Magnetron sputtering is a kind of physical vapor deposition (PVD). The general sputtering method can be used to prepare metals, semiconductors, insulators and other materials, and has the advantages of simple equipment, easy control, large coating area and strong adhesion. The magnetron sputtering method developed in the 1970s realizes high speed, low temperature and low damage. It is necessary to improve the ionization rate of the gas effectively because the sputtering is carried out at low pressure. Magnetron sputtering increases the plasma density and sputtering rate by introducing a magnetic field on the cathode surface of the target, which is constrained by the magnetic field on the charged particles.
Definition of magnetron sputtering
A closed magnetic field parallel to the target surface is added in the dipole sputtering. With the help of the orthogonal electromagnetic field formed on the target surface, the secondary electrons are bound to the specific area of the target surface to enhance the ionization efficiency, increase the ion density and energy, and realize the high-speed sputtering process.
Principle of magnetron sputtering
The working principle of magnetron sputtering is that under the action of electric field E, electrons collide with argon atoms in the process of flying to the substrate, which ionizes them to produce ar positive ions and new electrons; the new electrons fly to the substrate, and Ar ions accelerate to fly to the cathode target under the action of electric field, and bombard the target surface with high energy to make the target sputtering. In the sputtered particles, the neutral target atoms or molecules are deposited on the substrate to form a thin film, and the secondary electrons generated will be affected by electric and magnetic fields, and their trajectories are similar to a cycloid.
If the magnetic field is annular, the electrons will move around the target surface in the form of approximate cycloid. Their motion path is not only very long, but also bound in the plasma region close to the target surface, where a large amount of AR is ionized to bombard the target, thus achieving high deposition rate. With the increase of the number of collisions, the energy of the secondary electrons is exhausted, gradually away from the target surface, and finally deposited on the substrate under the action of electric field E.
Because the energy of the electron is very low, the energy transferred to the substrate is very small, resulting in the low temperature rise of the substrate. Magnetron sputtering is a collision process between incident particles and target. The incident particles undergo a complex scattering process in the target. They collide with the target atoms and transfer part of the momentum to the target atoms. The target atoms collide with other target atoms to form a cascade process. In this cascade process, some target atoms near the surface gain enough momentum to move outward, leaving the target to be sputtered out.
There are many kinds of magnetron sputtering. Each has its own working principle and application object. However, there is one thing in common: the interaction between magnetic field and electric field makes electrons spiral around the target surface, thus increasing the probability of electrons impacting argon to produce ions. The generated ions collide with the target surface under the action of electric field, thus splashing out the target.
The target source is divided into balanced type and non-equilibrium type. The coating of balanced target source is uniform, and the coating layer of unbalanced target source has strong adhesion to substrate. Balanced target source is mostly used for semiconductor optical film, while non-equilibrium target source is mostly used for wear decorative film. According to the distribution of magnetic field configuration, magnetron cathode can be divided into equilibrium state and unbalanced state.
The magnetic flux of the magnetic steel inside and outside the balanced state magnetic control cathode is approximately the same. The magnetic field lines of the two poles are closed to the target surface, which can well restrain the electron / plasma near the target surface, increase the collision probability and improve the ionization efficiency. Therefore, the glow discharge can be started and maintained at low working pressure and voltage. The utilization rate of the target material is relatively high The concept of unbalanced magnetron sputtering technology is that the magnetic flux of the outer magnetic pole of the magnetron cathode is greater than that of the inner magnetic pole, the two pole magnetic lines of force are not completely closed on the target surface, and some magnetic lines of force can extend to the substrate area along the edge of the target, so that some electrons can extend to the substrate along the magnetic lines, thus increasing the plasma density and gas ionization rate in the substrate area No matter the balance is not balanced, if the magnet is still, its magnetic field characteristics determine that the utilization rate of the general target is less than 30%.
In order to increase the utilization rate of target, rotating magnetic field can be used. However, the rotating magnetic field needs a rotating mechanism and the sputtering rate should be reduced. Rotating magnetic field is usually used for large or expensive targets. Such as semiconductor film sputtering. For small-scale equipment and general industrial equipment, static magnetic field target source is often used. It is easy to sputter metals and alloys with magnetron target source, and ignition and sputtering are very convenient.
This is because the target (cathode), plasma, and splashed parts / vacuum chamber can form loops. But if the insulator is sputtered, such as ceramic, the circuit is broken. So people use high-frequency power supply, adding a strong capacitance in the circuit. In this way, the target becomes a capacitor in the insulation circuit. But the high frequency magnetron sputtering power supply is expensive, the sputtering rate is very small, and the grounding technology is very complex, so it is difficult to use in large scale. To solve this problem, magnetron reactive sputtering was invented. It's using a metal target, adding argon and a reaction gas such as nitrogen or oxygen.
When the metal target collides with the part, due to the energy conversion, it combines with the reaction gas to form nitride or oxide. Magnetron reactive sputtering insulator seems easy, but it is difficult to operate. The main problem is that the reaction occurs not only on the surface of parts, but also on the surface of anode, vacuum chamber and target source. It can cause fire extinguishing, arc striking on the surface of target source and workpiece. The twin target source technology invented by LeiBao in Germany has solved this problem very well. The principle is that a pair of target sources are anode and cathode to eliminate oxidation or nitriding on the anode surface. Cooling is necessary for all sources (magnetron, multi arc, ion), because a large part of the energy is converted into heat. If there is no cooling or insufficient cooling, this heat will make the target source temperature reach more than 1000 degrees, thus melting the whole target source.