Surfaces can be probed with a variant of secondary ion mass spectrometry (SIMS) where the bombardment is with a sequence of single keV projectiles, each resolved in time and space, coupled with the separate record of the secondary ions (SIs) ejected from each projectile impact. The goal of this study was to demonstrate an efficient mode of SIMS where one obtains valid analytical information with a minimum of projectiles and hence a minimum of sample consumption. An inspection of the ejected SIs from individual bombardment events will reveal ??super efficient?? collision cascades i.e., events, where two or more secondary ions were emitted simultaneously. It has been shown that these coincidental emissions can provide information about the chemical composition of nano-domains. Previous studies using coincidence counting mass spectrometry (CCMS) indicated an enhancement of identifying correlations between SIs which share a common origin. This variant of SIMS requires an individual projectile impact thus causing SI emission from a surface area of ~5 nm in radius. Thus, in an event where two or moreSIs are ejected from a single projectile impact, they must originate from atoms and molecules co-located within the same nano-domain. Au nanorods covered by a 16-mercaptohexadecanoic acid (MHDA) monolayer were analyzed using this methodology. A coincidence ion mass spectrum was obtained for the MHDA monolayer covered Au nanorods which yielded a peak for a Au adduct. Similar results were obtained for a sample with a MHDA monolayer on a Au coated Si wafer. A series of samples consisting of Cu aggregates and AuCu alloys were investigated by SIMS to demonstrate that this technique is appropriate for characterizing nanoparticles. The mass spectra of these samples indicated that Au200 4+ is an effective projectile to investigate the surface of the target because it was able to penetrate through the poly(vinylpyrrolidone) (PVP) stabilizer that coated the surface of these nanoparticles. Coincidence mass spectra of the Cu aggregates yielded molecules colocated within the same nano-domain. Finally, this methodology was used to investigate surface structural effects on the occurrence of ??super-efficient?? events. The results indicated that it is possible to distinguish between two phases of ??-ZrP compounds although the stoichiometry remains the same.

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The chemical characterization of individual nanoparticles (NPs) </= 100 nm in diameter is one of the current frontiers in analytical chemistry. We present here, a methodology for the characterization of individual NPs by obtaining molecular information from single massive cluster impacts. The clusters used in this secondary ion mass spectrometry (SIMS) technique are Au4004+ and C60+. The ionized ejecta from each impact are recorded individually which allows to identify ions emitted from a surface volume of ~10 nm in diameter and 5-10 nm in depth. The mode of analyzing ejecta individually from each single cluster impact gives insight into surface homogeneity, in our case NPs and their immediate surroundings. We show that when the NPs (50 nm Al) are larger than the size of the volume perturbed by the projectile, the secondary ion emission (SI) resembles that of a bulk surface. However, when the NP (5 nm Ag) is of the size range of the volume perturbed by projectile the SI emission is different from that of a bulk surface. As part of this sub-assay volume study, the influence of neighboring NP on the SI emission was examined by using a mixture of different types of NPs (5 nm Au and 5 nm Ag). The methodology of using cluster SIMS via a sequence of stochastic single impacts yield information on the surface coverage of the NPs, as well as the influence of the chemical environment on the type of SI emission. We also present a case of soft landing NPs for laser desorption ionization mass spectrometry. NPs enhance the SI emission in a manner that maintains the integrity of the spatial distribution of molecular species. The results indicate that the application can be extended to imaging mass spectrometry.

Combining cesium (Cs) bombardment with positive secondary molecular ion detection (MCs+) can extend the analysis capability of Secondary Ion Mass Spectrometry (SIMS) from the dilute limit (<1%) to matrix elements. The MCs+ technique has had great success in quantifying the sample composition of III-V semiconductors as well as dopants and/or impurities; however, it has been less effective at reducing the matrix effect for IV compounds, particularly Si-containing compounds, due to Cs overloading at the surface during the analysis from the Cs primary ion beam. The Cs overloading issue is attributable to the mobility and relocation of the implanted Cs to the surface; this effect happens almost instantaneously. Once the surface is overloaded with Cs, the excess Cs begins to reneutralize the ionization Cs and, as a result, the MCs+ technique is ineffective at reducing the matrix effect. This research provides new insights for improving the MCs+ technique and elucidating the Cs mobility. A combination of multiple experimental techniques and theoretical modeling was implemented to assess the Cs retention, up-take, and distribution differences between group III-V and IV materials. Early experiments revealed a temperature-dependent component of the Cs mobility, prompting an investigation of this phenomenon. Therefore, we designed, built, and installed a variable temperature stage for our SIMS with temperatures ranging from -150 to 300 C. This enabled us to study the temperature-dependent component of the Cs mobility and the effect it has on the secondary ion emission processes. Additionally, a method was devised to quantify the amount of neutralization and ionization due to the relocated Cs. The results allow for a more thorough understanding of the material dependence on the Cs+-sample interaction and the temperature component of the Cs mobility. / Ph. D.

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An instrument, adding the capability to measure 3D volumetric chemical composition, has been constructed by me as a member of the Sánchez Nano Laboratory. The laboratory's in situ atomic force microscope (AFM) and secondary ion mass spectrometry systems (SIMS) are functional and integrated as one instrument. The SIMS utilizes a Ga focused ion beam (FIB) combined with a quadrupole mass analyzer. The AFM is comprised of a 6-axis stage, three coarse axes and three fine. The coarse stage is used for placing the AFM tip anywhere inside a (13x13x5 mm3) (xyz) volume. Thus the tip can be moved in and out of the FIB processing region with ease. The planned range for the Z-axis piezo was 60 µm, but was reduced after it was damaged from arc events. The repaired Z-axis piezo is now operated at a smaller nominal range of 18 µm (16.7 µm after pre-loading), still quite respectable for an AFM. The noise floor of the AFM is approximately 0.4 nm Rq. The voxel size for the combined instrument is targeted at 50 nm or larger. Thus 0.4 nm of xyz uncertainty is acceptable. The instrument has been used for analyzing samples using FIB beam currents of 250 pA and 5.75 nA. Coarse tip approaches can take a long time so an abbreviated technique is employed. Because of the relatively long thro of the Z piezo, the tip can be disengaged by deactivating the servo PID. Once disengaged, it can be moved laterally out of the way of the FIB-SIMS using the coarse stage. This instrument has been used to acquire volumetric data on AlTiC using AFM tip diameters of 18.9 nm and 30.6 nm. Acquisition times are very long, requiring multiple days to acquire a 50-image stack. New features to be added include auto stigmation, auto beam shift, more software automation, etc. Longer term upgrades to include a new lower voltage Z-piezo with strain-gauge feedback and a new design to extend the life for the coarse XY nano-positioners. This AFM-SIMS instrument, as constructed, has proven to be a great proof of concept vehicle. In the future it will be used to analyze micro fossils and it will also be used as a part of an intensive teaching curriculum.

Graduation date: 1993