WO2013092770A1

WO2013092770A1 - Method for removing deposits performed with varying parameters

Info

Publication number: WO2013092770A1
Application number: PCT/EP2012/076243
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 method for removing deposits from the surface of a solid body inside a plasma chamber and especially from the inner surface of a tube of a LPCVD system is described. During treatment of the deposits with an etching gas, especially F₂, the pressure is varied, especially from a higher pressure level to a lower pressure level. This allows the removal of deposits from areas close to the etching gas inlet to areas more remote from the etching gas inlet. It is especially possible to remove deposits from the inner surface of tubes over the whole length, even of tubes having a length of more than 1 m, e.g. 2 m tubes. Remote microwave sources are preferred sources to irradiate the etching gas (cleaning gas).

Description

Method for removing deposits performed with varying parameters

The present invention claims benefit of European patent application N°l 1195326.1 filed December 22, 2011 the whole content of which is incorporated herein for all purposes.

The present invention relates to a method for removing deposits which is useful particularly as a process for cleaning of treatment chambers, and especially for large plasma chambers.

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 using an etching gas or chemical vapor deposition using a CVD gas to form a treated substrate which, during the treatment, often is located on a support provided inside the treatment chamber. In other chambers, for example, low-pressure chemical vapor deposition chambers (LPCVD), wafers are introduced into the chamber arranged on a support. Often, they are assembled back-to-back such that only the other side is in contact with the deposition gas. After deposition, the support carrying the treated wafers is removed from the chamber, and another support is introduced into the chamber. The wafers often have a diameter of 200 mm up to 400 mm, and a trend to treat even larger wafers is observed.

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.

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.

WO 2011/051410 discloses the removal of silicon hydride from the surface of solid bodies.

In large LPCVD chambers, the inner walls may be constituted of large tubes which are up to 2 m long, and may be even longer, and which are suitable to treat wafers with a diameter of up to 400 mm and even more, up to 500 mm. At one end, a line is connected with a vacuum tube, the other end comprises a line for the introduction of the process gas. Plasma inside the tube can be achieved by a coil located on the outer surface of the chamber. Since, after the several deposition steps, the deposit formed on the walls must be removed to prevent damage of the wafers, habitually the tubes are removed from the chamber and are cleaned in a wet process, e.g. by contacting them with hydrofluoric acid. This is time consuming, and it is especially annoying that the process conditions (especially to obtain an optimal temperature profile during deposition) must be fine-tuned for each cleaned tube because the deposition is very sensitive in view of the process conditions. It often takes a whole working day to clean the tubes. The alternative would be to throw them away which also is not very agreeable.

The present invention now makes available in particular an efficient process for cleaning of treatment chambers and especially for the tubes of large LPCVD systems. As the term "LP"CVD indicates, the deposition in such systems is performed in a vacuum.

The invention concerns in consequence a method for removing deposits from the surface of a solid body inside a plasma chamber which comprises treating the deposits with an etching gas which method comprises at least one step wherein the etching gas is activated by a plasma and wherein at least partially during performing the at least one treatment step, the pressure inside the plasma chamber is varied ; and the etching gas comprises or consists of F₂ or COF₂. The variation of the pressure serves to imply different gas velocity to the etching gas ; consequently, the etching gas will pass through a shorter or longer space in a state of activation and thus can act on deposits closer or remote from the entry of the plasma-activated etching gas in the chamber or tube. To be noted that, as mentioned above, tubes with a length of 2 m and more are used for LPCVD processes.

A typical method to generate the plasma comprises exposing the etching gas to a high-frequency electrical field.

In a first aspect of the first particular embodiment, the frequency of the generated field is from 10 to 15 MHz. A typical frequency is 13.56 MHz.

In a second aspect of the first particular embodiment, the frequency of the generated field is from 40 to 100 MHz, preferably 40 to 80 MHz. A typical frequency is selected from 40 MHz and 60 MHz.

In a third aspect of the invention, frequency in the upper MHz range is applied, e.g. radiation having a frequency which is greater than 500 MHz. Preferably, a frequency is applied in the range of from 1 to 5 GHz. A microwave frequency of 2.45 GHz is especially suitable ; this is the frequency also applied in microwave ovens. The microwave source is a remote source, connected to the entrance of the treatment chamber. In proximity of the microwave source, the cleaning gas becomes dissociated. The reactive species are passed into the treatment chamber and perform the cleaning. It is assumed that often, the radicals are reactive for approximately 0.5 to 0.7 sec.

