Degradation Of Depth Resolution Research Articles

Cluster secondary ion mass spectrometry and the temperature dependence of molecular depth profiles.

The quality of molecular depth profiles created by erosion of organic materials by cluster ion beams exhibits a strong dependence upon temperature. To elucidate the fundamental nature of this dependence, we employ the Irganox 3114/1010 organic delta-layer reference material as a model system. This delta-layer system is interrogated using a 40 keV C(60)(+) primary ion beam. Parameters associated with the depth profile such as depth resolution, uniformity of sputtering yield, and topography are evaluated between 90 and 300 K using a unique wedge-crater beveling strategy that allows these parameters to be determined as a function of erosion depth from atomic force microscope (AFM) measurements. The results show that the erosion rate calibration performed using the known Δ-layer depth in connection with the fluence needed to reach the peak of the corresponding secondary ion mass spectrometry (SIMS) signal response is misleading. Moreover, we show that the degradation of depth resolution is linked to a decrease of the average erosion rate and the buildup of surface topography in a thermally activated manner. This underlying process starts to influence the depth profile above a threshold temperature between 210 and 250 K for the system studied here. Below that threshold, the process is inhibited and steady-state conditions are reached with constant erosion rate, depth resolution, and molecular secondary ion signals from both the matrix and the Δ-layers. In particular, the results indicate that further reduction of the temperature below 90 K does not lead to further improvement of the depth profile. Above the threshold, the process becomes stronger at higher temperature, leading to an immediate decrease of the molecular secondary ion signals. This signal decay is most pronounced for the highest m/z ions but is less for the smaller m/z ions, indicating a shift toward small fragments by accumulation of chemical damage. The erosion rate decay and surface roughness buildup, on the other hand, exhibit a rather sudden delayed onset after erosion of about 150 nm, indicating that a certain damage level must be reached in order to influence the erosion dynamics. Only after that onset does the depth resolution become compromised, indicating that the temperature reduction does not significantly influence parameters like ion-beam mixing or the altered-layer thickness. In general, the wedge-crater beveling protocol is shown to provide a powerful basis for increased understanding of the fundamental factors that affect the important parameters associated with molecular depth profiling.

Microstructure damage of titanium films by irradiation with fission fragments

Radiation damage caused by fission fragments to metal surfaces is an important research topic. Thin titanium foils were irradiated with a continuous wave beam of 132 MeV 132Xe +29 at the current intensity of 2 pnA. Pre- and post-irradiated surface topologies were investigated using atomic force microscopy and the observed defects were quantified by root mean square roughness, depth profile of the disordered zones, size and areal density of the voids, and discussed as a function of the applied fluencies (1–9) × 10 13 Xe/cm 2. The first ellipsoidal dislocation loops appeared at the fluence of 3.0 × 10 13 Xe/cm 2 with the areal density of 1.56 × 10 6/cm 2 that increased to 2.0 × 10 7 cm −2 when the dose rose to 9.0 × 10 13 Xe/cm 2. At this point also the first dislocation lines with the density of 1.3 × 10 7 cm −2 were seen. Our results suggest that the fission fragments might maximize large voids and dislocations and increase the degradation in depth resolution.

Confocal-rainbow volume holographic imaging system

The performance of broadband volume holographic imaging system in terms of depth selectivity is investigated. The mechanism for depth resolution degradation is explained. In order to overcome this resolution degradation, a novel imaging device, the confocal-rainbow volume holographic imaging system, is proposed. Modeling and experimental validation of the performance of this novel imaging system indicates that depth resolution <16 μm is achievable. The lateral resolution of this device is <2.5 μm along a field of view of 300 μm×100 μm.

A beneficial application of backside SIMS for the depth profiling characterization of implanted silicon

AbstractAfter ion implantation, annealing is needed to activate dopant and recover crystalline. However, in some annealing conditions there is a possibility that activation annealing forms defects in the implanted region, which may cause errors in SIMS depth profiles. The purpose of this study is to make clear the influence of defects generated during annealing on the conventional front‐side SIMS (FSS) depth profiles, and to provide proper measurement condition to get accurate depth profiles using back‐side SIMS (BSS) technique as another beneficial application. In this study, BF2 were implanted into crystalline silicon, and three different samples were treated, respectively, as implanted, 750 and 900° annealing. In an as‐implanted sample, BSS depth profiles of boron confirmed those obtained by FSS. On the other hand, in annealed samples, BSS depth profiles were steeper than those of FSS. This result suggests that the degradation of depth resolution in FSS cannot be explained only by the effect of ion mixing due to primary ions, but may be related with the crystalline structure in the implanted region. Therefore, BSS techniques can also be indispensable for getting more precise depth profiles in the implanted region of silicon. Copyright © 2010 John Wiley &amp; Sons, Ltd.

