Hollow cathode system for generating a plasma and method for operating such a hollow cathode system

Description

[0001] The invention relates to a hollow cathode system for generating a plasma, which is used, for example, in coating or processing substrate surfaces under vacuum conditions, and to a method for operating such a hollow cathode system.

[0002] Hollow cathode plasma sources are used for a variety of applications, such as plasma activation of PVD processes, excitation of PECVD processes, coating removal, cleaning or physicochemical modification of surfaces, the construction of arc evaporators, or as a heat source. The operating principle of a hollow cathode discharge consists of particularly efficient plasma excitation between opposing cathode surfaces, the space between which is traversed by a working gas (DE 10 2010 011 592 A1 and DE 10 2013 210155 A1). Ions from the plasma zone impact the inner cathode wall, thus providing electrons for discharge via secondary electron emission (hollow cathode glow discharge) or thermionic electron emission (hollow cathode arc discharge), although this is associated with various wear mechanisms.In both discharge types, ion bombardment causes the continuous sputtering of hollow cathodes. In the case of an arc discharge, where the hollow cathodes are typically tubular, the high operating temperatures lead to increased sputtering rates, evaporation, recrystallization, and material embrittlement, as well as chemically assisted erosion during reactive processes. Furthermore, the ignition phase is particularly prone to wear, as material spatter from cold arcs frequently occurs. The lifetime of such plasma sources is therefore only a few hours (depending on the operating characteristics and process conditions). This is far too short for continuous operation typical in industrial applications, lasting one or even two weeks.In general, the short cathode lifespan results in significant time loss, as replacing the wear parts requires interrupting the process and ventilating the process chamber (contamination of the process environment), which then necessitates reconditioning. Storing multiple complete plasma sources and decoupling them from the process chamber using vacuum dampers would be extremely complex and expensive, and therefore not an economically viable option.

[0003] Besides the issues of wear and lifespan, scaling up the plasma power is another problem. The discharge current in a hollow cathode arc discharge is limited by the maximum achievable emission current density of the hollow cathode material before reaching its melting temperature, as well as by the limited emission area – the discharge is concentrated in a so-called active zone, i.e., a specific axial region of the hollow cathode where the working gas pressure and mean free path of the plasma electrons are well matched. Therefore, a significant increase in discharge power generally cannot be achieved by increasing the load on a single hollow cathode source, but only through the costly implementation of multiple plasma sources operating in parallel.

[0004] From DE 10 2006 027 853 A1, a hollow cathode system is known in which a cathode tube, at least at the end of the gas outlet opening, is enclosed by an annular anode and an annular magnet coil. A higher plasma intensity can be achieved with such a hollow cathode system compared to conventional hollow cathode plasma sources. WO 2013 / 091927 A1 discloses plasma sources comprising two hollow cathodes, the outlet openings of the two hollow cathodes facing each other. A power supply unit is connected between the two hollow cathodes, providing an alternating voltage so that the two hollow cathodes alternately function as the cathode and anode of a hollow cathode arc discharge. With such a plasma source, a more intense plasma can also be generated between the two hollow cathodes compared to a plasma source with only one hollow cathode.However, even with the two hollow cathode systems described above, the operating time is limited by the lifespan of a cathode tube.

[0005] A third problem with hollow cathode systems lies in the immense amount of material required for the wear parts. The hollow cathodes, which need to be replaced frequently, are always replaced as a whole, even though the wear is essentially concentrated only in the active zone.

[0006] The invention is therefore based on the technical problem of creating a hollow cathode system for generating a plasma and a method for operating such a hollow cathode system, by means of which the disadvantages of the prior art can be overcome. In particular, it should be possible to achieve a longer operating time compared to the prior art by means of a hollow cathode system and a method according to the invention. Furthermore, it should be possible to change the intensity of a plasma generated by a hollow cathode system according to the invention by means of a hollow cathode system according to the invention.

[0007] The solution to the technical problem is achieved through objects having the features of claims 1 and 6. Further advantageous embodiments of the invention are set out in the dependent claims.

