- Email: [email protected]
Thermal annealing, irradiation, and stress in multilayers
Nuclear Instruments and Methods in Physics Research B 148 (1999) 227±231
Thermal annealing, irradiation, and stress in multilayers S. Fayeulle 1, A. Misra, H. Kung, M. Nastasi
*
Material Science and Technology Division, Los Alamos National Laboratory, Los Alamos, NM 87545, USA
Abstract The evolution of the microstructure of a TiN/B±C±N multilayered thin ®lm during thermal annealing and irradiation has been studied by low angle X-ray diraction and transmission electron microscopy. Stress has been determined by curvature measurements. After deposition, TiN is crystalline while B±C±N is amorphous. Thermal anneals in vacuum at 600±1000°C lead to an increase of the bilayer repeat length and to a phase separation at the interfaces. After the 600°C annealing, ion irradiation (Ar ions, 300 keV 1 ´ 1015 ions/cm2 ) causes an additional increase of the bilayer repeat length but a decrease of the quality of the interface. After annealing at 800°C or 1000°C, the irradiation causes a major decrease of the bilayer repeat length (more than 20% after annealing at 1000°C and irradiation). The stress is highly compressive after deposition (r )2000 MPa). After the 600°C annealing, the stress is totally relaxed (r 0) and becomes tensile after annealing at higher temperatures (r +1200 MPa after 800°C annealing, r +1500 MPa after 1000°C annealing). TEM con®rms the decrease of the bilayer repeat length after an irradiation of the samples subjected to high temperature anneal and reveal an increase of the roughness of the interfaces. These phenomena are discussed in terms of stress driven diusion during irradiation. Ó 1999 Elsevier Science B.V. All rights reserved. PACS: 61.80.Jh; 61.82.Rx; 81.15.Jj Keywords: Thermal annealing; Irradiation; Stress; Multilayers
1. Introduction The mechanical and electrical performances of multilayered structures made of alternating layers of dierent materials depend highly on their structural stability. The stability is related to the phases composing each bilayer, their nature and *
Corresponding author. Tel.: 001 505 667 7007; fax: 001 505 665 2992; e-mail: [email protected] 1 On leave from Laboratoire Materiaux-Mecanique Physique, UMR CNRS 5621 Ecole Centrale de Lyon, 69131 Ecully, France.
their composition, to the characteristics of the interfaces, and also to mechanical properties such as the state of stress existing in the ®lms. Ion irradiation and heat treatments of the multilayers are a very convenient way to study the stability as well as to examine their in¯uence on modi®cation or alteration of the material. Both thermal annealing [1±8] and ion irradiation [9±17] are capable of activating, among numerous other phenomena, phase formation and transformations, interdiusion or intermixing, structural relaxation or crystallization of amorphous layers, stress alteration, and interface roughening.
0168-583X/98/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 8 ) 0 0 7 7 4 - 5
228
S. Fayeulle et al. / Nucl. Instr. and Meth. in Phys. Res. B 148 (1999) 227±231
In this paper, we report the results of a study on the thermal and irradiation response of a ceramic/ ceramic multilayer. The system TiN/BCN has been chosen because of its technological potential as a hard coating. Low angle X-ray diraction and transmission electron microscopy (TEM) has been used to study microstructural changes and interdiusion between layers. 2. Experimental The thin ®lms were prepared using dc magnetron sputter deposition. A mixture of 25% argon and 75% nitrogen at a pressure of 3 mTorr was used to sputter the target materials of Ti and B4 C. The deposition rates were about 0.1 nm/s for TiN and about 0.2 nm/s for B±C±N. The samples were not biased and the depositions were done at a temperature lower than 100°C. The multilayers were made of 40 bilayers, each being 5 nm thick (2.5 nm TiN, 2.5 nm B±C±N) deposited on (1 0 0) silicon substrates. After deposition, the samples were annealed in vacuum, 5 ´ 10ÿ8 Torr, at temperatures upto 1000°C and irradiated with argon ions at an energy of 300 keV and a dose of 1 ´ 1015 ions/cm2 . Both range and straggling were calculated with the TRIM program and assuming the average density calculated by TRIM for the various compounds [18]. The value (range and straggling) was equal to 400 nm, larger than the thickness of the multilayer. The structure of the multilayers was studied using low angle X-ray diraction and TEM. The changes in the X-ray intensity of the ®rst order Bragg peaks of the layered structure are related to the interdiusivity. This technique has been widely used in the determination of small interdiusion coecients with high precision in various systems [19]. After deposition and after anneals, stress in the coatings was calculated from sample curvature measurements by using the Stoney equation [20,12].
