Abstract:
The invention is embodied in a plasma reactor having a vacuum chamber with a cylindrical side portion and a ceiling at a certain height above the top of the cylindrical side portion, a wafer-holding pedestal near the bottom of the vacuum chamber, gas injection ports near the cylindrical side portion and a vacuum pump, the reactor including a generally planar disk-shaped conductive ceiling electrode adjacent the ceiling, a helical coil antenna having a bottom winding near the top of the cylindrical side portion and a top winding generally corresponding to the second diameter near the planar disk-shaped conductive ceiling electrode, the helical coil antenna substantially spanning the height between the top of the cylindrical side portion and the ceiling, and a switch for individually connecting each one of the coil antenna, the ceiling electrode and the wafer pedestal to one of (a) a respective RF power source or (b) ground or (c) a floating potential (i.e., unconnected to any potential source). The invention is also embodied in a plasma reactor having a vacuum chamber, a wafer-holding pedestal near the bottom of the vacuum chamber, gas injection ports and a vacuum pump, the reactor including a conductive ceiling electrode at the top of the chamber, a coil antenna having a top generally coplanar with the ceiling electrode and a base generally coinciding with a circumference of the vacuum chamber, and a switch for individually connecting each one of the coil antenna, the ceiling electrode and the wafer pedestal to one of (a) a respective RF power source, or (b) ground or (c) a floating potential. In one embodiment, the reactor is a chemical vapor deposition plasma reactor.

Description:
RELATED APPLICATIONS 
     This application is a divisional under 37 C.F.R. § 1.53(b) associated with Ser. No. 08/766,053, filed Dec. 16, 1996, now U.S. Pat. No. 5,865,896 the entire contents of which are hereby incorporated by reference, and which is a continuation associated with Ser. No. 08/234,746 filed Apr. 26, 1994, now abandoned, which is a continuation in part of U.S. patent application, Ser. No. 08/113,776, filed Aug. 27, 1993 by Fairbairn and Nowak and is also related to U.S. patent application, Ser. No. 08/234,739 by Sahin et al. entitled “A Method of Operating a High Density Plasma CVD Reactor with Combined Inductive and Capacitive Coupling”. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The invention is related to a high density plasma CVD reactor having inductive and capacitive coupling modes which can be selected individually or in combination for chamber cleaning operations. 
     2. Background Art 
     Plasma reactors used for semiconductor processes such as chemical vapor deposition (CVD), etching, reactive ion etching and so forth typically employ either inductive coupling or capacitive coupling to strike and maintain a plasma Typically, an etch reactor employs capacitive coupling because the high ion energies characteristic of capacitively coupled plasmas are suitable for removing films such as, for example, silicon dioxide. The high ion energies arise from the high electric fields required to strike a capacitively coupled plasma For example, as disclosed in European Patent Application publication No. 0 552 491 A1 by Collins et al. entitled “Plasma Etch Process”published Jul. 28, 1993, a capacitively coupled reactor has parallel electrodes, including a grounded electrode in the ceiling and an RF biased electrode in the wafer pedestal. 
     However, the plasma density of a capacitively coupled plasma is not sufficient for performing simultaneous chemical vapor deposition (CVD) and in-situ sputtering at low pressures. Accordingly, a CVD plasma reactor of the type disclosed in U.S. patent application Ser. No. 08/113,776 entitled “High Density Plasma CVD and Etching Reactor” filed Aug. 27, 1993 by Fairbairn and Nowak employs inductive coupling to strike and maintain the plasma. In the Fairbairn and Nowak application, a domed-shaped helical coil in a dome-shaped chamber ceiling performs the inductive coupling to the plasma at a low chamber pressure (below 100 milliTorr), the dome shape providing a more uniform ion density distribution across the semiconductor wafer. Such an inductively coupled plasma is characterized by a high ion density and is therefore suitable for low pressure CVD plasma processing at high processing (e.g., deposition) rates. 
