Electrochemical study of titanium nitride films obtained by reactive sputtering

Thin Solid Films, 191 (1990) 305 316 PREPARATIONAND CHARACTERIZATION

305

E L E C T R O C H E M I C A L S T U D Y OF T I T A N I U M N I T R I D E F I L M S O B T A I N E D BY R E A C T I V E S P U T T E R I N G Y. MASSIANIAND A. MEDJAHED Laboratoire de Chimie des Matbriaux, Equipe Corrosion, Universitb de Provence, place Victor Hugo, 13331 Marseille (France)

P. GRAVIERAND L. ARGI~ME Laboratoire de Physique des Solides, Universitb de Provence, place Victor Hugo, 13331 Marseille (France)

L. FEDRIZZI Engineering Departement, University of Trento, Trento (Italy)

(Received December29, 1989;accepted April 24, 1990)

TiN coatings were prepared by reactive sputtering on glass substrates. The production parameters were chosen in order to obtain stoichiometric TiN. The electrochemical behaviour of the samples was investigated using both d.c. and a.c. electrochemical techniques in aqueous solutions: two sulphatic and one chloric. The chemical composition of the surface was analysed by Auger electron spectroscopy before and after the polarization tests. The TiN coating exhibited a very high free corrosion potential and a small anodic current, which indicates a very low reactivity in all the considered solutions. In the anodic range a current peak occurred which corresponds to the formation of an oxidized titanium layer with semiconducting properties.

1. INTRODUCTION Titanium nitride is a very interesting material for its optical and mechanical properties (high hardness); also, it exhibits a very low chemical reactivity. M a n y papers have dealt with the corrosion behaviour of titanium nitride in various media 1-9. The different elaboration processes lead to structural variations of TiN coatings and consequently to changes in the electrochemical properties, as showed in the various experimental papers. A study of TiN layers, obtained by ion implantation 3 has revealed very good behaviour in an aqueous NaC1 solution. A comparison of coatings elaborated by three different physical processes (physical vapour deposition (PVD), ion mixing and ion implantation)1 outlined their different microstructures in terms of porosity, columnar structure and homogeneity. Recently an electrochemical study, based on the effect of nitrogen implantation on titanium alloy, emphasized the diversity of the compounds obtained (amorphous phases a-TiN, ct-Ti, TiN) and, moreover, the difficulties in correlating the peaks of the anodic polarization curves with well-defined electrochemical reactions s. Elsener et al. 9 have shown that effective protection by TiN can only be obtained 0040-6090/90/$3.50

© ElsevierSequoia/Printed in The Netherlands

306

v. MASSIANIet al.

on substrates which undergo passivation. This is consistent with our study 6 which demonstrated poor protection of TiN coatings obtained on A R M C O iron owing to the presence of microdefects and columnar structures. All this research was carried out with electrochemical techniques which seem to be the best for studying the coating's behaviour in aggressive solutions. Generally these studies concerned TiN coatings obtained on metal substrates, so that, because of the presence of coating defects, the electrochemical responses included the substrate contribution. This contribution changed during the immersion time, as a consequence of the penetration (time dependent) of the electrolyte through the coating's defects. In order to evaluate the intrinsic electrochemical properties of the TiN coatings, without a possible contribution of the active substrate, it is necessary to use an inert substrate (for example glass). Some tests on inert substrates (quartz) have been performed 9, but only on the coatings obtained by the chemical vapour deposition (CVD) technique. The aim of this work is to study the electrochemical behaviour of titanium nitride obtained by P V D on an inert substrate (glass) in a large range of potentials using electrochemical techniques as well as Auger analysis of the anodic layers obtained after polarization. 2.

EXPERIMENTAL DETAILS

2.1. Preparation of the TiN films The TiN films were reactively sputtered with a d.c. triode system. The vacuum chamber was equipped with an oil diffusion p u m p and a liquid nitrogen trap and could be pumped down to a final pressure of 4 x 10-5 Pa. The sputtering gas was a mixture of nitrogen and argon of the highest available purity (99.9999/0). Nitrogen partial pressure was set to a value of 8 × 10- 3 Pa for stoichiometric samples and the total gas ( A r + N 2 ) pressure was 8 x 10 -2 Pa. The water-cooled titanium cathode had a purity of 99.8%. The plasma current density was 9 m A c m -2 and the deposition rate for a target-to-substrate distance of 140mm was 9 n m min -1. Substrates for electrochemical studies were rectangular float glass (3 m m x 6 m m × 25 mm). During film preparation the substrate temperature was 350 °C and the substrate holder was at a floating potential (about 40 V). The films were about 180 nm thick because, on account of stress, thicker films were not mechanically stable in glass substrates and broke up with time. 2.2. TiN film characterization 2.2.1. Electrical resistivity TiN films of different thicknesses were reactively sputtered onto square glass substrates. The resistivity measurements were performed with a four-point probe and the film thickness was determined with a multiple interferometer (precision, 10 nm). As shown by Fig. 1, representing resistivity vs. thickness, for thicknesses greater than 130 nm the film resistivity decreases slowly. For 195 nm the resistivity is equal to 47 txf2 cm; that is, close to the lowest values reported in the literature 1°-13. We can

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200

consider that films 180 nm thick are continuous and well representative of TiN coatings and so can be used for electrochemical investigation.

