Excimer laser induced electrical conductivity in polyimide: Applications to engineering, physics, and applied science
Ball, Zane Andrew
Sauerbrey, Roland A.
Doctor of Philosophy
Various aspects of the interaction of excimer laser pulses and polyimide were investigated, focusing on low fluences where laser induced electrical conductivity is observed. Multiple shot excimer ablation of polyimide was investigated, and at fluences between 20 mJ/cm$\sp2$ and 260 mJ/cm$\sp2,$ ablation would cease after a few hundred pulses. This cessation of ablation was found to be the result of a dramatic, fluence dependent increase in surface roughness accompanied by the formation of a conducting, carbonaceous layer on top the laser treated sample. The repetition rate of the laser could affect the electrical conductivity of the layer but had little effect on any other properties, implying that the electronic structure of laser induced conducting layer was highly temperature sensitive. The optical properties of the laser irradiated surface were also studied with regard to a transient decrease in reflectivity that is observed during and in the first few tens of nanoseconds after irradiation. Pump probe experiments using electronically delayed probe pulses were performed and measurements of reflectivity, transmissivity and scattering were made under identical conditions. The results, a decrease in reflectivity and scattering with a simultaneous increase in transmissivity, imply a transient refractive index phenomenon during the first 150 ns after excimer irradiation. A nonlinear current-voltage relation was observed near the critical point of the laser induced metal-insulator transition. The crossover behavior from nonlinear to linear regimes was studied in detail and interpreted in terms of a percolation theory of binary conductor-insulator mixtures. The nonlinearity and the crossover can, for the first time, be described simultaneously in terms of a new theory of space charge limited currents on percolation clusters.
Electronics; Electrical engineering; Engineering; Materials science