- Email: [email protected]
Influence of metal electrodes on crystal orientation of aluminum nitride thin films
ARTICLE IN PRESS
Vacuum 74 (2004) 699–703
Influence of metal electrodes on crystal orientation of aluminum nitride thin films Morito Akiyamaa,*, Keigo Nagaob, Naohiro Uenoa, Hiroshi Tateyamaa, Tetsuo Yamadab a
Institute for Structural and Engineering Materials, National Institute of Advanced Industrial Science and Technology, 807-1 Shuku, Tosu, Saga 841-0052, Japan b Ube Research Laboratory, Corporate Research & Development, Ube Industries Ltd., 1978-5 Kogushi, Ube, Yamaguchi 755-8633, Japan
Abstract We have investigated the influence of various bottom metal electrodes on the crystallinity and crystal orientation of aluminum nitride (AlN) thin films prepared on them in order to develop thin film bulk acoustic wave resonators. AlN films were prepared on 15 kinds of bottom metal electrode (Ag, Al, Au/Ti, Co, Cr, Cu, Fe, Mo, Nb, Ni, Pt/Ti, Ti, W, Zn, Zr) using rf magnetron reactive sputtering method. The crystallinity and crystal orientation of the AlN films strongly depend on the bottom metal electrodes. The AlN films prepared on the metal electrodes with the face centered cubic lattice structure show high c-axis orientation, except Ni. The AlN films prepared on Au/Ti and Pt/Ti show the highest crystallinity and orientation among them. The high crystallinity and orientation are due to the fact that the crystallinity of the Au/Ti and Pt/Ti electrodes is high, the surface roughness of the Au/Ti and Pt/Ti is low, and Au and Pt (1 1 1) planes match well with hexagonal AlN crystal structure. These results suggest that the crystallinity and orientation of AlN films are strongly influenced by bottom metal electrodes. r 2004 Elsevier Ltd. All rights reserved. Keywords: Aluminum nitride; Thin films; FBAR; Orientation; Crystallinity; Metal electrode; Sputtering
1. Introduction Quickly developed mobile telecommunication needs film bulk acoustic resonators (FBAR) for higher carrier frequencies [1]. A schematic view of FBAR is shown in Fig. 1 [1]. The resonance frequency of FBAR is mainly determined by the thickness of a piezoelectric layer and electrodes. The piezoelectric layer of 2.5 down to 0.5 mm would cover the very high frequency range from 2 to 10 GHz. Zinc oxide (ZnO) was taken as the *Corresponding author. E-mail address: [email protected] (M. Akiyama).
piezoelectric material due to its high effective coupling coefficient of 0.30 in the early 1980s [1]. However, ZnO has some drawbacks, such as low electrical resistance, low breakdown voltage and high dielectric losses, because ZnO is usually a ntype semiconductor. Aluminum nitride (AlN) would be a good piezoelectric material. With an achievable effective coupling coefficient of 0.25 [1], filter with a sufficiently large bandwidth could be produced for mobile telecommunication. Since the band gap of AlN is large (Eg=6.2 eV) [2], a high electrical resistance (1011–1013 O cm) [3], a high breakdown voltage (640 MV/m) [4] and low dielectric loss can
0042-207X/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2004.01.052
ARTICLE IN PRESS M. Akiyama et al. / Vacuum 74 (2004) 699–703
700
Table 1 Sputtering conditions
Fig. 1. Schematic view of FBAR.
Sputtering pressure Atmosphere Substrate temperature Sputtering rate Sputtering time rf power rf frequency Target substrate spacing
0.5 Pa Ar:N2=1:1 300 C 5.2 nm/min 4h 320 W 13.56 MHz >8.3 cm
be expected. Furthermore, AlN could achieve small thermal drifts, because the thermal expansion coefficient of AlN is low (c-axis parallel: 5.3 106/K, a-axis parallel: 4.2 106/K) [5]. AlN films can be expected to be competitive in sensor [6–8] and ultrasound applications [9] where low loss, low thermal drift and high signal-to-noise rations are demanded. For thin-film resonators, a polycrystalline c-axis oriented piezoelectric film is sufficient but it has to be grown on a metal electrode [1]. There may be no report on comparison with the influence of various bottom metal electrodes on the crystallinity and crystal orientation of AlN films prepared on them. In this paper we focus on the crystallinity and orientation of AlN films prepared on various bottom metal electrodes, because the crystallinity and orientation are important factors for the piezoelectric property of AlN films [10]. With AlN films an electromechanical coupling coefficient of 0.25 and low losses can be achieved, if strongly c-axis oriented AlN films are grown. AlN is, therefore, an excellent material for FBARs. We prepared AlN films on fifteen metal electrodes in order to find the most suitable bottom metal electrode for a FBAR. Because there is no report on the relationship between the crystal orientation of AlN films and bottom metal electrodes. In this paper we report on research into the most suitable bottom metal electrode and consider the nature of the crystal orientation mechanism of AlN films.
