Experimental investigations of nanosecond pulsed laser micromachining of metal using thin film micro sensors

Laser-material interactions during laser micromachining are extremely complicated. In order to improve the fundamental understanding of the laser micromachining process, it is essential to investigate the complex phenomena and mechanisms of the physical processes within and close to the region of the interaction. Especially it is important to accurately measure time-resolved temperatures of the workpiece during laser micromachining process.;Micro thin film thermocouples (TFTCs) with sufficiently high spatial and temporal resolutions to study the transient thermal response for laser micromachining were fabricated with standard microfabrication techniques. A novel technique for the batch fabrication of micro TFTC arrays, on an electroplated nickel workpiece, for the surface temperature measurement was developed, and optimization of materials for sensor and insulating layers was achieved. K-type micro TFTCs with a junction size of 25 mum x 25 mum were fabricated on the electroplated nickel workpiece. The sensitivity of micro TFTCs was estimated as 40.4 muV/°C and the response time was 28 ns.;The transient surface temperatures on nickel workpieces were measured during nanosecond pulsed laser micromachining. The result indicates clearly that an extremely steep temperature gradient exists immediately outside the laser spot. Moreover, the measured temperature distribution around the laser spot demonstrates the existence of a superheating area induced by the short pulse of laser beam with high energy fluence. The effect of laser energy fluence on the surface temperature was also investigated, and the results show the peak temperatures outside the laser spot are non-linear to the laser energy fluences. Topographical characterizations of laser micromachining with various laser energy fluences were undertaken to correlate the resulting geometry changes with surface temperature measurements. Plasma radiation effect on the surface temperature of workpiece was demonstrated.;In order to increase the maximum operation temperature and spatial resolution of micro TFTCs, C-type micro TFTCs with a junction size of 2 mum x 2 mum were fabricated. The results also show that there is a steep gradient of temperature in the radial direction and a superheated area around laser spot.