Oxide layer reduction and formation of an aluminum nitride surface layer during femtosecond laser surface processing of aluminum in nitrogen-rich gases

摘要

Femtosecond laser surface processing (FLSP) is a unique material processing technique that can produce self-organizedmicroanostructures on most materials including metals, semiconductors, and dielectrics. These structures havedemonstrated the enhancement of surface properties such as heat transfer and broadband light absorption. The chemicalcomposition and morphology of FLSP structures is highly dependent on processing parameters including backgroundgas composition, pressure, laser fluence, and number of laser pulses. When the laser processing is carried out in openatmosphere, a thick oxide layer forms on the FLSP surface structures due to the high reactivity of the surface with theenvironmental constituents immediately after laser processing. In this work, N_2 and forming gas are used during laserprocessing in an effort to form a metal nitride on the surface of aluminum. Aluminum nitride is a promising material forenhancing the heat transfer performance of surfaces because of its thermal conductivity, which can be as high as 285W/m-K, whereas aluminum oxide has a low thermal conductivity (30 W/m-K). Aluminum nitride incorporation intoFLSP surfaces has the potential to act as a passivation layer to decrease the oxygen content and increase the thermalconductivity of the surface. Nitrogen incorporation is studied by applying FLSP in air, N_2, and a 95% N_2/5% H_2 mixture. The chemical composition of the FLSP surfaces is determined by X-ray photoelectron spectroscopy (XPS) andenergy-dispersive X-ray spectroscopy (EDS). Cross-sectional analysis of the FLSP microstructures is performed usingion beam milling.