Key technical points of PI film baking and curing

Polyimide (PI): Properties and Applications

Polyimide (PI) boasts the highest flame retardancy rating (UL-94), excellent electrical insulation, mechanical strength, chemical stability, aging resistance, radiation tolerance, and low dielectric loss. These properties remain stable across a wide temperature range (-269°C to 400°C), making it an exceptional high-performance polymer. PI film is currently the best-performing insulating material globally, with extensive applications in the microelectronics field, including:

Stress-buffering coatings for redistribution layers

Enhancing adhesion of molding compounds

Protective passivation layers on completed ICs

Low-K separators between metal layers in integrated circuits

Chip bonding

Interlayer dielectrics

PI is typically applied in liquid form and then thermally cured into films or layers to achieve the desired properties. Precise temperature uniformity during the curing process is critical to avoid cracks or discoloration in the PI layers. Uniform color is particularly important for pattern recognition systems used in assembly, while low oxygen levels during processing contribute to brighter materials and better adhesion.

Types of Polyimide

Non-Photosensitive Polyimide

Photosensitive Ionic Polyimide

Photosensitive Ester Polyimide

Non-Photosensitive Polyimide

Non-photosensitive PI is cost-effective and easy to handle. The by-products generated during thermal curing are liquids, which typically do not deposit on the processing chamber walls. To convert the PI precursor into a stable PI film, extended baking at high temperatures (approximately 250°C to 450°C) is required for complete imidization. This process also removes the casting solvent, N-Methyl-2-pyrrolidone (NMP), and aligns the polymer chains for optimal electrical and mechanical performance.

Photosensitive Polyimide

Photosensitive PI offers the advantage of simplified processing compared to non-photosensitive PI because it eliminates the need for photoresist, reducing the number of processing steps. However, some photosensitive precursors can be challenging to fully volatilize during curing, leading to higher internal stress within the film compared to standard PI.

Ester-based photosensitive PI is more stable than ionic types, with a longer shelf life and better solubility in unexposed areas. Proper curing aims to:

Complete the imidization process

Optimize film adhesion

Remove residual solvents, gases, and photosensitive components

Effective evaporation of solvents and photosensitive elements ensures better control over the imidization process. Poor control can result in localized mechanical stress variations across the wafer, negatively impacting film adhesion. Additionally, oxygen in the environment can darken the PI film.

When multiple PI layers are applied during processing, transparency is crucial. Low-transparency PI layers may obscure alignment markers, complicating multilayer processing. Ensuring consistent clarity of the PI film is essential for precise layer alignment in complex manufacturing.