sputtering condition for preparing c-axis oriented AlN films is as follows [11]. Table 1 shows the detailed sputtering conditions. The target was a 76.2 mm-diam 99.999% pure aluminum. The film thickness of AlN films was about 1 mm. Substrates were silicon (1 0 0) wafers with oxide layer (thickness:1 mm). Fifteen Metal electrodes (Ag, Al, Au/Ti, Co, Cr, Cu, Fe, Mo, Nb, Ni, Pt/Ti, Ti, W, Zn, Zr) were prepared by dc sputtering using only Ar gas at room temperature. The thickness of the electrodes was about 300 nm. The titanium films deposited under gold and platinum were applied as adhesion layer, and the thickness was about 30 nm. Pressure in the vacuum chamber was maintained below 4 104 Pa before sputtering deposition, and then high-purity argon (99.999% purity) and nitrogen (99.999% purity) gases were introduced. Before the deposition process, the target was cleaned under the deposition conditions for 5 min. The crystal structure and crystallinity of AlN films were confirmed by X-ray diffraction (XRD; M03X-HF, Mac Science, Japan) using CuKa radiation. The crystal orientation of AlN films was evaluated by the full-width at half-maximum (FWHM) of the X-ray rocking curves of the (0 0 0 2) peaks of AlN films. The surface structures of AlN films were observed by scanning electron microscopy (SEM; S-4300, Hitachi, Japan). The surface morphology of metal electrodes was observed by atomic force microscope (AFM; Nanoscope IIIa).
2. Experimental
3. Results and discussion
AlN films were prepared in the rf magnetron sputtering system (CFS-4ES, Tokuda, Japan). The
The XRD patterns of the 15 metal electrodes were measured before preparing AlN films. The
ARTICLE IN PRESS M. Akiyama et al. / Vacuum 74 (2004) 699–703
XRD patterns show that Al, Ag, Au, Cu, Ni and Pt electrodes oriented to (1 1 1), Mo, Cr, Nb, W and Fe electrodes oriented to (1 1 0) , Ti, Co, Zn and Zr electrodes oriented to (0 0 2). All the metal electrodes showed highly crystal orientation. AlN films were prepared on the metal electrodes under the same sputtering conditions, and the XRD patterns of the AlN films were measured. The (0 0 0 2) peak intensity of the AlN films was drastically changed with the kinds of the metal electrodes, but all the AlN thin films indicated caxis orientation. These results suggest that the crystal orientation of the AlN films does not depend on the kinds of the metal electrodes, while the crystallinity strongly depends on the kinds of the metal electrodes. Table 2 shows the FWHM of the (0 0 0 2) peak rocking curves of the AlN films. The AlN films prepared on Au/Ti, Pt/Ti, Al, Ag and Cu with face centered cubic (fcc) lattice indicated high crystal orientation, except Ni. Especially, the AlN films prepared on the Au/Ti and Pt/Ti showed high crystal orientation and the FWHMs were 3.4 and 3.5 , respectively. While, the AlN films prepared on the metal electrodes with body centered cubic (bcc) and hexagonal close packed (hcp) lattices indicated low crystal orientation. The FWHMs of the AlN films prepared on Zn and Zr were 12.0 and 16.5 , respectively. From these results, the crystal orientation of AlN films is strongly influenced and the AlN films prepared on the fcc metal electrodes indicates high crystal orientation. We investigated the dependence of the crystal orientation of the AlN films on the crystallinity of the bottom metal electrodes to clarify the reason for the influence of the bottom metal electrodes. The dependence of the FWHM of the AlN films on the main peak intensity of the bottom metal electrodes is shown in Fig. 2. The FWHM
701
decreased with the increasing peak intensity of the metal electrodes. The peak intensity of the Au/ Ti and Pt/Ti was especially high, and that of the Cu and Ag also was comparatively high. But, the low peak intensity of the Al was exceptional for the tendency. We measured the surface roughness (rms) of the bottom metal electrodes by using AFM, because it is well known that the orientation of films is influenced by substrate morphology [8]. The relationship between the FWHM of the rocking curves of the AlN films and the surface roughness of the metal electrodes is shown in Fig. 3. The surface roughness of the Au/Ti and Pt/Ti electrodes were smoother than that of the other metal electrodes, and were 1.07 and 2.54 nm, respectively.
