Will the grain size and crystal orientation affect the quality of the sputtering targets?

Among the sputtering targets of the same composition, the targets having a smaller grain size have a faster deposition rate than those having a larger grain size.

In addition to affecting the sputtering rate, the grain size also affects the quality of the obtained film. If the grain size is too large and the sputtering time is short, the film layer may be poorly densified, resulting in oxidative release of the surface of the coated product.

The grain size has a smaller effect on uniformity. Studies have shown that for the four metal materials of the same material prepared by the same preparation process, changing the grain size from 0.5 to 3.3 mm through different heat treatment times does not change the uniformity of the film layer. Therefore, the grain size has little or no effect on the uniformity of film formation, but the uniformity of grain size can directly affect the uniformity of film formation.

In view of the constant consumption of the sputtering target, in addition to considering the uniformity of the same layer of the target, uniformity in the direction of the thickness of the target should also be considered. The grain size of different cross-sections is required to be as uniform as possible to ensure the uniformity of sputtering at different times.

The image below is a comparison of the microstructure of NiCr sputtering targets from different manufacturers. It can be seen from the photo of the crystal phase that the grain size and uniformity of the target a are better than that of the target b, so that the target a corresponds to a higher quality of the sputtering deposition film.

According to research by Energy Research of Japan, if the grain size of the titanium sputtering target is controlled to be less than 100 lm and the change in grain size is kept within 20%, the quality of the film obtained by sputtering can be greatly improved.

For polycrystals, the crystal grains of the crystal are arranged to some extent along certain special orientations. During the sputtering process of the sputtering target, the target atoms are easily sputtered along the direction in which the atoms are most closely arranged. The crystal orientation of the material has a great influence on the sputtering rate and film thickness uniformity. Therefore, the sputtering rate and film forming quality can be improved by changing the crystal structure of the sputtering target. 

For example, by controlling the processing process of the silicon sputtering target to make the crystal grains have a certain preferred orientation, the film thickness deviation of the film layer can be reduced from 10% to 5%.

Different materials have different crystal structures, so different molding and heat treatment methods should be used to make the target have grain orientation, thereby increasing the film formation rate and film quality of sputtering deposition.

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What is the mechanism of pulsed laser deposition?

Although the equipment of pulsed laser deposition (PLD) system is simple, its working mechanism is related to many complicated physical phenomena. It includes all physical interactions between the laser and the substance when the high-energy pulsed radiation strikes the solid target, the formation of plasma plumes and the transfer of the molten material through the plasma plume to the surface of the heated substrate. Therefore, PLD can generally be divided into the following three stages:

Interaction between laser radiation and the target

In this stage, the laser beam is focused on the surface of the target. When sufficient high energy flux and short pulse width are achieved, all elements of the target surface are rapidly heated to the evaporation temperature. At this point, the material in the target will be sputtered from the target. The instantaneous melting rate of the target is highly dependent on the flow of laser light onto the target. The melting mechanism involves many complex physical phenomena such as collisions, heat, excitation with electrons, delamination, and fluid mechanics.

Dynamics of molten matter

In the second stage, according to the law of aerodynamics, the sputtered particles have a tendency to move toward the substrate. The space thickness varies with the function cosn θ, and n>>1. The area of the laser spot and the temperature of the plasma have an important influence on the uniformity of the deposited film. The distance between the target and the substrate is another factor that affects the angular extent of the molten material. It has also been found that placing a baffle close to the substrate narrows the angular extent.

Deposition of molten material on the substrate

The third stage is the key to determining the quality of the film. The high-energy nuclides emitted hit the surface of the substrate and may cause various damages to the substrate. The high energy nuclide sputters some of the atoms on the surface, and a collision zone is established between the incident stream and the sputtered atoms. The film is formed immediately after the formation of this thermal energy zone (collision zone), which is the best place to condense particles. As long as the condensation rate is higher than the release rate of the sputtered particles, the heat balance condition can be quickly reached, and the film can be formed on the surface of the substrate due to the weakened flow of the molten particles.

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Related: The development of Pulsed Laser Deposition