     The main differences between capacitively coupled plasmas and inductively coupled plasmas are the following: An inductively coupled plasma has a plasma density which is up to two orders of magnitude higher, thereby providing higher processing rates. A capacitively coupled plasma has a higher electrode sheet voltage, higher self-biasing and higher plasma impedance, and therefore provides higher sputtering rates and greater risk of damage to the wafer. An inductively coupled plasma exhibits lower ion energy distribution, providing a lower risk of damage to the wafer. 
     A requirement of all plasma reactors, and especially plasma CVD reactors, is that they must be cleaned periodically to remove deposits and residue. For example, in a CVD reactor, during deposition of silicon dioxide onto a semiconductor wafer, silicon dioxide is deposited onto the chamber walls and ceiling as well as other parts of the reactor chamber, changing the characteristic of the chamber and giving rise to particulate contamination. Therefore, the reactor chamber must be cleaned periodically to remove all such deposits and residue therefrom. Removal of silicon dioxide deposits is accomplished by introducing a fluoride-containing etchant gas such as NF3 (for example) into the chamber and striking a plasma to perform a chamber cleaning operation. 
     One limitation of inductively coupled plasma reactors is that they require more time to perform a chamber cleaning operation due to their larger volumes, low operational pressure, non-conductive ceilings and lack of bias of the chamber wall surfaces. Certain residues, such as silicon dioxide, typically have relatively low etch rates at low ion energies, and therefore can be difficult to remove by inductively coupled plasmas. 
     One problem with such a chamber cleaning operation is that the various surfaces of the reactor chamber interior are not cleaned (etched) at the same rate, due to differences in location in the chamber and differences in materials. The plasma density and ion energies are not the same at all locations in the chamber, so that differences in cleaning (etch) rates are quite typical. Moreover during wafer processing (such as CVD processing), deposits build up more thickly on certain chamber surfaces than on others, so that the deposit or residue thickness is non-uniform throughout the chamber interior. For example, in the etch reactor of the Collins et al. European application referenced above, the etch-process residue is much thicker in the center of the ceiling than at the edge of the ceiling, due to plasma density non-uniformity. 
     As a result, some metal surfaces are thoroughly cleansed of residue or deposits before others, and the cleaned metal surfaces (e.g., aluminum) are etched while the remaining contaminants are removed from the other still unclean surfaces. For example, in the case of the flat ceiling electrode of the Collins et al. European application, the edge of the ceiling electrode quickly becomes clean while the center portion still has a thick residue coating remaining thereon. The exposed metal surface of the electrode edge shunts RF energy away from the still-unclean center portion, thereby preventing any further cleaning of the center portion. If a fluoride gas such as NF3 is employed to clean the chamber, the metal surfaces react with the etchant gas during the cleaning operation to form various aluminum fluorides, which redeposit onto various chamber surfaces, including the ones that are still unclean with other residues or deposits. Such aluminum fluorides etch very slowly or are virtually impervious to being etched (particularly in an inductively coupled plasma), thus masking the prior deposits (e.g., silicon dioxide) underneath. This deposition of aluminum fluorides during the cleaning operation prevents the removal of the remaining residues (e.g., silicon dioxide), so that the reactor chamber cannot be thoroughly cleaned, even by extending the duration of the chamber cleaning operation. A related problem is that some chamber dielectric surfaces (for example, quartz surfaces) are consumed rapidly during a chamber cleaning operation, particularly in an inductively coupled plasma, and must therefore be frequently replaced at great cost, a significant disadvantage. 
     A problem also exists in regards to the standard radial gas distribution system employed at the periphery of the base/bottom of the source region of the chamber, such as the system designated G1 in the aforementioned European Patent Application publication No. 0 552 491 A1. These systems typically have a gas distribution ring with inwardly facing radial holes. When these types of systems are used to emit gas, (e.g. silane), during a CVD process to form an oxide layer over the surface of the wafer (e.g. SiO 2 ), the emitted gas tends to diffuse equally in all directions, not just toward the wafer. In the cases where silane is being used as the emitted gas, the silane and oxygen from the plasma react together spontaneously. Since the chamber walls are closer to the gas outlet holes than most of the wafer (particularly for larger diameter wafers), deposition of SiO 2  over the interior surfaces of the vacuum chamber is greater than that on the wafer. This means that the reactor must be periodically removed from productive activity and the SiO 2  coating removed from its interior surfaces, a significant advantage. 