The pressure variation can be linear or non-linear.

The term "surface of a solid body inside a plasma chamber" preferably denotes the surface of parts inside the chamber, and it denotes especially the walls of the chamber. Parts inside the chamber are, for example, construction material and lines inside the chamber, and pump conducts. In a preferred embodiment, the method of the present invention is related to the removal of deposits inside the tubes of LPCVD apparatus. In view of this preferred embodiment, the invention will be described in detail.

Preferred tubes are those described above which have at least one line to introduce an etching gas (also denoted as "cleaning gas" in the frame of the present invention), and at least one line to connect to a vacuum pump.

Preferably, the lines are separated from another, e.g. the line or lines to introduce the etching gas are located on or close to one end of the tube, and the line or lines to connect to the vacuum pump are located on the other end of the tube.

Often, in the case of PECVD apparatus, the etching gas is transformed to a plasma by means of a coil or plates - which provide the electromagnetic frequency - around the outer wall of the tube. This is especially suitable for "lower" frequencies, e.g. frequencies of equal to or lower than 100 MHz.

According to the preferred embodiment of the invention, the cleaning of LPCVD apparatuses with tubes, the microwave frequency are generated, as mentioned above, by a microwave source with a frequency of equal to or greater than 500 MHz which provides radicals in the cleaning gas. The cleaning gas is then transferred into the tube to be cleaned. The microwave source preferably is a remote microwave source.

Contrary to other cleaning processes, it is not necessary to remove the tube from the LPCVD apparatus.

The at least one inlet for the etching gas is connected to a source of that gas, and the other at least one line is connected with a vacuum pump, or remains connect to it. The vacuum pump is brought into operation, and the line or lines to supply etching gas into the tube is or are opened. Depending on the pump power and the supply of etching gas per time unit, any desirable pressure can be generated inside the tube to be cleaned.

As mentioned above, the pressure is varied during at least a part of the treatment step ; the plasma remains activated during at least a part of the variation of the pressure. To achieve an increase in the pressure, the pump power can be reduced and/or the etching gas supply can be increased. To achieve a decrease in the pressure, the pump power can be increased and/or the etching gas supply can be decreased.

The pressure varies between a minimum pressure and a maximum pressure. Minimum pressure and maximum pressure may depend on the type of tube to be treated, e.g. from the inner diameter and the length of the tube. The inner diameter of the tube is such that wafers with the desired size can be treated, e.g. wafers having a diameter of 200 mm to 400 mm, and wafers having a diameter of up to 500 mm are not uncommon today. The length of the tube may vary, but often, tubes have a length of 2 meters and even more.

Generally, the minimum pressure during the cleaning treatment, especially during the tube cleaning treatment, is equal to or greater than 0.05 mbar, and preferably equal to or greater than 0.1 mbar. Preferably, in some cases, it is equal to or greater than 0.2 mbar. Generally, the maximum pressure during the cleaning treatment, especially during the tube cleaning treatment, is equal to or lower than 100 mbar, preferably, equal to or lower than 30 mbar, and very preferably, it is equal to or lower than 5 mbar. Still more preferably, it is equal to or lower than 3 mbar, most preferably, it is equal to or lower than 2.5 mbar, and especially preferably, it is equal to or lower than 2 mbar.

Preferably, the ratio of the upper level to the lower level of one pressure variation is equal to or greater than 1.5, more preferably, equal to or greater than 2. The ratio may be very high ; it is dependent on the plasma apparatus and of the tools used to produce the lower and upper level of the pressure. Often, the ratio of the upper level to the lower level of one pressure variation is equal to or lower than 50. For example, the pressure may be varied between a minimum pressure of 0.1 mbar to a maximum pressure of 3 mbar, or from 0.2 to 2 mbar.

In a preferred alternative, the initial pressure in the tube to be cleaned is equal to or greater than 1.5 mbar. It is especially preferably 2 mbar. At such a pressure, the cleaning gas removes deposits in the tube on a length from up to 50 cm to up to 1 m from the entry of the cleaning gas. By lowering the pressure, the regions of the tubes with a farther distance from the entry of the cleaning gas may be cleaned. A pressure of equal to or lower than 0.2 mbar is especially suitable here. In another preferred alternative, the initial pressure is comparatively low, e.g. equal to or lower than 0.2 mbar, to clean regions of the tube with a larger distance from the entry of the cleaning gas, and then, the pressure is allowed to rise, e.g. up to 2 mbar to clean the regions more close to the gas entry, especially those regions with a distance between 50 cm and 1 m.

