What is an ion source
Ion source (English name: Ion source) is a device that ionizes neutral atoms or molecules and draws ion beam from it. It is an indispensable component of various types of ion accelerators, mass spectrometers, electromagnetic isotope separators, ion implanters, ion beam etching devices, ion thrusters and neutral beam injectors in controlled fusion devices.
ion source
Ions can be generated during gas discharge, electron beam collision with gas atoms (or molecules), charged particle beam sputtering of working materials and surface ionization, and are drawn out into beams. Various types of ion sources have been developed according to different use conditions and purposes. The widely used sources are arc discharge ion source, PIG ion source, double plasma ion source and double Peng source, which are based on the gas discharge process and are generally called arc sources. High frequency ion source is produced by high frequency discharge in gas, and has a wide range of applications. The appearance of new heavy ion sources has significantly improved the charge state of heavy ions, among which the more mature are electron cyclotron resonance ion sources (ECR) and electron beam ion sources (EBIS). There are two types of negative ion sources with good performance, namely, charge transfer type and sputtering type. Under certain conditions, various ion sources based on the gas discharge process can provide a certain amount of negative ion beam.
Ion source is a subject with extensive application fields. In many basic research fields such as atomic physics, plasma chemistry, nuclear physics, etc., ion source is a very important and indispensable equipment.
Type of ion source
High frequency ion source
The high frequency discharge phenomenon in the rarefied gas is used to ionize the gas, which is generally used to generate low charge positive ions, and sometimes negative ions are also drawn from it to be used as negative ion sources.
In a high-frequency electric field, free electrons collide with atoms (or molecules) in the gas and ionize them. As a result of the multiplication of charged particles, an electrodeless discharge is formed and a large amount of plasma is produced. The discharge tube of high-frequency ion source is generally made of Pyrex glass or quartz tube. The high frequency field can be generated by the solenoid coil outside the tube, or by the annular electrode sheathed outside the tube. The former is called inductive coupling, and the latter is called capacitive coupling high-frequency oscillator. The frequency is 10~10 Hz, and the output power is more than hundreds of watts.
There are two ways to extract ions from high-frequency ion sources. One is to insert a tungsten wire at the top of the discharge tube as the positive electrode, install a negative electrode with a hole at the tail of the discharge tube, and make the hole into a tube shape, from which the ion current is drawn. Another way is to make the positive electrode into a hat shape and install it near the lead out electrode, while the discharge area is on the other side of it. No matter which extraction method is adopted, the metal electrode shall be wrapped with quartz or glass to reduce the recombination of ions on the metal surface.
When a constant magnetic field is added to the high frequency discharge area, the ion concentration in the discharge area can be increased due to the resonance phenomenon. Sometimes, non-uniform magnetic fields are added to the extraction area to improve the extraction.
Arc discharge ion source
In a uniform magnetic field, an ion source in which the cathode thermal emission electrons maintain the gas discharge. In order to reduce gas consumption, the discharge area is often closed. The anode is made into a cylinder, and the axis is parallel to the direction of the magnetic field. The magnetic field can well restrain the electron flow emitted by the cathode, ionize the atoms (or molecules) of the gas in the anode cavity, and form an arc column with high plasma density. The ion beam can be drawn out laterally perpendicular to the axis direction or along the axis direction.
PIG ion source
A powerful ion source with high yield
The ion source that generates reflection discharge under the constraint of external magnetic field is an improvement of arc discharge ion source. In the arc discharge ion source, an equipotential counter cathode is installed at the position symmetrical to the cathode at the other end of the anode, which makes the electron flow emitted by the cathode reflect and oscillate in the hollow anode, improving the ionization efficiency and changing the discharge mechanism. The cathode is generally made of tungsten blocks and heated by electron bombardment. It is called an intermediate hot cathode ion source. When the reflection discharge voltage is high, it can work in the cold cathode state. At this time, the structure of the ion source is simpler, which is called the cold cathode ion source. For the ion source with high power, the cathode is heated by the discharge to reach the electron thermal emission temperature, which is called the self heating cathode ion source.
