Patent Publication Number: US-9842744-B2

Title: Methods for etch of SiN films

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     The present application is a continuation of U.S. patent application Ser. No. 14/679,242 filed Apr. 6, 2015, now U.S. Pat. No. 9,343,327, which is a continuation of U.S. patent application Ser. No. 13/416,277 filed Mar. 9, 2012, now U.S. Pat. No. 8,999,856, which claims the benefit of U.S. Provisional Application No. 61/452,575, filed Mar. 14, 2011, the contents of which are hereby incorporated by reference in their entirety for all purposes. 
     The present application is also related to U.S. Nonprovisional patent application Ser. No. 13/088,930, filed Apr. 18, 2011, now U.S. Pat. No. 9,324,576; Ser. No. 13/251,663, filed Oct. 3, 2011, now abandoned; and U.S. Nonprovisional patent application No. 13/416,223, filed Mar. 9, 2012, now U.S. Pat. No. 9,064,815; the contents of which are each incorporated herein by reference in their entirety for all purposes. 
    
    
     BACKGROUND 
     Integrated circuits are made possible by processes which produce intricately patterned layers of materials on substrate surfaces. Producing patterned material on a substrate requires controlled methods for removal of exposed materials. Chemical etching is used for a variety of purposes including transferring a pattern in photoresist into underlying layers, thinning layers, or increasing lateral dimensions of features already present on the surface. Often it is desirable to have an etch process which etches one material faster than another. Such an etch process is said to be selective to the first material. As a result of the diversity of materials, circuits, and processes, etch processes have been developed with a selectivity towards a variety of materials. 
     Plasma deposition and etching processes for fabricating semiconductor integrated circuits have been in wide use for decades. These processes typically involve the formation of a plasma from gases that are exposed to electric fields of sufficient power inside the processing chamber to cause the gases to ionize. The temperatures needed to form these plasmas can be much lower than needed to thermally ionize the same gases. Thus, plasma generation processes can be used to generate reactive radical and ion species at significantly lower chamber processing temperatures than is possible by simply heating the gases. This allows the plasma to deposit and/or etch materials from substrate surfaces without raising the substrate temperature above a threshold that will melt, decompose, or otherwise damage materials on the substrate. 
     Exemplary plasma deposition processes include plasma-enhanced chemical vapor deposition (PECVD) of dielectric materials such as silicon oxide on exposed surfaces of a substrate wafer. Conventional PECVD involves the mixing of gases and/or deposition precursors in the processing chamber and striking a plasma from the gases to generate reactive species that react and deposit material on the substrate. The plasma is typically positioned close to the exposed surface of the substrate to facilitate the efficient deposition of the reaction products. 
     Similarly, plasma etching processes include exposing selected parts of the substrate to plasma activated etching species that chemically react and/or physically sputter materials from the substrate. The removal rates, selectivity, and direction of the plasma etched materials can be controlled with adjustments to the etchant gases, plasma excitation energy, and electrical bias between the substrate and charged plasma species, among other parameters. Some plasma techniques, such as high-density plasma chemical vapor deposition (HDP-CVD), rely on simultaneous plasma etching and deposition to deposit films on the substrate. 
     While plasma environments are generally less destructive to substrates than high-temperature deposition environments, they still create fabrication challenges. Etching precision can be a problem with energetic plasmas that over-etch shallow trenches and gaps. Energetic species in the plasmas, especially ionized species, can create unwanted reactions in a deposited material that adversely affect the material&#39;s performance. Thus, there is a need for systems and methods to provide more precise control over the plasma components that make contact with a substrate wafer during fabrication. 
     SUMMARY 
     Systems and methods are described for improved control of the environment between a plasma and the surfaces of a substrate wafer that are exposed to plasma and/or its effluents. The improved control may be realized at least in part by an ion suppression element positioned between the plasma and the substrate that reduces or eliminates the number of ionically-charged species that reach the substrate. Adjusting the concentration of ion species that reach the substrate surface allows more precise control of the etch rate, etch selectivity, and deposition chemistry (among other parameters) during a plasma assisted etch and/or deposition on the substrate. 
     In an embodiment, a method of selectively etching silicon nitride from a substrate comprising a silicon nitride layer and a silicon oxide layer is provided. The method includes flowing a fluorine-containing gas into a plasma generation region of a substrate processing chamber, and applying energy to the fluorine-containing gas to generate a plasma in the plasma generation region. The plasma comprises fluorine radicals and fluorine ions. The method also includes filtering the plasma to provide a reactive gas having a higher concentration of fluorine radicals than fluorine ions, and flowing the reactive gas into a gas reaction region of the substrate processing chamber. The method also includes exposing the substrate to the reactive gas in the gas reaction region of the substrate processing chamber. The reactive gas etches the silicon nitride layer at a higher etch rate than the reactive gas etches the silicon oxide layer. 
     In another embodiment, an etch process providing a higher etch rate of silicon nitride than an etch rate of silicon oxide is provided. The process includes generating a plasma from a fluorine-containing gas. The plasma comprises fluorine radicals and fluorine ions. The process also includes removing a portion of the fluorine ions from the plasma to provide a reactive gas having a higher concentration of fluorine radicals than fluorine ions, and exposing a substrate comprising a silicon nitride layer and a silicon oxide layer to the reactive gas. The reactive gas etches the silicon nitride layer at a higher etch rate than the reactive gas etches the silicon oxide layer. 
     Additional embodiments and features are set forth in part in the description that follows and will become apparent to those skilled in the art upon examination of the specification or may be learned by the practice of the invention. The features and advantages of the invention may be realized and attained by means of the instrumentalities, combinations, and methods described in the specification. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A further understanding of the nature and advantages of the present invention may be realized by reference to the remaining portions of the specification and the drawings, wherein like reference numerals are used throughout the several drawings to refer to similar components. In some instances, a sub-label is associated with a reference numeral and follows a hyphen to denote one of multiple similar components. When reference is made to a reference numeral without specifying an existing sub-label, it is intended to refer to all such multiple similar components. 
