Paper AS-WeA4
Application of TOF-SIMS and LEIS for the Characterization of Ultra-Thin Films

Wednesday, October 22, 2008, 2:40 pm, Room 207

The continuous scaling of semiconductor devices (film thickness, junction depth) to smaller dimensions increases the challenges for the established analytical methods. In addition, new and more complex materials are introduced into the devices. Secondary Ion Mass Spectrometry (SIMS) and more recently Time-of-Flight (TOF-) SIMS have become workhorses for the semiconductor industry. The depth distribution of implants as well as impurities is regularly monitored using Cs and O₂ sputter depth profiles. Meanwhile sputter energies as low as 200 eV are routinely used in order to achieve a high depth resolution. Even at these low sputter energies the transient effects prevent establishing a reliable depth scale and deriving quantitative information on the first few nm. In addition, concentrations exceed the dilute limit, thus further complicating the quantification process.In this situation, complementary techniques become increasingly important. One of the emerging techniques is Low Energy Ion Scattering (LEIS) using high sensitivity analyzers. It is extremely surface sensitive and provides quantitative information on the outermost atomic layer of a solid. Low energy (1 – 10 keV) noble gas ions (He, Ne, ...) are used as projectiles. The energy of the back-scattered ions from elastic collisions is characteristic for the masses of the target atoms and is measured using an electrostatic energy analyzer. Quantification is comparatively straightforward and bulk standards can be used to determine the sensitivity factors. In addition, information can be gained on the surface near depth distribution of elements (1 – 10 nm) by measuring the extra energy loss of ions scattered in deeper layers (non destructive, "static mode"). This can be combined with conventional sputter depth profiling using a separate beam of low energy noble gas ions ("dynamic mode").
 
In order to compare the performance of LEIS with TOF-SIMS results for the characterization of ultra-shallow layers we studied a variety of typical sample systems such as ultra-shallow As implants, high-k films, and diffusion barriers at different stages of ALD growth. The results are discussed with respect to information depth, ease of quantification and sensitivity. It can be shown that the static profiling approach of LEIS is extremely powerful in the first few nm of the profile whereas due to its higher sensitivity SIMS performs better after sputtering through the transient region.