Abstract:
A processing method and system are provided for processing a substrate with a plasma in the presence of an electro-negative gas. A processing gas is injected into a processing chamber. The gas includes a high electron affinity gas species. A surface is provided in the plasma chamber onto which the gas species has a tendency to chemisorb. The gas species is exposed to the surface, chemisorbed onto it, and the surface is exposed to energy that causes negative ions of the chemisorbed gas species, that interact in the plasma to release secondary electrons. A neutralizer grid may be provided to separate from the chamber a second chamber in which forms a low energy secondary plasma for processing the substrate that is dense in electrons and contains high energy neutrals of the gas species and high energy positive ions of processing gas. Pulsed energy may be used to excite plasma or bias the substrate. A hollow cathode source is also provided.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    Pursuant to 37 C.F.R. §1.78(a)(4), the present application claims the benefit of and priority to co-pending Provisional Application No. 62/142,778 (Attorney Docket No. TEA-053) filed on Apr. 3, 2015, and entitled ENERGETIC NEGATIVE ION IMPACT IONIZATION PLASMA, which is expressly incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention relates to a method and system for treating a substrate and, more particularly, to a method and system for processing of a substrate with a plasma that is formed in part with an electro-negative gas. 
       BACKGROUND OF THE INVENTION 
       [0003]    During semiconductor processing, plasma is often utilized to assist etching processes by facilitating the anisotropic removal of material along fine lines or within vias or contacts patterned on a semiconductor substrate. Examples of such plasma assisted etching include, for example, reactive ion etching (“RIE”), which is in essence an ion activated chemical etching process. 
         [0004]    Although RIE has been in use for decades, its maturity is accompanied by several negative features, including: (a) broad ion energy distribution (“IED”); (b) various charge- induced side effects; and (c) feature-shape loading effects (i.e., micro-loading). Particularly, a broad IED contains ions having either too little, or too much, energy to be useful. Further, overly energetic ions are susceptible to causing semiconductor substrate damage. Additionally, broad IED makes it difficult to selectively activate a desired chemical reactions, where side reactions are often triggered by ions of sub-optimal, undesired energy. 
         [0005]    Further, and over the course of the etching process, positive charge buildup on the semiconductor substrate may occur and repels ions that would otherwise be incident onto the semiconductor substrate. Alternatively, the charge buildup may produce local charge differences that affect currents on the substrate surface. Charge buildup may be due, in part, to the RF energy used to produce a negative bias on an otherwise non-conductive substrate used to attract positive ions from the plasma. Such RF frequencies are typically too high to allow a positive potential or near-neutral potential to exist for a sufficient amount of time to attract electrons for neutralizing the accumulated positive charges. Still further, non-uniform accumulation of charge across the surface of the substrate may create potential differences that may lead to currents on the semiconductor substrate that may very well damage the devices being formed. 
         [0006]    One known, conventional approach to addressing these problems has been to utilize neutral beam processing. A true neutral beam process takes place essentially without any neutral thermal species participating as the reactant, additive, and/or etchant. Instead, the process at the substrate is activated by the kinetic energy of incident, directional, and energetic neutral species. 
         [0007]    While neutral beam processes are not affected by flux-angle variation associated with the thermal species as in RIE, the use of neutrals has lead to absence of micro-loading efficiency. This lack of micro-loading results in a maximum etching efficiency of unity, in which one incident neutral nominally prompts only one etching reaction. Comparatively, in a RIE process the abundant thermal neutral etchant species may all participate in etching so that activation by one energetic incident ion may achieve an etch efficiency of 10, 100, and even 1000. 
         [0008]    In plasma processing of substrates, particularly in chemical etching systems and other etching systems but also in some deposition systems, electro-negative gas is added to the processing gas. But the desired beneficial effects for which the electro-negative gas is used have been difficult to fully realize. Such desired beneficial effects include the production of appropriately energetic electrons, the production of negative ions and neutral species of the electro-negative gas, and the enhancement of fast positive ions for treatment of the substrate and for the activation of chemical reactions for processing the substrate. 