[0008] A hollow cathode system according to the invention for generating a plasma belongs to that class of devices in which an electrical voltage is applied between a cathode tube and an anode assembly by means of a power supply unit. If, at the same time, the cathode tube is supplied with a working gas located in a first gas reservoir, an arc discharge can be formed between the cathode tube and the anode assembly within a vacuum chamber. A hollow cathode system according to the invention is characterized in that it has at least two cathode tubes which are electrically connected to one another. Preferably, the cathode tubes have a circular inner cross-section. However, the inner cross-section of the cathode tubes in a hollow cathode system according to the invention can alternatively have any other geometric shape.An electrically conductive contact between the individual cathode tubes can be formed, for example, by means of contact elements or, if the cathode tubes are arranged adjacent to one another, also by means of a touch contact. All cathode tubes of a hollow cathode system according to the invention thus always have the same electrical potential. Furthermore, in a hollow cathode system according to the invention, each cathode tube is assigned a separate actuator by means of which the quantity of gas provided in the first gas reservoir and / or the type of gas flowing through the cathode tube assigned to the actuator can be adjusted.

[0009] Because each cathode tube has a separate actuator for adjusting the amount of gas flowing through it, a hollow cathode arc discharge can be ignited separately from each cathode tube to the anode assembly. Each cathode tube of a hollow cathode system according to the invention is assigned the same anode assembly for generating a hollow cathode arc discharge. It is known from the prior art that the shape of a plasma can be shaped by placing several anode elements within a vacuum chamber. Therefore, in one embodiment, the anode assembly of a hollow cathode system according to the invention can also comprise several anode elements. In another embodiment, one element of the anode assembly is ring-shaped, which surrounds all cathode tubes at least on the side of the cathode tube gas outlet opening.

[0010] It is known from the prior art that auxiliary devices are typically used to ignite a hollow cathode, such as a heating coil wound around a hollow cathode and connected to an associated power supply unit to provide an electrical heating voltage. Such auxiliary devices can also include magnetic field coils or permanent magnets for generating a magnetic field in and around a hollow cathode, or a device for generating high-voltage pulses between the hollow cathode and the anode assembly. Each cathode tube of a hollow cathode system according to the invention can also be assigned such an auxiliary device for igniting an arc discharge. Any auxiliary device used in the prior art for igniting a hollow cathode arc discharge can be used.

[0011] It has already been explained that in a hollow cathode system according to the invention, a separate arc discharge can be ignited and maintained starting from each cathode tube, so that a plasma can also be formed by means of each cathode tube. All plasmas formed by means of the individual cathode tubes should penetrate as much of the same volume as possible.

[0012] In one embodiment, the tube axes of the at least two cathode tubes are therefore aligned parallel to each other or have an angle of a maximum of 5° to each other, with the gas from the gas reservoir flowing through the at least two cathode tubes in the same gas flow direction.

[0013] In order for the plasmas generated by means of the at least two cathode tubes to penetrate as much of the same volume as possible, it is also advantageous if adjacent cathode tubes have a distance of no more than 20 mm from each other.

[0014] A method according to the invention for operating a hollow cathode system, in which an anode device, a power supply device for providing an electrical voltage connected between the cathode tube and the anode device, and at least one first gas reservoir for providing a gas flowing through the cathode tube are used, is characterized in that at least two cathode tubes are used which are electrically connected to each other and wherein each cathode tube is assigned a separate actuator by means of which the quantity and / or type of gas flowing through the cathode tube assigned to the respective actuator is adjusted.

[0015] A hollow cathode system according to the invention can be operated in essentially two modes. In a first operating mode, an arc discharge is ignited by means of at least one first cathode tube and maintained until, for example, the service life of the at least one first cathode tube has expired. For this purpose, at least in the case of the first first cathode tube, a first quantity of gas flowing through the first cathode tube is set by means of a first actuator, which is suitable for igniting and maintaining an arc discharge between the first cathode tube and the anode assembly. When the at least one first cathode tube is worn out or its service life has expired, the arc discharge from the first cathode tube is extinguished and subsequently an arc discharge originating from at least a second cathode tube is ignited.In this way, the service life of a hollow cathode system according to the invention can be extended compared to the prior art, in which a vacuum chamber must be opened and the hollow cathode replaced after wear of a hollow cathode. In the procedure described above, an arc discharge is ignited once in a cathode tube and maintained until the operating time of the cathode tube has expired. Alternatively, the cathode tubes involved can also be activated alternately, one after the other, in partial cycles with respect to their service life, so that all cathode tubes involved always have an approximately identical state of wear, which has a positive effect on maintaining homogeneous process conditions.

