Low-energy Oxygen Bombardment Research Articles

Oxidation of Cobalt by Oxygen Bombardment at Room Temperature

Most experiments on thermal oxidation of cobalt have shown the parabolic growth rate consistent with the diffusion of cations through neutral cation vacancies. Here, we present experimental results on the oxidation of Co metal by low-energy oxygen bombardment at room temperature, which scales with the dose of implanted oxygen, Φ, as Φ1/6. This type of oxide growth, predicted theoretically for diffusion of cobalt cations by doubly charged cation vacancies, has not been observed previously in thermal oxidation of Co. Our results demonstrate that oxidation of Co, which involves formation of both monoxide, CoO, and spinel, Co3O4, oxide structures, can be indeed driven by doubly charged vacancies, as predicted theoretically, when oxidation conditions enhance both the production of point defects and the mobilities of cobalt cations and oxygen anions within cobalt oxide.

Oxidation of nickel surfaces by low energy ion bombardment

We have studied formation of oxides on Ni surfaces by low energy oxygen bombardment using X-ray photoemission spectroscopy (XPS) and secondary ion mass spectrometry (SIMS). Different oxidation states of Ni ions have been identified in XPS spectra measured around Ni 2p and O 1s core-levels. We have compared our results with thermal oxidation of Ni and shown that ion bombardment is more efficient in creating thin oxide films on Ni surfaces. The dominant Ni-oxide in both oxidation processes is NiO (Ni2+ oxidation state), while some Ni2O3 contributions (Ni3+ oxidation state) are still present in all oxidised samples. The oxide thickness of bombarded Ni samples, as determined by SIMS, was shown to be related to the penetration depth of oxygen ions in Ni.

Detailed analysis of the silicon surface under low-energy oxygen bombardment at atomic resolution

A detailed analysis of the amorphous-SiO${}_{2}$/crystalline-Si(011) interface formed by low-energy oxygen bombardment is performed by high-resolution Rutherford backscattering spectroscopy and spherical aberration-corrected scanning transmission electron microscopy (STEM). The atomic level analyses indicated a few nanometers of displaced Si atoms layer immediately below the synthetic SiO${}_{2}$ layer. A comparison between intensities of the high-angle annular dark field (HAADF) STEM image and the simultaneously acquired high-angle bright-field (HABF) STEM image shows that both intensities decreased at the existing layer of displaced Si atoms, and the relationship between these intensities cannot be explained by conventional image formation mechanisms. From detailed investigations using STEM imaging simulations, the HAADF STEM and HABF STEM images, which were simulated based on a realistic model, revealed the random distribution of a crystalline material and amorphous material at the interface; and this result agrees well with experimental results.

Low-energy oxygen bombardment of silicon by MD simulations making use of a reactive force field

In the field of Secondary Ion Mass Spectrometry (SIMS), ion–matter interactions have been largely investigated by numerical simulations. For MD simulations related to inorganic samples, mostly classical force fields assuming stable bonding structure have been used. In materials science, level-three force fields capable of simulating the breaking and formation of chemical bonds have recently been conceived. One such force field has been developed by John Kieffer [1] , [2] . This potential includes directional covalent bonds, Coulomb and dipolar interaction terms, dispersion terms, etc. Important features of this force field for simulating systems that undergo significant structural reorganization are (i) the ability to account for the redistribution of electron density upon ionization, formation, or breaking of bonds, through a charge transfer term, and (ii) the fact that the angular constraints dynamically adjust when a change in the coordination number of an atom occurs. In this paper, the modification of the force field to allow for an exact description of the sputtering process, the influence of this modification on previous results obtained for phase transitions in glasses as well as properties of particles sputtered at 250–1000 eV from a mono-crystalline silicon sample will be presented. The simulation results agree qualitatively with predictions from experiments or models. Most atoms are sputtered from the first monolayer: for an impact energy of 250 eV up to 86% of the atoms are sputtered from the first monolayer and for 750 eV, this percentage drops to 61%, with 89% of the atoms being sputtered from the first two monolayers. For sputtering yields, 250 and 500 eV results agree with experimental data, but for 750 eV sub-channelling in the pristine sample becomes more important than in experiments where samples turn amorphous under ion bombardment.

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.

Using accelerator techniques to verify details in SIMS profiles

Recent use of ultra-low energy (ULE) ion implantation in the semiconductor industry has placed increasing pressure on SIMS depth profiling capabilities. We have used nuclear reaction analysis (NRA) to calibrate implantation doses of boron ULE implants as an essential check of the SIMS accuracy. Comparison of NRA done in two different laboratories has revealed differences of up to 15%, making it essential to calibrate the measurements against an accurate standard. It has become evident in the last few years that the accepted SIMS depth profiling method for ULE B implants, low-energy oxygen bombardment with an oxygen leak, causes significant segregation of boron during the SIMS profile. High-resolution ERD measurements appear to be one of the few techniques other than SIMS that can resolve the near-surface details of the boron distribution. Careful comparisons of the ERD and SIMS profiles allow us to refine our SIMS correction procedures to produce the most accurate measurement of the boron distribution. Similarly, ULE As implants show significant segregation of As toward the native oxide interface as a result of annealing. To ensure that we measure these distributions accurately, MEIS has been employed to examine the As distribution within 10nm of the surface.

On the segregation of metals during low-energy oxygen bombardment of silicon

Segregation of metal impurities in Si under oxygen ion bombardment has been studied using secondary ion mass spectrometry. A strong evidence has been found for the thermodynamic driving force for segregation. This includes segregation of metal atoms at both interfaces of a buried SiO2 layer and the `antisegregation' behaviour of metal atoms having lower heats of oxide formation than Si. Segregation is enhanced at elevated temperatures when the metal diffusivity in amorphous Si is higher.