bilayer repeat length was found equal to 5.07 nm with an equivalent roughness at the interfaces equal to 0.65 nm (i.e., intermixing at the interfaces at the level of two to three atomic layers). Curvature measurements showed that the as-deposited stress was highly compressive, rad )2000 MPa. Sample curvature measurements as a function of annealing are presented in Fig. 1. These data show that stress relaxation is obtained for all anneals performed at temperatures greater than 200°C. Complete stress relaxation (r 0) is obtained after annealing for 1 h at 600°C. Anneals performed at higher temperatures produced tensile stresses, with a maximum stress of r +1500 MPa obtained at 1000°C. The eects of vacuum annealing and Ar ion irradiation on low angle X-ray diraction (evolution of the ®rst order Bragg peak) are presented in Fig. 2±4 (annealing at 600°C, 800°C, and 1000°C, respectively). In Fig. 2±4 peak 1 is from the asdeposited sample, peak 2 is obtained after vacuum annealing, and peak 3 is for the condition of thermal annealing followed by Ar ion irradiation. A general trend observed for all three annealing temperatures presented is that the ®rst order Bragg peak is always shifted to smaller angle and its intensity is always increased. The shift to lower angle is indicative of an increase in the bilayer length, while the increase in the intensity is characteristic of a sharpening of the interface, i.e. that phase separation occurred at the interfaces. The values of the bilayer repeat length and of the equivalent roughness after annealing are given in Table 1.
3. Results After deposition, TEM revealed that TiN was crystalline and B±C±N amorphous. Using low angle X-ray diraction [19] and simulation [21], the
Fig. 1. Change in residual stress after a 1 h thermal anneal in vacuum.
S. Fayeulle et al. / Nucl. Instr. and Meth. in Phys. Res. B 148 (1999) 227±231
229
From the data in Table 1, it is seen that the bilayer repeat length has a maximum expansion, 8.3%, after annealing at 600o C. Higher temperature annealing continued to produce an expansion in the bilayer repeat length. However, after the 1000o C annealing, the expansion was only 5%. The surface roughness decreases with increasing annealing temperature, reaching an optimum value after annealing at 800°C and 1000°C. The X-ray data from samples exposed to 1 ´ 1015 Ar/cm2 at 300 keV, after ®rst receiving a 1 h vacuum anneal, showed variable behavior with a strong dependence on stress. Irradiation of the 600°C annealed sample, Fig. 2, where the residual stress after annealing is equal to zero, resulted in a further increase of the bilayer repeat length to 5.65 nm (i.e. an increase of 11.4% compared to the asdeposited state). A decrease in the Bragg peak intensity is also observed which corresponds to an increased interface roughness of 0.7 nm. For the samples pre-annealed at 800°C and 1000°C, the
state of residual stress in the multilayer samples is tensile. Irradiation of the 800°C annealed multilayer (Fig. 3) caused a drastic decrease of the bilayer repeat length to 4.45 nm (a decrease of 18% compared to the annealed state). The intensity of the peak was also strongly decreased, corresponding to an interface roughness of 0.8 nm. A similar behavior, but more pronounced, was observed after irradiating the 1000o C sample, Fig. 4. The bilayer repeat length is decreased by 22%) and the X-ray data also indicates that an addition weaker peak has developed at about 2h 1.95°. The presence of these two peaks could correspond to a continuous variation of the bilayer repeat length from 5 to 4.18 nm. Cross-sectional TEM micrographs and diraction patterns taken from as-deposited multilayers, and a multilayer sample annealed at 800°C, followed by 1 ´ 1015 Ar/cm2 at 300 keV, are presented in Figs. 5 (a) and (b) respectively. The micrograph in Fig. 5(a) shows the sharp, smooth interfaces, between the crystalline TiN and amorphous BCN layers. The inset diraction pattern shows the presence of three superlattice diraction spots on either side of the fundamental spot. The ring pattern from the nano-scale crystalline TiN is not shown in Fig. 5(a). The number and presence of the superlattice diraction spots is indicative of the sharp interface between TiN and BCN layers. Examining Fig. 5(b), the eect of annealing and irradiation is apparent in both the micrograph and the diraction pattern. The image shows the
Fig. 2. Low angle X-ray diraction of TiN/BCN multilayers. Peak 1, as deposited sample; peak 2, after a 1 h anneal at 600°C, and peak 3, after annealing at 600°C plus an Ar irradiation of 1 ´ 1015 ions/cm2 at 300 keV.
