CN1861837A

CN1861837A - CVD device for deposit spathic silicon

Info

Publication number: CN1861837A
Application number: CNA2006100785248A
Authority: CN
Inventors: 金明洙; 金汉基; 郑锡宪
Original assignee: Samsung SDI Co Ltd
Current assignee: Samsung Display Co Ltd
Priority date: 2005-05-12
Filing date: 2006-05-08
Publication date: 2006-11-15
Also published as: JP2006319327A; KR20060117134A; KR100688837B1

Abstract

The disclosed CVD device for polycrystalline silicon sedimentation comprises: a chamber to form film on the substrate, a sprayer above the chamber to feed the reaction gas, a distributor with holes to distribute reaction gas, a catalyst hot wire unit to heat and decompose the injected gas, a chuck to arrange the substrate, some release holes, and a barrier wall as the chamber side. This invention can improve production capability and quality.

Description

CVD apparatus for depositing polycrystalline silicon

This application claims the benefit of korean patent application No. 2005-39926 filed by the korean intellectual property office at 12.5.2005, the disclosure of which is incorporated herein by reference.

Technical Field

The present embodiments relate to a Chemical Vapor Deposition (CVD) apparatus for depositing polycrystalline silicon, and more particularly, to a CVD apparatus for depositing polycrystalline silicon without a separate subsequent annealing process.

Background

Generally, when silicon (Si) is deposited on a glass substrate, the silicon is formed as polysilicon (P-Si) or amorphous silicon (a-Si).

Fig. 1A and 1B show the structures of polysilicon and amorphous silicon, respectively. Silicon is typically formed as polysilicon at temperatures above about 600 c and amorphous silicon at temperatures below about 600 c.

Compared with amorphous silicon, polysilicon has a stable structure and has excellent electrical, chemical and mechanical properties.

In addition, fig. 2A and 2B show a driver connection structure depending on the performance of silicon in a Thin Film Transistor (TFT). In the case of using amorphous silicon, a Printed Circuit Board (PCB) or an Integrated Circuit (IC) as a driver should be connected to the TFT, respectively, as shown in fig. 2B. On the other hand, when polysilicon is used, the driver can be provided integrally with the TFT as shown in fig. 2A, so that the size of the driver can be reduced.

Fig. 3 shows a TFT structure for driving a display device using polysilicon, in which a polysilicon region is shown by a dotted line. The polysilicon region is a region through which an on/off operation of the TFT is determined, for example, through which electrons and holes are transferred. In other words, the polysilicon region is related to TFT performance. Polysilicon TFTs generally have a more stable structure than amorphous silicon TFTs. Accordingly, the polysilicon TFT has advantages in that the TFT is rapidly operated due to high field effect mobility (cm/Vs) and can be driven with a low voltage. In addition, the structural stability of polysilicon enables uniform electrical properties to be obtained when fabricating TFTs, thereby eliminating the need for additional compensation circuitry.

As described above, devices using polysilicon have many advantages. However, a temperature higher than about 600 ℃ is required to form the polycrystalline silicon, and thus, the glass substrate cannot maintain its shape at this temperature. Therefore, a new method is required to apply polysilicon to a display device using glass as a substrate.

Recently, a Low Temperature Polysilicon (LTPS) method has been widely used to form TFTs at a low temperature. In the LTPS method, an amorphous silicon layer is deposited on a substrate at a relatively low temperature of less than about 450 ℃, and then laser and thermal annealing are used, so that the amorphous silicon layer is converted into polycrystalline silicon. That is, the LTPS method includes a dehydrogenation process and a laser (thermal) annealing process after forming an amorphous silicon layer.

However, such a conventional LTPS method has disadvantages in that the process is complicated and takes a lot of time to complete the process. In addition, the conventional LTPS method requires expensive equipment including a laser, etc.,thereby increasing the production cost of the display device and reducing its competitiveness. However, most displays are manufactured using the LTPS method. Therefore, a new method of forming polysilicon without using a laser is required.

Examples of the method not using a laser include a Sequential Lateral Solidification (SLS) method, a Metal Induced Crystallization (MIC) method, a Super Grain Silicon (SGS) method, and the like. However, these methods require other processes in addition to the deposition process, so that there are many problems in mass production using these methods.

