ToF-SIMS Depth Profiling Research Articles

Study of ionization processes during TOF-SIMS analysis by co-sputtering cesium and xenon

Because they are less sensitive to matrix effect than their homologues M+, the MCsn+ clusters are used for semi-quantitative depth profiling with ToF-SIMS. Unfortunately, their useful yields are low, particularly when the cesium surface concentration is high, leading to the drastic decrease of the positive ionization probability. In this paper, ToF-SIMS depth profiles of a bare silicon wafer were performed and the significant positive ions yields (e.g. Si+, SiCs+, Cs+) were monitored with respect to the varying cesium surface concentration. This analysis has been performed in the dual beam mode by diluting the cesium sputtering beam with xenon ions. This approach allows ToF-SIMS depth profiling with cesium beam concentrations varying from 0% (for pure xenon) to 100% (for pure cesium). At first, it was found that the Si+ and the Si2+ signals decrease exponentially with the increasing cesium beam concentration. The presence of optimums for the Cs+ and the MCsn+ signals was also observed. Finally, the tunneling model has been used to fit those maximums, which are a trade off between the increase of the cesium surface concentration and the decrease of the positive ionization probability.

Study of the Pd–Rh interdiffusion by ToF-SIMS, RBS and PIXE: Semi-quantitative depth profiles with MCs + clusters

In this paper, ToF-SIMS was used to study the Pd–Rh interdiffusion which has a great interest in brachytherapy, a cancer treatment. The secondary ion mass spectrometry was used in the semi-quantitative MCs + mode, by detecting the RhCs + and the PdCs + molecular ions under cesium bombardment. At first, different Rh x Pd y (from pure Rh to pure Pd) layers were deposited by PVD and were subsequently characterized by ToF-SIMS, RBS and PIXE. A linear relationship between the relative CsPd + yields and the Pd concentration into the Rh matrices was found. Moreover, the total sputtering yield increases linearly with the Pd concentration. Those relationships permitted to calibrate the ToF-SIMS depth profiles of annealed Pd/Rh layers and were successfully used to quantify the Pd–Rh interdiffusion.

Phosphorous degassing from poly silicon under thermal exposure: A ToF-SIMS depth profile investigation

In order to minimize the out-diffusion of phosphorous provided during the poly silicon doping procedure, a possible solution is capping the poly Si with a film with a relatively low P diffusivity with respect to the one in the poly. In fact, P diffusion in poly is normally enhanced by inter granular diffusion that in general is higher than the one due to the Si lattice (bulk diffusion). The aim of this work was to analyze, by ToF-SIMS depth profiling, poly Si samples that have been previously P doped by implantation and than subsequently thermally exposed, with and without a Si oxide capping layer, in order to study the effectiveness of this barrier to prevent the P depletion. The work shows the potential of ToF-SIMS depth profiling in being an efficient vehicle for revealing the goodness of this oxide capping approach in microelectronic applications.

Atomic distribution in quantum dots—A ToF-SIMS study

The atomic distribution in the monolayer of two different Mn-doped CdS quantum dots (QDs) was studied first time with ToF-SIMS. The model Cd:Mn QDs were immobilized on Au substrate by use of a self-assembled monolayer via 1,10-decanedithiol. Morphological analysis by SPM and TEM indicates larger particle size of in situ synthesizing CdS:Mn. ToF-SIMS depth profile and 3D-images reveal that Mn atoms reside on the surface of in situ synthesizing CdS:Mn and are uniformly embedded in capped CdS:Mn. The results obtained by SPM, TEM, and ToF-SIMS are comparable, indicating that ToF-SIMS might find potential applications in surface and interface study of semiconductor nanocrystals.

ToF-SIMS depth profiling of (Ga,Mn)As capped with amorphous arsenic: Effects of annealing time

The influence of annealing time on an amorphous As cap layer and the depth distribution of Mn atoms have been investigated. The results show that a ∼1600 Å thick As cap layer is completely desorbed after 3 h of annealing time. The depth distributions of Mn indicate that interstitial Mn atoms have diffused to the outer surface and being passivated. The thickness of the Mn passivation layer was around 90 Å.

