Apparatus and method for separating hard carbon layers

Abstract

The present invention relates to an apparatus and method for separating a carbon layer, in which an arc discharge is formed between an electron source (10) and an evaporation material (2) by a first current supply device (9), the negative pole of the first current supply device is electrically conductively connected to the electron source (10) and the positive pole of the first current supply device (9) is electrically conductively connected to the evaporation material (2), a permanent magnet system (5) and a magnetic coil (6) are arranged rotationally symmetrically around the evaporation material (2), and the evaporation material (2) is formed as a graphite rod, which is surrounded by at least one insulating element (4) at least at the end of the graphite rod to be evaporated.

Description

[Technical Field]

[0001] The present invention relates to an apparatus and method for coating a substrate with an extremely hard carbon layer based on arc evaporation of graphite in a vacuum chamber.

[0002] It is known that carbon layers can be produced with a wide variety of properties, mainly due to significant differences in structure. The layer hardness of carbon layers varies from hard and diamond-like to soft and graphite-like. In many experiments, the layer properties, especially the layer hardness, are related to the sp 3 Bonds and sp 2 It was found that the hardness depends on the bonding ratio and the hydrogen content. Furthermore, the highest hardness in the hydrogen-free layer is observed at very high sp 3 It is known that the hardness is achieved by the proportion of bonds. Ultra-hard carbon layers with Young's modulus of 400 GPa to 800 GPa are also called tetrahedrally bonded amorphous carbon (ta-C). Usually, the temperature of the substrate to be coated is adjusted to a high proportion of sp 3 It must be kept relatively low (below 300°C) so that the bond can be formed and very high layer hardness can be achieved.

[0003] Various methods are known for producing such tetrahedrally bonded superhard amorphous carbon layers. H. Schulz et al., "Applied Physics A, Pulsed Arc Deposition of Superhard Amorphous Carbon Films," Vol. 78, 2004, pp. 675-679, describe a method in which a vacuum arc on the cathode side is formed on a graphite cathode with a small bottom point, guided stochastically or by a magnetic field. Here, the carbon of the graphite cathode sublimes and becomes highly ionized due to the characteristics of the arc discharge. The ionized carbon particles separate onto the substrate to be coated and, due to their relatively high energy (in the range of about 10-40 eV), form a very high proportion of sp 3The carbon structure formed has bonds. However, due to the extremely high power density of the arc discharge at the graphite cathode, very small droplets called droplets, also called macroparticles, are generated. These droplets are relatively soft and can penetrate into the growing layer, impairing its properties. Magnetic filters that deflect the plasma flow emitted from the cathode and capture the droplets with an additional wall can significantly reduce the proportion of droplets in the separated layer. However, the coating rate is significantly reduced by such magnetic filtering.

[0004] The aforementioned document also describes a method in which pulsed laser radiation is used to ignite a pulsed, high-current arc discharge. Such laser-induced pulsed discharges can produce currents of several hundred amperes to over 1,000 amperes. However, due to the extremely high power density on the graphite surface, droplets are also ejected and deposited in the layer. Therefore, even in such a method, a magnetic filter can be used, which can significantly reduce the proportion of droplets in the layer. However, the technical costs involved in this method are extremely high due to the various components involved: the laser, the arc discharge, and the magnetic filter.

[0005] In the so-called laser ablation method, known for example from Steffen Weissmantel et al., "Surface & Coating Technology, Preparation of Superhard Amorphous Carbon Films with Low Internal Stress," Vol. 188 / 189, 2004, pp. 268-273, high-power laser pulses are directed at a graphite target in a vacuum, causing pulsed evaporation of carbon, which is then separated onto the substrate. Laser ablation has many similarities with cathode-side arc coating. In both cases, extremely high power densities are achieved on the graphite target, resulting in the generation of a highly ionized plasma. Relatively high-energy particles in the range of approximately 10-40 eV are generated here, and liquid droplets are also generated during laser ablation.

[0006] By using magnetron sputtering, it is possible to produce layers that are harder than those produced by simple evaporation of carbon. This is known, for example, from M. Stueber (FZ Karlsruhe) in the book "Wissenschaftliche Berichte, Magnetron-gesputtered superhardte, amorphe carbonstoffschichten mit gradiertem Schichtaufbau" (FZKA 5919, 1997). This also involves the production of layers with a relatively high bond sp 3 This is due to a certain percentage of high-energy particles in the vapor or plasma flow from the target to the substrate. In magnetron sputtering, the power density at the graphite target is significantly lower than in the arc discharge and laser methods. This is presumably the reason why magnetron sputtering cannot achieve the same high hardness in the carbon layer as in the arc discharge and laser methods described above. However, no liquid droplets, or only a few liquid droplets, can be introduced into these layers.

