Arc beam scanning to suppress anode overgrowth in PICVD systems.

Abstract

The present invention relates to a method for inhibiting anode overgrowth in an arc beam PICVD coating system, where an arc beam is scanned over at least a portion of a surface of an anode during at least a portion of a coating process of the arc beam PICVD coating system. Additionally, the present invention relates to an arc beam PICVD coating system for coating a part.

Description

[Background technology]

[0001] PICVD (plasma induced chemical vapor deposition) systems based on a DC plasma arc beam generated with an electron emission source (cathode) and a collector (anode) are typically operated in a low pressure gas atmosphere ranging between 0.1 mbar and 10 mbar. The gas mixture used in this process consists of a noble gas such as argon (Ar), hydrogen (H2), and a hydrocarbon gas. The hydrocarbon gas is the source of carbon atoms for the carbon-based film to be grown, the hydrogen ensures the growth in the diamond sp3 carbon bonding mode, and the Ar is necessary to operate the arc beam plasma. This plasma arc beam is stabilized by a magnetic field with magnetic field lines forming a non-convoluted path from the cathode to the anode. Such PICVD processes for producing diamond films on parts have been described in [1], [2], [3], [4], [5]. For brevity, the cathode and anode are typically arranged on opposite sides of a cylindrical reactor so that the arc beam forms a straight column from the cathode to the anode (see Figure 1a-c).

[0002] The magnetic field is generated by a coil (or multiple coils 10) arranged so that the magnetic field lines 10a are substantially parallel to the cathode to anode axis and the magnetic field strength is approximately homogenous.

[0003] In the hot plasma of the arc beam, the hydrocarbon gas molecules are excited and atomic hydrogen is formed. These species diffuse to the part located at a certain distance from the arc beam. On the surface of the part, these species react to form a carbon film. For a properly selected distance from the part to the arc beam, the temperature conditions for efficient growth of diamond films are met.

[0004] In a PICVD system, coating films are formed from excited species that condense on any surface. These excited species diffuse and move in the coating system. Depending on the surface temperature, different types of condensation reactions occur and different types of coatings grow on the surface. In a system for producing diamond-type coatings, the growth of sp3-type diamond films occurs in the presence of an appropriate amount of atomic hydrogen on surfaces with temperatures in the range of 700-900 ° C. These temperature conditions are met at a certain radial distance around the arc beam in the center of the system, when heated both by thermal radiation from the plasma of the arc beam and by exothermic surface reactions of mainly atomic hydrogen. At longer distances, the temperature is lower and graphite-type growth is favored. In areas of lower temperature, especially on the walls of water-cooled systems, polymer-type film growth is observed.

[0005] The rate of diamond film growth on a part located at a particular distance from the plasma arc beam depends critically on the distance from the center of the arc beam. In a typical configuration, as shown in Figures 1a and 1b, the part 1 is located at a fixed distance from the arc beam axis 9.

[0006] In such systems, for a stable operation of the arc beam, a cone-shaped anode electrode is preferentially used, ensuring axial symmetry, so as to ensure that the axis of the arc beam is defined on the central axis of the system. Electrons reaching the metal surface of the anode are accelerated by the positive anode sheath potential. A corresponding gain in energy is then lost at the metal anode surface. For typical arc currents of up to several hundred A, the power loss can be up to several kilowatts (kW). The size of the anode surface needs to be properly designed to ensure sufficient cooling to avoid overheating and melting of the anode. For a properly designed water-cooling channel inside the anode, a power loss of up to 100 W / cm2 can be achieved. 2 Power density levels in the range of 1000 to 10000 kW can occur on the conical anode surface. Total power losses in the anode can be several kW, resulting in a surface area of ​​several tens of square centimeters. 2Since the current density in the arc beam is highest at the center, the power density on the anode surface is also highest at the center and decreases with increasing radius. With increasing distance from the center of the anode, the internal water cooling area adjacent to the anode surface at a particular radius becomes larger (increasing as a square order with radius), while the power density from the arc beam decreases to a level much smaller than at the center of the arc beam. These two effects strongly reduce the anode surface temperature towards the outer diameter. The anode with an arc beam at the start of the coating process is shown in Figure 2a. The arc beam spreads over the entire metal surface.

[0007] Considering these facts, an insulating polymer type film may grow on the anode with the lowest surface temperature, especially at higher partial pressures of CxHy reactant gases or at lower arc currents. This polymer overcoat film starts first at the periphery and gradually grows over time toward the center of the anode cone. An anode with such overgrowth and a constricted arc beam in the vicinity of the anode is shown diagrammatically in Figure 2b.