Fig. 2. Dependence of FWHM of AlN thin films on main peak intensity of metal electrodes. The reflections of fcc, bcc and hcp lattices are marked with , J and B, respectively.
Table 2 FWHM of rocking curves of AlN thin films prepared on metal films and crystal structure of metal films Metal
Au/Ti
Pt/Ti
Al
Ag
Cu
Mo
Cr
Nb
W
Ni
Fe
Ti
Co
Zn
Zr
FWHM(deg.) Crystal structure
3.4 F
3.5 F
4.1 F
5.3 F
5.5 F
8.5 B
8.9 B
9.5 B
9.5 B
10.2 F
10.3 B
10.4 H
11.2 H
12.0 H
16.5 H
F: Face-centered cubic lattice, B: Body-centered cubic lattice, H: Hexagonal lattice.
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AlN film on Pt/Ti electrode shows epitaxial growth. The misfit between Al (1 1 0) and AlN (1 1 0 0) is 6%. However, we guess that the growth structure is quite different from AlN on Pt/Ti, because the existence of an amorphous layer on Al thin films is known [12]. Thus, we think that the crystal growth of AlN film on Al electrode is more similar to AlN film on glass than on the other metal electrodes [11,12].
Fig. 3. Relationship between FWHM of AlN thin films and surface roughness of metal electrodes. The reflections of fcc, bcc, hexagonal hcp lattices are marked with , J and B, respectively.
However, the dependence of the orientation on the surface roughness was not observed in Fig. 3. Because the orientation of the AlN films prepared on the Cr, Mo and Ni electrodes was low, although the surface roughness of the Cr, Mo and Ni electrodes was low. For investigating the difference of the orientation of the AlN films, the lattice misfits of AlN, Al and Pt were estimated. (0 0 0 1) AlN is a closepacked plane of hcp structure. Both Al (1 1 1) and Pt (1 1 1) are also close-packed structures. A closepacked plane of AlN grows on close-packed planes of the bottom metal electrodes. The atomic spacing of the Pt (1 1 1) and AlN (0 0 0 1) planes are shown in Fig. 4. The misfit between Pt (1 1 0) and AlN ð1 1 2% 0Þ is 11% and the misfit between Al (1 1 0) and AlN ð1 1 2% 0Þ is 8%. While the misfit between Pt (1 1 0) and AlN (1 1 0 0) is 3% when AlN atoms in a (0 0 0 1) plane are rotated 30 from Pt atoms around the AlN c-axis. Direct orientation on Pt is possibly caused by the 30 rotated growth. From these results, it is presumed that
Fig. 4. Atomic spacing of Pt (1 1 1) and AlN (0 0 0 1) planes.
Fig. 5. SEM images of AlN thin film surfaces prepared on Mo (a) and Pt/Ti (b) electrodes.
ARTICLE IN PRESS M. Akiyama et al. / Vacuum 74 (2004) 699–703
Fig. 5 shows the SEM images of the surfaces of the AlN films prepared on the Mo and Pt/Ti electrodes. The surface of the AlN films prepared on the Mo electrode was comparatively rough, and was formed by large particles. While, the surface of the AlN films prepared on the Pt/Ti electrode was smooth, and was formed by fine particles. These results mean that bottom electrode metals affect the growth mechanism of the AlN films prepared on them.
