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

layer, is investigated as a potential improvement of intermetallics formation with lead-
free solders especially under high temperature conditions. Formation of passivation layer:
This layer can serve to greatly reduce the transport of corrosive species to the underlying
metal's surface. It also serves as a solder mask to prevent flow of solder along the
interconnect traces. For die A, a thin layer of polyimide was used as the dielectric layer.
The detail process steps are described as follows:
- Polyimide adhesion promoter application: After the spin dry and dehydration bake, a
layer of VN651 Polyimide Promoter was spin coated at 3000 rpm for 30 seconds on
the wafer. The bonding of the polyimide promoter to the substrate was achieved
during the 1 min soft bake at 120 o C as the priming chemistry is activated by
temperature.
- HD PI2556 polyimide coating: A layer of 0.7μm polyimide was spin coated at 3500
rpm for 30 seconds, and soft baked at 120 o C oven for 5-10 min.
- Photolithographic process: step 3 to step 7 were repeated to form a layer of
photoresist which exposed each bump pad. The photoresist as well as the polyimide
coated on the bump pads was removed with AZ 400K during developing.
- Acetone was sprayed onto the wafer to strip off the remaining photoresist layer.
- Finally, the whole wafer was placed into a programmable oven to cure the polyimide
film. The cure heating cycle converts the polyamic acid to the insoluble imide form
and drives out remaining solvents. The curing profile consisted of :
Heating from room temperature to 200 o C, ramp 4 o C /minute in air
Hold time at 200 o C for 30 minutes in air
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