Study and improvement of the adhesion of chromium thin films deposited by magnetron sputtering

ELSEVIER

Thin Solid Films 258 (1995)

185-193

Study and improvement of the adhesion of chromium thin films deposited by magnetron sputtering V. Guilbaud-Massereau, L.M.C.T.S.

(IRA 320, Faculte des Sciences, Received

1 I May

A. Celerier, J. Machet

123 avenue Albert Thomas, 87060 Limoges cedex, France 1994; accepted

6 September

1994

Abstract

Pure and doped chromium coatings are deposited on glass substrates by magnetron sputtering. By varying the reactive gas content, tensile intrinsic stress transitions to compressive ones are observed. Scratch tests have demonstrated a good adhesion for doped chromium coatings which are in a compressive state. Doped nitrogen chromium films have the highest values of the critical load at which adhesion fails (17 N). Pure chromium coatings are in tensile stresses and the critical value is only 11 N. Adhesion and intrinsic stress dependences are shown. These results are correlated with average crystallographic grain size, preferential crystalline orientation, morphology and microhardness. Keywords:

Chromium;

Coatings; Physical vapour deposition;

Stress

1. Introduction

2. Experimental

Chromium coatings deposited by physical vapour deposition processes on glass or on metallic substrates are now frequently used in industrial applications. Their mechanical and optical properties could be modified by varying the deposition pressure and incidence angle of depositing atoms [ 1,2] and the substrate temperature [3]. But intrinsic stresses in thin chromium films are neverthless a problem which is still not resolved. In effect, with a high melting point (2149 K), chromium is considered as a material with a low mobility. It has been shown that this kind of material deposited at low deposition temperature presents important intrinsic tensile stresses [ 1,4-61. Stress states affect film performances and limit their uses and more particulary their adhesion. The study of the dependence of adhesion with intrinsic stresses is the purpose of this paper. After the description of the experimental procedure, the results concerning the adhesion and the intrinsic stresses will be presented. Then these results will be correlated with texture, morphology and microhardness of the films.

2.1. Deposition

0040-6090/95/%9.50 0 1995 SSDI 0040-6090(94)06360-5

Elsevier

Science

S.A. All rights

reserved

details procedure

Pure chromium and chromium-based coatings are deposited on square glass substrates (4 cm’) by cathodic magnetron sputtering. The target is made of cast chromium and its purity is 99.9%. Pure argon or argon-nitrogen mixtures of gases are used as sustaining gas discharge. The deposition reactor is a classical cylindrical vacuum chamber where the ultimate pressure is about 1O-4 Pa. Substrates are chemically and physically cleaned. For the chemical cleaning they are immersed in different solutions (see Section 3.1). The physical cleaning is realized in situ by biasing the substrates with a radiofrequency power supply (13.56 Mhz). A r.f.-biased electrode is made of conducting material and insulating substrates are attached to this electrode. By introducing pure argon in the vacuum chamber an electrical discharge is initiated. The ions created in the glow region and accelerated in the dark space, sputter the different impurities located at substrate surfaces. The parameters used for the physical cleaning are: total argon pressure,

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I Thin Solid Films 258 (1995) 185- 193

microscopy. Macro scale stress determination is performed by measuring the inherent curvature of the uncoated substrate and the strain-induced curvature of the coated substrate. The macroscale stresses C are calculated by the relation [7]: C=f

(1)

where E_v,, t, are respectively Young’s modulus, Poisson’s coefficient and the substrate thickness, tf is the film thickness, and R and R, are curvature radius of the substrate before and after deposition. It is easy to express R as a function of the length L covered by the stylus and its vertical shifting h. So I? can be expressed as a function of L and h: c = +4 E, t2,(h -ho) _3(1 _V,)f/ L2 We can write that the total stress is equal to the algebraic sum of the intrinsic stress (Ci) and a thermal one C,,:

300

c = ci f z,, 250 0

400

800

1200

1600

Cleaning time (s) Fig. 1. Substrate temperature evolution versus time (total pressure, 0.4 Pa; r.f. power density, biased voltage, - 650 V).

