Repair of Porous Methylsilsesquioxane Films using Supercritical Carbon Dioxide
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Repair of Porous Methylsilsesquioxane Films using Supercritical Carbon Dioxide Bo Xie and Anthony J. Muscat1 Department of Chemical & Environmental Engineering University of Arizona, Tucson, AZ 85721, U.S.A. 1 email: [email protected] ABSTRACT Porous methylsilsesquioxane (p-MSQ) films (JSR LKD 5109) were treated with alkyldimethylmonochlorosilanes having chain lengths of one, four, and eight carbon atoms dissolved in supercritical carbon dioxide at 150-300 atm and 50-60°C to repair oxygen ashing damage. Fourier transform infrared (FTIR) spectroscopy showed that trimethylchlorosilane (TMCS), butyldimethylchlorosilane (BDMCS), and octyldimethylchlorosilane (ODMCS) reacted with silanol groups on the surfaces of the pores producing covalent Si-O-Si bonds. Selfcondensation between alkylsilanols produced a residue on the surface, which was partially removed using a pure scCO2 rinse. The hydrophobicity of the blanket p-MSQ surface was recovered after silylation treatment as shown by contact angles >85°. The initial dielectric constant of 2.4 ± 0.1 increased to 3.5 ± 0.1 after oxygen plasma ashing and was reduced to 2.6 ± 0.1 by TMCS, 2.8 ± 0.1 by BDMCS, and 3.2 by ODMCS. INTRODUCTION Porosity reduces the dielectric constant of insulating thin films below 2.4, which is needed in microelectronic devices for the 45 nm technology node and beyond to lower power consumption and minimize cross talk between copper metal lines. Pores not only compromise the structural integrity but also expose the interior of the film to the processing environment. Plasma ashing using O2 and N2/H2 used to pattern Si-based low-k films induces damage in the form of silanol (SiOH) groups [1]. The silanols groups must be removed to reduce k and the pores of the films must be sealed to prevent contamination from subsequent processes. The goal of this study was to investigate the reactivity of a series of monochlorosilanes with surface silanol groups on porous methylsilsesquioxane (p-MSQ). The silanes were dissolved in supercritical carbon dioxide (scCO2) because of the processing advantages over conventional solvents. The CO2 and unreacted silanes or by-products are easily separated by reducing pressure lowering the environmental burden of processing. A wide range of silanes can be dissolved in scCO2. Supercritical CO2 has no surface tension so can wet any surface and is nonaqueous [2]. EXPERIMENTAL The details of the experimental setup have been described previously [3]. P-type (100) orientated Si wafers with a minimum resistivity of 0.5 ohm·cm containing as deposited and O2 ashed blanket p-MSQ films (JSR LKD 5109) were supplied by International Sematech. The silicon wafers were cleaved into 1.5 x 1.5 cm2 pieces for processing and analyzed with FTIR. The chemistries chosen for this study were TMCS [ CH 3 Si ( CH 3 )2 Cl ] (99+%), BDMCS
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[ CH 3 (CH 2 )3 Si (CH 3 ) 2 Cl ] (98%), and ODMCS [ CH 3 (CH 2 ) 7 Si(CH 3 ) 2 Cl ](97%). All chemicals were purchased from Sigma-Aldrich Co. and used as received. Samples were placed against the
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