[0008] Such polymer films are electrically insulating, and the overgrowth areas of the anode no longer contribute to the effective area of ​​the anode that conducts electrons from the arc beam. As a result, this active area of ​​the anode decreases over time, and for a fixed discharge current favorable for the coating process, the power density on the stationary metal anode surface rises, and the accumulated power loss gradually concentrates in the tip area of ​​the cone-shaped anode. As a result, the power density increases locally, and the power loss results in a locally higher heat load at a higher temperature. In addition, this constriction of the beam also increases the potential drop from the beam into the anode, which can result in a higher total power loss at a constant arc current. Part of the additional power loss due to the higher potential at the anode leads to higher power loss to the rest of the system, resulting in process temperature drift, which can negatively affect the process conditions for film growth on the surface of the part, with a deterioration in coating quality. In addition, it has been observed that the center of the arc beam can drift laterally on the anode cone, which can have some adverse effect on the radial uniformity of film growth on parts positioned around the geometric axis of the system.

[0009] To achieve higher coating rates, which is usually an economic advantage in coating applications, the process gas pressure can be increased as high as possible, but is usually limited by the desired film quality characteristics. However, with higher process pressures, the polymer overgrowth on the anode becomes larger and spreads to the center, which ultimately also limits the practically achievable coating rate in the system under production conditions.

[0010] A method to reduce or eliminate this overgrowth on the anode surface would be highly desirable, which would have no or minimal effect on the process conditions selected to obtain a coating film on the surfaces of the parts in the system. Summary of the Invention [Problem to be solved by the invention]

[0011] The present invention describes means and methods for inhibiting the formation of insulating overgrowth films on the anode surface. [Means for solving the problem]

[0012] The above problem is solved by a method having the features of claim 1 and by a system having the features of claim 10. [Brief description of the drawings]

[0013] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] Further means for improving the invention emerge from the following description of some embodiments of the invention, which are illustrated diagrammatically in the drawings. All features and / or advantages emerging from the claims, the specification or the drawings, including details of construction, spatial arrangements and process steps, may be essential to the invention both individually and in various combinations. It should be noted that the drawings are merely illustrative and are not intended to limit the invention in any way.

[0015] In accordance with the present invention, a method is provided for inhibiting anode overgrowth in an arc beam PICVD coating system in which an arc beam is scanned over at least a portion of a surface of an anode during at least a portion of a coating process of the arc beam PICVD coating system.

[0016] The anode overgrowth may include a layer of insulating components. As long as the anode overgrowth is very thin, it does not impair the flow of current. However, a thick anode overgrowth may limit the flow of current to non-coated areas or areas with a thinner coating. The arc beam PICVD coating system may include a means for establishing an arc beam configured to form a coating on the part at low pressure by plasma discharge. In the framework of the present invention, the scanning of the arc beam can be understood as the movement, relative to the anode, of the discharge zone that connects the plasma to the anode. This movement can be continuous or of varying speed. The surface of the anode may be the actual entire surface of the anode, or it may be considered as the part of the surface of the anode (only if it is not coated) that is conducting the current of the arc beam when there is no insulation on the formed anode and the arc beam is not moving. For example, the anode surface may be substantially the surface of the side of the anode that faces the cathode.

[0017] The method steps may be performed at least partially simultaneously or one after the other in chronological order, and the order of the method steps is not limited to the order given by the numbers, and thus individual steps may be performed in different orders.

[0018] The anode may be a substantially rotationally symmetric component and the scanning of the arc beam may be performed around an axis of rotational symmetry, or in other words, the anode may be formed as a substantially rotating solid body and the arc beam is moving around a central axis of the solid body.

[0019] Also, for scanning the arc beam, two pairs of coils may be used to generate an oscillating magnetic field component perpendicular to the axis of rotational symmetry to attract and / or deflect the arc beam and induce oscillation. In other words, if the magnetic field lines parallel to the electric field between the cathode and the anode are in the z direction, the magnetic field in the xy plane can deflect the arc beam at least in the x direction or the y direction. This provides the advantage that a wide variety of scanning patterns can be realized on the surface of the anode, thereby easily and effectively avoiding or reducing the formation of an insulating layer on the anode surface.

[0020] The scanning of the arc beam is considered to be controlled by a magnetic field, at least the strength or direction of the magnetic field being set based on the expected growth of the insulating layer on the anode. In other words, by setting the magnetic field to control the shape and movement of the arc beam, areas where large growth is expected if the arc beam is not moved on the anode surface may be exposed to the arc beam for a longer time during the deposition process. This has the advantage that the surface of the anode can be exposed to the arc beam as needed, so that the deposition of the insulating layer on the anode surface can be easily and effectively avoided or reduced.

[0021] The arc beam can be scanned in an elliptical path over the anode. In other words, if the z-axis is defined along the path between the anode and the cathode, the oscillations on the x-axis have a different amplitude than the oscillations on the y-axis. The elliptical path offers the advantage that non-circular overgrowth patterns can be efficiently avoided.