4. Conclusion We investigated the influence of the 15 bottom metal electrodes on the crystallinity and orientation of the AlN films prepared on them. All the AlN films indicated c-axis orientation, but the XRD peak intensity of the AlN films was drastically changed with the kinds of the metal electrodes. The AlN films prepared on the Au/Ti, Pt/Ti, Al, Ag and Cu with face centered cubic (fcc) lattice indicated high crystal orientation. While, the AlN films deposited on the metal electrodes with body centered cubic (bcc) and hexagonal close packed (hcp) lattices indicated low orientation. The dependence of the orientation of the AlN films on the crystallinity of the metal electrodes was also observed. Thus, bottom electrode metals strongly affect the growth mechanism of the AlN films prepared on them.
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Acknowledgements We thank Dr. Shobu of AIST kyushu center for valuable advice on crystal growth mechanism and for information on making thin films.
References [1] Dubois MA, Muralt P. Appl Phys Lett 1999;74:3032. [2] Ohta J, Fujioka H, Sumiya M, Koinuma H, Oshima M. J Cryst Growth 2001;225:73. [3] Strite S, Morkoc H. J Vac Sci Technol B 1992;10: 1237. [4] Butcher KSA, Tansley TL. J Appl Phys 2001;90: 6217. [5] Yim WM, Paff RJ. J Appl Phys 1974;45:1456. [6] Zheng L, Ramalingam S, Shi T, Peterson RL. J Vac Sci Technol A 1993;11:2437. [7] Akiyama M, Ueno N, Nonaka K, Tateyama H. Appl Phys Lett 2003;82:1977. [8] Akiyama M, Xu CN, Kodama M, Usui I, Nonaka K, Watanabe T. J Am Ceram Soc 2001;84:1917. [9] Shiosaki T, Hayashi M, Kawabata A. Proceedings of the 1982 Ultrasonics Symposium. San Diego: IEEE; 1982. p. 529. [10] Naik RS, Lutsky JJ, Reif RR, Sodini CG, Becker A, Fetter L, Huggins H, Miller R, Pastalan J, Rittenhouse G, Wong YH. IEEE Trans Ultrasonics Ferroelectrics Frequency Control 2000;47:292. [11] Akiyama M, Xu CN, Nonaka K, Shobu K, Watanabe T. Thin Solid Films 1998;315:62. [12] Yoshino Y, Inoue K, Takeuchi M, Ohwada K. Vacuum 1998;51:601.
Vacuum 74 (2004) 699–703
Influence of metal electrodes on crystal orientation of aluminum nitride thin films Morito Akiyamaa,*, Keigo Nagaob, Naohiro Uenoa, Hiroshi Tateyamaa, Tetsuo Yamadab a
Institute for Structural and Engineering Materials, National Institute of Advanced Industrial Science and Technology, 807-1 Shuku, Tosu, Saga 841-0052, Japan b Ube Research Laboratory, Corporate Research & Development, Ube Industries Ltd., 1978-5 Kogushi, Ube, Yamaguchi 755-8633, Japan
Abstract We have investigated the influence of various bottom metal electrodes on the crystallinity and crystal orientation of aluminum nitride (AlN) thin films prepared on them in order to develop thin film bulk acoustic wave resonators. AlN films were prepared on 15 kinds of bottom metal electrode (Ag, Al, Au/Ti, Co, Cr, Cu, Fe, Mo, Nb, Ni, Pt/Ti, Ti, W, Zn, Zr) using rf magnetron reactive sputtering method. The crystallinity and crystal orientation of the AlN films strongly depend on the bottom metal electrodes. The AlN films prepared on the metal electrodes with the face centered cubic lattice structure show high c-axis orientation, except Ni. The AlN films prepared on Au/Ti and Pt/Ti show the highest crystallinity and orientation among them. The high crystallinity and orientation are due to the fact that the crystallinity of the Au/Ti and Pt/Ti electrodes is high, the surface roughness of the Au/Ti and Pt/Ti is low, and Au and Pt (1 1 1) planes match well with hexagonal AlN crystal structure. These results suggest that the crystallinity and orientation of AlN films are strongly influenced by bottom metal electrodes. r 2004 Elsevier Ltd. All rights reserved. Keywords: Aluminum nitride; Thin films; FBAR; Orientation; Crystallinity; Metal electrode; Sputtering
1. Introduction Quickly developed mobile telecommunication needs film bulk acoustic resonators (FBAR) for higher carrier frequencies [1]. A schematic view of FBAR is shown in Fig. 1 [1]. The resonance frequency of FBAR is mainly determined by the thickness of a piezoelectric layer and electrodes. The piezoelectric layer of 2.5 down to 0.5 mm would cover the very high frequency range from 2 to 10 GHz. Zinc oxide (ZnO) was taken as the *Corresponding author. E-mail address: [email protected] (M. Akiyama).
