Heavy Ion Elastic Recoil Detection Research Articles

Summary of “IAEA intercomparison of IBA software”

The International Atomic Energy Agency has sponsored a formal intercomparison exercise for the seven depth profiling ion beam analysis codes, which are: GISA, RUMP, RBX, DEPTH, DataFurnace, SIMNRA and MCERD. This intercomparison is published in Nucl. Instr. and Meth. B [N.P. Barradas, K. Arstila, G. Battistig, M. Bianconi, N. Dytlewski, C. Jeynes, E. Kótai, G. Lulli, M. Mayer, E. Rauhala, E. Szilágyi, M. Thompson, Nucl. Instr. and Meth. B 262 (2007) 281–303] and summarised here. The codes implement all known physical effects and they are all evaluated. We demonstrate that there is agreement between codes often better than 0.1%; and also detailed agreement with real spectra, showing in particular that the SRIM 2003 stopping powers for Si are correct to 0.6% for 1.5MeV He. For the case of heavy ion elastic recoil detection (HI-ERD) the single scattering codes performed poorly for scattered particles, although recoiled particles were calculated correctly.

Optical properties and structure of HfO2 thin films grown by high pressure reactive sputtering

Thin films of hafnium oxide (HfO2) have been grown by high pressure reactive sputtering on transparent quartz substrates (UV-grade silica) and silicon wafers. Deposition conditions were adjusted to obtain polycrystalline as well as amorphous films. Optical properties of the films deposited on the silica substrates were investigated by transmittance and reflectance spectroscopy in the ultraviolet, visible and near infrared range. A numerical analysis method that takes into account the different surface roughness of the polycrystalline and amorphous films was applied to calculate the optical constants (refractive index and absorption coefficient). Amorphous films were found to have a higher refractive index and a lower transparency than polycrystalline films. This is attributed to a higher density of the amorphous samples, which was confirmed by atomic density measurements performed by heavy-ion elastic recoil detection analysis. The absorption coefficient gave an excellent fit to the Tauc law (indirect gap), which allowed a band gap value of 5.54 eV to be obtained. The structure of the films (amorphous or polycrystalline) was found to have no significant influence on the nature of the band gap. The Tauc plots also give information about the structure of the films, because the slope of the plot (the Tauc parameter) is related to the degree of order in the bond network. The amorphous samples had a larger value of the Tauc parameter, i.e. more order than the polycrystalline samples. This is indicative of a uniform bond network with percolation of the bond chains, in contrast to the randomly oriented polycrystalline grains separated by grain boundaries.

High-pressure reactively sputtered HfO2: Composition, morphology, and optical properties

Hafnium oxide films were deposited by high pressure reactive sputtering using different deposition pressures and times. The composition, morphology, and optical properties of the films, together with the sputtering process growth kinetics were investigated using heavy ion elastic recoil detection analysis, Fourier transform infrared spectroscopy, ultraviolet-visible-near infrared spectroscopy, x-ray diffraction, and transmission electron microscopy. The films showed a monoclinic polycrystalline structure, with a grain size depending on the deposition pressure. All films were slightly oxygen rich with respect to stoichiometric HfO2 and presented a significant amount of hydrogen (up to 6at.%), which is attributed to the high affinity for moisture of the HfO2 films. The absorption coefficient was fitted to the Tauc law, obtaining a band gap value of 5.54eV. It was found that the growth rate of the HfO2 films depends on the deposition pressure (P) as P−1.75. This dependence is explained by a diffusion model of the thermalized atoms in high-pressure sputtering. Additionally, the formation of an interfacial silicon oxide layer when the films were grown on silicon was observed, with a minimum thickness for deposition pressures around 1.2mbars. This interfacial layer was formed mainly during the initial stages of the deposition process, with only a slight increase in thickness afterwards. These results are explained by the oxidizing action of the oxygen plasma and the diffusion of oxygen radicals and hydroxyl groups through the polycrystalline HfO2 film. Finally, the dielectric properties of the HfO2∕SiO2 stacks were studied by means of conductance and capacitance measurements on Al∕HfO2∕SiO2∕Si devices as a function of gate voltage and ac frequency signal.

