Progress in manufacturing and processing of Al-Sc alloy targets

The development of the fifth-generation mobile communication technology (5G) puts forward higher requirements for radio frequency (RF) filters. Due to its higher frequency and larger bandwidth, bulk acoustic wave (BAW) filter will become the mainstream RF filter solution. Scandium doped aluminium nitride (AlScN) piezoelectric film has attracted much attention due to its excellent piezoelectric response coefficients and electromechanical coupling coefficients. The doped Sc concentration will directly affect the piezoelectric coefficient and electromechanical coupling coefficient of the film. The most common technique for producing AlScN thin films is magnetron sputtering of targets made of Al-Sc alloy. However, with the increase of Sc content, the solid-liquid solidification interval, the number of intermetallic compounds, and the brittleness of the material will all increase significantly. Additionally, it will result in component macro-segregation, uneven phase distribution, and poor processing deformability. Therefore, it is extremely difficult to manufacture and process Al-Sc alloy targets. The mainstream processes for preparing Al-Sc alloy targets are divided into direct reaction method and powder metallurgy method. This paper reviews the research progress in manufacturing and processing of Al-Sc alloy targets.


Introduction
The fifth-generation of mobile communication technology (5G) is a brand-new class of broadband mobile communication technology that offers fast speed, zero latency, and wide connection.The development of 5G communication technology has led to an urgent demand for radio frequency (RF) filters with higher frequency and wider bandwidth.Surface acoustic wave (SAW) filters and bulk acoustic wave (BAW) filters are the two main kinds.Figure 1 shows the application and trends of the SAW and BAW filters.Since the BAW filter is more suitable for the broadband and high frequency of 5G mobile, it will become the mainstream RF filter solution.
Undoped aluminium nitride (AlN) is typically selected as the piezoelectric thin film material for BAW filters due to its superior physical properties such as high stability, high thermal conductivity, and high acoustic wave transmission rate.Recently, scandium doped aluminium nitride (AlScN) has attracted much attention due to its significantly enhanced piezoelectric response coefficients and electromechanical coupling coefficients, which allows it to replace the role of AlN for RF filters.
AlScN thin films have been reported to be deposited using a variety of methods, including metal organic chemical vapor deposition [1][2] , molecular beam epitaxy [3][4] , and magnetron sputtering [5][6][7][8] .The most common approach for producing AlScN thin films is magnetron sputtering, which primarily involves co-sputtering of single metal targets and direct sputtering of Al-Sc alloy targets.However, the manufacturing and processing of Al-Sc alloy targets is extremely difficult, and there are few products that can meet market demand.Therefore, it is crucial to thoroughly research the manufacturing and processing of Al-Sc alloy targets.

Al-Sc Binary Phase Diagram
The binary Al-Sc phase diagram has been studied using thermodynamic simulations and experimental data ever since Savitsky et al. found the Al-Sc phase diagram in 1964 [9] .Early phase diagram studies focused on the Al-rich side, but less research on the Sc-rich side [10][11] .The research in recent decades has basically been slightly modified based on the initial Al-Sc phase diagram [12][13][14][15] .
Based on the new experimental data and considerations related to thermodynamics, Okamoto et al. suggested an updated version of the phase diagram [16] .However, the ambiguities of phase equilibria and the differences in thermodynamic data prohibited a reliable thermodynamic optimization of the system.
Cacciamani et al. investigated the Al-Sc binary system by combining experimental measurements and thermodynamic optimization [17] .They focused on the phase equilibrium with Sc content higher than 40 at%.The results confirmed some of Okamoto's theoretical speculations.Besides, two new invariant points have been found in the Sc-rich region: a eutectic reaction at 1185 ℃: (1) and a eutectoid one at 970 ℃. ( Based on these recent discoveries, the modern Al-Sc phase diagram is presented in figure 2 [18] .
Al and Sc have significantly different melting points, making it difficult for Sc to dissolve in solid form in Al.With the increase of Sc content, the solid-liquid solidification interval, the number of intermetallic compounds, and the brittleness of the material all increase significantly.It is more prone to problems such as macro-segregation, uneven phase distribution, and poor processing deformability.Al-Sc alloy targets are extremely difficult to manufacture and process as a result.

