Abstract

Physical and chemical transformations induced upon sintered aluminum nitride (AIN) ceramic materials under KrF excimer laser irradiation have been studied in order to improve its properties and make this advanced material interesting for microelectronic applications or high power electrical systems. Due to its high thermal conductivity (180 W.m^-1.K^-1 at 20 °C), its high electrical resistivity (10¹⁴ Ω.cm at 25 °C), a low thermal expansion coefficient (5.10^-6 K), a high temperature resistance (sublimation temperature 2500 °C), a very high hardness (1100 Kgf.mm^-2) and a relative chemical inertia, AIN is a very attractive material for industrial uses[1]. Experiments were performed using a LPX 220i KrF-248 nm- excimer laser able to deliver 0.4 J in 20 ns (FWHM) at 1-200 Hz repetition rate. Experimental conditions were chosen as following: incident fluence between 0.5 and 5 J/cm², up to 100 shots in atmospheric pressure ambient air. The material investigated was a commercial sintered ceramic crystallized in the wurtzite structure. After irradiation, depending on the fluence level, interaction processes induce color and topography changes in the impacted areas: - a reversible one, from the initial color (white/grey) to brown-yellow at low fluence (F< 0.5 J/cm²); - a irreversible one, from white to silver-grey at higher fluence (> 1J/cm2) with roughness and porosity modifications. This last process suggests a melting and vaporization of the surface and a one-step metallization process confirmed by different surface analysis techniques [2]. A molten layer is produced in the bulk material (200 nm) and the initial roughness (0.35 μm Ra) is decreased. Auger Electron Spectroscopy, Raman Spectroscopy and X-ray Photoelectron Spectroscopy gave results on composition and chemical bondings onto the extreme surface and in the molten layer. Low incidence angle X-ray diffraction method characterized elements and structure. These analysis revealed the obtaining of a electrical conductive path onto the substrate constituted in an Aluminum-rich layer. AES showed alumina with oxygen, carbon and nitrogen on foe top characteristic of a contamination layer before irradiation; after treatment, Al₂O₃ appears on foe top, Al in the melted depth (after sputtering). The formation of this aluminum layer is confirmed by XPS analysis where Al peak is shifted from 78.5 to 79.7 eV. On the Ruman spectra, foe three Al-N bonds desappeared after irradiations evidencing the metallic aspect of the surface. X-Y electrical resistance measurements were performed in order to confirm this metallization process using a classical four-probes measurement head The resistance (infinite on the initial material) drops to some Ω on the irradiated area (2 J/cm²; 20-50 shots) with a very sharp transition zone (a few microns). These experiments shown it is possible to obtain a conductive path by a chemical modification of the insolating ceramic surface with a very flexible way using an excimer laser, mask projection or micromarking by a scanning system. Measurements of the electrical resistivity evolution of the aluminum-rich conductive path with time under industrial conditions (ambient air, humidity, temperature) are in progress in the laboratory.

© 1998 IEEE