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The topography of a back-end wafer contains high-aspect vias that are unevenly distributed on a wafer with pitches ranging from 1:1 dense to isolated. The difference in via density causes thickness bias across the wafer, which causes the shift of resist depth of focus and critical dimension swing. The bottom anti-reflective coating (BARC) topography is formed basically after spin coating and reshaped through thermal reflowing when the coating is baked at high temperature to cure the film. In this paper, we discuss the thicknesess at isolated and dense via patterned areas in the spin-coating process and the baking process. Spin coating is a method commonly used to cast BARC and gap-fill materials on silicon wafers. The first stage of spin coating is the deposition of the coating liquid onto the wafer. The second stage is when the substrate is accelerated up to its final, desired rotation speed. The third stage is when the substrate is spinning at a constant rate and fluid viscous forces dominate the fluid thinning behavior. The fourth stage is when the substrate is spinning at a constant rate and solvent evaporation dominates the coating thinning behavior. Viscous flow and solvent evaporation occur throughout all stages. After spin coating, high-temperature baking cures the film and makes it insoluble in organic solvents that are used in the next spin-coating step. However, this step also gives the polymer a chance to undergo thermal flow because the bake temperature is much higher than the polymer's glass transition temperature (Tg). A chemical reaction occurs at the same time to form a polymer network that stops the thermal flow. The study of how far material can flow before the gel point of the polymer has been reached is presented in this paper. The calculation of the material transformation due to solvent evaporation and polymer rheometry is based on dynamic and kinetic effects. The work described in this paper showed how the parameters related to material properties and to the chemical reaction of crosslinking affected iso/dense bias. This work gives us guidance regarding what kind of materials and process conditions are good for planarization.
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