Appl. Phys. A 69 [Suppl.], S377–S380 (1999) / Digital Object Identifier (DOI) 10.1007/s003399900186 Applied Physics A Materials Science & Processing  Springer-Verlag 1999 Material effects in ultra-short pulse laser drilling of metals P.S. Banks * , M.D. Feit, A.M. Rubenchik, B.C. Stuart, M.D. Perry Laser Program, Lawrence Livermore National Laboratory, P.O. Box 808, L-477, Livermore, CA 94550, USA Received: 21 July 1999/Accepted: 31 August 1999/Published online: 28 December 1999 Abstract. Although hole drilling using ultra-short laser pulses has been shown to produce holes of excellent quality, we have observed that during the evolution of the hole, the morphology of the hole bottom has a dependence on fluence, number of shots, and polarization. We describe the nature of this structure within the hole and under what conditions it is observed. PACS: 79.20.Ds The use of femtosecond lasers allows materials processing of practically any material with extremely high precision and minimal collateral damage. Advantages over conventional laser machining (using pulses longer than a few tens of pi- coseconds) are realized by depositing the laser energy into the electrons of the material on a time-scale short compared with the transfer time of this energy to the bulk of the mate- rial, resulting in increased ablation efficiency and negligible shock or thermal stress. The improvement in the morphology by using femtosecond pulses rather than nanosecond pulses has been studied in numerous materials, from biological ma- terials to dielectrics to metals [1–7]. As part of the short-pulse material processing effort in metals, we have observed several phenomena that do not nec- essarily lend themselves to easy classification or explanation. In particular, even though high-quality holes can be drilled, several interesting features develop in the hole bottom. In this paper, we describe the evolution of this modulation in the hole crater for a range of incident laser fluences as well as function of time or shots. 1 Fluence and drill time effects Using a 120 fs, 6 W average power, 1 kHz Ti:sapphire laser system operating at 825 nm, holes were drilled at several an- gles of incidence in various metals, including stainless steel, * Corresponding author. (Fax: +1-925/422-5537, E-mail: pbanks@llnl.gov) COLA’99 – 5th International Conference on Laser Ablation, July 19–23, 1999 in Göttingen, Germany nickel alloys, and aluminum. We compared the resulting morphology for blind holes drilled in polished and rough- ened surfaces. Although the hole drilled on the rough surface begins with the crater bottom rougher and more modulated, after a few hundred shots the hole morphologies are essen- tially identical. Holes drilled under identical conditions look remarkably similar, indicating the presence of some mechan- ism which determines the scale lengths of the structure and modulation. The structures that we observed can be seen in Fig. 1, which shows SEM photomicrographs of holes drilled for 5000 shots at fluences from 0.25 to 2.7J/cm 2 (0.25, 0.5, 1.0, 1.5, 2.0, and 2.7J/cm 2 ). At low fluence, the crater bot- tom has some amount of texture, but the depth of modulation steadily increases and changes in nature as the fluence is increased. In fact, the hole shown in Fig. 1f shows the be- ginning of the formation of distinct, circular channels which will be discussed in more detail later. At the higher fluences (2–2.7J/cm 2 ), the scale of the modulation is similar to that shown by Chichkov et al. even though their results are for 390 nm light. The increase in surface roughness in the crater bottom is illustrated further in Fig. 2, which shows the measured depth (measured using the encoder on the z -axis of an optical mi- croscope with a small depth of field) from the workpiece surface to both the deepest point and the highest point in the crater bottom. The squares and pluses show the variation with fluence for the series shown in Fig. 1 (5000 shots) while the dots and x ’s crosses are for a series of only 125 shots per hole. As the hole is initiated at higher fluences, it is possible to develop features which extend well above the initial surface, which is indicated by the negative values in Fig. 2. As seen in this figure, there is a threshold fluence for the onset of the hole floor structure, as evidenced by the sepa- ration between the deepest and highest points: 1.5J/cm 2 for 125 shots and 0.5J/cm 2 for 5000 shots. For the 5 s exposures (5000 shots) in particular, the hole bottom becomes increas- ingly rough as the fluence increases above this threshold. It is also evident that there is some sort of saturation effect, particularly for the 5 s exposures where the deepest point of the hole is clamped at about 160 μ m above approximately 0.5J/cm 2 . Increasing the fluence beyond this point does not deepen the hole, but merely serves to increase the scale of the structure at the bottom of the hole.