Abstract:
A plasma reactor includes a chamber adapted to support an evacuated plasma environment, a passageway connecting the chamber to a region external of the chamber, the passageway being defined by spaced opposing passageway walls establishing a passageway distance therebetweeen, and a plasma-confining magnet assembly adjacent the passageway. The plasma-confining magnet assembly includes a short magnet adjacent one of the passageway walls and having opposing poles spaced from one another by a distance which a fraction of the gap distance, the short magnet having a magnetic orientation along one direction transverse to the direction of the passageway, and a long magnet adjacent the other one of the opposing passageway walls and generally facing the short magnet across the passageway and having opposing poles spaced from one another along a direction transverse to the passageway by a pole displacement distance which is at least nearly as great as the gap distance, the long magnet having a magnetic orientation generally opposite to that of the short magnet.

Description:
This application claims the benefit of provisional application No. 60/205,819, filed May 19, 2000. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention is related to plasma reactors in which the reactor chamber containing the plasma and the workpiece is maintained at a pressure less than atmospheric by pumping gas out of the chamber through a pumping annulus or throat. One problem is that the plasma process produces various chemical species which tend to contaminate the pumping apparatus and the surface of the pumping annulus as they are pumped out of the chamber. For example, plasma processes designed to etch silicon dioxide films, without etching silicon films or photoresist, typically employ a polymer precursor gas (such as a fluorocarbon gas) which tends to form a protective polymer film over silicon and photoresist surfaces that would otherwise be exposed to etchants in the plasma chamber. Such a film tends to not form over silicon dioxide surfaces, so that the etch selectivity to silicon dioxide is significantly improved. A problem is that the polymer accumulates as a very hard semi-permanent film on any surface exposed to the processing environment, including the pumping annulus and the pumping apparatus, unless somehow prevented from doing so. 
     A solution to the problem of polymer contamination is to permit the polymer film to deposit on surfaces within the primary process region over the workpiece, but block polymer precursor species from leaving the primary process region and specifically blocking the polymer precursor species from entering the pumping annulus of the reactor. This is accomplished by exploiting the fact that the polymer precursor species tend to be ionized in the plasma and may therefore be deflected by magnetic fields. Thus, plasma confinement magnets are placed around the entrance to the pumping annulus. As disclosed in U.S. Pat. No. 6,030,486, issued Feb. 29, 2000, to Loewenhardt et al., and U.S. patent application Ser. No. 09/773,409, filed Jan. 31, 2001 by Loewenhardt et al., a pair of horseshoe magnets may be placed across the entrance to the pumping annulus to deflect all ionized particles and thereby prevent them from entering the pumping annulus. The horseshoe magnets are relatively expensive. Other techniques for confining the ionized particles are also commonly used, such as baffle arrangements ahead of the pumping annulus. One such baffle arrangement entails imposing an “S-curve” in the wall at the entrance of the pumping annulus. The S-curve in the wall is such that there are no straight-line paths through the entrance of the pumping annulus. As a result, the particles entering the pumping annulus tend to collide with the wall of the annulus in the vicinity of the S-curve. For an appropriately proportioned S-curve, at least nearly all of the polymer precursor species collide with the wall in the vicinity of the S-curve and are deposited thereon before they can reach the interior of the pumping annulus or the pumping apparatus. 
     As technology advances, the workpiece size increases. For example, in microelectronic circuit fabrication, the workpiece is a silicon wafer whose diameter may reach or exceed 12 inches. This size increase leads to greater difficulty in maintaining uniform gas flow across the wafer surface, particularly at low chamber pressures. For very low chamber pressures and very large wafer diameters, the gas flow rate through the pumping annulus must be very high to maintain the low chamber pressure and uniform gas flow across the wafer surface. The larger the wafer diameter, the greater must be the gas flow rate through the pumping annulus. It has been found, for example, that for a 200 mm diameter wafer, in order to maintain a chamber pressure of 40 mT, a gas flow rate of about 100 cubic centimeters per second is required. In processing a larger wafer such as a 300 mm diameter wafer, an even lower chamber pressure may be required, such as 20 mT. In this case, a gas flow rate of about 200 cubic centimeters per second through the pumping annulus is required, and it is often difficult to maintain or even reach such a high flow rate in a conventional chamber sufficiently large to hold a 300 mm wafer. 
