Structural analysis of DC magnetron sputtered and spin coated thin films using RBS, TEM and X-ray reflectivity methods

Metallic thin films such as Au, Cr, Ag, etc., on silicon substrate have many technologically important applications as contact layers in microelectronic industry, as reflecting mirrors in synchrotron radiation research, etc. The native oxide layer on crystalline silicon surface inhibits wetting of few nm thick Au or Ag on native oxide/silicon systems. To obtain continuous thin metallic films (a few nm thick), a Cr layer was first deposited as a adhesion layer on the Si substrate. In this paper, Rutherford backscattering analysis (RBS) of Si/Cr/SiO₂/Si, Si/Au/SiO₂/Si, Si/Au/Cr/SiO₂/Si and Polystyrene (PS) polymer coated on some of these bi- or tri-layer structures has been reported. The X-ray reflectometry and transmission electron microscopy studies were carried out to complement the RBS measurements. The thickness, surface and interface roughness, and crystalline quality have been determined.     

HRBS/channeling studies of ultra-thin ITO films on Si

High-resolution Rutherford backscattering spectroscopy (HRBS)/channeling techniques have been utilized for a detailed characterization of ultra-thin indium tin oxide (ITO) films and to probe the nature of the interface between the ITO film and the Si(0 0 1) substrate. Channeling studies provide a direct measure of the lattice strain distribution in the crystalline Si substrate in the case of amorphous over layers. The measurements on DC magnetron sputtered ITO films have been carried out using the recently installed HRBS facility at the Centre for Ion Beam Applications (CIBA). The thickness of the ultra-thin (∼9.8 nm) ITO films was calculated from the HRBS spectra having an energy resolution of about 1.4 keV at the superimposed leading (In + Sn) edge of the ITO film. The films were near stoichiometric and the interface between ITO film and Si was found to include a thin SiO_x transition layer. The backscattering yields from (In + Sn) of ITO were equal in random and channeling directions, thereby revealing the non-crystalline nature of the film. Angular scans of HRBS spectra around the off-normal [1 1 1] axis clearly showed a shift in the channeling minimum indicative of compressive strain of the Si lattice at the SiO_x/Si interface. The observed strain was about 0.8% near the interface and decreased to values below our detection limits at a depth of ∼3 nm from the SiO_x/Si interface.     

Effect of ion beam irradiation on magnetic and structural properties of Pt/Cr/Co multilayers

This paper discusses the effect of ion beam irradiation on the magnetic and structural properties of Pt/Cr/Co multilayers. We observe Co-Cr-Pt ternary alloy phase formation in 1 MeV N⁺ ion irradiated [Pt (2.5 nm)/Cr (0.8 nm)/Co (3.0 nm)]_×6/Si multilayers for a fluence of 1 × 10¹⁶ ions cm⁻² and beyond. The observed phase formation is accompanied by an enhancement in the average grain size, surface roughness and coercivity. Monte Carlo simulation has been performed to study ion-induced defect evolution and atomic displacements to correlate the above observed effects.     

Strain-free Al0.08In0.018Ga0.902N/GaN heterostructure: Combined Rutherford backscattering/channeling and high resolution X-ray diffraction

A 160 nm Al_0.08In_0.018Ga_0.902N layer was grown by metal-organic chemical vapour deposition (MOCVD) on sapphire (0 0 0 1) with thick (>1 μm) GaN intermediate layer. The chemical compositions can be determined by Rutherford backscattering (RBS). The perpendicular and parallel strain of Al_0.08In_0.018Ga_0.902N layer was derived to be zero by using a combination of high resolution X-ray diffraction (HRXRD) and RBS/channeling. The conclusion is further evidenced by transmission electron microscopy (TEM).     

Influence of Cr-ions on the magnetic behaviour of FeCo film

Implantation of Cr-ions in Fe₇₀Co₃₀ thin film have been performed to modify its structural and magnetic properties. From the XRD results, the lattice constant as well as the grain size of the film is increasing with the ion fluence. Cr-ions (1 × 10¹⁷ ions/cm²) reduces the coercivity of the film from 140(3) Oe to 44(3) Oe. Coercivity of the film follows the exponential decay as a function of Cr-ions fluence. 35 keV (projectile range 13.5 nm) and 100 keV Cr-ions (projectile range 34.3 nm) have been used to understand the effects of magnetic Cr-ions and the effects of ballistic collision cascade on the MOKE signal. Similar changes on the coercivity behaviour of the film implanted with these two energies have been observed. It appears that the implantation process creates a solid solution of Cr in FeCo without any other additional treatment in the film. After 5 × 10¹⁶ Cr-ions, film exhibit four fold magnetic anisotropy.     

