Plasma etch process and plasma processing reactor

Abstract: A plasma reactor chamber (16) uses an antenna (30) driven by RF energy (LF, MF, or VHF) which is inductively coupled inside the reactor dome (17). The antenna (30) generates a high density, low energy plasma inside the chamber (16) for etching oxygen-containing layers overlying non-oxygen-containing layers with high selectivity. Auxiliary RF bias energy applied to the wafer (5) support cathode controls the cathode sheath voltage and controls the ion energy independent of density. Various magnetic and voltage processing enhancement techniques are disclosed, along with other etch processes, deposition processes and combined etch/deposition processes. Processing of sensitive devices without damage and without microloading is disclosed providing excellent yields. Etching of an oxygen-containing layer overlying a non-oxygen-containing layer can be achieved with high selectivity.

Description: The present invention relates to a plasma etch process. The invention is in the field of RF plasma processing reactors, and more particularly, of a plasma reactor which uses a radio frequency (RF) energy source for electromagnetically coupling the associated RF electromagnetic wave to the plasma, a silicon source being in contact with the plasma, and refers to processes carried out in such a reactor. The trend toward increasingly dense integrated geometries has resulted in components and devices of very small geometry which are electrically sensitive and susceptible to damage when subjected to wafer sheath voltages as small as approximately 200-300 volts due to energetic particle bombardment or radiation. Unfortunately, such voltages are of smaller magnitude than the voltages to which the circuit components are subjected during standard integrated circuit fabrication processes. Structures such as MOS capacitors and transistors fabricated for advanced devices have very thin (thickness < 20 nm (200 Angstroms)) gate oxides. These devices may be damaged by charge-up, resulting in gate breakdown. This can occur in a plasma process when neutralization of surface charge does not occur, by non-uniform plasma potential/or density, or by large RF displacement currents. Conductors such as interconnect lines may be damaged for similar reasons as well. Etching processes carried out in conventional plasma etch chambers are also becoming inadequate when very high aspect ratios, i.e., very deep and very narrow openings and trenches, must be formed in, or filled with, a variety of semiconductor materials. RF Systems Consider first prior art semiconductor processing systems such as CVD (chemical vapor deposition) and RIE (reactive ion etching) reactor systems. These systems may use radio frequency energy at low frequencies from about 10-500 KHz up to higher frequencies of about 13.56 - 40.68 MHz. Below about 1 MHz, ions and electrons can be accelerated by the oscillating electric field, and by any steady state electric field developed in the plasma. At such relatively low frequencies, the electrode sheath voltage produced at the wafers typically is up to one or more kilovolt peaks, which is much higher than the damage threshold of 200-300 volts. Above several MHz, electrons are still able to follow the changing electric field. More massive ions, however, are not able to follow the changing field, but are accelerated by steady state electric fields. In this frequency range (and at practical gas pressures and power levels), steady state sheath voltages are in the range of several hundred volts to 1,000 volts or more. An example of a prior art semiconductor processing system is disclosed in Document J. Vac. Sci. Technol. B9 (2) Mar/Apr. 1991, pp. 310-317. This document discloses a system for the application of a helicon source to plasma processing. It presents results of a study of the mode transitions in the helicon source when used in the geometry for plasma processing. The authors observe the basic characteristic of high density ( > 5 x 10 11 cm -3 in processing chamber at 500 W) and low plasma potential (~ 15 V) in this configuration. Magnetic Field-Enhancement A favorite approach for decreasing the bias voltage in RF systems involves applying a magnetic field to the plasma. This B field confines the electrons to the region near the surface of the substrate and increases the ion flux density and ion current and, thus, reduces the voltage and ion energy requirements. By way of comparison, an exemplary nonmagnetic RIE process for etching silicon dioxide might use RF energy applied at 13.56 MHz, an asymmetrical system of 10-15 liters volume, 6.6 Pa (50 millitorr) pressure and an anode area to wafer-support cathode area ratio of approximately (8-10) to 1, and develop substrate (cathode) sheath voltage of approximately 800 volts. The application of a magnetic field of 6 x 10 -3 Tesla (60 gauss) may decrease the bias voltage approximately 25-30 percent, from 800 volts to about 500-600 volts, while increasing the etch rate by as much as about 50 percent. However, the application of a stationary B field parallel to the substrate develops an E x B ion/electron drift and an associated plasma density gradient which is directed diametrically across the substrate. The plasma gradient causes non-uniform etching, deposition and other non-uniform film properties across the substrate. The non-uniformities may be decreased by rotating the magnetic field around the substrate, typically either by mechanical movement of permanent magnets, or by using pairs of electromagnetic coils which are driven in quadrature, 90 degrees out of phase, or by instantaneously controlling the current in pairs of coils to step or otherwise move the magnetic field at a controlled rate. However, although rotating the field reduces the non-uniformity gradient, typically some degree of non-uniformity remains. Furthermore, it is difficult to pack coils and, in particular, to pack two or more pairs of coils about a chamber and to achieve a compact system, especially when using a Helmholtz coil configuration and/or a multi-chamber system of individual magnetic-enhanced reactor chambers surrounding a common loadlock. A unique reactor system which has the capability to instantaneously and selectively alter the magnetic field strength and direction, and which is designed for use in compact multi-chamber reactor systems, is disclosed in commonly assigned U.S. Patent 4,842,683, issued June 27, 1989, in the name of inventors Cheng et al. Microwave/ECR Systems Microwave and microwave ECR (electron cylotron resonance) systems use microwave energy of frequencies >800 MHz and, typically, frequencies of 2.45 GHz to excite the plasma. This technique produces a high density plasma, but low particle energies which may be below the minimum reaction threshold energy for many processes, such as the reactive ion etching of silicon dioxide. To compensate, energy-enhancing low frequency electrical power is coupled to the substrate support electrode and though the substrate to the plasma. Thus, the probability of substrate damage is decreased relative to previous systems. However, microwave and microwave ECR systems operated at practical power levels for semiconductor substrate processing such as etch or CVD require large waveguides for power transmission, and expensive tuners, directional couplers, circulators, and dummy loads for operation. Additionally, to satisfy the ECR condition for microwave ECR systems operated at the commercially available 2.45 GHz, a magnetic field of 8.75 x 10 -2 Tesla (875 gauss) is necessary, requiring large electromagnets and large power and cooling requirements. Microwave and microwave ECR systems are not readily scalable. Hardware is available for 2.45 GHz, because this frequency is used for microwave ovens. 915 MHz systems are also available, although at higher cost. Hardware is not readily or economically available for other frequencies. As a consequence, to scale a 12.7 - 15.2 cm (5-6 in.) microwave system upward to accommodate larger semiconductor substrates requires the use of higher modes of operation. This scaling at a fixed frequency by operating at higher modes requires very tight process control to avoid so-called mode flipping to higher or lower order loads and resulting process changes. Alternatively, scaling can be accomplished, for example for a 5-6 in. microwave cavity, by using a diverging magnetic field to spread out the plasma flux over a larger area. This method reduces effective power density and thus plasma density. Further, ECR systems must be operated at very low pressures, on the order of 0.266 - 0.399 Pa (2-3 millitorr), because the density of the plasma generated in the system falls very rapidly above about 0.266 - 0.399 Pa (about 2-3 millitorr). This requires that a large volume of reactive gases be fed to the system, and also requires large vacuum exhaust systems to remove these large volume of gases. HF Transmission Line System EP application 91112917.9 filed July 7, 1991, entitled 'Plasma Processing Reactor' discloses a high frequency VHF/UHF reactor system in which the reactor chamber itself is configured in part as a transmission line structure for applying high frequency plasma generating energy to the chamber from a matching network. The unique integral transmission line structure permits satisfaction of the requirements of a very short transmission line between the matching network and the load and permits the use of relatively high frequencies, 50 to 800 MHz. It enables the efficient, controllable application of RF plasma generating energy to the plasma electrodes for generating commercially acceptable etch and deposition rates at relatively low ion energies and low sheath voltages. The relatively low voltage reduces the probability of damage to electrically sensitive small geometry semiconductor devices. The VHF/UHF system avoids various other prior art shortcomings, such as the above-described scalability and power limitations. Document J. Vac. Sci. Technol. A 5 (f) Jul/Aug 1987, pages 1585-1594 discloses a study of reactive ion etching related Si surface residues and subsurface damage, including their relationship to fundamental etching mechanisms. This article states that the selectivity of reactive ion etching of SiO 2 and Si 3 N 4 can over silicon can be improved by adding H 2 to the plasma gas for scavenging fluorine from the plasma. It is the object of the present invention to improve upon prior art etch systems and etch processes by increasing the selectivity of the etch process. In one aspect, the present invention, which overcomes prior art shortcomings, provides a plasma etch process according to independent claim 1, and apparatus according to independent claim 27. Further advantageous features, aspects and details of the invention are evident from the dependent claims, the description, examples and the drawings. BRIEF DESCRIPTION OF THE DRAWINGS Figs. 1-3 are schematic sectional views of several versions of a plasma reactor chamber system of the present invention. Fig. 4 is a diagram of a tuning circuit. Fig. 5 is a diagram of a load circuit. Fig. 6 is a diagram of a load circuit which applied RF input power. Fig. 7 is a diagram of a combination tuning circuit and load circuit. Fig. 