Welding of Au microwires by femtosecond laser irradiation

An alternative wire joining method is explored that utilizes femtosecond laser pulses directed towards the contact point of two pieces of 25-µm diameter Au bonding wires typically used in microelectronic wire bonding. The two pieces are in the form of wire loops bonded onto four terminals in a crossing pattern. In-situ four-wire measurement is used to measure the contact resistance at the contact point of the crossed wires before and after femtosecond laser irradiation. A greater decrease in contact resistance correlates to stronger weld between the two wires. Laser parameters of pulse energy and pulse number are varied between 20 to 40 µJ and 10 to 100 pulses to develop an initial process window. With a repetition rate of 1 kHz, the corresponding average power that the crossed wires received is between 20 to 40 mW. Over 90 % of all crossed wires samples showed drops in contact resistance from 39.8 % to 83.3 %, indicating that microwelding of gold microwires is possible with femtosecond laser pulses. A welding mechanism is proposed.