Fig. 3. Low angle X-ray diraction of TiN/BCN multilayers. Peak 1, as deposited sample; peak 2, after a 1 h anneal at 800°C, and peak 3, after annealing at 800°C plus an Ar irradiation of 1 ´ 1015 ions/cm2 at 300 keV.
Table 1 Bilayer repeat length and equivalent roughness of the interfaces after annealing 1 h in vacuum Annealing temperature (°C)
Bilayer repeat length (nm)
Roughness (nm)
As deposited 600 800 1000
5.07 5.49 5.45 5.34
0.65 0.5 0 0
230
S. Fayeulle et al. / Nucl. Instr. and Meth. in Phys. Res. B 148 (1999) 227±231
development of a wavy interface and the number of visible superlattice diraction spots has been reduced to 1. It should be noted that Fig. 5 is the TEM analog to peaks 1 and 3 of the X-ray data presented in Fig. 4. 4. Discussion
Fig. 4. Low angle X-ray diraction of TiN/BCN multilayers. Peak 1, as deposited sample; peak 2, after a 1 h anneal at 1000°C, and peak 3, after annealing at 1000°C plus an Ar irradiation of 1 ´ 1015 ions/cm2 at 300 keV.
Fig. 5. TEM cross-sectional microstructure from TiN(crystalline)/BCN(amorphous) multilayers. (a) as-deposited, (b) annealed 1 h at 800°C and Ar ion irradiated. Insets show the superlattice diraction spots.
The preceding thermal annealing data has shown that the as-deposited TiN/BCN multilayers do not react when heated to temperatures as high as 1000°C (i.e., they are immiscible) and that the sputtering synthesis technique used in making these multilayers produces a level of interface intermixing that can be reversed by heating. These two observations are consistent with previous thermodynamic and diusion analysis that estimated a positive heat of reaction between TiN and BCN, predicted that interdiusion is not expected, and that negative diusion or phase separation should occur at atomically mixed interfaces [10]. The latter point was proved in a recent set of experiments which examined the diusion response of samples ion mixed prior to thermal annealing [11]. The thermal annealing data also showed that the relaxation of compressive stresses is correlated with the removal of interface roughness and a decreasing bilayer repeat length. The ion irradiation data show for the ®rst time that residual tensile stresses have a similar in¯uence on interface mixing and/or surface roughening as has been previously demonstrated for TiN/ BCN multilayers under compressive stresses [22,23]. This is evident when comparing X-ray peaks 2 (thermal annealed) and 3 (thermal annealed plus Ar ion irradiation) in samples with tensile stresses after annealing, Figs. 3 and 4. The X-ray data show that ion irradiating tensile stressed multilayer results in a very large decrease in peak intensity. A major dierence between the irradiation response of multilayers under compressive vs. tensile stress is in the sign of the change in the bilayer repeat length; samples which are initially under compressive stress show an expansion (X-ray peak shifts to lower values of 2h) [10,11,22,23] while samples under tensile stress show a contraction (X-ray peak shifts to larger
S. Fayeulle et al. / Nucl. Instr. and Meth. in Phys. Res. B 148 (1999) 227±231
values of 2h). These trends are consistent with the irradiation stimulated atomic movement of material perpendicular to the plane, in response to the in plane force being applied by the residual stress. 5. Conclusions We have used low angle X-ray diraction and TEM to show the evolution of the microstructure of TiN/B±C±N multilayers during thermal annealing and ion irradiation. Thermal annealing reduces the residual compressive stresses present in the as-deposited structure, and for high annealing temperatures it leaves the multilayers in a state of compressive stress. Annealing also reduces interface roughness and/or promotes phase separation between TiN and B±C±N layers. Ion irradiations performed after thermal annealing increase the amount of interface roughness and/or intermixing, the magnitude of which increases with increasing pre-irradiation stress. In addition, Ar irradiation stimulates a change in the bilayer repeat length; samples under tensile stress show a contraction in length, while previous studies have shown that samples which are initially under compressive stress show an expansion in length. The changes in bilayer repeat length are attributed to irradiation stimulated atomic movement of material, in response to the in plane force being applied by the residual stress.