     One feature which severely limits the gas flow through the pumping annulus, and often prevents the gas flow from reaching the requisite level, are the baffle arrangements at the entrance to the pumping annulus, which has been referred to above. These necessarily induces turbulence that significantly impedes the gas flow. It appears that neither increasing the pump power nor “smoothing” the baffling are viable solutions to this problem. As for increasing the pump power, we have found that for a large wafer size (e.g., 300 mm diameter) and a target chamber pressure that is low (e.g., 20 mT), the impedance induced by the baffling is such that the requisite gas flow rate cannot be reached even if the pump power is increased. Also “smoothing” of the baffling or widening of the throat size of the pumping annulus in an attempt to increase the gas flow detracts from the ability of the baffle to block polymer precursor species, and therefore does not appear to be a practical solution to the problem. 
     What is needed is an inexpensive way of blocking polymer precursor species from the pumping annulus without impeding gas flow through the pumping annulus. 
     SUMMARY OF THE DISCLOSURE 
     A plasma reactor includes a chamber-adapted to support an evacuated plasma environment, a passageway connecting the chamber to a region external of the chamber, the passageway being defined by spaced opposing passageway walls establishing a passageway,distance therebetweeen, and a plasma-confining magnet assembly adjacent the passageway. The plasma-confining magnet assembly includes a short magnet adjacent one of the passageway walls and having opposing poles spaced from one another by a distance which a fraction of the gap distance, the short magnet having a magnetic orientation along one direction transverse to the direction of the passageway, and a long magnet adjacent the other one of the opposing passageway walls and generally facing the short magnet across the passageway and having opposing poles spaced from one another along a direction transverse to the passageway by a pole displacement distance which is at least nearly as great as the gap distance, the long magnet having a magnetic orientation generally opposite to that of the short magnet. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cut-away side view of a plasma reactor in accordance with a first embodiment of the invention employing a tri-magnet configuration. 
     FIG. 2 illustrates a variation of the embodiment of FIG. 1 in which plasma source power is inductively coupled. 
     FIG. 3 illustrates a variation of the embodiment of FIG. 1 in which the magnets are located inside the chamber. 
     FIG. 4 illustrates an embodiment in which all the magnets are located outside the chamber. 
     FIGS. 5A &amp; 5B are enlarged views of the embodiment of FIG.  1 . 
     FIG. 5C illustrates a simulation of the magnetic field employed in the embodiment of FIG.  1 . 
     FIGS. 6A AND 6B are enlarged views of the embodiment of FIG.  3 . 
     FIG. 6C illustrates a simulation of the magnetic field employed in the embodiment of FIG.  3 . 
     FIG. 6D is an enlargement of the magnetic field of FIG.  6 C. 
     FIG. 7 is an enlarged detail view of the embodiment of FIG.  1 . 
     FIG. 8 illustrates an individual unit of the horseshoe magnet assembly. 
     FIG. 9 illustrates an alternate embodiment of FIG. 4 with the positions of the magnets reversed. 
     FIG. 10 illustrates a variation of FIG.  9 . 
     FIG. 11 illustrates a variation of FIG.  10 . 
     FIG. 12 is a graph illustrating an optimum pole displacement of the horseshoe magnets. 
     FIG. 13 is a graph illustrating the diminishing trend in the magnetic filed. 
     FIGS. 14A,  15 A &amp;  16 A illustrate alternate magnetic fields using different tri-magnet configurations. 
