Abstract:
A magnetic field generator for confining plasma within a vacuum chamber is disclosed. The magnetic field generator produces a multi-pole magnetic field around a workpiece positioned within the vacuum chamber. The magnetic field generator is provided outside the vacuum chamber and comprises a plurality of segment type permanent magnets circularly arranged. The magnetic field generator further comprises a plurality of magnetic members to which the segment type permanent magnets are selectively attached.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to a magnetic field generator, and more specifically to such a generator for effectively confining magnetron plasma which is produced within a vacuum chamber in which a workpiece is positioned so as to be subject to plasma treatment. 
     2. Description of Related Art 
     It is known in the art to generate magnetron plasma, within an evacuated process chamber (viz., vacuum chamber), for use in implementing plasma treatment such as etching, film growth, etc. on the workpiece such as a semiconductor wafer provided in the chamber. 
     Prior to turning to the present invention, it is deemed advantageous to briefly describe, with reference to FIG. 1, a conventional magnetic field generator via which the plasma produced within the vacuum chamber is confined, When performing the plasma treatment (processing) on the workpiece, it is vital to effectively confine plasma in a manner to surround the workpiece so as to obtain designed results. 
     As shown in FIG. 1, a semiconductor wafer (viz., workpiece)  10  is positioned at the center area of a vacuum chamber  12  in a manner to face the surface of the workpiece upward on which the plasma treatment is to be carried out. Although not shown in FIG. 1, plasma is generated within the vacuum chamber  12  by way of conventional technology. The plasma generation per se is not directly concerned with the present invention, and thus the further description thereof will be omitted for the sake of simplifying the instant disclosure. 
     In order to effectively confine plasma around the workpiece  10  with forming no magnetic field above the wafer  10  (or with permitting very week magnetic field above the wafer), a magnetic field generator (or multi-pole magnet unit)  14  is provided outside the vacuum chamber  12 . The magnetic field generator  14  is comprised of a ring-like magnetic member (supporter)  16  and a plurality of segment type permanent magnets (hereinafter referred to as segment magnets)  18  attached to the inner side of the member  16 . More specifically, the segment magnets  18  are arranged such as to alternately change the magnetic polarities thereof in the circumferential direction of the supporter  16 , and thus, a multi-pole magnetic field is generated within the vacuum chamber  12  so as to confine the plasma around the workpiece  10 . Small arrows on the segment magnets  18  respectively indicate the magnetized directions of the magnets  18 , and curved lines  20  respectively indicate magnetic forth lines. 
     It is typical in the art, when constructing the magnetic field generator  14 , to make use of the permanent magnets  18  rather than electromagnets because the electric power consumption can be zeroed and the structure of the apparatus can be simplified. On the other hand, a study made by the inventor of the instant application has revealed that the plasma treatment rate (i.e., etch rate) on the wafer surface depends on the strength of the multi-pole magnetic field. However, there has been no proposal to control the multi-pole magnetic field which is generated using a multiple of permanent magnets. In other words, a permanent magnet type conventional magnetic field generator is unable to adaptively control the magnetic field strength. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a magnetic field generator wherein the strength of the multi-pole magnetic field generated thereby can adaptively be controlled. 
     One aspect of the present invention resides in a magnetic field generator for generating a multi-pole magnetic field around a workpiece positioned within a vacuum chamber. The magnetic field generator is provided outside the vacuum chamber and comprises a plurality of segment type permanent magnets circularly arranged. The magnetic field generator further comprises a plurality of magnetic members to which the segment magnets are selectively attached. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features and advantages of the present invention will become more clearly appreciated from the following description taken in conjunction with the accompanying drawings in which like elements or portions are denoted by like reference numerals and in which: 
     FIG. 1 is a plan view schematically showing a conventional magnetic field generator together with part of vacuum chamber, having been referred to in the opening paragraphs; 
     FIG. 2 is a plan view schematically showing a magnetic field generator together with part of a vacuum chamber according to a preferred embodiment of the present invention; 
     FIG. 3 is a sectional view taken along section line A-B of FIG. 1; 
     FIG. 4 is a plan view schematically showing a modification of the preferred embodiment of FIG. 2; 
     FIG. 5 is a plan view schematically showing another modification of the preferred embodiment of FIG. 2; 
     FIGS.  6 (A) and  6 (B) are schematically illustrating radial displacement of the members which comprise part of the magnetic field generator according to the preferred embodiment and the modifications thereof; and 
     FIGS.  7 (A) to  7 (C) are drawings for schematically explaining the manners of magnetic fluxes produced in magnetic members with different configurations, which members form part of the magnetic field generator of FIGS.  2  and  5 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference is now made to FIGS. 2 and 3 wherein one preferred embodiment of the present invention is schematically illustrated. 
