Microstructure and Nonstoichiometry of Barium Strontium Titanate thin films for dram Applications
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MICROSTRUCTURE AND NONSTOICHIOMETRY OF BARIUM STRONTIUM TITANATE THIN FILMS FOR DRAM APPLICATIONS S. STEMMER*, S. K. STREIFFER**, N. D. BROWNING*, A. I. KINGON*** *Department of Physics, University of Illinois at Chicago, Chicago, IL 60607-7059, [email protected] "**Argonne National Laboratory, Materials Science Division, Argonne, IL 60439-4838 ***Department of Materials Science and Engineering, North Carolina State University, Raleigh, NC 27695-7907 ABSTRACT In this paper we investigate the microstructural accommodation of nonstoichiometry in (BaSr,_X)Ti,+,O 3,Z thin films grown by chemical vapor deposition. Films with three different (Ba+Sr)/Ti ratios of 49/51 (y=0.04 in the notation of the formula above), of 48/52 (y = 0.08) and of 46.5/53.5 (y=0.15), were studied. High-resolution electron microscopy is used to study the microstructure of the BST films. High-spatial resolution electron energy-loss spectroscopy (EELS) is used to reveal changes in chemistry and local atomic environment both at grain boundaries and within grains as a function of titanium excess. We find an amorphous phase at the grain boundaries and grain boundary segregation of excess titanium in the samples with y=0.15. In addition, EELS is also used to show that excess titanium is being partially accommodated in the grain interior. Implications for the film electrical and dielectric properties are outlined. INTRODUCTION Thin films of (Ba•Srl-,)TiyO 3+1 (BST) are potential candidates as high permittivity dielectrics for very large scale integrated capacitor applications [1]. Therefore, many investigators have recently characterized the properties of these films, including dielectric response [21, leakage [31, and failure modes [4, 51, as well as the dependence of these properties on composition, film thickness, temperature, etc.. It has been found that the (Ba+Sr)/Ti ratio (y in the notation used above) strongly affects most film properties at a given ratio of Ba/Sr (i.e. x) and deposition temperature [6, 7, 8, 91. Therefore, the (Ba+Sr)/Ti ratio is one of the primary parameters used to date to control the dielectric and electrical behavior of the films. BST films are routinely grown by chemical vapor deposition with severely nonstoichiometric (Ba+Sr)/Ti ratios, so as to achieve acceptable leakage currents and lifetime. For example, BST films with x=0.7 have a maximum resistance degradation lifetime at approximately (Ba+Sr)/Ti = 48/52 [4], corresponding to y=0.083, although the maximum value of the dielectric constant is found at y=O 18]. Reasonable film behavior is generally achieved up to ratios of 46.5/53.5 or y=0.15, which greatly exceeds the solubility of excess Ti in bulk BST, of approximately y_
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