KR20070035565A - Atmospheric plasma treatment of gaseous hazardous emissions - Google Patents

Abstract

The present invention relates to a process for the conversion of gaseous or gaseous mixtures, in particular fluorinated gaseous hazardous emissions. According to the invention, at least one bond between two atoms of at least one molecule of a gas or gas mixture is broken under the influence of an electric and / or magnetic field applied to the gas or gas mixture. The gas or gas mixture stream is injected through the electric and / or magnetic field in a non-linear manner to increase the distance traveled by the gas molecules through the electric and / or magnetic field and to increase the effect of the conversion of the gas or gas mixture molecules in this way. do.

Atmospheric pressure plasma treatment of gaseous emissions, electric field, magnetic field, bursting effect of molecular bonds, nonlinear method

Description

Atmospheric plasma treatment of hazardous gaseous emissions {ATMOSPHERIC-PRESSURE PLASMA TREATMENT OF GASEOUS EFFLUENTS}

The present invention converts a first gas or gas mixture containing at least some molecules with at least one bond between two atoms constituting the molecule and thereby results in a liquid and / or solid product derived from such a conversion. To form a second gas or gas mixture which may contain, wherein at least one bond between two atoms of the molecule is ruptured under the action of an electric and / or magnetic field applied to the first gas or gas mixture will be.

Processes of this type which are applied to the destruction of hazardous emissions are known in particular from US Pat. No. 5,965,786.

Plasma is particularly applied to the removal of contaminants from emissions emitted by processes for the deposition and etching of thin layers for the manufacture of semiconductors. These hazardous emissions (fluoride gas, corrosive halide compounds, gaseous hydrides, organometallic precursors, etc.) are present in the exhaust from the primary vacuum pump at relatively high concentrations in a flow of 15 to 60 liters of nitrogen for each pump. do. In order to convert most of these large quantities of harmful molecules, microwave discharges at atmospheric pressure can cause large dissociative inelastic collisions (10 ¹² to 10 ¹⁵ cm). ^-3 ) is preferred over others.

One feature of atmospheric microwave plasmas is the relatively high average energy absorbed by heavy particles (neutral particles and ions). The temperature of the gas can actually reach 2727 ° C. (3000 K) to 6727 ° C. (7000 K) in the region of the axis of the dielectric chamber containing the discharge. The walls of such chambers (eg, dielectric tubes) remain at the lowest temperatures to match their physical integrity. The wall is also preferably cooled by circulation of a heat-carrying dielectric fluid in contact with the wall. Therefore, the radial temperature gradient exists in a decreasing state from the shaft portion to the peripheral portion. When the temperature drops, the density of the gas increases, less ionization occurs, and recombination of charged particles is promoted. As such, the electron density decreases at the same time as the temperature decreases from the shaft to the perimeter. Visually, it is found that the luminous intensity of the discharge is attenuated further away from the shaft. In some cases, the electron density is very low for axial positions less than the radius of the tube, and the discharge no longer fills the cross section of the radius of the tube. The discharge is then said to shrink.

The form of this radial distribution of electron density towards the periphery depends in particular on the operating parameters of the plasma, namely the nature and concentration of the various contaminant gases in nitrogen, the total flow rate and the microwave power. The shape of this radial distribution also depends on previously fixed parameters such as the inner diameter of the discharge tube and the nature of the material from which the discharge tube is made (particularly via thermal conductivity).

It can be understood that the radial distribution of the electron density and the gas temperature affects the heat exchange relationship between the gaseous medium and the wall of the tube and eventually the reliability of the tube. It has been found that some gases, such as helium and hydrogen, from nitrogen contents on the order of 1% promote radial expansion of the discharge and thus have an effect of increasing the gas temperature in the vicinity of the wall of the tube. In this way, the aging of the wall of the tube due to the thermal effect becomes significant.

It has also been found that other gases have the opposite effect and promote radial shrinkage of the discharge. In this case, it is generally observed that the plasma moves randomly within the cross section of the tube without remaining constant centered on the axis. When the plasma is decentralized and approaches the wall of the tube, it is temporarily exposed to very high gas temperatures as well as the action of electrons deviating from the thermodynamic equilibrium at much higher energies. This limiting case is the case where there is one or more very dense filaments which, when in contact with the wall for a sufficiently long period, create extremely localized stresses on the wall. Therefore, the risk of rupture of the wall by thermomechanical overload, the risk of point erosion of the wall of the tube by high energy fluoride species and also the point of contact of the plasma on the wall There is a risk of carbonization of the dielectric cooling fluid on the outer surface of the tube.