Hafnium oxide thin films deposited by high pressure reactive sputtering in atmosphere formed with different Ar/O 2 ratios

The physical and electrical properties of hafnium oxide (HfO 2) thin films deposited by high pressure reactive sputtering (HPRS) have been studied as a function of the Ar/O 2 ratio in the sputtering gas mixture. Transmission electron microscopy shows that the HfO 2 films are polycrystalline, except the films deposited in pure Ar, which are amorphous. According to heavy ion elastic recoil detection analysis, the films deposited without using O 2 are stoichiometric, which means that the composition of the HfO 2 target is conserved in the deposition films. The use of O 2 for reactive sputtering results in slightly oxygen-rich films. Metal-Oxide-Semiconductor (MOS) devices were fabricated to determine the deposited HfO 2 dielectric constant and the trap density at the HfO 2/Si interface ( D it) using the high–low frequency capacitance method. Poor capacitance–voltage ( CV) characteristics and high values of D it are observed in the polycrystalline HfO 2 films. However, a great improvement of the electrical properties was observed in the amorphous HfO 2 films, showing dielectric constant values close to 17 and a minimum D it of 2×10 11 eV −1 cm −2.

Teeth characterization using ion beam analysis

A collaboration project between the School of Dentistry and the Institute of Physics of the University of Sao Paulo has been established to measure elemental concentrations in teeth by proton induced X-ray emission (PIXE) and heavy ion elastic recoil detection analysis (HI-ERDA) techniques. Data on trace elements in human, bovine and swine teeth, analyzed by PIXE with a 2.4 MeV proton beam, were compared and concentrations for several elements were obtained with tens of mg/g sensitivity. HI-ERDA measurements employing a 52 MeV Cl beam were done to evaluate changes in elementary concentration in dental enamel after bleaching treatment with different products in 25 bovine incisors teeth. This non-destructive technique allowed the measurements of Ca, P, O and C concentrations above the limit of 100 mg/g.

The analysis of a thin SiO2/Si3N4/SiO2 stack: A comparative study of low-energy heavy ion elastic recoil detection, high-resolution Rutherford backscattering and secondary ion mass spectrometry

The analysis of thin films in the range of 10nm and less has become very important in microelectronics. The goal of this article is an evaluation of low-energy TOF-ERDA (time-of-flight elastic recoil detection analysis) in comparison with low-energy SIMS (secondary ion mass spectrometry) and HRBS (high-resolution Rutherford backscattering spectrometry), using a thin SiO2/Si3N4/SiO2 stack as a test vehicle. Comparisons are made on the depth resolution, its loss as a function of depth and the quantification accuracy.

Stress and stress relief in dielectric thin films – the role of hydrogen

Novel crack propagation modes are reported in amorphous silicon-rich oxide (SiOx) films deposited onto (100) silicon substrates and subjected to thermal annealing. These include the formation of straight cracks aligned with 〈001〉 directions in the underlying Si substrate as well as oscillating cracks aligned with 〈011〉 directions in the underlying Si substrate. The cracks are shown to form as the result of increasing tensile stress in the film caused by annealing, with the straight cracks forming at lower tensile stress levels than the oscillating cracks. The increase in tensile stress with increasing annealing temperature in the range 400–650°C is shown to be correlated with the hydrogen content of the films, as measured by heavy-ion elastic recoil detection.

Irradiation-induced damage in porous low-k materials during low-energy heavy-ion elastic recoil detection analysis

With the implementation of time-of-flight elastic recoil detection (ToF-ERD) for the analysis of thin films with high depth resolution using a standard ‘low-energy’ accelerator, routine application of ERD in semiconductor technology becomes possible. In case of irradiation-sensitive materials, like organosilicate low-k films, the energetic incident beam damages the sample during the measurement, resulting in loss of the lighter elements and, as a consequence, altering the sample composition.The ion beam induced damage is investigated for 19F, 35Cl, 63Cu, 79Br and 127I beams at energies of 6–16MeV and typical fluences for ERD analysis. By means of Fourier transform infrared (FTIR) spectroscopy a direct correlation between elemental losses and molecular broken bonds is obtained. The H losses can be described by the bulk molecular release model, with the associated release cross section for H linearly dependent on the energy deposited by the primary beam in the film.

