The properties of AlN prepared by plasma nitriding and plasma source ion implantation techniques

Surface and Coatings Technology 131 Ž2000. 345᎐349

The properties of AlN prepared by plasma nitriding and plasma source ion implantation techniques Yun-Keun Shima,U , Yoon-Kee Kima , K.H. Lee a , Seunghee Hanb a Materials Processing Lab, Institute for Ad¨ anced Engineering (IAE), Yongin P.O. Box 25, Kyunggi-do 449-820, South Korea Ad¨ anced Analysis Center, Korea Institute of Science and Technology, Cheongryang P.O. Box 131, Seoul 130-650, South Korea

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Abstract The formation of aluminum nitride ŽAlN. layer on 1050 aluminum substrate has been successfully performed by a low pressure plasma nitriding process and a plasma source ion implantation process. In the plasma nitriding ŽPN. process, a relatively thick and uniform stable h.c.p.-AlN layer of ; 20 ␮m has been obtained from a gas pressure of 400 Pa N2 gas at 480⬚C. In PSII technique, a metastable f.c.c.-AlN layer has been consistently observed in the implantation dose levels between 7 = 10 17 and 1.8= 10 18 ionsrcm2. In addition an apparent peak separation between metastable f.c.c.-AlN and f.c.c.-Al phase has been ˚ . of Cr K ␣ X-ray. 䊚 2000 Published by Elsevier Science B.V. observed in the XRD spectrum using a long wavelength Ž ␭ s 2.29 A All rights reserved. Keywords: AlN; Plasma nitriding; Plasma source ion implantation

1. Introduction In these days, aluminum alloys, which have attractive properties such as low specific weight and good machinability, have been considered as one of the most important materials in various industries w1,2x. However, poor wear property of Al alloys limits its use for industrial applications. Therefore, many researchers have tried to improve the surface properties of Al alloys by the formation of an AlN layer on Al alloys because AlN has a relatively high hardness Ž; HV 1400. with a melting point of 2490⬚C and good thermal conductivity Ž320 WrmK. w3x. In recent years many different technologies such as Plasma Source Ion Implantation ŽPSII., Plasma Nitriding ŽPN., conventional ion implantation, reactive ion sputtering deposition, and plasma deposition including ECR CVD process have been reported for the formation of AlN w1᎐9x. Among those technologies, plasma nitriding and PSII are relatively effective techniques for the various appliU

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cations due to their advantages for the surface treatment of three-dimensional objects w1᎐7x. In the process of AlN formation on Al alloys, aluminum oxide ŽAl 2 O 3 . phase can be easily formed on Al substrate because the reaction enthalpy of Al with oxygen is approximately 10 times higher than that of Al with nitrogen in a nitriding process. This is one of the critical problems in the process for the formation of AlN layer on Al surface because a thin layer of Al 2 O 3 prevents nitrogen atoms from reaching the surface of the Al substrate. The formation of a thick AlN layer is very difficult in conventional nitriding process using a d.c. power source because AlN film is an insulator with a high electrical resistivity of ; 10 7 ⍀rm w2᎐4x. Stock et al. w4x have reported the formation of 3᎐10 ␮m thick AlN layers using a plasma nitriding system equipped with a diffusion pump and using high purity Ž99.999%. N2 , H 2 , Ar gases during the nitriding process. They confirmed a uniform AlN layer in the films with a thickness of less than 3 ␮m. However, they observed serious cracks in the AlN film above a thickness of 3 ␮m because of the large difference of thermal expansion coefficients between Al alloys and the AlN layer.

0257-8972r00r$ - see front matter 䊚 2000 Published by Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 0 . 0 0 8 0 7 - 0

Y. Shim et al. r Surface and Coatings Technology 131 (2000) 345᎐349

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Table 1 Process parameter of plasma source ion implantation Process

PSII-1

Sputter cleaning

Time, 30 min; pressure, 6.6= 10y4 Pa; voltage, y500 V, Ar gas

PSII

7 0.13 60 N2 7 = 1016

Time Žmin. Pressure ŽPa. Voltage ŽykV. Gas Dose Žionsrcm.

