Externally excited torroidal plasma source with magnetic control of ion distribution

[0001] This application is a continuation-in-part of U.S. application Ser. No. 09/636,435, filed Aug. 11, 2000, entitled EXTERNALLY EXCITED MULTIPLE TORROIDAL PLASMA SOURCE by Hiroji Hanawa, et al., issued as U.S. Pat. No. 6,494,986 B1 on Dec. 17, 2002.

[0002] 1. Technical Field

[0003] The invention concerns plasma reactors used in processing workpieces in the manufacturing of items such as microelectronic circuits, flat panel displays and the like, and in particular plasma sources therefor.

[0004] 2. Background Art

[0005] The trend in microelectronic circuits toward ever increasing densities and smaller feature sizes continues to make plasma processing of such devices more difficult. For example, the diameter of contact holes has been reduced while the hole depth has increased. During plasma-enhanced etching of a dielectric film on a silicon wafer, for example, the etch selectivity of the dielectric material (e.g. silicon dioxide) to photoresist must be sufficient to allow the etch process to etch a contact hole whose diameter is ten to fifteen times its depth, without appreciably disturbing the photoresist mask defining the hole. This task is made even more difficult because the recent trend toward shorter wavelength light for finer photolithography requires a thinner photoresist layer, so that the dielectric-to-photoresist etch selectivity must be greater than ever. This requirement is more readily met using processes having relatively low etch rates, such as dielectric etch processes employing a capacitively coupled plasma. The plasma density of a capacitively coupled plasma is relatively less than that of an inductively coupled plasma, and the capacitively coupled plasma etch process exhibits good dielectric-to-photoresist etch selectivity. The problem with the capacitively coupled process is that it is slow and therefore relatively less productive. Another problem that arises in such etch processes is non-uniform plasma distribution.

[0006] In order to increase productivity or etch rate, higher density plasmas have been employed. Typically, the high density plasma is an inductively coupled plasma. However, the process precursor gases tend to dissociate more rapidly in such a high density plasma, creating a higher plasma content of free fluorine, a species which reduces the etch selectivity to photoresist. To reduce this tendency, fluorocarbon process gases such as C4F8 or C4F6 are employed which dissociate in a plasma into fluorine-containing etchant species and one or more polymer species which tend to accumulate on non-oxide containing surfaces such as photoresist. This tends to increase etch selectivity. The oxygen in the oxygen-containing dielectric material promotes the pyrolization of the polymer over the dielectric, so that the polymer is removed, allowing the dielectric material to be etched while the non-oxygen containing material (e.g., the photoresist) continues to be covered by the polymer and therefore protected from the etchant. The problem is that the increase in contact opening depth and decrease in photoresist thickness to accommodate more advanced device designs has rendered the high density plasma process more likely to harm the photoresist layer during dielectric etching. As the plasma density is increased to improve etch rate, a more polymer-rich plasma must be used to protect the non-oxygen containing material such as photoresist, so that the rate of polymer removal from the oxygen-containing dielectric surfaces slows appreciably, particularly in small confined regions such as the bottom of a narrow contact opening. The result is that, while the photoresist may be adequately protected, the possibility is increased for the etch process to be blocked by polymer accumulation once a contact opening reaches a certain depth. Typically, the etch stop depth is less than the required depth of the contact opening so that the device fails. The contact opening may provide connection between an upper polysilicon or metal conductor layer and a lower polysilicon conductor or metal layer through an intermediate insulating silicon dioxide layer. Device failure occurs, for example, where the etch stop depth is less than the distance between the upper and lower polysilicon or metal layers. Alternatively, the possibility arises of the process window for achieving a high density plasma without etch stop becoming too narrow for practical or reliable application to the more advanced device designs such as those having contact openings with aspect ratios of 10:1 or 15:1.

[0007] What would be desirable at present is a reactor that has the etch rate of an inductively coupled plasma reactor (having a high density plasma) with the selectivity of a capacitively coupled reactor. It has been difficult to realize the advantages of both types of reactors in a single reactor.

[0008] One problem with high density inductively coupled plasma reactors, particularly of the type having an overhead coil antenna facing the wafer or workpiece, is that as the power applied to the coil antenna is increased to enhance the etch rate, the wafer-to-ceiling gap must be sufficiently large so that the power is absorbed in the plasma region well above the wafer. This avoids a risk of device damage on the wafer due to strong RF fields. Moreover, for high levels of RF power applied to the overhead coil antenna, the wafer-to-ceiling gap must be relatively large, and therefore the benefits of a small gap cannot be realized.

[0009] If the ceiling is a semiconductive window for the RF field of an inductively coupled reactor, or a conductive electrode of a capacitively coupled reactor, then one benefit of a small wafer-to-ceiling gap is an enhanced electric potential or ground reference that the ceiling could provide across the plane of the wafer at a relatively small gap distance (e.g., on the order of 1 or 2 inches).

[0010] Therefore, it would be desirable to have a reactor not only having the selectivity of a capacitively coupled reactor with the ion density and etch rate of an inductively coupled reactor, but further having none of the conventional limitations on the wafer-to-ceiling gap length other than a fundamental limit, such as the plasma sheath thickness, for example. It would further be desirable to have a reactor having the selectivity of a capacitively coupled reactor and the etch rate of an inductively coupled reactor in which the ion density and etch rate can be enhanced without necessarily increasing the applied RF plasma source power.

[0011] The foregoing needs and desires have been met by the torroidal plasma source of the above-referenced parent application. The torroidal plasma source of the parent application is described below with reference to FIGS. 1-43. Very briefly, it involves a plasma reactor chamber with at least one external reentrant tube extending between ports on opposite sides of the chamber, and an RF power applicator such as an RF-driven magnetic core encircling a portion of the external tube. RF plasma current flow circulates through the tube and across the space between the ports. This space is the process region overlying the surface of a workpiece or semiconductor wafer to be processed, the workpiece surface being generally parallel to the RF plasma current flow.

[0012] One limitation of such a torroidal source is that the plasma ion density over the center of the workpiece or wafer tends to be less than the remaining regions. What is needed is some way of enhancing plasma ion density over the center of the wafer. One way to enhance it might be to use an overhead solenoid or coil that produces a cusp-shaped field confining plasma near the center. However, such a solenoid or coil necessarily has a significant radius relative to the small size of the center region having inferior plasma ion density. Therefore, such an overhead coil or solenoid tends to exert magnetic pressure on the plasma at a significant radial distance from the center and therefore tends to have less or little effect at the center itself, and thus may offer no improvement. Thus, what is needed is a way of enhancing plasma density over the wafer center that is effective within a small radial area around the center.

[0013] Another problem with the torroidal plasma source is that zones of high plasma ion density (“hot spots”) tend to appear near the wafer edge adjacent the center of each of the ports connected to the external tube. Accordingly, a way of eliminating such hot spots is needed that is non-invasive—that is, a way that does not create other non-uniformities in plasma ion distribution while eliminating the hot spots.

[0014] A further problem with the torroidal plasma source is that at least some of the recirculating plasma current induced in the reentrant tube can pass around the processing region by diversion through the pumping annulus below the plane of the wafer or workpiece. Such diversion reduces plasma ion density over the wafer, and therefore process performance can be greatly improved if diversion of plasma current through the pumping annulus can be prevented.

[0015] Still another problem is that the magnetic core encircling a portion of the external reentrant tube tends to crack or shatter when a relatively high level of RF power (e.g., several kiloWatts) is applied. This problem appears to arise from non-uniform heating of the core to due fringing magnetic flux. Therefore, a way of providing uniform distribution of magnetic flux in the core is needed.

[0016] A plasma reactor is described that includes a vacuum chamber defined by an enclosure including a side wall and a workpiece support pedestal within the chamber defining a processing region overlying said pedestal. The chamber has at least a first pair of ports near opposing sides of said processing region and a first external reentrant tube is connected at respective ends thereof to the pair of ports. The reactor further includes a process gas injection apparatus (such as a gas distribution plate) and an RF power applicator coupled to the reentrant tube for applying plasma source power to process gases within the tube to produce a reentrant torroidal plasma current through the first tube and across said processing region. A magnet controls radial distribution of plasma ion density in the processing region, the magnet having an elongated pole piece defining a pole piece axis intersecting the processing region.

[0017] While the pole piece can be a permanent magnet, the pole piece can instead be a magnetizable material, the magnet further including an electrical conductor wound around the elongate pole piece to form a coil whereby current flow through the coil produces a magnetic field.

[0018] The chamber enclosure can include a ceiling overlying and facing the workpiece support pedestal, the pole piece axis being external of and above the ceiling and optionally extending at least partially into the ceiling and terminating outside of said chamber. The processing region is defined between parallel planes of the pedestal and ceiling, the pole piece axis being generally perpendicular to the workpiece support. In this case, the ceiling can include a gas distribution plate.

[0019] The reactor can further include a second pair of ports near opposing sides of said processing region, a second external reentrant tube connected at respective ends thereof to the second pair of ports and overlying a portion of the first external reentrant tube, and a second RF power applicator coupled to the second reentrant tube for applying plasma source power to process gases within said second tube to produce a second reentrant torroidal plasma current through the second tube and across the processing region. The first and second tubes can cross one another at an angle such as a right angle.

Overview of the Plasma Reactor Chamber:

[0086] Referring to FIG. 1, a plasma reactor chamber 100 enclosed by a cylindrical side wall 105 and a ceiling 110 houses a wafer pedestal 115 for supporting a semiconductor wafer or workpiece 120. A process gas supply 125 furnishes process gas into the chamber 100 through gas inlet nozzles 130a-130d extending through the side wall 105. A vacuum pump 135 controls the pressure within the chamber 100, typically holding the pressure below 0.5 milliTorr (mT). A half-torroidal hollow tube enclosure or conduit 150 extends above the ceiling 110 in a half circle. The conduit 150, although extending externally outwardly from ceiling 110, is nevertheless part of the reactor and forms a wall of the chamber. Internally it shares the same evacuated atmosphere as exists elsewhere in the reactor. In fact, the vacuum pump 135, instead of being coupled to the bottom of the main part of the chamber as illustrated in FIG. 1, may instead be coupled to the conduit 150. The conduit 150 has one open end 150a sealed around a first opening 155 in the reactor ceiling 110 and its other end 150b sealed around a second opening 160 in the reactor ceiling 110. The two openings or ports 150, 160 are located on generally opposite sides of the wafer support pedestal 115. The hollow conduit 150 is reentrant in that it provides a flow path which exits the main portion of the chamber at one opening and re-enters at the other opening. In this specification, the conduit 150 may be described as being half-torroidal, in that the conduit is hollow and provides a portion of a closed path in which plasma may flow, the entire path being completed by flowing across the entire process region overlying the wafer support pedestal 115. Notwithstanding the use of the term torroidal, the trajectory of the path as well as the cross-sectional shape of the path or conduit 150 may be circular or non-circular, and may be square, rectangular or any other shape either a regular shape or irregular.

[0087] The external conduit 150 may be formed of a relatively thin conductor such as sheet metal, but sufficiently strong to withstand the vacuum within the chamber. In order to suppress eddy currents in the sheet metal of the hollow conduit 150 (and thereby facilitate coupling of an RF inductive field into the interior of the conduit 150), an insulating gap 152 extends across and through the hollow conduit 150 so as to separate it into two tubular sections. The gap 152 is filled by a ring 154 of insulating material such as a ceramic in lieu of the sheet metal skin, so that the gap is vacuum tight. A second insulating gap 153 may be provided, so that one section of the conduit 150 is electrically floating. A bias RF generator 162 applies RF bias power to the wafer pedestal 115 and wafer 120 through an impedance match element 164.

