Preferential Sputtering Effects Research Articles

Some Aspects of Quantitative Analysis of Ternary Alloys of Group III-Nitrides by Auger Electron Spectroscopy

In recent years, much attention has been paid to semiconductor materials based on group III nitrides and their alloys used as short wavelength light emitting materials. The properties of these materials are tightly related to their chemical composition. One of these materials is also the alloy of Al and Ga nitrides, AlxGa1−xN. To determine the elemental composition and its depth distribution, Auger electron spectroscopy (AES) is one of the most widely used techniques. Even though depth profiling using AES is already a standard process, there are still fields in which a further development is needed. One of them is accurate quantitative evaluation of Auger spectra in the case of nanometer thin film structures based on chemical compounds such as GaN, other group III/nitrides and their alloys. Whereas in the analysis of homogeneous alloys the accuracy of quantitative AES is relatively good, problems are encountered in analysis of compounds because in general the shape of Auger peaks is changed in comparison with the pure element. Hence, the quantities needed to evaluate elemental concentrations (Auger current and elemental sensitivity factors) cannot be determined from the amplitude of the Auger peaks but from the area below the peaks in N(E) spectra [1,2]. Other phenomena affecting the accuracy of quantitative analysis are the changes in morphology, structure and initial composition in the surface region caused by bombarding the surface by high energy ions used for depth profiling. To cope with this adverse fact it is necessary to know the changes induced by ions directly in the particular analysed material. These changes depend on both the parameters of the ion beam (energy, kind of ions, angle of incidence) and the analysed material itself and are the result of preferential sputtering (component sputtering yields) and other effects induced by interaction of the ion beam with the solid [3,4]. From among the various methods for determining the component sputtering yields, the most reliable quantitative results could by obtained using standard samples of comparable composition. If AES analysis involves sputtering and if the same experimental parameters are used in sputtering both the analysed and reference samples, then the experimentally determined elemental sensitivity factors account also for the component sputtering yields. For correct quantitative interpretation of spectra of chemical compounds and of their alloys it is necessary to have the relative sensitivity factors of elements and the sputtering yields measured for the material under analysis [5]. So far only scanty data have been published on these quantities for aluminium, gallium and their alloys [6] and they do not cover the required range of experimental parameters needed for wider utilization in experimental practice. The topic of this work has been experimental determination of the quantities needed for reliable and precise quantitative interpretation of the measured Auger spectra of materials based on gallium and aluminium nitrides and their ternary alloys. Relative elemental sensitivity factors and component sputtering yields have been measured of the components contained in these multicomponent materials.

Effect of Sputtering Parameters on Film Composition, Crystal Structure, and Coercivity of SmCo Based Films Deposited on Si (100) Substrates

The sputtering parameter mediated composition (SPMC) effect of 3.0-μm-thick SmCo-based films is experimentally and theoretically studied. The experimental results give a clear indication that the Sm concentration increases with the decreasing sputtering power or with the increasing Ar gas pressure, which are in agreement with the calculated values when the preferential sputtering effect is disregarded. The SPMC effect provides an opportunity for the same composite target to fabricate films with an Sm concentration varying from 13.8 at.% to 17.3 at.%, which is reasonable for the magnetic phase transformation (Sm2Co17 → SmCo7 → SmCo5) and the enhanced coercivity.

Correlation between sputtering parameters and composition of SmCo-based films for microelectromechanical system applications

About 3.0 μm thick SmCo-based films with additives of Fe, Cu, and Zr were deposited on Si substrates. Based on a developed semiempirical theoretical model, the dependence of the film composition on the sputtering parameters was discussed. The experimental results show that the Sm concentration increases with decreasing sputtering power or increasing Ar gas pressure, which are in good agreement with the calculated results when the preferential sputtering effect is disregarded. The effect of the sputtering parameters on the film composition provides an opportunity for the same composite target to fabricate films with Sm concentration varying from 13.8 to 17.3 at. %, which is reasonable for the permanent magnetic phase transformation (Sm2Co17→SmCo7→SmCo5). Furthermore, the observed TbCu7-type film shows a better crystal texture with a low remanence ratio for the hysteresis loops measured out plane to in plane of 0.08.