piezoelectric material due to its high effective coupling coefficient of 0.30 in the early 1980s [1]. However, ZnO has some drawbacks, such as low electrical resistance, low breakdown voltage and high dielectric losses, because ZnO is usually a ntype semiconductor. Aluminum nitride (AlN) would be a good piezoelectric material. With an achievable effective coupling coefficient of 0.25 [1], filter with a sufficiently large bandwidth could be produced for mobile telecommunication. Since the band gap of AlN is large (Eg=6.2 eV) [2], a high electrical resistance (1011–1013 O cm) [3], a high breakdown voltage (640 MV/m) [4] and low dielectric loss can
0042-207X/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2004.01.052
ARTICLE IN PRESS M. Akiyama et al. / Vacuum 74 (2004) 699–703
700
Table 1 Sputtering conditions
Fig. 1. Schematic view of FBAR.
Sputtering pressure Atmosphere Substrate temperature Sputtering rate Sputtering time rf power rf frequency Target substrate spacing
0.5 Pa Ar:N2=1:1 300 C 5.2 nm/min 4h 320 W 13.56 MHz >8.3 cm
be expected. Furthermore, AlN could achieve small thermal drifts, because the thermal expansion coefficient of AlN is low (c-axis parallel: 5.3 106/K, a-axis parallel: 4.2 106/K) [5]. AlN films can be expected to be competitive in sensor [6–8] and ultrasound applications [9] where low loss, low thermal drift and high signal-to-noise rations are demanded. For thin-film resonators, a polycrystalline c-axis oriented piezoelectric film is sufficient but it has to be grown on a metal electrode [1]. There may be no report on comparison with the influence of various bottom metal electrodes on the crystallinity and crystal orientation of AlN films prepared on them. In this paper we focus on the crystallinity and orientation of AlN films prepared on various bottom metal electrodes, because the crystallinity and orientation are important factors for the piezoelectric property of AlN films [10]. With AlN films an electromechanical coupling coefficient of 0.25 and low losses can be achieved, if strongly c-axis oriented AlN films are grown. AlN is, therefore, an excellent material for FBARs. We prepared AlN films on fifteen metal electrodes in order to find the most suitable bottom metal electrode for a FBAR. Because there is no report on the relationship between the crystal orientation of AlN films and bottom metal electrodes. In this paper we report on research into the most suitable bottom metal electrode and consider the nature of the crystal orientation mechanism of AlN films.
sputtering condition for preparing c-axis oriented AlN films is as follows [11]. Table 1 shows the detailed sputtering conditions. The target was a 76.2 mm-diam 99.999% pure aluminum. The film thickness of AlN films was about 1 mm. Substrates were silicon (1 0 0) wafers with oxide layer (thickness:1 mm). Fifteen Metal electrodes (Ag, Al, Au/Ti, Co, Cr, Cu, Fe, Mo, Nb, Ni, Pt/Ti, Ti, W, Zn, Zr) were prepared by dc sputtering using only Ar gas at room temperature. The thickness of the electrodes was about 300 nm. The titanium films deposited under gold and platinum were applied as adhesion layer, and the thickness was about 30 nm. Pressure in the vacuum chamber was maintained below 4 104 Pa before sputtering deposition, and then high-purity argon (99.999% purity) and nitrogen (99.999% purity) gases were introduced. Before the deposition process, the target was cleaned under the deposition conditions for 5 min. The crystal structure and crystallinity of AlN films were confirmed by X-ray diffraction (XRD; M03X-HF, Mac Science, Japan) using CuKa radiation. The crystal orientation of AlN films was evaluated by the full-width at half-maximum (FWHM) of the X-ray rocking curves of the (0 0 0 2) peaks of AlN films. The surface structures of AlN films were observed by scanning electron microscopy (SEM; S-4300, Hitachi, Japan). The surface morphology of metal electrodes was observed by atomic force microscope (AFM; Nanoscope IIIa).