[2]
[3]
[4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]
Acknowledgements This work was supported by the DOE Oce of Basic Energy Research, Division of Materials Science. References [1] P.D. Persans, A.F. Ruppert, B. Abeles, Y.J. Wu, V. Pantojas, K.S. Liang, G. Hughes, in: B.W. Dodson, L.J.
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231
Schowalter, J.E. Cunningham, F.H. Pollak (eds.), Layered Structures, Heteroepitaxy, Superlattices, Strain, and Metastability, vol. 160, Material Research Society, Pittsburgh, 1990, p. 547. A.F. Jankowski, in: B.W. Dodson, L.J. Schowalter, J.E. Cunningham, F.H. Pollak (eds.), Layered Structures, Heteroepitaxy, Superlattices, Strain, and Metastability, vol. 160, Material Research Society, Pittsburgh, 1990, p. 611. B. Rodmacq, J. Hillairet, J. Laugier, A. Chamberod, in: B.W. Dodson, L.J. Schowalter, J.E. Cunningham, F.H. Pollak (eds.), Layered Structures, Heteroepitaxy, Superlattices, Strain, and Metastability, vol. 160, Material Research Society, Pittsburgh, 1990, p. 589. V. Pelosin, B. Rodmacq, S.R. Teixeira, J. Hillairet, G. Carlotti, in: Stresses and Mechanical Properties IV, vol. 308, Material Research Society, Pittsburgh, 1993, p. 701. C.A. Ross, D.T. Wu, L.M. Goldman, F. Spaepen, J. Appl. Phys. 72 (1992) 2773. B. Park, PhD. thesis, Harvard University, Cambridge, MA, 1989. C. Janot, A. Bruson, G. Marchal, J. Physique 47 (1986) 1751. S.M. Prokes, F. Spaepen, Appl. Phys. Let. 47 (1985) 234. M. Nastasi, J. W. Mayer, J. K. Hirvonen, Ion-Solid Interactions: Fundamentals and Applications, Cambridge University Press, Cambridge, 1996. M. Nastasi, S. Fayeullea, Y.-C.Lu, H. Kung, Mat Sci and Engr., in press. S. Fayeulle, M. Nastasi, Appl. Phys. Lett. 73 (1998) 1077. C.A. Volkert, J. Appl. Phys. 70 (1991) 3521. E. Snoeks, T. Weber, A. Cacciato, A. Polman, J. Appl. Phys. 78 (1995) 4723. R. Nowak, Y. Horino, Y. Ando, S. Maruno, Appl. Phys. Lett. 68 (1996) 3743. E. Snoeks, K.S. Boutros, J. Barone, Appl. Phys. Lett. 71 (1997) 267. A. Jain, S. Loganathan, U. Jain, Nucl. Instr. and Meth. B 127/128 (1997) 43. A. Misra, S. Fayeulle, H. Kung, T.E. Mitchell, M. Nastasi, Appl. Phys. Lett. 73 (1998) 891. J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Range of Ions in Solids, Pergamon Press Inc., New York, 1985. A.L. Greer, F. Spaepen, in: L.L. Chang, B.C. Giessen (eds.), Synthetic Modulated Structures, Academic Press, New York, 1985, p. 419. G.G. Stoney, Proc. Roy. Soc. A 82 (1909) 172. J.H. Underwood, T.W. Barbee, Appl. Opt. 20 (1981) 3027. S. Fayeulle, P. Torri, M. Nastasi, Y.-C. Lu, H. Kung, J.R. Tesmer, Nucl. Instr. Meth. B 127/128 (1997) 198. S. Fayeulle, M. Nastasi, Y.-C. Lu, H. Kung, J.R. Tesmer, Appl. Phys. Lett. 70 (1997) 1098.
