Degradation Of Depth Resolution Research Articles

The study of ripple formation and degradation of depth resolution in GaAs and Al xGa 1−xAs structures

The present work investigates the characteristics of ripple formation in Al xGa 1−xAs samples under oxygen irradiation. The wavelength and the amplitude of the ripples have been determined under different conditions and are examined within the scope of two existing theoretical models: the model of Bradley and Harper [1] and the local incorporation model [12]. It is shown that oxygen bombardment of Al xGa 1−xAs (with 0 ≤ × ≤ 0.56) structures with an impact energy of 8 keV at 37° always produces ripples. Both the wavelength as well as the growth rate of the ripples are affected by the stoichiometric composition of the sample analyzed, the temperature, and the oxygen pressure near the sample. The work shows that the local incorporation mechanism contributes to the ripple growth and explains correctly the increase in ripple amplitude when introducing oxygen gas near the sample. Moreover, it also accounts for the material dependence and the strong reduction in ripple growth rate at the highest pressures. Neither of the two models can explain the dependence of the ripple formation as a function of the temperature.

Improvement of Cs-induced surface roughness for high depth resolution profiling of [formula omitted] multilayer films

The Cs ion bombardment during secondary ion mass spectrometry induces surface roughness such as precipitates and ripples on ZnSe. These ripples and precipitates cause the degradation of depth resolution. The size and number of precipitates become larger with the decreasing incidence angle of the Cs ion beam. Large Cs incidence angles and sample cooling are found to suppress the above surface roughness and realize high depth resolution profiling of CdZnSe ZnSe multilayer films.

Towards the ultimate limits of depth resolution in sputter profiling: Beam‐induced chemical changes and the importance of sample quality

AbstractRecent progress in materials characterization by sputter profiling is discussed, with particular emphasis on a critical evaluation of the parameters that determine the depth resolution. The net effect of beam‐induced material transport and sputtering can be studied in a straightforward manner using abrupt doping distributions embedded in the matrix of interest, parallel to its flat surface. It is shown that the quality of the data that can be obtained with modern instruments is sufficient to identify lack of sample quality in terms of either surface flatness or abruptness of the doping distributions. Except for the presence of special artefacts, the depth resolution is always improved by using as low a bombardment energy as possible. The optimum angle of beam incidence depends on the crystalline structure of the sample as well as on the chemical identity of the primary ions. With oxygen primary ions a rather complex situation is encountered. Compared to inert gas ions, an improvement or a degradation in depth resolution may be achieved, depending on the impurity–matrix system under study. The merits of sample rotation are also discussed.

Depth profiling with sample rotation: Capabilities and limitations

AbstractDepth profiling with sample rotation has become a frequently applied method to obtain the in‐depth distribution of composition in thin films with high resolution. Sample rotation strongly diminishes the effect of local variations of the ion beam intensity and of the sputtering yield, which are the main cause of increasing surface roughness during sputtering and therefore of profile broadening. Capabilities and limitations of rotational profiling are considered with respect to its dependence on various parameters, such as inhomogeneity of the ion beam instensity, ion incidence angle and rotation speed. In particular, it is shown that non‐linear components and excentric movement of the sample lead to periodic features in the profile and to non‐vanishing degradation of depth resolution with depth. For smooth sample surfaces, depth resolution improves with increasing ion incidence angle but is much less pronounced, as for profiling with stationary samples. For originally rough surfaces, the deterioration of depth resolution with increasing ion incidence angle is considerably reduced. The key parameter for optimized conditions in rotational profiling is the ratio of sputtering rate and rotation speed. The minimum necessary, useful rotation speed depends on this ratio and on the magnitude of other contributions to the depth resolution that are not affected by sample rotation.

