N. Thonnard

Applications of resonance ionization mass spectrometry

We describe here several types of mass spectrometry in which the mass selectivity is combined with a highly selective laser ionization method to achieve both high sensitivity and very high selectivity. These methods combine the most sensitive and highly selective laser ionization methods with mass selectivity in order to improve on both the sensitivity and the Z and A selectivity previously achievable in detecting atomic species. Applications of these methods include the dating of geophysical specimens, the on line analysis of rare short‐lived isotopes produced in high‐energy collisions, the detection of low levels of heavy metals or radioactive isotopes in biological samples, the detection of impurities in ultrapure materials, and a host of other applications. Because some versions of this method offer sensitivity to a hundred atoms of a particular isotope of an element in a macroscopic sample, there are new possibilities for fundamental studies of rare events. Several types of facilities for elemental and isotopic analysis will be described.

81Kr and85Kr in groundwater, Milk River aquifer, Alberta, Canada

Abstract The 85 Kr activity of well No. 9 is (2.1 ± 0.3) mBq/cm 3 STP ofKr or ≈0.3% of the modern atmospheric activity indicating that no young water component is present in this groundwater and that no contamination occurred in extracting a Kr gas sample for 81 Kr analysis. The 81 Kr concentration was measured by laser resonance ionization spectroscopy to be 82 ± 18% of the modern atmospheric concentration. Only 5000 atoms of 81 Kr were used in the final analytical step which represent ≈7% of the initial number present in a water sample of 50 1. An upper limit of 140 ka is calculated for the age of the water from a simple 81 Kr decay model. Of key importance for the future use of a 81 Kr dating technique is the conclusion that subsurface production of 81 Kr appears to be unimportant.

Resonance ionization spectroscopy and the detection of 81Kr

Abstract Development of resonance ionization spectroscopy (RIS) has made the concept of single-atom counting, whether radioactive or stable, practical for most elements of the periodic table. Tunable narrow-band lasers are used to efficiently and selectively excite and ionize a chosen element without interference from the much more abundant background, thereby achieving sensitivity at the level of a few atoms. Applications of the selectivity and sensitivity of RIS are numerous, including the direct analysis of solid, chemically processed liquid and noble gas samples. For example, 81 Kr, a 2.1 × 10 5 year half-life cosmogenic radioisotope was detected with less than 400 81 Kr atoms in the RIS analysis system.

Quantitative and sensitive profiling of dopants and impurities in semiconductors using sputter‐initiated resonance ionization spectroscopy

Sputter‐initiated resonance ionization spectroscopy (SIRIS) is an analytical technique with extremely high sensitivity, selectivity, dynamic range, and quantitation accuracy. SIRIS also provides good spatial resolution, and freedom from matrix effects on surfaces and at interfaces. In this paper we report the capability of SIRIS to quantitate, with high accuracy and high depth resolution, dopant and impurity concentrations in semiconductor devices at the 1013–1020 atoms/cm3 level. By utilizing layered GaAs/AlGaAs/GaAs samples grown by molecular beam epitaxy, a depth resolution of ∼2 nm has been demonstrated using 0.5 keV Ar+ primary ion energy. Depth profiles of boron in Si implants showed dynamic ranges up to 2×106. The correlation between B concentration and B SIRIS signal demonstrated high quantitation accuracy. The lateral imaging capability of SIRIS was demonstrated. We concluded that an optimized instrument could produce high depth‐resolved quantitative measurements over a 1012–1021 atoms/cm3 concen...

Sputter‐initiated resonance ionization spectroscopy: A matrix‐independent sub‐parts‐per‐billion sensitive technique applied to diffusion studies in SiO2–InP interfaces

Sputter‐initiated resonance ionization spectroscopy (SIRIS) is a very selective and extremely sensitive technique to analyze semiconductor materials. In the SIRIS technique, the abundant neutral atoms released by the sputtering process are ionized by precisely tuned laser beams and counted in a mass spectrometer. The resonance ionization spectroscopy (RIS) process virtually eliminates isobaric and other interferences and reduces matrix effects dramatically. Depth profiles of Si and In were measured in unannealed and rapid‐thermal annealed SiO2–InP samples using the SIRIS and secondary ion mass spectrometry (SIMS) techniques. The results show diffusion process of In through the SiO2 layer and of Si into the InP substrate, as well as the matrix independence of the SIRIS technique. A detection limit, defined as 2 standard deviation above background, of In in Si was measured at 300 parts per trillion (ppt) with a 0.75 μA, 0.8 μs wide ion beam pulse and less than 3 min analysis time at 30 Hz repetition rate. T...