Heavy ion elastic recoil detection analysis set up for electronic sputtering studies

Heavy ion elastic recoil detection analysis (ERDA) set up with a large solid angle (∼4.8 msr) Δ E−E position-sensitive telescope detector is developed at Inter University Accelerator Centre as a dedicated facility for the study of electronic sputtering of thin films under swift heavy ion (SHI) irradiation. The detector consists of a gas ionization chamber (Δ E) and a solid-state surface barrier detector (E) housed in a same assembly. The electronic sputtering yield (atoms/ion) is determined by analyzing on-line fluence-dependent ERDA data obtained from a variety of thin films. Large erosion (>105 atoms/ion) of carbon from a-C:H by 150 MeV Ag13+ ions, evolution of nitrogen (∼880 atoms/ion) from copper nitride and depletion of oxygen (∼1000 atoms/ion) from copper oxide film under 200 MeV Au15+ ion impact are studied and reported in this work. The electronic sputtering of these materials is discussed on the basis of the thermal spike model of SHI and solid interaction.

Compositional analysis of polycrystalline hafnium oxide thin films by heavy-ion elastic recoil detection analysis

The composition of polycrystalline hafnium oxide thin films has been measured by heavy-ion elastic recoil detection analysis (HI-ERDA). The films were deposited by high-pressure reactive sputtering (HPRS) on silicon wafers using an oxygen plasma at pressures between 0.8 and 1.6 mbar and during deposition times between 0.5 and 3.0 h. Hydrogen was found to be the main impurity and its concentration increased with deposition pressure. The composition was always slightly oxygen-rich, which is attributed to the oxygen plasma. Additionally, an interfacial silicon oxide thin layer was detected and taken into account. The thickness of the hafnium oxide film was found to increase linearly with deposition time and to decrease exponentially with deposition pressure, whereas the thickness of the silicon oxide interfacial layer has a minimum as a function of pressure at around 1.2 mbar and increases slightly as a function of time. The measurements confirmed that this interfacial layer is formed mainly during the early stages of the deposition process.

Hydrogen softening and optical transparency in Si-incorporated hydrogenated amorphous carbon films

High-resolution x-ray reflectivity (XRR) and heavy-ion elastic recoil detection were employed to study the role of hydrogen on the softening behavior observed in Si-incorporated hydrogenated amorphous carbon (Si-a-C:H) films synthesized by plasma-enhanced chemical-vapor deposition using tetramethylsilane (TMS) precursor in C2H2∕Ar plasma. An enhancement of the optical band gap and a massive reduction in the density of the films prepared at high TMS flow rate were revealed, respectively, by spectroscopic ellipsometry and XRR analysis with the development of a double critical angle. A hydrogenation process was responsible for a rise in the density of voids and an associated reduction in the connectivity of the carbon network and the release of its residual stress.

Physical properties of high pressure reactively sputtered TiO2

We present a study of the physical properties of TiO2 thin films deposited at 200°C on Si by high pressure reactive sputtering, a nonconventional deposition method. Just after deposition, the TiO2 films were in situ annealed in the deposition chamber at temperatures between 600 and 900°C in O2 atmosphere. Morphological, compositional, structural and electrical characterization of the samples was performed by means of several techniques, including transmission electron microscopy, heavy-ion elastic recoil detection analysis, infrared spectroscopy, x-ray and electron diffraction and capacitance-voltage measurements. Microscopy images show that the TiO2 films are polycrystalline, and that a SiO2 film spontaneously grows at the TiO2∕Si interface. The unannealed TiO2 films are oxygen rich, as shown by compositional measurements. By annealing this oxygen excess is released. For temperatures above 600°C the TiO2 films are stoichiometric. Infrared spectroscopy and diffraction measurements show that as-deposited films are a mixture of anatase and rutile grains. During annealing there is a phase transformation, and at 900°C the anatase phase disappears and only the rutile phase is found. The relative dielectric permittivity of the TiO2 film is calculated from capacitance-voltage measurements, and very high values in the 88–102 range are obtained.

