Relation between Envelope Shape of GHz/MHz Burst-mode Femtosecond Laser Pulses and Laser-processed Shapes in Copper Sintering

Abstract

This study investigated the relation between GHz/MHz burst-mode femtosecond laser pulse envelope shapes and laser-processed shapes of copper sintering. Using burst mode, a single pulse can be replaced by a cluster consisting of multiple pulses (intrapulses) with a time interval of 15.44 ns (MHz burst) or 197 fs (GHz burst). The envelope shape of the intrapulses in GHz/MHz burst mode can be adjusted. For copper sintering, we set the following laser processing conditions: 190 fs pulse width and a 10 kHz –1 MHz repetition rate with constant power of 4 W, or repetition rates of 200 kHz, 590 kHz, and 1 MHz for constant pulse energy of 4 μJ in burst and non-burst mode. The femtosecond laser pulses with and without the burst mode irradiated the copper powder. The envelope shape of the intrapulses in GHz burst mode varied between envelope control (EC) = -1 (increasing pulse energy in the intrapulse with time) and EC = 1 (decreasing pulse energy with time). After copper sintering, the laser processing traces were observed using a digital microscope. Additionally, after copper sintering, the sample was evaluated using Raman spectroscopy for information about the molecular and crystal structure. The Raman spectroscopy measurements showed that Cr precipitates from the copper powder mixture to the surface during processing. Additionally, it was demonstrated that the degree of oxidation of the processed material can be adjusted by changing the envelope shape.