AlScN Piezoelectric Film
AlN has a hexagonal wurtzite structure and C-axis preferred orientation.The strong (001) texture of the AlN layer is the prerequisite for obtaining good piezoelectric performance [19] .
The research of Akiyama et al. showed that the piezoelectric response coefficient d33 of AlN film will increase with the increase of doped Sc concentration, which confirms Takeuchi's theoretical calculations for the ScN structure [20][21] .Figure 3 showed the effect of temperature on piezoelectric response.A significant rise in the piezoelectric response coefficient was found for the substrate temperature of 400 °C, with the maximum value of 27.6 pC/N at the Sc concentration of 43% [22] .
The structure of the film becomes more and more disordered as the substrate temperature and Sc concentration rise.The creation of the cubic ScN phase with a rock-salt structure is caused by the rise of Sc content, which causes the decrease of piezoelectric properties.The substrate temperature and the N2 concentration in sputtering gas are important factors influencing the piezoelectric coefficient [23] .Especially, the N2 concentration is the most important control factor.Momida et al. used the first principles to study the relationship between the piezoelectric coefficient of ScxAl1-xN and the crystal structure [24] .The study found that with the increase of Sc content, the piezoelectric coefficient of ScxAl1-xN first increased until it reached the maximum peak and then decreased, and the corresponding crystal structure of ScxAl1-xN gradually changed from the wurtzite structure to the rock salt structure..
In recent years, researchers have conducted extensive research on improving the piezoelectric coefficients of AlScN thin films.As shown in figure 4, the results reported by Lu have extremely high Sc concentration and piezoelectric coefficient d33, and still maintain the c-axis crystal orientation [25] .Figure 4. Piezoelectric coefficients at different Sc concentrations [25] .
In addition, a great deal of work has been conducted on raising the electromechanical coupling coefficients kt 2 of AlScN thin films.It can be observed from figure 5 that kt 2 increased as the Sc concentration increases from 0 to 41%.kt 2 decreased significantly when the Sc concentration exceeded 47%.This decrease may be due to the phase transition from a piezoelectric wurtzite to a nonpiezoelectric cubic [26] .Figure 5. Electromechanical coupling coefficients at different Sc concentrations [26] .

Manufacturing and processing of Al-Sc alloy targets
Currently, the primary techniques for producing Al-Sc binary alloys are divided into three methods: metallothermic reduction [27][28] , molten salt electrolysis [29][30][31] , and direct reaction [32][33] .The raw material for metallothermic reduction is commonly scandium fluoride or scandium oxide, while the reductant is metal Al.Only 70-80% of the raw materials of Sc can be converted into the alloy using this method, and the conversion rate is not very high.The molten salt electrolysis method has problems such as low current efficiency and high energy consumption, and harmful gases will also be generated during the reaction process.Al-Sc intermediate alloys have traditionally been melted using the direct reaction method, which has a straightforward procedure and few technical requirements.However, due to the large difference in the melting points of Al and Sc, it is easy to cause uneven alloy composition, and the burning loss of Sc is serious.
The Al-Sc alloy prepared by the above methods has a low content of Sc, less than 2 wt%, and a high content of impurity elements, which is not suitable for high-end target products.Since the Al-Sc alloy target has extremely high requirements on uniformity and purity, conventional preparation methods cannot meet the existing needs.Therefore, it is necessary to explore the manufacturing and processing technology of Al-Sc alloy with higher purity.At present, the mainstream processes for preparing Al-Sc alloy targets are divided into direct reaction method and powder metallurgy method.
The global high-purity metal sputtering target market for integrated circuits is dominated by American and Japanese companies.The international companies such as JX Nippon Mining & Metals master the core technologies from the preparation of high-purity metal to the manufacturing and processing of target.Compared with the targets produced by well-known international companies, the quality of the target made in China still has great room for further optimization and improvement.There are still problems such as low density, uneven microstructure, and low material utilization.Therefore, there is still an urgent need to improve and upgrade the manufacturing and processing of Al-Sc alloy targets.