2. Experimental
3. Results and discussion
AlN films were prepared in the rf magnetron sputtering system (CFS-4ES, Tokuda, Japan). The
The XRD patterns of the 15 metal electrodes were measured before preparing AlN films. The
ARTICLE IN PRESS M. Akiyama et al. / Vacuum 74 (2004) 699–703
XRD patterns show that Al, Ag, Au, Cu, Ni and Pt electrodes oriented to (1 1 1), Mo, Cr, Nb, W and Fe electrodes oriented to (1 1 0) , Ti, Co, Zn and Zr electrodes oriented to (0 0 2). All the metal electrodes showed highly crystal orientation. AlN films were prepared on the metal electrodes under the same sputtering conditions, and the XRD patterns of the AlN films were measured. The (0 0 0 2) peak intensity of the AlN films was drastically changed with the kinds of the metal electrodes, but all the AlN thin films indicated caxis orientation. These results suggest that the crystal orientation of the AlN films does not depend on the kinds of the metal electrodes, while the crystallinity strongly depends on the kinds of the metal electrodes. Table 2 shows the FWHM of the (0 0 0 2) peak rocking curves of the AlN films. The AlN films prepared on Au/Ti, Pt/Ti, Al, Ag and Cu with face centered cubic (fcc) lattice indicated high crystal orientation, except Ni. Especially, the AlN films prepared on the Au/Ti and Pt/Ti showed high crystal orientation and the FWHMs were 3.4 and 3.5 , respectively. While, the AlN films prepared on the metal electrodes with body centered cubic (bcc) and hexagonal close packed (hcp) lattices indicated low crystal orientation. The FWHMs of the AlN films prepared on Zn and Zr were 12.0 and 16.5 , respectively. From these results, the crystal orientation of AlN films is strongly influenced and the AlN films prepared on the fcc metal electrodes indicates high crystal orientation. We investigated the dependence of the crystal orientation of the AlN films on the crystallinity of the bottom metal electrodes to clarify the reason for the influence of the bottom metal electrodes. The dependence of the FWHM of the AlN films on the main peak intensity of the bottom metal electrodes is shown in Fig. 2. The FWHM
701
decreased with the increasing peak intensity of the metal electrodes. The peak intensity of the Au/ Ti and Pt/Ti was especially high, and that of the Cu and Ag also was comparatively high. But, the low peak intensity of the Al was exceptional for the tendency. We measured the surface roughness (rms) of the bottom metal electrodes by using AFM, because it is well known that the orientation of films is influenced by substrate morphology [8]. The relationship between the FWHM of the rocking curves of the AlN films and the surface roughness of the metal electrodes is shown in Fig. 3. The surface roughness of the Au/Ti and Pt/Ti electrodes were smoother than that of the other metal electrodes, and were 1.07 and 2.54 nm, respectively.
Fig. 2. Dependence of FWHM of AlN thin films on main peak intensity of metal electrodes. The reflections of fcc, bcc and hcp lattices are marked with , J and B, respectively.
Table 2 FWHM of rocking curves of AlN thin films prepared on metal films and crystal structure of metal films Metal
Au/Ti
Pt/Ti
Al
Ag
Cu
Mo
Cr
Nb
W
Ni
Fe
Ti
Co
Zn
Zr
FWHM(deg.) Crystal structure
3.4 F
3.5 F
4.1 F
5.3 F
5.5 F
8.5 B
8.9 B
9.5 B
9.5 B
10.2 F
10.3 B
10.4 H
11.2 H
12.0 H
16.5 H
F: Face-centered cubic lattice, B: Body-centered cubic lattice, H: Hexagonal lattice.
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AlN film on Pt/Ti electrode shows epitaxial growth. The misfit between Al (1 1 0) and AlN (1 1 0 0) is 6%. However, we guess that the growth structure is quite different from AlN on Pt/Ti, because the existence of an amorphous layer on Al thin films is known [12]. Thus, we think that the crystal growth of AlN film on Al electrode is more similar to AlN film on glass than on the other metal electrodes [11,12].
Fig. 3. Relationship between FWHM of AlN thin films and surface roughness of metal electrodes. The reflections of fcc, bcc, hexagonal hcp lattices are marked with , J and B, respectively.