Retrospective sputter depth profiling using 3D mass spectral imaging.

A molecular multilayer stack composed of alternating Langmuir-Blodgett films was analyzed by ToF-SIMS imaging in combination with intermediate sputter erosion using a focused C60+ cluster ion beam. From the resulting dataset, depth profiles of any desired lateral portion of the analyzed field-of-view can be extracted in retrospect, allowing the influence of the gating area on the apparent depth resolution to be assessed. In a similar way, the observed degradation of depth resolution with increasing depth of the analyzed interface can be analyzed in order to determine the 'intrinsic' depth resolution of the method.

Roughening of silicon (1 0 0) surface during low energy Cs + ion bombardment

Nowadays, the use of low energy ion bombardment in secondary ion mass spectrometry (SIMS) is a mandatory step to obtain high depth resolution for the characterization of ultra shallow junctions. However, increment of roughness during ion bombardment leads to the degradation of depth resolution. The evolution of surface roughening and ripple formation on silicon at ambient temperature under 1 keV Cs + ion bombardment with and without sample rotation has been studied by means of atomic force microscopy. Ripple formation with a perpendicular orientation with respect to the Cs + beam direction has been detected, and their wavelength and correlation length have been monitored as a function of the experimental conditions. Roughness and wavelength increased with increasing ion fluence, while variations of ion flux showed little effect. The effect of sample rotation during ion bombardment led to a critical reduction of the surface roughness and disappearance of ripples.

Surface topography effects on glow discharge depth profiling analysis

AbstractGlow discharge optical emission spectroscopy (GD‐OES) has been shown to be of immense value in elemental depth profiling of thin or thick films on conductive or non‐conductive substrates. For aluminium, GD‐OES has been employed to examine locations of markers and tracers in anodic films, thereby assisting understanding of transport phenomena. In order to investigate the influence of surface topography on depth profiling analysis, anodic aluminium oxide films of various thicknesses, with incorporated electrolyte species, were produced on superpure aluminium substrates of controlled roughnesses. The distributions of incorporated species in the films were subsequently probed. Surface topography modifications and consequent depth resolution degradation were examined during depth profiling analysis performed by GD‐OES. The results reveal that the sputtering process leads to the roughening or smoothing of the surface topography of the specimen for a ratio of the film thickness to the amplitude of the substrate texture less, or greater, than 1 respectively. As a consequence of the surface topography dependence of the ion bombardment, analysis of thin films over rough surfaces suffers from depth resolution limitations due to sputtering‐induced topography changes, thereby limiting quantification of the resultant spectra. Copyright © 2010 John Wiley &amp; Sons, Ltd.

The Influence of Out-of-Focus Sample Regions on the Surface Specificity of Confocal Raman Microscopy

