  We report results of high-resolution TOF SIMS (time of flight secondary ion
mass spectrometry) depth profiling experiments on a nanolayered structure, a
stack of 16 alternating MgO and ZnO ~5.5 nm layers grown on a Si substrate by
atomic layer deposition. The measurements were performed using a newly
developed approach implementing a low energy direct current normally incident
Ar+ ion beam for sample material removal by sputtering (250 eV and 500 eV
energy), in combination with a pulsed 5 keV Ar+ ion beam at 60{\deg} incidence
for TOF SIMS analysis. By this optimized arrangement, a noticeably improved
version of known dual-beam (DB) approach to TOF SIMS depth profiling is
introduced, which can be called gentleDB. We apply the
mixing-roughness-information model to detailed analysis of experimental
results. It reveals that the gentleDB approach allows ultimate depth resolution
by confining the ion beam mixing length to about 2 monolayers. This corresponds
to the escape depth of secondary ions, the fundamental depth resolution
limitation in SIMS. Other parameters deduced from the measured depth profiles
indicate that a single layer thickness equals to 6 nm so that "flat" layer
thickness d is of 3 nm and interfacial roughness {\sigma} is of 1.5 nm thus
yielding d+2\bullet{\sigma}=6 nm. In essence, we have demonstrated that the
gentleDB TOF SIMS depth profiling with noble gas ion beams is capable of
revealing structural features of a stack of nanolayers, resolving its original
surface and estimating the roughness of interlayer interfaces, which is
difficult to obtain by traditional approaches.
