SIMS Depth Profiling Research Articles

Electrochemical and SIMS studies of cathodically formed hydride layers on titanium

The aims of the present work are to study the influence of the hydride layer (the formation of which during cathodic polarization has been known for quite a long time) on the rate of HER, to explore the applicability of SIMS to detect the presence of hydride and to determine the thickness of such layers. The present measurements show that the formation of hydride suppresses the HER, and that the suppression becomes constant as the hydride layer forms a continuous layer. This is not compatible with the previous literature where the formation of hydride has been reported to stimulate the HER. The present measurements show that the SIMS technique is well suited to detection of absorbed hydrogen in titanium. However, the present effort to use SIMS depth profile measurements to determine the thickness of the hydride layer was unsuccessful. The H − signal shows a smooth decrease from which it is impossible to determine the film thickness. Possible causes for this smooth decrease have been discussed.

Comparative XPS and SIMS depth profile analysis of Cu-implanted silicon: evidence of segregation effects

Copper concentration profiles in copper-implanted silicon have been analysed comparatively by XPS and SIMS techniques. The results are compared to theoretically computed concentration profiles, obtained by the TRIM procedure. Differences between experimental profiles and computed ones may be explained in terms of ion beam-induced segregation. The effect is strongly dependent on the sputtering rate: a shift and broadening of the concentration peak is induced in the XPS analysis, under relatively slow sputtering.

The ageing of Li implants in cadmium mercury telluride

AbstractSecondary ion mass spectrometry (SIMS) requires the preparation and measurement of standard samples in order to be considered a quantitative technique. This normally can be achieved by the production of ion‐implanted specimens in the matrix to be studied. The measured SIMS depth profile then can be integrated and compared with the known implanted dose in order to produce quantitative results. This paper reports a potential source of error in this method, discovered by studying the ageing of a lithium‐implanted cadmium mercury telluride (CMT) specimen over a six‐year period. Lithium has been shown to be a mobile impurity in CMT.It is shown that the original implant produced a satisfactory profile shape, but that room‐temperature diffusion subsequently degraded its shape over a period of five years. This finding is confirmed by the further action of annealing the implanted specimen at 250°C and studying the resulting profile. The change in the shape of the implant profile is discussed in relation to diffusion theory. SIMS images are shown to demonstrate the spatial inhomogeneity of the sample.

Theoretical and experimental studies of the broadening of dilute delta‐doped Si spikes in GaAs during SIMS depth profiling

AbstractDelta‐doped structures represent a powerful class of test structures to investigate the experimental and fundamental factors limiting the depth resolution obtainable in SIMS sputter depth profiling.In this work, theoretical studies of the effects on the broadening of an Si delta spike in GaAs as a function of the energy (1.4–4.4 keV) and angle of incidence (2°, 45° and 60° off‐normal) of the O2 sputter probe beam have been compared with recent experimental data. The theoretical calculations were carried out using the newly developed IMPETUS computer code, which simulates the depth profiling process by taking into account the combined effects of ballistic mixing (treating collisional mixing as a diffusion process), projectile incorporation into the matrix and sputtering. All of these are processes that always occur in any practical sputter depth profiling situation.The IMPETUS model can reproduce low‐energy Si depth profiles with great accuracy by using the well‐established TRIM calculated range, energy deposition and sputtering data and by making reasonable assumptions for the threshold energy for diffusion in addition to assuming a beam‐ and sputter statistics‐induced surface microtopography, which is described by a Gaussian area versus height distribution having a standard deviation σ = 0.8 nm. Significantly, it is shown that the effects of these parameters on the shape of the sputter profile are largely independent, with σ (accounting for microroughness) mainly affecting the leading edge and the threshold energy (determining mixing processes) the trailing edge of the sputter profile. Good agreement on the energy dependence of the broadening is also obtained. The expected improvement in depth resolution with increasing off‐normal bombardment angle is confirmed and can be quantified. The error in the experimental depth scale calibration based on a constant sputter rate, ignoring transient sputtering, is evaluated. Finally, the sputter depth profile observed for an Si delta spike in GaAs subjected to thermal annealing during growth by molecular beam epitaxy (MBE) can be reproduced accurately by considering diffusion broadening of the initial spike followed by a sputter profiling simulation.

