In this study the effects of oversize particle contamination in chemical mechanical polishing (CMP) slurries were investigated on the silica CMP process. The limits of light scattering technique were established in detecting coarse particles in a commercial silica CMP slurry using two different methods. The detection limits were set by observing the shift in particle size distribution curve or by the appearance of an additional peak in the particle size distribution curve of the baseline slurry when a known amount of coarser particles were added to it. Simultaneously, polishing tests were conducted by spiking the base slurry with coarser sol-gel silica particles at the established detection limits. It was observed that the contamination of larger particles not only created surface damage but also changed the material removal rate. The mechanism of polishing in the presence of larger size particles is discussed as a function of particle size and concentration.
The effects of alkaline ionic salts on silica chemical mechanical polishing ͑CMP͒ 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 regime͒. 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 (R max ) 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.Chemical mechanical polishing ͑CMP͒ has become the prime technique for achieving both local and global planarization in ultralarge-scale integrated ͑ULSI͒ circuits. [1][2][3][4] The recent advent of CMP as a major process technology has had a significant impact on both the development of more sophisticated processing tools and the formulation of novel slurries to further enhance polishing performance. 5 Optimum removal rate and surface finish are achieved through synergy of chemical and mechanical interactions at the padparticles-wafer interface. The near surface region of the silica wafer is modified by diffusion of water to form a chemically modified layer on the silica surface on which polishing is achieved by further indentation of particles. 6,7 Most of the knowledge in this field is based on previous studies on glass polishing. In addition, CMP processes have been designed empirically, with little scientific understanding of the underlying phenomena.Most metal oxides dispersed in water have a surface layer of the metal hydroxide which is amphoteric in nature and can become either positively or negatively charged depending on the pH, based on the following mechanism 8In the absence of surfactants or polymers, the stability of a dilute oxide suspension is maintained by electrostatic repulsion forces due to the surface charge of particles. The electrostatic repulsion force can be reduced by ionic screening or counterions which are held in an interfacial layer surrounding the particle. If this screening is sufficient, particles having necessary kinetic energy easily overcome this repulsive barrier and physically bond as a result of van der Waals attractive forces. Thus, at a given pH, one can destabilize most dilute oxide suspensions by increasing ionic strength using an inert electrolyte. 9 All CMP polishing slurries are composed of chemical additives...
Lateral frictional force measurement provides an understanding of pad-surface interactions in chemical mechanical polishing. The lateral forces have been found to be significantly dependent on the surface roughness of the silica, thus showing that in situ dynamic changes in the surface roughness could be observed. Studies of the effect of pH and ionic strength on polishing characteristics of silica are presented. The studies were conducted with particle-free slurries, so as to obtain a better signal-to-noise ratio. The rate of material removal as a function of pH and ionic strength was determined and correlated with the friction force measurements.
In this study, we have characterized the effects of abrasive properties, primarily particle size, on the Chemical Mechanical Polishing (CMP) of oxide films. Sol-gel silica particles with very narrow size distributions were used for preparing the polishing slurries. The results indicate that as particle size increases, there is a transition in the mechanism of material removal from a surface area based mechanism to an indentation-based mechanism. In addition, the surface morphology of the polished samples was characterized, with the results showing that particles larger than 0.5 μm are detrimental to the quality of the SiO2 surface.
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