Direct Reaction Method
The direct reaction method is a traditional way for preparing Al-Sc alloy.Specifically, pure Sc and Al are used as raw materials and melted in the crucible.Then the molten Al-Sc alloy is poured directly into the mold.This method is simple and easy to process.However, there is a substantial waste of raw materials, and the alloy is prone to segregation.
To assure the high purity of the material, Al-Sc alloy targets with the atomic fraction of Sc lower than 20% can be manufactured by the direct reaction process.However, there are still problems such as macroscopic composition segregation and many metallurgical defects.Therefore, the researchers modified the smelting process to address these potential issues.
Huang et al. obtained Al-Sc alloy with uniform composition, high purity, and low oxygen content by improving the doping method and repeated suspension melting [34] .The metal Al is added to the metal Sc in multiple doses, with each dose not exceeding 200% of the total Sc mass.Each melting process lasts for 5-15 minutes, and after cooling, the next mixing and melting process is continued until the desired Al-Sc alloy is obtained.The Al-Sc alloy is then subjected to processes such as pressure processing and heat treatment to obtain an internal microstructure with fine grains and uniformity, as shown in figure 6. Figure 6.Metallographic structure before (a) and after (b) pressure processing and heat treatment [34] .
Cai et al. carried out repeated arc melting and electromagnetic stirring under argon atmosphere, which can significantly improve the uniformity of alloy composition [35] .Yao et al. combined vacuum electromagnetic levitation melting with electron beam melting to ensure alloy purity and composition uniformity [36] .The structural and chemical composition of the solidified Al-Sc alloy ingot is uniform without obvious defects, and it can also reduce the oxygen content in the Al-Sc alloy.
By controlling the melt temperature, the microstructure uniformity can be improved.Ding et al. proposed to use pure Al and Al3Sc intermediate compound powders as raw materials [37] .To completely melt Al ingots while keeping the Al3Sc powders from melting, researchers placed them into crucibles made of alumina or zirconia and used medium frequency induction melting at the temperature of 700-900 °C.The high-temperature molten Al-Sc alloy is then rapidly cast into the mold, cooled, and solidified to produce a target billet that is nearly finished in size.The Al-Sc alloy target prepared by this method is directly obtained by melting and casting to obtain near final formed target blank, eliminating the intermediate pressure processing and heat treatment processes, and avoiding problems such as component segregation and coarse grains.The obtained target alloy has precise composition and uniform and fine microstructure.
In a typical casting process, the solidification process of the molten alloy usually proceeds from the mold wall to the center.For Al-Sc alloys with Sc content below 25 at%, the outermost regions will cool much faster than the central part of the casting, so the Al3Sc grains will exhibit a finer grain size than the central region.When the cooling rate is not controlled, the macro-segregation is severe, and the content of intermetallic compound Al3Sc increases, which will make the casting more and more brittle, and the casting is more prone to cracking in the subsequent processing.Myasnikov et al. recommend controlling the cooling rate so that the molten alloy completely fills the mold before macro-segregation occurs, keeping the cooling rate high enough to prevent segregation and slow enough to solidify and cool the casting without cracking the alloy [38] .The ingot is then subjected to hot rolling, hot forging, hot pressing and hot isostatic pressing to obtain a sputtering target with a uniform microstructure and no cracking.
When a sputtering target is manufactured using only the smelting method, the hardness of the entire target varies greatly, and the target is prone to cracking during sputtering.Morii et al. smelted highpurity Al and Sc together under inert gas or vacuum environment to obtain Al-Sc alloy ingot with low oxygen content [39] .The ingot is then heated to 500-1200 °C and forged with deformation of 50% to 95%.The hardness variation of the sputtering target can be reduced to less than 20%, which can effectively prevent cracking during sputtering.During the forging process, the Sc-rich phase (white part in figure 7) is cut off to form islands, so that uniform grain boundary pinning occurs, thereby reducing the hardness deviation.Figure 7. Metallographic structure of ingot after hot forging [39] .
Conventionally, it is difficult to obtain higher amounts of the ductile Al phase in alloys with higher Sc content.Because the alloys obtained by the casting method are in phase equilibrium and the microstructure has a lower Al content and phase volume ratio as predicted by the Al-Sc equilibrium phase diagram.Research has shown that using a specific amount of second phase in non-equilibrium alloys, such as the brittle intermetallic compound AlxScy phase, can maximize the content of ductile Al phase and minimize the content of brittle intermetallic compound AlxScy phase, while maintaining the overall composition percentage in the alloys.Van Heerden et al. obtained Al-Sc alloy targets with a higher content of ductile Al phase by preparing non-equilibrium alloys containing first phase and brittle second phase [40] .By adding metastable ductile phases such as "free aluminium" to the microstructure, the toughness and strength of the material are increased, chemical uniformity is improved, and the arcing and particle problems are reduced during the sputtering process.
In summary, through improving the doping method, using electromagnetic stirring, and controlling the melt temperature, the uniformity of the alloy composition and microstructure can be significantly improved.Repeated smelting can significantly reduce the impurity content and oxygen content of the target.And the degree of segregation can be significantly improved by using rapid cooling.Pressure processing can make the microstructure denser and the grain size more uniform and finer, while improving the sputtering performance of the target.Adding more ductile Al phase by preparing nonequilibrium alloys can reduce arcing and particle problems.