Anisotropic deformation of polystyrene particles by MeV Au ion irradiation

MeV Au irradiation leads to a shape change of polystyrene (PS) and SiO₂ particles from spherical to ellipsoidal, with an aspect ratio that can be precisely controlled by the ion fluence. Sub-micrometer PS and SiO₂ particles were deposited on copper substrates and irradiated with Au ions at 230 K, using an ion energy and fluence ranging from 2 to 10 MeV and 1 × 10¹⁴ ions/cm² to 1 × 10¹⁵ ions/cm². The mechanisms of anisotropic deformation of PS and SiO₂ particles are different because of their distinct physical and chemical properties. At the start of irradiation, the volume of PS particles decrease, then the aspect ratio increases with fluence, whereas for SiO₂ particles the volume remains constant.     

Interface strain study of thin Lu2O3/Si using HRBS

The interface of thin Lu₂O₃ on silicon has been studied using high-resolution RBS (HRBS) for samples annealed at different temperatures. Thin rare earth metal oxides are of interest as candidates for next generation transistor gate dielectrics, due to their high-k values allowing for equivalent oxide thickness (EOT) of less than 1 nm. Among them, Lu₂O₃ has been found to have the highest lattice energy and largest band gap, making it a good candidate for an alternative high-k gate dielectric. HRBS depth profiling results have shown the existence of a thin (∼2 nm) transitional silicate layer beneath the Lu₂O₃ films. The thicknesses of the Lu₂O₃ films were found to be ∼8 nm and the films were determined to be non-crystalline. Angular scans were performed across the [1 1 0] and [1 1 1] axis along planar channels, and clear shifts in the channeling minimum indicate the presence of Si lattice strain at the silicate/Si interface.     

Growth and surface morphology of ion-beam sputtered Ti-Ni thin films

Titanium-nickel thin films have been deposited on float glass substrates by ion beam sputtering in 100% pure argon atmosphere. Sputtering is predominant at energy region of incident ions, 1000 eV to 100 keV. The as-deposited films were investigated by X-ray photoelectron spectroscopy (XPS) and atomic force microscope (AFM). In this paper we attempted to study the surface morphology and elemental composition through AFM and XPS, respectively. Core level as well as valence band spectra of ion-beam sputtered Ti-Ni thin films at various Ar gas rates (5, 7 and 12 sccm) show that the thin film deposited at 3 sccm possess two distinct peaks at binding energies 458.55 eV and 464.36 eV mainly due to TiO₂. Upon increasing Ar rate oxidation of Ti-Ni is reduced and the Ti-2p peaks begin approaching those of pure elemental Ti. Here Ti-2p peaks are observed at binding energy positions of 454.7 eV and 460.5 eV. AFM results show that the average grain size and roughness decrease, upon increasing Ar gas rate, from 2.90 μm to 0.096 μm and from 16.285 nm to 1.169 nm, respectively.     

Sputtering of cryogenic films of hydrogen by keV ions: Thickness dependence and surface morphology

The sputtering yield induced by keV hydrogen ions measured at CERN and at Risø National Laboratory for solid H₂ and D₂ at temperatures below 4.2 K decreases with increasing film thickness from about 100 × 10¹⁵ molecules/cm². For a film thickness comparable to or larger than the ion range the data from Risø show a slight increase, whereas the yield from CERN continues to decrease up to very large film thicknesses, i.e. one order of magnitude larger than the ion range. The different behavior of the yield is discussed in terms of the probable growth modes of the films. The films produced at the Risø setup are quench-condensed films, while those produced at CERN are supposed to grow with large hydrogen aggregates on top of a thin bottom layer.     

