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

the nominal thickness of the resist near the center due to the dynamics of the spin coating
process. In order to achieve better contact between the photomask and the coating layer,
it was strongly recommended to remove the edge bead. Edge bead removal is performed
immediately after spin coat by directing a stream of acetone near the edge of the wafer
while it is spinning. Also, due to the opaqueness of this SolderBrace material, it was
necessary to scrape off some of the film along the outer edge of the wafer to expose some
features for accurate alignment.
5. Mask alignment and exposure: The mask (see Figure 3.8) was aligned to the first layer
pattern on the wafer. Once the mask was accurately aligned, the SolderBrace material
was exposed through the pattern on the mask with a high intensity ultraviolet light. 25
seconds continuous exposure time was used. If the sample were over exposed, the film
would have a very light pink/brown color after the post exposure bake step and the
apertures could not be completely developed out or the opening sizes would be very
small. If the sample was under exposure, it would cause a delamination problem due to
poor adhesion between the wafer and SolderBrace material, or the material residue could
flow back into the opening after development. These films were filled with silica, and the
light needed to "penetrate" through. Without a sufficiently strong light source, the light
would be reflected from the silic and the material underneath would not crosslink. The
fact that the resin flowed back suggested that the resin closest to the wafer was not cured.
If properly crosslinked, the film was a gel, unable to revert back to a liquid, regardless of
solvent or heat. In order to get the small features in the pattern, care had to be taken not
to over expose or under expose the material. Figure 3.9 is a depiction of different aperture
sizes obtained vs. exposure time.
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