Accurate measurement of sputtered depth for ion sputtering rates and yields: the mesh replica method

For the accurate measurements of crater depths, ion sputtering rates and ion sputtering yields in studies of sputter‐depth profiling using Auger electron spectroscopy (AES) or X‐ray photoelectron spectroscopy (XPS), a proposed mesh replica method has been evaluated. In this method, during ion sputtering, grids of between 50 and 400 mesh (per inch) are placed on the sample to retain unsputtered regions of the original surface to be used as reference. This enables a more accurate measurement of the depth to be made using a stylus profilometer close to the analytical region. The closer‐pitch meshes were thought to offer the prospect of measurements of higher accuracy. Calculations show that sputter deposits from the mesh sides may limit the mesh numbers used to 100 or those of a wider pitch for both stationary and rotated samples. A correlation with published data for stationary samples and new data for rotated samples confirms the calculations. In practice, it is difficult, without a special holder, to have intimate contact between the grid and sample. Such a holder is described. Further calculations concerning the shadowed profiles at the grid bar regions show that the grids may lift off the sample surface by 4–16 µm. This leads to non‐vertical crater walls in each mesh aperture. This effect, however, does not change the above conclusion on the mesh sizes to be used. In this range, the spurious appearance of Auger electrons emitted from the grid material is calculated to be less than 1%. This conclusion applies to the meshes evaluated here, which range in thickness from 13 to 29 µm. Thinner meshes may lead to the applicability of proportionately closer meshed grids in sputter‐profiling applications. Copyright © 2006 John Wiley & Sons, Ltd. The contribution of Martin P. Seah of the National Physical Laboratory is published with the permission of the Controller of HMSO and the Queen's Printer for Scotland.     

Improved sputter depth resolution in Auger composition‐depth profiling of polycrystalline thin film systems using single grain depth profiling

Today in‐depth profiling of microelectronics thin film systems is one of the important applications of Auger electron spectroscopy. It is used to monitor the elemental in‐depth composition after different manufacturing processes to control the quality of these processes. For instance, the layer interdiffusion and reactions with various process gases are analyzed. In addition, interface contaminations have to be controlled, because they strongly influence the properties of the whole thin film system. For polycrystalline layers, the depth resolution of sputter depth profiling is limited by the sputter yield differences attributed to grains having different crystalline orientations relative to the incoming ion beam. If depth profiling can be performed on single grains only, the poor depth resolution caused by these sputter yield differences can be avoided. Unfortunately, the approach works only on a few samples because single grains must be identified and have to have grain sizes that are in the dimensions of the layer thickness. Using methods of in situ sample preparation, however, allows application of single grain depth profiling to an extended range of thin film systems. Copyright © 2006 John Wiley & Sons, Ltd.     

Preferential sputtering observed in Auger electron spectroscopy sputter depth profiling of AlAs/GaAs superlattice using low‐energy ions

Auger electron spectroscopy (AES) sputter depth profiling of an ISO reference material of the GaAs/AlAs superlattice was investigated using low‐energy Ar⁺ ions. Although a high depth resolution of ～1.0 nm was obtained at the GaAs/AlAs interface under 100 eV Ar⁺ ion irradiation, deterioration of the depth resolution was observed at the AlAs/GaAs interface. The Auger peak profile revealed that the enrichment of Al due to preferential sputtering occurred during sputter etching of the AlAs layer only under 100 eV Ar⁺ ion irradiation. In addition, a significant difference in the etching rates between the AlAs and GaAs layers was observed for low‐energy ion irradiation. Deterioration of the depth resolution under 100 eV Ar⁺ ion irradiation is attributed to the preferential sputtering and the difference in the etching rate. The present results suggest that the effects induced by the preferential sputtering and the significant difference in the etching rate should be taken into account to optimize ion etching conditions using the GaAs/AlAs reference material under low‐energy ion irradiation. Copyright © 2005 John Wiley & Sons, Ltd.     

