Abstract:
An exemplary method and apparatus for abating reaction products from a vacuum processing chamber includes a reaction chamber in fluid communication with the vacuum processing chamber, a coil disposed about the reaction chamber, and a power source for supplying RF energy to the coil. The coil creates a plasma in the reaction chamber which effectively breaks down stable reaction products from the vacuum processing chamber such as perfluorocarbons (PFCs) and hydrofluorocarbons (HFCs) which significantly contribute to global warming. According to alternative embodiments, the plasma may be generated with grids or coils disposed in the reaction chamber perpendicular to the flow of reaction products from the vacuum processing chamber.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates generally to vacuum processing chambers and more particularly to a method and apparatus for abatement of reaction products such as perfluorocarbons and hydrofluorocarbons in vacuum processing chambers.  
       BACKGROUND OF THE INVENTION  
       [0002]     Various types of equipment exist for semiconductor processing such as plasma etching, ion implantation, sputtering, rapid thermal processing (RTP), photolithography, chemical vapor deposition (CVD), and flat panel display fabrication processes wherein etching, resist stripping, passivation, deposition, and the like, are carried out. For example, a vacuum processing chamber may be used for etching and chemical vapor deposition of materials on substrates by supplying an etching or deposition gas to the vacuum chamber and by application of radio frequency (RF) energy to the gas. Electromagnetic coupling of RF energy into the source region of a vacuum chamber is conventionally employed to generate and maintain a high electron density plasma having a low particle energy. Generally, plasmas may be produced from a low-pressure process gas by inducing an electron flow which ionizes individual gas molecules through the transfer of kinetic energy through individual electron-gas molecule collisions. Most commonly, the electrons are accelerated in an electric field, typically a radiofrequency electric field produced between a pair of opposed electrodes which are oriented parallel to the wafer.  
         [0003]     Plasma generation is used in a variety of such semiconductor fabrication processes. Plasma generating equipment includes parallel plate reactors such as the type disclosed in commonly owned U.S. Pat. No. 4,340,462, electron cyclotron resonance (ECR) systems such as the type disclosed in commonly owned U.S. Pat. No. 5,200,232, and inductively coupled plasma (ICP) or transformer coupled plasma (TCP™) systems such as the type disclosed in commonly owned U.S. Pat. No. 4,948,458.  
         [0004]     Due to the tremendous growth in integrated circuit production, the use of vacuum processing chambers has increased dramatically in recent years. The use of vacuum processing chambers may seriously affect the environment, however, because perfluorocarbons (PFCs) are widely used in plasma etch and plasma-enhanced CVD equipment. PFCs are highly stable compounds which makes them well suited for plasma processing. However, PFCs also significantly contribute to global warming and are not destroyed by scrubbers or other conventional emission control equipment used in vacuum processing chambers. Although there are many gasses which cause global warming, PFCs and hydrofluorocarbons (HFCs), both of which are referred to hereinafter as “fluorocarbons”, also used in plasma processing, have particularly high global warming potentials (GWPs). For example, CF 4 , C 3 F 8 , SF 6 , NF 3 , and C 2 HF 5  all have GWPs of over 3000, and C 2 F 6 , SF 6 , and CHF 3  have GWPs of over 12,000. By contrast, carbon dioxide, a well-known greenhouse gas, has a GWP of 1. In addition, because of their stability, PFCs have a very long lifetime. For example, the lifetimes of CF 4  and C 2 F 6  are 50,000 and 10,000 years, respectively. Thus, collectively, these process gasses can have a significant impact on the environment.  
         [0005]     To reduce the impact of PFCs on the environment, several conventional methods for abating PFCs from vacuum processing chambers have been proposed, including process optimization, chemical alternatives, and destruction/decomposition. Process optimization involves the refinement of system parameters to achieve the desired process while using the minimum amount of PFCs. Process optimization is desirable because it reduces chemical costs and emissions and may increase throughput and prolong the life of internal components of the reactor. However, process optimization does not provide a complete solution since it does not involve the abatement of PFCs once they are used in the system. Thus, although the amount of PFCs used is reduced by process optimization, the PFCs that are used are ultimately emitted into the environment.  
         [0006]     Chemical alternatives to using PFCs are desirable because they eliminate the problem of PFC emissions entirely. However, to date, research is still underway to uncover more effective and environmentally sound chemical alternatives.  
