Phase formation was studied in the Fe–Ge and Cr–Ge thin-film systems by means of Rutherford backscattering spectrometry and x-ray diffraction. In the Fe–Ge system, FeGe was the first phase to form while in the Cr–Ge system, Cr_11Ge_8 was found to form first. The results are compared with the predictions of the effective heat of formation model. Heats of formation were calculated using the Miedema model. The effect of the transformation enthalpy term Δ H ^tr, used to convert a semiconducting element into a hypothetical metallic one in the Miedema model, is also discussed.

Thermal stress can be engineered in a thin film system by depositing a film with a different thermal expansion to that of the substrate on the back of the substrate. In this work the effect of stress on TiSi2 formation was studied by depositing Si3N4 or SiO2 layers of different thickness on the backside of Si(100) wafers. One set of samples had 400 nm SiO2 on the back of the wafer and the other sets had 200 run and 400 nm Si3N4, respectively. Rutherford Backscattering Spectrometry (RBS) and X-ray diffraction (XRD) measurements showed that the reaction was faster in samples with Si3N4 on the back of the wafer. The TiSi2 formation kinetics was found to be linear with the cube root of the annealing time. The formation kinetics had an Arrhenius dependence on temperature. The apparent activation energy for all three types of samples was the same within experimental error. The growth rate of TiSi2 on wafers with Si3N4 layers on the back was between 3 (600degreesC) and 5 times (490degreesC) faster than that on wafers with SiO2 deposited on the backside.

Certain silicide phases are not very suitable as interconnects or contacts, since they normally grow non-uniformly with rough interfaces. In certain cases this limitation can be surmounted when the normal growth mechanism is altered by the use of a particular diffusion barrier layer. It is the purpose of this paper to show how an understanding of the basic silicide growth mechanisms together with the effect of the diffusion barrier on them, would extend the range of suitable silicides available for device fabrication.

In this study a nuclear microprobe (NMP) was used to analyse phase formation during reaction in Pt–Al lateral diffusion couples. Phase identification was done by Rutherford backscattering spectroscopy. These results were compared with phase formation during conventional thin film Pt–Al interactions. The co-existence of multiple phases in lateral diffusion couples is discussed with reference to the effective heat of formation (EHF) model.

The applicability of in situ, real-time RBS is demonstrated by characterizing the growth of thin Pd2Si films on Si 〈111〉 substrates using isothermal as well as non-isothermal annealing. In contrast to the currently fashionable in situ ramped resistance technique, it is possible to extract the activation energy from a single run with a constant heating rate. The results, which are in excellent agreement with the literature, will be compared for isothermal annealing, fitting an appropriate model for the growth process to data from a single run and a Kissinger-like analysis with different ramp rates. In situ, real-time RBS was also used to study marker motion during CrSi2 formation in the Si 〈100〉 /Pd2Si/Cr system. It is possible to distinguish between the following mechanisms: (1) CrSi2 formation via dissociation of the Pd2Si at the Pd2Si/Cr interface and subsequent reaction of Pd to form Pd2Si at the Si/Pd2Si interface and (2) CrSi2 formation by diffusion of Si from the substrate through the Pd2Si layer.

It is proposed that direct formation of epitaxial CoSi2 and NiSi2 as the first phase, is due to the interlayer between the metal and silicon acting as a diffusion barrier, which decreases the metal concentration at the growth interface. Such concentration controlled phase selection (CCPS) is explained thermodynamically by utilizing the effective heat of formation (EHF) model. This approach is also used to explain silicide formation with metal alloys. Concentration controlled phase selection (CCPS) is not only applicable to silicide formation but should in general enable materials scientists to form phases with desirable properties, by controlling the concentrations of the reactants at the growth interface. (C) 1997 American Institute of Physics.

A Nuclear Microprobe (NMP) was used to study void formation in thin film gold-aluminium systems. Microprobe Rutherford Backscattering Spectrometry (μRBS) was utilised to effectively obtain a three-dimensional picture of the void structure on the scale of a few nanometers in the depth dimension and a few microns in the in-plane dimension. This study illustrates the usefulness of the NMP in the study of materials and specifically thin-film structures.

The use of hydrogen as passivator in silicon solar cells is well known. The function of hydrogen is to occupy the dangling bonds in silicon that occur at defects, such its dislocations and grain boundaries. In this study we used the micro-ERDA (Elastic Recoil Detection Analysis) technique to determine the lateral and depth distribution of hydrogen in poly-crystalline silicon solar cell material. To discriminate against atmospheric hydrogen contamination, the solar cells were manufactured with deuterium as passivation. The ability of ERDA, using He-4 as primary ions, to discriminate between hydrogen and deuterium signals enabled us to study the location of both passivation deuterium and atmospheric hydrogen. Within the spatial resolution of the technique, the distribution of deuterium was found not to peak at the grain boundaries, but to be homogeneously distributed in the grains, although some grains did tend to have higher concentrations of deuterium than others.

An alloy consisting of a eutectic mix of aluminum and silicon to which traces of Sr and Mg have been added forms an important product of a major aluminum smelter (ALUSAF). Casts of this alloy, when cut and polished, display plate-like structures when viewed through an optical microscope. These structures are bound to have an effect on the physical properties of the alloy, for which strength is important. In this work we have used a nuclear microprobe to map areas of interest and determine the composition of the structures observed, using proton and alpha beams for proton-induced X-ray emission (PIXE) and Rutherford backscattering (RBS) analysis. The platelets were proved to be formed by the trace element strontium, a fact which has important metallurgical implications.

