The nature of uv ablation of organic polymers is discussed in terms of a pseudo-zeroth-order rate law of the form dx/dt = k0e−(Eact/kT), where Eact is assumed to be the strength of the weakest bonds in the polymer and T is the local temperature increase from the incident laser pulse. Equations derived from previous models that assumed nonthermal photodecomposition were duplicated from this photothermal model. Even for the simple case of single-photon absorption, nonideal behavior is affected by radiationless decay, pulse length, and thermal diffusion. These effects were probed. Results indicated that thermal diffusion may have a significant effect on the threshold fluence and to some degree on the shape of the etch depth versus fluence curve. Absorption dynamics (saturation and radiationless decay) appear to be the dominant factor in determining the functional dependence of etch depth on fluence. As a result of competition between absorption saturation and radiationless decay, the penetration depth is intensity dependent. High fluence as well as short temporal pulses (subnanosecond) penetrate more deeply into the polymer than predicted by simple Beer’s law absorption. The apparent existence of an optimum pulse length, for a given absorbing system, is another result of the absorption dynamics.
The effect of pulse repetition rate on polymer ablation was studied experimentally and theoretically for polyimide and 8230 photoresist, a polymethylmethacrylate (PMMA)-based polymer. Both the experimental data and the theoretical model showed a distinct tendency for increased repetition rate to decrease the ablation threshold, without substantially altering the absorption coefficient. This was attributed to heating of the sample sequentially by the laser pulses and the magnitude of the effect is proportional to the square of the absorption coefficient. Polyimide, with an absorption coefficient of , is expected to exhibit the repetition rate effect in the range of tens to hundreds of kilohertz; hence the effect has not heretofore been observed. The PMMA-based photoresist, on the other hand, exhibits the effect at a much lower repetition rate, owing to the smaller effective absorption coefficient.
Plasma etching of organic polymers typically involves the use of feed gas mixtures of oxygen with a fluorocarbon. The reactive etchants are generally accepted to be atomic fluorine and atomic oxygen. The roles of these etchants are discussed with special attention being given to the part played by atomic fluorine. Mechanisms are inferred theoretically from a molecular orbital study, and experimentally from the surface composition of plasma treated samples. Hydrogen abstraction by fluorine leads to enhanced etching of saturated polymer structures while addition of fluorine is the dominant mechanism for unsaturated moieties. Etch rate behavior for polyimide, polyisoprene, and polyethylene as a function of gas composition is discussed in terms of the affinity that fluorine has for the surface of the saturated and unsaturated polymers.
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