Polyimides are high temperature polymers that have been engineered with excellent mechanical, thermal, and electrical properties.
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 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:
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.
For a system specifically suited to curing polyimides, visit the YES-450PB Series and YES-VertaCure product pages.