- Email: [email protected]
Microstructure evolution of AlN films deposited under various pressures by RF reactive sputtering
Surface and Coatings Technology 166 (2003) 231–236
Microstructure evolution of AlN films deposited under various pressures by RF reactive sputtering Hao Cheng*, Yong Sun, Peter Hing School of Materials Engineering, Nanyang Technological University, Singapore 639798, Singapore Received 13 August 2002; accepted in revised form 16 October 2002
Abstract Wurtzite AlN (2H-AlN) films were deposited on p-type Si(100) by RF planar magnetron reactive sputtering under various pressures at a relatively low temperature (350 8C). X-Ray diffraction (XRD) and selected area electron diffraction (SAD) were used to study the microstructural features of the deposited films. XRD results showed that with the decrease in sputtering gas pressure, the preferred orientation of AlN films changed from (100) to (002). SAD results confirmed that a strong (100) texture existed in the films with (100) preferred orientation. Polycrystalline diffraction rings were recorded for the film where preferred orientation was not obvious. The AlN film deposited at 2 mtorr exhibited improved grain orientation along the c-axis. A further decrease in sputtering pressure to 1.4 mtorr produced an AlN film with two-domain structure instead of a perfect single crystal. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: AlN film; RF sputtering; Preferred orientation; X-Ray diffraction, Selected area electron diffraction
1. Introduction With novel thermal, chemical, mechanical, acoustic and optical properties, AlN thin films have received considerable interest as promising candidates for electronic materials for thermal dissipation, dielectric and passivation layers, surface acoustic wave (SAW) devices and photoelectric devices w1–4x. Many techniques, such as DCyRF sputtering w5,6x, chemical vapor deposition (CVD) w7x, laser chemical vapor deposition (LCVD) w8x, pulsed laser ablation (PLD) and molecular beam epitaxial (MBE) w9x, have been used to prepare AlN thin films on various substrates. The sputtering technique is promising under circumstances where low temperature deposition and conformal coatings are needed w10x. The performance of an AlN film is greatly influenced by its microstructure, which is strongly affected by deposition conditions. For example, films with various preferred orientations will show different piezoelectric behavior: (002) preferred orientation is shown to be better w11x. The heat dissipation capacity of AlN films is governed by such microstructure factors as morphol*Corresponding author. Tel.: q65-790-4614; fax: q65-790-0920. E-mail address: [email protected] (H. Cheng).
ogy, interface roughness and preferred orientation. These parameters individually will dominate the phonon scattering procedure in AlN films at certain temperature ranges and will thus influence heat transport properties of the deposited films w12x. Many efforts had been made in the past decade to study heat transport properties of thin films and devices in the sub-micro range, while attentions were paid to phonon scattering mechanism and measuring techniques w11–15x. Efforts to correlate heat transport properties of thin film materials to their deposition conditions are still lacking, partly because of the difficulty in characterizing the morphology and heat transport properties of films in the sub-micro range. As part of the effort made recently on this matter, in this paper we present our results on microstructural evolution of AlN films fabricated under various sputtering gas pressures. 2. Experimental AlN films were deposited with a 99.99% pure Al target and in 99.9995% purity nitrogen and argon mixtures. A planar magnetron sputtering system supplied by the Coaxial company in the UK was used for film deposition. The system consists of a cylinder chamber
0257-8972/03/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 2 . 0 0 7 7 1 - 5
232
H. Cheng et al. / Surface and Coatings Technology 166 (2003) 231–236
Table 1 Deposition conditions Substrate Substrate temperature Substrate-to-target distance Targets Gas Residual pressure RF power Pre-sputtering pressure Pre-sputtering RF power Sputtering pressure Gas flow rate
2=2 cm2 Si(100) 350 8C ;14 cm Al 99.99% purity AryN2mixture with 75% N2 -5=10y6 torr 300 W 20 mtorr 300 W 1.5–20 mtorr 12 sccm
with two 3-inch water-cooled target holders tilted at approximately 308 with respect to the normal of the horizontal substrate holder. An impedance-matching network was used to optimize the RF power input. Substrates in this work were 2=2 cm2 boron doped (100)"0.5 8 silicon wafers. The substrates were cleaned with 5% HF solution for 3 min, followed by rinsing in acetone and distilled water in an ultrasonic bath. High-purity Ar gas was introduced into the chamber after the chamber was evacuated to below 5=10y6 torr. Then the pure Al target was pre-sputtered for 10 min with the target shutter closed. During pre-sputtering the RF power and Ar pressure were kept at 300 W and 20
Fig. 1. Evolution of preferred orientation with the change of sputtering pressure.
