Process and device for purification of noxious exhaust gases by chemical conversion

p0001The invention relates to a method and a device for purification of exhaust gases with fluorine-containing pollutants, preferably, in particular installations for separating and ablating by plasma processes and chemical vapor deposition. These fumes contain inter alia fluorine-containing hydrocarbons or other fluorine compounds. Other processes will fall as a pollutant primarily on silanes. The exhaust gases are due to the high proportion of inert carrier gases such as nitrogen or argon mostly even non-flammable. The contaminants or their reaction products are toxic or promote the ozone depletion and the greenhouse effect due to their harmful effects in the Atmossphäre.

p0002For exhaust gas purification are known a number of methods. Very often the cleaning is done by the sorption of harmful gases from the exhaust gas, in which this example is performed by oxidizing, aqueous solutions (DE 3342 816 A1). Pollutants which are not or only slightly absorbed efficiently, can be removed from the exhaust gas by method of chemical reaction. Thus, for example fluorine compounds by reaction with heated by indirect heating SiO₂- Implement surfaces to easily continue disposable, volatile silicon fluorides (D / 254 723 5). The disadvantage here is the low efficiency of conversion of the pollutants such as caused by the cooling of the reaction surfaces through the exhaust. It has also been proposed to dispose of plasma process exhaust gases by passing over heated surfaces, where they are converted there into solid compounds (D / 2387975). The effectiveness of such a procedure is however reduced among others by sealing the surface by the secondary solid compounds over time dramatically.

p0003A plurality of exhaust gas purification method based on the thermal decomposition or oxidation of the pollutants in a combustion chamber. Are the pollutants itself is not flammable or are they just components of waste gases with a high proportion of inert gas, so they are for chemical conversion into a fuel gas flame, such as a natural gas or hydrogen / oxygen mixture is introduced (US 5,183,646). Malicious Sekündärstoffe the conversion are then, for example by sorption or washing processes, from the exhaust gas removed (US-A 288 9002). The emission is thus generally a multistage process in which sub-processes such as thermal decomposition or oxidation, cooling, sorption, hydrolysis and washing out solid reaction products run (034 689 3 B1). For this purpose, the exhaust gas is successively example, passed through a device having a combustion chamber and at least one further device, for example one that acts after the washing principle.

p0004There are also facilities for purifying exhaust gas have been proposed in which the exhaust gas is successively passed through a combustion chamber for combustion of the pollutants and a washing chamber, which are combined structurally into one unit (EP 89110875). A multi-stage cleaning process was implemented in a single reaction chamber in which the burned exhaust gas is passed through a finely divided liquid (sorption or coolant) or is associated with such a liquid film on the wall surfaces of the combustion chamber in contact (EN 43 200 44).

p0005However, the emission control in a combustion chamber with a fuel gas flame has a low efficiency in your cleaning effect, when applied for waste gases hydrofluorocarbons and other fluorine compounds. With reasonable consumption of fuel gas, the purified exhaust critically high levels of pollutants contain. Although a cooling of the reactor walls reduces the corrosion, but rather leads to the deterioration of the efficiency of the purification. An improvement in the efficiency of the cleaning in the direction of a low pollutant content in the purified exhaust gas, although can be achieved to some extent by increasing the amount of fuel gas relative to the amount of the supplied exhaust gas, however, results because of the increase of the fuel gas consumption to a critical deterioration of the economy of the exhaust gas purification.

p0006Since drain into the fuel gas flame with gas supply generally several reactions, the most important results under the effect of having supplied oxygen or combustion of the fuel gas (eg natural gas or hydrogen) for the purpose of thermal activation of harmful gases and the chemical conversion of harmful gases in hydrolyzable and absorbable . innocuous solid and volatile compounds, can not be expected due to the reaction kinetics that the desired conversion of harmful gases complete. This applies in particular when all the reactants (fuel gas, oxygen and harmful gas) are supplied in a stoichiometric ratio. As a result of the harmful gas Inertgasanteiles the reaction kinetics is adversely affected and therefore the implementation of pollutant gases further reduced proportionately.