[0088] Alternatively, the hollow conduit 150 may be formed of a machined metal, such as aluminum or aluminum alloy. Passages for liquid cooling or heating may be incorporated in the walls of the hollow conduit.

[0089] Alternatively, the hollow conduit 150 may be formed of a non-conductive material instead of the conductive sheet metal. The non-conductive material may be a ceramic, for example. In such an alternative case, neither gap 152 or 153 is required.

[0090] An antenna 170 such as a winding or coil 165 disposed on one side of the hollow conduit 150 and wound around an axis parallel to the axis of symmetry of the half-torroidal tube is connected through an impedance match element 175 to an RF power source 180. The antenna 170 may further include a second winding 185 disposed on the opposite side of the hollow conduit 150 and wound in the same direction as the first winding 165 so that the magnetic fields from both windings add constructively.

[0091] Process gases from the chamber 100 fill the hollow conduit 150. In addition, a separate process gas supply 190 may supply process gases directly in to the hollow conduit 150 through a gas inlet 195. The RF field in the external hollow conduit 150 ionizes the gases in the tube to produce a plasma. The RF field induced by the circular coil antenna 170 is such that the plasma formed in the tube 150 reaches through the region between the wafer 120 and the ceiling 110 to complete a torroidal path that includes the half-torroidal hollow conduit 150. As employed herein, the term torroidal refers to the closed and solid nature of the path, but does not refer or limit its cross-sectional shape or trajectory, either of which may be circular or non-circular or square or otherwise. Plasma circulates through the complete torroidal path or region which may be thought of as a closed plasma circuit. The torroidal region extends across the diameter of the wafer 120 and, in certain cases, has a sufficient width in the plane of the wafer so that it overlies the entire wafer surface.

[0092] The RF inductive field from the coil antenna 170 includes a magnetic field which itself is closed (as are all magnetic fields), and therefore induces a plasma current along the closed torroidal path described here. It is believed that power from the RF inductive field is absorbed at generally every location along the closed path, so that plasma ions are generated all along the path. The RF power absorption and rate of plasma ion generation may vary among different locations along the closed path depending upon a number of factors. However, the current is generally uniform along the closed path length, although the current density may vary. This current alternates at the frequency of the RF signal applied to the antenna 170. However, since the current induced by the RF magnetic field is closed, the current must be conserved around the circuit of the closed path, so that the amount of current flowing in any portion of the closed path is generally the same as in any other portion of the path. As will be described below, this fact is exploited in the invention to great advantage.

[0093] The closed torroidal path through which the plasma current flows is bounded by plasma sheaths formed at the various conductive surfaces bounding the path. These conductive surfaces include the sheet metal of the hollow conduit 150, the wafer (and/or the wafer support pedestal) and the ceiling overlying the wafer. The plasma sheaths formed on these conductive surfaces are charge-depleted regions produced as the result of the charge imbalance due to the greater mobility of the low-mass negative electrons and the lesser mobility of the heavy-mass positive ions. Such a plasma sheath has an electric field perpendicular to the local surface underlying the sheath. Thus, the RF plasma current that passes through the process region overlying the wafer is constricted by and passes between the two electric fields perpendicular to the surface of the ceiling facing the wafer and the surface of the wafer facing the gas distribution plate. The thickness of the sheath (with RF bias applied to the workpiece or other electrode) is greater where the electric field is concentrated over a small area, such as the wafer, and is less in other locations such as the sheath covering the ceiling and the large adjoining chamber wall surfaces. Thus, the plasma sheath overlying the wafer is much thicker. The electric fields of the wafer and ceiling/gas distribution plate sheaths are generally parallel to each other and perpendicular to the direction of the RE plasma current flow in the process region.

[0094] When RE power is first applied to the coil antenna 170, a discharge occurs across the gap 152 to ignite a capacitively coupled plasma from gases within the hollow conduit 150. Thereafter, as the plasma current through the hollow conduit 150 increases, the inductive coupling of the RE field becomes more dominant so that the plasma becomes an inductively coupled plasma. Alternatively, plasma may be initiated by other means, such as by RF bias applied to the workpiece support or other electrode.

[0095] In order to avoid edge effects at the wafer periphery, the ports 150, 160 are separated by a distance that exceeds the diameter of the wafer. For example, for a 12 inch diameter wafer, the ports 150, 160 are about 16 to 22 inches apart. For an 8 inch diameter wafer, the ports 150, 160 are about 10 to 16 inches apart.

Advantages of the Invention:

[0096] A significant advantage is that power from the RE inductive field is absorbed throughout the relatively long closed torroidal path (i.e., long relative to the gap length between the wafer and the reactor ceiling), so that RE power absorption is distributed over a large area. As a result, the RE power in the vicinity of the wafer-to-ceiling gap (i.e., the process region 121 best shown in FIG. 2, not to be confused with the insulating gap 152) is relatively low, thus reducing the likelihood of device damage from RE fields. In contrast, in prior inductively coupled reactors, all of the RF power is absorbed within the narrow wafer-to-ceiling gap, so that it is greatly concentrated in that region. Moreover, this fact often limits the ability to narrow the wafer-to-ceiling gap (in the quest of other advantages) or, alternatively, requires greater concentration of RF power in the region of the wafer. Thus, the invention overcomes a limitation of long standing in the art. This aspect enhances process performance by reducing residency time of the reactive gases through a dramatic reduction in volume of the process region or process zone overlying the wafer, as discussed previously herein.

[0097] A related and even more important advantage is that the plasma density at the wafer surface can be dramatically increased without increasing the RF power applied to the coil antenna 170 (leading to greater efficiency). This is accomplished by reducing the cross-sectional area of the torroidal path in the vicinity of the pedestal surface and wafer 120 relative to the remainder of the torroidal path. By so constricting the torroidal path of the plasma current near the wafer only, the density of the plasma near the wafer surface is increased proportionately. This is because the torroidal path plasma current through the hollow conduit 150 must be at least nearly the same as the plasma current through the pedestal-to-ceiling (wafer-to-ceiling) gap.

[0098] A significant difference over the prior art is that not only is the RF field remote from the workpiece, and not only can ion density be increased at the wafer surface without increasing the applied RF field, but the plasma ion density and/or the applied RF field may be increased without increasing the minimum wafer-to-ceiling gap length. Formerly, such an increase in plasma density necessitated an increase in the wafer-to-ceiling gap to avoid strong fields at the wafer surface. In contrast, in the present invention the enhanced plasma density is realized without requiring any increase in the wafer-to-ceiling gap to avoid a concomitant increase in RF magnetic fields at the wafer surface. This is because the RF field is applied remotely from the wafer and moreover need not be increased to realize an increase in plasma density at the wafer surface. As a result, the wafer-to-ceiling gap can be reduced down to a fundamental limit to achieve numerous advantages. For example, if the ceiling surface above the wafer is conductive, then reducing the wafer-to-ceiling gap improves the electrical or ground reference provided by the conductive ceiling surface. A fundamental limit on the minimum wafer-to-ceiling gap length is the sum of the thicknesses of the plasma sheaths on the wafer surface and on the ceiling surface.

[0099] A further advantage of the invention is that because the RF inductive field is applied along the entire torroidal path of the RF plasma current (so that its absorption is distributed as discussed above), the chamber ceiling 110, unlike with most other inductively powered reactors, need not function as a window to an inductive field and therefore may be formed of any desired material, such as a highly conductive and thick metal, and therefore may comprise a conductive gas distribution plate as will be described below, for example. As a result, the ceiling 110 readily provides a reliable electric potential or ground reference across the entire plane of the pedestal or wafer 120.

Increasing the Plasma Ion Density:

[0100] One way of realizing higher plasma density near the wafer surface by reducing plasma path cross-sectional area over the wafer is to reduce the wafer-to-ceiling gap length. This may be accomplished by simply reducing the ceiling height or by introducing a conductive gas distribution plate or showerhead over the wafer, as illustrated in FIG. 2. The gas distribution showerhead 210 of FIG. 2 consists of a gas distribution plenum 220 connected to the gas supply 125 and communicating with the process region over the wafer 120 through plural gas nozzle openings 230. The advantage of the conductive showerhead 210 is two-fold: First, by virtue of its close location to the wafer, it constricts the plasma path over the wafer surface and thereby increases the density of the plasma current in that vicinity. Second, it provides a uniform electrical potential reference or ground plane close to and across the entire wafer surface.

[0101] In order to avoid arcing across the openings 230, each opening 230 may be relatively small, on the order of a millimeter (e.g., hole diameter is approximately 0.5 mm). The spacing between adjacent openings may be on the order of a several millimeters.

[0102] The conductive showerhead 210 constricts the plasma current path rather than providing a short circuit through itself because a plasma sheath is formed around the portion of the showerhead surface immersed in the plasma. The sheath has a greater impedance to the plasma current than the space between the wafer 120 and the showerhead 210, and therefore all the plasma current goes around the conductive showerhead 210.

[0103] It is not necessary to employ a showerhead (e.g., the showerhead 210) in order to constrict the torroidal plasma current or path in the vicinity of the process region overlying the wafer. The path constriction and consequent increase in plasma ion density in the process region may be achieved without the showerhead 210 by similarly reducing the wafer-to-ceiling height. If the showerhead 210 is eliminated in this manner, then the process gases may be supplied into the chamber interior by means of conventional gas inlet nozzles (not shown).

[0104] One advantage of the showerhead 210 is that different mixtures of reactive and inert process gas ratios may be introduced through different orifices 230 at different radii, in order to finely adjust the uniformity of plasma effects on photoresist, for example. Thus, for example, a greater proportion of inert gas to reactive gas may be supplied to the orifices 230 lying outside a median radius while a greater proportion of reactive gas to inert gas may be supplied to the orifices 230 within that median radius.

[0105] As will be described below, another way in which the torroidal plasma current path may be constricted in the process region overlying the wafer (in order to increase plasma ion density over the wafer) is to increase the plasma sheath thickness on the wafer by increasing the RF bias power applied to the wafer support pedestal. Since as described previously the plasma current across the process region is confined between the plasma sheath at the wafer surface and the plasma sheath at the ceiling (or showerhead) surface, increasing the plasma sheath thickness at the wafer surface necessarily decreases the cross-section of the portion of the torroidal plasma current within process region, thereby increasing the plasma ion density in the process region. Thus, as will be described more fully later in this specification, as RF bias power on the wafer support pedestal is increased, plasma ion density near the wafer surface is increased accordingly.

