Methods for preparing high-quality powder for targets

Powder metallurgy process can produce excellent quality magnetic targets, in which powder quality affects the quality of the target to a large extent, the production of high-quality target has an important role. This paper mainly introduces the preparation methods of powder for targets, summarizes the preparation methods of fine powder and the characteristics of different preparation methods. Finally, it summarizes the shortcomings of the current research on powder for targets, proposes new research ideas, and prospects its future development.


Introduction
In recent years, with the rapid development of the semiconductor industry, the demand for targets is increasing.After the concept of giant magnetoresistance effect was proposed by M.N.Baibich et al [1] ., the target can be used in computer memory devices, and the magnetic random-access memory (MRAM) produced by subsequent research and development has outstanding performance, which requires the quality of the target used in memory, and the key to the quality control of the target is the preparation of high-performance powder.The preparation method of high-quality powder needs to be considered from different angles, first of all, the shape of the powder should be considered, the near-spherical powder can have higher fluidity, and the billet after sintering has higher density, but also consider the satellite powder and hollow powder contained in the powder, which will also have a certain impact on the density of the sintered billet, and should also pay attention to the oxygen increment of the target.The pulverizing process should ensure that as little oxygen element is mixed as possible, so that the powder has a low oxygen content.Finally, and of course the most important, the control of impurity elements, high purity powder is an important requirement for semiconductor materials, to ensure that the powder does not introduce impurity elements is crucial, such as magnetic target CoFeB, impurity elements Al, Cu, Si, etc., seriously affect the quality of the film, reduce the yield.Excellent quality powder can improve the quality of the target to a certain extent, and provide reference value for the production of high-quality targets.

Methods for preparing high-quality powder for targets
Powder preparation methods mainly include mechanical crushing method and atomization method.The mechanical crushing method includes ball milling method and air flow crushing method.The mechanical crushing method is simple in powder preparation but the yield of fine powder is low.Atomization methods include vacuum induction melting gas atomization method, electrode induction melting gas atomization method and plasma rotating electrode atomization method.Powder preparation method should be based on the actual demand, and the appropriate powder preparation method should be selected comprehensively.

Ball milling
Ball milling is an important method to prepare powder by mechanical crushing [2] .According to the research of Yang et al [3] ., the theoretical optimum process of powder preparation by ball milling method is as follows: the rotation rate is 76 %-88 % theoretical value, the medium filling rate is 0.3, and the pellet ratio is 0.6, which can be used to prepare high-quality powder.The particle size of the powder prepared by ball milling decreases with the increase of grinding time, but there will also be a large block, which makes the powder yield in a specific particle size range is low, and it is difficult to control the particle size of the powder (see figure 1).In addition, the sphericity is poor, most of them are irregular blocky, which also makes the powder flow weak, and it is possible to form holes in the interior after pressing and sintering directly, reducing the density of the pressed embryo.In order to obtain the powder with better sphericity by ball milling method, additional ball chemical sequence is required, which increases the complexity of the production process.In consideration of the preparation of targets, the good sphericity of the powder indicates the good fluidity of the powder, and the high loose density of the powder, and the target embryo obtained by subsequent pressing and sintering can have a higher pressing density of the embryo.However, ball milling powder with higher hardness will introduce impurity elements Fe and Cr, so the hardness of the material is not easy to be too high, while taking into account easy access to air, the material is not easy to oxidize, currently suitable for the preparation of high-quality powder with Cu-based targets.3 h, (c) 6 h, (d) 12 h, (e) 24 h [4] .Taking the planetary grinding method as an example, the production process is as follows [5] : before grinding, the grinding tank is cleaned and dried, and then loaded into the grinding pieces, grinding additives, protective liquid and other materials, the gas in the grinding tank is extracted and filled with argon, and the sound insulation shield is closed after installation to the specified position of the instrument, and the grinding is carried out according to demand.The production of powder by ball milling method has the characteristics of simple production process, fine processing and uniform mixing, easy to achieve large-scale production, etc.At the same time, it also has the following disadvantages, such as difficult production of spherical powder, long production cycle of powder, low yield of fine powder, and easy to generate heat.

