Silicon-containing transition-metal boride evaporation source

Abstract

The invention relates to an evaporation source comprising a target material, characterized in that the target material consists of one or more boride(s) selected from the group consisting of TiB2, W2B5, TaB2, VB2, NbB2, CrB2, MoB and HfB2 and one or more silicon-containing compound(s) selected from the group consisting of TiSi2, Ti5Si3, CrSi2, Cr5Si3, Cr3Si, CeSi2, SiB6, HfSi, Hf5Si4, Hf3Si2, NbSi2, Nb5Si3, MoSi2, Mo5Si3, Mo3Si, ZrSi, Zr2Si, Zr5Si3, Zr5Si4, Zr3Si2, TaSi2, Ta5Si3, Ta3Si, Ta2Si, W5Si3 and WSi2, in addition to unavoidable impurities, wherein the content of the silicon in the target material amounts to 1-30 at%. The invention also relates to a method for producing an evaporation source of this kind.

Description

[0001]Silicon-Containing Transition Metal Boride Evaporation Source The present invention relates to an evaporation source for physical vapor deposition for the deposition of silicon-containing transition metal boride layers. The present invention further relates to a method for producing such an evaporation source. Transition metal borides are known for their refractory properties, such as high melting temperatures (in many cases, over 3000 °C), broad phase stability, high thermal conductivity, and sufficient heat shock resistance. The ongoing interest in sustainable material applications and the longevity of high-performance components, such as machine elements in aircraft engines, calls for transition metal boride-based materials as a new class of protective coatings.The high temperature resistance of such coatings is a particular focus, and strategies are currently being developed to improve their weak oxidation resistance. It has been found that the oxidation resistance of the aforementioned coatings can be significantly improved by silicon enrichment (see unpublished PCT / EP2022 / 076328 and Thomas Glechner, "High temperature oxidation resistance of physical vapor deposited Hf-Si-B." 2±zthin films, in Corrosion Science, Vol. 205, 2022, https: / / doi.org / 10.1016 / j.corsci.2022.110413 ). The silicon enrichment was carried out on an experimental scale by placing silicon platelets on the surface of evaporation sources (targets) made of pure transition metal borides. The silicon content in the deposited layers was controlled by the number of platelets deposited. However, this process has the disadvantage that, although the silicon content can be varied, it can only be roughly adjusted. Furthermore, targets in industrially used coating systems are typically installed vertically, so simply depositing silicon platelets is not possible, as they would fall off. CN112323031 A relates to a corrosion-resistant layer consisting of 3 to 14 at% Si, 30 to 33 at% Ti, and 55 to 66 at% B.The layer is produced by simultaneous sputtering (co-sputtering) of two targets, one made of silicon and the other of titanium diboride. A disadvantage of this process, however, is that co-sputtering of two separate targets can lead to the formation of undesirable multilayers if the parts (substrates) to be coated are rotated. Applications that use co-sputtering therefore have the disadvantage that, to prevent the formation of multilayers, the substrates cannot be moved. This so-called stationary coating method, however, prevents the economical, industrial use of the technology to treat as many substrates as possible in a single pass (batch).A further disadvantage of co-sputtering is that in some PVD coating systems, the number of cathodes to which the targets are attached is limited, thus preventing the use of multiple targets made of different materials. JP2017218621 A relates to a target material containing Cr, Si, and B, and exhibiting a pure Si phase, a Cr2B phase, and a Cr solid solution phase. The Si content in the target material is 1 to 16.9 at%, and the B content is preferably 2.5 at% to 7 at% or less. The target material is produced by subjecting a powder mixture of silicon powder, boron powder, and chromium powder to a plasma sintering process with electrical discharge (SPS or spark plasma sintering). However, due to the direct use of silicon powder, with a melting point only slightly above 1400°C, this process is severely limited in terms of process temperatures.High temperatures in the range of up to 2000 °C and above are necessary for sufficient densification of target materials containing transition metal borides. However, under these conditions, the silicon liquefies, which can cause material to escape during the pressing process, making it impossible to control the precise composition of the target material. Furthermore, the resulting hydrostatic pressure can destroy the pressing tool. The object of the present invention is therefore to provide an improved evaporation source with a