High Etch Selectivity at High Etch Rates:

[0106] The invention solves the problem of poor etch selectivity which sometimes occurs with a high density plasma. The reactor of FIGS. 1 and 2 has a silicon dioxide-to-photoresist etch selectivity as high as that of a capacitively coupled plasma reactor (about 7:1) while providing high etch rates approaching that of a high density inductively coupled plasma reactor. It is believed that the reason for this success is that the reactor structure of FIGS. 1 and 2 reduces the degree of dissociation of the reactive process gas, typically a fluorocarbon gas, so as to reduce the incidence of free fluorine in the plasma region over the wafer 120. Thus, the proportion of free fluorine in the plasma relative to other species dissociated from the fluorocarbon gas is desirably reduced. Such other species include the protective carbon-rich polymer precursor species formed in the plasma from the fluorocarbon process gas and deposited on the photoresist as a protective polymer coating. They further include less reactive etchant species such as CF and CF₂ formed in the plasma from the fluorocarbon process gas. Free fluorine tends to attack photoresist and the protective polymer coating formed thereover as vigorously as it attacks silicon dioxide, thus reducing oxide-to-photoresist etch selectivity. On the other hand, the less reactive etch species such as CF₂ or CF tend to attack photoresist and the protective polymer coating formed thereover more slowly and therefore provide superior etch selectivity.

[0107] It is believed that the reduction in the dissociation of the plasma species to free fluorine is accomplished in the invention by reducing the residency time of the reactive gas in the plasma. This is because the more complex species initially dissociated in the plasma from the fluorocarbon process gas, such as CF₂ and CF are themselves ultimately dissociated into simpler species including free fluorine, the extent of this final step of dissociation depending upon the residency time of the gas in the plasma. The term “residency time” or “residence time” as employed in this specification corresponds generally to the average time that a process gas molecule and the species dissociated from the that molecule are present in the process region overlying the workpiece or wafer. This time or duration extends from the initial injection of the molecule into the process region until the molecule and/or its dissociated progeny are pass out of the process region along the closed torroidal path described above that extends through the processing zone.

[0108] As stated above, the invention enhances etch selectivity by reducing the residency time in the process region of the fluorocarbon process gas. The reduction in residency time is achieved by constricting the plasma volume between the wafer 120 and the ceiling 110.

[0109] The reduction in the wafer-to-ceiling gap or volume has certain beneficial effects. First, it increases plasma density over the wafer, enhancing etch rate. Second, residency time falls as the volume is decreased. As referred to above, the small volume is made possible in the present invention because, unlike conventional inductively coupled reactors, the RF source power is not deposited within the confines of the process region overlying the wafer but rather power deposition is distributed along the entire closed torroidal path of the plasma current. Therefore, the wafer-to-ceiling gap can be less than a skin depth of the RF inductive field, and in fact can be so small as to significantly reduce the residency time of the reactive gases introduced into the process region, a significant advantage.

[0110] There are two ways of reducing the plasma path cross-section and therefore the volume over the wafer 120. One is to reduce the wafer-to-showerhead gap distance. The other is to increase the plasma sheath thickness over the wafer by increasing the bias RF power applied to the wafer pedestal 115 by the RF bias power generator 162, as briefly mentioned above. Either method results in a reduction in free fluorine content of the plasma in the vicinity of the wafer 120 (and consequent increase in dielectric-to-photoresist etch selectivity) as observed using optical emission spectroscopy (OES) techniques.

[0111] There are three additional methods of the invention for reducing free fluorine content to improve etch selectivity. One method is to introduce a non-chemically reactive diluent gas such as argon into the plasma. The argon gas may be introduced outside and above the process region by injecting it directly into the hollow conduit 150 from the second process gas supply 190, while the chemically reactive process gases (fluorocarbon gases) enter the chamber only through the showerhead 210. With this advantageous arrangement, the argon ions, neutrals, and excited neutrals propagate within the torroidal path plasma current and through the process region across the wafer surface to dilute the newly introduced reactive (e.g., fluorocarbon) gases and thereby effectively reduce their residency time over the wafer. Another method of reducing plasma free fluorine content is to reduce the chamber pressure. A further method is to reduce the RF source power applied to the coil antenna 170.

[0112] FIG. 3 is a graph illustrating a trend observed in the invention in which the free fluorine content of the plasma decreases as the wafer-to-showerhead gap distance is decreased. FIG. 4 is a graph illustrating that the free fluorine content of the plasma is decreased by decreasing the plasma bias power applied to the wafer pedestal 115. FIG. 5 is a graph illustrating that plasma free fluorine content is reduced by reducing the RF source power applied to the coil antenna 170. FIG. 6 is a graph illustrating that the free fluorine content is reduced by reducing chamber pressure. FIG. 7 is a graph illustrating that plasma free fluorine content is reduced by increasing the diluent (Argon gas) flow rate into the tubular enclosure 150. The graphs of FIGS. 3-7 are merely illustrative of plasma behavioral trends inferred from numerous OES observations and do not depict actual data.

Wide Process Window of the Invention:

[0113] The chamber pressure is generally less than 0.5 T and can be as low as 1 mT. The process gas may be C₄F₈ injected into the chamber 100 through the gas distribution showerhead at a flow rate of about 15 cc/m with 150 cc/m of Argon, with the chamber pressure being maintained at about 20 mT. Alternatively, the Argon gas flow rate may be increased to 650 cc/m and the chamber pressure to 60 mT. The antenna 170 may be excited with about 500 Watts of RF power at 13 MHz. The wafer-to-showerhead gap may be about 0.3 inches to 2 inches. The bias RF power applied to the wafer pedestal may be 13 MHz at 2000 Watts. Other selections of frequency may be made. The source power applied to the coil antenna 170 may be as low as 50 KHz or as high as several times 13 MHz or higher. The same is true of the bias power applied to the wafer pedestal.

[0114] The process window for the reactor of FIGS. 1 and 2 is far wider than the process window for a conventional inductively coupled reactor. This is illustrated in the graph of FIG. 8, showing the specific neutral flux of free fluorine as a function of RF source power for a conventional inductive reactor and for the reactor of FIGS. 1 and 2. For the conventional inductively coupled reactor, FIG. 8 shows that the free fluorine specific flux begins to rapidly increase as the source power exceeds between 50 and 100 Watts. In contrast, the reactor of FIGS. 1 and 2 can accept source power levels approaching 1000 Watts before the free fluorine specific flux begins to increase rapidly. Therefore, the source power process window in the invention is nearly an order of magnitude wider than that of a conventional inductively coupled reactor, a significant advantage.

Dual Advantages of the Invention:

[0115] The constriction of the torroidal plasma current path in the vicinity of the wafer or workpiece produces two independent advantages without any significant tradeoffs of other performance criteria: (1) the plasma density over the wafer is increased without requiring any increase in plasma source power, and (2) the etch selectivity to photoresist or other materials is increased, as explained above. It is believed that in prior plasma reactors it has been impractical if not impossible to increase the plasma ion density by the same step that increases etch selectivity. Thus, the dual advantages realized with the torroidal plasma source of the present invention appear to be a revolutionary departure from the prior art.

Other Embodiments:

[0116] FIG. 9 illustrates a modification of the case of FIG. 1 in which the side antenna 170 is replaced by a smaller antenna 910 that fits inside the empty space between the ceiling 110 and the hollow conduit 150. The antenna 910 is a single coil winding centered with respect to the hollow conduit 150.

[0117] FIGS. 10 and 11 illustrate how the case of FIG. 1 may be enhanced by the addition of a closed magnetically permeable core 1015 that extends through the space between the ceiling 110 and the hollow conduit 150. The core 1015 improves the inductive coupling from the antenna 170 to the plasma inside the hollow conduit 150.

[0118] Impedance match may be achieved without the impedance match circuit 175 by using, instead, a secondary winding 1120 around the core 1015 connected across a tuning capacitor 1130. The capacitance of the tuning capacitor 1130 is selected to resonate the secondary winding 1120 at the frequency of the RF power source 180. For a fixed tuning capacitor 1130, dynamic impedance matching may be provided by frequency tuning and/or by forward power servoing.

[0119] FIG. 12 illustrates a case of the invention in which a hollow tube enclosure 1250 extends around the bottom of the reactor and communicates with the interior of the chamber through a pair of openings 1260, 1265 in the bottom floor of the chamber. A coil antenna 1270 follows along side the torroidal path provided by the hollow tube enclosure 1250 in the manner of the case of FIG. 1. While FIG. 12 shows the vacuum pump 135 coupled to the bottom of the main chamber, it may just as well be coupled instead to the underlying conduit 1250.

[0120] FIG. 13 illustrates a variation of the case of FIGS. 10 and 11, in which the antenna 170 is replaced by an inductive winding 1320 surrounding an upper section of the core 1015. Conveniently, the winding 1320 surrounds a section of the core 1015 that is above the conduit 150 (rather than below it). However, the winding 1320 can surround any section of the core 1015.

[0121] FIG. 14 illustrates an extension of the concept of FIG. 13 in which a second hollow tube enclosure 1450 runs parallel to the first hollow conduit 150 and provides a parallel torroidal path for a second torroidal plasma current. The tube enclosure 1450 communicates with the chamber interior at each of its ends through respective openings in the ceiling 110. A magnetic core 1470 extends under both tube enclosures 150, 1450 and through the coil antenna 170.

[0122] FIG. 15 illustrates an extension of the concept of FIG. 14 in which an array of parallel hollow tube enclosures 1250a, 1250b, 1250c, 1250d provide plural torroidal plasma current paths through the reactor chamber. In the case of FIG. 15, the plasma ion density is controlled independently in each individual hollow conduit 1250a-d by an individual coil antenna 170a-d, respectively, driven by an independent RF power source 180a-d, respectively. Individual cylindrical open cores 1520a-1520d may be separately inserted within the respective coil antennas 170a-d. In this case, the relative center-to-edge ion density distribution may be adjusted by separately adjusting the power levels of the individual RF power sources 180a-d.

[0123] FIG. 16 illustrates a modification of the case of FIG. 15 in which the array of tube enclosures 1250a-d extend through the side wall of the reactor rather than through the ceiling 110. Another modification illustrated in FIG. 16 is the use of a single common magnetic core 1470 adjacent all of the tube enclosures 1250a-d and having the antenna 170 wrapped around it so that a single RF source excites the plasma in all of the tube enclosures 1250a-d.

[0124] FIG. 17A illustrates a pair of orthogonal tube enclosures 150-1 and 150-2 extending through respective ports in the ceiling 110 and excited by respective coil antennas 170-1 and 170-2. Individual cores 1015-1 and 1015-2 are within the respective coil antennas 170-1 and 170-2. This case creates two mutually orthogonal torroidal plasma current paths over the wafer 120 for enhanced uniformity. The two orthogonal torroidal or closed paths are separate and independently powered as illustrated, but intersect in the process region overlying the wafer, and otherwise do not interact. In order to assure separate control of the plasma source power applied to each one of the orthogonal paths, the frequency of the respective RF generators 180a, 180b of FIG. 17 are different, so that the operation of the impedance match circuits 175a, 175b is decoupled. For example, the RF generator 180a may produce an RF signal at 11 MHz while the RF generator 180b may produce an RF signal at 12 MHz. Alternatively, independent operation may be achieved by offsetting the phases of the two RF generators 180a, 180b.

[0125] FIG. 17B illustrates how radial vanes 181 may be employed to guide the torroidal plasma currents of each of the two conduits 150-1, 150-2 through the processing region overlying the wafer support. The radial vanes 181 extend between the openings of each conduit near the sides of the chamber up to the edge of the wafer support. The radial vanes 181 prevent diversion of plasma from one torroidal path to the other torroidal path, so that the two plasma currents only intersect within the processing region overlying the wafer support.

