Thermal analysis of thin-film micromachining with lasers

Abstract

Laser micromachining of thin metallic films is promising to become an important processing step in the fabrications of electronic circuits. Important applications range from pattern generation of conductive paths to trimming of thin — film resistors. Since the thin films possess vaporization temperatures which generally are much higher than the melting temperature of the underlying dielectric substrate, short — duration pulses such as those available from Q-switched and cavity-dumped lasers must be used. Use of such short pulses minimizes potential thermal damage to the substrate and adjacent areas in the film. To predict the thermal state of the film and substrate, we have developed a theory which takes into account the heat loss due to conduction into the substrate, and the spatial distribution of the irradiating laser beam and its thermal effects in the film. Neglecting thermal gradients across the thickness of the film, we have been able to obtain a closed-form perturbation solution whose first-order solution accurately describes the temperature for many film thicknesses and pulse lengths. The second-order solution which accounts for conduction in a direction transverse to the laser propagation direction may contribute significatnly to the total temperature profile in some cases, particularly for pulse lengths of microsecond duration and longer. The theoretical analysis may be used to estimate the threshold intensity for vaporization of various thin films deposited on dielectric substrates. Estimation of threshold intensity for thin films of bismuth on Mylar and gold on glass is presented. This predicted threshold intensity for vaporization as derived from our first- and second-order solution compares favorably with limited published experimental results.