Audible acoustic wave real-time monitoring in laser processing of microelectronic materials

Audible acoustic wave generation during KrF excimer laser processing of microelectronic materials Si, Cu and Al is investigated. It is found that amplitude of the acoustic wave is closely related to laser pulse number and laser fluence. Due to the laser cleaning of surface contamination, the amplitude reduces to a steady level with laser irradiation up to a pulse number of 10. The first peak-to-peak amplitude of the acoustic wave at the steady condition is used to evaluate laser interaction with the materials. The amplitude analysis shows that there exists a threshold fluence. For laser fluence lower than the threshold, the amplitude is zero. For laser fluence higher than the threshold, the amplitude increases with laser fluence. Threshold fluences are 1.1, 1.35 and 1.2 J/cm² for Si, Cu and Al respectively. It also shows that the increase of the amplitude starts to saturate for laser fluence higher than 10 J/cm². According to ablation rate measurement, these thresholds of fluences are the same as the ablation thresholds of the materials. Saturation of amplitude increase is due to plasma shielding effect during the laser ablation of the materials. Theoretical calculation agrees well with the experimental results. Acoustic wave measurement provides a simple method to detect the threshold fluences of laser ablation and plasma shielding. By proper calibration, it can also be used as a real-time measurement of laser ablation rate. By applying appropriate pulse number, the laser processing of microelectronic materials can be controlled in-situ.