ANODE FOR PVD PROCESSES

Description

[0001] The invention relates to an anode and an anode arrangement in a device for carrying out physical vapor deposition processes, a method for producing such an anode and the use of the anode and the device, which greatly reduces or even prevents the degradation of the anode due to deposits.

[0002] In vapor deposition using PVD processes, the material used for coating is first converted into its gas phase through evaporation, sublimation, or sputtering, and then fully or partially ionized through discharge processes. Processes suitable for converting material into the plasma state include magnetron discharges and arc discharges.

[0003] The generation of the freely moving electrical charge carriers required for the arc can be achieved in a vacuum by field emission, thermionic emission, or targeted ionization of the cathode material. Targeted ionization of the cathode material is often used.

[0004] Before the actual coating process, care is taken to ensure the best possible adhesion properties for the coating material to the substrate. Since nitride layers, surface oxides, and adsorbates such as water and hydrocarbons have proven particularly problematic in this regard, it is common practice to remove these before the actual coating process.

[0005] WO 201 309 1802 A1 describes a method for surface coating using arc discharge. A problem is that cathode material clusters occasionally detach from the cathode during the coating process. This problem is addressed here by using magnetic fields that vary the point of interaction between the arc discharge and the electrode surface. However, the use of magnetic fields adversely affects the charged particles, resulting in a reduction in coating rates.

[0006] EP 3 196 331 A1 describes the application of adhesion-reducing coatings to moving substrates using a PVD process and substrate tension. The substrates are subjected to tension to minimize losses of the coating material to the inner surfaces of the PVD device used.

[0007] In PVD processes, material from an electrode is sputtered or vaporized. The respective electrode is consumed and thus degenerates during the process. This is always accompanied by a material transfer to the corresponding counter electrode. If the degenerating electrode is the cathode, the problem arises that unwanted growth of cathode material occurs on the anode. This growth changes the geometric shape of the anode and leads to changes in the distance between the electrode surfaces. This change in distance negatively influences the arc conditions and can even lead to the extinction of the arc due to a short circuit between the two electrodes.

[0008] For magnetron discharges, various solutions are known to prevent anode deposition through deposition processes. These include the hidden anode and the bipolar discharge.

[0009] In the hidden magnetron anode, the plasma is directed behind the cathode's view by a magnetic field, allowing only charged particles to reach the anode. This is not applicable to arc discharges, as the vaporized material is highly ionized and would no longer be available for coating if deflected by strong magnetic fields onto a hidden anode.

[0010] In a bipolar discharge, the coating on the anode is removed again in alternating operation as the cathode. This principle is also not applicable to arc discharges. The described growths pose a major problem, particularly in pulsed high-current carbon arcs, where the anode must be positioned close to the cathode to reliably ignite the discharge.

[0011] According to the current state of the art, the anodes used are primarily made of steel, copper, or titanium core material. The unavoidable growths of the graphitic cathode material are removed mechanically, for example, by scraping, grinding, turning, or blasting. The surface damaged by the mechanical removal of the cathode material is, ideally, subsequently polished. In document US 4,478,700 A, an electrically conductive layer is applied to a contaminated anode, thus restoring the electrical conductivity properties impaired by the contamination to their original electrical conductivity. Documents US 2012 / 298139 A1 and DE 10 2013 103762 A1 disclose metallic anodes or anodes with metallic layers.

[0012] The object of the invention is to propose an anode and an anode arrangement in a device, their use and the production of such an anode which greatly reduces or even prevents the degradation of the positively charged electrode due to deposits.

[0013] The problem is solved by the features of the independent claims. Preferred embodiments are disclosed in the dependent claims.

[0014] The anode according to the invention has a known shape. According to the invention, degradation is avoided or largely reduced by completely or partially covering the surface of the anode with an adhesion-reducing thin film of high electrical conductivity (hereinafter "thin film").

[0015] Thin films are understood to be flat areas of a substrate that differ structurally and / or materially from neighboring layers and the substrate on which they are deposited and whose thickness ranges from a few atomic layers, but at least one monolayer, up to 10 µm.

[0016] One method for surface modification by coating or applying a thin film to a workpiece is physical vapor deposition, or PVD. For the purposes of this document, the workpiece is understood as an object intended for coating and is referred to as the "substrate."

