S. Matteson

Triply-ionized B2 molecules from a tandem accelerator

Abstract Beams of 10B3+2, 11B23+, and 10B11B3+ ions have been observed to emerge from a tandem accelerator. B2− mo into the accelerator, and positive ions emerging from the machine were analyzed for mass per charge and total energy. For 10B11B, intensities of singly-, doubly-, and triply-charged molecules were measured as a function of N2 gas pressure in the accelerator terminal stripper canal. These intensities were found to exhibit the same qualitative behavior for all charge states, with the number of triply-charged molecules a factor of ~ 2 × 10−4 less than for the doubly-charged molecules. No quadruply-ionized molecules were seen. The observation of particles corresponding to the breakup products of the multiply-ionized molecules indicates that these species were decaying in flight, and are apparently metastable with lifetimes of ~ μs. Comparisons are made between these observations and molecular orbital calculations.

Impurity determination in electronic materials by accelerator mass spectrometry

Abstract An Accelerator Mass Spectrometry (AMS) facility has been developed in the Ion Beam Modification and Analysis Laboratory (IBMAL) at the University of North Texas (UNT) through a collaboration between UNT, Texas Instruments, Inc., the National Science Foundation, and the Office of Naval Research. The computer-controlled AMS instrument, which allows automated mass scans of stable isotopes in solid materials, removes molecular interferences which are present in Secondary Ion Mass Spectrometry (SIMS) allowing higher sensitivities for some elements than SIMS. A new low sample-contamination, raster-scanning, depth-profiling ion source has been constructed for this AMS facility. This source coupled to the AMS system should provide sensitivities of ppt (1 part in 10 12 or 10 10 atoms/cm 3 ) for many elements in the periodic table. The AMS system design is discussed including ion generation with the new ion source, ion analysis with the tandem accelerator and spectrometer, and ion detection hardware and software. Each of these functions may be viewed as a sub-assembly with a dedicated IBM computer controlling the appropriate functions. Results using the AMS facility to characterize trace element impurities in Si and CdZnTe electronic materials are discussed.

Accelerator mass spectrometry at the University of North Texas

An accelerator mass spectrometry system designed for analysis of electronic materials is being developed and installed on the University of North Texas 3 MV tandem accelerator (National Electrostatics Corporation 9-SDH). High-resolution magnetic (40° deflection, MΔM ≈ 350, maximum mass-energy product 69 MeVu) and electro static (45 ° deflection, E/q of 4.8 MeV, EΔE≈ 730) analysis, coupled with a 1.5 m time-of-flight path and total energy detection (surface barrier detector) forms the basis of the detection system. In order to provide stable element detection capability at the parts-per-trillion level in electronic materials (Si, GaAs, HgCdTe), a custom ion source, incorporating mass analysis of the sputtering beam, ultraclean slits, low cross-contamination and UHV capability, is being constructed.

The University of North Texas Ion Beam Modification and Analysis Laboratory

Abstract The University of North Texas Ion Beam Modification and Analysis Laboratory consists of three accelerators. These are: a 200 kV, 10 MA Cockcroft-Walton, a 2.5 MV single-ended Van de Graaff machine and a newly installed National Electrostatics Corporation Model 9-SDH-2, 3 MV tandem. The tandem accelerator was received in November of 1987 and acceptance tests were completed in December of 1987. The tandem — and the 2.5 MV Van de Graaff — are now both installed in a new 7000 ft 2 laboratory in the physics building. The tandem which will be used for many of the activities in the laboratory was purchased with a rf charge exchange (Alphatross) source and a SNICS-type cesium ion sputter source. Each of these sources injects through a 30° magnet into a 90° analysis magnet and then into the tandem accelerator. This configuration gives the high mass resolution which is needed for the AMS system. A table will be shown that overviews the analyzed beams that have been obtained to date with the tandem accelerator. A summary of the beam lines that are being constructed for all the activities of the laboratory will also be given. These include: accelerator mass spectrometry (AMS), nuclear reaction analysis (NRA), Rutherford backscattering and channeling (RBS&C), particle-induced X-ray emission (PIXE), high-energy ion implantation (HEII) and atomic collision physics.

A high-resolution electrostatic analyzer for accelerator mass spectrometry☆

Abstract Accelerator mass spectrometry (AMS) has the potential of unparalleled sensitivities for elemental impurity analysis in materials. To fully exploit the capabilities of AMS, a high-resolution electrostatic analyzer (ESA) is shown to be a necessary part of the detection system. An ESA with a resolution of E ΔE > 700 for ions with energies up to E q = 4.8 MeV has been designed and constructed. The system utilizes a 45° deflection in the ions trajectory with a radius of curvature of 1.2 m. The field is of toroidal geometry, which produces astigmatic focusing of the ion beam, so as to compensate for the lack of vertical focusing in the magnetic spectrometer.

