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)     

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.     

Characterization of oxide layers on amorphous Zr-based alloys by Auger electron spectroscopy with sputter depth profiling

Amorphous Zr-Cu-Ni-Al-[Ti, Nb] ribbons prepared by melt spinning under argon atmosphere were subjected to electrochemical investigations. Passive films developed at potentiostatic anodic polarization in sulphuric acid solution were investigated by Auger electron spectroscopy (AES) and sputter depth profiling.Changes in the shape of the Auger peaks have been analyzed by factor analysis of the spectra obtained during depth profiling. Pronounced changes in shape and position occur for the Zr, Al, and Ti Auger transitions, but not for Cu and Ni. At least three different peak shapes for O(KVV) were found and attributed to different oxygen binding states. The alloy composition has no significant effect on the thickness and composition of the oxide layer.In multi-element alloys preferential sputtering is a common phenomenon. In the steady state of sputtering, a significant depletion in Cu is found. At the oxide/metal interface, a distinct enrichment of copper is found for all alloys and treatments. The degree of this Cu enrichment depends on the pretreatment. It is higher for the electrochemically-passivated samples than for samples with oxide layers grown during melt spinning.     

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.     

Raster scanning depth profiling of layer structures

Implications in the use of the electronic gating scheme in depth profiling studies of layer structures by means of raster scanning secondary ion mass spectrometry are investigated. The profile of the sputtering crater and the intensity variation after break-through are calculated with the scan width and the gate width as parameters. Thick (5.6 μm) magnetic garnet layers grown an a non-magnetic garnet substrate were used for depth profiling measurements. A relative depth resolution of 1% could be obtained. Comparison of experimental results with calculated data shows excellent agreement.     

Studies of CdTe surfaces with secondary ion mass spectrometry,rutherford backscattering and ellipsometry

The composition and the stability of chemically etched, mechanically polished and oxidized surfaces of single crystals of cadmium-telluride were studied by secondary ion mass spectroscopy (SIMS), Rutherford backscattering (RBS) and ellipsometry. CdTe surfaces etched using a solution of bromine in methanol were found to be enriched in cadmium but a film, identified to be an oxide of tellurium, was observed to grow on it at room temperature and in air. The thickness of this film increased over long periods of timet linearly versus lnt. Mechanically polished samples and also chemically etched surfaces which were oxidized in a solution of hydrogen peroxide in amoniac were found to be stable.     

Sputtered neutral mass spectrometry (SNMS) as a tool for chemical surface analysis and depth profiling

An experimental system for mass spectrometry of supttered neutral particles involving a hf plasma operated in Ar at several 10⁻⁴ Torr is described. The potentialities of the method for quantitative surface analysis are reasoned. Depth profiling by sputtered neutral mass spectrometry is demonstrated for anodic oxide layers on Nb and Ta.     

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.     

Optoacoustic sensor head for depth profiling

-1 (for 0.25 mJ/cm²). The response function of the total experimental setup is derived from the optoacoustic signal of a black absorber, which is necessary to correct theoretical calculations with regard to the finite time response of the detection system. In the case of homogeneous samples, the theory of optoacoustic signal generation is verified, qualitative results are achieved with layered samples. Received: 14 October 1998 / Published online: 24 February 1999     

Collision cascade evolution in media with energy independent scattering: spatial,energy, and angular distribution

The evolution of an ion induced collision cascade in a solid medium is studied by means of a DPl-approximation to the linear transport equation. Infinite medium and half space geometries are considered. Special attention is given to the effect of the anisotropy of the energy independent scattering cross section. We present results on the spatial distribution of particles moving at different energies, and the energy and angle distribution at the target surface. The spatial distributions are found to obey simple scaling laws; the energy and angular distributions are independent of the form of the scattering cross section, unless it is very strongly forward peaked.     

Angular-resolved energy distribution of secondary ions emitted from a silicon target sputtered by a xenon ion beam

This paper reports preliminary results obtained on an experimental apparatus dedicated to the study of angular resolved energy distribution of particles emitted from a sputtered target. Secondary ions emitted during the bombardment of a silicon target by xenon ions at a primary energy of 10keV have been studied. In its low energy part the distribution reaches a maximum around 8eV, and then decreases according to an E ^–1 law. In the range 200eV to 1000eV, a second maximum appears whose height depends on the emission angle. Apart from this range, the angular distributions have a cosine square-like shape. On the contrary, the angular distribution of ions with energy between 200eV and 1000eV is pointed in a forward direction near the specular reflection direction of the ion beam. It is assumed that the measured ions correspond to two ionic populations: secondary ions sputtered according to the linear cascade theory and recoil silicon target ions.     

