WO2013092777A1

WO2013092777A1 - Plasma chamber apparatus and a method for cleaning a chamber

Info

Publication number: WO2013092777A1
Application number: PCT/EP2012/076252
Authority: WO
Inventors: Marcello Riva; Reiner Fischer; Gerd Walther
Original assignee: Solvay Sa
Priority date: 2011-12-22
Filing date: 2012-12-19
Publication date: 2013-06-27

Abstract

A plasma apparatus is described comprising a treatment chamber and a solid body inside the treatment chamber which apparatus further comprises means to insert at least one cleaning electrode inside the treatment chamber which cleaning electrode is suitable for plasma-supported chamber cleaning and has a lower surface than the removable deposition electrode, and which apparatus, inside the chamber, contains either at least one removable deposition electrode suitable for providing a plasma, or at least one cleaning electrode. A method to remove deposits using such a chamber is also described.

Description

Plasma Chamber Apparatus And A Method For Cleaning A Chamber

The present invention claims benefit of European patent application N° 11195331.1 filed December 22, 2011 the whole content of which is incorporated herein by reference for all purposes.

The present invention relates to a plasma chamber apparatus and a method for cleaning the chamber.

Plasma apparatus comprising treatment chambers are used in the semiconductor and photovoltaic industry to manufacture semiconductors, flat panel displays or photovoltaic elements. The manufacture generally comprises operations such as etching or chemical vapor deposition of a substrate which, during the treatment, is typically located on a support provided inside the treatment chamber.

During the manufacturing steps, in particular during chemical vapor deposition steps, materials are generally deposited not only on the substrate but also on interior parts of the chamber such as the chamber walls and counter electrodes. In order to prevent contamination problems during subsequent manufacturing runs, such materials are suitably removed.

EP-A-1138802 discloses that amorphous silicon deposited on inside parts of a treatment chamber can be cleaned thermally with fluorine as cleaning gas. This reference also teaches that silicon oxide or silicon nitride cannot be removed by this method.

The removal of deposits especially of plasma apparatus for the

manufacture of photovoltaic cells is difficult because the chambers of these apparatus have a large size, as have the electrodes. The chamber may for example have an inner volume of up to 6 m³ and even more. Accordingly, apparatus with such large chambers contain planar electrodes of remarkable size. They may have a surface of up to 3 m² and even more. These electrodes have larger power consumption per area, e.g. 5 to 15 kW per m², and they are used to provide plasma both the gases used to etch or coat the surface of the items to be treated and for exciting the etching gas to clean the surface of solid bodies inside the chamber. Nevertheless, due to the large electrode surface, the cleaning power of the plasma is not very high. The present invention now makes available, in a first aspect of the invention, in particular a plasma apparatus which allows effective removal of deposits on solid bodies inside the treatment chamber. The present invention, in a second aspect of the invention, also provides an efficient chamber cleaning process.

According to the first aspect of the present invention, a plasma

apparatus (1) is provided suitable to be operated with two different types of electrodes, namely with at least one deposition electrode and at least one cleaning electrode, which apparatus (1) comprises a solid body (3) inside the treatment chamber (2) which apparatus (1) further comprises means (5) to insert at least one cleaning electrode (6) inside the treatment chamber (2) which cleaning electrode (6) is suitable for plasma-supported chamber cleaning and has a lower surface than the at least one removable deposition electrode (4), and which apparatus (1), inside the chamber (2), contains either at least one removable deposition electrode (4) suitable for providing a plasma, or at least one cleaning electrode (6).

The plasma apparatus (1) is preferably a plasma apparatus for the manufacture of photovoltaic cells. Consequently, the chamber (2) preferably has an inner volume of up to 6 m³ and even more. Often, the inner volume is equal to or greater than 2 m³. Preferably, the apparatus (1) contains at least one, preferably one, large planar deposition electrode (4). The may have a surface of up to 3 m² and even more. Often, the surface of the deposition electrode (4) is equal to or greater than lm². These deposition electrodes have larger power consumption per area, e.g. 5 to 15 kW per m².

