FI84980C - Method and apparatus for removing substances from gas flowing under low pressure - Google Patents

Description

84980

The present invention relates to a method for removing substances from a gas flowing under a low pressure according to the preamble of claim 1.

The invention also relates to an arrangement for applying the method.

The method of the present invention and the arrangement for its application are suitable for use with Atomic Layer Epitaxy (ALE), Chemical Vapor Deposition (CVD) or plasma etching methods and other similar methods. This method is particularly well suited to the ALE method. It grows very thin films on the surface of substrates (glass plates) placed in a vacuum chamber. A vacuum of 1 mbar is drawn into the chamber with a vacuum pump and the chamber is heated to a temperature of about 500 ° C. Substances that form the desired molecular or atomic layer, such as zinc chloride and hydrogen sulfide, are then introduced into the chamber. Between the feeds of the different substances ..... the chamber is purged with nitrogen so that the different depositing substances do not mix with each other.

Slightly more reactants must always be fed into the chamber than are needed to form a film on the surface of the substrate to ensure that the film covers the entire surface of the substrate. Excess substances enter the vacuum pump through the suction duct of the chamber and form very small particles in the suction piping as they cool. The amount of excess material leaving the reaction chamber is typically about 100 g per day. Compounds that enter the pump consume the pump quickly. Thus, excess substances should be cleaned from the flow to the vacuum pump before they enter the pump.

The following factors must be taken into account when designing such a vacuum filter. The pressure drop caused by the filter must be as small as possible, and since the volume flow of the gas at low pressure is 2 84980 ri (about a thousand times the atmospheric pressure in the ALE process), the size of traditional filters based on paper, felt, etc. becomes large or requires filter media. which have good permeability, so that their resolution is correspondingly poor due to the large pore size. Due to the vacuum, it is difficult to use substances with a high vapor pressure in the filter as construction materials or auxiliaries. Therefore, traditional water washers and the like are very difficult to implement in practice under these conditions.

The following are some known filter solutions.

The cyclone separator is based on the vortex flow provided by the relationship between the mechanical dimensions of the device and the gas flow, from which the particles are separated by "centrifugal force". Such a filter has a low degree of separation of particles with a diameter below submicron (<1 μm) due to their low mass. Since the operation of the device is based on a certain type of flow, when changing the process flows, the mechanical dimensions of the device should also be changed. In addition, no clear sizing instructions are available for such sizing in the case of vacuum. When dimensioning the equipment, it is important to know the size distribution of the particles, which in this case is very difficult to determine in practice. Dimensioning is further complicated by the fact that when vacuum is pumped into the chamber, there are several decades of changes in flow rates, so that the filter is not in the dimensioned operating range. The advantages of a cyclone separator are the low pressure drop caused by the device and the simple structure.

Cold traps and condensers condense the substance from the gas flow to the cold surface. They are often used to remove water from a gas stream. If the condensed substance does not remain a liquid but becomes a solid, as would happen in the ALE process, the surface will fill quickly or the surface size should be unreasonably large. Cold traps do not prevent particles from drifting from the gripping surface back into the gas flow.

Traditional filters based on paper, felt, etc. are easily clogged with a high particle load. Clogging can be reduced by adequate filter sizing, which requires the use of large filter surfaces in vacuum technology. In this case, the size of the filter becomes unreasonably large. Alternatively, the permeability of the filter can be increased, but at the same time, of course, its filtration capacity is reduced and it is unable to stop the very small particles present in the ALE process. In addition, the filter significantly restricts the gas flow.

In the after-reaction chambers, substances which react with the substance to be purified are introduced into the stream to be purified. These devices are used to make highly life-threatening compounds less aggressive, e.g. in the semiconductor industry in process discharge lines before vacuum pumps. They do not filter particles from the flow, but they do.

The object of the present invention is to provide a method by means of which various substances can be efficiently purified from a gas flowing under low pressure.

The invention is based on increasing the particle size of the substance to be filtered by means of a post-reaction chamber, after which the flow is passed through a mechanical filter. The size of the particles continues to increase on the surface of the mechanical filter. To prevent the filter from clogging, its surface is cleaned mechanically.

More specifically, the method according to the invention is characterized by what is set forth in the characterizing part of claim 1.

The arrangement according to the invention is in turn characterized by what is set forth in the characterizing part of claim 13.

4,84980

The invention provides considerable advantages.

With this method and arrangement, the composition of the gas flowing in a vacuum can be changed so that the chemical compounds in the gas flow which are harmful to the vacuum pumps can be removed by means of the filter contained in the device. The solution achieves small size, long service life and high filtration capacity.

