Abstract:
A device and method for generating an electrical discharge are described. A first electrode ( 30 ) is operated to be a cathode relative to a second electrode ( 16 ). A gas is introduced into the chamber ( 14 ) by the first electrode ( 30 ). The first electrode ( 30 ) has a closed antechamber ( 32 ) with a metal wall ( 34 ). A tube ( 36 ) consisting of a different material than the wall ( 34 ) is provided through which the gas from the antechamber ( 32 ) is conducted into the chamber ( 14 ). A front portion of the tube ( 36 ) is embedded in the wall ( 34 ) of the antechamber ( 32 ). In its rear portion, the tube ( 36 ) has a free end projecting into the antechamber ( 32 ). A stable electrical discharge can be generated thereby in a particularly easy manner.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The invention relates to a device and method for generating an electrical discharge. In particular, the invention relates to the generation of an electrical charge within a chamber, such as a vacuum chamber for plasma treatment. 
         [0003]    2. Description of Related Art 
         [0004]    A plurality of devices and methods are known in which workpieces are treated within a vacuum chamber with an electrically generated plasma, for example in the form of plasma coating or plasma etching. 
         [0005]    For example, DE 10 2006 021 994 A1 describes a coating method and a device for coating bodies by means of magnetron sputtering. Magnetrons are connected to a common anode in a PVD magnetron coating chamber. Substrates are guided along the magnetrons on a turntable. A magnetron can be operated in pulses or with constant power by means of two power supplies. Directly before being coated, the substrate is subjected to a plasma etching treatment in which a negative potential is applied to the substrate which is bombarded with argon ions to clean and activate the surface. 
         [0006]    DE 10 2004 015 231 A1 describes a method and a device for treating substrate surfaces by bombarding with charge carriers. For surface treatment, a high-current gas discharge is formed between an electron emission device and an electrode connected as an anode. After evacuating a vacuum chamber, a hollow cathode installed in the wall of the chamber is activated with a heated tungsten tube, a cooled housing, an inlet device for the argon carrier gas, and an auxiliary electrode with a positive bias voltage which promotes the ignition and stabilization of a hollow cathode arc discharge. 
         [0007]    DE 10 2004 015 230 A1 describes a method and a device for enhancing a pulsed magnetron discharge. An electrode is arranged in a magnetron magnetic field and temporarily operated to be a cathode relative to a counter-electrode. In time intervals in which the first electrode is not operated as a cathode, an additional electron current from an additional electron source is conducted to the first electrode. Hollow cathodes with a hot cathode as well as wire or bolt cathodes of tungsten, or respectively lanthanum hexaboride/tungsten are cited as the electron sources. In one embodiment, argon gas flows through a tubular hot cathode. 
         [0008]    DE 100 60 002 A1 describes a device for surface treatment using a plasma generation chamber which is provided with a plasma generation electrode, and a substrate treatment chamber that is provided with a substrate support table. A plasma nozzle is used to as a generating area for a hollow anode discharge. 
         [0009]    In the article by A. Sherman “In situ removal of native oxide from silicon wafers”, J. Vac. Sci. Technol. B. 8, 1990, 4, 656, 657, a water-cooled hollow cathode consisting of molybdenum as the electron source is used for a DC discharge for CVD deposition on silicon. 
       BRIEF SUMMARY OF THE INVENTION 
       [0010]    It can be considered an object of the invention to propose a device and method by means of which an electrical discharge can be easily generated or supported in a chamber. 
         [0011]    The object is achieved by a device according to claim  1  and a method according to claim  11 . Dependent claims refer to advantageous embodiments of the invention. 
         [0012]    According to the invention, at least one first and one second electrode are provided in a chamber, preferably a vacuum chamber. At least temporarily, the first electrode can be operated as a cathode and the second electrode can be operated as an anode by an electrical power supply. 
         [0013]    A gas is conducted into the chamber through the first electrode operated as a cathode. According to the invention, first gas is fed into a closed antechamber which has a metal wall. From the antechamber, the gas passes through a tube into the chamber. 
         [0014]    According to the invention, the tube is aligned on the wall of the antechamber so that its front portion facing into the direction of the chamber, through which the gas passes into the chamber, is embedded in the wall of the antechamber, whereas its rear portion, into which the gas passes from the antechamber, projects freely into the antechamber. 