Powder Metallurgy Method
The number of intermetallic compounds, the solid-liquid solidification interval, and the material's brittleness all increase as Sc concentration grows.The direct reaction method is not suitable for the preparation of Al-Sc alloy targets with high Sc content.The powder metallurgy method is to form Al-Sc alloy targets by sintering powder at high temperature.It can release the internal stress in the target blank forming process, making large-scale forming of Al-Sc alloy targets with high Sc content possible.However, it is very likely to contain holes, which will cause arcing and particle problems during sputtering.Additionally, the purity, particle size and oxygen content of the powder are important factors affecting the quality of the target.

Maruko et al. prepared
Al-Sc alloy powders with an average particle size of less than 200 μm through gas atomization [41] .The smaller the average particle size, the faster the cooling rate, and the better it can suppress the precipitation of intermetallic compounds.Then hot pressing sintering or spark plasma sintering was performed at 800-1200 ℃.Sintering can be completed in a short time at low temperature with this method, and the composition distribution of powder produced by gas atomization can be easily maintained.Figure 8 compares the microstructure of alloys prepared by sintering and smelting processes.It can be observed that the precipitates in the sintered microstructure are relatively fine, which greatly improves the segregation phenomenon.Figure 8. Microstructure of sintering (a) and smelting (b) [41] .
The melt-spinning method was used by Wu et al. to prepare Al-Sc alloy powders [42] .The Al-Sc master alloy was made into thin strips using the melt-spinning method.Al-Sc alloy strips with low oxygen content, fine grains and uniform composition can be obtained by this way.Subsequently, the thin strips were pulverized into powders using the jet mill under argon atmosphere.The whole technological process is simple in operation, easy in industrialization and low in cost.And the oxygen mass content of the target is controlled within 0.05%.
Mechanical ball milling was employed by Liu et al. to create Al-Sc alloy powders with the 30% mass fraction of Sc [43] .Then they used spark plasma sintering technology to prepare Al-Sc alloy with high Sc content.The research results showed that spark plasma sintering can realize the rapid densification of Al-Sc alloy with high Sc content.The microscopic morphology at different sintering temperatures is shown in figure 9.When the sintering temperature was 450 °C, there were many porosities between the powder particles, which prevented the particles from completely densifying.The sintered samples were dense, without holes, and with no visible grain boundaries when the sintering temperature exceeded 500 °C.The relative density of the sintered sample can be increased to 92.19% by modifying the sintering process parameter.600 ℃ [43] .
Jia et al. suggested a method of preparing low segregation Al-Sc alloy targets [44] .The principle is to use the capillary interaction between dry tiny Al powder and slightly moist large Sc powder to obtain homogeneous adsorption of Al powder on the surface of Sc powder.A sintering neck between the Sc and Al powders is then formed through low-temperature sintering, resulting in a core-shell structure Al-Sc alloy powder coated with Al.When the Al-Sc alloy powder with this core-shell structure is created, there are no secondary segregation phenomena brought on by the differences between the two powders since the composition of the alloy powder is homogeneous.A low segregation Al-Sc alloy target can be made after further hot pressing sintering.