     FIGS. 14B,  15 B &amp;  16 B depict enlarged views of FIGS. 14A,  15 A &amp;  16 A respectively. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 illustrates a capacitively coupled plasma reactor whose walls define a vacuum chamber comprised of a cylindrical side wall  110 , a disk-shaped conductive ceiling  115  separated from the side wall  110  by an insulator ring  120 , the side wall  110  and ceiling  115  defining a chamber interior  125 , and a base assembly which includes a workpiece support pedestal  130  surrounded by a wall  134  to support a workpiece  135  such as a silicon wafer. A pumping annulus  140  extends below the chamber interior  125  and is coupled to a vacuum pump  145  via a butterfly valve  150 . The pumping annulus is enclosed by the pedestal wall  134  on one side and by the lower portion of the side wall  110 . Process gases are introduced into the chamber interior  125  from a process gas supply  155  via gas inlets  160 . Plasma source power is provided by an RF power generator  170  coupled through an impedance match circuit  175  to the workpiece support pedestal  130 . The conductive ceiling  115  is grounded and therefore serves as the return electrode for RF power delivered to the pedestal  130 . In addition, the chamber side wall  110  may be conductive and may be grounded to serve as an additional return electrode. Although a capacitively coupled plasma source is described, the invention is advantageously employed in combination with any plasma source, including inductively coupled sources and helicon sources, among others. 
     The pump  145  maintains the chamber interior  125  and the pumping annulus at an evacuated pressure below atmospheric which can approach vacuum conditions. The interior region surrounded by the pedestal wall  134  and the exterior region outside the sidewall  110  are free of plasma and are at atmospheric pressure. 
     In order to block ionized particles or plasma from entering the pumping annulus  140 , a set of three plasma confinement magnets is employed. The set of three magnets includes a pair of magnets  182 ,  184  connected by a steel bar  186  is housed within the interior region  132  of the wafer support  130 , and is preferably attached to the interior surface of the wall  134  of the workpiece support  130 . As will be described in greater detail later in this specification, the poles of the individual magnets  182 ,  184  are oriented oppositely so that the magnet pair  182 ,  184  and the steel bar  186  together form a horseshoe magnet, in which the top is the south pole and the bottom is the north pole, as indicated in FIG.  1 . On the opposite side of the pumping annulus  140  and facing the center of the horseshoe magnet is the third magnet, namely a single individual magnet  188  which is attached to the inner surface of the side wall  110 . The three magnets  182 ,  184 ,  188  constitute what is referred to in this specification as a tri-magnet apparatus. The distance across the “gap” of the pumping annulus is the difference between the radius of the lower sidewall section  110   a  and the radius of the pedestal wall  134 , and is about the same as the “gap” between the “inside” horseshoe magnet  182 ,  184 ,  186  and the “outside” individual magnet  188 . 
     Significantly, only the individual third magnet  188  requires special measures to protect it from the corrosive action of the plasma, because it is located within the interior chamber processing environment, e.g. the pumping annulus  140 , and is therefore exposed to the plasma. The horseshoe magnet  182 ,  184 ,  186  is on the atmospheric side of the support pedestal wall  134  and therefore is not exposed to the plasma. An advantage of this feature is that is relatively inexpensive to protect the individual magnet  188  because of its small size and simple structure. Such protection can be provided, for example, by a thin (50 mil) aluminum coating  190  around the individual magnet  188 . By comparison, if the more complex and larger horseshoe magnet  182 ,  184 ,  186  were placed on the plasma or vacuum side of the pedestal wall  134 , a much greater expense would be entailed, due to its more complex and larger structure. This is particularly true in embodiments (which are explored in more detail later in this specification) in which magnets on the vacuum side of either wall (either the side wall  110  or the pedestal wall  134 ) are mounted on removable/disposable chambers liners that protect the walls. In such embodiments, the cost of replacing the liner is increased by the cost of any plasma containment magnet mounted therein. Replacement cost of the horseshoe magnet is relatively high and for the individual magnet  188  is relatively low. Thus, a significant cost savings is realized in the invention, in which only the small individual magnet need be mounted on the vacuum side of the wall. 