2.2.2. Optical reflectivity The stoichiometry of TiN films can be controlled by the optical reflectivity because the film colour changes with nitrogen concentration. For each run the optical reflectivity of one film was measured and compared with a stoichiometric standard (the composition of the reference sample had been previously determined by Rutherford backscattering and was TiNo.97Oo.o 14C o.o 16).

2.2.3. X-ray diffraction X-ray diffraction was used to characterize the films. The X-ray diffractometer system was operating with Cu K0t radiation. Under conditions of no bias the sputtered TiN films had the NaCl-type structure with a preferred orientation of the (111) planes which were parallel to the film surface.

2.3. Electrochemical techniques TiN samples were embedded in epoxy resin and used as working electrode. The potentials are reported with respect to a saturated sulphate electrode (SSE). D.c. and a.c. electrochemical tests were performed with a 273 P A R C potentiostat-galvanostat, and a Look-in analyser (PARC 5208). The electrochemical impedance spectroscopy (EIS) measurements were acquired in a 100 k H z 5 m H z frequency range. Solutions were prepared with Millipore water (resistivity, greater than 16 Mf~ cm) and with high purity products. Corrosion behaviour tests were carried out in the three following aerated solutions: H2SO 4 (0.5M), Na2SO4 (0.5M) and NaC1 (0.51 M; 3 0 g l - l ) . The potentiodynamic polarization curves were obtained with a scan rate of 0.2 mV s - 1 starting from - 1000 mV (SSE). An extensive study of the anodic layer formed by polarization in sulphuric acid solution was performed, using EIS and Auger electron spectroscopy (AES).

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Y. MASSIANIe t al.

3. RESULTS 3.1. D.c. electrochemical studies

Figure 2 shows the evolution of the free corrosion potential E . . . . vs. time in the three considered solutions. All the curves exhibit a decrease towards more cathodic values, more notable for the sulphuric acid solution, signifying the dissolution of a slightly oxidized layer formed during the production process itself. After 1 h, E .... reached the values shown in Table I. O

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present an anodic peak (point P in Fig. 5) at a potential value which increases with pH and is independent of the nature of the anion present in solution (see the values for 1 N HaPO 4 in Table II). Such a peak has already been observed but its nature has not been explained s. Figure 5 shows a cyclic polarization curve in sulphuric acid solution. The current of the reverse scan is lower than that of the forward scan, indicating that the anodically formed layer inhibits the anodic oxidation. Figure 6 shows cyclic voltammetry curves in the potential range from - 4 0 0 to 1400 mV. The first scan presents an intense anodic peak which disappears in the

310

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T A B L E II E L E C T R O C H E M I C A L PARAMETERS

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subsequent scans. The amount of charge according to the anodic peak is about 6 mC c m - 2. This behaviour might be due to an oxidized species not reduced during the cathodic scan. A new voltammogram was obtained with a cathodic reversed potential of - 1000 mV, and the subsequent scan did not present an anodic peak. 3.2. A.c. electrochemical study The Nyquist impedance diagrams (Fig. 7) were obtained in potentiostatic mode at the following potentials: - 1 2 0 0 m V , - 4 0 0 m V , Ec.... 400mV, respectively indicated as A, B, C and D in Fig. 5. The impedance diagram of Fig. 7(a) shows an inductive loop at the lower end of the frequencies which is representative of proton reduction. The shape of the impedance diagram of Fig. 7(b), at the lowest frequencies, indicates the presence of a diffusion controlled process. Figure 7(c), obtained at Ec.... reveals a very high impedance. Experimentally it is very difficult to close the impedance diagram on the real axis. However, this trend is indicative of the electrochemical inertness of the titanium nitride. Figure 7(d) shows that the charge transfer resistance can be extrapolated to about 130kDcm 2. This value indicates that, even in this case

311

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3.3. Capacitance measurements T h e c a p a c i t a n c e m e a s u r e m e n t s were p e r f o r m e d in H 2 S O 4 s o l u t i o n in the p o t e n t i a l range from - 1 0 0 0 to 2000 m V after waiting for 5 m i n at the c h o s e n potential. The c a p a c i t a n c e values were c a l c u l a t e d for three frequencies (Fig. 8). T h e c a p a c i t a n c e values d e c r e a s e d n o t a b l y after the a n o d i c peak.