In-depth compositional analysis of ceramic (Bi2O3)0.75(Er2O3)0.25 by AES and XPS

The chemical composition of dense ceramics of erbia-stabilized -Bi₂O₃ was analyzed by Auger electron spectroscopy (AES) depth profiling using Ar⁺ ion sputtering. The relative sensitivity factors (rsf) and sputter rates of bismuth and erbium in this material have been determined by electron probe microanalysis (EPMA) and chemical analysis. These results, supplemented by data from angle resolved X-ray photoelectron spectroscopy (ARXPS), shows a bismuth enrichment at the surface. Evidence has been found for reduction of the bismuth-oxide at the outermost part of the surface layer.Dedicated to Professor Günther Tölg on the occasion of his 60th birthday     

Ion beam sputtering techniques for high-resolution concentration depth profiling with glancing-incidence X-ray fluorescence spectrometry

The application of ion beam sputtering in combination with glancing-incidence X-ray fluorescence spectrometry for high-resolution concentration depth profiling is presented. Two new techniques are described: first, in the “bevel-etching technique”, the sample depth profile is uncovered on the sample surface either by sputter etching with a gradient of the ion beam intensity or by varying the sputtering time by moving a shutter in front of the sample; second, in the “deposition technique”, samples are etched uniformly and the sputtered material is deposited on a moving substrate. The bevelled sample and also the material deposited on the substrate are characterized (laterally resolved) by glancing incidence X-ray fluorescence spectrometry. The apparatus and techniques are described in detail. Typical experiments showing the advantages of and problems with the two techniques are discussed. The achievable depth resolutions, 1.5 nm with the bevel-etching technique and 1.4 nm with the deposition technique, are comparable with the best results from other depth profiling methods.     

Preparation of samples for surface analysis

Analysis of single atomic layers at surfaces and interfaces is now possible routinely, using such techniques as Auger electron spectroscopy, X-ray photoelectron spectroscopy and secondary ion mass-spectroscopy. The handling of specimen surfaces before analysis is therefore critical to ensure a valid measurement of composition.     

Oxide and nitride protective layers on stainless steel studied by AES,WDS and XPS

Protective surface layers on AISI 321 stainless steel were prepared by thermal treatments at two different temperatures in air and two controlled atmospheres. Different oxide and/or nitride layers were formed. Surface morphology of the layers was investigated by scanning electron microscopy (SEM). Auger electron spectroscopy (AES) depth profiling of the samples was performed. Since depth profiling suggested layer thicknesses of the order of hundreds of nanometres, an attempt was made to obtain some fast, averaged information about the layer compositions using wavelength dispersive spectroscopy (WDS) at two different beam energies to obtain probing depths best suited to the layer thickness. X‐ray photoelectron spectroscopy (XPS) profiling of one layer was also performed to obtain information about the chemical states of the elements inside the layer. The analysed samples showed considerable differences with respect to their surface morphology, oxide/nitride layer thicknesses, compositions and layer–metal interface thickness. Copyright © 2007 John Wiley & Sons, Ltd.     

Status monitoring of ion sputter relevant parameters of an XPS depth profiling instrument

An X-ray photoelectron spectroscopy (XPS) instrument is utilized for sputter depth profiling of thin films. Relevant instrumental parameters are the ion gun sputter rate, the contamination level of the sputter ion gun, and the purity of the sputter ion gun gas supply as well as the vacuum quality of the instrument at the sample position. A long-term recording of these instrumental parameters ensures the reliability of the measured depth profile data. The ion gun sputter rate was estimated using the standard ISO conform depth profiling of a SiO₂ reference layer of known thickness. Two new procedures are developed to determine the other relevant parameters. Gases that are emitted by the ion sputter gun get implanted into a Si target. An analysis of the implanted gases allows judging on the contamination level of the sputter gun and the purity of the sputter gun gas supply. The vacuum condition of an XPS microprobe at the sample position is monitored by the recontamination of a sputtered Ti surface by adsorbed residual gas particles.     

Determination of an accurate depth distribution of sodium, calcium and aluminium in thin oxide layers using several methods

Summary Experience in obtaining accurate sodium, calcium and aluminium profiles in silicon dioxide using SIMS and Auger depth profiling is reported. With the knowledge of implantation energy and ion dose, it is possible to calculate and to realize well defined implantation profiles in special substrates with high accuracy. The technological demand is to measure this so called accurate profiles in implanted structures without alteration by the measurement. SIMS and Auger profiling have been tested in special applications to study the influence of ion sputtering on the depth distribution in membranes and to obtain accurate profiles. Experimental results are presented for the application of Auger profiling at sample edges and SIMS profiling using negative ions. In the case of Auger profiling a transformation routine was developed for using linescan and sputter profile results in combination.     

