- Stress buffer coating for Redistribution Layers
- 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. 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 is less expensive and easy to handle. By-products that occur during thermal curing are liquid, so they usually do not form depositions on the process chamber walls. To convert the polyimide precursors to a stable polyimide film, an elevated temperature (~250 C to 450 C) extended bake is required for complete imidization; it also drives off the N-methylpyrrolidone (NMP) casting solvents and orients the polymer chains for optimal electrical and mechanical properties.
Photosensitive polyimides offer the advantage of simpler processing by eliminating the need for photoresist compared to standard non-photosensitive polyimides. This reduces the number of process steps. For some types of photosensitive precursors, the photosensitive components can be difficult to evolve from the polyimide film. Residual photosensitive polyimide precursors can cause greater internal film-induced stress than those in a standard polyimide film. 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. The objectives of a proper cure process are to:
- Complete the imidization process,
- Optimize film adhesion performance
- Remove all residual solvents and extraneous gases, and
- Remove photosensitive components
The imidization process is better controlled when the solvents and photosensitive components are uniformly and efficiently evolving from the film. If the imidization rate is not controlled properly, there can be localized mechanical stress variations across the wafer, which can affect film adhesion to the substrate. Also, ambient oxygen darkens the polyimide film. This film transparency is critical when multiple polyimide layers are used during subsequent processing. For multi-layer processes, the alignment marks for the process sequence can be obscured by the layers of low transparency polyimide films.
5 Advantages of Using a Vacuum to Bake Polyimides:
- Uniform Solvent Evaporation – The constant vacuum and hot nitrogen mix pulls out the casting solvents uniformly in a laminar flow environment. The reduced pressure enables the efficient evolving of solvents, thereby eliminating the need for temperature dwell steps. (Older methods of curing polyimide relied on dwell steps to allow the substrate to heat up and boil out solvents).
- Optimal Temperature Control – Imidization rates can be better controlled when the casting solvents are efficiently evolving from the film. As a result, the controlled temperature ramp rates can be adjusted to provide a larger process window for the proper curing of a polyimide film.
- Consistency – At reduced pressures, the solvent is efficiently pulled off without any skin being formed on the polymer. There are no bubbles of solvents/ extraneous gases trapped in the polyimide film
- (Almost) No Oxygen – Using a vacuum achieves oxygen levels below 10ppm during processing. All of our polyimide bake ovens have double door seals, creating a nitrogen “buffer” between the outside atmosphere and the inside chamber.
- Cleaner Process – Laminar flow is achieved by pulling vacuum from one side of the oven and flowing in pre-heated nitrogen from the opposite side. The result is a Class 1 environment in which there is a laminar flow that is parallel to the orientation of the wafers.