defined composition for the improved production of silicon-enriched metal boride layers. This object is achieved by an evaporation source according to claim 1 and a method for producing the evaporation source according to claim 10. Advantageous developments of the invention can be found in the dependent claims, which can be freely combined with one another.In the state of the art, physical vapor deposition processes are widely used to produce the aforementioned layers. Physical vapor deposition (PVD) utilizes various techniques, such as evaporation, sputter deposition, or cathodic arc deposition (or arc-source evaporation). In this process, a solid starting material is converted into the vapor phase / plasma state using physical processes and then deposited onto a substrate. In cathodic sputtering, atoms are released from the solid by bombarding the surface of the evaporation source with high-energy ions. The atomized material is distributed throughout the coating chamber and deposited on the substrate to be coated.Such an evaporation source is also known in the art as a sputtering target. In arc evaporation, the solid is evaporated by a vacuum arc discharge and deposited on the substrate. Such an evaporation source is also known in the art as an arc cathode. In the context of the present disclosure, the term evaporation source refers to sputtering targets and arc cathodes, or targets for short. Such targets comprise at least the target material and, optionally, a backing plate attached thereto.To avoid problems during the deposition process, such as overheating caused by poor thermal conductivity of the target, process instability caused by the influence of the target on the quality of the vacuum during the coating process, or arc jamming during ablation of the target material caused by inhomogeneous electrical conductivity of the target, the target material must have the highest and most homogeneous density possible. Evaporation sources made of ceramic materials, such as borides, carbides, nitrides, and silicides, are therefore usually produced by hot pressing or spark plasma sintering. In the case of transition metal borides, which typically have high melting points in the range of approximately 2,100 to 3,300°C, extreme conditions are necessary, requiring temperatures in the range of at least 1,300°C, and in particular 1,500 to 2,000°C.200°C, as well as compression pressures in the range of 20 to 80 MPa, to achieve the required densification of the target material. As mentioned above, this complicates the enrichment of the target material with the comparatively low-melting silicon. The present invention succeeds in providing evaporation sources with a suitable, high density for PVD (physical vapor deposition) processes through the use of special starting materials in combination with special manufacturing processes. These sources have a defined silicon content and can be used to produce metal boride layers of a defined composition.According to the present invention, an evaporation source comprises a target material which consists of one or more borides selected from the group consisting of TiB2, W2B5, TaB2, VB2, NbB2, CrB2, MoB and HfB2 and one or more silicon-containing compounds selected from the group consisting of (melting points given in parentheses) TiSi2 (1480 °C), Ti5Si3 (2130 °C), CrSi2 (1490 °C), Cr5Si3 (1680 °C), Cr3Si (1770 °C), CeSi2 (1620 °C), SiB6 (1850 °C), HfSi (2142 °C), Hf5Si4 (2320 °C), Hf3Si2 (2480 °C), NbSi2 (1940 °C), Nb5Si3 (2520 °C), MoSi2 (2020 °C), Mo5Si3(2180 °C), Mo3Si (2020 °C), ZrSi (2210 °C), Zr2Si (1925 °C), Zr5Si3(2180 °C), Zr5Si4(2250 °C), Zr3Si2(2215 °C), TaSi2(2040 °C), Ta5Si3(2550 °C), Ta3Si (2350 °C), Ta2Si (2440 °C), W5Si3(2372 °C) and WSi2(2141 °C), together with unavoidable impurities and wherein the proportion of silicon in the target material is 1 at% to 30 at%.The key advantage of the invention is the provision of silicon in the target material in the form of high-melting silicon compounds. This overcomes the disadvantage of the greatly differing melting points of silicon and the boron-containing compounds that serve as the main component. In the present invention, high-melting silicon compounds with melting points close to 2000°C and above are used for the purpose of silicon enrichment. This avoids the occurrence of a liquid phase during the production of the evaporation source by hot compaction and simultaneously ensures sufficient compaction of the target material. This allows a defined silicon content to be set in the target material while simultaneously maintaining a high density of the target material. The starting materials are present side by side in the same target, which prevents the formation of multilayers during the deposition process.With the evaporation source of the present invention, it is possible to