Synthesis of silicon nitride films by ion-beam-enhanced deposition and their protective properties against oxidation

Materials Science and Engineering, A121 (1989) 519-523

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Synthesis of Silicon Nitride Films by Ion-beam-enhanced Deposition and Their Protective Properties Against Oxidation* SHIGEJI TANIGUCHI and TOSHIO SHIBATA Department of Materials Science and Processing, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565 (Japan) KYOTARO NAKAMURA Graduate School, Osaka University, Osaka (Japan) LIU X1ANG-HUAI, ZHENG ZHI-HONG, HWANG WEI and ZOU SHIHCHANG Ion Beam Laboratory, Shanghai Institute of Metallurgy, Academia Sinica, 865 Changning Road, Shanghai (China) (Received March 10, 1988)

Abstract Silicon nitride films were synthesized on coldrolled nickel coupons by concurrent electron beam evaporation of silicon and bombardment with nitrogen ions (ion-beam-enhanced deposition). IR, Rutherford backscattering, Auger electron spectroscopy, transmission electron spectroscopy, scanning electron microscopy, energy dispersive ellipsometry and spreading resistance measurements were performed for investigating the properties, composition, and structure of the films. Isothermal and cyclic oxidation tests were carried out on the uncoated nickel coupons and those coated with films of thicknesses up to lO000 ft at 1300 K in a flow of pure oxygen at atmospheric pressure. The films decrease the isothermal oxidation rates, the thicker films having a larger effect. However, films of thicknesses up to 3000 ft are not effective for cyclic oxidation. The scanning electron microscopy observation and energy dispersive X-ray analysis revealed that thin films rich in silicon remained in the NiO scale near the scale-substrate interface after both isothermal and cyclic oxidation. Further, much thinner films containing silicon were present on the outer surface of the NiO scales formed on the specimens coated with nitride films of thickness 10 000 ft. The oxidation mode initially follows an approximately linear law and approaches a parabolic law later.

1. Introduction It has been shown that ion implantation and ion beam mixing offer an alternative measure of *Invited paper. 0921-5093/89/$3.50

surface modification resulting in significant improvement in wear, oxidation and aqueous corrosion resistance of metals [1-3]. Ion-beam-enhanced deposition (IBED) is a combination of ion implantation and chemical vapour deposition (CVD) or physical vapour deposition (PVD) which has the advantages of improved adhesion to the substrate [4], ready control of composition and thickness of the film [4] and the possibility of synthesizing compound films and facilitating growth at relatively low temperatures [5]. This paper deals with the isothermal and cyclic oxidation behaviour of cold-rolled nickel coupons coated with silicon nitride films synthesized by IBED.

2. Experimental details 2. I. Growth of silicon nitride films Silicon nitride films were deposited on coldrolled nickel coupons with an Eaton Z-200 IBED system. It consists of an ion implanter delivering ions with intermediate energies of 20-100keV and currents of up to 6 mA, an electron beam evaporator with four crucibles and a rotating target holder with a water-cooling system. The base pressure in the target chamber w a s 10 - 7 Torr and the typical working pressure was 10 -6 Torr. A quartz crystal was used to monitor the growth rate of the film. The specimen temperature during the deposition was kept below 473 K with the water-cooling system. High-purity silicon and nitrogen gas were used as source materials. The ion species included © Elsevier Sequoia/Printed in The Netherlands

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1300 K, holding at this temperature for 72 ks (20 h) and cooling. The oxidation products were examined by XRD and scanning electron microscopy (SEM) equipped with an energy dispersive X-ray (EDX) analysis unit.

8(;

6o

5i

Si

Ni

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3. Resdts N

N

Si N 3

6 SPUTTER

9

12

15

TIME / ks

Fig. 1. AES profiles for nitrogen, silicon and nickel across the 5000 A nitride film formed on a cold-roiled nickel coupon by IBED, indicating an outermost silicon-rich layer, an Si3N 4 and a transition layer next to the nickel substrate.