         FIG. 1  shows a simplified cross-sectional view of a processing system that includes a processing chamber having a capacitively coupled plasma (CCP) unit and a showerhead according to an embodiment of the invention; 
         FIG. 2  shows a simplified perspective view of a processing system that includes a processing chamber having a CCP unit and a showerhead according to an embodiment of the invention; 
         FIG. 3  shows a simplified schematic of the gas flow paths of a pair of gas mixtures through a processing system according to an embodiment of the invention; 
         FIG. 4  shows a simplified cross-sectional view of a processing system that includes a processing chamber having a showerhead that also acts as an ion suppression element according to an embodiment of the invention; 
         FIG. 5  shows a simplified cross-sectional view of a processing system that includes a processing chamber with an ion suppression plate partitioning a plasma region from a gas reaction region according to an embodiment of the invention; 
         FIG. 6A  shows a simplified perspective view of an ion-suppression element according to an embodiment of the invention; 
         FIG. 6B  shows a simplified perspective view of a showerhead that also acts as an ion-suppression element according to an embodiment of the invention; 
         FIG. 7A  shows some exemplary hole geometries for openings in an ion-suppression element according to an embodiment of the invention; 
         FIG. 7B  shows a schematic of a hole geometry for an opening in an ion-suppression element according to an embodiment of the invention; 
         FIG. 8  shows an exemplary configuration of opposing openings in a pair of electrodes that help define a plasma region in a processing chamber according to an embodiment of the invention; 
         FIG. 9  is a simplified flowchart illustrating an exemplary method of selectively etching silicon nitride from a substrate comprising a silicon nitride layer and a silicon oxide layer according to an embodiment of the invention; and 
         FIG. 10  is a simplified flowchart illustrating an exemplary etch process providing a higher etch rate of silicon nitride than an etch rate of silicon oxide according to an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     Systems and methods are described for the generation and control of a plasma inside a semiconductor processing chamber. The plasma may originate inside the processing chamber, outside the processing chamber in a remote plasma unit, or both. Inside the chamber, the plasma is contained and separated from the substrate wafer with the help of an ion suppression element that is positioned between the plasma and the substrate wafer. In some instances, this ion suppression element may also function as part of a plasma generation unit (e.g., an electrode), a gas/precursor distribution system (e.g., a showerhead), and/or another component of the processor system. In additional instances, the ion suppression element may function primarily to define a partition between a plasma generation region and a gas reaction region that etches and/or deposits material on exposed surfaces of the substrate wafer. 
     The ion suppression element functions to reduce or eliminate ionically charged species traveling from the plasma generation region to the substrate. Uncharged neutral and radical species may pass through the openings in the ion suppressor to react at the substrate. It should be noted that complete elimination of ionically charged species in the reaction region surrounding the substrate is not always the desired goal. In many instances, ionic species are required to reach the substrate in order to perform the etch and/or deposition process. In these instances, the ion suppressor helps control the concentration of ionic species in the reaction region at a level that assists the process. 
     Exemplary Processing System Configurations 
     Exemplary processing system configurations include an ion suppressor positioned inside a processing chamber to control the type and quantity of plasma excited species that reach the substrate. In some embodiments the ion suppressor unit may be a perforated plate that may also act as an electrode of the plasma generating unit. In additional embodiments the ion suppressor may be the showerhead that distributes gases and excited species to a reaction region in contact with the substrate. In still more embodiments ion suppression may be realized by a perforated plate ion suppressor and a showerhead, both of which plasma excited species pass through to reach the reaction region. 
       FIGS. 1 and 2  show simplified cross-sectional and perspective views, respectively, of a processing system that includes both an ion suppressor  110  as part of a capacitively coupled plasma (CCP) unit  102  and a showerhead  104  that may also contribute to ion suppression. The processing system may also optionally include components located outside the processing chamber  100 , such as fluid supply system  114 . The processing chamber  100  may hold an internal pressure different than the surrounding pressure. For example, the pressure inside the processing chamber may be about 1 mTorr to about 100 Torr. 
     The CCP unit  102  may function to generate a plasma inside the processing chamber  100 . The components of the CCP unit  102  may include a lid or hot electrode  106  and an ion suppression element  110  (also referred to herein as an ion suppressor). In some embodiments, the lid  106  and ion suppressor  110  are electrically conductive electrodes that can be electrically biased with respect to each other to generate an electric field strong enough to ionize gases between the electrodes into a plasma. An electrical insulator  108 , may separate the lid  106  and the ion suppressor  110  electrodes to prevent them from short circuiting when a plasma is generated. The plasma exposed surfaces of the lid  106 , insulator  108 , and ion suppressor  110  may define a plasma excitation region  112  in the CCP unit  102 . 
     Plasma generating gases may travel from a gas supply system  114  through a gas inlet  116  into the plasma excitation region  112 . The plasma generating gases may be used to strike a plasma in the excitation region  112 , or may maintain a plasma that has already been formed. In some embodiments, the plasma generating gases may have already been at least partially converted into plasma excited species in a remote plasma system (not shown) positioned outside the processing chamber  100  before traveling downstream though the inlet  116  to the CCP unit  102 . When the plasma excited species reach the plasma excitation region  112 , they may be further excited in the CCP unit  102 , or pass through the plasma excitation region without further excitation. In some operations, the degree of added excitation provided by the CCP unit  102  may change over time depending on the substrate processing sequence and/or conditions. 
