FR2873045A1 - Gas conversion by chemical bond cleavage in an electric and-or magnetic field, e.g. for treatment of fluorinated effluents from semiconductor production, involves injecting gas into the field in a non-rectilinear manner - Google Patents

Abstract

Conversion of gas by breaking chemical bonds in an electric and/or magnetic field, involves injecting the gas through the applied field in a non-rectilinear manner so as to increase the path length of gas molecules in the field and thus increase the efficiency of bond cleavage. A method for the conversion of a gas or gas mixture (I) into another gas or gas mixture (II) containing liquid and/or solid products in which at least one bond between two atoms of the molecules in (I) has been broken by the action of an electric and/or magnetic field. This involves injecting a stream of (I) through the applied field in a non-rectilinear manner so as to increase the distance travelled by the gas molecules through the field and thus to increase the efficiency with which the bonds are broken. An independent claim is also included for a gas injection system for use in this method, comprising at least one gas injection channel, preferably situated in a plane at right angles to the tube whose upper end is closed.

Description

2873045 1

  The present invention relates to a method for destroying a gas or a mixture of gases, and in particular gaseous effluents, in which at least one bond between two atoms of at least one a molecule of said gas or mixture of gases is broken under the action of an electric and / or magnetic field to which said gas or mixture of gases is subjected.

  A process of this type is known in particular from US-A-5,965,786.

  Plasmas are particularly applied to the depollution of the discharges emitted by the deposition and etching processes of thin layers for the manufacture of semiconductors. These effluents (perfluorinated gases, corrosive halogens, gaseous hydrides, organometallic precursors, etc.) occur at the exhaust of the primary vacuum pumps with relatively high concentrations in a flow of 15 to 60 liters of nitrogen for each pump. To convert most of these large quantities of harmful molecules, the micro-wave discharges at atmospheric pressure are preferable to others because of their high electron density (1012 to 1015 cm-) to induce a large number of molecules. dissociative inelastic collisions.

  A characteristic of atmospheric microwave plasmas is the relatively high average energy taken by heavy particles (neutrons and ions). The temperature of the gas can indeed reach 3000 to 7000 K in the region of the axis of the dielectric chamber containing the discharge. The wall of this enclosure (for example a dielectric tube) must remain at a lower temperature compatible with its physical integrity. Also it is preferably cooled by the circulation in contact with a heat transfer dielectric fluid. There is therefore a radial temperature gradient decreasing from the axis to the periphery. As the temperature decreases, the density of the gas increases, the ionization is less likely and the recombination of the charged particles is favored. Thus, the electron density decreases at the same time as the temperature of the axis of the tube towards the periphery. Visually, it has been found that the light intensity of the discharge fades away from the axis. In some cases, the electronic density becomes very low for an axial position less than the radius of the tube and the discharge no longer fills the section of the latter. It is said that the discharge is contracted.

  The shape of this radial distribution decreasing towards the periphery of the electron density depends in particular on the operational parameters of the plasma: nature and concentrations of the different pollutant gases in the nitrogen, total flow, microwave power. It also depends on previously fixed parameters such as the internal diameter of the discharge tube, and the nature of the material constituting it (especially via the thermal conductivity).

  It is understood that the radial distribution of electronic density and temperature of the gas influences the heat exchange relationship between the gaseous medium and the tube wall, and thereby the reliability of the latter. It has been found that certain gases such as helium and hydrogen, from concentrations in the nitrogen of the order of a percent, have the effect of favoring the radial expansion of the discharge and therefore of increasing the temperature of the gas in the vicinity of the wall of the tube. Thus the aging of the latter by thermal effect is accentuated.

  It has also been found that other gases have the opposite effect and favor the radial contraction of the discharge. In this case, it is generally observed that the plasma does not remain constantly centered on the axis, but moves randomly in the section of the tube. When the plasma is decentered and gets closer to the wall of the tube, it is temporarily exposed to a very high gas temperature and the action of electrons out of equilibrium thermodynamic, even higher energy. The limiting case is when the plasma is one or more very dense filaments which, if they come into contact for a sufficient time of the wall, induce extreme localized stresses on the latter. There is then a risk of rupture of the wall by thermomechanical overload, of spot erosion of the tube wall by the high-energy fluorinated species, and also of carbonization of the dielectric cooling fluid on the outer surface of the tube opposite the point plasma contact on the wall.