Sputtering yield of chromium by argon and xenon ions with energies from 50 to 500 eV

A radioactive tracer technique is described for the quantitative measurement of the sputtering yield of a target material electroplated on a copper substrate. Sputtering yields of chromium by argon and xenon ions with energies from 50 to 500 eV are reported. The ion beams, having a current density ranging from 0.01 to 0.1 mA/cm² at an operating pressure of 2×10^–5 Torr, were produced by a low-energy ion gun. The sputtered atoms were collected on an aluminum foil surrounding the target. ⁵¹Cr was used as the tracer isotope. The results indicate that the radioactive tracer technique is sensitive enough in measuring the extremely small amount of sputtered material at low ion currents and low ion energies.     

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.     

Probing depth of the low energy cascade electrons from a transition metal

A surface probing depth of only 2 layers for low energy cascade electrons excited with a 3.2 keV primary electron beam is clearly demonstrated by model experiments with non-magnetic overlayers of Ta on magnetic substrates of Fe/Ni₈₀Fe₂₀. This result establishes a short probing depth of low energy electrons in transition metals generalizing the previously observed short magnetic probing depth for spin-polarized electrons in ferromagnets. The short probing depth sheds new light on a number of spectroscopic observations on ferromagnetic transition metals, and has important implications concerning surface magnetic properties and scattering processes of hot electrons in transition metals.     

Three-dimensional damage distributions of molecular ions implanted into silicon,as recorded by RBS/channeling combined with tomography

The depth distributions of damage of 1.4 MeV nitrogen molecular ions (N ₂ ⁺ ) implanted into Si crystals at doses slightly below the value for amorphization have been measured by means of standard RBS/channeling for different directions of impact. These damage distributions were fed into our modified tomographic program MO-TOR [1, 2], by which we could reconstruct the spatial distributions of nuclear energy transfer. These distributions are compared with the three-dimensional theoretical prediction of a modified TRIM code [3].It turns out that there exists a pronounced deviation from the purely ballistic damage distribution insofar as the reconstructed damage distribution is twice as broad in the lateral direction than predicted. This is essentially explained by deviations in flight geometry of molecular ions in comparison with single-atomic ones.     

Molecular depth profiling of polymers with very low energy ions

The need for a molecular depth profiling technique to study organic layers has become a strong incentive in the SIMS community in the last few years, especially with the recent successes obtained with cluster ion beam depth profiling. In this work, we have investigated a thoroughly different approach by using very low energy (down to 200 eV) monoatomic or diatomic ions to sputter organic matter. Quite surprisingly, we were able to retain specific molecular information on various polymers even at very high fluence.Polymethylmethacrylate (PMMA) and polyethylene terephthalate (PET) films were depth-profiled with 200 eV Cs⁺ and 500 eV O₂⁺ ions. With 200 eV Cs ions, the best profiles were obtained in the negative mode, due to a strong negative ionisation yield enhancement related to Cs retention in the polymer. A relatively high and stable signal from the most characteristic ions was measured all over the layer.With 500 eV O₂⁺, real molecular depth-profiles were also obtained in both the positive and the negative modes. Once again, the main characteristic fragments of PET or PMMA remain detectable with stable yields all over the profile.     

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.     

High-resolution depth profiling of ultrashallow boron implants in silicon using high-resolution RBS

Depth profiles of ultralow energy (0.2–0.5 keV) B ion implants in Si(0 0 1) samples are measured by high-resolution Rutherford backscattering spectroscopy. The boron profile does not show a narrow surface concentration peak which is usually observed in the measurement of secondary ion mass spectroscopy. The obtained boron profiles roughly agree with TRIM simulation even at 0.2-keV B ion implantation.     

Cesium redeposition artifacts during low energy ToF-SIMS depth profiling

H-terminated Si samples were preloaded with Cs by performing ToF-SIMS depth profiles (250 eV Cs⁺, 15 keV Ga⁺) until the steady state was reached both with and without a bias of +40 V applied to the ion extraction electrode. Xe⁺ depth profiles (350 eV Xe⁺, 15 keV Ga⁺) were obtained inside and around the Cs craters with and without applying the 40 V bias. The results indicate that the maximum of the Cs⁺ signal of the Xe⁺ depth profiles shifts to the surface if no bias is applied, either during the Cs⁺ sputtering or during the Xe⁺ sputtering (i.e., the profiles are broadest with both biases (Cs⁺ and Xe⁺) on and narrowest and closest to the surface if both biases are off). This effect can be explained by the electric field, caused by the bias, deflecting the sputtered low energy Cs⁺ ions back to the surface.