- Published on
The principle and characteristics of XPS, qualitative, quantitative, in-depth analysis and angle-resolved electron spectroscopy analysis
- Authors
- Name
- Universal Lab
- @universallab
Basic principles and characteristics
The XPS technology originated from the photoelectric effect discovered by the German physicist Hertz in 1887. That is, X-rays of a certain energy are irradiated onto the surface of the sample and interact with the surface atoms of the sample to be tested. When the energy of the photoelectrons is greater than the binding energy of the electrons outside the nucleus, it can excite the electrons in the atoms of the substance to be tested to detach from the atoms and become free electrons.By measuring the magnitude of the photoelectron binding energy of each element in the sample, the chemical composition, state and content of the elements on the sample surface can be identified, thereby conducting qualitative, quantitative analysis or in-depth dissection, etc.
The Application of XPS in Basic Research
Qualitative analysis:Qualitative analysis is to obtain information about the composition, chemical state, surface adsorption, surface state, surface valence electron structure, chemical structure of atoms and molecules, and chemical bonding conditions of the sample based on the position and shape of the measured spectrum. The main basis for the qualitative determination of elements is the characteristic energy value of the photoelectron lines of the constituent elements.
Identification of elemental composition:Each element has a unique set of energy levels. XPS technology identifies the composition of elements by measuring the binding energy of different elements in the spectrum. Uncertain samples on chemical composition, should make full spectrum scanning to all or most of the chemical elements on the surface of the preliminary determination. First, identify common elements of the spectral lines, especially C and O line; Second, identify the main elements in the sample line and strong about the time line; Finally, identify the remaining weak line. If it is the strongest line of unknown elements, to p, d, f line should be paid attention to the identification of its general for spin dual structure, interval between them should have a certain energy and strength than. Figure 2 shows the full-spectrum scan of the HfO2 thin film sample. It can be seen from the figure that the sample contains Hf and O elements, among which the binding energy peak of C comes from the C element used for calibration during the XPS test.
Chemical state analysis:The chemical state of a specific element can be determined through narrow-area scanning. If you want to study in a sample of known elements peak, high resolution scanning can be narrow area, in order to get more precise information, such as the binding energy of accurate position, accurate linear, accurate counting, etc., or decomposition of the back end or peak convolution back data processing, such as to identify the chemical state of element. If want to make sure that in figure 2 HfO2 film of Hf in the full spectrum of detailed information, can be in Hf peak near the strongest narrow spectrum scan. The narrow-band scanning results are shown in Figure 3. The binding energies corresponding to the two peaks are 17.50 eV and 19.18 eV, corresponding to Hf 4f7 and Hf 4f5 respectively. This is close to the binding energy of Hf4+ in HfO2 reported in the literature, thereby determining the chemical state of Hf in this sample.
Quantitative analysis:In XPS, the current application of quantitative analysis is mostly based on the ratio of the intensity of each peak in the energy spectrum, converting the observed signal intensity into the content of elements, that is, converting the peak area of the spectrum into the content of the corresponding elements. And quantitative analysis of many adopts element sensitivity factor method, the method of using a particular element spectral line intensity as the reference standard, other elements can be measured relative spectral line intensity, get the relative content of each element. In order to conduct a relatively accurate quantitative analysis of the content of each element in the sample, before the test, the equipment needs to be calibrated in accordance with the calibration of the XPS energy scale issued by the International Organization for Standardization. XPS quantitative analysis of the various elements in addition to the relative sensitivity factor is used to calculate the relative concentration of atom, to the same element in different chemical states of atoms relative concentration can also be analyzed. This type of analysis is somewhat challenging because the peak positions of the binding energy of atoms of the same element in different chemical states are very close. Instead of forming independent peaks, they are superimposed to form wide peaks. At this point, if one wants to obtain the relative content of these atoms by analyzing the peak intensity ratio of these atoms, the wide peak needs to be decomposed into the individual single peaks that make up it, that is, deconvolution. There are generally specialized software for deconvolution of spectral peaks. Although the computer program will set a set of optimal fitting parameters, in actual operation, the appropriate fitting parameters should be selected according to the problem under discussion.
Angle-resolved electron spectroscopy analysis:The depth to which photoelectrons escape 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 the electrons is d. By changing the Angle between the sample surface and the incident light beam, the detection depth of the incident light can be altered, making the detection depth shallower. As a result, the photoelectron signal from the outermost layer will be significantly enhanced compared to that from deeper layers. Using this feature, the ultrathin film on the surface of the sample of chemical information can be effectively detection, the vertical distribution of the ultra-thin sample chemical composition. In order to obtain the accurate information of the sample, the equipment should be calibrated before the test according to the linearity of the XPS strength scale published by ISO. XPS can the samples without the need to mechanical, chemical, or under the condition of ion etching, in many ways the use of Beer Lambert equations provide the thickness of the layer information, no damage depth profiling. By changing the geometric position of the experimental device, the energy of the incident electrons or the etching time, etc., the information of different depths of the sample is obtained. However, it should be noted that this method is applicable when the coating on the substrate is continuous and uniform with an ultra-thin thickness.
The features of XPS
XPS is a surface analysis of commonly used advanced analysis techniques, the analysis of the material in the process, not only can get the general chemical information, also can get the micro area and depth distribution of information, its characteristics are as follows:1.It has a wide testing range and can conduct qualitative and quantitative analysis on all elements existing on the surface except H and He.2.The test can obtain rich chemical information and can detect the sample surface without damaging it. 3.The spectral lines of the same energy level of adjacent elements are far apart, with little mutual interference, and the qualitative identification of elements is strong. 4. It can detect the chemical shift of elements, and thus is used for structural analysis and chemical bond research in materials. 5.It is a highly sensitive ultra-micro surface analysis technology with a detection depth of approximately 3 to 10 nanometers.
XPS sample charging issues and Solutions
During the XPS testing process, if the sample has poor insulation or conductivity, after being irradiated by X-rays, the positive charges generated on its surface cannot be replenished by electrons, resulting in charge accumulation and causing the measured binding energy to be higher than the normal value. Sample charged issues are difficult to eliminate, by one kind of methods commonly used solution has the following kinds: 1.Evaporate substances with good electrical conductivity, such as gold or carbon, on the surface of the sample. However, the thickness of the evaporated substance will have an impact on the determination of binding energy, and the evaporated substance may interact with the sample, thereby affecting the test results. 2.During the testing process, a low-energy electron neutralization gun is used to irradiate a large number of low-energy negative electrons onto the sample surface to neutralize the positive charges. However, how to control the irradiation electron current density without causing over-neutralization remains a major difficulty that needs to be solved. 3.In XPS analysis, the internal standard method is generally adopted to calibrate the test results. The commonly used method is the carbon internal standard method. Calibration is carried out using the C1s binding energy of the most common organic pollutant carbon in vacuum systems, which is 284.8 eV, or by using the binding energy of the known state-stable elements in the detection material for calibration. 4.In XPS quantitative analysis, relevant reference materials are indispensable. At present, China is just getting started in this regard. It is necessary to develop more standard substances based on industrial demands to promote the implementation of standards.
XPS technology is widely used in many fields such as materials, chemistry, solid-state physics, catalysis, microelectronics technology and metrology. The XPS technology can not only be used for qualitative analysis such as elemental composition identification and chemical state analysis and quantitative analysis of elements on the material surface, but also for in-depth analysis to study the longitudinal distribution of elements in the sample. With angle-resolved XPS technology, the thickness of ultra-thin film samples can also be measured.