Thermal annealing, irradiation, and stress in multilayers S. Fayeulle 1, A. Misra, H. Kung, M. Nastasi
*
Material Science and Technology Division, Los Alamos National Laboratory, Los Alamos, NM 87545, USA
Abstract The evolution of the microstructure of a TiN/B±C±N multilayered thin ®lm during thermal annealing and irradiation has been studied by low angle X-ray diraction and transmission electron microscopy. Stress has been determined by curvature measurements. After deposition, TiN is crystalline while B±C±N is amorphous. Thermal anneals in vacuum at 600±1000°C lead to an increase of the bilayer repeat length and to a phase separation at the interfaces. After the 600°C annealing, ion irradiation (Ar ions, 300 keV 1 ´ 1015 ions/cm2 ) causes an additional increase of the bilayer repeat length but a decrease of the quality of the interface. After annealing at 800°C or 1000°C, the irradiation causes a major decrease of the bilayer repeat length (more than 20% after annealing at 1000°C and irradiation). The stress is highly compressive after deposition (r )2000 MPa). After the 600°C annealing, the stress is totally relaxed (r 0) and becomes tensile after annealing at higher temperatures (r +1200 MPa after 800°C annealing, r +1500 MPa after 1000°C annealing). TEM con®rms the decrease of the bilayer repeat length after an irradiation of the samples subjected to high temperature anneal and reveal an increase of the roughness of the interfaces. These phenomena are discussed in terms of stress driven diusion during irradiation. Ó 1999 Elsevier Science B.V. All rights reserved. PACS: 61.80.Jh; 61.82.Rx; 81.15.Jj Keywords: Thermal annealing; Irradiation; Stress; Multilayers
1. Introduction The mechanical and electrical performances of multilayered structures made of alternating layers of dierent materials depend highly on their structural stability. The stability is related to the phases composing each bilayer, their nature and *
Corresponding author. Tel.: 001 505 667 7007; fax: 001 505 665 2992; e-mail: [email protected] 1 On leave from Laboratoire Materiaux-Mecanique Physique, UMR CNRS 5621 Ecole Centrale de Lyon, 69131 Ecully, France.
their composition, to the characteristics of the interfaces, and also to mechanical properties such as the state of stress existing in the ®lms. Ion irradiation and heat treatments of the multilayers are a very convenient way to study the stability as well as to examine their in¯uence on modi®cation or alteration of the material. Both thermal annealing [1±8] and ion irradiation [9±17] are capable of activating, among numerous other phenomena, phase formation and transformations, interdiusion or intermixing, structural relaxation or crystallization of amorphous layers, stress alteration, and interface roughening.
0168-583X/98/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 8 ) 0 0 7 7 4 - 5
228
S. Fayeulle et al. / Nucl. Instr. and Meth. in Phys. Res. B 148 (1999) 227±231
In this paper, we report the results of a study on the thermal and irradiation response of a ceramic/ ceramic multilayer. The system TiN/BCN has been chosen because of its technological potential as a hard coating. Low angle X-ray diraction and transmission electron microscopy (TEM) has been used to study microstructural changes and interdiusion between layers. 2. Experimental The thin ®lms were prepared using dc magnetron sputter deposition. A mixture of 25% argon and 75% nitrogen at a pressure of 3 mTorr was used to sputter the target materials of Ti and B4 C. The deposition rates were about 0.1 nm/s for TiN and about 0.2 nm/s for B±C±N. The samples were not biased and the depositions were done at a temperature lower than 100°C. The multilayers were made of 40 bilayers, each being 5 nm thick (2.5 nm TiN, 2.5 nm B±C±N) deposited on (1 0 0) silicon substrates. After deposition, the samples were annealed in vacuum, 5 ´ 10ÿ8 Torr, at temperatures upto 1000°C and irradiated with argon ions at an energy of 300 keV and a dose of 1 ´ 1015 ions/cm2 . Both range and straggling were calculated with the TRIM program and assuming the average density calculated by TRIM for the various compounds [18]. The value (range and straggling) was equal to 400 nm, larger than the thickness of the multilayer. The structure of the multilayers was studied using low angle X-ray diraction and TEM. The changes in the X-ray intensity of the ®rst order Bragg peaks of the layered structure are related to the interdiusivity. This technique has been widely used in the determination of small interdiusion coecients with high precision in various systems [19]. After deposition and after anneals, stress in the coatings was calculated from sample curvature measurements by using the Stoney equation [20,12].
