Raman spectroscopic investigation of stresses in microstructures of pyro-electric devices

摘要

Electronic microstructures show a great variety of geometric shapes like edges and stripes and features like columns which can cause large stress concentrations decreasing the reliability and life time of devices. Otherwise, the properties of materials e.g. ferroelectric layers, are dependent on mechanical stress. Raman spectroscopy provides the possibility to measure stresses in silicon with a spatial resolution of about 1 to 2 microns and with an accuracy of about 20 MPa using the piezo-spectroscopic effect. The frequency shift of Raman modes can be related to the mean hydrostatic stress in the material which is described by an empirical relation. Silicon dominates the microelectronic technology. The present contribution demonstrates how spectroscopic data measured on silicon in complex thin film structures can be interpreted in terms of stresses with a combined modeling of the measurement process and the stress field using the finite element method. Pyro-electric devices consisting of laterally structured lead zirconate titanate thin films and metal electrodes have been investigated. The films are deposited on silicon substrates or on poly-silicon membranes covering evacuated cavities to realize thermal isolation. Stresses in individual films and in the substrate near critical points of microstructures, e.g. at edges and near columns which carry a membrane, have been predicted theoretically by finite element calculations. A 2D finite element model was developed to describe experimental stress profiles. It was used to estimate film stresses in individual microstructures by a fitting procedure.