     Thus, there is a need for a reactor that deposits less CVD residue (e.g. SiO 2 ) on the interior chamber surfaces, and which therefore, requires less frequent cleaning. 
     SUMMARY OF THE DISCLOSURE 
     The invention is embodied in a plasma reactor having a vacuum chamber with a cylindrical side portion and a ceiling at a certain height above the top of the cylindrical side portion, a wafer-holding pedestal near the bottom of the vacuum chamber, gas injection ports near the cylindrical side portion and a vacuum pump, the reactor including a generally planar disk-shaped conductive ceiling electrode adjacent the ceiling, a helical coil antenna having a bottom winding near the top of the cylindrical side portion and a top winding generally corresponding to the second diameter near the planar disk-shaped conductive ceiling electrode, the helical coil antenna substantially spanning the height between the top of the cylindrical side portion and the ceiling, and a switch for individually connecting each one of the coil antenna, the ceiling electrode and the wafer pedestal to one of (a) a respective RF power source, (b) ground or (c) no potential (i.e., left unconnected or electrically floating). 
     The invention is also embodied in a plasma reactor having a vacuum chamber, a wafer-holding pedestal near the bottom of the vacuum chamber, gas injection ports and a vacuum pump, the reactor including a generally planar disk shaped conductive ceiling electrode at the top of the chamber, a generally dome shaped helical coil antenna having a truncated apex generally coplanar with the ceiling electrode and a base generally coinciding with a circumference of the vacuum chamber, and a switch for individually connecting each one of the coil antenna, the ceiling electrode and the wafer pedestal to one of (a) a respective RF power source, (b) ground or (c) no potential (floating). 
     The invention is further embodied in a chemical vapor deposition plasma reactor having a vacuum chamber, a wafer-holding pedestal near the bottom of the vacuum chamber and a vacuum pump, the reactor including deposition precursor gas injection ports connectable to a chemical vapor deposition precursor gas source, a generally planar disk-shaped silicon ceiling electrode at the top of the chamber, apparatus for applying RF energy to the interior of the chamber so as to maintain a plasma therein, and a switch for individually connecting selected ones of the RF energy applying apparatus, the ceiling electrode and the wafer pedestal to one of (a) a respective RF power source, (b) ground or (c) no potential (floating). 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a simplified cross-sectional elevational view of a plasma reactor embodying the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Overview 
     In one aspect the invention is embodied in a plasma reactor chamber having a dome-shaped helically coiled antenna in the side-portion of the ceiling surrounding a planar electrode in the center-portion of the ceiling. Means are provided for separately coupling RF energy to the coil antenna, the ceiling electrode and the wafer pedestal at the bottom of the chamber, so that any one or combination of the ceiling electrode, the coil antenna and the wafer pedestal can receive RF energy or be grounded or be unconnected (float). The coil antenna provides inductive coupling or capacitive coupling or both depending upon the type of plasma required, the ceiling electrode provides capacitive coupling and, when energized simultaneously, the ceiling electrode and coil antenna together provide combined capacitive and inductive coupling to a plasma inside the reactor chamber. During CVD processing, the coil antenna receives RF energy to provide an inductively coupled plasma while the ceiling electrode is grounded and an RF bias is applied to the wafer pedestal. During chamber cleaning operations, RF energy is applied to the ceiling electrode to strike a capacitively coupled plasma while the wafer pedestal is permitted to electrically float (to avoid damage to the wafer pedestal and associated hardware) and the coil antenna is grounded or permitted to float. Once the capacitively coupled plasma has been ignited, RF energy may be applied to the coil antenna to maintain the plasma at a higher plasma density, thus providing additional species and heat to enhance removal of silicon dioxide and similar compounds. 
