Phase transformation and preferred orientation in carboxylate derived ZrO 2 thin films on silicon substates
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Phase transformation and preferred orientation in ZrO 2 thin films, deposited on S i ( l l l ) and Si(100) substrates, and prepared by heat treatment from carboxylate solution precursors were investigated. The deposited films were amorphous below 450 °C, transforming gradually to the tetragonal and monoclinic phases on heating. The monoclinic phase developed from the tetragonal phase displacively, and exhibited a strong (111) preferred orientation at temperature as low as 550 °C. The degree of preferred orientation and the tetragonal-to-monoclinic phase transformation were controlled by heating rate, soak temperature, and time. Interfacial diffusion into the film from the Si substrate was negligible at 700 °C and became significant only at 900 °C, but for films thicker than 0.5 /xm, overall preferred orientation exceeded 90%.
I. INTRODUCTION Zirconium oxide (ZrO2) films are of current interest due to such unique properties as high refractive index, high hardness, and low extinction coefficient. Thin films of ZrO 2 are durable and resistant to high temperatures and harsh environments. Uses involving optical, dielectric, mechanical, and refractory applications for these films have been reported in the literature.1^1 An important characteristic of ZrO 2 (including the film) is the wide range of the polymorphic transformations exhibited under various heat-treatment conditions. At room temperature, bulk ZrO 2 with large grain size has the monoclinic structure. On heating to 1100 °C, the monoclinic phase transforms to the tetragonal phase, which is stable up to ~2285 °C, before it transforms to the cubic phase. Significant improvement in mechanical properties has been achieved by the toughening associated with the monoclinic-to-tetragonal phase transformation in this material.5 The structure of the ZrO 2 film is very sensitive to processing conditions. Films exhibiting either epitaxial growth or polycrystalline structure, in the different phase modifications, have been prepared by sputtering, chemical vapor deposition, evaporation, and solution techniques.3'4'6^9 Since the properties of ZrO 2 films are dependent on the crystallographic phase, a basic understanding of the phase evolution, as related to processing conditions, is essential. The objectives of this work, therefore, were to investigate the polymorphic phase transformation and preferred orientation in ZrO 2 films deposited on Si wafers from carboxylate solution precursors. Solution deposition was employed since it offers stoichiometric accuracy, controlled processing, and low cost. Silicon wafers were used as substrates since they form the J. Mater. Res., Vol. 7, No. 11, Nov 1992
basis of the microelectronics industry. Processing variables investigated were heating rate, processing temperature/time, and film thickness. II. EXPERIMENTAL Zirconyl trimethylacetate was synthesized by reacting zirconyl nitrate with ammonium trimethylacetate and was then dissolved in a mixture of propionic acid and amylamine to prepare a stable precursor solution. The oxide content o
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