bilayer repeat length was found equal to 5.07 nm with an equivalent roughness at the interfaces equal to 0.65 nm (i.e., intermixing at the interfaces at the level of two to three atomic layers). Curvature measurements showed that the as-deposited stress was highly compressive, rad )2000 MPa. Sample curvature measurements as a function of annealing are presented in Fig. 1. These data show that stress relaxation is obtained for all anneals performed at temperatures greater than 200°C. Complete stress relaxation (r 0) is obtained after annealing for 1 h at 600°C. Anneals performed at higher temperatures produced tensile stresses, with a maximum stress of r +1500 MPa obtained at 1000°C. The eects of vacuum annealing and Ar ion irradiation on low angle X-ray diraction (evolution of the ®rst order Bragg peak) are presented in Fig. 2±4 (annealing at 600°C, 800°C, and 1000°C, respectively). In Fig. 2±4 peak 1 is from the asdeposited sample, peak 2 is obtained after vacuum annealing, and peak 3 is for the condition of thermal annealing followed by Ar ion irradiation. A general trend observed for all three annealing temperatures presented is that the ®rst order Bragg peak is always shifted to smaller angle and its intensity is always increased. The shift to lower angle is indicative of an increase in the bilayer length, while the increase in the intensity is characteristic of a sharpening of the interface, i.e. that phase separation occurred at the interfaces. The values of the bilayer repeat length and of the equivalent roughness after annealing are given in Table 1.
3. Results After deposition, TEM revealed that TiN was crystalline and B±C±N amorphous. Using low angle X-ray diraction [19] and simulation [21], the
Fig. 1. Change in residual stress after a 1 h thermal anneal in vacuum.
S. Fayeulle et al. / Nucl. Instr. and Meth. in Phys. Res. B 148 (1999) 227±231
229
From the data in Table 1, it is seen that the bilayer repeat length has a maximum expansion, 8.3%, after annealing at 600o C. Higher temperature annealing continued to produce an expansion in the bilayer repeat length. However, after the 1000o C annealing, the expansion was only 5%. The surface roughness decreases with increasing annealing temperature, reaching an optimum value after annealing at 800°C and 1000°C. The X-ray data from samples exposed to 1 ´ 1015 Ar/cm2 at 300 keV, after ®rst receiving a 1 h vacuum anneal, showed variable behavior with a strong dependence on stress. Irradiation of the 600°C annealed sample, Fig. 2, where the residual stress after annealing is equal to zero, resulted in a further increase of the bilayer repeat length to 5.65 nm (i.e. an increase of 11.4% compared to the asdeposited state). A decrease in the Bragg peak intensity is also observed which corresponds to an increased interface roughness of 0.7 nm. For the samples pre-annealed at 800°C and 1000°C, the
state of residual stress in the multilayer samples is tensile. Irradiation of the 800°C annealed multilayer (Fig. 3) caused a drastic decrease of the bilayer repeat length to 4.45 nm (a decrease of 18% compared to the annealed state). The intensity of the peak was also strongly decreased, corresponding to an interface roughness of 0.8 nm. A similar behavior, but more pronounced, was observed after irradiating the 1000o C sample, Fig. 4. The bilayer repeat length is decreased by 22%) and the X-ray data also indicates that an addition weaker peak has developed at about 2h 1.95°. The presence of these two peaks could correspond to a continuous variation of the bilayer repeat length from 5 to 4.18 nm. Cross-sectional TEM micrographs and diraction patterns taken from as-deposited multilayers, and a multilayer sample annealed at 800°C, followed by 1 ´ 1015 Ar/cm2 at 300 keV, are presented in Figs. 5 (a) and (b) respectively. The micrograph in Fig. 5(a) shows the sharp, smooth interfaces, between the crystalline TiN and amorphous BCN layers. The inset diraction pattern shows the presence of three superlattice diraction spots on either side of the fundamental spot. The ring pattern from the nano-scale crystalline TiN is not shown in Fig. 5(a). The number and presence of the superlattice diraction spots is indicative of the sharp interface between TiN and BCN layers. Examining Fig. 5(b), the eect of annealing and irradiation is apparent in both the micrograph and the diraction pattern. The image shows the
Fig. 2. Low angle X-ray diraction of TiN/BCN multilayers. Peak 1, as deposited sample; peak 2, after a 1 h anneal at 600°C, and peak 3, after annealing at 600°C plus an Ar irradiation of 1 ´ 1015 ions/cm2 at 300 keV.
Fig. 3. Low angle X-ray diraction of TiN/BCN multilayers. Peak 1, as deposited sample; peak 2, after a 1 h anneal at 800°C, and peak 3, after annealing at 800°C plus an Ar irradiation of 1 ´ 1015 ions/cm2 at 300 keV.
