Characterization of Low-Fluence Excimer Laser Annealing of AlN Thin Films

A process for laser annealing aluminum nitride (AlN) thin films with a low-fluence excimer laser has been characterized. Residual stress inherited during film deposition can cause undesirable effects, such as delamination and cracking, in MEMS devices. Current methods of furnace annealing require 30 minutes or longer, but laser annealing can reduce this anneal time down to less than 1 minute. This makes laser annealing a valuable time saving tool in the fabrication process of MEMS devices.

The laser used in this study was a 40 ns KrF excimer laser with a 248 nm wavelength. A fluence of ~0.72 J/cm2 at a repetition rate of 4 Hz was selected to ensure no melting of the film. From a simple one dimensional analysis, we determined the average temperature of the film to be ~1100 K. We verified this analysis with a finite element model using FEMLAB® software and found the maximum temperature to be ~1450 K and the average temperature of the film to be ~1200 K which was close to the result from our simplified analysis. The film cools rapidly after the 40 ns pulse, so once the next pulse is applied, the film has already cooled back down to room temperature. Therefore, the temperature of the film cycles between the maximum temperature and room temperature for multiple pulses.

From experimentation, the best reduction in the strain gradient in our microcantilevers was observed with an application of 100 pulses of laser energy before release (from the substrate) with the laser parameters mentioned above. At higher applications (150-200 pulses), we observed film destruction upon release. The extent of the damage depends on the film and the laser parameters. For our film, 100 pulses was sufficient to anneal the film with minimal damage.
AFM scans and x-ray diffractometry were used to characterize the film before and after laser irradiation to ensure no damage to the film. By measuring the rocking curve before and after irradiation, we can compare the crystalline quality to determine the effect of the laser energy on the piezoelectricity of the film. An AFM scan of the film surface revealed no ablation of the film. Even after 150 pulses, no damage was observed, both on the surface as well as with the crystalline quality.