PSII-2

70 0.13 60 N2 7 = 1017

PSII-3

140 0.13 60 N2 1.6= 1018

In the PSII process, the existence of very thin natural Al oxide ŽAl 2 O 3 . layer on Al surface is not a serious matter for the formation of a stable AlN layer due to the high energetic nitrogen ions accelerated by high cathode voltage. These high energetic ions can easily break the oxide film and penetrate into the Al substrate w10x. However, the thickness of the AlN film is normally limited to a thickness of less than 1 ␮m because of the limited implantation energy and dose. In our experiment the structure of the AlN layer has been investigated to understand the structure of the layer associated with the growth mechanism of AlN film on the Al substrate. Previously, several experimental results have reported that the AlN layer has two different crystal structures such as hexagonal close packed structure Žh.c.p.. of Wurtzite-type and face centered cubic structure Žf.c.c.. of NaCl-type w8,12x. Generally h.c.p.-AlN crystal structure rather than f.c.c.-AlN has been observed in PSII and PN techniques. Setoyama et al. w11x expected that f.c.c.-AlN had a higher hardness and higher strength than h.c.p.-AlN film due to the higher volume density Ž; 18%. of f.c.c.-AlN. Some other researchers reported that the mixture of h.c.p. and f.c.c.-AlN structure has been observed in ion implantation process with increasing the implantation dose up to ; 9 = 10 17 ionsrcm2 w6x. In this paper, the properties of the AlN layer formed by PN and PSII will be discussed in terms of the structure of AlN film and the process parameters. X-Ray Photo electron Spectroscopy ŽXPS., X-Ray Diffraction ŽXRD., Scanning Electron Microscope ŽSEM., Auger Electron Spectroscopy ŽAES. and Wavelength Dispersive System ŽWDS. were used to analyze the AlN film.

PSII-4

35 0.13 20 N2 3.4= 1017

PSII-5

PSII-6

35 0.13 40 N2 3.4= 1017

35 0.13 60 N2 3.4 = 1017

2. Experimental details Specimens of aluminum ŽAl 1050. used as substrate material were prepared with plates Ž20 = 20 = 2 mm. and disks Ž ␾ 30 = 5 mm.. Substrates have been ground with 600 grit SiC paper and then ultrasonically cleaned before sputter cleaning. High purity N2 , Ar, and H 2 gases of 5 N Ž99.999%. were used with special oxygen filter to minimize the formation of Al oxide layer on the substrate. Experimental system of PSII is equipped with a diffusion pump for the base pressure of 1.3= 10y4 Pa and r.f. 250 W power is used to generate plasma. Sample has been sputtered and cleaned with a negative sample bias potential of y500 V using Ar gas in a pressure of 0.13 Pa for 30 min before the implantation process. Implantation dose of nitrogen ion was varied from 7 = 10 16 to 1.8= 10 18 ionsrcm2 with a d.c. biased voltage of y20 to y60 kV. The details of the experimental parameters are listed in Table 1. The low pressure plasma nitriding system has been used in the nitriding process with a base pressure of 6.6= 10y4 Pa using a diffusion pump and an oxygen purifier inserted in gas line to minimize the oxide formation on the substrate. The samples were heated up to 350⬚C to bake out the impurities from Al surface with a flowing of H 2 gas into the chamber to deoxidize a sample surface and were cleaned with a sputtering process using a gas mixture of 80% of Ar and 20% of H 2 gas for ; 30 min. Plasma nitriding has been performed with ; 400 Pa of N2 gas at 480⬚C for different processing time. The details of plasma nitriding parameters are listed in Table 2. The atomic compositions, chemical bonding states,

Table 2 Process parameters of the plasma nitriding Process

PN-1

Sputter cleaning

Time, 30 min; pressure, 100 Pa; voltage, y600 V; ArrH2 gas; temp., 350⬚C

Nitriding

Time Žh. Pressure ŽPa. Neg. voltage ŽV. Gas Temp. Ž⬚C.

3 400 500 N2 480

PN-2

6 400 500 N2 480

PN-3

8 400 500 N2 480

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Table 3 Atomic composition Žat.%. determined by WDS analysis of a cross-section of AlN sample prepared by plasma nitriding process

Atomic percent

Al Žat.%.

N Žat.%.

O Žat.%.

C Žat.%.

41.6

32.2

13.2

13.0

and nitrogen depth profiles of the specimens were analyzed by WDS, AES and XPS. The structure of the nitride compounds has been analyzed using a glancing angle X-ray diffraction ŽGAXRD. using Cu K ␣ X-ray ˚ . with an incident angle of 1⬚. In particular, Ž ␭ s 1.54 A ˚ . with an incident angle of 3⬚ Cr K ␣ X-ray Ž ␭ s 2.29 A analysis has been used to analyze a f.c.c.-Al substrate and a f.c.c.-AlN film because deflection angles of f.c.c.Al substrate are very close to those of f.c.c.-AlN in the range between 30 and 90⬚.

