Prevention of Metal Contamination in Sub 50 Nm SC1 Cleaning Process

The RCA cleaning process is a standard wet cleaning process for removal of contaminants from silicon wafer surface. As well known, the SC1 cleaning solution which consists of a mixture of NH₄OH (ammonium hydroxide), H₂O₂ (hydrogen peroxide) and H₂O is an efficient particle removal. Cleaning mechanism of SC1 cleaning solution on particles removal from silicon surface is slightly surface etching and lift off the particles from the Si surface by electrostatic repulsive force between particles and surface. SC1 cleaning process may etch a few angstrom (Å) level of silicon surface. However, metal contaminant which has a high redox potential is critical when bare silicon is exposed. Metal deposition on Si surface is induced by the oxidation reduction reaction between metal ion and Si surface. Metal ion can take electrons from Si surface and are reduced to become metal. Si surface beneath the metal particles is oxidized to become SiO₂ and is etched away by OH^- in SC1 solution. The metal pit is a form of localized etching that leads to the creation of small hole under the metal. Thus, the etching has a crucial effect on the formation of defects and removal of particles. The etch rate of Si increased with NH₄OH concentration whereas it decreased with H₂O₂ concentration. This indicates that metal pitting increased with NH₄OH concentration while H2O2 inhibits it. However, particle removal efficiency (PRE) decreased in high H₂O₂concentration. Particle removal is affected by etch rate and pH of cleaning solution. For high pH value in the SC1 solution, particles take on a strongly negative charge. The particles are repelled from a Si surface. This electrostatic interaction prevents recontamination by particles to Si surface. Therefore, it is important to optimize SC-1 cleaning process in Si surface cleaning process.

In this study, we investigated the relationship between PRE, surface etch rate and removal of metal contaminant with various concentrations of SC1 cleaning solution. The thermally oxidized 200 mm p-type Si (100) wafers were used. To identify relationship between etch rate and formation of defect, etch depth was measured for various composition of SC-1 solution by using atomic force microscopy. The Cu ions were used as a source of metal pitting to making Cu precipitate on Si surface. Si wafers were contaminated with Cu ions and particles by dipping method. Cu precipitate and particles on Si surface were counted using atomic force microscopy and a dark field optical microscope to confirm cleaning efficiency of SC1 solution. Etching of Si wafer surface in SC1 solution was evaluated as a function of composition of SC1 solution. It was shown that Cu precipitates decreased with H₂O₂ concentration in SC1 solution. But PRE also decreased. It was suggested that H₂O₂ reduced the formation of Cu precipitates and PRE was dependent of NH₄OH concentration. This study proposes cleaning process to minimize Cu precipitates with a high PRE in the optimized SC1 solution.