Microstructure, morphology and their annealing behaviors of alumina films synthesized by ion beam assisted deposition

Transcription

1 Nuclear Instruments and Methods in Physics Research B 206 (2003) Microstructure, morphology and their annealing behaviors of alumina films synthesized by ion beam assisted deposition Q.Y. Zhang a, *, W.J. Zhao a, P.S. Wang a, L. Wang b, J.J. Xu b, P.K. Chu c a Department of Physics, State Key Laboratory for Materials Modification by Laser, Ion and Electron Beams, Dalian University of Technology, Dalian , China b Institute of Materials Technology, Dalian Maritime University, Dalian , China c Department of Physics and Materials Science, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong, China Abstract Alumina films have been synthesized by ion beam assisted deposition (IBAD). X-ray photoelectron spectroscopy (XPS) and Rutherford back-scattering (RBS) analysis show that films synthesized by IBAD are stoichiometrical Al 2 O 3 films. Transmission electron microscopy (TEM), atomic force microscopy (AFM), and X-ray diffraction analysis (XRD) have been applied to characterize the microstructures and morphologies of Al 2 O 3 films and their annealing behaviors. We have found that the films are dominated by amorphous Al 2 O 3 phase when the substrate temperatures are lower than 800 C=6 h 1000 C=6 h 1200 C=2 h 500 C. The path of phase transition of film is amorphous Al 2 O 3! c-al2 O 3! c-al2 O 3 þ a-al 2 O 3! a-al 2 O 3. The film morphology is related with the phase transition of films during annealing. Ó 2003 Elsevier Science B.V. All rights reserved. PACS: 68.55; Jj Keywords: Ion beam assisted deposition; Al 2 O 3 film; Microstructure; Annealing behavior 1. Introduction Alumina combines many useful properties such as high dielectric constant, high thermal conductivity, relatively low refractive index, and transparency over wide range of wavelength [1 3]. Because of its excellent properties, they can find many applications in opto-electronics and microelectronics, which showed promising results as gate oxides and interpoly dielectrics in memory devices and planar waveguides [4 6]. * Corresponding author. Tel./fax: address: qyzhang@dlut.edu.cn (Q.Y. Zhang). Many deposition methods have been used to synthesize Al 2 O 3 films, including molecular beam epitaxy, chemical vapor deposition, magnetron sputtering, and ion beam assisted deposition (IBAD) [7 11]. IBAD is not only able to synthesize films at very low temperature, but also can improve the properties of thin films synthesized. The films synthesized by IBAD have smooth morphology, good adhesion of films to substrates, and high density. Traditionally, Al 2 O 3 films synthesized by IBAD were performed by electron beam evaporation with Ar ion beam assistance. In the present paper, we report on the synthesis of Al 2 O 3 films by using ion beam sputtering deposition with oxygen ion X/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi: /s x(03)