  A first solution for this type of problem is to use a tube of very high performance material such as aluminum nitride, with which this phenomenon of degradation becomes extremely rare without it being impossible to predict the occurrence, however . In particular, the parameters governing the phenomenon of contraction and filamentation are generally imposed by the characteristics of the recipes of the user's processes which can use various halogenated gases, a plasma gas such as argon and various adjuvants such as helium , hydrogen or other chemical additives, or even heavy rare gases, all in very variable and generally unknown proportions.

  As moreover the contact phenomenon of the plasma and the wall itself is completely random, it is therefore very difficult to guard against these phenomena which induce a risk of malfunction and therefore safety of the installation.

  Furthermore, the existence of a radial density gradient of the plasma also plays a large role in limiting the performance of the effluent destruction systems. Indeed, the peripheral zone of the enclosure is colder and depleted in electrons. Therefore, the dissociation of the pollutant gas molecules is less likely in this peripheral zone than in the central zone and their reformation from their fragments is favored (because of the relatively high absolute concentrations). A molecule of polluting gas passing through the enclosure while remaining in this low energy peripheral zone has a much lower probability of being dissociated than if it transited near the axis. It could be envisaged that said molecule, during its course, migrates towards the warmer central zone by diffusion, convection or turbulence. However, in nitrogen, the plasma column is relatively short and the flow rate is relatively high, taking into account the total flow of nitrogen at the outlet of the primary pumps, so that these processes of exchange of material have little time to be fulfilled.

  The invention makes it possible in particular to respond to the problems posed by microwave plasmas in an enclosure, in particular a tube: on the one hand, by opposing the variations in diameter and the random axial decentrations of the plasma to improve the endurance and reliability of the discharge tube.

  on the other hand, by forcing the pollutant gas molecules to travel considerably longer in the dense zones of the plasma in order to make better use of the excess of active species available on average in the system, and thus to increase the conversion efficiency reduced to the injected power.

  The process according to the invention is characterized in that the flow of gas or mixture of gases is injected through the electric and / or magnetic field in a non-rectilinear manner in order to increase the length traveled by the gas molecules through of said field and thus increase the destruction efficiency of the gas molecules or gas mixture.

  Preferably, the gas or gas mixture is injected into the field with a tangential amount of movement of the gas or gas mixture greater than the amount of axial movement of said gas or gas mixture; more preferably, the tangential momentum is much greater than the amount of axial movement.

  According to one aspect of the invention, at least a portion of the gas or gas mixture is injected with a tangential component of velocity in a tubular cavity before being subjected to the action of the electric and / or magnetic field.

  Preferably, the gas or gas mixture is injected via a plurality of injection having a tangential component.

  According to a preferred variant, the tangential injections are regularly distributed over the circumference.

  Different alternative embodiments are possible and in particular: the injections or gas mixtures are all located in the same plane; or the injections are located in different planes.

  The injections that are located in the same plane are regularly distributed in this plane.

  According to an alternative embodiment: at least one plane comprises only one injection; and / or at least one plane comprises two injections at 180; and / or at least one plane comprises three injections at 120; and / or at least one plane has four injections at 90.

  In general, the injection plane or planes are perpendicular to the axis of the tube or cavity subjected to the field. However, it is quite possible to perform these injections through orifices oriented so as to give a component of gas velocity parallel to the desired flow direction for the gas to or in the cavity.

  The operating conditions of the plasma devices located at the outlet of the pumps of the etching and deposition reactors (at atmospheric pressure or close to atmospheric pressure) must make it possible to absorb a total input flow rate greater than 80 liters per minute (slm) when the exhausts of several etching chambers are simultaneously connected to the depollution unit and operate simultaneously. The gas is then essentially nitrogen. To obtain a good conversion efficiency of the most stable molecules, such as PFCs, the total power required must generally be greater than 3 kW and cooling of the outer wall of the cavity, in particular of the discharge tube, is provided.

  The implementation of the invention generally allows the establishment of a system of hydrodynamic forces that tend to maintain axial symmetry of the system and prevent a random disturbance including electromagnetic or thermal nature does away with the plasma of the axial position.

