GAS ABATEMENT BY PLASMA

Abstract

A plasma abatement apparatus includes: a plasma device configured to generate a plasma stream from a plasma gas; an effluent stream aperture configured to convey the effluent stream into the plasma stream for treatment by the plasma stream; a first aperture positioned to deliver a reducing reactant to a first region of the plasma stream; and a second aperture positioned to deliver an oxidising reactant to a second region of the plasma stream, wherein the second region is located at a position of the plasma stream which is cooler than the first region.

Claims

1. A plasma abatement apparatus for treating an effluent stream from a semiconductor processing tool, said plasma abatement apparatus comprising: a plasma device configured to generate a plasma stream from a plasma gas; an effluent stream aperture configured to convey said effluent stream into said plasma stream for treatment by said plasma stream; a first aperture positioned to deliver a reducing reactant to a first region of said plasma stream; and a second aperture positioned to deliver an oxidising reactant to a second region of said plasma stream, wherein said second region is located at a position of said plasma stream which is cooler than said first region.

2. The apparatus of claim 1, wherein said first region is located at a position of said plasma stream which is hotter than said second region.

3. The apparatus of claim 1, wherein said first region is located at a position of said plasma stream which achieves a temperature of at least 1000° C.

4. The apparatus of claim 1, wherein said second region is located at a position of said plasma stream which achieves a temperature of no more than 1000° C.

5. The apparatus of claim 1, wherein said second region is located at a position of said plasma stream which achieves a temperature of at least 500° C.

6. The apparatus of claim 1, wherein said second region is located downstream of said first region.

7. The apparatus of claim 1, wherein said first region is located upstream of said second region.

8. The apparatus of claim 1, wherein said reducing reactant is premixed with said plasma gas prior to delivery to said plasma device.

9. The apparatus of claim 1, wherein said first region is located proximate said plasma device.

10. The apparatus of claim 1, said second region is located distal said plasma device.

11. The apparatus of claim 1, comprising a reaction chamber positioned to receive said plasma stream and wherein said second region is located within said reaction chamber.

12. The apparatus of claim 1, wherein said reducing reactant comprises at least one of: H.sub.2, NH.sub.3, a hydrocarbon such as propane, methane and the like, an alkaline earth metal such as beryllium, magnesium, calcium, strontium, barium, an alkaline earth salt an alkaline metal such as lithium, sodium, potassium, rubidium and an alkaline salt.

13. The apparatus of claim 12, comprising: a hydrogen generator configured to generate said H.sub.2 in situ by electrolysis.

14. The apparatus of claim 12, comprising at least one of: a power generator employed to generate said plasma stream; and a secondary power source configured to power said hydrogen generator.

15. The apparatus of claim 12, wherein said first aperture is configured to deliver an ammonia salt such as ammonium carbonate (NH.sub.4)2CO.sub.3, or the like to generate said NH.sub.3 by thermal decomposition within said first region.

16. The apparatus of claim 12, comprising: a heater configured to generate said NH.sub.3 by thermal decomposition of an ammonia salt such as ammonium carbonate (NH.sub.4)2CO.sub.3, or the like.

17. The apparatus of claim 1, wherein said oxidising reactant comprises at least one of O.sub.2 and O.sub.3.

18. A method of treating an effluent stream from a semiconductor processing tool, comprising: generating a plasma stream from a plasma gas; conveying said effluent stream into said plasma stream for treatment by said plasma stream; delivering a reducing reactant to a first region of said plasma stream; and delivering an oxidising reactant to a second region of said plasma stream which is located at a position of said plasma stream which is cooler than said first region.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0059] Embodiments of the present invention will now be described further, with reference to the accompanying drawings, in which:

[0060] FIG. 1 illustrates a DC plasma torch according to one embodiment;

[0061] FIG. 2 shows an arrangement similar to that of FIG. 1 but which utilizes an inductively coupled plasma torch;

[0062] FIG. 3 is similar to that shown in FIG. 1 but the reducing reactant is introduced inside or in close proximity to the cathode;

[0063] FIG. 4 shows an arrangement which is similar to that shown in FIG. 2 but with the reducing reagent being injected inside or in close proximity to the gas discharge of the insulated tube of the plasma torch; and

[0064] FIGS. 5A to 5E show abatement performance under different configurations.