Evaluation of the depth resolutions of Auger electron spectroscopic, X-ray photoelectron spectroscopic and time-of-flight secondary-ion mass spectrometric sputter depth profiling techniques

Concentration-depth profiles of sputter-deposited Si/Al multilayered specimens were determined by model fitting to measured data obtained by depth profiling, using Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary-ion mass spectrometry (TOF-SIMS). The model used for calculation of the concentration-depth profile accounts for the broadening (“smearing”) upon experimental depth profiling owing to the effects of atomic mixing, preferential sputtering, surface roughness and information depth of either the Auger electrons (for AES depth profiling) or the photoelectrons (for XPS depth profiling) or the secondary ions (for SIMS depth profiling). The depth resolution for each technique was derived directly from the values determined for the fitting parameters in the model.

Amorphous Ge quantum dots embedded in SiO2 formed by low energy ion implantation

Under ultrahigh vacuum conditions, extremely small Ge nanodots embedded in SiO2, i.e., Ge–SiO2 quantum dot composites, have been formed by ion implantation of Ge+74 isotope into (0001) Z-cut quartz at a low kinetic energy of 9keV using varying implantation temperatures. Transmission electron microscopy (TEM) images and micro-Raman scattering show that amorphous Ge nanodots are formed at all temperatures. The formation of amorphous Ge nanodots is different from reported crystalline Ge nanodot formation by high energy ion implantation followed by a necessary high temperature annealing process. At room temperature, a confined spatial distribution of the amorphous Ge nanodots can be obtained. Ge inward diffusion was found to be significantly enhanced by a synergetic effect of high implantation temperature and preferential sputtering of surface oxygen, which induced a much wider and deeper Ge nanodot distribution at elevated implantation temperature. The bimodal size distribution that is often observed in high energy implantation was not observed in the present study. Cross-sectional TEM observation and the depth profile of Ge atoms in SiO2 obtained from x-ray photoelectron spectra revealed a critical Ge concentration for observable amorphous nanodot formation. The mechanism of formation of amorphous Ge nanodots and the change in spatial distribution with implantation temperature are discussed.

Study of preferential sputtering and segregation effects on the surface composition of AlPdMn quasi‐crystals

AbstractUsing 2 keV He+ and Ne+ low‐energy ion scattering (LEIS), it was found that the Al/Pd concentration ratio at the surface of a nominally Al69.9Pd20.5Mn9.6 quasi‐crystal decreases to a steady‐state value under bombardment as a result of preferential sputtering. Sputtering of an annealed surface results in a significant increase in Mn concentration on the surface which remained at annealing temperatures below 575 K. Variations of the Mn/Pd and Al/Pd ratios have been measured by LEIS as a function of temperature in the range 295–975 K for clean‐annealed and sputtered surfaces. The results show that Al/Pd ratio does not significantly change from 295 to 575 K for both He+ and Ne+ but increases with sample temperatures up to 875 K. Copyright © 2008 John Wiley & Sons, Ltd.

Collisional and thermal effects on liquid lithium sputtering

The lithium sputtering yield from lithium and tin-lithium surfaces in the liquid state under bombardment by low-energy, singly charged particles as a function of target temperature is measured by using the Ion-surface Interaction Experiment facility. Total erosion exceeds that expected from conventional collisional sputtering after accounting for lithium evaporation for temperatures between 200 and $400\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$. Lithium surfaces treated with high-fluence D atoms are bombarded by ${\mathrm{H}}^{+}$, ${\mathrm{D}}^{+}$, ${\mathrm{He}}^{+}$, and ${\mathrm{Li}}^{+}$ at energies between 200 and $1000\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ and 45\ifmmode^\circ\else\textdegree\fi{} incidence. Erosion measurements account for temperature-dependent evaporation. For example, $700\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ ${\mathrm{He}}^{+}$ particles bombarding the D-treated liquid Li surface at room temperature result in a sputter yield of 0.12 Li/ion and at temperatures $\ensuremath{\sim}2.0{T}_{m}$ (where ${T}_{m}$ is the melting temperature of the sample), a yield near and above unity. The enhancement of lithium sputtering is observed to be a strong function of temperature and moderately on particle energy. Bombardment of a low-vapor-pressure lithium alloy (0.8 Sn-Li), used for comparison, also results in nonlinear rise of lithium erosion as a function of temperature. Measurements on both pure liquid Li and the alloy indicate a weak dependence with surface temperature of the secondary ion-induced secondary ion emission. Treatment of liquid Li surfaces with D, yields reduced sputtering under ${\mathrm{He}}^{+}$ impact by a factor of 5--6 when measured at room temperature due to preferential sputtering effects.