3.4. Auger electron spectroscopy characterization T o s t u d y the m o d i f i c a t i o n s of the surface layers o b t a i n e d d u r i n g a n o d i c p o l a r i z a t i o n , s o m e investigations using A E S were performed.

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Figures 9-11 show Auger depth profiles, presented as the peak-to-peak heights of the analysed elements vs. sputtering time. Figures 9, 10 and 11 correspond respectively to as-prepared TiN, TiN polarized for 2 h at 400 m V (before the anodic peak) and TiN polarized for 2 h at 1200 m V (after the anodic peak). The sputtering rate is about 2 nm m i n - a

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In all the profiles three signals were observed, corresponding to the oxygen response (O1), the titanium+nitrogen response (Til) since, in AES spectra, the nitrogen and titanium signals overlap and cannot be separated at the lowest energies), and the titanium response (Ti2) obtained at higher energies. The profile relevant to the as-prepared sample exhibits slight oxygen contamination (Fig. 9). Figures 9 and 10 are quite similar, but the sample polarized at 400 mV shows a very small oxygen hump up to 3 min of sputtering (Fig. 10), which indicates a slight oxidation of the coating. The depth profile of the TiN polarized at 1200 mV is completely different. The oxygen signal is very high up to 9 min of sputtering and during the same time a decrease in the Til signal with respect to its level in Figs. 9 and 10 can be observed. On the contrary, the decrease in the Ti2 signal is not so evident.

4. DISCUSSION The polarization curves and the impedance spectroscopy diagrams obtained in the sulphuric acid solution clearly indicate that around the free corrosion potential the main cathodic reaction is oxygen reduction, proton reduction occurring only at very high overpotentials. The very noble E . . . . values, observed in all the considered solutions (Table I), suggest that, using TiN as a coating, it is necessary to produce a defect-free coating in order to avoid galvanic corrosion, the substrate acting as an anode. The very high Rp and i.... values (Fig. 3 and Table II) indicate the high electrochemical inertia, which is effective also during long-time tests. Moreover, we can suppose that the impedance diagram obtained at E .... is going to reach the real axis at frequencies below 1 mHz, giving a charge transfer resistance of hundreds of kiloohms x square centimetre. Such a diagram corresponds to a process involving a faradaic impedance including a high charge transfer resistance in series with a diffusion impedance, the last being due to the oxygen diffusion process. In the anodic range the current values are very small, in all the considered solutions. Moreover, the Nyquist plot obtained in the anodic range in H2SO 4 (Fig. 7(d)) shows a charge transfer resistance of about 130 kf~ cm 2, indicating that a very slow dissolution occurred. The anodic polarization curves exhibit a peak which appears at different potential values from the considered solution. The Auger analyses performed after polarization tests, at potential values around the peak, in the sulphuric acid solution, reveal an increase in the oxygen signal and a simultaneous decrease in the nitrogen signal at 1200mV. On the contrary, the sample remains nearly unchanged when polarized at 400 mV. These results indicate the partial TiN transformation into an oxidized layer. Considering the potential at such a transformation occurs (740 mV), and in agreement with the Pourbaix diagram, we can suppose that the titanium oxide obtained is TiO2, covering the surface partially, or, as suggested by Ernsberger e t al. 14 for the low temperature oxidation behaviour of TiN, an oxynitride with titanium in an oxidation state between TiN and TiO2. The cyclic voltammetry curves indicate that the oxidized layer obtained after

