Abstract:
A closed drift ion source is provided, having an anode that serves as both the center magnetic pole and as the electrical anode. The anode has an insulating material cap that produces a closed drift region to further increase the electrical impedance of the source. The ion source can be configured as a round, conventional ion source for space thruster applications or as a long, linear ion source for uniformly treating large area substrates. A particularly useful implementation uses the present invention as an anode for a magnetron sputter process.

Description:
RELATED APPLICATION/CLAIM OF PRIORITY 
       [0001]    This application is related to and claims priority from provisional application Ser. No. 60/852,926, filed Oct. 19, 2006, which provisional application is incorporated by reference herein. 
     
    
     FIELD 
       [0002]    The present invention pertains to magnetically confined plasma and ion sources, in general, and to closed drift ion sources, in particular. 
       BACKGROUND 
       [0003]    The present invention relates to magnetically confined plasma and ion sources for industrial processes such as plasma treatment, sputtering and plasma etching and to electric propulsion devices for space applications. Many closed drift ion sources have been proposed for these applications and several remain commercially viable. Publicly available articles by Kim and Zhurin, Kaufman and Robinson provide good general background information and other relevant references that pertain to magnetically confined plasma and ion sources. As described in these articles, prior art closed drift ion sources have both an inner and outer magnetic pole with a separate annular anode located between these poles. A closed drift magnetic field passes over the anode between these two grounded or electrically floating poles. 
         [0004]    Extended Acceleration Channel Ion Sources 
         [0005]    Past literature has divided closed drift ion sources into two classifications: Extended Acceleration Channel and Anode Layer. Although the demarcation is not consistent, the common dividing line is the ratio of channel width to channel depth. If the depth exceeds the width dimension, the ion source is classed as an extended acceleration channel type. In both this class and the anode layer class, an ion accelerating electric field is created in a racetrack shape by magnetic field lines roughly orthogonal to the electric field. Outside of this racetrack electrons move relatively freely without the presence of a magnetic field. As the electrons enter the ion source and attempt to reach the anode however, they are impeded by the crossing magnetic field lines. This causes the electrons to gyrate around and move along these magnetic field lines. An additional motion is a drift at right angles to both the magnetic and electric fields. This is termed the Hall current and is the purpose for the racetrack shape of the confinement region. In Madocks U.S. Pat. No. 7,259,378, assigned to a common assignee with the present invention, these motions are discussed in detail. 
         [0006]    The Egorov U.S. Pat. No. 5,218,271 is typical of many extended acceleration channel sources in the prior art. Common to the prior art, this source has an annular anode with inner and outer high permeability magnetic poles. The Bugrova U.S. Pat. No. 6,456,011 B1 is of interest because this patent is directed to reducing the size of the ion source. The need for smaller, lighter ion sources is outlined. Bugrova reduces the source size by removing magnetic field generating components from the inner pole. The inner pole is still present but consists of only a high permeability material. The example given cites the outside diameter of the source to be 5 cm. 
         [0007]    Anode Layer Ion Sources 
         [0008]    Anode layer type ion sources are the second class of closed drift source. In anode layer sources, the closed channel depth is typically shorter or equal to the width. The closed drift published references discuss these sources. These sources have been commercialized for industrial uses. These sources were developed in Russia 40 years ago, and are largely considered public domain and few patents exist. However, U.S. Pat. Nos. 5,763,989 and 5,838,120 show typical configurations for an anode layer geometry. In the afore-mentioned U.S. Pat. No. 7,259,378, Madocks discloses an improved version of this source with pointed magnetic poles that focus the magnetic field in the magnetic gap. As can be seen in these and other anode layer ion sources, an annular anode is located between two, separate inner and outer magnetic poles. 
