Near-threshold photoablation characteristics of polyimide and poly(ethylene terephthalate)

Photoablation of polyimide (Dupont Kapton(TM)) and poly(ethylene terephthalate) (PET) (Dupont Mylar(TM)) were; studied in vacuum using 248 nm, 16 ns excimer laser pulses. A sensitive electron beam ionizer/quadrupole mass spectrometer detector was used to measure mass, translational energy, and angular distributions of the neutral photoproducts at fluences very close to the threshold (average material removal rates <100 Angstrom/pulse). The experiments were performed by combining results from many discrete sample spots, in order to minimize the changes in yield, energy, and surface topography caused by cumulative pulsing (''radiation hardening''). For both polyimide and PET, the dominant neutral photoproducts are fragments of the monomer, although there is a weak tail of intensity extending up to and beyond 1000 amu. The mean translational energy increases with increasing mass, due to the collisional ''seeded beam'' effect. The mean translational energy of the majority species (i.e., those below 200 amu) is around 1.6-4.9 eV for polyimide at 30 mJ/cm(2), and 0.9-1.5 eV for PET at 21 mJ/cm(2). The translational energy distributions of the heavier species are close to Boltzmann while those of the lighter species show small but reproducible deviations from Boltzmann form. The angular distribution of the CN product from polyimide is sharply peaked about the surface normal even in this near-threshold regime. (C) 1997 American Institute of Physics.