Sol-Gel Derived Silica Layers for Low- k Dielectrics Applications
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Sol-Gel Derived Silica Layers for Low-k Dielectrics Applications Sylvie Acosta1, André Ayral1, Christian Guizard1, Charles Lecornec2, Gérard Passemard3 and Mehdi Moussavi2 1 LMPM, UMR CNRS 5635, ENSCM, 8, rue de l’Ecole Normale, F34296 Montpellier cedex 5, France 2 LETI/DMEL/TCI, CEA-Grenoble, 17, rue des Martyrs, F38054 Grenoble cedex 9, France 3 ST Microelectronics, Central R&D, CEA-Grenoble, 17, rue des Martyrs, F38054 Grenoble cedex 9, France ABSTRACT Porous silica exhibits attractive dielectric properties, which make it a potential candidate for use as insulator into interconnect structures. A new way of preparation of highly porous silica layers by the sol-gel route was investigated and is presented. The synthesis strategy was based on the use of common and low toxicity reagents and on the development of a simple process without gaseous ammonia post-treatment or supercritical drying step. Defect free layers were deposited by spin coating on 200 mm silicon wafers and characterized. Thin layers with a total porosity larger than 70% and an average pore size of 5 nm were produced. The dielectric constant measured under nitrogen flow on these highly porous layers is equal to ∼ 2.5, which can be compared to the value calculated from the measured porosity, ∼ 1.9. This difference is explained by the presence of water adsorbed on the hydrophilic surface of the unmodified silica.
INTRODUCTION The continuous decrease of the integrated circuit dimensions has led to prospect for new low dielectric-constant materials for interlayer dielectric application [1]. Among the various dielectrics, porous silica is particularly attractive for low -permittivity electronics applications [2]. Various studies were carried out in order to prepare such dielectric layers by the sol-gel route [26]. The larger the porosity, the lower is the dielectric constant. Aimed K values lower than 2 require a silica porosity larger than 70% [7].The porosity of the sol-gel derived silica thin layers depends strongly on the synthesis, deposition and drying conditions [8,9]. Various strategies were applied in order to promote the preparation of highly porous silica layers : sol deposition near the gelation point [2,3], supercritical drying of the layers [4], use of low volatility solvents [5] or of the templating effect by liquid crystal mesophases [6,10], additional ammonia postdeposition treatment [5]. The synthesis route investigated for this study is based on the use of common and low toxicity reagents and on the development of a simple process without gaseous ammonia posttreatment or supercritical drying step. The optimization of the synthesis parameters in order to obtain highly porous silica layers is first described. The layer characteristics are then described and discussed.
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EXPERIMENTAL The chemical products used to prepare the sols were common and low toxicity compounds: Tetraethoxysilane (Si(OC2H5)4 ) as silica precursor ; acidic (10 -2 M HCl) and basic (NH3) aqueous solutions to hydrolyze the silicon alkoxide. The molar ratio water
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