8 is a diagram of another embodiment of a combination tuning circuit and load circuit. Fig. 9 is a diagram of still another embodiment of a combination tuning circuit and load circuit. Fig. 10 is a graph of etch/deposition rate versus bias voltage for silicon oxide and silicon. Fig. 11 is a wave form of DC bias voltage applied during etching. Fig. 12 is an alternate embodiment of a wave form of DC bias voltage applied during etching. Fig. 13 is a horizontal representation of a magnetic array at the dome of the reaction chamber of the invention. Fig. 14 A-D are diagrams of various shaped magnetic field lines of systems of the invention. Fig. 15 A-B are embodiments of Faraday shields useful in the systems of the invention. Fig. 16 is a block diagram of a control system for various components of the system of the invention. The present invention provides apparatus and process for plasma processing that allows improved selectivity for etching processes and variability of etching rates, and simultaneous deposition, etch and planarization of deposited layers. In the present chamber, preferably, LF/VHF (low frequency to very high frequency) RF power within the range 100 KHz to 100 MHz is used. More preferably, LF/HF power within the range 100 KHz to 10 MHz is used. Most preferably, MF (medium frequency) power is used within the range 300 KHz to 3 MHz. Preferably, the coupling means is a multiple turn, cylindrical coil antenna of uncoiled electrical length < λ /4 where λ is the wavelength of the high frequency RF excitation energy applied to the coil antenna during plasma operation. The present invention also encompasses means connected to the above antenna for tuning the antenna to resonance, as well as load means connected to the antenna to match the input impedance of the source to the output impedance of the means for supplying RF energy for the antenna. The tuning means may be a variable capacitance electrically connected between one end of the antenna and RF ground. The load means may be a variable capacitance electrically connected between the other end of the antenna coil and RF ground. RF energy may be applied via a tap at a selected location along the coil antenna. In another aspect, the present system includes a dielectric dome or cylinder which defines the source region. Preferably, the coil antenna surrounds the dome for inductively coupling the high frequency electromagnetic energy into the chamber. The article or substrate to be fabricated can be located within the source region or dome, within or closely adjacent the volume or the bottom turn of the antenna, or preferably, downstream of the antenna. The present invention also includes means for supplying gas to the chamber which comprises a gas inlet at the top of the dome, a first ring manifold at the base of the dome source region, and a second ring manifold surrounding the substrate support electrode, for selectively supplying processing diluent, passivation and other gases to the chamber. In yet another aspect of the present invention, an AC power supply and control system capacitively couples AC bias power, typically of the same or similar frequency as the source coil power, to a substrate support cathode, thereby effecting control of the cathode sheath voltage and ion energy, independent of the plasma density control effected by the source RF power. The system provides bias frequency selected to achieve a number of objectives. First, the upper frequency limit is selected to prevent "current-induced" damage (too high a frequency can cause charge-up damage to sensitive devices.) The lower frequency limit is selected in part to preclude "voltage-induced" damage. Lower frequency bias also yields higher substrate sheath voltages per unit bias power (less heating of substrates) and contributes less to plasma density and thus affords better independent control of ion density and energy. However, too low a bias frequency allows ions to follow the RF component of the substrate sheath electric field, thereby modulating ion energies. The result is a higher peak-to-average energy ratio and wider (double peak) ion energy distribution. Very low bias frequency causes insulator charge-up, inhibiting ion-induced processes during part of the bias frequency period. Conveniently, the preferred frequency ranges for satisfying the above considerations correspond to the preferred source frequency ranges as described above. The invention further includes control means for cyclically pulsing the DC bias voltage between low and high values selected, respectively, to form a passivation coating, such as a polymer coating, on a first selected material on a substrate to be etched for providing a relatively low etch rate of the substrate material, and a relatively higher etch rate and selectivity for a second material, e.g., a layer of second material overlying the substrate. In another aspect, the chamber is evacuated by a first vacuum pump means connected to the chamber proper and a second vacuum pump means connected to the dome, thereby establishing a vertical pressure differential across the dome for establishing a flow of neutral particles out of the dome, and wherein the voltage at the substrate support electrode is sufficient to overcome the pressure differential so that charged particles flow toward the chamber. Other aspects of the present invention include a conductive, Faraday shield of different configurations which is interposed between the coil antenna or other coupling means and the reaction chamber to prevent coupling of the electric field component of the high frequency electromagnetic energy into the chamber. Also, a high frequency reflector positioned surrounding the coil or other coupling means concentrates radiation of the high frequency energy into the chamber. Magnetic enhancement is supplied by peripheral permanent or electromagnet arrangements which apply a controlled static magnetic field parallel to the axis of the antenna, selected from uniform, diverging and magnetic mirror configurations, for controlling the location of and the transport of the plasma downstream relative to the substrate. Also, magnets may be mounted around the source and/or the chamber for applying a multipolar cusp field to the chamber in the vicinity of the substrate for confining the plasma to the substrate region while substantially eliminating the magnetic field across the substrate. In addition, a magnetic shunt may be positioned surrounding the substrate and the substrate support electrode for diverting any magnetic field from the substrate support electrode. The system construction permits scaling of its size by selecting the frequency of operation, while retaining low mode operation. In process aspects, the invention is embodied in a process for generating a plasma, comprising providing a vacuum chamber having a source region; supporting an article or substrate to be processed on an electrode in the chamber; supplying processing gas to the chamber; electromagnetically coupling RF energy into the chamber to control the plasma sheath voltage at the substrate support electrode; capacitively coupling RF energy into the chamber via the support electrode for controlling sheath voltage at the support electrode; and providing a source of silicon ions in contact with the plasma. The process also encompasses according to further aspects automatically and iteratively tuning the antenna to resonance and loading the input impedance thereof to the impedance of the RF energy supply for the antenna. In another aspect, the process for generating a plasma comprises providing a vacuum chamber having source and processing regions, and having walls, an electrode in the processing region and an electrode in the source region; connecting the electrode in the process region, the walls of the chamber and the source electrode electrically, with the processing region electrode being the cathode, the walls being the anode and the electrical connection of the source electrode being selected from ground, floating and RF or DC bias; supporting an article to be processed on the substrate support electrode; supplying processing gas to the chamber; using a cylindrical coil antenna connected to means of providing RF energy to the antenna, inductively coupling RF energy into the source region by means of the antenna for generating a plasma to process one or more materials on the article; and capacitively coupling RF energy into the chamber via the support electrode for controlling sheath voltage at the support electrode, and providing a source of silicon in the chamber. The source of silicon can be crystalline or polysilicon for example, silicon carbide, graphite and the like. Preferably, the silicon source is a third electrode located in the source region of the chamber, but the silicon source does not have to be an electrode, and can be elsewhere in the chamber providing it is in contact with the plasma. In another aspect, the antenna power and the bias power delivered to the electrode are controlled for selectively effecting anisotropic, semi-anisotropic and isotropic etching. The present processing chamber is particularly useful for etching a layer of a second material, most particularly an oxygen-containing layer such as an oxide, overlying a non-oxygen containing substrate, such as silicon. The use of a source of silicon ions in the processing chamber provides improved selectivity and etch profile enhancement. The process further encompasses cyclically driving the bias voltage to a low value selected to form an etch suppressing layer, such as a polymer, on the silicon and to a high value to etch the second material at a high rate relative to the substrate. In all cases, the present apparatus provides high etch rates and high selectivity required to produce high aspect ratio openings in the second layer down to the substrate. Processing in the present reactor also comprises sputter deposition of an oxide, such as silicon oxide or silicates, and a two-step process of, first, applying relatively low level RF power to the support electrode for depositing the oxide and, second, applying relatively higher level RF power to the support electrode for depositing silicon oxide, sputter faceting the silicon oxide and planarizing the layer. Specific process aspects include but are not limited to etching oxides, including etching contact holes in oxide formed over a substrate which does not contain oxygen, and etching via holes in oxide formed over aluminum or other substrates; high rate isotropic and anisotropic oxide etching; etching polysilicon conductors such as gates; photoresist stripping; anisotropic etching of, inter alia, single crystal silicon; anisotropic photoresist etching; low pressure plasma deposition of nitride and oxynitride; high pressure isotropic conformal deposition of oxide, oxynitride and nitride; etching metals, such as aluminum, tungsten and titanium, and compounds and alloys thereof such as tungsten silicide; and depositing various materials, at variable deposition rates, and including sputter facet deposition, locally and globally, and with planarization. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 1. OVERVIEW FIGS. 1-3 are schematic sectional views of several embodiments of a plasma reactor chamber system 10 for processing a semiconductor substrate 5 which uses an inductive plasma source arrangement, a magnetically-enhanced plasma source arrangement, a capacitively coupled bias arrangement and other aspects of the present invention. The three Figures illustrate preferred and alternative features of the system; three Figures are used because of drawing space limitations. The exemplary chamber is a modification of that depicted in the above mentiond co-pending incorporated European patent application, which include an integral transmission line structure. The salient features of the invention are applicable generally to plasma reactor chambers. Furthermore, it will be understood by those of skill in the art and from the description below that various features of the invention which cooperatively enhance the performance of the reactor system may be used separately or may be selectively omitted from the system. For example, the process conditions provided by the inductive plasma source arrangement and capacitively coupled bias source arrangement frequently eliminate any need for magnetic enhancement. The exemplary system 10 includes a vacuum chamber housing 11, formed of anodized aluminum or other suitable material, having sidewalls 12 and top and bottom walls 13 and 14. Anodized aluminum is preferred because it suppresses arcing and sputtering. However, other materials such as bare aluminum with or without a process-compatible liner of polymer or quartz or ceramic can be used. Top wall 13 has a central opening 15 between a lower chamber substrate processing section 16B defined between walls 12-12 and an upper chamber source section 16A defined by a dome 17. The dome 17 may be configured as an inverted single- or double-walled cup which is formed of dielectric material such as, preferably, quartz or several other dielectric materials, including alumina and alpha-alumina (sapphire). In the preferred arrangement shown in FIG. 1, the dome 17 comprises a cylindrical wall 17W of dielectric such as quartz and a cover or top 17T typically of aluminum or anodized aluminum. For processes such as high selectivity oxide etching, a silicon or silicon-containing top wall means, and silicon covered dome sidewalls are preferred. As shown in FIG. 1, the evacuation of the interior of the chamber housing 11 (chamber 16) is controlled by a throttle valve 18 (which regulates pressure independent of flow rate) in a vacuum line 19 which is connected to the bottom wall 14 and connects to a vacuum pumping system 21 comprising one or more vacuum pumps. As described in Section 11, the chamber components, including the chamber walls and dome, can be heated and/or cooled for enhanced process performance. For example, the dome 17 can be heated or cooled by a liquid or gas heat transfer medium, or heating elements can be used to heat the dome 17 directly. As described in Section 2 and depicted in FIG. 2, process gases, purge gases, diluents, etc., can be supplied to the chamber by three manifold injection sources, G1, G2, G3, located, respectively, at the base of the source (dome), the top plate 17T of the source, and peripheral to the wafer. The gases are supplied to the chamber 11, for example, typically from one or more sources of pressurized gas via a computer-controlled flow controller (not shown). At the main gas inlet manifold G1, the gases enter the internal vacuum processing chamber 16, as indicated at 22, through a quartz ring gas manifold 51, which is mounted on the inside of or integral with, the top wall 13. The manifold G1 preferably supplies etching gas and/or deposition gas at a slight upward angle to the chambers/chamber sections 16B and 16A for developing an etching and/or deposition plasma upon application of RF energy. A top manifold arrangement G2 in the top plate 17T of the dome 17 may be used to inlet reactant and other gases into the chamber 16. Also, a manifold arrangement G3 may be provided which is peripheral to the substrate to supply reactant and other gases. RF energy is supplied to the dome 17 by a source comprising an antenna 30 of at least one turn or coil which is powered by an RF supply and matching network 31. The antenna 30 preferably has a multiple turn cylindrical configuration. The antenna coil 30 defines a minimum conductor electrical length at a given frequency and a given source (coil) diameter and preferably has an electrical length less than one-quarter wavelength (< λ /4) at the operating frequency. By itself, the antenna 30 is not a resonator but is tuned to resonance as described below in Section 5 for efficient inductive coupling with the plasma source by Faraday's law of inductive coupling. Preferably, the gas flow from the chamber source section 16A is downward toward the substrate 5 to be processed and is then pumped radially outward from the substrate 5. To this end, an annular vacuum manifold 33 may be defined about the cathode transmission line structure 32, between the chamber wall 12 on one side and the outer transmission line conductor 320 on the other, and between the chamber bottom wall 14 on the bottom and a conductive pumping screen 29 on the top. The manifold screen 29 is interposed between the vacuum manifold 33 and the wafer processing chamber 16B and provides a conductive electrical path between chamber walls 12 and the outer conductor 320 of the cathode transmission line structure 32. The vacuum manifold 33 defines an annular pumping channel for implementing uniform radial pumping of exhaust gases from the periphery of the substrate 5. The vacuum manifold 33 communicates into the vacuum gas system line 19. The gas flow is along paths 22 from manifold G1 into the dome/source and/or along path 24 from manifold G2 into the dome/source and/or along paths 26 from manifold G3 radially inward toward the substrate 5. The overall gas flow is along path 34 from the upper chamber source section 16A toward the substrate 5, along path 36 from the substrate and through screen 29 into the gas outlet manifold 33, and along path 37 from the exhaust manifold 33 to the vacuum system 21. It should be noted that the conductive manifold screen 29 and the cathode transmission line structure are optional. Typically, at the low end of the frequencies of interest, the wavelength is very long and, thus, the transmission line structure is unnecessary. This contrasts with conventional RF system arrangements, in which the RF power is applied between two electrodes, typically the substrate support electrode 32C, the upper surface of which supports the substrate 5, and a second electrode which is the sidewalls 12, cop wall 13 and/or manifold G1 of the reactor chamber. Specifically, the antenna 30 is positioned outside and adjacent the dome 17 and the plasma chamber 16A for coupling the RF electromagnetic (em) energy into the source chamber 16A to induce electric fields in the process gas. By Faraday's Law of induction coupling, the changing B (magnetic) component of the energy energizes the process gas and thus forms a plasma in chamber 16 (numeral 16 collectively designates the chamber 16A and 16B and the plasma) characterized by relatively high density and low energy ions. The plasma is generated in the dome 17 concentrated in the small volume defined within the coil antenna 30. Active species, including ions, electrons, free radicals and excited neutrals, move downstream toward the substrate by diffusion and by bulk flow due to the prevailing gas flow described herein. Also, as described in Section 7, an appropriate magnetic field can be used to extract ions and electrons toward the substrate as described below. Optionally, but preferably, a bias energy input arrangement 41, FIG. 1, comprising a source 42 and a bias matching network 43, couples RF energy to the substrate support electrode 32C for selectively increasing the plasma sheath voltage at the substrata and thus selectively increasing the ion energy at the substrate. A reflector 47, which essentially is an open-bottom box, encloses the antenna 30 at the top and sides but not at the bottom. The reflector 47 prevents radiation of the RF energy into free space and thereby concentrates the radiation. and dissipation of the power in the plasma to enhance efficiency. As described in detail in Section 7, a Faraday shield 45, FIG. 3, may be positioned just inside, above and below the antenna 30 to permit the magnetic field coupling to the plasma but preclude direct electric field coupling, which could induce gradients or non-uniformities in the plasma, or accelerate charged particles to high energies. As described in Section 8, optionally, one or more electromagnets 81-81 FIGS. 2 and 3, or permanent magnets are mounted adjacent the chamber housing 11 for providing a magnetic field for enhancing the density of the plasma at the substrate 5, for transporting ions to the substrate, or for enhancing plasma uniformity. As is described fully in Section 4, the invention uses the magnetic component of inductively coupled electromagnetic energy, typically at frequencies much lower than microwave or microwave-ECR frequencies, to induce circular electric fields inside a vacuum chamber for generating a plasma characterized by high density and relatively low energy, without coupling potentially damaging high power RF energy through the substrate 5. In the preferred, illustrated downstream plasma source arrangement, the RF energy is fully absorbed remote from the substrate, with high plasma density, ensuring that the wave does not propagate to the substrate and thus minimizing the probability of damage. Selectively, and optionally, RF bias energy is applied to the substrate support electrode 32C for increasing the substrate sheath voltage and, thus, the ion energy, as required. Chamber 16 is capable of processing substrates including semiconductor wafers, by deposition and/or etching, using total chamber pressures of about 0.0133 Pa to about 6.6 kPa (about 0.1 mtorr to about 50 torr), and, for etching, typically 0.0113 Pa to 26.6 Pa (0.1 mtorr to 200 mtorr). The chamber can operate at pressures < 0.66 Pa ( < 5 mtorr) and, in fact, has run successfully at 0.266 Pa ( 2 mtorr). However, higher pressures are preferred for certain processes because of the increased pumping speed and higher flow rates. For example, for oxide etching, a pressure range of about about 0.66 Pa to about 6.6 Pa (about 5 mtorr to about 50 mtorr) is preferred. Such relatively high pressures require close spacing between the source and the substrate. The chamber can operate successfully at very suitable, close spacing, d, between the substrate 5 and the bottom turn of the antenna 30 of about 5 centimeters (2 inches) without charge-up damage to sensitive devices and, thus, is able to realize the advantages of such very close spacing: enhanced etch rates and selectivity; reduced bias voltage requirement and ion energy requirement for a given etch rate; and enhanced etch uniformity across the substrate. For example, reducing the spacing, d, between the wafer 5 and the source antenna 30 from 10 cm (4 in) . (which itself is close spacing) to 5 cm (2 in.) , has reduced the voltage requirement by half and has increased the uniformity from about 2.5 percent to about 1 percent. 