DESCRIPTION OF THE EMBODIMENTS

[0065] Before discussing the embodiments in any more detail, first an overview will be provided. Some embodiments provide an arrangement which improves the abatement efficiency of a plasma abatement apparatus. This is achieved by splitting the introduction of reactants into the plasma stream. In particular, a reducing reactant is delivered at a first position into the plasma stream and an oxidizing reactant is introduced at a second position in the plasma stream. The reducing reactant may be premixed with the plasma gas and so the first position may be the position where the plasma gas is first introduced or may be downstream of this position but upstream of the second position. This enables reducing reactants to be introduced at a higher temperature or more active region of the plasma stream and oxidizing reactants to be introduced at a lower temperature or less active region of the plasma stream. Introducing the oxidizing reactants at the lower temperature region of the plasma stream reduces the production of unwanted oxides while ensuring that the reduction of some compounds can still occur in the higher temperature region. Improved abatement performance is achieved with lower levels of unwanted by-products compared to introducing the reducing and oxidising reactants together.

[0066] Plasma Torch

[0067] FIG. 1 illustrates a DC plasma torch 100A according to one embodiment. A nozzled anode 3 is co-located with a coaxial cathode 2. A constant direct current power supply 1 is electrically coupled with the cathode 2 and the anode 3. A plasma gas carrier conduit 5B is positioned to deliver a plasma gas carrier 5A into a void between the cathode 2 and the anode 3. Downstream of the anode 3 is located a reaction chamber 8. An effluent stream conduit 9B is positioned to deliver an effluent stream 9A to a position between the anode 3 and the reaction chamber 8. A reducing reactant conduit 11B is positioned to deliver a reducing reactant 11A to a position between the anode 3 and the reaction chamber 8. An oxidizing reactant conduit 12B is positioned in the wall of the reaction chamber 8 to deliver an oxidizing reactant 12A into the reaction chamber 8. A controller 4 is coupled with the power supply 1 and the source of the plasma gas carrier 5A, the source of the reducing reactant 11A and the source of the oxidizing reactant 12A.

[0068] In operation, the controller 4 switches the power supply 1 to create a voltage difference between the cathode 2 and the anode 3 to initiate the plasma. Once the plasma is created it drives a constant direct current (DC) between the cathode 2 and the anode 3 (the power supply 1 operates as constant current power supply). The controller 4 causes the plasma gas carrier 5A to be delivered via the plasma gas carrier conduit 5B which creates a plasma plume 10 through a DC arc discharge between the cathode 2 and the anode 3. The plasma plume 10 extends into the reaction chamber 8. The discharge is sustained by the injection of the plasma gas carrier 5A.

[0069] The effluent stream 9A is conveyed to the plasma plume 10 by the effluent stream conduit 9B. The abatement reaction then takes place inside the reaction chamber 8, which is typically cylindrical in shape and provides thermal insulation. Hence, the effluent stream 9A mixes with the plasma plume 10 within the reaction chamber 8.

[0070] To assist the abatement reaction, the controller 4 controls the introduction of the reducing reactant 11A through the reducing reactant conduit 11B. Thus, the reducing reactant 11A mixes with the effluent stream 9A in a hotter zone 6 of the plasma plume 10. Typically, the hotter zone 6 experiences a temperature in excess of 1000° C. Optionally, a generating device 20 under the control of the controller 4 generates the reducing reactant 11A (such as H.sub.2 by electrolysis or NH.sub.3 by thermal decomposition).