Other methods of depositing silicon include Physical Vapor Deposition (PVD) and Chemical Vapor Deposition (CVD). The CVD method is widely used because it has good step coverage property when forming a thin film. In the CVD method, gaseous raw materials are decomposed and then reacted, thereby depositing a thin film. The silicon source gas used in the CVD method includes SiH₄、Si₂H₆、SiH₂Cl₂And the like. Generally, most of the feed gas is mixed with H₂、N₂And the like.

Among CVD methods for depositing silicon, a Plasma Enhanced Chemical Vapor Deposition (PECVD) method and a thermal CVD method are generally used. In PECVD, a plasma is used to decompose a raw material gas, thereby forming a thin film on a substrate. In thermal CVD, a raw material gas is decomposed using heat, thereby forming a thin film on a substrate.

Since the PECVD method uses plasma, the method can be applied to displays, solar cells, sensors, and the like, which are used at low temperature as a glass substrate. On the other hand, the thermal CVD method is performed at a relatively high temperature, so the method is widely used in the field of using Si or a metal substrate, or the like.

When silicon is deposited by thePECVD method, a source gas is deposited as SiHn on the substrate according to the following [ reaction equation 1], and a thin film is formed.

[ equation 1]

Deposited as SiHn on the substrate⁺Is bonded to another adjacent silicon atom, resulting in the silicon atom being formed as polysilicon. Therefore, the thin film formed by this method is not only poor in crystallinity but also increased in hydrogen content. The Si — H bonds in the thin film are easily separated by external energy, but this deteriorates the reliability of the device. In particular, this problem has a great influence on an Organic Light Emitting Diode (OLED), a Field Emission Display (FED), a solar cell, and the like, which utilize the optical properties of the material thereof.

As an example of an apparatus for forming a thin film on a substrate, there is a catalytic CVD apparatus that decomposes a reaction gas to be deposited using a catalyst.

Fig. 4 and 5 are sectional views showing a conventional catalytic CVD apparatus using a catalyst.

As shown in fig. 4, the CVD apparatus includes: a showerhead 12 disposed at an upper portion of the chamber 10 and injecting a reaction gas onto the substrate 18; a distributor 14 for uniformly distributing the reaction gas; a catalyst hot wire unit 16 for generating high temperature heat to heat and decompose the injected reaction gas; a chuck 19 on which the substrate 18 is mounted; the discharge holes 11 are used to discharge the reaction gas.

The dispenser 14 is formed in a plate-like shape, and is formed with a plurality of discharge holes 14a arranged at regular intervals. Therefore, the reaction gas injected through the showerhead 12 passes throughthe distribution holes 14a and is injected with a uniform distribution density.

The catalyst hot wire unit 16 includes a hot wire 16a, and the hot wire 16a is heated to a high temperature by a power supply. The hot wire 16a is typically made of tungsten. Referring to fig. 4, a catalyst hot wire unit 16 is installed in the chamber 10.

Therefore, in the conventional catalytic CVD apparatus, the reaction gas is introduced into the chamber 10 from the showerhead 12 through the distribution holes 14a of the distributor 14, so that the reaction gas is injected onto the substrate 18 with a uniform distribution density.

The reaction gas passing through the distributor 14 is converted into ions or radicals (radial) while passing through the high temperature hot wire 16a of the catalyst hot wire unit 16.

The reaction gas passing through the catalyst hot-wire unit 16 is completely converted into ions or radicals, which chemically react and physically change on the substrate 18, thereby completing deposition on the substrate 18.

Meanwhile, fig. 5 shows a CVD apparatus in which a catalyst hot wire unit and a shower head are integrated. This CVD apparatus includes: a showerhead 22 disposed at an upper portion of the chamber 20 and injecting a reaction gas onto the substrate 28; a distributor 24 for uniformly distributing the reaction gas; a catalyst hot wire unit 26 generating high temperature heat to heat and decompose the injected reaction gas; a chuck 29 on which the substrate 28 is mounted; the discharge holes 21 are used to discharge the reaction gas.

The catalyst hot wire unit 26 is installed on the spray head 22 and includes a hot wire 26a located below the distribution holes 24a of the distributor 24.

In the CVD apparatus having such a structure as shown in fig. 5, the reaction gas passing through the distributor 24 is converted into ions or radicals by the high-temperature catalyst hot-wire unit 26, and the ions or radicals undergo chemical reaction and physical change on the substrate 28, thereby completing deposition on the substrate 28.

