarbide:Transmissionelectron microscopy (TEM)

1. Introduction

Pure boron nitride (BN) primarily forms in three crystalline phases [ I ], ~ ~oft sp2-bonded ~raphite-like structure with hexagonal symme,try (h-BN), a hard sp3-bonded zinc blende structure with cubic symmetry (c-BN), and a hard sp 3bonded wurtzi~e structure with hexagonal symmetry (wBN). The latter two phases are metastable and analogous to diamond and lonsdaleite in the carbon system, respectively. When synthe~;,irmg thin films of BN, however, amorphous BN (a-BN) arid turbostratic-like BN (t-BN) phases, are commonly pre,~ent, t-BN can be considered as disordered hBN, with a different stacking of the hexagonal basal planes [2] and similar to that of turbostratic C [3 ]. The basal planes of h-BN have a regular...ABAB.., stacking sequence while the t-BN basal planes are randomly rotated around the c axis. Nevertheless, the t-BN layers reported in this work typically exhibit curvature of the basal planes. Thin films of BN have been studied with an increasing interest over the past few years and large efforts are being 0040-6090/96/$15.00 © 1996ElsevierScienceS.A. All rights reserved Pll S0040-6090 (96) 08 772-X

made in order to understand the phase formations, microstructure, and chemistry of the material. The main interest, ho~ever, has been focused on synthesizing c-BN that is known to have excellent mechanical and thermal properties [4] which makes it one of the most promising materials for wear and corrosion protection. BN films have been synthesized by plasma assisted chemical vapor deposition [ 5,6 ] and physical vapor deposition methods such as ion-beam assisted laser ablation [7,8], electron-beam evaporation [9,10] and reactive r.f. sputtering [ 11-13] from B and BN targets. Typically, these films consist of c-BN and h-BN phase mixtures. Phase characterizations has mainly been performed by Fourier transform infrared spectroscopy (FTIR), but also by electron energy loss spectroscopy (EELS) [ 14,15 ], Raman spectroscopy [16], transmission electron microscopy (TEM) [ 7,8,10,17 ], and X-ray photoelectron spectroscopy (XPS) [ 181. Radio-frequency (r.f.) sputter deposition offer the possibility of using not only conductive targets, but also insulating targets such as h-BN and pure B as a source of B in the

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preparation of BN films. However, low deposition rates and complex glow discharges are common to r.f. sputtering. These effects are less of a problem in the case of d.c. glow discharges where a better control over the important growth parameters such as particle fluxes and their energy (or energy distributions) in both neutral and ionized states at the subsmite are obtained. On the other hand, conductive targets are a necessity in the case of d.c. sputter depositions. B and BN can be made conducting by doping or adding compoundforming elements such as C.[ 19] The compound B4C (p < 102 [1 cm at room temperature) was recently used by us [ 17] as target material for reactive r.f. sputter deposition of cubic BN:C films. B4C thus shows potential as target material for reactive d.c. sputter deposition of BN:C films. In addition to diamond and compounds in the B-N, B-C, and C-N binary systems, ternary boron-nitrogen-carbon (B-N-C) materials show high hardness and variable conducting properties.[20] Both CVD [21,22] and high pressure-high temperature [23,24] methods have been used to prepare B-N--C compounds which in most cases consists of pyrolytic C and pyrolytic BN phase mixtures. However, two different film compositions (BC2H and BC4N) are known to exist in solid solutions.[20] Badzian found that mixed crystals of h-BN and graphite undergoes a phase transformation to c-BN and diamond under the conditions of high pressures ( 14 GPa) and temperatures (3 300 K) [24]. Sirota et al. showed by hot pressing of h-BN and B4C mixtures (5 kbar, 1 900-2 800 oC) that the solubility of B4C in c-BN is about 7 tool.% [25]. Higher concentrations of B4C in the mixture resulted in mixed c-BN and B4C phases. In this paper we report on the microstructure and composition in BN:C films prepared by reactive r.f. diode sputtering of a B4C target in mixed Ar and N2 discharges on Si(001 ) substrates. All BN:C films in this study had a B/N ratio close to unity. Films were prepared with three characteristic phase compositions, ¢ubi0 BN:C (c-BN), turbostratic BN:C (tBN), and homogeneous phase mixtures of c-BN and t-BN. For simplicity, the terminology c-BN and t-BN will be used throughout the paper, The structure and composition of asdeposited films were characterized using TEM including high-resolution TEM (HREM), electron diffraction, Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) and electron probe microanalysis (EPMA).