Limiting factors for secondary ion mass spectrometry profiling

Understanding the limitations of depth profiling with ion sputtering is essential for accurate measurements of atomically abrupt interfaces and ultra-shallow doping profiles. The effects of cascade mixing, sputtering statistics, ion-induced roughness, the inhomogeneity of ion beams, and sample rotation on the depth resolution of Si δ-doped, AlAs, and InAs monolayers in GaAs and an AlGaAs(5 nm)/GaAs(5 nm) superlattice were investigated. Atomic force microscopy (AFM) investigation of the ion-induced surface ripple formation on a GaAs substrate sputtered with 3 keV O+2 at angle of incidence θ=40° showed that ripples form rapidly below 200 nm depth. AFM measured root mean square roughness of Si δ-doped GaAs sputtered with 2 keV O+2 was 0.8 and 2.6 nm with and without sample rotation showing that ripples play a dominant role in depth resolution degradation at shallow depth under these conditions of bombardment. Sample rotation yielded the lowest full width at half-maximum, 4.1 nm for a Si δ layer at 120 nm depth corresponding to a depth resolution ΔZ=3.5 nm. Use of AFM enabled determination of the atomic mixing ΔZm and sputtering statistics ΔZss components of depth resolution to be identified directly for the first time. These components were 3.1 and 1.5 nm, respectively.

Comparison of rotational depth profiling with AES and XPS

After successful applications of depth profiling with sample rotation in AES and SIMS we used this technique in XPS, too. Until now, most of the experimental work with rotational depth profiling has been done by AES. It was conclusively shown that the technique enables high depth resolution which is independent of sputtered depth due to reduced ion-beam-induced topography effects. AES and XPS differ in some instrumental details and experimental conditions, however, the most important difference with respect to profiling is that of the analysed area, which is, even in the case of small-spot XPS instruments, much larger than in AES. This results in a decisive distortional influence of crater-edge effects on XPS depth profiles obtained on stationary and/or rotated samples, and is recognized in an increasing degradation of depth resolution with sputtered depth. The importance of alignment of the probe beam, analysed spot, ion beam and rotation axis is discussed and is demonstrated by comparison of the results of rotational depth profiling of a Ni/Cr multilayer structure with AES and XPS.

Auger electron spectroscopy and secondary ion mass spectrometry depth profiling with sample rotation

Recently, there has been a rapid increase in the application of multilayered structured materials, as opposed to bulk materials, in many areas of technological development. Accurate characterization of the structure and composition of advanced multilayers such as superlattices, quantum wells, contacts, and coatings is important for materials and device fabrication technology. Surface analysis techniques including Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy, and secondary ion mass spectrometry (SIMS) in conjunction with ion beam sputtering (sputter depth profiling) are at present the most widely used methods for characterization of modern multilayer thin film materials and devices. Ion-beam-induced surface topography, however, can limit depth resolution, and with SIMS, can also cause changes in the secondary ion yield. These changes are due to the high sensitivity of secondary ion yield to the local angle of incidence on sputter-roughened surfaces. Degradation of depth resolution and changes in secondary ion yields during sputter depth profiling have often limited studies of thin film interdiffusion, segregation, oxidation at interfaces, and impurity effects. Much theoretical and experimental work has been carried out to try to improve depth resolution including the use of low ion beam energy, high angle of incidence, and two ion guns. Recent studies of AES and SIMS with sample rotation have shown that depth resolution can be improved substantially and that constant secondary ion yields in SIMS can be achieved. We will first provide an overview of the studies made by various groups to improve depth resolution of metal multilayers using AES with rotation. Next we will review recent investigations of SIMS using sample rotation including studies of the effects of sample rotation on O 2 + ion-beam-induced topography, secondary ion yield, and the depth resolution of electronic, metallurgical and dielectric materials. The results presented demonstrate that SIMS with sample rotation provides constant secondary ion yield, and depth-independent depth resolution because sample rotation prevents ion-beam-induced roughness and reduces the effect of the inhomogeneity of low energy ion beams.

Secondary ion mass spectrometry analysis of aluminum films: Relative sensitivity factors and analytical considerations

Relative sensitivity factors (RSFs) have been measured for 20 elements in sputter deposited aluminum films using oxygen and cesium primary beams. E+, EAl+, EO+, and EAl O+ (where E is the element) were monitored for oxygen bombardment, and E−, EAl−, ECs−, and EAl Cs− for cesium bombardment. The results follow the RSF patterns observed for other matrices. Depth resolution degradation due to nonuniform sputtering of aluminum was examined. Oxygen bombardment produced smoother craters than cesium bombardment; sputtering rate and aluminum deposition temperature were also found to affect crater topography. The effect of surface oxygen on depth profiles has also been studied.