Applications of resonance ionization spectroscopy for semiconductor, environmental and biomedical analysis, and for DNA sequencing

Resonance Ionization Spectroscopy (RIS), an analytical technique with extremely high element specificity and sensitivity, is becoming recognized as an emerging field with wide applications. The sensitivity and selectivity of the RIS process is especially valuable for ultra-trace element analysis in semiconductor, geological, biological, and environmental samples, where the complexity of the matrix is frequently a serious source of interference. Using either Sputter-Initiated RIS (SIRIS) or Laser Atomization RIS (LARIS), it is possible to localize with high spatial resolution ultra-trace concentrations of a selected element to the sub-parts per billion level. The authors describe the implementation of RIS to solve a number of analysis problems and illustrate its salient characteristic with data from a wide range of applications. Results presented will include (a) concentration plots of ultra- trace elements in semiconductors and biological matrices, (b) characterization of dopants as a function of depth in semiconductors, (c) spatial distribution of natural uranium in bone and bone marrow, and (d) localization of stable isotope-labeled DNA in the development of faster DNA sequencing methods. The practical capabilities of SIRIS/LARIS to determine trace elements as a function of depth and lateral position in semiconductors and biological matrices are discussed.

Real-time optical diagnostics of graphene growth induced by pulsed chemical vapor deposition

The kinetics and mechanisms of graphene growth on Ni films at 720-880 °C have been measured using fast pulses of acetylene and real-time optical diagnostics. In situ UV-Raman spectroscopy was used to unambiguously detect isothermal graphene growth at high temperatures, measure the growth kinetics with ∼1 s temporal resolution, and estimate the fractional precipitation upon cooldown. Optical reflectivity and videography provided much faster temporal resolution. Both the growth kinetics and the fractional isothermal precipitation were found to be governed by the C2H2 partial pressure in the CVD pulse for a given film thickness and temperature, with up to ∼94% of graphene growth occurring isothermally within 1 second at 800 °C at high partial pressures. At lower partial pressures, isothermal graphene growth is shown to continue 10 seconds after the gas pulse. These flux-dependent growth kinetics are described in the context of a dissolution/precipitation model, where carbon rapidly dissolves into the Ni film and later precipitates driven by gradients in the chemical potential. The combination of pulsed-CVD and real-time optical diagnostics opens new opportunities to understand and control the fast, sub-second growth of graphene on various substrates at high temperatures.

An isotope separator for small noble gas samples

Abstract A Wien filter isotope enrichment system has been combined with a small turbomolecular pump to form a closed isotope separator for small noble gas samples. Atoms which leave the exit aperture of the plasma discharge ion source without being ionized are circulated back into the source through a feedback line. The system can be operated for several hours in a closed mode to collect up to 50% of the total number of atoms of a selected isotope (e.g. 81 Kr) out of a small gas sample of only 2 × 10 −3 cm 3 STP. Ions are implanted at 10 kV into an aluminized Kapton foil after a flight distance of 150 cm. A beam stabilization system centers the ion beam in two perpendicular directions onto a target aperture to maintain a high enrichment factor of at least 10 3 over extended periods of time. Calibration of the enrichment process is achieved by isotope dilution. The system is a key part of the sample processing for 81 Kr and 85 Kr analysis by laser resonance ionization spectroscopy for applications in isotope geophysics.

Analysis of 81Kr in groundwater using laser resonance ionization spectroscopy

A new analytical technique based on resonant ionization of krypton with a vacuum ultraviolet (VUV) laser source was used to determine low-level /sup 81/Kr concentrations in groundwater. The long half-life (210,000 years) and low concentration (1.3 x 10/sup 3/ /sup 81/Kr atoms per liter of modern water at 10/sup 0/C) make the detection of /sup 81/Kr by radioactive counting techniques extremely difficult. In this method, krypton gas was removed from water taken from an underground Swiss aquifer using standard cryogenic and chromatographic techniques. Stable krypton isotopes were then reduced by a factor of 10/sup 7/ by a two-stage isotopic enrichment cycle using a commercially available mass spectrometer. The enriched gas containing about 10/sup 8/ stable krypton atoms and about 10/sup 3/ atoms of /sup 81/Kr was implanted into a silicon disc. This disc was then placed in the high vacuum final counting chamber and the krypton was released by laser annealing. This chamber contained a quadrupole mass spectrometer which used a pulsed VUV laser source as the ionizer. The measured signal indicated that the sample contained 1200 (+-300) atoms of /sup 81/Kr.

Comparison of sputter‐initiated resonance ionization spectroscopy and laser atomization resonance ionization spectroscopy to localize tin‐labeled deoxyribose nucleic acid

Resonance ionization spectroscopy (RIS) is becoming recognized as an emerging field with wide applications. RIS is an analytical technique providing extreme element sensitivity and selectivity by detecting ionization produced from lasers tuned to excited states of the analyte. Using either sputter‐initiated RIS (SIRIS) or laser atomization RIS (LARIS), it is possible to localize and quantify with micrometer spatial resolution ultratrace concentrations of a selected element at or near the surface of solid samples. The sensitivity and selectivity of the RIS process is especially valuable for trace element analysis in biological media, where the complexity of the matrix is frequently a serious source of interference. We have compared both the SIRIS and LARIS technique to determine their characteristics to localize and quantify Sn‐labeled deoxyribose nucleic acid (DNA). We will present (a) data showing differences between SIRIS and LARIS response as a function of atomization parameter, substrate and analyte, ...