Mechanisms of Post-CMP Cleaning
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Mechanisms of Post-CMP Cleaning H. Liang, E. Estragnat, and J. Lee, University of Alaska Fairbanks; K. Bahten and D. McMullen, Rippey Corporation Abstract We investigate the post-CMP cleaning process with a tribological approach. A cleaning process involves three components: brush, wafer/disk, fluid undergoing three-body sliding contact between the brush, wafer, and particles from slurry. Having this in mind, we investigated cleaning mechanisms through experimental measurement of friction force and analyzed the contact condition for particle removal. Our investigation leads to the conclusions that the cleaning process is a boundary to elastohydrodynamic lubricating process that involves a constant contact between a brush and the wafer or disk surface. The motion of the brush nodule is such that the surface forces between the brush and workpiece change from an initial adhesion to sliding abrasion. These analysis leads to insight of particle removal mechanisms. Introduction One of the major defects after CMP is residual particles. The submicron and smaller particles cause catrostropic failure for memory disks and for ever-smaller sized IC chips. Any particles left behind can have a significant effect on the outcomes of downstrem process steps. These include creating imerfections ranging from bumps or pits to regions of excessive electrical resistance. The post-CMP cleaning is an important process in removing these particles. This process is usually accomplished with a deionized-water or dilute cleaning agent rinse. This process involves mechanical action by contacting one or more rotating roller brushes, while the wafer or disk itself is rotated and sprayed. In order to develop effective cleaning processes to meet the stringent requirements of the fabrication of advanced integrated circuits, and information stroage disketts, it is an essential to understand the mechanisms of particle removal. Previous studies have shown that the particles adhere to a surface primarily by Van der Walls forces, electrostatic attraction, or capillary action 1. The cleaning process is a hydrodynamic lubrication one. The thickness of the hydrodynamic fluid layer, as estimated, was around 3.7 1 P . On the contrary, numerical analysis concluded that the lift force in the hydrodynamic boundary layer of fluid is too small to lift particles off the surface2. The possible removal force is likely from the drag force between the brush and the wafer surface. Major research areas in cleaning include comparing the influence of the design of the brush on the efficiency of cleaning, the influence of chemistry on the cleaning effectiveness3 4 5 6 7 8 or the influence of the chemical properties of the solution used to soak the brush on the physical properties of the brush9. Although the post-CMP cleaning has been used widely as the standard manufacturing process, its mechamisms are not well understood. Obviously, a post-CMP cleaning process is a tribological process that involves three bodies rubbing against each other. We decide to take this approach in
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