     The plasma generating gases and/or plasma excited species may pass through a plurality of holes (not shown) in lid  106  for a more uniform delivery into the plasma excitation region  112 . Exemplary configurations include having the inlet  116  open into a gas supply region  120  partitioned from the plasma excitation region  112  by lid  106  so that the gases/species flow through the holes in the lid  106  into the plasma excitation region  112 . Structural and operational features may be selected to prevent significant backflow of plasma from the plasma excitation region  112  back into the supply region  120 , inlet  116 , and fluid supply system  114 . The structural features may include the selection of dimensions and cross-sectional geometry of the holes in lid  106  that deactivates back-streaming plasma, as described below with reference to  FIGS. 7A and 7B . The operational features may include maintaining a pressure difference between the gas supply region  120  and plasma excitation region  112  that maintains a unidirectional flow of plasma through the ion suppressor  110 . 
     As noted above, the lid  106  and the ion suppressor  110  may function as a first electrode and second electrode, respectively, so that the lid  106  and/or ion suppressor  110  may receive an electric charge. In these configurations, electrical power (e.g., RF power) may be applied to the lid  106 , ion suppressor  110 , or both. For example, electrical power may be applied to the lid  106  while the ion suppressor  110  is grounded. The substrate processing system may include a RF generator  140  that provides electrical power to the lid  106  and/or ion suppressor  110 . The electrically charged lid  106  may facilitate a uniform distribution of plasma (i.e., reduce localized plasma) within the plasma excitation region  112 . To enable the formation of a plasma in the plasma excitation region  112 , insulator  108  may electrically insulate lid  106  and ion suppressor  110 . Insulator  108  may be made from a ceramic and may have a high breakdown voltage to avoid sparking. The CCP unit  102  may further include a cooling unit (not shown) that includes one or more cooling fluid channels to cool surfaces exposed to the plasma with a circulating coolant (e.g., water). 
     The ion suppressor  110  may include a plurality of holes  122  that suppress the migration of ionically-charged species out of the plasma excitation region  112  while allowing uncharged neutral or radical species to pass through the ion suppressor  110  into an activated gas delivery region  124 . These uncharged species may include highly reactive species that are transported with less reactive carrier gas through the holes  122 . As noted above, the migration of ionic species through the holes  122  may be reduced, and in some instances completely suppressed. Controlling the amount of ionic species passing through the ion suppressor  110  provides increased control over the gas mixture brought into contact with the underlying wafer substrate, which in turn increases control of the deposition and/or etch characteristics of the gas mixture. For example, adjustments in the ion concentration of the gas mixture can significantly alter its etch selectivity (e.g., SiNx:SiOx etch ratios, metal:SiOx etch ratios, metal:SiNx etch ratios, Poly-Si:SiOx etch ratios, etc.). It can also shift the balance of conformal-to-flowable of a deposited dielectric material. 
     The plurality of holes  122  may be configured to control the passage of the activated gas (i.e., the ionic, radical, and/or neutral species) through the ion suppressor  110 . For example, the aspect ratio of the holes (i.e., the hole diameter to length) and/or the geometry of the holes may be controlled so that the flow of ionically-charged species in the activated gas passing through the ion suppressor  110  is reduced. The holes in the ion suppressor  110  may include a tapered portion that faces the plasma excitation region  112 , and a cylindrical portion that faces the showerhead  104 . The cylindrical portion may be shaped and dimensioned to control the flow of ionic species passing to the showerhead  104 . An adjustable electrical bias may also be applied to the ion suppressor  110  as an additional means to control the flow of ionic species through the suppressor. 
     The showerhead  104  is positioned between the ion suppressor  110  of the CCP unit  102  and a gas reaction region  130  (i.e., gas activation region) that makes contact with a substrate that may be mounted on a pedestal  150 . The gases and plasma excited species may pass through the ion suppressor  110  into an activated gas delivery region  124  that is defined between the ion suppressor  110  and the showerhead  104 . A portion of these gases and species may further pass thorough the showerhead  104  into a gas reaction region  130  that makes contact with the substrate. 
     The showerhead may be a dual-zone showerhead that has a first set of channels  126  to permit the passage of plasma excited species, and a second set of channels that deliver a second gas/precursor mixture into the gas reaction/activation region  130 . The two sets of channels prevent the plasma excited species and second gas/precursor mixture from combining until they reach the gas reaction region  130 . In some embodiments, one or more of the holes  122  in the ion suppressor  110  may be aligned with one or more of the channels  126  in the showerhead  104  to allow at least some of the plasma excited species to pass through a hole  122  and a channel  126  without altering their direction of flow. In additional embodiments, the second set of channels may have an annular shape at the opening facing the gas reaction region  130 , and these annular openings may be concentrically aligned around the circular openings of the first set of channels  126 . 
     The second set of channels in the showerhead  104  may be fluidly coupled to a source gas/precursor mixture (not shown) that is selected for the process to be performed. For example, when the processing system is configured to perform a deposition of a dielectric material such as silicon dioxide (SiO x ) the gas/precursor mixture may include a silicon-containing gas or precursor such as silane, disilane, TSA, DSA, TEOS, OMCTS, TMDSO, among other silicon-containing materials. This mixture may react in gas reaction region  130  with an oxidizing gas mixture that may include plasma excited species such as plasma generated radical oxygen (O), activated molecular oxygen (O 2 ), and ozone (O 3 ), among other species. Excessive ions in the plasma excited species may be reduced as the species move through the holes  122  in the ion suppressor  110 , and reduced further as the species move through the channels  126  in the showerhead  104 . In another example, when the processing system is configured to perform an etch on the substrate surface, the source gas/precursor mixture may include etchants such as oxidants, halogens, water vapor and/or carrier gases that mix in the gas reaction region  130  with plasma excited species distributed from the first set of channels in the showerhead  104 . 