         [0009]    For processing substrates having current high aspect ratio devices, generally a high energy flux of electrons, e.g., greater than 30 eV, is necessary. This energy level, and higher, reduce differential charging and minimize shading effects of the devices. Furthermore, the processing system must achieve high density plasmas (n e ) while controlling polymerization. While high RF sheath voltage is needed for creating and delivering energetic ions to the substrate, the RF sheath voltage must be sufficiently low so that energetic electrons may be dumped onto the wafer surface. 
         [0010]    Thus, there remains a need for a system that is configured to generate an ultra high density plasma without losing energetic electrons to trapping so as to dump the energetic electrons as the electrons are produced. 
       SUMMARY OF THE INVENTION 
       [0011]    The present invention overcomes the foregoing problems and other shortcomings and drawbacks of the prior art. While the present invention will be described in connection with certain embodiments, it will be understood that the present invention is not limited to these embodiments. To the contrary, this invention includes all alternatives, modifications, and equivalents as may be included within the spirit and scope of the present invention. 
         [0012]    According to principles of the present invention, a substrate with a plasma in the presence of an electro-negative gas having a high electron affinity gas species in the plasma chamber and exposing the high electron affinity gas species in the plasma chamber to a surface onto which the high electron affinity gas species has a tendency to chemisorb. The surface is then exposed to energy that causes the emission of negative ions of chemisorbed high electron affinity gas species into the plasma. The emitted negative ions may further produce secondary electrons when interacting with the plasma. The secondary electrons enhance the processing of the substrate. The exposing of the surface to energy may be accomplished by exposing of the surface to the plasma while applying a time average negative potential on the surface. 
         [0013]    In certain embodiments of the invention, the plasma may be ignited by applying a pulsed RF signal to an electrode operably coupled to the plasma chamber. In other embodiments, a pulsed voltage to the substrate alternates the bias of the substrate between a negative bias voltage level and a less negative or positive bias voltage level. 
         [0014]    In certain embodiments of the invention, a pulsed DC bias is applied to a substrate positioned on a substrate support in the processing chamber, and the substrate is periodically biased between first and second bias levels, the first bias level being more negative than the second bias level, wherein the substrate and substrate support, when biased at the first bias level, attracts mono-energetic positive ions from the plasma toward the substrate and operable to enhance a selected chemical etch process at a surface of the substrate. 
         [0015]    In some embodiments of the invention, a secondary plasma is energized in a portion of the chamber, or in a separate sub-chamber between the plasma and the substrate with the emitted energized electrons produced by the emitted negative ions. In further embodiments of the invention, a processing system is provided that includes a first plasma chamber configured to contain a first plasma and having a gas source coupled thereto for injecting a process gas and a high electron affinity species into the first plasma chamber. 
         [0016]    In some embodiments, secondary electron source is disposed within the first plasma chamber that includes a surface that has a tendency to chemisorb the high electron affinity species, which in turn, has a tendency to chemisorb onto that surface. A power source is provided to energize the surface, and may be coupled to the surface to supply a negative time-average potential to the secondary electron source to attract positive ions from the plasma to energize the surface. The energizing of the surface is effective to emit negative ions of chemisorbed high electron affinity species. In certain embodiments of the invention, a second plasma chamber is provided in fluid communication with the first plasma chamber. A neutralizer may be positioned between the first plasma chamber and the second plasma chamber. The neutralizer may have a plurality of openings therein configured to permit energetic electrons to pass while the negative ions impact the neutralizer to release an energetic electron. 
         [0017]    In certain embodiments, the surface of the secondary electron source is aluminum, while the high electron affinity species is oxygen gas and the neutralizer comprises silicon. In other embodiments, the surface of the secondary electron source may be doped-silicon, while the high electron affinity species includes at least one of fluorine gas, chlorine gas, bromine gas, tetrachlorosilane, and tetrafluorosilane, and the neutralizer comprises silicon. Other gases and surface materials are contemplated. 
         [0018]    In still further embodiments, a hollow cathode plasma source is provided for use in generating a high density plasma that has a wall configured to surround a plasma space in the chamber that includes the surface of the secondary electron source. 
