STM and AES study of the relation between ion sputter induced roughness and depth resolution

Scanning tunneling microscopy has been applied to characterize the enhancement of surface roughness by ion sputtering. A certified Ni/Cr multilayer film was employed as a target. It was found that the roughness induced by ion sputtering increased with the increase of sputtered depth, and that this increase in roughness accounted for most of the reduction in depth resolution in Auger depth profiling.     

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. Part of this paper was presented at the 3rd International Conference on Solid Surfaces, Vienna (September 1977)     

The simulation of nanoscale sputter depth profiles using molecular dynamics

The feasibility of using molecular dynamics (MD) for simulation of a nanoscale sputter depth profile experiment is examined for the idealised case of depth profiles of individual atomic layers in a Cu(1 0 0) target. Issues relating to the extraction of depth profile information from MD simulations are discussed in detail. The simulations examine the sputter erosion of static and azimuthally-rotated Cu(1 0 0) targets produced by 3 keV Ar projectiles incident at 25° from the surface. The simulated projectile fluence extends to 5 × 10¹⁵ cm⁻², and the mean value of the sputter depth, z, amounts to 8 Cu(1 0 0) monolayers (ML) or 15 Å. The simulations directly supply progressive layer erosion profiles (curves that depict the extent of sputter erosion of each atomic layer vs. total sputter depth). A fitting method is then used to extract smooth depth profiles for each atomic layer from these predicted erosion profiles. The depth profile characteristics (height, width, shift) for the first 10 layers of the target show a pronounced dependence on layer depth.     

Nickel-chromium interface resolution in Auger depth profiles

Auger depth resolutions for fine-grained crystalline Ni/Cr multilayers have been determined. Resolution is shown to be a function of ion species, ion energy, ion incident angle, and the presence of reactive species. Deterioration of interface resolution with depth is shown to be the result of cumulative surface roughness induced by the ion beam to an extent proportional to the ion velocity normal to the surface. The presence of a reactive species to create an amorphous surface layer is shown to inhibit the development of surface roughness. The best resolution was obtained using low energy Xe ions and large ion beam angles. The ultimate resolution is limited by the fabrication perfection of the standard, and not by the sputtering process or by the Auger mean free path length.     

Auger electron spectroscopy and ion sputter profiles of oxides on aluminum

To check the validity of those assumptions needed to relate the Auger peak to peak height (APPH) to quantitative elemental concentrations in oxide films, oxide films on aluminum were monitored by AES during ion-sputter-etching. Before and after film profiling with AES, contour maps of film thickness were obtained with ellipsometry. Ion-sputter-AES profiles are entirely misleading if interpreted as quantitative concentration profiles, but are very informative as to qualitative analysis and when properly understood reveal physical and chemical non-isotropy within the films.     

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

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.     

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.     

Dependence of Auger depth resolution and surface texturing on primary ion species

The dependence of Auger depth resolution and surface texturing on primary ion species was systematically investigated for polycrystalline Al, Mo, Ag and Ta films deposited on Si wafers, using 3 keV Ne⁺, Ar⁺ and Xe⁺ ions as projectiles. The resolution was found to depend strongly on ion species; the higher the ion mass, the better the resolution. The resolution improvement attained with Xe⁺ ions was dramatic for Al, becoming less pronounced for higher mass targets. As revealed by high-resolution scanning electron microscopy, Xe⁺ sputtering led to less-developed topographical structures of sputtered areas, which allowed us to conclude that ion sputtering with heavier ions roughens the surface less, resulting in a marked improvement in resolution.     

Implanted-fluorine depth profiles in molybdenum

Implanted-fluorine profiles in molybdenum have been accurately measured using¹⁹F (p,)¹⁶O resonant nuclear reaction atE _R=872.1 keV. A proper convolution-calculation method was used to extract the true distribution of fluorine from the experimental excitationyield curves. The range-distribution parameters,R _p and R _p, were thereby obtained and were compared with those obtained by Monte-Carlo simulation with a computer code developed recently in this gorup, and the TRIM'90 code. They show that the experimentalR _p values agree very well with the Monte-Carlo simulation values, while the experimental R _p values are larger than those obtained from the simulations.     