Jet milling
Air flow crushing is an important method for preparing fine powder with high purity and low oxygen content.When the feeding pressure and crushing pressure increase, the powder particle size decreases.With the increase of feeding amount, the particle size increases.The raw material of air crushing can be powder or small diameter particles.The particle size distribution of the original powder is relatively wide, and some powder particles with larger diameters exist (see Figure 2a).After the air flow crushing method, the sphericity of the powder is relatively good, the particle size is narrow, the dispersion is good, there is no agglomeration phenomenon, the sphericity of the powder is significantly improved, the powder fluidity is improved (see Figure 2b).For the production of powder, the air flow crushing method can ensure a narrow particle size distribution and a high degree of dispersion, which is conducive to the densification of the production target.In addition, the air crushing method can continuously produce powder with higher purity and specific particle size in large quantities, and the of processes such as minus screening can improve production efficiency to a certain extent.The powder produced by this method uses gas as the grinding medium, does not introduce other harmful impurities, and is suitable for the production of rare earth targets and Al-based targets.Air crushing production process [6][7][8] : raw materials are treated before grinding, the oxide layer is removed and dried, loaded into the feeding area, the gas in the equipment is extracted and the protective gas is filled, the high-speed air is turned on and the material is continuously fed, and the powder in the classification area is selected according to the required powder particle size.The production of powder by air crushing method can realize the integration of grinding and screening, reduce the production process, improve the production efficiency, and have the advantages of high grinding precision, high efficiency, energy saving and environmental protection.The equipment used by the air crushing method is expensive, and additional regular maintenance is required, further increasing the production cost, and at the same time, due to the high grinding precision and high-quality requirements for raw materials, additional management and monitoring methods are required.

Vacuum induction-melting gas atomization
Vacuum induction-melting gas atomization (VIGA) is the most commonly used atomization method for powder preparation.In the VIGA pulverization process [9] , powder size and particle size distribution are controlled by melt atomization temperature and atomization pressure.For the preparation of powder, the higher the temperature of the metal melt, the lower the viscosity of the metal melt, and the easier it is to be broken into fine droplets to form fine powder during atomization.However, when the melt temperature increases to a certain extent, the higher superheat increases the condensation time of the droplets, and it is easier to adhere to each other during atomization, increasing the probability of satellite powder, which is not conducive to the formation of the powder.The higher the pressure of the atomizing medium, the lower the negative pressure at the end of the diversion tube, the faster the gas flow rate, the faster the energy exchange rate between the gas and the droplet, and the smaller the powder particle size.The powder prepared by VIGA has high properties and purity (see figure 3).The powder has a high sphericity, the surface of the powder is smooth in the particle size range of 15~53 μm, and the sphericity of the powder decreases with the particle size increasing in the range of 53~180 μm.For the production of targets, the powder prepared by VIGA has good sphericity and surface finish, and can also obtain powders with different particle size distributions according to different atomization pressures, which is convenient for subsequent target densification production.The atomization process is carried out under the protection of inert gas to ensure that the powder has a low oxygen content, which is suitable for the production of most high-purity powders, but it cannot be used for ultra-high melting point materials such as titanium alloys.and (b,d)54~180 μm [10] . .
The powder produced by atomization method is easy to form a small-size powder and adhere to the surface of the large-size powder into a satellite-like powder structure, that is, satellite powder (see figure 5).The presence of a large amount of satellite powder in the powder will affect the sphericity of the powder and reduce the overall fluidity and loose density.Guan et al [9] .summarized two different theories to explain the occurrence principle of satellite powder, providing theoretical basis for reducing the occurrence of satellite powder.VIGA production process [12] : First of all, the surface oxides and impurities of the metal raw materials are disposed of by pickling, alkali washing and cleaning, and the samples are dried.After the treated metal raw materials are put into the electric furnace, the metal materials are melted into alloy liquid, and then transported to the tundish.The leakage hole at the bottom of the tundish flows out into the nozzle, and the gas is opened when the temperature reaches the requirement.The air flow is sprayed into the atomizing cylinder of the atomizing mechanism, and the alloy melt flowing down the nozzle is atomized into small droplets when it contacts the high-speed air flow, and forms alloy powder after cooling and solidification.VIGA has pure powder, low oxygen content, high fine powder recovery rate, and atomized high-pressure protective gas is supersonic gas-liquid two-phase flow state, can quickly reduce the powder temperature, get a good sphericity appearance, with the advantages of high sphericity.The spherical powder formed by rapid cooling in the protective gas during the atomization process has the characteristics of no segregation and low oxygen increment.The oxygen content, particle size and particle size distribution of different alloy powders can be adjusted by adjusting the vacuum degree, atomization pressure, metal melt temperature and other process parameters to optimize the pulverization process.