     We have found that it is necessary to widen the pumping annulus  140  in order to achieve the requisite gas flow rate. Conventionally, for smaller wafer sizes (e.g., 200 mm diameter) and higher chamber pressures (e.g., 140 mT), a fairly narrow pumping annulus gap of about 0.75 inch is sufficiently wide to provide the requisite gas flow rate. At the same time, the 0.75 inch gap is sufficiently narrow so that a simple magnetic confinement arrangement (e.g., two individual magnets facing each other across the gap) can block the plasma from the pumping annulus. A larger gap size has not been practical since it was not possible to block plasma across a significantly larger gap using a simple magnetic confinement arrangement. The problem is that for larger wafer sizes (e.g., 300 mm diameter), it is necessary to nearly triple the width of the pumping annulus to about 2.5 inches in order to reach a low chamber pressure such as 20 mT, whether or not the pump power is increased. Thus, the distance between the pedestal wall  134  and the outer side wall  110   a  at the pumping annulus  140  is 2.5 inches. Such a large gap prevents conventional plasma confinement techniques such as baffling or a pair of simple magnets across the gap from blocking the plasma from the pumping annulus. Thus, it has seemed impossible to process such large wafers without incurring significant risk of plasma flowing into the pumping annulus and reaching the pump. This problem is solved by the tri-magnet apparatus. 
     The tri-magnet apparatus can replace baffling (thus removing impediments to gas flow) and more importantly permits the gap to be widened to 2.5 inches (or possibly more) while effectively confining the plasma. We have found that one way of obtaining superior magnetic confinement of the plasma by the tri-magnet apparatus is to mount the individual magnet  188  on the interior surface of the side wall  110   a  in order to reduce the distance between it and the horseshoe magnet  182 ,  184 ,  186 . An advantage of this embodiment of the tri-magnet apparatus is that it is relatively inexpensive to protect the small individual magnet  188  from the plasma by means of encapsulation or a coating such as the aluminum coating  190 . The horseshoe magnet  182 ,  184 ,  186  is mounted on the exterior side of the wall  110  to avoid having to incur the greater expense of protecting the larger horseshoe magnet from the plasma. 
     As briefly alluded to above, we have found that it is only necessary that one of the two magnetic elements (the horseshoe magnet and the individual magnet  188 ) be mounted within the plasma in order to reduce the distance to the other magnetic element and thereby optimize performance. An advantage of the tri-magnet apparatus is that the smaller of the two elements (i.e., the individual magnet  188 ) can be selected as the one mounted within the plasma (i.e., on the vacuum side of the wall), so that the cost of protecting the one magnetic element exposed to the plasma is greatly reduced. For example, if disposable liners are employed covering the side wall and the pedestal wall, then any of the magnets of the tri-magnet assembly on the vacuum side of the side wall or pedestal wall would be encapsulated within the respective liner. The liners are replaced whenever wear and tear (or polymer deposition thereon) produces significant changes in plasma processing conditions (e.g., changes in the plasma load impedance). The cost of replacing the liner includes the cost of any magnets embedded therein. The individual magnet  188  is relatively inexpensive compared to the horseshoe magnet. Therefore, placing only the individual magnet  188  on the vacuum side of the enclosure provides a significant advantage in cost savings. 