Advanced ion energy loss models: Applications to subnanometric resolution elemental depth profiling

In principle, the depth distribution of the different chemical elements near the surface of solids can be determined quantitatively and absolutely with subnanometric depth resolution using medium energy ion scattering (MEIS), which is a refined version of Rutherford backscattering spectrometry (RBS). The energy resolution of current MEIS analyzers reveals spectral features that cannot be resolved using conventional RBS detectors. Thus, the usual data analysis framework based on a standard Gaussian approximation for the ion energy distribution in the target is applicable to regular RBS, but not generally to MEIS, in particular if one aims at subnanometric depth resolution. The observed asymmetry in the ion energy loss distributions is a direct consequence of the asymmetric character of inelastic energy transfers during individual atomic collisions and of the stochastic character of the resulting energy losses. We propose a model that accounts for the proper statistics of the small energy loss events and for an approximate electronic energy loss distribution during the backscattering event. The validity of this model is discussed and applied to the determination of HfO₂ and TiO₂ film thicknesses as well as to detect Al₂O₃ and HfO₂ intermixing. This final application case also illustrates the potentialities as well as some inherent limitations of MEIS. The model developed here has been made available to the public in the form of a software for MEIS data analysis.     

Auger electron spectroscopy investigation of sputter induced altered layers in SiC by low energy sputter depth profiling and factor analysis

The thickness of the altered layer created by ion bombardment of the 6H–SiC single crystal was determined by means of Auger electron spectroscopy (AES) depth profiling in conjunction with factor analysis. After pre-bombardment of the surface by argon ions with energies 1, 2 and 4 keV until the steady state, the depth profiles of the induced altered layers were recorded by sputtering with low energy argon ions of 300 eV. Since the position and shape of the carbon Auger peak depend on the perfection of the crystalline structure, they were used for depth profile evaluation by factor analysis. In this way the depth profiles of the damaged surface region could be estimated in dependence on the ion energy. As a result, the thickness of the altered layer of SiC bombarded with 1, 2 and 4 keV Ar ions using an incident angle of 80° as well as the corresponding argon implantation profile could be measured.     

Towards chemical state resolution in regularized depth profiles derived from ARXPS data taken on plasma-oxidized polystyrene

ARXPS data obtained from a plasma-oxidized polystyrene sample were evaluated by means of 3-parameter and 10-parameter depth profile models, with and without regularization. It was found that the partially regularized 3-parameter model gave equivalent results compared to the regularized 10-parameter model, but requiring one fifteenth of the computational effort.     

Subwavelength depth resolution in near-field microscopy

A generalized Radon transform is presented that relates, for the case of an evanescent wave that is incident upon a weakly scattering medium, the homogeneous components of the scattered field to the three-dimensional Fourier transform of the dielectric susceptibility. This relationship is used within the context of total internal reflection microscopy to reconstruct the depth structure of the dielectric susceptibility from simulated scattered field data.     

The effect of C60 cluster ion beam bombardment in sputter depth profiling of organic-inorganic hybrid multiple thin films

The effects of C₆₀ cluster ion beam bombardment in sputter depth profiling of inorganic-organic hybrid multiple nm thin films were studied. The dependence of SIMS depth profiles on sputter ion species such as 500 eV Cs⁺, 10 keV C₆₀⁺, 20 keV C₆₀²⁺ and 30 keV C₆₀³⁺ was investigated to study the effect of cluster ion bombardment on depth resolution, sputtering yield, damage accumulation, and sampling depth.     

Application of factor analysis to elemental detection limits in sputter depth profiling

Factor analysis of Auger spectra acquired during sputter depth profiling is superior to the conventional peak-to-peak amplitude method for determining elemental compositions, especially when the Auger signal strength is near the detection limit. The reason for the improvement is that factor analysis utilizes information from all the data channels in the Auger spectrum while the peak-to-peak amplitude method uses information from only two data channels. In addition, factor analysis can separate much of the spectral noise from the signal during processing, while peak-to-peak amplitude additively measures signal plus the range of the spectral noise. Procedures can also easily account for interfering species, even when their spectra are not known. In one example, at least a factor of five improvement in the minimum detection limit was achieved. In application to secondary ion mass spectrometry, only a marginal improvement in detection and precision was achieved. This is because our existing procedure (peak area measurement) already utilizes spectral information content fairly efficiently. However, factor analysis is capable of handling spectral interferences that the peak area method cannot.     