The first solution to this type of problem consists in using tubes made of very high performance materials such as aluminum nitride, whereby this deterioration is extremely rare and it is not impossible to predict the occurrence. In particular, the parameters governing shrinkage and filamentation are generally largely unknown, with varying proportions of various additives such as various halogenated gases, plasma gases such as argon and helium, hydrogen or other chemical additives, or It is set by the nature of the recipe of the user's method that can use a much heavier rare gas.

Moreover, the contact of the plasma with the wall is completely random in itself, so it is very difficult to protect against these phenomena which pose a risk of impairing the safety of the operation and therefore of the installation.

In addition, the presence of the radial density gradient of the plasma also plays a large role in limiting the performance of the system for the destruction of harmful emissions. In fact, the periphery region of the chamber is colder and depletes electrons. Eventually, dissociation of contaminant molecules occurs less than in the central region, and their reformation from the degradation is promoted (due to the fact that a relatively high absolute concentration exists). Contaminant gas molecules passing through the chamber while remaining within this low energy periphery region have a much lower probability of dissociation than if they passed near the axis. The molecule can be thought to move toward the hotter central region by diffusion, convection or turbulence during its passage. However, in nitrogen, the plasma column is relatively short and the rate of passage is relatively high considering the total flow of nitrogen leaving the primary pump, so that these processes for material exchange are time to complete. Have very little.

The present invention also relates to a generator of a gas such as fluorine (F ₂ ) obtained by cracking molecules such as NF _{3 in a} plasma. Such methods and related generators are described in International Patent No. PCT / FR05 / 01652, filed June 29, 2005, in the name of the applicant and incorporated herein by reference.

The present invention is in the following manner, namely

On the one hand, by countering fluctuations in the diameter of the plasma and random axial off-centering to improve the durability and reliability of the discharge tube,

On the other hand, contaminant gas molecules are significantly longer in the dense region of the plasma to not only increase the conversion efficiency for the injected output but also to better utilize the excess active species available on average within the system. By forcing the path to follow,

It makes it possible in particular to counter the problems presented by the microwave plasma in the chamber, in particular in the tube.

The method according to the invention provides an electric field in a non-linear manner in order to increase the distance that the stream of gas or gas mixture is moved by the gas molecules through the electric and / or magnetic field and thus increase the effect of the destruction of the molecules of the gas or gas mixture. And / or injected through a magnetic field.

Preferably, the gas or gas mixture is injected into the electric and / or magnetic field with the amount of tangential movement of the gas or gas mixture being greater than the amount of axial movement of the gas or gas mixture, and further the amount of tangential movement is Much greater than the amount of axial movement.

According to one feature of the invention, at least a portion of the gas or gas mixture is injected with the tangential velocity component into the cavity, preferably the tubular cavity, before the action of the electric and / or magnetic fields is applied.

Preferably, the gas or gas mixture is injected by a plurality of injections comprising tangential components.

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

Various modifications are possible, in particular,

The inlets or gas mixtures are all located in the same plane,

The injection section is located in a different plane.

Injections located in the same plane are regularly distributed in this plane.

According to one variant embodiment,

At least one plane has only one injection portion, and / or

At least one plane has two injection sections of 180 °, and / or

At least one plane has three injection sections of 120 °, and / or

At least one plane has four injection sections of 90 °.

In general, the injection plane or planes are perpendicular to the axis of the tube or cavity to which the electric and / or magnetic fields are applied. However, according to one variant of the invention, at least one of the injections is formed through an orifice oriented to provide a velocity component of the injection gas parallel to the desired flow direction for the gas towards or into the cavity. As such, in the case of a gas inlet into a cavity, in particular vertically arranged during its use and in which gas flows downward, in particular a tubular cavity, between 0 ° and 90 °, preferably 20 °, without forming such an injection part horizontally. It is desirable in some cases to form such an injection section in a direction inclined downwardly with respect to the vertical axis of the cavity between and 70 °, more preferably about 45 °.