Study of coating distribution onto metallic hollow particles

Porous lightweight materials have found increasing interest for various applications. Regular structured cellular materials might be accessible by sintering the hollow metallic particles. The properties of the final product depend to a large extent on the properties of original hollow metallic particles. Therefore, the preparation of reliable hollow spheres of defined parameters represents an important step towards the cellular solids production by means of powder metallurgy route. Electrochemical deposition of metallic coatings on the surface of hollow spherical particles facilitates the required modification of the surface corresponding to high mechanical quality. The copper hollow spheres were created on porous iron powder particles by electroless deposition. The suitable matrix for the preparation of relatively light but mechanically strong porous materials was obtained by electrochemical plating of copper hollow micro-particles by Ni layer. In this study ToF-SIMS depth profiling and SEM/EDX analysis with elements mapping are used to investigate the distribution of the coating components across the surface and subsurface layer of coated hollow particles.

Study of the Pd–Rh interdiffusion by three complementary analytical techniques: PIXE, RBS and ToF-SIMS

Pd diffusion in Rh has a great interest in the 103Pd* production intended to brachytherapy, a cancer treatment using radioactive implants. In order to study that diffusion, DC-magnetron sandwich layers of natural Pd and Rh were produced, annealed, and the Pd profiles were measured by ToF-SIMS. Nevertheless, because ToF-SIMS depth profiles suffer from a lack of quantification and depth calibration, the study was completed by RBS, PIXE. The combination of these techniques allowed us to convert the ToF-SIMS depth profiles from an intensity–time to a concentration–depth profile and thus to study the Pd diffusion in Rh.

Interpretation of TOF–SIMS depth profiles from ultrashallow high-k dielectric stacks assisted by hybrid collisional computer simulation

The TRIDYN collisional computer simulation has been modified to account for emission of ionic species and molecules during sputter depth profiling, by introducing a power law dependence of the ion yield as a function of the oxygen surface concentration and by modelling the sputtering of monoxide molecules. The results are compared to experimental data obtained with dual beam TOF–SIMS depth profiling of ZrO2/SiO2/Si high-k dielectric stacks with thicknesses of the SiO2 interlayer of 0.5, 1, and 1.5 nm. Reasonable agreement between the experiment and the computer simulation is obtained for most of the experimental features, demonstrating the effects of ion-induced atomic relocation, i.e., atomic mixing and recoil implantation, and preferential sputtering. The depth scale of the obtained profiles is significantly distorted by recoil implantation and the depth-dependent ionization factor. A pronounced double-peak structure in the experimental profiles related to Zr is not explained by the computer simulation, and is attributed to ion-induced bond breaking and diffusion, followed by a decoration of the interfaces by either mobile Zr or O.

Preparation of (TiTe2)x(Sb2Te3)y(TiTe2)3(Bi2Te3)z Superlattices using the Modulated Elemental Reactants (MER) Technique

AbstractDifficulty in preparing (Bi2Te3)x(Sb2Te3)y superlattices due to interdiffusion of Sb and Bi led to the study of interduffusion barriers. TiTe2 has been explored as an interdiffusion barrier to minimize the interdiffusion of Sb and Bi, as TiTe2 is not soluble in either Bi2Te3 or Sb2Te3. Preparation of (TiTe2)3(Sb2Te3)y(TiTe2)3(Bi2Te3)z superlattices has been achieved with varying x, y and z. The formation of the superlattices was studied as a function of annealing temperature and time. TOF-SIMS depth profiles were used to study the extent of interdiffusion in the samples. Unit cell control was achieved allowing for the preparation of an array of superlattices with varying periods with very good reproducibility.

Depth profile analysis of chemically amplified resist by using TOF-SIMS with gradient shaving preparations

The gradient shaving preparation was recently introduced as a new pretreatment technique for depth profiling. We can obtain a chemically undamaged slope with a length of several hundred micrometers by shaving linearly for a thickness ranging from a few nanometers to a few hundred nanometers. To analyze the chemical structure in organic compounds we performed depth profiles by doing line analysis along the sloped surface of various probes. In this work, the gradient shaving preparation was applied to analyze a chemically amplified photoresist which contains small amount of photoacid generator (PAG) and other additives by TOF-SIMS. The characteristic distribution of F − and CF 3SO 3 − fragments originated from PAG was determined. It was confirmed that the gradient shaving preparation allows TOF-SIMS depth profiling of small amount of organic materials in polymer films within the static limit.