2. Experimental details BN:C films were grown in a r.f. diode sputter deposition system evacuated to a base pressure of 10 -4 Pa. The system is equipped with an external solenoid that gives rise to a superimposed axial-symmetric magnetic flux {B[ < 40 G that is linearly dependent on the coil current (it). A full description of the deposition system is given elsewhere [ 13]. The target was a 99.5% pure B,,C disc and both Si(001) wafers and discs made of high-speed steel (HSS) were used as

substrate materials. Ar and N2 were used as sputtering and reactive gases, respectively. Films were grown to a total film thickness ranging between 0.1 and 1.3 I~m at different gas mixtures, substrate bias voltages, substrate temperatures, and magnetic field configurations. The total gas flow (QAr+ QN2) was kept at 50 sccm giving rise to a total gas pressure of 2.7 Pa while the nitrogen flow ratio, F = QN, I ( Q A r + QN,), was varied between 0 and 100%. The r.f. power at the target was kept constant at P, = 1 kW and the power at the substrate (Ps) was varied between 50 and 400 W resulting in a negative substrate d.c. bias (Vb) from 200 to 500 V. The deposition rate decreased approximately linearly with increasing Vb from 15 nm min - t (Vb--200 V) to 4 nm min-l (Vb f 5 0 0 V). Films were deposited at initial substrate temperatures (T ini') between 25 and 350 °C. Due to the ion bombardment during growth, the substrate temperature increased to steady-state temperatures (T n"~a) between 330 and 410 °C, within a few minutes after initiating growth. A detailed description of the deposition process and process conditions for as-deposited BN:C films (see Table 1) are given in Ref. [ 17]. The relative content of c-BN in the films were determined by the peak height ratio of the absorbance peaks in the FTIR transmittance spectra as Qc = I(c-BN)/I(h-BN) [ 17] where I(c-BN) is the peak height near 1065 cm- i associated to an IR active TO mode of sp3-bonded B-N in c-BN [26] and I(h-BN) the peak height near 1400 c m ! associated to an in-plane B-N sp 2 bond stretching mode in h-BN [27]. Moreover, sp2-bonded B-N also has an absorption near 800 cm-~ associated to an out-of-plane B-N-B bond bending mode [27]. The spectra of as-deposited BN:C films was obtained using a Pye Unicam PU 9516 IR-spectrometer, Microstructural aspects such as grain sizes, phases, and growth structures of the as~deposited films were characterized using TEM and electron diffraction. Both cross-sectional and plan-view TEM samples were studied using Philips CM 20 UT and JEOL 4000 EX electron microscopes operated at 200 and 400 kV, respectively. Samples for TEM were prepared using mechanical grinding followed by ion etching ( 10 keV Ar + ions) to electron-transparency using a Baltec RES 010 rapid ion etching system. Samples having a high relative content of c-BN were typically heavily stressed and suffered from severe film peeling during the ion thinning process. This effect was minimized using ion beam incidence angles of 7.5 ° and reducing the Ar + ion energy to 3 keV in the final stages of thinning. TEM samples were also prepared by scraping off the BN film onto a Cu grid. The state of chemical bonds in the films were studied by X-ray photoelectron spectroscopy (XPS). The spectra were obtained in a V.G. Scientific ESCALAB spectrometer using Mg Kl., ( 1253.6 eV) radiation in a vacuum chamber operating at a pressure lower than 9.3 × 10-s Pa. The analyzed area was about 0.2 mm 2. All spectra were recorded at a constant retardation ratio of 20. Charging corrections were made using the as-measured Fermi level of Ag and Au pieces on the metallic substrate holder using the Mg Kcx radiat ~on as a

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reference. The peaks were deconvoluted assuming Gaussian profiles according to the method described by Sherwood

[28]. The elemental composition of as-deposited BN:C films were measured by wavelength dispersive X-ray spectroscopy (WDS) using a Cameca SX 50 system in which up to five independent WDS detectors can be used simultaneously. This technique is also known as electron probe microanalysis (EPMA). The resolution of low-Z elements such as B, C, N, and O are approximately 0.1-0.5 at.%.