[0007] The drawback of the above-mentioned methods is that in many cases most of the droplets are trapped within the layer structure or significant technical costs are required to reduce the density and size of the droplets. Clearly, there is a relationship between the application of very high power densities in graphite targets, the high particle energies that can be achieved thereby, and the very high achievable layer hardness.

[0008] The technical problem of the present invention is therefore to provide a method and an apparatus for separating carbon layers, which can overcome the drawbacks of the prior art. In particular, by using the method and the apparatus according to the present invention, it should be possible to separate carbon layers having a very high hardness, with reduced droplet entrapment compared to the prior art.

[0009] The above-mentioned technical problem is solved by the subject matter having the features of claims 1 and 5. Further advantageous embodiments of the invention are evident from the dependent claims.

[0010] The device according to the present invention is based on elements known from arc evaporation using an anode. A first current supply powers an arc discharge between an electron source, which functions as a cathode, and an evaporation material, which functions as an anode. The anode material is significantly heated by electron bombardment and thus transitions to the vapor phase. Energy excitation of the vapor occurs through collisions between electrons of the arc discharge flowing to the anode and vapor particles. A so-called high-temperature hollow cathode can be used as the electron source. By high-temperature hollow cathode, we mean a hollow cathode through which a noble gas, such as argon, flows and which is heated to a temperature of at least 1000°C during operation. A magnet system consisting of an array of permanent magnets and a magnetic coil is arranged in a ring shape around the evaporation material connected as the anode. The magnetic field of such a magnet system guides electrons into the anode material and increases the electron density in the vicinity of the anode, which results in intensive heating of the anode material and higher ionization of the vapor emitted from the anode.

[0011] In the device according to the invention, the carbon-containing material of the anode is formed in the form of a cylindrical graphite rod, which can be placed inside a guide tube and subsequently pushed into the guide tube by a rear guide device depending on the evaporation rate, so that the position of the sublimation end face of the graphite rod does not change significantly during the process.

[0012] According to the invention, the evaporation material formed as a graphite rod is surrounded all around by at least one insulating element, at least at the rod end to be evaporated, thereby ensuring that only a small proportion of the heat generated by the arc discharge is dissipated in the graphite rod, for example via the guide tube.

[0013] In one embodiment, the at least one insulating element comprises a porous material lining the inner wall of the guide tube and having a melting point above 2000° C. The porous material thus has the shape of a hollow cylinder inside which the graphite rod to be evaporated is housed or into which it is subsequently pushed by a post-guiding device.

[0014] A particularly suitable porous material is graphite felt, for example. Graphite felt, within the meaning of the present invention, is understood to mean a woven or irregularly braided carbon fiber fabric. However, other materials are also possible, such as porous structures made of sintered aluminum oxide ceramic or metal foams made of molybdenum, tantalum, or tungsten. The graphite felt essentially fulfills two functions. On the one hand, it ensures thermal insulation between the graphite rod and the guide tube, thereby enabling the generation of high temperatures at the end face of the graphite rod to be evaporated, which allows for the sublimation of carbon. On the other hand, the graphite felt serves to electrically connect the graphite rod to be evaporated with the positive potential of the first current supply, which is conducted from the guide tube through the graphite felt to the graphite rod.

[0015] Preferably, the guide tube has a stop at its upper end to prevent the porous material, such as graphite felt, from being pushed out of the guide tube when the graphite rod is guided back in. The guide tube may be made of graphite, for example. However, metals and metal compounds, or ceramics with a melting point of at least 1000°C, are also suitable materials for the guide tube.

[0016] In an alternative embodiment, the evaporation material, which is formed as a graphite rod, is surrounded on its entire circumference, at least at the rod end to be evaporated, by at least two spaced-apart tubular heat reflectors. The heat reflectors are preferably made of a material with a high melting temperature above 2000°C, such as graphite, metals or metal compounds containing tantalum, molybdenum, or tungsten, or ceramics such as aluminum oxide or boron nitride. The spacing between the at least two tubular heat reflectors interrupts heat conduction and thus reduces the loss of thermal energy, thereby enabling stable evaporation of the graphite rod.

