Cee® Apogee™ 300 Bake Plate Manual
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4.2. The Skin Effect
Another disadvantage in normal oven baking results from baking
substrates from
the “outside in”. Since heat is applied to the outer
surface of the film first, a skin forms on the surface of the film thus
trapping solvents. Upon vaporizing, these solvents form blisters or
bubbles which results in adhesion loss or even bulk film failure. This
problem prevails in processes involving thick film resins, e.g. polyimides.
No skin effect occurs on a hotplate since hotplate baking heats the
substrate from the bottom up. This “inside out” approach offers
advantages for thick films since solvents in the film nearest the substrate
are baked off before the film surface seals over.
4.3. Hotplate Bake Variables and Methods
A typical bake process consists of preheating the surface to a known temperature, loading the
substrate onto the surface for a specific length of time and removing it promptly at the end of the
cycle. The selection of the temperature and time values used as well as the bake method
employed all affect the overall performance of the process.
4.4. Bake Temperature
The bake temperature used is dependent on several factors. The material and substrate being
baked as well as the results desired are key factors to be considered in developing a bake
process.
In general, hotplate baking will be performed at temperatures slightly higher than those used in
oven bake processes. The film being baked will reach a temperature somewhere between the
temperature of the hotplate and the ambient air above the film. As an example, with a hotplate
surface temperature of 115°C, a layer of photoresist on a silicon wafer will reach a final
temperature of about 105°C after a few seconds. Thicker substrates and/or substrates with lower
coefficients of thermal conductivity will require even higher temperatures to compensate for this
phenomenon.
