Experimental challenges of stress measurements with resonant micro-Raman spectroscopy

Micro-Raman spectroscopy allows one to measure stress in crystalline materials. The method is non-destructive and provides microscopic lateral resolution. In this paper we show that resonance excitation using an ultraviolet (UV) laser line strongly enhances the depth resolution in micro-Raman spectroscopy. The Raman line shift becomes a true picture of the stress-field in a 12 nm thick surface layer, whereas visible light averages over a depth of some hundreds of nanometers. We review the effects of defocusing and inhomogeneous scattering, and present results obtained from a processed wafer. Measurements of stress fields in a sample taken from a typical silicon integrated circuit process prove the strongly enhanced resolution. The UV light, therefore, allows one to resolve stress components averaged out by longer wavelength light, giving a much better way to pinpoint areas of critical stress levels that would likely lead to defects in subsequent processes in silicon microelectronic production.