The term "solid body" includes the inside walls of the apparatus.

Brief description of the drawings

Figure 1 shows a plasma apparatus (1) of the present invention having a removable planar deposition electrode (4) and means, namely openings (5), for inserting plasma rods (6) to perform the chamber cleaning.

Figure 2 shows the apparatus (1) wherein the planar deposition

electrode (4) is removed along with wafer (9), and plasma rods (6) are inserted. Detailed description of the invention

The apparatus of the present invention operates preferably according to an inductively coupled plasma (ICP). Such apparatus usually have either a planar electrode or a cylindrical one ; preferably, the apparatus of the present invention has a planar electrode. As mentioned above, these electrodes provide a high- density plasma, e.g. with an electron density from 10¹³ to 10¹⁶ cm^-3. As mentioned above, surfaces inside the treatment chamber, for example, walls, supports, lines or chucks are coated with undesired deposits during plasma deposition of layers on the items to be treated. Apparatus designed for the manufacture of solar panels have large sizes, as mentioned above ; they may have a volume up to 6 m³ and even more.

The apparatus of the invention preferably contains a planar electrode which can be removed from the apparatus during the chamber cleaning. To provide plasma, at least one, but preferably 2 or more plasma sources preferably in cylindrical form, especially in cylindrical form, with a lower surface area than that of planar electrode 4, are inserted into the plasma chamber of the apparatus. Often, 2, 3, 4, 5 or 6 or even more plasma sources, preferably plasma rods, with comparably low surface are inserted into the chamber. The advantage is that these sources with a comparatively low surface provide very efficient plasma even though the power is much lower than that used for the removed planar electrode. Often, operation of the plasma sources with 1 to 2 kW is sufficient to provide a very effective chamber cleaning.

Preferably, the cleaning electrode or electrodes are providing microwave plasma. Often, the electromagnetic frequency is equal to or greater

than 500 MHz. Preferably, the frequency is in the range of from 1 to 5 GHz.

The invention will now be described in detail in view of figures 1 and 2 which represent a preferred embodiment. It has to be noted that figure 1 and figure 2 are designed to operate with 3 power sources for chamber cleaning. Often, even 2 plasma sources provide fully satisfactory results as to chamber cleaning. Any openings not used for a power source can of course be sealed air tight by respective seals.

Figure 1 shows an apparatus 1 which is used for the manufacture of solar panels. During the manufacture, layers are applied via a CVD process inside chamber 2. a wafer (9) is carried by a support (3) which, if desired, may be rotating during deposition. A planar electrode (4) is located above and parallel to the wafer. The planar electrode (4) may have an area of 2 to 4 m², but may be even larger. A flexible coaxial cable (7) is connected with a microwave generator not shown. Openings (5) (the drawing contains three openings on opposite walls of the chamber (2), but it could comprise one pair of openings instead of three, or it could be two pairs of openings instead of three, or it could even be 4, 5, 6 or more pairs) are sealed during the deposition process to allow the application of a vacuum through line (8). Through line (6), etching gas or gas providing a coating under CVD conditions, e.g. silane gas or silane gas mixed with H₂, can be introduced into the chamber (2). After the deposition of the desired layer on wafer (9), the chamber (2) is evacuated, inert gas is introduced, and a fresh wafer is substituted for the treated wafer (9). After the treatment of a certain number of solar panel wafers, e.g. 10 to 25, the

chamber (2) must be cleaned. The planar electrode (4) is removed from the apparatus (1) together with coaxial cable (7). Seals are removed from the openings (5), and plasma rods (6) are inserted into the chamber (2).

An apparatus (1) having inserted three plasma rods (6) is shown in figure (2). The support (3) and a supporting plate (9) are visible, along with coaxial cables (7) forming the connection between a microwave generator (not shown in figure 2) and the plasma rods (6). The microwave generator may also provide an ICP inside the chamber (2) via the plasma rods (2). A frequency of 2.45 GHz (which is a commonly used microwave frequency) is very suitable. But of course, if desired, any other frequency range could be applied to generate plasma. The power imposed on the plasma rods (6) may be comparably low. For example, good cleaning results are achieved when a power as low as 1 to 1.5 kW are imposed on each of the plasma rods (6). This is very low if compared to e.g. 30 kW provided to the planar electrode (4).

