What Is Auger Electron Spectroscopy?

Auger electron spectroscopy (AES) is an analytical technique for surface science and materials science. So the technology is named mainly by analysis of the Auger effect. This effect occurs when the outer electrons of an excited atom jump to a lower energy level and the energy released is absorbed by other outer electrons to cause the latter to escape from the atom. This series of events is called the Auger effect, and the escaped electrons Called Auger Electronics. In 1953, Auger electron spectroscopy gradually began to be practically used to identify the chemical properties and composition analysis of the sample surface. Its characteristics are that Auger electrons come from the shallow surface and only bring out surface information, and the energy position of its energy spectrum is fixed, which is easy to analyze.

Auger electron spectroscopy (AES) is an analytical technique for surface science and materials science. So the technology is named mainly by analysis of the Auger effect. This effect occurs when the outer electrons of an excited atom jump to a lower energy level and the energy released is absorbed by other outer electrons to cause the latter to escape from the atom. This series of events is called the Auger effect, and the escaped electrons Called Auger Electronics. In 1953, Auger electron spectroscopy gradually began to be practically used to identify the chemical properties and composition analysis of the sample surface. Its characteristics are that the Auger electrons come from the shallow surface and only bring out the surface information, and the energy position of the energy spectrum is fixed, which is easy to analyze.

Chinese name: Auger Electron Spectroscopy
Foreign name: Auger Electron Spectroscopy (AES)

Excitation source: Use high-energy electron beam as excitation source
Fundamental: Physical principle
profession: analytical skills

Auger electron spectrum background history

In the last ten years, solid surface analysis methods have developed rapidly, and it is one of the most active branches in the field of analytical chemistry. Its development is closely related to the urgent need to understand various solid surface phenomena in related fields such as catalytic research, materials science, and microelectronic device development. The establishment of various surface analysis methods has created very favorable conditions for research in these fields.

Among the surface component analysis methods, Auger electron spectroscopy is the most important one in addition to electronic spectroscopy for chemical analysis. It has been widely used in chemistry, physics, semiconductor, electronics, metallurgy and other related research fields.

Although the French Auger first discovered the traces of Auger electrons in the Wilson Cloud Chamber as early as 1925, in 1953 Rand recognized the Auger electron spectrum for the first time from the secondary electron energy distribution curve. The Auger electron line has low intensity, and it is often submerged in the background of inelastically scattered electrons, so it is difficult to detect it. In the late 1960s, because of the use of the differential method of the electron energy distribution function and the electron-optical system using low-energy electron diffraction, a breakthrough was made in instrument technology for detecting Auger electrons. In 1969, the cylindrical electronic energy analyzer was applied to the AES spectrometer, which further improved the speed and sensitivity of the analysis. Since the 1970s, AES has rapidly developed into a powerful solid surface chemical analysis method.

Fundamentals of Auger Electron Spectroscopy

Auger Electron Spectroscopy Physics

The incident electron beam and matter can excite the inner electrons of the atoms to form holes. The energy released during the transition from the outer electron filling the hole to the inner layer may be emitted in the form of X-rays, which generates characteristic X-rays, or may cause another electron outside the nucleus to become a free electron. Auger Electronics.

Auger Electronics and X-Ray Production

The incident electron beam and matter can excite the inner electrons of the atom. The energy released during the transition of the outer electron to the inner layer may be released in the form of X-rays, which generates characteristic X-rays, or it may cause another electron outside the nuclear to become a free electron. This free electron is the Auger electron . For an atom, the excited state atom can only emit one kind of energy: the characteristic X-ray or Auger electron. Elements with a large atomic number have a high probability of emission of characteristic X-rays. Elements with a small atomic number have a high probability of Auger electron emission. When the atomic number is 33, the two types of emission have approximately the same probability. Therefore, Auger electron spectroscopy is suitable for the analysis of light elements.

If an electron beam excites an atomic K-layer electron into a free electron, the L-layer electron transitions to the K-layer, and the released energy excites another electron in the L-layer into an Auger electron. This Auger electron is called a KLL Auger electron . Similarly, the LMM Auger electrons are excited by the L layer electrons, and the M layer electrons are filled into the L layer

Auger electron emission principle diagram The released energy causes another M-layer electron to excite the Auger electron formed.

Auger transition

For free atoms, the electrons orbiting the nucleus are in some discrete "orbits", and these "orbits" form the shells of electrons such as K, L, M, and N. We use the concept of "energy level" to represent the amount of electron energy in a certain orbit. Due to the excitation of the incident electrons, the inner electrons are ionized, leaving a hole. At this time, the atom is in an excited state and unstable. An electron at a higher energy level falls into the vacancy of the inner energy level, and at the same time releases excess energy. This energy can be used to emit characteristic rays as photons, or it can be transferred to a third electron and emitted. This is Auger Electronics. The Auger transition is usually marked by the ray energy level. For example, KL ₁ L ₂ Auger electrons indicate that the initial K level was ionized. The electrons of L ₁ level fill the vacancies of K level. The excess energy is passed to an electron at L ₂ level and emitted. .

Auger electron spectrum energy formula

For atoms with atomic number Z, the Auger electron energy can be calculated using the following empirical formula:

E _WXY (Z) = E _W (Z) -E _X (Z) -E _Y (Z + ) -

In the formula, E _WXY (Z): an atom having an atomic number of Z, and energy of the Auger electron Y obtained by filling W holes with X electrons.

