  We report a detailed experimental and theoretical investigation of the effect
of residual strain, and strain relaxation, which manifests itself at the
Si/SiO$_{2}$ interfaces in commercial silicon-on-insulator (SOI) wafers. SOI
material is made of a single-crystal silicon overlayer (SOL) on top of an
insulator (buried SiO$_{2}$ layer) sitting on a handle silicon wafer. Infrared
reflectivity spectra show that the buried SiO$_{2}$ layer relaxes continuously
when thinning the SOL. At the same time the SOL surface roughness and the
linewidth of optical phonons in Si near the Si/SiO$_2$ interface (probed by
micro-Raman specroscopy) increase. In the as-delivered wafers, this comes from
a slight expansion of Si on both sides of the buried SiO$_{2}$ layer which,
conversely, is compressed. Thinning the SOL modifies these initial equilibrium
conditions. To get quantitative results, we have modeled all our Raman spectra
using a theory of inhomogeneous shift and broadening for optical phonons, which
takes into account the phonon interaction with the static strain fluctuations.
  From the variation of linewidth versus interface distance, we have found that
the mean squared strain continues to relax in the bulk of the wafer through a
depth on the order of several $\mu m$. We also show that the SOL surface
roughness is related to strain fluctuations near the Si/SiO$_2$ interfaces.