         [0009]    End Hall Ion Sources 
         [0010]    End hall ion sources are a variation of a closed drift ion source. In the end hall source, the inner magnet pole is lowered with respect to the outer pole to expose the sides of the annular anode. This is exemplified in both the Burkhart and Kaufman U.S. Pat. Nos. 3,735,591 and 4,862,032. With this geometry, a second electron confinement regime combines with a Penning style confinement of closed drift ion sources. The second confinement regime is mirror electron confinement in which electrons are partially confined along magnetic field lines by a gradient magnetic field. In the Burkhart and Kaufman patents and other prior art of this source type, e.g., Manley U.S. Pat. No. 5,855,745, the anode is again annular with the primary electron confining field lines passing from a central grounded or floating pole to an outer grounded or floating pole. 
         [0011]    In the Sainty U.S. Pat. No. 6,734,434, a different end hall ion source configuration is presented. In Sainty the anode is not annular and there is no central floating pole. The anode fills the central area of the ion source and the magnetic field passes through the anode. Important to Sainty, the center of the anode is electrically conductive and is coated to insure the central top anode surface remains conductive. Electrons flowing from a filament reach the anode through the magnetic mirror at the center of the anode rather than by crossing magnetic field lines. This significantly lowers the impedance an electron experiences in trying to reach the anode and is different than the present invention. 
         [0012]    Ion Sources with Sputter Magnetrons 
         [0013]    The combination of sputter magnetron cathodes and closed drift ion sources is known in several configurations. In Morrison, Jr., U.S. Pat. No. 4,361,472,  FIG. 13   b  shows a closed drift ion source connected as the anode to a sputter magnetron cathode. Morrison, Jr. teaches the use of separate power supplies to the cathode and anode (ion source) and the use of this tool in reactive sputtering. Scobey, U.S. Pat. No. 4,851,095, discloses another type of closed drift ion source using a sputter magnetron cathode to provide electrons to the ion source. In Scobey, separate power supplies are shown for the ion source and sputter magnetron cathode. In Manley, U.S. Pat. No. 5,855,745, an end Hall type ion source is used as the anode of a sputter magnetron cathode. Zhurin, U.S. Pat. No. 6,454,910, shows an Hall ion source with a sputter magnetron with separate power supplies for the ion source and sputter cathode. 
         [0014]    Anodes for Sputter Magnetrons 
         [0015]    Several prior art patents present apparatus for improved sputter magnetron anodes. In Meyer, U.S. Pat. No. 4,849,087, both inert and reactive gas is distributed in passageways though the anode. This is said to produce a stable plasma that uses the gases more efficiently. In this patent the anode is adjacent to the sputter magnetron and magnetic field lines are shown passing though the anode. Dickey, U.S. Pat. No. 5,106,474, teaches several anode configurations to maintain anode conductivity during magnetron sputtering of an insulating coating.  FIGS. 8 through 11  show anodes with an array of magnets to guide electrons from the sputter cathode to the anode. Countrywood, U.S. Pat. No. 6,110,540, discloses a conductive anode that maintains conductivity by flowing inert gas through a pinhole and creating a plasma at the anode. In  FIG. 7   c  of this patent the conductive anode is shown with plasma shaping magnets. 
       SUMMARY 
       [0016]    The present invention discloses a novel closed drift ion source having an anode that serves as both the center magnetic pole and as the electrical anode. In accordance with one aspect of the invention, the anode has a layer of an insulating material or an insulating cap that insures a closed drift region of electron confinement to increase stability and the electrical impedance of the source. In accordance with other aspects of the invention, the novel ion source can be configured as a round, conventional ion source such as for space thruster applications or it may be configured as an elongate, linear ion source such as is useful for uniformly treating large area substrates. One of several particularly useful implementations uses the present invention as an anode for a magnetron sputter process. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    The invention will be better understood from a reading of the following detailed description of embodiments of the invention in conjunction with the drawing figures, in which like reference designators are used to identify like elements, and in which: 
           [0018]      FIG. 1  is a longitudinal sectional view of a linear closed drift ion source with a sputter magnetron; 
           [0019]      FIG. 2  is an enlarged detail view of the anode region of the linear closed drift ion source shown in  FIG. 1 ; 
           [0020]      FIG. 3  is an isometric view of the linear closed drift ion source and sputter magnetron of  FIG. 1 ; and 
           [0021]      FIG. 4  is a sectional view of an annular closed drift ion source. 