ELECTROCHEMISTRY OF REACTIVELY SPUTTERED

TiN

315

the current peak at 740 mV cannot be reduced and hence that this oxide is quite stable in the considered solution. Generally, metallic oxides are semiconductors; consequently the TiN anodized surface was characterized in order to discover similar semiconducting properties. The capacitance measurements (Fig. 8) show a sharp decrease in the capacitance value after the anodic peak. This behaviour probably corresponds to the transformation of TiN in a TiOxNy oxidized layer and indicates that around the current peak there is a transformation from a metal-electrolyte interface to a semiconductorelectrolyte interface. Indeed, in this last case the decrease in the interface capacity is due to the more important role which the space charge capacitance assumes with respect to the Helmoltz capacitance. Moreover, the first photoelectrochemical measurements show that after the anodic peak an important increase in the photocurrent was observed ~5 and consequently indicate the semiconductor character of the interface. AES analyses, capacitance measurements and photoelectrochemical measurements allow us to conclude that the anodic peak corresponds to the transformation of TiN into an oxidized titanium semiconductor. This is consistent with the observations of Laidani et al. 16, which indicate that, in the case of TiN layer growth in N H 3 plasma, the semiconducting properties decrease, increasing the nitriding efficiency, the evolution of the semiconducting properties being probably due to the decrease in the oxidation of the layer. The reason for the shift of the peak towards more anodic potentials for solutions of increasing pH is not quite clear. Nevertheless, we can suppose that this shift could be related to a reduction in the anodic currents with the pH increase. In fact the formation of the oxidized titanium layer requires the dissolution of an adequate quantity of TiN. For the NaC1 and Na2SO 4 solutions such dissolution is low and consequently the anodic peak shifts.

5. CONCLUSION

With thin films prepared by reactive sputtering on a glass substrate it was possible to evaluate the intrinsic electrochemical properties of this material. In this way the electrochemical parameters of the TiN coating prepared by PVD (free corrosion potential, corrosion current, shape of the polarization curves etc.) were obtained. Knowledge of these parameters allows the efficiency of a coating deposited on an active substrate to be controlled by acting as a reference. This study confirms the chemical inertia of TiN prepared by PVD in the free solutions used. The high value of the free corrosion potential and the d.c. and a.c. electrochemical studies show that around Ecorr the only possible cathodic reaction is the dissolved oxygen reduction. In anodic polarization a very low dissolution of TiN was observed until the formation of an oxidized layer which is related on the curves to an anodic peak. AES and capacitance measurements confirm the presence of this oxidized layer (TiO2 covering partially the surf~/ce, or oxynitride). This layer has semiconducting properties and its formation occurred at high anodic potential (740mV in a sulphuric acid solution). On pure titanium the growth of the semiconductor oxide

316

v. MASSIANI et al.

l a y e r o c c u r r e d at a b o u t - 500 m V 17,1a. T h i s difference in p o t e n t i a l s m i g h t be d u e to the nitrogen bond. ACKNOWLEDGMENTS T h e a u t h o r s are g r a t e f u l to M. D a p o r o f the I n s t i t u t e o f Scientific a n d T e c h n o l o g i c a l R e s e a r c h o f T r e n t o (Italy) for t h e A u g e r c h a r a c t e r i z a t i o n . REFERENCES 1 A. Erdemir, W.B. Carter, R.F. HochmanandE. I. Meletis, Mater. Sci. Eng.,69(1985)89. T.A. M~intyl5., P. J. Helevirta, T. T. Lepist6 and P. T. Siitonen, Thin Solid Films, 126 (I 985) 275. A. Erdemir and R. F. Hochman, J. Mater. Energy Syst., 7 (3) (1985) 265. A. Telama, T. A. M~intyl5.and P. Kettunen, J. Vac. Sci. Technol. A, 4 (6) (1986) 2911. A. Raman, G. Nnaike, A. Choudhury and C. R. Das, Corros. Sci., 25 (1985) 107. Y. Massiani, J. Crousier, L. Fedrizzi, A. Cavalleri and P. L. Bonora, Surf. Coat. Technol., 33 (1987)

2 3 4 5 6

7 8 9 10 11 12 13 14 15 16 17 18

309. A. Wisbey, P. J. Gregson and M. Tuke, Biomaterials, 8 (1987) 477. A.M. De Becdelievre, D. Feche and J. De Becdelievre, Electrochim. Acta, 33 (1988) 1067. B. Elsener, A. Rota and H. Bfhni, Mater. Sci. Forum, 44-45 (1989) 29. W. Posadowski, L. Krol-Stepniewska and Z. Ziolowski, Thin Solid Films, 62 (1979) 347. M. Wittmer and H. Melchior, Thin Solid Films, 93 (1982) 397. K.Y. Ahn, M. Wittmer and C. Y. Ting, Thin Solid Films, 107 (1983) 45. G. Lemp~ridre and J. M. Poitevin, Thin Solid Films, 111 (1984) 339. C. Ernsberger, J. Nickerson, T. Smith, A. E. Miller and D. Banks, J. Vae. Sei. Technol. A, 4 (6) (1986) 2784. G. Picq and P. Vennereau, personal communication, 1989. N. Laidani, J. Perriere, D. Lincot, A. Gicquel and J. Amouroux, Appl. Surf Sei., 36 (1989) 520. T.D. Burleigh, Corrosion, 45 (6) (1989) 464. D.J. Blackwood and L. M. Peter, Electroehim. Aeta, 34 (1989) 1505.