3. Results and discussions Using a low pressure nitriding equipment, a stable AlN layer has been formed on the Al surface. As shown in Fig. 1 there are distinct peaks of h.c.p.-AlN

Fig. 1. XRD patterns of AlN films formed by the plasma nitriding process ŽCuK ␣ X-ray patterns at ␪ s 1⬚..

Fig. 2. AES depth profiles of AlN prepared by the plasma nitriding process.

and substrate of Al. Apparent AlN peaks, which correspond to the h.c.p. structure of JCPDS file ŽNo. 251133., have been observed at the samples which were treated for 3, 6 and 8 h in the PN process. Small peak of Al phase was shown in the XRD spectrum from a sample processed for 3 h and single phase of h.c.p.-AlN has been observed with increasing a processing time up to 8 h. These results are in good agreement with other previous results w2x. AES depth profile of the sample, which was plasma nitrided using a N2 gas with total pressure of 400 Pa and for 8 h, is shown in Fig. 2. The concentration of nitrogen and oxygen atoms in the AlN layer is expected to be approximately 40 and 30 at.%, respectively. Atomic compositions of the AlN layer Žshown in Fig. 3a. were also determined by WDS as listed in Table 3. The nitrogen atom concentration was 32.2 at.% and the oxygen atom concentration was 13.2 at.%. This indicated that significant oxygen content still exists in the AlN layer even though the sputtering has been performed for 30 min before the nitriding process. This may be interpreted that the oxidation process has proceeded simultaneously with the process of the formation of AlN and oxygen atoms existing in a form of

Fig. 3. SEM photograph of an AlN layer prepared by the plasma nitriding process.

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strate with increasing processing time. This is due to the fact that the large difference of thermal expansion coefficient between the AlN layer and Al substrate arises with increasing processing time and the possibility of grain growth of the Al substrate at the processing temperature of 480⬚C. Thus, the processing time of nitriding for the formation of AlN should be carefully controlled and the recommended thickness of the AlN layer is 10᎐20 ␮m in the plasma nitriding process. In the plasma source ion implantation ŽPSII. process, the conformation of AlN phase is not as easy as the observation of AlN peaks from the samples prepared by PN process in XRD spectrum. Fig. 4 shows the GAXRD patterns of AlN films prepared by a PSII

Fig. 4. The X-ray patterns for AlN layers prepared by PSII process. Ža. XRD patterns of the implanted layers on Al substrate using a CuK ␣ X-ray Žglancing angle ␪ s 3⬚.. Žb. XRD patterns of the implanted layers on Al substrate using a CrK ␣ X-ray Žglancing angle ␪ s 3⬚..

solid solution, or an amorphous phase of oxide in AlN layer. This is explained by the fact that no peaks of Al oxide have been observed in a XRD as shown in Fig. 1. In our experiment a thick AlN layer of ; 20 ␮m can be easily formed using low pressure nitriding equipment with a unipolar pulsed d.c. power, while other research groups obtained a limited thickness of AlN less than 10 ␮m w1,2,4x. Fig. 3 shows the cross-sectional images of AlN layers using SEM and the thickness of the AlN layer has been almost saturated up to ; 20 ␮m with a processing time of 3 h. However, AlN films become waved, cracked and detached from the sub-

Fig. 5. AES depth profile of implanted layer with four different dose levels. Ža. Nitrogen concentration profiles depend on the implantation dose. Žb. Nitrogen concentration profiles depend on the implantation energy.

Y. Shim et al. r Surface and Coatings Technology 131 (2000) 345᎐349

349

shown in Fig. 5b. The nitrogen depth profile of the AlN layer formed by PSII has been investigated using XPS as shown in Fig. 6. Three chemical states of Al have been observed in the form of Al 2 O 3 Ž75.6 eV., AlN Ž74.5 eV. and aluminum Ž72.9 eV.. From the surface to bulk Al peaks shift from chemical state of Al 2 O 3 to that of AlN and finally to that of Al. This reveals that the formation AlN phase occurred under the Al 2 O 3 layer in the PSII process.