Accordingly, the apparatus of the invention allows for effective cleaning in large plasma supported CVD chambers with a very low power consumption.

It has to be noted that figures 1 and 2 show an apparatus considered especially effective which easily may be modified, for example, by providing 1 , 2, 4 or more plasma rods or by different arrangement of the plasma rods inside the chamber, e.g. they need not be arranged in parallel.

A preferred plasma apparatus (1) comprises a deposition electrode (4) which has a power consumption of equal to or greater than 5 kW per m² and the at least one cleaning electrode (6) has a power consumption of equal to or greater than 0.5 kW per m² with the proviso that the power consumption per m² of the cleaning electrode (6) is lower than the power consumption of the deposition electrode (4).

The inside walls and any solid body, e.g. the support (3), in the plasma apparatus (1) preferably comprise or consist of a material selected from aluminum, aluminum alloy, stainless steel, ceramics, e.g. SiC. Another aspect of the present invention concerns an improved method for the removal of deposits in a plasma treatment chamber, especially a CVD apparatus. Preferably, the plasma treatment chamber is used for the manufacture of semiconductors, TFTs and solar panels, and especially for solar panels.

The method of the present invention relates to the removal of deposits from inside parts of a treatment chamber of a plasma apparatus which apparatus comprises at least one removable deposition electrode suitable for providing a plasma which apparatus further comprises means to insert at least one cleaning electrode for plasma supported wherein the at least one cleaning electrode is located inside the chamber comprising a deposit-removing step wherein the deposition electrode is removed from the treatment chamber, at least one cleaning electrode is inserted into the apparatus, a cleaning gas is introduced into the treatment chamber, plasma is provided by the at least one cleaning electrode, and deposits from inside parts of the treatment chamber are removed.

Preferably, the method of the invention is performed using a plasma apparatus as described above.

The means to insert the at least one cleaning electrode are preferably openings which are sealed during the CVD process to achieve a suitable vacuum. The seals are removed, of course, before the cleaning electrode(s) is (are) introduced to provide a plasma during the deposit-removing step

Preferably, the plasma applied during the deposit removal is an inductively coupled plasma. The deposition electrode preferably is a planar electrode which is also providing an inductively coupled plasma. The at least one cleaning electrode is preferably operated with electromagnetic microwave frequency. The frequency preferably is equal to or greater than 500 MHz. Especially preferably, the frequency to operate the at least one cleaning electrode is in the range of from 1 to 5 GHz. Preferred features, especially sizes and other parameters, e.g. applied power, are those given above.

Preferred cleaning gases are F₂ and COF₂ ; F₂ is especially preferred.

In the present invention, molecular fluorine (F₂) is used as an essential component of the gas.

According to one, preferred, embodiment, the gas consists or consists essentially of F₂. According to another embodiment of the invention, the gas consists or consists essentially of COF₂. The term "essentially" preferably denotes that the gas consists of equal to or more than 99 % by volume of F₂ or COF₂. In another aspect, a mixture comprising molecular fluorine and e.g. an inert gas, such as nitrogen, argon, xenon or mixtures thereof, in particular mixtures of nitrogen, argon and molecular fluorine, is used. In this case, the content of molecular fluorine in the mixture is typically equal to or less than 50 % molar. Preferably, this content is equal to or less than 20 % molar. Suitable mixtures are disclosed for example in WO 2007/116033 in the name of the applicant, the entire content of which is incorporated by reference into the present patent application.

According to one embodiment, gas mixtures comprising F₂ are applied as etching gas.

Often, the etching gas consists of 1 to 99 % by volume of F₂, 0 to 70 % by volume of N₂ and 0 to 20 % by volume of Ar with the proviso that the content of at least one of N₂ and Ar is equal or greater than 1 % by volume.

