Fusion Science and Technology / Volume 49 / Number 4 / May 2006 / Pages 728-736
Technical Paper / Target Fabrication
Vapor deposited PMDA-ODA poly(amic acid) and polyimide capsules have been produced with desirable material properties (high tensile strength, permeability, and elastic modulus), but the contributions of the process steps and their dependence on external control variables has not been investigated. We have combined numerical simulations with experimental measurements to model the steps of the vapor deposition process including monomer sublimation, vapor transport to the bounce pan, and poly-condensation on the substrate surfaces. The measured sublimation rates of PMDA and ODA monomer at temperatures that yielded stoichiometric poly(amic acid) (10-6 Torr deposition) are 1.2 × 10-7 gm/s PMDA (at 153° C) and 6.3 × 10-10 gm/s ODA (at 126° C) - a 180:1 PMDA:ODA molar ratio. These provide initial boundary conditions to simulate the thermal environment and vapor transport inside the deposition chamber at 1 × 10-2 Torr. A disproportionate loss of PMDA gas during transport to a stationary mandrel is shown by the numerical model to reduce the monomer stoichiometry to 9:1 PMDA:ODA. The transport-based loss depends strongly on the geometry of the substrate support, as is shown by modifying the substrate to change the flow pattern, which reduces this ratio to 1:1 PMDA:ODA above the mandrel. A separate model of the kinetics of monomer deposition and polymerization reactions was developed to correlate the gas concentrations above the substrate with the elemental concentrations comprising the film. This basic model was tested with rate constants based on reaction probabilities of one and equal deposition rates for two monomers in the absence of measured values and is sensitive to changes in vapor stoichiometry.