Fig. 2. Cross-sectional bright field and dark field images of AlN films.
H. Cheng et al. / Surface and Coatings Technology 166 (2003) 231–236
233
Fig. 3. SAD patterns of AlN films deposited at various pressures.
mtorr, respectively. Then nitrogen gas was introduced into the chamber and reactive sputtering was initiated. Only the gas pressure was changed for different batches while all the other parameters were kept constant. The detailed deposition parameters were summarized and listed in Table 1. A Shimadzu LabX-XRD-6000 X-ray diffraction instrument with CuKa radiation was used for XRD measurements. Grazing incidence scan was used for phase identification and conventional u –2u scan was used to study preferred orientation. To obtain more detailed information of microstructural evolution, SAD study was performed using a JEOL 2100 transmission Electron Microscopy (TEM) operated at 200 kV. Crosssection samples were used in this study, which were prepared by a conventional technique described elsewhere w16x. The technique involves gluing two samples
face to face, cutting, mechanical polishing, and dimpling followed by thinning in Arq ion milling. 3. Results and discussion All the deposited films were shown to be wurtzite AlN by grazing incidence XRD scanning. No cubic metastable phase was found. The evolution of preferred orientation was recorded by u –2u XRD results in Fig. 1. At high pressures (G7.5 mtorr), the films showed (100) preferred orientation. Decreasing the sputtering pressure to 5 mtorr caused the preferred orientation to become less distinct, as (100), (002) and (101) peaks appeared in the XRD pattern. At 2 mtorr, the (002) texture became obvious although a weak (100) peak was still apparent. Further decreasing the sputtering pressure to 1.4 mtorr changed the preferred orientation
234
H. Cheng et al. / Surface and Coatings Technology 166 (2003) 231–236
Fig. 4. The calculated diffraction pattern of AlN: (a) with w010x zone axis, (b) rotated 908 around the c-axis and (c) two-domain structure.
of the AlN film completely to (002). A similar trend was also reported for direct current (DC) sputtering AlN films w17x and was described as the rotation of the (002) plane axis from parallel to perpendicular to the substrate surface. The formation mechanism of the preferred orientation can be understood by the following discussion. It is well known that two kinds of Al–N bond exist in wurtzite AlN, named B1 and B2. {100} planes consist of only
B1 bond, while {002} and {101} planes consist of B1 and B2 bonds together. The bond energy of B2 is relatively smaller than that of B1 bonds and is easy to break, so that the energy required for sputtering particles to be deposited in the direction of the c-axis is larger w18x. Decreasing the pressure will reduce the scattering effect and subsequent kinetic energy loss of the reactive particles in the gas phase w19x. At low pressure, when
H. Cheng et al. / Surface and Coatings Technology 166 (2003) 231–236
the mean free path of the reactive particles is larger than the target–substrate distance, the particles will reach the substrate directly without collision and the energy of the sputtering particles is higher. Which is beneficial for the formation of bond B2, consequently, {002} planes grow faster. While at high pressure when the mean free path of the reactive particles is smaller than the target– substrate distance, the particles now experience one or many times collision before they reach the substrate and their energies were greatly reduced. The low kinetic energy condition is favorable for the formation of B1 bond hence {100} planes grow faster. The microstructure of resultant AlN films was further studied by TEM. Fig. 2 shows typical dark field (DF) and bright field (BF) images of AlN films deposited at various sputtering pressures. Fig. 3 shows the SAD patterns of these films. In Fig. 3a the brightest diffraction spots which showed six-fold symmetry confirmed that the film deposited at 20 mtorr had a strong (100) texture. The cross-sectional micrograph showed that the film had a columnar structure with well aligned columnar grains perpendicular to the substrate surface, as shown in Fig. 2a. For the film deposited at 5 mtorr, the circular character of the diffraction pattern was more obvious, indicating the presence of more randomly distributed AlN grains, as shown in Fig. 3b. It is also found that more voids exist between the columnar grains