8 Auger Electron Microscopy-2

Dr. Ajit K. Mahapatro

Learning Objectives

From this module students may get to know about the following

i. Introduction.

ii. Peak identification, chemical shift and quantitative analysis of AES.

iii. Depth profile of AES spectrum.

1. Introduction

The secondary electron emitted due to the incident electron beam produces a vacancy in the lower energy level (usually K shell). This vacancy can be filled by an electron from an outer shell with higher energy and there appears two possibilities when an electron with the higher energy fills this vacancy. First, it can emit the X rays through radioactive process, or, second, it can emit an electron through a non-radioactive process, generally termed as “Auger electron.” The Auger electron is produced by the knocking out of an electron in higher energy level by radiating energy when the vacant space is filled by an electron with higher energy.

Auger spectra is expressed in two different modes, direct mode and differential mode. A typical direct mode AES spectrum is a plot of intensity versus kinetic energy. The signals from Auger electrons are relatively weak compared the secondary electrons escaped from a solid surface, resulting the Auger peaks appear small in the background of direct mode spectrum. Generally, differential mode spectrum produces the first derivative of intensity using computer software and plotted versus kinetic energy. The differential mode spectrum is effectively reduce the background noise and enhances the signal to noise ratio resulting distinct Auger peaks. The AES is a powerful tool for surface chemical analysis. It can identify the chemical elements in the specimen layer within a range of several nanometers from the surface. Importantly, the peak positions of elements formed in XPS and AES spectra are sensitive to their chemical nature. For example, the positions of carbon peaks in carbon dioxide (CO2) are different from the appearance of peaks in the saturated hydrocarbons. This phenomenon is referred as the chemical shift AES spectra and provides extra information for the chemical analysis. The AES instrument is capable of generating depth profiles of the surface layer going towards the inner layers of bulk materials and reveal the spatial distributions of elements on the surface of bulk materials.

2. Spectrum Analysis

The concentric hemispherical analyzer (CHA) of the AES has a resolution for energy (∆E/E) over the whole range of the spectrum. The direct mode spectrum is expressed the energy distribution as the number of electrons times its kinetic energy [EN(E)]. The differential mode spectrum is represented by EdN(E) as the axis parameter. The Auger energy in the differential mode is represented by a sharp peak/dip position corresponding to an Auger signal. Figure 1 shows the energy of carbon KLL line in the differential mode is 272 eV and the equivalent carbon KLL peak energy in the direct mode is positioned in higher side because the exact peak energy is the differentiated curve passing the zero in the axis. This difference is ignored during analysis for elemental identification [1-2].

Figure 1: Principal Auger KLL peals of light elements, Be, B, C, N, O, and Na KL₂₃L₂₃ is the most visible KLL peak 

Qualitative analysis of the AES includes identifying elements from the location of the peaks in the energy spectrum. The light elements are identified from the dominated KLL Auger lines in the spectrum range. For elements of atomic number higher than 15, either LMM or MNN Auger lines dominant the spectrum. The principal Auger electron energies for different elements are represented in Figure 2. The dotted red lines indicate stronger and most characteristic peaks. The LMM lines are divided into triplets, resulting from the differences in subshells involved in the Auger process.

Figure   2:   Principal   Auger   electron   energies   of   KLL,   LMM,   and   MNN   

 
2.1 Peak identification

Qualitatively, the analysis for identifying the peaks in AES spectra is started by comparing the peaks appear in the experimental measurements with the standard peaks found in reference books (Figure 2) or computer databases. Two different X-ray sources can be used to distinguish between the Auger peaks and XPS peaks. The change in kinetic energy of  an Auger electron with energy of  primary X-rays is represented by the Auger peak. Thus, an Auger peak shifts in apparent binding energy in an XPS spectrum when we change the X-ray source. For example, an Auger peak shifts by 233 eV in the XPS spectrum by changing the radiation from MgKα (1253.6 eV) to AlKα (1486.6 eV).

2.2 Chemical Shifts

Chemical shifts also occur in AES spectra, and the chemical shifts can be significantly larger than the shifts in XPS. For example, the shift between Al oxide and Al metallic peak of AlKL2L3 is greater than 5 eV while the corresponding shift of Al2p binding energy is only about 1 eV. In addition, Auger peak shape may be affected by the chemical state. Also, the position and shape of oxygen KLL Auger peaks in different types of oxides is shown in Figure 3. The OKL₁L₂₃ and OKL₂₃L₂₃ peaks vary in separation by up to 5 eV and the shape of the OKL₂₃L₂₃ peak can be singlet or a doublet type.

Figure 3: Comparison of positions and shapes of O KLL Auger peaks in several solid.

