Fundamental Studies on the Mechanisms of Oxide CMP
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Fundamental Studies on the Mechanisms of Oxide CMP
Uday Mahajan, Seung-Mahn Lee and Rajiv K. Singh Department of Materials Science and Engineering and Engineering Research Center for Particle Science and Technology, University of Florida, Gainesville, FL 32611
ABSTRACT
In this paper, results of studies on the addition of salt to a polishing slurry, in terms of its effect on slurry stability, SiO2 polishing rate and surface roughness of the polished surface are presented. Three salts, viz. LiCl, NaCl and KCl were selected, and three concentrations were tested. Polishing rate measurements using these slurries show that adding salt leads to increased removal rate without affecting surface roughness significantly. Based on these results, we can say that the agglomerates formed by adding salt to the slurry are fairly soft and easily broken during the polishing process. In addition, turbidity and particle size measurements show that significant coagulation of the particles in the slurry occurs only at the highest salt concentration, and is fastest for LiCl and NaCl, with KCl showing the slowest coagulation. From these results, it can be concluded that the enhancement in polish rate is due to increased contact at the wafer-pad-slurry interface, and not due to formation of larger agglomerated particles in the slurry. This is because of reduced electrostatic repulsion between these three surfaces, due to the screening of their negative surface charge by the metal ions in solution, resulting in a higher wear rate.
INTRODUCTION
With the advent of multilevel metallization, Chemical Mechanical Planarization (CMP) has become one of the fastest developing areas in the microelectronics industry [1]. However, most of the knowledge in this field is based on previous studies on glass polishing [2]. In addition, a lot of CMP processes have been designed empirically, with little scientific understanding of the underlying phenomena. The effect of salt addition on polishing properties of slurries in particular is yet to be fully understood. Several salts are used in preparing slurries for CMP applications. They could be oxidizers, buffers or complexing agents. Addition of these salts is known to have an adverse effect on the stability of the slurry. This is chiefly due to the screening of the surface charge on the abrasive particles, which causes a compression of the electrical double layer around them. This results in a reduced repulsion between the particles, causing them to aggregate and settle in the slurry [3]. This settling is an undesirable phenomenon, as over a period of time these aggregates become hard to redisperse. These hard agglomerates can often damage the wafer surface, causing scratches and thereby reducing process yield. Although agglomeration of particles in high ionic strength environments has been extensively studied, a systematic investigation and correlation to polishing characteristics has not yet been carried out. In this study, we have reported the effect of addition of alkali metal salts (chlorides) on the
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