The Effects of pH Adjustors in Post Cu CMP Cleaning Solutions on Particle Adhesion and Removal

Cu has been widely accepted as an interconnection material in deep sub-micron multi-level device application because of its benefits such as the lower resistance, superior resistance to electro-migration and reducing RC time delay compared with aluminum [1]. The Cu interconnection formation is only possible by a novel damascene Chemical Mechanical Planarization (CMP) process. During Cu CMP, wafer surfaces are to be exposed to at least two different slurry solutions. After polishing, the removal of particles and trace metals remained on the wafer surfaces has become a great challenge since various hydrophobic low k dielectric materials are integrated with copper. Conventional HF and NH4OH cleaning chemistry do not work for Cu/low k materials. New cleaning chemistry should be designed to remove particle and metals with the prevention of watermarks on hydrophobic low k materials. Although we have studied the effect of pH on removal and adhesion of silica particles in slurry solutions [2], no study has yet reported on the effects of pH adjustors cleaning solutions on removal and adhesion of particles. The purpose of this study was to investigate the effect of different pH adjustors in post Cu CMP cleaning solutions on the adhesion and removal of alumina particles on Cu surfaces. Since the least adhesion force between abrasive particles and wafer surfaces is required for reducing the particle contamination, the adhesion force was measured in Cu cleaning solutions with various pH adjustors. All force curve measurements were carried out using an Atomic Force Microscope (AFM, PSIA, XE-100, Korea). AFM can determine the adhesion of individual particles by directly measuring the force required to remove them from a surface. The 40 μm diameter sized alumina particle (α phase, Micron Co., Japan) attached to a tipless silicon cantilever (MikroMasch, USA). The adhesion force was measured between particles and wafer surfaces in a liquid cell [3]. In the zeta potential measurements, γ (Degussa, 30nm) and α alumina (Buehler, 0.3μm) particle were used. For preparation of post Cu CMP cleaning solutions, various amine group additives were chosen as pH adjustors. In order to investigate the surface charge of α and γ alumina particle, the zeta potentials of the alumina particles were measured as a function of pH in 10M KCl solutions as shown in Fig. 1. Spherical alumina particles have α phase even though commercial Cu CMP slurries use γ phase alumina abrasive particle which their shape and size of the γ alumina particle are irregular. The zeta potentials of γ and α alumina particles were measured and found out to have identical behavior each other as a function of pH. They had the isoelectric points (IEP) at around pH 9.3. It justifies the use of spherical α alumina instead of γ alumina particle to measure the particle adhesion force. Fig. 2 shows the zeta potentials of γ alumina particle as a function of pH with addition of TMAH (tetramethylammonium hydroxide). TMAH was used as one of the pH adjustors. The interaction forces between particles and wafer surfaces were calculated based on the Derjaguin-LandauVerwey-Overbeek (DLVO) theory [4]. The cleaning efficiency of cleaning solutions was evaluated and related to the magnitude of adhesion forces in these cleaning solutions with different pH adjustors.