     In an alternative embodiment, RF power is applied to the wafer pedestal while a non-production (“dummy”) wafer is held on the pedestal, to strike a capacitively coupled plasma to clean the chamber. In this embodiment, the top electrode is preferably either grounded or has RF power applied to it. This method is also applicable using an inductively coupled plasma. The ceiling electrode is preferably heated to a temperature on the order of between approximately 260 and 270° C. to increase the chamber cleaning rate. 
     A capacitive plasma can be ignited by (i) applying RF power to the ceiling electrode alone, (ii) simultaneously applying power to the ceiling electrode and the coil, and (iii) simultaneously applying RF power to the ceiling electrode, the coil and the wafer pedestal. Each of these three different modes provides a unique advantage. Applying power to the ceiling electrode helps clean the grounded surfaces and ceiling surface fast. Applying power to both the ceiling electrode and the coil enhances cleaning of dielectric surfaces while cleaning the metal surfaces fast because of enhanced fluorine dissociation and higher plasma temperature. Applying RF power simultaneously to the ceiling electrode, the coil and the wafer pedestal reduces sputtering of the metal chamber wall surfaces by providing additional ground path(s) to the plasma. Thus, the choice of one of the three modes (i), (ii) and (iii) above may be made in accordance with the distribution of deposited oxide residue in the chamber. A “dummy” wafer may be kept on the wafer pedestal during chamber clean operations to protect the pedestal surface from etching. This is particularly preferred when operating in mode (iii) above. 
     It is not necessarily required to ground the ceiling electrode in order to ignite either an inductively coupled plasma or a capacitively coupled plasma 
     For an inductively coupled plasma, RF power is applied to the coils to ignite the plasma while maintaining a certain pressure level (typically between 30 and 300 milliTorr depending upon the applied RF power level). Once the plasma is ignited, the pressure may be reduced to less than 30 mTorr to maintain the plasma. 
     Description of the Reactor Structure 
     Referring to FIG. 1, a plasma reactor of the preferred embodiment has a vacuum chamber  10 , a vacuum pump  12  and gas injection nozzles  14  connected to a gas supply ring manifold  16  in a metal housing  18  supporting the injection nozzles  14 . The chamber  10  has a ceiling  20  consisting of a partial dome shaped or hemispherical annulus  22  surrounding a planar disk  24 . The dome shaped annulus  22  is an insulator such as quartz or a ceramic. The dome shaped annulus  22  supports a helically coiled RF antenna  26 , one terminal of which is connectable through a capacitor to ground by a switch  30  and the other end of which is connectable to an RF source  32  through a switch  34 . The structural details, including the length, of the helically coiled RF antenna  26  is disclosed in the above-referenced U.S. patent application by Fairbairn and Nowak, the disclosure of which is incorporated herein by reference. For example, the RF source  32  includes an RF generator and a conventional RF matching network (not shown). 
     The ceiling  24  is a conductor which is connectable to either an RF source  36  or ground through a switch  38 . The bottom of the chamber  10  may include a removable grounded conductive bottom annular ring or liner  40  which may be doped polysilicon or aluminum and a wafer pedestal  44  for holding a semiconductor wafer  45 . Preferably, a dielectric annulus  44   a  coplanar with the top surface of the wafer  45  surrounds the wafer  45  on the pedestal  44 . The wafer pedestal  44  may include an electrostatic chuck, cooling fluid channels and other conventional features not crucial to the present invention. The conductive annular ring  40  has a convex curved interior surface  40   a  facing the interior of the chamber  10  of approximately the same surface area as the ceiling electrode  24 . This feature facilitates capacitive discharge and exposes the chamber bottom ground surfaces toward the plasma. As a result, when an RF source is applied to the ceiling electrode  24  and the annular ring  40  is grounded, both are etched at about the same rate, a significant advantage. The removable ring annulus liner  40  is advantageous for periodic maintenance and chamber bottom cleaning. 