p0007The cleaning of fluorine-containing gases in a combustion chamber with a fuel gas flame requires specific procedure and equipment layout, if it is to be carried out both at a high efficiency of conversion of pollutants as well as with more favorable economics. The results are in use means the same satisfactory for all fluorine compounds. So the Effienz implementing eg tetrafluoromethane is unfavorable in a fuel gas flame.

p0008Moreover, particular specific needs! Give to the purification process, since in practice the cleaning of waste gases from CVD and plasma processes occur with different exhaust pollutants, except fluorine-containing compounds as well as silane. An adaptation of the same flue gas cleaning device having a combustion chamber at such different pollutant gases alone by adjusting the process parameters does not lead to satisfactory technical solutions.

p0009An increase of the fuel gas proportion in the combustion gas mixture compared to the stoichiometric ratio for all the fed reactants improves the conversion of pollutants, especially in the fluorine-containing hydrocarbons, however, leads to the discharge of unburnt also harmful combustion gases from the cleaning plant. A higher percentage of oxygen in the combustion gas mixture (EP 0347753) of the stoichiometric ratio causing one hand a better implementation of z. B. silane pollutants, but on the other hand leads to a critical deterioration of the pollutant conversion of fluorine-containing gases and thus unacceptably high levels of this pollutant in the cleaned exhaust gas ,

p0010<b>The invention is based on the object</b>, In the purification of exhaust gases with a method which works with a combustion chamber and a fuel gas flame, the efficiency of the cleaning, in particular in the cleaning of fluorine-containing exhaust gases to increase. It is also through one and the same method, to secure a high efficiency of cleaning for different toxic components of exhaust gases. The economy of the purification process can be improved by reducing the fuel gas consumption and longer uninterrupted operating times.

p0011<b>According to</b> the object by a method according to claim 1 and 2, and a device according to claim 3, 4 and 5 is achieved.

p0012The method assumes that incurred in the implementation of technological processes, particularly in CVD and plasma processes, waste gases, especially those with different pollutants that are to be cleaned in a preferably multistage process. exhaust containing partially or at least in a time interval, fluorine-containing hydrocarbons or other fluorine compounds. The purification of the exhaust gases takes place in a device having a combustion chamber and a burner for generating a combustion gas flame which serves for the heating or / and chemical conversion of pollutants.

p0013According to the invention the inner surfaces of the combustion chamber or / and additionally introduced into the combustion chamber surfaces are covered with a porous layer of silicon dioxide. In the fuel gas flame heated exhaust gas containing fluorine-containing pollutants is guided along the through which fuel gas flame also heated, said surfaces, wherein the heated fluorine-containing pollutants with the heated material of the porous layer to react. The deposition of the porous layer of silicon dioxide is carried out by thermal oxidation of a silane-containing gas into the combustion chamber itself, preferably sequentially temporal phases cleaning of exhaust gases containing fluorine-containing pollutants. The products resulting from chemical reaction with the hot silicon dioxide secondary volatile pollutants, in particular silicon fluorides, are hydrolyzed together with the burnt fuel gas and the other generated in the fuel gas flame reaction products of the noxious gases in a further process step with a sorbent optionally neutralized additionally.

p0014In the fuel gas flame, fluorine-containing compounds (for example C₂F₆, CF₄, CHF₃, NF₃) Already thermally decomposed partly or chemically reacted. One possible reaction is for example of C₂F₆ to CH₄ and HF, in particular if in the fuel gas flame, an excess of reducing gases such as hydrogen, is present. An essential starting point for the method is that can not be complete, such a chemical reaction reasons of reaction kinetics in the fuel gas flame. This circumstance is quantitatively crucial when thermally decomposable or difficult to hydrogen inert fluorine compounds, such as CF₄are contained in the exhaust gas. Not thermally decomposed in the fuel gas flame or chemically unreacted fluorine compounds, however, heated in the fuel gas flame, that is chemically activated. Likewise, the thermally activated by the thermal radiation and by convection of the hot combustion gases to said surfaces, situated silica for reaction. both activated reactants react inter alia to volatile silicon fluoride on the surfaces. The fluorine compounds of the exhaust gas are thereby more completely reacted, depending volkommener the contact of the heated exhaust gas stream with the inner surfaces of the combustion chamber and / or the additionally introduced into the combustion chamber surfaces. A prerequisite for this is a sufficient size, upper shed texture and arrangement of these surfaces, another is given by the porosity of the silica layer. The pores provide virtually an enlarged reaction surface for the heated pollutants.