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3. Results and discussion

The deposition process was optimized with respect to the maximum fraction of sp3-bonded B-N, as measured by IR spectroscopy, and c-BN films were obtained within the range of the deposition parameters given in Table 1. This section describes the microstmcture as observed by TEM and correlates it with measured composition as a function of the deposition parameters, nitrogen gas flow ratio (F), substrate temperature (T~n~), internal magnetic coil current (lc), and substrate bias (Vb) for relative contents of the cubic phase ranging from essentially single-phase c-BN (Sample A) to single-phase t-BN (Sample E). o

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3.1. Microstructure and composition of c-BN films

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BN:C films prepared within a narrow deposition parameter window consisted of almost single phase c-BN. Fig. I shows a typical IR spectrum of a cubic phase BN:C film (Sample A) prepared with F = 50%, T a"ul= 410 °C, Vb= 500 V, and Ic-" 6 A on a Si(001 ) substrate. Only one peak at 1065 cm- ' characteristic of sp3 bonded B-N is present in the spectrum, indicating a single-ph~e c-BN. Fig. 2 shows a cross-sectional bright-field TEM (XTEM) image with corresponding selected-area electron diffraction (SAD) pattern from Sample A. Typical features in this image are three distinct regions of the film growth structure with an initial ~ 6 nm thick amorphous region, a ~ 10 nrn thick

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Fig. !. TIR spectrumofa c-BNfilm (SampleA) showingonlyone peakat 1065 cm-I characteristicof sp3-bondedB-N.

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Fig, 3. Cross-sectional HREM images of Sample A obtained in the Sl [ 110] projection showing (a) the top surfaL;eand (b) the sabstrate-to- fihn interface of the BN:C film.

Fig. 2. Cross-sectionalTEM imageof Sample A obtainedin the Si [ 110] projection with the corresponding SAD pattem. The BN:C film phase evolution sequence is from an initial a-BN layer, a highly oriented t-BN layer withthe c axis parallel to the Si surface, to the c-BN layerexhibitingan outof-plane (110) preferred orientation. The SAD pattern is obtained at the substrate-to-film interfaceand indexedby overlappingthe patternsof [ 110]

and [ ! 12] projectionsof cubic-structure grains oriented with a common [ !10] directionperpendicularto the filmsurface, crystalline layer showing discontinuous lattice fringes with strong curvature (labelled as t-BN), and a ,-,200 nm thick polycrystalline layer (labelled as c-BN). The amorphous region shows contrast from two layers (eL Fig. 3) of which the upper layer is interpreted as a-BN. The layer closest to the substrate, however, is most likely a contamination layer. The SAD pattern was obtained using a SAD aperture ( 1 p,m in diameter) centered on the BN:C film, partly covering the Si substrate. Thereby, the SAD pattern also contains weak

diffraction spots originating from the Si substrate. The BN:C film contribution to the SAD pattern are indexed as c-BN exhibiting (110) preferred orientation. A small contribution from the thin t-BN interlayer is also seen in SAt) pattern labelled as (0002)t. The c-BN diffraction spots in the SAD pattern may be indexed by overlapping the patterns of [ 110] and [ ! 12 ] projections of cubic-structure grains oriented with a common [ 110] direction perpendicular to the film surface. The first ring in the resulting SAD pattern thus consists of six segments with high relative intensities originating fi~om cBN( 111 ) reflections. These segments are labelled as ( 111 )c. Fig. 3 shows cross-sectional high resolution electron microscopy (HREM) images of Sample A obtained at higher magnification of (a) the top surface and (b) the substrateto-film interface of the BN:C film. The resolved lattice fringes in Fig. 3(a) arc separated by 0.21 nm which is consistent with the c-BN( 111 ) lattice planes. Furthermore, the c-BN layer shows an out-of-plane (110) preferred orientation in the area investigated. This was also observed in the SAD pattern in Fig. 2. Fig. 3(b) shows two sets of parallel lattice

