Super Detailed Explanation:Principles, Characteristics, Qualitative and Quantitative Analysis, Depth Profiling, and Angle-Resolved X-ray Photoelectron Spectroscopy (XPS)

X-ray photoelectron spectroscopy (XPS) is a non-destructive measurement technique that detects surface information of materials by using a photon beam that penetrates 3-10 nm into the sample surface. XPS technology can effectively preserve the original structural information of the sample, and it can be used for qualitative and quantitative analysis of the elemental composition and content of the material surface, as well as for analyzing the chemical valence state, chemical bonds, and other information of the elements.

Angle-resolved XPS (ARXPS) can also be used to measure the chemical information within ultra-thin layers. Additionally, by using argon ion etching technology, it is possible to conduct depth profiling of material interiors beyond 10 nm, extending the analysis from the surface to a certain depth. XPS technology is used for elemental analysis, multiphase research, compound structure analysis, trace element analysis using enrichment methods, and identification of elemental valence states. It finds extensive applications in fields such as catalysis, metal corrosion, adhesion, batteries, and semiconductor materials and devices.

This article discusses the basic principles and characteristics of XPS, exploring its applications, features, common issues, and solutions in material science research from four aspects: qualitative analysis, quantitative analysis, depth profiling, and angle-resolved electron spectroscopy analysis.

Basic Principles and Characteristics

XPS technology originated from the photoelectric effect discovered by the German physicist Hertz in 1887. When X-rays with a certain energy (commonly used sources are Mg Kα-1253.6 eV or Al Kα-1486.6 eV) irradiate the sample surface, they interact with the surface atoms of the sample. When the photon energy exceeds the binding energy of the outer electrons, it can excite electrons from the atoms of the material being tested to become free electrons.

The emission process of photoelectrons is shown in Figure 1.

Figure 1. Emission process of photoelectrons after the sample is excited

** Applications of XPS in Fundamental Research**

** Qualitative Analysis**

Qualitative analysis involves obtaining information about the components, chemical states, surface adsorption, surface states, surface valence electron structures, chemical structures of atoms and molecules, and chemical bonding from the position and shape of the measured spectra. Elemental identification is primarily based on the characteristic energy values of the photoelectron lines of the constituent elements.

Elemental Composition Identification Each element has a unique set of energy levels. XPS technology identifies the elemental composition by measuring the binding energies of different elements in the spectrum.

For samples with uncertain chemical composition, a full-spectrum scan should be performed to preliminarily identify all or most of the chemical elements on the surface.

1. Identify the spectra of commonly present elements, especially the spectra of C and O;
2. Identify the strong spectral lines of the main elements in the sample and the related secondary strong spectral lines;
3. Identify the remaining weak spectral lines.

For the strongest spectral lines of unknown elements, the identification of p, d, and f spectral lines should consider their typical spin-orbit doublet structures, which should have specific energy intervals and intensity ratios. Figure 2 shows the full-spectrum scan of an HfO₂ thin film sample. From the figure, it is evident that the sample contains Hf and O elements, with the C binding energy peak arising from the calibration C element used during the XPS test.

Figure 2. Full-spectrum scan of an HfO₂ thin film sample

Chemical State Analysis

By conducting narrow region scans, the chemical states of specific elements can be determined.

To study the peaks of known elements in the sample, narrow region high-resolution scans can be performed to obtain more precise information, such as the exact position of binding energies, precise line shapes, and accurate counts. Through data processing like background subtraction or peak decomposition or deconvolution, the chemical states of the elements can be identified.

For example, to determine the detailed information of the Hf element in the full spectrum of the HfO₂ thin film sample shown in Figure 2, a narrow spectrum scan can be performed around the strongest peak of Hf. The narrow spectrum scan result is shown in Figure 3, where the two peaks correspond to binding energies of 17.50 eV and 19.18 eV, corresponding to Hf 4f₇ and Hf 4f₅, respectively. These values are close to the reported binding energies of Hf⁴⁺ in HfO₂, thus confirming the chemical state of Hf in the sample.

Figure 3. Narrow spectrum scan of Hf 4f in an HfO₂ thin film sample

Quantitative Analysis

In XPS, quantitative analysis is typically based on the ratio of the intensities of various peaks in the spectrum, converting the observed signal intensities into elemental content, i.e., converting the peak areas into the corresponding elemental content.

The quantitative analysis often uses the elemental sensitivity factor method, which uses the intensity of specific element lines as a reference standard, measures the relative intensity of other element lines, and calculates the relative content of each element. For accurate relative quantitative analysis of elements in the sample, the equipment needs to be calibrated according to the energy scale published by the International Organization for Standardization (ISO) (ISO 15472:2010).

Quantitative analysis in XPS can also be used to analyze the relative atomic concentration of the same element in different chemical states. This type of analysis is challenging because the binding energy peaks of atoms in different chemical states of the same element are very close and do not form independent peaks but overlap to form broad peaks. To obtain their relative content by parsing the peak intensities (areas) of these atoms, the broad peaks must be decomposed into their constituent single peaks, i.e., deconvolution. Specialized software is generally used for peak deconvolution. Although computer programs set the best fitting parameters, appropriate fitting parameters should be chosen based on the problem being studied. For further details, refer to the following articles:

1. XPS data processing software Avantage: Element identification, dealing with spectrum peak overlaps between elements, peak fitting for quantitative analysis
2. Avantage's unique NLLSF fitting function for analyzing complex XPS spectra to obtain more information!

