THRESHOLD BEHAVIOR IN POLYIMIDE PHOTOABLATION - SINGLE-SHOT RATE MEASUREMENTS AND SURFACE-TEMPERATURE MODELING

The ablation rate of Kapton(TM)-type polyimide has been measured as a function of incident fluence and excimer laser wavelength using a sensitive quartz-crystal microbalance (QCM). The experiments were performed such that the fluence and the ablated depth were known for each laser pulse, avoiding the need to average rate and fluence data over many pulses. By limiting the investigations to the low-fluence regimes near ablation threshold, high precision and detailed curve shapes were obtained. It was found that the ablation rate increases smoothly and exponentially with increasing fluence for 248, 308, and 351 nm wavelengths. This exponential behavior was modeled using an Arrhenius-type thermal rate equation. In contrast, the 193 nm curve is linear in fluence, displays a sharp threshold, and is consistent with a possible photochemical ablation mechanism. Using a sophisticated surface temperature modeling code, the maximum laser induced surface temperature at the fluence at which ablation can first be detected is found to be the same, approximately 850-degrees-C, for all four wavelengths. This ''ablation'' temperature is significantly higher than the approximately 500-degrees-C temperature at which Kapton(TM) starts to degrade under isothermal heating conditions.