Evaluation of polyimide coatings integrity by positron annihilation lifetime spectroscopy and electrochemical impedance spectroscopy

Positron annihilation lifetime spectroscopy (PALS) and electrochemical impedance spectroscopy (EIS) were used to investigate the existence of free volume cavities in polyimide coatings and the durability of these films for integrated circuits, as a function of their curing temperature. Impedance of the coatings vs. resistance to water uptake in the films was measured by EIS. A several orders of magnitude increase in the low frequency electrochemical impedance was observed for polyimide PI2566 (2,2'bis(3,4-dicarboxyphenyl) hexafluoroisopropylidene dianhydride + 4,4' oxydianiline) when the coating was cured at increasing temperatures, Water exposure decreases the low frequency impedance, indicating water penetrates and changes the dielectric and conduction properties of the film. PI2566 cured above 150 degrees C was resistant to diffusion of water after 900 h exposure. PALS showed the free volume fraction in PI2566 increased approximately 35% as a function of its curing temperature. PALS data showed three lifetimes and intensities in the polyimide specimens. There were three curing temperature regions in the free volume fraction and the average lifetime data corresponded to three imidization stages. The first region was associated-with the evaporation of solvent. The second region was due to the condensation reaction (loss of water from the polyimide). A third region was related to completion of imidization. The free volume cavities increased from 46 Angstrom(3) to 148 Angstrom with increasing curing temperature for PI2566. No large free volume cavities were found in PI2610D polyimide (pyromellitic dianhydride + oxydianiline) at curing temperatures above 150 degrees C.