WO2025168349A1 - Taal target - Google Patents

Abstract

The invention relates to a sputtering target produced by powder metallurgy, comprising between 25 and 75 at% Ta, with Al as the balance, and unavoidable metallic and nonmetallic impurities, characterized in that the sputtering target has an Al phase and a Ta phase and is free of intermetallic phases, and where the Al phase and the Ta phase are distributed uniformly in the sputtering target, measured both parallel to a target surface and perpendicularly to a target surface.

Description

TaAI target

The present invention relates to a tantalum (Ta)-aluminum (Al) target, which is particularly suitable as a sputtering target, and which was produced by powder metallurgy. The present invention further relates to a method for producing a TaAl target via a

5 powder metallurgy route.

Targets according to the invention can be used in many different physical vapor deposition processes (usually and also in the following a PVD process (physical vapor deposition = PVD)) with the help of which layers are deposited from the gas phase, such as the

10 Arc evaporation (cathodic arc deposition or arc-source evaporation technology) or cathode sputtering (already referred to above and hereinafter as sputtering or sputter deposition). In particular, the invention therefore relates not only, but particularly to sputter targets such as those used in a PVD sputtering process for depositing layers onto a substrate material intended for this purpose. Sputtering is a physical

15 Process in which atoms are released from a sputtering target by bombardment with energetic ions and pass into the gas phase.

Sputtering targets containing Ta and Al are already known from the state of the art. These can be produced either by melt metallurgy or powder metallurgy. If a sputtering target is produced by powder metallurgy, there are many different

20 possibilities, which must be selected according to the composition of the target and taking into account the properties of the integrated elements. Examples of these are manufacturing steps such as pressing, sintering, hot pressing (HP), hot isostatic pressing (HIP), spark plasma sintering (SPS), and various combinations thereof. However, with Ta-Al targets, it should be noted that, particularly during the steps of hot pressing (HP), hot isostatic pressing (HIP), or spark plasma sintering (SPS) (and combinations thereof), so-called tantalum aluminides, i.e., intermetallic compounds/phases of tantalum and aluminum (such as TaAH or Ta2Al), form in the target. This is because the temperature during these manufacturing steps is often less than 150°C below or even above the melting temperature of pure aluminum (melting temperature of pure

30 aluminum: 660°C), which accelerates diffusion processes and enables the formation of intermetallic phases. Tantalum aluminides increase the brittleness of the target material and reduce its thermal conductivity, leading to disadvantages of the

Public The formation of these intermetallic phases reduces the thermal conductivity of the sputtering target, so that effective cooling of the target during coating operations is not guaranteed. High proportions of the intermetallic phases can lead to the fracture of the target during use as a sputtering target, especially

5 by the pressure of the cooling water applied to the back of the target in some coating systems, either directly or via a flexible membrane. This effect increases with the wear of the target and the decreasing residual thickness of the target. Likewise, the intermetallic compounds impair a homogeneous deposition of the sputtering target. This is due to the different bonding states on

10 the different sputtering yield of the intermetallic phases compared to the pure elements.

The Japanese patent application JP2021110004 A describes a sputtering target material containing aluminum and one or more refractory metals M (selected from Ta, W, Nb, and Mo). The aluminum content in this target is between 1 at.% and 70 at.% and the

15 Refractory metal content between 30 at.% and 99 at.%. During the production of the inventive target of this application, a sintering step (preferably between 800-1250°C) is carried out, whereby the aluminum content melts, while the refractory metal content does not melt, thus increasing the density of the target material. Diffusion processes can occur between the two phases Ta and Al, resulting in the formation of intermetallic phases that can be detected by X-ray diffraction measurement (see Fig. 1: Diffraction pattern for the Ta-Al system with intermetallic phases according to the invention of JP2021110004).

In contrast, the sputtering target of the present invention (see Fig. 2: Phase analysis for the sputtering target according to the invention) shows no intermetallic phases.