substrate cleaning 3800 W m-‘; self

The intrinsic stress is due to crystallographic structure defects and coating morphology. The thermal stress is due to the difference in the thermal expansion coefficients of coating and substrate materials and is given by the relation [ 81: (3)

0.2 Pa; r.f. power density, 3800 W m-2 (autopolarisation bias, - 650 V). Furthermore this ion bombardment leads to the temperature of the substrates increasing. The cleaning time is adjusted so as to obtain the wanted substrate temperature. Fig. 1 shows the evolution of the substrate temperature versus the cleaning time. Temperature is measured by a Ni/NiCr thermocouple attached to the substrate. The obtained temperatures lie from 333 K to 473 K. The deposition process starts just after this physical cleaning when the wanted substrate temperature is obtained. For all the studied deposits, different parameters maintain the same values. These parameters are: total pressure, 0.13 Pa; sputtering power density, 700 W m _ ‘; target to substrate distance, 0.12 m; deposition rate, 2.5 nm s -I; film thickness, 3 urn.

2.2.

Coatings

characterizations

Film thickness is measured by a stylus .profilometer (DEKTAK IIA). Coating adhesion is investigated by means of a scratch tester (LSRH). Acoustic emission allows one to determine the critical load value. Moreover, failure mechanisms are observed by optical

where Ef and vf are Young’s modulus and Poisson’s coefficient of the film, mf and c(, are thermal expansion coefficients of the film and of the substrate, T, is the substrate temperature during deposition and Ta is the room temperature. For pure chromium coatings deposited on a glass substrate, these constants are [ 1,9] : Es = 70 GN m - 2; v,=O.22; t,= 1 mm; a,=92 x lO-7 K-‘; Ef=260 GN rne2; v,=O.31; t,=3 urn; elf= 62 x lo-’ K-‘. A positive value of C corresponds to a tensile stress and a negative value to a compressive one. A Shimadzu microhardness tester with a normal load of 0.15 N applied during 15 s is used to measure microhardness coatings. When films are realized both on glass and on stainless steel, the obtained microhardness values are the same. For each sample, 12 measures are realized. The coatings morphology is observed by scanning electron microscopy (SEM) on fractured cross-sections. Structure and crystallographic grain size are investigated by X-ray diffraction analysis using a DACO MP PW 1730 (X-Ray Philips Generator).

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/ Thin Solid Films 258 (1995) 18% I93

3. Experimental results and discussion 3.1. Pure chromium coatings

a

3.1.1. Results After different attemps, the glass substrates were chemically cleaned before their introduction in the deposition chamber. The following sequence was used: the substrates are first degreased in acetone, then they are immersed in a sulfochromic solution for 90 min, then rinsed in deionized water. Finally the substrates are dried with dry compressed nitrogen. When the coatings are obtained without in-situ precleaning of the substrate, the deposition temperature is low ( < 373 K), the adhesion is very poor and the chromium films peel off spontaneously from the glass substrates when they are taken off the deposition chamber. When the substrates undergo ion bombardment in a r.f. discharge initiated in an argon atmosphere their temperature before the deposition process increases. We observe that for deposition temperatures higher than 373 K and lower than 473 K, the films do not peel off spontaneously. This deterioration occurs a few days after their realization. For temperatures of the order of 473 K, the adhesion is greatly improved, but in this case the breaking did not take place at the film-substrate interface, but in the glass itself. We obtained some broken glass covered with chromium coating. Fig. 2 shows the aspect of the glass surface after this degradation. Thermal stresses induced when the substrate temperature varies from a deposition temperature equal to 473 K to room temperature is equal to - 0.2 GPa. Chromium coatings have a microhardness which is equal to 7 GPa and which is comparable with Munz and Gobel measurements: 5-7 GPa [3]. These values are not very different from the chromium bulk one of 8 GPa [lo]. Structure investigation by X-ray analysis shows the chromium coatings to have a preferential orientation according to [ 1lo] diffraction planes (Fig. 8(a)).The same orientation has been observed by Aubert et al.

