FI91422C - Process and apparatus for supplying liquid reagents to a chemical reactor - Google Patents

Description

91422

The present invention relates to a method according to the preamble of claim 1 for feeding liquid reagents to a chemical reactor in which the reagents are in the gas phase.

According to such a method, a preselected amount (material flow) of at least one reagent is fed to an evaporator, where it is allowed to evaporate, after which the steam is passed to a chemical reactor where the reagent is reacted with a substrate or other starting materials in the reactor.

The invention also relates to an apparatus according to the preamble of claim 9 for evaporating and dosing liquid reagents into a chemical reactor.

Thin films are prepared in chemical reactors in which the reagents are introduced into the reaction space in the gas phase and reacted with a planar substrate. Often, reagents use liquid substances at room temperature, which are evaporated in a separate evaporator before being introduced into the reactor.

25

Among the evaporation methods according to the prior art, mention is made in particular of the bubbling method applied in connection with CVD (Chemical Vapor Deposition) processes. Said method uses a source in which a carrier gas is bubbled through a constant volume flow through a liquid in a constant lamp state. When using this type of source, the dosing is controlled by the temperature of the liquid, which determines the vapor pressure, and by the volume flow of the carrier gas, at which the transfer rate is determined.

35 However, there are some drawbacks to this prior art. Thus, in order for the dosing to be accurate with a bubbling source as shown, the temperature of the liquid must be able to be kept constant at the desired temperature. Likewise, the volume flow of the carrier gas 2 must be accurately determined. Even if these parameters are in order, the degree of saturation of the carrier gas leaving the bubbler remains a key factor in the amount of material transferred, but it is difficult to control. It is influenced by 5 e.g. bubble size and the distance of the bubble within the liquid, i.e. the degree of filling of the spring. In addition, the temperature of the carrier gas fed to the source must be kept constant in order to keep the saturation constant. When changing the parameters of a source, it is not possible to accurately estimate the change in saturation level in advance, 10 so the desired changes in the amount of material leaving the source cannot be precisely controlled without extensive and thorough characterization of the source.

WO 91/19017 discloses a solution for evaporating liquid reagents which differs from the above.

The prior art method involves directing the reagents in a controlled and precisely controlled liquid flow to an expansion valve where the liquid reagent evaporates, after which the formed hydride is fed to a reactor maintained under reduced pressure. The size of the nozzle of the expansion valve is then available according to the pressure of the liquid reagent to eliminate variations in the liquid flow and to improve the uniformity of the gas phase flow. However, this solution does not work well either, as there are large and sudden fluctuations in fluid intake.

In addition, the nozzle opening of the expansion valve is prone to clogging and as a moving part sensitive to operating disturbances.

The object of the present invention is to obviate the drawbacks of the prior art and to provide a completely new solution for feeding liquid reagents to chemical reactors using gas-phase reagents.

In connection with the invention, it has been found that the amounts of liquid chemicals required in the manufacture of thin films are so small that it is difficult to pump them to the evaporator in a constant flow of 3 91422 due to the surface tension forces forming the droplets.

For this reason, the solution according to the present invention is based on the idea of using an evaporator capable of acting as a buffer reservoir of the liquid to be evaporated in order to smooth out the flow variations in the liquid stream feed. The evaporator according to the invention then consists at least in part of a porous, wettable material which prevents the formation of droplets by absorbing the liquid itself.

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

15

The apparatus according to the invention, in turn, is characterized by what is set forth in the characterizing part of claim 9.

20 For the purposes of this application, a porous, wettable material is a substance whose pore size is so small that the forces between the liquid and the pores (e.g. capillary forces and cohesive forces) are greater than the surface tension of the liquid, allowing the liquid to penetrate the pores and låpi. The maximum pore sizes vary depending on the reagent used, but are typically 0.1 nm, 100 nm, preferably about 1 to 30 nm. Preferably, the porous material is made of an inorganic material that is inert to the reagent, such as a ceramic or mineral material. Typical examples are various silicate-based materials. The porous body can also be made of graphite, the ability to conduct heat of which is advantageous in many respects when supplied according to the invention. Among the ingredients of the porous material, various carbides, such as silicon carbide, are mentioned.

The porous material can be shaped into any suitable geometric shape. The surface area / volume ratio of the material is selected according to how large a buffer stock is required for the vaporizing agent, i.e. how much variation occurs in the liquid feed. Usually, as will be shown below, the tendency for the liquid flow to be fed to the evaporator is to be kept very even, whereby the surface of the porous material is preferably quite large. In the example cat below, the vaporizing material is shaped like a rod, so that its cross-section in the direction of liquid intake is circular. However, if desired, the porous material can also be shaped, e.g., plate-shaped, conical or spherical. It should also be noted that the term "surface" refers to the geometric surface of a body of porous material, the so-called curtain surface.

