Low-K Porous Spin-On-Glass
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ABSTRACT Previously, the fabrication of air-gap structures for electrical interconnections was demonstrated using a sacrificial polymer encapsulated in conventional dielectric materials. The air-gaps were formed by thermally decomposing the sacrificial polymer and allowing the by-products to diffuse through the encapsulating dielectric. The diffusivity of the polymer decomposition products is adequate at elevated temperatures to allow the formation of air-gaps. This process was extended to form low dielectric constant, porous silica from commercially available methylsilsesquioxane (MSQ) by the addition of the sacrificial polymer to the MSQ. The porous MSQ film was thermally cured followed by decomposition of the NB at temperatures above 400'C. The dielectric constant of the MSQ was lowered from 2.7 to 2.3 by creating 70 nm pores in the MSQ. The voids created in the MSQ appeared to exhibit a closed-pore structure. INTRODUCTION
The development of low dielectric constant materials is critical to the fabrication of high-speed electrical interconnection for integrated circuits and electronic packaging. Methylsilsesquioxane (MSQ), CH 3SiOl 5, is a spin-onpolymer with a ladder structure formed by the hydrolysis of methyltriethoxysilane." 2 Spin-on-glasses (SOG) belonging to the methyl siloxane family are also available; however, their dielectric constants are sometimes not as low as MSQ. Cross linking (curing) takes place through reaction of the alkoxy silyl groups on the oligomers. 3-5 Cured MSQ films have low moisture content and show little sensitivity to humidity over the range of 40% to 80% relative humidity. Recently, it has been shown that air-gaps can be fabricated by using a family of polynorbornenes as sacrificial materials.6 The polynorbornene was patterned and encapsulated in plasma deposited silicon dioxide. The polynorbornene thermally decomposed at 400'C to 425'C, and the decomposition products permeated through the encapsulating silicon dioxide. Derivatives of norbornene polymer containing triethoxysilyl groups (NB) are particularly useful, just as in the case of MSQ and HSQ, because of the reactivity of the alkoxy silyl
55 Mat. Res. Soc. Symp. Proc. Vol. 565 ©1999 Materials Research Society
groups, especially for adhesion to metal and oxide surfaces and cross linking with other alkoxy silyl groups. In this study, we used the air-gap structure to investigate the decomposition of the sacrificial polymer followed by its use in the formation of nanoporous silsesquioxane foams. The foams were created by mixing the sacrificial polymer with commercially available spin-on-glasses (SOG) followed by curing of the SOG and decomposition of the polymer. Critical properties of the polymer include: complete decomposition (so as to leave clean cavities), high decomposition temperature (so that the SOG can be cured prior to decomposition), high glass transition temperature of the polymer (so as not to flow), and reactivity between the polymer and the SOG (so as to chemically attach the polymer
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