KR20210055348A - Apparatus and Method for Deposition of Thin Film - Google Patents

Abstract

A device for depositing a thin film according to one embodiment comprises: a chamber wherein a processing space for processing a substrate by generating plasma therein is formed; a gas supply device that injects process gas into the chamber; a susceptor installed so as to face the gas supply device and on which the substrate is mounted; and a power supply and matching part that connects to the gas supply device to provide a high-frequency RF power. As a substrate to be processed is provided, the present invention may be configured to supply a reducing gas to at least a part of a section including a reduction process by adsorbing a first source gas onto the substrate, and performing a film forming process by supplying a second source gas after the reduction process of reducing the first source gas by a plasma. Therefore, the present invention is capable of allowing the thin film having a uniform thickness and a dense film quality to be formed on the substrate on which a lower pattern is formed.

Description

Thin film deposition apparatus and method {Apparatus and Method for Deposition of Thin Film}

The present invention relates to a semiconductor film forming technology, and more particularly, to a thin film deposition apparatus and method.

As the size of the semiconductor integrated device becomes smaller and the shape becomes more complex, there is a demand for a technique for forming a thin film having a uniform and thin thickness on an underlying structure having a high step coverage and a large aspect ratio.

Accordingly, an atomic layer deposition (ALD) method is used in which a precursor and a reactant are injected in time-division while using a chemical vapor deposition reaction to accurately control the thickness of a thin film using a self-controlled reaction performed on the substrate surface. .

In addition, in order to improve the reaction rate between the precursor and the reactant and improve the film quality of the deposited thin film, a plasma ALD technology applied to the ALD process has been developed.

Nevertheless, with the miniaturization and densification of the semiconductor pattern, depositing a thin film having a uniform thickness on the concave portion, sidewall, and upper portion of the pattern remains a task to be solved.

An embodiment of the present technology can provide a thin film deposition apparatus and method capable of forming a thin film having a uniform thickness and a dense film quality.

A thin film deposition apparatus according to an embodiment of the present technology includes a chamber in which a processing space for processing a substrate by generating plasma is formed therein; A gas supply device for injecting a process gas into the chamber; A susceptor installed to face the gas supply device and on which the substrate is seated; And a power supply and matching unit connected to the gas supply device to provide high-frequency RF power, and as a substrate to be processed is provided, a first source gas is adsorbed onto the substrate and the first source gas is plasma After the reduction process by which the second source gas is supplied to perform the film formation process, it may be configured to supply the reducing gas to at least a part of the section after the reduction process.

A thin film deposition method according to an embodiment of the present technology includes an adsorption step of supplying a first source gas through a gas supply device above a chamber to adsorb the first source gas on a substrate; A reduction step of stopping supply of the first source gas and generating plasma while supplying a reducing gas to reduce the first source gas; And a film forming step of supplying a second source gas to react the first source gas with the second source gas.

According to the present technology, a thin film having a uniform thickness and a dense film quality can be formed on a substrate on which a lower pattern is formed.

Since it is possible to form a thin film having an excellent and uniform film quality through a low-temperature process on a dense and fine lower pattern, reliability of a subsequent process can also be improved.

1 is a configuration diagram of a thin film deposition apparatus according to an embodiment.
2 to 4 are timing diagrams for explaining a thin film deposition method according to embodiments.
5 is a view for explaining the characteristics of the thin film according to the reducing gas supply method.
6 is a diagram for explaining an impurity concentration in a thin film according to a reducing gas supply method.
7 is a view for explaining the uniformity and density of a thin film according to the reducing gas supply method.
8 is a flowchart illustrating a thin film deposition method according to an exemplary embodiment.
9 is a diagram for explaining characteristics of a thin film according to the number of process repetitions.
10 is a view for explaining the uniformity and density of a thin film according to the number of process repetitions.

Hereinafter, embodiments of the present technology will be described in more detail with reference to the accompanying drawings.

1 is a configuration diagram of a thin film deposition apparatus according to an embodiment.

Referring to FIG. 1, the thin film deposition apparatus 10 may include a chamber 100, a gas providing unit 200, and a power supply and matching unit 300.

The chamber 100 may include a body 110 with an open top and a gas supply device 120 configured to close the top of the body 110.

