  Test beam measurements have been carried out with a 3D sensor on a Timepix3
ASIC and the time measurements are presented. The measurements are compared to
those of a thin planar sensor on Timepix3. It is shown that for a
perpendicularly incident beam the time resolution of both detectors is
dominated by the Timepix3 front-end. The 3D detector is dominated by the
time-to-digital conversion whereas the analog front-end jitter also gives a
significant contribution for the thin planar detector. The 3D detector reaches
an overall time resolution of 567(6)ps compared to 683(8)ps for the thin planar
detector. For a grazing angle beam, however, the thin planar detector achieves
a better time resolution because it has a lower pixel capacitance, and
therefore suffers less from jitter in the analog front-end for the low charge
signals that mainly occur in this type of measurement. Finally, it is also
shown that the 3D and thin planar detector can achieve time resolutions for
large clusters of about 100ps and 250ps, respectively, by combining many single
hit measurements.
