Effect of annealing on the structural and mechanical properties of Ba0.7Sr0.3TiO3 thin films

(1)

Materials Science and Engineering A 426 (2006) 157–161

Effect of annealing on the structural and mechanical properties

of Ba

0.7

Sr

0.3

TiO

3

thin films

Te-Hua Fang

a

, Win-Jin Chang

b,∗

, Chao-Ming Lin

c

, Liang-Wen Ji

d

,

Yee-Shin Chang

d

, Yu-Jen Hsiao

e

a_{Institute of Mechanical and Electromechanical Engineering, National Formosa University, Yunlin 632, Taiwan} b_{Department of Mechanical Engineering, Kun Shan University, Tainan 710, Taiwan}

c_{Department of Mechanical Engineering, WuFeng Institute of Technology, Chiayi 621, Taiwan} d_{Department of Electro-Optics Engineering, National Formosa University, Yunlin 632, Taiwan} e_{Department of Materials Science and Engineering, National Cheng Kung University, Tainan 701, Taiwan}

Received 24 February 2006; received in revised form 26 March 2006; accepted 29 March 2006

Abstract

Mechanical properties and surface characterizations of Ba0.7Sr0.3TiO3thin films deposited on silicon substrate by metalorganic decomposition

(MOD) method under different annealing temperatures were investigated. Hardness, Young’s modulus and the contact stress–strain of the films were achieved by nanoindentation techniques. X-ray diffraction (XRD) and atomic force microscopy (AFM) were used to characterize the structure of Ba0.7Sr0.3TiO3thin films. The X-ray diffraction results showed that Ba0.7Sr0.3TiO3thin films exhibited a high (1 1 0)-orientation and presented

a pure perovskite-type structure. The grain size and surface roughness increased as the annealing temperature increased. As well, the hardness and Young’s modulus increased as the annealing temperature increased from 600 to 800◦C, with the best results obtained at 800◦C. In addition, contact stress–strain relationships and elastic recovery are also discussed.

PACS: 61.10.Nz; 62.20.Qp; 77.55.+f

Keywords: Nanoindentation; Metalorganic decomposition; BST; Young’s modulus; Hardness; Contact stress–strain

1. Introduction

Perovskite ferroelectric thin films, barium strontium titanate (BaxSr1−xTiO3, BST) have been of great interest for their use in

dynamic random access memories (DRAM), tunable microwave devices, infrared sensors and electro-optical devices[1–5]. BST thin films’ electrical and optical properties, such as, high dielec-tric constant, large electro-optical coefficient and low optical losses are critical for these applications[6–8].

Despite extensive investigations of several different proper-ties of BST thin films, there is still lacking sufficient research systematically correlating the mechanical and structural relationships on a nanometer-scale for designing advanced optoelectronic devices. From here on it is essential that study

∗_{Corresponding author.}

E-mail address:[email protected](W.-J. Chang).

continue on the mechanical characterizations of these thin films to recognize how the relative parameters affect the material structures and the properties for use in advanced applica-tions.

Modern nanoindentation testing equipment allows the mea-surement of loads versus penetration depth curves where the loads extend down to the range of micro-Newtons and the typical penetration depths are in the range of nanometers. This technique is well suited for examining the mechanical properties of thin films[9,10].

The characteristics of crystalline structure, surface rough-ness and nanomechanical properties of Ba0.7Sr0.3TiO3thin films

produced by metalorganic decomposition (MOD) method fol-lowed by rapid thermal annealing (RTA) were achieved by means of X-ray diffraction (XRD), atomic force microscopy (AFM) and nanoindentation technique. As well, the influ-ences of rapid thermal annealing of BST thin films is also discussed.

(2)

160 T.-H. Fang et al. / Materials Science and Engineering A 426 (2006) 157–161

Fig. 3. Surface roughness, Raand RMS of BST thin films.

thin films; a and R denote the contact radius and the indenter radius, respectively, and the quantity a/R represents the contact strain [22]. As can be seen in this figure, the films annealed at 800◦C exhibited a larger stress (33.64± 3.26 GPa) and a smaller strain (0.42± 0.03). This implies that a larger stress could bear a higher yielding behavior due to the crystalline struc-ture being improved. This result was agreed with the hardness results.