A study of theoretical depth resolution of GaAs/AlAs reference material with low energy inert‐gas ions by Monte Carlo simulation

AbstractA Monte Carlo (MC) simulation program written in C++ has been newly developed to describe the dynamic processes of depth profiling with low energy ions. This MC simulation was applied to the depth profiling of GaAs/AlAs reference material for Ne+, Ar+, and Xe+ ions to elucidate the depth resolution attained by surface analytical techniques. The result clearly predicts that there is a considerable difference between the depth resolutions estimated from the leading and trailing edges of Ne+ and Xe+ ions, whereas the difference is quite small for Ar+ ions.Systematic investigation of the dependence of theoretical depth resolution on primary ion energy has revealed that the preferential sputtering primarily caused by the difference in energy transfer to target atoms through elastic collisions between incident ions and target atoms results in the difference between the leading and trailing edges.The inclusion of other factors, e.g. preferential sputtering effect caused by the metallization of Al atoms on the topmost surface, etc. for further improvement of the MC simulation modeling before accommodating quantitative arguments on the depth resolution is strongly recommended. Copyright © 2006 John Wiley & Sons, Ltd.

In situ X-ray Photoelectron, Ultraviolet Photoelectron, and Auger Electron Spectroscopy Spectra from First-Row Transition-Metal Nitrides: ScN, TiN, VN, and CrN

X-ray photoelectron (XPS), ultraviolet photoelectron (UPS), and Auger electron spectroscopy (AES) spectra are presented from epitaxial, single-crystal transition-metal (TM) nitride (ScN, TiN, VN, and CrN) layers, that were grown in situ in an ultrahigh-vacuum (UHV) magnetron sputter deposition system attached to the analysis chambers. The samples are near-stoichiometric with N/Me ratios determined by Rutherford backscattering spectroscopy (RBS), and contain no bulk or surface impurities detectable by XPS, AES, or RBS. The spectra therefore represent reliable reference data. We also present XPS and AES data from sputter-etched samples in order to quantitatively determine the effect of preferential sputtering of nitrogen in TM nitrides. The sputter etching was performed under conditions typical for sputter cleaning of air-exposed samples until a steady state N/Me ratio is reached.

Auger electron spectroscopy sputter depth profiling technique for binary solids

In this work the Auger electron spectroscopy (AES) sputter depth profiling technique for binary solids has been discussed and exemplified by its application to reveal composition profiles generated in Ni 0.7Mo 0.3 by pre-bombardment with Ar +-ions of 1 through 5 keV. Two element-specific, surface-sensitive Auger transitions are employed to monitor the surface composition. The linear correlation between the Auger intensities provides the relative sensitivity factor for the quantification of AES data. A model proposed by Ho and co-workers is reformulated for the conversion of the measured temporal variation of surface concentration C s( t) into the original composition profile C( x). Radiation-enhanced diffusion caused by high-energy ion bombardment turns sputter depth profiling into an underdetermined inverse problem. On the other hand, experiments show that the strong preferential sputtering effect accompanying low-energy sputter may veil the detail of the composition profiles. An intermediate energy between about 100 and 300 eV is found to be optimum for the purpose of sputter depth profiling.

X-ray photoelectron spectroscopy studies of modified surfaces of α-Al2O3, SiO2, and Si3N4 by low energy reactive ion beam irradiation