in the nitrogen ion beam were N + and N 2 + . The nitride films were formed by bombardment with 40keV nitrogen ions with a ratio of N+:N2 + = 3:4 during the silicon film growth. The ion beam density used in our experiment was 56/~A cm -2. The deposition rate of silicon was 1 ./~ s -1. The atomic arrival rate ratio N:Si was 0.79:1. Previous experimental results showed that the film composition can be controlled and predicted by the atomic arrival rate ratio [6]. The IR absorption peak of the IBED nitride was located at a wavenumber of 840 cm- 1. The refractive index ranged from 2.2 to 2.6. The measurements of IR transmission and refractive index demonstrated that stoichiometric Si3N4 layers were formed in the films by IBED. An Auger electron spectroscopy (AES) diagram (Fig. 1) shows that the IBED nitride film consists of a silicon-enriched layer on the outer surface, an Si3N4 layer with uniform composition and a transition (or mixing) layer at the film-substrate interface. X-ray diffractometry (XRD) of the specimens coated with the nitride films showed no peaks corresponding to crystalline Si3N4. 2.2. Oxidation tests The oxidation behaviour of the specimens coated with nitride film was assessed by isothermal and cyclic oxidation tests in a flow of purified oxygen at atmospheric pressure. The isothermal oxidation tests were performed with an autorecording thermobalance at 1300 K for 100 ks.The cyclic oxidation tests were performed with a vertical furnace. One cycle consisted of heating to

3.1. Oxidation kinetics The results of the isothermal and cyclic oxidation tests are shown in Figs. 2 and 3, respectively, in the form of the mass gain per unit specimen area (AMA-~) plotted against the time or cycle. Duplicate runs were carried out under the same conditions in order to assess the reproducibility of the results. Figure 2 shows that the isothermal oxidation rate decreases as the film thickness increases, except for the lowest curve which is that for the specimen coated with a film of thickness 10 000 A. Films of all thicknesses are effective in decreasing the isothermal oxidation rate for up to 100 ks. Figure 3 shows that the 1000-3000 A films are not effective in decreasing the cyclic oxidation rate. They rather increase the rate. However, the 5000 and 10 000 A films decrease the cyclic oxidation rate. The former is more effective than the latter. Figures 4 and 5 show double-logarithmic plots of the isothermal and cyclic oxidation kinetics, respectively. The slopes of the curves for the nickel coupons in both figures indicate that the

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60

80

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Time / ks

Fig. 2. Isothermal oxidation kinetics of nickel coupons and those coated with nJtrid¢ films of various thicknesses at 1300 K in a flow of pure oxygen at atmospheric pressure.

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oxidation of nickel follows a parabolic law. However, the slopes for the coated specimens in Fig. 4 tend to increase as the film thickness increases from 0.85 for 2000/~ to 1.17 for 10 000 ,/k. As the oxidation time approaches 100 ks the slopes decrease to 0.65 for 2000/l and to 0.67 for 10 000 ,~. It is concluded from the above that the oxidation kinetics of the coated specimens follows an approximately linear law during the initial periods and approaches a parabolic law later. Figure 5 clearly shows this mode for the films of thicknesses up to 3000/l. The figure also shows that films of these thicknesses are not effective in decreasing the oxidation rate after 300 ks. On the other hand, the 5000 and 10 000 ,~ films are effective in decreasing the oxidation rate, though the slope for the film of 10 000/~ is still rather high at 0.78.

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150

100

0

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Fig. 3. Cyclic oxidation kinetics of the nickel coupons and those coated with the nitride films of various thicknesses at 1300 K in a flow of pure oxygen at atmospheric pressure.

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Fig. 4. Double logarithmic plots of the kinetic curves shown in Fig. 2.

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3.2. Morphologies of the oxidation products XRD of the specimens after isothermal and cyclic oxidation indicated that the oxides formed were NiO. No peaks corresponding to crystalline Si3N4 were found. This is probably due to the fact that the nitride film is buried in the NiO scale, as will be shown later. However, annealing of the 5000/l nitride film in argon at 1300 K for 1 ks showed peaks of ~-Si3N4. Figures 6(a)-6(c) show the outer surfaces of the specimens after isothermal oxidation at 1300 K for 100 ks; (a) uncoated, and coated with nitride film of (b) 3000/~ and (c) 10 000 t%,. The surfaces of NiO grains in Fig. 6(a) show partially stepped structures. Some NiO grains in Fig. 6(b) are associated with many pits which have geometrical shapes and distributions which vary from grain to grain. However, these kinds of pit were not found on the NiO grains in Fig. 6(c). Figure 7(a) shows an outer surface (top) and fractured edge (middle) of the NiO scale formed on the specimen coated with a 2000/~ nitride film,

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Fig. 5. Double logarithmic plots of the kinetic curves shown in Fig. 3.

Fig. 6. Outer surfaces of the NiO scales formed on the specimens. (a) Uncoated; (b) coated with a 3000 A nitride film; (c) coated with a 10000A nitride film. Oxidized at 1300K for 100 ks.