Oxygen to silicon ratio determination of SiO xH y thin films

The oxygen to silicon ratio of several SiO xH y thin films deposited by the electron cyclotron resonance plasma method was studied by several methods (heavy ion elastic recoil detection analysis, energy dispersive X-ray spectroscopy, Auger spectroscopy and infrared spectroscopy). Among these methods, other groups found that x scales linearly with the wavenumber of the Si–O–Si stretching vibration ( v st) by the relation x = 0.020 v st − 19.3. This equation has been used by many different groups to determine x of SiO x thin films, but we have found that in our ECR deposited films the above mentioned formula gives accurate results for x values higher than 1.5, but for Si richer films the formula overestimates the x value, with values well outside the 20% accuracy range. A possible explanation of this discrepancy may be the bonded hydrogen of the films: in the plasma deposited samples used in this study the hydrogen content was high, above 20 at.% for silicon-rich samples. As a consequence, the Si–O–Si groups were immersed in a more electronegative matrix than in the usual case (SiO x with a low hydrogen concentration) and thus the variation of the position of the stretching peak was less pronounced.

Heavy-ion elastic recoil detection analysis as a useful tool for tracking experimental modifications in bulk calcium silicates

Chemical modifications carried out on unique amorphous nano-structured calcium silicate have been traced by time-of-flight heavy-ion elastic recoil detection analysis (HERDA). It could be shown that this ion-beam analysis method allows not only surface but also depth analysis of the silicate samples and the modifications effected upon it. While providing a challenge for standard analysis methods, the highly porous, low-density nature of the calcium silicate proved to be an asset for the ion-beam analysis technique chosen. Presented are depth profiles giving elemental compositions and providing the bases for representative chemical formula for the silicates studied. It was proven that a study of the surface composition of this nano-structured silicate is sufficient for indicating the bulk composition of a sample of this material. Copyright © 2005 John Wiley & Sons, Ltd.

Influence of H on the composition and atomic concentrations of “N-rich” plasma deposited SiOxNyHz films

The influence of H on the composition and atomic concentrations of Si, O, and N of plasma deposited SiOxNyHz films was investigated. The bonding scheme of H was analyzed by Fourier-transform infrared spectroscopy. The composition and absolute concentrations of all the species present in the SiOxNyHz, including H, was measured by heavy-ion elastic recoil detection analysis (HI–ERDA). Samples were deposited from SiH4, O2, and N2 gas mixtures, with different gas flow ratios in order to obtain compositions ranging from SiNyHz to SiO2. Those samples deposited at higher SiH4 partial pressures show both Si–H and N–H bonds, while those deposited at lower SiH4 partial pressures show N–H bonds only. The Si–H and N–H bond concentrations were found to be proportional to the N concentration. The concentration of H was evaluated from the Si–H and N–H stretching absorption bands and compared to the HI–ERDA results, finding good agreement between both measurements. The deviation from H-free stoichiometric SiOxNy composition due to the presence of N–H bonds results in an effective coordination number of N to produce Si–N bonds lower than 3. By fitting the experimental composition data to a theoretical model taking into account the influence of N–H bonds, the actual concentration of N–H bonds was obtained, making evident the presence of nonbonded H. The presence of Si–H and Si–Si bonds was found to partially compensate the effect of N–H bonds, from the point of view of the relative N and Si contents. Finally, the presence of N–H bonds results in a lower Si atom concentration with respect to the stoichiometric film, due to a replacement of Si atoms by H atoms. This decrease of the Si concentration is lower in those films containing Si–H and Si–Si bonds. A model was developed to calculate the Si, O, and N atom concentrations taking into account the influence of N–H, Si–H, and Si–Si bonds, and was found to be in perfect agreement with the experimental data measured by HI–ERDA.

Characterization of sputtered SmCo thin films for light element contamination using RBS and HIERDA techniques

Samarium cobalt films were prepared on silicon substrates with and without a chromium buffer layer at room temperature and 600°C using direct current unbalanced magnetron sputtering. For obtaining ideal magnetic properties, the films should be free from impurities, such as O, Al and others. Rutherford backscattering spectrometry and heavy ion elastic recoil detection analysis were used to determine the composition and film thickness and to monitor the light element contamination across film thickness. X-ray diffractometer and superconducting quantum interference device were employed to characterize the structure and magnetic properties of the films, respectively. The results obtained led to an improved design of the ground shield and the use of a sorption pump to effectively minimize aluminium and oxygen concentration in the films, respectively.

