Oxygen Flooding Research Articles

In situ U–Pb dating of micro-baddeleyite by secondary ion mass spectrometry

We introduce a technique for U–Pb dating of baddeleyite using secondary ion mass spectrometry (SIMS) in situ analysis of ng-mass crystals that cannot be efficiently extracted by conventional mineral separation techniques. Average 207Pb/ 206Pb ages for Precambrian baddeleyite crystals are within < 0.3% of the respective isotope dilution thermal ionization mass spectrometry (ID-TIMS) ages. 206Pb/ 238U ratios are corrected for instrumental fractionation calibrated through linear regression in a Pb/U relative sensitivity vs. UO 2 +/U + calibration plot. Calibration is performed on separated baddeleyite crystals (∼ 100–200 μm in maximum dimension) mounted in random crystallographic orientation. 206Pb/ 238U ages for baddeleyite from Duluth gabbro (FC4b) and Kovdor are accurate within 1–2% when averaging 15–30 individual spot analyses and relative sensitivities calibrated on Phalaborwa baddeleyite. The relative difference of 206Pb/ 238U between large crystals and micro-baddeleyite from FC4b is within ∼ 1%. Comparison between silicate glass and baddeleyite, as well as replicate analysis of the same grains in different orientations relative to the incidence direction of the primary beam support previous evidence for bias in Pb/U sensitivity in baddeleyite due to variable crystal orientations. We successfully utilized oxygen flooding and a UO 2 +/U +-based calibration to significantly reduce orientation dependent bias.

The effect of oxygen during irradiation of silicon with low energy Cs+ ions

The effect of oxygen flooding during ultralow energy SIMS depth profiling of silicon with Cs+ primary ions is presented. New experimental data show the effective sputtering yield of silicon in the presence of oxygen, as well as the energy distribution of the secondary Si− ions. It is found that the component sputtering yield of Si is very sensitive to minute amounts of oxygen in the proximity of the sputtered surface. At these very low flooding pressures (in the 10−9–10−8mbar range), one cannot account for a full oxidation in the time frame of a typical measurement; it is concluded that in this particular case, which is technologically very relevant, small traces of oxygen will change the desorption characteristics of cesium as well as silicon. It shows that oxygen that is adsorbed to the silicon surface provides an attachment site for impinging cesium ions and thereby increases the retention capacity of cesium tremendously. Also, oxygen changes the secondary ion yield and can even promote the desorption of Si from the analysis surface.

Production and Applications of Metal-cluster-complex Ion Beams

A new ion source using massive molecules called metal cluster complexes has been developed. Metal cluster complexes are chemically-synthesized organometallic compounds, which have a wide range of chemical compositions with high molecular weight. The ion source is compact enough to be installed in commonly used secondary ion mass spectrometry (SIMS) systems. Using the ion source, sputtering characteristics of silicon bombarded with normally incident Ir4(CO)7+ ions were investigated. Experimental results showed that the sputtering yield at 10 keV was 36, which is higher than that with Ar+ ions by a factor of 24. In addition, SIMS analyses of boron-delta-doped silicon samples and organic films of poly(methyl methacrylate) (PMMA) were performed. Compared with conventional O2+ ion beams, Ir4(CO)7+ ion beams improved depth resolution by a factor of 2.5 at the same irradiation conditions; the highest depth resolution of 0.9 nm was obtained at 5 keV, 45° with oxygen flooding of 1.3×10-4 Pa. Furthermore, experimental results confirmed that Ir4(CO)7+ ion beams significantly enhanced secondary ion intensity in high-mass region.

SIMS analysis of xenon and krypton in uranium dioxide: A comparison of two models of gas-phase ionisation

