Low temperature curing of polyimide wafer coatings

Polyimide films are commonly used on wafers as passivation layers, stress buffer layers, dry etch masks, structural layers, and re-distribution layers for chip scale packaging and wafer level packaging. These films are cured in convection or diffusion ovens at high enough temperatures (350-400°C) to assure adequate mechanical and electrical properties. These high temperatures can change the electrical properties of the devices. If the cure temperature of these films were lowered there could be a reduction in the film(and wafer) stress as well as lower thermal budget for the devices. However, curing polyimide films at lower temperatures than 250°C has not been found to provide the mechanical, chemical, and dielectric properties required for the final device operation parameters or mechanical and dielectric protection. This paper describes the use of variable frequency microwave (VFM) for the curing of existing polyimides at more than 50-150°C lower than the standard convection cure temperatures while maintaining the necessary final mechanical and chemical film properties. These films can be etched, patterned, metalized and further processes to make multi-layer structures. Further, we report on the continuing development of optimized polyimide materials suitable for curing at temperatures less than 200°C with VFM. Experiments to determine the relationship between the molecular structure of the polyimides and the efficiency of microwave susceptibility are also described.