     As noted previously herein, flat ceiling electrodes (such as the type illustrated in the above-referenced European application of Collins et al.) tend to build up contamination (e.g., silicon dioxide) nonuniformly. In the case of an etch reactor with a flat ceiling, the deposit consists primarily of polymer which tends to deposit more on cooler unbiased surfaces, i.e., the outside edges of the ceiling electrode. In the case of a CVD reactor with a flat ceiling, most of the oxide deposit is located in the center of the ceiling electrode across from the wafer. This is due to the sputtering component of the deposition process induced by RF bias of the wafer pedestal. Consequently, in the CVD reactor, the outer portion of the ceiling electrode clears first during chamber clean, regardless of the plasma discharge mode. In order to overcome this problem, the ceiling electrode  24  of the reactor of the present invention (FIG. 1) is reduced to a size less than the diameter of the side wall (but preferably slightly larger than the wafer the ceiling electrode diameter could also be substantially the same size as the wafer), thus eliminating the portion of the ceiling electrode tending to a thinner residue accumulation and therefore to etching or overcleaning. 
     A related advantage is that the gently curved concave surfaces of the bottom aluminum liner  40  and the quartz dome annulus  22  eliminate sharp corners in the chamber interior which would otherwise be difficult to clean during the chamber clean operation. 
     Preferably, the materials of the chamber  10  are selected to minimize or eliminate metal surfaces which would otherwise contribute to formation of aluminum fluorides during chamber cleaning operations. Specifically, the ceiling electrode  24  and the bottom annular ring  40  may both be silicon, for example, polysilicon doped to a resistivity less than about 0.2 ohms per centimeter. An advantage of polysilicon is that any silicon removed therefrom during the chamber cleaning (etch) operation is volatile and readily pumped out of the chamber  10 . In order to further reduce the possibility of forming aluminum fluoride during chamber cleaning operations, the gas injection nozzles  14  are a ceramic material such as aluminum oxide or aluminum nitride. Moreover, a skirt  46  of quartz, silicon, polysilicon or ceramic covers the metal body  18 . Furthermore, the metal body  18  is grounded to reduce the possibility of arcing during chamber cleaning operations. Thus, in the preferred embodiment there are virtually no metal surfaces exposed to the chamber interior. The wafer pedestal  44  is preferably unconnected to any electric potential (i.e., electrically floating) during chamber cleaning operations so that very little etching thereof (or formation of aluminum fluoride therefrom) occurs during chamber cleaning operations. 
     The wafer pedestal  44  is connectable to ground or an RF source  50 , or may be electrically isolated, by a switch  52 . 
     One advantage of the reactor of FIG. 1 is that the coil antenna  26  may be energized simultaneously or after the ceiling electrode  24  has struck a high pressure capacitively coupled plasma in the chamber  10 . The advantage of energizing the coil antenna  26  at this point is that it provides an efficient means of maintaining the capacitively coupled plasma and it provides a certain amount of inductive and/or capacitive coupling to the plasma near the quartz annulus  22  so as to etch residues from the quartz annulus  22 . This coupling mode causes the quartz material of the annulus  22  to be etched at a rate less than that of a pure low pressure inductively coupled plasma mode, a significant advantage. Furthermore, the inductive coupling provided by the coil antenna  26  generates additional heat in the chamber  10 . High temperature greatly enhances the removal of silicon dioxide from the ceiling electrode  24  and other locations in the chamber  10 . It should be noted that the ceiling itself serves as a heat sink rather than a heat source because a lid  56  covering the ceiling electrode  24  has cooling jackets  58  running throughout its interior as shown in FIG.  1 . The cleaning rate of the ceiling electrode can, however, be accelerated by using an external heat source such as a resistive heater  60  to raise the temperature of the ceiling electrode to between 260 and 270 degrees C. for example. 