         [0015]    With an electrode designed in this manner, the front portion of the tube embedded in the wall is effectively thermally coupled to the wall of the antechamber so that heat arising in the antechamber is conducted through the wall of the antechamber, which preferably consists of an effectively thermally conducting metal such as copper. However, the rear end of the tube projects freely into the antechamber and is therefore much less effectively thermally coupled to the wall of the antechamber. Accordingly, the material of the tube can heat up at this location. 
         [0016]    When gas is guided through the tube into the chamber, a hollow cathode discharge can form in the tube, in particular a hollow cathode arc discharge. It has been shown that by strongly heating the free end of the tube that extends into the antechamber and into which the gas from the antechamber flows, electrons are emitted from the tube material at that location due to thermionic emission. The electrons support the formation of a hollow cathode arc discharge and pass together with the gas stream through the tube into the chamber. This yields high ionization suitable for plasma treatment in the chamber. 
         [0017]    In preferred embodiments, the tube consists of a different material than the wall of the antechamber in which it is embedded. This makes it possible to be able to select the respective material for the wall and the tube independent of each other corresponding to the desired function. The tube can for example be fittingly inserted or pressed into an opening formed in the material of the wall of the antechamber to achieve effective contact between the wall and the tube. 
         [0018]    The tube should preferably consist of a material that is resistant to temperatures which result from the arising electrical discharges. This can prevent excessive consumption. Different materials can be used for the tube material, especially metals and ceramic materials. Highly temperature-resistant materials are preferred, i.e., those that have a particularly high melting point above 1500° C. and preferably above 2000° C. In the preferred embodiments, the tube can for example consist of tantalum, tungsten or lanthanum hexaboride (LaB 6 ). Tantalum is particularly preferable as the material. A tube formed from these materials has proven resistant and scarcely shows consumption despite the high temperatures achieved from the arising discharges. 
         [0019]    It is useful for the material of the antechamber wall to be easily processable and deformable to form the wall, especially a solid wall. A wall material is preferred that has effective thermal conductivity, such as more than 100 W/(mK), preferably more than 200 W/(mK), particularly preferably more than 300 W/(mK), in order to dissipate the heat arising during the discharge. For example, aluminium can be used; however, copper or an alloy is preferred that consists at least primarily, i.e., more than 50 atomic percent, of copper. A material with a particularly high melting point is contrastingly not required for the material of the antechamber; for example, a metal can be used with a melting point of less than 1200° C. 
         [0020]    The tube can preferably have a round cross-section. More preferably it has an elongated shape, i.e., its axial length is longer than the outer diameter, preferably more than twice as large, more preferably more than three times as large as the diameter. For example, the tube can have a length of 10-40 mm, preferably 20-30 mm. For example dimensions of 4-15 mm, preferably 10 mm or less are conceivable as the outer diameter. The inner diameter can for example be less than 10 mm, preferably 6 mm or less. 
         [0021]    Further, it is preferred that at least one-half of the length of the tube freely projects into the antechamber. In the front portion, it is preferable for the tube to terminate at least substantially at the wall, i.e., not extend, or only extend slightly, out of the wall into the chamber. 
         [0022]    A feed tube can be provided for feeding gas into the antechamber. The feed tube can serve to introduce the gas without itself participating in the discharge. A front, free end of the feed tube can extend into the antechamber. Furthermore, the feeding tube can have equivalent or similar properties as the tube between the antechamber and chamber, i.e., a highly temperature-resistant material is preferred as the material such as tantalum, tungsten or lanthanum hexaboride, the feed tube can have a round cross-section and elongated shape and can project into the antechamber for more than one-half its length. In the rear portion, the feed tube is preferably embedded in highly thermally-conductive material. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0023]    In the following, an embodiment of the invention will be further described with reference to the drawings. In the drawings: 
           [0024]      FIG. 1  shows a schematic representation of a vacuum chamber with electrodes; 
           [0025]      FIG. 2  shows a cross-section of a first electrode of the chamber from  FIG. 1 ; and 
           [0026]      FIG. 3  shows a diagram of the voltage characteristic between the electrodes from  FIG. 1  over time. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0027]      FIG. 1  schematically portrays a PVD sputtering system  10  for coating substrates  12 . A magnetron cathode  16  with a sputtering target  18  consisting of a material to be sputtered is arranged in a vacuum chamber  14 . A rotatable substrate table  20  is located in the chamber  14 . Substrate holders  22  rotate on the substrate table  20  with substrates  12  arranged thereupon to be treated, of which only one substrate  12  is symbolically shown in  FIG. 1 . 