         [0007]     There are two basic categories for conventional destruction/decomposition techniques. The first category involves abatement performed on the atmospheric side of the system, either at each tool or on a large scale for multiple tools, after the gasses have passed through the pumping system. On the atmospheric side, there are several possibilities, including water scrubbers, resin beds, furnaces, flame-based bum boxes, and plasma torches. All of these except burn boxes and torches are ineffective against many of the highly stable PFC compounds.  
         [0008]     Burn boxes have been shown to be inefficient abatement devices, unless very large amounts of reactant gases such as hydrogen, methane, or natural gas are flowed through the burn box. This makes these devices very expensive to operate, and environmentally unfriendly.  
         [0009]     Plasma torches on the atmospheric side could be more effective; however, very large and expensive, not to mention dangerous, torch facilities would be required to abate what would be a small concentration of PFCs in a very high flow of tool effluent. This inefficiency is exacerbated by the addition of large amounts of nitrogen, used as a diluent in vacuum pumps and as a purge gas in many tool operations.  
         [0010]     The second general category of destruction/decomposition techniques involves abatement performed under a vacuum upstream of the pumping system. Plasma destruction is one method, for example, in which a device is employed to treat the exhaust from the tool upstream of the pump before nitrogen purge dilution has taken place. In plasma destruction, energy is applied to the reaction gasses to create a plasma in which the gasses are ionized. The PFCs become unstable at the high energy state, and are consequently broken down into smaller molecules which are less detrimental to the environment.  
         [0011]     Examples of devices for plasma destruction include the ETC Dryscrub and the Eastern Digital Post-Reaction Chamber (PRC). The ETC Dryscrub is a flat spiral chamber. The gases come in at the outer end of the spiral, circle around, and eventually exit through the center of the spiral. The spiral is an RF electrode operated at 100 kHz. The purpose of the reactor is to dissociate exhaust gases coming from a CVD system so that any remaining solid-producing gases are reacted onto the walls of the spiral. Tests performed on the ETC Dryscrub reactor, however, resulted in a relatively ineffective abatement of C 2 F 6 , the test gas. In addition, the main reaction product was another greenhouse PFC gas, CF 4 , and most of the secondary products were also greenhouse gasses. The Eastern Digital PRC is essentially the same as the ETC Dryscrub, and yields similar results. These devices are both inefficient at plasma abatement and have low plasma densities and dissociation rates.  
         [0012]     Another known method for plasma destruction, in which CF 4 , C 2 F 6 , and SF 6  may be abated, involves the use of a microwave plasma reactor. Microwave sources, however, are expensive and complex. They also have small skin depths, so they tend have an axial region where there is no plasma, through which unabated gases escape. This can be compensated by inserting a “plug”, but the plug then reduces the fluid conductance of the device, which in turn adversely affects the performance of the pumping system.  
       SUMMARY OF THE INVENTION  
       [0013]     According to an exemplary embodiment of the invention, an apparatus for abating fluorocarbons in gas reaction products from a vacuum processing chamber comprises a dielectric tube in fluid communication with an outlet port of the vacuum processing chamber, a coil disposed around the dielectric tube, and an RF source, connected to the coil, for supplying RF energy to the coil and destroying fluorocarbons in the gas.  
         [0014]     By applying RF power to the coil, an inductively coupled plasma (ICP) is generated in the dielectric tube which breaks down the reaction products from the vacuum processing chamber. The abatement device provides several advantages over prior systems. For example, in contrast to many prior designs, the abatement device can be made to be simple, compact, inexpensive, efficient, reliable, and require little or no operator or control system intervention. The abatement device also provides a high plasma density, high dissociation rate operation, and a skin depth which is adjustable through the frequency. This results in efficient abatement without compromising foreline conductance.  
