Back reflection analysis for sensing laser drilled through-glass vias

The properties of ultrathin glass, including high electrical resistivity, high thermal stability, low surface roughness, and flexibility, render the material particularly intriguing for applications in the semiconductor industry. Notably, when employed in conjunction with through-glass via (TGV) technology, it finds application in the form of flexible functional layers and multilayer materials, thereby facilitating the implementation of novel advanced integrated circuit packaging strategies and complex circuit boards. Due to the complexity of thin glass microhole drilling processes using pulsed lasers, there is a strong demand for fast monitoring sensors and electronics that control the process pulse by pulse during machining to achieve high processing quality and high reproducibility. This is particularly evident in cases where the borehole diameter is reduced to single-digit micrometer level, as inaccuracies such as variations in the glass thickness or pulse energy in this range become increasingly apparent. In this work, the principle of a monitoring sensor based on back-reflected laser pulses for drilling TGVs in ultrathin glass (t = 30 μm) using an ultrashort pulsed laser system is presented. The investigations include the analysis of the time-varying back reflection intensities and the comparison to the changing borehole geometry during the percussion drilling process at different pulse energies. Based on these investigations, algorithms for process control can be developed in the future, which allow a dynamic adjustment of the pulse energy or a pulse-precise termination of the process depending on the drilling progress.