The interior space of the chamber 100 may be a space in which a substrate W is processed, such as a deposition process. A gate G through which the substrate W is carried in and carried out may be provided at a designated position on the side of the main body 110. A through hole into which the support shaft 140 of the susceptor 130 on which the substrate W is seated may be formed on the bottom of the main body 110.

The susceptor 130 has an overall flat shape such that at least one substrate W is seated on the upper surface, and may be installed in a horizontal direction facing the gas supply device 120. The support shaft 140 is vertically coupled to the rear surface of the susceptor 130, and is connected to an external driving unit (not shown) through a through hole at the bottom of the chamber 100 to lift and/or rotate the susceptor 130. Can be configured to In one embodiment, the susceptor 130 may act as an electrode (second electrode).

A heater 132 is provided inside the susceptor 130 to control the temperature of the substrate W mounted thereon. The power supply unit 170 may be configured to supply power to the heater 132 so that the heater 132 generates heat.

Since the inside of the chamber 100 should be generally formed in a vacuum atmosphere, an exhaust port 150 may be formed at a designated location of the main body 110, for example, on the bottom surface. The exhaust port 150 may be connected to an external pump 160. The inside of the chamber 100 may be made into a vacuum state through the exhaust port 150 and gas generated after the process may be discharged.

1 illustrates an example in which the exhaust port 150 is formed on the bottom of the chamber 200, the exhaust port 150 may be formed on the side of the chamber 200.

The gas supply device 120 may be installed on the body 110 to face the susceptor 130. The gas supply device 120 may inject various process gases supplied from the gas providing unit 200 into the chamber 100. The gas supply device 120 may be selected from various types of gas supply devices such as a showerhead type, an injector type, and a nozzle type. In one embodiment, the gas supply device 120 may act as an electrode (first electrode).

The gas supply unit 200 may include a gas supply source 210, a valve 220, and a gas supply line 230.

The gas supply source 210 may include a plurality of gas supply sources. In one embodiment, the gas supply source 210 includes a first gas supply source 211 and a second gas supplying a first process gas, a second process gas, a third process gas, a fourth process gas, and a fifth process gas, respectively. A supply source 213, a third gas supply source 215, a fourth gas supply source 217, and a fifth gas supply source 219 may be included.

The valve 220 may be installed between the gas supply source 210 and the gas supply line 230 so that each process gas is supplied to or blocked by the gas supply line 230. The valve 220 is connected between the first to fifth gas supply sources 211, 213, 215, 217, 219 and the first to fourth gas supply lines 231, 233, 235, 237, respectively. Valves 221, 223, 225, 227, 229 may be included.

In one embodiment, the first process gas may be a first source gas, and the second process gas may be a carrier gas, for example, a carrier gas for the first source gas. In addition, the third process gas may be a second source gas, and the fourth and fifth process gases may be a purge gas and a reducing gas, respectively, but are not limited thereto.

The power supply and matching unit 300 may be configured to provide a high frequency power having a preset frequency band as a plasma power source. In addition, the output impedance of the high-frequency power source and the load impedance in the chamber 100 may be matched with each other to eliminate reflection loss due to reflection of the high-frequency power source from the chamber 100.

In one embodiment, the power supply and matching unit 300 may provide a high-frequency power having a center frequency of 27.12 MHz or more, or 60 MHz or more. When a high-frequency power with a center frequency of 13.56 MHz is supplied, the density of the thin film formed on the pattern appears softer at the lower side of the pattern compared to the upper portion of the pattern, whereas when a high-frequency power with a center frequency of 27.12 MHz or more is supplied, the upper and lower portions of the pattern A thin film having a dense and uniform film quality may be formed over the lower portion.

In order to form a thin film, the temperature in the chamber 100 is set at a low temperature, for example, 550° C. or less, preferably 400° C. or less, and the pressure is set to 1 to 10 Torr. Settle (W). Then, after making the inside of the chamber 100 into a vacuum state, a process gas, for example, a carrier gas and a first source gas are injected through the gas supply device 120.

Accordingly, the first source gas is adsorbed on the surface of the substrate W. After the adsorption process, the supply of the first source gas and the carrier gas is stopped, and the residual gas after adsorption may be removed through the first purge process.