Fig. 2. Photograph optical microscopy.

of glass

and

coating

surfaces

investigated

by

b

2w

Fig. 3. Fractographs of pure coatings ([N]/[Cr] = 7.4%).

(a) and nitrogen-doped

(b) chomium

[ 1l] and by Janda [ 121 for evaporated chromium films. Average crystallographic grain size is calculated with the half-height of diffraction peaks and is approximatively equal to 270 A. The morphology study of coating fracture surfaces investigated by SEM revealed a columnar growth perpendicular to the substrate surface. Fig. 3(a) shows a fractograph of pure chromium coating. 3.1.2. Discussion For substrate temperatures of the order of 473 K, we cannot say that chromium adhesion on a glass substrate is poor. On the contrary, this adhesion is excellent if the substrates are chemically and physically cleaned. However the fact to be realized is that ionic bombardment before deposition on the glass allows the elimination of impurities at its surface and creates a microhardness. The sputtered thickness for a cleaning time equal to 1440 s (24 min) is about 200 nm, but the improved adhesion after ionic bombardment could also be due to a very thin plasma polymerized layer bare substrate surface [ 13,141. The creation of free bondings allows one to obtain strong bondings between chromium atoms and glass ones. So we must suppose that chromium films induce important stresses which are able to lead to the glass decohesion. This hypothesis is confirmed by the internal stresses measurements. These stresses are tensile strains which are approximatively equal to 0.8 GPa. The stress states and their level in a film are correlated to the films’ structure and morphology. Tensile stresses generally appear in materials whose melting point is high like chromium and are characterized by a columnar growth [4,1 l] which are

188

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due to: grain boundary relaxation [ 11; and elimination of point defects, porosities [4]. So, to have good stability of pure chromium layers it is necessary to reduce tensile stresses. Thornton’s model [l] described evolution of structure layer versus total pressure and showed that if target to substrate distance is < 0.06 m and total pressure is about 0.1 Pa, films are in highly compressive stresses with columnar morphology. But in our case the target to substrate distance is constant and equal to 0.12 m, and if total pressure in our vacuum chamber is less than 0.13 Pa discharge instabilities are observed. It is also posssible to reduce tensile stress levels avoiding columnar growth. For that purpose substrate temperature may be increased so as to increase chromium atoms mobility. It has been shown [ 151 that pure chromium coatings obtained by evaporation on MgF, substrate at a total pressure which is equal to 4 x 10 - ’ Pa are in tensile stresses until substrate temperatures of 473 K. When the substrate temperature reaches 573 K the coatings are in compressive states. Therefore grain boundaries and defects built into the film are eliminated by film recrystallization during its deposition.

However we would like to obtain stable chromium films at a maximum substrate temperature equal to 473 K. This order of substrate temperature during the deposition process allows the growth of chromium grains whose size will be more important. So the relaxation of grain boundaries will still induce tensile stresses. Another possibility for modifying intrinsic stresses in chromium coatings is to incorporate a varying amount of impurities in the metallic film during its deposition [4,16]. For that purpose we suggest the introduction of a small nitrogen quantity during chromium deposition. 3.2. Nitrogen-doped chromium coatings 3.2.1. Results In this case, nitrogen-doped chromium coatings are realized by sputtering chromium in an argon-nitrogen mixture atmosphere. All the coatings are obtained at the same substrate temperature of 473 K. The total pressure in the reactor is constant and equal to 0.13 Pa. Nitrogen doping is characterized by the nitrogen flow rate QN, to the argon rate QA, ratio which are introduced in the deposition chamber. X-ray photoelectron spectroscopy (XPS) analysis allows one to determine

0.8

-0.8

OI 0

10

20

30

40

50

QN2/QAr (%) Fig. 4. [r\rl/[Cr] ratio versus QN2/QAr ratio (total pressure, 0.13 Pa; sputter power density, 700 W rn-? substrate temperature, 473 K).