[0016] The second operating mode of a hollow cathode system according to the invention is primarily not about extending its operating time, but about changing the intensity of a plasma generated by the hollow cathode system. It has already been explained that the plasmas of the individual cathode tubes of a hollow cathode system according to the invention penetrate essentially the same volume within a vacuum chamber. By additionally activating a second cathode tube when at least one arc discharge is already burning from a first cathode tube, or by deactivating a second cathode tube when an arc discharge is burning from at least one first and one second cathode tube, the intensity of the plasma in the volume penetrated by the plasma can be changed.In the second operating mode, an arc discharge towards the anode device is maintained at least temporarily from at least two cathode tubes simultaneously in order to form a stronger plasma compared to the case where an arc discharge is only burning from one cathode tube.

[0017] The invention is described in more detail below with reference to exemplary embodiments. The figures show: Fig. 1 a schematic representation of a hollow cathode system according to the invention; Fig. 2a, 2 a schematic sectional view of a first alternative cathode tube configuration; Fig. 3 a schematic sectional view of a second alternative cathode tube configuration; Fig. 4 a schematic sectional view of a third alternative cathode tube configuration; Fig. 5 a schematic sectional view of a fourth alternative cathode tube configuration.

[0018] In Fig. 1 A hollow cathode system 10 according to the invention is shown schematically. The hollow cathode system 10 comprises a first cathode tube 11a and a second cathode tube 11b, which are spaced apart from each other by a maximum of 20 mm. The tube axes 12a and 12b of the two cathode tubes 11a and 11b are aligned parallel to each other. The hollow cathode system 10 also includes an anode assembly 13 and a power supply assembly 14, which provides an electrical voltage for generating an arc discharge between the first cathode tube 11a and / or the second cathode tube 11b on one side and the anode assembly 13 on the other side. The two cathode tubes 11a and 11b and the anode assembly 13 are arranged within a vacuum chamber, which, for the sake of clarity, is not shown in the figure. Fig. 1 is shown.

[0019] During operation of the hollow cathode system 10, a working gas flows in the same direction through the first cathode tube 11a and / or through the second cathode tube. The working gas is supplied in a first gas reservoir 15 and is conveyed from the gas reservoir to the cathode tubes 11a and 11b via gas lines 16. A first actuator 17a is assigned to the first cathode tube 11a, and a second actuator 17b is assigned to the second cathode tube 11b. The amount of gas flowing through each cathode tube can be adjusted separately using the actuators 17a and 17b. The two cathode tubes 11a and 11b are electrically connected to each other by means of a contact element 18 and thus constantly exhibit the same electrical potential.The contact element 18 can, for example, also be designed as a base element made of an electrically conductive material, into which the cathode tubes 11a and 11b are screwed. The contact element 18 can be omitted if the two cathode tubes 11a and 11b are arranged so close together that at least a permanent contact is established between them. By means of the two actuators 17a and 17b, an arc discharge can thus be generated separately from each of the two cathode tubes 11a and 11b in the hollow cathode system 10, leading to the anode assembly 13.

[0020] In a first variant of the process, the first actuator 17a is used to adjust the gas flow through the first cathode tube 11a to a quantity suitable for generating an arc discharge between the first cathode tube 11a and the anode assembly 13. This ignites and maintains an arc discharge between the first cathode tube 11a and the anode assembly 13. A cathode tube participating in generating an arc discharge is subsequently referred to as an activated cathode tube. If, as previously described, an arc discharge is generated from the first cathode tube 11a to the anode assembly 13, then initially only the cathode tube 11a is an activated cathode tube, while the cathode tube 11b remains inactive. One or more substrates arranged in the vacuum chamber can thus initially be processed with the assistance of the plasma generated by the first cathode tube 11a.When the service life or operating time of the first cathode tube 11a has expired, the arc discharge between the first cathode tube 11a and the anode assembly 13 is first extinguished. Subsequently, by means of the second actuator 17b, a quantity of gas flowing through the second cathode tube 11b is adjusted to form an arc discharge between the second cathode tube 11b and the anode assembly 13. A corresponding arc discharge is then ignited and maintained, allowing the processing of one or more substrates within the vacuum chamber to continue with the assistance of the plasma generated by the second cathode tube 11b. In this way, the operating time of a hollow cathode system can be extended compared to the prior art without having to open the vacuum chamber.Alternatively, the cathode tubes 11a and 11b can be activated alternately, one after the other in partial cycles, so that cathode tubes 11a and 11b always have an approximately similar state of wear. Preferably, a second cathode tube is only activated after a first cathode tube has been deactivated. Alternatively, a second cathode tube can also be activated before or at the same time as the first cathode tube is deactivated.