Feasibility study of radioimmunoguided surgery of colorectal carcinomas using indium-111 CEA-specific monoclonal antibody

The study was undertaken to define the potential use of indium 111 carcinoembryonic antigen-specific antibody labelled [CEA F(ab')2] for the radioimmuno-detection of colorectal carcinoma using an intraoperative hand-held gamma probe. The use of a linear radioactive source allowed optimization of physical characteristics. The best results regarding sensitivity and resolution were obtained using a 5-mm thick tungsten alloy collimator. A simulation study with a liver phantom (22 MBq or 0.6 mCi) was performed to determine the effect of side scatter as opposed to direct background and showed that it is possible to detect small radioactive targets (3.7 KBq or 0.1 mu Ci) 4 cm from the phantom. A clinical study performed with ten patients showed that tumours with good uptake of CEA-specific antibody could be detected with sufficient contrast in two patients when the probe was used. Results of a biodistribution study performed after tumour fragment or normal tissue countings in a well counter showed high tumour uptake (above 8 x 10(-3) injected dose/g) and tumour-to-normal tissue ratios (between 2.5 and 20) in five patients. Results with the probe showed markedly lower ratios. There was no correlation between absolute tumour uptake and the count rates of tumour measured intraoperatively. This can be attributed to the degradation of depth resolution resulting from the high energy photopeak of gamma-emitting 111In.

Backside secondary ion mass spectrometry investigation of ohmic and Schottky contacts on GaAs

A backside secondary ion mass spectrometry (SIMS) technique is employed to examine seven distinctive metallic contacts on GaAs at various stages of formation. The contacts are formed on multilayered GaAs/AlGaAs structures grown by molecular beam epitaxy (MBE), each containing an AlGaAs etch-stop layer and AlGaAs marker layers for precise alignment of the SIMS depth profiles. After removal of the substrate, the contact structures are profiled from the backside to avoid depth resolution degradation, which results when sputtering through a nonuniform multilayered metallic contact. The seven contacts examined are TaSix, Au/Ge/Ni, Au/Ge/Pd, Ge/Pd, Si/Pd, In/Pd, and Si/Ni/Mg. The TaSix Schottky contact exhibits extensive interdiffusion. The Au/Ge/Ni contact reveals extensive GaAs consumption. Much less consumption occurs in the Au/Ge/Pd contact. The absence of Au in the Ge/Pd contact leads to an extremely abrupt contact interface, with the Ge concentration dropping by four orders of magnitude within ∼100 Å of the interface. The Si/Pd contact is similar, with evidence of a complex compound formation within 200 Å of the interface. An ∼200 Å InGaAs region is in evidence at the In/Pd contact interface. The Si/Ni/Mg p-type ohmic contact profiles illustrate the progressive development of NiSi with the Mg layer serving as a diffusion barrier. The results are supplemented by transmission electron microscopy (TEM), and compared to various models of ohmic contact formation.

Depth resolution of multilayer Cr/Ni thin film structures deposited on substrates with different roughness

The influence of surface topography on depth resolution of AES composition depth profiles was studied on multilayer Cr/Ni thin film structures deposited on silicon substrates. Surfaces with different roughness amplitude and angular distribution of microplanes were obtained by grinding and polishing the silicon substrates with abrasives of different particle size. The surface topography of the substrates was examined by Talysurf and Talystep methods and with electron microscopy. Substrates with the roughness amplitude, given by centre-line average R a from 0.012 to 2.300 μm, did not always have the same angular distribution of microplanes. A multilayer Cr/Ni thin film structure was sputter deposited on all these silicon substrates in the same run. We have found a correlation between the degradation of depth resolution and the increase of roughness amplitude only if the angular range in which microplanes are distributed is increased simultaneously. The depth resolution increases monotonously with sputtering depth in the range from 25 to about 400 nm on both smooth and rough surfaces.

Recent progress in high resolution depth profiling and interface analysis of thin films

Recent developments in high resolution depth profile analysis of thin film systems and interfaces are discussed in particular with respect to the minimization of sputter induced degradation of depth resolution. The development of interfering surface topography during the sputter removal can be significantly reduced by sample rotation. With this technique absolute depth resolution of a few nm being independent from the sputtered depth has been achieved for AES sputter depth profiling of Ni-Cr-sandwich systems. Extremely high depth resolution is established by low energy SNMS for ion bombarding energies in the 10 2 eV regime. Then very narrow structures of multilayer systems like W-Si with a periodicity of only 3–4 nm can be resolved with constant depth resolution across a total layer thickness of 250 nm.

