Method for evaporating cathode in vacuum arc evaporator

Abstract

The invention relates to a method for evaporating a cathode in a vacuum chamber by means of an arc evaporation process, in which method an arc is ignited and the speed and movement of a cathode spot of the arc on a predetermined trajectory on the surface of the cathode to be evaporated are influenced by a magnet. According to the invention, an arc is ignited at the center (12) of the cathode, after which the cathode spot moves outwards along an arcuate trajectory (13) and then along an annular trajectory (14) around the center of the target.

Description

Technical Field

[0001] The invention relates to a method for evaporating cathodes in a vacuum chamber by an arc evaporation process, in which an arc is ignited and the speed and movement of the cathode spot of the arc on a predetermined trajectory on the cathode surface to be evaporated are influenced by a magnet. Background Art

[0002] In arc evaporation, the material to be evaporated (which must be an electrically conductive material) is placed at cathode potential, while a special electrode is used as the anode, or the wall of the coating chamber is connected to the anode of a current source. To ignite the vacuum arc discharge, it is common practice to use an ignition mechanism to briefly short-circuit the anode potential to the cathode target, thereby generating a small ignition arc, which is sufficient to form and maintain a vacuum arc between the actual anode and the target and continue to burn independently. The vacuum arc causes a so-called cathode spot with a high energy density to impinge on the target. Depending on the material, the magnetic field and the current, the cathode spot moves on the target surface, causing the target material to transform from the solid state to the glassy state. When evaporating carbon in particular, due to the material properties, the cathode spot moves very slowly on the target surface, with the risk of getting stuck in one position. The local high currents caused by the significantly slow movement of the cathode spot can also lead to potential melting of the anode material. The slow movement of the cathode spot also leads to very uneven target consumption and short process times. Short process times occur in particular when extremely thin carbon layers need to be deposited. The carbon layer can have a wide variety of properties due to its different structures. The layer hardness of the carbon layer can vary from diamond-hard to graphite-soft. For the deposition of thin layers, only short process times are required, during which the cathode spot only sweeps a partial arc over the target. For example, a carbon layer in the range of 100 nanometers can be deposited in less than 1 minute or even less than 10 seconds. In any case, ignition at the outer edge of the target in the conventional way means that the cathode spot cannot describe a complete circular trajectory as desired. Moreover, utilizing only a portion of the target surface has the disadvantage that the target is only partially "consumed" and therefore needs to be replaced frequently. However, replacing the target leads to increased system downtimes, further increasing process costs. Summary of the invention

[0003] The object of the present invention is to provide a method in which the target material can be utilized more efficiently.

[0004] In order to solve this problem, it is proposed that in the described method, the arc is initially ignited at the center of the cathode, after which the cathode spot of the arc moves outward along an arc trajectory due to the existing unstable position, and then moves along a circular trajectory around the center of the target. This is achieved by maximizing the normal component of the magnetic field at the center of the target, which causes the cathode spot to move outward. In the edge area with a radius of, for example, greater than 35 mm and less than or equal to 50 mm, the radial component of the magnetic field is maximized, so that the cathode spot moves with an acceleration greater than 10 mm / s.

[0005] Preferred embodiments of the invention are described in the dependent claims.

[0006] The cathode spot thus describes an arc-shaped trajectory, in particular a trajectory that sweeps over an angle of greater than or equal to 160°. The method according to the invention is particularly suitable for targets consisting of carbon. BRIEF DESCRIPTION OF THE DRAWINGS

[0007] Further embodiments of the present invention are described with reference to the accompanying drawings.

[0008] Figure 1 A schematic diagram showing a target with an ignition wire attached for igniting an arc; and

[0009] Figure 2 A top view of the target is shown, preferably depicting the arcuate trajectory swept by the cathode spot and the stable end trajectory. DETAILED DESCRIPTION

[0010] like Figure 1 As shown, the arc is ignited by an ignition wire 10, which causes a short circuit on the surface of the target 11. By the ignited arc, the target material is evaporated, so that a plasma phase is formed in front of the target, which can condense on the sample located in the reactor chamber, thereby causing the formation of a layer. In this case, the target consists of a disc-shaped carbon body. However, in contrast to the prior art, according to the present invention, the arc is ignited in the center 12 of the target. However, this position is not an equilibrium position, so that due to the electromagnetic forces (Lorentz forces) present and based on the conductivity of the target material, the cathode spot is guided to a stable circular trajectory around the center of the target. Prior to this, the cathode spot describes an arc-shaped trajectory 13.

[0011] In a specific embodiment, a target material 11 made of graphite is used.

[0012] The target 11, i.e. the material with specific properties that evaporates during the process and deposits on the substrate surface, is mounted on a cathode in the vacuum chamber. The target has a diameter of 100 mm and a thickness of less than 20 mm.

[0013] The sample to be coated, i.e. the substrate, is transferred to a vacuum chamber. The vacuum chamber is evacuated by a connected pump system to a pressure range of less than 1x 10 -3 Pascal. Argon gas is introduced into the vacuum chamber using gas flow control hardware. The argon gas is ionized and accelerated towards the substrate by the voltage applied to the substrate. Due to the momentum transfer, superficial impurities and oxides on the substrate surface are removed and the substrate surface is activated. Now the sample is ready for the actual coating process. Depending on the process, the sample can also be heated to the appropriate temperature by a heater.

