Abstract

The advantages of ultrashort laser pulses for high quality micromachining have been early recognized. Indeed femtosecond laser are able to ablate any type of matter (metals, dielectrics..) and since the energy is deposited in the system in times shorter than the heat diffusion time in the solids, the ablation takes place “softly” with high final surface quality. Besides, femtosecond pulses - no matter the wavelength is - solve the difficult problem of the screening of the laser by the expanding ablation plasma which is the essential limit to nanosecond laser ablation with visible of IR light. However a problem inherent to laser drilling is the aspect ratio (length/diameter of holes) and the efficiency of deep hole drilling that can be expected. Indeed, in the case of industrial applications, this efficiency is the key feature that will control the throughput. In this work we have used high energy femtosecond pulses to explore the drilling efficiency in metals up to very high laser intensities. The experiments have been carried out at the CEA femtosecond laser facility at Saclay. The Ti-Sa laser that was used for the present work is delivering up to l0mJ per pulse at 800nm, with a repetition rate of 1 KHz. The pulse duration is 35 fs. The beam diameter at the sample is about of 20 ‘μm, leading to maximum fluences of 600 J/cm2. We have systematically measured the drilling efficiency for different types of metals (Tantalum, Molybdenum, and one SMA (Shape Memory Alloy), with thicknesses in the range 200pm to 4 mm. The samples were placed in air. The time to drill a hole through the samples was measured vs. the laser pulse energy. A typical result, obtained on Tantalum is shown on figure 1, for thicknesses in the range 250-1000 μm. Figure 2 shows the case of SAM samples, and the drilling time is measured vs the length for three different laser pulse energies (0.5,1,2

© 2007 IEEE