Depth profiling quantification

Destructive Chemical bonding Depth profiling Quantification Accuracy Detection limits Sampling depth Lateral resolution Imaging/mapping... 

Quantification Detection limits Depth profiling Penetration depth Depth resolution Lateral resolution... 

The most common application of dynamic SIMS is depth profiling elemental dopants and contaminants in materials at trace levels in areas as small as 10 pm in diameter. SIMS provides little or no chemical or molecular information because of the violent sputtering process. SIMS provides a measurement of the elemental impurity as a function of depth with detection limits in the ppm—ppt range. Quantification requires the use of standards and is complicated by changes in the chemistry of the sample in surface and interface regions (matrix efiects). Therefore, SIMS is almost never used to quantitadvely analyze materials for which standards have not been carefiilly prepared. The depth resoludon of SIMS is typically between 20 A and 300 A, and depends upon the analytical conditions and the sample type. SIMS is also used to measure bulk impurities (no depth resoludon) in a variety of materials with detection limits in the ppb-ppt range. 

The quantification algorithm most commonly used in dc GD-OES depth profiling is based on the concept of emission yield [4.184], Ri] , according to the observation that the emitted light per sputtered mass unit (i. e. emission yield) is an almost matrix-independent constant for each element, if the source is operated under constant excitation conditions. In this approach the observed line intensity, /ijt, is described by the concentration, Ci, of element, i, in the sample, j, and by the sputtering rate g, ... 

In contrast with the dc source, more variables are needed to describe the rf source, and most of these cannot be measured as accurately as necessary for analytical application. It has, however, been demonstrated that the concept of matrix-independent emission yields can continue to be used for quantitative depth-profile analysis with rf GD-OES, if the measurements are performed at constant discharge current and voltage and proper correction for variation of these two conditions are included in the quantification algorithm [4.186]. 

McPhail (1989) gives a detailed account of the experimental approach to depth profiling of semiconductors, including the quantification of the data. He illustrates the analysis of a silicon epilayer grown by molecular beam epitaxy (MBE) in which 11 boron-rich layers were incorporated by co-evaporation of boron. The intended structure is shown in Figure 4.8, and it was desirable to establish the concentration of boron in the layers, the inter-peak concentration and the sharpness of the doping transitions. 

The elemental composition, oxidation state, and coordination environment of species on surfaces can be determined by X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) techniques. Both techniques have a penetration depth of 5-20 atomic layers. Especially XPS is commonly used in characterization of electrocatalysts. One common example is the identification and quantification of surface functional groups such as nitrogen species found on carbon-based catalysts.26-29 Secondary Ion Mass spectrometry (SIMS) and Ion Scattering Spectroscopy are alternatives which are more surface sensitive. They can provide information about the surface composition as well as the chemical bonding information from molecular clusters and have been used in characterization of cathode electrodes.30,31 They can also be used for depth profiling purposes. The quantification of the information, however, is rather difficult.32... 

Instruments based on GD-MS coupling have been employed most commonly for the quantitative analysis of trace and ultratrace amounts in high-purity materials. However, it has been demonstrated that, as in GD-OES, quantitative depth profile analysis by GD-MS is possible [33]. At present, a GD-MS prototype which allows the depth quantification of thin layers on conducting or insulating materials is being developed for commercial purposes [34]. 

As a further chemical tracer method enabling ppm-resolution we have employed gas chromatography (GC). In combination with mass spectrometry this allows for material selective quantification of chemical impurities. The GC technique is based on the retention time of a given speeies on a functionalised column and varies with the respective molecular affinity to the column surfactants. By subsequent cycles of solvation and rinsing it is possible to obtain a depth profile of the ehemical impurities contained in the crystalline host [10]. The main requirements imposed by gas chromatography arc a sufficient solubility and separability of the analysed molecular species. For example, for tet-racene to be solvable to an adequate amount, saturation in toluene is reached at 150 ng/1 pL [31]. 

The primary result of an AES depth profile analysis is an Auger transition intensity versus sputter time presentation. Quantification leading to atomic concentrations is possible when RSFs or reference materials are available. A measurement of sputter rates which depend on the sputtered material enables... 

The following sections will deal first with the physical effects involved in postionization, then with the respective instrumentation, and finally with the quantification schemes in surface and depth profile analysis with SNMS. A critical comparison of the different SNMS techniques will be addressed where appropriate. 

SIMS refers to the mass spectrometry of ionized particles (secondary ions) emitted by a beam of primary ions bombarding a surface. SIMS provides a characterization of a target surface by means of mass spectra, depth profiles, and secondary ion images. Surface mass spectra allow the identification and quantification of all constituent elements, isotopes, and molecular species... 

Barozzi, M., Giubertoni, D., Anderle, M., Bersani, M. (2004) Arsenic shallow depth profiling accurate quantification in Si02/Si stack. Applied Surface Science, 231-232,632-635. 

RBS is a nondestructive nuclear method for characterizing thin films by analyzing the energy of backscattered ions such as H+ or He from the surface (Perriere, 1987 Kimura, 2006). RBS allows quantification of the surface composition without the need for reference standards and it provides depth profiling of individual elements (Haireche et al., 2013 Jeynes et al., 2012). 

An immediate distinction needs to be made between dynamic and static SIMS experiments. In the former, a primary ion current density of typically > 10 pA cm is used and this leads to rapid sputtering of material. The surface is eroded at a rate of order nm s and by following the intensity of chosen peaks in the mass spectrum as a function of time, a concentration depth profile can be constructed. In this mode SIMS can be very sensitive, with trace element detection limits in the ppm-ppb range. However, quantification is not straightforward. Secondary ion intensities are strongly matrix-dependent and extensive calibration procedures involving closely related standards of known composition and under identical experimental conditions must be used to extract quantitative concentrations. 

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