Sputter Profiling Research Articles

Theoretical and experimental studies of the broadening of dilute delta‐doped Si spikes in GaAs during SIMS depth profiling

AbstractDelta‐doped structures represent a powerful class of test structures to investigate the experimental and fundamental factors limiting the depth resolution obtainable in SIMS sputter depth profiling.In this work, theoretical studies of the effects on the broadening of an Si delta spike in GaAs as a function of the energy (1.4–4.4 keV) and angle of incidence (2°, 45° and 60° off‐normal) of the O2 sputter probe beam have been compared with recent experimental data. The theoretical calculations were carried out using the newly developed IMPETUS computer code, which simulates the depth profiling process by taking into account the combined effects of ballistic mixing (treating collisional mixing as a diffusion process), projectile incorporation into the matrix and sputtering. All of these are processes that always occur in any practical sputter depth profiling situation.The IMPETUS model can reproduce low‐energy Si depth profiles with great accuracy by using the well‐established TRIM calculated range, energy deposition and sputtering data and by making reasonable assumptions for the threshold energy for diffusion in addition to assuming a beam‐ and sputter statistics‐induced surface microtopography, which is described by a Gaussian area versus height distribution having a standard deviation σ = 0.8 nm. Significantly, it is shown that the effects of these parameters on the shape of the sputter profile are largely independent, with σ (accounting for microroughness) mainly affecting the leading edge and the threshold energy (determining mixing processes) the trailing edge of the sputter profile. Good agreement on the energy dependence of the broadening is also obtained. The expected improvement in depth resolution with increasing off‐normal bombardment angle is confirmed and can be quantified. The error in the experimental depth scale calibration based on a constant sputter rate, ignoring transient sputtering, is evaluated. Finally, the sputter depth profile observed for an Si delta spike in GaAs subjected to thermal annealing during growth by molecular beam epitaxy (MBE) can be reproduced accurately by considering diffusion broadening of the initial spike followed by a sputter profiling simulation.

The effect of atomic‐scale etch pit formation on depth resolution in sputter profiling

AbstractA very simplified model of atomic‐scale etch pitting during sputtering erosion for composition depth profiling is used to modify the Benninghoven–Hofmann approach to layer‐by‐layer sputtering. It is shown that analytically tractable defining equations result, with solutions that indicate that etch pitting slightly worsens depth resolution as the probability of producing deep etch pits increases.

Determination of depth resolution parameters from AES sputtering profiles of multilayer structures with varying single layer thicknesses

Determination of depth resolution parameters from AES sputtering profiles of multilayer structures with varying single layer thicknesses

Characterization of nitrogen-ion-implanted aluminium

Aluminium has been implanted with nitrogen ions at different temperatures. The implanted samples have been characterized by Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS) and electron energy-loss spectroscopy (EELS). Deconvolution procedures are needed to separate the influence of the ion sputter profiling by AES and XPS from the nitrogen-ion-beam-induced effects. The chemical state of Al, N, O and C was identified by deconvolution of the measured spectra. In general, there were double-peak structures observed for N 1s and O 1s, identified as contributions from nitrides and weakly bound nitrogen, and oxides and weakly bound oxygen, respectively. Auger analysis confirms the influence of the nitrogen ion fluence on the shape of the concentration distribution. The influence of temperature on the chemical state of implanted aluminium and on the concentration distribution is discussed.