  Among the advantages of the invention, it will be noted: the reduction of the average temperature of the wall thus making it possible to further space the preventive maintenance operations of the discharge tube.

  the maintenance of the plasma at a distance from the wall of the cavity (the tube, for example) avoiding localized elevations of temperature of this wall, which can reach temperatures of the order of 1000 C.

  The flow of the fluid according to the invention makes it possible to considerably lengthen the path of the gas in the active zone by impressing the flow preferably with a helical movement (when using an axially symmetrical cavity), and also by promoting the exchange of matter by turbulence between high and low energy zones of the plasma.

  In practice, it is preferable, especially when one wants to maintain the helical movement to respect a certain number of constraints. Preferably: It is first necessary to maintain the compactness of the device, without adding if possible significant additional space to the device comprising by the injection of gas according to the invention.

  It is also necessary to maintain a limited pressure drop on the flow of gas to be treated, imposed by the operating pressure at the outlet of the primary pump in the case of use for the destruction of effluents from a manufacturing reactor of semiconductors.

  In a general manner, the gas injection will preferably be tangential and carried out by means of one or more channels provided in the flange 2873045 for connecting the duct for supplying the effluent gas stream upstream of the tube. for discharge.

  In the case in particular of a helical movement of the gas, this gaseous motor flow used to obtain such a movement can be reduced to the aforementioned gaseous effluents from the exhaust of the primary pump. To maintain such movement stably, it is generally necessary that the tangential momentum of the gas is preferably substantially larger than its axial counterpart. This involves providing tangential inlet channels of the gas at the tube supply connection which are each of a section substantially smaller than the diameter of the discharge tube. This adds a significant component to the pressure drop of the device, which must not reach a value such that the total overpressure at the exhaust of the primary pump exceeds the practical limit allowed.

  However, effluent treatment systems are generally operated with variable capacity often continuously with one to four process reactors discharging at a time. To maintain the helical movement, in particular while respecting a maximum pressure drop, the diameter of the gas injection channels will be adapted to the treated flow.

  To adapt to variable flow rates in a wide range, it may for example be used for the launch of the vortex additional auxiliary engine gas flow, which will not necessarily be constrained by a maximum overpressure input. More specifically, the operation of a system for treating plasma effluents, in particular microwaves, generally requires the addition of one or more reactive auxiliary gases, for example air, oxygen, water vapor, etc ... brought for example in the form of compressed air. Very often also, for reasons related to the operation, this air flow is increased beyond the simple value necessary for the accomplishment of the chemical reactions of conversion of the pollutants. This additional air flow comes from the distribution network of the semiconductor manufacturing plant, under a pressure of several bars. It is therefore perfectly usable on small diameter orifices. In addition, the additional dilution introduced is largely offset by the increase in the specific destruction efficiency of the pollutants induced by the presence of the gas flow according to the invention, in particular the helical movement of these gases.

  In concrete terms, the injection system can take many forms. Tangential channels can lead to one level or many. The gas supply upstream of the injection channels (flow division) is arranged, in a manner known per se, so as not to add significant pressure drop.

  When using a dielectric tube, for example as described in US-A-5,965,786, the maximum internal diameter of the tube is imposed by the radial gradient electron density phenomenon of the discharge. When the value of the internal diameter of the tube is increased, all other things being equal, it can be seen that the pollutant conversion efficiency increases first of all because of the increase in the residence time with that of the section. However, beyond a certain value, the efficiency decreases because the section of the discharge fills a smaller and smaller fraction of the section of the tube and the radial extension of the peripheral cold zone increases. Thus an increasing proportion of polluting molecules are able to pass through the tube in a region of low reaction activity and the conversion efficiency of the device decreases.

  By adding a helical movement to the gases, it is possible to use a substantially larger dielectric tube internal diameter than that used without this gas movement, without incurring a significant drop in the conversion efficiency. The use of a larger diameter tube makes it possible to treat larger flows by increasing the power supplied to the plasma, without increasing the thermal stress on the tube and without increasing the pressure drop.

Example of embodiment.

  The device described in US Pat. No. 5,965,786 is accompanied by a helical gas injection system according to the invention.

  The invention will be illustrated in the figures which represent: FIG. 1, a sectional view of the gas injection system according to the invention; Figure 2 is a sectional view along A of the device of Figure 1; Figure 3 is a sectional view along B of the device of Figure 1; Figures 4 and 5, for the different measurement results.