The operating conditions of the plasma apparatus located at the outlet from the pump of the etching and deposition reactors (at atmospheric or near atmospheric pressure) are generally 80 when the exhausts from several etching chambers are simultaneously connected to the decontamination unit and operated simultaneously. It is possible to absorb the total flow at the inlet above slm (liter per minute). The gas then consists essentially of nitrogen. In order to obtain good conversion efficiency of the most stable molecules, such as PFCs, the total power required should generally exceed 3 kPa, provided cooling of the cavity, in particular the outer wall of the discharge tube.

The practice of the present invention is generally to establish a system of hydrodynamic forces that tend to maintain the axial symmetry of the system, and in particular that random disturbances of electromagnetic or thermal properties may cause the plasma from the axial position. Makes it possible to prevent displacement of the device.

Among the advantages of the present invention, the following advantages will be noted.

Reduction in the average temperature of the wall, which makes it possible in this way to further open the preventive maintenance work of the discharge tube,

Maintenance of the plasma away from the walls of the cavities (eg tubes), which prevents localized rise in temperature of these walls which can reach temperatures on the order of 1000 ° C.

The flow of the fluid according to the invention preferably exchanges material by providing helical movement to the flow (when a cavity with axial symmetry is used) and also by turbulent flow between regions of high plasma energy and low plasma energy. It is possible to significantly extend the path of the gas in the active region by promoting.

Indeed, compliance with any number of restraint requirements is particularly desirable when it is desired to maintain helical movement. Preferably,

The miniaturization of the device should first be preserved, if possible, without adding any significant additional volume to the device that does not comprise an injection of gas according to the invention,

The limited pressure loss imposed by the operating pressure of the exhaust from the primary pump in the case of use for the destruction of harmful emissions from the reactor making the semiconductor must also be preserved for the gas flow to be treated.

In a general manner, the gas inlet will preferably be tangential and will be formed by one or more channels provided in the flanges connecting the pipes that bring the stream of noxious emissions gas from upstream to the discharge tube.

Especially in the case of helical movement of the gas, this propellant gas stream used to obtain this movement can be reduced to the aforementioned gaseous hazardous emissions from the exhaust from the primary pump. In order to maintain this movement in a stable manner, the amount of tangential movement of the gas should generally be considerably greater than the amount of its axial movement. This involves providing a tangential inlet channel for the gas in the region of the connecting feed tube, each having a cross section significantly smaller than the diameter of the discharge tube. This adds a significant component to the pressure loss of the device where the total excess pressure in the exhaust from the primary pump must not reach any value such that the actual limit exceeded is allowed.

However, systems for treating hazardous emissions are generally used with variable capacities and often with one to four process reactors which discharge at the same time permanently. In order to maintain the helical movement, the diameter of the gas injection channel will be adapted to the flow to be treated, especially while observing the maximum pressure loss.

In order to adapt to a wide range of variable flow rates, it would be possible to use an additional auxiliary stream of propellant gas, for example, to initiate a vortex, which does not necessarily apply to the constraint of maximum excess pressure at the inlet. Will not. More precisely, operating a system for treating hazardous emissions by plasma, in particular by microwave, generally requires the addition of one or more auxiliary reactive gases such as air, oxygen, steam, etc., provided in the form of compressed air, for example. . In addition, very often, for reasons connected to operation, this air stream is increased beyond the simple values needed to achieve chemical reactions that convert pollutants. This additional air stream may come from the distribution network of the factory making the semiconductor at a pressure of several bar. It can be used perfectly well on such small diameter orifices. Moreover, the additional dilutions introduced are largely compensated by the increase in the specific efficiency of destruction to the contaminants caused by the presence of the gas streams according to the invention, in particular by the helical movement of these gases.

Specifically, the injection system takes several forms. Radial channels may appear at only one height or at several heights. Gas feeding (upstream of the stream) from the upstream to the injection channel is provided in a known manner so as not to add any significant pressure loss.

When a dielectric tube is used, for example as described in patent US-A-5,965,786, the maximum inner diameter of the tube is dictated by the electron density radial gradient phenomenon of the discharge. When the value of the inner diameter of the tube is increased, it is found that, with everything further being the same, the efficiency of the conversion of the contaminants first increases due to the increase in residence time with increasing cross section. However, beyond some value, the efficiency falls due to the fact that the cross section of the discharge fills the smaller and smaller fraction of the cross section of the tube and the radial extension of the peripheral cold zone increases. As such, increased proportions of contaminant molecules are likely to pass through the tubes in the region of low reaction activity and the conversion yield of the device is reduced.

