Reduction in edge beading defects using two-axis spin coating technology: an analytical, numerical, and experimental study.

Spin coating is used by several industries to apply thin but uniform liquid films to a given substrate. While this method produces very uniform coatings over much of the substrate, near the periphery of the substrate, defects in the coating, sometimes referred to as edge beading, often occur. This paper explores the usage of a two-axis spin coating technology that leads to a more uniform film thickness near the edge of the substrate. By rotating the substrate on an axis parallel to the substrate, centrifugal forces are generated that act both normal and parallel to the substrate. These forces are known as elevated gravity and act to even out any existing coating irregularities and also to drain liquid toward the edge of the substrate. This study investigates the effect of elevated gravity on the coating dynamics of a photoresist liquid film on a circular silicon wafer. Using lubrication theory, we derive a modified evolution equation that includes the effects of elevated gravity. We solve this equation numerically using implicit finite differences. Four layers of a photoresist film are spin coated on 3-inch circular wafers using a two-axes spin technology under various elevated gravity conditions. The thickness of the dried coating layers near the periphery of the substrate is measured using a SEM device. Both the experiments and the simulations show that edge beading defects can be substantially reduced for the 500g elevated gravity case compared to the normal 1g gravity case. [ABSTRACT FROM AUTHOR]