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Fig. 7. Specimens after the isothermal oxidation at 1300 K for 100 ks; coated with the nitride film of (a) 2000 ,~ and (b), (c) l0 000 A. (a) Outer surface (upper) and fractured edge (middle) of the NiO scale and an area near the scale-substrate interface. (b) A thin film containing silicon remains on the NiO scale and is partly curled up. (c) Mass of NiO crystals locally grown through the film on the NiO scale.

revealed by hammering the specimen edges, and an area near the scale-substrate interface (bottom). Figure 7(b) shows an outer surface (top) and a fractured edge (bottom) of the NiO scale formed on the specimen coated with a 10 000 ,~ nitride film. We can recognize that a thin film remains on the oxide and that part of it is curled up. EDX analysis showed that the thin film contains silicon. The same tendency was observed for the specimens coated with 5000 A nitride films. Using SEM for observation, it was impossible to confirm this kind of film formation for the specimens coated with nitride films of up to 3000 A. However, EDX analysis of the outer surface of the oxide showed the presence of a considerable amount of silicon. In the case shown in Fig. 7(a) the atomic ratio of silicon to nickel is about 38:62. This ratio is about 26:74 for Fig. 7(b). Figure 7(c), the same specimen as Fig. 7(b), shows that NiO crystals were growing through the surface film over small local areas, indicating that its partial fracture had taken place during oxidation. Figures 8(a)-8(c) show the outer surfaces of the specimens oxidized for 10 cycles (720 ks) at 1300 K. NiO grains formed on the nickel coupons

Fig. 8. Surfaces of the specimens after cyclic oxidation at 1300 K for I0 cycles (720 ks). (a) Uncoated nickel; (b) coated with a 3000 A nitride film; (c) coated with a 10 000 A nitride film.

Fig. 9. Surface (upper) and fractured edge (middle) of the NiO scales after cyclic oxidation at 1300K for 10 cycles (720 ks). Coated with nitride films of thickness (a) 3000 • and (b) 10 000 ]L

are associated with wrinkled structures and fine pits (Fig. 8(a)). This tendency became more pronounced in the specimens coated with 3000 ,~ nitride films (Fig. 8(b)). However, the NiO grains on the specimen coated with the l0 000/~ film show very different features characterized by the stepped surfaces and fine pits (Fig. 8(c)). Figures 9(a) and 9(b) show the outer surfaces (top) and fractured edges (middle) of the oxide scales after cyclic oxidation. The development of a columnar structure can be seen in the specimen coated with the 3000 A nitride film (Fig. 9(a)), while the NiO grains are not columnar for the specimen coated with the 10000]~ nitride film (Fig. 9(b)). It was impossible to confirm the presence and location of the original nitride film by the SEM examination shown above. Therefore, crosssections of the oxidized specimens were further examined. Detailed observations and EDX examinations of cross-sections of the specimens after isothermal and cyclic oxidation tests revealed that a thin layer rich in silicon remains in the NiO scale near the scale-substrate interface. This layer is shown by an arrow in Figs. 10(a) and 10(b) for the specimens coated with the nitride films of thick-

Fig. 10. Cross-section of the specimens after isothermal oxidation at 1300 K for 100 ks. A thin layer rich in silicon (arrowed) is located in the NiO scale near the scale-substrate interface. Coated with the nitride films of thickness (a) 3000 A and (b) 10 000 A. (c) The same specimen as (b) showing that a thin layer containing silicon remains on the outer surface of the NiO scale.

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Fig. 11. Cross-sections of the specimen after oxidation for 10 cycles (720 ks). A thin layer rich in silicon apparently at least a part of the original nitride film, remains in the NiO scale near the scale-substrate interface. (a)-(c) are in the order of increasing magnification for the same area.

ness 3000 and 10 000 ~, respectively. The presence of a very thin layer containing silicon at the outer surface of the NiO scale was revealed by the profile of silicon in Fig. 10(c). Similar thin layers rich in silicon partly remained after oxidation for 10 cycles as shown in Fig. 11 (arrowed). The original nitride film was 3000/~.

4. Conclusions The silicon nitride films deposited on coldrolled nickel coupons by IBED are effective for decreasing the isothermal oxidation rate at 1300 K for 100 ks. The original nitride film, or at least a part of it, remains in the NiO scale near the scale-substrate

interface, even after oxidation for 10 cycles (720 ks). Very thin films containing silicon were found to remain on the NiO scale on the specimens coated with 5000 and 10 000 ~, nitride films after isothermal oxidation for 100 ks. Nitride films with thicknesses less than 3000 do not effectively decrease the cyclic oxidation rate, while those of 5000 and l0 000/~ are effective.

Acknowledgments The authors are grateful to Mr. J. Nakata of Osaka University for his partial assistance in the experimental work and to Hanshin Research and Development Laboratories of Nissin Steel for AES. The present work is partially supported by the Chinese Academy of Sciences under special Grant 87-52.

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