25 The Chinese patent application CN112111714 A claims a process in which 86.5 to 87.5 wt.% (49 to 51 at.%) Ta powder is used, the remainder being aluminum powder and unavoidable impurities. The process involves a hot-press sintering step at 1050 to 1150°C (i.e., already well above the melting temperature of aluminum). Therefore, it can be assumed that, according to the phase analysis for the Ta-Al system,

30 intermetallic phases, which is not mentioned in the application.

Public The application only describes a uniform structure - a precise description of the microstructure of the target or of various phases in the target is not provided in the application.

Chinese patent application CN111945121 A also claims a method for producing a Ta-Al sputtering target. Here, the amount of aluminum is said to be 12.5 to 13.5 wt.%.

5 (48.9 to 51.1 at.% Al). The process according to the invention involves a two-stage hot isostatic pressing step, with the second step being carried out at a temperature of 750-950°C. Therefore, as already described above, it can be assumed that intermetallic phases will also form in this target. The application does not describe a microstructure.

10 Japanese patent application JPH09241835 A describes a sputtering target made of Al and M (M = Ta, Zr, Ti, Hf, Nb, Cr, W), with M present in an amount of 0.5 to 30 at.%. Here, the sputtering target is said to exhibit a concentration gradient of M along the thickness of the sputtering target.

Patent application EP 0 243 995 A2 discloses a method for producing a target for

15 describes cathode sputtering, where the target is composed of aluminum with lead, titanium, and/or tantalum. In the sole embodiment of this application, an aluminum/titanium target (in an atomic ratio of 1:1) is produced. However, the description does not disclose which quantities of lead, tantalum, and/or combinations of the elements lead, tantalum, and titanium are to be used.

20 The object of the present invention is to provide a Ta-Al sputtering target with a high density, which has a uniform or homogeneous distribution of a pure Al phase and a pure Ta phase both on the surface of the sputtering target and throughout the entire volume of the sputtering target. This enables uniform sputtering, so that the various phases are removed from the target simultaneously in the same amount. Likewise, the target according to the invention has no intermetallic phases, so that only pure Al and Ta phases are homogeneously distributed throughout the entire volume of the target. Thus, a sputtering target is provided that has a particularly high thermal conductivity, which ensures good cooling of the target during coating and is advantageous for the PVD process.

30 in turn allows a high performance during sputtering, which leads to correspondingly high

Public This leads to a faster growth rate of the deposited layers. Furthermore, there is a lower risk of target breakage.

Furthermore, it is an object of the invention to provide a manufacturing method for a Ta-Al sputtering target while avoiding the aforementioned disadvantages. The manufacturing method according to the invention

5 is intended to enable the production of a Ta-Al sputtering target with a high degree of purity and a homogeneous and fine-grained microstructure. Furthermore, the manufacturing process is intended to be cost-effective and reproducible.

The technical problem of the present invention is solved by a sputtering target having the features of claim 1 and a method having the features of claim 9.

10 Advantageous further developments of the invention can be found in the dependent claims, which can be freely combined with one another.

According to the present invention, the sputtering target comprises between 25 and 75 at.% Ta, balance Al, as well as unavoidable metallic and non-metallic impurities, and is characterized in that the sputtering target has an Al phase and a Ta phase and is free from

15 intermetallic phases, with the Al phase and the Ta phase being uniformly distributed in the sputtering target both parallel to a target surface and perpendicular to a target surface.

The Al phase and the Ta phase are the pure/elementary components AI (i.e. Al grains) and Ta (i.e. Ta grains), which are also used as initial components

20. This results in a two-phase microstructure in the sputtering target, which can be analyzed in a simple manner known to those skilled in the art using a metallographic section and evaluation under a light microscope or scanning electron microscope. The two-phase microstructure or two-phase structure of the sputtering target is understood to mean the presence of a pure Al phase and a pure Ta phase. However, additional phases such as oxides or pores can be present in the target material. The proportion of these phases should be kept as low as possible, as they can have a negative impact on the sputtering behavior, in particular the homogeneity of the deposition and the roughness of the layers. For example, oxides can promote the occurrence of local melting (arcing), which in turn leads to the formation of particles.

30 (droplets) and increased roughness of the layers. The microstructure is consistent or homogeneous over the entire volume of the sputtering target, i.e. both parallel and

Public also perpendicular to the target surface. There are no macroscopic concentration gradients with respect to the elements used in the target.