In order to generate ions of non gaseous elements, there are many ways to feed the elements into ion sources. The simple method is to use gas compounds, which can also be introduced into the vapor of the element. Some solid materials can also be plated on the cathode surface or anode cavity wall, and can be introduced into the discharge area by the sputtering effect in the discharge.
Double plasma ion source
An arc discharge ion source working in a non-uniform magnetic field. Its electrode system and magnetic system are carefully arranged, making the plasma produced by the discharge shrink twice (geometric pinch and magnetic pinch). It is widely used due to the large ion current intensity, high brightness and compact main structure.
High power double plasma ion source can generate positive ion beam above ampere level, which is an effective high current ion source. After the positive ions are neutralized, they are converted into neutral beams. The negative ion beam can be directly extracted from the double plasma ion source, or the positive ion beam can be extracted first, and then the negative ion can be obtained indirectly.
Shuangpeng Yuan
Synthesis of double plasma ion source and PIG ion source. The high-power double beam source is a single charge state high current ion source, which can draw ion current above ampere level.
Small devices are also used as multi charge heavy ion sources.
From the appearance structure, the double Peng source only adds a pair of cathode outside the anode of the double plasma ion source. However, from the discharge principle, the two ion sources are very different. The first three electrodes constitute a system similar to the double plasma ion source, which is regarded as an electron source. Since the cathode is applied with the same or more negative voltage as the intermediate electrode, electrons reflect and oscillate between the intermediate electrode and the cathode, improving ionization.
Charge transfer negative ion source
A device that uses positive ion beam to generate negative ions. A positive ion beam can be converted into a negative ion beam by interacting with the surface of a solid material or by trapping electrons through a gas target. The positive ion beam can be provided by a small double plasma ion source. If the high-frequency ion source is used, the negative ion beam can be obtained by lengthening the hole of the extraction electrode.
Sputter type negative ion source
By bombarding the working material with a positive ion beam, the negative ions of the material can be obtained. If cesium ion beam is used to sputter elements with higher electron affinity after Group IV of the periodic table, microampere negative ion beam current of the element can be obtained. If the hydrogen or argon ion beam is passed through a hole filled with a gaseous working material, a few microamps of negative ion beam of the material can be obtained.
Ion source technology is still developing. Ring double plasma ion source Large double Peng source has been able to provide 100 ampere level hydrogen positive ion flow magnetron negative ion source has been obtained ampere level hydrogen negative ion beam. Some small ion sources have the characteristics of low energy dispersion, low power consumption, low gas consumption and long life. Heavy ions with very high charge state have been obtained in experimental devices (such as electron cyclotron resonance ion source, electron beam ion source) that generate multi charge heavy ion beams. These new devices have been gradually adopted by cyclotron. The laser ion source which can produce high temperature plasma and uses inertial confinement also generates high charged ions.
In the heavy ion acceleration system using the combined acceleration method, the front stage accelerator accelerates the heavy ions with low charge state to the energy of MEV per nucleon, passes through the solid stripping film or the gas stripper, and strips off part of the orbital electrons. After increasing the charge state, the main accelerator accelerates continuously to obtain higher energy gain. In this combined acceleration system, the front accelerator and stripper can be regarded as a special heavy ion source system.
MS ion source
1. Electric shock ionization (EI)
The electron of certain energy directly acts on the sample molecule to ionize it, and the efficiency is high, which helps the mass spectrometer to obtain high sensitivity and high resolution. The ionization energy of organic compounds is about 10eV. When 50-100eV, most molecules have the largest ionization interface. At 70 eV energy, abundant fingerprints were obtained, and the sensitivity was close to the maximum. A strong molecular ion signal can be obtained by appropriately reducing the ionization energy, which is helpful for qualitative analysis in some cases.
