Abstract:
A gas distributor for an ion source includes a plate having a recess and a series of apertures spaced radially outward from the recess. The apertures define paths for the flow of a gas through the plate, and the gas distributor further includes a sacrificial element that is separate from the plate and that is receivable and seats within the recess. The sacrificial element forms an area of the gas distributor that is subjected to erosive forces during normal operations of the ion source, and therefore, prevents erosion of the surface of the plate. The sacrificial element is removable from the plate and replaceable with another sacrificial element during a procedure which neither requires the plate to be removed from the ion source nor the ion source to be disassembled.

Description:
FIELD OF THE INVENTION 
   The present invention relates to ion sources used, for instance, in apparatus and methods that deposit thin films on articles in vacuum chambers, and more particularly, the present invention relates to an ion source having an improved gas distributor plate. 
   BACKGROUND OF THE INVENTION 
   Ion sources, such as so-called gridless ion sources of the end-Hall-type, are used to produce ion beams for use, for instance, with sputtering targets or the like to deposit thin films on articles located within high vacuum coating chambers. An example of an end-Hall ion source is disclosed by U.S. Pat. No. 4,862,032 issued to Kaufman et al. Such an ion source typically includes an anode and cathode between which a potential is impressed to produce a flow of electrons. A magnetic field is established between the anode and cathode to define the path of the electrons, and a distributor plate is used to direct a flow of working gas to the anode and cathode so that the electrons bombard and collide with the neutral atoms or molecules of the working gas to create a conductive gas or plasma. 
   As disclosed by the Kaufman &#39;032 patent on column 3, lines 64–67, column 8, lines 54–55, and column 9, lines 34–54, such an ion source requires routine maintenance including cleaning and replacement of eroded parts. For example, the gas distributor plate erodes during normal operation since it is repeatedly struck by energetic ions. Most of the erosion of the gas distributor plate occurs at the center of the plate that faces the central opening of the annular anode. Replacement of the gas distributor plate prevents the erosion from creating an undesired hole in the center of the gas distributor plate and reduces contamination of the target. From a practical standpoint, it has been determined that a gas distributor should be replaced when its thickness at an eroded central location is reduced to approximately half of its original thickness. Thus, maintenance of such an ion source includes periodic dimensional measurement of the erosion depth of the gas distributor. 
   Measurement of the erosion depth of a gas distributor plate, which is typically located within the housing of an ion source assembly, is difficult to accurately perform when the ion source is assembled. Thus, this measurement can only accurately be determined when the ion source is disassembled, which is typically only performed when the ion source is due for a thorough cleaning. Of course, disassembly of the ion source is also required to replace the gas distributor. 
     FIGS. 1–7  provide an example of the steps required to disassemble a gridless ion source. An assembled gridless ion source  10  is shown in  FIG. 1 . Typically, the ion source  10  is mounted to a support bracket (not shown) via a socket  12  shown in  FIG. 2 . Thus, the ion source  10  must be disconnected from the socket  12 , and then the cathode  14  and cathode supports  16  are removed as shown in  FIG. 3 . The front support plate  18  is removed followed by the outer shell  20  as illustrated in  FIG. 4 . The front anode support  22  is then removed followed by the anode  24  and rear anode support  26  as illustrated in  FIGS. 5 and 6 . Thereafter, the gas distributor  28  is removed as shown in  FIG. 7  so that the extent of erosion at its center can be measured. If the amount of erosion is within acceptable limits the gas distributor  28  is re-installed. If not, a new gas distributor is installed. The time required to perform the above tasks typically requires about one hour of labor and is required to be performed more frequently than scheduled thorough cleanings of the ion source. 
   While the ion source, gas distributor, and maintenance procedures disclosed above may be satisfactorily for their intended purposes, there is a need for a gas distributor, ion source, and maintenance procedure therefor that permit ready and precise inspection of an eroded surface area of a gas distributor and that simplifies the steps required to place an ion source into an in-service condition with a gas distributor having desired surface qualities. To this end, the required labor and downtime of the ion source should be reduced, the material costs related to providing a distributor plate with a desired surface should be reduced, and accurate erosion measurements should be capable of being readily taken at frequent intervals within a minimum of time and requiring only a minimum of skills. 
   OBJECTS OF THE INVENTION 
   With the foregoing in mind, a primary object of the present invention is to provide a novel gas distributor for an ion source. 
   Another object of the present invention is to provide improved and simplified maintenance procedures for measuring the eroded area of a gas distributor and for placing the ion source in-service with a gas distributor having desired surface characteristics. 