In one embodiment, the first source gas is a halide-based silicon-containing gas such as DCS (DiChloroSilane; SiH2Cl2), HCDS (HexaChloroDisilane; Si2Cl6), and an amine-based silicon-containing gas such as DIPAS and BDEAS. Can be chosen. The carrier gas may be argon gas, but is not limited thereto. The primary purge gas may be argon gas, or nitrogen gas and argon gas, but is not limited thereto.

After purging the first source gas, a reduction process for the first source gas may be performed by applying a high frequency to the gas supply device 120 by the power supply and matching unit 300 while supplying the reducing gas. Through the reduction process, impurities, such as chlorine (Cl) and hydrogen (H2), in the reducing gas and the first source gas may be desorbed and discharged to the outside. In one embodiment, the reducing gas may be selected from among hydrogen containing gases.

Thereafter, argon gas, or nitrogen gas and argon gas may be supplied as the second purge gas to perform the second purge.

By removing impurities in the first source gas in advance during the reduction process of the first source gas, reactivity with the subsequently supplied second source gas may be improved.

Subsequently, a second source gas may be supplied as a reaction gas to perform a film formation process. When the second source gas is supplied, the reducing gas may be supplied together, and the power supply and application of the high-frequency power by the matching unit 300 may be blocked. When the second source gas is supplied, as the reducing gas is supplied together, impurities are removed by a reaction between the impurities included in the first source gas and the reducing gas, thereby improving reactivity between the first source gas and the second source gas. The second source gas _{may be selected from gases containing ammonia (NH 3} ), and a silicon nitride film (SiN) may be formed by a reaction between the first source gas and the second source gas. The ammonia-containing gas is more easily decomposed than other nitrogen-containing gases (for example, N2), and when applied as a second source gas, film formation efficiency can be improved.

After the film formation process, supply of the second source gas may be stopped, and reaction by-products and unreacted gas may be removed through a third purge process. Argon gas, or nitrogen gas and argon gas may be supplied as the third purge gas.

During the reduction process of the first source gas, by supplying the reducing gas, impurities in the first source gas may be combined with or desorbed from the reducing gas to be discharged to the outside. Accordingly, the reactivity between the first source gas and the second source gas from which impurities are primarily removed during the film formation process of supplying the second source gas is improved. In addition, since the reducing gas is also supplied when the second source gas is supplied, impurities contained in the first source gas are further desorbed, so that reactivity may be further improved.

In the present technology, in order to form a thin film at a low temperature of 550° C. or less, plasma is applied when the first source gas is supplied to excite the first source gas, while impurities are removed by the reducing gas. Subsequently, a second source gas is supplied and the first and second source gases react, thereby forming a film. In this case, impurities in the first source gas can be easily removed by supplying the reducing gas to at least a portion of the section including the reduction process.

The halide-based or amine-based first source gas supplied when the silicon nitride film is formed contains impurities (chlorine, hydrogen), and when a silicon nitride film is formed by using this, the thin film has low density and a step in proportion to the impurity concentration in the thin film. The coverage characteristics are poor. Accordingly, it is difficult to form a dense thin film while having a conformal thickness along the upper end, sidewall, and lower end of the pattern formed on the substrate surface. In particular, if the thickness of the thin film formed on the sidewall of the pattern is not uniform and not dense, a certain etch rate cannot be guaranteed during a subsequent etching process.

However, in the present technology, since the reducing gas is continuously and sufficiently supplied through at least a portion of the section including the reduction process of the first source gas, it is possible to form a dense silicon nitride film, and in particular, the denseness of the thin film formed on the pattern. Is increased, and step coverage characteristics can be secured.

On the other hand, after repeating the first process including the adsorption and reduction process described above for a preset L number of times, the second process including the film forming process is repeated a preset M number of times, and the first process and the second process are performed. By repeating the set N-th number of times, a more uniform and dense thin film can be formed.

2 to 4 are timing diagrams for explaining a thin film deposition method according to embodiments.

2 shows a thin film deposition method in which a film is formed by supplying a reducing gas during at least a partial section including a reduction process, for example, a reduction process and a film formation process.

Referring to FIG. 2, the thin film deposition process may include an adsorption step (T1 to T2), a reduction step (T2 to T4), a film formation step (T4 to T6), and a purge step (T6 to T7).

First, the substrate W mounted on the susceptor 130 may be heated to a preset temperature by the heater 132. In addition, after stabilizing the pressure in the chamber 100 to a preset pressure (for example, 1 to 10 Torr), the first source gas may be supplied together with the carrier gas at a preset pressure and flow rate (T1 to T2).