15

The absorbed reagent is vaporized from the porous material by transferring the necessary evaporation energy. Usually and preferably this is done by introducing thermal energy into the porous material from a suitable heat source. Steady 20 and efficient heat input to the evaporator is important because the reagent binds a large amount of heat upon evaporation and can be accomplished by using a constant temperature lamp source.

In a preferred embodiment of the invention, in order to provide evaporation, heating means are arranged around the porous material, which apply thermal energy to the material from which it is transferred to the liquid. As a result of the lamp energy, the liquid evaporates into the space surrounding the porous material, from where it is further conveyed to the reactor, e.g. by means of a carrier gas. So much of the lamp energy is introduced into the evaporator that the liquid is made to evaporate as a substantially uniform gas flow from the porous material.

As the heating means, for example, various radiant heaters can be used, such as electric resistance heaters. The heaters can also be arranged inside the porous material by forming openings or holes in which the heating members are placed. As mentioned above, the heating member is preferably maintained at a constant temperature.

According to a preferred embodiment, the porous material is formed around a central first, e.g. rod-shaped, heated lime in the form of a cylindrical jacket, from which heating the lamp is transferred to the porous material. Outside the cylinder jacket, a gap required for the steam flow 10 is arranged, and on the other side of this there is a second heating element in the form of a cylinder jacket, from which the lamp radiates to the porous material. In this embodiment, care must preferably be taken to ensure that the second heating element supplies so much thermal energy to the outer casing 15 of the evaporator that condensation of the steam on the inner surface of the casing is prevented.

The heating can also be arranged in such a way that a warm gas is introduced into the space surrounding the porous material (steam flow cell), e.g. a heated carrier gas, with which the vapors are conveyed to the reactor.

By forming the evaporator in one piece with the feed nozzles of the chemical reactor, the thermal energy is also transferred to the hoy-25 grate by conduction from the nozzle. Since the feed nozzles of a chemical reactor are often kept at a constant lamp temperature, it has proven advantageous to use the waste heat released from the nozzle to heat the evaporator.

It was mentioned above that according to an embodiment of the invention, the steam is introduced into the reactor by means of a carrier gas. In a preferred embodiment of the invention, the evaporator operates at the same pressure as the chemical reactor. Thus, in combination with the CVD apparatus 35 used in a normal air purifier according to the example shown below, the evaporator operates at atmospheric pressure, whereby both the reagent and the carrier gas are introduced into it at the same pressure.

6

If desired, the transport of the vaporized liquid can also be arranged by connecting the vaporizer directly to the application, i.e. the chemical reactor, and maintaining it at the same pressure as the vaporizer, whereby the reagent vapors move freely to the reactor,

The liquid can be introduced at any point in the porous material. Preferably, however, the porous material is arranged at least in a substantially vertical position, whereby the liquid is introduced into its upper part, from where it spreads downwards by gravity and laterally by capillary and cohesive forces and other similar physical forces. In the example below, the porous material is a vertically-arranged rod, the upper end of which is formed with a central axis opening into which the liquid can be fed. By feeding the liquid along the central axis, a uniform spread of the liquid over the cross-section of the rod is achieved, whereby the liquid evaporates evenly around the rod.

20

The rate of dosing and its accuracy are primarily dependent on the flow rate of the liquid fed. The amount of reagent to be evaporated and fed to the reactor as steam is controlled by controlling the pumping rate at which the liquid reagent is metered into the evaporator element. The pump must be such as to produce a uniform and controlled flow of liquid which is not dependent on the back pressure. Pumps of this type are, for example, manta and hose pumps, the capacity of which changes linearly as a function of the speed of the pump, whereby accurate dosing of the liquid volume is possible by changing the power of the pump.

As can be seen from the foregoing, the porous material acts as a buffer reservoir for the liquid reagent, eliminates the inertities of the liquid 35, and thus prevents variation in the evaporator outlet caused by droplet formation. According to the invention, the vaporizing material is operated in a continuity state of 7,91422 (steady state state). This case is described in more detail below:

In the evaporator element 5 according to the invention, the liquid is converted into vapor and leaves with the carrier gas at the same rate as it is pumped. In the continuous state, longitudinal and linear "markius" gradients are then generated inside the porous material, which depend on the pumping rate on the one hand and the lamp-10 state of the evaporator on the other hand, In the continuity state, the porous material contains the vaporizable liquid from the buffer storage. Its amount depends on the pumping speed of the liquid, the temperature of the heater, and the geometric dimensions of the 15 porous material. The buffer storage effectively compensates for short-term changes in the liquid intake, so the uniformity requirements for pumping are substantially relaxed.