Fig. 6. The XPS spectra of Al 1050 after implantation of 7 = 10 17 ionsrcm2 at 60 kV.

process. Using a Cr K ␣ X-ray it is possible to distinguish the AlN phase from the Al substrate as shown in Fig. 4b while with a Cu K ␣ X-ray it is not possible to distinguish the peaks of Al substrate from AlN layer as shown in Fig. 4a. In Fig. 4b, the f.c.c.-AlN Ž2␾ s 58.8⬚. peak has been obviously observed with increasing implantation dose of nitrogen ion from 7 = 10 16 to 1.8= 10 18 ionsrcm2 . This is in good agreement with a previous experimental result of Guzman et al. w8x in which they presume that the peaks of f.c.c.-AlN and f.c.c.-Al were very close to each other in the XRD spectra using a Cu K ␣ X-ray w7x. In our analysis a small AlN peak has been observed from the sample with a dose level of 7 = 10 16 ionsrcm2 using a Cr K ␣ X-ray but it was not clear. However, at the dose level above 3.4= 10 17 ionsrcm2 a clear AlN peak distinctly appeared as shown in Fig. 4b. The diffraction peak of f.c.c.-AlN appeared at 58.8⬚ corresponding to the interplanar spacing of ˚ which is the JCPDS file ŽNo. 46-1200.. Fig. 5 2.33 A, shows the AES depth profiles of the samples treated by the PSII processes. As shown in Fig. 5a a saturated nitrogen concentration of the sample treated by PSII process was revealed at the dose level above 7 = 10 17 ionsrcm2 which corresponds to ; 60 at.%. From the AES analysis it is expected that the formation of f.c.c.AlN phase is favorable above ion implantation dose of 7 = 10 17 ionsrcm2 . In addition, the nitrogen atom concentration with a dose level of 3.4= 10 17 ionsrcm2 is of ; 30 at.% in the AlN layer and this implies that approximately 30 at.% of nitrogen atoms are needed for the formation of AlN layer in the plasma source ion implantation process. Also, the effect of ion implantation energy is not critical for the formation of AlN phase compared to the influence of dose rate in this experiment condition. Implantation energies between 20 and 60 keV are reasonably favorable for the formation of AlN layer at a dose of 7 = 10 17 ionsrcm2 as

4. Conclusions The stable hcp-AlN layer with a thickness of ; 20 ␮m has been successfully formed by the low pressure plasma nitriding process at 480⬚C using a unipolar d.c. plasma source. In the PSII process, at least 30 at.% of nitrogen atoms, which correspond to a dose level of 3.4= 10 17 ionsrcm2 , are required for the formation of the AlN layer on the Al surface. Metastable f.c.c.-AlN with a depth of ; 100 nm has been observed in the XRD spectrum from AlN layer prepared by a plasma source ion implantation process. In addition the peak separation of AlN and f.c.c.-Al phase has been clearly observed in the XRD spectrum using a Cr K ␣ X-ray, ˚ .. which has longer wavelength Ž ␭ s 2.29 A

Acknowledgements This work has been partly financially supported by the Ministry of Commerce Industry and Energy. The authors would like to thank Miss J.H. Lee who gave gratefully helped us in the analysis of the samples. References w1x T. Arai et al., Proc. Int. Conf. Ion Nitriding 15᎐17 Ž1986. 37. w2x H.-Y. Chen, H.-R. Stock, P. Mayr, Surf. Coat. Technol. 64 Ž1994. 139. w3x E.I. Meletis, S. Yan, J. Vac. Sci. Technol. A 9 Ž1991. 2279. w4x H.R. Stock et al., Surf. Coat. Technol. 94᎐95 Ž1997. 247. w5x N. Renevier et al. Proceedings of the 6th International Conference on Plasma Surface Engineering Ž1996. 117. w6x A.P. Mathews et al., Nucl. Instrum. Math. B 59᎐60 Ž1991. 671. w7x J.R. Conrad et al., J. Appl. Phys. 62 Ž1987. 4591. w8x L. Guzman et al., Surf. Coat. Technol. 83 Ž1996. 284. w9x B. Reinhold and H.-J. Spies, Proceedings of the 1st International Automotive Heat Treating Conference, Puerto Vallarta, Mexico Ž1998. 213. w10x J.H. Booske et al., J. Mater. Res. 12 Ž1997. 1356. w11x M. Setoyama et al., Surf. Coat. Technol. 86᎐87 Ž1996. 225. w12x N.E. Christensen et al., Phys. Rev. B 47 Ž8. Ž1993. 4303.