Because the particle size of the substances to be filtered is increased in the reaction chamber before filtration, the filter material may be more porous. The flow resistance of the porous material is low, so that the filter does not increase the power requirement of the vacuum pump too much. Although porous material is used as the filter, there is very little particulate impurity in the flow through the filter and even small particles are bound by assembling them larger before mechanical filtration. Due to the mechanical cleaning of the filter, a smaller filter can be used. Normally, the filter must be dimensioned so that its flow resistance does not increase too much as the particles to be filtered adhere to the surface and pores of the material. When the filter material is cleaned during use, even a small filter achieves a long service life. The filter according to the present invention has a very simple structure and cheap, commonly available materials can be used as the filter materials. Thus, the manufacturing and operating costs of the filter are low.

The invention will now be examined in more detail with the aid of the accompanying drawings.

Figure 1 is a partial schematic representation of one embodiment of the present invention.

Figure 2 is a partial sectional view of the detail of Figure 1.

Figure 3 is a block diagram of another embodiment of the present invention.

5,84980

The filter consists of two cylindrical chambers 1 and 2. In the middle of the chamber 1 there is a mechanical filter 3 mounted on the shaft 7. The shaft 7 is arranged on the central axis of the filter chamber 1 and its upper end forms an outlet 8 inside the mechanical filter 3 leading outside the filter chamber 1 and further to the vacuum pump. . An actuator 18 is connected to the upper end of the shaft. The filter 3 is cylindrical and a scraper 4 is arranged on its side, which contacts the surface of the filter 3 parallel to the shaft 7. In the flow direction, the chamber 2 preceding the filter chamber 1 is a post-reaction chamber and is attached to the side of the filter chamber 1. The inlet end of the post-reaction chamber 2 has a gas inlet connection 5 to be filtered and a cleaning reagent supply connection 6 on the upper side thereof.

Figure 2 shows a cross-section of a mechanical filter 3 and a scraper 4 weighing the filter 3. The body li of the mechanical filter 3 is a perforated metal plate bent into the shape of a cylinder. Attached to the body 11 is a porous polyester gauze 12. On the flexible and thick gauze 12 there is a wear-resistant filter cloth 13. The scraper 4 weighing the surface of the filter 3 consists of a shaft 9 and surrounding scraper rings 10.

The filter works as follows. The gas containing impurities from the process enters the reaction chamber 2 via the inlet connection 5. The temperature of the gas as it enters the reaction chamber is about 50 to 200 ° C. The gas mixture from the ALE process may contain, for example, zinc chloride or hydrogen sulfide. These substances are in a gaseous state in the ALE process due to the low pressure and high temperature prevailing in the process. Zinc chloride becomes solid as it cools as it exits the process chamber. The particle size of the solid zinc chloride in the flow from the ALE process is very small. To increase the particle size, a cleaning reagent, for example water, is sprayed through the feed connection 6 into the gas stream coming from the process to the reaction chamber 2. Zinc chloride reacts with water to give solid zinc oxide and gaseous hydrogen chloride. The formation of particles is enhanced by the cooling effect of water.

6 84980

From the reaction chamber 2, the gas containing zinc oxide particles passes to the filter chamber 1. The vacuum caused by the vacuum pump sucks the gas mixture through the mechanical filter 3, whereby the zinc oxide particles remain on the surface of the filter cloth 13 forming the surface of the filter 3. Since the zinc oxide has accumulated into rather large particles, these easily adhere to the surface of the filter 3 and the filter cloth 13 can be sparse. Attached to the filter cloth 13 are water molecules that react with those zinc chloride molecules that have not reacted with water in the reaction chamber 2. The filter cloth 13 is selected so tightly that the particles to be filtered are mechanically stopped by its fibers. The particles stopped in the fabric 13 further bond to each other and form a brittle film on the surface of the filter 3. The material of the filter cloth 13 must be such that the particles to be filtered or the film formed therefrom do not adhere mechanically to the yarns or fibers of the material and do not adhere to it chemically.

Oxide particles adhering to the surface of the filter cloth 13 and the film formed therefrom clog the filter 3 during use and thus increase its flow resistance, in order to keep the flow resistance low, the filter cloth 13 is mechanically cleaned at certain intervals. A layer of zinc oxide is formed on the surface of the filter cloth 13, which consists of particles to be filtered, e.g. in the case of a water supply, reaction products. The fabric becomes clogged during use and before the clogging is detrimental to the process, i.e. the flow resistance of the mechanical filter 3 has become too high, debris is removed from the surface of the fabric.

