Study of Polyimide Ablation Using Sequential 308 NM Pulsed Laser Radiation

Studies of the ablation of polyimide accompanying two rapid sequential pulses of 308 nm laser radiation have been represented. An examination of the nature of the ablation threshold for etching of polyimide was performed. Assuming that sub-threshold radiation couples into the material thermally, the heat conduction equation for the system was solved in order to estimate resulting surface temperatures. For pairs of pulses whose combined fluence was near the single pulse fluence threshold for etching, separation of the pulses in time resulted in the initiation, cessation and reinitiation of soot deposition on the polymer surface. It is suggested that this temporally dependent soot deposition was caused by the formation of a vapour layer above the target surface, generated by the first pulse, that was rich in molecules such as CO, CN, C_2rm H_2, and C_2. The arrival of the second pulse induced a combustion process similar to that of a hydrocarbon flame which resulted in soot formation on the surface. Measurements of the change in surface reflectivity were also performed utilizing this double pulsing technique. This study revealed that both permanent and transient changes in surface reflectivity resulted when pulses above the ablation threshold irradiated the surface. The permanent change was due to physical modification of this polymer surface and deposition of ablated material while the transient change was likely due to scattering and absorption of the incident probe radiation by particles in the plume. An examination of the emission spectra and corresponding etch rates obtained when the irradiating laser fluences were an order of magnitude above the ablation threshold, revealed that as the pulses were separated, molecular emissions from CN and the continuum emission increased as did the etch rate. The molecular C_2 emissions decreased with increasing pulse separation. It is suggested that the increase in CN emission was due to the increased amount of material in the plume while the decrease in C _2 emission was due to the formation of soot particles which would then luminesce in the plume and increase the background continuum.