2. Chemical ionization (CI)
The defect of electron bombardment is that the molecular ion signal becomes very weak, even undetectable. Chemical ionization introduces a large amount of reagent gas, so that the sample molecules do not directly interact with ionized ions. Ionization is achieved by using active reaction ions, which may have a low reaction heat effect, so that the fragmentation of molecular ions is less than that of electron bombardment ionization. Commercial mass spectrometers generally use combined EI/CI ion sources. The reagent gas is generally methane gas, including N2, CO, Ar or mixed gas. Different partial pressures of reagent gas will change the strength of reaction ions, so the general source pressure is 0.5-1.0 Torr.
3. Atmospheric pressure chemical ionization (APCI)
Under atmospheric pressure, the chemical ionization reaction rate is higher, the efficiency is higher, and can produce rich ions. The ions generated under atmospheric pressure are transferred to a high vacuum (in the mass analyzer) by some means. Ni63 radiation ionization ion source was used in the early stage. Another design is corona discharge ionization, allowing the carrier gas velocity to reach 9L/S. It is necessary to reduce source wall adsorption and solvent molecular interference.
4. Secondary ion mass spectrometry (FAB/LSIMS)
In material analysis, people use high-energy primary particles to bombard the surface (metal palladium coated with samples), and then conduct mass spectrometry analysis for the secondary ions generated. There are mainly two ionization technologies: fast atom bombardment (FAB) and liquid secondary ion mass spectrometry (LSIMS), which use atomic beam and ion beam as high-energy primary particles respectively. Generally, liquid matrix loading samples (such as glycerin, thioglycerin, m-nitrobenzyl alcohol, diethanolamine, triethanolamine or a certain proportion of mixed matrix) are used. The main principle is that the molecules are protonated to form MH+ions, and some reactions will cause interference.
5. Plasma desorption mass spectrometry (PDMS)
Nuclear fission fragments of radioisotopes (such as Cf252) are used as primary particles to bombard the sample, and the metal foil (aluminum or nickel) is coated with the sample to bombard it from the back, transferring energy to make the sample analytically ionized. The ionization energy is much higher than that of FAB/LSIMS, which can be used to analyze polypeptides and proteins.
6. Laser desorption/ionization (MALDI)
The vacuum ultraviolet radiation with the wavelength of 1250-775 generates photoionization and desorption to obtain molecular ions and fragments with structural information, which are suitable for macromolecules with complex structures and are not easy to gasify, and auxiliary matrices are introduced to reduce excessive fragmentation. Generally, solid matrix is used, and the matrix sample ratio is 10000/1. Different matrices and wavelengths are used according to the analysis purpose.
7. Electro spray ionization (ESI)
Electrospray ionization uses a strong electrostatic field (3-5kv) to form highly charged mist like small droplets. After repeated solvent volatilization droplet splitting, a single multi charged ion is generated. In the ionization process, multiple protonated ions are generated.
Application of Ion Source - Ion Beam
A group of ions moving in almost the same direction at approximately the same speed.
Ion source is a device for obtaining ion beam. As we know, among all kinds of ion sources, the plasma ion source is the most widely used, that is, the electric field is used to draw ions from a mass of plasma. The main parameters of this type of ion source are determined by the density and temperature of the plasma and the quality of the extraction system. These ion sources include: Penning discharge type ion source radio frequency ion source, microwave ion source, dual plasma source, Fuliman ion source, etc. Another type of ion source that is widely used is the electron collision type ion source, which is mainly used in various mass spectrometers. In addition, there are surface ionization sources, photoionization ion sources, liquid metal ion sources and other types of ion sources.
Main parameters of ion beam
① Ion beam intensity
That is, the equivalent current intensity of the available ion beam, expressed in current unit A or mA.
② Percentage of useful ions
That is, the percentage of useful ion beam in total ion beam. Generally speaking, the total ion beam given by the ion source includes single charge ion, multi charge ion, various molecular ions and impurity element ions.