This paper considers the quantitative implications of out-of-focus regions on the lateral and depth resolution of Raman microscopy, with special regard for the surface specificity of the technique. It builds on work that has recently appeared in the literature which shows that with transparent samples, signals can originate throughout a large extended illumination volume, even though most of this region is out of focus with regard to the confocal aperture. This gives rise to weak but readily detectable spectral contributions from regions that are tens of micrometers from the point of tightest focus, an effect that is easily demonstrated if the laser is focused far above the sample surface. When we integrate the signals arising throughout this extended volume, the resulting total signal can be significant with respect to the Raman signals originating from the point of focus; this has obvious implications for surface specificity and depth resolution. Furthermore, as one moves the focal point through and above a sample surface, signals from thick transparent samples decay relatively slowly compared with thin or opaque ones, where the extended focal volume is irrelevant. This means that on moving above the surface of a thinly coated thick substrate during a confocal axial scan, the substrate-to-coating signal ratio increases dramatically, contrary to intuition. Consequently, confusing spectral artifacts arise if one focuses above the sample surface, either inadvertently when mapping an uneven sample, or deliberately in an attempt to improve surface specificity. In this work we show how a simple analytical model can predict the surface/substrate signal ratio as a function of distance above the surface. The model is validated using experimental data from monofilms and coated films. Furthermore, we show how this effect is not limited to the confocal axial profiling geometry. Similar effects are obtained when one scans laterally beyond the edge of mechanically prepared cross-sections due to an extended, out-of-focus laser field that can sample lateral regions far to the side of the optimum focus. This effect can lead to very confusing results, such as spectra from the substrate increasing in absolute intensity as one moves beyond the edge of the coating into the air. These observations, which as far as we are aware have not previously been reported, are rationalized using a simple ray-tracing description, which shows the potential for coupling light into the cross-section, which acts as a waveguide. These effects have a completely different origin than the well-known anomalies that are introduced by refraction and spherical aberration; even with a perfect, aberration-free system, the extended focal volume may cause significant degradation in depth resolution. Although the effects have been demonstrated with simple film systems, they have the potential to impact the results from Raman mapping and imaging of any samples that contain significant refractive index discontinuities, which can potentially cause refraction and waveguiding, or that have compositional depth gradients and an uneven sample surface.

The optimization of incident angles of low-energy oxygen ion beams for increasing sputtering rate on silicon samples

In order to determine an appropriate incident angle of low-energy (350-eV) oxygen ion beam for achieving the highest sputtering rate without degradation of depth resolution in SIMS analysis, a delta-doped sample was analyzed with incident angles from 0° to 60° without oxygen bleeding. As a result, 45° incidence was found to be the best analytical condition, and it was confirmed that surface roughness did not occur on the sputtered surface at 100-nm depth by using AFM. By applying the optimized incident angle, sputtering rate becomes more than twice as high as that of the normal incident condition.

Factorial analysis of cluster-SIMS depth profiling using metal-cluster-complex ion beams

A Ir 4(CO) 7 + primary ion beam, at energies from 2.5 keV to 10 keV, was used to profile boron-delta layers in Si to investigate the influences of atomic mixing and surface roughness on the degradation of depth resolution. Factorial analyses using the mixing-roughness-information (MRI) model indicated that the influence of the mixing increased as beam energy was reduced below 5 keV in the case of oxygen flooding. It was confirmed that the magnitude of the MRI surface roughness was different from that of the AFM surface roughness. The discrepancy in the magnitude of roughness was examined by considering the difference in sputtering depth as well as the definition of the MRI surface roughness.

Quantitative Molecular Depth Profiling of Organic Delta-Layers by C60 Ion Sputtering and SIMS

Alternating layers of two different organic materials, Irganox1010 and Irganox3114, have been created using vapor deposition. The layers of Irganox3114 were very thin ( approximately 2.5 nm) in comparison to the layers of Irganox1010 ( approximately 55 or approximately 90 nm) to create an organic equivalent of the inorganic 'delta-layers' commonly employed as reference materials in dynamic secondary ion mass spectrometry. Both materials have identical sputtering yields, and we show that organic delta layers may be used to determine some of the important metrological parameters for cluster ion beam depth profiling. We demonstrate, using a C(60) ion source, that the sputtering yield, S, diminishes with ion dose and that the depth resolution also degrades. By comparison with atomic force microscopy data for films of pure Irganox1010, we show that the degradation in depth resolution is caused by the development of topography. Secondary ion intensities are a well-behaved function of sputtering yield and may be employed to obtain useful analytical information. Fragments characteristic of highly damaged material have intensity proportional to S, and those fragments with minimal molecular rearrangment exhibit intensities proportional to S(2). We demonstrate quantitative analysis of the amount of substance in buried layers of a few nanometer thickness with an accuracy of approximately 10%. Organic delta layers are valuable reference materials for comparing the capabilities of different cluster ion sources and experimental arrangements for the depth profiling of organic materials.

