Layer detection in ultrashort pulsed multilayer laser ablation by analyzing ultrasonic process emission

We report on the capability of an ultrasonic sensor, consisting of a membrane-free optical microphone, to distinguish material transitions in ultrashort pulsed laser ablation of multilayer material systems for electronic applications. The acoustic emission during the ablation of printed circuit board materials is used to detect the material transitions via specific acoustic signatures, thus enabling layer detection in the ablation process. Due to a higher removal rate in polyimide as compared to copper an increase of acoustic energy at the material transition between the copper and polyimide layers results, which enables layer detection, this process event can be used for process control. Membrane-free optical microphones have outstanding properties in terms of high temporal resolution and high bandwidth, making them suitable sensors for monitoring ablation processes using ultrashort pulsed lasers. In detail, different levels of the emitted acoustic energy during the ablation of copper and polyimide layers in multilayer materials are analyzed in different frequency ranges via the acoustic signal in order to realize a material-selective ablation control, which represents a novelty in the field of monitoring ultrashort pulsed laser multilayer ablation processes. In addition to the layer detection of different materials, the presented investigations also illustrate the specific signatures of the emitted acoustic energy of ablation processes as a function of the layer thickness and its spatial emitting properties across the scanning field.