Correlation of local strain and temperature measurements in confocal Raman microscopy

In microsystem technologies, it is very common to employ Raman spectroscopy to monitor the strain generated during the fabrication of microelectromechanical systems and integrated circuits devices, an example being through-silicon vias. Typically, for strain analysis, the laser intensity is chosen to be sufficiently low to avoid the laser heating affecting the position of the Raman lines under consideration, because theoretically, the measured strain can have two origins: for one as a consequence of mechanical stress through Hooke's law, and for the other, as a consequence of temperature. The latter has often been overlooked throughout literature. Here, we revisit a couple of cases, for which tensile strain is detected, by comparing them to Raman analysis on samples of empty through-silicon vias, that were expected to be strain free. By simultaneously monitoring strain and the local temperature through the ratio of anti-Stokes to Stokes lines, it is found that at least part of the strain usually attributed to mechanical stress can be quantitatively attributed to laser-induced thermal expansion, even at relatively low laser intensities. We support our findings using a finite element model and present key implications on the interpretation of strain measurements in copper-filled through-silicon vias.

Automated summary

The study indicates that Raman spectroscopy can be used in microsystem technologies to monitor strain, however, the measured strain may originate from mechanical stress or temperature effects, with the latter often overlooked. By comparing with empty through-silicon vias samples, it was found that part of the strain can be quantitatively attributed to laser-induced thermal expansion.