VAMAS surface chemical analysis technical working party—an overview of project objectives, progress and the requirements for further work

AbstractAn update for 1989 is given for the VAMAS Surface Chemical Analysis Technical Working Party. The overall programme now has 26 projects targeted to solve key problems in the enabling base of AES, XPS, SIMS, SNMS and sputter depth profiling. Increased coordination of projects is leading to stronger project databases. Eleven of the projects have involved round robins, and details are provided of results where the analysis is complete and of project leaders where new participants are invited.

Optimization of the dynamic range of SIMS depth profiles by sample preparation

AbstractThe dynamic range is one of the most important figures of merit of SIMS measurements and equipment. The term refers to the concentration range of an analysed element—its concentration being high near the surface but decreasing with increasing depth—that can be measured by depth profiling. The state of the art is such that, under favourable conditions, 5–6 orders of magnitude are possible (measured on ion‐implanted boron in silicon). It is shown that with special sample preparation techniques (removal of the sputter crater environment) the dynamic range can be extended to 7 orders of magnitude. Experimental details of the preparative methods are given together with a critical evaluation of the methods.

Silicidation of Molybdenum and Titanium Double Layers

Silicidation of molybdenum and titanium double layers has been investigated by measuring sheet resistance, SIMS, and Auger depth profiles. Mo/Ti, , Ti/Mo, double layers, and Ti‐Mo cosputtered layer were annealed in Ar or atmosphere with a halogen lamp annealer or diffusion furnace. Listed in increasing order of sheet resistance, they are: Mo/Ti and , , Ti/Mo, Ti‐Mo cosputtered layer. These results were explained by SIMS observations. The variations of the sheet resistance of the annealed Mo/Ti double layers with annealing temperature were divided into three temperature regions. In the low annealing temperature region, sheet resistance of the unreacted Mo layer was dominant. In the intermediate region, sheet resistance of the titanium silicide layer formed by the reaction with Ti and Si substrate was dominant. In the high annealing temperature region, sheet resistance of the molybdenum and titanium silicide mixed layer formed by interdiffusion of Mo and Ti was dominant. The Mo/Ti double layer, annealed at 800°–900°C for 20s had the lowest sheet resistance and good etching resistance towards hydrofluoric acid.

Arsenic decay study in SIMS depth profile under O2+ beam bombardment.

SIMS is widely used for the process control in the field of LSI manufacture. The reason is that SIMS is the only surface sensitive technique to quantify the low concentration of impurities like Arsenic (As), Boron (B), and Phosphorus (P). The concentration of the impurities would typically be lower than ppb (parts par billion).In this paper, Arsenic distribution was measured by O2+ beam bombardment for different primary beam energies, incident angles and raster sizes. The sample where the Arsenic ion was implanted into SiO2 layer was prepared to study the pile-up of Arsenic into the interface of SiOx/Si. The following two items are the results of this study.(1) The arsenic distortion heavily increased with both higher beam energy and lower incident angle.(2) To avoid the As distortion effect, experiments must be done with O2+ primary beam which has the energy lower than 8 keV and incident angle higher than 45°.

Study of the Tnfluence of Structural Properties on Si and Be Doping of Heteroepitaxial InAsSb on GaAs-Coated Si Substrates for Infrared Photodiodes

ABSTRACTThe behavior of Si and Be impurities in InAsx Sb1−x (0.05<x<0.45) heteroepitaxial layers grown on GaAs-coated Si by MBE is characterised. Siis found to act as an n-type dopant for the growth conditions used. The calibration of the Si n-type doping as a function of the Sisource temperature is obtained by infrared absorption spectrometry, stripping Hall and SIMS depth profiling. At high doping levels the amphoterical characterof the Si impurities becomes evident. SIMS is used to investigate the dopant incorporation and the formation of diode structures. The influence of the mismatched epitaxy on the transport properties is investigated by stripping Hall measurements. In order to establish the correlation between crystalline properties of the epilayers and the ability to fabricate diodes, we compare the defect structure of InAs and In(As)Sb p-'i'-n diodes.