     The processing system may further include a power supply  140  electrically coupled to the CCP unit  102  to provide electric power to the lid  106  and/or ion suppressor  110  to generate a plasma in the plasma excitation region  112 . The power supply may be configured to deliver an adjustable amount of power to the CCP unit  102  depending on the process performed. In deposition processes, for example, the power delivered to the CCP unit  102  may be adjusted to set the conformality of the deposited layer. Deposited dielectric films are typically more flowable at lower plasma powers and shift from flowable to conformal when the plasma power is increased. For example, an argon containing plasma maintained in the plasma excitation region  112  may produce a more flowable silicon oxide layer as the plasma power is decreased from about 1000 Watts to about 100 Watts or lower (e.g., about 900, 800, 700, 600, or 500 Watts or less), and a more conformal layer as the plasma power is increased from about 1000 Watts or more (e.g., about 1000, 1100, 1200, 1300, 1400, 1500, 1600, 1700 Watts or more). As the plasma power increases from low to high, the transition from a flowable to conformal deposited film may be relatively smooth and continuous or progress through relatively discrete thresholds. The plasma power (either alone or in addition to other deposition parameters) may be adjusted to select a balance between the conformal and flowable properties of the deposited film. 
     The processing system may still further include a pedestal  150  that is operable to support and move the substrate (e.g., a wafer substrate). The distance between the pedestal  150  and the showerhead  104  help define the gas reaction region  130 . The pedestal may be vertically or axially adjustable within the processing chamber  100  to increase or decrease the gas reaction region  130  and effect the deposition or etching of the wafer substrate by repositioning the wafer substrate with respect to the gases passed through the showerhead  104 . The pedestal  150  may have a heat exchange channel through which a heat exchange fluid flows to control the temperature of the wafer substrate. Circulation of the heat exchange fluid allows the substrate temperature to be maintained at relatively low temperatures (e.g., about −20° C. to about 90° C.). Exemplary heat exchange fluids include ethylene glycol and water. 
     The pedestal  150  may also be configured with a heating element (such as a resistive heating element) to maintain the substrate at heating temperatures (e.g., about 90° C. to about 1100° C.). Exemplary heating elements may include a single-loop heater element embedded in the substrate support platter that makes two or more full turns in the form of parallel concentric circles. An outer portion of the heater element may run adjacent to a perimeter of the support platen, while an inner portion may run on the path of a concentric circle having a smaller radius. The wiring to the heater element may pass through the stem of the pedestal. 
       FIG. 3  shows a simplified schematic of the gas flow paths of a pair of gas mixtures through a processing system that includes both an ion suppressor plate and a showerhead. At block  305 , a first gas, such as a plasma generating gas mixture, is supplied to the processing chamber via a gas inlet. The first gas may include one or more of the following gases: CF 4 , NH 3 , NF 3 , Ar, He, H 2 O, H 2 , O 2 , etc. Inside the processing chamber, the first gas may be excited through a plasma discharge to form one or more plasma effluents at block  310 . Alternatively (or in addition to the in-situ plasma generation), a remote plasma system (RPS) coupled to the processing chamber may be used to generate an ex-situ plasma whose plasma excitation products are introduced into the process chamber. The RPS plasma excitation products may include ionically-charged plasma species as well as neutral and radical species. 
     Whether the plasma effluents are generated by an in-situ plasma unit, an RPS unit, or both, they may be passed through an ion suppressor in the processing chamber at block  315 . The ion suppressor may block and/or control the passage of ionic species while allowing the passage of radical and/or neutral species as the plasma activated first gas travels to the gas reaction region in the processing chamber. At block  320 , a second gas may be introduced into the processing chamber. As noted above, the contents of the second gas depend on the process performed. For example, the second gas may include deposition compounds (e.g., Si-containing compounds) for deposition processes and etchants for etch processes. Contact and reaction between the first and second gases may be prevented until the gases arrive at the gas reaction region of the process chamber. 
     One way to prevent the first and second gases from interacting before the gas reaction region is to have them flow though separate channels in a dual-zone showerhead (DZSH). Block  330  shows the activated first gas and second gas passing through a DZSH  33  that has a first plurality of channels that permit the activated first gas to pass through the showerhead without interacting with the second gas that passes through a second plurality of channels. After exiting the DZSH, the first and second gases may mix together in the gas reaction region of the processing chamber at block  335 . Depending on the process performed, the combined gases may react to deposit a material on the exposed surfaces of the substrate, etch materials from the substrate, or both. 
     Referring now to  FIG. 4 , a simplified cross-sectional view of a processing system  400  having a showerhead  428  that also acts as an ion suppression element is shown. In the configuration shown, a first gas source  402  for plasma generation is fluidly coupled to an optional RPS unit  404  where a first plasma may be generated and the plasma effluents transported into the processing chamber  406  through gas inlet  408 . Inside the processing chamber  406 , the gases may pass through holes  410  in a gas distribution plate  412  into a gas region  414  defined between the plate  412  and showerhead  428 . In some embodiments, this region  414  may be a plasma excitation/activation region where the gas distribution plate  412  and showerhead  428  act as first and second electrodes to further excite the gas and/or generate the first plasma. The holes  410  in the gas distribution plate  412  may be dimensionally or geometrically structured to deactivate back-streaming plasma. The plate  412  and showerhead  428  may be coupled with a RF power generator  422  that supplies a charge to the plate  412  and showerhead  428  to excite the gases and/or generate a plasma. In one embodiment, the showerhead  428  is grounded while a charge is applied to plate  412 . 
     The excited gases or activated gases in the gas region  414  may pass through showerhead  428  into a gas reaction region  416  adjacent a substrate  418  to etch material from the surface of the substrate and/or deposit material on the substrate&#39;s surface. The showerhead  428  may be a DZSH that allows the excited gases to pass from the gas region  414  into the gas reaction region  416  while also allowing a second gas (i.e., precursor gas/mixture) to flow from an external source (not shown) into the gas reaction region  416  via a second gas inlet  426 . The DZSH may prevent the activated/excited gas from mixing with the second gas until the gases flow into the gas reaction region  416 . 