  In FIG. 1, the gas injection device 1 has been modified with respect to the devices described in US Pat. No. 5,965,786 in which there was, for example, a single tangential injection of gas made in a lateral cylindrical opening of diameter substantially equal to that of the dielectric tube 5 where the plasma is produced (by means not shown in the figure). If we consider the vertical axis XA '(axis of the dielectric tube 5 and the gas injection cavity 4) the gas injections to be treated are carried out according to this example through the 2873045 11 piece 2 according to four injection ports 7, 8, 9 and 10 (Figures 1 and 2) located in a plane perpendicular to X-X '. These orifices are respectively extended by pipes respectively 11, 12, 13 and 14 to join the gas injection cavity 4. These four pipes and orifices are respectively oriented at 90 according to this example. See Figure 2 which is a section along the orthogonal plane A-A through the room 2. Above the gas injection cavity is the electrode 3 to turn on the plasma. In the orthogonal plane B-B (see section Figure 3) are a second series of orifices 20, 21 and gas injection pipes 22, 23 disposed respectively 180 to one another. In these orifices, a pressurized gas is injected under pressure (for example from 2 to 10 × 10 5 Pa) such as the compressed air still available in a manufacturing plant. This pressurized gas will have a driving effect to form the helical movement of the gas to be treated from the four orifices in the plane A-A.

  It is also possible to inject the process gas to be destroyed in the plane BB, but it is preferable to inject air, nitrogen, optionally an oxidizing gas favoring a reaction with the destroyed and under pressure molecules, preferably (between 1 and 10 x 105 Pa). All injection orientations of the different gases are possible, in particular orientations that were not made in a plane perpendicular to the axis of the tube, but at an angle less than 90 (co-current) or greater than 90 (against -current), etc ....

  The preliminary division of the total flow to feed the 4 channels 7, 8, 9 and 10 in the example in a uniform manner is usually done from a uniformization chamber (not shown) in which the gas flows are mixed. and whose conditions are becoming standardized, in which the main line opens out from the exhausts of the pumps. From this chamber there are four relatively symmetrical ducts departing from. As much as possible, the input flow and the divided output flows of this chamber must be parallel so as not to add losses.

  At a high flow rate (for example, four gram chambers simultaneously connected) such a flow rate, it is not necessary to use the injection channels of the auxiliary gas flow (plane BB) to have a tangential pulse sufficient to maintain the flow. helical movement of the gas. However, it is possible to inject via these channels 22, 23 a minimum flow of compressed air serving to supply the quantity of oxygen necessary for the completion of the conversion chemical reactions of the perfluorinated molecules.

  Destruction experiments were performed with a mixture of SF6 diluted in nitrogen at a representative concentration of 5000 parts per million by volume (ppmv). Oxygen is added as a reactive helper gas at about 1.5 times the volume amount of SF6 to be treated. Figure 4 shows the evolution of the destruction rate of SF6 as a function of the microwave power (net) supplied to the plasma, as well as the total pressure drop between the gas inlet in the uniformization chamber and the outlet gases after cooling in a heat exchanger (not shown in the figure) for cooling the gas exiting downstream of the dielectric tube where the discharge takes place.

  The performance of the same device (all things being equal) according to USP 5,965,786 without the invention and with the invention (that is to say with a part 2 having a single radial gas inlet of a diameter close to that of the tube 4 and a part according to the invention with its tangential injections of gas) are considerably improved.

  Indeed, according to the invention a destruction rate of 90% is obtained at a power of about 3000 W, and a destruction rate of 99% to 3500 W. Without the invention, it is not possible to process a flow of 80 liters / min (slm) with sufficient performance to be of practical interest.

  By using a device without implementation of the invention and with only 60 slm of flow, it takes more than 5500 W to destroy at 95% the same concentration of 5000 ppmv of SF6. Compared with the invention (60 slm and same mixture) it is sufficient less than 2500 W. Additional measurements at 80 slm showed that the result depends little on the concentration of SF6 between 1000 and 5000 ppmv.

  The pressure drop remains perfectly within the limits prescribed for industrial operation, with a certain margin for the case of unanticipated fluctuations that could result from certain operating conditions.