Determination of migration of ion-implanted helium in silica by proton backscattering spectrometry

Understanding the processes caused by ion implantation of light ions in dielectric materials such as silica is important for developing the diagnostic systems used in fusion and fission environments. Recently, it has been shown that ion-implanted helium is able to escape from SiO₂ films. To study this process in details, helium was implanted into the central part of a buried SiO₂ island up to a fluence of 4 × 10¹⁷ He/cm². The implanted helium could be detected in the SiO₂ island, if the oxide was insulated properly from the vacuum. The shape of the helium depth distributions was far from SRIM simulation because helium distributed in the whole 1 μm thick oxide layer. After the ion implantation, helium was observed only on the implanted spot. After nine months the implanted helium filled out the whole oxide island as it was expected from the high diffusivity.     

Depth profiling of Mg by 2010 keV resonance of Mg(p,p′γ)Mg nuclear reaction

Studies on the characteristics of 2010 keV resonance in ²⁴Mg(p,p′γ)²⁴Mg nuclear reaction for depth profiling Mg in thin films are reported. The resonance reaction, based on the detection of characteristic 1368 keV γ-rays, enables interference free measurement of Mg down to 2 × 10²⁰ atoms/cm³ and has a probing depth of about 20 μm. The width of the resonance extracted from excitation curves for thick (>180 nm) thermally grown elemental Mg films, by SPACES is about 350 ± 50 eV. The reaction has been used to depth profile Mg in a Mg/Ti/Mg/Si film which provides interesting information on interfacial mixing involving Ti layer and the underlying Mg layer.     

Fundamentals of surfactant sputtering

We introduce a new sputter technique, utilizing the steady-state coverage of a substrate surface with up to 10¹⁶ cm⁻² of foreign atoms simultaneously during sputter erosion by combined ion irradiation and atom deposition. These atoms strongly modify the substrate sputter yield on atomic to macroscopic length scales and therefore act as surfactant atoms (a blend of “surface active agent”). Depending on the surfactant-substrate combination, the novel technique allows enhanced surface smoothing, generation of novel surface patterns, shaping of surfaces and formation of ultra-thin films. Sputter yield attenuation is demonstrated for sputtering of Si and Fe substrates and different surfactant species using 5 keV Xe ions at different incidence angles and fluences up to 10¹⁷ cm⁻². Analytical approaches and Monte Carlo simulations are used to predict the sputtering yield attenuation as function of surfactant coverage. For sputtering of Si with Au surfactants we observe high sputter yields despite a steady-state surfactant coverage, which can be explained by strong ion-induced interdiffusion of substrate and surfactant atoms and the formation of a buried Au_xSi surfactant layer in dynamic equilibrium.     

Effect of swift heavy ion irradiation on the surface morphology of highly c-axis oriented LSMO thin films grown by pulsed laser deposition

A detailed investigation of the surface morphology of the pristine and swift heavy ion (SHI) irradiated La_0.7Sr_0.3MnO₃ (LSMO) thin film using atomic force microscope (AFM) is presented. Highly c-axis oriented LSMO thin films were grown on LaAlO₃ (1 0 0) (LAO) substrates by the pulsed laser deposition (PLD) technique. The films were annealed at 800 °C for 12 h in air (pristine films) and subsequently, irradiated with SHI of oxygen and silver. The incident fluence was varied from 1 × 10¹² to 1 × 10¹⁴ ions/cm² and 1 × 10¹¹ to 1 × 10¹² ions/cm² for oxygen and silver ions, respectively. X-ray diffraction (XRD) studies reveal that the irradiated films are strained. From the AFM images, various details pertaining to the surface morphology such as rms roughness (σ), the surface rms roughness averaged over an infinite large image (σ_∞), fractal dimension (D_F) and the lateral coherence length (ξ) were estimated using the length dependent variance measurements. In case of irradiated films, the surface morphology shows drastic modifications, which is dependent on the nature of ions and the incident fluence. However, the surface is found to remain self-affine in each case. In case of oxygen ion irradiated films both, σ_∞ and D_F are observed to increase with fluence up to a dose value of 1 × 10¹³ ions/cm². With further increase in dose value both σ_∞ and D_F decreases. In case of silver ion irradiated films, σ_∞ and D_F decrease with increase in fluence value in the range studied.     