[0071] The controller 4 controls the introduction of the oxidizing reactant 12A via the oxidizing reactant conduit 12B into a cooler zone 7 of the plasma plume 10, which is downstream of the hotter zone 6. Typically, the cooler zone 7 experiences a temperature which is greater than around 500° C. but which is lower than 1000° C.

[0072] Performance

[0073] FIGS. 5A to 5E show abatement performance under different configurations. One of the main challenges for abatement by means of thermal plasma torch is to achieve high efficiency of the abatement process so that the torch power can be reduced to its lowest possible amount. This results in a lower cost of operation due to the lower electrical energy demand. Another challenge in abatement by means of thermal plasma torches is to reduce the level of unwanted NOx emissions. Additionally, oxygen can act as an inhibitor of the plasma phase reactions due to its electro negative nature (i.e. it attracts bonding electrons and can quench the plasma state).

[0074] The arrangement such as that shown in FIG. 1 can improve the efficiency of the abatement process as this limits the abatement reaction within the hotter zone 6 to those related to the breakdown of C-F, S-F and F-F bindings and the reducing reactants converting F species into HF. The arrangement shown in FIG. 1 also helps to reduce the likelihood of oxygen-rich reactants reacting with N.sub.2 radicals at higher temperatures. By introducing the oxygen-rich reactants in the lower temperature zone 7, this results in lower amounts of NOx.

[0075] To illustrate this, FIG. 5A shows thermal equilibrium simulations of a mixture of CF.sub.4 and N.sub.2 (at concentrations of 1% and 99% respectively). The injection of different reagents has a stoichiometry to maximize the destruction and removal efficiency (DRE) of CF.sub.4. One abatement route for CF.sub.4 is its oxidisation into COF.sub.2. This is shown in FIG. 5A through the injection of just O.sub.2 into the hot zone 6 of the plasma stream 6. The COF.sub.2 is then hydrolysed into HF in the cold wet part of the abatement apparatus (not shown). As can be seen in FIG. 5B, the efficiency of the abatement can be improved if, rather than injecting O.sub.2 into the hot zone 6, instead just H.sub.2O is injected. As can be seen, this converts CF.sub.4 more easily into HF and CO.sub.2. However, as can be seen in FIG. 5B, there might be an attendant increase in NOx emissions versus the result shown in FIG. 5A, despite the lower power and temperature needed to abate CF.sub.4. This is possibly due to the more favourable routes of NOx creation where H.sub.2O radicals are present with N.sub.2 radicals as shown by the NO value constant increment versus temperature in FIG. 5B. As can be seen in both FIGS. 5A and 5B, just injecting O.sub.2 or just injecting H.sub.2O into the plasma stream 6 leads to unwanted by-products as well as undesirable levels of NOx.

[0076] However, as can be seen in FIGS. 5C and 5D, by splitting the addition of the reactants using the arrangement shown in FIG. 1, the abatement performance is improved significantly. As can be seen, if CF.sub.4 is reacted firstly with H.sub.2 in the hotter zone 6 and then with O.sub.2 in the cooler zone 7, an abatement may be achieved ideally with very few by-products such as CO and NOx, as shown in FIGS. 5C and 5B respectively. It is worth noting that the O.sub.2 simulations in FIG. 5D refer to O.sub.2 high temperature reactions with the by-products of the first reaction with hydrogen shown in FIG. 5C. This O.sub.2 step is required to oxidize some C.sub.XH.sub.YN.sub.Z by-products into less harmful compounds.

[0077] FIG. 5E shows that the concurrent injection of H.sub.2 and O.sub.2 does not provide the same results as the two-step abatement process illustrated in FIGS. 5C and 5D since, as can be seen, significant amounts of NOx and other unwanted by-products can still be generated.

[0078] Other Arrangements

[0079] FIG. 2 shows an arrangement similar to that of FIG. 1 but which utilizes an inductively coupled plasma torch 1008. In this arrangement, the plasma plume 10 is generated by ionizing the plasma gas carrier 5A which is injected into an insulated tube 16 via a plasma gas carrier conduit 15. Electromagnetic energy to sustain the discharge is supplied via a radio frequency power supply 13 coupled through coils 14. A matching box 17 is provided to match the load provided by the gas discharge 18.

