  Measuring the thermal conductivity of sub-surface buried substrates are of
significant practical interests. However, this remains challenging with
traditional pump-probe spectroscopies due to their limited thermal penetration
depths (TPD). Here, we experimentally and numerically investigate the TPD of
recently developed optical pump-probe technique steady-state thermoreflectance
(SSTR) and explore its capability for measuring the thermal properties of
buried substrates. The conventional definition of the TPD does not truly
represent the upper limit of how far beneath the surface SSTR can probe. For
estimating the uncertainty of SSTR measurements of a buried substrate a priori,
sensitivity calculations provide the best means. Thus, detailed sensitivity
calculations are provided to guide future measurements. Due to the steady-state
nature of SSTR, it can measure the thermal conductivity of buried substrates
typically inaccessible by traditional pump-probe techniques, exemplified by
measuring three control samples. We also discuss the required criteria for SSTR
to isolate the thermal properties of a buried film. Our study establishes SSTR
as a suitable technique for thermal characterizations of sub-surface buried
substrates in typical device geometries.