Preferably, the total proportion of the elemental Al and Ta phases in the sputtering target is at least 98%, particularly preferably at least 99%. The total proportion of elemental Al-

5 and Ta phases is determined by the purity levels of the powders used, as well as the proportion of measured metallic impurities in the sputtering target.

The presence of a pure Al phase as well as a pure Ta phase in a sputtering target according to the invention can be easily detected by X-ray diffraction (XRD) (taking into account the respective X-ray detection limit) using the relevant JCPDS (Joint Committee on Powder Diffraction Standards) cards.

The term intermetallic phases refers to phases that occur in binary, ternary, or multicomponent systems and that differ from the pure components. They often have crystal structures that differ from the pure components.

15 different crystal structures and proportions of non-metallic bond types. Intermetallic phases are primarily characterized by a narrow stoichiometric composition. Intermetallic phases are often brittle, thus possessing low toughness, which usually has a detrimental effect on the target material. Furthermore, intermetallic phases have the disadvantage that they exhibit different erosion rates across the target material compared to the elemental phases, which can influence the growth of the deposited layers.

The present sputtering target is free of intermetallic phases. This can be easily verified by X-ray diffraction (XRD) (taking into account the respective X-ray detection limit) using the relevant JCPDS cards.

25 can be proven.

As already explained above, such intermetallic phases occur preferentially in aluminum-containing targets when temperatures just below (less than 150°C) or above 660°C (melting temperature of pure aluminum) are used in the manufacturing process.

Public The term "unavoidable impurities" refers to manufacturing-related impurities of gases or accompanying elements that originate from the raw materials used. A distinction can be made between metallic and non-metallic impurities. Metallic impurities include, for example, Fe,

5 Co, Ni, Cr, Mn, Mo, W, Cu, Zn, Si, Ca, Ti etc. The proportion of such impurities in the sputtering target according to the invention is preferably in the range of less than 3000 pg/g (corresponds to 3000 ppm), preferably less than 2000 pg/g, particularly preferably less than 1500 pg/g. Non-metallic impurities can be gases such as C, O, N, H, S etc. The preferred range here is less than 2000 pg/g, preferably less than 1000 pg/g. Suitable methods for chemical

Elemental analysis is known to depend on the chemical element being analyzed. For the chemical analysis of the unavoidable impurities according to the invention, hot extraction analysis for the elements O or N (see ASTM E 1019:2018), combustion analysis for the elements C and S (see ASTM E 1019:2018), ICP-MS (inductively coupled plasma mass spectroscopy), or ICP-OES (inductively coupled plasma optical emission spectrometry) was used. The chemical analysis of the main components Al and Ta in the sputtering target according to the invention was performed using X-ray fluorescence spectroscopy (XRF).

The sputtering target made of the composite Ta-Al material is characterized by particularly good thermal conductivity, which in turn allows for high sputtering performance.

20 The inventive target enables particularly uniform removal for the deposition of thin layers using PVD. Both metallic layers and reactive nitride or carbide layers, and mixtures thereof, can be deposited. The TaAl-based layers can be used as metallic protective layers for electronic components in microprocessors, as metallic conductors, or in ceramic form (e.g., by adding nitrogen during the coating process) as hard material layers to minimize wear on tools or components.

Preferably, the sputtering target according to the invention consists of 25-75 at.% Ta, balance Al and a maximum proportion of unavoidable metallic impurities of < 3000 pg/g and

30 non-metallic impurities of < 2000 pg/g. If the Ta content is above 75 at.%, the plastic deformation of the TaAl target during densification can no longer be fully

Public This means the target becomes more porous, brittle, and therefore more susceptible to breakage. The amount of tantalum in the target is preferably between 40 and 60 at.%.

In particular, the oxygen content of the sputtering target according to the invention is below 1000 pg/g, particularly preferably below 800 pg/g. The oxygen content can be easily

5 hot extraction analysis.

In a preferred embodiment, the metallic purity of the sputtering targets is greater than 98 wt.%, preferably greater than 99 wt.%, even more preferably greater than 99.7 wt.%.