     In one preferred implementation of the reactor of FIG. 1, the radius R 1  of curvature of the interior surface  22   a  of the quartz annulus  22  was 325 cm (13″), the radius of curvature of the interior surface of the bottom ring  40  was 250 cm (9.5″-10″), the diameter D of the ceiling electrode  24  was 225 cm (9″), the diameter of the wafer pedestal  44  was 200 cm (8″) and the height of the ceiling electrode  24  from the pedestal  44  was 112 cm (4.5″). However, the invention is not limited to any particular design geometry. 
     Methods of Operating the Reactor 
     The invention, in another aspect, is embodied by certain methods of using the multi-mode coupled plasma reactor of FIG.  1 . Basically, the plasma reactor of FIG. 1 has three possible plasma coupling modes: (1) a predominantly inductively coupled plasma, (2) a predominantly capacitively coupled plasma, and (3) a mixture of both inductive and capacitive coupling simultaneously, depending upon the pressure regime, chemistry and applied RF power level. 
     In order to perform chemical vapor deposition of, for example, silicon dioxide onto a wafer  45  held on the pedestal  44 , the reactor is operated in the first mode to provide an inductively coupled plasma. A silicon precursor gas such as silane is introduced into the chamber  10  along with an oxygen containing gas, using the injection nozzles  14  for the silane. The pump valve  12   a  is positioned to maintain the chamber pressure at or below 100 milliTorr. The ceiling electrode  24  is grounded while 1000-5000 watts of RF energy at between 300 kHz and 14 MHz (preferably 2.0 MHz) is applied from the RF source  32  to the coil antenna  26 . The wafer pedestal may be either grounded or floating or an RF bias from the RF source  50  may be applied to the pedestal  44  to generate a suitable plasma sheath voltage over the semiconductor wafer  45  held on the pedestal  44  in accordance with the techniques disclosed in the above referenced application by Collins et al. and the above-referenced application by Fairbairn and Nowak. In some cases, it may not be necessary to ground the ceiling electrode in order to strike an inductively coupled plasma. 
     In order to perform a chamber clean operation for a chamber of a volume capable of holding an 8-inch diameter wafer (e.g., between 25 and 30 liters), the chamber pressure is set at between 0.5 and 5 Torr while between 0.5 and 3 liters per minute of a fluorine-containing gas (e.g., NF3, CF4, C2F6, etc.) and oxygen is pumped into the chamber  10 . The wafer pedestal is disconnected and allowed to electrically float by the switch  52 , the coil antenna is initially disconnected or grounded by the switches  30 ,  34  while between 1000 and 5000 watts of RF energy at between 300 kHz and 14 MHz (preferably 1.8 MHz) is applied to the ceiling electrode by the RF source  36  through the switch  38 . Once the plasma is struck, the coil antenna  26  may be connected to the RF source  32  by the switch  34  for the following three purposes: (1) the small component of capacitive coupling provided by the coil antenna  26  efficiently maintains the capacitively coupled plasma which has already been struck, (2) the proximity of the coil antenna  26  to the quartz annulus  22  enables etching or cleaning of residue or deposits from the annulus  22  and (3) the inductive coupling of the coil antenna  26  generates heat in the chamber  10 , which facilitates the removal of silicon dioxide residues from interior chamber surfaces such as the ceiling electrode  24  and the quartz material around the pedestal. 
     In order to perform a sputtering or etch operation with purely capacitive coupling, the reactor is operated as described in the preceding paragraph, with the exception that the coil antenna remains disconnected or grounded throughout. The capacitive coupling mode provides much faster etch rates because it is characterized by plasma sheath voltages on the order of 50 volts, while the inductive coupling mode is characterized by plasma sheath voltages on the order of only 5 volts. 
     Following the above-described chamber clean operation, adsorbed fluorine atoms need to be removed from the chamber interior. For this purpose, a hydrogen-containing plasma is struck in the chamber  10  in the same manner as described with reference to the chamber cleaning operation, with the exception that hydrogen gas (or a hydrogen-containing gas such as silane) is substituted for the NF3 gas. 
