Subplantation effect in magnetron sputtered superhard boron carbide thin films

D| OND R|L T[D T|R|ALS AND

ELSEVIER

Diamond and Related Materials 7 (1998) 835 838

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Subplantation effect in magnetron sputtered superhard boron carbide thin films S. Ulrich ~'*, H. Ehrhardt b, j. Schwan b R. Samlenski ~, R. Brenn ~ Forschungszentrum Karlsruhe GmbH, Postfach 3640, D-76021 Karisruhe, Germato, b Universiti~t Kaiserslautern, Erwin-SchrOdinger Str., D-6 7663 Kaiserslautern, Germany ¢ Materialforschun~szentrum und Fakultfit[~ir Physik, Universitiit Freiburg. D-79108 Freiburg, Germany

Received 5 August 1997; accepted 29 October 1997

Abstract Superhard amorphous boron carbide films with a film thickness of about 2 lam have oeen prepared by r.f.-magnetron sputtcring of a boron carbide target in a pure'argon discharge at a gas pressure of 1.6 × 10 -3 mbar. The flux ratio of the argon ions to boron and carbon atoms has been kept constant at 3.5, while the energy of the argon ions is varied by applying a d.c.-subst,'ate bias." The effect of argon ion implantation measured by Rutherford back scattering is discussed. When the argon ion energy is increased, the mechanical properties show extreme values at an argon ion energy of 74 eV, which can be explained quantitalively by knock-on subplantation. Stress up to 6.7 GPa and a micro-hardness up to 72 GPa are obtained. The hardness enhancement is correlated with the increase of stress. The influence of preferentiai sputtering of boron or carbon from the deposited B4C film can be neglected. © 1998 Elsevier Science S.A. Kevwords: Subplantation; Magnetron; Boron carbide: Thir, films

I. Introduction Boron carbide is an interesting material because of its high hardness, good mechaqical, tribological, electronic, and optical properties, and its high neutron absorption cross-section. Next to diamond and cubic boron nitride, it is the third hardest material at room temperalure. However, whereas the hardness of diamond and cubic boron nitride gradually decreases with rising temperature, boron carbide is characterized by its high thermal stability; above I I 0 0 ' C , it is, in fact, the hardest material. Boron carbide can be produced by various deposition techniques, which can be found in the literature. A variety of PVD techniques were used, such as diode sputtering [1,2], magnetron sputtering [3,4] and laser ablation of a boron cart:ide target [5] as well as vacuum arc deposition [6]. In typica~ PECVD or CVD experiments, the process gases used were BCI3+C.,.H~. [7 ~9], BCI3+CCI,~ [10] BBr3+C2H 2 [8,11- 13], BI3+C2H 2 [11], BsH,~+CzH2 [14], or BtoHt4+C2H 2 [14].

Boron carbide coatings are also of great interest because of their low atomic number, it is for this reason that they can be used as X-ray mirrors in multilayer systems built up of alternating layers of materials with high and low atomic numbers. Thus, the W/B4C layer system exhibits increased X-ray rcllcctivity of 0.1 .....100 nm for grazing to normal angles of incidence [15]. Blumenstock et al. [16] developed boron carbide tilms by ion beam deposition with high reflectivity in the ultraviolet range (67--121.6nm) under a normal incidence of more than 30%. Boron carbide is extremely resistant to wear and rather aggressive to the contact body generally used for grinding and polishing. Post-annealing in air allows the friction coefficient relative to 440C steel to be reduced from 0.7 to 0.07 [! 7]. Also, successful applications in electronics are known, lor instance in diodes with a hetero junction [18, 19] or in a lield effect transistor with a boron c a r b i d e boron junction [19].

2. Experimental details * Corresponding author. ~This paper was presented at the Diamond "97 Conference in Edinburgh, Scotland, August 3-8, 1997. 0925-9635/98/$19.00 <¢~1998 ElsevierScience B.V. All rights reserved. PI! S0925-9635(97)00306-3

All boron carbide thin films were produced by r.f.magnetron sputtering of a boron carbide target of B4C

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S. Uh'ich et a/. ' Di, momi a m / R e h m ' d Materials 7 1 1998) 835 838

stoichiometry in a pure argon atmosphere. The distance between target and substrate was 6 cm. The substrates were sputter-cleaned for 15 min by argon ion bombardement with an energy of 220 eV. All layers were deposited at room temperature on silicon (100) and hardened tool steel of DIN 1.2379 quality. More details of the experimental set-up are given elsewhere [20].