Reactions of N2+ ion beams with oxide surfaces of α-Al2O3(0001) single crystal and chemical vapor deposition (CVD) SiO2, and reactions of O2+ ion beams with a nitride surface of Si-rich CVD Si3N4 were investigated as a function of ion beam energy (200–1000 V) and dose (1×1015–1×1017/cm2). The thickness modified by the irradiation of a reactive low kinetic energy ion beam was measured using high resolution cross-sectional images of transmission electron microscopy (HR-XTEM), and the formation of new bonding induced by chemical reaction was analyzed by x-ray photoelectron spectroscopy (XPS). New bonding of Al–O–N on α-Al2O3(0001) started to be observed at 600 V N2+ ion energy and a dose of 1×1016/cm2, and Al–N bonding could be found at an ion beam energy of 1 keV. The thickness of the aluminum oxynitride layer after 800 V N2+ bombardment has been determined to be 10–50 Å by HR-XTEM analysis. In the case of CVD SiO2 surface modification, new bonding related to nitrogen was not clearly resolved in the XPS spectra, irrespective of the change of ion beam energy from 200 to 1000 V and ion dose from 1×1015 to 1×1017/cm2. However, widening of the full width at half maximum of Si 2p core-level XPS spectra for the modified SiO2 surface and the peak position of N 1s around 399 eV were evidence of the existence of nitrogen-related bonding like Si–O–N in the modified CVD SiO2 surfaces. Moreover, it was very interesting that the Si 2p peak of elemental Si appeared in the sample irradiated at a dose of 1×1017/cm2. Its occurrence was considered to be due mainly to the preferential sputtering effect, and was found to be largely dependent on the ion beam energy as well as on the ion dose. In the surface modification of low-pressure CVD Si3N4 by direct ken O2+ ion irradiation, Si–O–N bonding could be successfully created at an ion beam energy of 200 V and it evolved significantly at ion beam energies higher than 500 V. From the above results, low energy reactive ion beam irradiation can successfully create new bonding structures on oxide and nitride surfaces due to a surface chemical reaction like nitridation or oxidation, and is expected to be very useful for the formation of new ultrathin functional layers on ceramic surfaces.

Preferential sputtering effects in thin film processing

Predicting the partial sputtering yield (sputtered atoms of one element/incident energetic ion) for different elements during sputtering from multielement targets is not a straightforward task. It is commonly observed that ion bombardment of composite targets exhibits preferential sputtering. Such processes are normally studied by computer simulations since theory gives qualitative results only. This work presents a systematic computational study of the major ballistically induced preferential sputtering effects taking place during ion bombardment of composite materials. It is shown that in a vast number of cases ballistic preferential sputtering is a dominant process particularly at moderate temperatures where thermodynamic effects are suppressed. In particular, sputtering from composite targets, resputtering during ion assisted deposition of composite films, sputtering of thin films onto heavy substrates, ion assisted deposition onto different substrate materials, sputter depth profiling, etc. are technological processes shown to commonly exhibit preferential sputtering. It is further shown that a number of sputtering situations exhibit along with the preferential sputtering the so called sputter yield amplification effect. The latter consists in that the partial sputtering yield of a given species is larger than the sputtering yield of the pure material. As a consequence, the sputtering yield of certain materials can be enhanced substantially by introducing small concentrations (a few percent) of impurity species in the target. The work provides ample examples of various situations of academic and technological interest where preferential sputtering and sputter yield amplification occur. The major ballistic mechanisms leading to preferential sputtering and sputter yield amplification are also briefly discussed.

Compositional variations of sputter deposited Ti/W barrier layers on substrates with pronounced surface topography

Sputter deposited Ti/W barrier layers have been found to be Ti deficient with respect to the target composition, which is attributed to the preferential resputtering of Ti from the deposited films by energetic neutrals or ions from the discharge. On the other hand, the sputtering yield of most materials is known to be strongly dependent on the angle of incidence of the bombarding species. Due to this angular dependence the resputtering rate of Ti/W films will also be a function of the local orientation of the surface. The substrates of the IC circuits onto which the Ti/W barrier layers are deposited normally possess a pronounced surface topography. The purpose of this work is to study the deviation of the expected Ti/W concentrations at sloped surfaces as a function of the energy of the bombarding species as well as the atom to ion flux arrival ratio. It is shown that the Ti concentration in the films as a result of the preferential sputtering does exhibit substantial concentration variations across sloped surfaces at constant discharge parameters. The experiments are done in an Ar rf discharge, using a Ti/W target with 50 at.% Ti and 50 at.% W. The films have been analysed by Rutherford Backscattering Spectrometry (RBS) and energy-dispersive X-ray Spectroscopy (EDS). The experimental results are discussed and compared with dynamic simulations using the Transport of Ions in Matter (TRIM) code, which indicate that the loss of Ti in the deposited films is primarily due to ballistic preferential sputtering effects.

Compositional changes in alloys during ion bombardment at elevated temperatures

Near-surface compositional changes in alloys during ion bombardment have been studied theoretically. The employed scheme operates with a stable target, and the effects of preferential sputtering, collisional mixing, radiation-enhanced diffusion, and Gibbsian and radiation-induced segregation are allowed for. High-fluence composition profiles were determined directly from a nonlinear integro-differential equation, after insertion of feasible input, by means of an efficient iteration procedure developed recently. The dependence of the composition profile on input parameters such as target temperature and defect mobility has been examined for Ni–Cu, Ni–Ge and Ni–Pd alloys and compared to experimental results.