         [0019]    These and other embodiments of the invention will be readily apparent from the following detailed description of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0020]    The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the present invention and, together with a general description of the invention given above, and the detailed description of the embodiments given below, serve to explain the principles of the present invention. 
           [0021]      FIG. 1  illustrates a schematic view of a chemical processing system in accordance with one embodiment of the present invention. 
           [0022]      FIG. 2  is an enlarged, schematic view of the chemical processing system of  FIG. 1 , which is not to scale but illustrating a method of using the chemical processing system in accordance with one embodiment of the present invention. 
           [0023]      FIG. 3  is an enlarged, schematic view of the chemical processing system of  FIG. 1  that is not to scale but illustrating a method of using the chemical processing system in accordance with another embodiment of the present invention. 
           [0024]      FIG. 4  illustrates a schematic view of a chemical processing system including a hollow cathode and in accordance with another embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0025]    In the following description, to facilitate a thorough understanding of the embodiments of the present invention, and for purposes of explanation and not limitation, specific details are set forth, such as a particular geometry of a plasma processing system and various descriptions of the system components. However, it should be understood that the present invention may be practiced with other embodiments that depart from these specific details. Nonetheless, it should be appreciated that, contained within the description are features which, notwithstanding the inventive nature of the general concepts being explained, are also of an inventive nature. 
         [0026]    Referring now to the figures, and in particular to  FIG. 1 , a chemical processing system  50  according to one embodiment of the present invention is described in detail.  FIG. 2  includes an enlarged schematic of the chemical processing system  50 , which is not to scale. The chemical processing system  50 , as shown, is configured to perform space-charge neutralized neutral beam activated chemical processing of a substrate  52 . In that regard, the chemical processing system  50  includes a first plasma chamber  54  configured to form a first plasma  56  at a first plasma potential, Vp,1, and a second plasma chamber  58  configured to form a second plasma  60  at a second plasma potential, Vp,2, that is greater than Vp,1. The first plasma  56  may be formed by coupling power, such as direct current (“DC”) power or radio frequency (“RF”) power, into an ionizable gas injected into the first plasma chamber  54  from a process gas source  70 . However, power may be supplied by appropriate voltage source so long as the conductive surface  68  has a negative time-average potential. The second plasma  60  may be formed using electron flux (e.g., energetic electron (“ee”) current, jee) from the first plasma  56 . The chemical processing system  50  further comprises a substrate holder  62  that is configured to receive the substrate  52  thereon so as to expose the substrate  52  to the second plasma  60  at the Vp,2. 
         [0027]    The substrate holder  62  may be further configured to apply a direct current (“DC”) ground or floating ground to the substrate  52 . For example, the substrate holder  62  may include an electrode incorporated therein, or at least partially comprising the substrate holder  62 , that may be biased by a DC-pulse from a DC pulse generator  72 , such as is described in greater detail in U.S. application Ser. No. 13/837,391 (Docket No. TEA-62), entitled DC PULSE ETCHER. In other words, the electrode may be grounded through a negative DC voltage source  74  via, for example, a relay circuit  75 . 
         [0028]    The first plasma chamber  54  includes a plasma source  64  configured to ignite and maintain the first plasma  56 . The first plasma  56  may be heated by any conventional plasma source including, for example, an inductively coupled plasma (“ICP”) source, a transformer coupled plasma (“TCP”) source, a capacitively coupled plasma (“CCP”) source, an electron cyclotron resonance (“ECR”) source, a helicon wave source, a surface wave plasma (“SWP”) source, etc. While the first plasma  56  may be heated by any plasma source, it is desired that the first plasma  56  is heated by a method that produces a reduced or minimum fluctuation in its plasma potential, Vp,1. One suitable plasma source for this purpose is the ICP source, which is operable by practical techniques for producing the reduced or minimum Vp,1 fluctuation. 