When the thin film to be deposited is a silicon nitride film, the silicon precursor as the first source gas is a source gas containing silicon and chlorine, for example, a halide-based gas such as DCS and HCDS, and an amine such as DIPAS and BDEAS. (Amin) series gas can be used. Accordingly, silicon may be adsorbed on the surface of the substrate W. After adsorption, the supply of the carrier gas and the first source gas is stopped, and the unreacted gas may be removed by primary purging while supplying argon or a purge gas in which argon and nitrogen are mixed (T2 to T3).

Next, a reduction process of the first source gas may be performed (T3 to T4).

In order to reduce the first source gas, the first source gas and the reducing gas may be excited by applying a high-frequency power having a center frequency of 27.12 MHz or more or 60 MHz or more through the power supply and matching unit 300 while supplying the reducing gas to reduce the first source gas. have. The reducing gas may be selected from among hydrogen-containing waters. Through the reduction process, impurities (eg, chlorine, hydrogen) and reducing gas in the first source gas may react and desorb to be discharged to the outside.

Subsequently, a film forming process may be performed (T4 to T6).

For the film formation process, the application of high-frequency power is cut off, and unreacted gas can be removed by secondary purging while supplying argon or a purge gas in which argon and nitrogen are mixed (T4 to T5). In addition, a second source gas may be supplied to react with the first source gas (T5 to T6). The second source gas may be selected from gases containing ammonia (NH3), and a silicon nitride film may be formed on the substrate through this.

During the film forming process, the application of the high-frequency power is cut off, and a reducing gas can be additionally supplied. Impurities in the first source gas are reacted with or desorbed from the reducing gas by the reducing gas supplied during the film forming process, and are discharged to the outside, and the density of the thin film may be improved.

After film formation, reaction by-products and unreacted gases may be removed through a purge process (T6 to T7).

Impurities (Cl, H2) in the first source gas react with the reducing gas (H2) by at least a portion of the section including the reduction process, that is, the reducing gas supplied in the reduction process and the film formation process, inducing the formation of HCl, and purging. By removing through the processes (T4 to T5 and T6 to T7), foreign substances in the thin film can be efficiently removed and the film quality can be improved.

In addition to the reaction between the reducing gas excited by the plasma and the first source gas during the reduction process, impurities are removed, and the impurities are removed during the film formation process, the reactivity between the first and second source gases is improved, resulting in uniform and dense silicon. A nitride film may be formed.

When a thin film such as a silicon nitride film is formed on a semiconductor substrate on which a lower pattern having a high aspect ratio is formed by the method according to the present technology, a thin film having a uniform thickness can be formed over the lower part, sidewall, and upper part of the pattern. In addition, since it has excellent density and has a uniform etch rate, it is possible to sufficiently achieve the role of an underlayer film for a subsequent wiring layer or the like.

3 shows a thin film deposition method in which a film is formed by supplying a reducing gas through at least a partial section including a reduction process, for example, a whole process section after the reduction process. As shown in FIG. 3, it can be seen that the reducing gas is continuously supplied after the time point T3 at which the reduction process is started.

4 illustrates a thin film deposition method in which a reducing gas is supplied through at least a partial section including a reduction process, for example, a whole process section after the adsorption process, to form a film. As shown in FIG. 4, it can be seen that the reducing gas is continuously supplied after the time point T1 at which the adsorption process is started.

The thin film deposition method of FIGS. 3 and 4 is similar to the deposition method of FIG. 2 except for the supply pattern of the reducing gas, and thus a detailed description thereof will be omitted.

5 is a view for explaining the characteristics of the thin film according to the reducing gas supply method.

5 is a case in which only a purge gas (eg, argon gas) is supplied without supply of a reducing gas under the condition of applying plasma after the first source gas is supplied and purged (section T3 to T4 in FIGS. 2 to 4) ( a) When reducing gas (H2) is supplied to all sections except for the film formation process, and high-frequency plasma is applied along with supply of argon gas and reducing gas during the reduction process (b) and reducing gas throughout the entire section of thin film deposition The difference in the etch rate in the case (c) of applying high-frequency plasma along with supplying argon gas and reducing gas during the reduction process is shown.