         [0015]     According to alternative embodiments, the plasma may be generated with grids or coils disposed in the reaction chamber perpendicular to the flow of reaction products from the vacuum processing chamber. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]     The foregoing and other objects, features and advantages of the present invention will be more readily understood upon reading the following detailed description in conjunction with the drawings in which:  
         [0017]      FIG. 1  is an illustration of an abatement apparatus according to an exemplary embodiment of the invention;  
         [0018]      FIG. 2  is an illustration of one embodiment of a reactor tube;  
         [0019]      FIG. 3  is an illustration of one embodiment of the plasma reactor of  FIG. 1  which includes the reactor tube of  FIG. 2 ;  
         [0020]      FIG. 4  is an illustration of the exemplary matching network of  FIG. 3 ;  
         [0021]      FIG. 5  is an illustration of another embodiment of the plasma reactor of  FIG. 1 ;  
         [0022]      FIG. 6  is an illustration of another embodiment of the plasma reactor of  FIG. 1 ; and  
         [0023]      FIG. 7  is an illustration of an exemplary scrubber shown in  FIG. 1 . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0024]     Referring to  FIG. 1 , a fluorocarbon (e.g. perfluorocarbon and/or hydrofluorocarbon) abatement apparatus according to an exemplary embodiment of the invention generally comprises a plasma reactor  100  which includes an RF power source  102  for supplying power to the plasma reactor  100 . The abatement apparatus is preferably installed in the foreline  104  of each vacuum processing chamber at the point of use. According to a preferred embodiment, the abatement apparatus comprises a section of dielectric tubing replacing a section of foreline. The abatement apparatus shown in  FIG. 1  may thus be installed downstream of a vacuum processing chamber  106 . According to a preferred embodiment, a coil wrapped around the dielectric tube is driven with RF power in order to generate an inductively coupled plasma (ICP) within the foreline. The plasma breaks down reaction products from the vacuum processing chamber such as PFCs and HFCs.  
         [0025]     The plasma reactor  100  preferably includes a cooling mechanism  108 , such as a water cooler or a fan, for dissipating heat created by the plasma in the plasma reactor  100 . For example, the plasma reactor  100  may include a water jacket in which water is circulated to absorb heat in the plasma reactor  100 , or a fan.  
         [0026]     The abatement apparatus may also include a reactant mixing chamber  110  upstream of the plasma reactor  100  for mixing a reactant into the gas flow before it reaches the plasma reactor  100 . The reactant mixing chamber  110  is supplied with the reactant by a reactant source  112 . According to a preferred embodiment, water vapor is used as the reactant to supply hydrogen and oxygen to the reaction. The reactant may also include a compound such as H 2 , CH 4 , or other hydride to supply hydrogen to the reaction, and O 2  to supply oxygen to the reaction.  
         [0027]     A scrubber  114  can be installed downstream of the plasma reactor  100  to remove HF to prevent damage of the remaining foreline and/or mechanical pump by HF which is a highly corrosive gas. The scrubber  114  may include a material such as Si or W in the form of pellets, beads, chunks, lines, baffles, screens, etc., which reacts with HF. Downstream of the scrubber, an emission monitoring or sampling unit  116  may be provided to monitor the content of the effluent gasses.  
         [0028]      FIGS. 2 and 3  are illustrations of a plasma reactor  100 A according to a preferred embodiment of the invention. The plasma reactor  100 A may generally comprise a dielectric tube  120  through which reaction products from the vacuum processing chamber  106  flow. The dielectric tube  120  is preferably divided into three sections, as shown in  FIG. 2 . The middle section  122  preferably comprises quartz because of its ability to withstand high temperatures. Two end sections  124 , which are preferably made of glass, are sealed to the quartz tube  122 . The glass end sections  124  are also sealed to a metal foreline tube  126 , which can be a standard ISO NW50 flange. The glass end sections  124  are provided because it is difficult to bond quartz directly to metal.  
         [0029]     According to one embodiment, the tube shown in  FIG. 2  is about 13 inches long from flange to flβnge with an inner diameter of about 2 inches and an outer flange diameter of about 3 inches. The quartz section  122  may be about 10 inches long and the two glass sections  124  may each be 2 inches long. These dimensions are of course provided only as an example and are not intended to be limiting.  
         [0030]     As shown in  FIG. 3 , a coil  130  is provided to generate a high density plasma source which efficiently abates PFCs and other products from the vacuum processing chamber  106 . The coil  130  preferably encircles the inner quartz section  122  but not the glass end sections  124  so that the glass end sections  124  remain at a lower temperature.  
         [0031]      FIG. 3  shows the exemplary plasma reactor  100 A implemented as part of a vacuum processing apparatus. The foreline  104  of the apparatus is at low pressure and contains reaction products from the vacuum processing chamber. The pressure inside the dielectric tube may range from about 30 milliTorr to 3 Torr, for example, and preferably is about 200 milliTorr.  