[0017] In a method for separating a carbon layer according to the present invention, an arc discharge is formed between an electron source and an evaporation material using a first current supply device, with the negative pole of the first current supply device conductively connected to the electron source and the positive pole of the first current supply device conductively connected to the evaporation material. A permanent magnet system and at least one magnetic coil are arranged rotationally symmetrically around the evaporation material. The method according to the present invention is further characterized in that the evaporation material is formed as a graphite rod and is surrounded by at least one insulating element at least at the end of the graphite rod to be evaporated.

[0018] The device according to the invention and the method according to the invention make it possible to separate carbon layers on a substrate by arc evaporation using an anode in a vacuum chamber, and to maintain this separation process stably over a relatively long period of time, whereby carbon layers with extremely high hardness can be separated.

[0019] The present invention will be described in more detail below based on examples. [Brief explanation of the drawings]

[0020] [Figure 1] 1 is a schematic diagram of an apparatus according to the present invention; [Figure 2] 2 is a schematic enlarged view of the upper end portion of the guide tube 1 shown in FIG. 1. FIG. [Figure 3] 1 is a schematic diagram of an alternative device according to the present invention;

[0021] FIG. 1 shows a schematic diagram of an apparatus 100 according to the invention, in which the method according to the invention can also be carried out. The apparatus 100 according to the invention comprises a guide tube 1 arranged in a vacuum chamber (not shown in FIG. 1 ), in which an evaporation material 2 is stored. The evaporation material 2 is formed as a graphite rod and can be subsequently pushed into the interior of the guide tube by a rear guide device 3 during operation of the apparatus 100. According to the invention, the inner wall of the guide tube 1 is lined with a graphite felt 4 as an insulating element, so that the evaporation material 2 formed as a graphite rod is surrounded by the graphite felt 4 at least at the upper end of the graphite rod, from which the evaporation material 2 is evaporated. Furthermore, the guide tube 1 is surrounded by an annular permanent magnet system 5 and an annular magnetic coil 6, which together generate a rotationally symmetrical overall magnetic field around the tube axis 7 of the guide tube 1.

[0022] The permanent magnet system 5, magnetic coil 6, evaporator crucible 1, and rear guiding device 3 are at least partially enclosed by a housing 8, which serves to protect these components from impingement of the evaporation material or charged particles during operation of the apparatus 100. In one embodiment, the housing 8 is water-cooled to remove heat generated by the magnetic coil 6.

[0023] The first current supply device 9 supplies a current to form an arc discharge between the electron source 10 and the evaporation material 2, with the negative pole of the first current supply device 9 conductively connected to the electron source 10 and the positive pole of the first current supply device 9 conductively connected to the guide tube 1. The graphite felt 4 is conductive and mechanically contacts the guide tube 1 and the evaporation material 2, so that the evaporation material 2 also has the same potential as the guide tube 1. In this way, an arc discharge is formed between the electron source 10 connected as a cathode and the evaporation material 2 connected as an anode, which heats the evaporation material 2 and causes it to evaporate by sublimation.

[0024] In the embodiment shown in Fig. 1, the electron source 10 is configured as a high-temperature hollow cathode. Alternatively, any other electron source known from the prior art and used for arc evaporation with an anode is also suitable for the device according to the invention. The anode 11 and the magnetic coil 12 are arranged in an annular shape around the electron source 10 configured as a hollow cathode. For reasons of clarity, a current supply that ensures a current flow through the magnetic coil 12 is not shown in Fig. 1.

[0025] 1 also shows a second current supply 13, which generates a current through the magnetic coil 6, and a third current supply 14, the negative pole of which is electrically conductively connected to the electron source 10 and the positive pole of which is connected to the anode 11, thereby ensuring the ignition of a hollow cathode discharge. Furthermore, as is known from the prior art, a working gas flows through the hollow cathode, which is heated to temperatures of over 1000° C. during operation.