Thus, the pressure variation can be performed such that the pressure is varied from a lower level to a higher level. Alternatively, the pressure can be varied from a higher level to a lower level.

In an especially preferred embodiment, the chamber is a LPCVD chamber, the plasma source is a remote microwave source, and the electromagnetic frequency is equal to or greater than 500 MHz.

It is preferred to start with a higher pressure level and to proceed gradually to a lower level. In this manner, after starting the plasma, the surface close to the etching gas inlet is cleaned. While gradually decreasing the pressure, the plasmatically active etching gas front moves to surfaces farer apart from the etching gas entry. The pressure is then decreased such that all the inner surface is in contact with plasmatically active etching gas.

If desired, the process may be performed such that the pressure varies at least two times from a higher level to a lower level and, in between, at least one time from a lower level to a higher level, or vice versa.

Often, it is sufficient to introduce the etching gas into the tube, to generate a desired pressure, to start the plasma and to vary the pressure once from an upper level to a lower level.

The pressure preferably is regulated by regulating the power of a vacuum pump and/or by regulating the etching gas flow.

The time of treatment depends, for example, on the thickness and nature of the deposits, the desired level of cleaning, the power of the plasma, the gas pressure, the nature of the etching gas. It can be easily determined if the treatment time was sufficient by optically controlling the inner surface of the tubes after cleaning. The duration of one downward variation of the pressure (or, respectively, an upward variation) is very flexible. For example, the time span between the start of the variation of the upper level or lower level, and reaching the lower level or upper level, respectively, can be 1 second to 60 seconds ; if desired, it can even be longer, up to 10 minutes or more. The variation can be effected by operating a vacuum pump to vary the pressure downwards, or by opening a valve to supply etching gas and thus raise the pressure.

The variation of the pressure may be repeated until the desired degree of deposit removal is achieved.

It is possible to clean a 2 m tube over the whole length within one hour according to the process of the present invention.

The etching gas comprises at least one of F₂ and COF₂, or it consists of F₂, COF₂ or their mixture. Such gases and gas mixtures include, but are not limited to F₂, COF₂ ; F₂ or COF₂, further comprising at least one further etching gas selected from the group consisting of NF₃, SF₆, perfluoroalkanes and

perfluoroalkenes, e.g. CF₄, C₂F₆, C₃F₈, C₃F₆, C₄F₈, C₄F₆, hydrofluoroalkanes and hydrofluoroalkenes, e.g. CHF₃, C₂H₂F₄, C₂HF₅, or C₃H₂F₄, any mixtures comprising or consisting two or more thereof and mixtures containing at least one inert gases, e.g. at least one inert gas selected from the group consisting of N₂ and Ar.

COF₂ is preferably applied neat.

F₂ may be applied neat or preferably in mixtures with at least one of N₂ and Ar, ad preferably in mixtures containing F₂, N₂ and Ar.

Molecular fluorine (F₂) is particularly efficient for removal of deposits from surfaces of the tube. Fluorine gas has no global warming potential and may be used with relatively low energy consumption compared for example to conventionally used NF₃ cleaning gas, while efficiently removing the deposits. F₂ often is applied diluted by N₂, Ar or both. In binary mixtures with N₂ or Ar, the content of F₂ is preferably from equal to or greater than 10 % by volume to equal to or lower than 95 % by volume ; N₂ or Ar constitute the balance to 100 % by volume. Often, in ternary mixtures, the content of F₂ is equal to or greater than 10 % by volume and equal to or lower than 30 % by volume, the content of N₂ is equal to or greater than 55 % by volume and equal to or lower than 80 % by volume, and the content of Ar is equal to or greater than 5 % by volume and equal to or lower than 15 % by volume, and the contents of F₂, N₂ and Ar add up to 100 % by volume. In preferred ternary mixtures, the content of F₂ is equal to or greater than 10 % by volume and equal to or lower than 25 % by volume, the content of N₂ is equal to or greater than 60 % by volume and equal to or lower than 80 % by volume, and the content of Ar is equal to or greater than 5 % by volume and equal to or lower than 15 % by volume, and the contents of F₂, N₂ and Ar add up to 100 % by volume. Especially suitable mixtures consist of approximately 20 % by volume of F₂, approximately 70 % by volume of N₂ and approximately 10 % by volume of Ar. Here, the term "approximately" preferably denotes a range of 20±1 % by volume for F₂, 70±1 % by volume for N₂, and 10±1 % by volume for Ar.