Electrode induction-melting gas atomization
Electrode induction-melting gas atomization (EIGA) is an important method for preparing high purity metal powders (see figure 6).Compared with the gas atomization of vacuum induction melting, the melt in the process of electrode induction melting gas atomization does not directly contact the crucible and other moulds, and does not introduce other impurities, so the powder has a higher purity.
The research of Jin Ying et al [13] .showed that the atomization pressure had a great influence on the average particle size, fine powder yield, powder morphology, loose packing density and fluidity of the powder.With the increase of atomization pressure within a certain range, the powder particle size decreases continuously, and the powder yield of fine powder increases (see figure 7).However, the surface morphology of powder deteriorates, the sphericity decreases, the amount of satellite powder gradually increases, and the particle size of adhesive powder on the surface of satellite powder increases continuously.When the atomization pressure increases to a certain extent, some non-spherical powder particles will appear.13] .
The increase of atomization pressure also increases the proportion of hollow powder in the powder (see figure 8 and figure 9)..
Figure 9.Effect of gas pressure on apparent density and fluidity [13] .
EIGA production process [13] : Melt the required elements into the specified size of the alloy bar material, transfer the alloy bar material into the induction heating chamber, adjust the melting power, the alloy bar material is heated by the induction coil and melted, under the pressure difference between the induction heating chamber and the atomization chamber, the metal melt flows from the induction heating chamber into the atomization chamber, so that the metal melt breaks into droplets under the high-speed protective gas flow.It is cooled in the atomizing chamber and solidified into spherical powder, which is used after screening.EIGA can prepare most of the powder that can be prefabricated into alloy rods.At present, the powder used in titanium alloy targets is mainly prepared with EIGA.Because it is not in direct contact with the water-cooled crucible, the material will not be polluted, and the high chemical purity is guaranteed without introducing other impurities.The shortcomings are also more obvious, the atomization process needs to accurately control the melt state, high technical requirements, but also need to melt alloy rod material as the raw material for atomization powder, the performance of alloy rod material directly affects the performance of powder, alloy rod material segregation will cause powder composition uneven situation, at the same time, due to the atomization process, melt superheat is low.The powder does not have a long enough cooling time for spheroidization, so the sphericity of the powder is low, and the amount of satellite powder is large, and the low yield of fine powder is also a problem at present.

Plasma rotating electrode processing
The powder produced by plasma rotating electrode processing (PREP) has better sphericity compared to commonly used aerosol powders (see figure 10).Because the pulverizing principle is different, the powder no longer relies on high-speed airflow crushing but relies on centrifugal force crushing, making the powder sphericity is higher, the amount of satellite powder is less, the powder surface is relatively clean, and the powder quality can be ensured.
PREP produces a relatively small amount of hollow powder (see figure 11)..
PREP production process [14][15] : After the metal raw material is surface treated, the oxides and impurities are removed and washed and dried, the alloy bar material is melted into the specified size, the alloy bar material is put into the atomization chamber, the atomization chamber is vacuumed, the temperature of the protective gas in the atomization chamber is raised within a certain range, and then the cooling system is started, the pulverizing equipment is turned on, and the plasma gun power supply is started when the alloy bar material is rotated to a certain speed.The high-temperature plasma arc is generated between the alloy bar and the plasma gun, and the tiny droplets generated by the end face of the alloy bar fall into the cooling system and solidify into powder.
Compared with VIGA and EIGA, PREP has higher powder quality, higher powder chemical purity, better sphericity, smaller satellite powder and hollow powder, and can obtain a narrower powder particle size.At the same time, the particle size and particle size of the powder can be accurately controlled to prepare suitable powder.Because the powder is prepared directly from the alloy bar, the composition uniformity of the alloy bar has a great influence on the powder.Multi-component targets and materials that are not easy to prepare alloy bars are not suitable for this method, such as Al or CoFeB targets preparation.

Characteristics of different pulverizing methods
There are many ways to prepare powder before, and different methods have different characteristics, as shown in Table 1.The spherical degree of powder prepared by ball milling method is low, the oxygen increment after grinding is high, it is difficult to meet the production of high-quality target, suitable for use in the preparation process of powder performance requirements are low, the target purity requirements are not high; The air flow crushing method can be used to grind small particles or strips, and can also be used for secondary grinding of powder to improve the sphericity of powder, and to adjust the particle size and particle size distribution of powder to some extent.The powder prepared by aerosol method has higher purity and lower oxygen increment, and the sphericity of the powder has a certain guarantee, but the powder prepared by aerosol preparation has more satellite powder and hollow powder, although the proportion of the two powders can be reduced to a certain extent through the process, there will still be a certain amount of satellite powder and hollow powder; EIGA and PREP use rod material to prepare powder, which increases the process of preparing alloy rod material and increases the production cost to a certain extent.Compared with other powder production methods, the powder purity is higher and can meet the production requirements of high purity targets.

Figure 1 .
Figure 1.SEM micrographs of powder at different milling times: (a) unmilled labeled as 0 h, (b)3 h, (c) 6 h, (d) 12 h, (e) 24 h[4]  .Taking the planetary grinding method as an example, the production process is as follows[5]  : before grinding, the grinding tank is cleaned and dried, and then loaded into the grinding pieces, grinding additives, protective liquid and other materials, the gas in the grinding tank is extracted and filled with