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3. Results and discussion 3.1. Varkttion of argon gas pressure

First, a qualitative explanation of the results of particle flux analysis will be presented, and then the effects of variations in particle flux will be discussed. The ion flux consists mainly of singly charged argon ions. Their energy is generally composed of the pressure dependent plasma potential and the d.c.-substrate bias potential, which can be applied additionally. The ion current densities on the substrate are nearly independent of the target material, being in the order of l mA cm 2- and decreasing with rising argon gas pressure. The filmforming particles consist mainly of sputtered boron ~nd carbon atoms. This statement was confirmed aL,,: by Goehlich et al. [21]. Using laser-induced fluorescence spectroscopy, the authors determined the energy distribution of the sputtered atoms from the Doppler broadening of their emitted light. This energy distribution is described very well by the Thompson distribmion required theoretically. The most probable energy of the sputtered particles is half the surface binding energy, Eu. F,u" boron, E, amounts to 5.6 eV, and for carbon, 6 eV. If the surface binding energy is estimated by way of the binding enthalpy, a value of 6.2 eV is obtained in good agreement with experimental findings. At a bombardment energy of 1 keV, the sputter yields for boron and carbon amount to 0.54 and 0. ! 35, respectively. With the B4C stoichiometry taken into account, no preferential sputtering effects can be detected, in contrast to the studies of silicon carbide. At a gas pressure below 2 x 10-3 mbar, the mean free path is high. Consequently, the energy distribution of the film forming boron and carbon atoms can be described as a Thompson energy distribution. Increasing the working gas pressure by a factor of 10 reduces the energy of sputtered atoms as a result of collisions in the plasma. Variations of the argon gas pressure are important witl~ respect to process technology as they arc easy to achieve and, at the same time, greatly change the properties of a layer because of variation in the particle flux. This variation is difficult to interpret, as many parameters change at the same time. As the gas pressure rises, the energy of the ions and film-forming particles, the current density of the argon ions, and the flux ratio of argon ions to film-forming particle decrease. All

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F i r 1. Hardnc3~ o f boron carbide thin films (thickness: 2 lamj plotted vet ~us argon gas pressure. As the pressure increases, the neutral particle energy, the flux ratio o f argon ions to film forming particles, the plasma potential, and also the ion energy (at a constant substrate bias of - 10 V ) are decreased.

changes in particle flux cause the reduction of the deposited energy per film-forming particle. In the following experiment, the argon gas pressure is varied at low ion energy. Under these conditions, the subplantation effect is very weak, and no sputtering effects can occur on the substrate. When a low d.c.-substrate bias of - lOeV is applied in order to avoid electron currents, the ion energy at the low working gas pressure of 2 × 10 -3 mbar is 34 eV (applied substrate bias potential of 10eV+meastu'ed plasma potentenial of 24eV). When the working gas pressure is raised to !.7× 10-2 mbar, it is reduced to 27eV ( 1 0 e V + 1 7 e V ) due to the decrease of plasma potential. The decrease in ion energy, of the flux ratio and of deposited energy pet" lilm forming particle reduces the hardness of the boron carbide layers deposited from 43 GPa to 30 GPa ( Fig. I ). 3.2. Variation o f argon ion energy at low gas pre.~'sttre

In this section, the influence of the argon ion energy on the film properties is investigated. Experimentally, this was achieved by applying a d.c.-substrate bias. The ion energy resulted from the addition of the plasma potential of 24 eV to the applied substrate bias potential. The films were deposited in a pure argon atmosphere at a working gas pressure of 2 × 10 - 3 mbar at room temperature. An r.f.-power of 250 W was supplied. The films had the same stoichiometry as the target, namely B,~C. They were amorphous to all ion energies. RBS analysis was used to deiermine the argon content (0.7-2 at.%) and the metal impurities (<0.1 at.%) as a function of the argon ion energy. Fang et al. [22] used two-dimensional molecular dynamic computer simulations to study the intrinsic stress in sputter-deposited films. They found a stronger ion bombardment to increase stress, reduce the size of microvoids, and decrease their number. They also found

