Deposition of c-BN films by DC magnetron sputtering

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Diamond and Related Materials 7 (1998) 26-31

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Deposition of c-BN films by DC magnetron sputtering S. K o u p t s i d i s *, H. Ltithje, K . B e w i l o g u a , A. SchOtze, P. Z h a n g Fraunht~fer htstitute for Stoface Enghwering and Thin Fihns, Bienroder Weg 54E, D-38108 Braunschweig, Germany Received 4 December 1996: accepted 13 June 1997

Abstract

New results 'of the deposition of cubic boron nitride (c-BN) films by use of DC magnetron sputtering will be presented. This technique is of great interest lbr industrial applications, because of its high growth rates and upscaling potential. Boron carbide was used as target material with a suitable electrical conductivity for DC sputtering. The target as well as the substrate holder were connected to DC power. In the experimental set-up we used the unbalanced magnetron mode ( U B M ) attained with a magnetic coil surrounding the substrate table. A c-BN deposition process was developed based on a parameter transfer from a RF diode system. The c-BN content of the films will be discussed in relation to the different process parameters. Important

process parameters for the formation of the cubic phase were the ion current at the substrate table and the resulting ratio of ions to neutrals. Nearly single phase c-BN films were deposited on silicon and steel substrates. The elemental film composition and the film structure were characterized by electron probe microanalysis (EPMA), X-ray photoelectron spectroscopy (XPS) and infi'ared spectroscopy (IR). Nanoindentation and friction coefficient measurements revealed excellent mechanical and tribological properties of the films. (-5 1998 Elsevier Science S.A. Kevwm'ds: Deposition': Cubic boron nitride: Sputtering

I. Introduction Cubic boron nitride (c-BN) is a promising material fc," technical applications in different fields. Essential properties ~lrc the tfighest h,,,'dness next to diamond and non-rc~ctivity with ferrous metals. Therefore c-BN films seems to be a promising candidate for superhard, wear protecting coatings on cutting tools. Moreover, due to the electrical, chemical and thermal properties of c-BN many other applications are possible. Different PVD and PACVD techniques have been developed in the last few years in order to deposit c-BN layers. The use of PACVD methods [1,2] is nevertheless limited by a high temperature load on the substrates. Therefore PVD methods like ion plating [3,4], ion beam assisted deposition [5,6] and sputtering are of greater interest. In particular, the sputter technique has the highest potential for an economical deposition method. Conventional sputter methods i.n use for c-BN deposition are RF diode sputtering [7,8] and RF magnetron sputtering [9, 10]. The latter has the advantage of a film deposition with h.igher growth rates. The use of DC instead of RF power at the target is another interesting feature to * Corresponding author. 0925-9635/98/$19.00 (~, 1998 Elsevier Scieace S.A. All rights reserved. PIi S0925-9635(97)001 50-7

lower the device costs and increase the upscaling potential. Therefore DC magnetron sputtering seems to be a very promising deposition method for industrial applications. In the present paper we introduce the DC magnetron sputter technique as an interesting method for the deposition of c-BN layers. It is well known that hexagonal BN and boron are standard target materials for the sputter deposition of c-BN. These targets can be used only in RF diode arrangements or in RF magnetrons due to their iow electrical conductivity. As a precondition of using DC power we need an electrically conducting target material. Some possible candidates for discussion are the following metal borides: TiB,, LaB6 and ZrB12. However, up to now no successful deposition of c-BN containing films prepared with such target materials has been reported. We chose boron carbide ( B 4 C ) with an electrical conductivity of p < 102 f~ cm as our target material. One essential intention was a possible reduction of the impurity carbon in the film which can be achieved by forming of C - N bonds during reactive sputtering with nitrogen. We developed a DC magnetron sputter process for the deposition of cubic BN films based on experimental work which was done by use o; RF diode sputtering.

