AVS 46th International Symposium
    Applied Surface Science Division Tuesday Sessions
       Session AS-TuM

Paper AS-TuM5
Characterization of Ultra-thin (2-3nm) Oxide Films using Low Energy Cs Ion Beams

Tuesday, October 26, 1999, 9:40 am, Room 6A

Session: Ion Beam Analysis and Depth Profiling
Presenter: D.F. Reich, Physical Electronics
Authors: D.F. Reich, Physical Electronics
B.W. Schueler, Physical Electronics
J. Bennett, Sematech
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The thickness of silicon dioxide used as the transistor gate dielectric in most advanced memory and logic applications has decreased below 7nm, and is predicted to fall to 2-3nm for the next generation of devices. Characterization of thin oxide and nitrided oxide films by SIMS is currently achieved in quadrupole mass spectrometers using low energy primary Cs+ ions (0.75-2keV) at an angle of incidence of 60 degrees. Under these conditions, the preferred analysis method is to use MCs+ secondary ion monitoring in +SIMS, as this results in reduced matrix effects. For SIMS analysis of 2-3nm films, it is desirable to reduce the primary beam energy below 750eV, since the decay length and depth resolution even at this energy are comparable to the film thickness. Unlike the CsM+ results obtained with 0.75-2keV Cs+ beams at 60 degrees incidence, results with @<=@ 500eV Cs+ at angles from 60-75 degrees show matrix CsSi+ signals that are not in equilibrium in the near surface of the silicon substrate. By contrast, a 500eV Cs+ beam does result in constant matrix signals within the substrate when using ?SIMS. Interpretation of signals such as SiN- are, however, complicated by matrix effects at the oxide / silicon interface. The quadrupole SIMS instrument we are using employs ion columns with floating extractors. For the Cs column this results in useful primary beam currents of approximately 60nA at 500eV, and approximately 30nA at 250eV. Optimized off-axis extraction is possible using a split extractor when tilting the sample to vary the impact angle. To understand the CsM+ matrix signal variations at low primary energies, we report on measurements at Cs primary beam energies from 250-750eV, and at incidence angles of 60 and higher. Both +SIMS and -SIMS protocols are explored to understand optimum analytical conditions for characterization and quantification of 2-3nm ultra-thin oxides and oxynitrides.