Proton elastic scattering and proton induced γ-ray emission cross-sections on Na from 2 to 5 MeV

Differential cross-sections for proton elastic scattering on sodium and for γ-ray emission from the reactions ²³Na(p,p′γ)²³Na (E_γ = 440 keV and E_γ = 1636 keV) and ²³Na(p,α′γ)²⁰Ne (E_γ = 1634 keV) were measured for proton energies from 2.2 to 5.2 MeV using a 63 μg/cm² NaBr target evaporated on a self-supporting thin C film.The γ-rays were detected by a 38% relative efficiency Ge detector placed at an angle of 135° with respect to the beam direction, while the backscattered protons were collected by a Si surface barrier detector placed at a scattering angle of 150°. Absolute differential cross-sections were obtained with an overall uncertainty estimated to be better than ±6.0% for elastic scattering and ±12% for γ-ray emission, at all the beam energies.To provide a convincing test of the overall validity of the measured elastic scattering cross-section, thick target benchmark experiments at several proton energies are presented.     

Effect of 200 MeV Ag ion irradiation on structural and electrical transport properties of Fe3O4 thin films

Thin films of Fe₃O₄ have been deposited on single crystal MgO(1 0 0) and Si(1 0 0) substrates using pulsed laser deposition. Films grown on MgO substrate are epitaxial with c-axis orientation whereas, films on Si substrate are highly 〈1 1 1〉 oriented. Film thicknesses are 150 nm. These films have been irradiated with 200 MeV Ag ions. We study the effect of the irradiation on structural and electrical transport properties of these films. The fluence value of irradiation has been varied in the range of 5 × 10¹⁰ ions/cm² to 1 × 10¹² ions/cm². We compare the irradiation induced modifications on various physical properties between the c-axis oriented epitaxial film and non epitaxial but 〈1 1 1〉 oriented film. The pristine film on Si substrate shows Verwey transition (T_V) close to 125 K, which is higher than generally observed in single crystals (121 K). After the irradiation with the 5 × 10¹⁰ ions/cm² fluence value, T_V shifts to 122 K, closer to the single crystal value. However, with the higher fluence (1 × 10¹² ions/cm²) irradiation, T_V again shifts to 125 K.     

Investigation of conductive and transparent ITO/Ni/ITO multilayer films deposited by a magnetron sputter process

Transparent conducting ITO/Ni/ITO films were deposited by RF magnetron sputtering of Sn-doped In₂O₃ and DC magnetron sputtering of Ni on unheated polycarbonate substrates. Ni interlayers with thicknesses of 5, 10, and 20 nm were used as intermediate metallic layers.Changes in the work function and optical, electrical and structural properties of the films were examined with respect to Ni layer thickness. The work function was measured to be about 4.5 eV and was found to be independent of Ni layer thickness. However, the structural, optical, and electrical properties of the films were influenced by the Ni thickness. As-deposited ITO single layer films showed In₂O₃ diffraction peaks for the (2 2 2) and (4 0 0) planes, while after insertion of the Ni layer between ITO films, these diffraction peaks disappeared. The electrical resistivity decreased with the Ni intermediated film and the optical transmittance also decreased due to increased optical absorption. The figure of merit reached a maximum of 2.0 × 10⁻³ Ω⁻¹ for a 5 nm-thick inserted Ni film, which is greater than the value for as-deposited ITO films.     

Microbeam analysis of lateral inhomogeneity in depth penetration of Pd in porous silicon

Porous silicon (PS) is widely utilized in gas sensors. Palladium is a good choice to sensitize the surface of PS to hydrogen. Ideally for highest sensitivity of the sensor, all the pores of PS should be completely covered with palladium. Rutherford backscattering spectroscopy (RBS) technique is routinely used to determine depth profile of elements in thin layers. By using microbeam analysis as a complementary technique, we clearly observed the lateral image of penetrated Pd in depth. In this work, we used electrochemically anodized-silicon wafer doped with Pd by the electroless process. In our analysis, we intentionally characterized both the area of PS where Electric Field is applied during silicon etching (EF area) and the Field Free region (FF area) of the sample. Microbeam analysis show that EF area of the sample is covered with thicker palladium layer, but the pores are rarely filled in this area, while in the FF area of the sample, there is a thin palladium film on the silicon surface, but the pores are partially filled. We used a computational simulation method based on columnar pore structure and linear variation of Pd depth profile in pores and determined the lateral variation of Pd penetration in pores in both parts of the sample. These simulations showed that there are 180 ± 80 nm thick Pd layer with 300 ± 200 nm penetration in pores in EF region and 80 ± 40 nm thin Pd layer with 800 ± 200 nm penetrate in pores in FF region.     

