A thin film of aluminum, detected by x-ray photoelectron spectroscopy, is left at the surface of aluminum nitride (AIN) substrates exposed to a high intensity excimer laser beam of UV radiation. Due to the presence of this film, there is a decrease in the surface resistivity of the substrate with increasing number of laser pulses. In addition, line profilometry shows a decrease of the surface roughness with the number of pulses.
The thermal decomposition of AIN is assumed to take place in two stages. In the first, liquid aluminum is produced together with the evolution of gaseous nitrogen, and in the second, aluminum evaporates. Using a computer model to simulate the laser heating cycle, it is shown that the thickness of the aluminum film saturates at a given laser energy density. The saturation thickness is a strong function of the substrate absorption and reflectivity and, therefore, of the laser light frequency. The influence of the substrate roughness on the electrical resistivity of the aluminum film is discussed.
The application of this process to direct laser writing in high density hybrid circuits is illustrated. During hole drilling by excimer laser, a thin aluminum film is continuously produced at the hole walls. This process can also be employed for substrate planarization.