         [0029]    The first plasma chamber  54  further includes a DC conductive electrode  66  having a conductive surface  68  in contact with the first plasma  56 . The DC conductive ground electrode  66  is coupled to DC ground and operable as an ion sink driven by the first plasma  56  at the first plasma potential, Vp,1. 
         [0030]    Although not necessary, it is desirable for the conductive surface  68  to have relatively large surface area for contact with the first plasma  56 . The surface area of the conductive surface  68  is indirectly proportional to Vp,1. For example, the surface area may be greater than any other surface area in contact with the first plasma  56  and, indeed, the surface area may be greater than the total sum of all other conductive surfaces that are in contact with the first plasma  56 . Alternatively, the conductive surface  68  of the DC conductive ground electrode  66  may be the only conductive surface in contact with the first plasma  56 . The DC conductive ground electrode  66  may offer the lowest impedance path to ground. 
         [0031]    In accordance with the particular illustrative embodiment shown in  FIGS. 1 and 2 , the DC conductive ground electrode  66  may be constructed from an aluminum cathode, as explained in detail below, and is involved with the chemical process according to one embodiment of the present invention. 
         [0032]    The process gas system  70  injects the least one process gas into the first plasma chamber  54 , which may include an inert gas, conventionally argon, which is ignited into the first plasma  56 . In accordance with embodiments of the present invention, the process gas further includes at least a trace amount of a high electron affinity species, M2. The electron affinity species is selected to chemisorb or dissociatively chemisorb onto the conductive surface  68  of the cathode ground electrode  66 , which may be aluminum. For example, oxygen gas, O 2 , may be selected for use with an aluminum cathode ground electrode  66 . While the oxygen gas content of the process gas should not be limited to any particular concentration, the oxygen gas content generally may range from about 1% to about 99% by volume, for example, from about 1% to about 25%, or from about 1% to 10%. 
         [0033]    Plasma ignition within the first plasma chamber  54  causes the ionization of argon gas, Ar +  by releasing electrons, e − . Meanwhile, the oxygen gas, having a high affinity for the aluminum cathode ground electrode  66 , will dissociate and chemisorb onto the conductive surface  68 , increasing the secondary ion yield of the aluminum cathode ground electrode  66 . For achieving this increase in secondary ion yield, the chemisorb oxygen atoms on the conductive surface  68  remain on the aluminum surface until the bond is disrupted by radiation, which may be in the form of a photon, positive ion bombardment, and RF electrode emitted energetic-γ-electron bombardment, for example. 
         [0034]    As illustrated in  FIG. 1 , and more particularly in  FIG. 2 , a flux of argon ions (e.g., a first ion current, j i1 ) is attracted toward the aluminum cathode ground electrode  66 . When the argon ion impacts the aluminum cathode ground electrode  66 , a negative ion is emitted from the same, which in the instant embodiment is an oxygen ion, O − . Once within the first plasma  56 , the energetic oxygen ions (i.e., fastO − ) react with unionized, thermal argon to further fuel the plasma  56  and generate ion secondary electron emissions via the reaction provided below: 
         [0000]      fastO − +Ar→fastAr + +2 e   − +fastO
 
         [0035]    Ion secondary electron emission as provided by the above reaction for a  1 % oxygen gas content in the process gas may lead to a five- to ten-fold increase in number of electrons (up to about 1×10 12 ) as compared to the number of electrons produced in a conventional plasma comprised only of argon gas. Said another way, due to the secondary electron emissions noted above, the energetic electron flux (or electron current, j ee ) from the first plasma  56  to the second plasma  60  may be much greater than the flux of argon ions (e.g., a first ion current, j i1 ) attracted toward the aluminum cathode ground electrode  66 , i.e., j il &lt;j ee . 
         [0036]    At least a portion of the negative ions, O − , will be drawn toward the second plasma chamber  58 . In that regard, the first and second plasma chambers  54 ,  58  are separated by a neutralizer  76  that is driven by an electric field created by the difference in plasma potential between the first and second plasma chambers  54 ,  58 , ΔV= Vp,2−Vp,1 . The neutralizer  76  may be constructed from an insulator material, such as quartz, or a dielectric coated conductive material that is electrically floating and has a high RF impedance to ground. 