0

5

10

15

20

PW[Crl 6) Fig. 5. Intrinsic stresses versus [Nj/[Cr] ratio (total pressure, 0.13 Pa; sputter power density, 700 W m-*; substrate temperature, 473 K).

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et al. 1 Thin Solid Films 258 (1995) 185- 193

the [NJ/[ Cr] ratio in the films. The evolution of this ratio [Nj/[Cr] versus the flow rates ratio QN,/QAr is reported in Fig. 4. We can remark that for QN,/QAr = 0, the [N]/[Cr] ratio is different from 0. This is certainly due to adsorbed atmospheric nitrogen at the film surface which had not been completely eliminated by the pre-sputtering cleaning done before XPS analysis. All coatings are realized at the same substrate temperature so thermal stresses are constant and equal to - 0.2 GPa. Intrinsic stress evolution as a function of the concentrations ratio [Nj/[Cr] is reported in Fig. 5. A tensile to compressive stress transition is observed for a [N]/[Cr] ratio equal to 6.4%. When the concentrations ratio is higher than 9%, the compressive stress level is practically constant and equal to - 0.7 GPa. The evolution of the coating adhesion (critical load L,) versus [Nj/[Cr] ratio is reported in Fig. 6. We can notice an important correlation between the critical load values and the internal stresses nature. When coatings are in the tensile state, the critical load is equal to 11 N. For compressive stresses, the critical value is approximatively equal to 17 N.

3.2.2. Discussion The critical load obviously depends on the film adhesion on the substrate. It also depends on structure and mechanical properties of the film and more particulary on its microhardness. If we compare Figs. 5 and 6, we cannot conclude that adhesion depends only on the internal stresses in the films. So as to find an interpretation for the critical load evolution, coating microhardness has been measured and microstructure of the layer has been determined. Coating microhardness evolution versus the [Nj/[Cr] ratio is reported in Fig. 7. Microhardness regulary increases with [Nj/[Cr] ratio. These values are comparable to those obtained by Miinz and Gobel [3] who have realized CrN and Cr/CrN coatings by sputtering a pure chromium target in a nitrogen atmosphere. When ratio values vary from 6% to 9%, microhardness evolution does not present any discontinuity contrary to critical load evolution. In effect, the important variation of the critical load in this range of ratio values cannot be attributed to the microhardness variation.

30

20

18 24

16

6

0

0

5

10

15

20

I

0

5

15

20

[NlRCrl (%‘o)

[NlBCrl (%) Fig. 6. Critical load evolution versus [Nj/[Cr] ratio (total pressure, 0.13 Pa; sputter power density, 700 W m -*: substrate temperature, 473 K).

10

Fig. 7. Microhardness evolution 0.13 Pa; sputter power density, 473 K).

versus [Nj/[Cr] ratio (total pressure, 700 W m -‘; substrate temperature,

V. Guilbaud-Massereau

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25

30

35

Fig. 8. X-ray diffractograms 12.5%; i, 16%).

40

for different

45

et al. / Thin Solid Films 258 (1995) 185- I93

50

nitrogen-doped

56

chromium

60

65

70

7s

88

films ([Nj/[C r 1: a, 0%; b, 4.1%; c, 5.5%; d, 6%; e, 6.4%; f, 1.4%; g, 9%: h,

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spectra

for (a) pure chromium coating and (b) nitrogen-doped

cncrw

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Iin

chromium coatings ([NJ/[Cr] = 16%).