In the solution shown, the mechanical filter 3 is cleaned by rolling the surface of the fabric 13 forming the outer surface of the filter into a pile so that the debris on the surface falls off. The rolling is done by rotating the filter 3 with the actuator 18 against the scraper 4. When the filter 3 is rotated, the scraper rings 10 arranged on the shaft 9 of the scraper 4 press the filter cloth 13 against the metal plate 11 forming the body of the mechanical filter 7 84980 3. In this case, the thick polyester gauze 12 under the fabric 13 gives the mechanical filter 3 the elasticity required during rolling during rolling. Since the filter cloth 13 and the gauze 12 are flexible when pressed by the scraper rings 10, a wave is formed in the filter cloth 13 in front of the scraper 4. The film formed by the particles adhering to the surface of the fabric 13 breaks and the pieces splitting from the film detach from the fabric 13 due to the deformation and movement required by the wave even before the fabric 13 passes under the scraper rings 10. The scraper rings 10 must be such that they break the entire surface of the filter cloth 13, e.g. so that the ring 10 has surface-breaking protrusions perpendicular to the direction of rotation of the filter 4. Is important. that the scraper 4 actually rolls the surface of the mechanical filter 3 and does not drag it, because as it drags the scraper 4 would press the particles to be filtered into the filter material 12, 13, whereby the filter 3 would quickly clog. For the same reason, the filter material must be such that the membrane cannot adhere to its material. Typically, suitable materials are various plastic materials because of their low adhesion to other materials. In this case, the substance to be filtered does not adhere to the fibers of the filter cloth 13 and comes off easily when the filter is cleaned. A suitable filter fabric 13 in this example solution is the material ACA 463 from Suomen Silkkikutomo Oy.

The time between successive cleaning rolls should be chosen as long as possible. In this case, a thick layer of particles has time to form on the surface of the fabric (13), from which the waste comes off in large pieces.

In addition to the example described above, the present invention has several other embodiments.

Instead of water, of course, any other chemical substance which reacts in a suitable manner with the substance to be filtered can be used as the cleaning reagent. The correct amount of water or other cleaning reagent is important for the operation of the 8 84980 equipment. The feed rate is determined by the dew point due to the pressure and temperature of the exhaust gas of the vacuum pump. The reagent can only be fed to the extent that there is no risk of condensation in the vacuum pump. Thus, the amount of reagent to be fed may remain small. The amount of reagent to be fed is easiest to determine experimentally.

The particles are created in the reaction chamber 2 from the incoming gas stream, or the size of the particles contained in the gas stream is increased so large in the reaction chamber and on the surface of the filter 3 that they adhere to the surface of the sparse filter cloth 13 which does not significantly restrict the gas flow. This is accomplished in a number of different ways. The appropriate method of growing the particles must be selected according to the substance to be purified and the process conditions. The gas stream can be cooled, causing the compounds therein to condense into particles. Gases with high vapor pressure can be converted by the reagent into compounds whose vapor pressure is so low that they become particles under the conditions prevailing in the chamber (e.g., ZnO), or whose vapor pressure is so high that they pass through pumps as a gas (e.g., HCl). This is done by feeding chemicals suitable for the respective gases into the gas flow and creating favorable conditions for the chemical reaction. In addition to or in addition to the above-mentioned procedures, changes in temperature, or an electric or magnetic field, can be used to form the particles.

The amount of cleaning reagent used can be increased by means of the solution according to Figure 3. A commercial condensing device 14 is connected to the pipe 8 leading to the vacuum pump of a filter as described above. The gas entering the pump passes through the condensing device 14, where the reagent is removed by sealing. The condensed reagent is led through the valve 15 and the pipeline 16 to the evaporator 17 and from there on to the reaction chamber 2. Alternatively, the reagent circuit 6, 14, 15, 16, 17 can be dimensioned so that the reagent circulates continuously in a cycle, whereby the valve 15 is not required.

9 84980

Water or other reagent can be fed into the gas flow e.g. in the following ways:

The reagent is fed directly from the side of the gas inlet pipe 5 to be cleaned, whereby the problem may be that when the reaction products are insoluble in water, the supply pipe 6 grows clogged. Nozzles can be used if necessary to make the water more miscible with the gas, although the diffusion in the vacuum in the reaction chamber 2 is rapid and thus separate nozzles are often unnecessary. Crucial to the choice of feed method is the relationship between the mechanical dimensions of the reaction chamber 2 used and the free distance of the water molecules, from which the need for nozzles can be deduced. The water supply point is also significant because the reaction products grow on the walls of the inlet pipe 6 or the reaction chamber 2 and may grow on the pipe 5. In practice, the size of the reaction chamber 2 and the water supply point must be determined experimentally for the different processes.