Preparation of YBaCuO thin films on various substrates by laser ablation: Influence of the substrate temperature

High-quality YBa 2Cu 3O 7−δ films were prepared in situ by laser ablation on SrTiO 3 (100), on MgO (100) and on Si (111) + ZrO 2 buffer layer. We especially investigated the influence of the substrate temperature. In similar preparation circumstances zero resistivity has been achieved for films on SrTiO 3 up to 89.5 K, on MgO up to 84.6 K and on Si(+ZrO 2) up to 83.5 K. X-ray diffraction analysis was used to study the growth conditions of the films, and to determine the oxygen stoichiometry. The films deposited at substrate temperatures above 720°C show pure c-axis orientation with an oxygen deficiency δ<0.1. Interdiffusion between substrate and layer, analyzed with Auger sputter profiling, could not be observed. The grains of the films on Si+ZrO 2, studied by scanning electron microscopy, are caused by the polycrystalline buffer layer. These grains largely determine the physical properties of the deposited layer. Critical current measurements were carried out using wet chemically etched microbridges on the different substrates. At T=77 K the values are in the range from 3x10 6 A/cm 2 for films on SrTiO 3 to 2x10 3 A/cm 2 in the case of Si+ZrO 2. The extrapolated resistivity at T=0 is found to be proportional to the temperature width of the superconducting transition.

Comparison of profile tailing in SIMS analyses of various impurities in silicon using nitrogen, oxygen, and neon ion beams at near-normal incidence

We have performed a systematic SIMS study into the effect of (i) the chemical nature and (ii) the energy of the primary ions on the decay length λ which characterizes the exponential fall-off of impurity sputter profiles. The samples consisted of low resistivity, p-type Si covered with thin metallic overlayers. Bombardment was carried out at 2° off normal. Aspect (i) was investigated for tracers of Cu and Ga using N2+, O2+, and Ne+ primary ions at an energy of 5 keV/atom. The effect of the beam energy, aspect (ii), was studied for eight different tracer species and N2+primary ions at energies between 2 and 5 keV/atom. In the case of Ga, λ was found to be shorter with N2+or O2+primary ions (λ=7.0 and 7.5 nm, respectively) than with Ne+ (λ=12 nm). This effect is attributed to beam induced formation of Si3N4 or SiO2 layers, whereby the effective width of the internal distribution of intermixed Ga impurities in the Si subsystem is reduced significantly. In contrast to Ga, the decay length for Cu is smallest under bombardment with Ne+ (λ=16 nm), quite large with N2+(26 nm) and extremely large with O2+(2.2 μm). Segregation of Cu atoms at the Si3N4/Si and the SiO2/Si interface, respectively, is responsible for this depressed impurity removal rate. Within experimental accuracy the observed variation of the decay length with N2+energy E [keV/atom] can be written in the form λ=kEp, where k and p are element specific parameters which range from k=1.2 nm for Pb to 10 nm for Cu and from p=0.6 for Cu and Ag to 1.0 for Pb. The results are discussed with reference to conceivable shapes of the distribution of intermixed impurity atoms.

An Auger sputter profiling study of nitrogen and oxygen ion implantations in two titanium alloys

AbstractSamples of two titanium alloys, Ti–6Al–4V and Ti–15V–3Cr–3Sn–3Al, were ion implanted with a combination of nitrogen (N+) and oxygen (O+). For each alloy, implantation parameters were chosen to give implanted nitrogen concentrations of ∼10 or 50 at.%, from a depth of 100 nm to a depth of 400 nm. In all but one case, dual energy (200 keV and 90 keV) implantations of nitrogen were used to give a relatively uniform nitrogen concentration to a depth of 300 nm. In each case, oxygen was implanted at 35 keV, following the nitrogen implantation, to give an oxygen‐enriched region near the surface. The implanted samples were then examined by Auger electron spectroscopy (AES) combined with argon ion sputtering. In order to determine the stoichiometry of the nitrogen‐implanted regions, it was necessary to determine the N KVV contribution to the overlapping N KVV and Ti LMM Auger transitions. It was also necessary to correct for the ion‐bombardment‐induced compositional changes that have been described in an earlier study of titanium nitride thin films. The corrected AES depth profiles were in good agreement with theoretical predictions.