Depth Profiling

Due to the layered structure of the sample itself, such as coatings, oxidation, and passivation, there are chemical state differences in the depth direction.

The non-destructive methods mentioned earlier are limited to detecting compositional changes within 1-10 nm of the surface. To obtain information beyond 10 nm in depth, argon ion bombardment is used to etch the sample surface in the analysis chamber of the XPS equipment.

Depth profiling mainly studies the vertical distribution of elemental chemical information in the sample. By using an argon ion gun to perform argon ion sputtering and stripping the sample surface, controlling the appropriate sputtering intensity and time, the sample surface is etched to a certain depth, and then spectrum analysis is performed. To obtain accurate sputtering depths, standard materials with similar or identical thickness to the sample being tested are generally used to calibrate the sputtering rate, allowing calculation of the calibrated sputtering depth for the distribution of the corresponding elements based on the sputtering time.

To avoid interactions between the etching ion beam and the sample being tested, high-quality depth profiling results must be obtained under high vacuum conditions. Alternating between etching and spectrum acquisition provides the chemical information of the sample as a function of depth, greatly expanding the detection range of XPS.

Angle-Resolved Electron Spectroscopy Analysis

The escape depth of photoelectrons from the sample surface is related to the kinetic energy of the electrons. When the sample surface is perpendicular to the analyzer, the escape depth of electrons is 𝑑d. Changing the angle between the sample surface and the incident light beam changes the detection depth of the incident light, making the detection depth shallower. This way, the signal of photoelectrons from the outermost surface layer is greatly enhanced compared to the deeper layers.

Using this characteristic, the chemical information on the surface of ultra-thin film samples can be effectively detected, allowing the study of the vertical distribution of chemical components in ultra-thin samples. To obtain accurate information, the equipment should be calibrated according to the XPS intensity standard line linearity published by ISO (ISO 21270:2004).

XPS can provide coating thickness information using the Beer-Lambert equation without mechanically, chemically, or ion-etching the sample, allowing for non-destructive depth profiling. By changing the geometric position of the experimental setup, the energy of the incident electrons, or the etching time, information at different depths of the sample can be obtained. However, it should be noted that this method is suitable for cases where the coating on the substrate is continuous, uniform, and ultra-thin (less than 10 nm) in thickness.

Characteristics of XPS

XPS is an advanced analytical technique commonly used in surface analysis of materials. It provides comprehensive chemical information and can obtain data on micro-regions and depth distribution. Its specific characteristics are as follows:

1. Wide Test Range: XPS can qualitatively and quantitatively analyze all elements present on the surface, except for H and He.
2. Rich Chemical Information: It provides abundant chemical information during the testing process and can perform non-destructive surface analysis of samples.
3. Low Interference: The spectral lines of the same energy levels of adjacent elements are well-separated, reducing mutual interference, making element identification highly specific.
4. Chemical Shift Detection: It can detect the chemical shift of elements, which can be used for structural analysis and chemical bond studies in materials.
5. High Sensitivity: It is a highly sensitive ultra-trace surface analysis technique, with a probing depth of approximately 3-10 nm.

Charging Issues and Solutions in XPS Samples

During XPS testing, if the sample is insulating or has poor conductivity, positive charges will accumulate on its surface due to the lack of electron replenishment after X-ray irradiation, causing the measured binding energy to be higher than normal.

Charging issues are challenging to completely eliminate using a single method. Common solutions include:

1. Coating the Sample Surface: Coating the sample surface with a conductive material such as gold or carbon. However, the thickness of the coating affects the measurement of binding energy, and the coating material might interact with the sample, influencing the test results.
2. Electron Neutralization: Using a low-energy electron neutralization gun during testing to irradiate the sample surface with a large number of low-energy electrons to neutralize the positive charges. However, controlling the density of irradiated electron flow to avoid over-neutralization remains a significant challenge.
3. Internal Standard Method: Using the internal standard method to calibrate the test results. The commonly used method is the carbon internal standard method, using the C 1s binding energy of 284.8 eV from organic contaminants in the vacuum system for calibration, or using the binding energy of a known stable element in the sample for calibration.
4. Standard Reference Materials: In XPS quantitative analysis, relevant standard reference materials are essential. Currently, China is just beginning in this area, and more standard materials need to be developed according to industry needs to promote the implementation of standards.

XPS technology is widely used in various fields, including materials science, chemistry, solid-state physics, catalysis, microelectronics technology, and metrology. Utilizing XPS technology allows for qualitative and quantitative analysis of surface elements (such as elemental composition identification and chemical state analysis), depth profiling to study the vertical distribution of elements in samples, and thickness measurement of ultra-thin film samples using angle-resolved XPS technology.

References

Zhang Suwei, Yao Yaxuan, Gao Huifang, et al. Application of X-ray Photoelectron Spectroscopy in Material Surface Analysis [J]. Metrology Science and Technology (1): 5.