Enhanced Polymer Passivation Layer for Wafer Level Chip Scale ...

Cyclopentanone, a colorless liquid organic compound, is usually used in the production of
synthetic resins. As one of the most popular and stable solvents for SolderBrace material, it dries
slowly giving a nice uniform film after spin coating and soft baking. But drying slowly also
means longer drying times prior to UV exposure. Furthermore, cyclopenanone is considered an
organic solvent by regulatory agencies around the world, and while it can be used, it is better to
use a more environmentally friendly solvent. Acetone, another ordinary chemical, was also
selected as a possible solvent. However, it dried too quickly in air to have a stable coating
surface. Butylacetate, also known as butyl ethanoate, is capable of forming a uniform and
mirror-like surface. But, it requires much a longer curing time to get the surface dry for UV
exposure since the coating is much thicker than those formed by cyclopentanone and acetone.
SolderBrace materials with various viscosities and die adhesion characteristics were tested
and optimized to achieve dry coating and no delamination after developing. The lower the
viscosity, the faster the silica filler settled; the higher the viscosity, the thicker the film without
complete drying. Proper viscosity was the starting point to obtain a smooth and dry coating for
the rest of the lithography process. Adhesion, on the other hand, plays a crucial role in the final
developing step. Figure 3.5 shows delamination caused by poor adhesion between the wafer
surface and SolderBrace material.
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