By adding helical movement to the gas, it is possible to use an inner diameter of the dielectric tube that is significantly larger than that used without such movement of the gas in the absence of a significant decrease in conversion efficiency. The use of large diameter tubes allows larger flow rates to be processed while increasing the power provided to the plasma without significant thermal stress on the tubes and without large pressure losses.

The invention will be explained in the following figures.

1 is a cross-sectional view of a gas injection system according to the present invention.

2 is a sectional view along A of the apparatus of FIG.

3 is a cross-sectional view along B of the apparatus of FIG.

4 and 5 are diagrams of the different results of the measurements.

6 is a vertical sectional view of a dynamic injection head with a single step.

Figure 7 is a vertical sectional view of a dynamic injection head likewise with two stages.

In Fig. 1, the gas injection device 1 is formed in a lateral cylindrical opening having a diameter substantially the same as that of the dielectric tube 5 in which plasma is produced (by means not shown in the figure), for example, in only one tangential direction It was modified when compared to the apparatus described in US-A-5,965,786 with gas injection. Considering the vertically oriented axis X-X '(the axes of the dielectric tube 5 and the gas injection cavity 4), four injections are placed in a plane perpendicular to X-X'. It is formed in accordance with this example by penetrating part 2 through orifices 7, 8, 9, 10 (FIGS. 1 and 2). These orifices are respectively extended by channels 11, 12, 13, 14 to recombine with the injection cavity 4 for the gas. These four channels and orifices are each oriented at 90 ° according to this example. 2 is a cross-sectional view along a perpendicular plane AA through the part 2. An electrode 3 which enables the plasma to be activated is located above the injection cavity for the gas. A second set of orifices 20, 21 and gas injection channels 22, 23, each positioned at 180 ° to each other, is located in a right angle plane BB (see cross-sectional view in FIG. 3). Gases are injected into these orifices under a certain pressure (eg 2-10 × 10 ⁵ kPa) such as compressed air which is always available in the manufacturing plant. This pressurized gas will have a propulsion effect to form a helical movement of the gas to be treated exiting from four orifices in plane AA.

The process gas to be destroyed can also be injected into the plane BB, but injecting an oxidizing gas which promotes reaction with air, nitrogen and possibly under a pressure, preferably between 1 and 10 × 10 ⁵ kPa, Is preferred. All injection orientations of the various gases, in particular not formed in a plane perpendicular to the axis of the tube, but formed at an angle below 90 ° [co-current] or at an angle above 90 ° [counter-current] Orientation is possible.

In the example in a uniformly distributed manner, the preferential splitting of the total flow for feeding into the four channels 7, 8, 9, 10 usually results in a mixture of gas streams within it, equalizing its condition and exhaust from the pump. It is carried out from an equalizing chamber (not shown) in which the main channel from which appears. This chamber divides the four branch channels in a relatively symmetrical manner. If possible, the inflow flows and the split outflows from these chambers should be parallel to not add pressure loss.

At high flow rates (eg four chambers connected simultaneously), it is desirable to use a channel (planar BB) that injects an auxiliary stream of gas to have tangential impulses sufficient to maintain the helical movement of the gas. need. However, it is possible to inject through these channels 22, 23 a minimal compressed flow which serves to provide the amount of oxygen necessary to carry out the chemical conversion reaction of the perfluorinated molecules.

Fracture experiments were performed with a mixture of SF ₆ diluted with nitrogen at a typical concentration of 5000 parts per million by volume (ppm). Oxygen was added as auxiliary reactive gas at a rate approximately 1.5 times the amount of volume of SF ₆ to be treated. 4 shows the total pressure between the inlet for the gas into the equalization chamber and the outlet for the gas after cooling in a heat exchanger (not shown) that serves to cool the gas remaining downstream from the dielectric tube where the discharge occurs. The progression of the loss as well as the rate of breakdown of SF ₆ as a function of the microwave output (net) provided to the plasma is shown.