2. GAS PROCESSING SYSTEM As mentioned, the chamber incorporates multiple gas injection sources G1, G2, G3, FIG. 2, for the purpose of injecting reaction, purge, etc., gases at different locations to enhance a particular process according to the requirements of that process (etching, deposition, etc.) and the particular material(s) used in that process. First, the chamber includes a standard radial gas distribution system G1 at the periphery of the base/bottom of the source region 16B. In a presently preferred configuration, the G1 injection system comprises a quartz gas distribution ring 51 at the bottom of the source and a peripheral annular manifold 52 defining a distribution channel which feeds gas to the ring. The ring has inward facing radial holes 53-53 and, preferably, stepped sintered ceramic porous gas diffuser plugs 54-54 inserted in the holes to prevent hollow cathode discharge. The second gas injection arrangement, G2, comprises a grounded or floating or biased dome top plate 17T of material such as anodized aluminum having a center gas inlet hole 56 filled with a porous ceramic diffuser disk 57. The third gas injection source, G3, comprises a ring-shaped gas inlet manifold 58 mounted at the periphery of the substrate 5 (or a gas inlet incorporated into the clamp ring (not shown) used to hold the substrate in position against the substrate support pedestal). In etching openings through oxide-containing layers, such as silicon dioxide, over a non-oxygen-containing surface, including single crystal or polysilicon, desirably the silicon dioxide is etched at a much faster rate than the polysilicon or other substrate. Fluoride-containing etch gases are used as etchants. However, since fluoride etchants normally etch materials such as silicon dioxide and polysilicon, for example, at the same rate, the etch has little selectivity for oxide rather than silicon. However, during reactive ion etching, a protective polymer layer is formed on the sidewalls and bottom of the growing opening ; such a polymer is formed from carbon and fluorine and generally contains about 30% of carbon and about 60% of fluorine. This polymer however is dissociated in the presence of fluorine atoms. What would be desirable is to form a carbon-rich polymer containing over about 50% of carbon and less than about 40% of fluorine; We have found that if a scavenger for fluoride ions is placed in the reactor, fewer free fluoride ions are present in the plasma and fewer C-F bonds will be formed in the polymer film. Some success in prior art reactors has been obtained by adding a scavenger for fluoride ions to the plasma itself, such as silane or organosilanes, to provide free silicon in the plasma. However, since many different plasma ions are thus formed, the process is essentially "dirty", the nature of the etchants and polymers are diverse and non-uniform and thus the process is not entirely reproducible. Thus the present apparatus, which provides a "clean" source of silicon to scavenge free fluoride ions, is advantageous. In accordance with the invention, such a fluoride ion scavenger is a reactant for fluoride, which preferably is a source of silicon near or in the plasma. The fluoride scavenger can be a pure silicon, such as single crystal silicon, polysilicon, or a compound such as silicon carbide and the like, or preferably, the third electrode can be made of silicon or a silicon-containing material. Graphite can also be used to scavenge fluoride ions; for example, the third electrode can be made of graphite. During plasma etching in the present chamber, the oxygen present in silicon oxide films readily etches the polymer formed on the sidewalls and the bottom of the growing trench. However, when the trench depth reaches the polysilicon or other non-oxygen-containing substrate, no oxygen is present and the polymer remains on the polysilicon surface, protecting it from further etching. The preferred etchants herein are fluorocarbons such as CF 4 , C 2 F 6 and C 3 F 8 , which generate only carbon ions and fluoride ions. Other known fluoride etchants, such as CHF 3 , are less preferred because they also generate hydrogen ions in addition to carbon and fluorine ions. Thus in accordance with the present invention very high selectivity between an oxygen-containing layer and a non-oxygen-containing substrate or underlying layer, can be obtained, almost up to infinity, because carbon-rich polymer films do not decompose on the silicon surface or non-oxide containing substrates or layers in the absence of oxygen. These polymers are sensitive to oxygen, and when no oxygen is present, such as when the etchant has reached a non-oxygen-containing substrate, reduced decomposition of the polymer takes place and when coupled to a plasma with a reduced number of fluorine ions, a comparatively inert polymer passivating layer is formed that protects the underlying layer. Preferably the silicon source is placed near where the plasma is generated, so that silicon can scavenge fluoride ions as they are formed, and fewer fluoride ions will react with the surface of the substrate to be treated. For example, a silicon mesh can be suspended in the plasma area, or silicon can be placed near the walls or top of the reactor as part of the rf source. A silicon source can also be suspended near the surface of the substrate, but polymers having higher fluoride content may result. The silicon source can also be placed outside of the plasma area of the reactor, in which case it can be heated to a temperature that will form silicon ions to scavenge fluoride ions, e.g., at east about 150°C or higher. In that case, a means of regulating the temperature of the silicon source will also have to be provided in the reaction chamber. As alluded to above, various types of gases selected from etchant and deposition species, passivation species, diluent gases, etc., can be supplied to the chamber via one or more of the sources G1 through G3, to satisfy the requirements of particular etch and deposition processes and materials. For example, the present inductive source antenna 30 provides a very high density plasma and is very effective in dissociating the gases in the dome source region 16A of the chamber. As a consequence, when a polymer-forming species is supplied to the dome via G1, or G2, the highly dissociated species may coat the interior of the dome at the expense of coating the polysilicon and/or may be so fully dissociated that it does not adhere to the polysilicon surface which is to be protectively coated. A solution is to inlet etchant species such as C 2 F 6 or CF 4 into the source region 16A via G1 or G2 or via G1 and G3 and provide silicon scavenging of fluoride ions to form a carbon rich polymer preferentially on the substrate. Because of the high dissociation of the gases in the source region, fluorine-containing gases (even those in which the fluorine is tied up with carbon) typically produce free fluoride ions which etch silicon and silicon oxide equally and, thus, reduces the etch selectivity for oxide. When high selectivity is required, in additional to providing a silicon source in the plasma, a silicon-containing additive gas can be injected to further scavenge the free fluoride ions and diminish etching of non-oxygen-containing substrates. The etchant gas and the silicon-containing additive gas can be introduced separately via G1 and G2 or can be introduced as a mixture via G1 and/or G2. Suitable fluorine-consuming silicon-containing additive gases include silane (SiH 4 ), TEOS, diethylsilane and silicon tetrafluoride (SiF 4 ). The fluorine-consuming silicon source and polymer-forming additive gases can be used together in the same process to jointly enhance etch selectivity. 3. DIFFERENTIAL PUMPING FIG. 3 depicts an alternative vacuum pumping configuration. In addition to the vacuum pumping system 21 which is connected to or near the bottom of the chamber, a vacuum pump 39 is connected via line 38 to the source region 16A inside the dome 17. The flow rates of the pumping systems 39 and 21 are selected so they generate vertically across the source region 16B a pressure differential, ΔP p , which (1) opposes the transport of uncharged particles from the source 16A to the substrate 5, yet (2) is of lesser magnitude that the force, F DC, exerted by the bias voltage on charged particles such as electrons and ions. As a consequence of Δ P p , uncharged particles such as radicals do not reach the substrate 5, but rather flow predominantly out of the top vacuum connection line 38. As a consequence of F DC > ΔP p , charged electrons and ions flow predominantly to the processing region. This approach is useful, obviously, where it is desired to selectively keep uncharged particles, but not ions, out of the substrate processing region. That situation occurs, for example, (1) during etching which uses polymer-forming gas chemistry, but polymers are formed in the source region which adhere to the chamber sidewalls and/or do not adhere well to the desired substrate surface and/or (2) when fluorine radicals are formed in the source region. 4. RF POWER, ANTENNA AND BIAS SOURCES 1). Top or Antenna Source Referring to FIG. 1, preferably, the operating frequency of the RF power supply 31 for the top source 16A is selected to provide a dense plasma, to minimize damage to sensitive devices and to provide efficient inductive coupling of the RF power to the plasma. Specifically, the upper frequency of the operating range is limited to minimize "current-induced" damage. The lower limit of the operating frequency is selected for efficiency of RF power coupling to the plasma. Suggested limits are given above. 2). Bottom or Bias Source The AC power supply 42 for the substrate support cathode 32C, capacitively couples RF power to the plasma, thereby effecting control of various factors including cathode sheath voltage and ion energy, which are controlled independently of the plasma density control effected by the high frequency power. The bias frequency is selected to achieve a number of objectives. First, the upper frequency limit is selected to prevent current-induced charge-up damage to sensitive devices. A lower frequency is selected in part to preclude voltage-induced damage. Lower frequency bias also yields higher substrate sheath voltages per unit bias power (less heating) of substrates and contributes less to plasma density and, thus, affords better independent control of ion density and energy. However, too low a bias frequency allows ions to follow the RF component of the substrate sheath electric field, thereby modulating ion energies. The result would be a higher peak-to-average energy ratio and wider (peak-to-peak) ion energy distribution. Very low bias frequency causes insulation charge-up, inhibiting ion-induced processes during part of the bias frequency control. We have discovered that, conveniently, as noted hereinabove, the above considerations can be satisfied using bias frequency ranges which correspond to the source frequency ranges as set forth hereinabove. 