referring to fig1 an rf plasma reactor for processing a semiconductor wafer has a vacuum chamber 10 enclosed by a cylindrical side wall 12 , a ceiling 14 and a floor 16 . a wafer pedestal 18 supports a semiconductor wafer 20 which is to be processed . a plasma precursor gas is injected into the chamber 10 through a gas injector 22 from a gas supply 24 . plasma source power is coupled into the chamber 10 in any one of several ways . for example , the reactor may be a “ diode ” configuration , in which case rf power is applied across a ceiling electrode 26 and the wafer pedestal 18 . this is accomplished by connecting the pedestal 18 and the ceiling electrode 26 to either one of two rf power sources 28 , 30 . alternatively , a cylindrical side coil 32 wound around the chamber side wall 12 is connected to an rf power source 34 . alternatively to the foregoing , or in addition thereto , a top coil 36 is connected to an rf power supply . as is conventional , the wafer pedestal 18 may have its own independently controllable rf power supply 28 so that ion bombardment energy at the wafer surface can be controlled independently of plasma density , determined by the rf power applied to the coil 32 or the coil 36 . a vacuum pump 40 is coupled to the chamber 10 through a passage 42 in the floor 16 . the annular space between the periphery of the wafer pedestal 18 and the floor 16 forms a pumping annulus 44 through which the vacuum pump 40 evacuates gas from the chamber 10 to maintain a desired processing pressure in the chamber 10 . the pumping annulus 44 is coupled to the interior of the chamber 10 through an annular gap 46 between the periphery of the wafer pedestal 18 and the chamber side wall 14 . in order for the pump 40 to perform efficiently , the gap 46 is preferably free of obstructions . a conventional slit valve opening 50 of the type well - known in the art having a long thin opening in the chamber side wall 14 provides ingress and egress for a semiconductor wafer 52 to be placed upon and withdrawn from the wafer pedestal 18 . the walls 12 , 14 confining the plasma within the chamber 10 are subject to attack from plasma ions and charged radicals , typically , for example , by ion bombardment . such attack can consume the material in the walls 12 , 14 or introduce incompatible material from the chamber walls 12 , 14 into the plasma process carried out on the wafer 52 , thereby contaminating the process . such incompatible material may be either the material of the chamber wall itself or may be material ( e . g ., polymer ) previously deposited on the chamber walls during plasma processing , which can flake off or be sputtered off . plasma reaching the chamber walls can cause polymer deposition thereon . the openings from the interior portion of the chamber 10 , including the pedestal - to - side wall gap 46 and the slit valve opening 50 , permit the plasma to leak or flow from the chamber 10 . such leakage can reduce plasma density near the openings 46 , 50 , thereby upsetting the plasma process carried out on the wafer 52 . also , such leakage can permit the plasma to attack surfaces outside of the chamber interior . the flow of plasma into the pumping annulus 44 through the gap 46 permits the plasma to attack the interior surfaces or walls of the pumping annulus 44 . thus , the designer must typically take into account not only the materials forming the chamber ceiling 12 and side wall 14 , but in addition must also take into account the materials forming the pumping annulus , including the lower portion 56 of the side wall 14 , the floor 16 and the bottom peripheral surface 58 of the wafer pedestal 18 , which complicates the design . such a loss of plasma from the chamber 10 also reduces plasma density or requires more plasma source power to maintain a desired plasma density over the wafer 52 . in order to prevent plasma from flowing from the chamber 10 into the pumping annulus , a magnetic field perpendicular to the plane of the gap 46 and perpendicular to the direction of gas flow through the gap 46 is provided across the gap 46 . this is preferably accomplished by providing an opposing pair of magnetic poles 60 , 62 juxtaposed in facing relationship across the gap 46 . in the embodiment according to fig2 the magnetic pole 60 is the north pole of a magnet 64 located at the periphery of the wafer pedestal 18 while the magnetic pole 62 is the south pole of a magnet 66 next to the inner surface of the side wall 14 . the embodiment of fig2 may be regarded as an open magnetic circuit because the returning magnetic field lines of flux 68 in fig2 radiate outwardly as shown in the drawing . in order to prevent plasma from flowing from the chamber 10 through the slit valve opening 50 , a magnetic field perpendicular to the plane of the slit valve opening 50 and perpendicular to the direction of gas flow through the slit valve opening 50 is provided across the slit valve opening 50 . this is preferably accomplished by providing an opposing pair of magnetic poles 70 , 72 juxtaposed in facing relationship across the slit