However, the dependence of the orientation on the surface roughness was not observed in Fig. 3. Because the orientation of the AlN films prepared on the Cr, Mo and Ni electrodes was low, although the surface roughness of the Cr, Mo and Ni electrodes was low. For investigating the difference of the orientation of the AlN films, the lattice misfits of AlN, Al and Pt were estimated. (0 0 0 1) AlN is a closepacked plane of hcp structure. Both Al (1 1 1) and Pt (1 1 1) are also close-packed structures. A closepacked plane of AlN grows on close-packed planes of the bottom metal electrodes. The atomic spacing of the Pt (1 1 1) and AlN (0 0 0 1) planes are shown in Fig. 4. The misfit between Pt (1 1 0) and AlN ð1 1 2% 0Þ is 11% and the misfit between Al (1 1 0) and AlN ð1 1 2% 0Þ is 8%. While the misfit between Pt (1 1 0) and AlN (1 1 0 0) is 3% when AlN atoms in a (0 0 0 1) plane are rotated 30 from Pt atoms around the AlN c-axis. Direct orientation on Pt is possibly caused by the 30 rotated growth. From these results, it is presumed that
Fig. 4. Atomic spacing of Pt (1 1 1) and AlN (0 0 0 1) planes.
Fig. 5. SEM images of AlN thin film surfaces prepared on Mo (a) and Pt/Ti (b) electrodes.
ARTICLE IN PRESS M. Akiyama et al. / Vacuum 74 (2004) 699–703
Fig. 5 shows the SEM images of the surfaces of the AlN films prepared on the Mo and Pt/Ti electrodes. The surface of the AlN films prepared on the Mo electrode was comparatively rough, and was formed by large particles. While, the surface of the AlN films prepared on the Pt/Ti electrode was smooth, and was formed by fine particles. These results mean that bottom electrode metals affect the growth mechanism of the AlN films prepared on them.
4. Conclusion We investigated the influence of the 15 bottom metal electrodes on the crystallinity and orientation of the AlN films prepared on them. All the AlN films indicated c-axis orientation, but the XRD peak intensity of the AlN films was drastically changed with the kinds of the metal electrodes. The AlN films prepared on the Au/Ti, Pt/Ti, Al, Ag and Cu with face centered cubic (fcc) lattice indicated high crystal orientation. While, the AlN films deposited on the metal electrodes with body centered cubic (bcc) and hexagonal close packed (hcp) lattices indicated low orientation. The dependence of the orientation of the AlN films on the crystallinity of the metal electrodes was also observed. Thus, bottom electrode metals strongly affect the growth mechanism of the AlN films prepared on them.
703
Acknowledgements We thank Dr. Shobu of AIST kyushu center for valuable advice on crystal growth mechanism and for information on making thin films.
References [1] Dubois MA, Muralt P. Appl Phys Lett 1999;74:3032. [2] Ohta J, Fujioka H, Sumiya M, Koinuma H, Oshima M. J Cryst Growth 2001;225:73. [3] Strite S, Morkoc H. J Vac Sci Technol B 1992;10: 1237. [4] Butcher KSA, Tansley TL. J Appl Phys 2001;90: 6217. [5] Yim WM, Paff RJ. J Appl Phys 1974;45:1456. [6] Zheng L, Ramalingam S, Shi T, Peterson RL. J Vac Sci Technol A 1993;11:2437. [7] Akiyama M, Ueno N, Nonaka K, Tateyama H. Appl Phys Lett 2003;82:1977. [8] Akiyama M, Xu CN, Kodama M, Usui I, Nonaka K, Watanabe T. J Am Ceram Soc 2001;84:1917. [9] Shiosaki T, Hayashi M, Kawabata A. Proceedings of the 1982 Ultrasonics Symposium. San Diego: IEEE; 1982. p. 529. [10] Naik RS, Lutsky JJ, Reif RR, Sodini CG, Becker A, Fetter L, Huggins H, Miller R, Pastalan J, Rittenhouse G, Wong YH. IEEE Trans Ultrasonics Ferroelectrics Frequency Control 2000;47:292. [11] Akiyama M, Xu CN, Nonaka K, Shobu K, Watanabe T. Thin Solid Films 1998;315:62. [12] Yoshino Y, Inoue K, Takeuchi M, Ohwada K. Vacuum 1998;51:601.