3). Combined Operation of Antenna and Bias Sources A preferred feature of the invention is to automatically vary the bottom of the bias power supplied by the AC power supply 42 to maintain a constant cathode sheath voltage. At low pressures (<66.5 Pa (<500 mtorr)) in a highly asymmetric system, the DC bias measured at the cathode support 32C is a close approximation to the cathode sheath voltage. The bottom power can be automatically varied to maintain a constant DC bias. The bottom or bias power has very little effect on plasma density and ion current density. Top or antenna power has a very strong effect on plasma density and on current density, but a very small effect on cathode sheath voltage. Therefore, it is desired to use the top power to define plasma and ion current densities, and the bottom power to define cathode sheath voltage. Because the radio frequency of the source 31 driving the antenna 30 is nonetheless much lower than the frequencies used in microwave or microwave-ECR applications, the optional smaller magnets operated at lower DC current by less expensive power supplies can be used, with associated smaller heat loads. In addition, as is obvious from the above discussion, co-axial cable such as 31C can be used instead of wave guides. In addition, the plasma non-uniformities caused by the E x B electron drift in other magnetic-enhanced or assisted systems are absent here, because the applied magnetic fields (both the magnetic component of the RF field applied via the antenna 30 and any static magnetic field applied by magnets 81) are substantially parallel to the electric field at the cathode. Thus, there is no E x B drift in the system. A magnetic shunt path formed with a high permeability material may be used to allow a B field in the source (upper chamber 16A) but not at the substrate. Optionally, permanent or electromagnets may be placed in a multi-polar arrangement around the lower chamber 16B, typically in an alternating pole north-south-north-south...north-south arrangement, to generate a multi-cusp magnetic mirror at the source and/or chamber walls. The magnets may be vertical bar magnets or preferably horizontal ring magnets, for example. Such magnets may be used to reduce electron losses to the walls, thus enhancing plasma density and uniformity, without subjecting the substrate to magnetic fields. 4) Combination and Synchronism of RF Sources As indicated above, the preferred frequency of operation of the top or antenna RF source and the preferred frequency of operation of the bottom or bias RF source both fall conveniently into the same range. One optional configuration approach is to combine these two RF sources into one single source, instead of using two separate sources. More generally, the possibilities are to supply all three RF signals (including RF bias to the third or top electrode) from a single source, or to use one source for the antenna and bottom bias and a second source for the third electrode; or to use three separate RF sources. 5. ANTENNA TUNE AND LOAD 1) Tuning Typically, the antenna 30 is tuned to resonance by (1) varying the frequency of the generator 31 to resonate with the antenna; or (2) a separate resonating element, connected to the antenna for tuning to resonance. For example, this tuning element can be a variable inductance-to-ground or a variable capacitance-to-ground. It is desirable to build the system to the highest desirable resonant frequency to accommodate the decrease in resonant frequency when using capacitance or inductance tuning variables. Automatic tuning is preferred and may be executed by using an Impedance phase/magnitude detector to drive the tune/load variables. See FIG. 16 and Section 9. Alternatively, a reflected power bridge or VSWR bridge may be used to drive both tune and load variables, but iteration is required. 2) Loading Conductive, capacitive or inductive load means L can be used to match the source antenna 30 to the impedance of the RF generator 31 and the connecting co-axial cable 31C. For example, a tap or wiper may be ohmically contacted to the antenna close to or at the 50 ohm or 300 ohm or other generator output impedance location along the antenna. Alternatively, a variable inductance or a variable capacitance may be connected to the generator output impedance point 50 on the antenna. 3). Tune and Load Circuits Referring to FIGS. 4 to 9, preferably, a tuning means T is provided which is integral to the source antenna 30 to tune the source to resonance. Also, integral load means L is provided to match the input impedance of the source antenna 30 to the output impedance of the associated power generator 31 (or transmission line 31C). Referring to FIG. 4, in one aspect, the tune means T is a variable capacitance which is electrically connected between one end of the antenna 30 and RF ground. As shown in FIG. 5, in another aspect the load means L may be a variable capacitance which is electrically connected between one end of the antenna and RF ground. Also, the load means may be a variable position tap 60 which applies RF input power to the antenna. See FIG. 6. In a preferred combination shown in FIG. 7, the tuning means T is a variable capacitance which is electrically connected between one end of the antenna 30 and RF ground and the load means L is another variable capacitance which is electrically connected between the other end of the antenna and RF ground. In this arrangement, the RF input power can be applied to the antenna 30 via a tap, that is, by a tap 60 applied along the antenna 30 or at either end thereof. See FIG. 8. Alternatively, the RF power input connection 66 can be positioned at substantially the connection between the load variable capacitance L and the end of the antenna 30, as shown in FIG. 9. 6. SOURCE/BIAS PROCESS CONTROL The invention also incorporates the discovery that the etch rate of materials such as silicon dioxide is increased and the etch selectivity of silicon dioxide relative to materials such as silicon is increased by using a sufficiently high bias voltage to provide a high silicon dioxide etch rate and periodically pulsing the bias voltage to a low value. 1) Pulse/Modulated Bias-Enhanced Etch Rate and Selectivity Referring to FIG. 10, typically the etch rates of materials such as silicon dioxide, SiO 2 , increase with the bias voltage. Thus, increasing the bias voltage increases the etch rate of the oxide. Unfortunately, however, the etch rates of associated materials in the integrated circuit structure such as silicon/polysilicon also increase with the bias voltage. Thus, the use on a bias voltage of sufficient magnitude to provide a very high silicon dioxide etch rate also effects a silicon etch rate which (although it may be somewhat lower than the oxide etch rate) is undesirably high and reduces selectivity. Quite obviously, when etching silicon dioxide it is highly desirable to have the high oxide etch rate characteristic of high DC bias voltages, V h , combined with the relatively low silicon etch rate characteristic of low DC bias voltages, V l , and, thus, high oxide selectivity. Referring to DC bias voltage wave form 70 in FIG. 11, the seemingly contradictory goals expressed in the previous paragraph of combining the V h and V l characteristics are, in fact, achieved in polymer-forming etch processes (those processes which form an etch-suppressant polymer on materials such as silicon) by using a high base line DC bias voltage, V h , and periodically pulsing or modulating the voltage to a low value, V l . V l is at or below the cross-over point/voltage 68, FIG. 10, between silicon etching and silicon deposition, yet is at or above the oxide cross-over point/voltage. As a result, a protective polymer is deposited on the silicon to suppress etching thereof during return to the high rate etch voltage, V h , but no or insufficient deposition occurs on the oxide to significantly suppress the etching of the oxide at V h . Preferably, V l is characterized by deposition on the polysilicon, but at least slight etching of the oxide. In a presently preferred embodiment, the values of the parameters, V h (the high DC bias voltage), V l (the low DC bias voltage), P w (the pulse width of the low voltage, V l ), and P rp (the pulse repetition rate or combined width of the low voltage and the high voltage pulses) are, respectively, -400 V, -225 V, about 0.1 seconds, and about 1 second. 2) Dual Frequency Bias An alternative approach is depicted by DC bias voltage wave form 71 in FIG. 12. A relatively low frequency voltage variation is superimposed on the basic bias voltage frequency. For example, a slow frequency, T 2 , ≤ 25 KHz (preferably, 5 - 10 KHz) may be superimposed or mixed with the base radio frequency, T 1 , ≤ 2 MHz. Silicon oxide is an insulator; silicon/polysilicon, typically, has only a very thin native oxide layer. Thus, the low frequency T 2 DC bias voltage variations are not seen at the oxide surface because it charges up. However, the essentially uninsulated polysilicon responds to the low frequency T 2 in a manner similar to that described previously by forming a protective layer during the low voltage excursion 72 (V l ) of the low frequency, T 2 , cycle. This low frequency-formed layer inhibits etching during the variable high voltage excursions 73 of the high frequency, T 1 , cycles. As mentioned, the insulating nature of silicon dioxide prevents etch suppressing deposition thereon during the low voltage excursions of T 2 and the oxide etch proceeds unabated during the high voltage portions of the T 1 cycle. In short, a protective layer is formed on the silicon during the low voltage excursions 72 of the low frequency cycle, T 2 , suppressing silicon etching during the high voltage excursions 73 of the high frequency cycle, T 1 , which etches the oxide rapidly without deposition suppression. The result, similar to that for the pulse/modulated approach described above, is a high silicon oxide etch rate, a relatively low overall polysilicon etch rate and high etch selectivity for oxide. Please note, the pulsed/modulated approach is presently preferred to the dual frequency bias approach because of the precisely controlled nature of the former approach. 