deposit homogeneous layers with a defined composition, even during stationary processing. A further advantage of the invention is that silicides have a higher electrical conductivity than silicon carbides, nitrides, and oxides, which is important for the optimal functioning of the target. The target material can contain only a single boride or mixtures of several of the aforementioned borides. Preferably, the one or more borides are selected from the group consisting of TiB2, TaB2, CrB2, MoB, and HfB2, in particular HfB2, TiB2, and CrB2, and mixtures thereof. The target material can contain only a single silicon-containing compound or mixtures of several of the aforementioned silicon-containing compounds.Preferably, the one or more silicon-containing compounds are selected from the group consisting of Ti5Si3, Cr3Si, Cr5Si3, CeSi2, SiB6, HfSi, Hf3Si2, Hf5Si4, Nb5Si3, MoSi2, Mo5Si3, Mo3Si, ZrSi, Zr5Si3, Zr5Si4, Zr3Si2, TaSi2, Ta5Si3, Ta3Si, Ta2Si, W5Si3, and WSi2, and mixtures thereof. More preferably, the one or more silicon-containing compounds are selected from the group consisting of Ti5Si3, Cr3Si, CeSi2, SiB6, HfSi, Hf3Si2, Hf5Si4, and Nb5Si. 3,MoSi2, Mo5Si3, Mo3Si, Zr5Si3, ZrSi, TaSi2, Ta5Si3, Ta3Si, W5Si3, and WSi2, and mixtures thereof. Particularly preferred compositions of the target material are selected from the group consisting of CrB2 / CrSi2, CrB2 / MoSi2; HfB2 / CeSi2; HfB2 / MoSi2; TiB2 / MoSi2; TiB2 / TaSi2; TiB2 / Ti5Si3; TiB2 / Ti5Si3 / SiB6, and TiB2 / TiSi2. According to the present invention, unavoidable impurities refer to metallic and non-metallic elements that cannot be removed during the production of the starting materials, the powders of the borides and silicon-containing compounds, and are therefore present in the target material. The total values ​​for oxygen impurities in the starting materials are preferably in the range of 2,000–10,000 ppm. Other non-metallic impurities such as carbon or nitrogen are present in total concentrations of up to 2,000 ppm. Metallic impurities are present in total concentrations below 5,000 ppm, preferably below 2,000 ppm, in the starting materials.The chemical qualitative and quantitative analysis of the starting materials and the target material can be determined using ICP-OES and ICP-OMS. During densification of the material, mixed phases of the materials used may form. For example, in the combination of TiB2 / MoSi2, in addition to the TiB2 phase and the MoSi2 phase, a Mo2Ti3Si3 phase also forms in small amounts. The proportion of mixed phases in the target material is characteristic of the present invention and can be kept very low due to the preferred manufacturing process using spark plasma sintering. The proportion of mixed phases formed during the densification process can be quantified using EDX on cross-sections of the target on the SEM and is preferably less than 20% of the measured area proportion based on statistically verified measurements, more preferably less than 10%.For the purposes of defining the material composition of the evaporation source of the present invention, these mixed phases are considered to be included in the form of the starting materials. In other words, this means that, for example, a processed target material which, after hot-compacting a powder mixture of TiB2 / MoSi2, contains, in addition to the TiB2 phase and the MoSi2 phase in small amounts, also a Mo2Ti3Si3 phase, is defined as consisting of TiB2 / MoSi2. According to the invention, the proportion of silicon in the target material is in a range from 1 at% to 30 at%. The proportion of silicon can be adjusted by targeted selection and weighing of the starting materials. Preferably, the proportion of silicon in the target material is in a range from 8 to 20 at% and more preferably in a range from 10 to 15 at%. It is known that a small amount of boron can be lost during the physical vapor deposition of layers using boron-containing targets.To counteract this, the target material according to the invention can be enriched with SiB6. This allows the loss of boron during deposition to be compensated without introducing further metallic elements into the target material or a layer deposited therefrom. For these purposes, it is advantageous in the present invention if the evaporation source contains at least two silicon-containing compounds, one of which is SiB6. In this embodiment, the proportion of SiB6 is preferably in a range from 10 to 50 mol%. The proportion of SiB6 is particularly preferably in a range from 15 to 30 mol%. All data in the invention in mol% refer to 100 mol% of the total target material. In one embodiment, the target material of the present invention can thus consist of a ternary system of compounds of precisely one of the metals mentioned herein, boron, and silicon.In an alternative embodiment, the