20 The working range of the evaporator can be defined as follows: Μώ j = M ^ g in the continuity state

Mi », <25

Mlt> 0 where: 30 Μ »,, = mass flow rate of liquid pumped into the evaporator Μαη, ί = mass flow rate of vapor leaving the evaporator

Min, .max (T) = the maximum mass flow of liquid that a particular evaporator 35 can vaporize in the heater temperature T.

M „= content of evaporator liquid mass in steady state.

40 If Μ »,, is greater than Μ ^, ^ ίΤ), not the entire evaporator pumped δ

the pattu liquid has time to evaporate, but liquid droplets begin to form in the evaporator. depends not only on the temperature T

also on the dimensions and structure of the evaporator, more specifically on the heat flux that passes from the heater through the porous material of the heater to the liquid to be evaporated.

Figure 3 shows, by way of example, the experimentally determined curves of an evaporator having the structure according to the embodiment described below for water and ethanol and mixtures thereof.

The working area of the evaporator is the area delimited by the curve and the temperature axis. When choosing a working point, there should be 15 pages to keep you below the curve and at a sufficient distance from it.

Unless the substance to be vaporized itself imposes limitations on the temperature due to its thermal stability, the capacity of the vaporizer 20 can be increased by simply increasing the temperature of the heater.

In the extreme situation where M „= 0, the liquid evaporates accordingly when it is pumped upwards of the hybridizer while the porous material 25 remains dry. In this case, no buffer is formed inside the evaporator at all, and the evaporator does not compensate for short-term fluctuations in pumping. For this reason, it is advantageous to select the temperature at which the M * is suitably large as the operating point of the evaporator. When the temperature of the heater is changed while the pump-30 break rate remains constant, the evaporator enters a new continuous state that differs from the first in that M „(T2) & M„ (T,), provided that the working range of the evaporator is maintained. During the change phase & M * ,,. Mst also changes when the pumping rate is changed at constant temperature, 3 5 until a new continuous mode is reached, ie: Μ ,, ίΜ ^) M ,, (Myj ,,) · Mst changes with the effective evaporating surface.

9 91422

A particular advantage of the continuity mode is that since all the liquid to be fed enters the gas phase, no distillation takes place. Thus, in carrying out the invention, gas-phase mixtures can be obtained by mixing the respective liquids and evaporating the mixture. Due to the state of continuity, the co-rot of the steam is the same as that of the liquid mixture, except that it is not an azeotropic liquid mixture. This gives the prior art much greater variability in the production of thin films.

10

The invention will now be examined in more detail with the aid of the accompanying drawings, in which Fig. 1 shows a schematic diagram of a dispensing device according to the invention, Fig. 2 shows a side view of a evaporator used in the invention, Fig. 3 shows experimentally configured evaporators and ethanol and mixtures thereof, Fig. 4 is a schematic diagram of a normal air pressure (atmospheric pressure) CVD apparatus, and Fig. 5 is a diagram showing how reagents are fed to the nozzles of an APCVD apparatus by the method of the invention.

25

Figure 1 shows the components of a dosing and evaporation system according to the invention: a gas intake system 1 for a carrier gas, a vessel 2 for a liquid reagent, a pump 3 for precise dosing and an evaporator 4.

30

The carrier gas used is the same gas as in the reaction, preferably a gas inert to the reagent to be vaporized, such as nitrogen or argon, or air.

If appropriate, a reagent gas such as 35 oxygen can also be used. The gas is led to the evaporator 4, to which it can be fed under reduced pressure or at normal atmospheric pressure. Preferably, the gas is fed to the evaporator at the same pressure prevailing in the reactor. If necessary, the liquid reagent vessel 2 is kept under an atmosphere of inert gas to prevent oxidation of the reagent.

5 The control of the dosing system is based on the fact that in the continuity state the same amount of substance is removed from the hybridizer 4 as steam as what is pumped into it as a liquid. The controllability and accuracy of pump 3 are basic parameters of the reagent supply. As the pump 3, a peristaltic pump or a piston pump-10 is preferably used, the pumping speed of which can be controlled by controlling the amount of reagent to be vaporized and fed to the reactor as vapor. By simultaneously feeding liquid from two or more liquid sources and combining the liquid streams before the evaporator into a liquid mixture, the composition of the resulting liquid mixture - and thus the vapor mixture - can be obtained quite accurately by means of pumps.

