The challenge for secondary ion mass spectroscopy is to accurately measure the profile shape for low-energy implants within the first few nanometers as well as to precisely determine the junction depth in the structure after any thermal treatment. Even if knowledge of the exact profile shape is not required for dose measurement, this information becomes essential for process modeling. This article presents results on the accurate determination in depth distribution of shallow As and P implants in Si. Sub-keV impact energy is investigated for O-2(+) and Cs+ primary ions. An in situ laser interferometer providing a real-time record of the sputter rate is used for the depth scale calibration. Fundamental and instrumental effects limiting the depth resolution, the sensitivity, and the accuracy are discussed. (C) 2004 American Vacuum Society.