From the nanoindentation curves the elastic recovery was calculated[21]. The elastic recoveries of the thin films annealed at 600, 700 and 800◦C were 68.75, 81.02 and 86.67%, respec-tively. The elastic recovery of the BST thin films also increased as the annealing temperature was increased.

Fig. 4. Hardness and Young’s modulus of BST thin films measured at annealing temperatures of 600, 700 and 800◦C.

Fig. 5. Contact stress–strain relationships of BST thin films.

4. Conclusions

In summary, nanomechanical properties of Ba0.7Sr0.3TiO3

thin films under different annealing temperatures were stud-ied. The microstructures and characterizations of the films were improved as the annealing temperatures were increased. The BST thin film annealed at 800◦C exhibited better mechani-cal properties and a pure perovskite-type configuration with a (1 1 0)-preferential orientation. The hardness and Young’s mod-ulus of BST thin films annealed at 600–800◦C ranged from 1.95± 0.26 GPa to 2.66 ± 0.24 GPa, and from 39.37 ± 1.78 GPa to 80.2± 1.8 GPa, respectively. The results confirmed that the Young’s modulus and elastic recovery of the thin films are depen-dent strongly on the crystallinity of the films structure.

Acknowledgements

This work was partially supported by the National Science Council of Taiwan, under Grant nos. NSC94-2212-E-150-045 and NSC94-2212-E-150-046.

References

[1] A.I. Kingon, J.P. Maria, S.K. Streiffer, Nature 406 (2000) 1032. [2] N.A. Pertsev, V.G. Koukhar, R. Waser, S. Hoffmann, Appl. Phys. Lett.

77 (2000) 2596.

[3] C. Basceri, S.K. Streiffer, A.I. Kingon, R. Waser, J. Appl. Phys. 82 (1997) 2497.

[4] E. Ngo, P.C. Joshi, M.W. Cole, C.W. Hubbard, Appl. Phys. Lett. 79 (2001) 248.

[5] Y.H. Xu, J.D. Mackenzie, Integr. Ferroelectr. 1 (1992) 17. [6] O. Auciello, J.F. Scott, R. Ramesh, Phys. Today 51 (1999) 22. [7] K. Abe, S. Komatsu, Jpn. J. Appl. Phys. 32 (1993) 4186. [8] M.N. Kamalasanan, S. Chandra, Appl. Phys. Lett. 59 (1991) 3547. [9] M. Pang, D.F. Bahr, K.G. Lynn, Appl. Phys. Lett. 82 (2003) 1200. [10] T. Staecller, K. Schiffmann, Surf. Sci. 482–485 (2001) 1125.

(3)

T.-H. Fang et al. / Materials Science and Engineering A 426 (2006) 157–161 161

[11] W.C. Oliver, G.M. Pharr, J. Mater. Res. 7 (1992) 1564. [12] T.H. Fang, W.J. Chang, Microelectron. Eng. 65 (2003) 231.

[13] B.D. Cullity, Elements of X-ray Diffraction, Addison-Wesley, Reading, MA, 1978, p. 102.

[14] S.T. Tay, X.H. Jiang, C.H.A. Huan, T.S. Wee, R. Liu, J. Appl. Phys. 88 (2000) 5928.

[15] W.J. Lee, H.G. Kim, S.G. Yoon, J. Appl. Phys. 80 (1996) 5891. [16] M.W. Cole, C. Hubbard, E. Ngo, M. Ervin, M. Wood, R.G. Geyer, J.

Appl. Phys. 92 (2002) 475.

[17] T.H. Fang, S.R. Jian, D.S. Chuu, J. Phys. D: Appl. Phys. 36 (2003) 878. [18] J. Ruan, B. Bhushan, J. Appl. Phys. 76 (1994) 8117.

[19] S. Veprek, P. Nesladek, A. Niederhofer, F. Glatz, M. Jilek, M. Sima, Surf. Coat. Tech. 108/109 (1998) 138.

[20] M.Y. Gutkin, I.A. Ovid’ko, C.S. Pande, Rev. Adv. Mater. Sci. 2 (2001) 80.

[21] S.R. Jian, T.H. Fang, D.S. Chuu, J. Electron. Mater. 32 (2003) 496. [22] A.C. Fischer-Cripps, Nanoindentation, Springer-Verlag, New York,