In the steady state, the liquid is converted to vapor in the steady state element 4 and exits with the carrier gas 20 at the same rate as it is pumped. Hoyry has the same composition as a liquid. In this solution, the dosing line and its accuracy depend on the flow rate of the introduced liquid. Any variations in flow can be smoothed out.

25

Figure 2 shows a preferred embodiment of an evaporator. The evaporator comprises a tubular evaporation space 5, in the upper part of which a liquid supply connection 6 is formed and as a separate branch a carrier gas supply connection 7. A lowering connection 8 of the hybridized reagent is arranged in the lower part of the evaporation space 30.

A body 9 made of a concentric non-sintered ceramic material, such as a borosilicate glass, is arranged in the vapor space 5, the lower part of which is supported against the inner surface of the vapor space 5 and is connected at its top to an inlet pipe 10 arranged in the liquid supply connection 6.

Body 9 is porous in structure and has a pore size of about 10 nm. The inlet pipe is fitted in an opening formed in the upper end of the porous rod.

In this evaporator model, the liquid to be evaporated is pumped into the porous material 9, into which it is absorbed. The carrier gas is fed 5 through 7 to the evaporation space, where it flows past the porous material and transports the vaporized substance with it to the application. In the continuous state, longitudinal and radial "wetness" gradients are generated inside the porous material, as described above.

10

Electric resistance heaters 12 are arranged around the evaporation space 5 to introduce thermal energy into the system. The heaters are adapted to heat the porous rod piece 9 along its entire path. The aim is to transfer a sufficient amount of thermal energy to the body to vaporize the liquid reagent.

Example 20 The application of the method according to the invention to the growth of a flattening conductive thin film, fluorine-doped tin oxide (SnO 2; F), by the APCVD (Atmospheric Pressure Chemical Vapor Deposition) process is considered below. The liquid reagents used in the process are tin tetrachloride (SnCl4), methanol (CH3OH) and water (H2O). The process uses oxygen (O 2) as an additional oxidant and nitrogen (N 2) as a carrier gas to dilute the gas mixture. Trichlorobromomethane gas (CF3Br) is used for the fluorine doping.

Figure 4 schematically shows the APCVD apparatus. The main parts of the device are a conveyor belt 21 on which the growing media 22 is stacked, an oven chamber 23 and heaters 24, as well as reagent nozzles 25 for contacting the gaseous starting materials with the growing media 22. The growth of the thin film 35 takes place under the nozzles.

In the APCVD, the glass substrates 12 22 stacked on the conveyor belt 21 slowly move past the reagent nozzles 25 in an oven 24, which typically has a temperature of about 500 ° C. In the APCVD growth of tin oxide, there are usually two of these nozzles 25, which increases the possibilities of improving the properties of the thin film, since different growth processes can be run in the nozzles. The capacity of the device is also higher with two nozzles.

The forward speed of the belt 21 is typically about 25 cm / min.

10 The levels of chemicals introduced are exemplary:

Nozzle 1 SnCl4 15 ml / h H 2 O 3 0 ml / h CH 2 OH 25 ml / h 15 02 150 Sl / h N2 200 sl / h

Nozzle 2 SnCl4 60 ml / h H2o 30 ml / h 2 0 02 2 0 0 sl / h CF3Br 60 sl / h N2 200 sl / h

The amount of nitrogen required as a carrier gas and to dilute the reagent mixture depends on the geometry and dimensions of the furnace 24.

With the above reagent feeds, a 1 μιη: η thick SnO 2; F film grows at 500 ° C on a 30 cm wide substrate at a distance of about 25 cm per minute. The film has a square resistivity of 10 to 20 Ω / nelid and, due to the strong texture of the surface, effectively scatters 30 lights, visually displaying a milky haze.

Figure 5 schematically shows how yo. the process is preferably carried out utilizing the method of evaporation and dosing of liquid chemicals according to the invention.

The above liquid chemical amounts are so small that it is difficult to pump them into the evaporator in a constant flow due to the surface tension forces forming the droplets. The porous material 8 used in the evaporator 5-12 described above, denoted by reference numeral 26 in Fig. 5, absorbs the liquid itself and thus prevents variation in the evaporator outlet caused by the formation of droplets and the flow of liquid 5.