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M.P. Johansson et al. I Thin Solid Film,s 287 (1996) 193-201

f~i,ges ~cp~ated by 0.21 nm (tcp part) and 0.347 nm (middle part) corresponding to c-BN( 111 ) and t-BN(0002) lattice planes, respectively. The t-BN layer exhibited preferred orientation with the c axis parallel to the Si(001) surface while the out-of-plane (110) preferred orientation of the cBN is evident already at the onset to c-BN at the t-BN phase. However, it has not been possible to determine the sites of cBN nucleation at the t-BN phase. On the other hand, once formed the cubic phase grew without further phase transformation to the final film thickness of ~ 0.2 Ixm. The same deposition parameter window for growth of almost single phase c-BN on Si(001) substrates was also used on a HSS substrate. Fig. 4 shows the SAD pattern obtained using a SAD aperture ( 1 ~m in diameter) centered on the BN:C film of such a sample. This diffraction pattern is built up in a similar manner as described in Fig. 2. The first ring in the SAD pattern consists of six segments with high relative intensities that originates from c-BN(II 1) reflections (labelled as ( 111 ),:) and similar as above indicating a c-BN phase with (110) preferred orientation. The compositional analysis of Sample A showed B and N with relative contents of 44 at.% and 49 at.%, respectively (see Table 1). This sample also contained 5 at.% of C, a relatively small amount of Ar ( 1 at.%), and small amounts of other impurities ( < ! at.%) such as O, Fe, and Cr. Comparing the EPMA and IR results of the characterized films (see Table 1) and complementary films [ 17] havh~g different relative contents of the cubic phase, it was typically observed that films with a high relative content of c-BN (Q¢>_60%) contained < 10 at.% of C and ,-,0.5-1 at.% of Ar while lilms with Q,: <60% contained ~ 10-15 at.% of C and '-, 0-0.5 at.% of At. The increase in At' content was correlated to an increase in the substrate bias value and the corresponding increase in Ar ion energies. For the higher ion energies, the increased Ar projected ion range accompanying the momentum transfer associated with cubic-phase formation in BN [10,291 also resulted in enhanced trapping probability Ibr Ar [9]. The reason why the film C content is lower in BN:C films exhibiting a high relative content of c-BN is presently not known. Possible mechanisms for the reduction of C in BN:C films include chemical sputtering of volatile CN or possibly

CO species. Sj6str6m et al. suggested chemical sputtering of CN species when growing CN~ films at elevated substrate temperatures by reactive sputtering of pure C targets in mixed Ar and N2 discharges [ 30]. The state of chemical bonding in as-deposited BN:C samples was determined by XPS measurements. Fig. 5 ( a)-5 (c) shows the B I s, N I s, and C 1s core level spectra, respectively, from Sample A as obr,fined after Ar-ion sputter etching. The B Is and N Is spectra (see Fig. 5(a) and 5(b) ) were deconvoluted into single peaks centred at binding energies of 190.4 and 397.6 eV, respectively. However, the C 1s spectrum is best deconvoluted into three peaks with binding energies 282.5, 284.6, and 286.4 eV (see Fig. 5(c) ). The B Is, N Is, and C I s peak area intensity ratio was determined using sensitivity factors as given in Ref. [31 ] to yield B:N:C = 11:8:1. Although this intensity ratio underestimates the film N content (see Table 1) a reasonably good agreement between XPS and EPMA is obtained. Comparing the obtained binding energies of the B | s and N 1s spectra with standard spectra implicates BN as the most probable candidate [ 31 ]. Another possibility is the formation of C-N bonds. Marton et al. discussed, based on a comparison between the carbon binding energies in pyridine (C2HsN) and urotropine (C6Ht2N4), that the observed change in binding energies of these materials are due to polarization of the carbon bonds [ 32]. The carbon binding energy in urotropine is 286.9 eV and thus shifted 1.5 eV as compaxed to the nonpolar C-C bonds at 284.4 eV [ 31 ]. Furthermore, the nitrogen BIs

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Fig. 4. SAD patternof e-BN film deposited on a high-speed steel substrate exhibiting an out-of-plane ( ! 10) preferred orientation.