Cases Suitable for Large Diameter Wafers:

[0126] In addition to the recent industry trends toward smaller device sizes and higher device densities, another trend is toward greater wafer diameters. For example, 12-inch diameter wafers are currently entering production, and perhaps larger diameter wafers will be in the future. The advantage is greater throughput because of the large number of integrated circuit die per wafer. The disadvantage is that in plasma processing it is more difficult to maintain a uniform plasma across a large diameter wafer. The following cases of the present invention are particularly adapted for providing a uniform plasma ion density distribution across the entire surface of a large diameter wafer, such as a 12-inch diameter wafer.

[0127] FIGS. 18 and 19 illustrate a hollow tube enclosure 1810 which is a wide flattened rectangular version 1850 of the hollow conduit 150 of FIG. 1 that includes an insulating gap 1852. This version produces a wide “belt” of plasma that is better suited for uniformly covering a large diameter wafer such as a 12-inch diameter wafer or workpiece. The width W of the tube enclosure and of the pair of openings 1860, 1862 in the ceiling 110 may exceed the wafer by about 5% or more. For example, if the wafer diameter is 10 inches, then the width W of the rectangular tube enclosure 1850 and of the openings 1860, 1862 is about 11 inches. FIG. 20 illustrates a modified version 1850′ of the rectangular tube enclosure 1850 of FIGS. 18 and 19 in which a portion 1864 of the exterior tube enclosure 1850 is constricted.

[0128] FIG. 20 further illustrates the optional use of focusing magnets 1870 at the transitions between the constricted and unconstricted portions of the enclosure 1850. The focusing magnets 1870 promote a better movement of the plasma between the constricted and unconstricted portions of the enclosure 1850, and specifically promote a more uniform spreading out of the plasma as it moves across the transition between the constricted portion 1864 and the unconstricted portion of the tube enclosure 1850.

[0129] FIG. 21 illustrates how plural cylindrical magnetic cores 2110 may be inserted through the exterior region 2120 circumscribed by the tube enclosure 1850. The cylindrical cores 2110 are generally parallel to the axis of symmetry of the tube enclosure 1850. FIG. 22 illustrates a modification of the case of FIG. 21 in which the cores 2110 extend completely through the exterior region 2120 surrounded by the tube enclosure 1850 are replaced by pairs of shortened cores 2210, 2220 in respective halves of the exterior region 2120. The side coils 165, 185 are replaced by a pair of coil windings 2230, 2240 surrounding the respective core pairs 2210, 2220. In this case, the displacement D between the core pairs 2210, 2220 may be changed to adjust the ion density near the wafer center relative to the ion density at the wafer circumference. A wider displacement D reduces the inductive coupling near the wafer center and therefore reduces the plasma ion density at the wafer center. Thus, an additional control element is provided for precisely adjusting ion density spatial distribution across the wafer surface. FIG. 23 illustrates a variation of the case of FIG. 22 in which the separate windings 2230, 2240 are replaced by a single center winding 2310 centered with respect to the core pairs 2210, 2220.

[0130] FIGS. 24 and 25 illustrate a case providing even greater uniformity of plasma ion density distribution across the wafer surface. In the case of FIGS. 24 and 25, two torroidal plasma current paths are established that are transverse to one another and are mutually orthogonal. This is accomplished by providing a second wide rectangular hollow enclosure 2420 extending, transversely and orthogonally relative to the first tube enclosure 1850. The second tube enclosure 2420 communicates with the chamber interior through a pair of openings 2430, 2440 through the ceiling 110 and includes an insulating gap 2452. A pair of side coil windings 2450, 2460 along the sides of the second tube enclosure 2420 maintain a plasma therein and are driven by a second RF power supply 2470 through an impedance match circuit 2480. As indicated in FIG. 24, the two orthogonal plasma currents coincide over the wafer surface and provide more uniform coverage of plasma over the wafer surface. This case is expected to find particularly advantageous use for processing large wafers of diameters of 10 inches and greater.

[0131] As in the case of FIG. 17, the case of FIG. 24 creates two mutually orthogonal torroidal plasma current paths over the wafer 120 for enhanced uniformity. The two orthogonal torroidal or closed paths are separate and independently powered as illustrated, but intersect in the process region overlying the wafer, and otherwise do not interact or otherwise divert or diffuse one another. In order to assure separate control of the plasma source power applied to each one of the orthogonal paths, the frequency of the respective RF generators 180, 2470 of FIG. 24 are different, so that the operation of the impedance match circuits 175, 2480 is decoupled. For example, the RF generator 180 may produce an RF signal at 11 MHz while the RF generator 2470 may produce an RF signal at 12 MHz. Alternatively, independent operation may be achieved by offsetting the phases of the two RF generators 180, 2470.

[0132] FIG. 26 illustrates a variation of the case of FIG. 18 in which a modified rectangular enclosure 2650 that includes an insulating gap 2658 communicates with the chamber interior through the chamber side wall 105 rather than through the ceiling 110. For this purpose, the rectangular enclosure 2650 has a horizontal top section 2652, a pair of downwardly extending legs 2654 at respective ends of the top section 2652 and a pair of horizontal inwardly extending legs 2656 each extending from the bottom end of a respective one of the downwardly extending legs 2654 to a respective opening 2670, 2680 in the side wall 105.

[0133] FIG. 27 illustrates how a second rectangular tube enclosure 2710 including an insulating gap 2752 may be added to the case of FIG. 26, the second tube enclosure 2710 being identical to the rectangular tube enclosure 2650 of FIG. 26 except that the rectangular tube enclosures 2650, 2710 are mutually orthogonal (or at least transverse to one another). The second rectangular tube enclosure communicates with the chamber interior through respective openings through the side wall 105, including the opening 2720. Like the case of FIG. 25, the tube enclosures 2650 and 2710 produce mutually orthogonal torroidal plasma currents that coincide over the wafer surface to provide superior uniformity over a broader wafer diameter. Plasma source power is applied to the interior of the tube enclosures through the respective pairs of side coil windings 165, 185 and 2450, 2460.

[0134] FIG. 28A illustrates how the side coils 165, 185, 2450, 2460 may be replaced (or supplemented) by a pair of mutually orthogonal interior coils 2820, 2840 lying within the external region 2860 surrounded by the two rectangular tube enclosures 2650, 2710. Each one of the coils 2820, 2840 produces the torroidal plasma current in a corresponding one of the rectangular tube enclosures 2650, 2710. The coils 2820, 2840 may be driven completely independently at different frequencies or at the same frequency with the same or a different phase. Or, they may be driven at the same frequency but with a phase difference (i.e., 90 degrees) that causes the combined torroidal plasma current to rotate at the source power frequency. In this case the coils 2820, 2840 are driven with the sin and cosine components, respectively, of a common signal generator 2880, as indicated in FIG. 28A. The advantage is that the plasma current path rotates azimuthally across the wafer surface at a rotational frequency that exceeds the plasma ion frequency so that non-uniformities are better suppressed than in prior art methods such as MERIE reactors in which the rotation is at a much lower frequency.

[0135] Referring now to FIG. 28B, radial adjustment of plasma ion density may be generally provided by provision of a pair of magnetic cylindrical cores 2892, 2894 that may be axially moved toward or away from one another within the coil 2820 and a pair of magnetic cylindrical cores 2896, 2898 that may be axially moved toward or away from one another within the coil 2840. As each pair of cores is moved toward one another, inductive coupling near the center of each of the orthogonal plasma currents is enhanced relative to the edge of the current, so that plasma density at the wafer center is generally enhanced. Thus, the center-to-edge plasma ion density may be controlled by moving the cores 2892, 2894, 2896, 2898.

[0136] FIG. 29 illustrates an alternative case of the invention in which the two tube enclosures 2650, 2710 are merged together into a single enclosure 2910 that extends 360 degrees around the center axis of the reactor that constitutes a single plenum. In the case of FIG. 29, the plenum 2910 has a half-dome lower wall 2920 and a half-dome upper wall 2930 generally congruent with the lower wall 2920. The plenum 2910 is therefore the space between the upper and lower half-dome walls 2920, 2930. An insulating gap 2921 may extend around the upper dome wall 2920 and/or an insulating gap 2931 may extend around the lower dome wall 2930. The plenum 2910 communicates with the chamber interior through an annular opening 2925 in the ceiling 110 that extends 360 degrees around the axis of symmetry of the chamber.

[0137] The plenum 2910 completely encloses a region 2950 above the ceiling 110. In the case of FIG. 29, plasma source power is coupled into the interior of the plenum 2910 by a pair of mutually orthogonal coils 2960, 2965. Access to the coils 2960, 2965 is provided through a vertical conduit 2980 passing through the center of the plenum 2910. Preferably, the coils 2960, 2965 are driven in quadrature as in the case of FIG. 28 to achieve an azimuthally circulating torroidal plasma current (i.e., a plasma current circulating within the plane of the wafer. The rotation frequency is the frequency of the applied RF power. Alternatively, the coils 2960, 2965 may be driven separately at different frequencies. FIG. 30 is a top sectional view of the case of FIG. 29. FIGS. 31A and 31B are front and side sectional views, respectively, corresponding to FIG. 30.

[0138] The pair of mutually orthogonal coils 2960, 2965 may be replaced by any number n of separately driven coils with their winding axes disposed at 360/n degrees apart. For example, FIG. 32 illustrates the case where the two coils 2960, 2965 are replace by three coils 3210, 3220, 3230 with winding axes placed at 120 degree intervals and driven by three respective RF supplies 3240, 3250, 3260 through respective impedance match circuits 3241, 3251, 3261. In order to produce a rotating torroidal plasma current, the three windings 3210, 3220, 3230 are driven 120 degrees out of phase from a common power source 3310 as illustrated in FIG. 33. The cases of FIGS. 32 and 33 are preferred over the case of FIG. 29 having only two coils, since it is felt much of the mutual coupling between coils would be around rather than through the vertical conduit 2980.

[0139] FIG. 34 illustrates a case in which the three coils are outside of the enclosed region 2950, while their inductances are coupled into the enclosed region 2950 by respective vertical magnetic cores 3410 extending through the conduit 2980. Each core 3410 has one end extending above the conduit 2980 around which a respective one of the coils 3210, 3220, 3230 is wound. The bottom of each core 3410 is inside the enclosed region 2950 and has a horizontal leg. The horizontal legs of the three cores 3410 are oriented at 120 degree intervals to provide inductive coupling to the interior of the plenum 2910 similar to that provided by the three coils inside the enclosed region as in FIG. 32.

[0140] The advantage of the flattened rectangular tube enclosures of the cases of FIGS. 18-28 is that the broad width and relatively low height of the tube enclosure forces the torroidal plasma current to be a wide thin belt of plasma that more readily covers the entire surface of a large diameter wafer. The entirety of the tube enclosure need not be of the maximum width. Instead the outer section of the tube enclosure farthest from the chamber interior may be necked down, as discussed above with reference to the case of FIG. 20. In this case, it is preferable to provide focusing magnets 1870 at the transition corners between the wide portion 1851 and the narrow section 1852 to force the plasma current exiting the narrow portion 1852 to spread entirely across the entire width of the wide section 1851. If it is desired to maximize plasma ion density at the wafer surface, then it is preferred that the cross-sectional area of the narrow portion 1852 be at least nearly as great as the cross-sectional area of the wide portion 1851. For example, the narrow portion 1852 may be a passageway whose height and width are about the same while the wide portion 1851 may have a height that is less than its width.