2.3 Issues with insulating materials

In insulating materials, it is difficult to remove the incident electrons on the surface of the materials by pursuing electrical conduction through the material. This causes accumulation electrons and the sample’s surface of the sample becomes negatively charged during the AES. This accumulation of charges change the surface potential and further effects the detection process and the measured kinetic energy or binding energy could change by tens of eV compared to the already published table from database [2].It is difficult to overcome this surface charge problem with insulating samples for AES, as the electrons have to be removed from the insulating surface, rather than compensating for electron loss. AES does not work well with fully insulating materials. Generally, the methods used in AES for reducing the surface charge problems are,

(i) Reducing the primary electron beam energy. For example, the surface potential can be stabilized when the energy of the primary electron beam is reduced, and changes from 10 keV to about 2–5 keV.

(ii) Making use of a conductive coating on the sample’s surface. Coating the surface of the specimen with a thin layer of metal such as silver or gold to increase the conductivity to move the accumulater electronic charges near the area to be examined.

3. Quantitative Analysis

3.1 Peaks and sensitivity factors

Quantitative elemental analysis of electron spectroscopy and X-ray spectroscopy are almost similar to each other. The AES measures the intensities of the peaks and quantifies the concentrations of chemical elements on the surface of the sample. Most of the parameters required for estimation are not available in both the XPS and AES. Quantitatively, the atomic fraction of elements can be estimated theoretically from the intensities of electron signals by following the mathematical expression,

Xi = (I_i/S_i)/ ∑ I_j/S_j                (1)

where, Xi is the atomic fraction of a surface element, Ii is the specific peak intensity to be quantified, and Si and Sj are the sensitivity factors that can be determined experimentally for every element and varies according to the instrument and surface of the speciman. Generally, the sensitivity factors are taken from published handbooks with certain corrections according to the instrument characteristics, as it is not feasible to compile a set of in-house sensitivity factors. The intensities of XPS peaks is evaluated from the peak areas after subtracting the background and the chemical contamination on the surface of the sample affects the accuracy of using Equation (1).

The quantitative estimation using Equation 1is based on few assumptions and approximations. Cautions need to be exercised during calculations to prevent erroneous results. The intensity of AES peaks is considered as the peak-to-peak height in differential spectra. However, the peak-to-peak heights depends on the instrument resolution and the differentiation (modulation) process using computer software. This problem may be resolved by using the AES direct spectrum. The presence of chemical contamination on a surface of the sample affects the accuracy of using Equation 1. For an AES spectrum, the matrix composition affects the efficiency of the Auger emissions because the backscattered electrons in the matrix also excite the Auger electron emission. Here, the intensity of element ‘a’ present among multiple elements differs from the same amount of ‘a’ element in its pure solid form [2, http://www.chem.qmul.ac.uk].

4. Composition Depth Profile

The compositional changes with distance from the surface plane can be characterized with AES. The most common method for measuring a depth profile of composition is through sputter depth profiling using an ion gun. The ion gun can produce a flux of positively charged argon ions with current density and energy in the range of 1–50 μAmm⁻² and 0.5–5 keV, respectively. The atoms are ejected from surface of the sample by bombarding ion beams and by rastering ion beams directed at the surface, and develops crater. A profile of the compositions at the surface and near the surfaces could be known by examining the evolution of the AES and XPS spectra of through the crater at different depths.

For analyzing the depth profile quantitatively, it needs to measure the depth very accurately, in addition to estimate the concentrations. Several factors need to be taken into consideration for analyzing the resolution of depth measurement, including the following two factors.

1. Firstly, the etching rate for every atom is not constant for identical ion beam energy and ion densityies. Thus the preferential etching occurs for samples containing multiple elements. In samples comprising of several elements that are not uniformly distributed, certain areas would be etched faster than others depending on the preferential etching. This results a rough crater surface and increasing depth of etching could produce much crate structure. Here, the depth scale of the profile is based on the average etching rate and the measured concentration of elements with higher etching rates provide higher sputter yield, which would be higher than the exact concentration of the specimen.

2. Secondly, the incident ions can smear the interface during the depth profileing. Figure 4 represents the schematical diagram of the interface smearing during depth profiling. When an ion beam gets bombarded onto the surface of thin film sample with a smooth interface with the substrate, the depth profile of  element A often exhibits diffusion rather than sharp interface profile expected to be for real case.

 

Figure 4: Smearing of the interface with depth.

5.  Summary

• A typical AES spectrum is a plot of intensity versus kinetic energy and it is a plot of the first derivative of intensity versus the kinetic energy.

• The spectrum analysis, peak identification, chemical shift of AES spectrum and problem with insulating samples with schematic diagram has been studied.

• The quantitative analysis of AES and sensitivity calculation has been discussed. This quantifies the concentrations of chemical elements on a sample surface from the peak intensities of the spectra. In theory, the quantitative relationship between the intensities of electron signals and atomic fractions of elements can be calculated.

• AES can analyze the composition changes. The most commonly used method to obtain a depth profile of composition is sputter depth profiling using an ion gun.

References

• T E GALLON and J A D MATTHEW, Physics Department, University of York, Heslington, Auger electron spectroscopy and its application to surface studies.

• Materials Characterization Book, Yang Leng, 2008 John Wiley and Sons (Asia) Pte Ltd.