Achieving sub-nm depth resolution with EXtreme Low Impact Energy SIMS
Newest semiconductor chip manufacturing and further scaling down of CMOS devices push the limits of junction depths below the 10nm range, with a profile steepness of 1-2nm per decade. At such scale, the SIMS technique can be used to monitor in-depth distributions of dopants, provided that SIMS profiles can be measured with a depth resolution better than 1nm per decade.
A challenge for SIMS tools
This can only be achieved by using very low energy primary ion bombardment: that is, energies significantly lower than those applied so far (300-500 eV) are mandatory. There are 2 main limitations to implementing Extreme Low Impact Energy (EXLIE) sputtering conditions for SIMS:
- a dramatic drop in sputter yield is observed with impact energies below 250eV;
SIMS instruments have been optimized for operating with energies > 300eV, therefore, their primary ion columns do not deliver high current densities at lower impact energies, and only extremely low sputter rates (~0.1nm/mn) are available.
Recent innovations on the Cs+ and O2+ ion sources of the SC Ultra and IMS Wf have improved the primary beam density at very low impact energies, thereby giving access to sputter rates of 1nm/min for both Cs+ and O2+ @ 150eV.
The benefits of EXtreme Low Impact Energy
The transient sputtering processes (sputter and ion yield variation) decreases with the impact energy. This is demonstrated by data presented in Figure 1 for a BF2 2keV implant in Si sample measured at three different O2+ energies: 500, 250 and 150eV. The more realistic profile shape at the near surface (Gaussian shape) is obtained for the 150eV impact energy.
Accurate ultra shallow depth profiles
Depth profiles recorded under EXLIE conditions are shown for a selection of B, P and As in Si shallow implants.
It is noticeable that a profile dynamic range of more than 5 decades can be achieved even at extremely low impact energies, and the data show that the use of very low impact energies does not affect the dynamic range of the elements of interest. It must be emphasized that the use of EXLIE does not cancel matrix effect between SiO2 and Si. Because a silicon oxide layer is always present at the surface of silicon with a thickness not negligible compared to the depth of interest for dopant implants, accurate profile quantification still requires a dedicated data reducing algorithm.
Results for as-implanted B, P, and As profiles using Extremely Low Impact Energy (EXLIE) sputtering conditions and quantified with an appropriate algorithm have been successfully compared with HR-RBS and ERDA profiles .
 A. Merkulov et al, Advanced secondary ion mass spectroscopy quantification in the first few nanometer of B, P, and As ultra shallow implants, J. Vac. Sci. Technol. B 28, C1C48 (2010) .