[0141] The various cases described herein with air-core coils (i.e., coils without a magnetic core) may instead employ magnetic-cores, which can be the open-magnetic-path type or the closed-magnetic-core type illustrated in the accompanying drawings. Furthermore, the various cases described herein having two or more torroidal paths driven with different RF frequencies may instead be driven with same frequency, and with the same or different phases.

[0142] FIG. 35 is a version of the case of FIG. 17 in which the mutually transverse hollow conduits are narrowed as in the case of FIG. 20.

[0143] FIG. 36 is a version of the case of FIG. 24 but employing a pair of magnetic cores 3610, 3620 with respective windings 3630, 3640 therearound for connection to respective RF power sources.

[0144] FIG. 37 is a case corresponding to that of FIG. 35 but having three instead of two reentrant conduits with a total of six reentrant ports to the chamber. Having a number of symmetrically disposed conduits and reentrant ports greater than two (as in the case of FIG. 37) is believed to be particularly advantageous for processing wafers of diameter of 300 mm and greater.

[0145] FIG. 38 is a case corresponding to that of FIG. 38 but having three instead of two reentrant conduits with a total of six reentrant ports to the chamber.

[0146] FIG. 39 is a case corresponding to that of FIG. 35 in which the external conduits join together in a common plenum 3910.

[0147] FIG. 40 is a case corresponding to that of FIG. 36 in which the external conduits join together in a common plenum 4010.

[0148] FIG. 41 is an case corresponding to that of FIG. 37 in which the external conduits join together in a common plenum 4110.

[0149] FIG. 42 is a case corresponding to that of FIG. 38 in which the external conduits join together in a common plenum 4210.

[0150] FIG. 43 is a case corresponding to that of FIG. 17 in which the external conduits join together in a common plenum 4310.

Advantageous Features:

[0151] Constricting the torroidal plasma current in the vicinity of the wafer not only improves etch selectivity but at the same time increases the etch rate by increasing the plasma ion density. It is believed no prior reactor has increased etch selectivity by the same mechanism that increases etch rate or plasma ion density over the workpiece.

[0152] Improving etch selectivity by constricting the torroidal plasma current in the vicinity of the wafer or workpiece can be achieved in the invention in any one of several ways. One way is to reduce the pedestal-to-ceiling or wafer-to-ceiling height. Another is to introduce a gas distribution plate or showerhead over the wafer that constricts the path of the torroidal plasma ion current. Another way is to increase the RF bias power applied to the wafer or workpiece. Any one or any combination of the foregoing ways of improving etch selectivity may be chosen by the skilled worker in carrying out the invention.

[0153] Etch selectivity may be further improved in the invention by injecting the reactive process gases locally (i.e., near the wafer or workpiece) while injecting an inert diluent gas (e.g., Argon) remotely (i.e., into the conduit or plenum). This may be accomplished by providing a gas distribution plate or showerhead directly over and facing the workpiece support and introducing the reactive process gas exclusively (or at least predominantly) through the showerhead. Concurrently, the diluent gas is injected into the conduit well away from the process region overlying the wafer or workpiece. The torroidal plasma current thus becomes not only a source of plasma ions for reactive ion etching of materials on the wafer but, in addition, becomes an agent for sweeping away the reactive process gas species and their plasma-dissociated progeny before the plasma-induced dissociation process is carried out to the point of creating an undesirable amount of free fluorine. This reduction in the residence time of the reactive process gas species enhances the etch selectivity relative to photoresist and other materials, a significant advantage.

[0154] Great flexibility is provided in the application of RF plasma source power to the torroidal plasma current. As discussed above, power is typically inductively coupled to the torroidal plasma current by an antenna. In many cases, the antenna predominantly is coupled to the external conduit or plenum by being close or next to it. For example, a coil antenna may extend alongside the conduit or plenum. However, in other cases the antenna is confined to the region enclosed between the conduit or plenum and the main reactor enclosure (e.g., the ceiling). In the latter case, the antenna may be considered to be “under” the conduit rather than alongside of it. Even greater flexibility is afford by cases having a magnetic core (or cores) extending through the enclosed region (between the conduit and the main chamber enclosure) and having an extension beyond the enclosed region, the antenna being wound around the core's extension. In this case the antenna is inductively coupled via the magnetic core and therefore need not be adjacent the torroidal plasma current in the conduit. In one such case, a closed magnetic core is employed and the antenna is wrapped around the section of the core that is furthest away from the torroidal plasma current or the conduit. Therefore, in effect, the antenna may be located almost anywhere, such as a location entirely remote from the plasma chamber, by remotely coupling it to the torroidal plasma current via a magnetic core.

[0155] Finally, plasma distribution over the surface of a very large diameter wafer or workpiece is uniform. This is accomplished in one case by shaping the torroidal plasma current as a broad plasma belt having a width preferably exceeding that of the wafer. In another case, uniformity of plasma ion density across the wafer surface is achieved by providing two or more mutually transverse or orthogonal torroidal plasma currents that intersect in the process region over the wafer. The torroidal plasma currents flow in directions mutually offset from one another by 360/n. Each of the torroidal plasma currents may be shaped as a broad belt of plasma to cover a very large diameter wafer. Each one of the torroidal plasma currents may be powered by a separate coil antenna aligned along the direction of the one torroidal plasma current. In one preferred case, uniformity is enhanced by applying RF signals of different phases to the respective coil antennas so as to achieve a rotating torroidal plasma current in the process region overlying the wafer. In this preferred case, the optimum structure is one in which the torroidal plasma current flows in a circularly continuous plenum communicating with the main chamber portion through a circularly continuous annular opening in the ceiling or side wall. This latter feature allows the entire torroidal plasma current to rotate azimuthally in a continuous manner.

Controlling Radial Distribution of Plasma Ion Density:

[0156] FIG. 44 illustrates a plasma reactor similar to that illustrated in FIG. 17A having a pair of orthogonal external reentrant tubes 150-1, 150-2. RF power is coupled into the tubes by respective annular magnetic cores 1015-1, 1015-2 excited by respective RF-driven coils 170-1, 170-2, as described above with reference to FIG. 17A. However, in FIG. 44 the external tubes 150-1, 150-2 are rectangular, as in FIG. 24 rather than being round in cross-sectional shape. Moreover, the horizontal section of the lower tube 150-1 is not flat but rather has a dip 4410 at its middle. The dip 4410 permits the upper external tube 150-2 to nest closer to the reactor ceiling 110. This feature shortens the path length in the upper tube 150-2, thereby reducing plasma losses in the upper tube 150-2. In fact, the shape of the dip 4410 may be selected to at least nearly equalize the path length through the upper and lower external tubes 150-1, 150-2. The reactor of FIG. 44, like,the reactors of FIGS. 2 and 26, has a gas distribution plate 210 on the ceiling 110 (or forming the ceiling 110 itself) and overlying the wafer 120.

[0157] The dip 4410 is limited in that a vertical space remains between the top surface of the ceiling 110 and a bottom corner 4422 formed on the lower tube 150-1 at the apex of the dip 4410. The vertical space accommodates an electromagnet assembly 4430 that enhances plasma ion density over the center of the wafer 120. The electromagnet assembly 4430 includes a narrow elongate cylindrical pole piece 4440 formed of a magnetizable metal such as iron or steel (for example) and a coil 4450 of insulated conductive wire (e.g., copper wire) wrapped around the pole piece 4440. The cylindrical axis of the pole piece 4440 coincides with the axis of symmetry of the cylindrical chamber 100, so that the axis of the pole piece 4440 intersects the center of the wafer 120. The coil 4450 may be wrapped directly on the pole piece 4440 or, as illustrated in FIG. 45, may be wrapped around a mandril 4460 encircling the pole piece 4440. FIG. 45 shows that the coil 4450 may be wrapped around a section 4440-1 of the pole piece 4440 that extends above the ceiling 110. The lower section 4440-2 of the pole piece 4440 that is inside the ceiling 110 terminates within the gas manifold 220 of the gas distribution plate 210.

[0158] For efficiency, it is desirable to place the source of the plasma-confining magnetic field as close to the plasma as practical without disturbing gas flow within the gas distribution plate 210. For this purpose, the portion of the lower pole piece section 4440-2 that is inside the gas manifold 220 is a very narrow cylindrical end piece 4470 that terminates the pole piece 4440. The end piece 4470 extends the magnetic field lines of the pole piece 4440 near the bottom of the gas distribution plate to enhance the effect of the magnetic field on the plasma. The diameter of the end piece 4470 is sufficiently reduced so that it does not appreciably interfere with gas flow within the gas manifold 210. Moreover, such a reduced diameter brings the peak of the radial component of the magnetic field nearer the center axis.

[0159] FIG. 46 illustrates one case of the end piece 4470 having a tapered bottom 4475 terminated in a nipple 4477. FIG. 47 illustrates an case of the end piece 4470 in which the bottom 4476 is flat. FIG. 48 illustrates an case of the end piece 4470 in which the bottom 4478 is round.

[0160] In one implementation, pole piece 4440 has a, diameter of about 3.5 cm (such that the diameter of the approximately 60 turn coil 4450 is about 6 cm) and is about 12 cm long. The pole piece 4440 is extended about 2 cm (to a total of about 14 cm) with a smaller diameter extension of about 1 cm diameter. The bottom of the extension region of the pole piece 4440 is about 1.5 cm from the top of the plasma region. The material composition of pole piece 4440 is selected to have sufficiently high permeability (e.g., μ_r≧100) and high sat ration flux density (e.g. Bsat>1000 gauss) to maximize the magnetic flux density in the region below the pole piece 4440 with minimum magnetizing force and current. Note that because the magnetic path is “open” with pole piece 4440 (not closed within the pole piece), the effective permeability is reduced relative to the material permeability. Depending on the length/diameter ratio of the pole piece 4440, the μ_r “effective” is typically reduced to on the order of 10.

[0161] An optional shield 4479 of magnetic material such as iron shields plasma in the pair of tubes 150-1, 150-2 from the D.C. magnetic field of the electromagnet assembly 4430. The shield 4479 includes an overhead plate 4479a and a cylindrical skirt 4479b.

[0162] In the case of the gas distribution plate 210 illustrated in FIG. 45, a top plate 4480 is divided into radially inner and outer sections 4480a, 4480b, each having many small gas flow holes 4481 extending therethrough, the inner and outer sections having annular flanges 4482-1, 4482-2, 4482-3, 4482-4, forming vertical walls supporting the bottom surface of the ceiling 210 and forming therewith inner and outer gas manifolds 4483a, 4483b separated by a wall formed by the annular flanges 4482-2, 4482-3. In one case, there is no wall between the inner and outer gas manifolds, so as to avoid any discontinuity in gas distribution within the chamber that such a wall may cause. A gas mixing layer 4484 below the top plate 4480 diverts gas flow from a purely vertical flow direction and thereby induces multi-directional (or turbulent) gas flow that improves uniform mixing of gases of different molecular weights. Such diverting of the gas flow from a purely downward flow direction has the added benefit of suppressing high velocity gas flow effects, in which high velocity gas flow through gas distribution plate orifices directly over the wafer would form localized concentrations of process gas on the wafer surface that disrupt process uniformity. Suppression of high velocity gas flow effects enhances uniformity.

