Working principle and composition of ion probe microanalysis

Ion probe microanalysis
The scientific name of ion probe is Secondary Ion Mass Spectroscopy (SIMS). It is similar to electron probe in function, except that ion beam replaces electron beam and mass spectrometer replaces X-ray analyzer. Compared with EPMA, SISM has the following characteristics:
1. Since the penetration depth of ion beam on the solid surface (the depth of several atomic layers) is shallower than that of electron beam, the composition of such a very thin surface layer can be analyzed.
2. It can analyze light elements including hydrogen and lithium, especially hydrogen, which is a function that other instruments do not have.
3. It can detect trace elements (~50×10-9, the limit of electron probe is ~0.01%).
4. It can perform isotope analysis.

3-4-1 Basic Principles
The principle of the ion probe is to use an ion beam with an energy of 1 to 20 KeV to irradiate the solid surface, stimulate positive and negative ions (sputtering), and use a mass spectrometer to analyze these ions, measure the mass-to-charge ratio and intensity of the ions, and thus determine the type and quantity of elements contained on the solid surface.
1 Sputtering The
accelerated primary ion beam irradiates the solid surface, knocking out secondary ions and neutral particles, etc. This phenomenon is called sputtering. The sputtering process can be regarded as a series of independent collisions between a single incident ion and the atoms that make up the solid. The figure below illustrates the collision between the incident primary ion and the solid surface.

Part of the incident ions change their direction of motion after elastic or inelastic collision with the surface and fly into the vacuum, which is called primary ion scattering (as shown in Figure Ⅰ); another part of the ions directly transfer their energy to the surface atoms in a single collision, and drive the surface atoms out of the surface, causing them to be emitted with very high energy, which is called rebound sputtering (as shown in Figure Ⅲ); however, what happens on the surface in large quantities is that primary ions enter the solid surface and consume their energy on the lattice through a series of cascade collisions, and finally inject to a certain depth (usually several atomic layers). Once the solid atoms are collided, they will leave the lattice once they have enough energy and collide with other atoms again, increasing the number of atoms leaving the lattice, some of which affect the surface. When these affected surface or near-surface atoms have the energy and direction required to escape from the solid surface, they are emitted according to a certain energy distribution and angle distribution (as shown in Figure Ⅱ). Usually only atoms in 2-3 atomic layers can escape, so the emission depth of secondary ions is about 1nm. It can be seen that the emitted particles from the emission area undoubtedly represent the information of the near-surface area of ​​the solid, which is the basis for SISM to perform surface analysis.
There are many types of sputtering products caused by primary ion irradiation on the solid surface (see the figure below), among which secondary ions only account for a small part of the total sputtering products (about 0.01-1%). There are many factors that affect the sputtering yield. Generally speaking, the larger the atomic number of the incident ion, that is, the heavier the incident ion, the higher the sputtering yield; the greater the incident ion energy, the higher the sputtering yield. However, when the incident ion energy is very high, the depth of its penetration into the crystal lattice will increase, which will prevent the deep atoms from escaping the surface, and the sputtering yield will decrease instead.

2 Instrument composition
The ion probe mainly consists of three parts: primary ion emission system, mass spectrometer, and secondary ion recording and display system. The first two are in a vacuum chamber with a pressure of <10-7Pa. Its structural principle is shown in the figure.

① Primary ion emission system
The primary ion emission system consists of an ion source (or ion gun) and a lens. The ion source is a device that emits primary ions. It usually uses an electron beam of several hundred volts to bombard gas molecules (such as inert gases such as helium, neon, argon, etc.) to ionize the gas molecules and produce primary ions. Under the action of voltage, ions are ejected from the ion gun, and then the ion beam is focused by several electromagnetic lenses and irradiated on the surface of the sample to excite secondary ions. An extraction electrode with a voltage of about 1KV is used to introduce secondary ions into the mass spectrometer. The ions are accelerated by the extraction electrode, and their energy is: where e, m, and v are the charge, mass, and velocity of the ions, respectively, and V is the acceleration voltage.
② Mass spectrometer

The mass spectrometer consists of a sector-shaped electric field and a sector-shaped magnetic field. Secondary ions first enter a sector-shaped electric field, called an electrostatic analyzer. In the electric field, the ions move along a circular orbit with a radius of r, and the force generated by the electric field is equal to the centripetal force:

The radius of the motion orbit r is equal to mv2/eE, which is proportional to the energy of the ion. Therefore, the sector electric field can deflect ions of the same energy to the same extent. After being deflected by the electric field, the secondary ions enter the sector magnetic field (magnetic analyzer) for a second focusing. The Lorentz force generated by the magnetic flux is equal to the centripetal force:

Combining the above formulas, we can get the orbital radius r' of ions moving in the magnetic field:
It can be seen that ions with the same mass-to-charge ratio have the same movement radius. Therefore, after passing through the sector magnetic field, the ions are focused together according to the m/e ratio, and ions with the same m/e ratio are focused on the imaging surface at the C slit.
Ions with different mass-to-charge ratios are focused on different points on the imaging surface. If the C slit is fixed, the strength of the sector magnetic field is changed, and ions of different masses enter the detector through the C slit. The B slit is called the energy slit. By changing the width of the slit, secondary ions of different energies can be selected to enter the magnetic field.
③ Ion detection system
The ion detector is a secondary electron multiplier tube, which has curved electrodes inside. A voltage of 100-300V is applied between each electrode to accelerate the electron step by step. After passing through the mass spectrometer, the secondary ions directly collide with the primary electrode of the electron multiplier tube, generating secondary electron emission. The secondary electrons are attracted and accelerated by the secondary electrode, bombarding more secondary electrons on it, thus multiplying step by step and finally entering the recording and observation system.
The secondary ion recording and observation system is similar to that of the electron probe. It can display the secondary ion image on a cathode ray tube, give a surface distribution diagram of a certain element, or draw the secondary ion mass spectrum of all elements on a recorder.

Due to the characteristics of SISM, it can be applied to the following five aspects of analysis and research:
1. Surface analysis (including analysis of monomolecular layers). Some surface phenomena such as catalysis, corrosion, adsorption, and diffusion have been successfully analyzed and studied through SISM.
2. Depth profile analysis (analysis with a depth greater than 50nm). In thin film analysis, diffusion, and ion analysis, SISM is the most effective surface analysis tool for determining the depth concentration distribution of impurities and isotopes.
3. Surface analysis can provide information about the lateral distribution of elements and single quantitative information under appropriate conditions through ion imaging. At present, ion imaging has been used to study grain boundary precipitates, metallurgical and single crystal effects, lateral diffusion, mineral phase characteristics, and surface impurity distribution.
4. Micro-area analysis (micro-areas with a region diameter of less than 25μm) is used for trace analysis of elements, impurity analysis, analysis of suspended particles in the air, etc.
5. Volume analysis is the analysis of general properties of solids. Due to the many advantages of ion probes, their applications in semiconductors, metals, minerals, environmental protection, isotopes, and catalysts have been greatly developed since their introduction.
(1) Application in semiconductor materials
Since semiconductor materials have very high purity requirements and require the smallest analysis area, surface analysis and depth analysis are urgently required. Therefore, this is also the field where ion probes are most suitable for their use. Representative work includes:
• Impurity analysis of surfaces, interfaces and bulk materials
• Determination of ion implantation concentration and doping
• Application in actual analysis

(2) Application in metal materials
Ion probes are widely used in the surface, thin layer depth and trace analysis of metal materials.
• Determine the composition of the passive film, nitriding layer and oxide ink on the surface of various steels and alloys.
• Determine the mutual diffusion and penetration between various metals to understand their properties.
• Determine the composition of the precipitated phases, inclusions, carbides of steel and metals, and the segregation
of rare earth elements and boron, phosphorus, etc. on the grain boundaries of steel. • Determine the depth distribution of doping elements injected into the metal surface.
• Determine the composition of contamination and stains on the metal surface.
(3) Application in geology and minerals
Since ion probes do not require pre-separation of samples, the sample consumption is small, and they can be directly recorded using electrical methods, they are widely used in geology:
• Determine the content and distribution of trace elements in meteorites, as well as the abundance ratio of isotopes.
• Determine the rare earth elements and alkaline earth elements on the moon and compare them with the elements on the earth.
• Determine the diffusion of oxygen in feldspar, fluorine in lithium fluoride, and potassium in mica.
• Determine the composition of the oxide layer on the surface of the mineral to find the best mineral processing technology.
(4) Application in biological samples
• Determine the content of trace elements and the isotope abundance ratio of lithium in teeth and cartilage tissue.
• Study the relationship between the fluorine content in teeth and dental caries.
• Analyze the content of common elements such as calcium, potassium, boron, sodium, magnesium, and manganese in leaves to study the influence of element content.
(5) Application in the ceramic industry
• Determine the content and distribution of trace elements in phosphosilicate glass, boron nitride, and borosilicate glass.
• Analyze the diffusion of rare earth elements in nozzle bricks and their relationship with rare earth casting nodules.