[0017] The vacuum arc process described below is referred to by the terms "arc discharge," "electric arc," and "arc discharge," which are used synonymously in this document. These phenomena serve to generate a plasma. This occurs primarily under vacuum conditions.

[0018] For the purposes of this invention, a pumping station is a device for generating and maintaining a vacuum within a closed vessel.

[0019] In this document, electrodes are referred to as "anode" and "cathode" in the respective contexts. The cathode is defined as an electrode with an overall negative electrical charge state. Analogously, the anode is defined as an electrode with a predominantly positive electrical charge state.

[0020] In the following, a "hot electrode" refers to both an anode and a cathode, which are actively heated before and during their operating state until a defined temperature is reached. The individual electrode designs are referred to as "hot anode" and "hot cathode" in the following.

[0021] The device according to the invention comprises a vessel suitable for containing various process pressures, particularly in the vacuum range. At least two electrodes, which can be subjected to an electrical voltage from outside the vessel, are arranged inside the vessel, with at least one of the electrodes acting as the anode and at least one of the electrodes acting as the cathode. An arc discharge is generated between the cathode and the anode and maintained for a defined period of time.

[0022] These arc discharges result in material removal from the cathode and deposits on the anode. Surprisingly, it has been found that a thin film with high electrical conductivity used according to the invention significantly reduces the adhesive force between the deposits and the surface of the anode(s). Therefore, according to the invention, at least one of the anodes used is characterized by having an adhesion-reducing and electrically conductive thin film that completely or partially covers the surface of the anode.

[0023] The adhesion-reducing thin film with high electrical conductivity applied to the anodes has poorer adhesion properties with respect to the coating material than the uncoated electrode material.

[0024] The adhesion-reducing thin film with high electrical conductivity greatly reduces or even prevents the adhesion of deposits. At the very least, the removal of these deposits is made significantly easier. It is particularly advantageous if the adhesion-reducing thin film forms a closed surface in the areas where it is applied and is not interspersed with pores or other small perforations that would facilitate the adhesion of coating material.

[0025] The advantage of partially coating the electrode surface is the reduced production effort. Typically, the thin-film material is applied during a coating process. If an area of ​​the object to be coated is then concealed before the coating, a shadowed area is created that is not available for the coating. This allows for the targeted creation of surface areas that do not have a thin-film coating. These areas can be advantageously used to mount the anode.

[0026] The uncoated areas of the anode are advantageously arranged during operation in the arc arrangement such that the uncoated areas are concealed to prevent adhesion of cathode material to them.

[0027] The advantage of a complete coating is the simplified assembly effort, since, especially when using anodes with a cylindrically symmetrical cross-section, no special orientation of the anode within the arc arrangement needs to be taken into account, since the entire surface of the anode offers reduced adhesion for cathode material.

[0028] The cathode preferably consists entirely of an electrically conductive carbon modification or has an electrically conductive carbon modification at least on its surface. The electrically conductive carbon modification is preferably formed entirely or partially from graphite.

[0029] Since the cathode is subject to constant wear and the anode requires regular cleaning, in a preferred embodiment, at least one of the electrodes is arranged so that it is movable and / or replaceable. "Movable" in this context preferably means "rotatable," "pivotable," and / or "movable."

[0030] The thin film is preferably selected from a material suitable for high anode temperatures, since the anode is preferably designed as a "hot anode." It is advantageous if the thin film forms only with difficulty or, at best, no chemical bond with the coating material. For example, in the case of carbon-based coating material, the thin film is selected from a material that forms only with difficulty or, at best, no chemical bond with the carbon, e.g., only with difficulty or, at best, no carbides. This property contributes significantly to the creation of process-favorable adhesion properties between the coating material and the thin film.

[0031] The thin film is a layer of metal nitride. The nitride-forming metal is preferably selected from Ti, Zr, Hf, V, or mixtures of these metals. Nitrides with more than one metal in the compound are among the preferred designs if they have a specific electrical resistivity of more than 10 µΩ cm and up to 150 µΩ cm.