According to the invention which does not carry out the invention and has carried out the invention (i.e. with a part 2 having only one radial gas inlet along a diameter close to the tube 4 and with its tangential gas inlet) The performance of the same apparatus according to USP 5,965,786 (all of which are otherwise the same) with parts is significantly improved.

Indeed, according to the invention, a rate of breakdown of 90% is obtained at an output of approximately 3000 W, and a rate of breakdown of 99% is obtained at an output of 3500 W. Without the present invention, it would not be possible to treat a flow rate of 80 / liter / minute (slm) with sufficient performance to provide a practical value.

By using an apparatus that does not practice the present invention and with a flow rate of only 60 slm, an output exceeding 5500 W is required to destroy 95% of 5000 ppmv SF ₆ of the same concentration. This compares with the present invention (60 slm and the same mixture) where a power of less than 2500 W is required.

Further measurements at 80 slm showed that the results depend very little on the concentration of SF ₆ between 1000 and 5000 ppmv.

The pressure loss remains perfectly within the limits specified for industrial use with some margin for unintended fluctuations that may result from certain operating conditions.

In addition, radical changes were actually found in the spatial distribution of the plasma and its stability over time. The plasma remains well centered on the axis and has a clear radial extension that is smaller than in the case of injection without helical injection of gas. Visualization was performed with a camera through a transparent silica tube in lateral incidence. This visualization showed the absence of instability in the off-center state and the absence of adhesion of the plasma to the wall of the tube. An axial observation was also performed in the ceramic tube confirming the fixing properties of the plasma at the center of the cross section of the tube.

It has also been found that the amount of heat injected by the plasma is less in the state in which the invention is carried out than in the state without the invention.

It was possible to conduct fracture experiments under nominal conditions over several hours in a silica tube without any damage, especially frosting following chemical erosion on the surface by corrosive fluorinated compounds. . As a comparison, in the absence of the invention and under the same conditions, the silica tube is perforated by chemical erosion and / or local dissolution within minutes.

For total flow rates of 50 and 60 slm, the same injection procedure is used (process hazardous emissions enter through four tangential channels having a diameter of approximately 1/2 of the dielectric tube and compressed air is the channel 1, Entering through two channels 22, 23 having a diameter of approximately half of 8, 9, 10). The breakdown rate for microwave output is considerably better than at 80 slm, and the pressure loss is lower.

When the total flow is reduced below 50 slm, a stable helical movement can still be maintained in this feed configuration for flow rates as low as 30 slm. However, slightly lower stability is recognized in the gas flow.

Therefore, at lower flow rates, for example, using auxiliary injection channels 22 and 23 to provide additional propulsion and thereby maintain the helical movement of the gas while increasing the additional flow of air or nitrogen up to a total flow rate of 50 slm. It is preferable.

As such, when only one etching chamber is in operation (flow rate is approximately 20 slm), 30 slm of air or nitrogen is at a flow rate of 20 slm (1 piece) when the exhaust from the process equipment is one (1). Is added through.

10 slm of air or nitrogen is added through the auxiliary injection channel when the exhaust from the process equipment is at a flow rate of 40 slm (two etch chambers in operation).

FIG. 5 shows the change in failure rate and pressure loss as a function of net microwave power in the first case described above (20 + 30 slm). It should be noted that the curve is very similar at any concentration of perfluorinated gas, especially for the second case (40 + 10 slm) between 1000 and 5000 ppm.

A dynamic injection head with a single stage is shown in FIG. 6, which has a chamber 101 that equalizes the pressure of the gas or gas mixture.

The gas to be treated is injected via the channel 100 into the chamber 101 where the gas pressure is equalized. This chamber comprises an electrode block 104 which activates a plasma through which the gas to be processed passes through the injection section 106 and into the upper portion of the tube 105 and through the chamber 102 and the lid 102 of the body of the chamber 103. Is defined as a cylindrical crown 101. The lower portion of the tube 105 is widened at 107 to fit onto the dielectric tube (not shown).

A dynamic injection head with two stages is shown in FIG. 7, in which the same elements as in FIG. 6 bear the same reference numerals. The injection portion of the gas to be treated is formed via an orifice 201 on the upper portion, while the “bottom” orifice in which the auxiliary gas injection portion (nitrogen, argon) is in communication with the pressure equalization chamber 205 supplied through the channel 204. And formed through 203.