Preferential sputtering effects in the deposition of TiAl films by filtered cathodic arc deposition

A filtered cathodic arc deposition method has been used to deposit a range of TiAl films from alloy cathodes. The composition of the deposited films is dependent upon the negative bias applied to the substrate during deposition. The bias range studied was 0 to −200 V and the nominal cathode compositions studied were Ti : Al 75 : 25, Ti : Al 50 : 50, and Ti : Al : V 86 : 10 : 4 at.%. The decrease in Al content with applied substrate bias was modeled using a binary collision code assuming that (a) both Ti and Al species were energetic and (b) only the Ti species were energetic. The results indicate that bombardment induced segregation of Al is significant in these studies.

Ion bombardment induced segregation effects in VD x studied by SIMS and SNMS

Secondary ion mass spectrometry (SIMS) and complementary secondary neutral (particle) mass spectrometry (SNMS) investigations on VD x samples were carried out to study deuterium segregation effects induced by ion bombardment. It could be shown that the sputter process causes a distortion of the near-surface region, which leads, due to the extremely high mobility of deuterium in vanadium, to a deuterium segregation to the surface, i.e. a deuterium enrichment in the sputtered area, and at the same time to a deuterium impoverishment in the bulk (secondary preferential sputtering effect).

Ion-beam induced compositional changes in alloys at elevated temperatures

Near-surface compositional changes in alloys during ion bombardment have been studied theoretically. The scheme employed ensures pressure relaxation of the target, and the effects of preferential sputtering, collisional mixing, radiation-enhanced diffusion, and Gibbsian and radiation-induced segregation are allowed for. High-fluence composition profiles were determined directly from a nonlinear integro-differential equation by means of an efficient iteration procedure developed recently. The dependence of the composition profile on input parameters such as target temperature and defect mobility was examined for NiCu and NiPd alloys.

Segregation and preferential sputtering of Al on (100)

Using 1 - 2 keV and 1.5 - 3 keV beams, the measurement of the first layer composition of (100) at room temperature reveals a slight enrichment of Al on the surface. The enrichment of Al at room temperature also was confirmed by analysis using a 2 keV beam. It was found that Al atoms on the surface were preferentially sputtered for 1 - 2 keV and 1.5 - 3 keV beams. The surface concentration of Al increased with temperature in the range 300 - 620 K due to the combined effect of preferential sputtering, ion beam mixing and Gibbsian segregation. An equilibrium sputtering model has been developed which agrees with Al surface concentrations up to 473 K. However, at higher temperatures the surface Al concentrations are greater than those predicted by the model, indicating that the surface has not reached sputtering equilibrium.

SEGREGATION PHENOMENA ON PtxNi1−x LOW INDEX SINGLE CRYSTAL SURFACES STUDIED BY STM

Surface segregation changes the composition of alloy surfaces. It influences both the geometrical and the chemical structure of the surface. In this paper segregation phenomena are shown for low index single crystal surfaces of different PtNi alloys which can be seen only by scanning tunneling microscopy (STM). STM experiments performed with atomic resolution revealed the existence of subsurface dislocation networks. A closer study of the conditions of their existence allowed us to understand the effects of preferential sputtering and annealing on the segregation behavior (i.e. building up a rather stable altered layer and its disappearance only at elevated temperatures). In addition, local chemical ordering in small domains and shifted row reconstructions with a large and varying periodicity (i.e. phenomena that are hardly seen by other methods like e.g. LEED) have been observed.

Volumetric rendering of 3D SIMS depth profiles

Dynamic secondary ion mass spectrometry (SIMS) can be used to uncover unique information about interfaces and whole structures which are “buried” within a solid. During a depth profile of a solid, a sequence of SIMS images is acquired for each element under study The sequence is correlated into a vertical “stack” which contains digitised three dimensional (3D) elemental distributions. Although such distributions are distorted by surface roughness, preferential sputtering and SIMS matrix effects, there is still considerable structural information contained in the volume and much potential for further information retrieval as the above-mentioned distorting effects are addressedOne of the major tools used to assess distributional information within the 3D volume has been visual rendering software. Using Sunvision software (Sun Microsystems Inc.) and a small workstation, images of the volume can be constructed which display pixels either in a “maximum value” perspective or in perspectives where the density of each phase can be adjusted to maximise structural detail.Several examples of the technique will be shown.