         [0037]    The neutralizer  76  includes a plurality of openings  78  ( FIG. 2 ) extending therethrough, wherein each opening of the plurality  78  is shaped so as to permit the passage of the energetic electron flux, j ee , from the first plasma chamber  54  to the second plasma chamber  58 . The total area of the openings of the plurality  78  may be adjusted with respect to the conductive surface  68  of the aluminum cathode ground electrode  66  to ensure a relatively large potential difference, ΔV, while minimizing reverse ion current flux from the second plasma  60  to the first plasma  56 . The total area further ensures a sufficient ion energy for ions striking the substrate  52 . 
         [0038]    The openings of the plurality  78  may be fabricated by e-beam drilling techniques or other similar processes, and may be, in some embodiments, about 1 mm in diameter and 20 mm in height. 
         [0039]    While the diameter of the openings of the plurality  78  permits passage of the energetic electron flux, the diameter is generally no greater than the mean-free-path (“mfp”) of the O −  flux. As a result, O −  entering the neutralizer  76  will impact a wall of the neutralizer  76  and release an electron. The released electron further contributes to the energetic electron flux, j ee , and introduces a flux of neutral oxygen atoms, j n , into the second plasma chamber  58 . 
         [0040]    Thus, the total energetic electron flux, j ee , into the second plasma chamber  58  is much greater in the chemical processing system  50  according to this first embodiment than the energetic electron flux produced by a conventional, argon-based plasma process. This total j ee  is also configured to initiate and sustain the second plasma  60  within the second plasma chamber  58  at lower potential differences, ΔV, as compared to the known, conventional systems. 
         [0041]    Resultantly, the chemical processing system  50  and the method of use as described herein, provides a high density plasma (increased n e ) while reducing the RF sheath potential, which, in turn, reduces the trapping potential of the thermal electrons and increases the electron dump into high aspect ratio features in the substrate  52 . 
         [0042]    Turning now to the second plasma chamber  58  and generation of the mono-energetic space-charged neutralized beam, the second plasma chamber  58  includes a DC conductive bias electrode  80  having a conductive surface  82  thereon and electrically coupled DC voltage source  84  is in contact with the second plasma  60 . The conductive bias electrode  80  is configured to minimize or reduce fluctuations in ΔV. In use, the DC voltage source  84  is configured to bias the DC conductive bias electrode  80  at a positive DC voltage (+VDC), which causes the second plasma potential, V p,2  to be driven, and increased, by the voltage source (+VDC). While one DC conductive bias electrode  80  is shown in  FIG. 1 , the chemical processing system  50  may includes additional DC conductive bias electrodes if necessary or desired. 
         [0043]    Thermal electrons (“t e ”) may be generated within the second plasma chamber  58  when the second plasma  60  is ionized by the incoming energetic electron flux (or electron current j ee ) or when at least some of the energetic electrons, experience a loss of energy. Because of Debye shielding, a thermal electrons diffuse toward the DC conductive bias electrode  80  as a thermal electron current, j te , and generate a quiescent plasma zone proximate the electrode  80 . 
         [0044]    While the thermal electrons are diffusing toward the DC conductive bias electrode  80 , a second population of ions, of for example energetic or “fast” positive argon ions, is formed by the second plasma  60  and is directed to the substrate  52  as ion current, j i3 . These ions, along with the energetic electrons and neutral atoms of the electro-negative gas, participate in the chemical reaction, deposition, and/or etching at the substrate  52 . 
         [0045]    The chemical etching process at the substrate  52  is activated by the kinetic energy of the incident, directionally energetic neutral species, such as the flux of neutral oxygen atoms, i n , described above. In some embodiments, the reactive neutral species may also serve as the reactants or etchants. 