       
    
    
     DETAILED DESCRIPTION 
       [0022]      FIGS. 1 and 3  linear closed drift ion source  100  in a glass sputter assist application.  FIG. 3  shows an isometric view of closed drift ion source  100  with planar magnetron sputter source  110 . Closed drift ion source  100  is installed over glass  19  in a vacuum process chamber that is not shown in the drawing figures. In this application closed drift ion source  100  is connected as the anode for magnetron sputter cathode  110  across power supply  18 . Closed drift source  100  is supported over glass  19  by insulating brackets that are not shown. Glass  19  moves under cathode  110  and closed drift ion source  100  on conveyor rolls  20 . As best seen in  FIG. 2 , closed drift ion source  100  includes an anode  40  inside floating steel shunt  5 . Anode  40  comprises magnet  1 , anode body  2 , copper back plate  7  and anode top cover  3 . Magnet  1  is located inside anode body  2  such that magnet  1  and anode  40  are formed as an integrated structure. Cover  3  is affixed or fastened to copper anode body  2  by flat head screws that are not shown. Anode  40  is water cooled via a milled groove  8 . Copper back plate  7  is brazed over groove  8  to seal groove  8  to form a coolant, or in this embodiment water cavity. Inlet and outlet water fittings and piping are not shown. Shunt  5  is secured to floating housing  9  by fasteners not shown. Anode  40  is supported from housing  9  by insulating fasteners that are not shown. Anode  40  fits inside aluminum housing  9  and shunt  5  so that a dark space gap  14  is maintained around anode  40 . Housing  9  contains a gas cavity  11 . Gas  13  is brought into cavity  11  through opening  24  in steel back shunt  12 . Gas fittings and piping are not shown and are well known in the art. Gas  13  flows into dark space  14  around anode  40  through a linear array of distribution holes  10  in housing  9 . Shunt  5  is water cooled via welded or brazed-on piping  6 . Ceramic shield  4  is secured over magnet  1  on anode  40  by copper cover  3 . Shield  4  is not electrically conductive. 
         [0023]    In  FIG. 3  the elongate or linear nature of source  100  can be seen. Source  100  is shown as it is positioned over moving glass  19 . 
         [0024]    In operation, power supply  18  is turned on and magnetron cathode  110  ignites plasma  16 . Magnetron cathode  110  sputters material from target  23  onto glass  19 . Electrons  15  emanating from cathode  110  must reach anode  40  to return back to power supply  18 . As electrons  15  attempt to reach anode  40 , they are impeded by magnetic field lines  31  as more clearly seen in  FIG. 2 . To counter this resistance, power supply  18  creates a potential drop across the magnetic field  31  to encourage electron  15  flow to anode  40 . Gas  13  flowing out of dark space  14  encounters impeded electrons  15  in confinement region  33  and some portion of gas  13  is ionized. These newly created ions  22  then experience the electric field and are accelerated away from anode  40  toward glass  19 . Ions  22  bombard glass  19  and serve to increase the density of the thin film. If the ion source is placed upstream from the magnetron cathode, ion  22  bombardment serves to clean glass  19  surface prior to the sputter coating. A visually bright plasma  21  also is seen to emanate from source  100 . This plasma  21  is also beneficial for treating and modifying a substrate surface. 
         [0025]    The electron impeding magnetic field lines  31  passes though anode  40 , insulating layer or ceramic shield  4 , into the closed drift region  33  and then enters magnetic outer pole shunt  5 . Electrons  15  entering the ion source to reach anode  40 , are impeded by magnetic field lines  31  and begin to gyrate around these magnetic field lines. As the electrons  15  gyrate around the field lines  31 , they move relatively freely along field lines  31 . In the present invention, the electrons  15  are confined along field lines  31  by electrically floating surfaces at insulating cover  4  and shunt  5  surface  30 . As is known in the art, floating surfaces tend to charge to repel electrons. 
