Properties and Microstructures of Low-Temperature-Processable Ultralow-Dielectric Porous Polyimide Films

Coating of ultralow-dielectric (ultralow- k ) polymers on high-speed and high-frequency circuitries can increase signal propagation speed and reduce crosstalk. High-temperature foamed films have ultralow- k properties, but are ineffective in noise reduction because of the presence of dense skins. Vapor-induced phase separation was used to fabricate porous polyimide films in this investigation. Scanning electron microscopy revealed that the films are homogeneous without skin layers. The pore size can be controlled from less than a micron to several microns. Electrical characterization revealed that the relative dielectric constant is reduced to as low as 1.7 and is stable between 8.2 GHz and 18 GHz. Usage of this new ultralow- k material as a substrate can help to improve circuit speed by more than 40% compared to the dense Kapton substrate, making this material well suited for use in wide-band and high-frequency applications.