[0163] The gas mixing layer 4484 may consist of metal or ceramic foam of the type well-known in the art. Or, as shown in FIG. 49, the gas mixture layer 4484 may consist of plural perforation plates 4484-1, 4484-2 each having many mall gas orifices drilled therethrough, the holes in one perforation plate being offset from the holes in the other perforation plate. A bottom plate 4485 of the gas distribution plate 210 has many sub-millimeter gas injection holes 4486 (FIG. 50) drilled therethrough with large counterbored holes 4487 at the top of the bottom plate 4485. In one example, the sub-millimeter holes were between 10 and 30 mils in diameter, the counterbored holes were about 0.06 inch in diameter and the bottom plate 4485 had a thickness of about 0.4 inch. Inner and outer gas feed lines 4490, 4492 through the ceiling 110 furnish gas to the inner and outer top plates 4480a, 4480b, so that gas flow in radially inner and outer zones of the chamber may be controlled independently as a way of adjusting process uniformity.

[0164] It is believed that the radial component of the D.C. magnetic field produced by the electromagnet assembly 4430 affects the radial distribution of plasma ion density, and that it is this radial component of the magnetic field that can be exploited to enhance plasma ion density near the center of the chamber. It is believed that such enhancement of plasma ion density over the wafer center arises from the interaction of the D.C. magnetic field radial component with the plasma sheath electric field at the wafer surface producing azimuthal plasma currents tending to confine plasma near the wafer center. In absence of the D.C. magnetic field, the phenomenon of a reduced plasma ion density at the center of the chamber extends over a very small circular zone confined closely to the center of the wafer 120, because in general the reactor of FIG. 44 tends to have an exceptionally uniform plasma ion density even in absence of a correcting magnetic field. Therefore, correction of the center-low plasma ion density distribution requires a D.C. magnetic field having a relatively large radial component very near the center of the chamber or wafer 120. The small diameter of the magnetic pole piece 4440 produces a magnetic field having a large radial component very close to the center of the wafer 120 (or center of the chamber). In accordance with conventional practice, the center is the axis of symmetry of the cylindrical chamber at which the radius is zero. FIG. 51 illustrates the distribution of the magnetic field in an elevational view of the processing region over the wafer 120 between the wafer 120 and the gas distribution plate 210. The vectors in FIG. 51 are normalized vectors representing the direction of the magnetic field at various locations. FIG. 52 illustrates the magnetic flux density of the radial component of the magnetic field as a function of radial location, one curve representing the radial field flux density near the bottom surface of the gas distribution plate 210 and the other curve representing the radial field flux density near the surface of the wafer 120. The peak of the flux density of the radial magnetic field component is very close to the center, namely at about a radius of only one inch both at the ceiling and at the wafer. Thus, the radial component of the magnetic field is tightly concentrated near the very small diameter region within which the plasma ion density tends to be lowest. Thus, the distribution of the radial component of the D.C. magnetic field produced by the electromagnet assembly 4430 generally coincides with the region of low plasma ion density near the center of the chamber.

[0165] As mentioned above, it is felt that the radial component of the D.C. magnetic field interacts with the vertically oriented electric field of the plasma sheath near the wafer center to produce an azimuthally directed force that generally opposes radial travel of plasma. As a result, plasma near the center of the wafer is confined to enhance processing within that region.

[0166] A basic approach of using the electromagnet assembly 4430 in an etch reactor is to find a D.C. current flow in the coil that produces the most uniform etch rate radial distribution across the wafer surface, typically by enhancing plasma ion density at the center. This is the likeliest approach in cases in which the wafer-to-ceiling gap is relatively small (e.g., one inch), since such a small gap typically results in a center-low etch rate distribution on the wafer. For reactors having a larger gap (e.g., two inches or more), the etch rate distribution may not be center low, so that a different D.C. current may be needed. Of course, the electromagnet assembly 4430 is not confined to applications requiring improved uniformity of plasma ion density across the wafer surface. Some applications of the electromagnet assembly may require an electromagnet coil current that renders the plasma ion density less uniform. Such applications may involve, for example, cases in which a field oxide thin film layer to be etched has a non-uniform thickness distribution, so that uniform results can be obtained only by providing nonuniform plasma ion density distribution that compensates for the nonuniform field oxide thickness distribution. In such a case, the D.C. current in the electromagnet assembly can be selected to provide the requisite nonuniform plasma ion distribution.

[0167] As shown in FIG. 45, the plasma reactor may include a set of integrated rate monitors 4111 that can observe the etch rate distribution across the wafer 120 during the etch process. Each monitor 4111 observes the interference fringes in light reflected from the bottom of contact holes while the holes are being etched. The light can be from a laser or may be the luminescence of the plasma. Such real time observation can make it possible to determine changes in etch rate distribution across the wafer that can be instantly compensated by changing the D.C. current applied to the electromagnet assembly 4430.

[0168] FIG. 53 shows one way of independently controlling process gas flow to the inner and outer gas feed lines 4490, 4492. In FIG. 53, one set of gas flow controllers 5310, 5320, 5330 connected to the inner gas feed line 4490 furnish, respectively, argon, oxygen and a fluoro-carbon gas, such as C4F6, to the inner gas feed line 4490. Another set of gas flow controllers 5340, 5350, 5360 furnish, respectively, argon, oxygen and a fluoro-carbon gas, such as C4F6, to the outer gas feed line 4492. FIG. 54 shows another way of independently controlling process gas flow to the inner and outer gas feed lines 4490, 4492. In FIG. 54, a single set of gas flow controllers 5410, 5420, 5430 furnishes process gases (e.g., argon, oxygen and a fluoro-carbon gas) to a gas splitter 5440. The gas splitter 5440 has a pair of gas or mass flow controllers (MFC's) 5442, 5444 connected, respectively, to the inner and outer gas feed lines 4490, 4492. In addition, optionally another gas flow controller 5446 supplies purge gas such as Argon or Neon to the outer gas feed line 4492.

[0169] One problem in processing a large diameter wafer is that the torroidal or reentrant plasma current must spread out evenly over the wide surface of the wafer. The tubes 150 typically are less wide than the process area. The need then is to broaden the plasma current to better cover a wide process area as it exits a port 155 or 160. As related problem is that the reactor of FIG. 44 (or any of the reactors of FIGS. 1-43) can experience a problem of non-uniform plasma ion density and consequent “hot spot” or small region 5505 of very high plasma ion density near a port 155 or 160 of the reentrant tube 150, as shown in FIG. 55A. Referring to FIGS. 55A-56B, these problems are addressed by the introduction of a plasma current flow splitter 5510 at the mouth of each port (e.g., the port 155 as shown in FIG. 55A). The splitter 5510 tends to force the plasma current to widen while at the same time reducing plasma ion density in the vicinity of the region 5505 where a hot spot might otherwise form. The tube 150 can have a widened termination section 5520 at the port 155, the termination section 5520 having a diameter nearly twice as great as that of the remaining portion of the tube 150. The plasma current flow splitter 5510 of FIG. 55A is triangular in shape, with one apex facing the interior of the tube 150 so as to force the plasma current flowing into the chamber 100 from the tube 150 to spread out so as to better fill the larger diameter of the termination section 5520. This current-spreading result produced by the triangular splitter 5510 tends to widen the plasma current and reduces or eliminates the “hot spot” in the region 5505.

[0170] The optimum shape of the splitter 5510 depends at least in part upon the separation distance S between the centers of opposing ports 155, 160. If the splitter is too long in the direction of plasma flow (i.e., the vertical direction in FIG. 55A), then current flow along the divided path tends to be unbalanced, with all current flowing along one side of the splitter 5510. On the other hand, if the splitter 5510 is too short, the two paths recombine before the plasma current appreciably widens.

[0171] For example, in a chamber for processing a 12 inch diameter wafer, the separation distance S can be about 20.5 inches, with a tube width w of 5 inches, a tube draft d of 1.75 inches and an expanded termination section width W of 8 inches. In this case, the juxtaposition of the port 155 relative to the 12 inch wafer would be as shown in the plan view of FIG. 56C. In this particular example, the height h of the splitter 5510 should be about 2.5 inches, with the angle of the splitter's apex 5510a being about 75 degrees, as shown in FIG. 57. In addition, the length L of the termination section 5520 should equal the height h of the splitter 5510.

[0172] On the other hand, for a separation distance S of 16.5 inches, an optimum splitter 5510′ is illustrated in FIG. 58. The angle of the splitter apex in this case is preferably about 45 degrees, the triangular portion being terminated in a rectangular portion having a width of 1.2 inches and a length such that the splitter 5510′ has a height h of 2.5 inches. The height and apex angle of the splitter 5510 or 5510′ must be sufficient to reduce plasma density in the region 5505 to prevent formation of a hot spot there. However, the height h must be limited in order to avoid depleting plasma ion density at the wafer center.

[0173] FIGS. 59A and 59B illustrate splitters for solving the problem of plasma ion density non-uniformity near the entrance ports of a reentrant tube 2654 in which plasma current flow through each port is in a horizontal direction through the chamber side wall 105, as in the reactor of FIG. 26. Each splitter 5910 has its apex 5910a facing the port 2680.

[0174] FIGS. 60, 61 and 62 illustrate an implementation like that of FIG. 17A, except that the chamber side wall 105 is rectangular or square and the vertically facing ports 140-1, 140-2, 140-3 and 140-4 through the ceiling 110 are located over respective corners 105a, 105b, etc. of the rectangular or square side wall 105. A floor 6020 in the plane of the wafer 120 faces each port and, together with the corner-forming sections of the rectangular side wall 105, forces incoming plasma current to turn toward the processing region overlying the wafer 120. In order to reduce or eliminate a hot spot in plasma ion density in the region 6030, a triangular plasma current flow splitter 6010 is placed near each respective corner 105a, 105b, etc., with its apex 6010a facing that corner. In the implementation of FIG. 61, the splitter apex 6010a is rounded, but in other implementations it may be less rounded or actually may be a sharp edge. FIG. 63 illustrates a portion of the same arrangement but in which the edge 6010b of the splitter 6010 facing the wafer 120 is located very close to the wafer 120 and is arcuately shaped to be congruent with the circular edge of the wafer 120. While the splitter 6010 of FIG. 60 extends from the floor 6020 to the ceiling 110, FIG. 64 illustrates that the height of the splitter 6010 may be less, so as to allow some plasma current to pass over the splitter 6010.

[0175] As will be discussed in greater detail below with respect to certain working examples, the total path length traversed by the reentrant plasma current affects plasma ion density at the wafer surface. This is because shorter path length places a higher proportion of the plasma within the processing region overlying the wafer, reduces path length-dependent losses of plasma ions and reduces surface area losses due to plasma interaction with the reentrant tube surface. Therefore, the shorter length tubes (corresponding to a shorter port separation distance S) are more efficient. On the other hand, a shorter separation distance S affords less opportunity for plasma current flow separated at its center by the triangular splitter 5510 to reenter the center region after passing the splitter 5510 and avoid a low plasma ion density at the wafer center. Thus, there would appear to be a tradeoff between the higher efficiency of a smaller port separation distance S and the risk of depressing plasma ion density at the wafer center in the effort to avoid a plasma hot spot near each reentrant tube port.