In addition to the example described above, the surface of the mechanical filter 3 can be cleaned in other ways. The filter 3 or filter fabric 13 can be vibrated at a suitably selected frequency, or the filter body 11 can be made of a flexible material, whereby the filter 3 can be compressed in its axial direction 7 by a suitable actuator, e.g. In this case, the impurities adhering to the surface of the filter 3 are detached from the filter fabric 13. The scraper 4 can be annular, in which case it is conveyed around the filter 3 in the direction of this axis. Cleaning can be continuous or intermittent. The intermittent cleaning operation can be controlled with a time switch or by measuring the pressure difference across the filter, whereby the surface of the filter 3 is cleaned whenever the pressure difference becomes too large.

The most preferred shape of the mechanical filter 4 is a cylinder, because then its surface area can easily be made sufficiently large.

10 84 980

However, the shape of the filter is not critical to the operation of the device, so the filter may be of the desired shape, for example planar.

li

Claims (22)

1. Procedure for removing substances from blasting which flows under low pressure and originating e.g. from processes called Atomic Layer Epitaxy, Chemical Vapor Deposition or Plasma Etching, characterized in that - the gas is led from the process into a reaction chamber (2) where particles form the pollutants and allow the particles to grow in size, - the gas flow with the particles is led through a mechanical filter (3), wherein the contaminant particles adhere to the surface of the mechanical filter where their particle size may further grow and the surface of the mechanical filter (3) is cleaned to prevent the filter from clogging. Process according to claim 1, characterized in that the particles are formed and their size is increased by introducing cleaning reagents into the reaction chamber (2). 3. A process according to claim 1, characterized in that as a cleaning reagent, a substance which chemically reacts with impurities in the gas flow is used. Method according to claim 1, characterized in that an electric field is used in the formation of the particles. Process according to claim 1, characterized in that a magnetic field is used in the formation of the particles. Method according to claim 1, characterized in that the surface of the mechanical filter (3) is cleaned by scraping. Process according to claim 1, characterized in that the surface of the mechanical filter (3) is cleaned by shaking the filter. ie 84980 Method according to claim 1, characterized in that the surface of the mechanical filter is cleaned by compressing the filter (3) and then pulling it out to normal size. Process according to claim 1, characterized in that after the mechanical filtration in a condensing device (14), the cleaning reagents and other substances which can be condensed are removed from the gas flow. Method according to claim 1, characterized in that the surface ρέ of the filter is cyclically cleaned by measuring the pressure difference across the filter and the cleaning begins when the pressure difference has grown sufficiently large. 12. Device for removing substances from blast gases which flow under low pressure and which originate from e.g. the processes called Atomic Layer Epitaxy, Chemical Vapor Deposition or Plasma Etching, characterized by - a reaction chamber (2) into which the gas can be led into the formation of particles consisting of impurities and to increase their size, - a mechanical filter (3) which the gas flow with the particles can be passed through to collect the particles ρέ the surface of the filter (3), and - a device (4) which is displaceable relative to the filter for removing particles accumulated ρέ the surface of the mechanical filter (3) ) to prevent the filter from becoming clogged. Device according to claim 12, characterized by a channel (6) arranged in the reaction chamber (2) for supplying cleaning chemicals to the chamber (2). Device according to claim 12, characterized in that the cleaning device (4) of the filter (3) comprises a scraper 1, 84980 and a functional means (18) for rotating the mechanical filter (18). Device according to claim 12, characterized in that the cleaning device (4) of the filter (3) is a vibration device for vibrating the filter (3) or its surface. Device according to claim 12, characterized in that the cleaning device (4) of the filter (3) comprises at least one drive device, e.g. a pneumatic cylinder, for effecting a linear movement to compress the filter (3) and pull it out to full size again. Device according to claim 12, characterized in that the mechanical cleaning device (3) of the filter (3) comprises a shaft (9) located near the surface of the filter, the periphery of which has abrasive rings (10) pressing against the surface of the filter, and a drive means (18) for rotating the filter against the rings (10) and the shaft (9). Apparatus according to claim 12, characterized by a condensing device (14) in which the gas flow to be filtered can be led into the removal of liquid substances and moisture before the gas flow is led to a vacuum pump. 19. Device according to claim 12, characterized by a pressure measuring means for measuring the pressure difference across the mechanical filter (3). Device according to claim 12, characterized in that the mechanical filter (3) is a hollow cylinder. Apparatus according to claim 12, characterized in that the wall which passes through the filter (3) comprises a rigid, perforated filter body (11), an elastic medium (12) arranged on the surface of the body (11) and an actual filter element. material layer (13) arranged on the surface thereof. Apparatus according to claim 21, characterized in that the filler material layer (13) consists of a fabric made of a plastic material.