[0022] Advantageously, the arc beam can be scanned to follow a closed path on the anode, which can be easily achieved, for example, by an oscillating magnetic field, so that a Lissajous (or Lissajous)-like pattern can be formed on the surface of the anode.

[0023] Also, the amplitude of the arc beam path modulation can be between one-tenth and one-fifth of the anode diameter. This range of motion effectively reduces the deposition of an insulating layer on the surface anode.

[0024] It is further conceivable that the entire surface of the anode is exposed to the arc beam at least once during the coating process. In particular, the entire surface should be understood as the entire surface of the anode, which is usually covered by an insulating layer. If the anode surface is at least partially masked, for example by a holder, the entire surface is considered to be an unmasked area. In this way, the deposition of an insulating layer on the entire surface of the anode can be effectively suppressed.

[0025] Also, during the coating process, the arc beam can be directed for a longer period of time at areas of the surface of the anode where greater buildup of the insulating layer on the anode is expected in the absence of arc beam scanning, compared to areas where lower buildup of the insulating layer on the anode is expected in the absence of arc beam scanning. The expected buildup pattern is fed back into the control of the arc beam scanning, allowing the reduction in buildup of the insulating layer to improve from loop to loop.

[0026] In yet another aspect of the invention, an arc beam PICVD coating system for coating a part is provided, the arc beam PICVD coating system configured to carry out a method according to the invention.

[0027] The system according to the invention therefore offers the same advantages as those explained in detail with reference to the method according to the invention.

[0028] Further features and details of the invention can be obtained from the dependent claims, the description and the drawings. The features and details explained with respect to the method according to the invention naturally also apply in each case with respect to the system according to the invention and vice versa, so that cross-reference is or can always be made to the disclosure relating to the individual aspects of the invention.

[0029] The basic idea is a controlled cyclical motion of the arc beam across the anode surface. An initial very thin overgrowth may form on surface areas that are temporarily not in the arc beam during one motion cycle, but all parts of the surface area are continuously in the arc beam over the course of one complete motion cycle. The overcoat on these areas remains thin enough to develop insulation that inhibits electronic conduction. The slightly reduced conduction through these very thin layers may cause a small additional voltage drop on the arc beam as it subsequently moves across these areas during the motion cycle. This voltage drop leads to additional power loss and a localized surface temperature increase. This causes the overcoat layer to evaporate or disintegrate, leaving the area as a metallic surface after the arc beam has passed this section of the anode. The cyclical motion across the anode leaves the surface conductive and does not cause constriction of the arc beam at the anode.

[0030] Considering the rotational symmetry of the anode, the periodic motion pattern may be a circular motion of the arc beam around the central axis of the anode, which is shown diagrammatically in FIG. 3. The circular path of the arc beam motion can be realized by a set of coils outside the coating system chamber that generate in-plane magnetic field components to deflect the arc beam. FIG. 4a shows a top view of such a system, and FIG. 4b shows a side view. Preferentially, two pairs of coils on opposite sides of the system are used to generate magnetic field components in the x and y directions.

[0031] Appropriately modulated currents in these coils generate a horizontal magnetic field vector Bxy rotating in the horizontal plane around the central axis of the system. The same rotating magnetic field vector can be achieved if the coils are not vertical, simply by adapted amplitudes and corresponding phase relationships of the individual coil currents.

[0032] The diameter of the path is adjusted to minimize or eliminate the overgrowth layer on the anode that grows from the outer diameter toward the center. The diameter of rotation is not excessively large so that the arc beam distorts and adversely affects a stable coating process. If a non-circular overgrowth occurs with a static arc beam, an elliptical path with its axis adjusted to the overgrowth pattern can be easily produced by appropriate modulation of the horizontal coil current. If the overcoat is not centered on the anode axis, an appropriate static offset current can be applied to the coil to statically deflect the arc beam onto the anode center.

[0033] For a modulation period Tp=1 / f and modulation frequency f, the corresponding current signals for the x-coil and y-coil can be written as follows:

[0034]

number

[0035] The period of modulation must be chosen to be short enough so that the overgrowth formation during one period remains conductive yet extremely thin.

[0036] The thickness of the overcoat layer, estimated from experiments using a static arc beam, can typically be in the range of tens of microns after several hours of processing. As a result, the overcoat growth rate can be on the order of less than 1 nanometer per second. If the modulation period lasts for tens of seconds or less, the extremely thin overcoat layer remains on the order of only a few nanometers, which is still conductive enough for the arc beam electrons to reach the anode.