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288 286 284 282 280 Binding Energy (eV) Fig. 5. XPS core level spectra of Sample A showing the high-resolution pa'~s of (a) B Is, (b) N Is, and (c) C Is core level spectra.

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M,P. Johansson et aLI Thin Solid Films 287 (1996) 193-201

binding energy in urotropine is 399.4 eV. Pure CN~ (0.2 < X <0.35) films show two peaks in the C Is spectrum centered at 284.5 and 286.2 eV and two peaks in the N ls spectnnn centered a,~ 398.4 and 400.3 eV [33]. While we observe approximately the same C Is binding energy (see Fig. 5(c)), none of the above-mentioned peak in the N ls spectrum is observed (cf. Fig. 5(a)). Furthermore, only about 1 at.% of the total film C content is bonded with binding energies of 286.4 eV and this small amounts can only give rise to a tail in the N 1s spectrum. Thus, although the presence of C-N bonds cannot be excluded the origin of the peak at 286.4 eV in the C ls spectrum is more likely from single and/or double C-O bonds. The second peak in the C I s spectrum ( see Fig. 5 (c) ) with a binding energy of 284.6 eV is assigned to non-polar C-C bonds such as graphite while the third peak at 282.5 eV is due to a carbidic bond. A natural choice for the latter would be B4C. XPS measurement of the B4C target employed in this study showed a peak in the C l s spectrum centered at a binding energy of 281 eV, i.e. at 1.5 eV lower binding energy as compared with the closest C Is peak in Sample A. According to standard spectra of bulk B4C also a peak in the B I s spectrum at 186.5 eV [ 31] should be present which is not the case in our measurement (cf. Fig. 5 (a)). However, only about 2 at.% of C is bonded with binding energies of 282.5 eV which only give rise to a tail in the B I s spectrum. These results points to a deficient boron carbide compound like BC in which the bonding of both C and B are shifted towards higher binding energies. Based on the fact that the B I s and N Is spectra only yielded one peak each and taking into account the above discussion of the C Is spectrum, Sample A most likely consists of a phase mixture with BN as the dominant phase and possibly small amounts of compounds with C-.C and B-C bonds. The phase evolution of BN:C films on Si(001 ) substrates was from an initial amorphous layer over a highly oriented turbostratic structure with the c axis parallel to the substrate surface to a cubic structure having a (110) preferred orenration. This follows the model for preferred orientation in anisotropic materials under high biaxial compressive stress fields as proposed by McKenzie et al. [34]. These authors showed that the preferred orientation in diamond and c-BN films under high biaxial stresses will be such that a [111] direction, independently of its orientation, lies in the stress plane. However, we found a (110) texture in our ¢-BN films. The (110) orientation of a cubic structure is the only orientation that has two < 111 > axes lying in the stress plane which suggests that this is the most stable configuration in cubic phase BN:C. A preferred orientation of t-BN layer with the c axis lying in the stress plane as well as a film phase evolution sequence of substrate/t-BN/c-BN has also been predicted by McKenzie et al. [34] which are also in agreement with our results. Sequential BN phase formation has previously been observed in pure BN films on Si substrates grown by N~ ion assisted electron-beam evaporation of B [35] and ion-

assisted pulsed laser deposition from a hexagonal BN target [7].