V. Guilhaud-Massereau

et al. 1 Thin Solid Films 258 (1995) 185-193

At the same time, the X-ray study of the coatings does not show any discontinuity in film microstructure. Fig. 8 shows different X-ray spectra obtained for various [N] /[ Cr] ratios. We can only notice that preferential crystalline orientation in nitrogen doped chromium films varies from [ 1lo] to [200] when the [N]/[Cr] ratio increases from 0 to 16%. We can also remark that the diffraction angle corresponding to the 200 family planes decreases. Researchers [ 3,17-201 have shown by X-ray diffraction analysis that they obtain a Cr,N phase in coatings realized by reactive sputter deposition of chromium in an argon-nitrogen atmosphere. But in our case, we suppose that nitrogen is introduced in a chromium matrix by diffusion and creates a solid solution because of the low deposition temperature. This assumption is confirmed by ESCA analysis as indicated in Fig. 9. The 2~“’ 2p3/* photoelectron level energy is the same in pure or in doped chromium films. The crystallographic grain size decreases from 270 8, to 120 A when the [N]/[ Cr] ratio increases from 0 to 6.4% as shown in Fig. 10. For higher values, the grain size tends towards about 100 A. Finally, a SEM investigation of

\

i

0

5

10

15

20

UWCrl (%I Fig. 10. Average crystallographic grain size versus [N]/[Cr] ratio (total pressure, 0.13 Pa; sputter power density, 700 W m - ‘: substrate temperature, 473 K).

191

coating fractures shows that the morphology is very dense (Fig. 3(b)). This aspect is observed when coatings are in a compressive stress state [4,18]. To summarize these results, it can be said that the nature and the intensity of the intrinsic stress intensities depend on the nitrogen amount in the films. For nitrogen concentrations in the films lying from 0 to 9% nitrogen insertion during the film growth mainly occurs at grain boundaries and a few nitrogen atoms diffuse in a chromium matrix. Nitrogen atoms can be considered as impurities which inhibit grain boundary relaxation. Tensile intrinsic stress intensity is reduced. The nitrogen presence at grain boundaries also limits grain growth and their size decreases, leading to a dense morphology and an increase of the film microhardness. Therefore the critical load in this range of nitrogen concentration also increases a lot. This result could be explained by the reduction of tensile intrinsic stresses in the chromium coating itself. We could also think that nitrogen doping induces stronger bondings of the oxide substrate to a nitride than to pure chromium [21]. But it is necessary to remember that pure chromium bondings with a glass substrate in the same experimental conditions are very strong. However when pure chromium peels off the glass substrate, the breaking occurs in the glass and not in the metal-glass interface. For nitrogen concentrations in the films higher than 9%, the increase of the [N]/[Cr] ratio does not modify the films properties in an important way. We can notice only the evolution of the diffraction angle corresponding to the 200 family planes. We can suppose that in this range, grain boundaries are saturated, so nitrogen atoms diffuse in the chromium matrix and fill the octahedral sites of the chromium center cubic structure which leads to a lattice deformation. Seeing that the nitrogen amount at the grain boundaries is constant, intrinsic stresses, adhesion, microhardness and crystallographic grain size are almost constant. So as to confirm these results and these hypotheses, we have realized doped chromium films for different total pressures keeping a constant nitrogen flow rate (3 x 10 ’ Pa m3 s - ‘). The argon flow rate is adjusted so as to obtain a total pressure varying in the range 0.13 Pa to 0.4 Pa. The intrinsic stress evolution versus total pressure is reported in Fig. 11. For low pressures, chromium atoms do not undergo many collisions with argon atoms in the gaseous phase, so they fall on the growing film with an important energy and they can diffuse easily leading to a dense structure characterized by compression stresses. In this case it is necessary to note the importance of bombardment of the growing film by energetic neutrals which are produced by charge transfer and reflection at the cathode [22]. By increasing the total pressure the transition of compressive to tensile stresses is observed for a total pressure which is equal to about 0.2 Pa. Similar results have also been

192

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1.2

0.9

0.6 2 8 y: fi !i :: 0.3 ‘il

0

+lJ

-0.3

0 0.1

0.2

0.3

0.4

0.5

0.1

Total pressure (Pa)

0.2

0.3

0.4

0.5

Total pressure (Pa)

Fig. 11. Intrinsic stress evolution versus total pressure (sputtering power density, 700 W rnm2; substrate temperature, 473 K; nitrogen flow rate, 3 x 10m3 Pa m3 s-l).