[0021] In a second method variant, as described previously in the first method variant, the first actuator 17a is used to adjust the amount of gas flowing through the first cathode tube 11a, which is suitable for forming an arc discharge between the first cathode tube 11a and the anode assembly 13. Thus, initially only the first cathode tube 11a is activated, generating a plasma with which one or more substrates can be processed within the vacuum chamber. Later, when a stronger plasma is required for processing substrates within the vacuum chamber, the second actuator 17b is used to adjust the amount of gas flowing through the second cathode tube 11b, which is suitable for forming an arc discharge between the second cathode tube 11b and the anode assembly 13, thereby generating an additional plasma cloud.The current intensity for maintaining an arc discharge can be set separately for each cathode tube. Because the plasma clouds of the two cathode tubes 11a and 11b are at least partially interpenetrating due to their close proximity, a higher-powered overall plasma can be generated. If a lower-powered plasma is required later, the arc discharge can be extinguished by either of the two cathode tubes 11a or 11b that are activated at that time.

[0022] In the Fig. 1 The described embodiment of a hollow cathode system according to the invention comprises a total of two cathode tubes. However, a cathode tube configuration comprising more than two cathode tubes can also be used in a hollow cathode system according to the invention. There is no upper limit to the number of cathode tubes. The essential feature of the invention is simply that the cathode tubes are electrically connected to one another and that directly adjacent cathode tubes have a maximum distance of 20 mm between them.

[0023] In Fig. 2a Figure 1 shows a schematic cross-sectional view of an exemplary cathode tube configuration 20 according to the invention, which has three cathode tubes 21a to 21c. The cathode tubes 21a to 21c are electrically connected to one another by means of contact elements 28a to 28c. As already described for the device 10 from Figure 10, Fig. 1 As described above, according to the invention, each of the cathode tubes 21a to 21c is assigned a separate actuator with which the gas flow through an assigned cathode tube can be controlled separately. Thus, an arc discharge can also be formed separately from each of the cathode tubes 21a to 21c towards an anode device, so that a hollow cathode system according to the invention with a cathode tube configuration 20 can also be operated in the two process variants described above in the embodiment according to the above. Fig. 1 are described. The same applies to hollow cathode systems according to the invention with all cathode tube configurations described below. In Fig. 2b is the from Fig. 2a The known cathode tube configuration 20 is shown schematically again in cross-section. In the embodiment according to Fig. 2b The cathode tube configuration 20 is enclosed, at least at the end of the cathode tubes where the outlet opening of the cathode tubes is located, by an annular element 23 of the anode assembly belonging to the cathode tubes. By means of such an annular anode, a cathode tube of a hollow cathode system according to the invention can also be operated using the methods described, for example, in DE 10 2006 027 853 A1. An annular element 23 of an anode assembly can also be used with all previously and subsequently described cathode tube configurations, wherein the annular element encloses all cathode tubes of a cathode tube configuration, at least at the end of the cathode tubes where the gas outlet opening of the cathode tubes is located.

[0024] In all previously described embodiments, the cathode tubes of a hollow cathode system according to the invention are designed as separate tubes which are electrically connected to one another by means of contact elements. Fig. 3 Figure 30 schematically depicts a cross-sectional view of a cathode tube configuration 30, in which a block 38 made of an electrically conductive material has four cylindrical recesses 31a to 31d extending through its entire length. These four recesses 31a to 31d, with their cylindrical surfaces on the block 38, function as cathode tubes through which a working gas flows to form an arc discharge. Each recess 31a to 31d is assigned a separate actuator, allowing the gas flow through the corresponding recess to be controlled independently.

