In semiconductor technology, strain can be used as a concept to improve the mobility of the charge carriers in the channel region and boost the performance of the eventual device. Control over the precise strain levels in confined volumes is difficult and non-destructive local stress measurements remain a challenge for the semiconductor industry. Recently however, it was found that a periodic array of semiconductor fins can act as a photonic crystal where the incident light is focused into the channel material and light transmission into the surrounding oxide is forbidden. This confines the excitation light into the region of interest and enhances the Raman scattering over orders of magnitude. At the same time, the coupling of light into the surrounding material is blocked resulting in screening of parasitic signals coming from, for instance, the substrate. The result is that even though the region of interest makes up only a small fraction of the total probed volume, it dominates the Raman spectra. As such, this nanofocusing of light has re-enabled the application of Raman spectroscopy for the purpose of stress and composition measurements in structures far beyond the diffraction limit. Additionally, the reliable Raman characterization of anisotropic stress profiles requires the independent detection of TO and LO phonons, which is achieved through the use of a high-NA objective for the Raman microscope. In this work we show how this approach can be combined with intrinsic enhancement effects to reach quantitative stress metrology without the need for additional sample preparation. As a demonstration, measurements of the anisotropic biaxial stress along and across finFET structures with channel widths down to 20 nm are discussed. These fins have been designed to have uniaxial stress in the direction of the channel in order to enhance the transport properties, but our measurements show that a small but non-zero compressive stress component is also present in the direction across the channel. The results were validated using nano-beam diffraction measurements confirming the trends observed with Raman. This indicates that the combination of high-NA sample excitation and nano-focusing of the light into the sample results in quantitative stress measurements at the nanoscale using a non-destructive optical technique.

FIGURE 1. Raman scan along a 20 nm-wide finFET channel showing anisotropic biaxial stress components along with the scattering intensity.