Matrix effects in sims depth profiles of SiGe superlattices

Abstract The dependence of the secondary ion yield of Ge and Si atoms in Si1-xGex layers, where x varies from 0.005 to 0.75, has been measured for incident sputtering beams of O+2 and Cs+ of varying energy. For the O+2 beam, the positive ion yield per atom was constant for Ge concentrations 5%, whereas the negative ion yield decreased significantly.

Post‐implantation as an aid in scale calibration for SIMS depth profiling

AbstractIt is shown that low‐dose energetic ion implantation may greatly facilitate relative and even absolute depth‐scale calibration for SIMS depth profiling. Sample implantation has to follow all the processing steps to be investigated. The implantation conditions, such as species, fluence and energy, preferably have to be optimized for the particular problem at hand. Then the distribution profile of the implant is to be treated as a two‐dimensional marker, useful both for depth as well as for concentration calibration. A simple practical example demonstrates the power of the method.

SIMS depth profiling of Si/SiO2/Si layers by using self‐sputtered Ga+ from a gallium liquid metal ion source

AbstractSIMS depth profiles of Si/SiO2/Si systems have been carried out using a Ga+ primary beam tracing the Ga+ yield. This ion is found to be a good marker for thin oxide layers at an Si/Si interface. In the case of thicker (7–85 nm) buried SiO2 layers, various peaks are observed for Ga+ yield. The influence of primary beam energy on the observed profiles has been investigated. A qualitative model is proposed that explains the observed profiles in terms of phenomena, such as radiation‐induced segregation, occurring in the same depth range of the cascade collision.

Objective comparison of concentration profiles in solids by means of mathematical statistics

To compare concentration profiles in solids in an objective manner, a multi-dimensional generalization of the comparison of two means by Student'st-test is proposed. The approach is based on piecewise modelling the profiles using rougha priori information, estimating the model parameters and their covariance matrix and comparing the estimated parameters by use of multidimensional test variables. The piecewise modelling is treated in the most important cases in such a way that the partial functions are straight lines or cubic polynoms (splines) with continuity at all knots in each case. It enables us to compare whole profiles as well as the most relevant parts of them. Two ways of estimating the covariance matrix as a generalization of the variance estimation are discussed. The approach is useful for any kind of line profiles if rough information on the profile type is available. As an example SIMS depth-profiles of aluminum obtained from thin-layer systems after different technological heat treatments are considered. These profiles are modeled by three straight lines.

Optimized ion beam raster for SIMS sputter depth profiling

AbstractA computer‐driven ion beam deflection system is described. This system allows custom tailoring of the ion beam raster to optimize SIMS depth profile performance. The ion beam is deflected to produce an approximately elliptical crater, which better matches the acceptance geometry of the ion extraction optics. A crater with a larger and more evenly flat bottom is produced by varying the resident time of the ion beam within the raster area, thus allowing for the collection of more signal with better depth resolution. This is accomplished by creating an array of equally spaced sputter points on the sample, taking into account the orientation of the ion gun and the SIMS optics with respect to the sample normal. The resident time of the ion beam along the crater perimeter is extended and the raster and mass spectrometer are synchronized. The improvement in performance is shown using a strain layer superlattice test sample 720 nm thick, consisting of 40 double layers of Si/Si–30% Ge.