Scanning work function microscopy

When a sample is locally excited with a highly focused raster-scanned beam of keV electrons, the variations DeltaPhi of the work function across the surface can be monitored from the shift of the onset energy for secondary electron emission along a fixed energy scale. The performance of that "onset" technique of work function microscopy and its incorporation into scanning Auger microprobes is described. The potentialities of this extremely surface sensitive technique for structural and chemical microanalysis are demonstrated by different experimental examples comprising work function analysis of surface reactions, and sputter depth profiling with in-situ Auger and work function spectroscopy. Scanning work function microscopy for surface microanalysis is shown to supply a lateral resolution down to the 10 nm range with a detection limit below 10(-2) of a monolayer.     

Applications of work function microscopy in the onset mode

Work function spectroscopy (WFS) in a microprobe mode (scanning work function microscopy SWFM), or in conjunction with sputter depth profiling constitutes a useful supplementary method to other surface analytical techniques. The so-called onset technique of WFS utilizes the influence of the electronic work function of the sample on the onset of the energy distribution of the true secondary electrons. This technique can readily be incorporated into existing surface analytical instruments like Scanning Auger microprobes. WFS and Auger electron spectroscopy (AES) have been applied in-situ during sputter depth profiling of sulphur layers segregated on top of Cu(111), and of implantation profiles of Cs⁺ bombarded Si(111) with Ar⁺ ions of 1 keV. Because the onset technique for WFS takes advantage of the high intensity of the true secondary electrons, it is possible to use very low primary electron currents I_p. Employing a commercial instrument (PHI SAM 660) with a minimum spot size of 20 nm a lateral resolution of about 25 nm is achieved in the SWFM mode.     

X-ray microanalysis of thin film layered specimens containing light elements

Analysis of thin film layers on bulk substrates is carried out using a technique based on the (z) model of the depth distribution of X-ray emission. Both the composition and thickness of individual layers can be determined provided that the same element is not present in more than a single layer.The application of this method to the analysis of thin titanium-boron nitride bilayers on silicon or molybdenum substrates is discussed. X-ray intensities were measured by energy dispersive spectroscopy with a windowless or ultra thin window detector. The thickness of a 10 nm titanium layer could be estimated to within about ±1 nm, which is comparable with the depth resolution attainable by Auger sputter profiling.     

Are measured values of the Auger parameter always independent of charging effects?

In x‐ray photoelectron spectroscopy (XPS) the Auger parameter is often used to study the electronic properties of elements, particularly in insulator materials, because this parameter is assumed to be independent of charging effects. In this paper we report on subtle differences in sample structure and experimental conditions for which the sample potential may not remain constant during the measurements for some spectrometers or experimental arrangements; for such conditions the Auger parameter is not independent of charging. We compare a series of measurements with insulating plate substrates of Al₂O₃ on which different amounts of SnO₂ and Au were deposited. X‐ray photoelectron spectra were collected for different conditions of the sample that was placed either grounded or left floating on a metallic sample holder during measurement. It is found that the Auger parameter is independent of the experimental conditions for Au but substantial differences were found for deposited SnO₂. Surprisingly, measurement artifacts due to charging appeared in the Auger parameter for Sn when the sample holder was grounded but not when it was left floating. In the grounded samples differences up to 0.6 eV in the Auger parameter for Sn were found with respect to the actual value of this parameter measured with substrates where charging effects were not significant. Because no differences in peak broadening have been observed under different measurement conditions, it has been assumed that the shift was not caused by a conventional differential charging phenomenon. Considering the different response of the substrate and the deposited layer on stabilizing the charge when the sample is grounded, we have worked out a possible explanation to account for the observed artifacts. Instrumental specifications should be optimized very carefully, especially if (as here) relatively high charging shifts point to a non‐optimum self‐biasing of the surface potential at the insulating samples. Copyright © 2003 John Wiley & Sons, Ltd.     