[0080] FIG. 3 is similar to that shown in FIG. 1 but the reducing reactant 11A is introduced inside or in close proximity to the cathode 2 so that it mixes with the plasma gas carrier 5A to maximize its ionization and effectiveness as a reactant. A further advantage of this arrangement is that NH.sub.3 may be generated when N.sub.2 is employed as a plasma gas and this can reduce the baseline amount of NOx emission from the plasma torch 100C (such as when the effluent stream 9A does not include any compounds other than a purge compound).

[0081] FIG. 4 shows an arrangement which is similar to that shown in FIG. 2 but with the reducing reagent 11A being injected inside or in close proximity to the gas discharge 18 of the insulated tube 16 of the plasma torch 100D to provide similar advantages to those described with reference to FIG. 3 above.

[0082] Although in this example hydrogen is used as the reducing agent, it will be appreciated that other reducing agents such as an alkaline earth metal such as beryllium, magnesium, calcium, strontium, barium and the like and/or an alkaline earth salt may be used. Additionally, other reducing agents such as NH.sub.3, and/or a hydrocarbon such as propane, methane and the like may be used. As mentioned above, the NH.sub.3 may be generated by thermal decomposition of an ammonia salt such as ammonium carbonate (NH.sub.4)2CO.sub.3, or the like either within the first region 6 or in-situ by the generating device 20. Similarly, the hydrogen may be generated in-situ through electrolysis by the generating device 20.

[0083] Likewise, although in this example oxygen is used as the oxidizing reactant, it will be appreciated that other oxidizing reactants such as ozone.

[0084] Also, although the reducing reactant is shown being introduced between the anode 3 and the reaction chamber 8, it will be appreciated that this need not be the case and that the reducing reactant simply needs to be introduced at a location where the plasma stream is hotter than the location where the oxidizing reactant is introduced.

[0085] Additionally, although the embodiments have been described with reference to a DC plasma torch and an inductively coupled plasma torch, it will be appreciated that the same approach can be used for other plasma devices and sources of plasma such as, for example, a microwave plasma discharge.

[0086] Some embodiments address the optimization of thermal plasma abatement with the aim of reducing thermal NOx generation and increasing abatement reaction efficiency. Looking at thermal equilibrium simulations, it has become apparent that the abatement reactions involving the reduction of fluorinated species, F.sub.2, CFx and SFy into HF require much higher temperatures of the oxidation reaction of other abatement byproducts such as CO and CxHy. Some embodiments involve a thermal plasma abatement in two steps: 1) a hydrogen-rich reagent is injected in a much hotter reaction zone or premixed with torch plasma gas 2) the oxygen-rich reagents are injected further downstream in a relatively less hot zone. This provides for a split injection of two kinds of reagents in thermal plasma abatement.

[0087] Although illustrative embodiments of the invention have been disclosed in detail herein, with reference to the accompanying drawings, it is understood that the invention is not limited to the precise embodiment and that various changes and modifications can be effected therein by one skilled in the art without departing from the scope of the invention as defined by the appended claims and their equivalents.

REFERENCE SIGNS

[0088] power supply 1; 13 [0089] anode 2 [0090] cathode 3 [0091] controller 4 [0092] plasma gas carrier 5A [0093] plasma gas carrier conduit 5B; 15 [0094] hotter zone 6 [0095] cooler zone 7 [0096] reaction chamber 8 [0097] effluent stream 9A [0098] effluent stream conduit 9B [0099] plasma plume 10 [0100] reducing reactant 11A [0101] reducing reactant conduit 11B [0102] oxidising reactant 12A [0103] oxidising reactant conduit 12B [0104] coils 14 [0105] insulated tube 16 [0106] matching box 17 [0107] generating device 20 [0108] plasma torch 100A; 100B; 100C; 100D