Looking at the nanomechanics of electronic devices under the scanning electronic microscope

Abstract

Modern industry routinely uses sophisticated microscopic methods for quality control and for predicting the performance of manufactured products. These processes typically exploit optical emission and scattering phenomena—such as electron backscattered diffraction and energy-dispersive x-ray spectroscopy—using equipment attached to conventional scanning electron microscopy (SEM). In silicon-based metal oxide semiconductor (MOS) devices, residual stresses may remain from the manufacturing process owing to a mismatch in thermal expansion coefficients between constituent materials. Thermal fatigue and electromigration (displacement of atoms induced by electric current) phenomena can also affect the devices’ micromechanical state. Up to now, SEM has not been applied to these problems for two reasons. First, the effects of strain on electronic transitions arising from band-gap or optically active point defects, as well as their interaction with the electron probe, were not fully understood. Second, no comprehensive algorithm could be found that took into account the superposition of several chemical, physical, and mechanical effects (i.e., temperature gradients, self-absorption, chemical gradients) to extract the stress information from the luminescence spectrum. Yet, the spatial resolution of the SEM would be highly advantageous in residual stress assessment. We recently developed a spectroscopic method for analyzing materials and devices in the SEM that works by visualizing surface and subsurface stress fields. Further, we have used the technique to solve a number of important industrial problems, for example, the stress stored in interconnect structures (like inFigure 1. An SEM image (a) and a CL/PS stress image (b) of the immediate neighborhood of the crack tip in a gallium nitride crystal. The stress map represents the spatial variation of the trace of the stress tensor acquired with a spatial resolution of 30nm.