Monitoring of excimer laser ablation of multi-layer thin films by acoustic emissions

In micromachining, excimer laser ablation was a key process for producing featured by removing parts of a photoresist layer. One hurdle was that the seed layer (e.g., copper), on which the photoresist was spun, was easily attacked or damaged in laser ablation of the photoresist. To overcome this drawback, an acoustic emission transducer (AET) was coupled to the X-Y stage of an excimer laser system to acquire surface acoustic waves (SAWs) arising from pulsed laser-material interaction. The characteristics of such a process could then be investigated by analysing this feedback signal. The sequences of acoustic emission signals were transformed to frequency domain by employing fast Fourier transformations (FFTs). Analysis of the frequency spectrum showed that there were few dominant frequencies correlating with the ablation process through one material to another. Specifically, the amplitude of the dominant frequency was a function of the number of pulses, where an abrupt change could be found when laser beam approached the interface of two layers. The exact number of shots machining through one material was indicated by properly calibrating the function at given laser parameters. In turn, the etch rate of machined material could be caculated by averaging the thickness of this material with number of shots. Thus, a real-time monitoring scheme of complex laser micromachining process could be realised on the basis of taking SAWs as feedback signals