Fig. 12. Critical load evolution versus total pressure (sputtering power density, 700 W m-*; substrate temperature, 473 K; nitrogen flow rate, 3 x lOA Pa m3 ss’).

obtained for total pressure between 0.2-0.4 Pa [23]. But in our case it is important to remark that when total pressure increases the ratio PN2/PAr decreases and that the energy of the chromium atoms at the substrate surface also decreases. The film structure becomes columnar leading to the appearance of tensile stresses. As a consequence, the adhesion of the film decreases as indicated in Fig. 12 and becomes very low (2 N). At the same time microhardness is reduced. So when total pressure increases, bombardment effects to growing films by energetical neutrals and the decrease of the ratio PN2/PAr are significant.

To avoid this columnar structure, the doping of a chromium film with nitrogen during the deposition process allows the grain size to decrease and rigid grain boundaries to be obtained. By varying the proportion of nitrogen in the film it is possible to modify the stress level and to pass from a tensile state to a compressive state. In these conditions, films present a very good adhesion on glass and they are very hard. It must also be pointed out that the structure and the morphology, and as a consequence, the stress level depend not only on the film composition but also on the total pressure and on the temperature of the growing film. Indeed these parameters modify the energy of the chromium atoms falling on the growing film.

4. Conclusion It is difficult to obtain adherent pure chromium films on glass substrates. This is principaly due to important internal tensile stresses. These stresses are caused by a columnar structure of the films with large grain boundaries whose relaxation gives rise to tensile stresses.

References [l]

J.A. Thornton and D.W. Hoffman, Thin Solid Films, 171 (1989) 5-31. [2] H. Yamadera and Y. Taga, Appl. Phys. Lett., 55 (1989) 1080. [3] W.D. Mtinz and J. Gobel, Surface Engineering, 3(l) (1987) 47-51.

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R. Abermann, Vacuum, 41(4-6) (1990) 1279-1282. Y. Goto and Y. Taga, J. Appl. Phys., 67( 1990) 1030. Y. Taga and Y. Goto, Thin Solid Films, 1931194 (1990) 164. V. Stambouli, 0. Burat, D. Bouchier and G. Gauthier, Surf Coat. Technot., 43144 (1990) 137. M. Janda and 0. Stefan, Thin Solid Films, 112 (1984) 127-137. M. Okamoto, Jpn. J. Appl. Phys., 29(5) (1990) 930-933. J Asawari, J. Mater. Sci., 25 (1990) 135771365. A. Aubert, R. Gillet, A. Gaucher and J.P. Terrat, Thin Solid Films, 108 (1983) 165-172. M. Janda, Thin Solid Films, 142 (1986) 37-45. K. Suzuki, K. Matsumoto and T. Takatsuka, Thin Solid Films, 80 (1981) 67-76. J.M. Burstand, J. Appl. Phys., .52(7) (1981) 4795-4800. G. Thumer and R. Aberman, Thin Solid Films, 192 (1990) 2777285.

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[16] R.W. Hoffman, Thin Solid Films, 34 (1976) 185-190. [ 171 0. Knotek, W. Bosch, M. Atza, W.D. Mung, D. Hoffman and J. Goebel, High Temp., High Press., 18 (1986) 435442. [ 181 P.M. Fabis, R.A. Cooke and S. Mc.Donough, J. Vat. Sci. Technol. A, 815) (1990) 3809-3818. [ 191 J.P. Terrat, A. Gaucher and H. Hadj-Rabah, Surf. Coat. Technol., 45 (1991) 59-65. [20] M. Charbonnier, M. Roche and J.P. Terrat, Adu. X-Ray Anal., 35 (1992). [21] David R. Lide, Handbook of Chemistry and Physics, 73rd edn., CRC Press, Boca Raton, FL, 1992- 1993. [22] J.A. Thornton and A.S. Penford, in J.L. Vossen and W. Kern (eds.), Cylindrical magnetron sputtering, Thin Film Processes, Academic Press, New York, 1978. [23] J.A. Thornton, J. Tabock and D.W. Hoffman, Thin Solid Films, 16(1979) 134.