Application of the analytical methods REM/EDX, AES and SNMS to a chlorine induced aluminium corrosion

Summary Scanning electron microscopy (SEM) with energy dispersive X-ray detection (EDX), Auger electron spectroscopy (AES) and sputtered neutral mass spectrometry (SNMS) have been used to characterize a chlorine induced corrosion of an aluminium metallisation. SEM/EDX detects the characteristic X-rays, that are emitted from the first few micrometers beneath the specimens surface after inner shell ionisation by the primary electrons. AES detects the alternatively ejected Auger electrons, that are generated within the topmost atomic layers of the sample. To obtain elemental concentration depth profils, the surface layers are removed by ion sputtering. Whereas AES detects the composition of the remaining surface, SNMS measures sputtered fluxes and does not suffer from preferential sputtering. As demonstrated by the example of a chlorine induced aluminium corrosion, these analytical methods are complementary with respect to quantification, chemical information and information depth. Only by simultaneous use measuring artifacts are detectable and able to be excluded from interpretation.     

Interface of nanoparticle-coated electropolished stents

Nanostructures entail a high potential for improving implant surfaces, for instance, in stent applications. The electrophoretic deposition of laser-generated colloidal nanoparticles is an appropriate tool for creating large-area nanostructures on surfaces. Until now, the bonding and characteristics of the interface between deposited nanoparticles and the substrate surface has not been known. It is investigated using X-ray photoelectron spectroscopy, Auger electron spectroscopy, and transmission electron microscopy to characterize an electropolished NiTi stent surface coated by laser-generated Au and Ti nanoparticles. The deposition of elemental Au and Ti nanoparticles is observed on the total 3D surface. Ti-coated samples are composed of Ti oxide and Ti carbide because of nanoparticle fabrication and the coating process carried out in 2-propanol. The interface between nanoparticles and the electropolished surface consists of a smooth, monotone elemental depth profile. The interface depth is higher for the Ti nanoparticle coating than for the Au nanoparticle coating. This smooth depth gradient of Ti across the coating-substrate intersection and the thicker interface layer indicate the hard bonding of Ti-based nanoparticles on the surface. Accordingly, electron microscopy reveals nanoparticles adsorbed on the surface without any sorption-blocking intermediate layer. The physicomechanical stability of the bond may benefit from such smooth depth gradients and direct, ligand-free contact. This would potentially increase the coating stability during stent application.     

Growth of TiC films by Pulsed Laser Evaporation (PLE) and characterization by XPS and AES

Summary Thin films of TiC with a thickness of some 100 nm have been grown on Si(100) substrates by Pulsed Laser Evaporation (PLE). Advantages of PLE in comparison with more conventional growth methods e. g. PVD or CVD are reported. The feasibility of growing stoichiometric thin films of TiC by PLE was investigated. These films produced have been analysed in situ by X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES). XPS results and Auger sputter depth profiles indicate that the films grown between RT and 500°C are stoichiometric TiC. Film/substrate interdiffusion is observed at 600°C substrate temperature and higher.     

Incident angle and energy dependences of low‐energy Ar+ ion sputtering of GaAs/AlAs multilayered system

Dependences of the depth resolution in Auger electron spectroscopy sputter‐depth profiling of a GaAs/AlAs superlattice reference material on the incident angle and energy of primary Ar⁺ ions were investigated. The results revealed that the depth resolution is improved for the lower primary energy as a square root of the primary energy of ions at both the incident angles of 50° and 70° , except for 100 eV at 50° , where the significant deterioration of the depth resolution is induced by the preferential sputtering of As in AlAs, and the difference in the etching rate between GaAs and AlAs. The deterioration of the depth resolution, i.e. the difference in the etching rate and the preferential sputtering, observed for 100 eV at 50° was suppressed by changing the incident angle of ions from 50° to 70° , resulting in the high‐depth resolution of ～1.3 nm. The present results revealed that the glancing incidence of primary ions is effective to not only reducing the atomic mixing but also suppressing the difference in the etching rates between GaAs and AlAs and the preferential sputtering in the GaAs/AlAs multilayered system. The results also suggest that careful attention is required for the optimization of conditions of sputter‐depth profiling using GaAs/AlAs superlattice materials under low‐energy ion irradiation. Copyright © 2009 John Wiley & Sons, Ltd.