valve opening 50 . in the embodiment according to fig3 the magnetic pole 70 is the north pole of a magnet 74 extending across the bottom edge of the slit valve opening 50 while the magnetic pole 72 is the south pole of a magnet 76 extending along the top edge of the slit valve opening 50 . the embodiment of fig3 may also be regarded as an open magnetic circuit because the returning magnetic field lines of flux 78 in fig3 radiate outwardly as shown in the drawing . one potential problem with the returning lines of magnetic flux 68 ( fig2 ) and 78 ( fig3 ) is that some returning flux lines extend near the wafer 52 and may therefore distort or perturb plasma processing of the wafer 52 . in order to minimize or eliminate such a problem , a closed magnetic circuit ( one in which returning magnetic lines of flux do not extend into the chamber ) is employed to provide the opposing magnetic pole pairs 60 , 62 and 70 , 72 . for example , in the embodiment of fig4 and 5 , the opposing magnetic poles 60 , 62 across the gap 44 are each a pole of a respective horseshoe ring magnet 80 , 82 concentric with the wafer pedestal 18 . the horseshoe ring magnet 80 has the north pole 60 and a south pole 81 while the horseshoe ring magnet has the south pole 62 and a north pole 83 . the poles 60 , 81 of the inner horseshoe ring magnet 80 are annuli connected at their inner radii by a magnetic cylindrical core annulus 85 . similarly , the poles 62 , 83 of the outer horseshoe ring magnet 82 are annuli connected at their outer radii by a magnetic cylindrical core annulus 86 . the magnetic circuit consisting of the inner and outer horseshoe ring magnets 80 , 82 is a closed circuit because the lines of magnetic flux between the opposing pole pairs 60 , 62 and 81 , 83 extend straight between the poles and , generally , do not curve outwardly , at least not to the extent of the outwardly curving returning lines of flux 68 , 78 of fig2 and 3 . in the embodiment of fig4 a , 4b and 6 , the opposing magnetic poles 70 , 72 across the slit valve opening 50 are each a pole of a respective long horseshoe magnet 90 , 92 extending along the length of the slit valve opening 50 . the long horseshoe magnet 90 extends along the top boundary of the slit valve opening 50 while the other horseshoe magnet extends along bottom edge of the slit valve opening 50 . the advantage of the closed magnetic circuit embodiment of fig4 is that the magnetic field confining the plasma does not tend to interfere with plasma processing on the wafer surface . in the embodiment of fig7 and 8 , the lower annuli 81 , 83 of the two horseshoe ring magnets 80 , 82 are joined together as a single annulus by a magnetic core annulus 96 , so that the horseshoe ring magnets 80 , 82 constitute a single horseshoe ring magnet 94 having a north pole 60 and a south pole 62 . the core annulus 96 extends across the pumping annulus 44 and can be protected by a protective coating 98 such as silicon nitride . in order to allow gas to pass through the pumping annulus 44 , the core annulus 96 has plural holes 100 extending therethrough . one advantage of the invention is that plasma ions are excluded from the pumping annulus 44 . this is advantageous because the pumping annulus interior surfaces can be formed of any convenient material without regard to its susceptibility to attack by plasma ions or compatibility of its sputter by - products with the plasma process carried out on the wafer . this also eliminates reduction in plasma density due to loss of plasma ions through the pumping annulus . another advantage is that gas flow through the pedestal - to - side wall gap 46 is not obstructed even though plasma is confined to the interior chamber 10 over the wafer . furthermore , by so confining the plasma to a smaller volume ( i . e ., in the portion of the chamber 10 directly overlying the wafer 52 ), the plasma density over the wafer 52 is enhanced . a further advantage is that stopping plasma ions from exiting through the slit valve opening 50 eliminates loss of plasma density over portions of the wafer 52 adjacent the slit valve opening 50 . in one example , each of the magnetic pole pair 60 , 62 has a strength of 20 gauss for a distance across the gap 46 of 5 cm , while each of the magnetic pole pair 70 , 72 has a strength of 20 gauss for a width of the slit valve opening 50 of 2 cm . while the invention has been described with reference to preferred embodiments in which the plasma confining magnets are protected from attack from plasma ions and processing gases by being at least partially encapsulated in the chamber walls or within the wafer pedestal or within a protective layer , in some embodiments ( as for example , the embodiment of fig6 ) the magnets may be protected by being located entirely outside of the chamber walls . alternatively , if the reactor designer is willing to permit some plasma interaction with the magnets , magnets may be located inside the chamber in direct contact with the plasma , although this would not be preferred . 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 .