The influence of bombardment energy on preferencial sputtering and depth profile analysis of mercury cadmium telluride (MCT) with SNMS, AES and XPS: J W Storm, M Altebockwinkel, L Wiedmann and A Benninghoven, Physialisches Institut der Universität, Wilhelm-Klemm-Str 10, D-4400 Münster, FRG

The influence of bombardment energy on preferencial sputtering and depth profile analysis of mercury cadmium telluride (MCT) with SNMS, AES and XPS: J W Storm, M Altebockwinkel, L Wiedmann and A Benninghoven, Physialisches Institut der Universität, Wilhelm-Klemm-Str 10, D-4400 Münster, FRG

Incidence Angle Dependence of the Silicon Near-Surface Contamination Caused by CF4 Reactive Ion Beam Etching

The incidence angle dependence of silicon near-surface contamination caused by CF4 Reactive Ion Beam Etching was studied using on-line variation of SiF4 partial pressure and Auger Sputter Profiling. Initial and steady state etch rates both vanish at grazing incidence and have a smooth maximum at 20–30 deg. It is shown that whatever is the incidence angle, a C, F-blocking overlayer grows involving an etch rate attenuation by a factor which is only slightly dependent on the incidence angle. As the angle increases, with a 500 eV beam, the thickness of the C, F contamination layer decreases monotonically from 1.9 nm down to a constant value of about 1nm achieved beyond a critical angle of 45 deg. Then a “C, F-reactive mixed layer” is seen over the silicon the etching of which is dominated by surface mechanisms. Conversely with the lower angles, the thickness of the “carbonaceous incorporated layer” becomes larger than that of the mixed layer, and volume mechanisms dominate the silicon removal.

Low temperature diffusion effects on microstructural changes in thick gold films on silicon

The formation of a silicon oxide layer and the microstructural changes in thick sputtered deposited gold films on Si(111) substrates were studied by scanning electron microscopy, transmission electron microscopy and X-ray diffraction methods. Redistribution of silicon, gold and oxygen was analysed by Auger electron spectroscopy and secondary-ion mass spectrometry sputter profiling and electron probe microanalysis. It was found that structures formed during low temperature annealing in air up to 250°C consist of several separated zones. In the outer structure zone a thick porous silicon oxide layer with a needle-like structure is honeycombed with columnar gold grains, thus forming a mixed zone. The thickness of the gold and silicon oxide zone increases with increasing temperature until the substantial gold grain growth at the Au Si interface zone impedes the release of silicon atoms and their further outdiffusion and oxidation.

Ohmic Contacts on N-Type Hg0.4Cd0.6Te.

ABSTRACTThe electrical characteristics of In, Sn, Au and Pt contacts on n-type Hg0.4Cd0.6Te formed in the presence and absence of prior In2+ implantation have been examined. Measurements of specific contact resistance made using a Transmission Line Model have shown that the unimtlanted In/Hg0.4Cd0.6 and Sn/Hg0.4Cd0.6 junctions gave values of pc = 3.0x10−3 to 4.0x10−3 ohm.cm2. Auger sputter profiles of the asdeposited In/Hg0.4Cd0.6 and Sn/Hg0.4Cd0.6 interfaces have shown a significant in-diffusion of the metal overlayer. The influence of shallow In2+ implantation prior to metallization was an increase in pc which occurred above a dose of 1013 ions/cm2. In contrast, Pt and Au formed Schottky barrier diodes on n-type Hg0.4Cd0.6 with øb=0.69eV for Pt and øb=0.79eV for Au. With prior In2+ implantation, both Pt and Au contacts exhibited an ohmic behaviour with pc= 2x10−1 ohm.cm2. These results have significance in the fabrication of devices for 1.0 -2.5μm optical communications.

Structural and Electrical Properties of Tin and Carbon Co-Implanted Silicon

AbstractThe alloy system Six(SnyC1-y)1-x was investigated. In this work, samples were prepared by co-implantation of tin and carbon ions into silicon wafers with dosage range 1015 − 1016cm−2, followed by rapid thermal annealing. Rutherford backscattering channeling, Auger sputter profiling, and secondary ion mass spectrometry were employed to study the crystallinity, chemical composition and depth profiles. A near perfect crystallinity for 0.5% at. of tin and carbon was achieved. To study the electrical properties in the implanted materials, diode I-V measurements were performed. The data show near ideal p-n junctions in the co-implanted region. This work demonstrates promising features of group IV semiconductor synthesis by ion implantation.

