Wonseop Choi

Effect of Slurry Ionic Salts at Dielectric Silica CMP

The effects of alkaline ionic salts on silica chemical mechanical polishing ~CMP! have been studied. Particle size, zeta potential, and stability via turbidity tests have been characterized. Particle size and size distributions have been found to increase with ionic strength for three types of alkaline ionic salts due to the decrease in the magnitude of the zeta potential of silica slurry due to the addition of alkaline ionic salts. Slurry stability measured by turbidity tests showed two regimes of slurry stability ~i.e., stable regime and unstable regime!. For the stable slurry regime, the increase in ionic strength leads to an increase in friction force and material removal rate; however, for the unstable slurry regime, the addition of ionic salts results in a decrease in the measured friction force and material removal rate. Surface root-mean-square roughness and maximum depth of surface damage (Rmax) are shown to increase with particle size and size distribution. Investigation into the effect of ionic salts on the polishing mechanism reveals both a chemical and mechanical aspect to polishing silica wafers with silica slurries containing alkaline ionic salts.

Effects of Slurry Particles on Silicon Dioxide CMP

force during polishing. 14,15 This paper presents further validation of the polishing mechanism using sol-gel silica slurries. The effects of particle size and solids loading on friction force and surface finish were studied to delineate the dynamic contact of particles during polishing.

A Tribochemical Study of Ceria-Silica Interactions for CMP

Shallow trench isolation (STI) allows tighter device packing and reduced chip area for isolation. STI is critically dependent on the global planarity that is only possible using chemical mechanical polishing (CMP). Ceria-based slurries are considered the most promising candidates for STI CMP. Despite decades of use in glass polishing, the unique characteristics of ceria slurries are not well understood. In this study, we have conducted force measurements and tribological tests using an atomic force microscope (AFM) and a scanning electron microscope to investigate pH-dependent ceria-silica and silica-silica interactions that occur during CMP. Our studies confirm the effect of hydrolysis at high pH during silica-silica abrasion. An additional physicochemical contribution to ceria-silica polishing is identified and discussed. Furthermore, a strong correlation was observed between the AFM based studies and in situ friction force measurements during CMP.

Effect of pH on ceria-silica interactions during chemical mechanical polishing

To understand the ceria–silica chemical mechanical polishing (CMP) mechanisms, we studied the effect of ceria slurry pH on silica removal and surface morphology. Also, in situ friction force measurements were conducted. After polishing; atomic force microscopy, x-ray photoelectron spectroscopy, and scanning electron microscopy were used to quantify the extent of the particle–substrate interaction during CMP. Our results indicate the silica removal by ceria slurries is strongly pH dependent, with the maximum occurring near the isoelectric point of the ceria slurry.

pH and Down Load Effects on Silicon Dioxide Dielectric CMP

Understanding the pH and down pressure effects is critical in elucidating the chemical and mechanical mechanisms in chemical mechanical polishing (CMP). This paper describes the variation in polishing rate by nonagglomerated silicon dioxide particles. The repulsive interaction force, solubility of amorphous silica, and total contact area at the pad-particles-wafer interface are important factors in determining polishing performance. In situ friction force measurements are used to detect the variation of interfacial contact during polishing. Surface finishes and interaction force of silica/silica were investigated using atomic force microscopy.

Roles of Colloidal Silicon Dioxide Particles in Chemical Mechanical Polishing of Dielectric Silicon Dioxide

Chemical mechanical polishing (CMP) is carried out using slurry particles in contact with a wafer and a pad. The size and distribution of particles between the wafer and the pad play a crucial role in achieving desired CMP performance. Polishing rates and friction forces were measured as a function of particle size and solids loading, and surface finishes of silica wafers polished with colloidal silica particles were analyzed to validate the polishing mechanism. On the basis of polishing rate, friction force and surface finish, polishing occurring at the pad-particles-wafer interface was analyzed and an interfacial contact model was proposed. Understanding the polishing mechanism using colloidal particles makes it possible to achieve desired CMP performance.

Estimation of Fractional Surface Coverage of Particles for Oxide CMP

The surface coverage of particles between pad and wafer has been studied as a function of down load, particle size, and concentration in order to investigate oxide chemical mechanical polishing (CMP) mechanism. In situ friction force measurements have been conducted to investigate the interfacial contact behavior during polishing. In the absence of particles, friction force increased with increasing down load. In the presence of particles, friction force increased with an increase in down load and solids loading. An equation derived from Amontons second law was used to determine the fraction of abrasive particles that actually come into contact with the wafer. The fractional surface coverage of abrasive particles was independent of down load and increased with increasing solids loading and decreasing particle size. Determination of the amount of abrasive particles effectively involved in polishing will make clearer the oxide CMP polishing mechanisms improving slurry design leading to further optimization of the CMP process.

Effects of Particle Concentration in CMP

This paper reports on characterization of the surface coverage of particles by in-situ lateral friction force measurement during chemical mechanical polishing. The lateral friction force apparatus was made to operate close to real CMP conditions. For these experiments a sapphire wafer of constant surface roughness was used. For both 2psi and 4psi down force we observed increase in lateral friction forces with increasing solid loading. The lateral friction forces have been found to be significantly dependent on the contact area at the wafer-pad-slurry interface, thus showing that in-situ dynamic friction force changes in the surface coverage of particles. From these results, we conclude that the enhancement of frictional force is due to increased contact area at the wafer-pad-slurry interfaces. The lateral friction force measurement can provide an understanding of wafer-pad-slurry interactions.

Dynamic Contact Characteristics During Chemical Mechanical Polishing (CMP)

In chemical mechanical polishing (CMP), it is critical to understand dynamic contact at the pad-particles-wafer interface for desired CMP performance. The dynamic contact is dependent on process variables (platen velocity and down pressure) and particle characteristics (size and concentration), which in turn affect friction force. In this study, we have characterized the dynamic contact at the pad-particles-wafer interface as a function of platen velocity and down pressure. In situ lateral friction force measurements were carried out for silica slurry / sapphire wafer system in order to investigate the dynamic contact during polishing. As solids loading increases, the slope in the friction force vs. platen velocity curve changes from a negative to a positive value. Friction force increases with down pressure for different solids loading conditions. Consequently, friction force is determined as a function of down pressure and platen velocity, validating a dynamic contact mechanism during CMP.