S. Kouptsidis et al. / Diamondand Related Materials 7 (1998) 26-31

With this device, we started some time ago, the deposition of c-BN films by use of a h-BN target [8]. In a second step we changed to a B4C target and investigated experimental parameters tbr the c-BN growth [11,12]. In the present work we concentrated on the process transfer from the RF devic~ to the DC magnetron. Important transfer parameters for controlling the c-BN growth were the ion current at the substrate and the ratio of ions to neutrals. The deposition parameters of the c-BN films, as well as some essential film properties will be discussed in detail. In this work we investigated the basic knowledge of c-BN deposition by use of DC magnetron sputtering for a further transfer to an industrial plant. Results of deposition experiments using an ABS coater ( H T C 1000 from Hauzer) will be published.

of 170 mm. We fixed a thermocouple at the surface of the table to measure and control the deposition temperature. The substrate table was connected to DC power as well as the target. Surrounding the substrate table we installed a magnetic coil, 200 mm in diameter carrying 300 windings. With a coil current of 5 A we obtained maximal magnetic fields in the range of 80 G in the centre of the coil. By use of the magnetic coil, we operated in the unbalanced magnetron mode ( U B M ) with a displacement of the plasma density from the target to the substrate area resulting in a higher ion current at the substrate table. Ar and N2 were used as sputter gases controlled by a mass flow system. We used doubleside polished Si( 100)-wafers and high speed steel plates as standard substrate materials. No special precleaning of the substrates was done beside a sputter cleaning in pure argon at the beginning of each process. The composition of the films was characterized by electron probe microanalysis (EPMA). Film structure and bindings were analyzed by infrared spectroscopy (IR) and X-ray photoelectron spectroscopy (XPS). For investigations of mechanical and tribological film properties we pertbrmed nanoindentation and pin-on-disk measurements.

2. Experimental details The experimental work was performed on a Balzers BAS 450 machine which is schematically shown in Fig. I. The vacuum chamber was of round geometry. 0.5 m in diameter with a resulting volume of 10131. Base pressures of I x 1 0 - 6 m b a r can be achieved by use of a turbomolecular pump. Boron carbide was used as an electrically conducting target with a size of 254 m m x 127 mm. The sputter device was working in a magnetron arrangement attained with a set of permanent magnets behind the target to increase the growth rate. The substrate table, a round steel plate with a diameter of 80 mm, was mounted opposite the target at a distance

3. Results Sputtering of BN films with a B4C target requires the addition of nitrogen to the sputter gas. Therefore we used reactive gas mixtures of nitrogen and argon, usually

B4C-target (dc) target coil

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opposite side coil gas inlet (NJAr) Fig. 1. Scheme of the DC magnetron arrangement for c-BN sputter deposition.

S. Kouptsidis et al. /Diamond and Rehtted Materials 7 (1998) 26-31

28

with a total gas flow of 50 sccm and a resulting process pressure of 5 × 10-3 mbar. Experimental work was done to find process conditions to get stoichiometric BN and to reduce the carbon content in the films. Fig. 2 shows the elemental composition of the sputtered films measured by EPMA as a function of the nitrogen content in the sputter gas. The nitrogen content was calculated by the gas flow ratio [N2]/([N2]+[Ar]). For a nitrogen content in the gas. above 10% we achieved stoichiometric conditions with a ratio of B/N near 1. It should be mentioned that for a nitrogen content higher than 50%, instabilities of the deposition process occurred due to target poisoning. In general, the increase of the nitrogen content in the gas led to a reduced carbon content in the films. With 20% nitrogen as a usual experimental gas mixture we were able to reduce the carbon content from 20 at% in the target to values under 10 at% in the films. Other impurities such as oxygen, hydrogen and argon were observed in smaller amounts in the elemental film content and will not be considered in this diagram. SIMS measurements verified the results of EPMA and revealed furthermore that the elemental film distribution was constant over the whole film depth. A typical deposition process was divided into the cleaning part of target and substrate and into the real deposition part. The substrates were sputter cleaned before the deposition by argon ion bombardment for 15 min with an etching rate of 0.5 Ftm h- ~. At the same time, the target was sputter cleaned in order to remove nitrogen contaminations from the previous process until