Ti diffusion in chalcogenides: a ToF-SIMS depth profile characterization approach

Ge 2Sb 2Te 5 (GST) is one of the most promising chalcogenide materials for phase-change memory devices, which are potential candidates for the future generation of non-volatile memories. The goal of this work was to investigate the inter-diffusion properties of the Ti–GST system upon annealing, by means of ToF-SIMS depth profile characterization. A double stack (Ti–GST–Ti) and a single stack (Ti–GST) have been considered, both as virgin samples (as-deposited) and post-annealed at 673 K for 10 s in pure N 2 atmosphere. Two basic results have been found: (a) the annealed sample looked clearly affected by Ti diffusion throughout the whole GST layer; (b) the initial GST layer composition was dramatically altered as Te diffused as well as reacted with Ti, forming a Ti–Te compound (most likely as Te 2Ti). As a consequence, pronounced inter-diffusion of Ti and Te has been observed, resulting in a substantial reduction of the Te content of the original GST layer.

ToF-SIMS depth profiling analysis of the uptake of Ba2+ and Co2+ ions by natural kaolinite clay.

The sorption behavior of Ba(2+) and Co(2+) ions on a natural clay sample rich in kaolinite was studied using time-of-flight secondary ion mass spectrometry (ToF-SIMS). Depth profiling at 10-A steps was performed up to a 70-A matrix depth of the clay prior to and following sorption. The results showed that Co(2+) is sorbed in slightly larger quantities than Ba(2+), with significant numbers of ions fixed on the outermost surface of the clay. Depletion of the ions K(+), Mg(2+), and Ca(2+) from the clay lattice was observed to accompany enrichment with Co(2+) and Ba(2+) ions. The data obtained using X-ray powder diffraction (XRPD) and scanning electron microscopy (SEM) indicated insignificant structural and morphological changes in the lattice of the clay upon sorption of both Ba(2+) and Co(2+) ions. Analysis using energy dispersive X-ray spectroscopy (EDS) showed that the average atomic percentage (+/-S.D.) of Ba and Co on kaolinite surface were 0.49 +/- 0.11 and 0.61 +/- 0.19 , respectively, indicating a limited uptake capacity of natural kaolinite for both ions.

Depth profile analysis of chemically amplified resist by using TOF-SIMS with gradient shaving preparations

The gradient shaving preparation was recently introduced as a new pretreatment technique for depth profiling. We can obtain a chemically undamaged slope with a length of several hundred micrometers by shaving linearly for a thickness ranging from a few nanometers to a few hundred nanometers. To analyze the chemical structure in organic compounds we performed depth profiles by doing line analysis along the sloped surface of various probes. In this work, the gradient shaving preparation was applied to analyze a chemically amplified photoresist which contains small amount of photoacid generator (PAG) and other additives by TOF-SIMS. The characteristic distribution of F− and CF3SO3− fragments originated from PAG was determined. It was confirmed that the gradient shaving preparation allows TOF-SIMS depth profiling of small amount of organic materials in polymer films within the static limit.

Ti diffusion in chalcogenides: a ToF-SIMS depth profile characterization approach

Ge2Sb2Te5 (GST) is one of the most promising chalcogenide materials for phase-change memory devices, which are potential candidates for the future generation of non-volatile memories. The goal of this work was to investigate the inter-diffusion properties of the Ti–GST system upon annealing, by means of ToF-SIMS depth profile characterization. A double stack (Ti–GST–Ti) and a single stack (Ti–GST) have been considered, both as virgin samples (as-deposited) and post-annealed at 673 K for 10 s in pure N2 atmosphere. Two basic results have been found: (a) the annealed sample looked clearly affected by Ti diffusion throughout the whole GST layer; (b) the initial GST layer composition was dramatically altered as Te diffused as well as reacted with Ti, forming a Ti–Te compound (most likely as Te2Ti). As a consequence, pronounced inter-diffusion of Ti and Te has been observed, resulting in a substantial reduction of the Te content of the original GST layer.

ToF-SIMS depth profiling of Hf and Al composition variations in ultrathin mixed HfO 2/Al 2O 3 oxides

Ultrathin (a few nm) mixed HfO 2/Al 2O 3 oxides deposited by ALCVD on Si substrates were analyzed with low energy (250 eV) Cs + and Xe + ions by ToF-SIMS in the dual beam mode. The analysis beam was 15 keV Ga +. Several layers were measured, with different metal ratios (Hf:Al) and different number of deposition cycles. We observed a significant enhancement of the Al (and AlO x clusters) signal at the surface along with a Hf (and HfO x clusters) depletion at the film surface. However, the thinnest films were found to contain much more Hf than the thickest ones. Several ion ratios were used (e.g. HfO +/AlO + or HfO 2 −/AlO 2 −) in order to quantify the Hf:Al concentration ratio in the films. We found that these ratios correlate well with each other, with the metal deposition ratio, and with XPS ratios, enabling us to reconstruct the Al and Hf composition variation in the film.