         [0026]    The insulating layer or cover  4  is important to the present invention and is against the teaching of Sainty in U.S. Pat. No. 6,734,434. With insulating layer or cover  4 , electrons  15  cannot reach anode  40  by simply moving along magnetic field lines. In the present invention electrons must cross magnetic field lines  31  in confinement region  33  to reach anode  40 . Forcing electrons  15  to cross field lines  31  creates a higher impedance and therefore higher energy ions. 
         [0027]    In operation, linear closed drift ion source  100  generates a dense, uniform linear plasma  21  and ion beam  22  as seen in  FIGS. 1 and 3 . Plasma  21  and ion beam  22  are each uniform due to Hall direction forces and the closed drift configuration of the source  100 . Electrons  15 , trapped in magnetic confinement region  33 , move around in a racetrack along the Hall direction. At the ends of linear ion source  100 , the shunt  5  is rounded to maintain electron  15  confinement in a closed drift. 
         [0028]    Referring back to  FIG. 2 , an advantage of the present invention over prior art closed drift ion sources is apparent. The primary magnetic field lines  31  pass through the anode  40  and insulating layer or cover  4 , forcing electrons  15  to cross magnetic field lines  31  to reach anode  40 . The primary magnetic field lines  31  are defined as the field lines forming the electron impeding closed drift region  33 . 
         [0029]    In prior art closed drift ion sources the primary magnetic field passed from an outer pole to an inner pole over an annular anode. 
         [0030]    In accordance with the present invention, a central, non-annular anode  40  either houses the magnetic means  1  or the primary magnetic field  31  passes through anode  40 . 
         [0031]    In prior art ion source of U.S. Pat. No. 6,734,434 the primary magnetic field lines also pass through the anode. However, in U.S. Pat. No. 6,734,434 electrons are able to reach the anode without crossing magnetic field lines. U.S. Pat. No. 6,734,434 implements a mirror electron impedance between the anode and the cathode. 
         [0032]    Further in accordance with the present invention operation, a ceramic, non-conductive layer or cover  4  blocks electrons  15  from reaching anode  40  along magnetic field lines  31 . In ion source  100 , electrons  15  must cross magnetic field lines  31  to reach anode cover  3 . As is known in the art, electron  15  impedance across magnetic field lines  31  is higher than the impedance of a mirror magnetic field. This higher impedance results in important benefits. 
         [0033]    One benefit is that the higher impedance produces a higher voltage across the closed drift region producing more energetic ions emanating out of the source. 
         [0034]    An additional benefit is that electrons flowing toward anode  40  are more efficiently impeded and this results in more efficient ionization of gas  13 . 
         [0035]    The addition of ceramic insulating layer or cover  4  over anode  40  also benefits operation of present invention applied to a long linear ion source. Without cover  4 , the anode  40  would be exposed to axial electron current flow similar to that of U.S. Pat. No. 6,734,434. In operating a long, linear source  100  without cover  4 , the ion current emanating out of the source is not uniform. This is visually seen as a non-uniform glow across the length of the source. In particular the electron current appears to be greatest at the ends of the source ( FIG. 3 ,  109 ) where bright spots are visible. With cover  4 , the plasma glow  21  uniformity across the source is noticeably improved indicating a more uniform linear ion beam  22  emanates out of the ion source. 
         [0036]    As an anode for a sputter magnetron cathode, linear ion source  100  has several advantages and uses. 
         [0037]    One such advantage is that closed drift ion source  100  can replace an existing anode in a magnetron sputter cathode system. No new or additional power supplies are needed. Therefore the present invention can be easily and economically retrofitted into existing large area sputter systems. 
         [0038]    Another such advantage is that source  100  bombards the glass with a dense, uniform ion beam  22  and plasma  21  over the full glass width. 
         [0039]    A further advantage is that unlike earlier point source plasma anodes, the present invention produces a linear ion beam capable of uniformly treating the full substrate width. 
         [0040]    One advantageous use is that by placing closed drift ion source  100  ahead of or before the magnetron, source linear plasma  21  and ion beam  22  can clean and prepare the substrate surface for sputter coating. 
         [0041]    Another advantageous use is that by placing the closed drift ion source  100  after the magnetron, source plasma  21  and ion beam  22  can help to densify the sputtered coating and/or prepare the surface for the next sputtered film in a multi film process. 
