In‐plane solvent diffusion in a soluble polyimide lift‐off structure

The mechanism that governs the in‐plane solvent (N‐methyl pyrrolidinone) diffusion in a soluble polyimide [3,3’,4,4’‐benzophenone tetracarboxylic acid dianhydride and diaminophenylindane (BTDA‐DAPI)] lift‐off structure was investigated. Using a metal‐on‐polyimide test pattern along with a surface profilometer, the solution process was shown to occur in three stages. A thermal‐activated diffusion process which follows the t1/2 Fick’s law was observed as solvent molecules diffuse laterally (parallel to the film plane) underneath the metal pad, which serves as a solvent diffusion barrier, into the soluble polyimide film. The diffusion coefficients are described by the expression: D=0.0005 exp(−3600 cal/RT) cm2/s, over the 40–85 °C temperature range. Behind the diffusion front was a region of solvent/polyimide (PI) swollen gel with increasing solvent concentration away from the front. After a maximum solvent concentration was reached, the swollen gel began to disintegrate into the solvent bath. This gradually...