Reasonable detection limits in secondary ion mass spectrometry (SIMS) analysis of inert gases in solids may be achieved by gas-phase ionisation. For Xe in UO 2 Desgranges and Pasquet (D&P) have recently reported that the ion intensity I Xe + can be enhanced significantly using O 2 + ion bombardment in combination with an oxygen jet directed at the sputtered area. A similar effect was reported by Portier et al. for Kr in UO 2. The enhancement was attributed to ionisation by an interaction with oxygen atoms and molecules (D&P model). The proposed mechanism is at variance with a recently outlined model of gas-phase ionisation involving charge transfer between the ejected atoms and the incoming primary ions. The purpose of this study was to clarify these contradictory views by a reanalysis of the original data. Access to all relevant original data was kindly provided by L. Desgranges. In contrast to expectation based on the D&P model, step-wise increases in the flow rate of the oxygen jet did not result in an immediate response of I Xe + . Instead, transient yield changes were observed, with characteristic rise times that were even longer for Xe + than for uranium specific signals. This observation invalidates the idea that ionisation of Xe atoms is due to interaction with oxygen molecules of the jet. Recalling well known transient phenomena in ion-bombardment induced photon emission, it is argued that the enhancement effect may be associated with the increase in the oxidation state of the sample, as a result of which the fraction of Xe (or Kr) atoms leaving the sample in an electronically excited state is presumably increasing, thus enlarging the effective cross section for charge transfer. Alternatively, the enhancement may be due to a lowering of the ejection velocity of rare gas atoms, possibly caused by the increase in near-surface sample oxidation. A second problem with the D&P study is the use of I U + as a reference signal. Literature data as well as new results reveal that UO 2 + and UO + secondary ions dominate the SIMS spectrum of oxidised uranium at energies <60 eV, the U + fraction amounting to only 1% or less. Other issues are (i) the significant variation of the sample erosion rate that occurred upon deliberate changes of the mean primary ion current density, (ii) the associated bombardment induced, progressive oxygen depletion of the sample, (iii) the presence of a background superimposed on the Xe + signals, a significant effect at low current densities and (iv) the space–charge broadening of the primary ion beam at high beam currents. Eliminating all these interfering factors, the (reduced) ionisation probabilities of Xe + were found to be in accordance with gas-phase ionisation by charge exchange, even in the pressure of excessive oxygen flooding.

Roughness development in the depth profiling with 500 eV O 2+ beam with the combination of oxygen flooding and sample rotation

Roughness development is one of the most often addressed issues in the secondary ion mass spectrometry (SIMS) ultra-shallow depth profiling. The effect of oxygen flooding pressure on the roughness development has been investigated under the bombardment of 500 eV O 2 + beam with simultaneous sample rotation. Oxygen flooding had two competing effects on the surface roughening, i.e., enhancement of initiating roughening and suppression of roughening development, which were suggested to be described by the onset depth z on and transient width w tr of surface roughening. Both z on and w tr decreased as oxygen flooding pressure increased. As the result, surface roughening was most pronounced at the intermediate pressure from 4.4E−5 Pa to 5.8E−5 Pa. The surface roughening is negligible while without flooding or with flooding at the saturated pressure. No flooding is preferable for depth profiling ultra-shallow B implantation because of the better B profile shape and short analysis time.

Segregation under low-energy oxygen bombardment in the near-surface region

We found that a transient region of SIMS profiles was suppressed to less than the native oxide in silicon, with O 2 + bombardment energy of less than 0.2 keV under an incident angle of 0° with respect to the surface normal without oxygen flooding. However, gallium segregated significantly due to oxidation caused by O 2 + bombardment in this condition. We also found that the segregation decreased as the incident angle increased, and that it disappeared at the angle of around 40°, which was verified by comparing SIMS profiles with the HR-RBS profile. These results suggested that the angle of around 40° was the critical angle to prevent segregation. The transient region was almost the same at angles of 0–40°. Therefore, we consider that the energy of 0.2 keV at the angle of around 40° under O 2 + bombardment without oxygen flooding is the optimum SIMS condition for depth profiling in the near-surface region. On the other hand, the profile shift of arsenic depending on the angle was quite different as compared with gallium, but the shift was a minimum at the same critical angle. We expect that more accurate profiles for other impurities can be obtained using this SIMS condition.

Cluster SIMS using metal cluster complex ions

Metal cluster complexes are chemically synthesized organometallic compounds, which have a wide range of chemical compositions with high molecular weight. Using a metal cluster complex ion source, sputtering characteristics of silicon bombarded with normally incident Ir 4(CO) 7 + ions were investigated. Experimental results showed that the sputtering yield at 10 keV was 36, which is higher than that with Ar + ions by a factor of 24. In addition, secondary ion mass spectrometry (SIMS) of boron-delta-doped silicon samples and organic films of poly(methyl methacrylate) (PMMA) was performed. Compared with conventional O 2 + ion beams, Ir 4(CO) 7 + ion beams improved depth resolution by a factor of 2.5 at the same irradiation conditions; the highest depth resolution of 0.9 nm was obtained at 5 keV, 45° with oxygen flooding of 1.3 × 10 −4 Pa. Furthermore, it was confirmed that Ir 4(CO) 7 + ion beams significantly enhanced secondary ion intensity in high-mass region.