2 358 Q.Y. Zhanget al. / Nucl. Instr. and Meth. in Phys. Res. B 206 (2003) beam assistance. We have studied the microstructures and morphologies of the films, their annealing behaviors and the compositions of the films synthesized. 2. Experiment The deposition of Al 2 O 3 films was carried out in an IBAD system, in which the films can be synthesized by ion beam sputter deposition of aluminum in the oxygen atmosphere and bombarded simultaneously by low energy oxygen ions. Al 2 O 3 films were deposited on Si(1 0 0) substrates and optical glasses at the substrate temperature ranging from 70 C (without heating) to 500 C (using a heater). The silicon wafers were cleaned by the standard cleaning method. The sputtering ions were Ar ions with energy of 3 kev and beam current of 30 ma. The energy of oxygen ions was 500 ev. The base vacuum of the system was in the order of 10 3 Pa. Before deposition, Ar ion sputtering was used to clean the surfaces of samples for 15 min. During deposition, the oxygen flow was changed from 1.8 to 6.0 sccm and the working pressure was kept at Pa. To investigate the annealing behaviors of Al 2 O 3 films, Al 2 O 3 films were annealed at the temperatures of 800 and 1000 C for 6 h and 1200 C for 2 h in the atmosphere. Transmission electron microscopy (TEM), X- ray diffraction (XRD), and atomic force microscopy (AFM) were used to examine the plane-view morphology and to determine the structure of films. Glance-angle incident XRD was carried out with incident angle of 0.5. X-ray photoelectron spectroscopy (XPS) and Rutherford back-scattering (RBS) analysis have been applied to characterize the compositions of the films synthesized. 3. Results and discussion 3.1. Composition analysis of films synthesized Al 2 O 3 films were synthesized at different oxygen flows. Analysis results showed that films synthesized at the oxygen flow ranging from 1.8 to 6.0 sccm were all stoichiometrical and the oxygen flow did not influence the compositions of films, but changed the deposition rate. With the increase of oxygen flow from 1.8 to 6.0 sccm, the deposition rate decreased from about 2.0 to 1.0 lm/h. The reason of deposition rate decrease at high oxygen atmosphere is probably that the pure Al sputtering target is more seriously oxidized at high oxygen atmosphere and oxidized Al lowers the sputtering rate. The quantitative result by fitting the RBS spectrum gave the ratio of O to Al is of 3.1:2.0 and quantitative XPS analysis showed that the ratio of O to Al is of 3.0:2.0. The binding energies measured by XPS are and 75.2 ev for O 1s and Al 2p, respectively. The Al 2 O 3 films synthesized by IBAD are transparent in the range of the visible and near-infrared region. With the increase of deposition temperature, the refractive index of Al 2 O 3 films increases from 1.65 to 1.71 as measured by ellipsometry. Infrared absorption measurements showed that there is no evidence of OH impurity in the films Microstructures and annealingbehaviors of Al 2 O 3 films Fig. 1 shows the TEM images and their electron diffraction patterns of Al 2 O 3 films as deposited by IBAD and thermal annealing at 800 and 1000 C for 6 h, and 1200 C for 2 h. The films as deposited by IBAD are typical amorphous. With the increase of the deposition temperature from 70 to 500 C, no obvious morphology change and no obvious crystalline phases have been observed. In other words, Al 2 O 3 films synthesized below 500 C are dominated by amorphous. After annealing at 800 C for 6 h, however, the films become to poly crystalline films. By indexing the electron diffraction pattern, we find the films are c-al 2 O 3 phase in the grain size of nm. For the case of samples annealed at 1000 C for 6 h, the microstructure of the films is still dominated by c-al 2 O 3 phases and the grains grow up to several hundreds nm. XRD examination revealed that a few mounts of a-al 2 O 3 phase have appeared in the films as shown in Fig. 2. When annealing at 1200 C for 2 h, the films transit to pure a-al 2 O 3, phase in the grain size of 5 10 lm.

3 Q.Y. Zhanget al. / Nucl. Instr. and Meth. in Phys. Res. B 206 (2003) Fig. 1. TEM images and their electron diffraction patterns of Al 2 O 3 films synthesized by IBAD: (a) as deposited and thermal annealing, (b) at 800 C for 6 h, (c) at 1000 C for 6 h and (d) at 1200 C for 2 h. Table 1 Structures and annealing behaviors of Al 2 O 3 films synthesized by various PVD methods Deposition Structure ) Annealing behaviors RF reactive sputtering [12] DC reactive sputtering [13] RF magnetron sputtering [14] Thermal evaporation [15] a + c (<60 C) ) c (800 C/2 h) ) c + a (800 C/24 h) ) c + a (1000 C/2 h) ) c + a (1200 C/2 h) a (100 C) ) c (1150 C/164 h) c (500 C) ) c (1200 C) a (150 C) ) a (1200 C/2 h) a (500 C) ) h (1200 C/2 h) a (<150 C) ) c + a (570 C/8 h) ) c + h (670 C/32 h) ) c + d + h (825 C/12 h) ) d + h + a (870 C/32 h) ) a (1170 C/17 h) Fig. 2. XRD spectra of Al 2 O 3 films. Based on above results, we can summary the path of phase transition of the Al 2 O 3 films obtained by IBAD as amorphous Al 2 O 3! 800 C=6 h 1000 C=6h 1200 C=2h c-al 2 O 3! c-al2 O 3 þa-al 2 O 3! a-al2 O 3. Table 1 gives the microstructures of Al 2 O 3 films obtained by other methods and their thermal annealing behaviors. In the table, we can see that the thermal annealing behavior of the films obtained