A sputtering target according to the invention preferably has a density of more than 95% of the theoretical density. A density of more than 97% of the theoretical density is particularly advantageous. The theoretical density is the maximum achievable density of the Ta-Al target, provided that no internal cavities or impurities, as well as no intermetallic phases, are present. The higher the density of the target, the more advantageous its properties. Targets with a lower density have a higher proportion of pores. These pores can be formed during the manufacturing process (e.g., during machining).

15 impurities, such as lubricants, can be absorbed. These impurities would later negatively affect the quality of the deposited layers. Furthermore, low-density targets tend to absorb water from the environment, which can lead to vacuum processes that are difficult to control. Furthermore, the thermal conductivity of low-density material is lower than that of higher-density material.

The theoretical density of metallic composites can be calculated arithmetically from the densities of the individual pure metals and their weight fractions. The density can be determined, for example, using the Archimedes method by determining the weight of the target in air and in water.

According to a preferred embodiment, the sputtering target according to the invention has a

25 average grain size of the Ta phase (i.e., the Ta grains in the Ta phase) and Al phase (i.e., the Al grains in the Al phase) of less than 80 pm each, preferably an average grain size of less than 63 pm each, particularly preferably less than 45 pm each. An average grain size of the Ta and Al phases of less than 80 pm leads to particularly uniform sputtering behavior and thus to the deposition of particularly homogeneous layers.

Public The mean grain size of the phases can be easily determined by a line-section method, e.g. according to ASTM E112, on a metallographic section.

In a preferred embodiment, the sputtering target according to the invention has a textured structure with a grain size ratio of at least 1.2, preferably at least 1.4.

5 Texturing is achieved by uniaxial forming, preferably by forging, of the powder-metallurgically produced sputtering target. The grain stretching perpendicular to the forming direction is calculated from the ratio of the mean chord lengths of the Al grains, whereby the mean chord lengths of the Al grains are measured parallel to the target surface as well as perpendicular to the target surface. For this purpose,

10, the mean chord length of the aluminum grains in the microstructure is determined, firstly along the grain extension (perpendicular to the forming direction) and secondly perpendicular to it (parallel to the forming direction). Finally, the ratio between the two determined mean chord lengths is calculated.

The microstructure images for determining grain elongation are taken on metallographic

Fifteen cross-sections were taken, the surfaces of which were oriented parallel to the forming or forging direction. At 100x magnification, lines were placed equidistantly from edge to edge of the image. The mean chord length of the Al grains was measured in both directions (forming and normal to it), and the grain elongation was calculated.

20 By installing a sputtering target according to the invention in different coating systems and for coating substrates of different geometries, different geometric requirements are placed on a sputtering target according to the invention. Such a target can be in the form of a flat target, for example, a plate or disc, in the form of a rod, in the form of a tubular target, or as another complex-shaped body.

A sputtering target according to the invention is preferably a round flat target.

In a preferred embodiment, the sputtering target (as a round flat target) has a diameter of more than 280 mm. This size places special demands on an optimal and homogeneous microstructure. For the forming of round blanks

30 with these dimensions, a powder mixture must be provided that provides a homogeneous, demixing-free mixture of the elementary components of the

Public This is difficult due to the large differences in the densities of the elements (2.7 g/cm ³ for Al and 16.7 g/cm ³ for Ta). On the other hand, a high forging force must be applied, which allows the achievement of a high density of the composite material, while observing moderate (at least 150°C below the

5 melting point of pure aluminum) forging temperatures to avoid the formation of intermetallic phases between Ta and Al.

The process according to the invention for producing a powder metallurgically produced Ta-Al sputtering target, which comprises between 25 and 75 at.% Ta, the remainder Al, as well as unavoidable metallic and non-metallic impurities, is characterized by the following

10 steps:

Mixing the Al powder and the Ta powder to form a powder mixture;

Cold pressing of the powder mixture into a shaped body;

Forging the shaped body at temperatures below the melting temperature of aluminum, which results in densification of the target,

15 characterized in that the sputtering target has an Al phase and a Ta phase and is free of intermetallic phases, and wherein the Al phase and the Ta phases are uniformly distributed in the sputtering target both parallel to a target surface and perpendicular to a target surface.