③ Energy divergence
Due to the different thermal motion and extraction location of ions, the energy of ion beam given by the ion source is discrete to the required single energy. It is generally hoped that the divergence is as small as possible, especially in high-precision ion beam applications.
④ Focusing performance of beam
It is expressed in terms of cross section and angle of ion beam. A poorly focused ion beam will cause a large number of ions to be lost in the transmission process. The final obstacle to obtain an ion beam with good focusing characteristics is the electrostatic repulsion force between ions in the beam. In order to overcome this obstacle, ions should be made to obtain higher energy as soon as possible.
⑤ Efficiency of ion source
The proportion of working materials extracted in the form of ion beam in the total consumption of working materials.
⑥ Working life
The time after the installation of the ion source.
Device for accelerating ion beam
The energy of the ion beam obtained from the ion source generally ranges from hundreds of electron volts to tens of thousands of electron volts. Because the ion beam with high extraction voltage to obtain high energy is limited by breakdown, the ions must be accelerated in the electric and magnetic fields. Such devices are called accelerators (see particle accelerators). Various accelerators can enable ions to obtain high energy (such as hundreds of gigahertons), or slow down the ions to obtain ion beams with low energy (such as dozens of electron volts) but high current intensity.
Application of ion source
① Ion doping and ion beam modification
Since the 1960s, people have implanted a certain amount of boron, phosphorus or other elements into semiconductor materials to form doping. The doping depth can be controlled by changing the ion energy; The doping concentration can be controlled by the integrated ion current intensity. The repeatability and reliability of ion implantation method are better than that of diffusion method. Ion implantation doping has become an important link in the production of semiconductor large-scale integrated circuits. It has become an inevitable trend to replace the old diffusion process with ion implantation in some devices.
The results of ion implantation in the modification of metal materials are very attractive. In the modification of common metals by ion implantation, the hardness, corrosion resistance and fatigue strength of metals can be improved, and the wear rate of metals can be reduced. Some insulating materials, such as pottery, glass and organic materials, undergo important changes in properties after being irradiated by ion beam, and obtain new applications.
The process of ion beam irradiation and doping is a non thermal equilibrium process, so new materials that cannot be obtained by general metallurgical and chemical methods can be obtained by this method. The small accelerator "ion implanter" with low energy (50-400keV), which is specially used for ion implantation, has become a special equipment. Its volume is equivalent to an electron microscope or a high-voltage oscilloscope, and it is very convenient to use and maintain. Ion beam has been widely used in the research of diamond-like carbon materials, high temperature superconductors, magnetic materials, photosensitive materials, etc. A new metallurgy - "ion implantation metallurgy" is emerging.
② Ion beam analysis
The interaction between ions with certain energy and substances will cause them to emit electrons, photons, X-rays, etc., and may also cause elastic scattering, inelastic scattering and nuclear reaction, resulting in rebound ions, recoil nuclei γ X-ray, hydrogen nucleus, tritium nucleus, particle and other nuclear reaction products can provide information about the composition, structure and state of the material. Using this information to analyze samples is generally referred to as ion beam analysis. Among the ion beam analysis methods, the relatively mature ones include back scattering analysis, X-ray fluorescence analysis, nuclear reaction analysis, channeling effect (see channeling effect and blocking effect) and other analysis methods. In addition, low energy ion beam can also be used for surface composition analysis, such as ion scattering spectroscopy (ZSS), secondary ion mass spectrometry (SZMS), etc. Ultra sensitive mass spectrometry (accelerator mass spectrometry), charged particle activation analysis, ion excitation spectroscopy, and ion excitation Auger electron spectroscopy are under development. MV level accelerator for ion beam analysis has special commercial equipment.
③ Ion beam processing
Low energy ion beams are widely used in industrial processing, such as ion thinning, ion polishing, ion beam drilling, ion beam etching, ion beam sputtering metal films, etc.