     We have found that care must be taken to ensure that the distance between the individual magnet  188  and the horseshoe magnet  182 ,  184 ,  186  be sufficiently small and the strengths of each of the three magnets  182 ,  184 ,  188  be sufficiently great to provide a strong magnetic field across the opening to the pumping annulus to deflect plasma ions. Otherwise, an insufficient magnetic field may actually promote the travel of plasma into the pumping annulus  140 . In order to prevent this, the gap or distance between the two magnetic elements (i.e., the horseshoe magnet and the individual magnet  188 ) may be reduced, or the strengths of the magnets  182 ,  184 ,  188  be increased. Thus, while it would appear the gap size may be increased (to increase gas flow) provided the magnetic strength is increased, a greater magnetic strength is not desireable because of the risk of increasing magnetic fields on the wafer or workpiece  135 , a which would increase the probability of device damage. Thus, it would not appear the gap can be significantly increased. 
     However, we have discovered a way of maintaining sufficient magnetic field strength across the gap (i.e., across the pumping annulus  140 ) without having to increase the strengths of the individual magnets  182 ,  184 ,  188 . Specifically, we have discovered that a larger gap size (a larger pumping annulus) may be accommodated without appreciable loss of magnetic field strength across the gap by increasing the distance between the two magnets  182 ,  184  of the horseshoe magnet. This distance is referred to herein as the pole displacement (PD) of the horseshoe magnet (and depicted in FIG.  1 ), and will be discussed in detail later in this specification. Such an increase in the pole displacement requires that the steel backing  186  be lengthened accordingly. Thus, as the gap size increases, the magnetic field strength across the gap may be preserved by increasing the pole displacement of the horseshoe magnet. 
     The magnetic field directly above the horseshoe magnet turns out to be less than that directly above the individual magnet  188 , as will be explored later in this specification. Accordingly, in order advantageously to minimize the magnetic field adjacent the periphery of workpiece  135 , and thus avoid plasma nonuniformities across the wafer, it is preferable that the horseshoe magnet  182 ,  184 ,  186  be the portion of the tri-magnet assembly which is nearest the workpiece support pedestal  130 , and that the individual magnet  188  be on the opposite side of the pumping annulus  140  (i.e., furthest from the workpiece support pedestal). 
     The vertical displacement (VD) between the top of the horseshoe magnet  182 ,  184 ,  186  and the plane of the wafer  135  may be increased. This reduces magnetic field strength at the wafer  135  or, equivalently, permits the magnets  182 ,  184 ,  188  to be stronger (e.g., to allow a larger gap across the pumping annulus  140  for a greater gas flow rate) without a concomitant increase in magnetic field strength at the wafer. Adjusting the vertical displacement typically entails vertically shifting the entire tri-magnet apparatus to achieve a desired vertical displacement for a given pole displacement. 
     The tri-magnet apparatus can facilitate a number of ways to improve the gas flow rate through the pumping annulus  140 . First, the replacement of baffling with magnetic confinement improves the gas flow. Secondly, increasing the gap across the pumping annulus  140  without a concomitant loss in magnetic field strength is facilitated by increasing the pole displacement of the horseshoe magnet. Third, the gap may be further widened while preserving magnetic field strength across the gap by increasing the strength of the individual magnets, in which case a corresponding increase in the magnetic field at the wafer is avoided by increasing the vertical displacement between the tri-magnet configuration and the plane of the wafer  135 . 
     We have found that the tri-magnet configuration makes possible for the first time the plasma processing of 300 mm diameter wafers at a low chamber pressure on the order of only 20 mT with good gas flow across the wafer. This is because the tri-magnet configuration enabled us to improve gas flow by widening the pumping annulus gap from 0.75 inches to 2.5 inches while magnetically blocking plasma from traveling through the pumping annulus  140 . Moreover, the magnetic confinement of the plasma by the tri-magnet apparatus eliminates any need for baffling expedients, thus removing that impediment to gas flow. Sufficient magnetic field strength was due in part to a sufficiently large pole displacement of the horseshoe magnet. Suppression of magnetic field strength in the plane of the wafer was accomplished by a sufficiently large vertical displacement between the wafer plane and the tri-magnet assembly. Having thus broken the barrier for processing wafers as large as 300 mm at low pressure (20 mT), it is anticipated that the ability to scale the tri-magnet configuration to cope with ever-widening pumping annulus gaps as described above will continue to push the barrier back even further. Such scaling is facilitated by adjusting the pole displacement of the horseshoe magnet  182 ,  184 ,  186 , adjusting the magnetic strength of the magnets  182 ,  184 ,  188  and concomitantly adjusting the vertical displacement between the tri-magnet assembly and the wafer plane, as referred to above. 