To address the issue of low target density, Wang et al. combined Sc powder with the second phase compound Al3Sc powder [45] .Since the melting point of Sc (1541 °C) and Al3Sc (1320 °C) are both high, the powder sintering temperature can be increased to 1000-1300 °C, which improves the density of the Al-Sc alloy target.Additionally, the second phase can be disseminated more evenly and dispersedly because the Al3Sc powder is added directly.Yao et al. prepared Al-Sc alloy targets through two hot-pressing sintering methods, which can effectively reduce the generation of brittle phases [46] .The first low-temperature hot pressing sintering allows the Al-Sc target to be pre-sintered to achieve initial densification, and the low sintering temperature will not produce many brittle alloy phases.The second high-temperature hot pressing sintering has a low deformation rate, and even if some alloy phases are generated, it will not cause cracking of the target.
In summary, the fabrication process for powder includes gas atomization, ball milling and meltspinning.An appropriate procedure can be used to produce Al-Sc alloy powder with fine grain size, consistent composition, and low oxygen concentration.The density of the target can be increased by selecting powders with appropriate composition.In addition, optimizing the sintering process can reduce the brittleness of the target and improve its processing performance.

Conclusions
The development of 5G communication technology has led to an urgent demand for RF filters with higher frequency and wider bandwidth.Due to its superior piezoelectric characteristics and electromechanical coupling coefficient, AlScN can take the place of AlN in RF filters.The mainstream method of producing AlScN thin films is magnetron sputtering, which raises a strong need for Al-Sc alloy targets.
However, the manufacturing and processing of Al-Sc alloy targets is quite challenging due to the physical characteristics of Sc.As the content of Sc increases, problems such as macro-segregation of components, uneven phase distribution, and poor processing deformation ability are more likely to occur.In addition, the content of Sc will also affect the piezoelectric coefficient and electromechanical coupling coefficient of AlScN film.
Since the Al-Sc alloy target has extremely high requirements on uniformity and purity, conventional preparation methods cannot meet the existing needs.At present, the mainstream processes for manufacturing and processing Al-Sc alloy targets are divided into direct reaction method and powder metallurgy method.
The direct reaction method is simple and easy to process.However, there are issues such as substantial raw material loss, severe segregation, and numerous metallurgical defects.The degree of segregation and microstructure uniformity can be considerably improved by improving the doping method, optimizing the smelting process, and rationally applying pressure processing methods.
The powder metallurgy method can make large-scale forming of Al-Sc alloy targets with high Sc content possible.However, the purity, particle size and oxygen content of the powder will greatly affect the quality of the target.By selecting appropriate powder preparation processes and optimizing sintering processes, Al-Sc alloy targets with low segregation, high density, and uniform microstructure can be obtained.
The global high-purity metal sputtering target market for integrated circuits is dominated by American and Japanese companies.When compared to the targets manufactured by well-known international companies, the quality of the targets produced in China has significant space for additional optimization and improvement.There are still problems such as low density, uneven microstructure, and low material utilization.Therefore, the manufacturing and processing of Al-Sc alloy targets is still one of the key research projects that urgently needs to be improved and upgraded.

Figure 1 .
Figure 1.Application and trends of the SAW and BAW filters.