p0015By surface reaction occur next innocuous substances, zBKohlendioxid, secondary pollutants such as silicon fluoride, for example, mentioned. These secondary pollutants must be eliminated, along with other, possibly resulting in the fuel gas flame reaction products, such as hydrogen fluoride, from the exhaust gas. This is done in the simplest case in an aqueous sorbent. In this example, silicon fluoride and hydrogen fluoride is hydrolyzed. If the sorption agent is a neutralizing agent, for example, KOH or Ca (OH)₂, Added so formed salts of Flßsäure which react slightly basic, are not corrosive and can be disposed of easily.

p0016In the course of purifying exhaust gas with the fluorine-containing pollutant, the stock is consumed in porous silicon oxide on said surfaces of the combustion chamber. The consumption increases with the amount of exhaust gas to be purified and with the percentage proportion of fluorine-containing pollutants. According to this consumption of the silicon oxide layer must be regenerated. Given silane or silane-containing gas is introduced into a regeneration phase into the burner. The silane or silane-containing gas is reacted in the hot, oxygen-fuel gas flame in a volume of reaction in silica and water vapor. The resulting silicon dioxide is deposited on the said surfaces of the combustion chamber from the porous layer. The regeneration of the surfaces therefore takes place by this procedure in the gas cleaning system itself. It is only supplied with a silane-containing gas instead of the fluorine-containing exhaust gas at this stage. Optionally, in addition, the composition and / or the amount of the combustion gas mixture to the two sequentially occurring procedures is adjusted. For example, the proportion of oxygen in the silicon dioxide deposition is set larger than in the case of heating the fluorine-containing exhaust gas.

p0017In the simplest case, a silane-containing gas or directly silane can be supplied during the regeneration phase by switching valves instead of fluorine-containing gas.

p0018According to the invention in particular, the regeneration of the surfaces in the combustion chamber, in which the purification of exhaust gases with fluorine-containing pollutants and the sequential deposition of porous silicon oxide is carried out on said surfaces of the combustion chamber by the introduction of exhaust gases of different composition at different CVD or plasma processes of semiconductor technology incurred.

p0019Thus, for example sequentially in a pollutant among others silane obtained exhaust gases from a CVD apparatus for deposition of silicon on semiconductor wafers, and a plasma etching device for etching of semiconductor wafers, in which a pollutant is obtained a fluorocarbon, are supplied to the exhaust purification apparatus consecutively.

p0020During a cleaning phase of exhaust gas containing fluorine-containing pollutants, in particular fluorocarbons, consumed at the chemical reaction material of the porous silica layer. During the subsequent purification phase of the exhaust gas with silanes as the main pollutants of this exhaust gas is not only cleaned, but this phase in this case serves simultaneously for the regeneration of the silicon oxide layer on the surfaces of the combustion chamber.

p0021An important application for the novel process is the emission of CVD or plasma coating systems, in which after coating processes, the inner surfaces of these systems by means of plasma etching cleaning. This fall in a plant for the technological fabrication of semiconductor circuits in natural consequence of gases containing fluorine-containing pollutants and with silane-containing pollutants in, for its cleaning only process parameters of the emission control device must be adapted. This is for example an increased feed of fuel gas mixture into the burner of the exhaust gas purification device during deposition of silicon on the semiconductor wafers in the coating system.

p0022Are the amounts of the silane-containing exhaust gases produced during the coating process in the CVD or plasma coating systems, is not sufficient to provide for their purification and chemical reaction the supply of silicon dioxide on the surfaces of the combustion chamber, which is necessary to reduce the amount of, in plasma etching processes resulting fluorine-containing gases to clean in the next phase, then this deficit can be offset in the exhaust gas easily by feeding an additional quantity of silane.

p0023The inventive method can also be applied when fluorine-containing and silane in the exhaust gas pollutants together anfallen.Die silanes namely chemically reacted in the volume of the fuel gas flame, the fluorine compounds preferably on the hot surfaces of Brennkammer.An these surfaces in this case is the use of silicon dioxide and the deposition, ie, the regeneration simultaneously.