Oxygen flooding and sample cooling during depth profiling of HfSiON thin films

A combination of oxygen flooding and. Cs primary ion bombardment can suppress the enhancement of the secondary ion signal at the surface and at the interface of a thin HfSiON layer on a Si substrate. The surface concentration of both Cs and O during Cs primary ion bombardment with oxygen flooding was higher than that without oxygen flooding, as confirmed by X-ray photoelectron spectroscopy. When the sample was cooled to about −150 °C, the enhancement of the secondary ion signal could be suppressed at a lower oxygen pressure.

Factorial analysis of cluster-SIMS depth profiling using metal-cluster-complex ion beams

A Ir 4(CO) 7 + primary ion beam, at energies from 2.5 keV to 10 keV, was used to profile boron-delta layers in Si to investigate the influences of atomic mixing and surface roughness on the degradation of depth resolution. Factorial analyses using the mixing-roughness-information (MRI) model indicated that the influence of the mixing increased as beam energy was reduced below 5 keV in the case of oxygen flooding. It was confirmed that the magnitude of the MRI surface roughness was different from that of the AFM surface roughness. The discrepancy in the magnitude of roughness was examined by considering the difference in sputtering depth as well as the definition of the MRI surface roughness.

High depth resolution SIMS analysis using metal cluster complex ion bombardment

SIMS depth profiles were measured using metal cluster complex ions of Ir4(CO)7+ as a primary ion beam in order to obtain high depth resolution. Depth resolution was evaluated as a function of primary ion species, energy and incident angle using a multiple boron delta-doped silicon sample. The depth resolution obtained using cluster ion bombardment was considerably better than that obtained by oxygen ion bombardment under the same bombardment condition due to reduction of atomic mixing in the depth. The best depth resolution was 0.9 nm under the bombardment condition of 5 keV, 45° with oxygen flooding, which approaches the value measured with state of the art SIMS analyses. However, depth resolution was not improved by decreasing the cluster ion energy (less than 5 keV), even though the roughness of the sputtered surface was suppressed. The limit of depth resolution improvement may be caused by a carbon cover-layer that prevents the formation of surface oxide that buffers atomic mixing. To overcome this issue, it will be necessary to eliminate carbon from the cluster ion.

Cesium ion sputtering with oxygen flooding: Experimental SIMS study of work function change

We investigated the work function (WF) change of a silicon surface being under cesium ion bombardment and simultaneous oxygen flooding with various oxygen pressures at the sample surface. It was found that WF of Cs + ion sputtered Si decreases under oxygen flooding. This decrease provides an essential grow of secondary ion yields of some negative ions, sputtered from Si. At the same time Si − ion yield decreases approximately in two times. In the paper we have discussed possible explanations of our experimental data: we considered a surface composition change, formation of surface dipoles and work function change caused by oxygen adsorption, and their relationships between each other.

Detrital zircon dating of the Palaeoproterozoic Himmerkinlahti Member, Posio, northern Finland; lithostratigraphic implications

The Himmerkinlahti Member is a thin, poorly exposed metaconglomerate – metaquartzite unit in the SE part of the Palaeoproterozoic Kuusamo Belt, Posio, northern Finland (Figs. 1 & 2). It was originally included into the middle part of the Karelian supracrustal rocks of the belt (Laajoki, 2000), but later on its lithostratigraphic position was considered problematic (Laajoki & Wanke, 2002; Laajoki, 2005). In order to have more light to this question, zircons from a granule-pebbly metaquartzite sample (4/190KL91) with abundant opaques and a sericite-rich (pseudo)matrix (see Fig. 7e in Laajoki, 2000) were separated for U-Pb dating at the Geological Survey of Norway. Dr. Ansgar Wanke picked and mounted them at the University of Oulu. The ion microprobe analyses were performed using the Nordic Cameca IMS 1270 at the Swedish Museum of Natural History, Stockholm. The spot diameter for the 4nA primary O2ion beam was ca. 25 μm and oxygen flooding in the sample chamber was used to increase the production of Pb+ ions. Four counting blocks comprising a total of twelve cycles of the Pb, Th and U species were measured in each spot. The mass resolution (M/DM) was ca. 5000. The raw data was calibrated relative to Geostandards zircon 91500 reference, which has an age of 1065 Ma (Wiedenbeck et al., 1995) and corrected for background at mass 204.2 and modern common lead (T=0; Stacey & Kramers, 1975). For further details of the analytical procedures see Whitehouse et al. (1997, 1999). The excel-programs by Whitehouse were used for data reduction, and the Isoplot/Ex programs by Ludwig (2001) for line fitting and concordia plots. Decay constants follow the recommendations of Steiger & Jager (1977).