In preferred embodiments, the content of F₂ is equal to or greater than 1 % by volume, and it is preferably equal to or lower than 30 % by volume, and especially preferably, it is equal to or lower than 20 % by volume. The content of Ar and/or N₂ is the balance to 100 % by volume. In ternary mixtures, the content of Ar is preferably equal to or greater than 5 % by volume, and equal to or lower than 15 % by volume, and the content of N₂ is preferably equal to or greater than 5 % by volume and equal to or lower than 70% by vol.

A particular mixture consists essentially of about 10 % vol Argon, 70 % vol nitrogen, and 20 % vol F₂.

In a particular embodiment of this aspect, the content of molecular fluorine, in the mixture with an inert gas as described above, especially in binary mixtures, is more than 50 % molar. Preferably, this content is equal to or more than 80 % molar, for example about 90 % molar. In this particular embodiment, argon is a preferred inert gas. A mixture consisting of about 90 molar % molecular fluorine and about 10 molar % argon is more particularly preferred. In this particular embodiment of this aspect, the content of molecular fluorine in the mixture with an inert gas as described above is equal to or lower than 95 % molar.

According to another embodiment, the etching gas consists of COF₂, or it comprises COF₂.

Often, the etching gas comprises or consists of 1 to 99 % by volume of COF₂, 0 to 70 % by volume of N₂ and 0 to 20 % by volume of Ar with the proviso that the content of at least one of N₂ and Ar is equal or greater than 1 % by volume. A ternary mixture comprising COF₂, Ar and N₂ may be applied. The content of COF₂ is equal to or greater than 1 % by volume, and it is preferably equal to or lower than 30 % by volume, and especially preferably, it is equal to or lower than 20 % by volume. The content of Ar and/or N₂ is the balance to 100 % by volume. In quaternary mixtures, the content of Ar is preferably equal to or greater than 5 % by volume, and equal to or lower than 15 % by volume, and the content of N₂ is preferably equal to or greater than 5 % by volume and equal to or lower than 70 % by volume. Neat COF₂ is preferred over mixtures, be they binary or ternary, which comprise COF₂.

A particularly preferred mixture consists essentially of about 10 % vol

Argon, 70 % vol nitrogen, and 20 % vol F₂.

Molecular fluorine for use in the present invention can be produced for example by heating suitable fluorometallates such as fluoronickelate or manganese tetrafluoride. Preferably, the molecular fluorine is produced by electrolysis of a molten salt electrolyte, in particular a potassium

fluoride/hydrogen fluoride electrolyte, most preferably KF.2HF.

Preferably, purified molecular fluorine is used in the present invention. Purification operations which are suitable to obtain purified molecular fluorine for use in the invention include removal of particles, for example by filtering or absorption and removal of starting materials, in particular HF, for example by absorption, and impurities such as in particular CF₄ and 0₂. Typically, the HF content in molecular fluorine used in the present invention is less

than 10 ppm molar. Typically, the fluorine used in the present invention contains at least 0.1 molar ppm HF.

In a preferred embodiment, purified molecular fluorine for use in the present invention is obtained by a process comprising (a) electrolysis of a molten salt, in particular as described above, to provide crude molecular fluorine containing HF, particles and optional impurities ;

(b) an operation to reduce the HF content relative to the HF content of crude molecular fluorine, comprising for example an adsorption on sodium fluoride and preferably reducing the HF content in the molecular fluorine to the values mentioned here before ;

(c) an operation to reduce the particle content relative to the particle content of crude molecular fluorine, comprising for example passing a fluorine stream containing particles through a solid absorbent such as for example sodium fluoride.

The molecular fluorine, in particular produced and purified as described here before, can be supplied to the method according to the invention, for example, in a transportable container. This method of supply is preferred when mixtures of fluorine gas with an inert gas in particular as described above are used in the method according to the invention.

Alternatively, the molecular fluorine can be supplied directly from its manufacture and optional purification to the method according to the invention, for example through a gas delivery system connected both to the silicon hydride removal step and to the fluorine manufacture and/or purification. This embodiment is particularly advantageous, if the gas used in the method according to the invention consists or consists essentially of molecular fluorine.