     The capacitive discharge mode is more favorable for chamber cleaning due to a higher voltage drop across the sheet in the vicinity of surface to be cleaned. This results in a higher energy of the ions impinging on the surface, thus facilitating breakdown of the silicon-oxide bond. However, the reactor may be cleaned in either regime, capacitive or inductive coupling. Matching of cleaning rates in different areas of the reactor can be achieved by proper clean regime selection. In fact, a two step clean process may be employed consisting of an inductive mode plasma clean step followed by an capacitive mode plasma clean step. Alternatively, the order of these two steps may be reversed. 
     In the capacitive clean mode the pressure has to be relatively high (greater than 0.5 Torr) to prevent excessive sputtering of the chamber walls. In the low pressure regime, plasma coupling is primarily inductive and minimal wall surface biasing occurs. The inductive discharge mode leads, however, to generally higher plasma temperatures. A capacitively coupled plasma is very efficient for cleaning the electrodes. It is the inductive enhancement of the generally capacitively coupled plasma described above that helps to clean the quartz dome and other areas of the chamber not directly biased by the capacitive discharge. 
     The wafer pedestal can be biased, grounded or floating. It is beneficial to be able to so control the pedestal potential in order to either clean the pedestal from residues or, alternatively, to protect it from overcleaning and damage. 
     A key advantage of the Chamber Leaning process described above is that for a given reactor temperature the cleaning process is five to ten times faster than the cleaning process for an inductively coupled high density plasma reactor of the type described in the above-referenced application by Fairbairn and Nowak. 
     While the invention has been described with reference to a preferred embodiment in which separate individual RF sources are furnished for the ceiling electrode  24  and the coil antenna  26 , the coil antenna and the ceiling electrode may be connected together to a common RF source whenever both are to receive RF energy simultaneously. 
     Other methods of using the reactor of FIG. 1 are useful. By adjusting the ratio of RF power applied to the ceiling electrode  24  and the coil antenna  26 , the reactor of FIG. 1 may be operated in a broad pressure range from below 10 milliTorr to 15 Torr, the lower pressure regimes favoring a predominantly inductively coupled plasma and the high pressure regimes favoring a predominantly capacitively coupled plasma 
     The problem of depositing excessive amounts of CVD residue (e.g. SiO 2 ) on the interior chamber surfaces is solved in the embodiment of the invention shown in FIG. 1 by the periodically circumferentially spaced radially inwardly-directed gas feed nozzles  14  each connected through the cylindrical chamber side wall to the gas supply manifold  16  therein. The nozzles  16  extend from the chamber side wall to points close to the periphery of the wafer  45 , but do not overlie it. This arrangement allows the release of CVD gas (e.g. silane) very near the wafer  80 , rather than adjacent the chamber sidewalls, thus increasing deposition on the wafer  45 , and decreasing deposition on the chamber sidewalls. 
     It is noted that structural elements, such as the nozzles  14 , placed near the wafer periphery are subject to heating. If the nozzles  14  are heated while a CVD gas, such as silane, is flowing through them, a possibility exists that amorphous silicon residues could form in the interior of the nozzles  14 . This could result in nozzle failure. However, heating does not present a problem with the nozzle arrangement shown in FIG. 1 because the chamber wall  102  acts as a heat sink to the nozzles  14 , cooling the nozzles  14  well below the temperature at which silane tends to break down to from the amorphous silicon. 
     It is further noted that structural elements placed near the periphery of the wafer could adversely affect the density of the plasma. However, the nozzles  14  present such a small cross-section to the plasma that there is little loss of the plasma density. In addition, the nozzles  14  extend close to, but not over the wafer  45 , to minimize any impedance between the plasma and the wafer  45 . 
     In order to achieve a uniformity of deposition across the surface of the wafer  45 , at least four, and as many as twelve nozzles  14 , could be spaced around the periphery of the wafer  45 . For example, with an 8-inch wafer surface, more than eight periodically spaced nozzles are preferred. 
     While the invention has been described in detail by specific reference to preferred embodiments, it is understood that variations and modifications thereof may be made without departing from the true spirit and scope of the invention.