Extension of micro-Raman spectroscopy for full-component stress characterization of TSV structures

In this paper, we extend the micro-Raman technique for measurement of all three normal stress components around the through-silicon via (TSV) structure, as compared with the traditional technique of measuring the sum of the normal stresses. In this technique, a high numerical aperture (NA) objective is used to observe the two additional lattice vibration modes (Mode 1 and 2) together with the conventional longitudinal optical mode (Mode 3) so that all three normal stress components can be determined separately. [1] In our micro-Raman system, all the three vibration modes become measurable with numerical aperture (NA) exceeding 0.40 and with spectral intensities increasing with NA. The relatively weak signal strengths of Mode 1 and 2 necessitate careful calibrations of the signal-to-noise ratio and the optimization of the laser power level. This led to a choice of NA of 0.85 with 100× objective in the experiments. This method was applied for a full characterization of the stress components in a 2 by 2 array of blind copper TSVs with 5 μm diameter, 55μm depth and covered with 2μm oxide. The accuracy of Raman measurements was calibrated as a function of polarization configuration, laser power (2mW to 10mW). The experimental results show a good agreement with FEA simulation.