[0014] For the subsequent coating process, an argon gas flow was introduced into the vacuum chamber at an argon pressure of less than 1 Pascal. An ignition voltage of 88 V was applied to the target. The trigger pointer at the anode potential for ignition was pointed to the target surface so that the contact point was about 40 mm from the center of the target, thus creating a short circuit.

[0015] When the trigger pointer is moved away from the target surface, a short circuit forms an arc, which vaporizes the target material, thereby forming a conductive gas phase (plasma) in the vacuum chamber, which enables the plasma to burn between the cathode and the anode (chamber wall). After the plasma is ignited, a stable process can be achieved with a target voltage of approximately 20 volts to 25 volts and a target current of less than 80 amps.

[0016] The cathode spot is set in motion by the improved magnet located behind the target and the electric field (E field) generated by the self-induction of the target current. The magnetic field is optimized in such a way that the normal component of the magnetic field is maximized in the central area of ​​the target, while the radial component of the magnetic field is maximized at a radius greater than 35 mm and less than or equal to 50 mm. As a result, the cathode spot ignited from the center of the target is guided to an arc-shaped trajectory that sweeps a radius greater than or equal to 160° on a stable edge trajectory, and is accelerated there to a speed greater than 10 mm / s.

[0017] The arc initially travels on an arc-shaped trajectory 13, i.e., from a radius of about 22 mm at the center of the target outward to a radius of about 45 mm (see Figure 2 ). The initial direction of this arc trajectory is statistically arbitrary, so that the arc can run in different directions after each ignition. Once it reaches an equilibrium position at a radius of about 40 mm, the arc begins to move in a normal circular motion with a quasi-static radius on the target surface. Due to the improved magnet configuration, the arc takes about 30 seconds (about 10 mm / s) to complete one circle.

[0018] The process time depends on the deposition rate and is usually less than 1 minute. Depending on the process, the sample is cooled to the specified temperature. The system is then vented and the sample is transferred out of the vacuum chamber.

[0019] However, if, under the same process conditions, the arc is ignited on the target non-centrally at 35 mm to 45 mm from the cathode center, i.e. for a point on a circular trajectory where the cathode spot assumes an equilibrium position and moves on a circular trajectory, the arc requires 180 seconds to 240 seconds to make one revolution around the circular trajectory, i.e. it moves at a speed of about 1 mm / s. In the case of short coating times (<1 minute), this results in the cathode spot moving only over a small portion of the circular trajectory in the conventional process, resulting in uneven removal of the target and the need for more frequent replacement, resulting in undesirably long downtimes of the system.

[0020] The invention is particularly suitable for the vapor deposition of very thin carbon layers with a thickness in the order of 100 nanometers. The carbon layer is applied in a short time of a maximum of 1 minute, preferably less than 10 seconds. During this time, the arc can only move over a small part of the target surface. In the conventional practice of igniting the arc at the outer edge of the target, only a part of the circular trajectory is crossed during carbon evaporation, resulting in an erosion profile only on a part of the target surface. The invention eliminates the disadvantage that the target must be replaced even if it is only partially worn. By igniting the arc in the middle of the target and then moving it outward along the arc trajectory, it can be ensured that the target is removed evenly by evaporation.

[0021] The magnetic field guiding the arc is optimized for the speed and equilibrium position of the arc. The strength of the magnetic field strongly influences the speed of the arc, so that it generally runs faster on the target. The arc represents an intense negative charge, which burns between the target and the anode arranged around it. If the arc moves slowly, this current flow can cause the anode to melt. This can be minimized or even prevented by moving faster. In addition, the arc covers a longer distance in a short time.

[0022] The equilibrium position of the arc also depends on the magnetic field. By modifying the magnetic field, the trajectory of the arc on the target surface can be set.

Claims

1. A method for evaporating a cathode by an arc evaporation process in a vacuum chamber, wherein an arc is ignited and the speed and movement of the cathode spot of the arc on a predetermined trajectory on the cathode surface to be evaporated are influenced by magnets, characterized in that The arc is ignited at the center (12) of the cathode, after which the cathode spot moves outward along an arcuate trajectory (13) and then along a circular trajectory (14) around the center of the target.

2. The method according to claim 1, wherein: The magnetic field on the target surface is improved so that the cathode spot is caused to move outward along an arc path by maximizing the normal component of the magnetic field at the center of the target, and a stable end position is achieved on a circular trajectory with a radius greater than 35 mm and less than or equal to 50 mm by maximizing the radial component of the magnetic field, while the acceleration of the cathode spot is greater than 10 mm / s.

3. The method according to claims 1 and 2, characterized in that The angle swept by the arc-shaped trajectory (13) is greater than or equal to 160°.

4. The method according to claim 1, 2 or 3, characterized in that: The cathode is composed of carbon (graphite).

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