3.2. Microstructure and composition of BN:C phase mixtures BN:C films prepared outside the narrow deposition parameter window for growth of c-BN, typically consisted of a homogeneous phase mixture of c-BN and t-BN or of pure t-BN. Fig. 6 shows a typical IR spectrum of a mixed phase film (Sample B in Table I) prepared with F---60%, T~nal- 330 °C, Vb=550 V, and 1c=6 A on a Si(001) substrate. The two peaks at "-800 cm -~ and ~ 1400 c m - 2 respectively, are characteristic of sp2-bonded B-N and the peak at ~ 1070 cm- t of sp3-bonded B-N which indicates mixed c-BN and t-BN phases. The relative content of c-BN was estimated to Qc ~ 61%. Fig. 7 shows a typical HREM image of Sample B. The film is non-textured nanocrystalline with grains of c-BN and tBN. The grains show lattice fringes sets of 0.21 nm and 0.348 nm characteristic of c-BN( l l 1 ) and t-BN(0002), respectively. One of the c-BN grains (marked) is imaged close to the [ 110] projection. No crystalline BC phase was observed. Also, as observed by XPS, in the samples with lower content ofc-BN (Samples E and F), nearly all boron is bonded to nitrogen and the fraction of C-C and B--C bonds is small or absent. Wurtzite-structure BN:C (w-BN) was only observed in conjunction with the Si substrate and under deposition conditions yielding low relative content of c-BN. Fig. 8 shows a cross-sectional HREM image of a film deposited with T "'"l = 4 l0 °C (Sample D). Two sets of parallel fringes are seen in the BN:C film. The larger spaced fringes (0.347 nm) have a curvature much like the fringes seen in Fig. 3(b) and are indexed as t-BN. The less spaced fringes (0.195 nm) are observed in two directions and corresponds to w-BN (101/) lattice planes. Films prepared with Vb between 300 and 500 V (bias series) were found to consist of a homogeneous phase mixture of c-BN and t-BN whereas films prepared with biases

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Wavenumbers(cm') Fig. 6. FTIR spectrum of a mixed c-BN and t-BN film (Sample B) with

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M.P. Johansson et al, / Thin Solid Films 287 (1996) 193-201

199

Fig. 9. Plan-view HREM images of (a) Sample C showing grains of c-BN imaged in the [ 110] projection and (b) Sample E showing typical lattice fringes of t-BN.

Fig. 7. Plan-view HREM image of mixed c-BN and t-BN film (Sample B), Grains are indicated in the figure showing lattice fringe sets of 0.21 nm and 0.348 nm corresponding to c-BN(Ill) and t-BN(0002) lattice planes, respectively.

Fig. 8. Cross-sectional HREM image of Sample D obtained in the Si [ ! 10] projection showing a grain of w-BN.

Vh< 300 V only revealed single phase t-BN. The relative content ofc-BN increased with increasing bias to a maximum of Qc ~ 61% at Vb= 500 V and then decreased at higher

biases. Fig. 9(a) is a HREM image of Sample C prepared with Vb----440Vshowing grains ofc-BN imaged in the [ 110] projection. The c-BN( 111 ) lattice fringes spacing was 0.21 nm and each grain contained microtwins on the ( 111 ) lattice planes. The c-BN grains were separated by t-BN. Fig. 9(b) is a HREM image of Sample E prepared with Vt,= 300 V showing discontinued lattice fringes with strong curvature which is typically observed for the t-BN layers in this work (see also Fig. 3, Fig. 7 and Fig. 8). The average lattice fringe spacing is ~0.35 nm which is consistent with t.BN(0002) lattice planes. The effect of Vb on the grain size was studied by TEM using dark-field imaging. Fig. 10(a)-10(c) are typical TEM images from Samples B, C, and E, respectively, showing brig~t-field images with corresponding SAD patterns al,~ddark-field images using the first diffraction ring in the diffraction pattern. The SAD patterns were indexed as a phase mixture of e-BN and t-BN with similar relative intensity (Samples B and C) whereas the SAD pattern for Sample E is indexed as t-BN. The grain size as estimated from darkfield images increased with increasing bias to a maximum of ~ 50 nm at Vb= 440 V (Sample C) and then decreased at higher biases. Samples prepared with varying coil current, I¢, (flux series) showed a homogeneous phase mixture of c-BN and t-BN. The relative content of c-BN (Q¢) if'creased with increasing 1¢to a maximum ofQ¢ ~ 60% (Ic = 6 A) in Sample B. The grain size increased with increasing I~ to a maximum of 60 nm (I¢ -- 10 A), which was the highest 1,~studied. Films prepared with F > 6 % (N2 flow ratio series) and within the temperature series showed a nano- to sub-microcrystalline film structure with homogeneous phase mixture of c-BN and t-BN. The relative content of c-BN increased