A maximum of 5 dosing pumps 27, 5 evaporator elements 26, 8 flow meters 28 or mass flow 10 rectifiers and 3 chemical containers 29 are required for implementation; in addition, of course, hoses for gases and liquids are suitable. There are two nozzles (31 and 32). If a mixture of water and methanol in a suitable ratio is pumped into the nozzle 31 instead of separate water and methanol sources, 1 evaporator, 1 15 nitrogen flow meter / mass flow precipitate can be removed from the system. This is possible because the evaporator operates in a continuity state where the distillation phenomena do not change the composition of the effluent mixture compared to that introduced.

It is advantageous to provide the evaporator according to the invention as an integrated part of the gas intake nozzle of the CVD apparatus, since the waste lamp energy of the constant-lamp nozzle can then be applied to the porous material to evaporate the liquid reagent. This solution can replace the separate heating elements of the hoist-25 timmer.

Claims (12)

A process for feeding liquid reagents to a chemical reactor in which the reagents are in the vapor phase, according to which a preselected material flow of the reagents is fed into an evaporator (4), in which the reagents are evaporated, and then 10 is conducted. the vapor of a chemical reactor, when the reagents are reacted with the substrate or other starting substances contained in the reactor, characterized by allowing the reagents to be sucked up into a porous material (9) in the evaporator and to the reagents in the porous material the required vaporization heat, the evaporator; (4) is operated in a stationary state, the liquid flow volume and the amount of vapor heat supplied to the porous material being adapted to each other so that at least as much vapor is evaporated from the porous material as it is supplied. this and the evaporator (4) are supplied with a liquid mixture of at least t Wave reagents, whereby with evapora towers operating in stationary state, produce a vapor which at least substantially corresponds to the composition of the liquid mixture. 2. A method according to claim 1, characterized in that the volume area of the liquid reagent is adapted to at least approximately correspond to the amount of vaporized reagent fed into the reactor per unit time. 35 Process according to Claim 1 or 2, characterized in that the volume of the porous material (9) is selected in such a way that no throughput of the required liquid 18 will occur at the temperature in question. 4. A method according to claim 3, characterized in that the capacity of the porous material (9) is increased by increasing the heat flow directed towards the material. A method according to any one of the preceding claims, wherein a porous material formed into an elongate piece is used, characterized in that the porous material (9) is held substantially in vertical position, whereby the liquid field is fed to the upper part of the material and allowed spread through the material under the influence of gravity and the physical forces acting between the liquid and the porous material. 15 Process according to one of the preceding claims, characterized in that as porous material (9) an unsintered ceramic material or graphite is used. Apparatus for supplying liquid reagents to a chemical reactor, wherein the reagents are in the vapor phase, comprising at least one cold (2) for a liquid reagent, a said cold connected pump (3) for aspirating the desired amount liquid reagent from the cold, an evaporator (4) connected to the pressure side of the pump (3) for evaporation of the liquid flow from the pump, which evaporator is connected to the chemical reactor for feeding the vapor phase reagents into the reactor, characterized by the evaporator (4) comprises a vaporization chamber (5) provided with an inlet (6) for the liquid reagent, an inlet (7) for a carrier gas and an outlet (8) for the vaporized reagent, a piece arranged in the vapor chamber (5) (9) of a wettable porous material which contacts the inlet (6) of the liquid reagent and a heat energy source (12) arranged in conjunction with the evaporator compartment (5) applying heat to the porous piece to evaporate the liquid reagent which has been passed to the piece. 5 Apparatus according to claim 7, in which the evaporator (4) is provided with a cold (1) for carrier gas, wherein gas can be fed from the cold to the evaporator, so that the evaporated reagent can be passed from the evaporator to a chemical reactor. (21 - 25), characterized in that the porous piece (9) is arranged in the evaporation chamber in such a way that a gas-flow channel adjacent the gas outlet (5) is formed between the piece (9) and the evaporation chamber (5). evaporated from said piece (9) streams. Device according to claim 7 or 8, characterized in that the porous piece (9) consists of a ceramic or a mineral material or graphite, with a pore size of 0.1 to 100 nm, preferably about 1 to 30 nm. Device according to any one of claims 7-9, characterized in that the porous piece is formed into an elongated rod (9) which has a substantially circular cross-section and the longitudinal axis of the spring is inclined in at least approximate vertical steel, wherein the inlet ( 6) for the liquid reagent is associated with the upper spirit of the piece. Device according to any one of claims 7-10, characterized in that the heat energy source consists of heat resistors (12) arranged around the evaporation chamber (5). 12. Device according to any of claims 7-10, characterized in that the evaporator (4) is integrated with the gas supply nozzles of a CVD system, whereby the heat energy from the nozzles can be directed towards the porous material.