         [0042]    Advantageously, oxygen gas can be delivered directly into closed drift ion source  100  as the gas  13 . The oxygen is then activated by the electrons  15  confined in closed drift region  33 . This can reduce sputter target  23  poisoning in a reactive sputter process. 
         [0043]    One additional advantage is that anode  40  is ‘hidden’ behind shunt  5  and plasma  21  and tends to remain conductive even during an insulating reactive sputter process. 
         [0044]      FIG. 4  shows a section view of another embodiment of the present invention. Ion source  200  is an annular source for space thruster or industrial ion source applications. In source  200 , the central magnet pole of the closed drift circuit is anode  203  such that the central magnet pole and the anode are formed as an integrated structure. Anode  203  fits in the center of ceramic electromagnet spool  214  and is held in place by an insulating fastener through back shunt  215  not shown. Electromagnet  202  is wound on electrically insulating spool  214 . Gas  13  flows through port  201  in back shunt  215  into void  213 . From this void  213 , gas  13  flows into multiple distribution vias  212 ,  211  and  210  in spool  214 . After passing through vias  210 , gas  13  flows into discharge cavity  219 . External pole  204  is an annular tube around spool  214 . Both anode  203  and external pole  204  are plasma coated with ceramic, insulating coatings  206  and  205  respectively. Importantly, a conducting portion  220  of center pole anode  203  is exposed to the discharge cavity  219 . 
         [0045]    An electron source  207  such as a hollow cathode or a filament supplies electrons  209  to create ions  216  and to neutralize ion  216  beam. Ion source power supply  208  is connected to anode  203 . In operation, electrons leaving electron source  207  attempt to reach the electrically conductive surface  220  of anode  203 . In the closed drift region  217 , electrons are impeded by the radial magnetic field  218  and by insulating coatings  206  and  205 . Trapped energetic electrons  209  ionize gas  13  in the closed drift region  217  in source  200 . The newly created ions  216  are then ejected from the source due to the electric field in the closed drift region  217 . The result is a dense ion beam emanating out of source  200 . 
         [0046]    In accordance with the principles of the invention, source  200  combines or integrates the inner, central magnetic pole and the anode into one component  203 . All prior art closed drift ion sources have separate inner pole and anode components. In addition, all prior art anodes are annular in shape. 
         [0047]    The combination of the inner pole and anode functions and the simplified anode shape have several advantages. One such advantage is that closed drift thruster sources for space applications can be made smaller and lighter. Minimizing source size and weight is a critical design concern for these applications. A further advantage is that the source is lower cost as a separate anode and anode support structure are not needed. The simplicity of source  200  makes it attractive for both industrial and space applications. 
         [0048]    The present invention discloses two exemplary embodiments using the principles of the present invention. The two embodiments are indicative of the many variations possible using the principles of the present invention. 
         [0049]    The closed drift ion source can be configured as round ion source or as a linear ion source with length exceeding 3 meters. 
         [0050]    The closed drift ion source can be an anode for a sputter magnetron. In this application a single power supply between the cathode and ion source or two power supplies can be used. In the case of two power supplies, one is connected between the cathode and ground and the other between the anode and ground. 
         [0051]    The electron source for the closed drift ion source can be a hollow cathode. 
         [0052]    The power supplies can be DC, pulsed DC, AC or RF. With AC or RF, a blocking capacitor can be added to maintain a DC bias on the cathode or anode. 
         [0053]    The ion source anode can contain ferromagnetic material to conduct the magnetic field, a magnet or can be constructed from non-magnetic material. The inventive criterion is that the primary closed drift magnetic field lines pass through the anode. 
         [0054]    The invention has been described in terms of specific embodiments that have been shown and described. It will be apparent to those skilled in the art that various changes and modifications can be made to the embodiments and the variations that have been described herein without departing from the scope of the invention. It is not intended that the scope of the invention be limited by the embodiments and variations shown and/or described herein, but that the scope of the invention be limited only by the claims appended hereto.