Solvent diffusion in porous low- k dielectric films
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E9.11.1
Solvent diffusion in porous low-k dielectric films Denis Shamiryan and Karen Maex IMEC, Kapeldreef 75, Leuven, 3001, Belgium, also at Electrical Engineering department of K.U.Leuven ABSTRACT Porous materials are being investigated as low dielectric constant (low-k) materials. While porosity decreases the k-value of a material by decreasing its density, it simultaneously allows unwanted adsorption and diffusion of chemicals inside the porous matrix. To investigate this, different porous low-k materials, specifically silicon oxycarbide (SiOCH), methylsilsesquioxane (MSQ), and a polymer, were exposed to polar (ethanol) and non-polar (toluene) solvents. A difference in diffusion of polar and non-polar solvents would be an indication of the density of polar centers which attract polar molecules (such as water) and increase the dielectric constant of a film. The diffusion coefficient for toluene at room temperature was found to be approximately 2×10-5 cm2/sec for MSQ (40% porosity), 10-7 cm2/sec for SiOCH (7% porosity), 2×10-8 cm2/sec for the polymer. The observed diffusion can be described by a model of a viscous flow in a porous medium. The toluene/ethanol diffusion coefficient ratios were 4.4, 1.3, 1 for MSQ, SiOCH, and the polymer, respectively. The difference in toluene/ethanol diffusion can potentially be used to screen a material’s affinity for water adsorption. INTRODUCTION Resistance-capacitance (RC) delay in advanced interconnection must be reduced in order to improve device performance. After introduction of Cu, materials with a low dielectric constant k (low-k materials) [1] are evaluated as a replacement for SiO2. The k-value of a material can be decreased by decreasing its density which implies introduction of porosity. Being beneficial for the k-value reduction, porosity simultaneously allows unwanted adsorption and diffusion of chemicals inside the porous matrix. For example, the presence of water molecules inside a porous film significantly increases the k-value as water has a high k (about 80) due to high polarity of H2O molecules. Although the majority of low-k materials are hydrophobic, they could contain limited amount of water. Polar water molecules stick to polar centers inside a porous film. The amount of such centers might reflect the material’s affinity for water adsorption. Presence of polar centers inside a porous film results in different diffusion rates of polar and non-polar solvents through the film. This effect is used in chromatography [2,3] for separating mixture of unknown components. Non-polar solvents diffuse faster through a porous film than polar solvents, as movement of non-polar molecules is not affected by presence of polar centers in the porous medium. This observation can be applied to study porous films using diffusion of polar (ethanol in our case) and non-polar (toluene in our case) solvents. While chromatography uses diffusion of unknown solvents through known porous media to study solvents, we use diffusion of known solvents through unknown porous media (low-k fil
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