Damage by radicals and photons during plasma cleaning of porous low-k SiOCH. II. Water uptake and change in dielectric constant

Abstract

Porous dielectric materials provide lower capacitances that reduce RC time delays in integrated circuits. Typical low-k materials include porous SiOCH—silicon dioxide with carbon groups, principally CH3, lining the pores. With a high porosity, internally connected pores provide pathways for reactive species to enter into the material. Fluorocarbon plasmas are often used to etch SiOCH, a process that leaves a fluorocarbon polymer on the surface that must later be removed. During cleaning using Ar/O2 or He/H2plasmas,reactions of radicals that diffuse into the SiOCH and photons that penetrate into the SiOCH can remove –CH3 groups. Due to its higher reactivity, cleaning with Ar/O2plasmas removes more –CH3 groups than He/H2 plasmas, and so produce more free radical sites, such as –SiO2• (a –SiO2–CH3 site with the –CH3 group removed).Upon exposure to humid air, these free radical sites can chemisorb H2O to form hydrophilic Si–OH which can further physisorb H2O through hydrogen bonding to form Si–OH(H2O). With the high dielectric constant of water, even a small percentage of water uptake can significantly increase the effective dielectric constant of SiOCH. In this paper, we report on results from a computational investigation of the cleaning of SiOCH using Ar/O2 or He/H2plasmas and subsequent exposure to humid air. The authors found that plasma cleaning with He/H2 mixtures produce less demethylation than cleaning with Ar/O2plasmas, as so results in less water uptake, and a smaller increase in dielectric constant. The water that produces the increase in dielectric constant is roughly half chemisorbed and half physisorbed, the latter of which can be removed with mild heating. Sealing the pores with NH3plasma treatment reduces water uptake and helps prevent the increase in dielectric constant.