The reduction of the change of secondary ions yield in the thin SiON/Si system

For the analyses of gate insulating materials of thin silicon oxy-nitride (SiON) and dielectric films, SIMS is one of the available tool along with TEM and ESCA, etc. Especially, to investigate the distribution of dopant in the thin films, SIMS is appreciably effective in these techniques because of its depth profiling capability and high sensitivity. One of the problem occurring in this SIMS measurement is the change of secondary ion yield at the interface as well as in the layers with different chemical composition. To solve this problem, some groups have researched the phenomenon for SiO 2/Si interface [W. Vandervorst, T. Janssens, R. Loo, M. Caymax, I. Peytier, R. Lindsay, J. Fruhauf, A. Bergmaier, G. Dollinger, Appl. Surf. Sci. 203–204 (2003) 371–376; S. Hayashi, K.Yanagihara, Appl. Surf. Sci. 203–204 (2003) 339–342; M. Barozzi, D. Giubertoni, M.Anderle, M. Bersani, Appl. Surf. Sci. 231–232 (2004) 632–635; T.H. Buyuklimanli, J.W. Marino, S.W. Novak, Appl. Surf. Sci. 231–232 (2004) 636–639]. In the present study, profiles of boron and matrix elements in the Si/SiON layers on Si substrate have been investigated. The sensitivity change of Si and B profiles in SiON layer become smaller by using oxygen flood than those without oxygen flood for both O 2 + and Cs + beam. At the range of 0–25 at.% of N composition, 11B dosimetry in SiON layer implanted through amorphous Si depends on N composition. This trend could be caused by the sensitivity change of 11B, or it indicates real 11B concentration change in SiON lyaer. N areal density determined by Cs + SIMS with oxygen flooding also shows linear relationship with N composition estimated by XPS.

Surface roughening and erosion rate change at low energy SIMS depth profiling of silicon during oblique [formula omitted] bombardment

Surface roughening of boron δ -doped Si samples under low energy (0.5 keV/ O 2 + , 44 ° and 54 ° , and 1.0 keV/ O 2 + , 48 ° ) O 2 + bombardment at oblique incidence with and without oxygen flooding was studied with atomic force microscopy (AFM) and secondary ion mass spectrometry (SIMS). The erosion rate, the surface topography and the depth resolution as a function of depth have been measured. Changes in secondary ion yields have been correlated with changes in surface topography. It is found that the surface roughness depends on impact energy and incidence angle without flooding. The roughness decreases with decreasing impact energy. For the same energy (0.5 keV/ O 2 + ), the wavelength increases slightly with increasing angle of incidence and the roughness increases with increasing angle of incidence. With flooding, the roughness can be efficiently avoided. The best conditions to avoid roughness when analysing ultra shallow profiles with our magnetic sector instrument is 0.5 keV/ O 2 + , 44 ° with flooding. A procedure for the depth calibration of the profiles revealed that surface roughness causes an erosion rate change as measured using the shift of the position of the measured B peaks with and without flooding. The consequences of the roughness in terms of depth resolution of the profiles are analysed with and without flooding. Moreover, we show that the value of the Gaussian broadening parameter of the depth resolution function is closely related to the final dispersion of the heights in the crater bottom.

SIMS depth profiling of boron ultra shallow junctions using oblique O 2+ beams down to 150 eV

An epitaxial Si grown multi-layer stack consisting of boron delta-doped layers separated by 6.4 nm thick undoped films has been profiled using a Cameca IMS Wf magnetic SIMS. Using low energy oblique O 2 + beam, the boron depth resolution is improved from 1.66 nm/decade at 500 eV down to 0.83 nm/decade at 150 eV. At very low impact energy O 2 + bombardment induces a near full oxidation of silicon and oxygen flooding is then no more needed in the analytical chamber to get a smooth sputtering of silicon at 45° incidence angle.

Boron ultra low energy SIMS depth profiling improved by rotating stage

Optimization of oblique incidence ultra low energy O 2 + SIMS analysis of ultra shallow boron distributions has been investigated varying the atmosphere in the analysis chamber (ultra high vacuum or oxygen flooding) and evaluating the effect of a rotating stage allowing a 20 rpm rotation during the analysis. The impact of the different analytical approaches to the ripple formation on the crater bottom has been investigated on a boron delta doped silicon sample by AFM analysis. The combined use of a 0.5 keV O 2 + beam at 68° of incidence with oxygen flooding and stage rotation of 20 rpm gave a decay length of 2.0 nm/decade at 60 nm depth without any appreciable detection of variation of sputtering rate.