The only two models which make definite and unambiguous predictions regarding first phase formation during solid-state interaction are the Walser-Bené and effective heat of formation (EHF) models. Although these models appear to have certain features in common, such as models appear to have certain features in common, such as the choice of the liquidus minimum composition as a measure of atomic mixing at the interface, a careful analysis and comparison reveals fundamental differences between them, mainly owing to the fact that the EHF model also makes use of heats of formation. The EHF model in effect gives a quantitative reason for the relatively large margin of success of the Walser-Bené rules. A major feature of the EHF model is that it often predicts that there is not much to choose thermodynamically between the formation of several phases, whereas the Walser-Bené rules can only predict a single phase. In 53 out of 84 binary systems investigated it has been found that there is complete agreement between the predictions of the two models, whereas for 14 systems the Walser-Bené and EHF model predict the same first phase, the EHF model also predicts other phases, which usually are confirmed by experimental measurements. In seven binary systems, namely Al-Au, Al-Fe, Al-Gd, Au-Cd, Au-V, Au-Zn and Pt-Ti, the EHF model correctly predicts the first phase while Walser-Bené does not. There are also 10 systems where both models fail to predict the correct phase mainly because the liquidus minimum of the binary system is not well defined or the thermodynamically favoured phase has problems in nucleating.

The effective heat of formation (EHF) model is used to predict compound phase formation and stability in metal-silicon systems. The model defines an effective heat of formation Delta H', which is concentration dependent and shows a linear dependence on the concentration of the limiting element at the growth interface. For instance if CrSi2 (Cr0.33Si0.67) formation is considered and the effective concentration of the interacting atomic species is 20 at.% Cr and 80 at.% Si, then Cr is the limiting species. The effective concentration at an interface is taken to be the composition of the liquidus minimum of the binary system, as the greatest mobility of the atoms at an interface and therefore the most effective mixing, is expected to take place at this composition. It is also shown how the EHF model can be used to predict the phase formation sequence in thin him systems and to explain the influence of impurities and diffusion barriers on compound formation. The anomalous and somewhat contradicting results for initial formation of titanium silicide phases can also be evaluated by using the EHF model.

A nuclear microprobe has recently been commissioned at the Van de Graaff group of the National Accelerator Centre (NAC) at Faure, South Africa. The data acquisition system has been developed and adapted locally to meet specific local requirements, and is well suited to the application of three dimensional RBS. Due to the design of the software, a specimen can be efficiently analyzed in a minimum of beam time, thus optimizing facility utilization and minimizing beam damage. This paper will describe the application of the data acquisition system to three dimensional RBS, and the implementation of the technique will be illustrated with reference to a silicon integrated circuit.

In this investigation buried Sb dopant profiles in single crystal silicon have been formed from evaporated layers using laser annealing. For irradiations carried out in air, severe oxidation of the surface layers inhibited epitaxy. Oxygen concentrations as high as 5 × 1017 atoms/cm2 (equivalent to about 105 nm SiO2) were measured. It was found that both the thin (less than 3 nm) Sb layer and the free volume in the a-Si, deposited by evaporation onto a cold substrate, need to be present for this degree of oxidation to take place. However, when silicon was evaporated onto a substrate heated to 350 °C, dense packing of the silicon atoms was obtained and even for irradiations in air good epitaxy (minimum yield of 7%) and no oxidation occurred. To form buried Sb profiles, laser energies only slightly higher than the threshold for epitaxy were used to prevent excessive spreading due to an increase in liquid state diffusion obtained at higher energies. Under these conditions the width of the buried Sb profile was typically about 120 nm, and up to 90% of the Sb atoms were found to occupy lattice sites.

First phase formation has been determined in Cu binary thin film systems with Ti, Zr, Mg, Sb, Pd, and Pt using transmission electron microscopy and Rutherford backscattering spectrometry. CuTi, CuZr, CuMg2, Cu2Sb, Cu3Pd, and Cu3Pt are the first phases to form upon annealing the Cu/metal bilayers. The effective heat of formation model is used to predict first phase formation in 14 Cu/metal systems.

The effective heat of formation model enables heats of formation to be calculated as a function of concentration. By choosing the effective concentration at the growth interface to be that of the liquidus minimum, the model correctly predicL. first phase formation for 14 binary systems for which experimental data was found, except for the Au-Cu system which does not have a well-defined minimum on the liquidus curve.

The anodic oxidation of the silicides CoSi, CoSi 2 , CrSi 2 , Ni 2 Si, NiSi, NiSi 2 , Pd 2 Si, PtSi, TiSi 2 and ZrSi 2 was studied by using Rutherford backscattering of 2MeV alpha particles. The room temperature oxidation was carried out at a constant current density of 8.9 mA cm −2 using n -methylacetamide (2% H 2 0 and 1% KNO 3 ) as electrolyte. No oxidation of Pd 2 Si and PtSi was detected. Pure SiO 2 layers were grown on CoSi, CoSi 2 , Ni 2 Si, NiSi and NiSi 2 at a much lower rate than on Si〈100〉 and with a thickness increase per volt of 0.6 ± 0.03 nm V −1 . Mixed layers of SiO 2 /metal oxide were grown on CrSi 2 , TiSi 2 and CrSi 2 . All oxidations occurred at the expense of the silicide layer. It is also shown how the purity of the SiO 2 layer formed can be predicted from thermodynamic considerations.