Pt/Ti/SiO 2 /Si substrates
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L. M. Casas Army Research Laboratory, Electronic and Power Sources Directorate, AMSRL-EP-EC-M, Fort Monmouth, New Jersey 07703-5601 (Received 6 April 1994; accepted 3 March 1995)
Pt/Ti/SiO 2 /Si structures have been studied to investigate the structural, chemical, and microstructural changes that occur during annealing. Grain growth of the as-deposited Pt columns was observed after annealing at 650 °C, and extensive changes in the Pt microstructure were apparent following a 750 °C anneal for 20 min. In addition, two types of defects were identified on the surfaces of annealed substrates. Defect formation was retarded when the surface was covered with a ferroelectric film. Concurrent with the annealing-induced Pt microstructure changes, Ti from the adhesion layer between the Pt and the SiO2 migrated into the Pt layer and oxidized. It was shown with spectroscopic ellipsometry and Auger electron spectroscopy that for long annealing times, the titanium oxide layer can reach the Pt surface. Consequently, at the processing temperatures utilized in preparing many ferroelectric thin films, the substrate is not completely inert or immobile. The changes associated with Ti migration could be especially problematic in techniques that require the substrate to be heated prior to film deposition.
I. INTRODUCTION The development of ferroelectric devices that are integrated with Si based technology has produced a need for electrode materials that also act as reaction barrier layers during the processing of the ferroelectric material. This need for electrically conductive barrier layers is particularly evident in the systems that employ Pb-based perovskites since processes that form a fully crystallized ferroelectric phase require temperatures above 500 °C, and PbO and Si react at temperatures as low as 500 °C to form lead silicate phases. 1 ^ Not only do the PbO and Si react to form a lead silicate phase, but pores are also formed at the film/substrate interface, which causes deformation of the film surface and an increase in the surface roughness. This type of reaction leaches Pb from the ferroelectric and will severely degrade the observed electrical properties. While Pt does not react with Pb-based perovskites over the temperature range of interest for the processing of ferroelectric films, it cannot be used directly on Si as a barrier layer electrode because of its own reactivity with Si at temperatures as low as 400 °C.5'6 The resultant diffusion of Si into the Pt to form a platinum silicide causes a loss in the integrity of the Pt layer when Si reaches the surface of the Pt. A breach in the Pt reaction barrier layer allows the diffusion of Si into the
a)Present
address: Laboratoire de Ceramique, Ecole Polytechnique Federale de Lausanne, Lausanne, Switzerland, CH-1015.
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J. Mater. Res., Vol. 10, No. 6, Jun 1995
http://journals.cambridge.org
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ferroelectric film and also allows the diffusion of Pb into the underlying Si substrates.7 Because of the difficulties encountered with bareSi and Pt-coa
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