Transient phenomena and impurity relocation in SIMS depth profiling using oxygen bombardment: pursuing the physics to interpret the data

High detection sensitivity in bulk analysis or depth profiling by secondary ion mass spectrometry (SIMS) can only be achieved, for positively charged ions, if the nearsurface regions of the sputter eroded sample are fully oxidized. Using oxygen primary ions, a stationary oxidation state is established after some time of bombardment during which period the sputtering yield decreases and the ionization probability increases. The physical and chemical processes occurring during the transient period are reviewed with emphasis on the results for impurity analysis in silicon, i.e. the matrix material that has been studied most thoroughly in the past. The transient decrease in sputtering yield gives rise to a depth scale offset and an associated apparent shift of impurity profiles towards the surface. The effect is largest at normal beam incidence, ca. 1 nm /keV (O + 2 ), in which case silicon is fully oxidized. The transition depth, i.e. the depth sputtered before achieving a stable ion yield is about twice as large and increases as the impact angle is turned away from normal. The shift and the depth scale offset can be measured safely using thin (delta) layers of isotopically pure tracers, for example 30 Si in 28 Si. Boron delta layers can serve as secondary standards because this impurity behaves almost the same as the host silicon atoms. The profile shift observed with other common dopants may contain contributions due to unidirectional relocation, often driven by segregation away from the surface. At O + 2 energies below about 0.7 keV the transition depths fall below the thickness of typical native oxides, so that the transient changes in sputtering yield and ionization probability can disappear. The transient phenomena may be described by a simple sputtering-oxidation model that connects the depth scale offset to other observable parameters like the initial and final sputtering yield, the transition fluence and the oxide thickness.

Stoichiometric changes of Si, CoSi 2 and TiSi 2 during low energy oxygen bombardment in combination with oxygen bleed-in

Rutherford backscattering spectrometry was used to investigate the near surface compositional changes and the internal layer variations of Si, CoSi 2 and TiSi 2 during 12 keV O 2 + bombardment with and without oxygen bleed-in. Experiments have been carried out with the angle of incidence between the sputter beam and the normal at 26 and 40°. The results show the incorporation of oxygen, leading to oxide formation, preferential sputtering of Si in the silicides and the formation of a surface oxide during oxygen flooding.

Stoichiometry changes during low energy oxygen bombardment

A basic understanding of the sputter induced artifacts during low energy oxygen ion beam bombardment is of great importance for a correct evaluation of the results of suptter depth profiling in SIMS. In this paper the sputter depth profiling of Si, CoSi 2 and TiSi 2 under normal oxygen bombardment has been investigated. With the combined sputter/RBS set-up and in combination with ex-situ XPS measurements, the oxidation and near-surface compositional changes in the transient region have been explained.

Oxidation of silicon by low energy oxygen bombardment

High resolution Rutherford backscattering and channeling has been used to study the formation of surface oxides during room temperature bombardment of silicon with oxygen in a secondary ion mass spectrometry system. Stoichiometric SiO2 is formed at angles of incidence (to the surface normal)≤25° and the angular dependence is adequately modeled using the profile code. A linear dependence of oxide thickness on energy is obtained in the energy range 3–40 keV (per oxygen ion) and this is consistent with trim code calculations. The suboxide damage has also been measured and studied during annealing. Our data are consistent with a simple model of oxygen build up and formation of strong Si–O bonds during room temperature bombardment. Once a buried SiO2 layer is reached and Si bonds are saturated, oxygen can migrate in SiO2 to extend the oxide towards the surface.

Sputtering phenomena of CoSi2 under low energy oxygen bombardment

Sputter depth profiling with AES, XPS and SIMS suffers from sputter induced artifacts such as preferential sputtering, segregation phenomena and recoil displacements. A combined in situ sputter RBS system has been used to study the sputter process into detail. In this work the beam induced oxidation of CoSi2 during low energy oxygen irradiation is investigated. The oxygen bombardment of CoSi2 offers a very complex behavior as a function of dose where we first observe a gradual Co depletion subsequently followed by a replenishment of this depleted region. Additional XPS and TEM analysis of the bombarded CoSi2 shows that the transient behavior can be correlated with changes in oxidation and different phase formations.

Characterization of sharp phosphorus dopant features in silicon by secondary ion mass spectrometry

The in-depth distribution of a sharp P dopant feature in Si grown by vapor phase epitaxy has been determined by secondary ion mass spectrometry under various bombardment conditions. It is shown that with a low energy primary 0+2 beam supplemented by oxygen gas flooding of the sample surface both sub-ppm detection limits and good depth resolution can be obtained simultaneously. This combination is not possible by using either primary Cs+ bombardment, giving only good detection limits, or low energy oxygen bombardment, resulting in a good depth resolution but unfavorable detection limits.

Ion-enhanced adhesion of thin gold films

The adhesion of gold films to many surfaces of technological importance, such as glass and silicon, has in recent years been substantially improved by the use of ion bombardment. Ions may also play an important role in the film deposition process, depending on the method employed. Low-energy oxygen bombardment during deposition gives rise to a strong bonding effect, whilst medium-energy inert-gas ion bombardment can promote interfacial mixing. High-energy ion and electron post-bombardment of gold films give rise to another bonding mechanism which is thought to result in electronic interactions at the film-substrate interface.