         [0032]     A bellows  134  may be provided between the foreline  104  and the plasma reactor  100  and may be attached to the foreline  104  with a flange  136 . The bellows  134  provides strain relief to the dielectric tube  120  of the plasma reactor  100 A. Thus, any strain caused by movement of the foreline  104  with respect to the plasma reactor  100 A may be alleviated with the flexible bellows  134  so that the dielectric tube  120  of the plasma reactor  100 A is not damaged. The dielectric tube  120  can be further protected by securely attaching it to the rigid enclosure  136  in which it is located. The walls of the rigid enclosure  136 , which may be metal and which are fixed to the dielectric tube  120 , provide additional resistance to strains imparted on the dielectric tube  120  by movement of the foreline  104 .  
         [0033]     The apparatus may also include an RF shielding seal  138  at the junction between the reactor tube  120  and the rigid enclosure  136  in which the reactor tube  120  is located. The RF shielding seal  138  prevents RF radiation generated by the coil  130  from interfering with nearby electronic devices. An RF feed through connector  140  may be installed on the enclosure  136  to transmit RF power to the interior of the enclosure  136 .  
         [0034]     To control the power applied through the coil  130  and to adjust the resonant frequency of the coil  130 , a matching network  142  can be provided. The matching network  142  is preferably simple and inexpensive. As shown in  FIGS. 3 and 4 , the matching network  142  may include a first variable capacitor C 1  connected at one side to ground and at the other side to the RF power source  102  and to one end of the coil  130 . A second variable capacitor C 2  may be connected between ground and the other end of the coil  130 . The capacitance of the variable capacitors C 1  and C 2  may be adjusted in any suitable manner, such as with knobs  143 , to adjust the circuit resonance frequency with the frequency output of the RF generator  102  and to cancel the inductive reactance of the coil  130 . Impedance matching maximizes the efficiency of power transfer to the coil  130 . Those skilled in the art will recognize that other types of matching networks  142  can be used in conjunction with the present invention.  
         [0035]     When the coil  130  is powered by the power source  102 , two examples of the chemical reactions of two PFCs, C 2 F 6  and SF 6  are as follows:  
       EXAMPLE 1  
       [0036]     C 2 F 6 →CF 3 +CF 3    
         [0037]     O 2 →O+O  
         [0038]     COF 2 →COF+F  
         [0039]     CO 2 →CO+O  
         [0040]     F 2 →F+F  
         [0041]     COF+O→CO 2 +F  
         [0042]     CF 3 +O→COF 2 +F  
         [0043]     CF 3 +CF 3 +M→C 2 F 6 +M  
         [0044]     O+O+M→O 2 +M  
         [0045]     COF+F+M→COF 2    
         [0046]     CO+O+M→CO 2 +M  
         [0047]     F+F+M→F 2 +M  
         [0048]     CO+F+M→COF  
       EXAMPLE 2  
       [0049]     SF 6 +O 2 →SO 2 +3F 2    
         [0050]     SF 6 +O 2 →SO 2 F 2 +2F 2    
         [0051]     According to one embodiment of the invention, a reactant injector  110  is provided upstream of the plasma reactor, as shown in  FIG. 1 . The reactant injector  110  delivers chemicals with which to react away the PFC compounds, which otherwise could simply recombine downstream of the plasma reactor. The preferred embodiment is a water vapor injector. The hydrogen from the water reacts with the fluorines of the PFCs to produce HF. The HF may then be removed either by the scrubber chamber  114  upstream of the pump or by a conventional scrubber on the atmospheric side. The oxygen reacts with carbon, sulphur and/or nitrogen to produce CO x , SO x  and NO x , which are less harmful global warming gases than PFCs, and which may be removed from the emission stream by scrubbers. Various hydrides may also be produced and removed by scrubbing, as well as polymers and inorganic solids which will deposit on the reactor walls. The reactor, therefore, is preferably designed to be easy to clean or replace. Water vapor is preferred as the reactant because it is much less expensive than the reactants H 2 , CH 4  or other hydrides and O 2  commonly used in abatement systems to supply hydrogen and oxygen to the reaction. The flowrate of the total reactant (e.g. water vapor, H 2 , CH 4  and/or O 2 ) supplied by the reactant source  112  may be approximately equal to the flowrate of the process gasses from the vacuum processing chamber, for example 50-1000 cubic centimeters at standard temperature and pressure per minute. The reactant or reactants are preferably supplied in an amount effective for minimizing recombination of the dissociated PFC&#39;s.  