         [0046]    In using the chemical processing system  50  in accordance with one embodiment of the present invention, at a particular time interval, such as in accordance with a desired waveform, the relay circuit  75  coupled to the substrate holder  62  may be switched so as to apply a pulsed DC bias to the substrate  52 . For example, a pulsed negative bias may be applied to the substrate  52  during which positive ions are drawn toward the substrate  52 . Pulsed periods of less negative bias (even positive bias) applied to the substrate  52  between the intervals of negative bias draws at least a portion of the energetic electrons from the second plasma  60  toward the substrate  52 . As a result, the DC pulse bias achieves a mono-energetic ion excitation of the substrate  52  during the negative bias and an energetic electron dump via a more positive bias onto the substrate  52  to neutralize positive charge accumulation on the substrate  52 . 
         [0047]    In other embodiments of the present invention, not specifically shown herein, an RF voltage source may be electrically coupled to the substrate holder  62  and is operable to apply an RF energy pulse that draws a portion of the second plasma  60  toward the substrate. The particle balance throughout the plasma  60  drawn toward the substrate  52  enforces an equal number of electrons (e.g., electron current, j e2 ), ions (e.g., ion current, j i3 ), as well as the neutral species (e.g., neutral current, i n ) to impact the substrate  52 . This charge balance manifests as a space-charge neutralized neutral beam directed to substrate  52  configured to activate a chemical process at the substrate  52 . 
         [0048]      FIG. 3  illustrates another chemical processing system  50 ′ in accordance with another embodiment of the present invention. For illustrative convenience, like reference numerals having primes thereafter designate corresponding components of the embodiments. 
         [0049]    According to this particular embodiment, the process gas system  70  ( FIG. 1 ) includes argon gas as well as a high electron affinity species. While the high electron affinity specie illustrated herein is diatomic chlorine gas, it would be understood that other gases may also be used, including, for example, fluorine gas (F 2 ), bromine gas (Br 2 ), tetrachlorosilane (SiCl 4 ), and tetrafluorosilane (SiF 4 ), although chlorine gas (Cl 2 ) is expressly described herein. While the argon gas is ionized into the plasma  56 ′, the diatomic chlorine gas dissociatively chemisorbs onto the conductive surface  68 ′ of the DC conductive electrode  66 ′, which in accordance with this embodiment is constructed, at least in part, from silicon. Argon ions, attracted toward the silicon cathode ground electrode  66 ′, impact the conductive surface  68 ′ so that the surface  68 ′ emits a negative ion, herein, an energetic chloride ion, Cl − . 
         [0050]    Once the energetic chloride ion (i.e., fastCl − ) is in the first plasma  56 ′, it will react with unionized thermal argon gas to further fuel the first plasma  56  and generated ion secondary electron emissions via the reaction provided below: 
         [0000]      fastCl − +Ar→fastAr + +2 e   − +fastCl
 
         [0051]    Use of about 1% chlorine gas has been used to increase the plasma density (i.e., number of electrons) as compared to conventional argon gas plasma. 
         [0052]    Other energetic chloride ions may diffuse toward the neutralizer  76 ′, as described previously, to neutralize the chloride ions and further emit energetic electrons into the second plasma  60 ′. Neutral chlorine atoms, energetic electrons, and ion current may participate in the chemical processing of the substrate  52 ′ in manner similar to what was described above with reference to  FIG. 2 . 
         [0053]    Turning now to  FIG. 4 , a hollow cathode plasma source  100  (hereafter “hollow cathode”) for use in a surface emitted negative ion neutral beam chemical processing system  102  is shown and described in accordance with one embodiment of the present invention. The chemical processing system  102  may be somewhat analogous to the chemical processing systems  50  ( FIG. 1 ) described above in that the chemical processing system  102  includes a first plasma chamber  104  and a second plasma chamber  106 . However, in the instant embodiment, a top portion of the first plasma chamber  104  is includes, at least partially, the hollow cathode  100 . 
         [0054]    Generally, the hollow cathode  100  may be a DC conductive electrode that, for example and as described in detail above, may be constructed from a highly doped silicon material or as aluminum cathode. Alternatively still, the DC conductive electrode may comprise hafnium, copper, graphite, a thermal paralytic graphite, or other materials known to those of ordinary skill in the art. The hollow cathode  100  may be cooled, such as a by a plurality of cooling channels  108  configured to received a coolant, to a temperature that ranges from about 195° C. to about 0° C. In the case of very low temperatures, a cryogenic coolant may be used, such as liquid nitrogen. The hollow cathode  100  may be grounded via a DC voltage source  114 . 