However, such a conventional catalytic CVD apparatus using a catalyst has three problems due to a reaction between the catalyst and a reaction gas: catalyst degradation, particle generation, limiting process parameters.

First, the catalyst deterioration is as follows.

When the catalyst hot wire is heated to a high temperature, the bent region or the power receiving region of the catalyst hot wire has a relatively low temperature. When the process is performed in this state, the catalyst in these regions reacts with the reaction gas and produces tungsten silicide (WSi)₂). Resulting tungsten silicide (WSi)₂) Electrically different from tungsten, causing a localized exotherm as the temperature of the catalyst increases. In addition, tungsten silicide (WSi) is produced₂) Harder than tungsten, so that the thin film is easily broken due to external impact, thereby reducing the reliability of the system.

Second, particles are produced as described below.

Raw material gas (SiH)₄) Deposited on all surfaces of the chamber, as well as on the substrate. In the case where the adhesion between the walls of the chamber and the silicon thin film is weak, the deposited silicon thin film becomes particles as the process proceeds, thereby having an influence on the process. By reaction with H contained in the process gas₂The gas reacts to produce particles. To prevent particle generation, adhesion should be improved. For example, adhesion may be increased by heating the walls of the chamber at a temperature above 100 ℃,typically above 300 ℃. However, in the conventional CVD apparatus, the walls of the chamber are in contact with the outside, so that it is impossible to heat the walls at such a high temperature.

Third, the process parameters are limited as follows.

In a CVD apparatus using a catalyst, the quality and structure of the catalyst are very important. That is, the uniformity and characteristics of the thin film can vary depending on the distance between the showerhead and the catalyst. However, in such a CVD apparatus, the distance between the showerhead and the catalyst cannot be adjusted, thereby limiting the improvement in the performance of the thin film.

In the CVD apparatus shown in fig. 5, the catalyst hot-wire unit is integrated with the shower head, and a control element for supplying power to the shower head and the catalyst is required. However, this structure is complicated to manufacture.

In addition, there is a problem in that the showerhead is changed in order to change the structure of the catalyst. Therefore, it is difficult to apply such a limitation of property improvement to a large-sized display requiring a thin film excellent in performance and thickness uniformity.

Disclosure of Invention

Accordingly, it is an aspect of the present embodiment to provide a CVD apparatus for depositing polysilicon using a glass substrate without a separate subsequent annealing process.

It is another aspect of the present embodiment to provide a CVD apparatus in which catalyst deterioration is prevented, particle generation is minimized, and process parameters are not defined.

It is still another aspect of the present embodiment to provide a CVD apparatus for forming a silicon thin film of high quality and low hydrogen content.

The foregoing and/or other aspects of the present embodiment are achieved by providing a CVD apparatus comprising: a chamber having walls and configured to form a thin film on a substrate; a showerhead positioned at an upper portion of the chamber and configured to inject a reaction gas onto the substrate; a distributor formed with distribution holes located downstream of the reaction gas injected from the showerhead into the chamber and between the showerhead and the substrate, configured to uniformly distribute the reaction gas; a catalyst hot wire unit for heating and decomposing the reaction gas injected through the distribution holes of the distributor; a substrate; a chuck positioned below the showerhead and the catalyst hot wire unit in the chamber, the substrate being mounted on the chuck; one or more discharge holes for discharging the reaction gas; and a shield wall provided as a sidewall of the chamber and formed with a heater for suppressing generation of particles.

According to another aspect of the embodiment, the catalyst hot wire unit includes a gas passage through which the purge gas is introduced, such that the reaction gas and the purge gas are separately introduced into the chamber. Here, the gas channel is transversely formed to supply purge gas to the joint portion of the catalyst hot wire unit.

According to an aspect of the embodiment, the CVD apparatus further comprises a channel for supplying purge gas between the inner wall of the chamber and the shield wall, wherein the channel is formed transversely with respect to the showerhead so that the purge gas flows along the inner wall of the chamber, and the purge gas is discharged through the exhaust channel.

According to an aspect of the embodiment, the heater includes a hot wire inside the shield wall.

According to an aspect of the embodiment, the purge gas comprises H₂、Ar、N₂And He.