In a preferred embodiment, the interior parts are those of a treatment chamber for manufacture of semiconductors, flat panel displays (TFTs) or photovoltaic elements (solar panels).

The method according to the invention is particularly suitable for cleaning organic, e.g. fluorosubstituted organic polymers, and inorganic deposits.

Prominent examples for inorganic deposits which can be removed according to the process of the present invention are SiON, amorphous Si, micro crystalline and crystalline Si, Si0₂, phosphosilicate glass ("PSG"), amorphous,

microcrystalline and crystalline Si hydrides, Ti , TaN or W.

Typically, inorganic deposits had been deposited on the surface of the solid body by chemical vapor deposition, especially in process chambers used for the manufacture of photovoltaic elements. Such processes are well known to the expert. Just to give an example, a referral is made to EP-A-1138802 which discloses a plasma CVD process with silane and hydrogen to form an amorphous silicon layer. Preferably in the method according to the invention, the gas pressure for the cleaning step is generally from 0.1 to 100 mbar, often from 0.2 to 10 mbar and preferably from 0.2 to 3 mbar. In the first particular embodiment of the method according to the invention, the residence time of the cleaning gas is generally from 1 to 180 s, often from 1 to 70 s and preferably from 1 to 60 s.

The power applied to generate the plasma by means of the plasma rods is preferably 0.5 to 3 kW per m².

The chamber cleaning may be supported by an additional remote plasma.

Another aspect of the invention concerns a process for the manufacture of a product selected from the group consisting of a semiconductor, a flat panel display and a solar panel wherein at least one treatment step for the manufacture of the product is carried out in a treatment chamber and an inorganic deposit is deposited on interior parts of the treatment chamber, which process comprises cleaning said interior part by the method according to the invention. Typical products are selected from a semiconductor, a flat panel display and a photovoltaic element such as a solar panel. The deposition is performed in a vacuum. The cleaning is performed as described above.

The deposit may be any metal or metal compound. It is preferably selected from the group consisting of Si, Si02, and silicon hydride.

Should the disclosure of any of the patents, patent applications, and publications that are incorporated herein by reference be in conflict with the present description to the extent that it might render a term unclear, the present description shall take precedence.

The examples here after are intended to illustrate the invention without however limiting it.

Examples

Example 1 : Plasma cleaning with F₂

The apparatus applied is a plasma CVD apparatus with an internal volume of 6 m³. The apparatus contains two pairs of openings through which 2 plasma rods can be inserted. The electrode used in the deposition step (deposition electrode) is operated according to the ICP principle, it is of the planar type, and it has a surface of approximately 3 m². The deposition electrode is removably arranged in the chamber. The openings are sealed to be air tight.

The power applied to the deposition electrode is approximately 30 kW. In the manufacture of a solar panel a chemical vapor deposition step using silane gas and H₂ and doping gases containing PH₃ is carried out to deposit a silicon containing layer on a panel substrate mounted on a support within a treatment chamber having inside walls made of aluminum alloy. Depending upon deposition conditions and concentration of reagents, it is observed that after the PECVD step, microcrystalline or amorphous Si:H deposits are present on the inside walls and on the deposition electrode of the chamber. The concentration of H in the Silicon Hydride is between 10 % and 25 % in the amorphous phase, whilst it is between 3 % and 10 % in the microcrystalline phase. After removing the panel substrate from the chamber, the deposition electrode is removed, the seals are removed from the openings for the plasma rods, and two plasma rods are inserted into the chamber ; the plasma rods are operated with a frequency of 2.45 GHz. A gas consisting essentially of F₂ is introduced at 35 slm into the chamber at a pressure of 2 mbar, and a plasma is ignited ; the power applied to the plasma rods is approximately 1.25 kW to 2.5 kW per each rod. After 3 min treatment, the microcrystalline and amorphous Si:H layer is substantially removed from the chamber walls and from the support.

Example 2 : Chamber cleaning using a gas mixture consisting of COF₂

Example 1 is repeated successfully, but using neat COF₂ as chamber cleaning gas.