     The excited gas may flow through a plurality of holes  424  in the DZSH, which may be dimensionally and/or geometrically structured to control or prevent the passage of plasma (i.e., ionically charged species) while allowing the passage of activated/excited gases (i.e., reactive radical or uncharged neutral species).  FIG. 7A  provides exemplary embodiments of hole configurations that may be used in the DZSH. In addition to the holes  424 , the DZSH may include a plurality of channels  426  through which the second gas flows. The second gas (precursor gas) may exit the showerhead  428  through one or more apertures (not shown) that are positioned adjacent holes  424 . The DZSH may act as both a second gas delivery system and an ion suppression element. 
     As described above, the mixed gases may deposit a material on and/or etch a material from the surface of the substrate  418 , which may be positioned on a platen  420 . The platen  420  may be vertically movable within the processing chamber  406 . The processing of the substrate  418  within the processing chamber  406  may be affected by the configurations of the holes  424 , the pressure within the gas region  414 , and/or the position of the substrate  418  within the processing chamber. Further, the configuration of the holes  424  and/or pressure within the gas region  414  may control the concentration of ionic species (plasma) allowed to pass into the gas excitation region  416 . The ionic concentration of the gas mixture can shift the balance of conformal-to-flowable of a deposited dielectric material in addition to altering the etch selectivity. 
     Referring now to  FIG. 5 , a simplified cross-sectional view of another processing system  500  having a plate  512  (i.e., ion suppressor plate) that acts as an ion suppression element is shown. In the configuration shown, a first gas source  502  is fluidly coupled to an RPS unit  504  where a first plasma may be generated and the plasma effluents transported into the processing chamber  506  through gas inlet  508 . The plasma effluents may be transported to a gas region  514  defined between the ion suppressor plate  512  and the gas inlet  508 . Inside the gas region  514 , the gases may pass through holes  510  in the ion suppressor  512  into a gas reaction/activation region  516  defined between the ion suppressor  512  and a substrate  518 . The substrate  518  may be supported on a platen  520  as described above so that the substrate is movable within the processing chamber  506 . 
     Also as described above, the holes  510  may be dimensionally and/or geometrically structured so that the passage of ionically charged species (i.e., plasma) is prevented and/or controlled while the passage of uncharged neutral or radical species (i.e., activated gas) is permitted. The passage of ionic species may be controllable by varying the pressure of the plasma within gas region  514 . The pressure in gas region  514  may be controlled by controlling the amount of gas delivered through gas inlet  508 . The precursor gas (i.e., second gas) may be introduced into the processing chamber  506  at one or more second gas inlets  522  positioned vertically below or parallel with ion suppressor  512 . The second gas inlet  522  may include one or more apertures, tubes, etc. (not shown) in the processing chamber  506  walls and may further include one or more gas distribution channels (not shown) to deliver the precursor gas to the apertures, tubes, etc. In one embodiment, the ion suppressor  512  includes one or more second gas inlets, through which the precursor gas flows. The second gas inlets of the ion suppressor  512  may deliver the precursor gas into the gas reaction region  516 . In such an embodiment, the ion suppressor  512  functions as both an ion suppressor and a dual zone showerhead as described previously. The activated gas that passes through the holes  510  and the precursor gas introduced in the processing chamber  506  may be mixed in the gas reaction chamber  516  for etching and/or deposition processes. 
     Having now described exemplary embodiments of processing chambers, attention is now directed to exemplary embodiments of ion suppressors, such as ion suppressor plates  412  and  512  and showerhead  428 . 
     Exemplary Ion Suppressors 
       FIG. 6A  shows a simplified perspective view of an ion-suppression element  600  (ion suppressor) according to an embodiment of the invention. The ion suppression element  600  may correspond with the ion suppressor plates of  FIG. 4  and/or  FIG. 5 . The perspective view shows the top of the ion suppression element or plate  600 . The ion suppression plate  600  may be generally circular shaped and may include a plurality of plasma effluent passageways  602 , where each of the passageways  602  includes one or more through holes that allow passage of the plasma effluents from a first region (e.g., plasma region) to a second region (e.g., gas reaction region or showerhead). In one embodiment, the through holes of the passageway  602  may be arranged to form one or more circular patterns, although other configurations are possible. As described previously, the through holes may be geometrically or dimensionally configured to control or prevent the passage of ion species while allowing the passage or uncharged neutral or radical species. The through holes may have a larger inner diameter toward the top surface of the ion suppression plate  600  and a smaller inner diameter toward the bottom surface of the ion suppression plate. Further, the through holes may be generally cylindrical, conical, or any combination thereof. Exemplary embodiments of the configurations of the through holes are provided in  FIGS. 7A-B . 
     The plurality of passageways may be distributed substantially evenly over the surface of the ion suppression plate  600 , which may provide even passage of neutral or radical species through the ion suppression plate  600  into the second region. In some embodiments, such as the embodiment of  FIG. 5 , the processing chamber may only include an ion suppression plate  600 , while in other embodiments, the processing chamber may include both a ion suppression plate  600  and a showerhead, such as the showerhead of  FIG. 6B , or the processing chamber may include a single plate that acts as both a dual zone showerhead and an ion suppression plate. 
       FIG. 6B  shows a simplified bottom view perspective of a showerhead  620  according to an embodiment of the invention. The showerhead  620  may correspond with the showerhead illustrated in  FIG. 4 . As described previously, the showerhead  620  may be positioned vertically adjacent to and above a gas reaction region. Similar to ion suppression plate  600 , the showerhead  620  may be generally circular shaped and may include a plurality of first holes  622  and a plurality of second holes  624 . The plurality of first holes  622  may allow plasma effluents to pass through the showerhead  620  into a gas reaction region, while the plurality of second holes  624  allow a precursor gas, such as a silicon precursor, etchants etc., to pass into the gas reaction region. 