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Table i c-BN deposition parameters Deposition parameters Substrate voltage Substrate current density Target power Nitrogen content Process pressure Temperature

the target voltage reached a constant level. The details of the process parameters for the deposition of c-BN films are summarized in Table 1. Infrared spectroscopy (IR), which is the most commonly used technique to quantitatively estimate the c-BN content, was carried out. Fig. 3 shows an IR spectrum of a typical 0.2 lam thick c-BN layer deposited on silicon with the deposition parameters listed in Table 1. This film clearly consists of nearly single phase c-BN with respect to the absorption bands at 1100 cm- ~ (c-BN) and 1400, 800 cm- i (h-BN). Other evidence of the cubic phase formation has been obtained by a transmission electron diffraction pattern [17]. Further experimental work was done to analyze the influence of the process parameters on the formation of the cubic phase. Fig. 4 shows for example the c-BN content, expressed as the ratio Ac-BN/Ah.~Nof the c-BN absorption at 1100 cm- ~ and the h-BN absorption at 1400 c m - ~, as a function of the substrate voltage. In agreement with other works [13] we observed a threshold in the substrate voltage near - 2 2 0 V where the growth of the cubic phase starts. The decrease of the ratio ,4c_llN,/Ah.llr~ for higher substrate voltages is caused by resputtering resulting in a decrease in film thickness and a resputter threshold of - 4 0 0 V . However, we obtained a process window for the growth .

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of the cubic phase in the subs(rate voltage range from - 220 to - 320 V. Similar process windows were found for the other essential sputter parameters for the c-BN growth. The parameter findings of Table 1 were carried out as a compromise of conditions for the cubic phase growth and of conditions for a stable running process. This means especially that the increase of target power and nitrogen content had a strong influence on the target running and poisoning. An interesting fact for industrial applications is the low deposition temperatures of about 300 ~C. As a result of the experimental work of parameter variations we found that the substrate current density seems to be the most important process parameter to influence the growth of the cubic phase. By use of a DC powered substrate table it was possible to measure the substrate current. It turned out that substrate current densities >2 mA cm -2 were necessary to prepare c-BN containing films. Considering the corresponding deposition rate of 8 nm min -~, estimated with a Dektak profilometer, we obtained flux ratios of ions to neutrals (deposited boron atoms) of ~ 10. This result is a good fit to the c-BN data collection of Reinke and co-workers [ 14]. Cubic BN films prepared by DC magnetron sputtering are well adherent on silicon substrates up to a thickness of 0.5 lam. Thicker films on silicon and even thin films on steel suffer from delamination caused by film stress and air humidity influence. Sputtering with a boron carbidetarget led as described to a carbon incorporation in the deposited layers of about 10at%. We performed X-ray photoelectron spectroscopy ( XPS ) measurements to reveal the binding state of carbon in the films. Fig. 5 shows the C ls XPS spectra of a typical c-BN film including the fitted curves.

282

Obviously, the C - C bond at 284.6 eV is the predominant binding estimated to be approximately 60%, as shown" in Table 2. Remaining binding states of carbon can be associated with respect to XPS data collections to C-B, C - N and C - O bonds. The latter might be estimated too high due to surface contaminations. Otherwise, after substrate surface cleaning by ion beam sputtering characterization was difficult because of binding damage. Beside the question of carbon we analyzed the spectra of B is and N Is. With the atomic sensitive factor we calculated the elemental film content of boron and nitrogen to tile same values as measured by EPMA. Of course, it turned out that boron and nitrogen were mainly B-N bonded. It should be mentioned that the results of the XPS measurements describe mainly the surface condition of the films. However, we assume a similar bchaviour of the binding states for the whole film thickness according to other authors [18]. Information about the mechanical and tribological properties of c-BN films is of fundamental interest for technical applications. We performed nanoindentation measurements [15] to obtain the hardness of the deposited films. Experiments were done with a diamond pyramid under different loads. From loading and unloading ct:rves we determined the film hardness. Fig. 6 shows the film hardness as a function of the indention depth of two typical c-BN samples. Measured values of Table ." Binding states of carbon Energy leVI

Binding

Calculated content 1%)

282.0 284.6 286.4 288.4

C B C -C C-N C O

8 58 16 18

S. Kouptsidis eta/. / Diamond and Related Materials 7 (1998) 26-31

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normal force /~=F,rlFN, as a function of the normal load. Four different samples of nearly the same thickness and c-BN content were measured. It turned out that friction coefficients varied from a excellent value of /t=0.05 (FN=0.3 N) for sample ltl to highest values of /,t=0.32 (FN=0.3 N) for sample /,t4. AFM measurements were done to measure the surface roughness of the samples. In detail we measured roughnesses of R = 1.2 nm for sample/~1 and R = 4 . 4 nm for sample/~4. Thus, the value of the friction coefficient depends on the surface roughness. Correlations between the roughness and the deposition conditions are still unclear.