     FIG. 2 is a top plan view of a magnetic field generator (or multi-pole magnet unit)  22  and a vacuum chamber  24  (although only the vertical wall thereof  24   a  is shown). A workpiece  26  such as a semiconductor wafer is positioned within the vacuum chamber  24 . In FIG. 2, a top wall of the vacuum chamber  24  is omitted for the convenience of illustrating the inside of the chamber  24 . FIG. 3 is a cross section taken along section line A-B of FIG.  2 . 
     As shown in FIG. 3, an upper and lower plate-like electrodes  28   a  and  28   b  are provided within the vacuum chamber  24 . Although not shown in FIG. 3, a high frequency alternate voltage is applied to the two electrodes  28   a  and  28   b  so as to generate a plasma therebetween. An open arrow  29  indicates the direction of a high frequency electric field at a moment when the upper and lower electrodes  28  and  28   b  are respectively positive and negative. The workpiece  26 , which is positioned on the lower electrode  28   b , is subject to the plasma treatment such as etching, film growth, etc. The plasma generation per se is well known in the art, and the present invention is not directly concerned therewith, and as such, further description thereof will not be given for the sake of simplifying the instant disclosure. 
     The magnetic field generator  22  (FIG. 2) is installed in a manner to surround the vacuum chamber  24  so as to generate a multi-pole magnet field within the chamber  24  in order to confine magnetron plasma. The magnetic field generator  22  generally comprises a plurality of segment magnets  28  and a plurality of magnetic members  30 . As shown in FIG. 2, the segment magnets  28  are alternately attached to the magnet members  30  which are arranged such as to be adjacent with each other forming a ring-like configuration. As best shown in FIG. 3, the magnetic members  30  are respectively supported at the upper and lower ends thereof by way of a pair of ring-like members  32   a  and  32   b , each of which is made of a non-magnetic material such as aluminum, stainless steel, brass, synthetic resin, etc. 
     In the instant embodiment, the numbers of the magnetic members  30  and the segment magnets  28  are respectively 32 and 16 (see FIG.  2 ). The present invention is however in no way limited to such numbers. By way of example, the number of the magnetic members  30  may be in the range from 8 to 64, while the number of the segment magnets  28  is equal to or less than that of the magnetic members  30 . The key of the present invention resides in the fact that the magnetic field generator  22  is comprised of a plurality of magnetic members  30 . 
     The arrow attached to each of the segment magnets  28  indicates the magnetized direction thereof, and a plurality of curves  34  denote each a magnetic force line between the adjacent segment magnets  28 . 
     As shown in FIG. 2, the segment magnets  28  are arranged such that the polarities thereof are alternately changed, which arrangement leads to the generation of multi-pole magnetic field around the workpiece  26  (or circumferentially along the inner wall  24   a  of the vacuum chamber  24 ). The magnetic flux density of the multi-pole magnetic field strength may be in a range from 0.005 to 0.2T (200 to 2000G), and preferably might range from 0.03 to 0.045T (300 to 450G) by way of example. In such a case, the strength of multi-pole magnetic field is substantially zero (or very week) in the vicinity of the center of the workpiece  26 . 
     According to the inventor&#39;s experiment conducted under the following conditions, a magnetic flux density in the vicinity of the inner wall  24   a  of the chamber  24  was 0.03T (300G). The experimental conditions were such that the diameter D of a circle inscribed in the segment magnets  28  was 450 mm. Further, each of the magnetic members  30  had a trapezoidal cross section in a direction perpendicular to the central axis of the magnetic field generator  22 . Still further, each segment magnet  28  had a width W of 40 mm, a thickness TM of 7 mm, and a height L of 120 mm, and the upside and base of the trapezoid was 45.7 mm and 47.4 mm. Still further, each of the magnetic members  30  had a thickness TY of 9 mm and a height L of 120 mm. In this experiment, each segment magnet  28  was a rare-earth magnet with a residual magnetization flux density of 1.3T, and each magnetic member  30  was made of low-carbon steel S 15 C, and the supporter  32   a ,  32   b  was made of aluminum. 
     The segment magnet  28  is in no way limited to a special one, and may take the form of a rare-earth magnet, a ferrite magnet, an alnico magnet, etc., while the magnetic member  30  may be made of pure iron, carbon-steel, iron-cobalt stainless steel, etc. On the other hand, the supporter  32   a  ( 32   b ) may be made of non-magnetic material such as aluminum, stainless steel, brass, resin, etc. 
