Features of sputtering coating:

(1) For any material to be coated, as long as it can be made into a target, sputtering can be achieved;
(2) The thin film obtained by sputtering is well combined with the substrate,
(3) The film obtained by sputtering has high purity and good density;
(4) The sputtering process has good repeatability and controllable film thickness. At the same time, a thin film with uniform thickness can be obtained on a large-area substrate.
 

Sputtering method:

DC sputtering: suitable for good conductors
RF sputtering: suitable for insulators, semiconductors and conductors, etc.
Magnetron sputtering: low deposition temperature and high rate
Reactive sputtering: the thin film material is different from the target material
Ion beam sputtering: target and substrate are extremely irrelevant to acceleration.
 

1. DC sputtering

 
DC sputtering, also known as cathode sputtering or two-pole sputtering, is suitable for various alloy films with good conductivity.
 

1.1 The influence of working air pressure on film quality

 
When the sputtering pressure is low, the atoms incident on the surface of the substrate have not undergone many collisions, so their energy is higher, which is beneficial to improve the diffusion ability of the atoms during deposition and increase the density of the deposited tissue.
The increase of the sputtering pressure reduces the energy of the incident atoms, which is not conducive to the densification of the film structure.
 

1.2 Disadvantages of DC sputtering

 
Cannot independently control various process parameters, such as cathode voltage, current, and sputtering pressure;
The pressure used is high (about 10Pa), the sputtering rate is low, and the film quality (density, purity) is poor.
 

2. RF sputtering

 
A sputtering deposition method suitable for various metallic and non-metallic materials
 

2.1 The self-biasing effect of the target

 
Since the speed of electrons is much greater than that of ions, any part near the plasma is at a negative potential compared to the plasma.
 
Imagine that there is no charge accumulation on one electrode at the beginning. Driven by radio frequency voltage, it can be used as an anode to accept a large number of electrons, and as a cathode to accept ions. In a positive half cycle, the electrode will accept a large number of electrons and make itself negatively charged. In the next negative half cycle, it will accept a small amount of slower-moving ions, so that part of its negative charge will be neutralized. After such a few cycles, the electrode will carry a certain amount of negative charge and present a certain negative potential to the plasma.
 
Therefore, this large potential difference makes the target electrode actually under a negative bias, which drives the plasma to hit the target electrode after acceleration, thereby forming continuous sputtering on the target.
 

2.2 The characteristics of the radio frequency sputtering method

It can generate self-bias effect to achieve bombardment and sputtering of the target material and deposit on the substrate;
 
The process of spontaneously generating negative bias is independent of whether the target material used is a conductor. However, when the target material is a metal conductor, the power supply must be capacitively coupled to the target material to isolate the path of charge flow, thereby forming a self-bias;
 
Compared with the case of DC sputtering, the radio frequency sputtering method can directly couple energy to the electrons in the plasma, so its working pressure and corresponding target voltage are lower.
 

3. Magnetron sputtering equipment

 
Magnetron sputtering method has unique advantages because of its higher deposition rate (an order of magnitude higher than other sputtering methods) and lower working pressure.
 

3.1 Two disadvantages of general sputtering deposition methods:

 
a. The deposition rate is lower than the evaporation method
 
b. The required working pressure is high, otherwise the mean free path of electrons is too long, and the discharge phenomenon is not easy to maintain, resulting in a higher possibility of film contamination.
 

3.2 Features of magnetron sputtering

 
a. The working pressure is low, the deposition rate is high, and the possibility of film contamination is reduced;
 
b. Low target voltage required to sustain discharge
 
c. The bombardment energy of electrons on the substrate is small, which can reduce the damage of the substrate and lower the deposition temperature;
 
d. It is easy to realize low-temperature deposition of thin films on substrates such as plastics.
 

3.3 Disadvantages of magnetron sputtering

 
The sputtering of the target material is not uniform, and the uniformity of the film needs to be further improved;
 
It is not suitable for the sputtering of ferromagnetic materials. If ferromagnetic materials are sputtered, there is little leakage of magnetic field, and no magnetic field lines pass through in the plasma.
 

4. Ion beam sputtering equipment

4.1 Features of ion beam sputtering

 
The gas impurity is small and the purity is high, because the sputtering is carried out under a higher vacuum condition.
 
Since there is no plasma near the substrate, a series of problems such as plasma bombardment that cause the substrate temperature rise, electron and ion bombardment damage, etc., will not occur.
 
Because the ion beam energy, beam current size and beam direction can be accurately controlled, and the sputtered atoms can directly deposit thin films without collision, the ion beam sputtering method is very suitable as a research method for thin film deposition.
 

Sputtering target

Pure metals and alloys: prepared by smelting or powder metallurgy, with good purity and density.
Compound: Prepared by powder hot pressing method, the purity and density are relatively poor.