Swift iodine ion modification of the structural and magnetotransport properties of Fe/Cr systems

Fe/Cr/Fe trilayers and (Fe/Cr)₂₀ multilayers prepared under ultrahigh vacuum conditions by thermal evaporation were irradiated with 200 MeV I¹³⁺ ions in the fluence range between 1 × 10¹¹ and 8 × 10¹² I/cm². The structural properties of the Fe/Cr/Fe trilayers and (Fe/Cr)₂₀ multilayers were measured by X-ray reflectivity (XRR) and conversion electron Mössbauer spectroscopy (CEMS). Magnetic exchange coupling between the Fe layers through the Cr spacer layer was observed by SQUID magnetization measurements. Magnetoresistance effect was measured using four probe method at room temperature. The XRR spectra showed an increase of the interface roughness versus increasing irradiation fluence in the multilayers, while in the trilayers smoothening of the interfaces in the sample irradiated with fluence equal to 4 × 10¹¹ I/cm² and very slight change for other fluences were observed. Improving of the interface structure in the trilayers at this fluence was observed also by CEMS. Moreover the Mössbauer spectra also confirm roughening of the interfaces as a function of fluence for multilayers. Before irradiation an antiferromagnetic coupling fraction dominated in all samples. After irradiation the changes of magnetic coupling were different in both types of samples. The trilayers were less sensitive to the irradiation fluence than multilayers and an increase of the antiferromagnetic fraction at small fluences was observed. In the multilayers a continuous decrease of the antiferromagnetic fraction as a function of fluence was evidenced. Vanishing of the antiferromagnetic coupling, observed for the largest fluence, resulted in the decrease of magnetoresistance effect in the Fe/Cr multilayers.     

Growth of silicon nitride films by bombarding amorphous silicon with N ions: MD simulation

In this study, the molecular dynamics simulation method was employed to investigate the growth of silicon nitride films by using N⁺ ions, with energies of 50, 100, 150 and 200 eV, to bombard an amorphous silicon surface at 300 K. After an initial period of N⁺ bombardment, saturation of the number of N atoms deposited on the surface is observed, which is in agreement with experiments. During subsequent steady state deposition, a balance between uptake of N by the surface and sputtering of previously deposited N is established. The Si(N_x) (x = 1-4) and N(Si_y) (y = 1-3) bond configurations in the grown films are analyzed.     

10 MeV Au ion irradiation effects in an MgO-HfO2 ceramic-ceramic (CERCER) composite

Room temperature ion irradiation damage studies were performed on a ceramic composite intended to emulate a dispersion nuclear fuel. The composite is composed of 90-mole% MgO and 10-mole% HfO₂. The as-synthesized composite was found to consist of Mg₂Hf₅O₁₂ (and some residual HfO₂) particles embedded in an MgO matrix. X-ray diffraction revealed that nearly all of the initial HfO₂ reacted with some MgO to form Mg₂Hf₅O₁₂. Ion irradiations were performed using 10 MeV Au³⁺ ions at room temperature over a fluence range of 5 × 10¹⁶-5 × 10²⁰ Au/m². Irradiated samples were characterized using both grazing incidence X-ray diffraction (GIXRD) and transmission electron microscopy (TEM), the latter using both selected-area electron diffraction (SAED) and micro-diffraction (μD) on samples prepared in cross-sectional geometry. Both GIXRD and TEM electron diffraction measurements on a specimen irradiated to a fluence of 5 × 10²⁰ Au/cm², revealed that the initial rhombohedral Mg₂Hf₅O₁₂ phase was transformed into a cubic-Mg₂Hf₅O₁₂ phase. Finally, it is important to note that at the highest ion fluence used in this investigation (5 × 10²⁰ Au/m²), both the MgO matrix and the Mg₂Hf₅O₁₂ second phase remained crystalline.