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

Abstract

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 led to a completely dissolved region behind the swollen gel. The rate of solvent diffusion was very close to that of dissolution. Both were governed by the peak PI curing temperature, solvent temperature, and the types of solvent used. Based on this study, the total time required to lift off evaporated metal on top of the stencil can be calculated from the diffusion coefficients and the induction time obtained at each temperature.