Heavy ion elastic recoil detection analysis of silicon-rich silica films

Heavy ion elastic recoil detection (ERD) is shown to be a powerful tool for studying the deposition and processing of Si-rich silica films deposited by plasma enhanced chemical vapour deposition (PECVD). Such films are of interest for the fabrication of SiO 2 layers containing silicon nanocrystals and are commonly deposited using SiH 4 and N 2O source gases. Heavy ion ERD reveals that as-deposited films can contain high concentrations of N, potentially reducing the efficacy of nanocrystal formation due to competing reactions between Si and N. This is believed to explain why high excess-Si concentrations are required to produce luminescent nanocrystals in such layers. Analysis of samples following high temperature (1200 °C) annealing also reveal the loss of H from the layers and highlight an experimental artefact associated with such high temperature processing.

Efficient Monte Carlo simulation of heavy ion elastic recoil detection analysis spectra

The interpretation of ion beam analysis data requires accurate simulation of the output spectra. The recent trend towards Monte Carlo (MC) simulation, particularly for techniques using heavy ions, follows in recognition of limitations in traditional analytical approaches. MC modeling is ideally suited to reproducing the pronounced broadening and tailing effects of multiple and plural scattering typically seen in heavy ion elastic recoil detection analysis spectra. The efficiency of the MC modelling process can be improved by exploiting kinematic relationships and experimental parameters to reduce the time spent following ion paths which will not lead to detection events of interest. The efficiency enhancement approach used is different to that of the Helsinki group [Nucl. Instr. and Meth. B 174 (2001) 163]. Another difference is that the code developed at RMIT models the scattered ion spectra in addition to the recoil data. These spectra can be important in cases where the beam species is also present in the sample and therefore the scattered ion contribution needs to be separated from the recoil spectra, or where signals from scattered ions and recoils of similar atomic mass cannot be resolved. An example of the latter case is presented where the simulation is used to interpret the overlapped data. The code has also been used to study the contribution of plural and multiple scattering to the shape of the experimental output spectra. It is shown that the spectral shape is strongly dependent upon the inclusion of these effects.

Surface smoothing of thin HTc YBa2Cu3O7−δ superconducting films by high-energy iodine ions

Abstract A variety of techniques were applied to probe for surface modifications and structural changes in thin high T c YBa 2 Cu 3 O 7− δ (YBCO) superconducting films prepared onto MgO substrates after time of flight heavy ion elastic recoil detection analysis (HERDA). Films have been prepared by ex-situ processing of a spin-coated organic sol–gel precursor. Evidence is provided for the smoothening of the surface of the films accompanied with an amorphisation of the YBCO films in the HERDA spot region. The virgin surface roughness of 30–50 nm could be reduced to 1–2 nm after bombarding the film for a short time with pA current. However, large scale surface roughness on the 200 nm scale remained unchanged. Importantly, the integrity of the superconductor films did not change significantly during HERDA allowing simultaneous measurements of high resolution Y, Cu, Mg and C depth profiles. Further research will utilise the HERDA depth profiling capability for carbon and copper since the copper stoichiometry and fraction of carbon as an impurity are crucial for an understanding of the performance of the YBCO films.

Ion beam studies of TiN xO y thin films deposited by reactive magnetron sputtering

Titanium oxynitride compounds exhibit interesting properties for applications in fields ranging from protective/decorative coatings to solar panels. The properties of TiN x O y are related to the oxide/nitride ratio and can be tailored playing with this ratio. In this work we studied the influence of substrate bias voltage and flow rate of reactive gases (a mixture of N 2 and O 2) on the properties of TiN x O y films. The films were deposited on steel substrates at a constant temperature of 300 °C by r.f. reactive magnetron sputtering. The depositions were carried out from a pure Ti target. The composition throughout the entire thickness was determined by Rutherford backscattering spectrometry. To obtain information on the profile of light elements (O, N) and detect the presence of hydrogen on the films, heavy ion elastic recoil detection analysis was performed. The results indicate a nearly constant stoichiometry through the entire analysed depth. The colouration varied from the shiny golden for low oxygen contents (characteristic of TiN films) to dark blue for higher oxygen contents. The electrical resistivity of the samples was obtained at room temperature and the values varied from approximately 120 μΩ cm for a sample with very low oxygen content to values up to 350 μΩ cm, for the highest oxygen contents.