in this film (Fig. 2 b), compared to the films with (100) preferred orientation. The formation of such microstructure can be attributed to the coexistence of (100), (002) and (101) surface planes (Fig. 1) and the development of columnar grains. According to the Van der Drift model, during the formation of a columnar structure, crystal grains oriented with their slower growing direction normal to the surface are terminated while faster growing directions are preserved as they intersect the grain boundaries w20x. Because the difference in the growth rates on different lattice planes, films with more randomly distributed grains has been shown to grow in such a less densely packed structure compared with the films with well aligned grains. It is the anisotropic growth rate on different lattice planes that is responsible for the formation of more faceted surface morphology (Fig. 2) in the deposited film w21x. Decreasing the sputtering pressure to 2 mtorr, although strong misorientation can still be found (as indicated by the elongated bars and rings) the grain orientation along c-axis was improved. Further decrease of the pressure to 1.4 mtorr, the resultant film showed a dense and well aligned columnar structure, as shown in Fig. 2c. The SAD pattern was changed to a pattern resembling a single crystal. Analysis of the diffraction pattern showed that the film grown in a two-domain mode instead of a perfect one-domain single crystal mode w22x. The indexed diffraction spots are shown in Fig. 3d. Most of the spots belong to the diffraction of w010x zone axis except the few marked by arrows, which are shown to
235
come from another kind of AlN domain rotated 308 (or 908 according to the six-fold symmetry of wurtzite AlN, while the indices of the spots may be different for the two rotations) with respect to the former domain. The theoretical diffraction patterns were calculated to explain this experimental result more clearly. Fig. 4a is the calculated diffraction pattern of w010x zone axis. Fig. 4b shows the resultant pattern by rotating the w010x zone axis around the c-axis (w002x) by 908. Fig. 4c shows the diffraction pattern of a possible two-domain structure composed of domains rotated 908 with respect to each other, as shown in Fig. 4a,b. The pattern agrees well with the experimental result and confirms the twodomain growth mode. Such a two domain-growth mode can be attributed to the step and terrace structures of Si(100) surface and is shown to be energetically favorable. Off-axis (vicinal) substrates have been shown to be more favorable for a single-domain growth mode and can improve film quality w23x. 4. Conclusions Wurtzite AlN films were deposited under various sputtering pressures at a relatively low temperature (350 8C). The evolution of microstructure was studied by XRD and SAD techniques. It was found that with a decrease of sputtering pressure the preferred orientation changed from (100) to (002). Similarly, the crosssectional microstructure changed from a well-aligned columnar structure to a columnar structure with more voids and finally to a dense and well-aligned columnar structure. SAD analysis showed that the highly (002) orientated films were single crystal like. The films grew by the two-domain growth mode instead of the perfect single crystal growth mode. The evolution and variation of preferred orientation with pressure can be explained by the dependence of energy and mobility of adatoms on pressure and the anisotropic growth rate of different lattice planes in the deposition process. References w1x A. Kumar, H.L. Chan, J.J. Weimer, L. Sanderson, Thin Solid Films 308y309 (1997) 406–409. w2x A. Giardini, A. Mele, T.M. DiPalma, C. Flamini, S. Orlando, R. Teghil, Thin Solid Films 295 (1997) 77–82. w3x N. Tanaka, H. Okano, T. Usuki, K. Shibata, Jpn. J. Appl. Phys. 33 (1994) 5249–5254. w4x J.H. Edgar, C.A. Carosella, C.R. Eddy, D.T. Smith, J. Mater. Sci. Mater. Electronics 7 (1996) 247–253. w5x S. Muhl, J.A. Zapien, J.M. Mendez, E. Andrade, J. Phys. D: Appl. Phys. 30 (1997) 2147–2155. w6x G. Carlotti, G. Gbbiotti, F.S. Hickernell, H.M. Liaw, G. Socino, Thin Solid Films 310 (1997) 34–38. w7x A.V. Dobrynin, J. Appl. Phys. 85 (1999) 1876–1882. w8x G. Radhakrishnan, J. Appl. Phys. 78 (1995) 6000–6005. w9x S. Tanaka, R.S. Kern, J. Bentley, R.F. Davis, Jpn. J. Appl. Phys. 35 (1996) 1641–1647. w10x R. Bathe, R.D. Vispute, D. Habersat, et al., Thin Solid Films 398y399 (2001) 575–580.
