Preparation and Characterization of High Porosity Si0 2 Xerogels for Low k Dielectrics
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ABSTRACT Due to its low dielectric constant, high porosity SiO 2 is a potential intermetal dielectric (IMD) film for sub-half micron devices. High porosity SiO 2 xerogels with a low dielectric constant were prepared by a sol-gel method. Basic and mixtures of SiO 2 sols (acid/base) aqueous particulate suspensions were employed using TEOS as a precursor. Porosity values up to 55% could be obtained for the silica xerogels calcined at 250'C. The porosity values were estimated by means of nitrogen adsorption (BET). The dielectric constant was directly measured using an impedance analyzer and also calculated from the nitrogen adsorption measurements. Both techniques were in good agreement and resulted in a dielectric constant value of 2.32 for xerogels having the highest porosity. INTRODUCTION Low k (i.e., low dielectric constant) materials, which substitute to the conventional IMD have become imperative for the reduction of parasitic capacitance between multi-metal layers. Solgel derived porous SiO 2 thin films have great potential as IMD, because a very low dielectric constant can be achieved from its porosity. In particular, one of the porous SiO 2 gels, aerogels, has extremely high porosity, up to 95%, and its dielectric constant is close to 1 [1-5]. However, the mechanical properties are limited by a supercritical drying procedure. Furthermore, this process is very energy intensive, often dangerous, and not easily adaptable to continuous thin films forming operations. On the other hand, most porous materials are incompatible with microelectronics since pore sizes must be at least one order of magnitude smaller than microelectronics features. Aerogels have usually pore sizes in the same order as microelectronics feature sizes. Instead of aerogels, xerogels with intermediary porosity could be used as dielectrics for electronic applications. So the high cost of a supercritical drying process can be avoided and the
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211 Mat. Res. Soc. Symp. Proc. Vol. 565 ©1999 Materials Research Society
films can possess appropriate mechanical strengh for device fabrication. Hong et al. [6] prepared SiO 2 xerogel thin films with a low dielectric constant by a two step acid-base catalyst procedure and successive surface modification with trimethylchlorosilane (TMCS). Only 15% porosity was achieved without surface modification but with that modification the porosity increased to 50%. They do not mention the values of the pore sizes. These porosity values correspond to measured dielectric constant of 3.95 and 2.45, respectively. On the other hand, the organic polymers have dielectric constants typically lower than 3.0 and can be as low as 1.9. However, their thermal stability is mostly beneath 400'C. This is lower than the temperature (450'C) used by most semiconductor manufacturers during post-metallization sintering [7]. Although the maximum temperature used in device f
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