| The effects of key process parameters were investigated in vapor deposition, thermal treatment, photoexposure and development of vapor deposition polymerization of auto-photosensitive polyimide thin films. The principal goals were to demonstrate proof-of-concept through identification of preferred operating windows for each step of the short process flow and ultimate success in patterning performance. Films were vapor deposited in a dual thermal evaporation system with independent control of monomer fluxes and substrate temperature. Screening experiments with different dianhydride - diamine monomer combinations revealed that the ethyl-substituted diamine provided good photosensitivity and processability. A study of the effects of monomer ratio and substrate temperature and overall patterning capability showed that the best film compositions were achieved using a 0.85 diamine : dianhydride monomer flux ratio. Polyamic acid formation extent increased substantially with increasing substrate temperature up to 353 K, and then decreased with further increases in temperature to 383 K; this decrease is associated with re-evaporation of diamine monomer. Accurate film thickness and optical properties for cross-linked polyimide films were determined using a spectroscopic ellipsometry model incorporating a series of Tauc-Lorentz oscillators to model a signature light absorption in the wavelength region from 550 to 650 nm. Photo-exposure response curves generated for two different pre-exposure soft-bake anneal treatments showed that the contrast and sensitivity values obtained for the two-step, higher temperature recipe are higher than those for the single-step anneal by more than a factor of four and a factor of two, respectively. The sensitivity for the two-step anneal recipe was on the order of 260 mJ/cm2, substantially higher than the 30–100 mJ/cm2 sensitivity values typically reported for negative-tone photoimageable polyimides. Field Emission Scanning Electron Microscopy (FESEM) images showed good patterning capability for large features, but high-resolution images revealed evidence of high film porosity at the sidewall. A post-development thermal conversion study indicated films were completely imidized after one hour over the temperature range of 573–623 K. Film thickness losses of 15% were reported for fully imidized films, which is a significant reduction in comparison to the 40–50% losses observed with many commercially available spin-on analogs. |
