Femtosecond laser interactions with Co/Al multilayer films
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Femtosecond laser interactions with Co/Al multilayer films Yoosuf N. Picard1,2, David P. Adams3, and Steven M. Yalisove1,2 Dept. of Materials Science and Engineering, University of Michigan, Ann Arbor, MI 48109-2176, U.S.A. 2 Center for Ultrafast Optical Science, University of Michigan, Ann Arbor, MI 48109-2099, U.S.A. 3 Advanced Manufacturing Processes Lab, Sandia National Laboratories, Albuquerque, NM 87185-0959, U.S.A. 1
ABSTRACT Reactive multilayer films of Co and Al were irradiated using femtosecond and nanosecond pulse-length lasers. While no ignition of a self-propagation reaction occurred during the laser machining studies, we observe considerable differences in the morphology and extent of damage induced by femtosecond and nanosecond lasers. Using scanning electron microscopy, we show that single metal layers can be removed at the micron scale with negligible damage to the underlying layers using femtosecond pulse-length lasers. INTRODUCTION The advantage of limited thermal dissipation into the material being machined has made femtosecond lasers useful for controllably machining explosive materials [1,2]. Recent work has extended this application to the cutting of reactive multilayer foils that can be used for roomtemperature soldering components [3]. Such reactive foils are composed of alternating layers of two different metals that, when intermixed, form a stable phase through a highly exothermic reaction [4]. Ignition of these foils occurs once adequate interdiffusion of atoms between layers has occurred and adequate heat is produced from the reaction to propagate a reaction front through the entire foil. In this study, we seek to cut completely through a reactive Co/Al multilayer foil without the ignition of a self-propagating reaction. Co/Al multilayer thin films were chosen because a self-propagating, high temperature reaction has been observed by our group when ignited using an electronic match. Co/Al has a moderate enthalpy of formation equal to -60 kJ/mole of atoms [5] and a high adiabatic reaction temperature. This material pair is also of potential use for joining [6], because it forms a B2 crystal structure upon reaction. A B2 structure is often ductile.
EXPERIMENTAL DETAILS Co/Al multilayers were deposited in a Unifilm, Inc. sputter deposition system having a base pressure of 5 x 10-9 Torr. Co and Al thin film densities were first determined using x-ray reflectivity in separate experiments, and layer thicknesses were chosen to lock in a 1:1 stoichiometry in the multilayers. Films were grown using a 10 mTorr argon working pressure to total thicknesses of 7.5 µm. Foils of different design (varying bilayer thickness) were deposited onto Si substrates having a thermal oxide and removed from the deposition system for delamination and mounting as freestanding foils. Mounted freestanding foils confirmed a self-
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propagating reaction when ignited using an applied current and were used for determination of reaction rate. Steady state propagation speeds were measured and shown to var
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