[0176] This tradeoff is ameliorated or eliminated in the case of FIGS. 65A, 65B and 66, by using a triangular splitter 6510 that extends at least nearly across the entire width W of the termination section 5520 of the port and is shaped to force plasma current flow away from the inner edge 6610 of the port and toward the outer edge 6620 of the port. This feature leaves the port separation distance S unchanged (so that it may be as short as desired), but in effect lengthens the plasma current path from the apex 6510a of the splitter to the center of the wafer 120. This affords a greater opportunity for the plasma current flow split by the splitter 6510 to rejoin at its center before reaching the wafer or center of the wafer. This feature better avoids depressing plasma ion density at the wafer center while suppressing formation of plasma hot spots at the reentrant tube ports.

[0177] As illustrated in FIGS. 65A, 65B and 66, each splitter 6510 presents an isoceles triangular shape in elevation (FIG. 65B) and a rectangular shape from the top (FIG. 65A). The side view of FIG. 66 reveals the sloping back surface 6610c that extends downwardly toward the outer edge 6620 of the port. It is the sloping back surface 6610c that forces the plasma current toward the back edge 6620 thereby effectively lengthening the path from the top of the apex 6510a to the wafer center, which is the desired feature as set forth above. The rectangular opening of the port 150 is narrowed in the radial direction (the short dimension) by the sloped wall or sloping back surface 6610b from about 2″ at top to about ¾″ at the bottom. This pushes the inner port edge about 1-¼″ radially farther from the wafer (thus achieving the desired increase in effective port separation distance). In addition, the port 150 has the full triangular splitter 6510 in the azimuthal direction (the long or 8″ wide dimension of the opening 150).

[0178] The plasma current splitter 5510 or 6510 may have coolant passages extending therethrough with coolant ports coupled to similar ports in the reactor body to regulate the temperature of the splitter. For this purpose, the plasma current splitter 5510 or 6510 is formed of metal, since it easily cooled and is readily machined to form internal coolant passages. However, the splitter 5510 or 6510 may instead be formed of another material such as quartz, for example.

[0179] FIG. 67 illustrates another way of improving plasma uniformity in the torroidal source reactor of FIG. 24 by introducing a set of four annular electromagnets 6710, 6720, 6730, 6740 along the periphery of the reactor, the windings of each electromagnet being controlled by a magnet current controller 6750. The electric currents in the four electromagnets may be driven in any one of three modes:

[0180] (1) in a first mode, a sinusoidal mode, the coils are driven at the same low frequency current in phase quadrature to produce a magnetic field that rotates about the axis of symmetry of the reactor at the low frequency of the source;

[0181] (2) in a second mode, a configurable magnetic field mode, the four electromagnets 6710, 6720, 6730, 6740 are grouped into opposing pairs of adjacent electromagnets, and each pair is driven with a different D.C. current to produce a magnetic field gradient extending diagonally between the opposing pairs of adjacent electromagnets, and this grouping is rotated so that the magnetic field gradient is rotated to isotropically distribute its effects over the wafer;

[0182] (3) in a third mode, the four electromagnets are all driven with the same D.C. current to produce a cusp-shaped magnetic field having an axis of symmetry coinciding generally with the axis of symmetry of the reactor chamber.

[0183] As shown in FIG. 1, a pumping annulus is formed between the cylindrical wafer support pedestal 115 and the cylindrical side wall 105, gases being evacuated via the pumping annulus by the vacuum pump 135. Plasma current flow between the opposing ports of each reentrant tube 150 can flow through this pumping annulus and thereby avoid flowing through the processing region between the wafer 120 and the gas distribution plate 210. Such diversion of plasma current flow around the process region can occur if the chamber pressure is relatively high and the wafer-to-ceiling gap is relatively small and/or the conductivity of the plasma is relatively low. To the extent this occurs, plasma ion density in the process region is reduced. This problem is solved as shown in FIGS. 68 and 69 by the introduction of radial vanes 6910, 6920, 6930, 6940 blocking azimuthal plasma current flow through the pumping annulus. In one implementation, the vanes 6910, 6920, 6930, 6940 extend up to but not above the plane of the wafer 120, to allow insertion and removal of the wafer 120. However, in another implementation the vanes may retractably extend above the plane of the wafer to better confine the plasma current flow within the processing region overlying the wafer 120. This may be accomplished by enabling the wafer support pedestal 115 to move up and down relative to the vanes, for example. In either case, the vanes 6910, 6920, 6930, 6940 prevent plasma current flow through the pumping annulus, and, if the vanes can be moved above the plane of the wafer 120, they also reduce plasma current flow through the upper region overlying the pumping annulus. By thus preventing diversion of plasma current flow away from the processing region overlying the wafer, not only is plasma ion density improved in that region but process stability is also improved.

[0184] As mentioned previously herein, the magnetic core used to couple RF power to each reentrant tube 150 tends to crack or shatter at high RF power levels. It is believed this problem arises because magnetic flux is not distributed uniformly around the core. Generally, one winding around the core has a high current at high RF power levels. This winding can be, for example, a secondary winding that resonates the primary winding connected to the RF generator. The secondary winding is generally confined to a narrow band around the core, magnetic flux and heating being very high within this band and much lower elsewhere in the core. The magnetic core must have a suitable permeability (e.g., a permeability between about 10 and 200) to avoid self-resonance at high frequencies. A good magnetic core tends to be a poor heat conductor (low thermal conductivity) and be readily heated (high specific heat), and is therefore susceptible to localized heating. Since the heating is localized near the high current secondary winding and since the core tends to be brittle, it cracks or shatters at high RF power levels (e.g., 5 kilowatts of continuous power).

[0185] This problem is solved in the manner illustrated in FIGS. 70 through 74 by more uniformly distributing RF magnetic flux density around the annular core. FIG. 70 illustrates a typical one of the magnetic cores 1015 of FIG. 17A. The core 1015 is formed of a high magnetic permeability material such as ferrite. The primary winding 170 consists of about two turns of a thin copper band optionally connected through an impedance match device 175 to the RF generator 180. High current flow required for high magnetic flux in the core 1015 occurs in a resonant secondary winding 7010 around the core 1015. Current flow in the secondary winding 7010 is about an order of magnitude greater than current flow in the primary winding. In order to uniformly distribute magnetic flux around the core 1015, the secondary winding 7010 is divided into plural sections 7010a, 7010b, 7010c, etc., that are evenly distributed around the annular core 1015. The secondary winding sections 7010a, etc., are connected in parallel. Such parallel connection is facilitated as illustrated in FIGS. 71A and 71B by a pair of circular copper buses 7110, 7120 extending around opposite sides of the magnetic core 1015. Opposing ends of each of the secondary windings 7010a, 7010b, etc., are connected to opposite ones of the two copper buses 7110, 7120. The copper buses 7110, 7120 are sufficiently thick to provide an extremely high conductance and low inductance, so that the azimuthal location of any particular one of the secondary winding sections 7010a, 7010b, etc. makes little or no difference, so that all secondary winding sections function as if they were equidistant from the primary winding. In this way, magnetic coupling is uniformly distributed around the entire core 1015.

[0186] Because of the uniform distribution of magnetic flux achieved by the foregoing features, the primary winding may be placed at any suitable location, typically near a selected one of the plural distributed secondary winding sections 7110a, 7110b, 7110c, etc. However, in one implementation, the primary winding is wrapped around or on a selected one of the plural distributed secondary winding sections 7110a, 7110b, 7110c, etc.

[0187] FIG. 72 is a representation of the distributed parallel inductances formed by the parallel secondary winding sections 7010a, 7010b, etc., and FIG. 73 shows the circular topology of these distributed inductances. In order to provide resonance at the frequency of the RF generator 180, plural distributed capacitors 7130 are connected in parallel across the two copper buses 7110, 7120. The plural capacitors 7030 are distributed azimuthally around the magnetic core 1015. Each capacitor 7030 in one implementation was about 100 picoFarads. The equivalent circuit of the distributed inductances and capacitances associated with the secondary winding 7010 is illustrated in FIG. 24.

[0188] Referring to FIG. 71B, the secondary winding sections 7010a, 7010b, etc., can have the same number of turns. In the case of FIG. 71B, there are six secondary winding sections 7010a-7010f, each section having three windings. The skilled worker can readily select the number of secondary winding sections, the number of windings in each section and the capacitance of the distributed capacitors 7030 to achieve resonance at the frequency of the RF generator 180. The copper band stock used to form the primary and secondary windings around the core 1015 can be, for example, 0.5 inch wide and 0.020 inch thick copper stripping. The two copper buses 7110, 7120 are very thick (e.g., from 0.125 inch to 0.25 inch thick) and wide (e.g., 0.5 inch wide) so that they form extremely low resistance, low inductance current paths. The core 1015 may consist of a pair of stacked 1 inch thick ferrite cores with a 10 inch outer diameter and an 8 inch inner diameter. Preferably, the ferrite core 1015 has a magnetic permeability □=40. The foregoing details are provided by way of example only, and any or all of the foregoing values may require modification for different applications (e.g., where, for example, the frequency of the RF generator is modified).

[0189] We have found that the feature of distributed inductances illustrated in FIGS. 71A and 71B solves the problem of breakage of the magnetic core experienced at sustained high RF power levels (e.g., 5 kiloWatts).

[0190] FIG. 75 illustrates the equivalent circuit formed by the core and windings of FIGS. 71A and 71B. In addition to the primary and secondary windings 170 and 7010 around the core 1015, FIG. 75 illustrates the equivalent inductive and capacitive load presented by the plasma inductively coupled to the core 1015. The case of FIGS. 70-75 is a transformer coupled circuit. The purpose of the secondary winding 7010 is to provide high electric current flow around the magnetic core 1015 for enhanced power coupling via the core. The secondary winding 7010 achieves this by resonating at the frequency of the RF generator. Thus, the high current flow and power coupling via the magnetic core 1015 occurs in the secondary winding 7010, so that virtually all the heating of the core 1015 occurs at the secondary winding 7010. By thus distributing the secondary winding 7010 around the entire circumference of the core 1015, this heating is similarly distributed around the core to avoid localized heating and thereby prevent shattering the core at high RF power levels.

[0191] The distributed winding feature of FIGS. 71A and 71B can be used to implement other circuit topologies, such as the auto transformer circuit of FIG. 76. In the auto transformer circuit of FIG. 76, the winding 7010 around the core 1015 is distributed (in the manner discussed above with reference to FIGS. 70-74) and has a tap 7610 connected through the impedance match circuit 175 to the RF generator 180. The distributed capacitors 7030 provide resonance (in the manner discussed above). As in FIG. 70, the core 7010 is wrapped around the reentrant tube 150 so that power is inductively coupled into the interior of the tube 150. The circuit topologies of FIGS. 75 and 76 are only two examples of the various topologies that can employ distributed windings around the magnetic core 1015.

[0192] In one implementation, the impedance match circuits 175a, 175b employed frequency tuning in which the frequency of each RF generator 180a, 180b is controlled in a feedback circuit in such a way as to minimize reflected power and maximize forward or delivered power. In such an implementation, the frequency tuning ranges of each of the generators 180a, 180b are exclusive, so that their frequencies always differ, typically on the order of a 0.2 to 2 MHz difference. Moreover, their phase relationship is random. This frequency difference can improve stability. For example, instabilities can arise if the same frequency is used to excite plasma in both of the orthogonal tubes 150-1, 150-2. Such instabilities can cause the plasma current to flow through only three of the four ports 155, 160, for example. This instability may be related to the phase difference between the torroidal plasma currents in the tubes. One factor facilitating plasma stability is isolation between the two plasma currents of the pair of orthogonal tubes 150-1, 150-2. This isolation is provided mainly by the plasma sheaths of the two plasma currents. The D.C. break or gap 152 of each of the reentrant tubes 150-1, 150-2 also enhances plasma stability.