   SUMMARY OF THE INVENTION 
   More specifically, the present invention provides a gas distributor for an ion source. The gas distributor is a plate having a recessed central portion and a series of apertures spaced radially outward from the recessed central portion. The apertures define paths for the flow of a gas through the plate within the ion source. The gas distributor also accommodates a sacrificial element that is separate from the plate and that is receivable and seats within the recessed central portion of the plate. The sacrificial element positioned in the recessed central portion on top of the gas distributor plate is subjected to erosive forces during normal operations of the ion source, and therefore, prevents erosion of the surface of the gas distributor plate. The sacrificial element is removable from the gas distributor plate and replaceable with another sacrificial element during a procedure which neither requires the gas distributor plate to be removed from the ion source nor the ion source to be disassembled. 
   According to another aspect of the present invention, an ion source having an anode, cathode and gas distributor plate is provided. The gas distributor plate directs a flow of gas in a discharge area defined by the anode and cathode, and a separate sacrificial element is receivable within a recess formed on the gas distributor plate. The sacrificial element prevents erosion of the gas distributor plate during normal operation of the ion source and can be replaced without disassembly of the ion source. 
   According to yet another aspect of the present invention, a method is provided for inspecting and restoring a gas distributor plate that is mounted within an ion source and that is subject to erosive forces during normal operation of the ion source. The method includes the step of removing from the ion source a separate first sacrificial element located on the gas distributor plate. The removal of the sacrificial element is accomplished while the gas distributor plate is mounted within the ion source and while the ion source is in an assembled condition. Preferably, the method also includes re-positioning the first sacrificial element or positioning a separate second sacrificial element on the gas distributor plate while the gas distributor plate is mounted within the ion source and while the ion source is in an assembled condition. Thus, the depth of erosion can be inspected and a desired surface of the sacrificial element can be restored within a minimum of time. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other objects, features and advantages of the present invention should become apparent from the following description when taken in conjunction with the accompanying drawings, in which: 
       FIGS. 1–7  are perspective views of an assembled and partially disassembled gridless ion source according to the prior art; 
       FIG. 8  is an isometric view, partially broken away in cross-section, illustrating an end-Hall ion source according to the prior art; 
       FIG. 9  is a schematic diagram of energization and control circuitry of the ion source of  FIG. 8 ; 
       FIG. 10  is a cross-sectional view of an upper portion of the ion source of  FIG. 8 ; 
       FIG. 11  is a plan view of a gas distributor according to the present invention; 
       FIG. 12  is a side elevational view of the gas distributor plate of  FIG. 11 ; and 
       FIG. 13  is a side elevational view of the sacrificial element according to the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   The present invention relates to a gas distributor  30  as best illustrated in  FIGS. 11–13 . As will be discussed in greater detail, the gas distributor  30  includes a plate  32  having a recessed central portion  34  for receiving a separate sacrificial element  36 . Thus, when the gas distributor  30  is installed within an ion source, the sacrificial element  36  can be removed and replaced without requiring disassembly of the ion source. Such a procedure can be accomplished in approximately less than two minutes. 
   Turning to the structure of a typical ion source, such as a gridless end-Hall ion source, the disclosure of U.S. Pat. No. 4,862,032 is herein incorporated by reference. To this end, the ion source  40  illustrated in  FIGS. 8 and 9  include a cathode  42  spaced from an annular anode  44  having a frustoconical inner peripheral wall  46 . An electromagnet  48  is located on a side of the anode  44  opposite the cathode  42  and creates a magnetic field between the cathode  42  and anode  44  that decreases in strength from the anode  44  to the cathode  42 . A gas distributor  50  is located adjacent the anode  44  between the anode  44  and electromagnet  48 . The gas distributor  50  has a circular pattern of apertures  52  that are located beneath the anode  44  and that are spaced outwardly relative to a central opening  54  of the annular anode  44  that faces the gas distributor  50 . 
   A potential is applied between the anode  44  and cathode  42 , for instance by an alternating current supply  56 , thereby producing a flow of electrons (depicted in  FIG. 10  by “−”) in a direction from the cathode  42  toward the anode  44 . The electromagnet  48  is energized, for instance by a direct current source  58 , to create a magnetic field as shown by lines  60  in  FIG. 10 . A gas flow controller  62  operates a valve  64  to control the flow of a working gas to the gas distributor  50 . The working gas has neutral atoms or molecules (depicted in  FIG. 10  as “0”) and is fed through the apertures  52  in the gas distributor  50  so that the gas is uniformly fed into a discharge region within the annular anode  44 . The electrons strike the neutral atoms or molecules thereby producing ions (depicted in  FIG. 10  as “+”). The mixture of electrons and ions forms a desired conductive gas or plasma. 
   According to the present invention, the gas distributor  50  described in the above example is replaced with the gas distributor  30  according to the present invention. See  FIGS. 11–13 . The gas distributor plate  32  has a series of apertures  38  similar to that of distributor  50 . However, unlike distributor  50 , the gas distributor plate  32  according to the present invention has a recessed central portion  34  that receives and holds a separately formed sacrificial element  36 . 