     The plurality of first holes  622  may be through holes that extend from the top surface of the showerhead  620  through the showerhead. In one embodiment, each of the plurality of first holes  622  may have a smaller inner diameter (ID) toward the top surface of the showerhead  620  and a larger ID toward the bottom surface. In addition, the bottom edge of the plurality of first holes  622  may be chamfered  626  to help evenly distribute the plasma effluents in the gas reaction region as the plasma effluents exit the showerhead and thereby promote even mixing of the plasma effluents and precursor gases. The smaller ID of the first holes  622  may be between about 0.5 mm and about 20 mm. In one embodiment, the smaller ID may be between about 1 mm and 6 mm. The cross sectional shape of the first holes  622  may be generally cylindrical, conical, or any combination thereof. Further, the first holes  622  may be concentrically aligned with the through holes of passageways  602 , when both an ion suppression element  600  and a showerhead  620  are used in a processing chamber. The concentric alignment may facilitate passage of an activated gas through both the ion suppression element  600  and showerhead  620  in the processing chamber. 
     In another embodiment, the plurality of first holes  622  may be through holes that extend from the top surface of the showerhead  620  through the showerhead, where each of the first holes  622  have a larger ID toward the top surface of the showerhead and a smaller ID toward the bottom surface of the showerhead. Further, the first holes  622  may include a taper region that transitions between the larger and smaller IDs. Such a configuration may prevent or regulate the passage of a plasma through the holes while permitting the passage of an activated gas. Such embodiments may be used in place of or in addition to ion suppression element  600 . Exemplary embodiments of such through holes are provided in  FIG. 7A . 
     The number of the plurality of first holes  622  may be between about 60 and about 2000. The plurality of first holes  622  may also have a variety of shapes, but are generally round. In embodiments where the processing chamber includes both a ion suppression plate  600  and a showerhead  620 , the plurality of first holes  622  may be substantially aligned with the passageways  602  to facilitate passage of the plasma effluents through the ion suppression plate and showerhead. 
     The plurality of second holes  624  may extend from the bottom surface of the showerhead  620  partially through the showerhead. The plurality of second holes may be coupled with or connected to a plurality of channels (not shown) that deliver the precursor gas (e.g., deposition compounds, etchants, etc.) to the second holes  624  from an external gas source (not shown). The second holes may include a smaller ID at the bottom surface of the showerhead  620  and a larger ID in the interior of the showerhead. The number of second holes  624  may be between about 100 and about 5000 or between about 500 and about 2000 in different embodiments. The diameter of the second holes&#39; smaller ID (i.e., the diameter of the hole at the bottom surface) may be between about 0.1 mm and about 2 mm. The second holes  624  are generally round and may likewise be cylindrical, conical, or any combination thereof. Both the first and second holes may be evenly distributed over the bottom surface of the showerhead  620  to promote even mixing of the plasma effluents and precursor gases. 
     With reference to  FIG. 7A , exemplary configurations of the through holes are shown. The through holes depicted generally include a large inner diameter (ID) region toward an upper end of the hole and a smaller ID region toward the bottom or lower end of the hole. The smaller ID may be between about 0.2 mm and about 5 mm. Further, aspect ratios of the holes (i.e., the smaller ID to hole length) may be approximately 1 to 20. Such configurations may substantially block and/or control passage of ion species of the plasma effluent while allowing the passage of radical or neutral species. For example, varying the aspect ratio may regulate the amount of plasma that is allowed to pass through the through holes. Plasma passage may further be regulated by varying the pressure of the plasma within a region directly above the through holes. 
     Referring now to specific configurations, through hole  702  may include a large ID region  704  at an upper end of the hole and a small ID region  706  at a lower end of the hole with a stepped edge between the large and small IDs. Through hole  710  may include a large ID region  712  on an upper end and a large ID region  716  on a lower end of the hole with a small ID region  714  therebetween. The transition between the large and small ID regions may be stepped or blunt to provide an abrupt transition between the regions. 
     Through hole  720  may include a large ID region  722  at the upper end of the hole and small ID region  726  at a lower end of the hole with a tapered region  724  that transitions at an angle θ between the large and small regions. The height  728  of the small ID region  726  may depend on the overall height  727  of the hole, the angle θ of tapered region  724 , the large ID, and the small ID. In one embodiment, the tapered region  724  comprises an angle of between about 15° and about 30°, and preferably about 22°; the overall height  727  is between about 4 mm and about 8 mm, and preferably about 6.35 mm; the large ID is between about 1 mm and about 4 mm, and preferably about 2.54 mm; the small ID is between about 0.2 mm and 1.2 mm, and preferably about 0.89 mm, so that the height  728  of the small ID region  726  region is between about 1 mm and about 3 mm, and preferably about 2.1 mm. 
     Through hole  730  may include a first ID region  732  at the upper end of the hole, a second ID region  734  concentrically aligned with and positioned vertically below first ID region  732 , and a third ID region  736  concentrically aligned with and positioned vertically below second ID region  734 . First ID region  732  may comprise a large ID, second ID region  734  may comprise a small ID, and third ID region  736  may comprise a slightly larger ID than second ID region  734 . Third ID region  736  may extend to the lower end of the hole or may be outwardly tapered to an exit ID  737 . The taper between the third ID region  736  and the exit ID  737  may taper at an angle θ 3 , which may be between about 15° and about 30°, and preferably about 22°. The second ID region  734  may include a chamfered edge that transitions from the first ID region  732  at an angle θ 1 , which may be between about 110° and about 140°. Similarly, the second ID region  734  may include a chamfered edge that transitions into the third ID region  736  at an angle θ 2 , which may also be between about 110° and about 140°. In one embodiment, the large ID of first region  732  may be between about 2.5 mm and about 7 mm, and preferably about 3.8 mm; the small ID of second ID region  734  may be between about 0.2 mm and about 5 mm, and preferably about 0.4 mm; the slightly larger ID of third ID region  736  may be between about 0.75 mm and about 2 mm, and preferably about 1.1 mm; and the exit ID may be between about 2.5 mm and about 5 mm, and preferably about 3.8 mm. 
