STRESS GENERATED IN POLYIMIDE BY EXCIMER-LASER IRRADIATION

Stress transients are generated in polyimide by irradiation with excimer-laser pulses at radiant exposures between 3 X 10(-3) and 10(2) J/cm2 . The duration and peak stress of these transients are measured using piezoelectric film transducers. For all the wavelengths tested (193, 248, 308, 351 nm) we determine three ranges of radiant exposure within which different physical mechanisms govern the stress generation. The scaling of stress with radiant exposure depends on wavelength only in the low fluence regime. In this regime the stresses observed are attributed to subsurface thermal decomposition at 351 and 308 nm and to photodecomposition at 248 and 193 nm. At higher radiant exposure the stress generation is governed either by the thermal expansion of the gaseous ablation products or by the formation and expansion of a dense plasma. The boundary between these two regimes is identified from the variation of the mechanical coupling coefficient with radiant exposure. The results also indicate that heat conduction contributes significantly to the coupling of energy into the target, leading to a reduction in the stresses generated and to an increase in etch depth when ablation is achieved.