     FIG. 2 illustrates a variation of the embodiment of FIG. 1 in which plasma source power is inductively coupled from an overhead coil antenna  210  connected through an impedance match circuit  215  to an RF power generator  220 . The RF power generator  170  is typically used in the embodiment of FIG. 2 to control the bias power on the wafer  135 . 
     FIG. 3 illustrates a variation of the embodiment of FIG. 1 in which not only is the individual magnet  188  placed inside the pumping annulus but also the entire horseshoe magnet  182 ,  184 ,  186  is placed inside the pumping annulus. While the individual magnet  188  is placed on the surface of the side wall lower portion  110   a  inside the pumping annulus, the horseshoe magnet  182 ,  184 ,  186  is placed on the surface of the pedestal wall  134  inside the pumping annulus. As described above with reference to FIG. 1, the individual magnet  188  is protected by a thin aluminum film  190 . Likewise, the horseshoe magnet is protected by an overlying aluminum film  310  as shown in FIG.  3 . 
     FIG. 4 illustrates an embodiment in which all three magnets  182 ,  184  and  188  are placed outside of the pumping annulus  140 . Specifically, the horseshoe magnet  182 ,  184 ,  186  is placed on the surface of the pedestal wall  134  outside the pumping annulus and the individual magnet is placed on the surface of the side wall lower portion  110   a  outside the pumping annulus  140 . In FIG. 4, none of the magnets require any protective coating and therefore this embodiment is inexpensive. However, in the embodiment of FIG. 4 the distance between the magnet  188  and the horseshoe magnet  182 ,  184 ,  186  is greater than in the other embodiments, and therefore a greater pole displacement of the horseshoe magnet is required. 
     FIG. 5A illustrates how the embodiment of FIG. 1 is implemented using liners oh the walls defining the pumping aperture  140 . Specifically, a removable liner  510  covers the cylindrical side wall  110 . The liner  510  includes a radially extending shelf  515  within which the individual magnet  188  is held. The liner  510  is formed of a hard material such as aluminum. A liner  520  covers the pedestal wall  134 . Such liners maybe quickly removed and replaced whenever required due to corrosive surface wear or accumulation of contaminants, which would otherwise lead to an unacceptable change in reactor characteristics or degradation in performance. In the embodiment of FIG. 5A, the horseshoe magnet  182 ,  184 ,  186  is placed behind the pedestal wall  134 , so that it does not require any special protective measures. 
     FIG. 5B illustrates certain normalized dimensions that can be used to implement either the embodiment of FIG. 5A or the embodiment of FIG.  1 . These normalized dimensions were employed in a computer simulation of the magnetic field generated across the pumping annulus gap, and FIG. 5C illustrates the results of that simulation. In the simulation, it is assumed each of the magnets  182 ,  184 ,  188  is formed of many Samarium Cobalt magnets, as will be discussed below with reference to FIG.  14 . The simulation of FIG. 5C is instructive in that it shows a continuous magnetic field line of 80 Gauss extending across the entire gap of the pumping annulus  140 . Thus, for any charged particle to penetrate, it would have to overcome the deflective force of 80 Gauss. FIG. 5C also shows how the field strength decreases near the wafer  135 . FIG. 5D is a magnified version of FIG. 5C near the wafer  135 , and shows a maximum magnetic field strength near the wafer  135  of about 6 Gauss. 