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Fig. 10, Plan-viewTEM imagesof (a) SampleB, (b) SampleC, and (¢) SampleE showingbright-and dark-field(DF) imageswiththe correspondingSAD pa)terns, The DF imagesin (a) end (b) wereobtainedusingII" ( I I 1)c and in c) by g - ( 10),. with increasing N2 in the gas mixture accompanied by a decrease in the deposition rate which is most likely due to nitridization of the target surface. Films prepared with F<6%, however, showed single-phase t-BN. No or little variation of the grain sizes was observed within the parameter range for the two series. 4. Conclusions BN:C films were grown on Si(001 ) substrates by reactive r,f. diode sputter deposition of B4C targets in mixed At-N2

discharges. Films deposited within a narrow deposition parameter window contained a cubic phase BN:C layer with a (110) preferred orientation. The film phase evolution sequence was from an initial a-BN layer, a highly oriented tBN with the c axis parallel to the substrate surface layer, to the c-BN layer. Approximut.ely S at.% of C was present in these films and mainly present with the chemical bonds C---C and B-C. BN:C films prepared outside the narrow deposition parameter window for growth of c-BN consisted of a homogeneous phase mixture of c-BN and t-BN. These films were non-

M.P. Johansson et al. /Thin Solid Films 287 (1996) 193-201

textured and nano- to sub-micro-crystalline. The film C content decreased with increasing relative content of c-BN.

Acknowledgements 'Ilais work was carried out under the Brite Euram project No. BE 5745 and within the Materials Research Consortium "Thin Film Growth" (financed jointly by the Swedish Board for Technical Development, NUTEK and the Swedish Natural Research Council, NFR). Prof. J.-E Sundgren, Department of Physics, Link6ping University, Sweden is acknowledged for contributing to the evaluation of the XPS results. Dr. L.R. Wallenberg, National Centre for HREM, Chemical Centre, Lund, Sweden is acknowledged for his kind assistance in operating the JEOL 4000 EX electron microscope.

References [ 1] R.W.G. Wyckoff, Vol. 1, 2nd edn., Wiley, New York, 1963, p. 184. [2] J. Thomas, Jr., N.E. Weston and T.E. O~onnor, J. Am. Chem. Soc., 84 (1963) 4619. [3] X.J. Ning, P. Pirouz and K.P.D. Lagerlof, J. Mater. Res., 5 (1900) 2865. [41 R.C. DeVries, Cubic Boron Nitride: Handbook of Properties, General Electric Company, Report No. 72 CRD 178, 1972. [5] D,C. Cameron, M.Z. Karim and M.S.J. Hashmi, Thin Solid Films, 236 (1993) 96. 161 A. Weber, U. Bringmann, R. Nikulski and C.-P. Klages, Diamond Relat. Mater, (1993) 201. [71 A.K. Ballal, L. Salmanca-Riba, C.A. Taylor !! and G.L. Doll, Thin Solid Fihns, 224 (1993) 46. [81 T.A. l~rtedmann, K.F. McCarty, E.J. Klaus, J.C. Barbour, W.M. Cliff, H.A. Johnsen, D.L. Medlin, M.J. Mills and D.K. Ottesen, Thin Solid Films, 287 (1994) 48. [91 D.R. MeKenzie, W.D, MeFall, W.G. Salnty, C.A. Davis mid R.E. Collins, Diamond Relat. Mater., 2 (1993) 970. [1oi D.J. Kester and R. Messier, J. Appl. Phys., 72 (1992) 504. [111 M. Mieno and T. Yoshida, Jpn. J. Appl. Phys., 7 (1990) L1175. [121 M. Murakawa and S. Watanabe, Surf. Coat. Technol., 43/44 (1990) 128.