[0032] In order to produce a closed surface with sufficient adhesion between the thin film and the core material of the anode on the one hand and to avoid detachment or abrasion of the thin film on the other hand, a layer thickness of the thin film of more than 0.1 µm and less than 3.5 µm, of more than 0.5 µm and less than 3.2 µm is particularly preferred and of 1 µm and less than 3 µm is particularly preferred, is selected.

[0033] The vacuum vessel of the arc arrangement, in which the coating processes take place, is preferably connected to a pumping station for generating and monitoring a vacuum by means of a vacuum-compatible connection.

[0034] In a further embodiment, at least one vacuum-compatible electrical feedthrough is passed through the shell of the vacuum vessel. This feedthrough is suitable for conducting electrical currents and for electrically connecting at least one of the electrodes to a voltage source located outside the vacuum vessel by means of electrical leads.

[0035] In a further preferred embodiment, the device for arc arrangement is characterized in that at least one gas-tight, sealable opening is provided in the vacuum vessel wall, which allows objects to be introduced into and removed from the vessel. A holder for receiving at least one object is arranged inside the vessel. The holder is conductive but insulated from the housing and can be supplied with an electrical voltage from outside the vessel.

[0036] In a further preferred embodiment, a holder for receiving at least one substrate is arranged inside the vessel, wherein the holder is designed to be electrically insulated from the substrate and the housing and only the substrate can be subjected to an electrical voltage from outside the vessel.

[0037] The use of an anode and a device with the anode according to the invention as described above in a coating system for coating substrates is preferred because this significantly increases the operating time of the device until the anode needs to be changed.

[0038] It has been proven that the thin film advantageously has a very low roughness. This is due to the reduced adhesion support provided by a particularly smooth surface. Furthermore, a particularly smooth electrode surface results in fewer points of extremely strong current flow that could adversely affect the electrode surface. To achieve a very low roughness of the thin film, polishing the surface has proven helpful.

[0039] It is particularly preferred to polish the electrodes until they reach a mirror finish before applying the thin film, as this allows for a uniformly controlled application of the thin film with high electrical conductivity.

[0040] A method for producing an anode for use in the coating system, in particular for carrying out PVD processes, is therefore characterized in that at least the electrode intended as the anode undergoes the process steps of "polishing" and "coating", wherein the electrode is polished before the application of the thin film until the value of the mean roughness of the polished surface preferably reaches a value between 0.1 µm and 0.3 µm, and particularly preferably a value between 0.01 µm and 0.1 µm, wherein the application of the thin film takes place after polishing and the thin film is also polished after its application until the roughness of the polished thin film surface reaches a value between 0.01 µm and less than 0.1 µm.

[0041] After the polishing process is complete, typical roughness values ​​of the polished surface of the thin film of Ra < 0.1 µm or a surface roughness of Rz < 0.7 µm are measured using roughness measuring instruments. In the case of a lapped surface, typical roughness values ​​of Ra < 0.01 µm or a surface roughness of Rz < 0.07 µm are measured using tactile roughness measuring instruments after the lapping process. In addition to tactile roughness measuring instruments, optical devices such as laser microscopes can also be used.

[0042] Preferably, the "polishing" and "coating" process steps are performed multiple times. After performing the "polishing" and "coating" process steps at least once, they can be repeated either not at all or in any order.

[0043] The invention is described below in several embodiments and illustrated in the accompanying figures.

[0044] Fig. 1shows, as a first exemplary embodiment, a cross-section of an anode (614) coated with an adhesion-reducing thin film with high electrical conductivity. The adhesion-reducing thin film with high electrical conductivity is partially applied to the core material of the anode (601) with a preferred layer thickness of 0.1 µm to 3.5 µm (624). In this example, the anode is omitted from the coating in the area where the anode fits into an electrode holder with a blind-hole-shaped recess (604).

[0045] Fig. 2 shows schematically as a second embodiment the cross section of an anode (614) completely coated with an adhesion-reducing thin film with high electrical conductivity, which anode is held (605) at the end face and is electrically conductively contacted.

[0046] In one embodiment, a graphite cathode and an anode are arranged in a vacuum chamber equipped with sensors for monitoring the process conditions and a pumping station.