236
H. Cheng et al. / Surface and Coatings Technology 166 (2003) 231–236
w11x R.S. Naik, R. Reif, J.J. Lutsky, C.G. Sodini, J. Electrochem. Soc. 146 (1999) 691–696. w12x A. Balandin, K.L. Wang, J. Appl. Phys. 84 (1998) 6149–6153. w13x A.D. McConnell, S. Uma, K.E. Goodson, in: G.P. Celata, et al. (Eds.), Proceedings of the International Conference on Heat Transfer and Transport Phenomena in Microscale, Begell House, New York, 2000, pp. 413–419. w14x W. Holmes, J.M. Gildemeister, P.L. Richards, V. Kotsubo, Appl. Phys. Lett. 72 (1998) 2250–2252. w15x S. Bauer, B. Ploss, J. Appl. Phys. 68 (1990) 6361–6367. w16x S. Ohuchi, P.E. Russel, J. Vac. Sci. Technol. Sect A 5 (1987) 1630–1634. w17x B. Wang, Y.N. Zhao, Z. He, Vacuum 48 (1997) 427–429.
w18x X.H. Xu, H.S. Wu, C.J. Zhang, Z.H. Jin, Thin Solid Films 388 (2001) 62–67. w19x M. Penza, M.F. De Riccardis, L. Mirenghi, M.A. Tagliente, E. Verona, Thin Solid Films 259 (1995) 154–162. w20x B. Rauschenbach, J.W. Gerlach, Cryst. Res. Technol. 35 (2000) 675–688. w21x J.D. torre, G.H. Gilmer, D.L. Windt, et al., Technical Proceedings 1999 International Conference on Modeling and Simulation of Microsystems, MSM 99, 1999. w22x U. Kaiser, P.D. Brown, I. Khodos, C.J. Humphreys, H.P.D. Schenk, W. Richter, J. Mater. Res. 14 (1999) 2036–2042. w23x V. Lebedev, J. Jinschek, J. Krausslich, ¨ ¨ U. Kaiser, B. Schroter, W. Richter, J. Crystal Growth. 230 (2001) 426–431.
Microstructure evolution of AlN films deposited under various pressures by RF reactive sputtering Hao Cheng*, Yong Sun, Peter Hing School of Materials Engineering, Nanyang Technological University, Singapore 639798, Singapore Received 13 August 2002; accepted in revised form 16 October 2002
Abstract Wurtzite AlN (2H-AlN) films were deposited on p-type Si(100) by RF planar magnetron reactive sputtering under various pressures at a relatively low temperature (350 8C). X-Ray diffraction (XRD) and selected area electron diffraction (SAD) were used to study the microstructural features of the deposited films. XRD results showed that with the decrease in sputtering gas pressure, the preferred orientation of AlN films changed from (100) to (002). SAD results confirmed that a strong (100) texture existed in the films with (100) preferred orientation. Polycrystalline diffraction rings were recorded for the film where preferred orientation was not obvious. The AlN film deposited at 2 mtorr exhibited improved grain orientation along the c-axis. A further decrease in sputtering pressure to 1.4 mtorr produced an AlN film with two-domain structure instead of a perfect single crystal. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: AlN film; RF sputtering; Preferred orientation; X-Ray diffraction, Selected area electron diffraction
1. Introduction With novel thermal, chemical, mechanical, acoustic and optical properties, AlN thin films have received considerable interest as promising candidates for electronic materials for thermal dissipation, dielectric and passivation layers, surface acoustic wave (SAW) devices and photoelectric devices w1–4x. Many techniques, such as DCyRF sputtering w5,6x, chemical vapor deposition (CVD) w7x, laser chemical vapor deposition (LCVD) w8x, pulsed laser ablation (PLD) and molecular beam epitaxial (MBE) w9x, have been used to prepare AlN thin films on various substrates. The sputtering technique is promising under circumstances where low temperature deposition and conformal coatings are needed w10x. The performance of an AlN film is greatly influenced by its microstructure, which is strongly affected by deposition conditions. For example, films with various preferred orientations will show different piezoelectric behavior: (002) preferred orientation is shown to be better w11x. The heat dissipation capacity of AlN films is governed by such microstructure factors as morphol*Corresponding author. Tel.: q65-790-4614; fax: q65-790-0920. E-mail address: [email protected] (H. Cheng).