         [0028]    A bias power supply  31  is provided on the substrate table  20  by means of which the substrates  12  are held as recipients at a negative potential, compared to the conductive wall of the chamber  14 . 
         [0029]    The chamber  14  has a gas outlet  24  by means of which a vacuum is generated in the interior of the chamber  14  by a pump system (not shown). Working gas, preferably argon, as well as additionally also reactive gas such as nitrogen depending on the desired treatment, can be fed through a gas inlet  26 . 
         [0030]    With the coating system  10  shown in  FIG. 1 , the gas inlet  26  is connected to the interior of the chamber  14  by a special electrode  30  so that the gas flows through the electrode  30  into the chamber. An electrical power supply  33  is provided between the electrode  30  and the magnetron cathode  16 . 
         [0031]    The PVD coating system  10  is only schematically portrayed in this context with a few of the elements contained therein. PVD coating systems and in particular systems for magnetron sputtering are known per se. The substrates  12  are coated by igniting a plasma in the interior of the chamber  14 . Ions of the plasma sputter the target  18 . Components of the plasma deposit on the surface of the substrates  12 . During the coating, the electrode  30  can be operated to be an anode relative to the magnetron cathode  16 . 
         [0032]    In addition to the elements shown in  FIG. 1 , sputter systems normally have additional elements such as additional sputter cathodes, in particular magnetron cathodes of the unbalanced type (UBM). The detailed description and explanation of other elements with which a person skilled in the art is familiar will not be addressed in this context. For example, WO 98/46807 A1 shows a PVD coating system of which the electric configuration with a bias power supply, separate power supplies for a number of magnetron cathodes and an additional power supply between the metal wall of the chamber  14  and a separate anode are described. WO 2009/132822 describes a PVD system and coating methods implemented therewith in which at least some of the sputter cathodes are used with high-energy pulses in HPPMS mode. These documents and the systems and methods described therein are incorporated by reference with regard to potential elements of a sputtering system and its electrical circuitry. 
         [0033]    To clean and activate the surface of the substrates  12  before the actual coating in the sputtering procedure, preferably ion etching is first performed in which charged particles act on the surface of the substrates  12 . In the portrayed example in  FIG. 1 , this etching occurs by an electrical discharge which forms between the magnetron cathode  16  and the electrode  30  on the gas inlet  26 . The electrode  30  is operated as a cathode, whereas the electrode  16  is operated as anode (due to its use as a cathode in the subsequent coating procedure, the electrode  16  is referred to as “magnetron cathode”). 
         [0034]    The electrode  30  in the portrayed example is placed on the wall of the chamber  14  approximately in the middle. It is electrically insulated from the electrically conductive chamber wall  14  by an insulation layer  15 . The electrode  30  and the magnetron cathode  16  operated as a counter electrode are arranged opposite each other in the chamber  14  so that the substrates are located in the area between. 
         [0035]      FIG. 2  shows a schematic cross-section of the structure of the electrode  30 . The electrode  30  comprises a base plate  17  and a dome  34  placed thereupon which are formed from electrically conductive material. In a preferred embodiment, in particular the dome  34  has a thick copper wall which encloses an antechamber  32 . The dome  34  is placed directly on the baseplate  17  so that they electrically contact each other. Accordingly, the antechamber is surrounded by a wall made of a consistently conductive material so that the wall always has a uniform electrical potential during operation. Preferably, a liquid cooler for the electrode  30  can be provided on the base plate  17 . 
         [0036]    The antechamber  32  is sealed from the interior of the chamber  14  and is connected thereto only by a tube, or respectively tubule  36 . The gas is supplied through a channel  38  in the wall of the base plate  17 , in this case, preferably argon as the working gas. 
         [0037]    The supplied gas passes through the channel  38  in the base plate  17  as well as through a feed tubule  40  into the interior of the antechamber  32 . The gas then passes from the antechamber  32  through the tubule  36  into the interior of the chamber  14 . 
         [0038]    As shown in  FIG. 2 , the tubule  36  is a part separate from the wall of the antechamber  32  from which a front section facing the interior of the chamber  14 , that approximately corresponds to one-third of the overall length of the tubule  36 , is embedded in the wall  34  of the antechamber  32 . 