   As discussed previously, the surface portion of a gas distributor that faces the anode is subject to erosive forces during normal operation of the ion source. Typically, this surface corresponds to a central portion of the gas distributor that faces the central opening  54  of an annular anode  44 . Thus, the sacrificial element  36  of the present invention forms the part of the distributor  30  that will be eroded during normal operation of the ion source. The sacrificial element  36  is removable from the distributor plate  32  without requiring the distributor plate  32  to be removed from the ion source. Thus, complete or partial disassembly of the ion source is not required to remove and/or replace the sacrificial element  36 . 
   In the illustrated embodiment, the gas distributor plate  32  is a disc having a diameter “D”, for example, of about 3 inches and a thickness “T”, for example, of about 0.10 to about 0.12 inch. Preferably, about sixteen apertures  38  are uniformly spaced in a circular array concentric to a recessed central portion  34  formed on a top surface of the gas distributor plate  32 . The recess  34  can be formed, for instance, by machining a flat-bottom, circular hole into the center of the top surface of the gas distributor plate  32 . The recess  34  can have, for example, a diameter “A” of about 0.7 inch and a depth “B” of about 0.06 inch. Thus, the depth “B” of the recess  34  is preferably about equal to the thickness “E” of the sacrificial element  36  and is preferably about half of the thickness “T” of the surrounding sections of the plate  32 . 
   Preferably, the sacrificial element  36  has dimensions permitting it to be slip fit into the recess  34  of the plate  32 . The sacrificial element  36  should be held firmly in place within the recess  34  yet be capable of being readily removed therefrom. For example, the illustrated embodiment of the sacrificial element  36  is disc shaped corresponding to the shape of the recess  34  and can have, for instance, a diameter “C” of slightly less than 0.7 inch and a thickness “E” of about 0.06 inch. Of course, all of the above referenced dimensions, shapes, patterns and the like of the plate  32  and sacrificial element  36  can be modified as desired. 
   The gas distributor plate  32  can be made of graphite, non-magnetic stainless steel, molybdenum, tantalum, or any relatively strong non-magnetic material. The sacrificial element  36  can be made of graphite, non-magnetic stainless steel, molybdenum, tantalum, or any other material that is compatible with a user application. The plate  32  and sacrificial element  36  can be made of the same or different material. Thus, for example, a molybdenum element  36  can be used with a non-magnetic stainless steel distributor plate  32 . This provides an advantage in that the user can select the best material for the sacrificial element  36  for his/her particular needs without having to disassemble an ion source and replace the entire gas distributor. 
   In addition, preferably the sacrificial element  36  has identical opposite faces,  66  and  68 , as manufactured. Thus, after one of the faces has been eroded beyond a pre-determined limit, the sacrificial element  36  can be re-positioned within the recess  34  in an inverted position to thereby permit the opposite face to be subject to the erosive forces. 
   The maintenance procedure for an ion source having a gas distributor according to the present invention is greatly simplified. To this end, a tool (not shown), such as tweezers, an elongate tool with an exposed adhesive tip, or the like, is simply inserted into an assembled ion source and is used to grasp the sacrificial element  36  seated on the plate  32 . The element  36  is quickly withdrawn from the ion source and a new element is slip fit with the tweezers or like tool into the recess  34  of the distributor plate  32  mounted within the ion source. Alternatively, the original sacrificial element  36  can be re-positioned within the recess  34  in an inverted or non-inverted position. Such a procedure should take less than two minutes of labor and should save about an hour of downtime relative to replacing/inspecting prior art gas distributor plates. 
   Preferably, the ion source has an annular anode with a central opening that faces the gas distributor  30  of the present invention. The recess  34  of the gas distributor plate  32  is aligned with and faces the central opening of the annular anode. This location corresponds to the section of the plate  32  to which erosive forces will be directed. Thus, the sacrificial element  36  is located in the recess  34  and protects the plate  32  from undesired erosion. The central opening of the annular anode provides accessibility to the sacrificial element  36  and recess  34 . Thus, the sacrificial element  36  is withdrawn from the recess  34  and ion source through the annular anode and is replaced and/or repositioned on the gas distributor plate  32  via the central opening of the annular anode. 
   During normal operations of the ion source, the sacrificial element  36  will be eroded. The above described maintenance procedure can be performed at frequent intervals without significant, or any, downtime of the ion source. Erosion of the removed element  36  can be accurately measured after the sacrificial element  36  is removed form the ion source to determine whether or not it can be further utilized. In addition, each element  36  has two sides, and when one side is eroded beyond a pre-determined limit, it can be flipped over and re-positioned on the distributor plate  32  so that its opposite side can be subject to erosion. Thus, both sides of the element  36  can be eroded thereby providing further material cost savings. 
   While a preferred gas distributor, ion source and maintenance procedure therefor have been described in detail, various modifications, alterations, and changes may be made without departing from the spirit and scope of the distributor and method according to the present invention as defined in the appended claims.