Quantitative fundamental SIMS studies using 18O implant standards

The use of dilute ‘minor-isotope’ 18O implant reference standards for quantification of surface oxygen levels during steady-state SIMS depth profiling is demonstrated. Some results of two types of quantitative fundamental SIMS studies with oxygen ( 16O) primary ion bombardment and/or oxygen flooding (O 2 gas with natural isotopic abundance) are presented: (1) Determination of elemental useful ion yields, UY(X ±), and sample sputter yields, Y, as a function of the oxygen fraction c O measured in the total flux emitted from the sputtered surface. Examples include new results for positive secondary ion emission of several elements (X = B, C, O, Al, Si, Cu, Ga, Ge, Cs) from variably oxidized SiC or Ge surfaces. (2) The dependence of exponential decay lengths λ(Au ±) in sputter depth profiles of gold overlayers on silicon on the amount of oxygen present at the sputtered silicon surface. The latter study elucidates the (element-specific) effects of oxygen-induced surface segregation artifacts for sputter depth profiling through metal overlayers into silicon substrates.

Narrow surface transient and high depth resolution SIMS using 250 eV O 2+

Ultrashallow junctions in semiconductors and multi-quantum wells (MQW) in lasers demand high depth resolution for accurate depth profiling. SIMS has been widely used in depth profiling and the use of ultralow-energy SIMS has demonstrated a narrower surface transient and an improvement in depth resolution. In this work, we use an ATOMIKA 4500 SIMS depth profiler with O 2 + primary ions at an ultralow-energy ( E p) of 250 eV and incidence angles ( θ) between 0 and 70° without oxygen flooding. A sample with 10 delta layers of Si 0.7Ge 0.3 nominally grown 11 nm apart is used. We observe that for applications like characterizing ultrashallow junctions, θ ∼ 0° provides the narrowest surface transient ( z tr) of 0.7 nm, which is marginally better than at θ ∼ 40° with z tr of 1.0 nm. The depth resolution denoted by the full width at half maximum (FWHM) of the 70Ge + peaks is comparable for both θ ∼ 0 and 40° at 1.6 and 1.4 nm, respectively. However, in the case of MQW profiling, whereby the quantum wells are normally located deeper, θ ∼ 40° is preferable. At this angle, the average sputter rate of 47 nm min −1 nA −1 cm −2 is significantly higher, more than double that at θ ∼ 0° and a better depth resolution with decay length ( λ d) of 0.64 nm compared to 0.92 nm at θ ∼ 0°. Moreover, the dynamic range possible is also better at θ ∼ 40°. θ ∼ 60 ο is not ideal, even though there is no sign of the onset of roughening. Although the higher sputter rate is an advantage, the depth resolution deteriorates as the profile gets deeper.

Room temperature migration of boron in crystalline silicon during secondary ion mass spectrometry profiling

Recently we have demonstrated that substitutional boron in crystalline silicon can migrate for long distances even at room temperature (RT) and below during secondary ion mass spectrometry (SIMS) profiling. The phenomenon is suppressed after amorphization or by cooling the sample. The above data force to reconsider the observations obtained in the last decades by SIMS in light of possible long-range migration artifacts. Here we show that the use of oxygen flooding (OF) during the analysis enhances the injection of self-interstitials (I) responsible for the migration of B, producing profiles that are more broadened and less accurate than in ultrahigh vacuum. On the other hand, by properly controlling OF, we have obtained new insights on the mechanisms of B diffusion and interaction with intrinsic traps at RT. Moreover, by cooling the sample the migration of B is suppressed even while using OF, allowing measurements of boron deltas in c-Si of unprecedented level of accuracy and sensitivity. Finally, we have clarified the role of the migration phenomena on the profiling of ultra low energy B implants for ultra shallow junctions.

Observation of small gas-phase metal–oxygen dianions

Novel small metal–oxygen dianions have been produced in the gas phase by sputtering of metallic targets with simultaneous oxygen flooding. These exotic species were observed in a mass spectrometer for ion flight times of >10 μs. Both Cs + bombardment of an elemental Be metal surface and of a copper-based Cu(Be) alloy foil, with exposure of these sputtered surfaces to O 2 gas, produced the novel dianions of Be n - 1 O n 2 - with n = 4, 6. Be 3 O 4 2 - at m/ z 45.5 and 46.5 was unambiguously identified by its isotopic abundance. In addition, CuBe 2 O 4 2 - at m/ z 72.5 and 73.5 was detected for O 2 flooding of Cs + sputtered Cu(Be).