TOF-SIMS study of modified polymer surfaces

Amongst various surface modification techniques, plasma source ion implantation (PSII) has been used to study the effect of modification on polymer surface properties. Polyimide foils were exposed in an inductively coupled rf (13.56 MHz) plasma system while applying a negative high voltage pulse to the sample stage. The influence of low-pressure plasmas of oxygen, nitrogen, or inert gases on the chemical composition, topography, and electrical properties of polyimide surfaces was studied in detail. The ion energy was varied from 10 to 50 keV for the PSII experiments. Depending on the ion energy, ion dose, and ion species, the surface resistivity of the film was reduced by several orders of magnitude. Ion bombarded layers were characterized by TOF-SIMS, SEM, and AFM. TOF-SIMS spectra of the modified polyimides showed an enhancement of the low mass fragments and carbon species. Electrical property improvement, as measured by high resistance electrometer, was linked to the chemical changes occurring at the polyimide surface, as identified by TOF-SIMS depth profile.

Lateral and Vertical Quantification of Spin-Coated Polymer Films on Silicon by TOF-SIMS, XPS, and AFM

PMMA and PVC were spin coated on silicon wafers using a range of polymer concentrations. This resulted in a corresponding range of film thicknesses and, for ultrathin film layers, various degrees of surface coverage. The thickness was determined by AFM profilometry and was used to correlate the time scale of a dynamic TOF-SIMS depth profile analysis to a depth scale. The degree of surface coverage was determined from AFM imaging and was used to correlate polymer secondary-ion yields of a static TOF-SIMS analysis to the degree of surface coverage. TOF-SIMS imaging was found to be inferior to AFM imaging with regard to determining the degree of surface coverage of polymers on silicon surfaces. PVC and PMMA both form separated domains on silicon when using spin-coating solutions with a polymer concentration below a material-dependent threshold. In the intermediate concentration range between domain formation and complete surface coverage, networks form. At low concentrations, the deposited polymer material was heterogeneously distributed in the plane of the film as well as vertically to the substrate, resulting in relatively thick hillocks distributed over a limited surface area. At higher concentrations, where network formation is observed, the polymer is heterogeneously distributed in the plane of the film with a large degree of surface coverage, and the average small thickness of the polymer network is fairly constant. A linear correlation was observed between the secondary-ion yield and the degree of surface coverage for both PVC and PMMA, which constitutes the calibration curves for the lateral quantification.

Low energy dual beam depth profiling: influence of sputter and analysis beam parameters on profile performance using TOF-SIMS

In the dual beam mode for TOF-SIMS depth profiling, the high energy analysis beam performs the analysis in the center of the sputter crater. Usually, the depth resolution is determined only by the parameters of the low energy sputter beam eroding the sample given that its sputter rate—and therefore, its contribution to the atomic mixing—is large compared to the sputter rate of the analysis beam. Typically, in shallow depth profiling the ratio of the sputter rates R is larger than 100. In microarea depth profiling however, the area to be analyzed is comparatively small. If the analysis current is kept high in order to get a good dynamic range, the sputter rate of the analysis beam is increased and its contribution to the atomic mixing becomes significant. We have investigated the dependence of the depth resolution on the sputter rate ratio for different sputter and analysis beam energies, using delta layer samples of B and Ge in Si. From these results, the value of R, below which the mixing of the analysis beam can degrade the depth resolution, can be estimated. On the other hand, the amount of additional mixing in applications that require low values of R like 3D microanalysis can be derived as well.

XPS and TOFSIMS studies of shallow Si/Si 1− xGe x/Si layers

We present here a combined study of XPS and TOFSIMS on the oxidation of Si/Si 1− x Ge x /Si layers with x = 0.2 and top Si thickness from 0.5 to 3 nm. The oxidation conditions correspond to the formation of a 2, 4 or 6 nm thick SiO 2 layer. XPS shows that, the oxidation of Ge does not occur and that the actual oxide thickness measured from the Si2p peak is in very good agreement with the expected thickness except for the thinnest top layer. The samples with the thinnest top layer show a fully oxidized cap layer and the oxidation continued through the SiGe layer to produce an oxidized layer. These results were largely confirmed by TOFSIMS depth profile measurements with a dual beam system (IONTOFIV) which show no differences comparing the four thickest cap layers. The samples with the initially very thin Si top layers show a much thicker oxide layer estimated to ~ 20 nm, and Ge segregation is also observed.