Explanation of pronounced impurity migration in SIMS depth profiling

Explanation of pronounced impurity migration in SIMS depth profiling

Surface Protection during Plasma Hydrogenation for Acceptor Passivation in InP

AbstractVarious dielectric and metallic films were examined as H-permeable surface protection layers on InP during H2 or D2 plasma exposure for passivation of acceptors in the InP. Plasma deposited SiNx, SiO2, and a-Si(H) films ranging in thickness from 85 to 225 Å were used to protect p-InP during D2 plasma exposure at 250°C. Optimum protective layer thicknesses were determined by a trade-off between the effectiveness of the layer to prevent P loss from the wafer surface and the ability to diffuse atomic H or D at a rate greater than or equal to that in the underlying InP. SIMS and capacitance-voltage depth profiling were used to determine the extent of D in-diffusion and acceptor passivation respectively. Sputter deposited W and e-beam evaporated Ti films ~100 Å thick were also evaluated. The W coated sample yielded similar results to those with dielectric films in that acceptors in p-InP were passivated to a similar depth for the same plasma exposure. The 100 Å Ti film, however, did not allow the D to diffuse into the InP substrate. It is surmised that the Ti film trapped the D, thus preventing diffusion into the substrate.

Depth profiles of Ta2O5/SiO2/Si structures: a combined X-ray photoemission, Auger electron, and secondary ion mass spectroscopic study

We prepared thin films of tantalum oxide on SiO2/Si substrates by thermal oxidation of tantalum. The different oxide layers and their interfaces were characterized by SIMS, AES, and XPS. Characteristic structures were obtained after different oxidation procedures. The comparative discussion of AES and SIMS depth profiles makes possible an unequivocal characterization of the reactive interfaces between the oxides of Ta and Si. The Ta2O5/SiO2 interface in particular shows non-stoichiometries which depend on the oxidation procedures and which determine the performance characteristics of pH-sensitive Ta2O5 field-effect transistors.

Characteristics of ion beam mixed and alloyed AuGe/Ni ohmic contacts to n-GaAs

The morphology of contacts which have been subjected to an ion implantation step has shown great improvement as compared to that of contacts prepared without this step. We have performed SIMS and AES studies to reveal the properties of top and underlying layers. From the SIMS and AES depth profiles it follows that a nickel/germanium/arsenic mixed region has formed, which is spread uniformly throughout the metallization layers, in contrast to NiGe and NiAs regions formed in the contacts which were annealed only. The lowest specific contact resistance, obtained by using a circular test pattern, was 5 × 10 −6 and 1 × 10 −5 Ω cm 2 for the unimplanted and implanted contacts, respectively.

Quantification of sputter depth profiles by means of wedge crater sputtering—a new technique for depth scale calibration

AbstractAn essential part of depth profile quantification is the conversion of sputtering time into depth, for which a simple method is described. It requires an ion gun providing a sufficient fine focusable ion beam. A suitable beam control produces a wedgelike distribution of ion current density in a raster field which is large compared to the ion beam cross‐section. A “wedge crater” is sputtered in this way in addition to the “normal crater” for depth profiling. The wedge crater profile obtained of surface profilometer measurements provides the sputtering time–depth relation. Moreover the sputtering rate as a function of depth may be deduced from wedge crater profile. Application of the method to SIMS depth profiling in multilayer thin film systems shows its usefulness in depth scale calibration and intensity correction. Small expenditure and universal applicability are advantages in comparison with other methods.

XPS and SIMS characterization of metal oxide/amorphous silicon—carbon interfaces

AbstractFor different a‐SiC:H/metal oxide (TCO) layers—used for thin film solar cells—migration of metal atoms into the amorphous film, SiOx formation at the interface and reduction of TCO were investigated by SIMS and XPS depth profiling. Zinc oxide seems to be most suitable as a TCO and blocking layer, as the SiOx formation which limits transport of charge carriers is rather low compared to that from SnO2 contacts, and as no Zn is detected in a‐SiC:H in contrast to SnO2 and indium tin oxide ITO with 0.3% Sn and 20 ppm In, the latter acting as counterdopand in p+ a‐SiC:H layers. From correlations between SI yields and elemental concentrations determined by XPS, the quantification of different elements, as well as silicon oxide and metal migration can be deduced from SI intensities. In particular Si+ enhancement is a sensitive indicator for SiOx formation. By these correlations SIMS depth profiling enables fast, quantitative and highly sensitive investigations of numerous samples for optimization of preparation conditions. This is demonstrated in detail for a‐SiC:H/ZnO.