Micromachinining with tailored pulse parameters

Abstract

Experiments on different metals and silicon were conducted to optimize removal rate or surface finish with nanosecond pulses of different parameters. A special fiber laser allows independent adjustment of pulse parameters while keeping beamquality constant. Fiber lasers in MOPA configuration enable a tool for micro-machining that allows to independently adjust pulse parameters while keeping the beam quality constant. Such a fiber laser was developed delivering an average power of 11 W and a maximum pulse energy of 0.5 mJ for a wide range of pulse parameters at diffraction limited beam quality. The pulse duration is continuously adjustable in the range from 10 ns to cw, the repetition rate is continuously adjustable from from 10 kHz to 1 MHz while the pulse energy can be adjusted continuously up to 0.5 mJ. All parameters can be independently varied and the pulse shape is almost a rectangular. A scanner was adapted to the output of the fiber laser to determine removal rates by scanning the focused laser beam multiple times over an area of 1.8mm by 1.8mm. A 160mm and 80mm telecentric focusing lens was used resulting in 45?m, respectively 25 ?m spot size and a maximum power density of 280 MW/cm2 and 460 MW/cm2. Ablation experiments were carried out on stainless steel, nickel and silicon varying the repetition rate in the range from 20kH.z to IOOkHz, the pulse duration in the range from 20ns to 650ns and the scanner speed in the range from 100mmls to 800mmls. For stainless steel and nickel the average power was set to 2.25W using the 25?m spot size, while for silicon, the energy was kept constant at 45mJ for varying pulse duration. Fairly similar pulse parameters for maximum removal rate were found for all three materials. For stainless steel, a pulse duration of 320 ns, a repetition rate of 20 kHz, and a pulse energy of 113?J resulted in the maximum removal rate of 0.46 mm3/min, while for nickel the optimum parameters were found with a pulse duration of 160ns, a repetition rate of 40 kHz and a pulse energy of 112?J For silicon 320ns pulse duration at 100kHz and 45 mJ resulted in optimum removal. In single shot experiments a dramatic influence of pulse duration was found that for silicon with a clear optimum for removal rate and clean processing. This was not so apparent for the ablation of metals, even so a different set of parameters must be used to obtain best surface quality or maximum removal rate. It was found that for maximum removal rate, the scanner speed must be synchronized with repetition rate to achieve a pulse to pulse overlap of at least 70% and that increasing the repetition rate requires shorter pulses, e.g. 320ns at 20 kHz or 80ns at 100kHz. An optimum range of power density at the work piece was also identified to be in the range of 35MW/cm2 to 70MW/cm2. Lower densities are below ablation threshold, while higher power densities result in plasma shielding within the pulse.