Highly selective SiO 2/Si reactive ion beam etching withlow energy fluorocarbon ions

Highly selective SiO 2/Si reactive ion beam etching was achieved from a newspecific ion gun, the electrostatic reflex ion source operated with either CHF 3 or CF 4 An extensive fragmentation of the neutrals may be obtained. The beam energy dependence of SiO 2 and silicon etch rates as studied using profilometry and on-line variations of SiF 4 partial pressure. Surface modifications of single-crystal silicon were characterized by Auger sputter profiling and metal-Si contact electrical evaluation. It is shown that both etch rates decrease with decreasing energy, whereas the SiO 2:Si selectivity increases. The latter becomes infinite as the silicon etch rate vanishes at a particular energy W(0), about 150eV. A highly selective etching may thus be achieved with low energy fluorocarbon ions. At 200 eV, the selectivity and SiO 2anisotropic etch rate are respectively 55 and 105 nm min −1 mA −1 cm −2 with CHF 3, and 30 and 80nmmin −1 mA −1 cm −2 with CF 4. Similarly, the conditions that minimize both the residue contamination and the single-crystal damage are fulfilled at W (0). Therefore a convenient trade-off between etch rates, selectivity and damage may be achieved by a proper choice of the ion energy slightly above W (0).

AES depth profiling of a new type of multilayer structure composed of Cr/Ni layers of various thicknesses

A new multilayer structure for the systematic study of sputter depth profiling and interdiffusion was designed and sputter-deposited on a smooth silicon substrate. It is composed of alternating chromium and nickel thin films with a thickness increasing from 15 nm to 200 nm in the direction from the multilayer surface to the silicon substrate and with a total thickness of 1.1 microm. The advantages of this structure are: (1) determination of the depth resolution in sputter profiling in the wide range 15 nm-1.1 microm, (2) prompt and precise ion gun calibration at low and high sputtering rates, (3) possibilities of extracting interdiffusion parameters by different methods. AES sputter depth profiles obtained for as-deposited Cr/Ni multilayers and after annealing at 400°C, 500°C and 800°C are presented and discussed. Numerical evaluations of interface width data allow an estimation of diffusion parameters.

Ion beam characterization and treatment

Almost every process that occurs when an ion beam hits a solid has been pressed into service as a technique in materials science. Some ions bounce off: backscattering is a routine technique for near-surface analysis in many laboratories. Atoms of the sample are knocked out: sputter profiling is used as an adjunct to many surface science measurements, and analysis of the ejected atoms yields information about the composition of the sample. Ions slow down in a solid, depositing energy in the sample and causing radiation damage. This makes ion beams useful in the development of radiation-resistant materials and has to be understood in order to apply other ion beam techniques. Finally, the ion stops and becomes incorporated into the sample, which is known as “implantation.” As well as being a vital industrial technique in the manufacture of semiconductor devices, implantation can be used in materials science wherever it is useful to change the composition of a near-surface layer. The implanted species doesn’t have to be soluble in the sample.

Origin of defects in (111) HgTe grown by molecular beam epitaxy

Films of HgTe grown in the (100) orientation by molecular beam epitaxy typically possess better electrical properties than do (111) films. Films with the latter orientation, when grown under conditions which lead to well-streaked RHEED patterns, still contain a columnar microstructure with a high density of twins and dislocations. Data taken by transmission electron microscopy and Auger sputter profiling suggest that these films suffer from island nucleation caused by adsorption of Hg onto the CdTe buffer layers. Comparisons are made with (111) samples grown with excess Hg flux and with (100) films, and prospects for improved (111) nucleation are discussed.