S. Ulrich el aL ./Diamoml and Rektted Materials 7 (1998) 835 838

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that the argon inclusions consists mainly of neutral argon atoms from the gas phase. The authors calculated an argon content in the order of 1%. In our experience, the number of argon inclusions decrease linearly from 2 at.% at 25 eV to 0.7 at.% a't 115 eV with increasing ion energy, which is in good agreement with the discussed theoretical results. A higher ion energy causes a higher surface diffusion and, in this way, a smaller number of microvoids in which argon atoms can be deposited. The absolute levels of the argon content are between 2 at% and 0.7 at%, which is the expected order of magnitude. Also, Chiang [23] used r.f.-magnetron sputtering to produce boron carbide films, and RBS analyses to determine comparable argon inclusions between 2 and 3 at% for the amorphous boron carbide layers. In the crystalline boron carbide layers, which he had obtained at substrate temperatures above 950 ('. the argon content dropped to 0. ! at.% or even 0.5 at.%. At a higher substrate bias, more metal atoms from the substrate holder are sputtered because the sputter yield increases with increasing ion energy. The metal atoms are ionized in the plasma apd, according to an RBS analysis, between 0%o and 1%, are incorporated in the layer. This small quaotity has hardly any influence on the mechanical properties, but can be suppressed nevertheless by coating the substrate holder with boron carbide prior to the actual deposition of the layer. Fig. 2 shows the stress in 250 nrn thick boron carbide films plotted as a function of argon ion energy. Each point in the diagram represents the mean value of six samples, with eazh substrate being measured by a profilometer in two directions normal to each other. Above approximately 200 nm, stress is nearly independent of layer thickness, because the interface stress is losing its influence. The difference in lattice constants of boron carbide, and the silicon substrate is 4% at room temperature and 0.4% at 1000 C [24]. A low stress component of the interface can be expected due to the amorphous structure of the boron carbide films and the low difference of the lattice constants of the two interface materi-

837

als. As the films were deposited at room temperature, the thermal stress component can be neglected. No systematic studies of stress in boron carbide thin films have been reported in the literature. Only Chiang [23] calculated a stress level of 2.5 GPa for a crystalline boron carbide layer produced at 950 C without substrate bias. In our experiment, the ion saturation current density amounts to 0.64 m A c m -z, corresponding to 3.95 × 10 ~s argon atoms per cm 2 and second. The flux of the film forming particles amovats to 1.12 atoms per cm 2 and second at a argon gas pressure of 2 × 10 -3 mbar. On this basis, a flux ratio of argon ions co fi!m forming boron and carbon atoms can be calculated to be 3.5. At this flux ratio, the peak stress amounts to 6.7 GPa as a function of the argon ion energy at 74 eV, which can be explained on the basis of the subplantation theory developed by Robertson [25], Davis [26] and Lifshitz et al. [27]. Increasing the ion energy results in and enhancement of forward sputtering [26] and in an increasing penetration probability [251 and, conse: quently, in higher stress o [26] higher densification [25]. Beyond an ion energy of 74eV, however, relaxation processes caused by increased mobility bombardment with higher energetic ions will dominate. As a consequence, the total stress will decrease. Robertson calculated in his paper [25] that the difference between his model and the stress model of Davis [26] is not significant for an ion energy range between 25 eV and 120 eV. Furthermore, he showed the linearity between densification, fraction of sp 3 bonded atoms and stress of a-C and a-C:H thin lilms. The curve shown in Fig. 2 corresponds to the subplantation model of Robcrtson [25] for the case of stress in dependence of the argon ion energy EA~, and the llux ratio of argon ions q)a~' to lihn forming boron cl)~ and carbon atoms (l)c:

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S. Uh'ich et al.

Diamond and Related Materials 7 (!o08) 835 838

gemeinschaft ( D F G ) (Projekt No. EH 23/30-1) carried out under the auspices of the trinational " D - A - C H " German, Austrian and Swiss cooperation on the "'Synthesis of Superhard Materials".

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thermal spike parameters [25,26] of 0.016 and 0.013 and a relative activation energy for the relaxation processes of Eo = 1.59 eV. It should be remarked that these formulae contain two constants that could be also interpreted as fitting parameters for the shape of the curve: (1) 0.014 and (2) 0.016. 0.014. E~ 5/3 As stress in the boron carbide layers increases, the hardness is also enhanced, as is evident from Fig. 3. Ion bombardement moves boron and carbon atoms into subsurface layers on interstitial positions by "knockon" subphmtation. This results in a variation of the optimum bonding angles and bonding lengths, which introduces stress. A compressive stress causes lower bond lengths and consequently, according to a theory by Liu and Cohen [28], the bulk modulus and the

hardness increase.

4. Summary and conclusions R.t:-magnetron sputtering and argon ion bombardement at room temperature allows the formation of superhard amorphous boron carbide thin films with a layer thickness of 2 )am and with a maximum hardness of 72 GPa. The dependence of stress on the argon ion energy was explained by the subplantation model of Robertson for the case of forward sputtering. This means that subplantation is a general effect, observed also in the case of the formation of highly tetrahedral amorphous carbon and cubic boron nitride.

Acknowledgement

The authors gratefully acknowledge the financial support o1" the present work by the Deutsche Forschungs-

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