The deposits which can be removed by the process of the present invention can be organic deposits or inorganic deposits.

Organic deposits are, for example, formed when molecules with C-F bonds are applied as etching agents, or when organic compounds are applied as precursors of polymeric coatings, e.g. in a plasma assisted process of anisotropic etching. For example, unsaturated hydrofluorocarbon molecules are suitable for this purpose as described in WO 2010/007064. In anisotropic etching, it is desirable to form polymeric coatings in certain areas of items to be treated to protect these coated areas against etching. In this case, or when molecules having C-F bonds are applied as etchant, deposits from fluorinated polymers form not only in the desired ranges of the item to be treated but also on interior surfaces of the plasma chamber and inside the tube. F₂, if desired diluted by N₂ and/or Ar, is highly suitable to remove organic deposits by applying the process of the invention.

The method of the invention is also suitable to remove inorganic deposits inside the tube. Inorganic deposits can be removed if the etching agent forms volatile reaction products with the deposits. For example, Si deposits, Si0₂ deposits or W deposits form gaseous SiF₄ or gaseous WF₆, respectively, with F₂ used as etchant. 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, amorphous,

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

The removal of Si is a major field of applying the process. 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.

In the present invention, molecular fluorine (F₂) is used as a preferred etchant of the etching gas.

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 the method according to the invention, the solid body generally comprises or consists of an electrically conductive material such as for example aluminum, or aluminum alloys in particularly aluminum/magnesium alloys, stainless steel, or of ceramics, e.g. SiC, quartz or AI₂O₃. The tubes of LPCVD, which are preferably treated according to the invention, are made from ceramics, especially from quartz.

The microwave source may be mounted onto a flange which has the same geometry as the front door through which the tube is loaded. The flange which may be fixed to a davit, crane or lifting device, may have weight compensation and can be pressed to tube entrance instead of the front door. The vacuum which is provided serves for a solid connection to the tube.

The invention concerns also a process for the manufacture of a product wherein at least one treatment step for the manufacture of the product is carried out in a treatment chamber and deposits are formed on interior parts of the treatment chamber which process comprises cleaning said interior part of the chamber by the method according to the invention. The tubes are described above.

Typically, the manufacture of the product comprises at least one chemical vapor deposition step, e.g. to form SiON, amorphous Si, microcrystalline and crystalline Si, Si0₂, amorphous, microcrystalline and crystalline Si hydrides, TiN, TaN or W, or a step of forming a polymeric coating, especially a forming a fluorosubstituted polymer, as described above, onto a substrate. Typical products are selected from a semiconductor, a flat panel display and a photovoltaic element such as a solar panel ; preferably, a tube of a CVD apparatus is cleaned, especially the tube of a PLCVD apparatus for the manufacture of photovoltaic elements.

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 : Cleaning of a LPCVD tube with F₂

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 panel substrates mounted on a support ; the support is introduced automatically into the quartz tube of an LPCVD treatment chamber. The tube is suitable to treat 400mm wafers and has a length of 2 m. Coils arranged around the tube are used to generate a plasma inside the tube. A deposition gas is passed into the tube through a line on one end of the tube. Depending upon deposition conditions and concentration of reagents, it is observed that after the LPCVD step, microcrystalline or amorphous Si deposits are present on the inside walls of the tube which may further comprise some hydrogen. After removing the panel substrates and their support from the tube, the line used for the delivery of deposition gas is connected with a line for the delivery of F₂ as etching gas (cleaning gas). The vacuum pump (which is connected to the tube via a line on the end opposite to the etching gas inlet) is activated, F₂ is supplied, the plasma is activated and vacuum pump power and F₂ supply are adjusted such that the pressure inside the tube is approximately 2 mbar. The inner surface of the quartz tube close to the etching gas inlet is cleaned. Slowly, the pressure is reduced to approximately 0.2 mbar. The deposits are now cleaned progressively until the deposits over the whole length of the 2 m tube are removed. In less than one hour, the LPCVD process can be started again. A fine tuning of the temperature program is not necessary.

Example 2 : Plasma cleaning with F₂ mixed with inert gas.

Example 1 is repeated. After removing the panel substrates from the chamber, a gas mixture consisting of molecular fluorine (20 %) and

nitrogen (70 %) and Ar (10 %) is introduced into the chamber, the vacuum pump is started, and the pressure is regulated to be approximately 2 mbar. Once again, plasma is started. The pressure is slowly and progressively regulated to a minimum pressure of 0.2 mbar during which the whole length of the 2 m tube is cleaned.