     The transition (blunt, stepped, tapered, etc.) between the large ID regions and small ID regions may substantially block the passage of ion species from passing through the holes while allowing the passage of radical or neutral species. For example, referring now to  FIG. 7B , shown is an enlarged illustration of through hole  720  that includes the transition region  724  between the large ID region  722  and the small ID region  726 . The tapered region  724  may substantially prevent plasma  725  from penetrating through the through hole  720 . For example, as the plasma  725  penetrates into the through hole  720 , the ion species may deactivate or ground out by contacting the walls of the tapered region  724 , thereby limiting the passage of the plasma through the through hole and containing the plasma within the region above the through hole  720 . The radical or neutral species, however, may pass through the through hole  720 . Thus, the through hole  720  may filter the plasma  720  to prevent or control the passage of unwanted species. In an exemplary embodiment, the small ID region  726  of the through holes comprises an ID of 1 mm or smaller. To maintain a significant concentration of radical and/or neutral species penetrating through the through holes, the length of the small ID region and/or the taper angle may be controlled. 
     In addition to preventing the passage of plasma, the through holes described herein may be used to regulate the passage of plasma so that a desired level of plasma is allowed to pass through the through hole. Regulating the flow of plasma through the through holes may include increasing the pressure of the plasma in the gas region above the ion suppressor plate so that a desired fraction of the plasma is able to pass through the ion suppressor without deactivating or grounding out. 
     Referring now to  FIG. 8 , a simplified illustration of a CCP unit  800  is shown. Specifically, the CCP unit  800  shown includes a top plate  802  and a bottom plate  804  that define a plasma generation region  810  in which a plasma is contained. As previously described, the plasma may be generated by an RPS (not shown) and delivered to the plasma generation region  810  via through hole  806 . Alternatively or additionally, the plasma may be generated in the CCP unit  800 , for example, by utilizing top plate  802  and bottom plate  804  as first and second electrodes coupled to a power generation unit (not shown). 
     The top plate  802  may include a through hole  806  that allows process gas and/or plasma to be delivered into the plasma generation region  810  while preventing back-streaming of plasma through the top plate  802 . The through hole  806  may be configured similar to through hole  730  having first, second, and third ID regions ( 820 ,  822 , and  824  respectively), with a chamfered edge between adjacent regions ( 828  and  829 ) and a tapered region  826  transitioning between third ID region  824  and an exit ID. The tapered region  826  between third ID region  824  and the exit ID and/or the chamfered edge between second and third ID regions ( 822  and  824  respectively) may prevent back-streaming of plasma by deactivating or grounding ion species as the plasma penetrates into the through hole  806 . 
     Similarly, the bottom plate  804  may include a through hole  808  that allows the radical or neutral species to pass through the through hole while preventing or controlling the passage of ion species. The through hole  808  may be configured similar to through hole  720  having a large ID region  830 , a small ID region  832 , and a tapered region  834  that transitions between the large ID region  830  and the small ID region  832 . The tapered region  834  may prevent the flow of plasma through the through hole  808  by deactivating or grounding ion species as previously explained while allowing radical or neutral species to pass there through. 
     To further prevent passage of the plasma through the through holes,  806  and/or  808 , the top plate  802  and/or bottom plate  804  may receive a charge to electrically bias the plasma and contain the plasma within plasma generation region  810  and/or adjust an ion concentration in the activated gas that passes through the bottom plate. Using top plate  802  and bottom plate  804  in CCP unit  800 , the plasma may be substantially generated and/or maintained in the plasma generation region  810 , while radical and neutral species are delivered to a gas reaction region to be mixed with one or more precursor gases to etch material from or deposit material on a substrate surface. 
     Exemplary Processes 
     In accordance with some embodiments of the invention, an ion suppressor as described above may be used to provide radical and/or neutral species for etch or deposition processes. In one embodiment, for example, an ion suppressor is used to provide fluorine radicals to selectively etch silicon nitride. Using the fluorine radicals, an etch rate selectivity of silicon nitride to silicon oxide of as high as about 80:1 or more can be obtained. One use for such a process is to remove silicon nitride in a replacement gate process. A silicon nitride gate may be selectively removed without removing exposed silicon oxide regions such as gate oxide. The silicon nitride may be replaced with a gate material such as metal. 
     The ion suppressor may be used to provide a reactive gas having a higher concentration of radicals than ions. Because most of the charged particles of a plasma are filtered or removed by the ion suppressor, the substrate is typically not biased during the etch process. Such a process using radicals and other neutral species can reduce plasma damage compared to conventional plasma etch processes that include sputtering and bombardment. Embodiments of the present invention are also advantageous over conventional wet etch processes where surface tension of liquids can cause bending and peeling of small features. 