After the first source gas is supplied, the reducing gas and impurities in the first source gas are desorbed through a reduction process and discharged to the outside, thereby improving reactivity with the subsequently supplied second source gas (b). However, even after impurities in the first source gas are removed through a reduction process, residual impurities may exist. During the film formation process of supplying the second source gas, the second source gas and the impurities in the first source gas may react and be partially removed, but if a reducing gas is additionally supplied, the residual impurities in the first source gas are further reacted with the reducing gas. And can be desorbed. Accordingly, the reactivity between the first source gas and the second source gas can be improved and the film quality can be made dense (c).

Therefore, as can be seen from FIG. 5, the film quality in the case of supplying the reducing gas (b, c) is denser than that in the case where the reducing gas is supplied (b, c), and furthermore, the film quality in the case of supplying the reducing gas throughout the entire process section is the most dense, and thus the etch rate You can see that it appears low.

6 is a diagram for explaining an impurity concentration in a thin film according to a reducing gas supply method.

In FIG. 6, the horizontal axis represents the depth from the surface 1 of the deposited thin film, and the vertical axis represents the concentration of impurities (Cl) in the deposited thin film.

Looking at the impurity concentration from the surface of the thin film to a significant depth (1 to 200), when comparing the thin film (a) formed without supplying reducing gas and the thin film (b) formed by supplying reducing gas to discharge impurities, It can be seen that the impurity concentration of the thin film is remarkably low when the reducing gas of the thin film is supplied.

7 is a view for explaining the uniformity and density of a thin film according to the reducing gas supply method.

In the case of supplying only a purge gas (eg, argon gas) without supply of a reducing gas under a common condition of applying plasma after the first source gas supply and purge (section T3 to T4 in FIGS. 2 to 4), the film formation process In the case of supplying reducing gas (H2) to all sections except for and applying high-frequency plasma along with supplying argon gas and reducing gas during the reduction process, and applying reducing gas throughout the entire section of thin film deposition, argon is applied during the reduction process. The difference in the deposition profile and the etch rate in the case of applying a high-frequency plasma while supplying a gas and a reducing gas is shown.

Comparing the deposition profiles, it can be seen that the step coverage (S/C=Side/TOP) characteristics are the best when the reducing gas is applied over the entire section of depositing the thin film. It can be seen that the step coverage after etching and the top/sidewall etch rates are excellent in step coverage when a reducing gas is applied throughout the entire section of thin film deposition, and a conformal and dense thin film having a uniform etch rate can be formed. .

8 is a flowchart illustrating a thin film deposition method according to an exemplary embodiment.

As described above, in the present technology, the first step (S10) in which adsorption (S101), purge (S103), and reduction (S105) are cycled once is repeated L (an integer greater than 1)-cycle, and purge (S201) and The second step (S20) in which the film formation (S203) is made into one cycle can be repeated in an M (an integer greater than or equal to 1)-cycle. Purging may be performed following the second process S20 (S30).

The first process (S10) of the L-cycle and the second process (S20) of the M-cycle may be a unit film formation cycle, and the unit film formation cycle, that is, the first process (S10) and the second process (S20) are N ( An integer greater than or equal to 1)-cycle can be repeated to form a thin film having a target thickness.

9 is a diagram for explaining characteristics of a thin film according to the number of process repetitions.

9 is a first process (S10) is performed 1-cycle, the second process (S20) is performed 1-cycle, and then the first process (S10) and the second process (S20) are repeatedly performed N-cycle. The etch rate (a) of the thin film and the unit film formation cycle in which the first process (S10) is performed with an L (an integer of 2 or more)-cycle, and the second process (S20) is performed with an M (an integer of 2 or more)-cycles are N (1). It shows the etch rate (b) of the thin film formed by performing the above integer) times. As in the case of (b), it can be seen that the thin film formed by repeating each cycle a plurality of times has excellent density and thus a low etch rate.

10 is a view for explaining the uniformity and density of a thin film according to the number of process repetitions.

FIG. 10 is a first process (S10) performing 1-cycle, a second process (S20) performing a second process (S20) 1-cycle, and then a first process (S10) and a second process (S20). A thin film formed by repeatedly performing N-cycles, and a unit film formation cycle in which the first process (S10) is performed with an L (an integer of 2 or more)-cycle, and the second process (S20) is performed with an M (an integer of 2 or more)-cycles The difference in the deposition profile and the etch rate of the thin film when formed by performing N (an integer of 1 or more) times is shown.

