Laser microvia drilling and ablation of silicon using 355 nm pico and nanosecond pulses

Laser ablation of silicon has become an intense research topic due to the rapidly growing interest in laser processing in the photovoltaics and electronics industries. Different types of lasers are being used for edge isolation, grooving, drilling among other applications, with the pulse width ranging from the ultrashort femtosecond regime up to long microsecond pulses. The results may vary significantly depending on the wavelength and pulse width delivered by the laser source. In this study, two frequency triplicated Nd:YVO4 lasers, delivering pulses of width 9 to 12 ps and 9 to 28 ns, were used to drill holes and form grooves in silicon wafers. The thickness of the wafers was 200 µm. Groove depth and geometry were measured using an optical 3D profiling system. Results revealed that the material removal rate was greatly influenced by the pulse energy and repetition rate when the nanosecond pulsed laser beam was used. With picosecond laser beam the volumetric material removal rate remained rather constant in the range of 100 to 500 kHz, but the groove width and depth varied. Scanning and transmission electron microscopy were used to characterize the drilled holes. Microstructures were investigated by selected area electron diffraction patterns. According to the measurements, nanosecond pulses induce not only thermal, but also mechanical damage to the hole walls, while picosecond processing only results in a thin HAZ layer, which is partially covered with amorphous nanoparticles. (19 refs.)