Ruthenium thin films are deposited by atomic layer deposition (ALD) from bis(N,N'-di-tert-butylacetamidinato)ruthenium(II) dicarbonyl and O2. Highly conductive, dense, and pure thin films can be deposited when oxygen exposure, EO, approaches a certain threshold (Emax). When EO > Emax the film peels off due to the recombinative desorption of O2 at the film/substrate interface. Analysis by atomic probe microscopy (APM) shows that the crystallites are nearly free from carbon impurity (<0.1 at.-%), while a low level of carbon (<0.5 at.-%) is segregated near the grain boundaries. APM also shows that a small amount of O impurity (0.3 at.-%) is distributed uniformly between the crystallites and the grain boundaries.