Interference effects in laser micromachining of thin films on silicon

Abstract

Some recent designs of new electronic circuits on silicon have been implemented by laser removal of thin metallic film from the composite structure. The presence of an insulating dielectric region between the thin metallic film and the silicon substrate gives rise to a complex laser-micromachining process whose understanding is crucial to successful applications. In this paper we explore the laser-machining process at 1.06 μm for a tantalum nitride thin-film resistors material on top of a dielectric film on silicon. We experimentally verify the existence of strong optical interference effects in laser machining of these resistors and determine their influence on various laser-trimming parameters. Good correlation is obtained between an analytical model for these interference effects and the experimental data. For structures containing a single SiO2 layer with thickness up to 1 μm between the tantalum film and silicon, it is observed that from a laser-trimming standpoint, a SiO2 thickness of 9000 or 5400 Å is preferred to all others. A selection of either one of these thickness will lead to a wider laser-trimming-process window. These results could, therefore, be viewed as structure design guidelines for laser trimming by material removal at 1.06 μm.