Confocal Raman microspectroscopy with dry objectives: A depth profiling study on polymer films

In this work, we test simple models proposed to predict two characteristic features when performing depth profiling with dry-optics confocal Raman microspectroscopy (CRM): the decay in the collected intensity and the degradation of depth resolution with focusing depth due to laser refraction. With this aim, we carried out experiments on transparent thick polystyrene and poly(methyl methacrylate) films, in which we tracked the collected Raman intensity as a function of focusing depth. These results are interpreted in the context of the model proposed by Batchelder. We also investigated the Raman response on a series of well-defined planar interfaces, generated by contact between thin poly(methyl methacrylate) films (45, 94 and 145 μm thickness) coated onto a much thicker piece of poly(butyl methacrylate). These results allowed us to test the enlargement in focus length with focusing depth predicted by the models of Everall and Batchelder. We found that with minor modifications that keep the simplicity of the original treatment, the model of Batchelder reproduces reliably fine features of the Raman intensity profiles. The results of this work show that these simple models cannot only be used to assist data interpretation but also to predict quantitatively Raman intensity variations in depth profiling experiments.

Depth profiling using C 60+ SIMS—Deposition and topography development during bombardment of silicon

A C 60 + primary ion source has been coupled to an ion microscope secondary ion mass spectrometry (SIMS) instrument to examine sputtering of silicon with an emphasis on possible application of C 60 + depth profiling for high depth resolution SIMS analysis of silicon semiconductor materials. Unexpectedly, C 60 + SIMS depth profiling of silicon was found to be complicated by the deposition of an amorphous carbon layer which buries the silicon substrate. Sputtering of the silicon was observed only at the highest accessible beam energies (14.5 keV impact) or by using oxygen backfilling. C 60 + SIMS depth profiling of As delta-doped test samples at 14.5 keV demonstrated a substantial (factor of 5) degradation in depth resolution compared to Cs + SIMS depth profiling. This degradation is thought to result from the formation of an unusual platelet-like grain structure on the SIMS crater bottoms. Other unusual topographical features were also observed on silicon substrates after high primary ion dose C 60 + bombardment.

Multiplicity and contiguity of ablation mechanisms in laser-assisted analytical micro-sampling

Laser ablation is implemented in several scientific and technological fields, as well as a rapid sample introduction technique in elemental and trace analysis. At high laser fluence, the ejection of micro-sized droplets causes the enhancement of the surface recession speed and depth resolution degradation as well as the alteration of the sampling stoichiometry. The origin of such large particles seems to be due to at least two different processes, phase explosion and melt splashing. Experimental evidence for both was found in metallic matrices, whereas non-metallic samples showed more complex phenomena like cracking. The spatial distribution of the beam energy profile is responsible for significant differences in the ablation mechanism across the irradiated region and for heterogeneous sampling. Under Gaussian irradiance distribution, the center of the crater, where the irradiance is the highest, experienced a fast heating with rapid ejection of a mixture of particles and vapor (spinodal breakdown). The crater periphery was subjected to more modest irradiation, with melt mobilization and walls formation. The overall resulting particle size distribution was composed of an abundant nano-sized fraction, produced by vapor condensation, and a micro-sized fraction during melt expulsion.

Surface roughening of silicon under ultra-low-energy cesium bombardment

Ripple formation on silicon due to oblique (ultra-)low-energy cesium bombardment was studied by measuring the degradation in depth resolution during sputter profiling of an antimony doped delta layer sample (six Sb deltas with an interlayer spacing of 5 nm). Additional insight into the evolution of surface roughness was obtained by recording depth dependent changes in matrix negative-ion signals. The surface topography of the sputtered crater bottoms was investigated ex situ by scanning electron microscopy. At beam energies between 0.25 and 1 keV, ripple formation was found to take place at “critical” angles exceeding 50–60°. Depending on the impact angle θ, the onset of ripple growth could be observed at depths as small as about 2 nm, i.e. immediately after saturating the sample with cesium. The rate of ripple growth increased with increasing θ. Almost complete loss of resolution of the Sb deltas occurred at angles about 10° above the critical angle, at depths as low as 15–20 nm.