     FIG. 6A illustrates how the embodiment of FIG. 3 is implemented using liners on the walls defining the pumping aperture  140 . Specifically, a removable liner  610  covers the cylindrical side wall  110 . The liner  610  supports the individual magnet  188 , which is encapsulated in the protective aluminum coating  190 . The liner  610  is formed of a hard material such as aluminum. A liner  620  covers the pedestal wall  134 . In the embodiment of FIG. 6A, the horseshoe magnet  182 ,  184 ,  186  is placed inside the liner  620 , so that the liner  620  protects the horseshoe magnet from the plasma. 
     FIG. 6B illustrates certain normalized dimensions that can be used to implement either the embodiment of FIG. 6A or the embodiment of FIG.  3 . These normalized dimensions were employed in a computer simulation of the magnetic field generated across the pumping annulus gap, and FIG. 6C illustrates the results of that simulation. In the simulation, it is assumed each of the magnets  182 ,  184 ,  188  is the same as that described above with reference to the simulation of FIG.  5 C. The simulation of FIG. 6C is instructive in that it shows a continuous magnetic field line of 100 Gauss extending across the entire gap of the pumping annulus  140 . Thus, for any charged particle to penetrate, it would have to overcome the deflective force of 100 Gauss. This field is stronger than the one obtained in the simulation of FIG. 5C because, for the same gap distance, the horseshoe magnet is somewhat closer to the individual magnet  188 . FIG. 6C also shows how the field strength decreases near the wafer  135 . FIG. 6D is a magnified version of FIG. 6C near the wafer  135 , and shows a maximum magnetic field strength near the wafer  135  which is less than that obtained in the simulation of FIG.  5 C. The difference is due at least in part to the greater radial distance of the horseshoe magnet from the wafer in the embodiment of FIG.  6 A. 
     FIG. 7 illustrates how the horseshoe magnet  182 ,  184 ,  186  at the pedestal  135  and the individual magnet  188  at the side wall  110  are formed from many individual solid rectangular magnet pieces  710  arranged side-by-side and defining an annulus. FIG. 18 illustrates an individual unit of the horseshoe magnet including an upper magnet piece  810 , a lower magnet piece  820  and a steel backing piece  830  connecting the two magnet pieces  810 ,  820 . In an actual implementation, each magnet piece was a solid rectangular Samarium Cobalt block with dimensions and magnetic orientation as illustrated in FIG.  8 . Preferably, a non-magnetic metal such as aluminum is placed in the space bounded on three sides by the upper and lower magnet pieces  810 ,  820  and the backing piece  830 . 
     FIG. 7 clearly shows that the horseshoe magnet  182 ,  184 ,  186  is a ring magnet extending 360 degrees around the circumference of the radially inner surface of the pedestal wall  134 . Similarly, the individual magnet  188  is a ring magnet extending 360 degrees around the circumference of the radially outer surface of the side wall lower portion  110   a.    
     FIG. 9 illustrates an alternative tri-magnet assembly in which the positions of the horseshoe magnet  182 ,  184 ,  186  and the individual magnet  188  are reversed, the horseshoe magnet being at the side wall  110  and the individual magnet being at the pedestal  130 . In the embodiment of FIG. 9, all magnets are located outside of the pumping annulus  140 . 
     FIG. 10 illustrates a variation of FIG. 9 in which the individual magnet  188  is placed inside the pumping annulus  140  on the pedestal wall  134  and is protected by a thin aluminum coating  190 . 
     FIG. 11 illustrates a variation of FIG. 10 in which both the individual magnet  188  and the horseshoe magnet are placed inside the pumping annulus on the pedestal wall  134  and the side wall  110 , respectively. The individual magnet  188  is protected by the aluminum coating  190  while the horseshoe magnet is protected by an aluminum coating  195 . 
     FIG. 12 is a graph illustrating an optimum pole displacement of the horseshoe magnet  182 ,  184 ,  186  as a function of pumping annulus gap size. The scale of the graph of FIG. 12 is not necessarily linear nor proportional and the graph is meant to illustrate that, for a desired minimum magnetic field strength across the gap, the pole displacement must generally increase as the gap size increases. 