7. FARADAY SHIELD Referring to FIGS. 1 and 15, to decrease the steady state electrostatic field, the chamber optionally incorporates a Faraday shield 45. In one embodiment shown in FIG. 15A, the structure is what we term a "single" Faraday shield 45S comprising a cylindrical array of grounded spaced, axially extending posts or bars, etc., surrounding the dome walls 17W and the antenna 30. The single shield may vary from large spacing configurations to those having very small gaps between the shield sections. FIG. 15B depicts a so-called "full" Faraday shield 45F comprising a pair of concentric shields spaced so that the bars of one overlap the gaps of the other and vice versa. This precludes line of sight paths for the electric field lines through the shield and thereby shunts the electrostatic field. Although various configurations of the Faraday shields 45S and 45F are possible, the presently preferred configuration is the outwardly flanged, electrically conductive, open-ended cylinder configuration depicted in vertical cross-section in FTG. 1. The single or double wall apertured field surfaces 46, 47, 48 extend around the top, inner (source) and bottom sides, respectively, of the antenna 30 while a ground side 49 (which may be solid) is positioned at the cutside of the antenna 30. This configuration allows the axially-directed, magnetic component of the em wave from the antenna 30 to induce closed loop electric fields in and parallel to the plane of the antenna 30, which generate the plasma. However, the shield 45 capacitively shunts the direct electric field component to ground and prevents the direct electric field component of the high frequency electromagnetic energy from coupling to the plasma. Without the shield 45, the varying voltage along the antenna 30 would couple to the plasma in accordance with Maxwell's equations for capacitive displacement current coupling. This may induce non-uniformities and gradients in the plasma density and in the energy across the substrate 5 and result in process non-uniformity and high energy charged particles. In short, a discontinous or slitted or sectioned Faraday shield minimizes the shorting effect of the shield on the changing em field from the coil, reduces eddy current losses, and allows coupling of the radio frequency, axially directed fringing magnetic field to the plasma for inducing closed loop electric fields which generate the plasma, but precludes direct coupling of the electric field (which varies along the antenna) to the plasma and, thereby, precludes any associated loss of plasma uniformity and process uniformity for high energy charged particles. 8. MAGNETIC FIELD CONFINEMENT AND ENHANCEMENT 1) Confinement To reduce losses (decreased plasma density) at the walls 17W of the cylinder/dome source, a magnetic arrangement is provided which generates a peripheral annular (shallow) field. In a preferred arrangement, shown in the FIG. 13 horizontal section representation, this field is provided by a closely-spaced "bucket" or cylindrical multi-polar array of axially-oriented permanent magnets or electro-magnets 76-76, each of which is magnetized across its small dimensions to form a closed, alternating pole, peripheral -N-S-N-S-magnetic field B. The multi-polar array generates a multi-cusp magnetic mirror 77 at the dome wall. Alternatively, the array may be horizontal ring magnets. Such magnets reduce alectron losses to the walls 17W, thus enhancing plasma density and uniformity without subjecting the substrate to magnetic fields. Referring to FIG. 3, in another aspect, the plasma in the substrate proccessing region 16B can be decoupled from the plasma in the generating or source region 16A by positioning a generally planar grid of magnets at the bottom of the source region/top of the processing region. The magnetic grid comprises closely-spaced generally parallel magnetic bars 78-78 which, like the above-described bucket arrangement, are magnetized NS across their small dimension to provide a planar configuration -NS-NS-NS- magnetic field with the field lines originating at one bar and terminating at the next. The resulting generally planar magnetic filter 79 across the opening 1.5 of the source confines the magnetic field to the plane/region of the plate and does not penetrate into either the source or substrate region. Due to the relationship F = qV x B, high energy/high velocity electrons in the source are bent back or repelled by this magnetic filter 79 to a greater extent than are ions, and are not able to penetrate to the substrate processing region. This reduces the density of high energy electrons in the processing region 16B and decreases the plasma density in that region. The processing and source regions are decoupled. 2) Enhancement As mentioned above, one or more (preferably, at least two) permanent or electromagnets 81-81, FIG. 3, may be used to define a static, generally axial magnetic field orthogonal to and through both the horizontal plane of the antenna coils and the electric fields induced by the radio frequency RF radiating antenna. Preferably, as described below, one of three field-types is used: uniform, divergent or magnetic mirror. Referring to FIG. 14A, a homogenous, axial uniform magnetic field 82 applied orthogonally to the substrate 5 by the magnets 81-81 restricts the motion of the electrons to the walls. Because of the inability of ions to follow high frequency field variations, the ions follow the electron deficiency, and are concentrated in the plasma over the substrate. For maximum efficiency, this and other static magnetic fields can be tuned to resonance with the high frequency electromagnetic field: omega = 2 π f = Be/m, where B is the magnetic flux density and e and m are the electron charge and mass, respectively. An axially divergent field 83 is depicted schematically in FIG. 14B. By the conservation of magnetic moment, the axial gradient of the magnetic field converts circular translational energy to axial translational energy and tends to drive the electrons and ions from the stronger field regions to the weaker regions thereof. Diverging magnetic fields can be used to push the electrons and ions from the plasma generating regions and to concentrate the plasma at the substrate. Referring to FIGS. 14C and 14D, there are shown, respectively, a bulging or aiding magnetic field 84 (FIG. 14C) and a cusp-shaped or opposing field 85 (FIG. 15D). The effect of each of these so-called "magnetic mirror" fields is similar to that of the axially divergent field: charged particles are driven from the relatively strong field regions (at the ends) toward the relatively weak central region. Selectively positioning the magnet(s) and selecting and varying the strength of the fields provided by the single magnet or cooperating magnets shapes the associated uniform, diverging, or magnetic mirror field in controlled fashion to increase the density of the plasma at the substrate. For magnetic mirror fields, the preferred substrate position for maximum plasma density enhancement is closely adjacent to or at the bulge or cusp, to provide maximum plasma density enhancement. 9. CONTROL SYSTEM The following descriptions are used here in reference to the control system depicted in FIG. 16: Psp: Power set point P f : Forward power Measured by directional coupler located at/inside power supply P r : Reflected power " " |Z| : Magnitude of impedance <phi : Phase of impedance Tsp: Tune set point Lsp: Load set point Tfb: Tune feedback (measured) Lfb: Load feedback (measured) FIG. 16 is a block diagram of an exemplary system for controlling the various components including the power supplies. Here, a system controller 86 is interfaced to the antenna power supply 31, an impedance bridge 87, the antenna 30, a bias power supply 42, an impedance bridge 88, the matching network 43, and the cathode 32. The process parameters antenna power and DC bias, selected for ion flux density and ion energy, are supplied as input to the controller 86. The controller 86 may also control other parameters such as gas flow(s), chamber pressure, electrode or wafer temperature, chamber temperature, and others. The controller 86 may preset initial tune 1 and load 1 conditions by issuing signals on Tsp 1 and Lsp 1 lines connected to the antenna 30. The controller 86 may also preset initial tune 2 and load 2 conditions by issuing signals on Tsp 2 and Lsp 2 lines connected to the matching network 43. Typically, these conditions are selected to optimize plasma initiation (gas breakdown). Power may be applied first to either the antenna 30 or to the cathode 32, or applied simultaneously to both. The controller 86 issues power set points on the Psp 1 line to the antenna power supply 31 and on the Psp 2 line to the bias power supply 42 simultaneously or sequentially (in either order). Avalanche breakdown occurs rapidly in the gas, generating a plasma. The controller 86 monitors forward power (P f1 ) and reflected power (P r1 ) to/from the antenna 30, and monitors forward power (P f2 ) and reflected power (P r2 ) to/from the cathode 32. DC bias (cathode to anode DC voltage) is also monitored as shown by the controller 86. The controller 86 adjusts the coil tune 1 and load 1 parameters by issuing set points on lines Tsp 1 and Lsp 1 , based on either (a) forward power P f1 and reflected power P r1 , or (b) impedance magnitude ¦Z 1 ¦ and impedance phase <phi 1 . The bridge 87 furnishes impedance magnitude and phase angle information to the controller 86. The antenna 30 is matched when reflected power P r1 is substantially zero and when the impedance (magnitude and phase ¦Z 1 ¦ < phi) is the complex conjugate of the coil power supply output impedance. (The zero reflected power condition and the conjugate impedance condition occur simultaneously, so either reflected power may be minimized or impedance may be matched, with the same result. Alternatively, VSWR (voltage standing wave ratio) or reflection coefficient may be minimized). The controller 86 adjusts the cathode 32 and the matching network 43 tune 2 and load 2 parameters by issuing set points on the Tsp 2 and Lsp 2 lines, based on either (a) forward power P f2 and reflected power P r2 or (b) impedance magnitude ¦Z 2 ¦ and impedance phase <phi 2 . The bridge 88 furnishes impedance magnitude ¦Z 2 ¦ and phase <phi 2 information to the controller 86. Matching occurs when, similarly to antenna matching, reflected power P r2 is essentially zero, and when impedance (magnitude and phase ¦Z 2 ¦ <phi 2 ) is the complex conjugate of the bias power supply 42 output impedance. The DC bias is monitored by the controller 86, which varies the bias power supply's output power to obtain the desired measured DC bias. The controller 86 subtracts the measured value of DC bias from the desired value of DC bias. If the difference is negative, the bias power supply 42 output is increased. If the difference is positive, the bias power supply output is decreased (higher bias power supply output generates a more negative DC bias). Proportional, proportional-integral, or proportional-integral-derivative control or other control may be used in accordance with this method. Alternatively, instead of the preferred embodiment of adjusting bias power supply 42 outpout to maintain a constant DC bias, a constant bias power supply output may be used. 10. TRANSMISSION LINE STRUCTURE Proper co-axial/transmission line design requires both a feed via a low characteristic impedance, a short transmission line from the matching network to the wafer and a return path along the transmission line. This design requirement is satisfied by the integral transmission line structure 32 depicted in FIG. 1 which comprises the cathode 32C, concentric annular conductor 320, and a non-porous low loss insulator 32I which surrounds the cathode 32C and insulates the cathode from the concentric annular conductor 320 and displaces process gases which otherwise might break down. For example, Teflon TM or quartz materials are preferred because they have high dielectric strength, low dielectric constant and low loss. The input side of this structure is connected to the matching network in a manner described below. The insulated cathode 32C and the outer conductor 320 provide separate current paths between the matching network 43 and the plasma. One reversible current path is from the matching network along the outer periphery of the cathode 32C to the plasma sheath at the chamber (electrode) surface. The second reversible path is from the plasma along the upper inside section of the chamber walls 12 then along the conductive exhaust manifold screen 29 and via the inside of the outer conductor 320 to the matching network. Please note, the exhaust manifold screen 29 is part of the uniform radial gas pumping system, and the return path for the RF current. Thus, the chamber incorporates a transmission line structure that couples power from the matching network 31 to the plasma. That transmission line structure (1) preferably is very short compared to a quarter wavelength at the frequencies of interest or, alternatively, is approximately equal to an integral half wavelength, to prevent undesirable transformation of the plasma impedance; (2) has a characteristic Z o selected to suppress losses due to the presence of standing waves on the line between the plasma and the matching network; and (3) uses an outside conductor path cross-sectional dimension which is not substantially larger than that of the center conductor. 