ogy, interface roughness and preferred orientation. These parameters individually will dominate the phonon scattering procedure in AlN films at certain temperature ranges and will thus influence heat transport properties of the deposited films w12x. Many efforts had been made in the past decade to study heat transport properties of thin films and devices in the sub-micro range, while attentions were paid to phonon scattering mechanism and measuring techniques w11–15x. Efforts to correlate heat transport properties of thin film materials to their deposition conditions are still lacking, partly because of the difficulty in characterizing the morphology and heat transport properties of films in the sub-micro range. As part of the effort made recently on this matter, in this paper we present our results on microstructural evolution of AlN films fabricated under various sputtering gas pressures. 2. Experimental AlN films were deposited with a 99.99% pure Al target and in 99.9995% purity nitrogen and argon mixtures. A planar magnetron sputtering system supplied by the Coaxial company in the UK was used for film deposition. The system consists of a cylinder chamber
0257-8972/03/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 2 . 0 0 7 7 1 - 5
232
H. Cheng et al. / Surface and Coatings Technology 166 (2003) 231–236
Table 1 Deposition conditions Substrate Substrate temperature Substrate-to-target distance Targets Gas Residual pressure RF power Pre-sputtering pressure Pre-sputtering RF power Sputtering pressure Gas flow rate
2=2 cm2 Si(100) 350 8C ;14 cm Al 99.99% purity AryN2mixture with 75% N2 -5=10y6 torr 300 W 20 mtorr 300 W 1.5–20 mtorr 12 sccm
with two 3-inch water-cooled target holders tilted at approximately 308 with respect to the normal of the horizontal substrate holder. An impedance-matching network was used to optimize the RF power input. Substrates in this work were 2=2 cm2 boron doped (100)"0.5 8 silicon wafers. The substrates were cleaned with 5% HF solution for 3 min, followed by rinsing in acetone and distilled water in an ultrasonic bath. High-purity Ar gas was introduced into the chamber after the chamber was evacuated to below 5=10y6 torr. Then the pure Al target was pre-sputtered for 10 min with the target shutter closed. During pre-sputtering the RF power and Ar pressure were kept at 300 W and 20
Fig. 1. Evolution of preferred orientation with the change of sputtering pressure.
Fig. 2. Cross-sectional bright field and dark field images of AlN films.
H. Cheng et al. / Surface and Coatings Technology 166 (2003) 231–236
233
Fig. 3. SAD patterns of AlN films deposited at various pressures.
mtorr, respectively. Then nitrogen gas was introduced into the chamber and reactive sputtering was initiated. Only the gas pressure was changed for different batches while all the other parameters were kept constant. The detailed deposition parameters were summarized and listed in Table 1. A Shimadzu LabX-XRD-6000 X-ray diffraction instrument with CuKa radiation was used for XRD measurements. Grazing incidence scan was used for phase identification and conventional u –2u scan was used to study preferred orientation. To obtain more detailed information of microstructural evolution, SAD study was performed using a JEOL 2100 transmission Electron Microscopy (TEM) operated at 200 kV. Crosssection samples were used in this study, which were prepared by a conventional technique described elsewhere w16x. The technique involves gluing two samples
face to face, cutting, mechanical polishing, and dimpling followed by thinning in Arq ion milling. 3. Results and discussion All the deposited films were shown to be wurtzite AlN by grazing incidence XRD scanning. No cubic metastable phase was found. The evolution of preferred orientation was recorded by u –2u XRD results in Fig. 1. At high pressures (G7.5 mtorr), the films showed (100) preferred orientation. Decreasing the sputtering pressure to 5 mtorr caused the preferred orientation to become less distinct, as (100), (002) and (101) peaks appeared in the XRD pattern. At 2 mtorr, the (002) texture became obvious although a weak (100) peak was still apparent. Further decreasing the sputtering pressure to 1.4 mtorr changed the preferred orientation
234
H. Cheng et al. / Surface and Coatings Technology 166 (2003) 231–236
Fig. 4. The calculated diffraction pattern of AlN: (a) with w010x zone axis, (b) rotated 908 around the c-axis and (c) two-domain structure.