  In addition, there is indeed a radical change in the spatial distribution of the plasma and its stability over time. The plasma remains well centered on the axis and has a smaller apparent radial extension than in the case of the injection without helical injection of the gases. One visualized with a camera through a transparent tube in silica in lateral incidence: this visualization showed the absence of instabilities with decentering and the absence of sticking of the plasma to the wall of the tube. Axial shots have also been made in a ceramic tube which confirm the fixity of the plasma at the center of the section of the tube.

  It has also been found that the amount of heat radiated by the plasma is less with the invention than without the invention.

  2873045 14 Destruction experiments under nominal conditions could be continued for several hours in a silica discharge tube, without any damage found on the walls of the tube, in particular etching after surface etching by the fluorinated compounds corrosive. In comparison, without implementation of the invention and under the same conditions, a silica tube is pierced by chemical erosion and / or local melting in a few minutes.

  For total flow rates of 50 and 60 slm, the same mode of injection is used (process effluents entering through the 4 tangential channels of diameter approximately half that of the dielectric tube and injection of compressed air by the two channels 22, 23 about half the diameter of the channels 1, 8, 9 and 10). The rate of destruction brought back to the microwave power is significantly better than 80 slm and the pressure drop decreases.

  When reducing the total flow below 50 slm, it is still possible in this power configuration to maintain a stable helical movement for a flow rate as low as 30 slm. However, there is a little less stability in the flow of gases. . At low flow rate, it is therefore preferable to use the auxiliary injection channels 22, 23 to provide additional motive power for the maintenance of the helical gas movement, by increasing the additional air or nitrogen flow rate at the same time. For example, when only one etching chamber is in operation (flow rate of approximately 20 slm), 30 slm of air or nitrogen is added via the auxiliary injection channels when exhaust of process equipment discharges 20 slm (one) 2873045 10 slm of air or nitrogen is added through the auxiliary injection channels when exhaust of the process equipment delivers 40 slm (two Figure 5 shows the evolution of the destruction rate and the pressure drop as a function of net microwave power, in the first case above (20 + 30 slm). Note that the curves are very similar for the second case (40 + 10 slm) regardless of the concentration of perfluorinated gas, especially between 1000 or 5000 ppm.

  The invention described above is not limited to surface wave plasmas but concerns any atmospheric microwave plasma maintained in a cavity, in particular a dielectric tube, whether from a resonant cavity or at the inside a microwave circuit, for example in a hollow rectangular guide.

2873045 16

Claims (12)

  1. A method for destroying a gas or a mixture of gases and especially fluorinated gaseous effluent, wherein at least one bond between two atoms of at least one molecule of said gas or mixture of gases is broken under the action of an electric and / or magnetic field to which said gas or gas mixture is subjected, characterized in that the flow of gas or mixture of gases is injected through the electric and / or magnetic field in a non-rectilinear manner in order to increase the length traversed by the gas molecules through said field and thus increase the destruction efficiency of the gas molecules or gas mixture.   2. Method according to claim 1, characterized in that the gas or gas mixture is injected into the field with a tangential amount of movement of the gas or gas mixture greater than the amount of axial movement of said gas or mixture of gases.   3. Method according to claim 2, characterized in that the tangential momentum is much greater than the amount of axial movement.   4. Method according to claims 1 to 3, characterized in that at least a portion of the gas or gas mixture is injected with a tangential component of velocity in a tubular cavity before being subjected to the action of the electric field and / or magnetic.   5. Method according to claim 4, characterized in that the gas or gas mixture is injected via a plurality of injection comprising a tangential component.   6. Method according to claims 1 to 5, characterized in that the tangential injections are regularly distributed over the circumference.   7. Method according to one of the preceding claims, characterized in that the injections are located in the same plane.   8. Method according to one of the preceding claims, characterized in that the injections are in different planes.   9. Method according to one of the preceding claims, characterized in that the injections in the same plane are regularly distributed.   10. Method according to one of claims 1 to 9, characterized in that the injections of gas to be treated are made in a first plane and an injection of engine gas such as air, nitrogen or oxygen is achieved in a second plane, preferably parallel to the first.   11. Gas injection apparatus for carrying out the method according to one of claims 1 to 10, characterized in that this apparatus comprises at least a first gas injection channel, preferably located in a perpendicular plane. to the axis of said tube whose upper end is closed.   12. Apparatus according to claim 11, characterized in that it comprises at least a second gas injection channel, preferably located in a plane perpendicular to the axis of the tube.