Table 1 Bilayer repeat length and equivalent roughness of the interfaces after annealing 1 h in vacuum Annealing temperature (°C)
Bilayer repeat length (nm)
Roughness (nm)
As deposited 600 800 1000
5.07 5.49 5.45 5.34
0.65 0.5 0 0
230
S. Fayeulle et al. / Nucl. Instr. and Meth. in Phys. Res. B 148 (1999) 227±231
development of a wavy interface and the number of visible superlattice diraction spots has been reduced to 1. It should be noted that Fig. 5 is the TEM analog to peaks 1 and 3 of the X-ray data presented in Fig. 4. 4. Discussion
Fig. 4. Low angle X-ray diraction of TiN/BCN multilayers. Peak 1, as deposited sample; peak 2, after a 1 h anneal at 1000°C, and peak 3, after annealing at 1000°C plus an Ar irradiation of 1 ´ 1015 ions/cm2 at 300 keV.
Fig. 5. TEM cross-sectional microstructure from TiN(crystalline)/BCN(amorphous) multilayers. (a) as-deposited, (b) annealed 1 h at 800°C and Ar ion irradiated. Insets show the superlattice diraction spots.
The preceding thermal annealing data has shown that the as-deposited TiN/BCN multilayers do not react when heated to temperatures as high as 1000°C (i.e., they are immiscible) and that the sputtering synthesis technique used in making these multilayers produces a level of interface intermixing that can be reversed by heating. These two observations are consistent with previous thermodynamic and diusion analysis that estimated a positive heat of reaction between TiN and BCN, predicted that interdiusion is not expected, and that negative diusion or phase separation should occur at atomically mixed interfaces [10]. The latter point was proved in a recent set of experiments which examined the diusion response of samples ion mixed prior to thermal annealing [11]. The thermal annealing data also showed that the relaxation of compressive stresses is correlated with the removal of interface roughness and a decreasing bilayer repeat length. The ion irradiation data show for the ®rst time that residual tensile stresses have a similar in¯uence on interface mixing and/or surface roughening as has been previously demonstrated for TiN/ BCN multilayers under compressive stresses [22,23]. This is evident when comparing X-ray peaks 2 (thermal annealed) and 3 (thermal annealed plus Ar ion irradiation) in samples with tensile stresses after annealing, Figs. 3 and 4. The X-ray data show that ion irradiating tensile stressed multilayer results in a very large decrease in peak intensity. A major dierence between the irradiation response of multilayers under compressive vs. tensile stress is in the sign of the change in the bilayer repeat length; samples which are initially under compressive stress show an expansion (X-ray peak shifts to lower values of 2h) [10,11,22,23] while samples under tensile stress show a contraction (X-ray peak shifts to larger
S. Fayeulle et al. / Nucl. Instr. and Meth. in Phys. Res. B 148 (1999) 227±231
values of 2h). These trends are consistent with the irradiation stimulated atomic movement of material perpendicular to the plane, in response to the in plane force being applied by the residual stress. 5. Conclusions We have used low angle X-ray diraction and TEM to show the evolution of the microstructure of TiN/B±C±N multilayers during thermal annealing and ion irradiation. Thermal annealing reduces the residual compressive stresses present in the as-deposited structure, and for high annealing temperatures it leaves the multilayers in a state of compressive stress. Annealing also reduces interface roughness and/or promotes phase separation between TiN and B±C±N layers. Ion irradiations performed after thermal annealing increase the amount of interface roughness and/or intermixing, the magnitude of which increases with increasing pre-irradiation stress. In addition, Ar irradiation stimulates a change in the bilayer repeat length; samples under tensile stress show a contraction in length, while previous studies have shown that samples which are initially under compressive stress show an expansion in length. The changes in bilayer repeat length are attributed to irradiation stimulated atomic movement of material, in response to the in plane force being applied by the residual stress.
[2]
[3]
[4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]
Acknowledgements This work was supported by the DOE Oce of Basic Energy Research, Division of Materials Science. References [1] P.D. Persans, A.F. Ruppert, B. Abeles, Y.J. Wu, V. Pantojas, K.S. Liang, G. Hughes, in: B.W. Dodson, L.J.
[18] [19] [20] [21] [22] [23]
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