of the AlN film completely to (002). A similar trend was also reported for direct current (DC) sputtering AlN films w17x and was described as the rotation of the (002) plane axis from parallel to perpendicular to the substrate surface. The formation mechanism of the preferred orientation can be understood by the following discussion. It is well known that two kinds of Al–N bond exist in wurtzite AlN, named B1 and B2. {100} planes consist of only
B1 bond, while {002} and {101} planes consist of B1 and B2 bonds together. The bond energy of B2 is relatively smaller than that of B1 bonds and is easy to break, so that the energy required for sputtering particles to be deposited in the direction of the c-axis is larger w18x. Decreasing the pressure will reduce the scattering effect and subsequent kinetic energy loss of the reactive particles in the gas phase w19x. At low pressure, when
H. Cheng et al. / Surface and Coatings Technology 166 (2003) 231–236
the mean free path of the reactive particles is larger than the target–substrate distance, the particles will reach the substrate directly without collision and the energy of the sputtering particles is higher. Which is beneficial for the formation of bond B2, consequently, {002} planes grow faster. While at high pressure when the mean free path of the reactive particles is smaller than the target– substrate distance, the particles now experience one or many times collision before they reach the substrate and their energies were greatly reduced. The low kinetic energy condition is favorable for the formation of B1 bond hence {100} planes grow faster. The microstructure of resultant AlN films was further studied by TEM. Fig. 2 shows typical dark field (DF) and bright field (BF) images of AlN films deposited at various sputtering pressures. Fig. 3 shows the SAD patterns of these films. In Fig. 3a the brightest diffraction spots which showed six-fold symmetry confirmed that the film deposited at 20 mtorr had a strong (100) texture. The cross-sectional micrograph showed that the film had a columnar structure with well aligned columnar grains perpendicular to the substrate surface, as shown in Fig. 2a. For the film deposited at 5 mtorr, the circular character of the diffraction pattern was more obvious, indicating the presence of more randomly distributed AlN grains, as shown in Fig. 3b. It is also found that more voids exist between the columnar grains in this film (Fig. 2 b), compared to the films with (100) preferred orientation. The formation of such microstructure can be attributed to the coexistence of (100), (002) and (101) surface planes (Fig. 1) and the development of columnar grains. According to the Van der Drift model, during the formation of a columnar structure, crystal grains oriented with their slower growing direction normal to the surface are terminated while faster growing directions are preserved as they intersect the grain boundaries w20x. Because the difference in the growth rates on different lattice planes, films with more randomly distributed grains has been shown to grow in such a less densely packed structure compared with the films with well aligned grains. It is the anisotropic growth rate on different lattice planes that is responsible for the formation of more faceted surface morphology (Fig. 2) in the deposited film w21x. Decreasing the sputtering pressure to 2 mtorr, although strong misorientation can still be found (as indicated by the elongated bars and rings) the grain orientation along c-axis was improved. Further decrease of the pressure to 1.4 mtorr, the resultant film showed a dense and well aligned columnar structure, as shown in Fig. 2c. The SAD pattern was changed to a pattern resembling a single crystal. Analysis of the diffraction pattern showed that the film grown in a two-domain mode instead of a perfect one-domain single crystal mode w22x. The indexed diffraction spots are shown in Fig. 3d. Most of the spots belong to the diffraction of w010x zone axis except the few marked by arrows, which are shown to
235
