The ability to analyze the chemical structure and composition of solid surfaces on a length scale of nanometers is key in supporting the development of devices with nanometer dimensions.
Time-of-flight secondary ion mass spectrometry (TOF-SIMS) with focused ion beams can deliver information on composition and molecular structure with lateral resolutions down to a few hundred nanometers. Sensitivity and lateral resolution are, however, interdependent. The number of molecular species in the area of a 100 nm by 100 nm pixel is about 5 x 104, and the number of secondary ions that can be detected from this area is limited by the ionization probability in the sputtering event (typically <10-3). We address this limitation by use of very highly charged instead of singly charged ions as projectiles in TOF-SIMS [1-3]. Recent progress in ion source development at LLNL has made beams of slow (Ekin=1 keV to 1 MeV) Xe44+ or Au69+ available for ion-solid interaction studies . Such highly charged ions deposit tens to hundreds of keV of electronic excitation energy into nanometer size target volumes. Both secondary ion yields and ionization probabilities, i. e., the number of secondary ions emitted per unit of sputtered material from oxidized surfaces, increase strongly as a function of projectile charge. Yields of detected secondary ions are over two orders of magnitude higher than for singly charged ion sputtering. Detection of two or more secondary ions from the same impact event enables the application of coincidence counting techniques for analysis of correlations in secondary ion emission. The area probed by individual projectiles has a diameter of about 10 nm [1, 3]. In this approach, no direct image of surface species is obtained. However, analysis of correlations from many impact events removes the limitation due the small number of surface species in each impact area. Consequently, information on chemical composition and homogeneity of target surfaces becomes available on a length scale of about 10 nm. In our presentation, we will present results on nano-environments of gramicidin S molecules in solid matrixes and on nano-clusters of transition metals on SiO2/Si surfaces.
This work was performed under the auspices of the U. S. Department of Energy by Lawrence Livermore National Laboratory under contract No. W-7405-ENG-48.
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Dr. Thomas Schenkel
Chemistry & Materials Science Department, Lawrence Livermore National Laboratory
P. O. Box 808, L-414, 7000 East Ave., Livermore, CA 94550
Phone: 925-423-0106; Fax: 925-422-5940