[0037] The periodic motion of the arc beam can unintentionally affect the growth rate of the desired diamond film on the part located around the arc beam pillar. The growth rate of the film on the part depends on the distance from the arc beam axis, and increases / decreases as the distance from the beam axis decreases / increases. With an arc beam rotating about a system axis (merged with a static arc beam axis), the growth rate on the part surface is modulated with the rotational frequency of the beam's circular motion. As long as this modulation is small compared to the growth rate with a static arc beam and the number of modulation cycles throughout the coating process is large, these modulations have minimal effect on the film formation, and the overall thickness remains unaffected because the growth variations from the modulation cycles are averaged over the many modulation cycles. In addition, certain non-sinusoidal modulation schemes can better minimize part thickness variations while still suppressing overgrowth on the anode, where sinusoidal modulation may leave some variation.

[0038] In a typical example, the diameter of the anode may be on the order of 50-100 mm. Initially, the arc beam spreads over the entire area of ​​the anode. Over time, with a static arc beam, depending on the process conditions, such as the partial pressure of the C-containing gas, the total pressure, and the arc beam current level, the overcoat may grow from the outer diameter of the anode to a ring with an inner diameter less than half the diameter of the anode, leaving a metal area on the anode less than a quarter of the total anode area at the start of the process. With an arc beam modulation amplitude of typically 10 mm, the overgrowth area can be reduced to only a small rim at the outer diameter of the anode, leaving almost the entire anode surface metal. Since there is some increase in the discharge voltage with increasing modulation amplitude, the choice of modulation amplitude is a compromise between eliminating the overgrowth and potentially affecting the process parameters in a way that may be detrimental to the membrane manufacturing process.

[0039] Figures 5 and 6 show the effect of the method and system according to the present invention. Both figures are schematic plan views of photographs showing a cone-shaped anode (100) used in a coating process. In Figure 5, the method and system according to the present invention are used, and Figure 6 shows the result of the coating process without the method and system according to the present invention. In comparison, it can be seen that the overgrowth area (101) in Figure 5 according to the method and system according to the present invention is much smaller. It should be noted that the thickness of the insulating layer on the anode (100) is larger without the method and system according to the present invention (not shown). In particular, when the arc beam is scanned over at least a portion of the surface of the anode (100) during at least a portion of the coating process of the arc beam PICVD coating system, a large uncoated area (102) remains after the coating process is completed.

[0040] The above description of the embodiments describes the invention exclusively in the context of examples. Naturally, the individual features of the embodiments can be freely combined with one another, where technically appropriate, without departing from the scope of the invention. [1]:Karner, Pedrazzini, Bergmann, US 5753045 [2]:"High current dc arc (HCDCA) technique for diamond deposition", J.Karner, M.Pedrazzini, C.Hollenstein, Diamond and Related Materials 5, (1996), 217-220 [3]:"Verfahren zur Diamantbeschichtung", D.Franz, J.Karner, WO 03 / 031675 A2 [4]:Franz, Karner, US 7192483 [5]:Karner, Pedrazzini, Bergmann, US 5902649

Claims

1. 1. A method for inhibiting anode overgrowth in an arc beam PICVD coating system, comprising: scanning an arc beam over at least a portion of a surface of the anode during at least a portion of a coating process of the arc beam PICVD coating system.

2. 2. The method of claim 1, wherein the anode is a substantially rotationally symmetric component and the scanning of the arc beam is performed about an axis of rotational symmetry.

3. 3. The method according to claim 1 or 2, characterized in that for scanning the arc beam, two pairs of coils are used to generate oscillating magnetic field components perpendicular to the axis of rotational symmetry, thereby attracting and / or deflecting the arc beam and causing it to oscillate.

4. A method according to any one of claims 1 to 3, characterized in that the scanning of the arc beam is controlled by a magnetic field, and at least the strength or direction of the magnetic field is set based on the expected growth of an insulating layer on the anode.

5. A method according to any one of claims 1 to 4, characterised in that the scanning of the arc beam follows an elliptical path over the anode.

6. A method according to any one of the preceding claims, characterised in that the scanning of the arc beam follows a closed path over the anode.

7. A method according to any one of the preceding claims, characterised in that the amplitude of the arc beam path modulation is between one tenth and one fifth of the diameter of the anode.

8. A method according to any one of the preceding claims, characterized in that the entire surface of the anode is exposed to the arc beam at least once during the coating process.

9. 9. The method according to claim 1, characterized in that during the coating process, the arc beam is directed for a longer time at areas of the surface of the anode where a greater buildup of insulating layer on the anode is expected in the absence of scanning of the arc beam compared to areas where a lower buildup of insulating layer on the anode is expected in the absence of scanning of the arc beam.

10. 10. An arc beam PICVD coating system for coating a part (1), characterized in that the arc beam PICVD coating system is configured to carry out the method according to any one of claims 1 to 9.

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