Other aspects of the present embodiment are achieved by providing a CVD apparatus comprising: a chamber having walls and configured to form a thin film on a substrate; a showerhead positioned at an upper portion of the chamber and configured to inject a reaction gas onto the substrate; a distributor formed with distribution holes located downstream of the reaction gas injected from the showerhead into the chamber, between the showerhead and the substrate, and configured to uniformly distribute the reaction gas; a catalyst hot wire unit for heating and decomposing the reaction gas injected through the distributor; a chuck on which a substrate is formed, the chuck being located below the spray head and the catalyst hot wire unit; one or more discharge holes for discharging the reaction gas; and a gas passage in the catalyst hot wire unit, the gas passage allowing the reaction gas and the purge gas to be separately introduced into the chamber.

Drawings

These and/or other aspects and advantages of the embodiments will become apparent and more readily appreciated from the following description of the preferred embodiments, taken in conjunction with the accompanying drawings of which:

FIGS. 1A and 1B show the structures of polysilicon and amorphous silicon, respectively;

fig. 2A and 2B illustrate a driver connection structure depending on the property of silicon in a TFT;

fig. 3 is a cross-sectional view of a TFT structure for driving a display device using polycrystalline silicon;

FIG. 4 is a sectional view of a conventional CVD apparatus using a catalyst;

FIG. 5 is a sectional view of still another conventional CVD apparatus using a catalyst;

FIG. 6 is a cross-sectional view of a CVD apparatus according to an embodiment;

fig. 7 is a sectional view showing gas flows in the CVD apparatus according to the embodiment.

Detailed Description

Hereinafter, preferred embodiments will be described with reference to the accompanying drawings, which are provided to enable those skilled in the art to easily understand the preferred embodiments.

Fig. 6 is a cross-sectional view of a CVD apparatus according to an embodiment. The CVD apparatus according to the embodiment includes: a chamber 30 in which a thin film is formed on a substrate 38; a showerhead 32 located at an upper portion of the chamber 30 for injecting a reaction gas onto the substrate 38; a distributor 34 formed with distribution holes 34a for uniformly distributing the reaction gas; a catalyst hot wire unit 36 for heating the reaction gas injected through the distribution holes 34a of the distributor 34 and converting the reaction gas into ions and radicals; a chuck 39 on which the substrate 38 is mounted; the discharge holes 31 for discharging the reaction gas have the same structure as that of the conventional CVD apparatus.

Here, the CVD apparatus according to an embodiment further includes a shielding wall (shielding wall)42 which serves as a side wall of the chamber 30 and suppresses particle generation.

The shield wall 42 prevents the silicon thin film deposited on the inner wall of the chamber 30 from generating particles. To suppress particle generation, the shield wall 42 is heated at a temperature higher than about 100 ℃, and preferably, the shield wall 42 is heated at a temperature higher than about 300 ℃.

According to an embodiment, the shield wall 42 is provided with a high temperature heater. As an example of the heater, the high temperature hot wire 42a may be disposed inside the shielding wall 42.

When power is supplied to the hot wire 42a, the hot wire 42a is heated. According to an embodiment, the hot wire 42a can raise the temperature of the shield wall 42 to about 100 ℃ to about 400 ℃, thereby minimizing particle generation in the process.

That is, since the shield wall 42 is provided in the chamber 30, when the converted raw material gas such as SiH is supplied₄At the time of deposition, the source gas is deposited on the inner surface of the shield wall 42. In many embodiments, since the shield wall 42 is heated by the hot wire 42a at a temperature of about 400 ℃, the adhesion between the shield wall 42 and the deposited silicon thin film is increased, thereby preventing the silicon thin film from becoming particles. Thus, particle generation is minimized.

Meanwhile, the catalyst hot wire unit 36 is formed with a gas passage 36b through which the purge gas is introduced through the passage 36 b.

The gas channel 36b can be formed transversely in the catalyst hot wire unit 36 so as to supply purge gas to the joint portion (see "a" in fig. 6) of the catalyst hot wire 36 a.

Here, the purge gas includes, for example, H₂、Ar、N₂And He, etc. In one embodiment, H₂Used as purge gas, but the embodiment is not limited to H₂. Alternatively, other gases may be used as purge gases.

According to an embodiment, the reactive gas is injected through the distribution holes 34a and the purge gas is injected through the gas passages 36b, so that the reactive gas and the purge gas are separately and individually injected into the chamber 30 (see fig. 7).