4 360 Q.Y. Zhanget al. / Nucl. Instr. and Meth. in Phys. Res. B 206 (2003) by IBAD is similar to that of the Al 2 O 3 films obtained by RF reactive sputtering Morphologies and roughness of Al 2 O 3 films The morphologies of Al 2 O 3 films as deposited by IBAD and annealed at different temperatures have been measured by using AFM as shown in Fig. 3. No obvious crystalline character can be observed on the morphology of Al 2 O 3 film as deposited and the surface of Al 2 O 3 film is very smooth. The morphology of Al 2 O 3 film annealed at 800 C is similar to that as deposited, but the surface roughness of Al 2 O 3 film increases as shown in Fig. 4. After annealed at 1000 C for 6 h, obvious crystallization with regular grains can be observed on the morphology of Al 2 O 3 film surface. The grains of Al 2 O 3 film on the surface are in size of nm, which coincides with the observation by TEM in Al 2 O 3 film annealed at 1000 C. In the same time, the surface roughness increases up to 3 4 nm, which is about four times of surface roughness of Al 2 O 3 film as deposited and annealed at 800 C. After annealed at 1200 C for 2 h, regular grains in nm disappear and the larger grains can be found on the surface of Al 2 O 3 film. The grains of Al 2 O 3 film on the surface are about several micrometers in size, which is similar to the results obtained by TEM observation in Al 2 O 3 film annealed at 1200 C. The surface roughness of Al 2 O 3 film annealed at 1200 C is about nm, which is about 80 times of that as deposited and annealed at 800 C. According to the results obtained by AFM, we can conclude that the morphology of Al 2 O 3 films can be obviously changed with the increase of annealing temperature up to 1200 C and the surface roughness of Al 2 O 3 films is mainly dominated by the phase transition appeared in the films. When the phase transition of amorphous Al 2 O 3 film to c-al 2 O 3 happens at 800 C, the volume and density of Al 2 O 3 change little because the grain size of c-al 2 O 3 is less than 100 nm, so that the surface roughness of Al 2 O 3 film increases a few. Fig. 3. AFM images of Al 2 O 3 films (a) as deposited and annealed at (b) 800 C for 6 h, (c) 1000 C for 6 h and (d) 1200 C for 2 h.

5 Q.Y. Zhanget al. / Nucl. Instr. and Meth. in Phys. Res. B 206 (2003) (3) The surface roughness of Al 2 O 3 film synthesized by IBAD at the temperature lower than 500 C is lower than 1 nm. After annealed at C in atmosphere, the surface roughness increases from 1 to 80 nm. The change of film morphology is related to the microstructure of Al 2 O 3 films during annealing. Acknowledgements Fig. 4. Roughness of Al 2 O 3 films changed with annealing temperature, where R RMS is root-mean-square roughness and R a is arithmetic average roughness. After annealed at 1000 C for 6 h, the grains of c- Al 2 O 3 in the films grow up, which induces the obvious increase of surface roughness. Due to the appearance of phase transition from c-al 2 O 3 to a- Al 2 O 3, the cell volume and density of Al 2 O 3 films are considerably changed so that the surface roughness of Al 2 O 3 film increases rapidly. 4. Conclusions (1) Al 2 O 3 films synthesized by IBAD at the oxygen flow range of sccm are stoichiometrical and the oxygen flow does not clearly influence the compositions of films. (2) Al 2 O 3 films synthesized by IBAD at the temperature lower than 500 C are amorphous. The path of phase transition of Al 2 O 3 films annealed 800 C=6 h at C is amorphous Al 2 O 3! 1000 C=6 h 1200 C=2 h c-al 2 O 3! c-al2 O 3 þ a-al 2 O 3! a-al 2 O 3. This work is supported by the National Natural Science Foundation of China under grant no and by the Education Ministry of China. References [1] E. Dorre, H. Hubner, Alumina, Springer, Berlin, [2] K.H. Zaininger, A.S. Waxman, IEEE Trans. Electron Devices 16 (1969) 333. [3] J.A. Aboaf, J. Electrochem. Soc. 114 (1967) 948. [4] G.S. Higashi, C.G. Fleming, Appl. Phys. Lett. 55 (1989) [5] K.P. Paude, V.K. Nair, D. Gutierrez, J. Appl. Phys. 54 (1983) [6] A. Polman, J. Appl. Phys. 82 (1997) 1. [7] S. Maniv, W.D. Westwood, J. Vac. Sci. Technol. 17 (1980) 743. [8] A. Belkind, A. Freilich, R. Scholl, Surf. Coat. Technol (1998) 558. [9] K. Sawada, M. Ishida, T. Nakamura, N. Ohtake, Appl. Phys. Lett. 52 (1988) [10] M. Ishida, I. Katakabe, T. Nakamura, N. Ohtake, Appl. Phys. Lett. 52 (1988) [11] R. Serna, M. Jimenez de Castro, J.A. Chaos, C.N. Afonso, I. Vickridge, Appl. Phys. Lett. 75 (1999) [12] R.G. Frieser, J. Electrochem. Soc. 113 (1966) 357. [13] J.A. Thornton, J. Chin, Ceram. Bull. 56 (1977) 504. [14] C.A.T. Salama, J. Electrochem. Soc. 117 (1970) 913. [15] A.L. Dragoo, J.J. Diamond, J. Am. Ceram. Soc. 50 (1967) 568.