Crack Development in Pulsed Laser-Deposited PZT Thin Films
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CRACK DEVELOPMENT IN PULSED LASER-DEPOSITED PZT THIN FILMS H. M. LEE*+, T. J. CHUANG', C. K. CHIANG', L. P. COOK* AND P. K. SCHENCK* "National Institute of Standards and Technology, Gaithersburg, MD. +University of Maryland, College Park, MD ABSTRACT The development of cracks in a PZT thin film prepared by pulsed laser deposition on an unheated Pt-coated silicon substrate, and subsequently crystallized by post-deposition annealing, was investigated as a function of film thickness. As deposited, the film was amorphous. The film was heated at 600 'C to produce predominantly ferroelectric crystalline PZT. Spacing, width and morphology of cracks in the film followed a regular progression in which crack area decreased with decreasing film thickness. Data on area shrinkage, as deduced from crack area, were fit equally well as either a linear or a parabolic function of film thickness. It is suggested that crystallization-induced stresses rather than thermalgradient related stresses, were dominant in the formation of the cracks, and that these stresses were modified by interaction with the substrate. INTRODUCTION The laser deposition method is an important alternative route for processing lead zirconate-titanate (PZT) thin films because it can be made compatible with silicon wafer processing, and it is a practical way of transferring material with known bulk properties to thin film form1 '2 . Using a pulsed excimer laser, PZT thin films have been successfully deposited on platinum-coated silicon wafers3 . When the substrate was held at room temperature, the as-deposited films were amorphous. Post-deposition heat-treatment at 550600 °C led to the formation of crystallized PZT films with ferroelectric properties. In previous studies 4 ' 5 , we have found that films crystallized by post-deposition annealing always contained mud-crack-like fissures of varying size. In thicker films the cracks were relatively regular and interconnected, whereas in thinner films the cracks became more irregular. As the film thickness decreased, the width of cracks was reduced and the cracks were no longer connected, the crack pattern changing from an interconnected network of cracks to numerous individualized small cracks. We observed that the spacing and width of the larger cracks could be correlated with the film thickness. Since the existence of such cracks will seriously affect the ferroelectric properties of PZT films and thus the reliability in microelectronic devices, we have examined in more detail the crack behavior of these films in order to improve their processing. EXPERIMENTAL The preparation of the pulsed laser-deposited PZT thin film discussed in this paper was as follows. Briefly, it was prepared by using an ArF excimer laser (193 nm). The deposition was done at a repetition rate of 10 Hz and a nominal pulse width of 23 ns. The laser beam was focused to produce a fluence of 10-30 J/cm 2 . A commercial PZT ceramic (47% PbTiO 3 and 53% PbZrO3 ) was used as the target. An unheated silicon [100] substrate Mat. Res. Soc. Symp. Proc. V
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