This structure prevents the reaction gas and the purge gas from being diluted and from having a negative influence on the process. In addition, purge gas is injected into the chamber 30 from the side of the catalyst hot wire unit 36, thereby preventing reaction gas from reacting at a low temperature portion, for example, at a bonding portion (see "a" in fig. 6) of the catalyst hot wire 36a of the catalyst hot wire unit 36, thereby preventing catalyst degradation.

In addition, a passage 44 is formed between the inner wall of the chamber 30 and the shield wall 42 to supply purge gas.

The passage 44 can be formed transversely with respect to the showerhead 32 so that purge gas injected through the passage 44 flows along the inner wall of the chamber 30 and is discharged through the discharge holes 31. In addition, the passage 44 communicates with the distributor 34. Also, the channel 44 has a structure that allows purge gas to flow through the catalyst hot wire unit 36 and along the inner wall of the chamber 30.

Accordingly, when the purge gas injected from the channel 44 flows along the inner wall of the chamber 30, the non-heated wall of the chamber 30 prevents the deposition of a thin film thereon, thereby preventing particle generation.

The operation of the CVD apparatus having such a structure according to the embodiment is as follows.

The reaction gas is introduced into the chamber 30 from the showerhead 32 through the distribution holes 34a of the distributor 34.

The reaction gas passing through the distributor 34 is converted into ions or radicals by the high-temperature catalyst hot wire 36a of the catalyst hot wire unit 36.

The reaction gas passing through the catalyst hot wire 36a is completely converted into ions or radicals. Here, ions or radicals undergo chemical reactions and physical changes and are then deposited on the substrate 38.

In the CVD apparatus according to another embodiment, the shield wall 42 installed in the chamber 30 is controlled to have a substantially constant temperature so as to pass a process gas such as SiH₄To improve adhesion of the silicon thin film deposited on the substrate. The increase in adhesion of the silicon thin film deposited on the shield wall 42 due to reaction and etching effects in the process suppresses generation of particles from the thin film. Preferably, the shield walls 42 installed in the chamber 30 are controlled to have a substantially constant temperature below about 400 ℃.

This configuration overcomes the limitation that the temperature of the walls of the chamber 30 adjacent the exterior cannot be increased to about 400 c.

In addition, when the chamber 30 is cleaned, the temperature of the shielding walls 42 is controlled to maximize the cleaning effect.

In the CVD apparatus using a catalyst according to the embodiment, the reaction gas collides with the catalyst hot-wire unit 36 heated at a high temperature, so that Si completely decomposed according to the following reaction equation 2 is deposited on the substrate.

Reaction equation 2:

therefore, Si is easily bonded to adjacent silicon atoms, so that the thin film has good crystallinity and the hydrogen content is reduced.

The conventional LTPS method using a laser additionally includes a dehydrogenation process, as compared to the process of the CVD apparatususing a catalyst according to an embodiment.

The conventional LTPS method has disadvantages in that the process is complicated and it takes much time to complete the process. In addition, the conventional LTPS method requires expensive equipment such as a laser and a CVD system for deposition and dehydrogenation. Therefore, the conventional LTPS method consumes a large amount of investment and maintenance expenses, thereby increasing the production cost of the apparatus and thus reducing price competitiveness.

On the other hand, as shown in fig. 6, the CVD apparatus using a catalyst of the present embodiment can directly form polycrystalline silicon without additional equipment, thereby significantly reducing process time and process cost.

According to an embodiment, the process is performed at a temperature below about 400 ℃, such that glass may be used as the substrate.

According to the embodiment, the device manufacturing technology is unique, so that the polysilicon deposition technology and the deposition method thereof are unique, thereby ensuring the competitiveness of the device.

According to the embodiment, even if glass is used as a substrate, polysilicon can be formed without a process such as an annealing process.

In addition, catalyst deterioration is prevented in the process, particle generation is minimized, and thus yield is increased.

Also, the resulting film has good crystallinity and reduced hydrogen content.

In addition, the time period between preventive maintenance required for the chamber is extended, thereby improving productivity.

While the present embodiments have been described in connection with certain exemplary embodiments, it will be understood by those skilled in the art that the present embodiments are not limited to the disclosed embodiments, but, on the contrary, are intended to cover various modifications within the spirit and scope of the appended claims and their equivalents.