target material of the present invention can consist of compounds containing, in addition to boron and silicon, several metals, preferably two of the aforementioned metals as a quaternary system, in the respective bound form. The target material of the evaporation source according to the invention typically has a very high density. Preferably, the density is at least 95% of the theoretical density, preferably 98% to 100% of the theoretical density. The theoretical density applicable to the respective composition of the target material is, by definition, calculated from the molar fraction-weighted average of the theoretical densities of the individual phase components. The actual density of the target material is determined using Archimedes' principle.During the coating process, the evaporation sources are thermally exposed to plasma, an electric arc, and, if necessary, a heater in the coating chamber. To prevent the evaporation sources from overheating, they are typically cooled from the back. Cooling can be achieved either by direct water cooling of the back of the target material or by indirect cooling via a rigid support element in the form of a backplate connected to the target material. The support element can be applied in a single-step process during the same compaction process in which the target material is compacted / sintered, from a specially prepared powder mixture or from an already solid backplate / bulk material, or later, after the compaction process of the target material has been completed, as bulk material using a suitable high- or low-temperature joining process.Materials that are particularly suitable for the support element are pure metals such as titanium, copper, molybdenum, tungsten, tantalum, or alloys or composites of these metals. Molybdenum-, tungsten-, or tantalum-based alloys are preferred, whereby the term base alloy refers to an alloy that contains, in addition to other metals, more than 50 at% of the base metal of the same name. Molybdenum-based alloys, tungsten-based alloys, or tantalum-based alloys are also suitable as materials for a support element because they exhibit the advantageous combination of properties of sufficiently high thermal conductivity, a high modulus of elasticity (i.e., high rigidity), and a relatively low coefficient of thermal expansion. With regard to industrial mass production, support elements made of molybdenum, titanium, molybdenum-based alloys, and titanium-based alloys are particularly preferred.The evaporation source of the present invention is suitable for industrial application in coating systems. Evaporation sources according to the invention can be either plate-shaped (with a round or rectangular format) or tubular. In a preferred embodiment, the evaporation source is designed with a back plate as a support element, particularly preferably with the following dimensions: - round formats typically have a diameter of 50 mm to 350 mm, a thickness of 5 to 40 mm, with support elements of corresponding diameter and a thickness of 2 to 15 mm, - rectangular formats typically have a width of 50 mm to 300 mm, a thickness of 5 mm to 40 mm, and a length of up to 1000 mm. In special embodiments, these formats can be multi-part, with several segments of the compacted target material being arranged on a common support element.An associated support element typically has the following dimensions: 50–300 mm wide, 2–10 mm thick, and up to 1,000 mm long. Joining techniques for connecting the target material to the support element are known in the art. Typically, the target material is joined to the support element using indium, but also using hard or soft solders known in the art. Examples of soft solders are tin-silver-based and tin-copper-based solders. Examples of hard solders are silver, brass, and phosphorus solders. In a preferred embodiment, the target materials are joined to the backplates using a silver-based solder containing titanium, copper, and / or nickel. The way these solders are applied is irrelevant.There are two options for supplying the solder: the use of a commercially available solder foil, which is placed between the target material and the backplate during the soldering process, or the application of the solder-forming elements using a spraying process, such as cold gas spraying (CGS), either on the backplate or on a corresponding surface of the target material, which are then later bonded together. Another method of bonding the target material to a suitable backplate can be achieved during the actual compaction process in the spark plasma sintering or hot-press system. For this purpose, the backplate is placed in the system either as an already compacted component or in the form of powder of its components and compacted simultaneously with the target material.Another connection method is the