Pulsed laser repair of adhered surface-micromachined polycrystalline silicon cantilevers

Abstract

A technique for repairing adhered surface-micromachined polycrystalline silicon (polysilicon) structures using pulsed lasers has been developed, experimentally investigated, and modeled. Advantages of using lasers to repair adhered microfabricated structures include the ease of implementation, the ability to apply the laser energy locally to the adhered region, and the non-contact nature of the process. The amount of thermal heating is typically less than 100 °C and does not damage polysilicon structures. Pulsed laser repair has been successful for undoped and n-doped polysilicon microcantilevers that had adhered to the substrate during chemical etching of the sacrificial layers and subsequent drying. This paper reviews experiments investigating the use of Ti:sapphire and Nd:YAG laser systems to repair failed polysilicon microcantilevers. The first experiments used a Ti:sapphire laser system, and later experiments using a Nd:YAG laser produced repair yields up to 100% for 10 μm wide cantilevers that were as long as 1 mm with laser fluences of 70 mJ/cm2. While the Ti:sapphire laser experiments indicate contributions from both non-thermal and thermal mechanisms for the repair process, the Nd:YAG laser experimental results are consistent with a thermal mechanism, specifically thermal expansion. A thermomechanical model is presented which enables the prediction of the laser operating conditions needed for laser repair. The model calculates the increase in strain energy due to heating a cantilever to a temperature higher than the substrate, and equilibrium crack lengths are determined by equating the adhesion energy and the strain energy release rate. The equilibrium crack lengths predicted by the model agree qualitatively and are within 15% of measured crack lengths. Increased adhesion energy decreases the equilibrium crack lengths for both single and multiple pulse laser operations. The laser repair process is predicted to increase the free lengths of micromachined polysilicon cantilevers from that achieved immediately after the drying process.