     FIG. 13 is a graph illustrating the diminishing trend in the magnetic field at the wafer plane as the vertical displacement of the tri-magnet assembly below the wafer plane is increased. The scale of the graph of FIG. 13 is not necessarily linear nor proportional. This trend is exploited in minimizing the magnetic field on the wafer in the foregoing embodiments. 
     FIGS. 14A,  15 A and  16 A depict different tri-magnet configurations. The tri-magnet apparatus of FIG. 14A has a longer pole displacement of the horseshoe magnet, a larger gap and a smaller individual magnet  188  than that of FIG.  15 A. The tri-magnet configuration of FIG. 16A has the smallest gap. The magnetic field strength across the gap is largest in FIG.  16 A and smallest in FIG. 14A, due at least in part to the larger gap of FIG.  14 A. Comparison of FIGS. 14A,  15 A and  16 A appears to indicate that the longer pole displacement of FIG. 14A at least partially compensates for the larger gap length. Moreover, it appears that variation in length of the individual magnet  188  has little appreciable effect on the magnetic field. Thus, it appears that little penalty is incurred by the relatively small size of the individual (third) magnet  188 . This is advantageous because the small size of this magnet provides a significant cost savings. 
     FIGS. 14B,  15 B and  16 B show that in each of these embodiments, the magnetic field strength in the wafer plane is advantageously low, namely about 8 Gauss or less. This minimizes the likelihood of device damage to the workpiece from plasma ions interacting with the magnetic field at the wafer plane. 
     In summary, the tri-magnet assembly provides effective magnetic confinement of the plasma across the very large gap that is required for low pressure processing of large wafers. It accomplishes this in a highly cost effective way heretofore not possible in that only a single small magnet ring (the individual magnet  188 ) disposed on the vacuum side of the side wall is embedded in a removable liner. The more expensive horseshoe magnet on the opposite side of the gap—at the pedestal—is not on the vacuum side of the pedestal wall and therefore does not affect the cost of replacement of the liner covering the pedestal wall, a significant advantage. The tri-magnet assembly can be scaled to permit enlargement of the gap as needed. This scaling is accomplished principally by adjusting (increasing) the pole displacement of the horseshoe magnet. If it is necessary to reduce the magnetic field in the wafer plane, further (independent) adjustment is accomplished by changing (increasing) the distance between the tri-magnet assembly and the wafer plane. By placing the horseshoe magnet on the wafer side of the gap instead of the individual magnet, the magnetic return field facing the wafer plane is tighter and does not extend as far toward the wafer, thus minimizing the magnetic field in the wafer plane. 
     While the invention has been described with reference to a preferred embodiment in which the tri-magnet apparatus confines plasma from a pumping annulus surrounding a wafer support pedestal, the invention may also find use in confining plasma from passageways other than a pumping annulus. For example, the passageway may be a slit valve of the chamber. In this case, the two elements of the tri-magnet assembly (i.e., the horseshoe magnet and the individual magnet) would be placed on opposing sides of the passageway. If chamber liners are used on or near the passageway, then one or both of two elements of the tri-magnet assembly may be placed in a respective chamber liner so that the two elements face each other across the passageway. 
     While the invention has been described with reference to an embodiment in which the larger magnet is a horseshoe magnet, in other embodiments the larger magnet may not be a horseshoe magnet but instead may appear to be a very long bar magnet. Such a long bar magnet may be constructed by placing two small bar magnets on opposite ends of a long magnetic backing piece, without a “horseshoe” shape. Nevertheless, the distance between the opposite poles of the long magnet would be governed by the principles set forth above governing the pole displacement of the horseshoe magnet. 
     While the invention has been described in detail by specific reference to preferred embodiments, it is understood that variations and modifications thereof may be made without departing from the true spirit and scope of the invention.