         [0055]    A process gas system  115  is operably to inject process gases into the hollow cathode  100  for plasma formation. The process gas system  115  may further include high electron affinity species in accordance with embodiments of the present invention. A pump  117  may also be coupled to the chemical processing system  102  for drawing a vacuum and evacuating chemicals from the first and second chambers  104 ,  106 . 
         [0056]    Turning now to the details of the hollow cathode  100 , the dimensions may be based, in part, in relation to a plasma sheath  110 . For instance, a height, h, of the hollow cathode  100  is selected such that a distance between a dielectric neutralizer  112  and the plasma sheath  110 ,  1 , is much less than the ionization mfp of the selected high electron affinity species such that the surface emitted ion (e.g., O −  of  FIG. 2  or Cl −  of  FIG. 3 , above) may cross the first plasma  116  to the dielectric neutralizer  112  without suffering a statistical gas-phase collision. 
         [0057]    The dielectric neutralizer  112  may be manufactured in a manner that is similar to the neutralizer  76  ( FIG. 2 ). That is, the dielectric neutralizer  112  may comprise a plurality of openings  118  to permit passage of the electric electron flux while being less than the mfp of the surface emitted ions so as to neutralize the surface emitted ions and release an energetic electron flux and a flux of neutral atoms into the second plasma  120  within the second plasma chamber  106 . In that regard, the dielectric neutralizer  112  may be electrically floating with a high RF impedance to ground. 
         [0058]    Turning now to the second plasma chamber  106  the total energetic electron flux into the second plasma chamber  104  is much greater in the chemical processing system  102  according to this illustrative embodiment than the energetic electron flux produced by a conventional, argon-based plasma process. This increased energetic electron flux is also configured to initiate and sustain the second plasma  120  within the second plasma chamber  106  at lower potential differences, ΔV, as compared to the known, conventional systems. 
         [0059]    Accordingly, a mono-energetic space-charged neutralized beam may be generated in the second plasma chamber  106 . 
         [0060]    Thermal electrons may be generated within the second plasma chamber  106  when the second plasma  120  is ionized by the incoming energetic electron flux (or electron current) or when at least some of the energetic electrons, experience a loss of energy. Because of Debye shielding, a thermal electrons diffuse toward the conductive, grounded electrode  122  as a thermal electron current, and generate a quiescent plasma zone proximate the electrode  122 . 
         [0061]    While the thermal electrons are diffusing toward the grounded electrode  122 , a second population of ions formed by the second plasma  120  is directed to the substrate  126  as ion current. These ions, along with the energetic electrons, participate in the chemical reaction, deposition, and/or etching at the substrate  126 . 
         [0062]    In using the chemical processing system  102  in accordance with one embodiment of the present invention, at a particular time interval, such as in accordance with a desired waveform, the relay circuit  128  coupled to electrode associated with the substrate holder  132  is switched so as to apply a pulsed DC bias to the substrate  126 . For example, a pulsed negative bias may be applied to the substrate  126  during which positive ions are drawn toward the substrate  126 . Pulsed periods of less negative bias (even positive bias) applied to the substrate  126  between the intervals of negative bias draws at least a portion of the energetic electrons from the second plasma  120  toward the substrate  126 . As a result, the DC pulse bias achieves a mono-energetic ion excitation of the substrate  126  during the negative bias and an energetic electron dump via a more positive bias onto the substrate  126  to neutralize positive charge accumulation on the substrate  126 . 
         [0063]    While the present invention has been illustrated by description of various embodiments and while those embodiments have been described in considerable detail, those skilled in the art will readily appreciate that many modifications are possible in the exemplary embodiment without materially departing from the novel teachings and advantages of this invention. The invention in its broader aspects is therefore not limited to the specific details and illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the spirit or scope of the present invention.