Since the thin film formed by repeating each cycle multiple times has a dense film quality, a thin film having a uniform thickness is formed on the upper and sidewalls of the lower pattern (SC=100.0%), and the upper and sidewall etch rates are the same (0.69) during etching. /0.69), so it can be confirmed that the thickness uniformity (SC=100.0%) after etching is guaranteed.

As such, those skilled in the art to which the present invention pertains will be able to understand that the present invention can be implemented in other specific forms without changing the technical spirit or essential features thereof. Therefore, the embodiments described above are illustrative in all respects and should be understood as non-limiting. The scope of the present invention is indicated by the claims to be described later rather than the detailed description, and all changes or modified forms derived from the meaning and scope of the claims and their equivalent concepts should be construed as being included in the scope of the present invention. do.

10: thin film deposition apparatus
100: chamber
200: gas supply unit
300: power supply and matching part

Claims (20)

A chamber in which a processing space for processing a substrate by generating plasma is formed therein;
A gas supply device for injecting a process gas into the chamber;
A susceptor installed to face the gas supply device and on which the substrate is seated; And
Includes; a power supply and matching unit connected to the gas supply device to provide a high frequency RF power,
As the substrate to be processed is provided, after a reduction process of adsorbing a first source gas on the substrate and reducing the first source gas by plasma, a second source gas is supplied to perform a film formation process, and the reduction process is performed. A thin film deposition apparatus configured to supply a reducing gas to at least a portion of the section including. The method of claim 1,
The thin film deposition apparatus configured to continuously supply the reducing gas after the reduction process. The method of claim 1,
A thin film deposition apparatus configured to supply the reducing gas during the film formation process. The method of claim 1,
The reducing gas is a thin film deposition apparatus configured to be continuously supplied after the adsorption process and the adsorption process. The method of claim 1,
A thin film deposition apparatus configured to not apply the high frequency power when the second source gas is supplied. The method of claim 1,
A thin film deposition apparatus in which the temperature in the chamber is set to 550° C. or less. The method of claim 1,
In the reduction process, the power supply and the matching unit provide a high-frequency RF power having a center frequency band of 27.12 MHz or more. The method of claim 1,
The first process including the adsorption and reduction process is repeated a preset L (an integer greater than or equal to 1) number of times, and then the second process including the film forming process is repeated a preset M (an integer greater than or equal to) number of times, and the second process is repeated. A thin film deposition apparatus configured to repeat the first process and the second process an N-th (an integer greater than or equal to 1) number of times. The method of claim 1,
The first source gas is a silicon-containing gas, and the reducing gas is a hydrogen-containing gas. The method of claim 1,
The second source gas is an ammonia-containing gas. An adsorption step of adsorbing the first source gas on a substrate by supplying a first source gas through a gas supply device above the chamber;
A reduction step of stopping supply of the first source gas and generating plasma while supplying a reducing gas to reduce the first source gas; And
And a film forming step of reacting the first source gas and the second source gas by supplying a second source gas. The method of claim 11,
In the first process including the adsorption step and the reduction step, after repeatedly performing the L-th (an integer greater than or equal to) number of times, the second process including the film forming step is repeatedly performed for the M-th (an integer greater than or equal to) number of times, and the A thin film deposition method in which the first process and the second process are repeatedly performed N times (an integer greater than or equal to 1). The method of claim 11,
The reducing gas is a thin film deposition method that is continuously supplied after the reducing step. The method of claim 11,
The film forming step is a thin film deposition method performed while supplying the reducing gas. The method of claim 11,
The reducing gas is a thin film deposition method configured to be continuously supplied after the adsorption step and the adsorption step. The method of claim 11,
A thin film deposition method for blocking the high frequency power when the second source gas is supplied. The method of claim 11,
The thin film deposition method in which the temperature in the chamber is set to 550° C. or less. The method of claim 11,
The reducing step includes generating the plasma by applying a high-frequency RF power having a center frequency band of 27.12 MHz or more. The method of claim 11,
The first source gas is a silicon-containing gas, and the reducing gas is a hydrogen-containing gas. The method of claim 11,
The second source gas is an ammonia-containing gas.

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