[0193] While the D.C. break or gap 152 in each of the orthogonal tubes is illustrated in FIG. 44 as being well-above the chamber ceiling 110, it may in fact be very close to or adjacent the ceiling. Such an arrangement is employed in the implementation of FIG. 77, in which the case of FIG. 55A is modified so that the termination section 5520 electrically floats so that its potential follows oscillations of the plasma potential. This solves a problem that can be referred to as a “hollow cathode” effect near each of the ports 155, 160 that creates non-uniform plasma distribution. This effect may be referred to as an electron multiplication cavity effect. By permitting all of the conductive material near a port to follow the plasma potential oscillations, the hollow cathode effects are reduced or substantially eliminated. This is achieved by electrically isolating the termination section 5520 from the grounded chamber body by locating a D.C. break or gap 152′ at the juncture between the reentrant tube termination section 5520 and the top or external surface of the ceiling 110. (The gap 152′ may be in addition to or in lieu of the gap 152 of FIG. 44.) The gap 152′ is filled with an insulative annular ring 7710, and the termination section 5520 of FIG. 77 has a shoulder 7720 resting on the top of the insulative ring 7710. Moreover, there is an annular vacuum gap 7730 of about 0.3 to 3 mm width between the ceiling 110 and the termination section 5520. In one implementation, the tube 150 and the termination section 5520 are integrally formed together as a single piece. The termination section 5520 is preferably formed of metal so that internal coolant passages may be formed therein.

[0194] FIGS. 44-77 illustrate cases in which the uniformity control magnet is above the processing region. FIG. 78 illustrates that the magnet pole 4440 may be placed below the processing region, or under the wafer support pedestal 115.

[0195] An etch process was conducted on blanket oxide wafers at a chamber pressure of 40 mT, 4800 watts of 13.56 MHz RF bias power on the wafer pedestal and 1800 Watts of RF source power applied to each reentrant tube 150 at 11.5 MHz and 12.5 MHz, respectively. The magnetic field produced by the electromagnet assembly 4430 was set at the following levels in successive steps: (a) zero, (b) 6 Gauss and (c) 18 Gauss (where the more easily measured axial magnetic field component at the wafer center was observed rather than the more relevant radial component). The observed etch rate distribution on the wafer surface was measured, respectively, as (a) center low with a standard deviation of about 2% at zero Gauss, (b) slightly center fast with a standard deviation of about 1.2% at 6 Gauss, and (c) center fast with a standard deviation of 1.4%. These examples demonstrate the ability to provide nearly ideal compensation (step b) and the power to overcompensate (step c).

[0196] To test the effective pressure range, the chamber pressure was increased to 160 mT and the electromagnet's field was increased in three steps from (a) zero Gauss, to (b) 28 Gauss and finally to (c) 35 Gauss (where the more easily measured axial magnetic field component at the wafer center was observed rather than the more relevant radial component). The observed etch rate was, respectively, (a) center slow with a standard deviation of about 2.4%, center fast with a standard deviation of about 2.9% and center fast with a standard deviation of about 3.3%. Obviously, the step from zero to 28 Gauss resulted in overcompensation, so that a somewhat smaller magnetic field would have been ideal, while the entire exercise demonstrated the ability of the electromagnet assembly 4430 to easily handle very high chamber pressure ranges. This test was severe because at higher chamber pressures the etch rate distribution tends to be more severely center low while, at the same time, the decreased collision distance or mean free path length of the higher chamber pressure makes it more difficult for a given magnetic field to effect plasma electrons or ions. This is because the magnetic field can have no effect at all unless the corresponding Larmour radius of the plasma electrons or ions (determined by the strength of the magnetic field and the mass of the electron or ion) does not exceed the plasma collision distance. As the collision distance decreases with increasing pressure, the magnetic field strength must be increased to reduce the Larmour radius proportionately. The foregoing examples demonstrate the power of the electromagnet assembly to generate a sufficiently strong magnetic field to meet the requirement of a small Larmour radius.

[0197] Another set of etch processes were carried out on oxide wafers patterned with photoresist at 35 mT under similar conditions, and the current applied to the electromagnet assembly 4430 was increased in five steps from (a) 0 amperes, (b) 5 amperes, (c) 6 amperes, (d) 7 amperes and (e) 8 amperes. (In this test, a current of 5 amperes produces about 6 gauss measured axial magnetic field component at the wafer center.) At each step, the etch depths of high aspect ratio contact openings were measured at both the wafer center and the wafer periphery to test center-to-edge etch rate uniformity control. The measured center-to-edge etch rate differences were, respectively, (a) 13.9% center low, (b) 3.3% center low, (c) 0.3% center low, (d) 2.6% center high and (e) 16.3% center high. From the foregoing, it is seen that the ideal electromagnet current for best center-to-edge uniformity is readily ascertained and in this case was about 6 amperes.

[0198] A set of etch processes were carried out on blanket oxide wafers to test the efficacy of the dual zone gas distribution plate 210 of FIG. 44. In a first step, the gas flow rates through the two zones were equal, in a second step the inner zone had a gas flow rate four times that of the outer zone and in a third step the outer zone had a gas flow rate four times that of the inner zone. In each of these steps, no current was applied to the electromagnet assembly 4430 so that the measurements taken would reflect only the effect of the dual zone gas distribution plate 210. With gas flow rates of the two zones equal in the first step, the etch rate distribution was slightly center high with a standard deviation of about 2.3%. With the inner zone gas flow rate at four times that of the outer zone, the etch rate distribution was center fast with a standard deviation of about 4%. With the outer zone gas flow rate at four times that of the inner zone, the etch rate distribution was center slow with a standard deviation of about 3.4%. This showed that the dual zone differential gas flow rate feature of the gas distribution plate 210 can be used to make some correction to the etch rate distribution. However, the gas flow rate control directly affects neutral species distribution only, since none of the incoming gas is (or should be) ionized. On the other hand, etch rate is directly affected by plasma ion distribution and is not as strongly affected by neutral distribution, at least not directly. Therefore, the etch rate distribution control afforded by the dual zone gas distribution plate, while exhibiting some effect, is necessarily less effective than the magnetic confinement of the electromagnet assembly 4430 which directly affects plasma electrons and thus ions.

[0199] The dependency of the electromagnet assembly 4430 upon the reentrant torroidal plasma current was explored. First a series of etch processes was carried out on blanket oxide wafers with no power applied to the torroidal plasma source, the only power being 3 kiloWatts of RF bias power applied to the wafer pedestal. The electromagnet coil current was increased in four steps of (a) zero amperes, (b) 4 amperes, (c) 6 amperes and (d) 10 amperes. The etch rate distribution was observed in the foregoing steps as (a) center high with a standard deviation of 2.87%, (b) center high with a standard deviation of 3.27%, (c) center high with a standard deviation of 2.93% and (d) center high with a standard deviation of about 4%. Thus, only a small improvement in uniformity was realized for a relatively high D.C. current applied to the electromagnet assembly 4430. Next, a series of etch processes was carried out under similar conditions, except that 1800 Watts was applied to each of the orthogonal tubes 150-1, 150-2. The electromagnet coil current was increased in six steps of (a) zero amperes, (b) 2 amperes, (c) 3 amperes, (d) 4 amperes, (e) 5 amperes and (f) 6 amperes. The etch rate distribution was, respectively, (a) center low with a standard deviation of 1.2%, (b) center low with a standard deviation of 1.56%, (c) center high with a standard deviation of 1.73%, (d) center high with a standard deviation of 2.2%, (e) center high with a standard deviation of 2.85% and (f) center high with a standard deviation of 4.25%. Obviously the most uniform distribution lies somewhere between 2 and 3 amperes where the transition from center low to center high was made. Far greater changes in plasma distribution were made using much smaller coil current with much smaller changes in coil current. Thus, the presence of the reentrant torroidal plasma currents appears to enhance the effects of the magnetic field of the electromagnet assembly 4430. Such enhancement may extend from the increase in bias power that is possible when the torroidal plasma source is activated. In its absence, the plasma is less conductive and the plasma sheath is much thicker, so that the bias RF power applied to the wafer pedestal must necessarily be limited. When the torroidal plasma source is activated (e.g., at 1800 Watts for each of the two orthogonal tubes 150-1, 150-2) the plasma is more conductive, the plasma sheath is thinner and more bias power can be applied. As stated before herein, the effect of the D.C. magnetic field may be dependent upon the interaction between the D.C. magnetic field and the electric field of the plasma sheath, which in turn depends upon the RF bias power applied to the pedestal. Furthermore, the reentrant torroidal plasma currents may be attracted to the central plasma region due to the aforementioned postulated interaction between D.C. magnetic field and the electric field of the plasma sheath, further enhancing the plasma ion density in that region.

[0200] The effects of the port-to-port separation distance S of FIG. 55A were explored in another series of etch processes on blanket oxide wafers. The same etch process was carried out in reactors having separation distances S of 16.5 inches and 20.5 inches respectively. The etch rate in the one with smaller separation distance was 31% greater than in the one with the greater separation distance (i.e., 6993 vs 5332 Angstroms/minute) with 1800 Watts applied to each one of the orthogonal tubes 150-1, 150-2 with zero current applied to the electromagnet assembly 4300 in each reactor.

[0201] The effects of the port-to-port separation distance S of FIGS. 55-56 were also explored in another series of etch processes on oxide wafers patterned with photoresist. With 3.7 amperes applied to the electromagnet assembly 4300 having the smaller source (16.5 inch) separation distance S, the etch rate was 10450 Angstroms/minute vs 7858 Angstroms/minute using the larger source (20.5 inch) separation distance S. The effect of increasing power in the reactor having the greater (20.5 inches) separation distance S was explored. Specifically, the same etch process was carried out in that reactor with source power applied to each of the orthogonal tubes 150-1, 150-2 being 1800 Watts and then at 2700 Watts. The etch rate increased proportionately very little, i.e., from 7858 Angstroms/minute to 8520 Angstroms/minute. Thus, the effect of the port-to-port separation distance S on plasma ion density and etch rate cannot readily be compensated by changing plasma source power. This illustrates the importance of cases such as the case of FIGS. 65A, 65B and 66 in which a relatively short port-to-port separation distance S is accommodated while in effect lengthening the distance over which the plasma current is permitted to equilibrate after being split by the triangular splitters 5440.

[0202] The pole piece 4440 has been disclosed a being either a permanent magnet or the core of an electromagnet surrounded by a coil 4450. However, the pole piece 4440 may be eliminated, leaving only the coil 4450 as an air coil inductor that produces a magnetic field having a similar orientation to that produced by the pole piece 4440. The air coil inductor 4450 may thus replace the pole piece 4440. Therefore, in more general terms, what is required to produce the requisite radial magnetic field is an elongate pole-defining member which may be either the pole piece 4440 or an air coil inductor 4450 without the pole piece 4440 or the combination of the two. The diameter of the pole-defining member is relatively narrow so as to appropriately confine the peak of the radial magnetic field.

[0203] While the invention has been described in detail by specific reference to preferred cases, it is understood that variations and modifications thereof may be made without departing from the true spirit and scope of the invention.