Polyimide Cure

Polyimides are high temperature engineering polymers utilized by semiconductor manufacturers for their excellent mechanical, thermal, and electrical properties.

Applications include:

• Adhesion promotion of molding compounds
• Stress buffer coating
• Protective passivation layer on top of a completed IC
• Low-K separator between metal layers in ICs
• Chip bonding
• Interlayer dielectric

Polyimides are usually applied in liquid form and then thermally cured as a thin film or layer to achieve the desired properties. Typically, polyimides are cured in a vacuum oven so effluents can be removed.

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.

Three types of polyimide:

• Non-photosensitive polyimide
• Photosensitive, ionic type polyimide
• Photosensitive, ester type polyimide

Non-Photosensitive Polyimide

Non-photosensitive polyimide is less expensive and easy to handle. By-products that occur during thermal curing are liquid, so they do not quickly form hard coatings.

Photosensitive Polyimide

Photosensitive material achieves better process performance, including higher resolutions, wider margins in photolithography steps, and longer shelf lives. By using photosensitive polyimide, the number of process steps can be significantly reduced. Ester bond types are more stable than ionic types and have the longest shelf life. Solubility of unexposed areas is also better with ester type polyimide.

5 Advantages of Using a Vacuum to Bake Polyimides:

1. Cleaner Process. The vacuum is pulled from the bottom of the oven, and preheated nitrogen enters from above and goes through a restraining stainless steel flat plate filter. This induces vertical flow and the constant removal of particles from the substrate.

2. Uniform Solvent Evaporation. The constant vacuum and hot nitrogen mix pulls out the solvent with more efficiency. It also provides a gentle laminar flow to remove particles. (Older methods of heating polymide relied on dwell steps to allow the substrate to heat up and boil out solvent).

3. Consistency. Using a vacuum provides tighter control of your process. Processing conditions are very repeatable because of reduced pressure combined with temperature profile of ramp baked.

4. Temperature Uniformity. Using a vacuum allows an adjustable air-mixing ratio for chamber cooling to adapt the tool for best performance over a broad range of operating temperatures. (Vacuum + pre-heated nitrogen gives control of process atmosphere).

5. (Almost) No Oxygen. Using a vacuum achieves oxygen levels below 10ppm during processing. And the newer polyimide bake ovens all have double door seals, creating a nitrogen "buffer" between the outside atmosphere and the inside chamber. Plus, as a bonus, a vacuum process significantly reduces the amount of nitrogen flow required.

For a system specifically suited to curing polyimides, visit the YES-450PB Series product pages.