X-Ray

Thin Solid Films 516 (2008) 8479 – 8486 www.elsevier.com/locate/tsf

Stress, texture and microstructure of zirconium thin films probed by X-ray diffraction
J. Chakraborty a,⁎, K. Kishor Kumar a , S. Mukherjee a , S.K. Ray b
a

Facilitation Center for Industrial Plasma Technology, Institute for Plasma Research, B15-17/P, GIDC, Electronic Estate, Sector-25, Gandhinagar, Pin-382044, India bDepartment of Physics and Meteorology, Indian Institute of Technology Kharagpur, Kharagpur, West Bengal, Pin.-721302, India Received 19 April 2007; received in revised form 19 April 2008; accepted 24 April 2008 Available online 2 May 2008

Abstract Zirconium (α-Zr) thin films (thicknesses: 12 nm–240 nm) have been deposited on glass substrates by dc magnetron sputtering. Anisotropic broadening ofdiffraction lines has been observed for all films when probed by X-ray diffraction. Williamson–Hall method of X-ray line profile analysis shows average crystallite sizes of a few nanometers and high average microstrains for all films. Diffraction stress analysis of Zr films reveals relatively high biaxial compressive stresses that are found to decrease with increasing film thickness. The level ofstress has been compared to the theoretical yield strength of Zr films obtained by a model calculation. All Zr films show (0002) fiber texture as dominant texture component except 240 nm thick films. Ratios of strain-free lattice parameters (co/ao) determined for all Zr films are higher than the reported co/ao ratio for bulk zirconium. Observation of anisotropic diffraction line broadening in thetextured Zr films with high in-plane compressive stress indicates anisotropy in the shape of individual crystallite in all films. © 2008 Elsevier B.V. All rights reserved.
Keywords: Zirconium; X-ray diffraction; Stress; Texture

1. Introduction Zirconium (Zr) is a nuclear material with hexagonal close packed crystal structure (α-Zr) at room temperature. In the past decades, structural andmechanical properties of bulk zirconium and its alloys have been extensively studied [1–8]. Also, polycrystalline zirconium films are used in thin multilayers for various applications [9–11]. Such thin films are often deposited under thermodynamically nonequilibrium conditions in a deposition chamber by various methods like sputtering, evaporation etc. Consequently, microstructures of polycrystallinethin films are often characterized by large number of defects (mainly high dislocation density) which results in large microstrain (inhomogeneous strains) and also high residual stresses

⁎ Corresponding author. Present address: John F. Welch Technology Center, GE Global Research, 122 EPIP, White Field Road, Bangalore, PIN 560066, India. Tel.: +91 80 40123259. E-mail address:jay.chakraborty@ge.com (J. Chakraborty). 0040-6090/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2008.04.096

within the films [12–14]. Such high residual stress can be detrimental for the mechanical stability of the films. Determination of residual stresses in polycrystalline thin films by X-ray diffraction (from the macroscopic peak shift) is an important and active area ofresearch [15,16]. Also, wellestablished methods of X-ray line profile analysis are available for the determination of the microstructural parameters (crystallite size, defect densities, microstrain etc.) [17,18]. Nevertheless, it seems, compared to bulk zirconium, there exists only a very few reports on the structural properties of thin zirconium films [19–22]. Also, diffraction stress analysis ofzirconium thin films have not yet been investigated till date. Apart from high microstrain and macrostress, thin films are also characterized by crystallographic texture (nonrandom distribution of grain orientations) [23–26]. Crystallographic textures in polycrystalline thin films also depend on the films thicknesses, residual stresses and other microstructural parameters [23–26]. In the present...