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Question:
I have heard that it is better to cure polyimide films in an oxygen free environment. First of all, is that true and secondly, how do you ensure that your chamber is oxygen free and leak-tight?
Answer:
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By:
Ken Sautter |
Critical steps in any cure process include complete removal of residual solvents, uniform temperature distribution, pressure control, ability to maintain dry inert atmosphere, and control of heating and cooling rates. Our YES-PB Series of ovens are used for polymide curing plus particle reduction in most applications. Unlike an atmospheric process that uses large amount of nitrogen to displace the oxygen, YES ensures "complete" removal (less than 10 ppm over background) by using vacuum nitrogen interchanges and a laminar flow process with nitrogen.
Polyimides are usually applied in liquid form and then thermally cured as a thin film or layer to achieve the desired properties. Polyimides are high temperature engineering polymers utilized by semiconductor manufacturers for their excellent mechanical, thermal, and electrical properties.
Precise temperature uniformity is essential in order to avoid cracks in the polyimide layer and color variations. Color uniformity is important for the pattern recognition systems used in assembly, and low oxygen values during this process help to achieve bright material and good adhesion. Variable programmable temperature ramp rate controlling capability using our sophisticated PID controllers minimizes temperature overshoot and under shoot and maintains precise temperature uniformity
Curing in an oxygen-free environment is critical to having a strong flexible film. Oxygen will break the long chains of the polymer and make a brittle film. The YES-PB systems are designed from the ground up to ensure a proper atmosphere for curing these films.
Oxygen level monitoring is measured in the exhaust stream of the chamber. The monitor readings are fed back to the system control software where two levels of alarm can be established. Typical oxygen levels during processing are 5-10ppm. A warning alarm can be set by the user to a non-critical level that indicates a change in the system say 20 ppm. A second level can also be established which would indicate a major problem.
To maintain the extremely low O2 levels, the YES-PB systems utilize sub-atmospheric vacuum, nitrogen flow to maintain 200-300Torr pressure and a double door gasket. The biggest source of leaks in a typical vacuum system is the large door where product enters the system. The YES-PB systems have a double door gasket. In between the two gaskets, low pressure N2 is flowed. If the inner seal should fail, the seal will "leak" nitrogen, not atmosphere thus maintaining low O2 levels. | 
