Wallis F. Calaway

Yields and kinetic energy distributions of sputtered neutral copper clusters

Abstract Neutral copper clusters up to Cu 15 were observed from the sputtering of polycrystalline copper by normally incident 3.6 keV Ar + . This is the first observation of sputtered neutral clusters larger than Cu 5 . The sputtered neutral species were detected by laser post-ionization time-of-flight mass spectrometry. The absolute sputtering yield of Cu 2 was determined to be 0.05 Cu 2 /Ar + . Sputtering yields of the larger clusters relative to the atom were estimated from their postionized concentrations and found to range from 10 −4 for Cu 3 to 10 −9 for Cu 11 . The kinetic energy distributions of Cu, Cu 2 and Cu 3 were also measured. The energ distribution of the atoms agreed very well with the expected distribution from the Sigmund-Thompson model. The maxima in the energy distributions were in the order Cu 3 > Cu 2 > Cu. All three distributions had approximately the same high-energy dependence. Neither the single nor the multiple collision models that have been proposed to describe the cluster energy distributions appear to agree with our measurements.

New findings on the sputtering of neutral metal clusters

Neutral copper and aluminum clusters containing up to 20 and 12 atoms, respectively, were observed in the sputtering of the polycrystalline metals by 3.75 or 3.9 keV Ar+ ions. The clusters were postionized by 6.4 eV photons from an ArF excimer laser and were mass analyzed in a time-of-flight spectrometer. The yields of the clusters were estimated from their postionized concentrations and are shown for the first time to exhibit a power-law dependence on the number of atoms in the cluster. The kinetic energy distributions of Cu through Cu6 were measured, and the cluster distributions were found to resemble the atom distribution, in agreement with our earlier copper and aluminum data. Collision-cascade-based models cannot predict the yield and kinetic energy distribution data. Several alternate models are considered, but none is found to be satisfactory for explaining the large cluster emission. For small (n < 3) clusters, the superposition of two or more mechanisms may help to explain the observations.

Analyzing individual presolar grains with CHARISMA

Isotopic analysis of heavy elements in individual stardust grains is important in testing and constraining theories of stellar nucleosynthesis. These analyses are challenging in that the grains are very small, the largest being perhaps a few microns in diameter, and contain only trace concentrations of heavy elements, generally on the order of ppm. In addition, isotopic analysis requires the suppression of isobaric interferences. We describe a unique instrument, based on resonant ionization mass spectrometry, that has successfully characterized such grains for the past several years, and report on some recent upgrades that significantly enhance the instrumental capabilities. The fundamental principles and operational details are discussed, along with illustrative results and plans for future modifications.

Velocity and electronic state distributions of sputtered Fe atoms by laser‐induced fluorescence spectroscopy

Velocity distributions and relative populations in the fine‐structure levels of the a 5DJ ground state of Fe atoms, produced by sputtering with 3 keV argon ions, have been investigated by Doppler‐shifted laser‐induced fluorescence. The laser system employs a single‐mode, scanning ring dye laser, amplified by a sequence of three excimer‐pumped flowing dye cells. Frequency doubling in a KD*P crystal was used to produce high energy (>0.5 mJ) pulses of narrowband tunable UV output near 300 nm. Laser power influence on effective velocity bandwidth was investigated. Favorable light‐collection geometry minimized distortion of the velocity spectra from apparatus‐averaging effects. In impurity flux diagnostic applications in fusion devices, substantial spatial averaging may occur. In the latter case, the narrow velocity bandwidth (70 m/s, transform limit) of the present laser system is particularly useful.

Kinetic energy distributions of sputtered neutral aluminum clusters: A1-A16

Abstract Neutral aluminum clusters sputtered from polycrystalline aluminum were analyzed by laser postionization time-of-flight (TOF) mass spectrometry. The kinetic energy distributions of Al through Al 6 were measured by a time-of-flight technique. The interpretation of laser postionization TOF data to extract velocity and energy distributions is presented. The aluminum cluster distributions are qualitatively similar to previous copper cluster distribution measurements from our laboratory. In contrast to the steep high energy tails predicted by the single- or multiple-collision models, the measured cluster distributions have high energy power law dependences in the range of E −3 to E −4.5 . Correlated collision models may explain the substantial abundance of energetic clusters that are observed in these experiments. The possible influences of cluster fragmentation on the distributions are discussed.

Neutral copper cluster sputtering yields: Ne+ Ar+ and Xe+ bombardment☆

Abstract The sputtering of neutral metal clusters was investigated by measuring relative sputtering yields of copper clusters ejected from polycrystalline copper under 3.9 keV bombardment by Ne+, Ar+ and Xe+ ions at normal incidence. The yields of clusters from Ne4 bombardment were lower than those from Ar+ bombardment, and yields from Ar+ bombardment were lower than those from Xe+ bombardment. The sputtering yield ratios Ne+/Ar+ and Xe+/Ar+ were measured to be 0.56 and 1.08. The size distribution of the sputtered clusters can be fit by a power law dependence with exponents of −8.1, −8.2 and −6.2 for Ne+, Ar+ and Xe+, respectively. The similarity of the exponents of the Ne+ and AI+ power law fits indicates that the sputtering yields for these two primary ions are similar, while that for Xe+ is substantially higher, in contrast to the sputtering yield ratio data. The difference between the two measurements can be explained by assuming a systematic uncertainty in the sputtering yield ratio measurements that makes the measured ratios lower than the true values. Assuming a value at the high end of the experimental Ne+ sputtering yield range, the exponents of the power law fits exhibit a linear dependence on the total sputtering yield.