Claims

1. A chemical vapor deposition apparatus comprising:

a chamber having walls;

a showerhead positioned at an upper portion of the chamber and configured to inject a reaction gas onto a substrate;

a distributor formed with distribution holes located downstream of the reaction gas injected from the showerhead into the chamber and between the showerhead and the substrate;

a catalyst hot wire unit configured to heat and decompose the reaction gas injected through the distribution holes of the distributor;

a chuck positioned below the showerhead and the catalyst hot wire unit in the chamber, on which the substrate is mounted;

wherein the chamber comprises one or more exhaust holes provided to exhaust the reaction gas;

and the shielding wall is used as the indoor side wall and is positioned between the wall of the chamber and the chuck.

2. The chemical vapor deposition apparatus of claim 1, wherein the shield wall comprises a heater configured to inhibit particle generation.

3. The chemical vapor deposition apparatus of claim 1, wherein the catalyst hot-wire unit further comprises a gas passage through which a purge gas is introduced into the chamber separately from a reaction gas.

4.The chemical vapor deposition apparatus according to claim 3, wherein the gas channel is formed laterally to supply the purge gas to a combined portion of the catalyst hot wires of the catalyst hot wire unit.

5. The chemical vapor deposition apparatus of claim 1, further comprising a passage between an inner wall of the chamber and the shield wall.

6. The chemical vapor deposition apparatus of claim 5, wherein the channel is formed laterally with respect to the showerhead such that purge gas flows along an inner wall of the chamber; discharging the purge gas through the discharge holes.

7. The chemical vapor deposition apparatus of claim 3, further comprising a passage between an inner wall of the chamber and the shield wall.

8. The chemical vapor deposition apparatus of claim 7, wherein the channel is formed laterally with respect to the showerhead and a purge gas is flowed along an inner wall of the chamber, the purge gas being discharged through the discharge holes.

9. The chemical vapor deposition apparatus of claim 2, wherein the heater comprises a hot wire positioned within the shield wall.

10. The chemical vapor deposition apparatus of claim 2, wherein the shield wall is heated by the heater to a temperature of from about 100 ℃ to about 400 ℃.

11. The chemical vapor deposition apparatus of claim 7, wherein the shield wall comprises a heater, and the shield wall is heated by the heater to a temperature from about 100 ℃ to about 400 ℃.

12. The chemical vapor deposition apparatus of claim 3, wherein the purge gas comprises H₂、Ar、N₂And one of HeOr a plurality thereof.

13. A chemical vapor deposition apparatus comprising:

a chamber having walls;

wherein the chamber comprises one or more exhaust holes provided to exhaust the reactant gases;

a gas channel in the catalyst hot wire unit, the gas channel introducing a purge gas into the chamber separately from the reaction gas.

14. The chemical vapor deposition apparatus according to claim 13, wherein the gas channel is laterally formed to supply the purge gas to a combined portion of the catalyst hot wires of the catalyst hot wire unit.

15. The chemical vapor deposition apparatus of claim 13, further comprising a channel for supplying the purge gas.

16. The chemical vapor deposition apparatus of claim 15, wherein the channel is formed laterally with respect to the showerhead such that the purge gas flows along an inner wall of the chamber and the purge gas is discharged through the discharge holes.

17. The chemical vapor deposition apparatus of claim 14, further comprising a channel for supplying a purge gas.

18. The chemical vapor deposition apparatus of claim 17, wherein the channel is formed laterally with respect to the showerhead such that a purge gas flows along an inner wall of the chamber and the purge gas is discharged through the discharge holes.

19. The chemical vapor deposition apparatus of claim 13, further comprising a shield wall disposed transverse to the chamber and formed with a heater.

20. The chemical vapor deposition apparatus of claim 19, wherein the heater comprises a hot wire positioned within the shield wall.

21. The chemical vapor deposition apparatus of claim 19, wherein the shield wall is heated by the heater to a temperature from about 100 ℃ to about 400 ℃.

22. The chemical vapor deposition apparatus of claim 20, wherein the shield wall is heated by the heater to a temperature from about 100 ℃ to about 400 ℃.

23. The chemical vapor deposition apparatus of claim 13, wherein the purge gas comprises H₂、Ar、N₂And He.