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an excellent hardness of about 80 GPa for films with a thickness of 0.2 lam are in good agreement to literature data [ 16]. Further investigations were done to measure macroscopic friction coefficients by pin-on-disk testing. We used steel (100Cr6) balls rotating under a normal load against the c-BN films. All friction tests were carried out in air with a humidity between 40 and 50%. Fig. 7 shows the friction coefficient of several c-BN samples, defined as the ratio of the tangential force and the sample sample w e d sample ---B--- sample

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Acknowledgement

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We would like to thank our colleagues R. Bethke, Dr. P. Willich and Dr. K, Taube for EPMA, SIMS and nanoindentation measurements. XPS measurements were carried out in the department of Physical Chemistry at the University of Hamburg. This work was partly sponsored by the Brite Euram project (BE 5745).

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A DC magnetron sputter process was developed for the deposition of c-BN films. This technique seems to be more useful for industrial applications than other known deposition methods. Experiments were carried out with a boron carbide target and a magnetic coil surrounding the substrate using the unbalanced magnetron mode (UBM). Reactive sputterirg with nitrogen led to a carbon reduction from 20 at% in the target to values lower than l0 at% in the films. XPS measurements exhibited carbon is predominately C - C bonded. Infrared spectroscopy pointed out that nearly single phase c-BN films were deposited on silicon and steel substrates. Variations of the main deposition parameters allowed the definition of a process window for the growth of the cubic phase. We found out that the growing of the cubic phase depends strongly on the ion current at the substrate table and the ratio of ions to neutrals. The deposited films with a thickness of 0.2 ~tm revealed excellent mechanical and tribological properties of hardness ( ~ 80 GPa) and friction coefficients (p =0.05 with FN =0.3 N).

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References 2

normal load [N] Fig. 7. Friction coel~cients of several c-BN films measured by pin-on-disk testing.

[I] A. Weber, U. Bringmann, R. Nikuiski, C,-P. Kiages. Surf. Coat. Technol, 2 (1993) 201. [2] M. Kuhr, S. Reinke, W. Kulisch, Surf. Coat. Technol. 74/75 (1995) 807.

S. Kouptsidis et al. / Diamond and Related Materials 7 (1998.) 26-31

[3] T. lkeda, Y. Kawate, Y. Hirai. J. Vac. Sci. Technol. A 8 (1990) 3168. [4] M. Murakawa, S. Watanabe, Surf. Coat. Technol. 43,"44 (1990) 128. [5] D.J. Kester, R. Messier, J. Appl. Phys. 72 ( 19921 504. [6] N. Tanabe. T. Hayashi, M. iwaki, Diamond Relat. Mater. I (1992) 151. [7] M. Mieno, T. Yoshida, Surf. Coat. Technol. 52 (1992) 87. [8] K. Bewilogua, J. Buth, H. H~bsch, M. Grischke, Diamond Relat. Mater. 2 (1993) 1206. [9] V.Y. Kulikovsky, L.R. Shaginyan, V.M. Vereschaka, N.G. Hatynenko, Diamond Relat. Mater. 4 (1995) I 13. [1(1] S. Ulrich, J. Scherer, J. Schwan, 1. Barzen, K. Jung, H. Ehrhardt, Diamond Relat. Mater. 4 (1995) 288. [11] H. Ltithje, K. Bewilogua, S. Daaud. M. Johansson, L. Hultman, Thin Solid Films 257 (1995) 40.

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[12] A. Scht~tze, K. Bewiiogua, H. LQthje. S. Kouptsidis, Diamond Relat. Mater. 5 (1996) ! 130. [131 S. Kidner, C.A. Taylor !!, R. Clarke, Appl. Phys. Lett. 64 (141 (1994) 1859. [14] S. Reinke. M. Kuhr, W. Kulisch, Diamond Relat. Mater. 3 (1994) 341. [151 J.H. Edgar, Properties of Group 11I Nitride, lnspec, London, 1994. [161 M. Fryda, ~/,. Taube, C.-P. Klages, Vacuum 41 (1990) 1291. [17] M. Johansson, Thesis, Link6ping University, 1997, to be prepared. [181 M. Johansson, L. Hultman, S. Daaud, K. Bewilogua, H. LiJthje, A. SchOtze, S. Kouptsidis, G.S.A.M. Theunissen, Thin Solid Films 287 (1996) 193.