         [0039]    The material of the tubule  36  differs from the material of the wall of the antechamber  32 , in particular from the material of the dome  34 . The dome  34  is made of copper as a highly thermally-conductive material which, however, is not highly temperature-resistant. In contrast, the tubule consists of a highly temperature-resistant material, preferably tantalum. 
         [0040]    In the preferred embodiment, the tubule  36  has a length of 25 mm, an inner diameter of 4 mm and an outer diameter of 6 mm. By being embedded in the relatively thick copper wall  34 , the front portion of the tantalum tubule  36  is effectively coupled thermally to a relatively large thermal reservoir so that heat is quickly removed therefrom. 
         [0041]    The rear end of the tantalum tubule  36  freely projects into the interior of the antechamber  32 . The rear portion of the tantalum tubule  36  projecting freely into the antechamber  32  corresponds to more than one-half of the entire length of the tubule. As a result, the rear end of the tubule  36  is not directly thermally coupled to the wall  34  of the antechamber  32  so that any heat arising there is not directly removed through the wall  34 . 
         [0042]    The feed tube  40  also projects into the antechamber  32  in the depicted example. The feed tube  40  also consists of tantalum and can have approximately the same dimensions as the tubule  36 . The front section of the feed tubule  40  projects freely into the interior of the antechamber  32 . It serves to introduce the working gas into the antechamber  32  and accordingly has the function of a nozzle but does not participate in the discharge. 
         [0043]    When the system  10  is in etching mode according to the sketch in  FIG. 1 , the power supply  33  operates as a current source with a set current. This initially leads to the application of a voltage of, for example, 400-900V. When argon is simultaneously fed as the working gas through the channel  38  and the feed tubule  40 , a hollow cathode discharge initially forms, thus causing the tubule  36  to be heated. In the process, a temperature distribution arises over the length of the tubule  36  in which the rear end of the tubule  36  has a significantly higher temperature than the front end because the heat at the front end is removed through the wall  34 . 
         [0044]    If, because of the heating, the material of the tubule  36  enters a temperature range in which a significant emission of electrons from the material arises from the thermoelectric effect, these electrons primarily leave the rear end of the tubule  36  and lead to the formation of a hollow cathode arc discharge. The electrons pass through the tubule  36  with the gas stream into the interior of the chamber  14 . Within the interior of the chamber  14 , the electrons are attracted by the magnetron cathode  16  operated as an anode and accelerated in its direction. Due to the accelerated electrons, the ionization of components within the interior of the chamber  14  is enhanced, in particular to form argon ions by means of which the surface of the substrate  12  can be treated. 
         [0045]    It has been observed that the tubule  36  only experiences relatively slight wear, or respectively consumption. The discharge triggered at that location is very stable and easily reproducible, in particular after achieving a heating temperature of approximately 2500° C. 
         [0046]      FIG. 3  shows an example of a characteristic curve of the voltage V over time t applied between the electrodes  30 ,  16 . At that location, an ignition voltage V 1  of for example approximately 700 V arises from the power supply  33 . After a while, the hollow cathode discharge is triggered at point  42  so that the voltage drops to a significantly lower value of for example 50 V. 
         [0047]    The tubule  36  heats up in particular at its rear end until, after an interval of time, the effect of thermionic emission from the tantalum material of the tubule  36  at its rear end is quite noticeable at time  44  which changes the discharge into a hollow cathode arc discharge, and a slight rise to a third voltage V 3  occurs. The third voltage V 3  can for example be 65 V and accordingly lies slightly higher than the second voltage V 2 . 
         [0048]    The continued discharge at voltage level V 3  has proven to be extraordinarily stable. 
         [0049]    A number of changes, or respectively alternatives are possible to the depicted embodiments. In addition to the electrode  30 , one or more additional electrodes of this kind can be provided in particular in the system according to  FIG. 1 . Preferably, a separate additional power supply can be provided for each of the electrodes, and more preferably, each can be connected to a separate counter electrode. As with the arrangement of the electrodes  16 ,  30  shown in  FIG. 1 , it is furthermore preferable for the holder for the substrates  12  to be arranged between the electrodes in order for the arising plasma to have an effect on the substrates  12 . 
         [0050]    Whereas in the above example, the depicted system  1  is a sputtering system, a treatment by the discharge between the electrodes  16  and  30  can also be used independently thereof in any other chamber, or respectively any other type of plasma treatment system.