diffusion bonding of a backplate as bulk material with the compacted target material as bulk material in a hot-press or SPS system. The present invention also relates to a method for producing the evaporation sources described herein, in particular the target material. The method according to the invention for producing an evaporation source described herein comprises at least the following steps: - Providing a powder mixture of the target material comprising at least one boride and at least one silicon-containing compound - Uniaxially compacting the powder mixture in a pressing tool at temperatures such that sintering of the powder mixture into a solid occurs without the formation of liquid phases. To produce the powder mixture, the at least one boride and the at least one silicon-containing compound are used in the form of their commercially available powders.The average particle diameter of these starting powders, defined by the D50 value of a size distribution measurement using laser diffraction particle size analysis, is typically 1 μm to 50 μm, preferably 2 μm to 20 μm, for the borides and typically 2 μm to 100 μm, preferably 2 μm to 20 μm, for the silicon-containing compounds. To adjust the desired chemical composition and homogeneity, the starting powders are weighed in the required ratio and mixed for a few minutes, preferably in a free-fall mixer with the addition of isopropanol and grinding balls. The homogenized powder mixture is dried in an oven if necessary, poured into a graphite tool, which is then placed in a hot press or SPS system and compacted uniaxially using the upper and lower punches of the press / SPS system.Uniaxial compaction typically occurs at a pressure in a range of 20 to 80 MPa, preferably at a pressure in a range of 25 to 60 MPa, at temperatures in the range of 1,300°C to 2,200°C, preferably at temperatures in the range of 1,450°C to 2,200°C, more preferably in the range of 1,500°C to 2,200°C, more preferably in the range of 1,650°C to 2,200°C, and more preferably in the range of 1,800°C to 2,200°C. This process is referred to in the technical field of the present invention as hot pressing (HP). As mentioned, it is also possible to apply electricity to the powder mixture during the pressing process. As a subtype of hot pressing process, during this spark plasma sintering process the powder mixture is heated in a very short period of time by the current flowing through it due to the electrical resistance of the powder mixture.The use of this process helps suppress the formation of further compounds (mixed phases) of the starting components and enables the production of high-density targets in very short process times. Therefore, the compaction process of the target material is preferably carried out in an SPS system under the application of electricity. The powder mixture is preferably brought to operating pressure and temperature within 5 to 90 minutes, preferably 10 to 60 minutes, in particular 10 to 30 minutes, under simultaneously increasing pressure and compacted under these conditions over a period of 5 to 90 minutes, preferably 15 to 60 minutes, such that the powder mixture sinters into a solid without the formation of liquid phases. Further advantages and usefulness of the invention will become apparent from the following production examples. Production Examples 1 to 28: To produce the targets, the starting powders are weighed in the ratios specified in the table above after converting the mol% to wt%. They are homogenized and mixed in a free-fall mixer with the addition of steel balls in isopropanol. The mixture is then dried in an oven, filled into a graphite tool, and installed in a Spark Plasma Sintering system. The mixture is then heated to the specified operating temperature within approximately 30 minutes, while simultaneously building up the specified operating pressure. Under these conditions, the mixture was uniaxially compacted for approximately 1 hour. Compaction occurred without the formation of any liquid phases. As can be seen from the table, the compacts all exhibit very high densities and are therefore very suitable as evaporation sources. The compact was then bonded to the specified support element.In the case of a Mo backplate, this was manufactured separately and bonded via brazing using a Ticusil® foil as the solder medium, or using Ti, Cu, and Ag as the CGS layer. In the case of a copper backplate, this was bonded to the compacted target material using indium bonding. In this process, the plate-shaped support element and the back of the compacted target material are each wetted with molten indium, then the wetted areas are aligned with each other and cooled to room temperature. During this process, the molten indium solidifies and creates a bond between the support element and the compacted target material.