Laser-based secondary neutral mass spectroscopy: Useful yield and sensitivity☆

Abstract Multiphoton ionization (MPI) by pulsed, tunable lasers provides a sensitive means for detection of neutral atoms due to the high efficiency achievable both in the ionization and subsequent detection. Substantial selectivity can be achieved by excitation between energy levels of the atom of interest. This resonant MPI technique can access all atomic states of any particular atom including its ground and metastable levels. In principle all elements may be ionized through judicious selection of the color of the excitation laser light. In practice resonance ionization has been experimentally demonstrated for nearly every element. A variety of problems exist in order to optimally apply resonance ionization spectroscopy (RIS) to the detection of sputtered neutral atoms, however. Several of these problems and their solutions are examined in this paper. First, the possible useful yields obtainable and the dependence of useful yield on various laser parameters for this type of sputtered neutral mass spectrometer (SNMS) are considered. Second, the choice of a mass spectrometer and its effect on the instrumental useful yield is explored in light of the unique ionization region for laser based SNMS. Finally a brief description of noise sources and their effect on the instrumental sensitivity is discussed. That it is possible to combine in one instrument both high useful yields and high sensitivity for the detection of minority species (either very dilute surface constituents or species sputtered in highly excited states) will be demonstrated with results of Fe implanted Si samples in the surface analysis by resonance ionization of sputtered atoms (SARISA) instrument. SARISA accomplishes the necessary noise reduction without signal loss through the extraction of the photoions into a sector-field time-of-flight (TOF) mass spectrometer. In standard, isochronous operation, energy and angular spreads at the point of ionization are compensated in flight to produce well-resolved TOF mass spectra. Noise sources (photons, metastable and scattered atoms) escaping through transparent grids are strongly suppressed.

Sputtering of neutral and ionic indium clusters

Secondary neutral and secondary ion cluster yields were measured during the sputtering of a polycrystalline indium surface by normally incident ∼4 keV Ar+ ions. In the secondary neutral mass spectra, indium clusters as large as In32 were observed. In the secondary ion mass spectra, indium clusters up to In18+ were recorded. Cluster yields obtained from both the neutral and ion channel exhibited a power law dependence on the number of constituent atoms n in the cluster, with the exponents measured to be −5.6 and −4.1, respectively. An abundance drop was observed at n=8, 15, and 16 in both the neutral and ion yield distributions, suggesting that the stability of the ion (either secondary ion or photoion) plays a significant role in the observed distributions. In addition, our experiments suggest that unimolecular decomposition of the neutral cluster may also play an important role in the measured yield distributions.

Trace surface analysis: 30 ppb analysis with removal of less than a monolayer. Fe and Ti impurities in the first atomic layer of Si wafers

Abstract Recent trends, exemplified by the stringent demands of the semiconductor industry, demonstrate a need for surface analysis at progressively lower impurity levels. Because the total number of impurity atoms available for measurement in the first atomic layer is severely limited, a successful trace analysis technique must efficiently utilize the impurity atoms available as well as achieve good signal to noise. We report here the results of a new Surface Analysis by Resonant Ionization of Sputtered Atoms (SARISA) apparatus which has a demonstrated ability to measure Ti and Fe impurities in the surface atomic layer of a Si wafer at the 30 ppb level. The Fe results are particularly instructive since Secondary Ion Mass Spectrometry (SIMS), because of the isobaric overlap of Fe and Si2, has achieved no better than 200 ppb with removal of nearly 100 atomic layers. The measurements were made with a 2 mm2 ion spot. A measured collection efficiency of 1 Fe atom counted per 100 Fe atoms sputtered could also be achieved. Apparatus improvements will be described which should increase the collection efficiency to 1 atom counted per 10 sputtered.

Ion and neutral atomic and cluster sputtering yields of molybdenum

The yield of neutral and ionized Mo, Mo2, and Mo3 sputtered from a Mo target by 4‐keV Ar+ has been measured in the surface analysis by resonance ionization of sputtered atoms (SARISA) machine. Ionization spectroscopy combined with time‐of‐flight (TOF) secondary ion mass spectrometry (SIMS) allowed us to obtain for the first time absolute sputtering yields and ionization fractions of sputtered atoms and metal clusters. Unlike sputtered atomic species, Mo clusters have been found to be sputtered with large ion fractions. The sputtering yield of Mo clusters is very sensitive to oxygen on the surface, i.e., even small amounts of oxygen on the surface identified by Mo+ and MoO+ peaks in the SIMS spectrum, reduce the cluster yield substantially. A broad structureless absorption band was observed for sputtered Mo2 molecules indicating substantial rovibronic excitation as predicted by theoretical models.