The effect of ceria coatings on the high-temperature oxidation of iron-chromium alloys

The effect of sputtered ceria coatings (0.2–30 nm thick) on the oxidation of Fe25Cr alloys at 1100 °C has been studied using the 18O/secondary ion mass spectrometry technique. Samples were oxidized either in 16O 2 only or sequentially first in 16O 2 and then in 18O 2 for periods up to 20 h. Oxidation rates were reduced with increasing ceria thickness up to 4 nm; there was no significant further reduction in rate for thicknesses of 4 nm or more. The secondary ion mass spectrometry sputter profiles indicated that scales were mainly Cr 2O 3; iron was present in the outer part of the oxide layer at levels of 1% or less. With increasing ceria coating thickness the maximum in the cerium sputter profile moved from the vicinity of the alloy-oxide interface towards the gas-oxide interface. When Fe25Cr was sequentially oxidized, 18O was found to have diffused towards the alloy-oxide interface. The secondary ion mass spectrometry data suggest that when ceria is located within the scale there has been a change in mechanism from predominantly cation to predominantly anion diffusion.

SIMS/AES characterization of the initial oxidation of an iron—chromium—molybdenum alloy and its base metals at room temperature

AbstractLow‐temperature oxidation of a FeCrMo alloy and their base metals was investigated by static and dynamic positive and negative SIMS combined with AES Ar+ sputter profiling at 5 keV primary beam energy. In situ oxidation was carried out for 0.3–10000 Langmuir (L) oxygen exposure after sputter cleanning. Static SIMS revealed oxidation of chromium at the outer surface at very low exposures (&lt;0.5 L). With increasing exposures, mainly Fe2O3 was formed at the surface. Dynamic SIMS and AES concentration profiles revealed a multilayer structure with an outer Fe2O3 layer followed by a layer rich in chromium oxide. Oxidized molybdenum was found throughout the film and at the oxide/metal interface.

The temperature dependence of grain boundary segregation in NiIn bicrystals studied with auger electron spectroscopy

Auger electron spectroscopy has been used to study indium grain boundary segregation in symmetric «110å tilt boundaries and randomly oriented boundaries in bicrystals as well as polycrystals of NiIn containing 0.l–1.4 at. % In. Quench-induced non-equilibrium segregation of In, caused by fluxes of In vacancy complexes to the interfaces, influenced the results for quenching temperatures above 1000–1100 K. A careful analysis of sputtering profiles supplied additional informations for the discussion with respect to current models of QINS. From the equilibrium segregation behaviour for the symmetric boundaries both the segregation enthalpies ( ΔH se = 38 ± 3 kJ/mol and ΔH seg = 39 ± 3 kJ/mol) and the mean segregation entropy [ ΔS seg = (0 ± 0.5) R] have been determined for the first time. The segregation free energy for 970 K is ΔG seg = 50 ± 5 kJ/mol for the polycrystals. The symmetric atomic arrangement along the tilt boundaries is assumed to be responsible for the low segregation entropy.

Aluminum deposition on polyimides: The effect of i n s i t u ion bombardment

The chemistry of the Al polyimide interface is examined by x-ray photoelectron spectroscopy sputter profiling. Al deposited on polyimide films without an in situ Ar backsputter shows a clearly defined 50-Å Al2O3 layer just prior to the polyimide. This layer is identified by the O/Al atom ratio at 1.5, and the binding energy of the Al 2p transition. There is a clear separation of the Al/Al2O3/polyimide layers in the sputter profiles. Deposition of Al on polyimide surfaces after Argon backsputtering produces a diffuse Al/polyimide interface with no Al2O3 present. There is evidence in the Al 2p spectra for Al–C or Al–O–C type bonds, while the C 1s spectrum clearly has a metal carbide component. Increased adhesion of Al to polyimide surfaces with Ar backsputtering may be due to the differences in chemistry observed in these two instances.