sputtering deposition technology
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Sputtering coating refers to the technique of bombarding the surface of the target material with charged particles in a vacuum chamber, hitting the atoms and other particles in the target material through particle momentum transfer, and precipitating them on the substrate to form a film. Sputtering coating technology has the advantages of large area rapid deposition, good adhesion between film and matrix, high sputtering density, few pinholes, good film controllability and repeatability, etc., and any substance can be sputtering, so it has developed rapidly and been widely used in recent years.
The sputtering mechanism
When the incident ion bombards the target surface, some of its energy is transferred to the surface lattice atoms, causing the atoms in the target to move. Some atoms gain energy and shift from the lattice and overcome the surface barrier to sputtering directly. Some cannot break away from the lattice, can only do in situ vibration and affect the surrounding atoms, the result of the target temperature rise; While some atoms gain enough energy to produce a recoil, which will shift the neighboring atoms' collision, and the recoil will continue to produce high-order recoil, a process called cascade collision. The result of cascade collision is that some atoms reach the surface and overcome the barrier to escape, which forms cascade sputtering and this is the sputtering mechanism. When the recoil atom density is not high in the range of cascade collision, the dynamic recoil atom collisions can be ignored, which is called linear cascade collision.
Diode sputtering is the earliest and simplest basic sputtering method. The dc diode sputtering device is composed of anode and cathode. The target made of film material (conductor) is used as the cathode, and the workpiece frame placed on the plated part is used as the anode (ground). The spacing between the two poles is generally from centimeters to about ten centimeters. When the electric field intensity reaches a certain value in the vacuum chamber, abnormal glow discharge occurs between the electrodes. Ar + ions in plasma are accelerated and bombarded on the cathode target, and target atoms are deposited on the substrate to form thin films.
Three pole sputtering
The diode sputtering method is simple, but the discharge is unstable and the deposition rate is low. In order to increase the sputtering rate and improve the film quality, a tri-pole sputtering device was made by adding a heated cathode on the basis of a diode sputtering device.
In tripole sputtering, the plasma density can be controlled by changing the electron emission current and acceleration voltage. The bombarding energy of ion on target material can be controlled by target voltage, and the contradiction between target voltage, target current and pressure in sputtering is solved.
The disadvantage of tripole sputtering is unstable discharge and uneven film thickness caused by uneven plasma density. To this end, an auxiliary anode was added on top of the tripole sputtering to form a quadrupole sputtering.
Magnetron sputtering is also called high speed low temperature sputtering. Working gas ions (such as Ar+) in plasma under the constraint of magnetic field and enhancement, under the acceleration of the target cathode electric field, bombard the cathode material, so that atoms or molecules on the material surface fly away from the target surface, and after passing through the plasma area, they will be deposited on the substrate surface, migrate and finally form a film.
Compared with the diode sputtering, the deposition rate of magnetron sputtering is high, the substrate temperature is low, the film quality is good, the repeatability is good, and it is convenient for industrial production. Its development has led to a great revolution in film preparation technology.
Magnetron sputtering sources must have two basic conditions in structure:
(1) establish a magnetic field perpendicular to the electric field;
(2) the direction of the magnetic field is parallel to the surface of the cathode and an annular magnetic field is formed.
It can be seen from the schematic diagram of planar magnetron target structure that the magnetron sputtering source is actually a magnet placed behind the cathode target of diode sputtering, and the magnet generates a horizontal magnetic field on the target surface. When ions bombard the target, secondary electrons are released. These electrons have a long motion path and are bound by the electromagnetic field in the plasma region near the target surface and circle around the runway. In this region, a large amount of Ar + is ionized by frequent collisions to bombard the target, thus achieving high-speed sputtering. After several collisions, the energy of electrons gradually decreases, and they gradually move away from the target surface, and finally fly to the anode substrate with very low energy, which makes the temperature rise of the substrate also lower. Due to the increase of the binding effect of the orthogonal electromagnetic field on the electrons, the discharge voltage (500 ~ 600V) and the air pressure (10-1 Pa) are much lower than the dc diode sputtering.
Reactive magnetron sputtering
Taking metals, alloys, low-valent metal compounds or semiconductor materials as target cathodes, they react with gas particles to generate compound films in the process of sputtering or in the process of depositing film on the substrate surface, which is reactive magnetron sputtering. Reactive magnetron sputtering is widely used in the mass production of compound films because:
(1) high purity of target materials (single-element target or multi-element target) and reaction gases (oxygen, nitrogen, hydrocarbons, etc.) used in reactive magnetron sputtering is conducive to the preparation of high purity compound films.
(2) by adjusting the process parameters in reactive magnetron sputtering, compound films with chemical or non-chemical ratios can be prepared and their properties can be regulated by adjusting the composition of the films.
(3) in the process of reactive magnetron sputtering deposition, the substrate heats up less, and the substrate is not required to be heated at high temperature in the film-making process, so there are fewer restrictions on the substrate materials.
(4) reactive magnetron sputtering is suitable for the preparation of large area uniform film, and can achieve the single machine annual output of millions of square meters of coating industrial production.
In 1985, Window et al. first introduced the concept of non-equilibrium magnetron sputtering and gave the principle design of non-equilibrium magnetron sputtering plane target. For a magnetron sputtering target, the magnetic field intensity of the outer ring is equal to or close to that of the central magnetic pole, which is called "balanced magnetron sputtering target". A nonequilibrium magnetron sputtering target is formed when the magnetic field of one magnetic pole increases or decreases with respect to a part of the opposite polarity. By means of non-equilibrium magnetron sputtering, the plasma on the cathode target surface is guided to the range of 200mm to 300mm before the sputtering target by additional magnetic field, and the substrate is immersed in the plasma. In this way, on the one hand, the sputtered particles are deposited on the surface of the substrate to form a film, and on the other hand, the plasma bombards the substrate to play an ion-assisted role, greatly improving the quality of the film. In addition to a high sputtering rate, non-equilibrium magnetron sputtering can output more ions to the coating area, and the ion concentration is proportional to the discharge current of the sputtering target. At present, the technology is widely used to prepare various hard films. The magnetic field of non - equilibrium magnetron sputtering can be divided into closed field and non - closed field. The closed magnetic field can control the electrons to move along the magnetic force line only in the magnetic field, avoiding the loss of electrons on the vacuum chamber wall.