[0025] As previously demonstrated, hollow cathode systems typically include auxiliary devices, such as a heating coil with an associated power supply unit, by means of which an arc discharge can be ignited at a cathode tube. In all previously described embodiments of the invention, such an auxiliary device for igniting an arc discharge can also be assigned to the cathode tubes. For example, in embodiments where the cathode tubes are designed as separate tubes, as in the embodiments according to the Figuren 1 bis 2b Each cathode tube has a separate heating coil wound around it, or each cathode tube has a separate heating element. In the embodiment according to Fig. 3 For example, a heating coil could be wrapped around block 38.

[0026] In Fig. 4 Figure 40 schematically shows a cross-sectional view of a cathode tube configuration 40, comprising four cathode tubes 41a to 41d, which are electrically connected to one another by means of contact elements 48a to 48d. A plasma can be generated within a vacuum chamber using the cathode tubes 41a to 41d, according to the procedures described above, and this plasma can be used to process at least one substrate within the vacuum chamber. A further cathode tube 49 is arranged in the center of the four cathode tubes 41a to 41d. This further cathode tube 49 is electrically insulated from the four cathode tubes 41a to 41d and is equipped with a separate power supply for generating an arc discharge. During operation of a hollow cathode system according to the invention, which comprises the cathode tube configuration 40, an arc discharge is first ignited by the cathode tube 49, thereby generating a plasma.The plasma generated by cathode tube 49, however, is produced with a lower intensity than the plasmas that can be produced by cathode tubes 41a to 41d. The plasma generated by cathode tube 49 is also not primarily involved in processing a substrate, but mainly serves to provide charge carriers in the space between the cathode tubes and the anode assembly, which facilitate the ignition of at least one of the cathode tubes 41a to 41d.

[0027] Because the plasma generated by means of the cathode tube 49 has only a low intensity, the cathode tube 49 is also subject to less wear compared to the cathode tubes 41a to 41d. Therefore, an arc discharge can be maintained by means of the cathode tube 49 throughout the entire operating time of a hollow cathode system according to the invention with cathode tube configuration 40, while the cathode tubes 41a to 41d are activated successively or alternately.

[0028] It has already been explained that in a hollow cathode system according to the invention, at least two cathode tubes are involved, in which, for example, during operation of the hollow cathode system, at least one first cathode tube can be activated while at least one second cathode tube is inactive. During the period in which the second cathode tube is inactive, either no gas at all can flow through the second cathode tube, or a small amount of gas can flow through the second cathode tube, which is not sufficient to form an arc discharge between the second cathode tube and the associated anode assembly.Flowing a small amount of gas through the second cathode tube, compared to the amount required to generate an arc discharge, serves to purge the second cathode tube. This prevents it from becoming clogged with particles that may be deposited, for example, from the activated first cathode tube or from a coating material used to coat a substrate within the vacuum chamber. The working gas supplied in the first gas reservoir, which also flows through a cathode tube to generate an arc discharge, can be used as the purge gas for a non-activated cathode tube. Alternatively, a different gas, preferably an inert gas, supplied in a second gas reservoir, can be used to purge the cathode tube.

[0029] If a different gas is used to purge a cathode tube than to ignite and maintain an arc discharge, the type of gas flowing through a particular cathode tube can be set, for example, using the same actuator that controls the gas flow rate through that tube. Alternatively, the type of gas flowing through a particular cathode tube can also be set using a separate actuator.

[0030] While in the exemplary embodiments according to the Figuren 1 , 2a , 2b and 4 the cathode tubes as separate tubes and in the exemplary embodiment according to Fig. 3 which are formed as cylindrical recesses in a block of material, is in Fig. 5A longitudinal section schematically depicts a cathode tube configuration 50, which forms a hybrid of separate cathode tubes and recesses in a material block. The cathode tube configuration 50 comprises a base element 58 made of an electrically conductive material, in which two cylindrical recesses 59a and 59b have been machined, extending through the entire length of the base element 58. However, the recesses 59a and 59b do not extend over the entire length required for a cathode tube. Therefore, cathode tube sub-elements 51a and 51b are also attached to the base element 58, preferably by means of a detachable connection, such that the tube axis 52a of the cathode tube sub-element 51a and the cylinder axis of the cylindrical recess 59a are identical, as is the tube axis 52b of the cathode tube sub-element 51b and the cylinder axis of the cylindrical recess 59b.Alternatively, the cylindrical axis of a recess and the tube axis of an associated cathode tube sub-element can be offset to, for example, deflect radiation of any kind from the interior of the vacuum chamber. As a detachable connection, the cathode tube sub-elements 51a and 51b can, for example, have an external thread that is screwed into the base element 58. Thus, the recess 59a and the associated cathode tube sub-element 51a together form a first cathode tube, and the recess 59b and the associated cathode tube sub-element 51b form a second cathode tube. The cathode tube configuration 50 comprises, by way of example, only two cathode tubes, but in alternative embodiments, it can also include more than two cathode tubes.