Claims

CLAIMS 1. Evaporation source comprising a target material, characterized in that the target material consists of one or more boride(s) selected from the group consisting of TiB2, W2B5, TaB2, VB2, NbB2, CrB2, MoB and HfB2 and one or more silicon-containing compound(s) selected from the group consisting of TiSi2, Ti5Si3, CrSi2, Cr5Si 3,Cr3Si, CeSi2, SiB6, HfSi, Hf5Si4, Hf3Si2, NbSi2, Nb5Si3, MoSi2, Mo5Si3, Mo3Si, ZrSi, Zr2Si, Zr5Si3, Zr5Si4, Zr3Si2, TaSi2, Ta5Si3, Ta3Si, Ta2Si, W5Si3, and WSi2, along with unavoidable impurities, and wherein the proportion of silicon in the target material is 1 at% to 30 at%.

2. Evaporation source according to one of the preceding claims, wherein boride(s) and silicon-containing compound(s) are present as separate phases in the target material, and wherein the proportion of mixed phases is below 20%, preferably in a range of 1 to 10%.

3. The evaporation source according to claim 1, wherein the target material comprises at least two silicon-containing compounds, and one of the silicon-containing compounds is SiB6.

4. The evaporation source according to claim 3, wherein the proportion of SiB6 in the target material is in a range of 10 to 30 mol%. 5.Evaporation source according to one of the preceding claims, wherein the one or more borides is / are selected from the group consisting of TiB2, TaB2, CrB2, MoB, and HfB2, preferably HfB2, TiB2 and CrB2 and mixtures thereof and the one or more silicon-containing compound(s) is / are selected from the group consisting of Ti5Si3, Cr3Si, Cr5Si3, CeSi2, SiB6, HfSi, Hf3Si2, Hf5Si4, Nb5Si3, MoSi2, Mo5Si3, Mo3Si, ZrSi, Zr5Si3, Zr5Si4, Zr3Si2, TaSi2, Ta5Si3, Ta3Si, Ta2Si, W5Si3 and WSi2 and mixtures thereof.

6. Evaporation source according to one of the preceding claims, wherein the target material is selected from the group consisting of CrB2 / CrSi2, CrB2 / MoSi2; HfB2 / CeSi2; HfB2 / MoSi2; TiB2 / MoSi2; TiB2 / TaSi2; TiB2 / Ti5Si3; TiB2 / Ti5Si3 / SiB6; and TiB2 / TiSi2.

7. Evaporation source according to one of the preceding claims, wherein the target material forms a ternary system consisting of boron, silicon, and a metal. 8.Evaporation source according to one of the preceding claims, wherein the target material has a density of at least 95% of the theoretical density.

9. Evaporation source according to one of the preceding claims, wherein the evaporation source comprises a support element.

10. A method for producing an evaporation source according to one of the preceding claims, comprising the following steps: - providing a powder mixture of the target material comprising at least one boride and at least one silicon-containing compound - uniaxially compacting the powder mixture in a pressing tool at such temperatures and at a pressure that sintering of the powder mixture into a solid occurs without the formation of liquid phases, and optionally - bonding the solid to a support element.

11. The method according to claim 10, wherein the method is a spark plasma sintering method in which the powder mixture is subjected to an electric current during compaction. 12.A process according to any one of the preceding claims, wherein the compaction takes place at a pressure in a range of 20 to 80 MPa and a temperature in the range of 1300°C to 2200°C.

13. A process according to any one of the preceding claims, wherein the compaction takes place at a pressure in a range of 25 to 60 MPa and a temperature in the range of 1500°C to 2200°C.

14. A process according to any one of the preceding claims, wherein the compaction takes place at a pressure in a range of 25 to 60 MPa and a temperature in the range of 1650°C to 2200°C.

15. A process according to any one of the preceding claims, wherein the compaction takes place at a pressure in a range of 25 to 60 MPa and a temperature in the range of 1800°C to 2200°C.

16. A process according to any one of the preceding claims, wherein the average particle size of the starting powders is 1 μm to 50 μm for the borides and 2 μm to 100 μm for the silicon-containing compounds.

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