The dynamic injection head is mounted directly and vertically on the ceramic tube on which the plasma is established.

The heads shown in FIGS. 6 and 7 allow for circular movement with coaxial downward displacement in the tube so that the plasma generated to provide enhanced protection of the tube is not inadvertently attached to the wall and is sufficiently separated from the wall. To provide. The ceramic tube 5 protected in this way causes its thermal load to be reduced by 25 to 35%, which results in a significantly lower oil temperature than when there is no downward circular movement of the gas.

The cooling system oil does not degrade in contact with the hot ceramic wall (the absence of carbon-containing deposits on the outer wall (oil side) of the tube demonstrates the effect of the device and the uniformity of the "skin temperature" of the tube). .

It was possible to reduce the frequency of preventive care of the device.

For the effective functioning of the dynamic injector, it is generally necessary to inject at a minimum gas flow rate of approximately 2 to 60 l / m depending on the geometry of the head (number of injectors, diameter of the injector, angle of incidence, etc.).

In order to remain in a permanent "vortex" situation in the region of the plasma, the total flow rate must continue to be adjusted by adding additional flow of nitrogen in another neutral gas (from 0 to 50 L / m) (flow rate to be processed) Calculated according to the number of chambers, several chambers are connected to the system in parallel).

In all cases, the sum of the flow rates of the primary pump and additional nitrogen to be treated must be greater than the minimum operating flow rate of the plasma, which in all cases cannot be less than 2 l / m.

The invention described above is not limited to surface wave plasma and is in a cavity, in particular in a dielectric tube, in a hollow rectangular guide, in which it is either from a resonating cavity or in the interior of a microwave circuit. To any atmospheric microwave plasma maintained.

Claims (15)

May convert a first gas or gas mixture containing at least some molecules having at least one bond between the two atoms constituting the molecule and thereby contain liquid and / or solid products derived from such conversion A method of forming a second gas or gas mixture, wherein at least one bond between two atoms of the molecule ruptures under the action of an electric and / or magnetic field applied to the first gas or gas mixture, The stream of gas or gas mixture may increase the distance traveled by the gas molecules through the electric and / or magnetic field and thus increase the effect of rupture of the binding of molecules on the gas or gas mixture and / or in a nonlinear manner. Injecting through a magnetic field. 2. The method according to claim 1, wherein the first gas or gas mixture is in particular a mixture comprising fluorinated gaseous hazardous emissions such as PFC, HFC or similar gas. 3. The method of claim 1, wherein the first gas or gas mixture comprises molecules having a bond between a fluorine atom and another atom capable of producing molecular fluorine by passage into an electric and / or magnetic field. . The gas or gas mixture according to claim 1, wherein the gas or gas mixture is introduced into an electric field and / or a magnetic field, with the amount of tangential movement of the gas or gas mixture being greater than the amount of axial movement of the gas or gas mixture. Injecting. The method of claim 4, wherein the amount of tangential movement is much greater than the amount of axial movement. 6. The method of claim 1, wherein at least a portion of the gas or gas mixture is injected into the cavity with a tangential velocity component before the action of the electric and / or magnetic fields is applied. 7. . 7. The method of claim 6, wherein the gas or gas mixture is injected by a plurality of inlets comprising tangential components. 8. A method according to any one of the preceding claims, wherein the tangential injection portion is distributed regularly over the circumference. 9. The method according to claim 1, wherein the infusion is located in the same plane. 10. 10. The method of any one of claims 1-9, wherein the infusion is in a different plane. The method of claim 1, wherein the infusions located in the same plane are regularly distributed. 12. The second of claim 1, wherein an injection portion of the gas to be treated is formed in the first plane, and an injection portion of propellant gas, such as air, nitrogen or oxygen, is preferably parallel to the first plane. Formed in the plane. 13. A gas injection device for carrying out the method according to any one of the preceding claims, wherein at least one first is located in a plane perpendicular to the axis of the tube in which its upper end is in a closed state. And a gas injection channel. The apparatus of claim 13, comprising at least one second gas injection channel, preferably located in a plane perpendicular to the axis of the tube. 15. An apparatus according to claim 13 or 14, comprising a dynamic injection head having two heights, which also includes an orifice (203) for injecting auxiliary gas.

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