     As mentioned above, it is preferable that the magnetic field strength is substantially zero in the vicinity of the center of the workpiece  26 . It is, however, practically sufficient if the magnetic field in the space in which the workpiece  26  is to be positioned is week to such an extent not to adversely affect the plasma treatment of the workpiece. In the above-mentioned experiment, a magnetic flux density less than 420 μT (4.2 G) was present in the vicinity of the circumference of the workpiece  26 . 
     FIG. 4 illustrates a first modification of the above-mentioned preferred embodiment of the present invention, wherein the magnetic members  30 , which respectively carry no segment magnets  28 , are removed or not installed. With this modification, it is possible to reduce the magnetic field strength in the vicinity of the inner wall  24   a  of the vacuum chamber  24 . The inventor of the present invention conducted another experiment under the same conditions as mentioned above while the magnetic members  30  each having no segment magnet  28  are removed as shown in FIG.  4 . The experimental result indicated that the magnetic flux density in the vicinity of the inner wall of the chamber  24  was able to be reduced to 0.023T (230G). 
     FIG. 5 shows a second modification of the above-mentioned preferred embodiment, in which eight pairs of segment magnets  28  are arranged in a manner to be attached to the magnetic members  30  at intervals of two. This arrangement can be realized without difficulty by relocating the magnetic members  30  with and without the segment magnets  28 . To this end, it is necessary to detachably install the magnetic members  30  to the supporters  32   a  and  32   b . In the case shown in FIG. 5, an eight-pole magnetic field is generated within the vacuum chamber  24 , and the magnetic force lines extend further toward the center of the chamber  24  compared with the embodiment shown in FIG.  2 . Accordingly, with this modification, the magnetic field strength can be increased in a space surrounding the workpiece  26  relative to the case shown in FIG.  2 . 
     As an extension of the second modification, it is possible to provide four groups of segment magnets  28  with each group consisting of four magnets  28  arranged in proximity with each other. In this case, four-pole magnetic field is generated, As the number of magnetic poles decreases, the magnetic fluxes extend further inside the vacuum chamber  24 . 
     The magnetic members  30 , each of which accompanies the segment magnet as shown in FIGS. 2,  4  and  5 , are movably installed in the supporter  32   a  and  32   b  in an outward radial direction. In especial, the magnetic members  30  of FIG. 4 are movably installed in the supporter  32   a  and  32   b  in both outward and inward radial directions. It is understood that the radial displacement of the magnetic members  30  enables it possible to control the multi-pole magnetic field strength within the vacuum chamber  24 . 
     FIG.  6 (A) schematically shows a combination of one magnetic member  30  and one segment magnet  28  before being moved in a radial direction, while FIG.  6 (B) shows the combination has been radially displaced. 
     According to still another experiment conducted by the inventor of the present invention, all the magnetic members  30  of FIG. 2, each of which carries the segment magnet, were radially displaced by 20 mm in an outward direction. The experimental results indicated that the magnetic flux density in the vicinity of the inner wall  24   a  of the vacuum chamber  24  was be able to be reduced to 0.01T (100G). 
     In order to prevent undesirable magnetic flux losses between the magnetic members  30  especially shown in FIGS. 2 and 5, it is highly preferable to tightly contact them so as to leave no air gap therebetween. To this end, each magnetic member  30  is desirably configured such as to have a trapezoidal or fan-shaped cross section in a direction perpendicular to the central axis of the magnetic field generator, as shown in FIGS.  7 (A) and  7 (B). By way of example, in case the magnetic member  30  has a rectangular cross section (FIG.  7 (C)), the magnetic fluxes decreases due to magnetic saturation caused by air gaps with the result of undesirable reduction of the multi-pole magnetic field strength. 
     However, it is possible to positively use the air gaps between the magnetic members  30  in order to control the multi-pole magnetic field strength. That is to say, if the magnetic members  30  (FIGS.  2  and  5 ), each of which has no segment magnet thereon, are radially displaced in the outward direction, the air gaps are generated between the adjacent magnetic members  30 . Thus, the strength of the multi-pole magnetic field is reduced within the vacuum chamber  24 . In other words, such displacement of the magnetic members  30  is able to exhibit the same effects as detaching them from the magnetic field generator  22 . 
     The foregoing descriptions show one preferred embodiment and several modifications thereof. However, other various variants are apparent to those skilled in the art without departing from the scope of the present invention which is only limited by the appended claims. Therefore, the embodiments and modification shown and described are only illustrated, not restrictive.