离子束作用下的光学表面粗糙度演变研究

 为了获得超光滑光学表面,介绍了离子束作用下改善表面粗糙度的抛光方法,并通过相关的实验进行了验证。光学材料是典型的硬脆材料,在加工过程中的表面粗糙度要经历复杂的演变过程。离子束加工作为光学镜面加工中的最后一道工序,如果在修正面形的同时,能够有效地改善表面粗糙度,那么离子束加工的性能就可以得到更好的延伸。分析了离子束作用下的粗糙度演变机理,在此基础上提出了倾斜入射抛光和牺牲层抛光技术2种改善表面粗糙度的方法,并使用原子力显微镜进行了测量。实验结果表明：以45°倾斜入射抛光熔石英样件,其粗糙度由初始的0.67nm RMS减小到0.38nm RMS;涂上牺牲层的材料表面粗糙度由0.81nm RMS减小到0.28nm RMS,倾斜入射抛光和牺牲层抛光技术能够有效地改善表面粗糙度。     

Progress in instrumentation,data reduction,and depth profiles in Auger electron spectroscopy

Progress in the use of Auger electron spectroscopy is discussed. Specifically limits to spatial resolution in the scanning Auger microprobe resulting from backscattered electrons is illustrated. Determination of chemical state by peak shapes and energies are discussed along with the use of correction factors in quantitating Auger electron data. Finally, the use of inverse Laplace transforms of angle-resolved electron spectroscopy data to generate composition depth profiles of sputtered GaAs is illustrated. It is concluded that Gibbsian surface segregation during sputtering caused depletion of As near the surface of GaAs.     

Implanted-fluorine depth profiles in silver gallium diselenide

Range distributions for fluorine ions in ¹⁹F⁺-implanted silver gallium diselenide (AgGaSe₂) in an energy range of 80–350 keV were measured by using the ¹⁹F(p, )¹⁶O resonant nuclear reaction at E_R=872.1 keV with width =4.2 keV. A proper convolution calculation method was used to extract the true distributions of fluorine from the experimental excitation-yield curves. The experimental range-distribution parameters, R_p and R_p, were compared with those obtained from Monte Carlo simulation codes. PACS 79.20.Rf; 66.30.Jt; 61.72.Ww     

Novel matching lens and spherical ionizer for a cesium sputter ion source

The beam optics of a multi-sample sputter ion source, based on the NEC MCSNICS, has been modified to accommodate cathode voltages higher than 5 kV and dispenses with the nominal extractor. The cathode voltage in Cs sputter sources plays the role of the classical extractor accomplishing the acceleration of beam particles from eV to keV energy, minimizing space charge effects and interactions between the beam and residual gas. The higher the cathode voltage, the smaller are these contributions to the emittance growth. The higher cathode voltage also raises the Child’s law limit on the Cs current resulting in substantially increased output. The incidental focusing role of the extractor is reallocated to a deceleration Einzel lens and the velocity change needed to match to the pre-acceleration tube goes to a new electrode at the tube entrance. All electrodes are large enough to ensure that the beam fills less than 30% of the aperture to minimize aberrations. The improvements are applicable to sputter sources generally.     

Improvement of depth resolution in XPS analysis of fluorinated layer using C60 ion sputtering

Depth profile of C₆₀ ion-used X-ray photoelectron spectroscopy (XPS) was studied on fluorinated organic layers with different thicknesses. We found that the depth resolution decreased, the sputtering rate went down and the surface turned rough as the layer thickness increased. This is because carbon-rich layer was formed on the surface by cross-linking reaction of the polymer and/or accumulation of degraded C₆₀ through continuous sputtering. Surprisingly, the high sputtering rate drastically improved the resolution of the analysis. The rate over 48.7 nm/min did not show any deterioration on the depth resolution, the sputtering rate and surface smoothness.