Depth Resolution In Sputter Profiling Research Articles

Depth resolution in sputter profiling revisited

Based on a brief review of the well-established framework of definitions, measurement and evaluation principles of the depth resolution in sputter profiling for interfaces, delta layers, single layers and multilayers, an extension to additional definitions is presented, which include the full-width-at-half-maximum of layer profiles and non-Gaussian depth resolution functions as defined by the Mixing-Roughness-Information depth (MRI) model. Improved evaluation methods for adequate analysis of sputter depth profiles as well as improved definitions of depth resolution are introduced in order to meet new developments in ToF-SIMS and GDOES, and in cluster ion sputtering of so-called delta layers in organic matrices. In conclusion, the full-width-at-half-maximum definition and measurement of depth resolution, Δz(FWHM), is found to be more appropriate than the traditional Δz(16–84%) in order to characterize depth profiles of single layers and multilayers, because it is also valid for non-Gaussian depth resolution functions. Copyright © 2016 John Wiley & Sons, Ltd.

Simulation of sputter-induced roughness for depth profiling of thin film structures.

Sputtering induced surface roughening is the dominant factor that degrades depth resolution in sputter profiling of polycrystalline film samples. Due to the dependence of the sputtering yield on the crystallographic orientation, ion beam incidence angle and composition, the local sputtering rate differs from grain to grain. A simple computer program based on a model of Marton and Fine can simulate such a roughness development within one layer, an improved version can even be applied for interfaces. A further extension of the program using a model of Hauffe includes effects like shadowing and enhanced peak erosion leading to surface smoothing.

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.

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.

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.

Influence of the impact angle on the depth resolution and the sensitivity in SIMS depth profiling using a cesium ion beam

We have measured the falloff of the SIMS signal of surface impurities (C, O, and F) on Si and GaAs to determine the angular dependence of the depth resolution in sputter profiling under 8-keV Cs+ bombardment. The decay lengths characterizing the exponential tails decreased with increasing impact angle. The relative gain in depth resolution is largest in cases where the resolution is poor at normal beam incidence. The sputtering yield was found to increase approximately as 1/cos2θ, whereas the secondary ion yields of Si− and As− decreased by several orders of magnitude as θ was increased from 0 to 60°. This effect is attributed to a strong angular dependence of the stationary cesium coverage of the ion-bombarded samples. Some information concerning the efficiency for ion-induced desorption is also derived from the experiments.

The depth resolution of sputter profiling

Depth profiles of impurity markers and overlayers obtained by sputter-sectioning techniques such as secondary ion mass spectrometry (SIMS) and Auger electron spectroscopy (AES) are modeled by a diffusion theory incorporating atomic mixing and preferential sputtering effects. The model explains very well the broadening and shifts frequently observed in experimental depth profiles.

The depth dependence of the depth resolution in sputter profiling

AbstractIt is shown that data from different authors on the depth dependence of the depth resolution can be compared only when both the instrumental parameter (β) and the intrinsic parameter (α) are taken into account. These two essential parameters are shown to be unique for every given ‘set of instrument and sample’. A linear z‐dependence was derived in a first approximation from experimental data. In this case it is shown that a power approximation in a (log (Δz/z)vs log z) plot, can have any value for the exponent between −1 and 0, dependent on the z‐interval considered. The z‐interval for which a given value of the exponent is obtained, in turn depends on α and β. The influence of the acceptance area and the primary ion energy on the intrinsic parameter α and the influence of the beam inhomogeneity on the parameter β are discussed. The implications of a non‐linear z‐dependence on the interpretation of experimental data are considered.

Theoretical assessments of major physical processes involved in the depth resolution in sputter profiling

Abstract Two of the more important physical processes which militate against high depth resolution capabilities for ion-induced sputter sectioning associated with compositional analysis techniques are surface topography development and recoil atomic mixing. This review describes earlier, simplistic, theoretical modelling of such processes and describes new approaches based upon empirical evidence of the nature and magnitude of these processes. It is shown that, in general, the depth resolution of the sputtering technique will be a complex function of the depth probed and that both “broadenings” and “shifts” in depth evaluation are to be expected.

Distortion of depth profiles during sputtering: I. General description of collisional mixing

The present work concerns collisional mixing which is one of the factors limiting the depth resolution of sputter profiling. A general treatment comprising the effects of recoil implantation and cascade mixing on the distortion of an impurity profile in a bulk matrix is presented. The statistics of the distorting events is shown to be a close analogue to that of the stopping of charged particles in random matter, and describable in terms of the Bothe-Landau theory of energy loss; a general expression is given for the Green's function transforming a given impurity profile into an apparent (or distorted) profile. In the diffusion approximation, the present theory yields a well defined shift and smearing of an impurity profile without introduction of a (more or less arbitrary) effective minimum energy for displacement.

Depth resolution of sputter profiling investigated by combined Auger-x-ray analysis of thin films

Conversion of conventional “Auger sputter profiles” into the “depth profiles” has been accomplished by detecting characteristic X-rays produced simultaneously with Auger electrons. We discuss the sputtering through the interface between GeSi films with respect to the following points: determination of the original interface position on the depth scale; asymmetry of the shape of the Auger profiles, and broadening of the interface region due to atomic mixing for different ion masses and energies (He + to Xe +, 0.5 and 5 keV). We found that the best depth resolution of 2.5 nm is obtained for 0.5 keV Xe + which increases to more than 10 nm for He + at 5 keV.

The depth resolution of sputter profiling

It is shown that the bulk radiation damage accompanying sputtering events sets ultimate limits to the depth resolution attainable in sputter profiling. These limits have been reached in a few cases but most published experimental resolutions are dominated either by instrumental effects or deterioration of depth resolution caused by surface-topography changes. The radiation-damage induced mixing is called “cascade mixing”. Guidelines for selection of projectile species and energies to minimize such mixing are given and numerical estimates for attainable depth resolutions are presented. Finally, the influence of cascade mixing is assessed relative to that of recoil implantation.

Depth resolution in sputter profiling: Evidence against the sequential layer sputtering model

The sequential layer sputtering model originally developed by Benninghoven is analysed critically. The validity of the model is checked by comparison with the results of a variety of recently reported depth profiling studies on sandwich layer structures. It is shown that with adequately performed measurements the parameter a representing the “monolayer thickness” is of subatomic dimension and up to an order of magnitude smaller than the relevant lattice constant. Accordingly a is merely a fitting parameter without any physical significance. This result clearly rules out the validity of the model. The cause of non-statistical emission is discussed briefly.

Depth resolution in sputter profiling

The dependence of depth resolution on sputter depth due to various parameters is theoretically estimated. Comparison with experimental work of different authors shows the validity of the proposed model.