Analysis of the discrete stages of the formation of polyimide films by vapor deposition and their effects on the film's properties.

摘要

The process for vapor depositing and curing PMDA-ODA polyimide films was empirically designed and optimized to fabricate capsules with 1 mm diameter and 1-mum thick walls, with acceptable surface roughness, and mechanical properties. In order to adapt this process to produce larger-scale films, the operating parameters have to be reoptimized. In past studies, the vapor deposition polymerization (VDP) process was refined empirically and it is not clear whether it was fully optimized because the contribution and importance of individual steps in this complicated process was not fully understood. This study quantified the coupling of the sublimation, transport, deposition, and surface reaction steps using parametric measurements and developed and tested a computational model of the VDP process.; The parametric studies of operating conditions determined that: (1) The sublimation rates of PMDA and ODA varied by up to 80% for the same set of operating temperatures. (2) The deposition rates of each monomer decreased by 75% over a lateral distance of 4 cm and followed an axisymmetric profile. (3) All Solid PMDA and ODA films had 30 to 300 nm rms roughness values and some contained voids or columnar features. (4) PMDA/ODA/PAA films deposited at (10-7 Torr) were fully dense, but the surface roughness varied by up to 300 nm rms. (5) All films contained PMDA-ODA amic acid bonds, regardless of the microstructure, and (6) Films deposited at 10-3 Torr had the smoothest finish and highest concentration of PAA for a 1:1 PMDA:ODA moles sublimating.; The Computational Fluid Dynamics (CFD) calculations of vapor transport, deposition, and surface reactions showed that: (1) Refining the substrate geometry doubled the monomer flux to the substrate, without affecting the film quality. (2) The sticking coefficient for both PMDA and ODA is 1. (3) Transport of monomer vapor is the rate limiting step, (4) The concentration of each monomer in the deposited film is a function of the equipment design. (5) An excess concentration of either monomer on the substrate will inhibit the polymerization reaction.