       FIG. 9  is a simplified flowchart illustrating an exemplary method of selectively etching silicon nitride from a substrate comprising a silicon nitride layer and a silicon oxide layer according to an embodiment of the invention. The method includes flowing a fluorine-containing gas into a plasma generation region of a substrate processing chamber ( 902 ). The fluorine-containing gas may include HF, F 2 , NF 3 , CF 4 , CHF 3 , C 2 F 6 , C 3 F 6 , BrF 3 , ClF 3 , SF 6 , or the like. Other embodiments may include other halogen-containing gases that do not include fluorine, such as Cl 2 , HBr, SiCl 4 , and the like, in place of the fluorine-containing gas. In the exemplary method of  FIG. 9 , the fluorine-containing gas may also include one or more oxygen sources such as O 2 , O 3 , N 2 O, NO, or the like. Using oxygen can increase an etch rate of the silicon nitride with minimal impact on an etch rate of the silicon oxide. The fluorine-containing gas may also include one or more inert gases such as H 2 , He, N 2 , Ar, or the like. The inert gas can be used to improve plasma stability. Flow rates and ratios of the different gases may be used to control etch rates and etch selectivity. In an embodiment, the fluorine-containing gas includes NF 3  at a flow rate of between about 5 sccm and 500 sccm, O 2  at a flow rate of between about 0 sccm and 5000 sccm, He at a flow rate of between about 0 sccm and 5000 sccm, and Ar at a flow rate of between about 0 sccm and 5000 sccm. One of ordinary skill in the art would recognize that other gases and/or flows may be used depending on a number of factors including processing chamber configuration, substrate size, geometry and layout of features being etched, and the like. 
     The method also includes applying energy to the fluorine-containing gas to generate a plasma in the plasma generation region ( 904 ). As would be appreciated by one of ordinary skill in the art, the plasma may include a number of charged and neutral species including radicals and ions. The plasma may be generated using known techniques (e.g., RF, capacitively coupled, inductively coupled, and the like). In an embodiment, the energy is applied using a CCP unit at a source power of between about 15 W and 5000 W and a pressure of between about 0.2 Torr and 30 Torr. The CCP unit may be disposed remote from a gas reaction region of the processing chamber. For example, the CCP unit and the plasma generation region may be separated from the gas reaction region by an ion suppressor. 
     The method also includes filtering the plasma to provide a reactive gas having a higher concentration of fluorine radicals than fluorine ions ( 906 ). The plasma may be filtered using an ion suppressor disposed between the plasma generation region and the gas reaction region of the substrate processing chamber. The ion suppressor may include a plurality of channels that allow passage of the fluorine radicals and neutral species between the plasma generation region and the gas reaction region. The ion suppressor may be configured to remove some or all of the ions passing from the plasma generation region. In an embodiment, for example, a significant portion of the ions may be removed such that the reactive gas is substantially free from ions. 
     The method also includes flowing the reactive gas into a gas reaction region of the substrate processing chamber ( 908 ). In an embodiment, the ion suppressor may be configured as a showerhead, and the reactive gas exiting the ion suppressor may flow into the gas reaction region proximate to the substrate. Alternatively, the reactive gas exiting the ion suppressor may flow through a showerhead or another gas distributor and into the gas reaction region. 
     The method also includes exposing the substrate to the reactive gas in the gas reaction region of the substrate processing chamber ( 910 ). In an embodiment, the temperature of the substrate may be between about −10° C. and 200° C., and the pressure in the substrate processing chamber may be between about 0.2 Torr and 30 Torr. One of ordinary skill in the art would recognize that other temperatures and/or pressures may be used depending on a number of factors as explained previously. The reactive gas etches the silicon nitride layer at a higher etch rate than the reactive gas etches the silicon oxide layer. 
       FIG. 10  is a simplified flowchart illustrating an exemplary etch process providing a higher etch rate of silicon nitride than an etch rate of silicon oxide according to an embodiment of the invention. The process includes generating a plasma from a fluorine-containing gas, the plasma comprising fluorine radicals and fluorine ions ( 1002 ). As explained above, the plasma may be formed in a plasma generation region of a substrate processing chamber that is separate from a gas reaction region. The process also includes removing a portion of the fluorine ions from the plasma to provide a reactive gas having a higher concentration of fluorine radicals than fluorine ions ( 1004 ). The portion of the fluorine ions may be removed using an ion suppressor. The process also includes exposing a substrate comprising a silicon nitride layer and a silicon oxide layer to the reactive gas, where the reactive gas etches the silicon nitride layer at a higher etch rate than the reactive gas etches the silicon oxide layer ( 1006 ). 
     It should be appreciated that the exemplary processes illustrated in  FIGS. 9-10  are not limited to use with the processing chambers illustrated in  FIGS. 1-5  or the ion-suppression elements illustrated in  FIGS. 6A, 6B, 7A, 7B, and 8-9 . Rather, processes in accordance with embodiments of the invention may be performed using other hardware configurations. Further, the specific steps illustrated in  FIGS. 9-10  provide particular methods in accordance with embodiments of the present invention. The steps outlined above may be continuously repeated by system software, and other sequences of steps may be performed according to alternative embodiments. For example, the steps outlined above may be performed in a different order. Moreover, the individual steps illustrated in  FIGS. 9-10  may include multiple sub-steps that may be performed in various sequences as appropriate to the individual step. Furthermore, additional steps may be added or removed depending on the particular application. One of ordinary skill in the art would recognize many variations, modifications, and alternatives. 
     It should be noted that the methods and apparatuses discussed throughout the specification are provided merely as examples. Various embodiments may omit, substitute, or add various steps or components as appropriate. For instance, it should be appreciated that features described with respect to certain embodiments may be combined in various other embodiments. Furthermore, embodiments may be implemented by hardware, software, firmware, middleware, microcode, hardware description languages, or any combination thereof. When implemented in software, firmware, middleware, or microcode, the program code or code segments to perform the necessary tasks may be stored in a computer-readable medium such as a storage medium. Processors may be adapted to perform the necessary tasks. The term “computer-readable medium” includes, but is not limited to, portable or fixed storage devices, optical storage devices, sim cards, other smart cards, and various other mediums capable of storing, containing, or carrying instructions or data.