         [0052]     Downstream of the plasma reactor  100  may be a scrubber chamber  114  or section of foreline which contains materials that react with the HF, such as Si or W in the form of pellets, beads, chunks, liners, baffles, screens, etc., as shown in  FIGS. 1 and 7 . In  FIG. 7 , the scrubber chamber  114  includes an inner section  115  of porous HF reactive materials such as Si or W in the form of mesh, gravel, etc. The scrubber chamber  114  also includes an outer wall  117  which comprises HF reactive material. The provision of such a chamber  114  reduces the likelihood of damage of the remaining foreline and/or mechanical pump by HF, which is a highly corrosive gas. Although HF is easily handled by a scrubber on the atmospheric side, the lifetime of the vacuum plumbing and pumps can be increased significantly by providing a scrubber chamber  114  upstream of the vacuum plumbing and pumps.  
         [0053]     According to another embodiment of the invention, the plasma reactor may comprise conductive elements inside the foreline to generate a plasma. As shown in  FIG. 5 , the elements of the plasma reactor  100 B may be in the form of conductive grids  150  through which the gases flow. Adjacent grids  150  are oppositely charged with an RF generator  152  to generate a capacitive plasma. The reaction products, e.g., PFCs, are unstable in the high energy plasma, and are consequently reacted into smaller, less harmful molecules as described above. The grids  150  are preferably planar with the plane of the grid oriented perpendicular to the flow of reaction products from the vacuum processing chamber. To generate a capacitive plasma, at least two grids  150  are used. To further enhance the effectiveness of the plasma reactor, additional grids  150  can be provided. The additional grids alternate in polarity so that each adjacent pair of grids  150  acts as a capacitor. The grids  150  may comprise a plasma resistant material for a long lifetime. Alternatively, the grids may comprise a consumable material or materials for enhanced abatement capacity.  
         [0054]     The grids  150  are preferably disposed in a chamber  154  which has a larger cross sectional area than the cross sectional area of the foreline. In this way, the fluid conductance of the plasma reactor  100 B is not compromised. In addition, a fan or other cooling device can be provided to dissipate heat created by the conductive grids  150 . As in the previous embodiment, a matching network  156  can be provided to maximize or control the power delivered to the grids  150 .  
         [0055]     According to another embodiment of the invention, an exemplary plasma reactor  100 C comprises at least one and preferably two or more transformer coupled plasma (TCP™) coils. The TCP coils  160 , as shown in  FIG. 6 , have a generally spiral, planar configuration and are preferably oriented such that the plane of the coil is perpendicular to the flow of reaction products. The TCP coils  160  are coupled, preferably through a matching network  166 , to an RF generator  162 . By resonating a radiofrequency current through the coils  160 , a planar magnetic field is induced which induces a generally circular flow of electrons within a planar region parallel to the plane of the coil  160 . The circulating electrons create a plasma by ionizing individual gas molecules through the transfer of kinetic energy from individual electron-gas molecule collisions.  
         [0056]     The reaction products, e.g., PFCs, are unstable in the high energy plasma, and are consequently reacted into smaller molecules as described above. Preferably, at least two coils  160  are provided so that the period of time during which the reaction products are in the plasma state is sufficiently long to effectively break down the reaction products. The provision of additional coils  160  thus increases the size of the region which the plasma occupies so that for a given flowrate, the reaction products are in the plasma state for a longer time. The TCP coils can comprise plasma resistant materials for a long lifetime. Alternatively, the TCP coils can comprise a consumable material or materials for enhanced abatement capacity. So that the fluid conductance of the apparatus is not compromised, the chamber  164  in which the TCP coils  60  are located can have an enlarged cross sectional area perpendicular to the flow. A fan or other cooling mechanism can be provided to dissipate heat generated by the coils  160  of the plasma reactor  100 C.  
         [0057]     Those skilled in the art will recognize that exemplary embodiments of the present invention provide significant advantages in the abatement of reaction products such as fluorocarbons from a vacuum processing chamber. For example, in contrast to many prior designs, the reactor can be made to be simple, compact, inexpensive, efficient, reliable, and require little or no operator or control system intervention. The plasma reactor also provides a high plasma density, high dissociation rate operation, and a skin depth which is adjustable through the frequency. This results in efficient abatement without compromising foreline conductance. Also, in the event that an abatement device fails, only one tool is affected, rather than an entire section of a processing system.  
         [0058]     The above-described exemplary embodiments are intended to be illustrative in all respects, rather than restrictive, of the present invention. Thus the present invention is capable of many variations in detailed implementation that can be derived from the description contained herein by a person skilled in the art. All such variations and modifications are considered to be within the scope and spirit of the present invention as defined by the following claims.