An ultra-clean microprobe ion source for atomic mass spectrometry

Abstract The design and initial implementation of an ultra-clean microprobe ion source (Chimera), which is under construction at the University of North Texas in collaboration with Texas Instruments, is presented. The source, which is intended for trace impurity analysis of stable isotopes, combines the features of existing secondary ion mass spectrometry (SIMS) microprobe instruments with the molecular discrimination and single-ion detection capabilities of accelerator mass spectrometry (AMS). Issues of cesium ion microbeam formation and cleanliness, secondary ion collection and transmission efficiency are discussed. Novel features of the Chimera include high-resolution magnetic analysis of the primary Cs+ ion beam ( m Δm > 200) , high-purity silicon slits and an ultra-high vacuum analysis chamber with unique silicon-based extraction optics and sample holder.

Depth profiling analysis of semiconductor materials by accelerator mass spectrometry

Abstract With the continual reduction in semiconductor device size, the identification and location of impurities is extremely important. A Trace Element Accelerator Mass Spectrometry (TEAMS) system is being used for analysis of elemental and compound semiconductors. The TEAMS system, which was developed by the University of North Texas (UNT) and Texas Instruments, Inc. (TI), may be used for both bulk and depth profiling measurements. Computer control of magnetic and electrostatic analyzers allows automated mass scans of stable isotopes in solid materials. A low sample contamination ion source is based upon clean Si components for ion optics and magnetic analysis of the Cs sputtering beam. The TEAMS system removes molecular interferences, which are present in Secondary Ion Mass Spectrometry (SIMS), by electron stripping of molecular ions to high charge states. As a result, the TEAMS system will allow higher sensitivities for some elements than SIMS. At present, bulk sensitivities of sub ppb (1 part in 109 or 1013 atoms/cm3) are possible for many elements in the periodic table. The TEAMS system design and operation are briefly discussed. Depth profiling results for F, P, and As implanted Si are demonstrated.

APPLICATIONS OF ACCELERATOR MASS SPECTROMETRY TO ELECTRONIC MATERIALS

Abstract Applications of accelerator mass spectrometry (AMS) to stable-element detection in electronic materials are being explored. Conventional AMS hardware at the University of Arizona has been used to profile shallow semiconductor structures (ion-implanted samples) and to establish minimum values of system efficiency for several ions in Si and/or GaAs. A custom instrument under development at the University of North Texas has been used to generate molecule free mass spectra which can be directly compared with secondary-ion mass spectrometry (SIMS) data. Elemental fragments associated with molecular dissociation after passage through the accelerator are shown to constitute a source of system “background” which can be removed through clever selection of charge states.

Molecular-interference-free accelerator mass spectrometry

Abstract The University of North Texas tandem accelerator (NEC 9SDH) has been configured as an accelerator mass spectrometry (AMS) system for ultrasensitive detection of impurities in materials. This instrument has been used to acquire mass spectra of materials of interest in semiconductor technology. Investigation of the phenomenon of Coulomb dissociation of molecular ions during passage through the stripper canal of the tandem accelerator is reported, particularly for ions of injected mass of approximately 31 u and 56 u arising from the sputtering of clean silicon with cesium. Peaks in the spectra at these masses could be evidence of 31 P or 56 Fe. However, as is well known from secondary ion mass spectrometry (SIMS), silicon substrates produce large currents of molecular ions such as 30 Si 1 H, 29 Si 1 H 2 and 28 Si 1 H 3 at 31 u and 28 Si 2 at 56 u. These molecular ions are shown to dissociate and appear with momenta and energies which are both unique and predictable. The results suggest that AMS will be a useful tool for truly elemental analytical surveys of materials as well as trace-element detection.

Atomic mass spectrometry of materials

Abstract Texas Instruments and the University of North Texas (UNT) are collaborating on the design of an accelerator mass spectrometry (AMS) system dedicated primarily to the analysis of impurities in electronic materials and metals. An AMS beamline consisting of high-resolution magnetic ( M d M > 350) and electrostatic ( E d E > 700) analysis followed by a surface barrier detector has been installed on the NEC 9SDH pelletron at UNT, and a “clean” ion source is under development. An existing ion source (NEC Cs sputter source) has been used in conjunction with the AMS beamline to generate computer controlled molecule-free mass analyses of solid samples. Through a careful choice of isotopes and charge states a robust algorithm can be developed for removing molecular interferences from the mass analysis for essentially all materials. Examples using graphite, Si and CdZnTe are discussed.