[0026] In one embodiment, the second current supply 13 is configured to periodically change the intensity of the current flowing through the magnetic coil 6. In this embodiment, the second current supply 13 can supply a current with a pulsed current intensity, so this current supply is also referred to as a pulsed current supply. It has been confirmed that changes in the intensity of the current flowing through the magnetic coil 6 result in changes in the directional distribution of vapor propagation of the evaporated evaporation material 2. This change in the vapor propagation characteristics depending on the coil current intensity is reproducible. Depending on the combination of the magnetic fields of the permanent magnet system 5 and the magnetic coil 6, the vapor propagation direction can preferably radiate along the axis 7 or at an angle inclined relative to this axis. The reason the propagation characteristics are affected by the magnetic field is presumably because the vapor flow formed is highly ionized and the plasma flow is deflected in the magnetic field.

[0027] In the device and method according to the invention, the effect that the directional distribution of vapor propagation is influenced by the magnitude of the coil current strength can be utilized. Thus, by periodically changing the strength of the current flowing through the magnetic coil 6, the layer thickness distribution on the substrate to be deposited can be easily influenced. If the strength of the current flowing through the magnetic coil, and thus the magnetic field strength of the overall magnetic field, is increased, the vapor propagation of the evaporated evaporation material 2 is directed more in the direction of the axis 7, whereas by decreasing the strength of the current flowing through the magnetic coil, and thus the magnetic field strength of the overall magnetic field, the main vapor propagation direction is formed at an angle to the axis 7 that can be more than 60°.

[0028] FIG. 2 shows the upper part of the guide tube 1 of FIG. 1 in more detail. As can be seen in FIG. 2, the guide tube 1 has an upper stop 15, which prevents the graphite felt 4 from being pushed upward out of the guide tube 1 when the evaporation material 2, which is formed as a graphite rod, is guided back. FIG. 2 also shows that the graphite felt 4 partially evaporates near the evaporation end of the graphite rod during operation of the device according to the invention, forming a funnel-shaped gap between the non-evaporated graphite felt 4 and the evaporation material 2, which is formed as a graphite rod. However, this gap has no effect on either the arc discharge or the evaporation of carbon. As already mentioned above, the lining of the inner wall of the guide tube 1 with the graphite felt 4 according to the invention allows for stable evaporation of carbon by arc evaporation using an anode.

[0029] In the following, some specific values ​​are given purely as examples in order to explain which hard carbon layers can be separated by the device according to the invention and the method according to the invention.

[0030] In the first test apparatus, a graphite rod with a diameter of 10 mm is evaporated in a vacuum chamber as the evaporation material 2. The graphite rod is positioned in a guide tube 1 with an inner diameter of 25 mm. The inner wall of the guide tube 1 is lined with graphite felt 4 so that the graphite felt 4 is in contact with the inner wall of the guide tube 1 over its entire circumference and with the graphite rod over its entire circumference. First, an arc discharge is ignited between the electron source 10 formed as a hollow cathode and the anode 11 by a third current supply device 14. The current intensity here is 60 A. At this time, argon, the working gas, flows through the hollow cathode. The pressure in the vacuum chamber is 1*10 -3After the current supplies for the magnetic coils 6 and 12 are switched on, the first current supply 9 is also switched on and their current strength is gradually increased from 0 A to 110 A within 5 minutes. After that, the current strength of the third current supply 14 is reduced to 10 A and the gas flow through the hollow cathode is also reduced until the pressure in the vacuum chamber reaches 7*10 -4 The pressure is reduced to 1000 mbar. The rear guide 3 is activated and the evaporation material 2, which is formed as a graphite rod, is pushed upward at a speed of 5 cm / h. The first substrate, made of steel, is exposed to the carbon vapor for 6 minutes. During this time, a 1 μm thick carbon layer separates on the first substrate. Measurements using the nanoindentation method show that this layer has an extremely high hardness of 55 GPa and an elastic modulus of 490 GPa.

[0031] In the second test apparatus, a graphite rod with a diameter of 12 mm is evaporated in a vacuum chamber as the evaporation material 2. The graphite rod is positioned in a guide tube 1 with an inner diameter of 40 mm. The inner wall of the guide tube 1 is lined with graphite felt 4 so that the graphite felt 4 is in contact with the inner wall of the guide tube 1 over its entire circumference and with the graphite rod over its entire circumference. First, an arc discharge is ignited between the electron source 10 formed as a hollow cathode and the anode 11 by a third current supply device 14. The current intensity here is 60 A. At this time, argon, the working gas, flows through the hollow cathode. The pressure in the vacuum chamber is 1*10 -3 After the current supplies for the magnetic coils 6 and 12 are switched on, the first current supply 9 is also switched on and their current strength is gradually increased from 0 A to 200 A within 5 minutes. After that, the current strength of the third current supply 14 is reduced to 10 A and the gas flow through the hollow cathode is also reduced until the pressure in the vacuum chamber reaches 7*10 -4The pressure is reduced to 1000 mbar. The rear guide 3 is activated and the evaporation material 2, which is formed as a graphite rod, is pushed upward at a speed of 15 cm / h. A second substrate made of steel is exposed to the carbon vapor for 1.5 minutes. During this time, a 1.2 μm thick carbon layer separates on the second substrate. Measurements using the nanoindentation method show that the layer has an extremely high hardness of 74 GPa and an elastic modulus of 680 GPa.

