  The properties of artificially grown thin films are often strongly affected
by the dynamic relationship between surface growth processes and subsurface
structure. Coherent mixing of X-ray signals promises to provide an approach to
better understand such processes. Here, we demonstrate the continuously
variable mixing of surface and bulk scattering signals during real-time studies
of sputter deposition of a-Si and a-WiS2 films by controlling the X-ray
penetration and escape depths in coherent grazing incidence small angle X-ray
scattering (Co-GISAXS). Under conditions where the X-ray signal comes from both
the growth surface and the thin film bulk, oscillations in temporal
correlations arise from coherent interference between scattering from
stationary bulk features and from the advancing surface. We also observe
evidence that elongated bulk features propagate upward at the same velocity as
the surface. Additionally, a highly surface sensitive mode is demonstrated that
can access the surface dynamics independently of the subsurface structure.