11. CHAMBER TEMPERATURE CONTROL Temperature control features which may be incorporated in the reactor chamber system 10 include, but are not limited to, the use of a fluid heat transfer medium to maintain the internal and/or external temperature of the gas inlet manifolds above or below a certain value or within a certain range; resistive heating of the cathode 32C; fluid heat transfer heating or cooling of the cathode 32C; the use of gas heat transfer medium between the substrate 5 and the cathode 32C; the use of a fluid heat transfer medium to heat or cool chamber walls 12-14 and/or dome 17; and mechanical or electrostatic means for clamping the substrate 5 to the cathode 32C. Such features are disclosed in commonly assigned U.S. Patent No. 4,872,947, issued October 10, 1989, and commonly assigned U.S. Patent No. 4,842,683, issued June 27, 1989. For example, a recirculating closed loop heat exchanger 90 can be used to flow fluid, preferably dielectric fluid, through the block and pedestal of the substrate support/cathode 32C, as indicated schematically by flow path 91, to cool (and/or heat) the substrate support. For silicon oxide etching, dielectric fluid temperatures of, for example, -40°C are used. As mentioned above, the heat transfer between the substrate 5 and the substrate support 32 is enhanced by an inert gas heat transfer medium such as helium at the substrate-support interface. The chamber walls and the dome can be heated and/or cooled by air convection (blown air) and/or a dielectric fluid heat exchanger. For example, a closed circuit heat exchanger 92 recirculates dielectric fluid at a controlled temperature ranging from heating to cooling, for example, +120°C to -150°C, along path 93 through the chamber sidewalls. Similarly, the dome sidewalls 17W and top 17T can be heated and/or cooled by heat exchangers 94 and 96 which recirculate fluid along paths 95 and 97, respectively. In an alternative dielectric heat control system, the antenna coil 30 is positioned between the double walls 17W of the dome, immersed in the recirculating dielectric fluid. In another alternative approach for dielectric fluid neat control of the dome, the coils of the antenna 30 are encapsulated in high temperature plastic or TEFLON TM , heat conductive thermal grease is applied between the encapsulated antenna and the dome, and the coil, which is hollow, is heated and/or cooled by flowing the dielectric fluid through the coil. Because RF energy is also applied to the coil and because of the proximity to the source plasma, the dielectric oil must have good dielectric and insulating properties and a high boiling point, in addition to having high specific heat and density for efficient heat transfer at acceptable flow rates. One suitable dielectric fluid is Siltherm, available from DuPont. 12. THREE ELECTRODE CONFIGURATION Referring to FIG. 1, in a presently preferred embodiment, the chamber incorporates a unique, three-electrode arrangement which affords novel process control and enhancement. The arrangement comprises a cathode (preferably the substrate support electrode 32), an anode (preferably the chamber side and bottom walls) and a third electrode which is (or includes) the dome top plate 17T. As shown in FIG. 1, the top electrode may be floating, grounded or connected to an RF power supply 40. The third electrode advantageously includes various configurations and can be formed of various materials: conductive material, preferably aluminum; dielectric-coated materials such as anodized aluminum; silicon or silicon-containing conductive material such as aluminum-silicon alloy; or may include a sacrificial silicon member 17S such as, but not limited to, a silicon wafer. 1) Grounded Third Electrode The grounded top plate 17T improves the ground reference surface for the bias voltage (relative to the conventional reference provided by the walls 12) and as a consequence enhances ion extraction from the source 16A to the processing region 16B and so increases process rates (such as etch rates). In addition, the grounded top plate improves the coupling of the plasma (generated in the source) with the wafer. 2) Biased Third Electrode The use of an RF-biased third electrode in combination with supplying free silicon to the source plasma (which may include using an electrode which includes or is covered with a silicon-containing member), enhances various processing characteristics including etch rate and selectivity. Aided by the strong dissociation characteristics of the source plasma, the silicon enters the gas phase and combines with/scavenges free fluoride ions. (The dissociation characteristics of the source plasma results in the high concentrations when fluorine-containing gas chemistry is used, for example, to etch oxide. This increases the etch rate of oxide but also increases the etch rate of the associated substrate materials such as polysilicon and, thus, reduces the oxide to polysilicon selectivity). The fluoride ion scavenging by the free silicon forms a carbon-rich polymer. The result is increased oxide etch rate, increased oxide selectivity relative to polysilicon, and enhanced oxide etch anisotropy and vertical profile and decreased microploading. In addition, the free silicon affects the polymerization reaction to form a carbon-rich polymer, which is readily etched when oxygen is present, as etching an oxygen-containing layer, with enhanced suppression when no oxygen is present, such as when the opening reaches a non-oxygen-containing layer or substrate such as polysilicon, and results in increased oxide selectivity relative to the non-oxygen-containing substrate. 13. PROCESSING EXAMPLES Examples of processes which can be advantageously carried out in the present reactor are given below. Example 1. Etching Polysilicon over Silicon Oxide Polysilicon over silicon oxide on silicon wafers was etched in our three-electrode chamber using pressure within the range about 0.266 Pa to about 2.66 Pa (about 2 mtorr to about 20 mtorr); 50 cc chlorine (Cl 2 ) etchant gas flow rate (manifold G1 only); source power of 1500 watts; bias voltage on -20 volts; and a grounded top electrode (without silicon), providing a polysilicon etch rate of 350 to 400 nm per minute (3500 -4000 Angstroms per minute), a vertical etch profile, and a > 100:1 selectivity of polysilicon to oxide. Example 2. Etching Silicon Oxide over Polysilicon Silicon oxide over polysilicon was etched in our three-electrode chamber using pressure 0.266 - 4 Pa (2 - 30 mtorr); gas chemistry flow rates CHF 3 , 30 - 60 sccm; CO or CO 2 , 6 - 18 sccm; and Ar, 100 - 200 sccm (inlet manifold G1 only); source power of 2000 watts; bias voltage of 200 volts; top electrode 17T with a silicon disk 17S mounted thereto and biased by RF energy of 2 MHz and 1000 watts. The silicon oxide was etched at a rate of 800 nm per minute (8000 Angstroms per minute) with 50:1 selectivity of oxide to polysilicon. Alternatively, the silicon-containing body may be supplemented by a silicon coating on the quartz dome walls 17W. Examples 3-12. Etching Oxygen-Containing Layers over Oxygen-Free Substrates A series of etch processes was carried out in the present reactor etching oxide-containing layers such as silicon oxide over a variety of non-oxygen-containing substrates. After patterning the oxide layer using conventional photolithographic techniques, etching using C 2 F 6 was carried out in the present reactor using a grounded third silicon electrode. The results are summarized in Table I below: Example Thickness of Silicon Oxide Substrate Selectivity 3 500 - 1000 nm (5000-10,000 Å) polysilicon 30:1 4 500 - 1000 nm (5000-10,000 Å) WSi 25:1 5 PECVD Si 3 N 4 15:1 6 500 - 1000 nm (5000-10,000 Å) LPCVD Si 3 N 4 15:1 7 500 - 2000 nm (5000-20,000 Å) TiN/Al 15:1 8 500 - 2000 nm (5000-20,000 Å) Al 30:1 9 500 nm (5000 Å) single crystal silicon 30:1 Another series of etches using CF 4 were performed except that borophosphosilicate glass (BPSG) was substituted as the oxide-containing layer. The results are summarized below iin Table II: Example Thickness of BPSG glass Substrate Selectivity 10 500 - 1000 nm (5000-10,000 Å) WSi 30:1 11 500 - 2500 nm (5000-25,000 Å) p+doped single crystal silicon 30:1 12 500 - 2500 nm (5000-25,000 Å) n+doped single Crystal silicon 30:1 The selectivity can be adjusted by changing gas flows, source power and the silicon plate condition to optimize the selectivity for a particular silicon oxide or glass on a particular substrate. Selectivities of up to 100:1 are achievable in the present reactor using a silicon scavenger for fluorine ions. 14. OTHER FEATURES 1) Plasma Control A preferred feature of the invention is to automatically vary "bottom" power to maintain a constant cathode (substrate) sheath voltage. At low pressures (<66.5 Pa (<500 mtorr)) in a highly asymmetric system, the DC bias measured at the cathode is a close approximation to the cathode sheath voltage. Bottom power can be automatically varied to maintain a constant DC bias. Bottom power has very little effect on plasma density and ion current density. Top or antenna power has a very strong effect on plasma density and on current density, but very small effect on cathode sheath voltage. Therefore, it is desired to use top power to define plasma and ion current densities, and bottom power to define cathode sheath voltage. 