[0032] An alternative device 300 according to the invention is shown schematically in Figure 3. The device 300 shown in Figure 3 has a structure that is largely identical to the device 100 according to the invention of Figure 1. The components in Figure 3 have the same reference numbers as the components in Figure 1, and therefore also have the same structure and the same principles of operation.

[0033] The apparatus 300 of Figure 3 differs from the apparatus 100 of Figure 1 in that the evaporation material 2, which is formed as a graphite rod, is not surrounded by graphite felt but is surrounded, at least at the end of the graphite rod to be evaporated, by three spaced-apart tubular heat reflectors 16, 17, and 18, which function as thermal insulating elements. The tubular heat reflectors 16, 17, and 18 maintain heat at the end of the graphite rod to be evaporated, allowing the graphite rod to stably evaporate.

[0034] A further difference arises in the manner of electrical contacting of the evaporation material 2, which is configured as a graphite rod: in the device 300, the graphite rod is contacted via a contact element 19, which in this way forms an electrically conductive connection to the positive pole of the current supply device 9. The contact element can be configured, for example, as a sliding contact.

Claims

1. 1. An apparatus for separating a carbon layer onto a substrate to be coated, comprising: The device includes a first current supply device (9) that forms an arc discharge between an electron source (10) and an evaporation material (2), a negative pole of the first current supply device is conductively connected to the electron source (10), and a positive pole of the first current supply device (9) is conductively connected to the evaporation material (2); The device comprises a permanent magnet system (5) and at least one magnetic coil (6) arranged rotationally symmetrically around the evaporation material (2), the evaporation material (2) is formed as a graphite rod and is surrounded by at least one heat insulating element (4; 16; 17; 18), at least at the rod end of the graphite rod that is to be evaporated, The evaporation material (2) formed as a graphite rod is placed inside a guide tube (1), The guide tube (1) has an upper stopper (15). An apparatus characterized in that

2. An apparatus as described in claim 1, wherein the inner wall of the guide tube is lined with a porous material having a melting point above 2000°C, at least at the end of the graphite rod to be evaporated.

3. 3. The device of claim 2, wherein the porous material comprises graphite felt (4).

4. 2. The device according to claim 1, wherein the evaporation material (2) formed as a graphite rod is surrounded at least at the end of the graphite rod to be evaporated by at least two tubular heat reflectors (16; 17; 18) spaced apart from one another.

5. 3. The device according to claim 1, wherein the electron source (10) is configured as a hollow cathode.

6. 6. The device according to claim 1, further comprising a second current supply device (13) electrically conductively connected to the magnetic coil (6), the second current supply device (13) being configured as a pulsed current supply device.

7. 1. A method for depositing a carbon layer on a substrate to be coated, comprising: A first current supply device (9) is used to generate an arc discharge between the electron source (10) and the evaporation material (2); The negative pole of the first current supply device is conductively connected to the electron source (10), and the positive pole of the first current supply device (9) is conductively connected to the evaporation material (2); A method in which a permanent magnet system (5) and a magnetic coil (6) are arranged rotationally symmetrically around the evaporation material (2), The evaporation material (2) is formed as a graphite rod and is surrounded by at least one heat insulating element (4; 16; 17; 18), at least at the rod end of the graphite rod that is to be evaporated, The evaporation material (2) formed as a graphite rod is placed inside a guide tube (1) having an upper stopper (15). A method characterized by:

8. The method of claim 7, wherein the electron source (10) is formed as a hollow cathode.

9. 9. The method according to claim 7, further comprising the step of electrically connecting a second current supply (13) to the magnetic coil (6), the second current supply (13) being configured as a pulsed current supply.

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