Influence of interfacial depth on depth resolution during GDOES depth profiling analysis of thin alumina films

AbstractFactors controlling the depth resolution in radio frequency glow discharge optical emission spectroscopy (rf‐GDOES) depth profiling analysis of thin films have been investigated using anodic alumina films of varying thicknesses with delta function marker layers of ∼2 nm width located at various depths from the oxide surface. In rf‐GDOES depth profiling analysis, where a relatively large area of ∼4 mm in diameter is analysed, it was found that the depth resolution is determined mainly by the large‐scale variation of sputtering rate across the crater. Despite the relatively large area analysed, the uniformity of sputtering rate across the crater was constant to within 1% under the optimum Ar pressure except for near‐edge regions of ∼0.4 mm width, where the sputtering rate was ∼6% higher than in the central crater region. Owing to this relatively minor non‐uniformity of sputtering rate, the depth resolution degrades approximately linearly with depth. A depth resolution of ∼1.3 nm was realized to depths of ∼25 nm, where the degradation of depth resolution due to the non‐uniformity in sputtering rate is insignificant. The depth resolution is close to the ultimate depth resolution in sputter depth profiling, claimed to be 0.7– 1.0 nm, and arises from the minimal atomic mixing associated with film sputtering with Ar+ions of very low energy, ∼50 eV, in rf‐GDOES analysis. Copyright © 2001 John Wiley &amp; Sons, Ltd.

Auger electron spectroscopy investigations of the effect of degradation of depth resolution and its influence on the interdiffusion data in thin film Au/Ag, Cu/Ag, Pd/Au and Pd/Cu multilayer structures

Auger depth profile investigations of interdiffusion in the bilayer and multilayer Au/Ag, Cu/Ag, Pd/Au and Pd/Cu thin film structures in the temperature range 423–573K have been reported. Bilayers and multilayers have been obtained by successive evaporation onto the SiO2 and Cu substrates with different surface topographies. The thickness of bilayers was changed from 50 to 200nm, whereas the thickness of particular layers of the multilayer structure was changed from 20 to 80nm. Interdiffusion data have been derived from the change of the slope of the profile at the interface of two layers, from the rate of rise of diffusant concentration inside the upper layer and from the change of the concentration amplitude in the multilayer structure. In all cases, the depth resolution was monitored and its influence on the interdiffusion data was studied.

Influence of argon pressure on the depth resolution during GDOES depth profiling analysis of thin films

The influence of Ar pressure on depth resolution during glow discharge optical emission spectroscopy (GDOES) depth profiling has been examined through the use of 358 nm thick anodic alumina films grown over flat aluminium surfaces. The films are ideal standards for the present purpose, being amorphous and highly uniform in thickness, with a flat and sharply-defined metal/film interface. The depth resolution is influenced strongly by the Ar pressure, which, in turn, is related to the shapes of the craters generated by cathodic sputtering. For a given power, an optimum Ar pressure results in a flat-bottomed crater and the highest depth resolution is realized. In the present example, the highest depth resolution, expressed in terms of width of the transition of the aluminium profile at the metal/film interface, i.e. ∼7 nm, was achieved at a pressure gauge voltage of 5.60 V (0.41 mbar) and a power of 40 W. Increase or decrease of Ar pressure from the optimum leads to rapid degradation of depth resolution. Copyright © 2000 John Wiley & Sons, Ltd.

Non-uniform sputtering and degradation of depth resolution during GDOES depth profiling analysis of thin anodic alumina films grown over rough substrates

Degradation of depth resolution during glow discharge optical emission spectroscopy (GDOES) depth profiling analysis of thin films, formed on relatively rough substrates, has been investigated using preconditioned aluminium substrates of controlled surface roughness. The anodic alumina films, -120 nm thick, were depth profiled partially and examined carefully by atomic force microscopy and by transmission electron microscopy of ultramicrotomed sections. It is revealed clearly that depth profiling sputters ridges, associated with the rough surface, more rapidly than valleys, thereby degrading depth resolution. Further, such observations show that film sputtering proceeds very smoothly, with relatively little damage to the remaining film material. The excellent depth resolution realized during GDOES depth profiling analysis of films grown on mirror-finished, microscopically flat substrates is clearly associated with the highly uniform sputtering of amorphous anodic alumina.

Non‐uniform sputtering and degradation of depth resolution during GDOES depth profiling analysis of thin anodic alumina films grown over rough substrates

Non‐uniform sputtering and degradation of depth resolution during GDOES depth profiling analysis of thin anodic alumina films grown over rough substrates