p0024The method is performed with a device which with an element located at one side of the combustion chamber burner, suitably a ring burner, besteht.Die anordning the axis of the combustion chamber in the space is substantially of a preferably rotationally symmetrical combustion chamber for the efficiency of the process not relevant. The ring burner is a fuel gas and oxygen or a fuel gas mixture including: natural gas / oxygen or hydrogen / oxygen). Am Brenner, a fuel gas flame forms. Through a feed, preferably in the center of the ring burner, the exhaust gases, with particular fluorine and hydride pollutants fed. By a central supply the exhaust gas is enclosed on all sides of the fuel gas flame, an important condition for effective exhaust gas heating in the interest of chemical degradation and / or the chemical reaction of components of the supplied pollutants. The walls of the combustion chamber are offset from the outer limits or panels is thermally insulated, as a result, the wall surfaces of the combustion chamber are heated. They heat up aufhohe temperatures

p0025At the exit of the combustion chamber is ensured by openings or gaps that the burned combustion gases and decomposition or reaction products of the harmful gases are supplied directly or via a suction further, non-thermal processes part of the emission control. In the simplest case this is for example a washing section with an aqueous sorbent, optionally additionally comprising a neutralizing agent. The so purified gas stream then enters the ventilation system or open space.

p0026According to the invention the inner surfaces of the combustion chamber can be by mechanical, chemical or Electrochemical machining in its surface opposite to the effectively active, which results from the macroscopic dimensions of the combustion chamber parts is increased. To improve the conditions for the reaction between the activated fluorine-containing pollutants and covered with silica surfaces further, additional parts are arranged in the hot gas stream to the fuel gas flame on the walls and / or in space of the combustion chamber. The surfaces of these additional components increase the total area, the more increase in the effective reaction surface is ereicht for the deposition of silicon dioxide and thus as a reaction surface for the activated fluorine-containing noxious gases available steht.Eine, even if the surfaces of the additionally arranged parts are processed in said manner ,

p0027The increase in the effective surface of the combustion chamber, or disposed in their parts can take place by a plurality of processing manners. In the simplest case, such an enlarged surface is already achieved by a rough turning, whereby grooves are formed in the surfaces of the parts. By chemical or electrochemical etching, the microscopic wirsame upper times is increased by that forms pores. but it can be applied to said target and porous, screen-like or mesh-like coatings or mesh on the surfaces of the combustion chamber or / and the additional parts. As materials and korosionsbeständige metals or ceramics are suitable to heat. Such coatings also improve the adhesion of, deposited silicon dioxide layers on these surfaces.

p0028Through the cited instructions for design of the device is achieved in that between the gas molecules of the heated pollutants and the heated surfaces of the combustion chamber and in addition, arranged parts, a higher number of impacts is achieved. This is a prerequisite for it comes with high security for the chemical reaction between the activated pollutant molecules and the activated silicon dioxide on said surfaces.

p0029A higher number of collisions between the gas molecules heated pollutants and the heated silica surfaces can also be achieved in that the additionally arranged in the combustion chamber parts are made in their geometric shape such that the hot gas stream or units of the same on the way from the fuel gas flame to washing range is forced on several occasions to change its direction. This can be achieved for example with concentric rings, which lateral surfaces are multi-angled relative to the axis of the combustion chamber zigzag-shaped. In the thus formed annular ducts for hot gases, a multiple impact the majority of the gas molecules is achieved in the wall surfaces only when the flow of hot gases is gestaltent turbulent. Known ways to do this: a sufficient flow rate, enough narrow annular channels, high combustion gas temperature and roughened Oberflächen.Letzteres has already called for in the interests of both the creation of large contact surfaces for the reaction.

p0030<b>The invention will in the following with reference to an exemplary process</b> and reference to the drawings Fig. 1 a preferred embodiment of the device <b>explained in more detail.</b> Fig. 1 shows the device in a schematic longitudinal section.

