Strain at SiSiO₂ interfaces studied by Micron-Raman spectroscopy

Micro-Raman spectroscopy of optical phonons is applied to analyse local stress at microstructured SiSiO₂ interfaces. The short penetration length of visible light in silicon and focussing the laser beam allow local measurement in the submicron range. Special emphasis is put on the observation of different phonon modes in order to determine the direction and symmetry of strain. The polarisation selection rules of Raman backscattering are calculated taking into account the enlarged solid angle of detection through a microscope object-lens. Weak symmetry forbidden phonon modes are observed due to this enlarged angle. The measured mode splitting supplies information on the anisotropic character of strain. The selection rules allow a distinction of uniaxial and biaxial strain by observation of different phonon modes in Raman backscattering from cleavage planes. The additionally produced surface causes just partial stress relaxation. Scanning depth profiles of phonon frequencies reveals homogeneous strain into the silicon to a depth of approximately 1 ωm. The effects of implanted layers and free carries on phonons are discussed as well.