Example 3 : Chamber cleaning using a F₂/N₂/Ar gas mixture

Example 1 is repeated successfully using a gas mixture consisting of 20 % by volume of F₂, 70 % by volume of N₂ and 10 % by volume of Ar.

Claims

C L A I M S

1. A plasma apparatus (1) suitable to be operated with two different types of electrodes, namely with at least one deposition electrode and at least one cleaning electrode, comprising a treatment chamber (2), a solid body (3) inside the treatment chamber (2) which apparatus (1) further comprises means (5) to insert at least one cleaning electrode (6) inside the treatment chamber (2) which cleaning electrode (6) is suitable for plasma-supported chamber cleaning and has a lower surface than the at least one removable deposition electrode (4), and which apparatus (1), inside the chamber (2), contains either at least one removable deposition electrode (4) suitable for providing a plasma, or at least one cleaning electrode (6).

2. The plasma apparatus (1) of claim 1 wherein the at least one cleaning electrode (6) provides plasma by applying a frequency of 2.45 GHz.

3. The plasma apparatus (1) of claim 1 or 2 wherein the at least one deposition electrode (4) has a planar geometry.

4. The plasma apparatus (1) of anyone of claims 1 to 3 which contains one deposition electrode (4).

5. The plasma apparatus (1) of anyone of claims 1 to 4 wherein the at least one cleaning electrode (6) has a cylindrical geometry.

6. The apparatus (1) of anyone of claims 1 to 5 wherein the apparatus is a high-density plasma apparatus.

7. The plasma apparatus (1) of claim 6 wherein the electron density supplied by the deposition electrode (4) is from 10¹ to 10¹⁶ cm^-3.

8. The plasma apparatus (1) of anyone of claims 1 to 7 wherein the deposition electrode (4) has a power consumption of equal to or greater than 5 kW per m² and the at least one cleaning electrode (6) has a power consumption of equal to or greater than 0.5 kW per m² with the proviso that the power consumption per m² of the cleaning electrode (6) is lower than the power consumption of the deposition electrode (4).

9. The plasma apparatus (1) according to anyone of claims 1 to 8, wherein the solid body comprises or consists of a material selected from aluminum, aluminum alloy, stainless steel and SiC.

10. A method for removing deposits from inside parts of a treatment chamber of a plasma apparatus which comprises at least one removable deposition electrode suitable for providing a plasma which apparatus further comprises means to insert at least one cleaning electrode for plasma-supported chamber cleaning wherein the at least one cleaning electrode is located inside the chamber comprising a deposit-removing step wherein the deposition electrode is removed from the treatment chamber, at least one cleaning electrode is inserted into the apparatus, a cleaning gas is introduced into the treatment chamber, plasma is provided by the at least one cleaning electrode, and deposits from inside parts of the treatment chamber are removed.

11. The method according to claim 10, wherein at least two cleaning electrodes are applied during the deposit-removing step.

12. The method according to claims 10 or 11, wherein the treatment comprises generating inductively coupled plasma from the cleaning gas.

13. The method according to anyone of claims 10 to 12, wherein the cleaning gas comprises F₂ or COF₂, preferably F₂.

14. The method of anyone of claims 10 to 13 wherein the cleaning gas further contains at least one inert gas selected from the group consisting of N₂ and Ar.

15. The method according to anyone of claims 10 to 14, wherein the gas pressure is from 0.1 to 100 mbar.

16. The method according to anyone of claims 10 to 15, wherein the power consumption of the at least one cleaning electrode is equal to or greater than 0.5 kW per m², and equal to or lower than 3 kW per m².

17. The method according to anyone of claims 10 to 16, wherein the inside parts of the apparatus comprise or consist of a material selected from aluminum, aluminum alloy, stainless steel and SiC.

18. A process for the manufacture of a product selected from the group consisting of a semiconductor, a flat panel display and a solar panel wherein at least one treatment step for the manufacture of the product is carried out in a treatment chamber and an inorganic deposit is deposited on interior parts of the treatment chamber, which process comprises cleaning said interior part by the method according to anyone of claims 10 to 17.

The process of claim 18 wherein silicon hydride is deposited.