A powder mixture suitable for use in the process according to the invention is prepared by mixing the appropriate amounts of Ta and Al. The powders are poured into a suitable mixing device and mixed until a homogeneous distribution of the components in the powder mixture is ensured. The morphology of the powders used and the particle size distribution range of the powders used are selected to form a homogeneous, segregation-free mixture.

25 The powder mixture thus produced is filled into a die or tube for the compaction step, in this case cold pressing, and then pressed into a shaped body. This is preferably carried out in a cold isostatic step (where hydraulic pressure acts on the powder from all sides). During pressing in the die, the body is compacted uniaxially in a tool.

Public The shaped body is then compacted below the melting temperature of aluminum by forging, preferably in a single forging step. Hydraulic forging with semi-open, open, or closed dies is preferably used. In a preferred embodiment, the

5 The forging temperature is set at 350 to 450°C. The forgings are heated in a furnace for 1-3 hours in a protective gas atmosphere to prevent oxidation or nitriding of the Al and Ta components of the forging. The forging die itself is preheated to a temperature of around 150 to 250°C before forging.

The sputtering target produced in this process usually has a density of

10 at least 95% of the theoretical density, preferably at least 97% of the theoretical density. The forging step deliberately creates a textured structure with a grain stretch of at least 1.2, preferably 1.4. As already described above, the grain stretch is calculated from the ratio of the mean chord lengths of the Al grains, whereby the mean chord lengths of the Al grains are measured parallel to the target surface as well as perpendicular to the target surface. During the forging process, in addition to the densification of the material, flow also occurs from the center to the edge of the forging die, which in this context leads to the reorientation and stretching of the grains or grain agglomerates.

The final processing of the forged blank into a target according to the drawing with the

20 required dimensions and tolerances are achieved by machining processes such as turning and/or milling.

In a preferred embodiment, the shaped body produced by the process according to the invention has a diameter of more than 280 mm. This size places particular demands on an optimal and homogeneous microstructure. As already explained above, a powder mixture must be provided for a shaped body with these dimensions that ensures a homogeneous, demixing-free mixture of the elemental components of the material. This is difficult due to the large differences in the densities of the elements (2.7 g/cm ³ for Al and 16.7 g/cm ³ for Ta). Furthermore, a high forging force must be applied, which makes it impossible to achieve

30 a high density of the composite material while observing moderate (at least

Public Forging temperatures (150°C below the melting point of pure aluminum) are permitted to avoid the formation of intermetallic phases between Ta and Al.

Further advantages and benefits of the invention will become apparent from the following description of an embodiment with reference to the accompanying figures.

5 Of the figures show

Fig. 1: Phase diagram of the Ta-Al system (Source: ASM Handbook Vol. Ill, Alloy Phase

Diagrams 1992);

Fig. 2: X-ray diffractogram of a sample of a TaAl-

Sputtering targets, pure Al phase and pure Ta phase detectable;

10 Fig. 3: Microstructure of a TaAl sputtering target according to the invention (in the

magnification 100x) in a light micrograph;

Fig. 4: Microstructure of the TaAl sputtering target according to the invention from Fig. 3 in 200x magnification.

15

Tantalum powder with a particle size of less than 63 pm (D90 measurement according to Malvern) and aluminum powder with a particle size of less than 63 pm (D90 measurement according to Malvern) were used as raw materials. Care was taken to select powders with low levels of impurities, especially low amounts of oxygen and iron.

20. The powders were placed in a closed container in a ratio of 45 at.% Ta and 55 at.% Al and mixed in a free-fall mixer. Once a homogeneous mix has been achieved, the powder mixture is poured into a tube mold and cold-isostatically pressed using a hydraulic press at a pressure of approximately 200 MPa to form a disc-shaped green body with a relative density of approximately 90% and an outer diameter of approximately 300 mm. The green body thus produced was face-turned on its end faces. The green body was then heated to a temperature of 360 °C and compacted by forging. The die was preheated to approximately 180 °C. A single forging step was sufficient to achieve the target thickness of 23 mm. After compaction by forging, the target was machined into its round shape with

30 with an outer diameter of approximately 298 mm and a thickness of 19 mm. The resulting

Public Target was used in an industrial PVD sputtering system and could be sputtered at high rates.