[0031] As previously explained, wear in a hollow cathode occurs primarily within a locally confined active zone, which, in a hollow cathode system with a cathode tube configuration 50, is located within the cathode tube sub-elements 51a and 51b. Therefore, with a cathode tube configuration 50, only worn cathode tube sub-elements need to be replaced, rather than the entire cathode tube, resulting in material savings. Significant material savings are achieved when the length of the recesses 59a and 59b is at least 30% of the sum of the lengths of the recess and the corresponding cathode tube sub-element.

Claims

1. Hollow cathode system for generating a plasma, comprising at least two cathode tubes (11a; 11b), an anode device (13), a power supply device (14) for providing an electrical voltage applied between the cathode tubes (11a; 11b) and the anode device (13) and at least one gas reservoir (15) for providing a gas that flows through the cathode tubes (11a; 11b), and wherein each cathode tube (11a; 11b) is assigned a separate actuator (17a; 17b), by means of which the amount of gas that flows through the cathode tube (11a; 11b) assigned to the actuator (17a; 17b) can be set, characterized in that the at least two cathode tubes (11a; 11b) are electrically conductively connected to one another.

2. Hollow cathode system according to Claim 1, wherein the tube axes (12a; 12b) of the at least two cathode tubes (11a; 11b) are aligned parallel to one another or are at an angle to one another of no more than 5°.

3. Hollow cathode system according to Claim 1 or 2, wherein the direction of gas flow through the at least two cathode tubes (11a; 11b) is the same.

4. Hollow cathode system according to one of the preceding claims, wherein adjacent cathode tubes (11a; 11b) are at a distance from one another of no more than 20 mm.

5. Hollow cathode system according to one of the preceding claims, wherein at least one element (23) of the anode device is of an annular form, wherein the at least one element (23) of the anode device encloses the at least two cathode tubes (21a; 21b; 21c).

6. Method for operating a hollow cathode system in which an anode device (13), a power supply device (14) for providing an electrical voltage applied between at least two cathode tubes (11a; 11b) and the anode device (13) and at least one gas reservoir (15) for providing a gas that flows through the cathode tubes (11a; 11b) are used, wherein each cathode tube (11a; 11b) is assigned a separate actuator (17a; 17b), by means of which the amount and / or the type of gas that flows through the cathode tube (11a; 11b) assigned to the respective actuator (17a; 17b) is set, characterized in that the at least two cathode tubes (11a; 11b) are electrically conductively connected to one another.

7. Method according to Claim 6, wherein, at least for a first cathode tube (11a), a first amount of gas that flows through the first cathode tube (11a) and is suitable for igniting an arc discharge between the first cathode tube (11a) and the anode device (13) is set by means of a first actuator (17a).

8. Method according to Claim 7, wherein, at least for a second cathode tube, a second amount of gas that flows through the second cathode tube and is not suitable for igniting an arc discharge between the second cathode tube and the anode device is set by means of a second actuator.

9. Method according to Claim 6, wherein, at least for a first cathode tube (11a), for which an arc discharge is formed between the first cathode tube (11a) and the anode device (13), the arc discharge is ended and wherein, at least for a second cathode tube (11b), for which no arc discharge is formed between the second cathode tube (11b) and the anode device (13), an arc discharge is ignited.

10. Method according to one of Claims 7 to 9, wherein an arc discharge from at least two cathode tubes (11a; 11b) to the anode device (13) is maintained simultaneously with amperages that can be set separately for each cathode tube.

11. Method according to Claim 6, wherein the type of gas that flows through the cathode tube (11a; 11b) assigned to the respective actuator (17a; 17b) is also set by means of the separate actuators (17a; 17b).