Laser micro-forming using picosecond pulse durations

Multi-pass LμF was conducted using the Fianium system, operating at 1059 to 1064 nm wavelength and 500 kHz repetition rate with a spot diameter of approximately 35 μm. With increasing multiple irradiations a cumulative increase in bend angle was observed, as shown in Figure 100.

Figure 100 - Cumulative bend angle variation with successive irradiations (1000 x 300 x 50 μm stainless steel AISI 302, 35 μm beam diameter, 500 kHz repetition rate, 10 mm/s traverse speed).

In addition to a cumulative increase in bend angle, an ablated groove was also observed along the length of the scan path, the aspect ratio of which became too large to resolve the depth by WLI after multiple irradiations, as shown in Figure 101.

Figure 101 - WLI image of ablated groove after 6 irradiations (1000 x 300 x 50μm stainless steel AISI 302, 1500 mW, 35 μm beam diameter, 500 kHz repetition rate, 10 mm/s traverse speed).

This ablated groove can be attributed to the use of multiple irradiations in conjunction with fluences above those of the experimentally determined values of Φth. An experiment was

subsequently conducted in which the traverse speed was increased and the ablated groove depth and bend angle was monitored, the results of which are shown in Figure 102.

Figure 102 - Bend angle and ablation depth variation with increasing speed, 50 and 75 μm thickness top and bottom respectively (1000 x 300μm stainless steel AISI 302, 35 μm beam diameter, 500 kHz repetition rate, 1.5 W average power).

From Figure 102 it is evident that the ablated groove has a detrimental effect on bend angle up to a point, in this instance when deeper than approximately 2 μm. The optimum traverse speed at which a suitable combination of bend angle and z-depth was achieved was found to be 35 mm/s, with the degree of pulse overlap illustrated in Figure 103.

Figure 103 - Number of pulses per spot and degree of pulse overlap (35 μm beam diameter, 500 kHz repetition rate, 35 mm/s traverse speed).

Whilst it was possible to limit the ablation depth for single scans, multiple irradiations resulted in an increase in ablation depth. This phenomenon could be attributed to a conditioning of the irradiated surface after an initial irradiation [124], increasing the absorption for subsequent scans.

As highlighted in Figure 100, multiple irradiations were essential to achieving a large range of deformation. As such an alternative to multiple irradiations along a single path was required to obtain a variation in bend angle. One such method investigated involved a combination of varying power and a hatched scan strategy. The hatch consisted of four single irradiation paths, scanned sequentially in alternate directions. Each irradiation path was spaced 30 μm apart, allowing for little or no overlap, as shown in Figure 104.

Figure 104 - Scanning electron microscope image of ablated groove after hatch irradiation strategy at a) 1500 mW, b) 1250 mW, c) 1000 mW and d) 750 mW (1000 x 300 x 50 μm stainless steel AISI 302, 35 μm beam diameter, 500 kHz repetition rate, 35 mm/s traverse speed).

From Fig 104 it is evident that melt and re-solidification has occurred in the irradiated areas, becoming more noticeable with increasing laser power. This has also been observed by Singh et al. [99,125] upon the application of high intensity, low repetition rate nanosecond pulsed radiation to magnetic sliders in hard disk drives. It should be noted that the stress induced by this melt and re-solidification has the potential to contribute to the net bending angle.

Through variation of laser power with a single line scan strategy, controlled and repeatable micro-adjustment was achieved. The application of a hatched scan strategy increased the range over which micro-adjustment could be achieved whilst keeping the ablated groove to within approximately 2 μm depth, as shown in Figure 105.

Figure 105 - Bend angle variation with increasing power, 50 and 75μm thickness top and bottom respectively (1000 x 300 μm stainless steel AISI 302, 35 μm beam diameter, 500 kHz repetition rate, 35 mm/s traverse speed).

In this chapter, thin sheet thermal LμF for the micro-adjustment of actuator style components was demonstrated using picosecond duration pulses with no absorptive coatings required. A full empirical study was conducted, with the effect of pulse overlap, laser power and irradiation strategy investigated.

The thermal LμF technique presented in this chapter combines short pulse durations with high repetition rates and offers a method of generating localized heat build-up on the top surface of micro-scale components, allowing for controlled and repeatable micro-adjustment. Through a combination of irradiation strategies a large range of deformation is achievable, as shown in Figure 105.

The use of 20 ps pulse durations limits the heat diffusion depth to within a suitable range (i.e. half the sheet thickness) on the top surface of the component, whilst not being so short as to cause significant material removal by ablation. It was found that the repetition rate and therefore degree of pulse overlap must be high enough to ensure sufficient build up in temperature on the surface of the component for thermal forming.

The process has significant potential for the post-fabrication micro-adjustment of functional components in micro-electronic devices. Challenges include minimising ablation along the irradiated scan path and obtaining a larger range of deformation through optimized process parameters.

Conclusions