D.K. Marble

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.

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.

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.

L-shell X-ray production by 2-12 MeV carbon ions in fifteen selected elements from copper to lead

L-shell X-ray production cross sections for 29Cu, 31Ga, 32Ge, 35Br, 39Y, 42Mo, 47Ag, 50Sn, 57La, 60Nd, 64Gd, 67Ho, 70Yb, 79Au and 82Pb were measured for carbon ions in the 2-12 MeV energy range. The copper to yttrium data were previously measured using a Si(Li) detector with a beryllium window, while the molybdenum to lead X-rays were counted with a windowless Si(Li) detector, whose efficiency was determined by various normalization techniques. The measured X-ray cross sections are compared to predicted X-ray cross sections from the first Born and the ECPSSR theories. Using single-hole fluorescence yields, at low projectile velocities the first Born approximation overpredicts the data by as much as one order of magnitude while the ECPSSR theory at worse underestimates the data by about a factor of four. For the highest projectile velocities relative to target L-shell electrons, both theories converge toward the data to almost within experimental uncertainties. After modifying the fluorescence yield to account for multiple outer-shell ionization, the ECPSSR theory is brought into better agreement with the data for light targets but still overestimates the data for heavier targets. Also, the 2 MeV measurements using doubly ionized carbon ions are still significantly underestimated by the ECPSSR theory even after accounting for multiple ionization. Our multiple ionization correction assumes that the target foil thickness is thin enough to insure single collision conditions. However, in order to obtain acceptable signal-to-noise ratios, the target foils used in this experiment were too thick to provide single collision conditions. For thicker foils, ions are stripped inside the target resulting in an appreciable fraction of higher charge states that enhance the ionization of outer shells and the effective fluorescence yields. This effect could account for the remaining discrepancy between the ECPSSR theory and the data.

Fabrication of silicon‐based optical components for an ultraclean accelerator mass spectrometry negative ion source

Article discussing the fabrication of silicon-based optical components for an ultraclean accelerator mass spectonomy negative ion source.

Model of the contamination effect in ion-induced electron emission

Abstract Ion-induced electron emission yields from contaminated surfaces are well known to be enhanced relative to the yields from atomically clean surfaces. Under the bombardment of energetic ions, the surfaces become sputter-cleaned with time and the yields from the samples are reduced accordingly. The time dependent reduction of yields observed are shown to be due to various effects such as desorption of contaminant atoms and molecules by incident ions and adsorption of residual gas onto previously clean sites. Experimental results obtained in the present work show the lower, saturated yield (γs) to be a function of residual gas pressure (P) and the fluence (oi) of the ion. We present a dynamic equilibrium model which explains the increase in yields for surface gas contamination, the decrease in yields for contaminant desorption, and the pressure/fluence dependence in the time required to reach γs. The predictions of the model agree well with the observations of γs as a function of the ratio of gas flux to ion flux, and the electron yields of clean and gas covered surfaces.

The University of North Texas atomic mass spectrometry facility for detection of impurities in electronic materials and metals

Abstract An accelerator mass spectrometry (AMS) facility is being developed at the University of North Texas through a collaboration between UNT and Texas Instruments Inc. The computer controlled AMS instrument will presently allow automatic mass scans of stable isotopes in solid materials using a conventional NEC SNICS ion source. Even though the SNICS ion source contaminates the sample, the AMS instrument allows molecular interference-free mass scans to be obtained with a higher sensitivity than SIMS for some elements, A new low sample contamination microbeam ion source under construction should allow sensitivities of ppt (1 part in 1012 or 1010 atoms/cm3) for any element in the periodic table, as well as sputter depth profiling and secondary electron imaging of a sample.

M-shell X-ray production in gold, lead, bismuth, thorium and uranium by 70–200 keV protons☆

Abstract M-shell X-ray production cross sections have been measured for thin-foil targets (thickness in μg/cm2) of 79Au (7.0), 82Pb (10.6), 83Bi (4.1), 90Th (7.9) and 92U (8.8) for 70–200 keV incident protons. These data are compared to other measurements at higher energies and to the first Born (plane-wave Born approximation for direct ionization and Oppenheimer-Brinkman-Kramers-Nikolaev approximation for electron capture) and ECPSSR (energy-loss and Coulomb deflection effects, perturbed stationary state approximation with relativistic correction for both direct ionization and electron capture) theories. The electron capture, included for completeness, contributed less than 1%. The ECPSSR underpredicts the data at all energies, while the first Born, which overpredicts the data, seems to approach agreement for low Z2. The data follow a trend established in this laboratory by earlier work [R. Mehta et al., Phys. Rev. A26 (1982) 1883] at higher energies.