2) Differential Bias As an alternative to biasing the substrate 5 with respect to ground, the bias marching network 43 and the top plate 17T can be "ungrounded" and referenced to one another, as indicated by the dotted connection 50, FIGS. 1 and 2. Referring to FIG. 2, the top plate is driven differentially and balanced so that the voltage V T-SS between the top plate and the substrate is approximately twice the magnitude of the voltage V T-W between the top plate and the wall 12, and approximately twice the magnitude of the voltage V SS-W between the substrate and the wall. This balanced differential drive reduces the interaction of the plasma with the walles and increases the interaction, ion extraction, between the source region 16A and the processing region 16B. 3) Alternative Configurations The inventive plasma reactor system is depicted in FIG. 1 in the conventional orientation, that is vertically, with the substrate 5 residing on an electrode 32 (cathode) and the antenna 30 surrounding the dome 17 above the electrode. For convenience, it was referred to the power supplied to the antenna 30 as "antenna" or "source" or "top" power and that supplied to the electrode/cathode 32 as "bias" or "bottom" power. These representations and designations are for convenience only, and it is to be understood that the described system may be inverted, that is, configured with the electrode 32 on top and an antenna located below this electrode, or may be oriented in other ways, such as horizontally, without modification. In short, the reactor system works independently of orientation. In the inverted configuration, plasma may be generated at the antenna 30 and transported upwardly to the substrate 5 located above the antenna in the same manner as described in the specifications. That is, transport of active species occurs by diffusion and bulk flow, or optionally assisted by a magnetic field having an axial gradient. This process does not depend on gravitational forces and thus is relatively unaffected by orientation. The inverted orientation may be useful, for example, to minimize the probability of particles formed in the plasma generation region in the gas phase or on a surface, falling onto the substrate. Gravity then reduces the probability of all but the smallest of such particles moving upward against a gravitational potential gradient to the substrate surface. 4) High and Low Pressure Operation and Variable Spacing Our chamber design is useful for both high and low pressure operation. The spacing, d, between the substrate support cathode 32C and the plane of the bottom coil or turn of the antenna 30 may be tailored for both high and low pressure operation. For example, high pressure operation at 66.5 Pa - 6.65 k Pa (500 mtorr - 50 torr) preferably uses spacing d ≤ about 5 centimeters, while for lower pressure operation over the range <0.0133 Pa - 66.5 Pa (< 0.1 mtorr - 500 mtorr), a spacing d of over 5 centimeters may be preferable. The chamber may incorporate a fixed spacing d, as shown, or may utilize variable spacing designs such as interchangeable or telescoping upper chamber sections. The reactor system 10 is useful for processes such as high and low pressure deposition of materials such as silicon oxide and silicon nitride; low pressure anisotropic reactive ion etching of materials such as silicon dioxide, silicon nitride, silicon, polysilicon, glass and aluminum; high pressure plasma etching of such materials; and CVD faceting involving simultaneous deposition and etchback of such materials, including planarization of substrata topography These and other processes for which reactor system 10 may be used are described in EP applicatioon 91112905.4 entitled VHF/UHF PLASMA PROCESS FOR USE IN FORMING INTEGRATED CIRCUIT STRUCTURES ON SEMICONDUCTOR WAFERS", filed on July 31, 1991, in the name of the applicant. 15. APPARATUS EXAMPLES A present working embodiment of our system incorporates the source configuration and the antenna configuration depicted in FIG. 1. The 12.7 cm (5-inch) high quartz source chamber 17 has a diameter of 30.48 cm (12 inches). The 2 MHz, 33.02 cm (13-inch) diameter, 10.16 cm (4-inch) high, 13 turn coil antenna is terminated at both ends (with variable capacitors L and T which are grounded), spaced about 0.64 cm (about 0.25 inch) from (below) the ground plane, and surrounds the source. Reactive load matching is supplied by the variable capacitor L (10-3000 picofarad variable cap, rated 5kV). Also, capacitive tuning of the antenna to resonance is provided by a tuning capacitor T (5-100 picofarad, 15kV rating). Operation using source RF energy of 2 kilowatt, 2 MHz provides a plasma which extends to the substrate, which is 5.08 cm (2 inches) downstream (beneath the source). This provides a plasma density of 1-2 x 10 12 /cm 3 and ion saturation current density of 10-15 mA/cm 2 downstream at the substrate. A bottom or bias of 2 MHz, 600 watts applied to a 12.7 cm (5-inch) substrate positioned on the support electrode approximately 5.1 cm (2 inches) below (downstream) of the antenna provides a 200 volt cathode sheath voltage. Those of usual skill in the art will appreciate that the present invention is not limited to the use of a dome per se. Rather, it applies to substantially any configuration having a source region and processing region. This includes, for example, the "stepped", domed chamber configuration depicted in the drawings as well as a more conventional non-stepped configuration in which the source and processing region or chamber sections are substantially of the same cross-section. 16. PROCESSES As indicated above, the above-described reactor embodying the present invention is uniquely useful for numerous plasma processes such as reactive ion etching (RIE), high pressure plasma etching, low pressure chemical vapor deposition (CDV) including sputter facet deposition and planarization, and high pressure conformal isotropic CVD. Other applications include, but are not limited to, sputter etching, ion beam etching, or as an electron, ion or active neutral plasma source.

Claims: A plasma etch process comprising: a) providing a vacuum chamber (16) for generating a plasma, b) supporting an article (5) to be processed on an electrode (32c) in the chamber, c) supplying a fluorine-containing etch gas to the chamber, d) inductively coupling RF energy from an RF power supply into the chamber for generating a high density plasma for processing said article, characterized by e) providing a source of silicon or graphite (17s) aside from said etch gas, said source being in contact with said plasma for scavenging fluorine from said plasma. A process according to claim 1, wherein said source is heated to maintain a surface thereof to be reactive. A process according to claim 1 or 2, wherein said source (17s) is electrically biased to maintain said surface thereof to be reactive. A process according to any one of the preceding claims, wherein said source (17s) is a solid source of silicon or graphite. A process according to any one of the preceding claims, further comprising capacitively coupling RF energy into the chamber via the support electrode (32c). A process according to any one of the preceding claims, wherein the source (17s) is a silicon-containing electrode positioned in the chamber overlying the electrode supporting the article. A process according to claim 6, further comprising RF biasing said silicon-containing electrode (17s). A process according to any one of the preceding claims, wherein the temperature of said source is regulated. A process according to any of the preceding claims, wherein the RF coupling is made via a coil antenna (30) through a dielectric wall (17w) of said chamber (16) into said chamber so as to generate said plasma. A process according to claim 9, wherein said dielectric wall (17w) adjacent to said coil is heated. A process according to any one of the preceding claims wherein the source (17s) is situated in the plasma region of the chamber (16). A process according to any one of the preceding claims wherein the antenna power and the bias power delivered to the support electrode (32c) are varied to effect anisotropic, semi-anisotropic and isotropic etching. An etch process according to any one of the preceding claims wherein the article (5) to be processed comprises a non-oxygen-containing material and an overlying oxygen-containing material. An etch process according to any one of the preceding claims wherein said fluorine-containing etch gas is a fluorocarbon. An etch process according to any one of the preceding claims wherein the article (5) comprises an non-oxygen-containing material selected from the group consisting of single crystal silicon, polysilicon, silicon nitride, tungsten silicide, titanium nitride and aluminum. An etch process according to any one of the preceding claims, wherein the article (5) further comprises an oxygen-containing material selected from the group consisting of silicon oxide and glass. An etch process according to any one of the preceding claims, wherein a passivating polymer containing more than 50% of carbon and 40% or less of fluorine is formed on the substrate (5). An etch process according to claim 1, wherein said article being a substrate (5) having a first surface area of a first material not containing oxygen and an overlying second surface area comprising a second material comprising oxygen, and wherein said etch gas is a processing gas that contains fluorine and carbon, wherein said processing gas etches said second material and forms a polymer containing by weight more than 50% of carbon and 40% or less of fluorine over exposed portions of said first surface area. An etch process as recited in claim 18, further comprising generating a plasma to assist said processing. An etch process as recited in any of claims 18 or 19, wherein said first material comprises silicon. An etch process as recited in any of claims 18 to 20, wherein said first material consists essentially of polysilicon. An etch process as recited in any of claims 18 to 21, wherein said second material additionally comprises silicon. A process according to any one of claims 1 to 17, wherein said source (17s) is a silicon-containing gas. A process according to any one of claims 1 to 17 or 23, wherein said source of silicon or graphite (17s) is disposed above a top surface of said article (5) being processed. A process according to any one of claims 1 to 17 or 23 to 24, further comprising RF biasing said source of silicon or graphite (17s). A plasma etch process according to any one of the preceding claims, further comprising providing a solid scavenger comprising silicon or graphite for scavenging fluorine (17s). A plasma processing reactor, comprising: a vacuum chamber (16); a source of processing gas (G1) to said vacuum chamber; a support for a workpiece (5) to be processed (32c); at least one RF power supply (40,42); a coil antenna (30) disposed above said workpiece (5) for inductively coupling an RF signal from said RF power supply into said chamber to generate a plasma from said processing gas; characterized by a source of graphite or silicon (17s) aside -from said workpiece in contact with said plasma for scavenging fluorine from said plasma. A reactor according to claim 27, wherein said source (17s) is a solid source of graphite or silicon. A reactor according to claim 27 or 28, wherein said source (17s) is temperature controlled. A reactor according to any one of claims 27 to 29, further comprising means (90) for heating said source (17s). A reactor according to any one of claims 27 to 30, wherein said source (17s) is disposed above a top surface of said workpiece (5) to be processed. A reactor according to any one of claims 27 to 31, wherein said source (17s) is an electrode supplied with a second RF signal from said at least one RF power supply (40,42).

Inventors: COLLINS KENNETH S, GROECHEL DAVID W, KESWICH PETER R, MARKS JEFFREY, WONG JERRY YUEN-KUI, YANG CHAN-LON, COLLINS, KENNETH S., GROECHEL, DAVID W., KESWICH, PETER R., MARKS, JEFFREY, WONG, JERRY YUEN-KUI, YANG, CHAN-LON
Applicants: APPLIED MATERIALS INC, APPLIED MATERIALS, INC.
Application year: 1992
Main classification: H01J37
IPC codes: C23C16, H01J37