201

[131 K. Bewilogua, .L Buth, H. Htibsch and M. Grischke, Diamond. Relat. Mater., 2 (t993) 1206. [141 D.R. McKenzie, D.J.H. Cockayne, D.A. Muller, M. Murakawa, S. Miyake, S. Watanabe and P. Fallen, ./. Appl. Phys., 70 ( i 991 ) 3007. [151 D.L. Medlin, T.A. Friedmann, P.B. Mirkarimi, K.F. McCarty and M.J. Mills, Mater. Res. Soc. Syrup. Prec., 311 (1993) 373. [161 O. Brafman, G. Lengyel, S.S. Mitra, P. Gielisse, J.N. Plendl and L.C. Mansur, Solid. State. Commun., 6 (1968) 523. [171 H. Liithje, K. Bewilogua, S. Daaud, M. Johansson and L. Hultman, Thin Solid Films, 257 (1995) 40. [18] R. Trehan, Y. Lifshitz and J.W. Rabalais, ./. Vac. Sci. Technol., A8 (1990) 4026. [191 Boron Compounds with Group IVa Elements, Landholt B6rnstein, Sect. 9.14.9. [2o1 R. Riedel, Adv. Mater., 6 (1994) 549. [21] F. Saugnac, F. Teyssandier and A. Marchand, J. Am. Ceram., 75 (1992) 161. [22] A.R. Badzian, T. Niemyski, S. Appenheimer and E. Olkusnik, in F.A. Gkaski (ed.), Prec. of the 3rd hit. Conf. on Chemical Vapor Deposition, American Nucl. Soc., Hinsdale, IL, 1972, p. 747. [23] T.V. Dubovik and T.V. Andreeva, J. Less Common Met., 117 (1986) 265. [241 A.R. Badzian, Mater. Res. Bull., J6 ( 1981 ) 1385. [25] N.N. Sirota, M.M. Zhok, A.M. Mazurenko and A.I. Olekhnovich, Ser. Fiz. Mat. Nauk., 2 (1977) I 1I. [26] P.J. Gielisse, S.S. Mitra, J.N. Plendl, R.D. Griffis, L.C. Mansur, R. Marshall and E.A. Pascce, Phys. Rev., 155 (1967) 1039. [27] R. Geick, C.H. Perry a~d G. Rupprecht, Phys. Rev., 146 (1966) 543. [28] P. Sherwood, in D. Rriggs and M.P. Seah (eds.), Practical Surface Analysis by Auger ,:. :t X-ray Photo Electron Spectroscopy, Wiley, New York, 1983, p. 170. [29] P.B. Mirkarimi, K.F. McCarty, D.L. Medlin, W.G. Wolfer, T.A. Friedmann and E.J. Klaus, J. Mater. Res., 9 (1994) 2925. [30] H. Sj6striJm, I. lvanov, M. Johansson, L. Hultman, J.-E. Sundgren, S.V. Hainsworth, T.F. Page and L.R. Wallenberg, Thin Solid Fibns, 246 (1994) !03. [311 C.D. Wagner, W.M. Riggs, L.E. Davis, J.F. Moulder and G.E. Mullenberg, Handbook of X.ray Photoelectron ; pectroscopy, Perkin-. Elmer Corporation, Minnesota, 1979. 132] D. Marten, K.J. Boyd, A.H. AI-Bayati, S.S. Todorov and J.W. Rabalais, Phys. Rev. Let[., 73 (1994) 118. 1331 H. SjOstr0m, L. Hal[man, J.-E. Sundgren, S.H. Hamsworth, T.F. Page and G.S.A.M Theunissen, J. Vac. Sci. Technol., A14 (1996) 56. [341 D.R. McKenzie, N.A. Marks, P. Guan, B.A. Pailthorpe, W.D. McFall and Y. Yin, Energetic condensation as a means af inducing the growth ¢~'libns containing high pressure phases, School of Physics, University of Sydney, Australia, 1994. [351 D.J. Kester, K.S. Alley, R.F. Davis and K.L. More, J. Mater. Res., 8 ', 1993) 1213.