Example 3 : In situ plasma cleaning with COF₂

Example 1 is repeated using COF₂ as cleaning gas. The COF₂ is preferably applied neat.

Example 4 : Cleaning of a LPCVD tube with F₂

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 panel substrates mounted on a support ; the support is introduced automatically into the quartz tube of an LPCVD treatment chamber. The tube is suitable to treat 400mm wafers and has a length of 2 m. A deposition gas is passed into the tube through a line on one end of the tube. Depending upon deposition conditions and concentration of reagents, it is observed that after the LPCVD step, microcrystalline or amorphous Si deposits are present on the inside walls of the tube which may further comprise some hydrogen. After removing the panel substrates and their support from the tube, a remote microwave source irradiating at 2.45 GHz on a flange fitting to the opening door of the PLCVD tube is arranged to cover the front door of the plasma tube. The microwave source is connected with a line for the delivery of F₂ as etching gas (cleaning gas) which is passed through the microwave source. The vacuum pump (which is connected to the tube via a line on the end opposite to the etching gas inlet) is activated, F₂ is supplied, the microwave plasma is activated and vacuum pump power and F₂ supply are adjusted such that the pressure inside the tube is approximately 2 mbar. The inner surface of the quartz tube close to the etching gas inlet (up to about 50 cm to 1 m) is cleaned by the F radicals produced by the microwave source and delivered into the tube. Slowly, the pressure is reduced to approximately 0.2 mbar. The deposits are now cleaned progressively even in a distance of more than about 1 m from the inlet of the cleaning gas until the deposits over the whole length of the 2 m tube are removed. In less than one hour, the LPCVD process can be started again. A fine tuning of the temperature program is not necessary.

Claims

C L A I M S

1. A method for removing deposits from the surface of a solid body inside a plasma chamber which comprises treating the deposits with an etching gas which method comprises at least one step wherein the etching gas is activated by a plasma and wherein at least partially during performing the at least one treatment step, the pressure inside the plasma chamber is varied, and wherein the etching gas comprises or consists of F₂ or COF₂.

2. The method of claim 1 wherein the etching gas is activated by plasma.

3. The method of claim 1 or 2 for removing deposits from the inner surface of a tube of an LPCVD chamber.

4. The method of claim 3 wherein the LPCVD chamber is used for the manufacture of substrates having a diameter of up to 200 mm, preferably up to 400 mm.

5. The method of claim 3 or 4 wherein the tube has a length of equal to or more than 1 m and equal to or less than 3 m.

6. The method according to claim 2, wherein the pressure is varied downwards from an upper level to a lower level.

7. The method according to anyone of claims 1 to 6, wherein the etching gas consists of neat COF₂.

8. The method according to anyone of claims 1 to 6, wherein the etching gas is a mixture of F₂ and at least one inert gas preferably selected from the group consisting of nitrogen and argon.

9. The method according to claim 8, wherein the etching gas is a mixture of F₂, N₂ and Ar, and wherein the F₂ content of the etching gas is from equal to or more than 10 % by volume to equal to or less than 30 % volume, the N₂ content is from equal to or more than 55 % by volume to equal to or less 80 % by volume, and the Ar content is from equal to or greater than 5 % by volume to equal to or less than 15 % by volume.

10. The method according to anyone of claims 1 to 7, wherein the pressure is from equal to or greater than 0.1 mbar to equal to or lower than 2.5 mbar.

11. The method according to anyone of claims 1 to 10 wherein the chamber is a LPCVD chamber, the plasma source is a remote microwave source, and the electromagnetic frequency is equal to or greater than 500 MHz.

12. The method according to anyone of claims 3 to 11, wherein the tube is part of a treatment chamber for manufacture of photovoltaic elements.

13. The method according to anyone of claims 1 to 12, which further comprises providing F₂ for use in the method by electrolysis of a molten salt electrolyte.

14. The method according to anyone of claims 1 to 13 wherein the plasma source is a remote microwave source.

15. A process for the manufacture of a product, preferably a solar panel, wherein at least one treatment step for the manufacture of the product is carried out in a treatment chamber, wherein organic or inorganic deposits are deposited on interior parts of the treatment chamber, which process comprises cleaning said interior part by the method according to anyone of claims 1 to 14.