p0031<b>The inventive device</b> essentially consists of a cylindrical combustion chamber (1) made of stainless steel. It has 18 cm diameter and 90 cm long. This combustion chamber is positioned a thermally insulated by means of supports (2,3,4,5) in an outer casing (6). In the area of ​​the end face (7) of the combustion chamber (1) is a ring burner (8) is arranged, which the fuel gas mixture of hydrogen and oxygen via a supply (9) is supplied. The ring burner (8) has a diameter of 25 mm. On annular channel (10), the fuel gas flame (11) is formed. The exhaust gas with the fluorine-containing and silane-containing pollutants to the burner (8) supplied via the central feed (12). It passes through the bore (13) centrally into the fuel gas flame (11).

p0032In the inner wall of the combustion chamber is a made of heat resistant, corrosion-resistant metal cylindrical body (14) inserted, which has 4 wavy ribs having an axial wavelength of 40 mm in a radial wave height of 20 mm. Two similarly constructed cylindrical body (15) and (16) to brackets (17), (18), (19) and (20) in the space of the combustion chamber between fuel gas flame (11) and cover (21) is used the combustion chamber. The stream of hot gases is divided in this manner in the region of the addition surfaces disposed in two cylindrical flow passages, whose radial width is approx 25 mm. The gas flow is deflected in the region of the body four times in his direction (arrows 22,23,24,25).

p0033On the cylindrical bodies meshed sieves are secured with a wire thickness of 0.5 mm made of heat resistant steel in order to increase the effective surface.

p0034Between the cylindrical mantle of the combustion chamber (1) and its, the burner remote from the end face (21) is a 30 mm wide annular gap (26) for the exit of the burnt hot gases from the combustion chamber in a space (27). In the area of ​​the space (27) is a tubular connection for the suction (28) through which the combustion gases are transferred to a washing device for carrying out further sub processes of the multi-stage exhaust gas cleaning.

Example for process control:

p0035In a plasma CVD coating apparatus falls in the first part of a technological cycle for the deposition of silicon dioxide on silicon wafers 60 l / min gas. The exhaust gas is composed of 57 l / min of nitrogen and 3L / min silane as vorwiegendem pollutant. In the second part of the technological cycle, the internal components of the plasma CVD coating plant to be cleaned by means of a modified process control of contaminating silicon layers by a plasma etching process. This process is with CF₄ and oxygen as the process gas conducted. The resulting flue gas consists out of 48 l / min argon predominantly of 2 l / min CF₄ and silicon tetrafluoride as pollutants.

p0036In a first cleaning phase, for the accruing during the coating exhaust gas (57 l / min of nitrogen and 3 liters / min silane) this via the feed (12) introduced into the fuel gas flame, by admission of 24 l / min of hydrogen and 18 l / min of oxygen in the feed (9) is maintained. As toxic pollutant silane is predominantly burned. Silica separates out from aufbesagten surfaces of the combustion chamber.

p0037In the second part of the technological cycle falls when cleaning the coating system sequentially to the exhaust gas containing fluorine-containing compounds. In the corresponding sequential exhaust gas purification phase is in the exhaust-gas flame, which is maintained by feeding in l / min of hydrogen and L / min of oxygen, through the feed (12) the exhaust gas (48 l / min argon and 2 l / min CH₄ and in a smaller amount of other pollutants) eingelassen.Das exhaust gas, especially the fluorine compounds are heated in the fuel gas flame to approximately 1400 ° C. It pulls (in Fig. 1 in the direction of arrow 22 to 25) by the said heated portions of the combustion chamber and comes with the surfaces into intimate Kontakt.Im result of a surface reaction with the silicon dioxide on the surfaces is CF₄ reacted to volatile hydrogen fluoride.

p0038The hot combustion gases with the secondary reaction products from the two sequentially occurring emission phases, for predominantly silane and predominantly fluorine-containing gases, pass through the nip (26) in the space (27) above the combustion chamber, are collected there and the suction (28) a washing device supplied. In the washing device an aqueous sorbent is effective. The hot exhaust gases are cooled to about 50 ° C. The hydrogen fluoride is either water or a basic solution (KOH, K₂CO₃ etc.) absorbed.

p0039In the above description, the behavior only for process-typical, mainly occurring substances is described.

p0040The process has to be chemically very different behaving pollutants high detergency. In particular, the pollutant content of fluorine-containing compounds in the exhaust air of the exhaust gas purifying device is reduced aufweniger than 10 ppm.