  We have employed a mesh experiment for back-illuminated (BI) CCDs. BI CCDs
possess the same structure to those of FI CCDs. Since X-ray photons enter from
the back surface of the CCD, a primary charge cloud is formed far from the
electrodes. The primary charge cloud expands through diffusion process until it
reaches the potential well that is just below the electrodes. Therefore, the
diffusion time for the charge cloud produced is longer than that in the FI CCD,
resulting a larger charge cloud shape expected.
  The mesh experiment enables us to specify the X-ray point of interaction with
a subpixel resolution. We then have measured a charge cloud shape produced in
the BI CCD. We found that there are two components of the charge cloud shape
having different size: a narrow component and a broad component. The size of
the narrow component is $2.8-5.7 \mu$m in unit of a standard deviation and
strongly depends on the attenuation length in Si of incident X-rays. The
shorter the attenuation length of X-rays is, the larger the charge cloud
becomes. This result is qualitatively consistent with a diffusion model inside
the CCD. On the other hand, the size of the broad component is roughly constant
of $\simeq 13 \mu$m and does not depend on X-ray energies. Judging from the
design value of the CCD and the fraction of each component, we conclude that
the narrow component is originated in the depletion region whereas the broad
component is in the field-free region.