Direct measurement of small diffusion coefficients with secondary ion mass spectroscopy

Sputter sectioning in combination with secondary ion mass spectroscopy enables the determination of very small diffusion coefficients which are not attainable with classical sectioning techniques. The exceedingly good depth resolution of the sputter sectioning and the high sensitivity of the mass spectroscopy allow to resolve penetration profiles of solutes in the 10-nm range at the ppm level. Two perturbing effects, inherent to the method and limiting its sensitivity are discussed: degradation of depth resolution by surface roughening and atomic mixing, and near surface distortion of profiles by transient erosion effects. Degradation of depth resolution was minimized by use of single crystalline specimens and low energy sputtering with reactive ions. To overcome the near surface distortions a special sample preparation technique has been developed, resulting in single crystalline specimens with one or more inserted layers of the solutes to be diffused. The application of the method is demonstrated by examples of thermal- and irradiation-induced diffusion of nickel in copper, and the main errors are discussed.

Interpretation of AES depth profiles of porous Al anodic oxides

This paper discusses the effects of microtopographical structures on the Auger electron spectroscopic (AES) depth profiles of porous Al anodic oxides. In addition to a shadowing effect, the microtopographical structures in the oxide are responsible for a number of complications in the analysis, e.g., (1) reduction of Auger signal amplitude, (2) change of depth scale, (3) degradation of depth resolution (broadening effect), and (4) complicated change of surface topography during ion sputtering. The morphology of the oxide surface before and after ion sputtering was studied by means of a high resolution scanning transmission electron microscope (STEM), and the result was correlated with the AES profile data to make a rational interpretation of the profiles of Al, O, Ar, Cu and P in the anodic films of 2024 Al alloy formed in H 3PO 4 solutions. The effect of electron beam is also discussed.

The development of surface shape during sputter‐depth profiling in Auger electron spectroscopy

AbstractIon bombardment is an important process in surface analysis, not only for surface cleaning, but also for obtaining composition‐depth profiles through thin films and interfaces. Although the use of depth profiling is well established for initially flat surfaces and the problems associated with ion‐induced artefacts such as surface topography, preferential sputtering and redeposition are well recognized, the additional problems which occur when non‐flat surfaces are bombarded have been neglected. For example, depth profile analysis of rods, wires, fibres, etc., using Auger electron spectroscopy is now of interest for many commercial purposes. However, the erosion of such geometries is non‐uniform since different angles of incidence are presented to the incoming ion beam. In this paper, computer simulations are presented which illustrate the erosion of such geometries and a quantitative analysis is given for the resulting degradation in depth resolution as a function of sample and electron beam diameter.

Depth resolution degradation of sputter-profiled InP/InxGa1−xAsyP1−y interfaces caused by cone formation

Both InP and InxGa1−xAsyP1−y surfaces with trace metal contaminations are unstable with respect to cone formation when sputter etched by Ar+. Such gross surface texture severely degrades the depth resolution of sputter profiling techniques. Cone formation can be prevented by avoiding contamination of the material being profiled with metals sputtered from various surfaces in the analysis system. An uncorrected width of 140 Å was determined using techniques which optimize sputter depth profiling for an In0.74Ga0.26As0.60P0.40-InP heterojunction grown by liquid phase epitaxy.

The determination of composition depth profiles using spherical erosion and scanning Auger electron spectroscopy

Accurate composition depth profiles which are essential to many investigations of surface coating and surface treatment processes are at present obtained by sequential ion bombardment and surface analysis. Successive layer removal by ion bombardment is satisfactory when the depth of interest is less than 1 μm; above this it is time consuming and induces surface damage which causes problems of interpretation. This paper describes an accurate method of depth profiling which does not rely on ion bombardment. A very fine taper section of the specimen surface regions is produced by the erosion of a hemispherical crater with a spherical tool. Scanning Auger electron spectroscopy is then used to conduct analysis across this taper face. Initially, line scans to detect the elements of interest are performed across the diameter of the crater; these are followed by individual point analyses at the major places of interest for purposes of quantification. A linear depth profile is then derived using the crater dimensions and simple geometry. This method of depth profiling is rapid because it eliminates ion bombardment; it is also non-destructive and applicable to all types of materials. Further, it completely avoids the uncertainties of interpretation which arise from depth resolution degradation caused by the differential sputtering of elements during long term ion bombardment. A composition depth profile through a TiN coating on steel is used to illustrate the method.