[0047] The vacuum chamber is additionally provided with an opening that can be sealed gas-tight, particularly vacuum-tight. Inside the vacuum chamber, there is a holder for the application of the substrates intended for coating. The holder itself is electrically insulated from the process chamber wall and from the substrate. After their electrical contact, the substrates are connected as auxiliary anodes during the process.

[0048] Previously, with the opening of the vacuum chamber wall sealed gas-tight, the process pressure in the vacuum chamber is adjusted to approximately 10 -6 < hPa using the connected pumping station. An electrical voltage of approximately 200 V is applied between the main electrodes.

[0049] The laser of the ignition device generates a pulsed laser spot on the surface of the graphite cathode over a period of approximately 50 ns. This generates carbon ions and electrons, and the electric arc ignites for a duration of approximately 330 µs. After this time, a new pulse of laser spot is generated at a location that is different from the previous one. This process scans the entire cathode surface.

[0050] The anode used is coated with an adhesion-reducing and electrically conductive TiN nitride layer. The layer thickness is approximately 2 µm.

[0051] To apply the thin film with high electrical conductivity to the anode surface, it was first mechanically polished. The mechanical polishing or lapping of the anode surface was performed until a mirror finish was achieved, following the following steps: 1. Preparation of the uncoated anode by grinding with an abrasive of grain size between 80 - 120, 2. Removal of coarse scratches by grinding with "medium" grain size 180-220, 3. Pre-polishing with a polishing wheel of grain size 280 - 320 and 4. Polishing with suitable polishing paste and a suitable polishing wheel.

[0052] The grain sizes of the polishing and abrasives mentioned refer to the classification according to the FEPA standard. Reference symbol

[0053] 601 Anode 604 Electrode holder as blind hole-shaped recess 605 Electrode holder as end-face holder 614 Partially coated anode 624 Fully coated anode

Claims

1. Anode suitable for physical vapor deposition processes, the surface of the anode being completely or partially covered with an adhesion-reducing thin film of high electrical conductivity, the anode being designed as a hot anode and the thin film with high electrical conductivity being suitable for high anode temperatures, characterized in that the adhesion-reducing thin film has a thickness between 0.1 µm and 3.5 µm, the adhesion-reducing thin film being a ceramic nitride layer.

2. Device for physical vapor deposition processes with an anode arrangement, comprising a vessel which is suitable for enclosing various process pressures, in the interior of which at least two electrodes are arranged which can be subjected to an electrical voltage from outside the vessel, wherein at least one of the electrodes functions as an anode and at least one of the electrodes functions as a cathode, and an arc discharge is generated between the cathode and the anode and can be maintained over a defined period of time, wherein at least one of the anodes is an anode according to claim 1.

3. Anode according to claim 1 or device according to claim 2, wherein the material of the ceramic nitride layer is a metal nitride selected from the metals Ti, Zr, Hf and V or a mixture thereof.

4. Device or anode according to claim 3, wherein the selected metal nitride has an electrical resistance between 10 µΩ cm and 150 µΩ cm.

5. Device according to claim 4, wherein the cathode consists entirely of an electrically conductive carbon modification or that it has an electrically conductive carbon modification at least partially on its surface.

6. Device according to claim 5, wherein the electrically conductive carbon modification is graphite.

7. Device according to claim 2, wherein the distance between the electrodes is adjustable, wherein at least one of the electrodes is designed to be movable.

8. Device according to any of claims 2 to 7, wherein at least one of the electrodes is designed to be replaceable.

9. Use of a device according to any of claims 2 to 8 as a coating system for coating substrates applied to a holder.

10. Method for producing an anode according to claim 1 for use in a device according to any of claims 2 to 8, wherein the electrode provided as an anode undergoes the method steps of polishing and coating, wherein, before the application of the thin film, the electrode is polished until the value of the mean roughness of the polished surface reaches between 0.01 µm and 0.3 µm, wherein the thin film is subsequently applied and, after application, the thin film is left as it is or is polished until the value of the mean roughness of the polished thin film surface reaches a value of greater than 0.01 µm and less than 0.1 µm.

11. Method according to claim 10, wherein the method steps of "polishing" and "coating" are carried out multiple times.

12. Method according to claim 11, wherein the method steps of "polishing" and "coating" are repeated in any order after being carried out once.