E _W (Z) -E _X (Z): Energy released when X electrons fill W holes.

E _Y (Z + ): the energy required to ionize the Y electrons.

Because the Y electron is ionized when there is already a hole, the ionization energy is equivalent to the ionization energy of an atom having an atomic number between Z and Z 1. Where = 1 / 2-1 / 3. According to the formula (10.6) and the electron ionization energy of each element, the energy of each Auger electron can be calculated to make a spectrum manual. Therefore, as long as the energy of the Auger electron is measured, and the existing Auger electron energy chart is compared, the composition on the surface of the sample can be determined.

Auger Electron Spectra

Because the energy of the primary electron beam is much higher than the energy of the inner orbits of the atom, multiple inner electrons can be excited, and various Auger transitions can occur. Therefore, there are multiple sets of Auger peaks on the Auger electron spectrum. Although the qualitative analysis is complicated, relying on multiple Auger peaks will make the qualitative analysis highly accurate, and it can perform a multi-element qualitative analysis other than hydrogen and helium. At the same time, semi-quantitative analysis of elements can also be performed using the linear relationship between the intensity of Auger electrons and the atomic concentration in the sample. Auger electron spectroscopy is a highly sensitive surface analysis method. Its information depth is 1.0-3.0nm, and its absolute sensitivity can reach ^10-3 monoatomic layer. Is a very useful analysis method.

Auger electron spectrum Auger current

The Auger current measured from the surface of a pure solid is approximately 10 ^-5 I _p , where I _p is the incident electron beam current. Auger current can be calculated in principle by estimating the ionization cross section, but it is affected by many factors. The calculation is complicated and does not fit well with the experiment. In actual measurement, in order to maximize the Auger current, an appropriate E _P / E _W ratio must be selected. E _P is the energy of the incident electron, and E _W is the energy of the inner energy level that was initially ionized. If E _{P <} E _W is not enough to ionize the W level, Auger electron output is equal to zero. If E _P E _W , the time of human-electron interaction with the atom is insufficient, which is not conducive to increasing Auger output. The ratio of E _P / E _{W which} can obtain the maximum Auger electronic output is about 2-6. When grazing with a small angle of incidence, the effective "detection volume" can be increased, and more surface atoms can be ionized, thereby increasing the Auger yield. Generally speaking, the optimal angle of incidence is 10 ° -30 °.

Auger Electron Spectroscopy Auger Surface Analysis

Auger electrons also undergo frequent inelastic scattering when operating in solids. The only thing that can escape from the surface of the solid is Auger electrons generated by several layers of atoms on the surface. The energy of these electrons is generally in the range of 10 to 500 electron volts. The average free path is very short, about 5-20 angstroms, so the Auger electron spectroscopy is only examining the solid surface layer. Auger electron spectroscopy usually uses an electron beam as the radiation source. The electron beam can be focused and scanned. Therefore, Auger electron spectroscopy can be used for surface micro-analysis, and Auger element images can be obtained directly from the fluorescent screen. It is a powerful tool for investigating solid surfaces in modern times. It is widely used in the analysis of various materials and in the fields of catalysis, adsorption, corrosion and wear.

Auger Electron Spectroscopy

Auger energy spectrometer includes electron optical system, electronic energy analyzer, sample placement system, ion gun, ultra-high vacuum system. The following only introduces the core electron optical system and electronic energy analyzer.

Auger Electron Spectrometer

Auger Electron Spectroscopy Electro-optical System

The electron optical system is mainly composed of an electron excitation source (hot cathode electron gun), electron beam focusing (electromagnetic lens) and a deflection system (deflection coil). The main indexes of the electron optical system are the three indexes of incident electron beam energy, beam intensity and beam diameter. The minimum area for AES analysis basically depends on the minimum beam spot diameter of the incident electron beam; the detection sensitivity depends on the beam intensity. These two indicators are usually somewhat contradictory, because a smaller beam diameter will significantly reduce the beam current, so a compromise is generally required.

Auger Electron Spectrum

This is the heart of AES, and its role is to collect and separate electrons of different kinetic energy. Due to the extremely low Auger electron energy, special equipment must be used to achieve the required sensitivity of the instrument. Many Auger spectrometers use a device called a tube analyzer.

The body of the analyzer is two concentric cylinders. The sample and the inner tube are grounded at the same time. A negative deflection voltage is applied to the outer tube. The inner tube has a circular electronic inlet and outlet. The excited electron gun is placed in the inner cavity of the lens barrel analyzer (also can be placed in the lens barrel). Outside the analyzer). The electrons with a certain energy emitted from the sample enter the two-cylinder sandwich from the entrance position. Due to the deflection voltage applied to the outer tube, the electrons finally enter the detector from the exit. If the deflection voltage on the outer cylinder is continuously changed, Auger electrons with different energies can be sequentially received on the detector. The electrons output from the energy analyzer enter the pulse counter after passing through the electron multiplier and preamplifier. XY recorder or fluorescent screen display Auger spectrum Auger electron number N with electron energy E distribution curve

A combination of a tube analyzer and an electron beam scanning circuit can form a scanning Auger microscope. The working method of the electron gun is similar to that of a scanning electron microscope. The two-stage lens reduces the electron beam spot to 3 micrometers. The scanning system controls the electron beam to synchronize scanning on the sample and the phosphor tube screen. The Auger electron signal detected by the tube analyzer The electron multiplier is used to modulate the light of the phosphor screen after amplification, so that Auger electron images can be obtained.