Figure 2 shows the X-ray diffraction pattern of this example. For the evaluation of the diffraction patterns, the JCPDA cards 00-0040788 (Ta) (corresponds to the Ta phase), 00-004-

5 0787 (AI) (corresponds to the Al phase) is used. From this figure, it can be seen that these are pure phases - 2Theta angles for pure Ta at 38.5; 55.5; 69.5; 82.5, 95; 107.5;

121.5, and 2Theta angles for pure Al at 38.5; 44.5; 65.78; 82.5; 137.5. This figure shows that no intermetallic phases (such as Ta2Al, TaAl, or TaAH) are formed. The peaks arise due to the pure Al phase and the pure Ta phase.

10 Fig. 3 and Fig. 4 show the light micrographs of this example at a magnification indicated by the scale bar. The microstructure shows Ta grains or Ta grain agglomerates (dark gray) and Al grains (light color). The black color indicates pores caused by the powder metallurgical production or artifacts that occurred during the micrograph.

15

Public

Claims

1. A powder metallurgically produced sputtering target comprising between 25 and 75 at.% Ta, the remainder being Al, as well as unavoidable metallic and non-metallic impurities, characterized in that the sputtering target has an Al phase and a Ta phase and is free of intermetallic phases, and wherein the Al phase and the Ta phase are uniformly distributed in the sputtering target both parallel to a target surface and perpendicular to a target surface. 2. Sputtering target according to claim 1, characterized in that the proportion of Ta is between 40 and 60 at.%. 3. Sputtering target according to one of the preceding claims, wherein the maximum proportion of metallic impurities in the sputtering target is < 3000 ppm. 4. Sputtering target according to one of the preceding claims, wherein the maximum proportion of non-metallic impurities in the sputtering target is < 2000 ppm. 5. Sputtering target according to one of the preceding claims, wherein the density of the sputtering target is at least 95%. 6. Sputtering target according to one of the preceding claims, wherein the sputtering target has an average grain size of the Ta phase and the Al phase of less than 80 pm each. 7. Sputtering target according to one of the preceding claims, wherein the sputtering target has a textured structure with a grain elongation of at least 1.2, wherein the grain elongation is calculated from the formation of a ratio of the mean chord lengths, which are measured parallel to the target surface and perpendicular to the target surface, of the Al grains in the Al phase. 8. Sputtering target according to one of the preceding claims, characterized in that the sputtering target is a round flat target and has a diameter of more than 280 mm. 9. A process for producing a Ta-Al sputtering target comprising between 25 and 75 at.% Ta, balance Al, as well as unavoidable metallic and non-metallic impurities, via the powder metallurgy route, characterized by the following steps: Mixing the Al powder and the Ta powder to form a powder mixture; Cold pressing of the powder mixture into a shaped body; Forging the shaped body at temperatures below the melting temperature of aluminum, whereby densification of the target occurs, characterized in that the sputtering target has an Al phase and a Ta phase and is free of intermetallic phases, and wherein the Al phase and the Ta phase are uniformly distributed in the sputtering target both parallel to a target surface and perpendicular to a target surface. 10. A method for producing a Ta-Al sputtering target according to claim 9, characterized in that the forging takes place at a temperature between 350°C and 450°C. 11. Method according to one of claims 9 or 10, characterized in that a further mechanical processing step takes place after forging. 12. Method according to one of claims 9 to 11, characterized in that the shaped body is round both after cold pressing and after forging and has a diameter of more than 280 mm. 13. Method according to one of claims 9 to 12, characterized in that the target after forging has a textured structure with a grain stretch of at least 1.2, wherein the grain stretch is calculated from the formation of a ratio of the mean chord lengths, which are measured parallel to the target surface and perpendicular to the target surface, of the Al grains in the Al phase.