The Mechanism of Low pH Silica Based Oxide Slurries
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1249-E04-07
The Mechanism of Low pH Silica Based Oxide Slurries Michael L. White, Lamon Jones and Richard Romine Cabot Microelectronic Corp. 870 N. Commons Dr. Aurora, Il 60504
Abstract The mechanism of oxide polishing at low pH in the presence of an organic cation is discussed. The role of the cation is thought to involve increasing the nucleophilicity of the silanolate active site on the particle surface by lowering the hydration state. Additionally, the activation energy of the reaction may be lowered by charge attraction between the particle and wafer surface and by increased hydrophobic interactions. The use of high pH oxide slurries such as Semi-Sperse® SS25 to polish various forms of silicon dioxide wafers at high rates has been prevalent for many years. However, typically such an approach uses fumed silica at silica concentrations exceeding 10%. Much lower removal rates are obtained with colloidal silica at similar concentrations. For instance, fumed silica yields a removal rate of 2700 Å/min on TEOS while colloidal silica has a removal rate of 410 Å/min at pH 12. At a pH less than 7, the removal rate for colloidal silica is a mere 90 Å/min. However, the addition of cationic rate additives can increase the TEOS removal rate to 3000 Å/min at the same CMP process and at less than half the solids of fumed silica based slurries while maintaining low defectivity (DCN/DCO < 50 at 225 nm). This represents a removal rate of over 30 times that of colloidal silica without the rate additives. The mechanism of the rate acceleration is believed to involve increased silanolate nucleophilicity in the presence of the organic cation and the inversion of the negative charge on the particle surface yielding a coulombic attraction between the particle and the wafer, thus lowering the activation energy and dramatically increasing the removal rate. This mechanism is supported by zeta potential titrations on the particle and streaming potential measurements on the charge on the wafer surface.
Introduction
Experimental 4” and 8” diameter, TEOS, BPSG, thermal oxide, silicon nitride and silicon wafers were polished on a IPEC Planar 472, a Logitech IC M51 or a Mirra 3400 at various down forces, table speeds and slurry flow rates. Removal rates were determined by ellipsometry on a Filmetrics F50 or a KLA-Tencor UV-1050. pH was measured with an Accumet Ap61 meter. Formulations were prepared by mixing colloidal silica under high shear with the appropriate chemical additives. Defectivity (DCO and DCN) were measured via a KLA-Tencor SP1 TBI. Zeta potential was measured on a Dispersion Technologies DT-1200. Particle sizes were determined by a Malvern Zetasizer 3000HSA in a 1 mM NaCl buffer solution. Streaming potential was measured on an Anton Paar EKS100.
Discussion A number of organic cations were studied in an attempt to increase the removal rate of colloidal silica at pH’s lower than 7. We have found that primary, secondary and tertiary amines and quaternary ammonium along with phosphonium salts can be used as rate accelerants. In
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