come from another kind of AlN domain rotated 308 (or 908 according to the six-fold symmetry of wurtzite AlN, while the indices of the spots may be different for the two rotations) with respect to the former domain. The theoretical diffraction patterns were calculated to explain this experimental result more clearly. Fig. 4a is the calculated diffraction pattern of w010x zone axis. Fig. 4b shows the resultant pattern by rotating the w010x zone axis around the c-axis (w002x) by 908. Fig. 4c shows the diffraction pattern of a possible two-domain structure composed of domains rotated 908 with respect to each other, as shown in Fig. 4a,b. The pattern agrees well with the experimental result and confirms the twodomain growth mode. Such a two domain-growth mode can be attributed to the step and terrace structures of Si(100) surface and is shown to be energetically favorable. Off-axis (vicinal) substrates have been shown to be more favorable for a single-domain growth mode and can improve film quality w23x. 4. Conclusions Wurtzite AlN films were deposited under various sputtering pressures at a relatively low temperature (350 8C). The evolution of microstructure was studied by XRD and SAD techniques. It was found that with a decrease of sputtering pressure the preferred orientation changed from (100) to (002). Similarly, the crosssectional microstructure changed from a well-aligned columnar structure to a columnar structure with more voids and finally to a dense and well-aligned columnar structure. SAD analysis showed that the highly (002) orientated films were single crystal like. The films grew by the two-domain growth mode instead of the perfect single crystal growth mode. The evolution and variation of preferred orientation with pressure can be explained by the dependence of energy and mobility of adatoms on pressure and the anisotropic growth rate of different lattice planes in the deposition process. References w1x A. Kumar, H.L. Chan, J.J. Weimer, L. Sanderson, Thin Solid Films 308y309 (1997) 406–409. w2x A. Giardini, A. Mele, T.M. DiPalma, C. Flamini, S. Orlando, R. Teghil, Thin Solid Films 295 (1997) 77–82. w3x N. Tanaka, H. Okano, T. Usuki, K. Shibata, Jpn. J. Appl. Phys. 33 (1994) 5249–5254. w4x J.H. Edgar, C.A. Carosella, C.R. Eddy, D.T. Smith, J. Mater. Sci. Mater. Electronics 7 (1996) 247–253. w5x S. Muhl, J.A. Zapien, J.M. Mendez, E. Andrade, J. Phys. D: Appl. Phys. 30 (1997) 2147–2155. w6x G. Carlotti, G. Gbbiotti, F.S. Hickernell, H.M. Liaw, G. Socino, Thin Solid Films 310 (1997) 34–38. w7x A.V. Dobrynin, J. Appl. Phys. 85 (1999) 1876–1882. w8x G. Radhakrishnan, J. Appl. Phys. 78 (1995) 6000–6005. w9x S. Tanaka, R.S. Kern, J. Bentley, R.F. Davis, Jpn. J. Appl. Phys. 35 (1996) 1641–1647. w10x R. Bathe, R.D. Vispute, D. Habersat, et al., Thin Solid Films 398y399 (2001) 575–580.
236
H. Cheng et al. / Surface and Coatings Technology 166 (2003) 231–236
w11x R.S. Naik, R. Reif, J.J. Lutsky, C.G. Sodini, J. Electrochem. Soc. 146 (1999) 691–696. w12x A. Balandin, K.L. Wang, J. Appl. Phys. 84 (1998) 6149–6153. w13x A.D. McConnell, S. Uma, K.E. Goodson, in: G.P. Celata, et al. (Eds.), Proceedings of the International Conference on Heat Transfer and Transport Phenomena in Microscale, Begell House, New York, 2000, pp. 413–419. w14x W. Holmes, J.M. Gildemeister, P.L. Richards, V. Kotsubo, Appl. Phys. Lett. 72 (1998) 2250–2252. w15x S. Bauer, B. Ploss, J. Appl. Phys. 68 (1990) 6361–6367. w16x S. Ohuchi, P.E. Russel, J. Vac. Sci. Technol. Sect A 5 (1987) 1630–1634. w17x B. Wang, Y.N. Zhao, Z. He, Vacuum 48 (1997) 427–429.
w18x X.H. Xu, H.S. Wu, C.J. Zhang, Z.H. Jin, Thin Solid Films 388 (2001) 62–67. w19x M. Penza, M.F. De Riccardis, L. Mirenghi, M.A. Tagliente, E. Verona, Thin Solid Films 259 (1995) 154–162. w20x B. Rauschenbach, J.W. Gerlach, Cryst. Res. Technol. 35 (2000) 675–688. w21x J.D. torre, G.H. Gilmer, D.L. Windt, et al., Technical Proceedings 1999 International Conference on Modeling and Simulation of Microsystems, MSM 99, 1999. w22x U. Kaiser, P.D. Brown, I. Khodos, C.J. Humphreys, H.P.D. Schenk, W. Richter, J. Mater. Res. 14 (1999) 2036–2042. w23x V. Lebedev, J. Jinschek, J. Krausslich, ¨ ¨ U. Kaiser, B. Schroter, W. Richter, J. Crystal Growth. 230 (2001) 426–431.