Auger depth profiling of thick insulating films by angle lapping

It is demonstrated that the chemical composition of thick and multilayer insulating films can be easily depth profiled by Auger electron spectroscopy by laterally scanning an angle-lapped region that reveals the entire depth to be profiled. Conventional depth profiling of such structures is often difficult because of unstable sample charge-up conditions that occur when Auger measurements are made during ion milling. The angle-lap technique is nondestructive, in that it allows one to reexamine the entire profile whenever desired. It is thus particularly suitable for investigation of samples with initially unknown or trace impurities. Also, this technique is free of depth-resolution degradation and differential sputter yield effects usually associated with long-term ion milling of thick films. The absolute depth resolution of the angle-lap–Auger profiling method is independent of the film thickness and is equal to the electron beam diameter plus sample manipulator resolution divided by the angle-lap lateral magnification. An ultimate depth resolution of ∠35 Å would be possible using a 0.5-μm-diam beam, a 0.2° lap, and a 0.5-μm manipulator resolution. This is approaching the depth resolution associated with the finite escape depth of Auger electrons. Using a 10-μm electron beam and a 0.5° angle lap in the present study, a depth resolution of about 1000 Å was obtained, as predicted. The oxygen concentration in oxygen-rich polysilicon films measured by this technique agrees with that measured by an Electron Probe Microanalyzer. The Auger data also suggest that all oxygen atoms in the films are bonded to silicon in the form of SiO2.

Abstract: Backscattering spectrometry

Backscattering spectrometry is the microanalysis of the surface and near−surface regions of materials utilizing energetic (typically 1−2 MeV) He and H ions as the analytical probe. 1−4 The ions in the collimated beam provided by a particle accelerator penetrate into the target and are scattered at various depths. Analysis of the energy spectrum of the particles backscattered from the target provides information in three areas: identification of atomic mass of the target constituents, the target composition, and depth profiles. Mass identification is provided by the kinematics of the energy transfer between the projectiles and target atom in the collision process. For MeV He ions, the mass resolution is ±1 atomic mass unit (amu) up to about mass 40, and deteriorates to about ±10 amu for the heaviest elements. Quantitative analysis of the number of target atoms per cm2 is provided by the scattering cross section which connects the measured yield of backscattered particles to target composition. In compositional analysis, backscattering only provides atomic ratios, for information on the chemical nature of the material, x−ray diffraction or other tools with chemical sensitivity must be used. Backscattering techniques are most useful to examine heavy elements in a light−element matrix as the sensitivity is rather poor for light elements unless proton resonance techniques are used. Depth information is provided by the energy loss of the projectiles on their inward and outward paths through the sample. The energy loss of the projectiles per unit path length depends on the energy of the particle as well as on the projectile and target. Compilations (5) and reviews of stopping cross sections are available. For MeV 4He ions, the depth resolution is about 200−300 Å for depths up to 0.5 μ. At greater depths, energy straggling causes a degradation in depth resolution. Surveys of the applications of backscattering spectrometry are provided in Ref. 1−4. The technique has been used to analyze ion−implanted samples, thin−deposited films, metal−metal thin film interdiffusion, metal film−bulk silicon reactions, solubility of impurities in bulk samples, oxidation processes and other phenomena occurring in the near−surface region of materials. A typical beam spot is 1 mm2 in area, but for convenient handling, samples should preferably have a size of 1/2−1 cm2. The uniformity of the sample over the area of the beam spot is critical. Since backscattering analysis has no laterial resolution, the samples have to be checked with scanning electron microscopes or with optical microscpoes to verify this assumption. The primary strength of backscattering analysis is the ability to provide quantitative information on atomic composition ratios and on absolute thicknesses of films in the range of several thousand Angstroms without resorting to standards. Routine accuracies for this information are of the order of ±5%.